Catalyst Characterization Science. Surface and Solid State Chemistry 9780841209374, 9780841211209, 0-8412-0937-5

Table of contents :
Title Page ......Page 1
Half Title Page ......Page 3
Copyright ......Page 4
ACS Symposium Series......Page 5
FOREWORD......Page 6
PdftkEmptyString......Page 0
PREFACE......Page 7
1 Arsenic Poisoning of Hydrodesulfurization Catalysts......Page 8
Experimental......Page 9
Results and Discussions......Page 11
Literature Cited......Page 19
2 A Pilot Plant Reactor-Surface Analysis System for Catalyst Studies......Page 21
Description of Apparatus......Page 22
Study of Methanol Synthesis Catalyst......Page 27
Discussion......Page 30
Literature Cited......Page 31
3 Correlation Between Spectroscopic Measurements and Catalytic Behavior of Selective Oxidation Catalysts......Page 32
In - Situ Raman Experiments......Page 33
Redox Processes and Solid State Transformations in Bismuth Molybdates......Page 34
Catalytic Behavior and Phase Composition of Bismuth-Iron Molybdates......Page 35
Identification of Functionally Discrete Oxide Ions in Bi2MoO6......Page 39
Literature Cited......Page 42
4 Application of Surface Analysis Techniques in the Study of Catalyst Systems......Page 43
Experimental......Page 44
1) Off-Axis Reaction Cell......Page 46
2) Silicon Anode X-Ray Source......Page 48
3) Application of XPS Using Silicon Anode X-Ray Source, Scanning Auger Microprobe, and Reaction Facility in a Copper/Aluminum Catalyst System......Page 52
Literature Cited......Page 62
5 Valence State of Rhenium in Reduced Bimetallic Catalysts With and Without Alkali Metals......Page 63
Experimental......Page 64
Results and Discussion......Page 65
Literature Cited......Page 71
6 Frequency Response Chemisorption Studies of Carbon Monoxide Hydrogenation Catalysts......Page 73
Background......Page 74
Experimental......Page 75
Results and Discussion......Page 76
Literature Cited......Page 84
7 Spectroscopy of Metal-Titanium Dioxide Systems......Page 85
Results......Page 86
Discussion......Page 89
Literature Cited......Page 91
8 Isotopic Tracers in Catalysis: Aromatics from n-Paraffins over Te-NaX Zeolite......Page 93
Results......Page 94
Discussion......Page 97
Acknowledgments......Page 100
Literature Cited......Page 102
9 Titanium Dioxide Single-Crystal and Powder Surfaces in the Presence and Absence of Platinum An Auger Electron Spectroscopic and Electron-Stimulated Desorption Study......Page 103
Results......Page 104
Discussion......Page 109
Literature Cited......Page 113
10 Clusters: Molecular Surfaces......Page 116
Literature Cited......Page 127
11 Iron Fischer-Tropsch Catalysts: Surface Synthesis at High Pressure......Page 129
Experimental......Page 130
Results......Page 132
Literature Cited......Page 137
12 Low-Energy Ion-Scattering Spectroscopy: Applications to Catalysts......Page 138
Qualitative Aspect of LEISS......Page 139
Quantitative Aspects of LEISS......Page 140
Applications to Catalysts......Page 142
Literature Cited......Page 148
13 X-ray Photoelectron and X-ray Absorption Spectroscopic Characterization of Cobalt Catalysts Reduction and Sulfidation Behavior......Page 149
Experimental......Page 150
Results and Discussion......Page 151
Literature Cited......Page 156
14 Studies of the Kinetics and Mechanisms of Ammonia Synthesis and Hydrodesulfurization on Metal Single-Crystal Surfaces......Page 158
Experimental......Page 159
Ammonia Synthesis......Page 160
The Hydrodesulfurization of Thiophene......Page 162
Conclusions......Page 166
Literature Cited......Page 168
15 Oxygen Interactions and Reactions on Palladium(100): Coadsorption Studies with C2H4, H2O, and CH3OH......Page 169
ETHYLENE ADSORPTION AND REACTION......Page 170
REACTIONS OF H2O AND OH......Page 174
METHANOL REACTIONS......Page 176
Discussion......Page 179
Literature Cited......Page 180
16 Surface Reactions on Clean Platinum and Rhodium at Low and High Pressures......Page 181
NO and NH3 Decomposition......Page 183
The NO + CO Reaction on Pt......Page 185
Summary......Page 187
Literature Cited......Page 188
17 Hydrocarbon Synthesis and Rearrangement over Clean and Chemically Modified Surfaces......Page 189
Discussion......Page 190
Kinetics of Structure Insensitive Reactions Over Clean Single Crystal Surfaces......Page 192
Kinetics Over Chemically Modified Single Crystal Surfaces......Page 194
Literature Cited......Page 201
Carbon Monoxide Adsorption on the Sulfur Modified Ni(100) Surface......Page 203
Hydrogen Sulfide Adsorption and Decomposition on the Clean and S-Covered Pt (lll) Surface......Page 204
Sequential Dehydrogenation of Methanethiol on the Pt (lll) Surface......Page 206
Ethylene Adsorption and Decomposition on Pt (lll) Surface......Page 210
Literature Cited......Page 212
19 Selective Epoxidation of Ethylene Catalyzed by Silver Mechanistic Details Revealed by Single-Crystal Studies......Page 214
Experimental......Page 215
Results and Discussion......Page 216
Reaction Kinetics and Mechanism......Page 218
The Role of Chlorine Promoters......Page 220
Mechanism......Page 223
Literature Cited......Page 224
20 Surface Structure and Reaction Dynamics in Catalysis......Page 226
Experimental......Page 227
Results......Page 228
Discussion......Page 236
Conclusions......Page 239
Literature Cited......Page 240
21 Laser-Induced Thermal Desorption with Fourier Transform Mass Spectrometric Detection......Page 242
Laser-Induced Temperature Jumps and Molecular Desorption......Page 243
FTMS and Laser Desorption Results......Page 247
Acknowledgments......Page 253
Literature Cited......Page 255
22 Structure of Bimetallic Catalysts Application of Extended X-ray Absorption Fine Structure Studies......Page 256
Analysis of EXAFS Data......Page 257
Ruthenium-Copper and Osmium-Copper Clusters......Page 258
Rhodium-Copper Clusters......Page 264
Platinum-Iridium Clusters......Page 265
Iridium-Rhodium Clusters......Page 267
Literature Cited......Page 268
23 Surface Chemistry and Catalysis on Some Platinum-Bimetallic Catalysts......Page 270
Results......Page 271
Discussion......Page 280
Literature Cited......Page 281
24 Determination of the Atomic and Electronic Structure of Platinum Catalysts by X-ray Absorption Spectroscopy......Page 283
Experimental Methods......Page 284
EXAFS Data Analysis......Page 285
Analysis of XANES Data......Page 288
Literature Cited......Page 295
25 The Effect of Support-Metal Precursor Interactions on the Surface Composition of Supported Bimetallic Clusters......Page 297
Experimental Procedures......Page 298
Results......Page 299
Discussion......Page 303
Conclusions......Page 306
Literature Cited......Page 307
26 Surface Characterization and Methanation Activity of Catalysts Derived from Binary and Ternary Intermetallics......Page 308
Physico-Chemical Characterization......Page 309
Surface and Bulk Characterization of Binary Alloys (NixSiy and NixThy)......Page 310
Correlation Between Surface Structure and CO Conversion Activity......Page 315
Literature Cited......Page 319
27 Secondary Ion Mass Spectroscopic Studies of Adsorption and Reaction at Metal Surfaces Correlations with Other Surface-Sensitive Techniques......Page 320
Experimental......Page 322
Literature Cited......Page 329
28 Electron Microscopy and Diffraction Techniques for the Study of Small Particles......Page 331
Transmission electron microscopy (TEM)......Page 332
Scanning transmission electron microscopy (STEM)......Page 333
Microanalysis......Page 334
Scanning reflection electron microscopy (SREM)......Page 336
Microdiffraction in a STEM instrument......Page 337
Statistical information from single crystal patterns......Page 339
Literature Cited......Page 341
29 Atomic Imaging of Particle Surfaces......Page 343
Results......Page 344
Geometric Effects......Page 347
Acknowledgments......Page 350
Literature Cited......Page 351
30 Microanalysis of a Copper-Zinc Oxide Methanol Synthesis Catalyst Precursor......Page 353
Results......Page 354
Discussion......Page 358
Literature Cited......Page 362
31 Characterization of Catalysts by Analytical Electron Microscopy......Page 363
Analytical Electron Microscopy Techniques......Page 364
Specimen Preparation......Page 366
Digital Imaging Using X-ray Signals......Page 367
Results and Discussion......Page 368
Acknowledgments......Page 372
Literature Cited......Page 374
32 Multitechnique Characterization of Supported Metals......Page 376
Experimental......Page 377
Results and discussion......Page 378
Conclusion......Page 385
Literature Cited......Page 386
33 Diffraction from Supported Metal Catalysts......Page 387
Results.......Page 389
Acknowledgments......Page 391
Literature Cited......Page 392
34 Vibrational Analysis of Adsorbed Molecules......Page 393
Principles of the Vibrational Analysis......Page 394
Electronic Structure Calculations......Page 398
Comparison Between Theory and Experiment......Page 402
Literature Cited......Page 404
35 IR Spectroscopic Characterization of Adsorbed Species and Processes on Surfaces......Page 405
Experimental Methods......Page 406
Experimental Results......Page 408
Literature Cited......Page 421
36 Computerized IR Studies of Cobalt-Molybdenum-Aluminum Oxide Hydrodesulfurization Catalysts......Page 423
Results......Page 424
Discussion......Page 431
Conclusions......Page 433
Literature Cited......Page 435
37 Fourier Transform IR Studies of Surface Adsorbates and Surface-Mediated Reactions......Page 436
Experimental Section......Page 437
Results and Discussion:......Page 440
Conclusion......Page 448
Literature Cited......Page 449
38 IR Photoacoustic Spectroscopy of Silica and Aluminum Oxide......Page 450
Background......Page 451
Experimental......Page 452
Results......Page 453
Discussion......Page 460
Conclusion......Page 462
Literature Cited......Page 463
39 Carbon Monoxide Oxidation on Platinum: Coverage Dependence of the Product Internal Energy......Page 465
Literature Cited......Page 470
40 The Role of Intercalates in Heterogeneous Catalysis......Page 472
Experimental......Page 475
Results......Page 476
Literature Cited......Page 483
41 Dynamics of Benzene in X-Type Zeolites......Page 485
Results & Discussion......Page 486
Literature Cited......Page 497
42 Magnetic and Mössbauer Characterization of Iron-Zeolite and Iron and/or Ruthenium on Doped-Carbon Catalysts......Page 498
Background Information on the Principles of Magnetism and Mössbauer Spectroscopy......Page 499
The Fe and Fe-Co/Zeolite Systems......Page 504
Fe on Boron-Doped Carbons......Page 507
Fe and Fe2Ru on Carbons Using Fe3(CO)12 and Fe2Ru(CO)12......Page 513
Literature Cited......Page 516
Supplementary Literature Cited......Page 517
Theory......Page 518
Experimental......Page 522
Results/Discussion......Page 523
Summary......Page 528
Literature Cited......Page 532
44 In Situ Spectroscopic Studies of Oxygen Electrocatalysis Involving Transition Metal Macrocycles......Page 534
UV-Visible Reflectance Spectroscopy......Page 536
Mössbauer Effect Spectroscopy (MES)......Page 538
Acknowledgments......Page 544
Literature Cited......Page 548
45 IR Spectroscopy as an In Situ Probe for Molecular Structure in Electrocatalytic and Related Reaction......Page 549
Cell design......Page 550
Information accessible......Page 551
Hydrogen adsorption......Page 552
Electrocatalytic oxidations......Page 555
Bonding and interactions in adsorbed CO layers......Page 558
Molecular Adsorption......Page 561
Literature Cited......Page 563
46 Surface Spectroscopy of Platinum-Cadmium Sulfide-Perfluorosulfonate Polymer Systems......Page 565
Results and Discussion......Page 566
Literature Cited......Page 572
47 New Catalysts and New Electrolytes for Acid Fuel Cells......Page 574
New Catalysts......Page 575
Surface Chemistry of Pt − Ti Alloys......Page 576
Characterization of TiO2 Promoted Pt Catalysts......Page 577
Current Status of New Catalysts......Page 578
Literature Cited......Page 579
48 Carbonaceous Surfaces: Modification, Characterization, and Uses for Electrocatalysis......Page 581
Experimental......Page 582
Results And Discussion......Page 584
Literature Cited......Page 593
Author Index......Page 595
A......Page 597
Β......Page 598
C......Page 599
D......Page 601
Ε......Page 602
G......Page 603
H......Page 604
I......Page 605
M......Page 606
Ρ......Page 608
R......Page 610
S......Page 611
Τ......Page 613
V......Page 614
Ζ......Page 615

Citation preview

Catalyst Characterizatio

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

ACS

SYMPOSIUM

SERIES

288

Catalyst Characterization Science Surface and Solid State Chemistry Marvin L. Deviney, EDITOR Ashland Chemical Company

Joh Exxon Research&Engineering Company

Developed from a symposium sponsored by the Divisions of Petroleum Chemistry, Inc. and Colloid and Surface Chemistry at the 188th Meeting of the American Chemical Society, Philadelphia, Pennsylvania, August 26-31, 1984

American Chemical Society, Washington, D.C. 1985

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Library of Congress Cataloging in Publication Data Catalyst characterization science. (ACS symposium series, ISSN 0097-6156; 288) "Developed from a symposium sponsored by the Divisions of Petroleum Chemistry, Inc. and Colloid and Surface Chemistry at the 188th Meeting of the American Chemical Society, Philadelphia, Pennsylvania, August 26-31, 1984." Includes bibliographies and indexes. 1. Catalysts—Congresses. 2. Catalysis—Congresses. I. Deviney, Marvin L., 1929 L., 1947. III. American Chemica Division of Petroleum Chemistry Chemical Society. Division of Colloid and Surface Chemistry. V. American Chemical Society. Meeting (188th: 1984: Philadelphia, Pa.) VI. Series. QD505.C386 1985 ISBN 0-8412-0937-5

541.3'95

85-20081

Copyright © 1985 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each chapter in this volume indicates the copyright owner's consent that reprographic copies of the chapter may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc., 27 Congress Street, Salem, MA 01970, for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating a new collective work, for resale, or for information storage and retrieval systems. The copying fee for each chapter is indicated in the code at the bottom of the first page of the chapter. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. Registered names, trademarks, etc., used in this publication, even without specific indication thereof, are not to be considered unprotected by law. PRINTED IN THE UNITED STATES O F AMERICA

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

ACS Symposium Series M . Joan Comstock, Series Editor Advisory Robert Baker U.S. Geological Survey Martin L . Gorbaty Exxon Research and Engineering Co.

Robert Ory USDA, Southern Regional Research Center Geoffrey D. Parfitt Carnegie-Mellon University

Roland F. Hirsch U.S. Department of Energy

James C . Randall Phillips Petroleum Company

Herbert D. Kaesz University of California—Los Angeles

Charles N . Satterfield Massachusetts Institute of Technology

Rudolph J. Marcus Office of Naval Research

W. D. Shults Oak Ridge National Laboratory

Vincent D. McGinniss Battelle Columbus Laboratories

Charles S. Tuesday General Motors Research Laboratory

Donald E. Moreland USDA, Agricultural Research Service

Douglas B. Walters National Institute of Environmental Health

W. H . Norton J. T. Baker Chemical Company

C. Grant Willson IBM Research Department

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

FOREWORD The ACS SYMPOSIUM SERIES was founded in 1974 to provide a

medium for publishin format of the Serie IN CHEMISTRY SERIES except that, in order to save time, the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form. Papers are reviewed under the supervision of the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously published papers are not accepted. Both reviews and reports of research are acceptable, because symposia may embrace both types of presentation.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

PREFACE .A.S T H E GROWTH O F INDUSTRIAL heterogeneous catalysis continues to accelerate, the role and contributions of surface scientists and mechanism specialists become increasingly vital. Scientists in these fields are making major efforts to keep pace by developing the fundamental techniques to provide the basic knowledge needed to sustain rapid progress in basic and applied catalysis. Many recen catalytic materials and reactio advances in our understanding of catalytic phenomena. This book highlights a large number of these major new developments in catalyst characterization science, involving both surface and solid state chemistry. We would like to thank the Colloid and Surface Chemistry Division and the Petroleum Chemistry Division of the American Chemical Society for their support and encouragement. Acknowledgment is also made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this activity. The encouragement of Ashland Chemical Company, Exxon Research and Engineering Company, and General Motors Research Laboratory is gratefully acknowledged, and special thanks is expressed to James D . Idol, Andrew Kaldor, and John Larson. The excellent cooperation of the many authors and coauthors of the chapters included in this book is sincerely appreciated. Finally, we would like to thank our families, particularly our wives, Marie Deviney and Wanda Gland, for their patience and continuous support. M A R V I N L. D E V I N E Y

J O H N L. G L A N D

Ashland Chemical Company Columbus, Ohio 43216

Exxon Research & Engineering Company Annandale, New Jersey 08801

xi

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

1 Arsenic Poisoning of Hydrodesulfurization Catalysts Ruth N. Merryfield, Lloyd E. Gardner, and George D. Parks Phillips Petroleum Company, Bartlesville, OK 74004

X-ray photoelectro emission spectroscopy (MES), and activity tests show that arsenic poisons hydrodesulfurization (HDS) catalysts by affecting the chemical nature of the sulfided catalyst. Activity tests show that Co-Mo/ Al2O3 and Mo/Al2O3 catalysts are deactivated when arsenic is added to the catalyst, either as a contaminant from the reactor feed or as As2O5 by laboratory impregnation. XPS shows one form of arsenic, As+5, on the calcined catalyst and two forms, probablyAs+3and Aso, on the sulfided catalyst. XPS also shows sintering of the molybdenum on the sulfided catalyst. We have used MES to study the effect of arsenic on the Co-Mo-S phase (believed to be active for HDS). Arsenic does not destroy this structure, but alters its electronic state. The arsenic appears to be interacting strongly with the cobalt, possibly filling the anion vacancies with atoms or clusters. A r s e n i c p o i s o n i n g o f c a t a l y s t s , p a r t i c u l a r l y r e f o r m i n g and hydrot r e a t i n g c a t a l y s t s , i s a l o n g s t a n d i n g problem. Interest i n shale o i l r e f i n i n g emphasized t h i s problem, as s h a l e o i l s o f t e n c o n t a i n 20-40 ppm a r s e n i c . I n t h i s s t u d y we have used s e v e r a l methods t o c l a r i f y t h e n a t u r e o f a r s e n i c p o i s o n i n g on h y d r o d e s u l f u r i z a t i o n (HDS) c a t a l y s t s . HDS a c t i v i t y t e s t s were used t o determine t h e e x t e n t o f p o i s o n i n g . X-ray p h o t o e l e c t r o n s p e c t r o s c o p y (XPS), X-ray d i f f r a c t i o n (XRD), and Môssbauer e m i s s i o n s p e c t r o s c o p y (MES) have been used t o s t u d y m e t a l s on t h e c a t a l y s t and t o i d e n t i f y s p e c i f i c compounds where p o s s i b l e . C0-M0/AI2O3 c a t a l y s t s have been s t u d i e d e x t e n s i v e l y , b o t h f o r t h e i r s t r u c t u r e and r e a c t i o n mechanisms, and many s t u d i e s have been r e p o r t e d i n t h e l i t e r a t u r e ( 1 - 1 4 ) . However, t h e HDS a c t i v i t y i s not c o m p l e t e l y understood and many c o n f l i c t i n g views have been r e p o r t e d . No attempt i s made here t o e x p l a i n the HDS mechanism,

0097-6156/85/0288-0002$06.00/0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

1.

MERRYFIELD ET AL.

Arsenic Poisoning of Hydrodesulfurization Catalysts

but o n l y t o d e s c r i b e t h e s t a t e o f a r s e n i c on these c a t a l y s t s and i t s e f f e c t on a c t i v i t y . Experimental C a t a l y s t P r e p a r a t i o n . Most samples were p r e p a r e d u s i n g a K e t j e n a l u m i n a (1/16 i n c h e x t r u d a t e w i t h 280 m /g s u r f a c e a r e a and 0.71 ml/g pore volume) and t h e i n c i p i e n t wetness t e c h n i q u e f o r impregna­ tion. C a t a l y s t s were s t i r r e d on a h o t p l a t e u n t i l v i s i b l y d r y , d r i e d o v e r n i g h t i n a 100°C oven, and c a l c i n e d i n a i r a t 500°C f o r t h r e e h o u r s . Ammonium paramolybdate, c o b a l t n i t r a t e , and a r s e n i c p e n t o x i d e s o l u t i o n s were u s e d , w i t h d r y i n g and c a l c i n i n g a f t e r each a d d i t i o n . Molybdenum was always added f i r s t , f o l l o w e d by c o b a l t where a p p l i c a b l e , t h e n t h e a r s e n i c . On c a t a l y s t s w i t h h i g h l o a d i n g s o f a r s e n i c , some a r s e n i c was l o s t d u r i n g c a l c i n a t i o n and s u l f i d i n g . C a t a l y s t compositions a i n T a b l e I . A n o t h e r sample w i t h 9.9% Co and 8.5 % As f o r use as a r e f e r e n c e m a t e r i a l . 2

T a b l e I . C a t a l y s t C o m p o s i t i o n by Χ-Ray F l u o r e s c e n c e

Catalyst Μ0/ΑΙ2Ο3 Μ0/ΑΙ2Ο3 + As C0-M0/AI2O3 C0-M0/AI2O3 + As C0-M0/AI2O3 + As

(Used C a t a l y s t )

(Wt. %)

Mo 8.9 7.9 9.6 8.6

Co

2.4 2.2

3.9

8.4

1.9

3.6

As 3.6

A r s e n i c was added t o an American Cyanamid HDS-2 c a t a l y s t f o r comparison w i t h a used c a t a l y s t c o n t a i n i n g 3.6% A s . T h i s used c a t a l y s t was a l s o an American Cyanamid HDS-2 c a t a l y s t w h i c h had been i n s e r v i c e i n a r e f i n e r y d i s t i l l a t e HDS u n i t f o r about s i x y e a r s . A g a i n , X - r a y f l u o r e s c e n c e determined c o m p o s i t i o n s a r e i n Table I . The same K e t j e n a l u m i n a d e s c r i b e d e a r l i e r was used f o r t h e Môssbauer e x p e r i m e n t s . The samples were prepared i d e n t i c a l l y , w i t h the f o l l o w i n g e x c e p t i o n s . The e x t r u d a t e was ground t o 20-40 mesh b e f o r e i m p r e g n a t i o n , and 0.5 gram samples were prepared u s i n g 2 mCi o f Co->7. The samples were p r e p a r e d t o g i v e 8.9% Mo and 1.2% Co (Co/Mo = 0.21). These samples were n o t a n a l y z e d , b u t t h e a r s e n i c c o m p o s i t i o n s based on t h e p r e p a r a t i o n a r e g i v e n i n T a b l e I V . C a t a l y s t S u l f i d i n g . The c a l c i n e d samples were s u l f i d e d p r i o r t o XPS e x a m i n a t i o n by p u r g i n g t h e sample a t room t e m p e r a t u r e w i t h n i t r o ­ gen, h e a t i n g t o 149°C, then s w i t c h i n g t o 10% H2S i n hydrogen and r a i s i n g t h e temperature g r a d u a l l y o v e r a f o u r hour p e r i o d t o 316°C. A f t e r c o o l i n g i n H2S/H2, t h e sample was f l u s h e d i n n i t r o g e n and p l a c e d i n a g l o v e box. There i t was loaded onto t h e XPS sample h o l d e r and t r a n s p o r t e d t o t h e s p e c t r o m e t e r i n an a i r - t i g h t c a r r i e r . S u l f i d i n g f o r t h e Môssbauer e x p e r i m e n t s was s i m i l a r . A l l c o n d i ­ t i o n s were i d e n t i c a l e x c e p t an 8% H2S/H2 b l e n d was used. Figure

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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CATALYST CHARACTERIZATION SCIENCE

4

1 shows t h e r e a c t o r used f o r c a l c i n i n g and s u l f i d i n g t h e r a d i o a c t i v e samples. The sample was t r a n s f e r r e d t o t h e Môssbauer c e l l w i t h o u t exposure t o a i r and t r a n s p o r t e d t o t h e s p e c t r o m e t e r . P r e s u l f i d i n g f o r t h e a c t i v i t y t e s t s was a c c o m p l i s h e d by f i r s t h e a t i n g t h e c a t a l y s t i n n i t r o g e n a t 204°C. 10% H2S/H2 was i n t r o ­ duced a t t h i s temperature and a l l o w e d t o f l o w over t h e c a t a l y s t f o r f i v e h o u r s . The temperature was then r a i s e d g r a d u a l l y t o 371°C and h e l d f o r an a d d i t i o n a l f i v e h o u r s . The c a t a l y s t was c o o l e d in nitrogen. XPS Measurements. XPS measurements were performed u s i n g a P h y s i c a l E l e c t r o n i c s Model 548 e l e c t r o n s p e c t r o m e t e r w i t h A l k a r a d i a t i o n (1486.6 e V ) . The s p e c t r o m e t e r was i n t e r f a c e d t o a H e w l e t t - P a c k a r d 21ΜΧ computer f o r d a t a a c q u i s i t i o n and m a n i p u l a t i o n . The i n s t r u m e n t was o p e r a t e d a t about 2 χ 10"^ t o r r , w i t h t h e samples b e i n g i n t r o ­ duced i n t o t h e u l t r a h i g 1 χ 10'6 t o r r . Bindin 84.0 eV. A t h i n f i l m o f g o l d was e v a p o r a t e d onto t h e sample a f t e r a complete s e t o f s p e c t r a had been o b t a i n e d , and a n o t h e r s e t o f s p e c t r a was then t a k e n . On t h e s u p p o r t e d samples, b i n d i n g e n e r g i e s were r e f e r e n c e d t o t h e A l 2s peak a t 119.6 eV, as d e t e r m i n e d by g o l d r e f e r e n c i n g . The s u r f a c e c o n c e n t r a t i o n s g i v e n i n T a b l e I I I a r e d e t e r m i n e d r e l a t i v e t o t h e A l 2s peak as 100 u s i n g S c o f i e l d ' s c r o s s s e c t i o n s ( L 5 ) and t h e method o f c a l c u l a t i o n d e s c r i b e d by C a r t e r et a l . ( 1 6 ) . P r e t r e a t m e n t o f t h e samples was performed i n t h e prechamber of t h e s p e c t r o m e t e r , e x c e p t f o r t h e s u l f i d i n g d e s c r i b e d p r e v i o u s l y . C a t a l y s t s were c a l c i n e d i n a i r a t 500°C f o r one h o u r , o r reduced i n hydrogen a t 310°C o r 350°C f o r up t o f o u r h o u r s . The prechamber was then evacuated and t h e sample i n t r o d u c e d i n t o t h e s p e c t r o m e t e r w i t h o u t exposure t o t h e atmosphere. F o r t h e XPS work, r e f e r e n c e m a t e r i a l s were examined t o e s t a b l i s h binding energies f o r the v a r i o u s arsenic o x i d a t i o n s t a t e s . A r s e n i c m e t a l , AS2O3, As2S2 5 AS2S3, a l l from V e n t r o n , and AS2O5 from J . T. Baker Chemicals were used. The a r s e n i c m e t a l powder was imbedded i n i n d i u m f o i l f o r e x a m i n a t i o n . An a r s e n i c m i r r o r formed on a r e ­ a c t i o n f l a s k was a l s o examined. HDS A c t i v i t y Measurements. HDS a c t i v i t y measurements were made i s o t h e r m a l l y i n a 3/4 i n c h i . d . h i g h p r e s s u r e t r i c k l e bed r e a c t o r . C a t a l y s t s were ground t o 20-40 mesh and d i l u t e d w i t h alundum (37.5cc alundum t o 12.5cc c a t a l y s t ) . A f t e r p r e s u l f i d i n g , l i g h t c y c l e o i l (a c r a c k i n g p r o d u c t b o i l i n g between 177°C and 343°C and c o n t a i n i n g 1.7 wt % s u l f u r ) was i n t r o d u c e d a l o n g w i t h hydrogen (7 moles h^/mole f e e d ) . Most o f t h e s u l f u r i n t h e o i l was p r e s e n t as benzothiophenes and d i b e n z o t h i o p h e n e s . The r e a c t i o n was r u n a t 600 p s i g and 4.0 LHSV. A temperature s u r v e y was made from 257°C t o 357°C a t 14°C i n t e r v a l s over a s i x t y hour p e r i o d . Môssbauer, The Môssbauer e m i s s i o n s p e c t r o s c o p y measurements were made u s i n g t h e Co^7 doped c a t a l y s t as a s t a t i o n a r y s o u r c e . The moving a b s o r b e r was F e * e n r i c h e d K4Fe(CN)6·3H2O. Both t h e C o and t h e a b s o r b e r were o b t a i n e d from New England N u c l e a r . The con7

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

5 7

1.

MERRYFIELD ET AL.

Arsenic Poisoning of Hydrodesulfurization Catalysts

s t a n t a c c e l e r a t i o n mode Môssbauer s p e c t r o m e t e r was c a l i b r a t e d u s i n g a source o f C o i n p a l l a d i u m and an a d s o r b e r o f e n r i c h e d Fe57 i r o n foil. The c a t a l y s t samples were loaded i n t o a g l a s s c e l l w i t h a one i n c h d i a m e t e r b e r y l l i u m window. S p e c t r a were accumulated f o r a t l e a s t 24 hours and f i t by computer f o r L o r e n t z i a n c u r v e s u s i n g a l e a s t squares f i t . R e s u l t s and D i s c u s s i o n s 5 7

A c t i v i t y T e s t s . F i g u r e 2 shows r e s u l t s o f a c t i v i t y t e s t s f o r a commercial American Cyanamid HDS-2 c a t a l y s t w h i c h had been i n use f o r about s i x y e a r s . The c a t a l y s t was sampled a t v a r i o u s depths and r e s u l t s f o r t h r e e samples c o n t a i n i n g 0.01% As, 0.6% As, and 3.6% As show a d e c r e a s e i n a c t i v i t y w i t h i n c r e a s i n g a r s e n i c c o n t e n t . A s i m i l a r c a t a l y s t t o w h i c h 3.9% a r s e n i c had been added i n t h e l a b o r a t o r y was t e s t e d an a c t i v i t y of a fresh catalys The a c t i v i t y l o s s o f t h e used c a t a l y s t c o n t a i n i n g 3.6% As c o r r e s p o n d s c l o s e l y w i t h t h a t f o r t h e prepared sample, i n d i c a t i n g t h a t a r s e n i c added by i m p r e g n a t i o n a c t s l i k e t h a t d e p o s i t e d under a c t u a l o p e r a t i n g c o n d i t i o n s . When t h e used c a t a l y s t s were r e g e n e r a t e d i n a i r a t 482°C, t h e a r s e n i c was n o t removed. The molybdenum on a l u m i n a c a t a l y s t was a l s o t e s t e d f o r a c t i v i t y w i t h and w i t h o u t a r s e n i c . A l t h o u g h t h i s c a t l y s t has a much lower i n t r i n s i c a c t i v i t y f o r HDS, t h e r e s u l t s i n F i g u r e 4 show t h a t 3.6% a r s e n i c almost completely d e a c t i v a t e s the c a t a l y s t . The s m a l l amount o f a c t i v i t y r e m a i n i n g i s t h a t e x p e c t e d f o r AI2O3 a l o n e . Thus a r s e n i c a l s o d e a c t i v a t e s c a t a l y s t s w i t h o u t c o b a l t promoters. XPS. S e v e r a l b u l k m a t e r i a l s and one supported sample were examined by XPS t o e s t a b l i s h b i n d i n g e n e r g i e s f o r t h e a r s e n i c . These v a l u e s , g i v e n i n T a b l e I I , c o r r e s p o n d c l o s e l y t o those r e p o r t e d i n t h e l i t e r a t u r e (17-19). The b i n d i n g energy found f o r AS2O5 on a l u m i n a i s comparable t o t h a t found on t h e b u l k AS2O5, i n d i c a t i n g t h a t t h e v a l u e s f o r supported a r s e n i c s h o u l d be s i m i l a r t o t h o s e f o r t h e bulk m a t e r i a l s . Table I I .

XPS B i n d i n g E n e r g i e s

As m e t a l As m i r r o r e d on f l a s k AS2O3 AS2O5

As 3d Be(eV) 41.9 41.7 45.2 45.6

f o r A r s e n i c Reference M a t e r i a l s

As2S2(As4S4)

As 3d Be(eV) 42.4

AS2S3 AS2O5/AI2O3

42.8 45.5

XPS s p e c t r a were o b t a i n e d f o r t h e c a t a l y s t s i n t h e c a l c i n e d , s u l f i d e d , and sometimes i n t h e reduced s t a t e , as d e s c r i b e d b e f o r e . T a b l e I I I g i v e s t h e b i n d i n g e n e r g i e s and r e l a t i v e s u r f a c e concen­ t r a t i o n s f o r t h e M0/AI2O3 and C0-M0/AI2O3 c a t a l y s t s , w i t h and w i t h out a r s e n i c . Data f o r t h e used c a t a l y s t s , w h i c h a r e n o t l i s t e d i n t h e t a b l e , a r e s i m i l a r t o those f o r t h e c a t a l y s t s prepared i n the l a b o r a t o r y .

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

5

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

R e l a t i v e Surface

Concentration,

a

6.04 1.70 3.15

7.15 1.72 4.00

6.99 1.45 3.22

9.93 1.92 4.71

RSC

r a t i o e d t o A l 2S as 100.

398.8 398.7 394.9

398.9 399.3 395.2

399.3 398.1 395.3

399.1 399.0 395.2

Mo3P BE(eV)

a.

5.44 1.50 2.55

4.94 1.24 2.16

a

15.4 15.9 4.7 18.9

45.1 44.4 42.0

45.9 44.8 41.9

RSC

Co-As/AlpO^ ( C o p r e c i p i t a t e d ) Dried 45.6 Presulfided 41.2 Reduced 44.8 40.9

C o - M o / A l ^ + As Calcined Presulfided

Co-Mo/Al?(h Calcined Presulfided

M0/AI2O3 4- As Calcined Presulfided

M0/AI2O3 Calcined Presulfided

As 3d BE(eV)

T a b l e I I I . XPS R e s u l t s on P r e p a r e d Samples

782.3 777.9 781.7 777.6

782.3 782.1 779.1

782.5 782.2 779.1

a

{23.2

17.1 16.2

{1.98

1.96

{2.21

2.29

Co 2P BE(eV) RSC

161.9

162.1

162.2

162.2

162.2

BE(eV)

S 2P a

12.1

10.1

12.5

8.7

11.9

RSC

1218 1226 1218 1226

1218 1218 1225

1217 1218 1224

As Auger KE(eV)

οm

Q m

1

Ν

2

m

χ > >

sο

î

ON

1.

MERRYFIELD ET AL.

Arsenic Poisoning of Hydrodesulfurization Catalysts

REACTOR F i g u r e 1.

Sample r e a c t o r and Môssbauer c e l l .

PERCENT HDS

1

Ο 240

= J

260

1

280

' 300

' 320

—

1

340

360

TEMPERATURE(DEGREES C) F i g u r e 2.

HDS a c t i v i t y o f p o i s o n e d p l a n t

catalysts.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

1

8

CATALYST CHARACTERIZATION SCIENCE

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

1.

MERRYFIELD ET AL.

Arsenic Poisoning of Hydrodesulfurization Catalysts

R e s o l u t i o n o f the Mo 3p peaks showed molybdenum t o be p r e s e n t as Mo 6 f o r t h e c a l c i n e d c a t a l y s t s , and a m i x t u r e o f Mo ^ and Mo ^ f o r s u l f i d e d c a t a l y s t s . The Mo 3p peaks were used s i n c e b o t h a r s e n i c and s u l f u r peaks i n t e r f e r e w i t h t h e Mo 3d d o u b l e t . Although some a u t h o r s have been a b l e t o r e s o l v e a peak a t t r i b u t e d t o Mo ^ on s u l f i d e d c a t a l y s t s ( 8 - 9 ) , we were a b l e t o r e s o l v e o u r peaks u s i n g o n l y two component peaks. Curve r e s o l v i n g c o n s i s t e n t l y showed t h a t about 70% o f t h e molybdenum s i g n a l i n t h e s u l f i d e d c a t a l y s t s was due t o Mo+ . T a b l e I I I shows t h a t b o t h c o b a l t and a r s e n i c a f f e c t t h e Mo/Al r a t i o as measured by XPS. These elements cause t h e Mo/Al r a t i o on b o t h c a l c i n e d and s u l f i d e d c a t a l y s t s t o d e c r e a s e . A g a i n t h i s i s due e i t h e r t o c o v e r i n g o f molybdenum by promoters o r more l i k e l y t o some change i n molybdenum d i s p e r s i o n induced by t h e p r o m o t e r s . I t i s l i k e l y t h a t these m e t a l s d i s p l a c e molybdenum from s u r f a c e s i t e s on t h e a l u m i n a , s a c t i o n o f t h e molybdenu well dispersed. The Mo/Al r a t i o measured by XPS always d e c r e a s e d upon s u l f i d i n g . B u l k a n a l y s i s showed no change i n molybdenum c o n c e n t r a t i o n upon s u l f i d i n g , i n d i c a t i n g t h a t no molybdenum was l o s t d u r i n g s u l f i d i n g . The drop i n i n t e n s i t y can b e s t be e x p l a i n e d by assuming t h a t t h e molybdenum s u l f i d e form i s s i n t e r i n g i n t o l a r g e c l u s t e r s . The model o f K e r k h o f and M o u l i j n (20) was used t o i n t e r p r e t t h i s d a t a . We assumed t h a t a l l o f t h e Mo ^~~on c a l c i n e d and s u l f i d e d c a t a l y s t s was p r e s e n t as a w e l l d i s p e r s e d monolayer. C r y s t a l l i t e s i z e s were c a l c u l a t e d based on t h e change i n Mo/Al r a t i o upon s u l f i d i n g ( t h i s method e l i m i n a t e s e r r o r s due t o i n a c c u r a c i e s i n p h o t o i o n i z a t i o n c r o s s s e c t i o n s ) . C r y s t a l l i t e s i z e s r a n g i n g from 2nm t o 4nm were c a l c u l a t e d f o r t h e s u l f i d e d s t a t e . These s i z e s s h o u l d be viewed w i t h c a u t i o n , due t o u n c e r t a i n t y i n e l e c t r o n mean f r e e paths and d i s p e r s i o n o f the oxide species. Incomplete d i s p e r s i o n o f Mo ^ o r p a r t i a l c o v e r i n g o f t h e M0S2 c r y s t a l l i t e s by promoter atoms would lead t o erroneous r e s u l t s . These c r y s t a l l i t e s i z e s a r e l a r g e r than those proposed by Topsoe e t a l . (21) based on i n f r a r e d and EXAFS d a t a . A l t h o u g h t h e e r r o r s mentioned above may a f f e c t t h e a b s o l u t e s i z e c a l c u l a t e d , i t does n o t a l t e r t h e c o n c l u s i o n t h a t some degree o f s i n t e r i n g i s o c c u r r i n g on s u l f i d i n g . An i n t e r e s t i n g e f f e c t i s n o t i c e d i f one l o o k s a t t h e r e l a t i v e d e c r e a s e i n t h e Mo/Al r a t i o upon s u l f i d i n g f o r each c a t a l y s t . When c o b a l t i s p r e s e n t t h i s r a t i o drops by about 20% a f t e r s u l f i d i n g (independent o f t h e presence o f a r s e n i c ) . Without c o b a l t , t h e r a t i o i s d e c r e a s e d by about 33%. I t t h e r e f o r e appears t h a t c o b a l t h e l p s t o lower t h e amount o f s i n t e r i n g w h i c h o c c u r s upon s u l f i d i n g ( a l t h o u g h i t causes some s i n t e r i n g o f t h e o x i d i c form o f t h e c a t a ­ lyst). S u l f i d i n g has no e f f e c t on XPS Co/Al r a t i o s . The As 3d peak on c a l c i n e d c a t a l y s t s n o r m a l l y appeared as a s i n g l e peak c o r r e s p o n d i n g t o A s ^ ( F i g u r e 5 ) . S u l f i d i n g u s u a l l y gave two peaks, t h e peak a t h i g h e r b i n d i n g energy p r o b a b l y c o r r e s ­ ponding t o A s 3 a r s e n a t e ) which i s t i e d up w i t h t h e support and d i f f i c u l t t o reduce. The peak a t lower b i n d i n g energy c o r r e s ­ ponds c l o s e l y t o t h e v a l u e f o r z e r o v a l e n t a r s e n i c . The A s / A l r a t i o d e c r e a s e s when t h e sample i s s u l f i d e d . I t was found on r e c a l c i n a t i o n o f t h e used c a t a l y s t t h a t t h e A s / A l r a t i o r e t u r n e d t o t h e v a l u e +

+

+

+

4

+

+

+

+

o

r

a

n

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

9

CATALYST C H A R A C T E R I Z A T I O N SCIENCE

10

50.00

45.00

40.00

35.00

Binding Energy (eV)

Figure

5. X P S s p e c t r a

f o r A s o n C0-M0/AI2O3

catalyst.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

1.

MERRYFIELD ET AL.

Arsenic Poisoning of Hydrodesulfurization Catalysts

found on t h e o r i g i n a l c a l c i n e d sample, i m p l y i n g t h a t s i n t e r i n g was r e s p o n s i b l e f o r t h e lower a r s e n i c s i g n a l on t h e s u l f i d e d sample. The coimpregnated sample w i t h h i g h e r l e v e l s o f c o b a l t and a r s e n i c on a l u m i n a was s t u d i e d because t h e c o n c e n t r a t i o n o f t h e m e t a l s was h i g h enough t o g i v e XRD p a t t e r n s . The o n l y phase i d e n ­ t i f i e d i n o t h e r c a t a l y s t s by XRD was gamma a l u m i n a . XRD showed the f o r m a t i o n o f a l l o y s (CoAs and C 0 2 A S ) when t h e c o p r e c i p i t a t e d sample was reduced i n hydrogen. However, on s u l f i d i n g , XRD gave o n l y a CoAs p a t t e r n w i t h broadened and s l i g h t l y d i s p l a c e d peaks, i n d i c a t i n g some d i s t o r t i o n o f t h e c r y s t a l l a t t i c e and a s m a l l e r c r y s t a l l i t e s i z e . The i n t r o d u c t i o n o f s u l f u r a p p a r e n t l y d i s r u p t e d the a l l o y f o r m a t i o n . XPS r e s u l t s f o r t h e c o p r e c i p i t a t e d c a t a l y s t , g i v e n i n T a b l e I I I , show a b i n d i n g energy o f 40.9 eV f o r t h e As 3d peak on t h e hydrogen reduced c a t a l y s t . A s l i g h t l y h i g h e r v a l u e , 41.2 eV, i s found f o r t h e s u l f i d e d sample. Môssbauer. The r a d i o a c t i v were examined by MES a f t e r c a l c i n i n g and a l s o a f t e r p r e s u l f i d i n g , b o t h w i t h and w i t h o u t a r s e n i c . T a b l e IV g i v e s t h e quadrupole s p l i t ­ t i n g , ΔΕ, and t h e isomer s h i f t , δ, measured f o r t h e two s e t s o f d o u b l e t s found i n each spectrum. On t h e s u l f i d e d sample, t h e d o u b l e t w i t h t h e s m a l l e r quadrupole s p l i t t i n g c o r r e s p o n d s t o t h e Co-Mo-S phase d e s c r i b e d by Topsoe e t a l . (11,22,23). The o t h e r d o u b l e t i s assigned t o the c o b a l t i n the alumina. As a r s e n i c i s added, a d e c r e a s e i s seen i n t h e quadrupole s p l i t t i n g f o r t h e Co-Mo-S phase ( F i g u r e 6 ) . A d e c r e a s e i n t h e i n t e n s i t y o f t h e Co:Al2Û3 phase i s a l s o o b s e r v e d as t h e a r s e n i c c o n c e n t r a t i o n i s i n c r e a s e d .

T a b l e I V . Môssbauer Parameters f o r S u l f i d e d C0-M0/AI2O3 C a t a l y s t s . g. As Added on 0.5 g C a t a l y s t 0.000 0.016 0.032 0.048 a. b.

Co-Mo-S D o u b l e t AE _δ_^ 1.09 0.19 1.02 0.21 0.83 0.18 0.79 0.18 a

Co:Al203 D o u b l e t ΔΕ* §} 2.12 0.89 2.22 0.77 2.16 0.84 2.16 0.82

Quadrupole s p l i t t i n g , i n mms"* Isomer s h i f t , i n mms"*

S i n c e t h e s t r u c t u r e o f t h e Co-Mo-S phase i s n o t known, i t i s d i f f i c u l t t o propose v e r i f i a b l e e x p l a n a t i o n s f o r t h e changes i n the Môssbauer spectrum due t o a r s e n i c . However, s e v e r a l g e n e r a l o b s e r v a t i o n s might be made. The Co-Mo-S phase i s n o t d e s t r o y e d by a r s e n i c , i . e . , t h e a r s e n i c i s n o t p r e v e n t i n g t h e f o r m a t i o n o f the phase d u r i n g s u l f i d i n g . The c o b a l t i s n o t c h a n g i n g o x i d a t i o n s t a t e s and t h e geometry o f t h e c o b a l t i s e s s e n t i a l l y t h e same, b u t a change i n e l e c t r i c f i e l d a t t h e c o b a l t n u c l e u s i s i n d i c a t e d . T h i s change c a n be a t t r i b u t e d t o t h e presence o f t h e a r s e n i c , apparently i n t e r a c t i n g s t r o n g l y with the c o b a l t , p o s s i b l y f i l l i n g t h e a n i o n v a c a n c i e s i n t h e Co-Mo-S s t r u c t u r e .

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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catalysts.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

1.

M E R R Y F I E L D ET A L .

Arsenic Poisoning of Hydrodesulfurization Catalysts 13

Conclusions HDS a c t i v i t y t e s t s show c o n c l u s i v e l y t h a t a r s e n i c p o i s o n s the Co-Mo/ AI2O3 and the M0/AI2O3 c a t a l y s t s . XPS b i n d i n g e n e r g i e s i n d i c a t e a z e r o v a l e n t form o f a r s e n i c p r e s e n t on the s u l f i d e d c a t a l y s t s (as w e l l as an A s 3 f o r m ) . When s u l f i d e d samples a r e r e c a l c i n e d , a l l o f the a r s e n i c r e t u r n s t o the o x i d i z e d s t a t e . Môssbauer spec­ troscopy i n d i c a t e s that the a r s e n i c i s i n t e r a c t i n g s t r o n g l y w i t h the c o b a l t and t h a t w h i l e the Co-Mo-S phase s t i l l e x i s t s , i t s e l e c t r o n i c s t r u c t u r e has been a l t e r e d by the a r s e n i c . These d a t a i n d i c a t e that the a r s e n i c i s a l t e r i n g the e l e c t r o n i c s t r u c t u r e of the a c t i v e s i t e s , perhaps by o c c u p y i n g a n i o n v a c a n c i e s w i t h a r s e n i c atoms o r c l u s t e r s . S i m i l a r a n i o n v a c a n c i e s have been proposed by V a l y o n and H a l l (13) f o r t h e unpromoted molybdenum on a l u m i n a c a t a ­ lyst. These v a c a n c i e s c o u l d be b l o c k e d by a r s e n i c j u s t l i k e those i n the promoted c a t a l y s t +

Acknowledgments The a u t h o r s w i s h t o acknowledge the f o l l o w i n g i n d i v i d u a l s f o r t h e i r a s s i s t a n c e i n t h i s s t u d y : B i l l L e r o y f o r c a r r y i n g out the Môssbauer s t u d i e s , Ed Farmer f o r o b t a i n i n g the XPS s p e c t r a , Mike B r i g g s and Bobby Dodd f o r c o n d u c t i n g the HDS a c t i v i t y t e s t s , and M a r v i n J o h n s o n , Gary Nowack and P e t e r Gray f o r h e l p f u l d i s c u s s i o n s . Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Lipsch, J . M. J . G; Schuit, G. C. Α.; J . Catal., 1969, 15, 163. Friedman, R. M.; Declerck-Grimee, R. I.; F r i p i a t , J . J . ; J . Electron Spectrosc. Relat. Phenom., 1974, 5, 437. Brinen, J . S.; Armstrong, W. D., J . Catal., 1978, 54, 57. P h i l l i p s , R. W.; Fote, Α. Α.; J . Catal., 1976, 41, 168. Walton, R. Α., J . Catal., 1976, 44, 335. Delvaux, G.; Grange, P.; Delmon, B.; J . Catal., 1979, 56, 99. Gajardo, P.; Mathieux, Α.; Grange, P.; Delmon, B.; Appl. Catal., 1982, 3, 347. Patterson, Τ. Α.; Carver, J . C.; Leyden, D. E.; Hercules, D. M.; J . Phys. Chem., 1976, 80(15), 1700. Okamoto, Y.; Shimokawa, T.; Imanaka, T.; Teranishi, S.; J . Catal., 1979, 57, 153. Gates, B. C.; Katzer, J . R,; Schuit, G. C. Α.; "Chem. of C a t a l y t i c Processes," McGraw-Hill, Inc., New York, 1979. Topsoe, H.; Clausen, B. S.; Candia, R.; Wivel, C.; Morup, S.; J . Catal., 1981, 68, 433. Chung, K. S.; Massoth, F. E.; J . Catal., 1980, 64, 332. Valyon, J . ; H a l l , W. K.; J . Catal., 1983, 84, 216. Massoth, F. E.; Muralidhar, G.; Shabtai, J . ; J . Catal., 1984, 85, 53. S c o f i e l d , J . H., J . Electron Spectrosc. Relat. Phenom., 1976, 8, 129. Carter, W. J . ; Schweitzer, G. K.; Carlson, Τ. Α.; J . Electron Spectrosc. Relat. Phenom., 1974, 5, 827. Bahl, M. K.; Woodall, R. O.; Watson, R. L.; I r g o l i c , K. J . ; J . Chem. Phys.; 1976, 64(3), 1210.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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18. 19. 20. 21.

22. 23.

Taylor, J . Α.; J . Vac. S c i . Technol.; 1982, 20(3), 751. Brundle, C. R.; and Seybold, D.; J . Vac. S c i . Technology, 1979, 16(5), 1186. Kerkhof, F. P. J . M.; Moulijn, J . Α.; J . Phys. Chem., 1979, 83(12), 1612. Candia, R.; Clausen, B. S.; Bartholdy, J . ; Topsoe, Ν. Y.; Lengeler, B.; Topsoe, H.; Proc. 8th Int. Congr. Catal., 1984, Vol. I I , p. 375. Wivel, C.; Candia, R.; Clausen, B.; Morup, S.; Topsoe, H,; J . Catal. 1981, 68, 453. Breysse, M.; Bennett, Β. Α.; Chadwick, D.; Vrinat, M.; B u l l Soc. Chim. Belg., 1981, 90, 1271.

R E C E I V E D December 6, 1984

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2 A Pilot Plant Reactor-Surface Analysis System for Catalyst Studies T. H. Fleisch Amoco Research Center, Standard Oil Company (Indiana), Naperville, IL 60566

A system has bee combined studie catalyst surface properties. Key elements of the system are a computer-controlled pilot plant with a plug flow reactor coupled in series to a minireactor which is connected, via a high vacuum sample transfer system, to a surface analysis instrument equipped with XPS, AES, SAM, and SIMS. When interesting kinetic data are observed, the reaction i s stopped and the test sample is transferred from the minireactor to the surface analysis chamber. Unique features and problem areas of this new approach will be discussed. The power of the system w i l l be illustrated with a study of surface chemical changes of a CuO/ZnO/Al2O3 catalyst during activation and methanol synthesis. Metallic Cu was identified by XPS as the only Cu surface site during methanol synthesis.

The development o f modern s u r f a c e c h a r a c t e r i z a t i o n t e c h n i q u e s has p r o v i d e d means t o s t u d y t h e r e l a t i o n s h i p between t h e c h e m i c a l a c t i v i t y and t h e p h y s i c a l o r s t r u c t u r a l p r o p e r t i e s o f a c a t a l y s t s u r f a c e . E x p e r i m e n t a l work t o u n d e r s t a n d t h i s r e a c t i v i t y / s t r u c t u r e r e l a t i o n s h i p has been o f two t y p e s : f u n d a m e n t a l s t u d i e s on model c a t a l y s t systems (1,2) and postmortem a n a l y s e s o f c a t a l y s t s w h i c h have been removed from r e a c t o r s ( 3 , 4 ) . Experimental a p p a r a t u s f o r t h e s e s t u d i e s have i n v o l v e d s m a l l volume r e a c t o r s mounted w i t h i n (1) o r appended t o (5) vacuum chambers c o n t a i n i n g analysis instrumentation. A l t e r n a t e l y , c a t a l y s t samples have been removed f r o m remote r e a c t o r s v i a t r a n s f e r a b l e sample mounts (6) o r an i n e r t gas g l o v e box ( 3 , 4 ) . Very l i t t l e r e s e a r c h has attempted t o r e l a t e r e a c t i o n k i n e t i c s t o c a t a l y s t s u r f a c e p r o p e r t i e s a s a f u n c t i o n o f time under a c t u a l r e a c t o r o p e r a t i n g c o n d i t i o n s . The g o a l o f t h e 0097-6156/85/0288-0015$06.00/0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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p r e s e n t d e s i g n was t o c o u p l e a p i l o t p l a n t r e a c t o r t o a s u r f a c e a n a l y s i s i n s t r u m e n t i n a way w h i c h would a l l o w t h e r e a c t i o n k i n e t i c s o f a l a r g e c a t a l y s t volume t o be r e l a t e d t o c a t a l y s t s u r f a c e c h e m i c a l p r o p e r t i e s a s determined by X-ray P h o t o e l e c t r o n Spectroscopy (XPS o r ESCA) and b o t h S t a t i c and Scanning Auger M i c r o s c o p y (AES/SAM). D e s c r i p t i o n o f Apparatus G e n e r a l D e s c r i p t i o n . The major subsystems o f t h e p i l o t p l a n t / s u r f a c e a n a l y s i s system a r e a c o m p u t e r - c o n t r o l l e d p i l o t p l a n t r e a c t o r and a sample i n t r o d u c t i o n t r a n s f e r system m o d i f i e d to i n c l u d e a m i n i r e a c t o r w h i c h a t t a c h e s t o a P e r k i n - E l m e r PHI Model 550 ESCA/SAM i n s t r u m e n t ( F i g u r e 1 ) . F o r a g i v e n e x p e r i m e n t , a p p r o x i m a t e l y 50 grams o f t h e c a t a l y s t o f i n t e r e s t a r e l o a d e d i n t o a plug flow reacto the same c a t a l y s t ( a p p r o x i m a t e l sample h o l d e r i s i n s e r t e syste c h a r a c t e r i z a t i o n . The sample h o l d e r w i t h c a t a l y s t d i s c i s then t r a n s f e r r e d under h i g h vacuum c o n d i t i o n s from t h e i n t r o d u c t i o n system i n t o t h e m i n i r e a c t o r . A m e t a l / m e t a l s e a l s e p a r a t e s t h e h i g h p r e s s u r e r e a c t o r s i d e from t h e h i g h vacuum sample t r a n s f e r s i d e . A f t e r t h e p l u g f l o w r e a c t o r , m i n i r e a c t o r , and c o n n e c t i n g t u b i n g a r e a t o p e r a t i n g t e m p e r a t u r e , r e a c t a n t gas f l o w i s s t a r t e d . R e a c t i o n p r o d u c t s a r e m o n i t o r e d by a gas chromatograph ( F i g u r e 1 ) . When an i n t e r e s t i n g k i n e t i c b e h a v i o r i s o b s e r v e d , the r e a c t a n t f l o w i s stopped and t h e c a t a l y s t d i s c i s t r a n s f e r r e d back i n t o t h e ESCA/SAM system f o r s u r f a c e c h a r a c t e r i z a t i o n . The c y c l e o f exposure t o r e a c t i o n c o n d i t i o n s f o l l o w e d by s u r f a c e c h a r a c t e r i z a t i o n i s c o n t i n u e d u n t i l t h e experiment i s completed. P i l o t P l a n t . The p i l o t p l a n t c o n s i s t s o f two gas i n l e t systems, a p l u g f l o w r e a c t o r , and a gas chromatograph. The p l u g f l o w r e a c t o r i s f a b r i c a t e d from an 86 cm s t a i n l e s s s t e e l tube (3.2 cm O.D. and 2.3 cm I.D.). An A n a l o g D e v i c e s MACSYM 2 computer c o n t r o l s p r e s s u r e , f l o w r a t e , and p i l o t p l a n t r e a c t o r and m i n i r e a c t o r t e m p e r a t u r e s . T h i s computer c o n t r o l i n c o n j u n c ­ t i o n w i t h a number o f a l a r m s t h a t t r i g g e r r e a c t i o n shutdown p e r m i t s u n a t t e n d e d o p e r a t i o n . Gas chromatographic a n a l y s i s o f r e a c t i o n p r o d u c t s i s automated w i t h a gas sampling v a l v e and an HP 5880A l e v e l f o u r t e r m i n a l . Sample I n t r o d u c t i o n and T r a n s f e r System. The sample i n t r o d u c t i o n and sample t r a n s f e r system i s a lengthened v e r s i o n o f t h e PHI Model 15-720B i n t r o d u c t i o n system w h i c h c o n s i s t s o f a polymer b e l l o w s - c o v e r e d h e a t i n g and c o o l i n g p r o b e , a t r a n s f e r a b l e sample h o l d e r , an e i g h t - p o r t d u a l - a x i s c r o s s , and the m i n i r e a c t o r i n t e r f a c e p o r t and t r a n s f e r probe ( F i g u r e 2 ) . There i s a l s o a t r a n s f e r v e s s e l p o r t w i t h t h e n e c e s s a r y t r a n s f e r probe f o r i n t r o d u c t i o n o f a i r s e n s i t i v e samples. They a r e n o t p a r t o f t h e r e a c t o r / s u r f a c e a n a l y s i s system. The d u a l c r o s s and a t t a c h e d hardware a r e supported by t h e probe d r i v e mechanism w h i c h f l o a t s on a b l o c k d r i v e n v e r t i c a l l y and t r a n s v e r s e l y by two m i c r o m e t e r s . These m i c r o m e t e r s p l u s t h e probe d r i v e mechanism a l l o w X-Y-Z

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

FLEISCH

Pilot Plant Reactor-Surface Analysis System

ESCA/SAM system Sample transfer Gas

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Minireactor

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Computer

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Minireactor and interface chamber

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Analysis chamber

Transfer probe (for minireactor)

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F i g u r e 2. Sample i n t r o d u c t i o n and sample t r a n s f e r system. (Reproduced w i t h p e r m i s s i o n from R e f . 7. C o p y r i g h t 1984, Academic P r e s s . )

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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sample p o s i t i o n i n g i n t h e a n a l y s i s chamber. Pumping f o r the i n t r o d u c t i o n system i s p r o v i d e d by a B a l z e r s Model TSUI10 t u r b o m o l e c u l a r pump backed by a m e c h a n i c a l pump. T h i s l a t t e r pump a l s o p r o v i d e s d i f f e r e n t i a l pumping f o r f o u r s e t s o f T e f l o n sliding seals. F i g u r e 3 shows an e x p l o d e d v i e w o f the t r a n s f e r a b l e sample h o l d e r on w h i c h a 1.2 cm d i a m e t e r p r e s s e d c a t a l y s t d i s c i s mounted. W i t h the sample h o l d e r engaged i n the h o t / c o l d probe and t h e d u a l c r o s s e v a c u a t e d , t h e h o l d e r t e m p e r a t u r e can be v a r i e d from -170°C t o +650°C. When t r a n s f e r r i n g the sample h o l d e r from the h o t / c o l d p r o b e , a s p l i t p i n on t h e t r a n s f e r probe i s p r e s s e d i n t o t h e h o l e i n t h e s i d e o f t h e sample h o l d e r . The h o t / c o l d probe i s t h e n r e t r a c t e d t o d i s e n g a g e the h o l d e r and p r o v i d e c l e a r a n c e t o move the h o l d e r i n t o the i n t e r f a c e chamber. By t r a n s f e r r i n g the sample from the h o t / c o l d probe i n t o a r e a c t o r r a t h e r than r e a c t i n p r o b e , the sample i n t r o d u c t i o Day-to-day a n a l y t i c a l wor performe g r e a c t i o n experiments are i n progress. R e a c t i o n I n t e r f a c e and M i n i r e a c t o r . The r e a c t i o n i n t e r f a c e i s a f i v e p o r t s t a i n l e s s s t e e l vacuum chamber w i t h t h e m i n i r e a c t o r and sample d r i v e mechanism mounted on the upper and l o w e r 11.4 cm d i a m e t e r f l a n g e s , r e s p e c t i v e l y ( F i g u r e s 4a,4b). At r i g h t a n g l e s t o t h e v e r t i c a l a x i s a r e 7 cm d i a m e t e r p o r t s f o r sample a c c e s s , a v i e w p o r t , and a pumping p o r t p l u s p r e s s u r e r e l i e f v a l v e . A g a t e v a l v e s e p a r a t e s the r e a c t i o n i n t e r f a c e chamber from the d u a l c r o s s . I n the i n t e r f a c e chamber the sample h o l d e r i s t r a n s f e r r e d from the t r a n s f e r probe i n t o a s t a i n l e s s s t e e l cup as shown i n F i g u r e 4a· V e r t i c a l m o t i o n o f the sample h o l d e r i n t o the m i n i r e a c t o r i s p r o v i d e d by a b e l l o w s - s e a l e d p l a t e a t t a c h e d t o a s c r e w - d r i v e n frame. A t o r q u e wrench i s used t o a p p l y a s e a l i n g f o r c e o f 300 pounds t o a S i e r r a c i n - H a r r i s o n type 24105 g o l d - c o a t e d m i n i - s e a l w h i c h s e a l s t h e h i g h p r e s s u r e m i n i r e a c t o r from the h i g h vacuum i n t e r f a c e chamber ( F i g u r e 4 b ) . H e l i u m p r e s s u r e t e s t i n g u s i n g a UTI 100C q u a d r u p o l e mass s p e c t r o m e t e r showed no d e t e c t a b l e h e l i u m l e a k a g e a t 100 p s i p r e s s u r e and a r e a c t o r t e m p e r a t u r e o f 600°C. The m i n i r e a c t o r i t s e l f i s a s t a i n l e s s s t e e l c y l i n d e r w i t h a 5.6 cm o u t s i d e d i a m e t e r and 2.24 cm i n s i d e d i a m e t e r w h i c h i s t h e r m a l l y i s o l a t e d from i t s 11.4 cm mounting f l a n g e by a t h i n - w a l l e d c o n i c a l s e c t i o n ( F i g u r e 4)· A gas i n l e t and o u t l e t p o r t i s s i t u a t e d d i r e c t l y above the sample. S i x 150 w a t t c a r t r i d g e h e a t e r s i n the m i n i r e a c t o r w a l l p r o v i d e adequate power t o h e a t the r e a c t o r f r o m room t e m p e r a t u r e t o 600°C i n 20 m i n u t e s . The m i n i r e a c t o r t e m p e r a t u r e i s c o n t r o l l e d by the MACSYM 2 computer u s i n g e i t h e r o f two t h e r m o c o u p l e s , one embedded i n t h e r e a c t o r w a l l o r one i n p r e s s u r e c o n t a c t w i t h a l e d g e machined i n t o the sample h o l d e r ( F i g u r e s 3,4). In steady s t a t e o p e r a t i o n , t e m p e r a t u r e r e a d i n g s f r o m the two thermocouples a g r e e w i t h i n 2°C. Operating temperature s t a b i l i t y i s w i t h i n ±1°C and the t e m p e r a t u r e d i f f e r e n c e between p i l o t p l a n t r e a c t o r and m i n i r e a c t o r i s a l w a y s l e s s than 2 C. e

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Pilot Plant Reactor-Surface Analysis System

transfer pin

F i g u r e 3. E x p l o d e d v i e w o f t h e sample h o l d e r and c a t a l y s t d i (Reproduced w i t h p e r m i s s i o n from Réf. 7. Copyright 198U, Academic P r e s s . )

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

FLEISCH

Pilot Plant Reactor-Surface Analysis System

Study of M e t h a n o l S y n t h e s i s C a t a l y s t A s t u d y o f a methanol s y n t h e s i s c a t a l y s t i s used t o demonstrate the u s e f u l n e s s o f the new p i l o t p l a n t / m i n i r e a c t o r / s u r f a c e a n a l y s i s system. T h i s work i s d e s c r i b e d i n more d e t a i l e l s e w h e r e (7)· I n d u s t r i a l methanol s y n t h e s i s c a t a l y s t s a r e based on CuO/ZnO/Al 0 o r CuO/ZnO/Cr 0 c o m p o s i t i o n s . R. G. Herman e t a l . (8) s t u d i e d these c a t a l y s t systems i n g r e a t d e t a i l and suggested a Cu+1 s o l u t i o n i n ZnO as a c t i v e phase where Cu+1 n o n - d i s s o c i a t i v e l y chemisorbs and a c t i v a t e s CO and ZnO a c t i v a t e s H . I n the range of 15 t o 85% CuO i n the c a t a l y s t , up t o 16% Cu+1 became d i s s o l v e d i n the ZnO (9) and Cu+1 has been w i d e l y a c c e p t e d as a c t i v e s i t e ( 1 0 ) . Recently, however, Raney Cu-Zn c a t a l y s t s have been shown t o be v e r y a c t i v e methanol s y n t h e s i s c a t a l y s t s ( 1 1 ) . The a c t i v e component f o r t h e s e Raney c a t a l y s t a c t i v i t y maximum a t 9 The c a t a l y s t i n t h i s study was a commercial c a t a l y s t (C18HC, U n i t e d C a t a l y s t , Inc.) w i t h 42% CuO, 47% ZnO and 10% A 1 0 . The f e e d gas c o n s i s t e d of 73% H , 25% CO and 2% C 0 . The c h e m i c a l s t a t e o f Cu was s t u d i e d by XPS ( A l Κα e x c i t a t i o n , hv«1486.6eV) u s i n g the Cu 2p and Cu (Ι^Μι^Μι^) Auger l i n e . With those two l i n e s Cu+2, Cu+1 and Cu can e a s i l y be d i s t i n g u i s h e d employing a s o - c a l l e d c h e m i c a l s t a t e p l o t (CSP) w i t h the Cu 2 p , b i n d i n g energy on the a b s c i s s a ( d e c r e a s i n g from l e f t t o r i g h t ) and the Cu (Ι^Μι^Μι^) k i n e t i c energy on the o r d i n a t e as shown i n F i g u r e 5 (13,14). The p o s i t i o n of Cu o f the u n t r e a t e d c a t a l y s t i n the CSP ( p o i n t 1) c l e a r l y i d e n t i f i e s i t as CuO. After s y n t h e s i s gas c o n v e r s i o n a t 250°C o n l y m e t a l l i c Cu i s seen on the c a t a l y s t s u r f a c e ( p o i n t 3 ) . A t 100°C, Cu+2 becomes reduced t o Cu+1 but no methanol f o r m a t i o n i s o b s e r v e d . Z i n c o x i d e does not become reduced d u r i n g methanol s y n t h e s i s . The s m a l l changes i n the CSP ( F i g u r e 5) a r e due t o d r y i n g . Thus, the w o r k i n g c a t a l y s t s u r f a c e i s suggested t o c o n s i s t o f m e t a l l i c Cu, ZnO, and A 1 0 , S p e c i a l a t t e n t i o n was p a i d t o the d e t e c t i o n o f r e s i d u a l Cu+1 q u a n t i t i e s accompanying the m e t a l l i c Cu. The r e l a t i v e amounts of Cu+1 and Cu were determined by c u r v e - f i t t i n g the Cu (LMM) s p e c t r a u s i n g the P h y s i c a l E l e c t r o n i c s V e r s i o n 6 c u r v e - f i t t i n g program. The c a t a l y s t showed r e d u c t i o n of Cu+2 i n t o a m i x t u r e of Cu+1 and Cu a f t e r r e d u c t i o n i n H a t 250°C f o r one hour ( F i g u r e 6) as e v i d e n c e d by the two r e s o l v e d peaks i n the Cu (LMM) spectrum a t 568.0 and 570.3 eV w h i c h a r e c h a r a c t e r ­ i s t i c o f Cu and Cu+1, r e s p e c t i v e l y , and by the d i s a p p e a r a n c e of the Cu+2 2p s a t e l l i t e s t r u c t u r e . I t c o u l d be shown t h a t l e s s t h a n 2%, i f any, o f the t o t a l Cu c o u l d be p r e s e n t i n the +1 o x i d a t i o n s t a t e d u r i n g methanol f o r m a t i o n . However, when the c a t a l y s t was b r i e f l y exposed t o a i r (1 m i n u t e ) , a few p e r c e n t o f Cu+1 r e a d i l y formed ( 7 ) . Thus, any k i n d of o x i d a t i o n environment has t o be a v o i d e d between methanol s y n t h e s i s and c a t a l y s t a n a l y s i s . O t h e r w i s e , a p p r e c i a b l e amounts o f Cu+1 w i l l be d e t e c t e d . 2

3

2

3

2

2

3

2

2

3

2

3

2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CO

c

0)

0)

ο c

>

1

as received

/

': /

;

A Β C

Cu

2

3

r.u + 2 J

/ 11·/

/ Cu

•/Cu + K

2

3

/\

1

935 934 933 932 931

/

Cu

CU2Û

CuO

2p /2 binding energy, eV

938 937 936

913

914

915

916

917

α> c α> 919 ο c 918

J?

2

921

922

> E> 920

>

923

F i g u r e 5. C h e m i c a l s t a t e changes o f Cu and Zn i n a commercial C u 0 / Z n 0 / A l 0 c a t a l y s t i n f e e d gas (H /C0/C0 = 73/25/2, 2 atm).

2p3/2 binding energy, eV

1026 1025 1024 10231022 1021 1020 1019

2 100°C, 1 hr 3 250°C, 10 hr

η m η m

C/3

δ

Ν

2

m

5

Η η as >

C/3

π

2.

FLEISCH

Pilot Plant Reactor-Surface Analysis System

23

F i g u r e 6. Changes i n Cu 2p and C u i l ^ M ^ M ^ ) e l e c t r o n s p e c t r a upon i n s i t u reduction i n H . (Reproduced w i t h p e r m i s s i o n from Ref. 7. C o p y r i g h t 1984, Academic P r e s s . ) 2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CATALYST C H A R A C T E R I Z A T I O N SCIENCE

24 Discussion

Two major q u e s t i o n s r e g a r d i n g t h e a p p l i c a b i l i t y o f t h i s system remain t o be d i s c u s s e d . F i r s t , how r e p r e s e n t a t i v e i s t h e c a t a l y s t sample i n t h e m i n i r e a c t o r o f t h e c a t a l y s t i n t h e p i l o t p l a n t ? And second, how does t h e i n t e r r u p t i o n o f t h e r e a c t i o n i n f l u e n c e k i n e t i c behavior? The f i r s t q u e s t i o n can be answered e a s i l y . A t l o w c o n v e r s i o n and i s o t h e r m a l c o n d i t i o n s no s i g n i f i c a n t d i f f e r e n c e s i n t h e c a t a l y s t c o m p o s i t i o n as f u n c t i o n o f c a t a l y s t bed p o s i t i o n a r e u s u a l l y o b s e r v e d . Thus, t h e s u r f a c e a n a l y s i s d a t a from t h e c a t a l y s t i n t h e m i n i r e a c t o r s h o u l d be r e p r e s e n t a t i v e n o t o n l y o f the bottom s e c t i o n b u t o f a l l t h e c a t a l y s t i n t h e p l u g f l o w r e a c t o r . I n t h e case o f t h e methanol f o r m a t i o n s t u d i e s r e p o r t e d i n t h i s p a p e r , c o n v e r s i o n was low (,(A), ^ ( E ) , v^if^), i ^ ( F ) . A l l of t h e s e v i b r a t i o n s are Raman a c t i v e , w h i l e o n l y the t r i p l y degenerate modes are IR a c t i v e . By comparison w i t h o t h e r molybdates, i t has 2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

34

CATALYST C H A R A C T E R I Z A T I O N SCIENCE

been determined t h a t a l l of the h i g h frequency bands (around 800 cm ) are a s s o c i a t e d w i t h the s t r e t c h i n g modes of molybdenum-oxygen p o l y h e d r a ( v^(A) and ^ ( F ^ ) ) . The bending modes (^oW and v,{?^)) are observed between 300 and 400_^cm · I n a d d i t i o n , Bi^MoO^ d i s p l a y s a weak IR band near 600 cm w h i c h i s a s s i g n e d t o a Mo-0B i mode* As shown i n T a b l e V, r e d u c t i o n of Bi^MoO^ w i t h butene f o l l o w e d by r e o x i d a t i o n w i t h oxygen-18 produces no immediate s h j f t s i n the t h r e e h i g h frequency Raman bands at 853, 798 and 720 cm which are a s s i g n e d t o Mo-0 s t r e t c h i n g modes i n the s o l i d * In contrast to t h i s , s h i f t s are observed i n the h i g h frequency bands when the c a t a l y s t i s reduced w i t h propylene o r methanol and r e o x i d i z e d w i t h oxygen-18. Therefore, exchange of l a t t i c e oxygen about the molybdenum-oxygen p o l y h e d r a occurs only a f t e r o x i d a t i o n of propylene and methanol t o a c r o l e i n and f o r m a l d e h y d e , r e s p e c t i v e l y , but not a f t e r buten oxidatio butadiene Thi clearl s u b s t a n t i a t e s our e a r l i e associated with bismut a b s t r a c t i n g f u n c t i o n i n bismuth molybdate w h i l e oxygens a s s o c i a t e d w i t h molybdenum i n s e r t i n t o the a l l y l i c i n t e r m e d i a t e d u r i n g s e l e c t i v e propylene o x i d a t i o n t o a c r o l e i n .

T a b l e V.

Raman S p e c t r a of P a r t i a l l y Reduced

Bi MoO 2

(

18 Reoxidized with

0-Oxygen

M a j o r Band P o s i t i o n s (cm *) a f t e r :

Reductant

First I n i t i a l Cycle

Second T h i r d Cycle Cycle

Fourth Cycle

Fifth Cycle

Sixth Cycle

Propylene**

844 803 725

835 792 715

832 792 715

830 790 713

830 790 712

830 786 710

830 785 708

Butene**

844 803 725

841 801 724

841 802 724

841 802 725

840 798 722

840 798 722

840 797 723

Methanol

844 803 725

839 803 721

839 801 720

838 802 719

836 799 717

832 787 709

831 787 709

a)

Degree of r e d u c t i o n = 50 μmoles [0] v a c a n c i e s c y c l e ; Raman s p e c t r a taken at 430°C.

b)

Reduced and r e o x i d i z e d a t 430°C.

c)

Reduced a t 400°C, r e o x i d i z e d a t 430°C.

per m

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

per

3. BRAZDIL ET A L .

Selective Oxidation Catalysts

35

C l o s e r e x a m i n a t i o n o f t h e ^i^ftoQ^ s t r u c t u r e a l l o w s a more s p e c i f i c i d e n t i f i c a t i o n o f these c a t a l y t i c a l l y a c t i v e oxygens. N e u t r o n d i f f r a c t i o n ( 1 3 ) shows t h a t o x i d e i o n s o f t h i s s t r u c t u r e can be p l a c e d i n t o t h r e e g e n e r a l c a t e g o r i e s based on t h e i r bonding t o b i s m u t h and molybdenum. One type o f oxygen i s bound o n l y t o bismuth and b r i d g e s bismuth i o n s i n t h e B i ^ O ^ l a y e r . A second type of oxygen i s s i t u a t e d a t t h e a p i c e s or the MoO^ o c t a h e d r a and b r i d g e s bismuth and molybdenum i o n s . The t h i r d type o f oxygen b r i d g e s molybdenum i o n s a t t h e c o r n e r s o f the octahedra. Based on t h i s d e t a i l e d understanding o f t h e s t r u c t u r e and r e c o g n i z i n g t h e need f o r c l o s e s p a t i a l p r o x i m i t y of t h e two c a t a l y t i c f u n c t i o n s , i t i s r e a s o n a b l e t o a s s i g n t h e 0?-H a b s t r a c t i n g f u n c t i o n t o oxygens w h i c h b r i d g e bismuth and molybdenum. Oxygens w h i c h i n s e r t i n t o t h e a l l y l i c i n t e r m e d i a t e a r e l i k e l y t o be those a t t h e c o r n e r s o f t h e octahedra. I t can f u r t h e r be reasoned, based on t h e unique a b i l i t y of o ^ 3 S electrolyte t h a t t h e two lone pair moieties are responsibl dioxyge of t h e c a t a l y s t under r e a c t i o n c o n d i t i o n s . The s t r u c t u r e o f t h e s e a c t i v e s i t e s i s s c h e m a t i c a l l y i l l u s t r a t e d i n F i g u r e 3. B i

o x v

e n

O' = Oxygen responsible for α - H abstraction ·" = Oxygen associated with Mo; responsible for oxygen insertion into the allylic Intermediate • = Proposed center for 0 reduction and dissociative chemisorption 2

F i g u r e 3. Schematic d e p i c t i o n o f t h e a c t i v e s i t e s t r u c t u r e on the s u r f a c e o f B i M o 0 . . o

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

36

CATALYST C H A R A C T E R I Z A T I O N SCIENCE

C o n c l u d i n g Remarks As i l l u s t r a t e d h e r e , t h e t o o l s o f s u r f a c e s c i e n c e have o n l y r e c e n t l y provided the c a t a l y s i s researcher the opportunity t o r i g o r o u s l y t e s t and c h a l l e n g e p o s s i b l e c a t a l y t i c mechanisms on a molecular level. Previously, mechanistic evidence has been i n d i r e c t and c o n j e c t u r e d . S u r f a c e and s o l i d s t a t e a s p e c t s o f c a t a l y t i c processes c o u l d not r e a d i l y be e v a l u a t e d and c h a l l e n g e d . S u r f a c e s c i e n c e has now p r o v i d e d t h e o p p o r t u n i t y t o examine i n c r e a s i n g l y complex c a t a l y s t systems and p r o c e s s e s . No l o n g e r must the c a t a l y s t s c i e n t i s t be bound t o the study o f s i m p l e model systems w h i c h o n l y r e m o t e l y resemble a c t u a l , r e a l l i f e c a t a l y s t s . The promise o f s u r f a c e s c i e n c e , combined w i t h t r a d i t i o n a l c a t a l y t i c i n v e s t i g a t i n g , i s t h a t i n depth m o l e c u l a r l e v e l u n d e r s t a n d i n g o f real catalysts will produce t h e knowledge n e c e s s a r y t o a c h i e v e major advances i n c a t a l y t i scienc d technology Acknowledgments We w i s h t o thank Dr. diffraction results discussions·

Raymond G. and P r o f .

T e l l e r f o r the x - r a y and n e u t r o n Edward K o s t i n e r f o r f r u i t f u l

Literature Cited 1.

Eischens, R. P.; Pliskin, W. Α.; Francis, S. Α.; J. Chem. Phys. 1954, 22, 1786. 2. Grasselli, R. Κ.; Burrington, J. D.; Adv. Catal. 1981, 30, 133. 3. Grasselli, R. K.; Burrington, J. D.; Brazdil, J. F.; Discuss. Chem. Soc. 1981, 72, 203. 4. Grasselli, R. K.; Brazdil, J. F.; Burrington, J. D.; Proc. 8th Int. Cong. Catal., Berlin (West), Vol. V, p. 369, (1984). 5.

Carson, D.; Coudurier, G.; Forissier, M.; Vedrine, J. C.; J. Chem. Soc., Faraday Trans. 1, 1983, 79, 1921. 6. Brazdil, J. F.; Suresh, D. D.; Grasselli, R. K.; J. Catal., 1980, 66, 347. 7.

Egashira, M.; Matsuo, K.; Kagawa, S; Seiyama, T.; J . Catal., 1979, 58, 409.

8. Jeitschko, W.; Sleight, A. W.; McClellan, W. R.; Weiher, J. F.; Acta Cryst., 1976, B32, 1163. 9. Krenzke, L. D.; Keulks, G. W.; J. Catal., 1980, 64, 295. 10. Ueda, W.; Moro-oka, Y.; Ikawa, T.; J . Chem. Soc., Faraday Trans. 1, 1982, 78, 495. 11. Machiels, C. J.; Sleight, A. W.; Proc. 4th Int. Conf. on the Chemistry and Uses of Molybdenum; 1982; 411. 12. Matsuura, I.; Schut, R.; Hirakawa, K.; J. Catal., 1980, 63, 152. 13. Teller, R. G.; Brazdil, J. F.; Grasselli, R. K.; Acta Cryst., in press. 14. Verkerk, M. J.; Hammink, M. W. J.; Burggraaf, A. J.; J. Electrochem. Soc., 1983, 130, 70. RECEIVED March 20, 1985

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

4 Application of Surface Analysis Techniques in the Study of Catalyst Systems D. D. Hawn, R. C. Cieslinski, andH.E.Klassen Analytical Laboratories, The Dow Chemical Company, Midland, MI 48667 The application o (XPS) and Auger Electro solution of complex real world catalyst problems is often difficult and frustrating. Three recent developments in this laboratory have greatly aided in the solution of such problems. These are 1) a dual anode x-ray source using a silicon target, 2) an off-axis, low cost reaction facility for carrying out simple preparative treatments, and 3) installation of a dedicated high-performance Scanning Auger Microprobe (SAM) system. Advantages of silicon x-radiation include the access of aluminum and magnesium core level (1s) lines and the corresponding (KLL) Auger transitions for chemical state identification and improved quantitation, because these lines are at least 10 times more intense than the corresponding (2p) or (2s) lines. The construction of an off-axis reactor has produced a simple, versatile and inexpensive system easily adapted to any vacuum system. The role of AES and SAM in catalyst research will also be highlighted by examples. The a p p l i c a t i o n o f s u r f a c e a n a l y t i c a l t e c h n i q u e s , most n o t a b l y X-ray P h o t o e l e c t r o n S p e c t r o s c o p y (XPS) and Auger E l e c t r o n S p e c t r o s c o p y (AES), o r i t s s p a t i a l l y r e s o l v e d c o u n t e r p a r t , Scanning Auger M i c r o ­ a n a l y s i s (SAM), i s o f g r e a t v a l u e i n u n d e r s t a n d i n g the performance of a c a t a l y s t . However, t h e r e s u l t s o b t a i n e d from any o f t h e s e t e c h n i q u e s a r e o f t e n d i f f i c u l t t o i n t e r p r e t , e s p e c i a l l y when o n l y one t e c h n i q u e i s used by i t s e l f . In t h i s a r t i c l e we d e s c r i b e n o v e l approaches aimed a t making s u r f a c e a n a l y t i c a l d a t a e a s i e r t o o b t a i n and i n t e r p r e t . These i n ­ clude: 1) a s m a l l , l o w c o s t r e a c t i o n f a c i l i t y d e s i g n e d t o work o f f - a x i s t o a commercial XPS system, 2) a s i l i c o n anode x - r a y source f o r access t o h i g h e r b i n d i n g energy p h o t o e l e c t r o n l i n e s , and 3) t h e use o f d e d i c a t e d SAM i n c o n j u n c t i o n w i t h XPS i n c a t a l y s t problems. Examples showing the u t i l i t y o f each will be d i s c u s s e d . 0097-6156/ 85/ 0288-O037$06.00/ 0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CATALYST C H A R A C T E R I Z A T I O N SCIENCE

38 Experimental

XPS s p e c t r a were o b t a i n e d u s i n g a P e r k i n - E l m e r P h y s i c a l E l e c t r o n i c s (PHI) 555 e l e c t r o n s p e c t r o m e t e r equipped w i t h a double pass c y l i n ­ d r i c a l m i r r o r a n a l y z e r (CMA) and 04-500 d u a l anode x - r a y s o u r c e . The x - r a y source used a c o m b i n a t i o n m a g n e s i u m - s i l i c o n anode, w i t h c o l l i m a t i o n by a shotgun-type c o l l i m a t o r ( 1 ) . AES/SAM s p e c t r a and photomicrographs were o b t a i n e d w i t h a P e r k i n - E l m e r PHI 610 Scanning Auger M i c r o p r o b e , w h i c h uses a s i n g l e pass CMA w i t h c o a x i a l lanthanum hexaboride (LaBe) e l e c t r o n gun. R e a c t i o n s were c a r r i e d o u t u s i n g a f a c i l i t y mounted o f f - a x i s t o the l o a d l o c k o f t h e PHI 555 e l e c t r o n s p e c t r o m e t e r . This device u t i l i z e s a r a d i a n t h e a t e r t o h e a t a sample and mount i n s i d e a g l a s s r e a c t o r . T h i s d e v i c e i s shown i n F i g u r e 1. I t c o n s i s t s o f a c e n t e r s e c t i o n c o n s t r u c t e d from two Pyrex g l a s s - t o - m e t a l a d a p t e r s on 2.75 i n c h Confiât f l a n g e s j o i n e one i n c h i n d i a m e t e r b i n c h Confiât f l a n g e a l l o w feedthroug ga , e v a c u a t i o n , and chromel/alumel thermocouple. The r e a c t o r i s sepa­ r a t e d from t h e sample l o a d l o c k by a h i g h vacuum gate v a l v e . A 0.125 i n c h t h i c k sample h o l d e r i s moved from t h e s t a n d a r d t r a n s p o r t / a n a l y s i s r o d o f t h e PHI system t o t h e h e a t e r zone o f t h e r e a c t o r v i a a 0.25 i n c h diameter s t a i n l e s s s t e e l r o d . A d o v e t a i l mount i s used to r e c e i v e t h e sample h o l d e r on t h e PHI probe. The 0.25 i n c h r o d i s s e a l e d by a p a i r o f g r a p h i t e - i m p r e g n a t e d T e f l o n s e a l s , r i d i n g i n a h o u s i n g c o n s t r u c t e d on a 2.75 i n c h Confiât b l a n k f l a n g e . The space between t h e s e a l s i n pumped, as i s t h e r e a c t o r v i a t h e gas o u t l e t , by a m e c h a n i c a l r o t a r y pump equipped _ w i t h l i q u i d n i t r o g e n t r a p . The e n t i r e assembly o p e r a t e s from 10 t o r r t o 1 atmosphere o f p r e s s u r e when employing t h e t u r b o m o l e c u l a r pump w h i c h pumps t h e PHI load lock. H e a t i n g o f t h e mount i s accomplished by a m o d i f i e d h a l o g e n p r o j e c t i o n b u l b , w i t h t h e sample mount temperature r e g u l a t i n g t h e a p p l i e d lamp power v i a a d i g i t a l temperature c o n t r o l l e r . A stain­ l e s s s t e e l i n s u l a t e d c l a m - s h e l l e n c l o s u r e houses t h e lamp, and has p r o v i s i o n s f o r lamp c o o l i n g and an i n t e g r a l r e f l e c t o r . Temperatures to 600°C i n 1 atmosphere o f hydrogen can be a c h i e v e d a t t h e sample mount. R e a c t i v e gases a r e p r e h e a t e d b e f o r e passage over t h e sample by t r a v e l l i n g t h e l e n g t h o f t h e r e a c t o r d i r e c t l y i n f r o n t o f t h e h a l o g e n lamp b e f o r e making c o n t a c t w i t h t h e sample. Alternately, gas c a n be p r e h e a t e d i n a tube f u r n a c e b e f o r e e n t r y i n t o t h e r e a c ­ tor. However, i n t h i s case care s h o u l d be e x e r c i s e d t o n o t exceed 450°C d u r i n g a r e a c t i o n . The g l a s s - t o - m e t a l a d a p t e r s used here f o r t h e c e l l body a r e i n t e n d e d o n l y t o be used t o 400°C, w h i l e t h e g l a s s i t s e l f can s a f e l y w i t h s t a n d 500°C. Due t o t h e f o c u s s i n g power o f t h e lamp onto t h e mount, a sample temperature o f 600°C c a n be a c h i e v e d a t t h e sample w h i l e t h e g l a s s o r g l a s s - t o - m e t a l adapters remain s i g n i f i c a n t l y below t h i s temperature. F o r t h e copper/aluminum c a t a l y s t a n a l y s e s l a t e r d e s c r i b e d , t h e sample mount was t r a n s f e r e d from t h e XPS system f o l l o w i n g t r e a t m e n t and a n a l y s i s t o an i n e r t atmosphere d r y box w i t h o u t a i r exposure. From t h e r e i n d i v i d u a l p e l l e t s were t r a n s f e r r e d t o t h e SAM f o r sub­ sequent a n a l y s i s w i t h o u t a i r exposure. The r e v e r s e p r o c e s s was employed f o r t h e next r e a c t i o n c y c l e . 6

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

HAWN ET A L .

Surface Analysis Techniques

f F i g u r e 1. Schematic Diagram o f t h e o f f - a x i s r a d i a n t heated r e a c t o r . A. c e l l body; B. l i n e a r / r o t a r y motion f e e d t h r o u g h ; C. t r a n s p o r t r o d ; D. p r o j e c t o r b u l b ; E. r e f l e c t o r ; F. i n s u ­ l a t e d s t a i n l e s s s t e e l e n c l o s u r e ; G. a i r c o o l i n g p o r t ; H. gas i n l e t ; I . gas o u t l e t / p u m p i n g p o r t ; J . c h r o m e l / a l u m e l thermo­ c o u p l e ; K. h i g h vacuum gate v a l v e ; L. sample mount.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CATALYST C H A R A C T E R I Z A T I O N SCIENCE

40 Discussion

1) O f f - A x i s R e a c t i o n C e l l The a b i l i t y t o t r e a t c a t a l y s t m a t e r i a l s under c o n t r o l l e d c o n d i t i o n s f o l l o w e d by t r a n s f e r t o t h e a n a l y s i s environment w i t h o u t a i r expo­ s u r e , has been an i m p o r t a n t f a c t o r i n t h e s o l u t i o n o f many c a t a l y s t problems. E a r l y d e s i g n s o f such components have i n v o l v e d the d i r e c t h e a t i n g of the m a t e r i a l i n a p r e p a r a t i o n f a c i l i t y d i r e c t l y adjacent t o t h e s p e c t r o m e t e r , w i t h subsequent t r a n s f e r o f t h e m a t e r i a l be­ tween t h e s e two chambers. A more s o p h i s t i c a t e d m o d i f i c a t i o n o f t h i s approach, as d e s c r i b e d by Ganschow and co-workers ( 2 ) , u t i l i z e s a sample module w h i c h can be s h u t t l e d among s e v e r a l i n t e r c o n n e c t e d a n a l y t i c a l chambers and a r e a c t i o n f a c i l i t y . A l t e r n a t e d e s i g n s have i n v o l v e d t r a n s f e r o f the sample t o a remote f a c i l i t y f o r r e a c t i o n ( 3 , 4 ) , o r s e a l i n g o f t h e sample i n a r e a c t o r housed i n t h e UHV a n a l y s i s chamber ( 5 , 6 ) . been assembled by Ganscho h e r e i n has t h e advantage o f b e i n g i n e x p e n s i v e and e a s i l y r e p l a c e d o r repaired. W i t h i t s o f f - a x i s d e s i g n , t h e s p e c t r o m e t e r can s t i l l be used w h i l e a r e a c t i o n i s b e i n g c a r r i e d o u t . The d e s i g n a l s o p r e ­ s e n t s a b u f f e r between t h e r e a c t o r and a n a l y s i s chamber, thus l i m i t i n g c o n t a m i n a t i o n o f t h e a n a l y s i s chamber by t h e r e a c t o r . Adverse w a l l r e a c t i o n s a r e l i m i t e d , s i n c e the a r e a i n t h e heated zone i s g l a s s , and o n l y t h e sample and i t s h o l d e r a r e m a i n t a i n e d a t the r e a c t i o n t e m p e r a t u r e . A carbon s u p p o r t e d molybdenum c a t a l y s t used f o r the p r o d u c t i o n of L i q u i f i e d P e t r o l e i u m Gas (IPGs) demonstrates t h e u s e f u l l n e s s and h i g h temperature c a p a b i l i t i e s o f t h i s f a c i l i t y . F i g u r e 2 compares the molybdenum (3d) s p e c t r a f o l l o w i n g extended t r e a t m e n t s i n h e l i u m and hydrogen a t 500°C. F o r t h e s t a r t i n g c a t a l y s t , the Mo(3d 5/2) p h o t o l i n e i s c e n t e r e d a t 232.4 eV b i n d i n g energy when r e f e r e n c e d t o the g r a p h i t i c s u p p o r t carbon ( I s ) p h o t o l i n e a t 284.3 eV. This b i n d i n g energy i s c o n s i s t e n t w i t h t h a t r e p o r t e d f o r M o ( V I ) , o r Mo0 (4*7). F o l l o w i n g h i g h temperature exposure t o h e l i u m ( F i g u r e 2 B ) , a s e r i e s o f l o w e r b i n d i n g energy peaks were o b s e r v e d , w i t h Mo(3d 5/2) components c e n t e r e d a t 231.1 and 228.9 eV b i n d i n g energy. These a r e a t t r i b u t e d t o Mo(V) and M o ( I V ) , r e s p e c t i v e l y , based on p r e v i o u s l y r e p o r t e d l i t e r a t u r e v a l u e s ( 4 ) and work performed i n t h i s l a b o r a ­ t o r y . I n a d d i t i o n , a h i g h e r b i n d i n g energy component a t 234.2 eV i s observed i n t h i s spectrum, and i s a t t r i b u t e d t o molybdate (Mo04=). We a l s o observed a marked i n c r e a s e i n p o t a s s i u m a t t h e s u r f a c e f o l ­ lowing t h i s treatment. In this c a s e , t h e Mo (IV)/Mo (VI) and Mo(V)/Wo(VI) r a t i o s a r e 0.2 and 0.13 r e s p e c t i v e l y a f t e r 6 hours o f treatment. Extended r e a c t i o n t i m e s ( t o 12 h o u r s ) do not s i g n i f i ­ cantly affect this ratio. R e d u c t i o n i n hydrogen a t 500°C produces s u b s t a n t i a l l e v e l s o f Mo, as evidenced by t h e appearance o f a peak a t 227.8 eV b i n d i n g energy. A g a i n Mo(IV) i s o b s e r v e d , b u t no molybdates were observed. T h i s r e d u c t i o n was observed t o proceed r a p i d l y a t f i r s t , so t h a t , a f t e r 6 h o u r s , the Mo(IV)/ Mo(VI) and Mo/Mo(VI) r a t i o s were 0.80 and 2.20, r e s p e c t i v e l y . Extended times ( t o 9 h o u r s ) d i d n o t g r e a t l y a f f e c t t h e s e r a t i o s . P a t t e r s o n and co­ workers ( 4) observed s i m i l a r b e h a v i o r f o r a s u p p o r t e d molybdenum c a t a l y s t , i n w h i c h case l i t t l e f u r t h e r r e d u c t i o n o f Mo(VI) t o Mo was observed a f t e r 7 hours i n hydrogen. I n t h i s c a s e , t h e presence o f 3

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

H AWN ET AL.

Surface Analysis Techniques

A. initial

239

235

231

227

Binding Energy

F i g u r e 2. Molybdenum (3d) XPS s p e c t r a o f a molybdenum on carbon c a t a l y s t f o l l o w i n g v a r i o u s t r e a t m e n t schemes; A. c a t a ­ l y s t as p r e p a r e d ; B. f o l l o w i n g 500°C f o r 6 hours i n h e l i u m ; C. f o l l o w i n g 500°C f o r 6 hours i n hydrogen.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CATALYST C H A R A C T E R I Z A T I O N SCIENCE

42

M o ( I V ) , and i t s o p t i m i z a t i o n on the c a t a l y s t s u r f a c e , i s an impor­ t a n t c o n s i d e r a t i o n i n the development o f t h i s c a t a l y s t , as Mo(IV) i s b e l i e v e d t o be the a c t i v e component f o r s e v e r a l c a t a l y t i c p r o c e s s e s (8). 2)

S i l i c o n Anode X-Ray Source A l t h o u g h XPS has been a p p l i e d w i t h success i n a wide v a r i e t y o f c a t a l y s t p r o b l e m s , c e r t a i n s i t u a t i o n s a r i s e where i n t e r f e r e n c e s o f photoelectron lines from d i f f e r e n t elements make the technique d i f f i c u l t t o use. An i n t e r e s t i n g example i s the copper/aluminum system, i n w h i c h case the copper (3s) and (3p) l i n e s i n t e r f e r e w i t h the aluminum(2s) and (2p) l i n e s ( F i g u r e 3 ) . These l i n e s cannot be i s o l a t e d from each o t h e r by u s i n g a d i f f e r e n t x - r a y l i n e , as i s commonly done w i t h Auger l i n e s o v e r l a p p i n g XPS p h o t o l i n e s . I n t h e s e s p e c i a l c a s e s , the b e s t s o l u t i o n i s t o employ a h i g h e r energy x - r a y s o u r c e t o a c c e s s deeper A l ( l s ) p h o t o l i n e i n the S e v e r a l c o n s i d e r a t i o n s must be made when s e l e c t i n g an anode material. F o r r o u t i n e a n a l y t i c a l work, one anode o f a d u a l anode s o u r c e s h o u l d be magnesium because o f the narrow x - r a y l i n e w i d t h and e x t e n s i v e l i t e r a t u r e base. A n o t h e r c o n s i d e r a t i o n i s the energy range o f the a n a l y z e r , as p h o t o e l e c t i o n l i n e s w i t h k i n e t i c e n e r g i e s e x c e e d i n g the k i n e t i c energy range o f the a n a l y z e r would be i n a c c e s ­ sible. T h e r e f o r e , i n the PHI 555 system w i t h an energy range o f 2400 e l e c t r o n v o l t s , g o l d (Ma=2122.9 e V ) , z i r c o n i u m (La=2042.4 e V ) , and s i l i c o n (Ka=1739.4 eV) would be p o s s i b l e c h o i c e s . C a s t l e and co-workers have d i s c u s s e d the use o f S i K a ( 9 ) and Z r L a ( 1 0 ) , w h i l e the use o f AuMa has been demonstrated by Wagner ( 1 1 ) . A n o t h e r c o n s i d e r a t i o n i s the n a t u r a l l i n e w i d t h and s a t e l l i t e s t r u c t u r e o f the x - r a y l i n e used. T i t a n i u m (TiKof=4510.9 eV) has seen l i m i t e d use (12) f o r n o n - d e s t r u c t i v e d e p t h p r o f i l i n g , b u t the o b s e r v e d s p e c t r a are c o m p l i c a t e d by the T i K a s a t e l l i t e s t r u c t u r e and the l a r g e n a t u r a l l i n e w i d t h o f 2.0 eV ( 1 3 ) . S i K a i s a good c h o i c e because o f i t s energy and narrow l i n e w i d t h o f 1.0 eV (9). F i g u r e 4 shows some o f the element core l e v e l s w h i c h can be e x c i t e d w i t h magnesium (Kcf=1253.6 e V ) , aluminum (Ka=l486.6 eV) and s i l i c o n (Ka=1739.4 eV) s o u r c e s . The h i g h c r o s s s e c t i o n o f 6.01 f o r the A l ( l s ) p h o t o l i n e , as measured r e l a t i v e t o the f l u o r i n e ( I s ) p h o t o l i n e u s i n g S i K a x - r a d i a t i o n , i s s u b s t a n ­ t i a l l y g r e a t e r t h a n t h a t o f the A l ( 2 p ) p h o t o l i n e , a t 0.170. In a d d i t i o n , the r e s u l t i n g Auger t r a n s i t i o n s , such as the Al(KLL) s e r i e s , a l l o w the development o f Auger parameters f o r t h i s s e r i e s . F i g u r e 5 shows such an Auger parameter p l o t f o r a s e r i e s o f aluminum compounds. Due t o c r o w d i n g , s e v e r a l v a l u e s g i v e n i n T a b l e I are o m i t t e d from t h i s p l o t . Most o f the compounds are grouped t o the l o w e r l e f t , whereas aluminum m e t a l i s a t the upper r i g h t . I n t e r m e d i a t e between t h e s e are sodium z e o l i t e and z i n c a l u m i n a t e (ZnAl 0 ). A l t h o u g h the range covered by the A l ( l s ) l i n e f o r the compounds i s n e a r l y the same as t h a t f o r the A l ( 2 p ) l i n e (5.4 eV as compared t o 4.3 e V ) , the added d i m e n s i o n o f the Auger (KL23L23) l i n e a l l o w s d i f f e r e n t i a t i o n o f compounds not d i s t i n g u i s h a b l e u s i n g t h e i r A l ( 2 p ) l i n e s a l o n e ( f o r example, A1(0H)3 and A I C I 3 ) . B e r y l l u i m (7 m i c r o n ) , s i l i c o n d i o x i d e (10 m i c r o n ) , and aluminum (2 m i c r o n ) were t r i e d as x - r a y windows. B e r y l l u i m was the b e s t 2

4

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

HAWN ET A L .

Surface Analysis Techniques

133

129

88

84

125 121 Binding Energy

117

113

80

72

68

76

Binding Energy

F i g u r e 3. XPS s p e c t r a o f a l u m i n a - s u p p o r t e d copper c a t a l y s t , showing i n t e r f e r e n c e o f XPS p h o t o l i n e s : A. copper (3s) and aluminum ( 2 s ) r e g i o n s ; B. copper (3p) and aluminum (2p) regions.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CATALYST C H A R A C T E R I Z A T I O N SCIENCE

Relative Intensity 1800 Si Κ α 1600 •Br(2p )

Al(1s)

3/2

ΑΙ Κ α 1400

Mg Κ α 1200 Nads) 1000

-o

800

600

400

AI(KLL) "ir(LMM)

Mg(KLL)

Na(KLL)

200 Al(2s) Al(2p) ' Br(3d)

Mg(2s) Mg(2p)

Na(2s) Na(2p)

F i g u r e 4. P o s i t i o n s and r e l a t i v e i n t e n s i t i e s o f XPS and Auger transitions using s i l i c o n , aluminum, and magnesium x-ray sources.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

4.

HAWN ET AL.

-1566

45

Surface Analysis Techniques

-1564

-1562

-1560

-1558

Al(1s) Binding Energy

F i g u r e 5. Auger c h e m i c a l s t a t e p l o t f o r aluminum compounds as o b t a i n e d u s i n g s i l i c o n Κα x - r a d i a t i o n .

Table I .

C h e m i c a l S t a t e Data F o r V a r i o u s Aluminum Compounds Ob­ t a i n e d U s i n g S i l i c o n Κα X - R a d i a t i o n (1739.4)

Compound

Al(ls)

Al V-AI2O3 A1 0 A1(0H) A1(P0 ) A1(N0 ) Al C l Al I Al F Na(A)Zeolite ZnAl 0 1:1 Cu/Al

1558.3 1562.4 1561.3 1562.1 1562.8 1562.0 1562.3 1561.4 1563.7 1561.5 1561.6 1562.1

2

3

3

4

3

3

3

3

2

4

3

Al(KL

2 3

L 3) 2

364.1 353.2 353.8 353.4 354.7 353.8 354.1 354.5 355.3 353.6 351.6 353.5

N E

Al(2p)*

AUKL23L23W 1393.3 1386.2 1386.7 1386.0 1384.7 1385.7 1385.4 1385.0 1384.1 1385.9 1387.8 1386.0

2951.6 2948.6 2948.0 2948.1 2947.5 2947.7 2947.7 2946.4 2947.8 2947.4 2949.4 2948.1

72.2 75.3 74.7 74.9 75.8 75.2 75.8 76.0 77.0 74.4 74.7

--

" U s i n g Mg Κα r a d i a t i o n B i n d i n g energy r e f e r e n c e d t o A u ( 4 f 7/2) l e v e l a t 83.8 eV f o r t h i n l a y e r o f g o l d e v a p o r a t e d onto sample.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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c h o i c e s i n c e t h e h i g h e s t count r a t e s were o b s e r v e d , p r o d u c i n g Ag(3d 5/2) count r a t e s 26 p e r c e n t t h a t o f t h e magnesium anode w i t h a 2μπι aluminuim window a t t h e same a n a l y z e r pass energy. S i l i c o n d i o x i d e and aluminum windows lowered t h e observed count r a t e s on s i l v e r t o 4 p e r c e n t o f t h a t o f magnesium. When u s i n g aluminum as t h e window m a t e r i a l , ghost l i n e s due t o copper were 25 p e r c e n t o f t h e i n t e n s i t y o f t h e Ag(3d 5/2) p h o t o l i n e . Some c r a c k i n g o f t h e S i 0 window was o b s e r v e d w i t h u s e , presumably due t o h e a t d i s t o r t i o n . The n a r r o w e s t f u l l - w i d t h - a t - h a l f - m a x i m u m peak h e i g h t (FWHM) f o r Ag(3d 5/2) was 1.36 eV, i n w h i c h case some assymetry o f t h e l i n e was o b s e r v e d due t o s e p a r a t i o n o f t h e SiKu"i,2 components. T h i s was n o t a problem i n r o u t i n e use. The Ag (3d 5/2) FWHM o b s e r v e d u s i n g t h e same a n a l y z e r pass energy and magnesium Κα x - r a d i a t i o n was 0.96 eV. 2

3)

A p p l i c a t i o n o f XPS U s i n g S i l i c o n Anode X-Ray S o u r c e , S c a n n i n g Auger M i c r o p r o b e , C a t a l y s t System The use o f t h i s anod catalys analysi a commercially a v a i l a b l e p e l l e t i z e d c a t a l y s t with a bulk composition o f 84 w e i g h t p e r c e n t CuO, 14% A 1 0 , 1% N a 0 , and 1% g r a p h i t e b i n d e r . T h i s c a t a l y s t has t h e form o f h a r d g l o s s y p e l l e t s a p p r o x i m a t e l y 3 mm i n d i a m e t e r by 3 mm i n l e n g t h . The c a t a l y s t i s w i d e l y used i n t h e reduced form f o r h y d r o l y s i s o f p r i m a r y amines. P e r i o d i c regenera­ t i o n o f t h i s c a t a l y s t involves m i l d r e o x i d a t i o n t o burn o f f r e s i d u a l h y d r o c a r b o n s , f o l l o w e d by r e - r e d u c t i o n . The g o a l o f t h i s work was t o u n d e r s t a n d t h e s u r f a c e c o m p o s i t i o n a l changes w h i c h o c c u r r e d during repeated regenerations. R e f e r r i n g a g a i n t o F i g u r e 3, i t was i m p o s s i b l e t o d i s c e r n t h e r o l e o f aluminum i n t h i s c a t a l y s t due t o t h e i n t e r f e r e n c e o f copper p h o t o l i n e s w i t h t h e A l ( 2 s ) and A l ( 2 p ) photolines. F i g u r e 6 compares t h e XPS s u r v e y scans o b t a i n e d from t h e a s - r e c e i v e d m a t e r i a l , and a f t e r r e d u c t i o n i n a hydrogen/helium gas m i x t u r e a t 200°C f o r 12 h o u r s . B o t h s p e c t r a were o b t a i n e d u s i n g SiKa x - r a d i a t i o n . The most n o t a b l e d i f f e r e n c e s between t h e s e two s p e c t r a a r e t h e i n c r e a s e d i n t e n s i t y o f t h e A l ( l s ) and N a ( l s ) p h o t o ­ l i n e s , and t h e l o s s o f s a t e l l i t e s t r u c t u r e i n t h e copper (2p) r e g i o n due t o r e d u c t i o n o f C u ( I I ) s p e c i e s t o C u ( I ) o r Cu m e t a l . The l a r g e c a r b o n ( I s ) i n t e n s i t y was s u r p r i s i n g c o n s i d e r i n g t h e low l e v e l o f g r a p h i t e added as b i n d e r . That carbon i s s e g r e g a t e d t o t h e p e l l e t s u r f a c e i s c l e a r l y i n d i c a t e d by comparison o f t h e c a r b o n ( I s ) / aluminum ( I s ) a t o m i c r a t i o s i n rows A o r C-F t o rows Β o r G i n T a b l e I I . I n t h i s c a s e , t h e powdered m a t e r i a l i s i n t e n d e d t o be representative of the bulk c a t a l y s t . These powders were d u s t e d onto c o n d u c t i v e tape f o r a n a l y s i s , and t h e r e f o r e t h e carbon-to-aluminum r a t i o s may be s l i g h t l y i n e r r o r due t o t h e sample p r e p a r a t i o n . The e x c e p t i o n a l l y h i g h C ( l s ) / A l ( l s ) r a t i o f o r t h e p e l l e t i z e d s t a r t i n g m a t e r i a l r e s u l t s from t h e p r e s e n c e o f s u r f a c e hydrocarbons d e p o s i t e d d u r i n g p r o c e s s i n g o r subsequent h a n d l i n g . The carbon ( I s ) peak was v e r y b r o a d due t o t h i s c o n t r i b u t i o n a t 284.6 eV b i n d i n g energy, as w e l l as g r a p h i t i c type carbon a t 284.3 eV. The carbon ( I s ) peak f o r t h e powdered s t a r t i n g m a t e r i a l was n a r r o w e r due t o t h e reduced c o n t r i b u t i o n o f t h i s 284.6 eV component t o t h e spectrum. F o l l o w i n g r e d u c t i o n and o x i d a t i o n , t h e carbon was p r e d o m i n a n t l y g r a p h i t i c i n n a t u r e , i n w h i c h case t h e h y d r o c a r b o n contaminants were q u i c k l y burned o f f d u r i n g t r e a t m e n t . A l s o note t h e s u b s t a n t i a l 2

3

2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

HAWN ET A L .

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A. Cu/AI, initial

1600

1400

1200

1000

800

600

400

200

0

Binding Energy

F i g u r e 6. XPS wide scans o f a commercial copper/aluminum e x t r u d e d c a t a l y s t o b t a i n e d u s i n g s i l i c o n Κα x - r a d i a t i o n : A. u n t r e a t e d c a t a l y s t ; B. f o l l o w i n g 200°C f o r 12 hours i n 1 0 % hydrogen/90% h e l i u m gas m i x t u r e .

American Chemical Society Library 1155 16th St., N.W.

In Catalyst Characterization Science; Deviney, M., et al.; Washington, ACS Symposium Series; American Chemical D.C. Society:20036 Washington, DC, 1985.

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T a b l e I I . E l e m e n t a l Atomic R a t i o s f o r Copper/Aluminum C a t a l y s t F o l l o w i n g V a r i o u s Treatments as Obtained U s i n g S i l i c o n Κα X-Radiation

Condition A Starting Material Β Starting Material C (R) D (R),(0) Ε (R),(0), F

00,(0)

(R),(0),(R) (0),(R) G Same as F

Cu(2p 3/2) Al(ls)

Na(ls) Al(ls)

0(ls) Al(ls)

C(ls) Al(ls)

Pellet

2.0

0.9

9.6

28.2

Powdered Pellet Pellet

1.0 0.6 1.6

0.3 1.8 3.1

3.7 3.2 6.7

4.8 10.7 10.3

Pellet

1.5

2.3

5.0

9.5

Pellet Powdered

0.4 0.5

2.2 0.3

2.1 2.4

5.1 2.3

P r e p a r a t i o n Code: (R) 12 hours a t 200°C i n 10% H /He (0) 12 hours a t 200°C i n 10% 0 /He 2

2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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HAWN ET AL.

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i n c r e a s e i n sodium a t the p e l l e t s u r f a c e f o l l o w i n g r e d u c t i o n and o x i d a t i o n , d e m o n s t r a t i n g the m o b i l i t y o f sodium, e s p e c i a l l y under reducing conditions. The lower atomic r a t i o s f o r t h e o t h e r compo­ nents i n the c a t a l y s t r e s u l t i n p a r t from s u r f a c e coverage by sodium as d i s c u s s e d l a t e r . H i g h r e s o l u t i o n s c a n n i n g Auger microprobe (SAM) a n a l y s i s i n d i ­ c a t e d t h a t the h i g h s u r f a c e carbon was r e l a t e d t o the presence o f exposed g r a p h i t e i s l a n d s on the s u r f a c e o f the c a t a l y s t p e l l e t s . F i g u r e 7A shows a secondary e l e c t r o n (SEM) p h o t o m i c r o g r a p h o f t h e s u r f a c e o f the u n t r e a t e d c a t a l y s t . P o i n t mode Auger s p e c t r a o b t a i n e d from the d a r k e r a r e a s i n t h i s p h o t o m i c r o g r a p h showed o n l y carbon t o be p r e s e n t . I n comparison, s p e c t r a o b t a i n e d from t h e grey a r e a s e v i d e n t i n t h i s p h o t o m i c r o g r a p h showed t h e presence o f c a r b o n , oxygen, copper, sodium, and aluminum. F i g u r e 7B shows t h e carbon (KLL) Auger map c o r r e s p o n d i n g t o the a r e a shown i n 7A. Note the correspondence o f dark c o n t e n t areas e v i d e n t i F o l l o w i n g i n i t i a l r e d u c t i o n a t 200°C, photomicrographs showed the development o f d i s t i n c t l i g h t - c o l o r e d i s l a n d s on t h e c a t a l y s t s u r f a c e , i n a d d i t i o n t o t h e d a r k (carbon) i s l a n d s and grey a r e a s p r e v i o u s l y observed ( F i g u r e 8A). P o i n t mode s p e c t r a (8 kV, 10 nanoamp p r i m a r y beam) o b t a i n e d i n t h e s e l i g h t areas i n d i c a t e d p r i m a r i l y copper and oxygen t o be p r e s ­ ent, s i m i l a r t o p o i n t mode s p e c t r a o b t a i n e d from g r e y areas i n t h e p h o t o m i c r o g r a p h . Auger s p e c t r a a c q u i r e d from the e n t i r e a r e a imaged i n F i g u r e 8A i n d i c a t e d t h a t sodium and oxygen were t h e main s u r f a c e components. The sodium (KLL) Auger map o b t a i n e d i n the (peakbackground) /background mode c o r r e s p o n d i n g t o F i g u r e 8A i s shown i n F i g u r e 8B. Note the correspondence o f the l i g h t c o l o r e d a r e a s i n the SEM image t o h i g h sodium a r e a s observed i n F i g u r e 8B. The oxygen (KLL) Auger map ( F i g u r e 8) shows s i g n i f i c a n t c o r r e l a t i o n w i t h t h e sodium Auger map, s u g g e s t i n g t h a t t h e s e l i g h t c o l o r e d a r e a s observed i n F i g u r e 8A are sodium o x i d e ( N a 0 ) . The c o n f l i c t between n i l sodium observed i n t h e p o i n t mode i n l i g h t a r e a s o f t h e SEM p h o t o m i c r o g r a p h , and s u b s t a n t i a l sodium i n the sodium maps i n t h e s e a r e a s , p o i n t s t o t h e m i g r a t i o n o f sodium caused by the p r i m a r y e l e c t r o n beam. Even though the p r i m a r y beam c u r r e n t s used here were low (10 n A ) , the beam c u r r e n t d e n s i t y i n the p o i n t mode was h i g h ( a p p r o x i m a t e l y 0.5 amps p e r square c e n t i m e t e r ) . D e r e a l i z a t i o n o f the p r i m a r y beam by r a s t e r i n g , o r by d e f l e c t i o n as i s done d u r i n g mapping, produced s u f f i c i e n t l y lower beam c u r r e n t d e n s i t y , thus a l l e v i a t i n g sodium m i g r a t i o n . The i s l a n d i n g o f sodium o x i d e as observed i n t h i s case i s most l i k e l y caused by t h e i n a b i l i t y o f sodium o x i d e t o wet the reduced copper s u r f a c e . These i s l a n d s were observed o n l y d u r i n g the i n i t i a l r e d u c t i o n s t e p ; a u n i f o r m s u r f a c e d i s t r i b u t i o n o f sodium was ob­ s e r v e d f o l l o w i n g subsequent o x i d a t i o n , i n w h i c h case t h e o x i d e n a t u r e o f s u r f a c e would be more w e t t a b l e . R e f e r r i n g a g a i n t o T a b l e I I and the XPS r e s u l t s , the h i g h N a ( l s ) / A l ( l s ) atomic r a t i o observed f o l l o w i n g r e d u c t i o n and o x i d a t i o n (row D) i s a r e s u l t o f more u n i f o r m coverage o f the s u r f a c e by sodium. When r u n n i n g h y d r o l y s i s r e a c t i o n s i n the l i q u i d phase, i t was observed t h a t the sodium c o u l d be washed from the bed by f l u s h i n g w i t h water and m o n i t o r i n g the change i n pH o f the bed e f f l u e n t . I f t h i s p r e c a u t i o n was not o b s e r v e d , i n i t i a l low y i e l d s o f p r o d u c t were observed u n t i l sodium was e n t i r e l y removed from the bed. 2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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F i g u r e 7. Secondary e l e c t r o n m i c r o g r a p h and carbon (KLL) map of untreated c a t a l y s t : A. p h o t o m i c r o g r a p h o b t a i n e d a t 8kV, O.lnA p r i m a r y beam; B. c o r r e s p o n d i n g carbon (KLL) Auger map o b t a i n e d i n (peak-background)/ background mode, a t 8kv, lOnA p r i m a r y beam.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

HAWN ET A L .

Surface Analysis Techniques

F i g u r e 8. Secondary e l e c t r o n p h o t o m i c r o g r a p h , sodium ( K L L ) , and oxygen (KLL) Auger maps o f c a t a l y s t f o l l o w i n g s i n g l e r e d u c t i o n : A. SEM p h o t o m i c r o g r a p h o b t a i n e d w i t h 8 k v , 0.1 nA p r i m a r y beam; B. c o r r e s p o n d i n g sodium (KLL) auger map o b t a i n e d i n (peak-background) /background mode; C. c o r r e s p o n d i n g oxygen (KLL) Auger map o b t a i n e d i n (peak-background )/background mode w i t h 8kv, ΙΟηΑ p r i m a r y beam. In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Photomicrographs o b t a i n e d f o l l o w i n g the second o x i d a t i o n showed the appearance o f s m a l l c r a c k s throughout the p e l l e t s . In a c t u a l use, these c r a c k s would be d e t r i m e n t a l s i n c e they are the p r e c u r s o r to p e l l e t f r a c t u r e and subsequent compaction, w h i c h would r e s u l t i n i n c r e a s e d p r e s s u r e drop a c r o s s the r e a c t o r . E x a m i n a t i o n o f aluminum ( I s ) and copper (2p) s p e c t r a suggest t h a t a strong metal-support i n t e r a c t i o n occurs i n t h i s c a t a l y s t . Aluminum ( I s ) s p e c t r a o b t a i n e d a f t e r v a r i o u s t r e a t m e n t s are shown i n F i g u r e s 9 and 10. The i n i t i a l c a t a l y s t A l ( l s ) spectrum i s composed o f two peaks, w i t h the h i g h e r b i n d i n g energy component a t 1562.4 eV c o n s i s t e n t w i t h aluminum o x i d e . The lower b i n d i n g energy component i s c e n t e r e d a t 1560.4 eV, w h i l e the A l ( K L L ) t r a n s i t i o n i s l o c a t e d a t 352.2 eV b i n d i n g energy. F o l l o w i n g repeated r e d u c t i o n and o x i d a ­ tion, two a d d i t i o n a l components appear i n the aluminum ( I s ) spectrum, a t b i n d i n g e n e r g i e s o f 1558.0 eV and 1555.6 eV. Although the 1558.0 eV componen m e t a l i s not e x p e c t e d unde I n a d d i t i o n , the plasmo s i m i l a r t o those observed f o r the A l ( 2 s ) l i n e , are not observed i n t h i s case. T h i s s h o u l d be compared t o the A l ( l s ) spectrum f o r the powdered c a t a l y s t , shown i n F i g u r e 10C, w h i c h shows a s i n g l e component a t 1562.4 eV. The A l ( K L L ) Auger t r a n s i t i o n was observed a t 353.3 eV b i n d i n g energy, s u g g e s t i n g t h a t the b u l k aluminum i s p r e s e n t as an aluminum o x i d e ( y - A l 0 ) . As i n the powdered c a t a l y s t , no changes i n e i t h e r the A l ( l s ) o r A l ( K L L ) Auger s p e c t r a were observed f o r p e l l e t s w h i c h were c l e a v e d and s u b j e c t e d t o the same t r e a t m e n t c y c l e s , so t h a t A 1 0 was the o n l y aluminum compound observed. The copper (2p) s p e c t r a f o r t h i s c a t a l y s t o b t a i n e d w i t h mag­ nesium Κα x - r a d i a t i o n showed similar unusual behavior, as i l l u s t r a t e d i n F i g u r e 11. Whereas the powdered m a t e r i a l showed o n l y C u ( I I ) o x i d e t o be p r e s e n t f o l l o w i n g o x i d a t i o n and copper m e t a l f o l l o w i n g r e d u c t i o n , the s u r f a c e e x h i b i t e d p r e d o m i n a t l y copper m e t a l f o l l o w i n g r e d u c t i o n , and two copper components a t 932.4 eV and 934.8 eV e i t h e r i n i t i a l l y o r f o l l o w i n g o x i d a t i o n . The 934.8 eV component i s a t t r i b u t e d t o copper ( I I ) c a r b o n a t e , and i s supported by the p r e s e n c e of an a d d i t i o n a l peak i n the carbon ( I s ) spectrum a t 289.6 eV b i n d i n g energy, i n d i c a t i v e of carbonate. The 932.4 eV copper peak, a l o n g w i t h the copper (LMM) auger t r a n s i t i o n a t 335.9 eV b i n d i n g energy, suggests a copper ( I ) s p e c i e s . T h i s i s a l s o sup­ p o r t e d by the low i n t e n s i t y o f the shake-up l i n e s r e l a t i v e t o the Cu(2p) l i n e s . Note p a r t i c u l a r l y t h a t t h i s b e h a v i o r was not observed i n powder m a t e r i a l o r i n c l e a v e d p e l l e t s s u b j e c t e d t o the same treatments. S i m i l a r experiments w i t h copper d i s p e r s e d on A 1 0 d i d not show any u n u s u a l b e h a v i o r o f the A l ( l s ) o r Cu(2p) p h o t o l i n e s . In t h i s c a s e , the copper c o u l d be e a s i l y c y c l e d between CuO under o x i d a t i v e c o n d i t i o n s , t o Cu m e t a l d u r i n g r e d u c i n g c o n d i t i o n s . We observed o n l y a s l i g h t s h i f t ( on «-h i

ο

m m ο ο

φ CO

cd —ι M

l

u u

c

_l

500

I

L I

L u wJ l_

—I

1000 mass

I I 1500

I

ν L_

(amu)

F i g u r e 7. T i m e - o f - f l i g h t mass s p e c t r a shewing r e s u l t s o f p l a t i n u n c l u s t e r r e a c t i o n s w i t h benzene. The lower t r a c e i s c l e a n metal without r e a c t a n t . The upper t r a c e i s w i t h the pulsed a d d i t i o n o f .21 % benzene i n h e l i u n . The n o t a t i o n i n d i c a t e s the nunber o f adducts on each metal c l u s t e r . The metal c l u s t e r are a l l two photon i o n i z e d , w h i l e t h e observed products are s i n g l e photon i o n i z e d , hence the enhancement o f the product over metal s i g n a l s . Reproduced from Ref. 17. Copyright 1985, American Chemical S o c i e t y .

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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s p e c i e s which s a t i s f y the c o o r d i n a t i o n requirements of the system. I t should a l s o be c l e a r t h a t more d e t a i l e d e l e c t r o n i c s t r u c t u r a l probes are required before we can speak with assurance about the chemical behavior of these systems. I t i s e v i d e n t , however, t h a t the s t u d y of c l u s t e r s , p a r t i c u l a r l y of the t r a n s i t i o n m e t a l s , as models f o r molecular surfaces has emerged as an e x c i t i n g s c i e n t i f i c regime. Literature Cited 1. 2. 3.

4.

5.

6. 7. 8. 9. 10. 11.

12. 13.

K . J . Klabunde and Y . Tanaka, J . M o l . C a t a l y s i s 21, 57 (1983). G . A . O z i n , F a r . Symp. Chem. Soc. 14, 7 (1980); W. S c h u l z e , F . Frank, K . - P . C h a r l e , and B . Tesche, B e r . Bunsenges. Phys F o r a summary clusters see E. L . Muetterties, C h e m i c a l and E n g i n e e r i n g News, 30 Aug. 1982, pp. 28-42. Also P. C h i n i , J . Organometal. Chem. 200, 37 (1980). S. Leutwyler, A . Herrmann, L . Woste and E . Schumacher, Chem. P h y s . 48, 253 (1980); M. M. Kappes, R. W. Kunz and E . Schumacher, Chem. Phys. L e t t . 91, 413 (1982). See a l s o : W. D. Knight, R. Monat, E . R. D i e t z and A . R. G e o r g e , P h y s . Rev. L e t t . 40, 1324 (1978). T . G . D i e t z , M. A . Duncan, D . E . Powers and R. E . S m a l l e y , J . Chem. Phys. 74, 6511 (1981); D. E . Powers, S. G . Hansen, M. E . G e u s i c , A. C . P u i u , J . B . H o p k i n s , T . G . D i e t z , M. A . Duncan and R. E . S m a l l e y , J . Phys. Chem. 86, 2556 (1982); V . E . Bondebey and J . H. E n g l i s h , J . Chem. Phys. 76, 2165 (1982). For reviews of t h i s work, see: M. D. Morse and R. E . S m a l l e y , B e r . Bunsenges. Phys. Chem. 88, 228 (1984); R. E . Smalley, Laser Chem. 2, 167 (1983). T. G. D i e t z , M. A. Duncan, R. E . Smalley, D. M. Cox, J . A. Horsley and A. K a l d o r , J . Chem. Phys. 77, 4417 (1982). E . A . R o h l f i n g , D. M. Cox, R. Petkovic-Luton and A. K a l d o r , J . Phys. Chem., i n p r e s s . E . A . R o h l f i n g , D. M. Cox and A . K a l d o r , J . Chem. Phys. 81, 3322, 1984. A . E l G o r e s y and G . Donnay, S c i e n c e 161, 363 (1968). J . Muhlbach, K. Sattler, P. Pfau and E . Recknagel, Phys. L e t t . 87A, 415 (1982); W. D . K n i g h t , K. Clemenger, W. A. de Heer, W. A . Saunders, M. Y . Chou and M. L . Cohen, Phys. Rev. L e t t . 52, 2141 (1984). E . A . R o h l f i n g , D . M. Cox and A . K a l d o r , J . Phys. Chem. 87, 4497 (1984). E . A. R o h l f i n g , D. M. Cox and A. K a l d o r , Chem. Phys. L e t t . 99, 161 (1983); E . A. R o h l f i n g , D. M. Cox, A . K a l d o r and Κ. H. Johnson, J . Chem. P h y s . , in p r e s s .

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

10. KALDOR ET AL. 14.

15.

16.

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123

K. Lee and J . C a l l o w a y , " E l e c t r o n i c S t r u c t u r e of Small Iron C l u s t e r s , " Submitted for p u b l i c a t i o n ; C . Y. Yang, Κ. H. Johnson, D. R. Salahub, J . Kaspar and R. P. Messmer, Phys. Rev. Β 24, 5673 (1981). D. M. C o x , D. J . T r e v o r , R. L . Whetten and A . K a l d o r , to be p u b l i s h e d . F o r a r e v i e w of t h i s and other work on t r a n s i t i o n metal c l u s t e r s , see: R. L . Whetten, D. M. C o x , D. J . T r e v o r and A . K a l d o r , Surf. S c i . , in press. An a l t e r n a t i v e approach i s to mix r e a c t a n t and c l u s t e r s p r i o r to expansion. See: S. J . R i l e y , E . K. Parks, G . C . Nieman, L . G . Pobo and S. Wexler, J . Chem. P h y s . 80, 1360 (1984); S. J . R i l e y , Ε . K . P a r k s , L . G . Pobo and S. W e x l e r , B e r . Bunsenges. Phys. Chem. 88, 287 (1984). D. J . T r e v o r K a l d o r , J . Am

RECEIVED March 20, 1985

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

11 Iron Fischer-Tropsch Catalysts: Surface Synthesis at High Pressure D. J. Dwyer Corporate Research Science Laboratories, Exxon Research & Engineering Company, Annandale, NJ 08801 An XPS investigatio before and after exposure to realistic reaction condi tions is reported. The iron catalyst used in the study was a moderate surface area (15M2/g) iron powder with and without 0.6 wt.% K2CO3. Upon reduction, surface oxide on the fresh catalyst is converted to metallic iron and the K2CO3 promoter decomposes into a potassium-oxygen surface complex. Under reaction conditions, the iron catalyst is converted to iron carbide and surface carbon deposition occurs. The nature of this carbon deposit is highly dependent on reaction conditions and the presence of surface alkali. In r e c e n t y e a r s t h e c o u p l i n g o f a t m o s p h e r i c p r e t r e a t m e n t o r r e a c t o r s y s t e m s d i r e c t l y t o UHV s u r f a c e a n a l y s i s s y s t e m s h a s become common place. This combination o f t e c h n i q u e s has e s t a b l i s h e d a c l e a r r e l e v a n c y f o r UHV s u r f a c e s c i e n c e i n t h e a r e a o f c a t a l y s i s . It p e r m i t s b o t h d e t a i l e d m e c h a n i s t i c s t u d i e s o v e r w e l l d e f i n e d model s u r f a c e s , as w e l l as c h a r a c t e r i z a t i o n o f i n d u s t r i a l c a t a l y s t s i n t h e i r true activated form. One c a t a l y s t s y s t e m w h i c h h a s r e c e i v e d t h i s type of a t t e n t i o n i s the i r o n Fischer-Tropsch catalyst.(1-5) T h e i r o n c a t a l y s t i s a c o m p l e x m a t e r i a l whose c o m p o s i t i o n i s s o m e what d y n a m i c . The c a t a l y s t i s g e n e r a l l y p r e p a r e d a s a h i g h s u r f a c e a r e a o x i d e (Fe Û3) w i t h t h e a d d i t o n o f b o t h t e x t u r a l ( S i O o , Cu) and c h e m i c a l (K) p r o m o t e r s . P r i o r t o use the c a t a l y s t i s s u b j e c t e d t o various pretreatments which involve either reduction i n H2 or d i r e c t c o n t a c t w i t h CO/H2 m i x t u r e s . The p u r p o s e o f t h e s e p r e t r e a t ments i s t o s y n t h e s i z e a w o r k i n g s u r f a c e w h i c h e x h i b i t s a d e s i r a b l e c a t a l y t i c response. C o n t r o l o f t h e s u r f a c e s y n t h e s i s s t e p i s a key technological challenge in industrial c a t a l y s i s . In t h i s p a p e r we r e p o r t how a h i g h p r e s s u r e r e a c t o r / U H V e l e c t r o n s p e c t r o m e t e r s y s t e m can be used to monitor changes in surface composition that accompany these pretreatments. The two c a t a l y s t s s t u d i e d were moderate s u r f a c e a r e a powders ( 1 5 M 2 / g ) w i t h and w i t h o u t 0 . 6 wt.% 2

0097-6156/85/0288-0124$06.00/0

© 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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125

K2CO3. The r e s u l t s i n d i c a t e t h a t t h e w o r k i n g c a t a l y t i c s u r f a c e i s a c a r b i d e d form o f i r o n which i s s y n t h e s i z e d under C 0 / H 2 . It i s a l s o found, that the type of carbon deposit that f o r m s on t h e surface i s s e n s i t i v e t o the presence of surface a l k a l i . Experimental The e x p e r i m e n t a l a p p a r a t u s shown s c h e m a t i c a l l y i n F i g u r e 1 h a s b e e n described elsewhere.^) I t c o n s i s t s o f a medium p r e s s u r e m i c r o r e a c t o r c o u p l e d t o a n u l t r a - h i g h vacuum s y s t e m e q u i p p e d t o p e r f o r m x-ray photoelectron spectroscopy. The XPS s y s t e m c o n s i s t e d o f a L H S - 1 0 e l e c t r o n e n e r g y a n a l y z e r a n d a d u a l a n o d e x - r a y s o u r c e (Mg and Al). The m i c r o - r e a c t o r was a s m a l l UHV c o m p a t i b l e tube furnace. The r e a c t o r ' s i n t e r n a l v o l u m e was a p p r o x i m a t e l y 10 c c a n d t h e w a l l s were g o l d p l a t e d f o r i n e r t n e s s . The r e a c t o r was d e s i g n e d such t h a t t h e sample an gas m i x i n g t a k e s p l a c p r e s s e d i n t o a g o l d mesh b a c k i n g m a t e r i a l w h i c h i n t u r n was m o u n t e d on a g o l d s a m p l e b o a t . The s a m p l e a n d b o a t c o u l d b e moved d i r e c t l y f r o m t h e r e a c t o r i n t o t h e UHV s y s t e m v i a a m a g n e t i c a l l y coupled motion feed through. The i r o n p o w d e r was p r e p a r e d by r e d u c i n g u l t r a - h i g h purity Fe 0o i n a s e p a r a t e t u b e f u r n a c e . The r e d u c t i o n was c a r r i e d o u t t o c o m p l e t i o n a t 675K, 1 atm H 2 f o r a p p r o x i m a t e l y 24 h o u r s . The s u r f a c e o f t h i s p r y o p h o r i c m a t e r i a l was t h a n p a s s i v a t e d by e x p o s u r e t o 1% o x y g e n i n a h e l i u m c a r r i e r f o r 2 h o u r s . The p a s s i v a t e d p o w ­ d e r was c h a r a c t e r i z e d b y x - r a y d i f f r a c t i o n (XRD) a n d o n l y α-iron was detected. XPS a n a l y s i s of the surface of t h i s material r e v e a l e d o n l y Fe^Oo p r e s e n t . These r e s u l t s s u g g e s t t h a t t h e i r o n bulk i s covered witn a r e l a t i v e l y t h i n oxide s k i n . 2 . 5 g r a m s o f t h i s p a s s i v a t e d m a t e r i a l was c o a t e d w i t h .015 grams o f K 2 C 0 3 t h r o u g h a s t a n d a r d a q u e o u s i m p r e g n a t i o n technique. The amount o f a l k a l i was c h o s e n t o m a t c h t h e 0 . 6 % by w e i g h t c a l l e d f o r i n many i r o n c a t a l y s t p r e p a r a t i o n s . ( 7 ) This loading of K2C03 i s t h o u g h t t o p r o d u c e t h e maximum p r o m o t i o n a l e f f e c t . The i m p r e g ­ n a t e d c a t a l y s t was a i r d r i e d a t 3 3 5 Κ f o r 12 h o u r s t o r e m o v e e x c e s s water. The p h y s i c a l s u r f a c e a r e a s o f t h e t w o s a m p l e s ( w i t h a n d without K0CO3) w e r e m e a s u r e d by a s t a n d a r d BET m e t h o d after a second hydrogen r e d u c t i o n . The a l k a l i t r e a t e d c a t a l y s t h a d a s u r ­ f a c e area o f 16M2/gram and t h e u n t r e a t e d c a t a l y s t a s u r f a c e a r e a o f 18 M 2 / g r a m . Assuming complete d i s p e r s i o n o f t h e a l k a l i and an i r o n surface s i t e density of 1 0 1 5 s i t e s / c m 2 , t h e a l k a l i surface coverage on t h e p r o m o t e d c a t a l y s t i s a p p r o x i m a t e l y 1 / 3 o f a m o n o l a y e r . 2

The g a s e s u s e d w e r e p u r c h a s e d p r e m i x e d i n a l u m i n u m c y l i n d e r s to avoid carbonyl formation. The h i g h p u r i t y g a s m i x t u r e was further p u r i f i e d by a z e o l i t e w a t e r t r a p a n d a c o p p e r carbonyl trap. The g a s p r e s s u r e i n t h e r e a c t o r was m e a s u r e d w i t h a c a p c i t a n c e manometer and t h e f l o w m o n i t o r e d w i t h a mass f l o w c o n t r o l ­ ler. The t y p i c a l g a s f l o w r a t e s w e r e 15 c c / m i n ( S T P ) a n d t h e maximum c o n v e r s i o n was « 1% b a s e d on i n t e g r a t i o n of hydrocarbon products. The h y d r o c a r b o n p r o d u c t s w e r e a n a l y z e d by g a s c h r o m a t o g ­ r a p h y ( t e m p e r a t u r e programmed c h r o m o s o r b 1 0 2 , F I D ) .

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Apparatus. Schematic o f Experimental 1.

MEDIUM PRESSURE REACTOR/X-RAY PHOTOELECTRON SPECTROMETER

Figure

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Iron Fischer- Tropsch Catalysts

127

Results In t h i s s e c t i o n t h e r e s u l t s o f two s t u d i e s a r e s u m m a r i z e d . The f i r s t s t u d y was c a r r i e d o u t w i t h an u n p r o m o t e d i r o n p o w d e r . The second study used t h e potassium promoted i r o n powder. A detailed r e p o r t o f t h e f i r s t s t u d y h a s been p r e v i o u s l y p u b l i s h e d . ( 6 ) The key f e a t u r e s of t h i s study are summarized here to f a c i l i t a t e a comparison between the catalytic response of the promoted and unpromoted powder. Unpromoted Iron Powder. The XPS s p e c t r u m o f t h e f r e s h l y prepared i r o n p o w d e r i s shown i n F i g u r e 2 a . The i r o n 2 p 3 ^ 2 p h o t o l i n e i s c e n t e r e d at 710.7 eV a n d t h e o x y g e n l i n e i s c e n t e r e d a t 5 2 9 . 7 e V . The p o s i t i o n a n d i n t e n s i t i e s o f t h e s e l i n e s a r e c o n s i s t e n t w i t h a s u r f a c e l a y e r o f F e 2 0 3 on t h e i r o n p o w d e r . ( 8 ) In a d d i t i o n t o t h e i r o n o x i d e , a s m a l l amoun the surface of the c a t a l y s t s a m p l e was moved t o t h e r e a c t o r a n d r e d u c e d i n H 2 a t 6 2 5 Κ f o r approximately 2 hours. This pretreatment, a s shown i n F i g u r e 2b, was s u f f i c i e n t t o r e d u c e t h e s u r f a c e o x i d e t o m e t a l ( B . E . Fe2p3^2 = 706.6 eV). The o n l y d e t e c t i b l e i m p u r i t i e s on t h e s u r f a c e o f t h e c a t a l y s t a f t e r r e d u c t i o n were t r a c e amounts o f s u l f u r , c a r b o n and oxygen. U s i n g s t a n d a r d XPS c r o s s s e c t i o n s i t was e s t i m a t e d that these i m p u r i t i e s were less than 1 atom % o f t h e XPS sampling volume. A f t e r r e d u c t i o n and s u r f a c e c h a r a c t e r i z a t i o n , t h e i r o n sample was moved t o t h e r e a c t o r a n d b r o u g h t t o t h e r e a c t i o n c o n d i t i o n s (7 a t m , 3 : 1 H 2 : C 0 , 540 K ) . Once t h e r e a c t o r t e m p e r a t u r e , g a s f l o w a n d pressure were s t a b i l i z e d ( * 10 m i n . ) t h e catalytic activity and selectivity were monitored by o n - l i n e gas chromatography. As p r e v i o u s l y r e p o r t e d , t h e i r o n powder e x h i b i t e d an i n d u c t i o n p e r i o d i n which the c a t a l y t i c a c t i v i t y i n c r e a s e d with t i m e . The c a t a l y s t reached steady state activity after approximately 4 hours on line. This i n d u c t i o n p e r i o d i s b e l i e v e d t o be t h e r e s u l t o f a c o m p e t i t i o n f o r s u r f a c e c a r b o n between bulk c a r b i d e f o r m a t i o n and hydrocarbon s y n t h e s i s . ( 6 , 9 ) Steady s t a t e s y n t h e s i s i s reached o n l y a f t e r the surface region of the c a t a l y s t i s f u l l y c a r b i d e d . To verify that steady state catalytic activity had been a c h i e v e d , t h e c a t a l y s t was a l l o w e d t o o p e r a t e u n i n t e r r u p t e d f o r a p ­ proximately 8 hours. The c a t a l y s t was t h e n r e m o v e d f r o m t h e r e a c ­ t o r a n d t h e s u r f a c e i n v e s t i g a t e d by X P S . The r e s u l t s a r e shown i n Figure 2c. The t w o m a j o r c h a n g e s i n t h e XPS s p e c t r u m w e r e a s h i f t i n t h e i r o n 2 p 3 ^ 2 l i n e t o 7 0 6 . 9 eV a n d a new c a r b o n I s l i n e c e n ­ t e r e d at 283.3 eV. T h i s c o m b i n a t i o n o f i r o n and c a r b o n lines i n d i c a t e s t h e f o r m a t i o n o f an i r o n c a r b i d e p h a s e w i t h i n t h e XPS sampling volume.(J5) In f a c t a f t e r e x t e n d e d o p e r a t i o n , XRD o f t h e i r o n s a m p l e i n d i c a t e d t h a t t h e b u l k had b e e n c o n v e r t e d t o FecC2 c o m m o n l y r e f e r r e d t o a s t h e Hagg c a r b i d e . ( _ 7 ) It appears t h a t the bulk and s u r f a c e a r e f u l l y c a r b i d e d u n d e r d i f f e r e n t i a l reaction conditions. The s t e a d y s t a t e r a t e s o f h y d r o c a r b o n s y n t h e s i s o v e r t h e c a r ­ b i d e d i r o n s u r f a c e a r e g i v e n i n Table I. The r e a c t i o n r a t e s h a v e been n o r m a l i z e d t o t h e p h y s i c a l s u r f a c e a r e a of t h e s t a r t i n g i r o n powder [18 M 2 / g ] and a r e r e p o r t e d i n m o l e c u l e s / c m 2 s e c . A turnover

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

732.0

522.0 307.0

702.0 542.0

l

I

I

Figure 2. XPS s p e c t r a o f i r o n p o w d e r a f t e r e x p o s u r e t o v a r i o u s environment, a) F r e s h l y prepared a i r exposed powder, b) A f t e r h y d r o g e n r e d u c t i o n 2 a t m H 2 a t 6 2 5 K. c) After steady s t a t e operation f o r 8 h r , 3:1 H2:C0 7 a t m , 540 K.

BINDING ENERGY eV

4X

2X

0(1s)

L

b

•A

ι 277.0

8

c

-F /· /

C(1S)

Table

I.

Steady

State

Rates Iron

540 K, Iron Carbon

129

Iron Fischer- Tropsch Catalysts

11. DWYER

Number

of

Hydrocarbon

Synthesis

Over

Carbide

H?:C0,

3:1

7

Potassium

Carbide

Rate (Molecules/cnr X

2.4

X

1.6

X

7.4

X

10,11

1.8

X

10

Promoted

Rate (Molecules/cnr

sec)

,12 10; 12 10 ,12 10;

4.5

atm

4.1

X

10;

3.2

X

10

3.2

X

10;

1.9

X

10

9.6

X

10

sec)

number h a s n o t b e e n r e p o r t e d s i n c e t h e i r o n s u r f a c e s i t e d e n s i t y o f t h e c a r b i d e d m a t e r i a l i s unknown. If a s i t e d e n s i t y of 1 0 1 5 / c n r i s c h o s e n t h e s t e a d y s t a t e m e t h a n a t i o n r a t e i s on t h e o r d e r o f 10"3 molecules/site sec. This turnover frequency i s considerably lower than that reported in e a r l i e r studies (range .05 to 2).(2,4,9) This r e s u l t suggest that only a small f r a c t i o n of the i r o n carbide surface i s active. Potassium Modified Iron Powder. Surface analysis (XPS) of the freshly prepared Dotassiurn m o d i f i e d surface i s given i n Figure 3a. The i r o n 2 p 3 ' 2 b i n d i n g e n e r g y i s l o c a t e d a t 7 1 0 . 6 eV a n d t h e dominant oxygen Is l i n e i s a t 5 2 9 . 7 eV. These v a l u e s a r e a g a i n consistent with a s u r f a c e l a y e r of Fe 0o. In a d d i t i o n t o the s u r f a c e o x i d e , K2CO3 i s a l s o p r e s e n t on t n e s u r f a c e o f t h e c a t a lyst. The p r e s e n c e o f t h e c a r b o n a t e i s i n d i c a t e d by a p o t a s s i u m 2 p 3 / 2 p e a k a t « 2 9 3 eV a n d a c a r b o n a t e c a r b o n l i n e a t « 2 8 9 e V . A h i g h b i n d i n g e n e r g y s h o u l d e r i s a l s o p r e s e n t on t h e o x y g e n Is l i n e b u t an e x a c t b i n d i n g energy i s d i f f i c u l t t o measure due t o the overlap with the strong iron oxide s i g n a l . These r e s u l t s a r e i n g e n e r a l a g r e e m e n t w i t h t h o s e r e p o r t e d by B o n z e l a n d c o - w o r k e r s ( 5 ) f o r K0CO3 t r e a t e d i r o n f o i l s . The p o t a s s i u m t r e a t e d m a t e r i a l was t h e n moved t o t h e m i c r o r e a c t o r and r e d u c e d under c o n d i t i o n s i d e n t i c a l t o t h o s e used f o r t h e unpromoted i r o n . F i g u r e 3b c o n t a i n s t h e XPS s p e c t r u m o f t h e modified surface after hydrogen reduction. Once a g a i n t h e iron 2 p 3 / 2 peak i s c e n t e r e d a t 7 0 6 . 6 eV i n d i c a t i n g t h e r e d u c t i o n o f t h e iron oxide to m e t a l l i c i r o n . The m a i n o x i d e o x y g e n I s s i g n a l a t 5 2 9 . 7 eV i s a l m o s t t o t a l l y removed f r o m t h e s p e c t r u m s u g g e s t i n g complete reduction of the o x i d e . In t h e p o t a s s i u m 2p a n d t h e c a r bon I s r e g i o n o f t h e s p e c t r u m i m p o r t a n t c h a n g e s t a k e p l a c e . The k e y f e a t u r e s a r e t h a t t h e p o t a s s i u m 2p s i g n a l i n c r e a s e s r e l a t i v e t o the carbonate carbon signal ( B E « 2 8 9 ) , and t h e appearance of a 2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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s t r o n g oxygen Is l i n e at 531.9 eV. These r e s u l t s a r e c o n s i s t e n t w i t h t h e p a r t i a l d e c o m p o s i t i o n o f K2CO3 i n t o a s u r f a c e KOH l a y e r a s d e s c r i b e d by B o n z e l a n d c o - w o r k e r s . ( 2 j J > ) The c h a n g e i n t h e r a t i o of carbonate carbon to potassium signal i n t e n s i t y suggests that some o f t h e c a r b o n a t e c a r b o n i s l o s t d u r i n g t h e r e d u c t i o n . The oxygen Is feature at 531.9 eV i s i n d i c a t i v e of the formation chemisorbed KOH.(5) The f r e s h l y r e d u c e d p o t a s s i u m m o d i f i e d s u r f a c e was t e s t e d f o r catalytic activity under c o n d i t i o n s i d e n t i c a l t o those used for unpromoted m a t e r i a l (540K, 3:1 Ho.'CO, 7 a t m ) . The p r o m o t e d iron powder e x h i b i t e d an i n d u c t i o n p e r i o d w h e r e i n t h e c a t a l y s t a c t i v i t y increased with time. H o w e v e r , t h e i n d u c t i o n p e r i o d was c o n s i d e r ­ ably shorter ( « 1 hr) than that observed over the unpromoted surface. T h i s r e s u l t may i n d i c a t e a m o r e r a p i d c a r b i d i n g o f t h e iron surface region. The s t e a d y s t a t e h y d r o c a r b o n s y n t h e s i s r a t e s are given in Table I p r o d u c t i o n , t h e most s i g n i f i c a n a n d u n p r o m o t e d m a t e r i a l s i s t h e much l o w e r m e t h a n a t i o n rate over the promoted s u r f a c e . On a p h y s i c a l s u r f a c e a r e a b a s i s (reduced promoted i r o n 15rrr/g) t h e m e t h a n a t i o n a c t i v i t y o v e r t h e promoted c a t a l y s t i s s u p p r e s s e d by a l m o s t an o r d e r o f m a g n i t u d e . The r a t e s o f f o r m a t i o n o f o t h e r m o l e c u l e s i n t h e C j t o Cg r a n g e w e r e a l s o suppressed but to a decreasing degree with increasing chain length. F o r e x a m p l e , t h e p r o d u c t i o n o f m e t h a n e was approximately 13% o f t h e u n p r o m o t e d r a t e . The r a t e o f Cg p r o d u c t i o n was 53% o f t h e unpromoted rate. O n c e t h e s t e a d y s t a t e a c t i v i t y h a d b e e n v e r i f i e d by c o n t i n u o u s o p e r a t i o n f o r e i g h t h o u r s w i t h o u t l o s s o f a c t i v i t y , t h e s a m p l e was removed from t h e r e a c t o r f o r s u r f a c e a n a l y s i s . The XPS r e s u l t s a r e given i n Figure 3c. The m a j o r c h a n g e i n t h e XPS s p e c t r u m i s t h e l a r g e i n c r e a s e i n the carbon Is s i g n a l i n t e n s i t y . Close inspection o f t h e c a r b o n r e g i o n r e v e a l s t h a t two d i s t i n c t c a r b o n s i g n a l s a r e present. The m a j o r p e a k i s c e n t e r e d a t 2 8 5 . 7 , t h e o t h e r a t 2 8 3 . 3 eV. In a d d i t i o n , t h e i r o n 2 p 3 ^ 2 i s c e n t e r e d a t 7 0 6 . 9 e V . Once a g a i n , we b e l i e v e t h a t t h e 2 8 3 . 3 eV c a r b o n f e a t u r e a n d t h e 7 0 6 . 9 eV i r o n s i g n a l are c l e a r i n d i c a t o r s of i r o n carbide f o r m a t i o n . The key q u e s t i o n , however, i s t h e n a t u r e of t h e i n t e n s e carbon f e a t u r e centered at 285.7. Previous work h a s s u g g e s t e d t h a t potassium i n c r e a s e s t h e r a t e o f g r a p h i t e d e p o s i t i o n on i r o n s u r f a c e s u n d e r reaction conditions.(1,3) We b e l i e v e t h a t t h e c a r b o n s p e c i e s i s n o t g r a p h i t i c but i s an a d s o r b e d h y d r o c a r b o n phase ( g r o w i n g c h a i n s o r h i g h m o l e c u l a r w e i g h t p r o d u c t s ) on t h e s u r f a c e . The argument for this assignment is two-fold. First, t h e 2 8 5 . 7 eV binding energy is identical to that measured for octacosane (C23H50) a d s o r b e d on a i r o n foil(6). Second, i s t h e mass s p e c t r u m of m a t e r i a l d e s o r b e d f r o m t h i s s a m p l e upon h e a t i n g t o 4 2 5 Κ i n t h e vacuum s y s t e m . I t c o n s i s t s o f a c r a c k i n g p a t t e r n s t a r t i n g a t mass 15 a n d c o n t i n u i n g a t m u l t i p l e s o f mass 14 a s h i g h a s t h e mass s p e c t r o m e t e r p e r m i t t e d (200 amu). This cracking pattern i s c l e a r l y that of l i n e a r saturated hydrocarbons s i m i l a r t o polymethylene (nB a s e d on t h e a t t e n u a t i o n o f t h e i r o n 2p 1 s i g n a l and assuming a mean f r e e p a t h f o r t h e i r o n e l e c t r o n s o f 1 . 5 t o 2 n a n o m e t e r s , it i s estimated that the carbon o v e r l a y e r i s at l e a s t 1.8 t o 2.5 nano-

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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meters t h i c k . It i s interesting to note, h e a v y b u i l d up o f m a t e r i a l o n t h e c a t a l y s t , remains at steady state.

that in spite the c a t a l y t i c

of this activity

Conclusions^ This XPS investigation of small iron Fischer-Tropsch catalysts b e f o r e and a f t e r t h e p r e t r e a t m e n t and e x p o s u r e t o s y n t h e s i s gas has y i e l d e d the f o l l o w i n g information. Relatively mild reduction cond i t i o n s ( 3 5 0 ° C , 2 a t m , Hg) a r e s u f f i c i e n t t o t o t a l l y r e d u c e s u r f a c e o x i d e on i r o n t o m e t a l l i c i r o n . Upon e x p o s u r e t o s y n t h e s i s g a s , the m e t a l l i c i r o n surface i s converted to i r o n c a r b i d e . During this transformation, the c a t a l y t i c response of the m a t e r i a l inc r e a s e s and f i n a l l y r e a c h e s s t e a d y s t a t e a f t e r t h e s u r f a c e i s f u l l y carbided. The a d d i t i o n o f a p o t a s s i u m p r o m o t e r a p p e a r s t o a c c e l e r ate the c a r b i d a t i o n of achieved somewhat e a r l i e r c a u s e s a b u i l d up on c a r b o n a c e o u s m a t e r i a l on t h e s u r f a c e o f the c a t a l y s t s which i s b e s t c h a r a c t e r i z e d as p o l y m e t h y l e n e .

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

Bonzel, H. P.; Krebs, H. J. Surface Science, 1981, 109, L527. Krebs, H. J . ; Bonzel, H. P. Surface Science, 1982, 88, 269. Bonzel, H. P.; Chem. Ing. Tech, 1982, 54, 908. Dwyer, D. J . ; Somorjai, G. A. J. Catalysis, 1970, 52, 291. Bonzel, H. P.; Broden, G.; Krebs, H. J. Applications of Surface Science,1983, 16, 373. Dwyer, D. J . ; Hardenbergh, J. H. J. Catalysis, 1984, 87, 66. Storch, H. H.; Golumbic, N.; Anderson, R. B. "The FischerTropsch and Related Synthesis," John Wiley: New York, 1951. Hirohawa, H.; Okee, H. Talanta, 1979, 26, 855. Vannice, M. A. J. Catalysis, 1975, 37, 449.

RECEIVED May 2, 1985

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

12 Low-Energy Ion-Scattering Spectroscopy: Applications to Catalysts James C. Carver, S. Mark Davis, and Duane A. Goetsch Exxon Research & Development Laboratories, Baton Rouge, LA 70821

Over the l a s t niques such as study the surfaces of catalysts with varying degrees of success. Low Energy Ion Scattering Spectroscopy (LEISS), also known as Ion Scattering Spectroscopy (ISS), has i n general not received as much attention as the other techniques, although LEISS offers unique surface s e n s i t i v i t y to the topmost atomic layer of a catalyst or, under d i f f e r e n t conditions, to the second or third layer. We have used LEISS to study supported metal, metal oxide, and metal s u l f i d e catalysts. With single c r y s t a l layered sulfides such as MoS2, conditions were adjusted so that f i r s t layer analysis was possible. The top layer was sulfur and the second layer was Mo. Support of MoS2 on alumina causes d i s t i n c t l y different spectra from the unsupported material which suggests that the c r y s t a l orientations d i f f e r . In addition, contaminants such as Na or K, which sometimes can unexpectedly dominate a surface, are also c l e a r l y seen by LEISS, while other substances such as carbon, which may be abundant on a surface, are difficult to see because their scattering cross-section i s so small. In such cases combination of LEISS with XPS or SIMS can provide unique and invaluable information about catalysts. D u r i n g t h e p a s t s e v e r a l y e a r s , S u r f a c e S c i e n c e has begun t o have a major impact on t h e f i e l d o f c a t a l y s i s . ( 1 , 2 ) Numerous t o o l s have been developed f o r a p p l i c a t i o n t o s t u d y i n g c a t a l y s t s . One of these t o o l s , Low Energy Ion S c a t t e r i n g S p e c t r o s c o p y ( a l s o known as ISS o r L E I S S ) , p r o v i d e s unique d a t a a r i s i n g from t h e top one o r two l a y e r s o f a s u r f a c e . However, t h i s t e c h n i q u e has been used o n l y s p a r i n g l y f o r t h e s e k i n d s o f s t u d i e s . ( 3 ) I t s l a c k o f p o p u l a r i t y may be due t o e x p e r i m e n t a l difficulties a s s o c i a t e d w i t h examining i n s u l a t o r s , o r perhaps because o f ambiguous d a t a i n t e r p r e t a t i o n f o r p r a c t i c a l samples. The b a s i c 0097-6156/85/0288-0133$06.00/0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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t h e o r i e s have been d i s c u s s e d a t l e n g t h , but f o r the most p a r t these t h e o r i e s have c o n c e n t r a t e d on metals and semiconduc­ tors. ( 4 ^ ) In a d d i t i o n , much o f the e a r l y e x p e r i m e n t a l d a t a were o b t a i n e d a t r a t h e r h i g h i o n f l u x e s and w i t h r a t h e r poor vacuum.(6) We r e c o g n i z e now t h a t m i s l e a d i n g d a t a can a r i s e from such a p r o c e d u r e , and i n s t e a d , LEISS s h o u l d be done i n a UHV system u s i n g nA i o n c u r r e n t s and w i t h a low energy beam.(7_) A l s o , sample c h a r g i n g can c o m p l i c a t e the s i t u a t i o n g r e a t l y . As w i t h any charged p a r t i c l e t e c h n i q u e , s u r f a c e c h a r g i n g can be a l i m i t i n g problem when a t t e m p t i n g to do LEISS on i n s u l a t i n g s u r ­ f a c e s such as c a t a l y s t s . ( 7 _ ) W i t h proper charge compensation and c a r e f u l c o n t r o l of the i o n beam c u r r e n t , these l i m i t a t i o n s can be overcome and LEISS can be a v e r y p o w e r f u l t o o l f o r c a t a l y s t studies. LEISS d a t a a r e p r e s e n t e d on a v a r i e t y o f c a t a l y s t s t o demonstrate the unique nique. LEISS can p r o v i d but a l s o y i e l d new d a t a r e q u i r i n g r e i n t e r p r e t a t i o n of o t h e r data. LEISS can seldom s t a n d a l o n e , e s p e c i a l l y w i t h i n s u l a ­ tors. However, we f i n d LEISS t o be e x t r e m e l y u s e f u l i n r e s e a r c h on c a t a l y s t s . Experimental Low Energy Ion S c a t t e r i n g experiments were done on a Le y bo I d Heraeus spectrometer which uses a h e m i s p h e r i c a l a n a l y z e r w i t h a lens. The i o n gun was c o n t i n u o u s l y c o n t r o l l a b l e from about 200 eV t o 5000 eV w i t h i o n c u r r e n t s a t the sample r a n g i n g from about lnA/(cm) t o s e v e r a l yA/(cm) . As complementary d a t a , Secondary Ion Mass Spectrometry and X-ray P h o t o e l e c t r o n Spectroscopy were performed u s i n g the LH i n s t r u m e n t . He i o n s were used as the p r i m a r y s o u r c e , a l t h o u g h ^He , ^Ne , and A r i o n s were a l s o available. The l a b o r a t o r y s c a t t e r i n g a n g l e f o r i o n s c a t t e r i n g i s 120°. +

+

+

Q u a l i t a t i v e Aspect of LEISS Low Energy Ion S c a t t e r i n g Spectroscopy i s q u i t e s i m p l e i n p r i n ­ ciple. The p r o c e s s depends on the d o m i n a t i o n o f a s i n g l e b i n a r y elastic collision. F i g u r e 1 i l l u s t r a t e s the fundamental p r i n c i ­ p l e s upon which LEISS i s based. An incoming i o n w i t h a mass o f M a t an energy of E and a t a v e l o c i t y of V c o l l i d e s w i t h a solid surface. The p r i m a r y i o n i s then s c a t t e r e d from t h a t s u r f a c e a t some a n g l e (Θ) w i t h an energy ( E ) and a v e l o c i t y ( V ) w h i c h are determined by the t a r g e t atom i t s t r i k e s . This t a r g e t atom, which has a mass of M. and i s i n i t i a l l y a t r e s t , r e c o i l s i n t o the s o l i d a t an a n g l e (φ) and a v e l o c i t y V , c a r r y i n g w i t h i t an energy E . Using c l a s s i c a l r e l a t i o n s h i p s , the f o l l o w i n g e q u a t i o n can be g e n e r a t e d : Q

Q

Q

f

1

t

fc

E»/E

2

0

» [M /(M +M )] [cos Θ + 0

0

t

[(M^M^-sin

2

Θ]

1 / 2

]

2

(1)

T h e r e f o r e , i t i s c l e a r t h a t i f t h e r e are o n l y s i n g l e c o l l i s i o n s and t h e r e a r e no energy l o s s e s except k i n e m a t i c , then the s p e c t r a a r e q u i t e s i m p l e and s t r a i g h t f o r w a r d . However, we a r e

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

12.

CARVER ET

AL.

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d e a l i n g w i t h i o n s as the primary beam, and thus we have t o d e a l w i t h i o n n e u t r a l i z a t i o n of the e x i t i n g beam. The d e t a i l s o f t h i s phenomenon a r e d i s c u s s e d elsewhere and w i l l o n l y be b r i e f l y reviewed h e r e . ( 5 ) Only a few percent of the i o n s w h i c h impinge onto the s u r f a c e remain i o n s upon s c a t t e r i n g . Much o f the n e u t r a l i z a t i o n o c c u r s by an Auger type process i n which the i n c o m i n g i o n g a i n s an e l e c t r o n from the t a r g e t . The l o n g e r the i n t e r a c t i o n t i m e , the g r e a t e r the chance f o r n e u t r a l i z a t i o n . Thus, as the energy of the p r i m a r y i o n i s lowered, i t i s more l i k e l y t o be n e u t r a l i z e d . In a d d i t i o n , s c a t t e r i n g from the second o r t h i r d l a y e r s i n c r e a s e s the r e s i d e n c e time o f the p r i ­ mary i o n and enhances i t s chances of n e u t r a l i z a t i o n . When s c a t t e r i n g from these deeper l a y e r s o c c u r s o r when n o n - k i n e m a t i c energy l o s s e s o c c u r , a s h o u l d e r on the lower energy s i d e o f the p r i m a r y peak can be seen. I f double s c a t t e r i n g o c c u r s , then a s h o u l d e r on the h i g h e the primary i o n ' s energ more from deeper l a y e r s and more s p u t t e r i n g o c c u r s In F i g u r e 2 we can observe the i n c r e a s e d background between peaks and a s l i g h t skewing o f the s t r o n g e r peaks toward lower energy* Though not shown h e r e , at higher i o n c u r r e n t s even more pronounced skewing o c c u r s and some double s c a t t e r peaks b e g i n t o a r i s e * (7_) F i n a l l y , when d e a l i n g w i t h i n s u l a t o r s , the a n g l e s Θ and φ are a l t e r e d as shown i n F i g u r e 3* I f the s u r f a c e has a p o s i t i v e c h a r g e , the incoming p o s i t i v e i o n i s r e p e l l e d c a u s i n g the i o n t o curve e i t h e r away from the s u r f a c e or t o be focused through the f i r s t l a y e r t o deeper l a y e r s * The s c a t t e r e d i o n s and s p u t t e r e d i o n s a l s o are r e p e l l e d from the s u r f a c e c a u s i n g them to g a i n energy* T h e r e f o r e , s i n c e the s c a t t e r i n g angle Θ i s no l o n g e r w e l l d e f i n e d , the e x a c t peak p o s i t i o n can d e v i a t e from i t s theoretical position* F u r t h e r , the v e l o c i t y o f the e x i t i n g i o n will be a l t e r e d from s i m p l e theory causing an a d d i t i o n a l d e v i a t i o n from the expected peak p o s i t i o n * A l s o , the d e v i a t i o n from t h e o r y w i l l not n e c e s s a r i l y be l i n e a r f o r a g i v e n sample* Charge compensation w i t h a low energy e l e c t r o n f l o o d gun a l l o w s one to o b t a i n m e a n i n g f u l s p e c t r a even though the measured peak p o s i t i o n s do not always c o i n c i d e w i t h the expected positions* The s p u t t e r peak i s d i m i n i s h e d and the s c a t t e r peaks emerge from the background when a f l o o d gun i s used on an i n s u l a t i n g sample.(7) Although peak i d e n t i f i c a t i o n i s not a b s o l u t e , i n g e n e r a l , when coupled w i t h XPS o r SIMS, s u r f a c e s p e c i e s can be d e t e r m i n e d . Q u a n t i t a t i v e A s p e c t s of LEISS The i n t e n s i t y f o r LEISS u s i n g a noble gas as the p r i m a r y i o n source i s q u i t e low s i n c e , as p r e v i o u s l y d i s c u s s e d , most incoming i o n s a r e n e u t r a l i z e d upon s c a t t e r i n g and thus go undetected. F u r t h e r , the r e l a t i v e i n t e n s i t y of the v a r i o u s components i s a s t r o n g f u n c t i o n of the i n c i d e n t i o n energy as shown i n F i g u r e 2. These changes i n i n t e n s i t y can be a t t r i b u t e d to s e v e r a l f a c t o r s as d e s c r i b e d below: X

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In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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137

where 1^ = s c a t t e r e d i o n i n t e n s i t y from the i t h s p e c i e s , I = i n c i d e n t beam i n t e n s i t y , - atomic f r a c t i o n o f i t h s p e c i e s a t t h e s u r f a c e , do/d9 = d i f f e r e n t i a l s c a t t e r i n g c r o s s - s e c t i o n o f the i t h s p e c i e s , ISP - i o n s u r v i v a l p r o b a b i l i t y , - geometric o r shadowing f a c t o r , Τ - a n a l y z e r t r a n s m i s s i o n f a c t o r , D d e t e c t o r e f f i c i e n c y , d6 s o l i d a n g l e o f acceptance f o r the analyzer* The i n c i d e n t i o n beam i n t e n s i t y can be measured, and t h e r e a r e s e v e r a l t a b u l a t i o n s of c r o s s - s e c t i o n c a l c u l a t i o n s * (8) A l s o , t h e a n a l y z e r parameters, T, D, and d9 can be determined* The t h r e e a s p e c t s of t h i s e q u a t i o n , w h i c h a r e n o t w e l l u n d e r s t o o d nor e a s i l y d e t e r m i n e d , i n c l u d e the number o f atoms of a p a r t i c u l a r k i n d , the i o n s u r v i v a l p r o b a b i l i t y , and t h e shadowing o r g e o m e t r i c term* The f i r s t q u a n t i t y i s q u i t e o f t e n t h a t which you would l i k e t o d e t e r m i n e * Hie second two are often d i f f i c u l t t i m p o r t a n t when t r y i n t r y i n g t o determine the l o c a t i o n o f a d s o r b a t e s . ( 9 ) However, shadowing f o r p o l y c r y s t a l l i n e samples, though i m p o r t a n t , i s v e r y d i f f i c u l t to deal with q u a n t i t a t i v e l y * Ion n e u t r a l i z a t i o n ( o r i o n s u r v i v a l ) can dominate t h i s t e c h n i q u e * A number of t h e o r i e s have a r i s e n t o account f o r t h i s phenomenon, but a l l seem t o i n c l u d e b o t h Auger t r a n s i t i o n s and resonance t u n n e l i n g p r o c e s s e s as the dominant means o f i o n neutralization*(4,5) For v e r y slow i o n s , as i n LEISS, Auger t r a n s i t i o n s seem t o be more i m p o r t a n t * The i o n s u r v i v a l p r o b a b i l i t y a t c o n s t a n t s c a t t e r i n g a n g l e can be d e f i n e d as f o l l o w s (assuming o n l y Auger t r a n s i t i o n s ) : Q

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where b can be c o n s i d e r e d a c h a r a c t e r i s t i c n e u t r a l i z a t i o n con­ s t a n t , (10) l a r g e l y independent o f energy* The term b^ does i n c l u d e a s m a l l c o n t r i b u t i o n from the i n c i d e n t energy o f the i o n beam, but under the range of e n e r g i e s n o r m a l l y encouraged i n LEISS ( i . e . , -500-2000 e V ) , t h i s c o n t r i b u t i o n i s i n s i g n i f i c a n t . (3,4,10) A p p l i c a t i o n s to Catalysts Surface Sensitivity. U s i n g t h e s e i d e a s , one can g a t h e r i n f o r m a t i o n on a c a t a l y s t which can i n d i c a t e e x a c t l y w h i c h elements a r e on the s u r f a c e and which a r e i n second o r deeper layers. To i l l u t s t r a t e t h i s i d e a , c o n s i d e r MoS * T h i s m a t e r i a l has a l a y e r e d s t r u c t u r e which has Mo sandwiched between s u l f u r layers* LEISS d a t a r e p o r t e d elsewhere show t h a t a t low e n e r g i e s « 6 0 0 eV) almost no Mo i s observed * ( 11 ) At h i g h e r e n e r g i e s , such as o f t e n r e p o r t e d i n the l i t e r a t u r e , we see a pronounced s i g n a l from Mo* S p u t t e r i n g does o c c u r f o r these samples, but t h i s i n c r e a s e d Mo s i g n a l i s not due t o t h a t e f f e c t . The i m p l i ­ c a t i o n i s t h a t LEISS d a t a must be t a k e n a t v e r y low v o l t a g e s t o r e f l e c t only f i r s t layer contributions. One of the concerns i n d o i n g LEISS on c a t a l y s t s i s t h a t the i n f o r m a t i o n w i l l be of l i t t l e use because e i t h e r the f i r s t l a y e r may be p r i m a r i l y carbon o r the s u r f a c e may be too rough. 2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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T h e r e f o r e , i n o r d e r t o be sure t h a t our d a t a were m e a n i n g f u l , we a l s o examined p o l y c r y s t a l l i n e MoS as w e l l as MoS supported on alumina* In a d d i t i o n , the supported MoS cases was promoted w i t h o t h e r metals such as Q> o r N i , which are commonly used i n commercial h y d r o t r e a t i n g c a t a l y s t s * A g a i n , as seen i n the s i n g l e c r y s t a l c a s e , the s p e c t r a f o r p o l y c r y s t a l l i n e MoS i s dominated by s u l f u r a t 500 v* At h i g h e r e n e r g i e s , we see the Mo s i g n a l b e g i n to show through* However, the s u p p o r t e d MoS shows s i g n i f i c a n t Mo even a t 500 v, s u g g e s t i n g t h a t the MoS i s not o r i e n t e d b a s a l p l a n e up but p o s s i b l y edge up.(11) When Go o r N i i s added t o the Mo on the a l u m i n a , we f i n d t h a t the Mo i s unchanged* More q u a l i t a t i v e l y , we can p l o t the r e c i p r o c a l o f the p e r ­ p e n d i c u l a r v e l o c i t y v e r s u s the l o g o f the i o n s u r v i v a l p r o b a ­ bility. Ions a r i s i n g from d i f f e r e n t depths s h o u l d have d i f ­ ferent slopes or i n t e r c e p t r e s u l t i n g l i n e correspond t h a t i s s e n s i t i v e t o the e l e c t r o n i c s t r u c t u r e and the d e p t h d i s t r i b u t i o n o f the s c a t t e r i n g c e n t e r s * C l e a r l y , geometric c o n s i d e r a t i o n s ( o r shadowing) p l a y an Important r o l e i n t h e s e e xperiments* The r a d i u s of the shadow cone o f the i n c i d e n t i o n can be r e p r e s e n t e d by an e q u a t i o n which i n c l u d e s energy* However, d i r e c t i o n a l l y the e f f e c t w i l l be the same as neu­ t r a l i z a t i o n and thus w i l l o n l y exaggerate the r e s u l t s * ( 1 2 ) W i t h t h e s i n g l e c r y s t a l MoS system, we found t h a t s c a t t e r i n g from the S atoms changed v e r y s l o w l y w i t h r e s p e c t t o the i o n energy, w h i l e the molybdenum i n t e n s i t y changed s h a r p l y w i t h energy. Such an a n a l y s i s i s c o n s i s t e n t w i t h the i d e a t h a t the s u l f u r i s a t the s u r f a c e . Ions s c a t t e r e d from the molybdenum (which i s i n the second l a y e r ) are p r e f e r e n t i a l l y n e u t r a l i z e d , p a r t i c u l a r l y a t low energy. A t 500 eV, g r e a t e r than 90% of the s c a t t e r i n g i n t e n s i t y o r i g i n a t e s from f i r s t l a y e r c o l l i s i o n s , w h i l e a t 2000 eV about 70% of the i n t e n s i t y a r i s e s from f i r s t l a y e r s c a t t e r ­ ing. When t h i s same a n a l y s i s i s a p p l i e d to the supported MoS systems, the molybdenum and the s u l f u r have s i m i l a r i n t e n ­ sities. The A l s i t e s seem to be almost c o m p l e t e l y c o v e r e d . These d a t a a r e c o n s i s t e n t w i t h MoS assuming a d i f f e r e n t s u r f a c e s t r u c t u r e when supported than i t does i n a b u l k form. 2

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M e t a l s D i s p e r s i o n from LEISS. Since He s c a t t e r i n g i s very s e l e c t i v e t o the outermost s u r f a c e l a y e r , one s h o u l d a n t i c i p a t e t h a t LEISS would be a v a l u a b l e t o o l f o r s t u d i e s o f metals d i s p e r s i o n f o r supported c a t a l y s t s . For low m e t a l concen­ trations on h i g h a r e a s u p p o r t s , th e ( m e t a l / s u p p o r t ) LEISS intensity ratio s h o u l d be d i r e c t l y p r o p o r t i o n a l t o m e t a l s dispersion. Recent s t u d i e s i n our l a b o r a t o r y have c o n f i r m e d that expectation. In F i g u r e 4, the P t / A l r a t i o from LEISS i s shown f o r a s e r i e s of P t / A l 0 c a t a l y s t s w i t h v a r i a b l e Pt c o n t e n t (0.2-2.0 wt.%) following treatments w i t h 0 a t 500°C and 600°C. For c a t a l y s t s t r e a t e d a t 500°C, the P t / A l i n t e n s i t y r a t i o v a r i e s l i n e a r l y w i t h m e t a l l o a d i n g . However, a f t e r the 600°C t r e a t m e n t , the P t / A l i n t e n s i t y r a t i o i s lowered s u b s t a n t i a l l y , e s p e c i a l l y a t h i g h e r Pt l o a d i n g s . T h i s change i n the P t / A l i n t e n s i t y r a t i o r e f l e c t s a r e s t r u c t u r i n g o f the m e t a l component d u r i n g h i g h 2

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In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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temperature oxygen t r e a t m e n t s . In p a r t i c u l a r , the growth o f l a r g e r p l a t i n u m and/or p l a t i n u m o x i d e c r y s t a l l i t e s appears t o take p l a c e . These c a t a l y s t s were a l s o examined by 0 c h e m i s o r p t i o n t o t e s t the v a l i d i t y of our i n t e r p r e t a t i o n of the LEISS d a t a . A p l o t o f these r e s u l t s i s a l s o shown i n F i g u r e 4. Catalysts t r e a t e d w i t h oxygen at 500°C, and subsequently reduced a t 200500°C, showed h i g h oxygen uptakes (O/Pt - 1 . 1 ) , w h i c h i n d i c a t e e s s e n t i a l l y complete Pt d i s p e r s i o n . However, f o r c a t a l y s t s t r e a t e d w i t h oxygen a t 600°C, the oxygen c h e m i s o r p t i o n c a p a c i t y was s i g n i f i c a n t l y reduced (O/Pt -0.26-1.0). In F i g u r e 4 a l l c a t a l y s t s , r e g a r d l e s s of t r e a t m e n t , f a l l on the same p l o t . W i t h i n the u n c e r t a i n t y of the LEISS i n t e n s i t y r a t i o s an e x c e l ­ l e n t 1:1 c o r r e l a t i o n e x i s t s , c o n f i r m i n g t h a t LEISS i s s e n s i t i v e o n l y t o the f r a c t i o n of exposed p l a t i n u m and thus i s a good probe o f d i s p e r s i o n . 2

Morphology S t u d i e s . Anothe syste WOg/A^O-j. S e v e r a l m a t e r i a l s which v a r i e d i n p e r c e n t monolayer coverage of t u n g s t e n o x i d e a l l showed a d i s c e r n a b l e A l peak (see Figure 5). In a d d i t i o n , t h e r e was a s i g n i f i c a n t Na contamina­ t i o n on these samples. Whereas XPS and Laser Raman s p e c t r o s c o p y had shown a l i n e a r i n c r e a s e i n W/Al up to monolayer c o v e r a g e , (18) LEISS shows t h a t the W/Al r a t i o i s almost c o n s t a n t up t o monolayer coverage where the Na a p p a r e n t l y begins t o dominate the s u r f a c e ( F i g u r e 6 ) . Beyond monolayer coverage, Raman has shown t h a t b u l k - l i k e WOo forms on the s u r f a c e . We a l s o see a s i g n i f i c a n t change i n the LEISS d a t a . The l a c k o f c o n s i s t e n c y between XPS and LEISS i s important s i n c e b o t h t e c h n i q u e s s h o u l d measure s u r f a c e c o n c e n t r a t i o n s . However, the t h e o r y f o r e s t i ­ mating c r y s t a l l i t e s i z e from XPS breaks down f o r v e r y s m a l l p a r t i c l e s . ( 1 4 , 1 5 ) The s m a l l WO^ c r y s t a l l i t e may a l s o be growing i n t o r a f t - l i k e morphologies. For t h i n r a f t s , XPS would s t i l l show almost a l i n e a r i n c r e a s e of W/Al i n t e n s i t y r a t i o s , a l t h o u g h the a c t u a l number of s u r f a c e tungstens would change much l e s s . The f a c t t h a t a change i n s t r u c t u r e t o WO3 or A l ^ W O ^ ^ i s reflected i n the LEISS d a t a shows t h a t the technique is s e n s i t i v e t o s t r u c t u r a l changes. T h e r e f o r e , these d a t a suggest t h a t no s i g n i f i c a n t change i n the s u r f a c e s t r u c t u r e seems t o occur between samples t h a t have l e s s than one monolayer e q u i v a l e n t and c a t a l y s t s which have a p p r o x i m a t e l y one monolayer e q u i v a l e n t of t u n g s t e n . T h i s f a c t i s shown even more c l e a r l y upon p l o t t i n g the ISPs versus the r e c i p r o c a l of the v e l o c i t y . As shown i n F i g u r e 7, a sample c a l c i n e d a t 500° C (which s h o u l d be w e l l under a monolayer) and a sample c a l c i n e d a t 950°C (which s h o u l d be r i g h t a t a monolayer) seem t o be q u i t e s i m i l a r . In contrast, the sample c a l c i n e d a t 1050°C w h i c h s h o u l d be A1 (W0^)^, shows q u i t e d i f f e r e n t s l o p e s and i n t e r c e p t s (see F i g u r e 7;. Though o n l y the n e u t r a l i z a t i o n aspect of the change i n s c a t t e r i n g i n t e n s i t y i s t r e a t e d h e r e , i t would be wrong t o i m p l y t h a t o t h e r f a c t o r s , such as shadowing and the t a r g e t atom's e l e c t r o n c o n f i g u r a t i o n , do not p l a y a r o l e . C l e a r l y , they do, but t h e i r e f f e c t s do not a l t e r our c o n c l u s i o n s . Thus, i t appears t h a t LEISS may a l s o be q u i t e v a l u a b l e as a q u a l i t a t i v e probe f o r changes i n s u r f a c e morphology.(16) 2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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142

I

I I

16.0 14.0 12.0 10.0 W/Al 8.Oh W0 MONOLAYER X

6.0 4.0 2.0 ·-

m

6

400

500

^

— ^

^ r ^ Q - ^

650 800 950 CALCINATION TEMPERATURE

1050

F i g u r e 6. Ion I n t e n s i t y R a t i o s f o r 10% WO3/AI0O3 as a F u n c t i o n o f C a l c i n a t i o n Temperature.

a

-4 r

-4 r •Ç^r-=~ TUNGSTEN

-5 Y

-6 μ

ALUMINUM

-5 Y

TUNGSTEN

-6 ALUMINUM

-7 Y

-7

-8

\

\

OXYGEN LEGEND

-10

·A AΘΔ -

-9

W(500° CALC.) W(950° CALC.) Al(950° CALC.) Al(500° CALC.) 0(950° CALC.) 10

15

\

LEGEND •φ· - Al

-10

•

-W

0

- 0 10

20

15 -1

V (au)

_1

F (au)

F i g u r e 7. Ion S u r v i v a l P r o b a b i l i t y f o r 10% W 0 / A 1 0 ( a ) C a l c i n e d a t 500°C and 900°C; (b) C a l c i n e d a t 1050°C. 3

2

3

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

20

12.

CARVER ET AL.

Low-Energy Ion-Scattering Spectroscopy

Conclusions LEISS has proven t o be an i m p o r t a n t t o o l f o r studying catalysts. The t e c h n i q u e can be d e c e i v i n g l y s i m p l e , b u t f o r m e a n i n g f u l s u r f a c e a n a l y s i s g r e a t c a r e must be t a k e n . V e r y low energy i o n beams a t low c u r r e n t s a r e r e q u i r e d f o r e x c l u s i v e l y f i r s t layer data. C a t a l y s t s which i n g e n e r a l a r e i n s u l a t o r s r e q u i r e proper charge compensation. Q u a n t i t a t i o n i s p o s s i b l e on a r e l a t i v e s c a l e and peak i d e n t i f i c a t i o n s h o u l d be c o n f i r m e d by another t o o l such as XPS o r SIMS. W i t h t h e s e p r e c a u t i o n s , LEISS i s c a p a b l e o f d e t e r m i n i n g t h e c o m p o s i t i o n o f t h e outermost p o r t i o n of a c a t a l y s t . I t can p r o v i d e complementary d a t a , y e t more o f t e n i t p r o v i d e s unique i n f o r m a t i o n due t o i t s extreme surface s e n s i t i v i t y . Acknowledgment s The a u t h o r s w i s h t o thank R. G. M i s i t a , H. D. W i c k e r , J . J . C a s s i d e y , J . L. B l a k e s , and S. R. Coleman f o r t h e i r e x p e r i m e n t a l a s s i s t a n c e , I . E. Wachs f o r h i s a s s i s t a n c e on t h e t u n g s t e n work, and K. L. R i l e y f o r h i s u s e f u l comments. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

P. Grange, Catal. Rev.-Sci. Eng., 21, 135(1980). For example, (a) H. Jeziorowski, H. Knozinger, E. Taglauer, and C. Vogdt, J . Catal., 80, 286(1983). R. L. Chin, D. M. Hercules, J . Phys. Chem., 86, 3079(1982). H. D. Hagstrum, Phys. Rev., 96, 336(1954). B. R. Haight, L. C. Feldman, T. M. Buck, and W. M. Gibson, Phys. Rev. B, 30, 734(1984). R. F. Goff and D. P. Smith, J . Vac. S c i . Technol., 7, 72(1970). H. F. Helbig, P. J . Adelmann, A. C. M i l l e r , and A. W. Czanderna, Nucl. Inst. and Methods, 149, 581(1978). G. C. Nelson, Sandia Laboratory Report, 79-0712 (1979). S. H. Overbury and P. C. S t a i r , J . Vac. S c i . Technol. A, 1, 1055(1983) D. P. Woodruff, Surface Science, 111, L219(1982). S. M. Davis, J . C. Carver, and A. Wold, Surface Science, L12, 124(1983). L. Marchut, T. M. Buck, G. H. Wheatley, and C. J . McMahon, J r . , Surface Science, 141, 549(1984). S. S. Chan, I. E. Wachs, and L. L. Murrell, J . Catal., i n press. S. M. Davis, J . Catal., to be submitted. S. C. Fung, J . Catal. 58, 454(1979). R. C. McCune, J . Vac. S c i . Technol, 18, 700(1981), and other references therein discuss similar use of LEISS.

R E C E I V E D March 11,

1985

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

143

13 X-ray Photoelectron and X-ray Absorption Spectroscopic Characterization of Cobalt Catalysts Reduction and Sulfidation Behavior D. G . Castner and P. R. Watson1 Chevron Research Company, Richmond, C A 94802-0627

We have observed spectroscopically, on a real-time basis, the changes i n catalyst structure and compos i t i o n which occur during reduction and s u l f i d a t i o n reactions. This c a p a b i l i t y was demonstrated by examining the of bulk Co3O4 (~20 nm Co3O4 p a r t i c l e s ~500 aggregates) and 10% Co/SiO2-923 (≤5 nm Co3O4 p a r t i c l e s ) with X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). In s i t u experiments were done i n a catalyst-treatment system and a controlled atmosphere cell. CoO was observed as an intermediate i n the H2 reduction of the Co3O4 p a r t i c l e s to Co. The smallest Co3O4 p a r t i c l e s were the hardest to reduce to metallic cobalt. Sulfidation of the Co3O4 p a r t i c l e s with H2S/H2 proceeded through CoO and Co intermediates, forming Co9S8. On the supported catalysts the d i r e c t reaction of CoO to Co9S8 was observed. In contrast to the reduction r e s u l t s , the smaller Co3O4 p a r t i c l e s were the easiest to sulfide. The performance of a supported metal or metal s u l f i d e catalyst depends on the d e t a i l s of i t s preparation and pretreatment. For petroleum r e f i n i n g applications, these catalysts are activated by reduction and/or s u l f i d a t i o n of an oxide precursor. The amount of the c a t a l y t i c component converted to the active phase and the d i s persion of the active component are important factors i n determining the c a t a l y t i c performance of these materials. This investigation examines the process of reduction and s u l f i d a t i o n on unsupported C o o and silica-supported C o 0 catalysts with d i f f e r e n t C o 0 dispersions. The C o 0 p a r t i c l e sizes were determined with electron microscopy, X-ray d i f f r a c t i o n (XRD), and 3

3

4

4

3

3

4

4

1Current address: Department of Chemistry, Oregon State University, Corvallis, OR 97331

0097-6156/85/0288-0144$06.00/0 © 1985 American Chemical Society

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CASTNER AND WATSON

XPS and XAS Characterization of Cobalt Catalysts

XPS. The changes i n catalyst composition during reduction and s u l f i d a t i o n were determined by XAS and XPS. Experimental Catalyst Preparation. The unsupported C o o was obtained from Johnson Matthey Chemicals (Puratronic grade) and was used as received. The supported catalysts with nominal Co loadings of 10 wt % were prepared by p o r e - f i l l impregnation with an aqueous solution of Co(N0 ) on Davison Grade 62 and 923 s i l i c a supports. The two supported catalysts w i l l be referred to as Co/Si0 -62 and Co/Si0 -923. The s i l i c a supports were f i r s t calcined at 500°C f o r two hours, then impregnated, equilibrated i n a capped bottle f o r f i v e days, dried under vacuum (~300 torr) at 100°C f o r four hours, and f i n a l l y recalcined at 250°C for one hour and 450°C f o r two hours. 3

3

4

2

2

2

XPS Analysis. The ultrahigh vacuum (UHV) catalyst treatment surface analysis system employed to characterize and treat the cobalt catalysts has been described previously (1_ 2). The cata­ l y s t treatment and data analysis procedures have also been described (1_). B r i e f l y , the samples were treated i n quartz reac­ tors and then transferred under UHV into a modified HewlettPackard 5950A ESCA spectrometer for analysis. Peak areas were normalized with theoretical cross-sections (3) to obtain r e l a t i v e atomic compositions. f

XAS Analysis. A l l of the XAS experiments were done on l i n e VII-3 at the Stanford Synchrotron Radiation Laboratory (SSRL) with the electron storage ring operating at 3 GeV and ~40 mAmps. The spec­ tra were collected i n the transmission mode using a wiggler i n s e r t i o n device, a double S i (220) c r y s t a l monochromator, and N f i l l e d ion chambers. Both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) mea­ surements a t the cobalt Κ edge (7709.5 eV) were made. Before and a f t e r each reduction or s u l f i d a t i o n experiment detailed high reso­ l u t i o n XANES and EXAFS spectra were taken. During the treatments low resolution XANES spectra were taken approximately every two minutes. Both temperature-programmed (~5°C/min.) and isothermal experiments were done i n a controlled atmosphere c e l l . The XAS c e l l was a modified version of a c e l l used i n infrared spectro­ scopy experiments (_4) and w i l l be described i n d e t a i l elsewhere (5_) · B r i e f l y , the c e l l was modified by replacement of the Pt wire heater with two 500-watt quartz halogen lamps, replacement of the s a l t windows with mylar windows, addition of water cooling to the outer stainless steel jacket, enlargement of the diameter of sam­ ple wafers from 1.27 cm to 1.91 cm, and reduction i n size of the entire c e l l so i t would f i t into the experimental hutches at SSRL. The c e l l was capable of operation a t temperatures from -196 to over 650°C at pressures from 10~ to 10 t o r r . A gas manifold was used to flow the selected gas (He, H , or 2% H S/H ) through the c e l l and a turbomolecular pump was used to evacuate the c e l l . 2

8

3

2

2

2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

145

146

CATALYST CHARACTERIZATION SCIENCE

This paper presents the results from the XANES spectra taken during temperature-programmed reduction and s u l f i d a t i o n experiments. A l l spectra were normalized to a step height of unity after removing a linear pre-edge background. The zero point of the energy scale was set to the f i r s t i n f l e c t i o n point of Co f o i l . Each normalized XANES spectra was then decomposed into the contributions from various cobalt species by least squares f i t t i n g . The reference spectra for C o 0 , CoO, Co, and Co Sg used for this decomposition are i n Figure 1· The reference spectra were taken at the same resolution conditions as the spectra taken during reduction and s u l f i d a t i o n . An example of the decomposition i s shown i n Figure 2. 3

4

9

Electron Microscopy Analysis. Transmission and scanning electron microscopy (TEM and SEM) studies were performed on a JEOL 100CX TEMSCAN. The unsupporte sided sticky tape and were obtained i n the SEM mode at 40 and 60 KeV. Ground powder from the s i 0 ~ P P ° * samples was embedded i n epoxy, then microtomed with a diamond knife to obtain sections ~600 Â thick. These thin sections were put on 3-mm TEM grids and coated with a carbon layer (

N

2

+

H0 2

(3)

-*

NH +

H0

(4)

3

3

N

2

H

2

2

+

H0

(5)

2

3

NH

3

->

NH

3

+

N 0

+

2

2

NH

3

+

CH^

—•

NH

3

+

CH

+

4

(6)

NO

+

H0 2

(7)

N

+

H0

(8)

2

HCN 0

2

+ ~>

2

H

(9)

2

HCN

+

H0 2

+

C0

(10)

2

These should be simple unimolecular or bimolecular reactions y i e l d i n g a single or at most two reaction paths. Rates may there­ fore be f i t using Langmuir-Hinshelwood (LH) rate expressions. For A —> products t h i s should be

H

•

while f o r a bimolecular r e a c t i o n , A + Β —*• products, the simplest rate expression should be r

=

(12)

j


-H bond cleavage i s also indicated by the 2

2

3

χ

N

3

3

3

3

3

3

3

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

18. K O E S T N E R E T A L .

Clean and Sulfur-Modified Metal Surfaces

Pt(111) + CH SH(sat) 3

Electron Energy Loss Eo = 3 to 4eV

T = 110K A

Temperature Programmed Desorption β = 10 K/sec

H.x1 2

1000

2000 Energy Loss ( c m M -

3000

0 5.0 10.0 Type Κ Thermocouple Voltage (mV)

Figure 2. HREELS spectra i l l u s t r a t i n g sequential CH3SH decomposition on the P t ( l l l ) surface. The HREELS spectra were taken following treatment at the temperatures indicated on the reference TPD spectra shown i n the r i g h t panel.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

203

204

CATALYST C H A R A C T E R I Z A T I O N SCIENCE

disappearance of the S-H stretching (2750 cm ) and bending (820 cm" ) modes. The H TPD spectrum i n Figure 2 indicates that adsorbed SCH^ loses a d d i t i o n a l hydrogen with heating to 400K. The H desorption peaks at 210 and 305K have nearly equal areas i n d i c a t i n g that the second adsorbed intermediate has an average thioformaldehyde (SCH ) stoichiometry. The HREELS spectrum i n Figure 2 taken a f t e r heating to 400K generally corresponds to a matrix i s o l a t e d (8) SCH . However the S-C stretch i n adsorbed SCH i s dramatically red s h i f t e d from 1050 cm" for the matrix i s o l a t e d species to 670 cm i n d i c a t i n g a highly perturbed adsorbed SCH species* The assignment of the 670 cm mode to the u(C-S) v i b r a ­ t i o n f o r the adsorbed species follows from the low CH character of t h i s mode (a deuterium s h i f t of .03). Analogous s h i f t s to 560 cm" have been observed i n u(C-SX f o r an inorganic c l u s t e r Os(n -CHgS)([0] (PPh ) ) (9) with -bonded CH S ligand Th 400 cm" red s h i f t i i n t e r a c t i o n between th small f r a c t i o n of the SCH-j layer ( 10%) decomposes near 350K to give CH^ which desorbs and atomic s u l f u r which remains on the surface. The NEXAFS spectra i n Figure 3 taken at the C K-edge allows us to estimate the t i l t angles f o r the adsorbed SCH^ and SCH« species. Spectra were recorded at grazing (20 ) and normal (90* ; incidence a f t e r heating a multilayer of CH^SH on P t ( l l l ) to either 230K (SCH~) or 380K (SCH ). Only one molecular resonance appears near 288.OeV f o r the SCH^ layer and i s assigned to a C ( l s ) 0.03), C0 production i s predominated by a mechanism which proceeds through the same rate-determining step as the epoxidation path. Thus e x i s t the very strong k i n e t i c s i m i l a r i t i e s f o r EtO and C0 production (2,3,6,25). The other path to C0 i s less understood (25) and only predominates under extreme conditions, but i s more s e n s i t i v e to θ^. 2

q

2

2

2

2

The data i n Figs. 2 and 3 suggest a reaction which requires a d e l i c a t e balance between adsorbate coverages, consistent with a Langmuir-Hinshelwood mechanism. More extensive data of t h i s type (24-27) indicate that molecularly adsorbed ethylene and 0 are the c r i t i c a l species, consistent with the mechanism proposed below. 2

The Role of Chlorine Promoter Figure 4 shows the e f f e c t coverag (θ^τ) upo the rates, s e l e c t i v i t y and θ at a set of f i x e d pressure and temper­ ature conditions on AgOlO? (26). The e f f e c t s are q u a l i t a t i v e l y i d e n t i c a l to those seen by adding trace quantities of chlorinated hydrocarbons to the feed i n real-world Ag c a t a l y s t s : the s e l e c t i v i t y to EtO i s promoted a t , however, some loss i n o v e r a l l a c t i v i t y (1-2,8,26,32-36). This confirms that the promoter e f f e c t i s due to d i r e c t CI atom - Ag i n t e r a c t i o n s , and i s unrelated to the support material (e.g. A 1 0 ) . Note that the major e f f e c t occurs f o r chlo­ rine coverages between the ρ(2X1) structure ( θ ^ = 0.5) and the c(4x2) structure ( θ ^ = 0.75). The high coverage t o r those changes and i t s c o r r e l a t i o n with d i s t i n c t overlayer structures indicates that an ensemble rather than e l e c t r o n i c factor plays the dominant role i n promotion (26). 2

3

Note i n F i g . 4 that the oxygen adatom coverage i s already com­ p l e t e l y suppressed by = 0.25, at which point the reaction rates have hardly changed. This further suggests that 0 rather than 0 plays the dominant role i n the reaction mechanism. The decay i n 0 with almost p e r f e c t l y r e f l e c t s the dramatic decrease i n the d i s s o c i a t i v e s t i c k i n g p r o b a b i l i t y f o r 0 with chlorine coverage (26,27). We further found that, i n t h i s coverage range, the adsorp­ t i o n of molecular (peroxo-) 0 i s hardly affected by (27), con­ s i s t e n t with the minor effects of chlorine on the epoxidation rate for 0 < 0.25 (Fig. 4). 2 > a

q

2

2

C 1

The s o p h i s t i c a t i o n with which one can monitor the effects of a surface modifier i n a c a t a l y t i c reaction i s demonstrated i n F i g . 5. Using a whole series of measurements such as F i g . 4, with a broad range of temperatures and reactant pressures near 563 Κ, Ρ = Λ

150 t o r r , and P„ = 20 t o r r , we were able to determine d i r e c t l y the v a r i a t i o n s with i n the apparent a c t i v a t i o n energies (E^) and reaction orders (m,n) f o r both EtO and C0 production (27). At a given chlorine coverage, the reaction rates are w r i t t e n as: 2

m

TON = ν exp[-E /RT] P ^ P a

n t

.

The r e s u l t s are shown i n F i g . 5. In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

(1)

CAMPBELL

Selective Epoxidation of Ethylene

Figure h. The e f f e c ts of chlorine adatom coverage ( ) upon the r a t e s , s e l e c t i v i t y , and θ on Ag(llO)' at ^90 Κ and P Q = 150 t o r r and P == U.1 t o r r . C l

0

E T

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2

218

CATALYST CHARACTERIZATION

SCIENCE

• Ο

ftC l 0.9

UJ Û

w i t h

s t a n d i n

f o r

t h e

V AB b ^AA V BBV *b S interatomic (bulk) distance, i i i ) the ε.. s can be calculated from the corresponding sublimation energies ^ and/or heat mixing, using a correction f o r the compression or expansion of the interatomic distances, when the a l l o y i s formed from pure metals; t h i s correc1

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

23.

PONEC

269

Platinum-Bimetallic Catalysts

t i o n i s calculated by using the "6-12" p a i r - p o t e n t i a l , i v ) the d i f f e rences i n the thermal ( i n t e r n a l ) entropy upon segregation are neglected and the configurâtional entropy i s calculated as f o r i d e a l (random mixing) systems. The procedure i s otherwise the same as developed by Williams and Nason (11), only the Ω-parameters are ob­ tained from the corresponding expressions and not by averaging as the authors (11) suggest. Experimental determination of the surface composition i n noni d e a l systems, i n which the gradients extend over several layers inwards the c r y s t a l i s as d i f f i c u l t as the exact c a l c u l a t i o n s . Therefore, one has to make again rather unpleasant assumptions. Van Langeveld (10) i n h i s thesis f i r s t calculated the concentration i n the f i r s t and the second surface layer of the a l l o y s and when he saw the second layer d i f f e r e d only marginally from the bulk, he determined the surface concentration by AES, assuming that only the f i r s t layer i s d i f f e r e n fro th bulk h th relevan equation i s as follows: IÇA)_ I

(

B

m

)

£

j* "

P

. "t a

X

l * °-

^ulk

•J(i-« )+(ni) e.g. Pt/Au, Pt/Ag, (endothermic a l l o y formation) or Pt/Re, Pd/Ag, Pd/Au (moderately exothermic). The best method a v a i l a b l e at the moment, to look at the e l e c ­ t r o n i c structure of t r a n s i t i o n metals and t h e i r a l l o y s , i s probably

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CATALYST CHARACTERIZATION SCIENCE

270

1.0

Figure 1. Surface Pt content as a function of the bulk content i n Pt/Cu a l l o y s . Shaded: predicted by theory f o r Τ - 300-1000K and non-ideal a l l o y s . i d e a l a l l o y s . Points f , own data ob­ tained by AES; other symbols - various data from the l i t e r a t u r e . (Reproduced with permission from Ref.10. North-Holland Publ.Co.)

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

23.

PONEC

Platinum-Bimetallic Catalysts

271

the photon excited electron spectroscopy. With synchroton r a d i a t i o n (10-150 eV) or with more c l a s s i c a l sources of UV r a d i a t i o n , the valence bands can be studied i n many d e t a i l s , while higher energy (X-ray) photons are suitable to check the behaviour of the core l e v e l s . Studied by these methods, Pt/Cu a l l o y s o f f e r the following picture: i ) there i s d e f i n i t e l y no electron transfer (12-14) from Cu to P t , as assumed i n the o l d Rigid Band Theory and i n i t s c a t a l y t i c a l applications i n the theory by Dowden (16). i i ) Even f o r this rather strong-exothermically formed system of a l l o y s , the changes i n the e l e c t r o n i c structure of Pt and Cu are s u r p r i s i n g l y subtle. The 5d valence electrons of Pt can be consi­ dered as uninfluenced at a l l by a l l o y i n g (12-14) and the valence (3d) and core levels (2p) electrons of Cu show a lower B.E. (B.E. hv " " E j . ; r e l a t i v e to the Fermi energy). This lowering i s more pro­ nounced f o r the core l e v e l s due to better screenin Cu. Indeed, the e f f e c t i s most l i k e l y due to the f i n a l state r e l a x a ­ tion/screening (see the remark on p.7297 i n ref.14). i i i ) There are strong indications that the valence band states show a strong mixing of Pt 5d and Cu 3d states (13,14). Further, some data have been interpreted as showing a lower degree of 3d/4s h y b r i ­ d i z a t i o n on Cu i n a l l o y s than i n pure Cu metal (12). Summarizing, the changes i n the e l e c t r o n i c structure of Pt and Cu atoms caused by a l l o y i n g are rather marginal. At l e a s t , those which are observable by the electron spectroscopy. How are the e l e c t r o n i c structure changes r e f l e c t e d by adsorption? I t can be expected that the e l e c t r o n i c structure changes would be r e f l e c t e d by the heats of adsorption of suitable chosen molecules. Indeed, Shek et a l (17) report that one maximum i n the thermal desorption p r o f i l e of CO s h i f t s to lower temperatures when the Cu content of a l l o y s increases. I f the v a r i a t i o n s i n the entropy changes upon adsorption can be neglected (probably - they can) t h i s would indicate a lower heat of adsorption of CO on a l l o y s than on Pt: from abt. 33 Kcal/mol on pure Pt.to 26 Kcal/mol f o r an a l l o y with abt. 20% Cu. However, before we accept t h i s conclusion as the d e f i n i t i v e one, a word of caution i s necessary. Due to the CO-CO interactions the heats of adsorption depend on the coverage Θ. Shek et a l (17) compared Pt and a l l o y s at a constant dosage and t h i s could mean that the coverage of Pt was (at the same dosage) lower on Pt than on a l l o y s , and consequently - the heat of adsorption higher. Shek et a l report the above mentioned data f o r the (111) faces of Pt and a l l o y s . They studied also the (110) faces, but there the e f f e c t of a l l o y i n g i s masked by the reconstruction of the surface upon C0adsorption (18). A l l o y s used by Shek et a l (18) were prepared by d i s s o l v i n g ad­ sorbed Cu layers into the bulk, by heating the sample. Complica­ tions with a varying roughness can not be excluded and i t i s known that such changes would change the heat of adsorption as w e l l (19). In f a c t , such effect has been probably observed (20). I t would be desirable to have data on monocrystals e q u i l i b r a t e d or - better grown at a much higher temperature. Unfortunately due to the side

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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e f f e c t s of ordering i t i s not easy to produce the Pt/Cu monocrystals. Therefore, Noordermeer et a l (21) studied a r e l a t e d system, which i s easier to obtain, the PdCu (111) monocrystal plane. These authors come to the conclusion that a l l o y i n g does not change the p o s i t i o n of the CO-desorption peaks (see figure 2) and only the population of various states (peaks) vary. Another way to monitor the expected changes i n the metal e l e c t r o n i c structure i s to look at the adsorbed molecules, which are s e n s i t i v e i n t h e i r properties to the changes i n the e l e c t r o n i c structure of surface metal atoms. Such a molecule i s CO and the f r e ­ quency of the CO s t r e t c h v i b r a t i o n s ( v(CO)) i s a s e n s i t i v e detector of the d i r e c t - and back-donation upon adsorption of CO. I t has been reported, that ν(CO) decreases f o r the V I I I group metal by a l l o y i n g of Pd with Ag (22), N i with Cu (23), but also when mixing N i with Co (24). This has been f i r s t explained (25) as an i n d i c a t i o n f o r an increased backdonation du t d electro s h i f t C -* P t Cu N i and consequentl was rather u n s a t i s f y i n g (althoug y sprea g popular). Cobalt caused namely the same kind of e f f e c t as Cu and yet i t could be hardly suspected of donating electrons to N i . Indeed» i t appeared that the reason f o r a lower V(VIII grp., CO) value with a l ­ loys i s simply the d i l u t i o n of the CO-layer. The following experi­ ments elucidated the problem. When CO i s adsorbed on, say P t , i n t e r a c t i o n of dipoles of i n ­ d i v i d u a l molecules i s r e p u l s i v e , and i t decreases the heat of ad­ sorption and increases the v(Pt,CO) frequency. This e f f e c t on f r e ­ quency i s a resonance e f f e c t ; when a 1 C0 layer i s d i l u t e d by ^CO or C^O, e t c . , the i n t e r a c t i o n i s much weaker (26,27). ^CO " cules i n the f u l l layer of 1 C0 simulate, thus, a c t u a l l y the empty s i t e s . I f Pt surface layer i s d i l u t e d by Cu atoms, not covered by C0, or covered, but by ^CO v i b r a t i n g at a d i f f e r e n t frequency, then the v(Pt, CO) value at 0(Pt,CO)+ 1 must be with Pt/Cu a l l o y s lower than the corresponding value of v( CO,Pt) on pure Pt. The question i s - does a l l o y i n g cause an e f f e c t a d d i t i o n a l to t h i s de­ crease by d i l u t i o n ? The answer can be obtained when V(Pt.* CO) i s monitored at 0(CO,Pt)-> 1 as a function of χ, χ - CO/( CO+ CO), the molar r a t i o of C0 i n the adsorbed layer. One follows the v ( P t , C 0 ) on pure Pt and on one or more a l l o y s and extrapolates V to χ ·> o. The remaining e f f e c t , i . e . , the difference at χ + 0 of v ( P t , C0) between pure Pt (or on other metal) and i t s a l l o y s , i s the maximum room l e f t f o r possible e l e c t r o n i c structure changes being r e f l e c t e d by the v(Pt, CO) frequency. Toolenaar, Stoop et a l performed such measurements, being i n ­ spired by e a r l i e r papers (26,27) and they have seen that with Pt/Cu the room f o r possible e l e c t r o n i c structure e f f e c t s was v i r t u a l l y zero (28) (see figure 3). Also with Pt/Re and with very strongly exo­ thermic Pt/Sn a l l o y s , the " r e s i d u a l " e f f e c t at χ ο was rather small (29). The effect was only pronounced with Pt/Pb a l l o y s , f o r reasons not known at the moment. However, even with Pt/Pb the maxi­ mum possible " e l e c t r o n i c " e f f e c t was of a size comparable with that of the e f f e c t of varying CO-coverage. The conclusion i s easy then: investigations using adsorbed CO as a detector d i d not reveal any appreciable change i n the e l e c t r o n i c structure of underlaying metal Pt atoms. 2

m o l e

2

12

12

12

2

12

l2

13

l2

12

2

12

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Platinum-Bimetallic Catalysts

—τ—ι—ι 300

1

1 500

273

1

1

'

ϊ 700 Τ(Κ)

Figure 2. Above: thermal desorption of CO adsorbed on P d A g ( l l l ) , exposures at 250K; 0.15, 8.3, 0.6, 1.5, 3.0 and 6 nbar.s. In agreement with the LEED and IR data, the peak and shoulders correspond to the multiple and single bound (on top and i n the v a l l e y s adsorbed) CO. In the middle: thermal desorption of CO adsorbed on P d A g ( l l l ) ; exposures at 250K; 0.15, 0.3, 0.6, 1.2, 3.0, 6.0 and 12 nbar.s. In agreement with IR data, only the single bound CO i s present i n t h i s strongly d i l u t e d ( * 10% P d f ) a l l o y s . Below: thermal desorption of CO adsorbed on P d C u ( l l l ) ; exposures at 250K; 0.15, 0.3, 0.6, 1.2, 3.0 and 6 nbar.s. On t h i s Pd r i c h a l l o y ( P d * 70%) the same types of adsorption are present as on Pd.(ref.21) (Reproduced with permission from Ref.21. North-Holland Publ.Co.) s u r

s u r f

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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CATALYST CHARACTERIZATION SCIENCE

With P t , the strongly p r e v a i l i n g type of CO adsorption i s the "on top" adsorption. Only at very high coverages i s some CO moved by mutual repulsions into a less favourable p o s i t i o n , into the v a l l e y s . However, with Pd or with N i , CO prefers at low coverages to be bound to several Pd or Ni atoms, s i t t i n g i n the v a l l e y among several Pd or N i atoms (multiply bound CO). When a second metal i s introduced i n t o N i or Pd (Ag or Pt) the adsorption into the m u l t i p l y bound CO state i s suppressed, demonstrating that the number of available atoms (ensemble size) i s c r i t i c a l for the way i n which "C" of CO i s bound to the surface (14). I t can be reasonably expec­ ted that the same holds f o r "C" of hydrocarbons. C a t a l y t i c e f f e c t s of a l l o y i n g of Pt with Cu. C a t a l y t i c e f f e c t s are represented by f i g u r e 4. We observe that the s e l e c t i v i t y f o r hydrogenolysis decreases and that of isomerization increases with i n ­ creasing temperature. Thi i n the a c t i v a t i o n energie t i o n and dehydrocyclization; the a c t i v a t i o n energy of isomerization should be then abt. 25 Kcal/mol higher than that of hydrogenolysis (30). A c t i v a t i o n energy i s found to be independent of the a l l o y com­ p o s i t i o n : i . e , the curves of s e l e c t i v i t i e s as functions of tempera­ ture, are only s h i f t e d i n p a r a l l e l to higher temperatures with a higher Cu-content. Another piece of information a v a i l a b l e concerns the surface intermediates. By using the l a b e l l i n g and by monitoring the reaction of a molecule which i s an "archtype" for two types of complexes the following has been established (31-33): i ) d i l u t i o n of Pt by Cu increases the r e l a t i v e contribution to isomerization of the 5-C-intermediates (something l i k e an adsorbed methylcyclopentane) i n comparison with that of 3C-intermediates. i i ) The con­ t r i b u t i o n of various types of the 3C-(ay) and 2C-(a3) intermediates to the o v e r a l l reaction i s independent of the Cu-content, but with Cu i n c r e a s i n g , the proportion increases to which the 3C-(ay) i n t e r ­ mediates are hydrogenolysed (as compared with t h e i r isomerization). On some important side e f f e c t s . When an a l l o y i s brought into contact with a gas which shows a higher a f f i n i t y f o r one of the a l l o y components, a gas induced segregation takes place. Such a pro­ cess has been also found f o r the Pt/Cu a l l o y s . The authors (34) established, that by hydrocarbon adsorption, Pt i s p r e f e r e n t i a l l y covered by a carbon(aceous) l a y e r , but under t h i s layer the Pt concentration i s higher than under vacuum (figure 5). However, t h i s f i n d i n g does not change p r i n c i p a l l y the p i c t u r e of the surface one had e a r l i e r . While Pt i s p r e f e r e n t i a l l y covered, and only at very high C-depositions also Cu may become p a r t i a l l y covered, the presence of Cu exerts another important e f f e c t , namely on the behaviour of the car­ bon (aceous) layer. Auger spectra reveal (35) that Ρt, more than other metals (Rh, Ni) which are good for hydrogenolysis, converts the surface carbon(aceous) layer i n a highly unreactive (blocking, protective) graphite layer. However, d i l u t i o n of Pt by Cu slows down t h i s g r a p h i t i z a t i o n (figure 6) and, most l i k e l y , other metals might have a s i m i l a r e f f e c t as w e l l . This i s c e r t a i n l y a point which must be considered i n the discussion of the c a t a l y t i c e f f e c t s .

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

23. PONEC

Platinum-Bimetallic Catalysts 2060-1

V(cm«) co

2060

2040

2020

tt

% co l2

Figure 3. Wavenumbers of the high-frequency " C 0 " IR abs. band maxima of CO on P t , as a function of x ( C 0 ) . A l l samples of Pt/Cu on AI2O3, at 0(CO,Pt)+ 1. ο P t , ΔPt 76% • Pt 42% . Pt 31%, % the average (- bulk) concentration. (Reproduced with permission from Ref.28. Acad.Prèss Inc.) 12

(a)

isomerisation

100 61 29

20)

280

300

320

sΊ.8

340 T(°C)

Figure 4. S e l e c t i v i t y i n isomerization, dehydrocyclization and hydrogenolysis (cracking) of Pt/Cu a l l o y s (on S1O2). Bulk compo­ s i t i o n of a l l o y s (% Pt) i n d i c a t e d . (Reproduced with permission from Ref.30. Chem.Sοc.London) In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CATALYST CHARACTERIZATION SCIENCE

Figure 5, The molar f r a c t i o n x of Pt i n the topmost atomic layer of the a l l o y as a function of the bulk molar f r a c t i o n of Pt-xb. Curved f u l l l i n e : the best f i t through the experimental AES data f o r surfaces i n vacuum. The shaded area indicates the range of the steady state molar f r a c t i o n of P t , estimated by using d i f f e r e n t growth-models f o r the carbon(aceous) layers, calculated f o r the topmost layer of Pt/Cu alloys i n contact with ethene, at ambient temperature. (Reproduced with permission from Ref.34. North-Holland Publ.Co.) s

α

Figure 6. C h a r a c t e r i s t i c part of the carbon KW AES spectra a f ­ ter 55 min. exposure of 2.10"^ nbar ethene at ambient temperature* Pt and C peak are not separated i n these spectra; nevertheless the C-spectrum shows a pronounced graphite structure on pure Pt (c) and much more of the c a r b i d i c (or molecular) C on 42% (a) and 63% (b) Pt a l l o y s . (Reproduced with permission from Ref.34. North-Holland Publ.Co.)

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Discussion A decrease i n the s i z e of Pt ensembles achieved by a l l o y i n g with a very i n e r t second component l i k e Ag or Au, leads to a decrease of hydrogenolytic s e l e c t i v i t y (36-38). This i s a common feature of various VIII group l b a l l o y s (1) ; at least when they are prepared i n a w e l l defined form (as, e.g., i n (37)). However, a decrease of the Pt-ensemble s i z e by a l l o y i n g with Cu leads to an increase i n the hydrogenolytic s e l e c t i v i t y . This has been r a t i o n a l i z e d by assu­ ming that Cu i s - somehow - involved i n the formation and conversion of the surface intermediates (30). The knowledge accumulated since the time this hypothesis has been formulated allows us to be a l i t t l e b i t more s p e c i f i c i n suggesting the p i c t u r e of the Pt/Cu working surfaces. The three changes i namely: i ) the 3C-complexe given temperature, i i ) The MCP-ring-opening produces more n-hexane than on pure P t , and i i i ) the formation of 5C-surface complexes i s promoted. These three changes are the same as changes caused by diminishing the Pt (pure Pt) p a r t i c l e s i z e . What happens then by diminishing the p a r t i c l e size? The small Pt p a r t i c l e s are less s e l f poisoned by the r e a c t i o n , since i t i s more d i f f i c u l t to cover a curved (or stepped, with monocrystals) surface of small p a r t i c l e s , as compared with large, f l a t planes (39). With very small p a r t i c l e s , the reaction intermediates bound to the edge atoms f i n d i t more d i f ­ f i c u l t to get the neighbouring metal atoms d i r e c t l y involved i n the conversions of the intermediates, etc. On the other hand these neighbours have a better chance not to be covered by a continuous carbon(aceous) layer and they can intervene i n d i r e c t l y i n the con­ versions of the intermediate, f o r example by mediating a free trans­ port of Η-atoms, from and to the intermediate. This i s to be com­ pared with the s i t u a t i o n on the f l a t planes where a continuous carbon(aceous) layer can grow e a s i l y and where the neighbours of an uncovered Pt atom are most l i k e l y prevented from having any interference d i r e c t l y or i n d i r e c t l y with the conversion of the surface intermediates. Consider now reactions of, say, n-hexane at increasing tempera­ tures, i . e . , at r e v e r s i b l y increasing coverage of the surface by a carbonaceous layer. I t i s c e r t a i n l y conceivable that when the sur­ face i s being progressively covered, the remaining i s o l a t e d Pt atoms tend to isomerize hexane, rather than to s p l i t i t by hydrogenolysis. Among other reasons, t h i s i s because the progress of dehydrogenation of intermediates and the f i n a l back-hydrogenation of the fragments, i s slowed down by the carbonaceous layer (the same s i t u a t i o n a r i s e s when Pt atoms are i s o l a t e d by i n e r t Ag or Au atoms). Further, the i s o l a t e d V I I I group atoms or the smallest ensembles of V I I I group atoms usually show a promoted isomerization and a slowed down hydrçgenolytic s p l i t t i n g and t h i s could be an i n t r i n s i c property of the smallest ensembles. However, the s i t u a t i o n i s probably d i f f e r e n t with Cu. The Cu-H bond strength i s s u f f i c i e n t to make a s p l i t t i n g of a C-H bond by a p a i r Pt-Cu, i n p r i n c i p l e , possible. Cu-^fomg can f u r ther mediate a rather free transport cf Η-atoms to and from the intermediates, possibly promoting by that the C-H bond s p l i t t i n g and fragment hydrogénation.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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More severe conditions, i . e . , a higher T, is needed to exclude Cu from this kind of interfering and this situation would lead to a parallel shift of the selectivity curves to higher temperatures, exactly as observed experimentally (see figure 4). When the Pt/Cu catalysts are compared with Pt at the same temperature, an increased hydrogenolytic selectivity is observed. A metal like Re or Ir, when alloyed with Pt, can play a very similar role like Cu. As mentioned above, when a Pt-atom is surrounded by Ag, Au, Sn, etc., that i s , by atoms considerably less active than Cu, very well isolated Pt atoms or small Pt ensembles can be created, which, even without any assistence of the neighbouring Ag, Au, Sn, etc., w i l l tend to isomerize hexane rather than to split i t . However, when the alloys are not well homogenized, as is frequently the case with alloys on some supports, sufficient larger ensembles of Pt, but now more difficult to be selfpoisoned (like the smallest Pt particles), may coexist next to Au for higher hydrogenolyti easily misinterpreted as a consequence of an alloy - support (non specifiedl) electronic interaction. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Ponec, V Adv.Catal. 1983, 32, 149 Clarke, J.K.A. Chem.Rev. 1975, 75, 291 Sinfelt, J . H . Acc.Chem.Res. 1977, 10, 15 Sachtler, W.M.H.; van Santen, R.A. Adv.Catal. 1977, 26, 69 Ponec, V. Catal.Revs.Sci.Engn. 1975, 11, 1 Ponec, V . ; Sachtler, W.M.H. J.Catal. 1972, 24, 250 Ponec, V. Proc.of the IXth Intl.Vacuum Congr.and Vth Intl.Conf. on Surf.Sci., Madrid, 1983, p.121 Kelley, M; Ponec, V. Progress in Surf.Sci. 1981, 11, 139 Ponec, V. In "Electronic Structure and Reactivity of Solid Surfaces"; Derouane, E . G . ; Lucas, Α.Α., eds.; Plenum Press, 1976, p.537 van Langeveld, A.D.; Ponec, V. Applic.of Surf.Sci. 1983, 16, 405 Williams, F . L . ; Nason, D. Surf.Sci. 1974, 45, 377 Kleimann, G.G.; Sundaram, V.S.; Barreto, C . L . ; Rogers, J.D. Solid State Commun. 1979, 32, 919 Shek, M.L.; Stefan, P.M.; Lindau, I . ; Spicer, W.E. Phys.Rev.B 1983, 27, 7277 Shek, M.L.; Stefan, P.M.; Lindau, I . ; Spicer, W.E. Phys.Rev.Β 1983, 27, 7288 Hultgren, R.; Desai, P.D.; Hawkins, D.T.; Gleiser, M.; Kelley, K.K. In "Selected Values of Thermodynamic Properties of Binary Alloys"; Amer.Soc.Metals, Ohio, 1973 Dowden, D.A. J.Chem.Soc. 1950, 242 Shek, M.L.; Stefan, P.M.; Lindau, I . ; Spicer, W.E. Surf.Sci. 1983, 134, 438 Shek, M.L.; Stefan, P.M.; Lindau, I . ; Spicer, W.E. Surf.Sci. 1983, 134, 399 and 427 Erley, W.; Ibach, H . ; Lehwald, S.; Wagner, H. Surf.Sci. 1979, 83, 585 See the analysis of various data in ref.1 of this paper

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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21. Noordermeer, Α.; Kok, G.A.; Nieuwenhuys, B.E. Surf.Sci. submit­ ted paper for ECOSS VI, England,1984 22. Primet, M.; Mathieu, M.V.; Sachtler, W.M.H. J.Catal. 1976, 44, 324 23. Dalmon, J . A . ; Primet, M.; Martin, G.A.; Imelik, B. Surf.Sci. 1975, 50, 95 24. Van Dijk, W.L. M.Sc.Thesis, Leiden,1974 25. Blyholder, G. J.Phys.Chem. 1964, 68, 2772 26. Hammaker, R.M.; Francis, S.A.; Eischens, R.P. Spectrochim. Acta 1965, 21, 1295 27. Crossley, Α.; King, D.A. Surf.Sci. 1977, 68, 528 28. Toolenaar, F.J.C.M.; Stoop, F . ; Ponec, V. J.Catal. 1983, 82, 1 29. Bastein, A.G.T.M.; Toolenaar, F.J.C.M.; Ponec, V. J.Catal. in print 30. De Jongste, H.C.; Kuijers, F . J . ; Ponec, V. Proc. VIth Congr. on Catal., London, eds.; Chem.Soc., London 31. De Jongste, H . C . ; Ponec, V. Proc. VIIth Congr.on Catal., Tokyo, 1980; Seiyama, T.; Tanabe, K. eds.; Kadansha, Tokyo, 1981, Vol.1, p.186 32. De Jongste, H.C.; Ponec, V . ; Gault, F.G. J.Catal. 1980, 63, 395 33. Botman, M.J.P.; De Jongste, H.C.; Ponec, V. J.Catal. 1981, 68, 9 34. Van Langeveld, A.D.; Van Delft, F.C.M.J.M.; Ponec, V. Surf.Sci. 1983, 134, 665 35. Van Langeveld, A.D.; Van Delft, F.C.M.J.M.; Ponec, V. Surf.Sci. 1983, 135, 93 36. Van Schaik, J.R.H.; Dessing, R.P.; Ponec, V. J.Catal. 1975, 38, 273 37. Sachtler, J.W.A.; Somorjai, G.A. J.Catal. 1983, 81, 77 38. Vogelzang, M.W.; Botman, M.J.P.; Ponec, V. Faraday Soc. Dis­ cussions, 1982, 72, 33 39. Lankhorst, P.P.; De Jongste, H.C.; Ponec, V. In "Catalyst Deactivation"; Delmon, B.; Froment, G.F. eds.; Elsevier, Amsterdam, 1980, p.43 RECEIVED

March 20, 1985

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

24 Determination of the Atomic and Electronic Structure of Platinum Catalysts by X-ray Absorption Spectroscopy 1

1

2,4

25 ,

6 2,

F. W. Lytle , R. B. Greegor, E. C. Marques , V. A. Biebesheimer , D. R. Sandstrom, J. A. Horsley, G. H. Via , and J. H. Sinfelt 3

3

3

1

The Boeing Company, Seattle, WA 98124 Department of Physics, Washington State University, Pullman, WA 99164 Exxon Research & Engineering

2 3

X-ray absorption spectroscopy was used for in situ characterization of active, supported Pt catalysts. The extended fine structure (EXAFS) was used to deter­ mine bond distances, coordination number and disorder. The near edge (XANES) was used as an indication of electronic state. Significant results include, 1) a reversible change of shape of clean supported metal clusters as a function of temperature, 2) supported Pt clusters have more disorder or strain compared to the bulk metal, and 3) a clear determination of the bonds between the catalytic metal atoms and the oxygen atoms of the support.

X-ray absorption spectroscopy combining x-ray absorption near edge fine structure (XANES) and extended x-ray absorption f i n e structure (EXAFS) was used to extensively characterize Pt on Cabosil c a t a l y s t s . XANES i s the r e s u l t of electron t r a n s i t i o n s to bound states of the absorbing atom and thereby maps the symmetry - selected empty manif o l d of electron states.

I t i s s e n s i t i v e to the e l e c t r o n i c config-

uration of the absorbing atom. When the photoelectron has s u f f i c i e n t k i n e t i c energy to be ejected from the atom i t can be backseattered by neighboring atoms. The quantum interference of the i n i t i a l

4

Current address: Monsanto Company, St. Louis, MO 63166 Current address: Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 1000 Berlin 33, Federal Republic of Germany Current address: The Boeing Company, Seattle, WA 98124

5

6

0097-6156/85/0288-0280$06.00/0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

24.

LYTLE ET AL.

281

Structure of Platinum Catalysts

electron wave state and backseattered wave produces a modulation of the absorption cross s e c t i o n , i . e . , EXAFS.

EXAFS data can provide

information on bond distances, coordination numbers, disorder and types of ligand f o r the f i r s t few coordination spheres.

Although

both XANES and EXAFS depend upon a measurement of x-ray absorption cross section the e l e c t r o n i c and s t r u c t u r a l information i s due to electron t r a n s i t i o n s and/or s c a t t e r i n g where the source of electrons i s an atomic species w i t h i n the sample.

This element i s chosen by

the coincidence i n energy of the x-ray probe and the absorption edge of i n t e r e s t .

In t h i s element s p e c i f i c i t y l i e s the power of the tech­

nique f o r c a t a l y s t characterization niques, x-ray spectroscop support.

The support only appears i n the data as i t i n t e r a c t s with

the atoms examined.

A recent review by Lee, et a l . has discussed the

general use of the EXAFS technique and i t s l i m i t a t i o n s . ( 1 )

We have

reviewed c a t a l y t i c applications of both EXAFS and XANES.(2-3) In the f o l l o w i n g , s t r u c t u r a l data are obtained f o r Pt atoms and t h e i r near neighbors on active c a t a l y s t s under c o n t r o l l e d conditions. XANES i s used to i n d i c a t e the d i r e c t i o n and amount of d-electron flow between the Pt c a t a l y s t and i t s ligands, EXAFS to measure near neigh­ bor s t r u c t u r a l parameters.

We f i n d EXAFS/XANES to be a s e n s i t i v e and

subtle i n d i c a t o r of small changes i n the environment of c a t a l y t i c atoms · Experimental Methods The preparation and c h a r a c t e r i z a t i o n by chemisorption (H/Pt =0.9

for

1 wt.% c a t a l y s t and 1.0 f o r 0.5 wt.% c a t a l y s t ) of the c a t a l y s t s have been described.(40

The data were obtained i n the c a t a l y s t c e l l pre­

viously described (2) e i t h e r by transmission measurements, measuring In I / I , or by the fluorescent EXAFS technique (5) measuring O

—

Γ»/I r

Ο

where I , I and I _ are the i n t e n s i t y of the incident x-rays, transO

r

mitted x-rays and, Pt L-fluorescent x-rays, r e s p e c t i v e l y . For Pt at 0.5 - 1.0 wt.% concentration the transmission and fluorescent tech­ niques are about of equal merit.

The fluorescent EXAFS technique

becomes advantageous f o r elements of lower atomic number or at lower concentration (to 10 ppm i n favorable cases).

The measurements were

made at Stanford Synchrotron Radiation Laboratory.

In a l l cases a

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CATALYST CHARACTERIZATION SCIENCE

282

Si(220) double c r y s t a l monochromator was used with entrance s l i t (1 mm h i g h 20 m from the source p o i n t ) chosen t o g i v e a bandpass o f 2 eV at the P t 1> edge, 11,563.7 eV. (6) The o p e r a t i o n o f the c a t a l y s t c e l l allowed i n s i t u r e d u c t i o n , chemisorption o r c a t a l y s i s w h i l e m a i n t a i n i n g the temperature i n the d e s i r e d range from 1300 t o 90 K. In p r a c t i c e once a d e s i r a b l e c a t a l y s t c o n d i t i o n was achieved x-ray measurements o f t e n were taken a t temperature and a f t e r quenching t o 90 Κ i n an attempt t o minimize thermal smearing o f the EXAFS d a t a . For each measurement 2 t o 3 scans were averaged over a t o t a l time o f ^ 40 minutes. Temperatures were measured and kept constant a t t h e temperatures i n d i c a t e d t o +5°C An example o f raw data f o r 2.5 urn t h i c k P t f o i l a t 298 Κ i are shown, with the zero o f energy a t 11,563.7 eV, the p o s i t i o n o f the L . edge. Note the sharp XANES f e a t u r e a t the 1» edge i n con­ t r a s t w i t h the L j j edge which w i l l become more evident l a t e r when the graph i s expanded. ττι

1ιτ

I I 3

EXAFS Data A n a l y s i s We have d e s c r i b e d i n d e t a i l our technique o f data a n a l y s i s . ( 2 - 4 ) B a s i c a l l y , the EXAFS, X(K) X(K) ^ A j O C ) s i n J2KR.J + φ (K)J

(1)

i

V

K)

β

/KR

2

fj j ) V

K)

exp

(

"

2k2

2

°j )

(2)

i s F o u r i e r transformed

i/2^

c o r e

l e v e l

s

( )

t 0

t n e

empty 5d and 6s states

( A J - 0 + 1 ) ; however, the t r a n s i t i o n to 6s has a much lower prob­ a b i l i t y and i s not expected to contribute measurably.

In previous

work (10) i t was shown that the i n t e n s i t y of the peak at the L J - Q absorption edge was approximately proportional to the d-electron vacancies.

A series of compounds of one element i l l u s t r a t e d that the

increase i n the peak i n t e n s i t y of the compound compared with the pure element was proportional to an i o n i c i t y estimate of the number of 5d electrons removed from the element by formation of chemical bonds.(11) In t h i s same work XANES s e n s i t i v i t y to i n t e r a c t i o n with the support and to 0

2

chemisorption was also demonstrated f o r Pt or I r supported

on Α1 0 · 2

3

Gallezot, et a l . (12) demonstrated s i m i l a r e f f e c t s f o r Pt

on z e o l i t e s .

Mansour, et a l . (13) investigated Pt on SiO^ and Al^O^

as a function of

reduction temperature.

A l l three studies (refs.

11-13) agreed that the increase i n area of the peak at the Pt L edge(s) indicated that reduced, supported Pt was electron d e f i c i e n t compared with the bulk metal. i s shown to be over s i m p l i f i e d . electrons than bulk Pt.)

(In that which follows t h i s conclusion At high temperature P t / S i 0 has more 2

The most simple explanation i s that Pt 5d

electrons form bonds with a v a i l a b l e ligands on the support.

Short,

et a l . (14) have used the same technique to explore the strong metal support i n t e r a c t i o n (15) (SMSI) of Pt on T i 0 . 2

A small e f f e c t was

noted i n the EXAFS and XANES between the normal and SMSI c a t a l y s t conditions.

An i n t e r e s t i n g r e s u l t i n l i g h t of references 11-13 was

that the L^.^ edge peak was diminished i n i n t e n s i t y and width compared

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

286

CATALYST CHARACTERIZATION SCIENCE

to bulk Pt i n contrast with the p a r t i c u l a r l y wide ^ on S i 0 or A^O^.

ι τ ι

peak observed

A serious caveat of a l l the above r e s u l t s (refs.

2

11-14) i s that the c a t a l y s t was conditioned at a p a r t i c u l a r tempera­ ture and then cooled to room temperature or below f o r the x-ray measurement.

As w i l l be shown there i s a s i g n i f i c a n t temperature

e f f e c t on the XANES of a Pt c a t a l y s t i n H shown how the L ^

2

or He.

Horsley (16) has

x-ray absorption edge resonances can be modeled

by Χα-SW molecular o r b i t a l c a l c u l a t i o n s of a c l u s t e r composed of Pt or I r and i t s f i r s t neighbors.

Both chemical compounds and c a t a l y s t

clusters were modeled and there was good agreement with experiment. Both Horsley (16) and Mansour edges must both be considere d-state occupancy. The XANES region f o r the Pt L ^ 0.5% c a t a l y s t i s shown i n figure 4.

edges i n Pt metal and the I t i s evident that there i s an

edge resonance ("white l i n e " ) at the L ^ ^ j edge which i s much dimin­ ished at the L ^

edge.

The reason for t h i s difference was

first

pointed out by Mott (17) and i s based upon atomic dipole s e l e c t i o n rules ( A J - 0 + 1).

In a Pt atom the empty 5d state has J • 5/2

symmetry, hence a t r a n s i t i o n i s expected at the L J J J edge ( J = but not at the L ^

edge ( J = 1/2).

3/2)

Brown, et a l . (18) discussed

t h i s problem i n d e t a i l and concluded that i n Pt metal the empty dband i s predominately J » 5/2.

Mattheiss and Dietz (19) (MD) i n ­

vestigated the problem with a more accurate r e l a t i v i s t i c model which included h y b r i d i z a t i o n of the 5d with the broad 6s and 6p bands. Their r e s u l t d i f f e r s i n d e t a i l but generally substantiates the r e s u l t of Brown, et a l . (18)

Mansour, Cook and Sayers (20) have applied

the MD r e s u l t to Pt c a t a l y s t s , developing a procedure to determine changes i n d-electron occupancy. MD (19) determined the equations f o r dipole t r a n s i t i o n s i n ­ cluding the s p l i t t i n g of the f i n a l d-3tates due to s p i n - o r b i t cou­ pling.

These r e s u l t s were then applied to r e l a t i v i s t i c t i g h t binding

energy band c a l c u l a t i o n s for Pt and Au where the appropriate electron symmetries had been projected out.

Selected emission and absorption

data were shown to be i n agreement with c a l c u l a t i o n s .

Pertinent to

the present work i s the r e s u l t

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

24.

LYTLE ET AL.

287

Structure of Platinum Catalysts

0.015

SURFACE ATOMS

0.010

CM b

BULK PLATINUM

0.005

0000

0

10

Figure 3. Comparison of r e l a t i v e mean squared thermal motion f o r bulk and surface Pt atoms as a function of temperature. The t r i a n g l e s mark the catalyst surface atoms, the c i r c l e s are f o r bulk P t .

20

Figure 4. Comparison of Pt metal ( 0.5% Pt/Cabosil i n H ( ) and He ( 2

30

) ^ Ι Ι , Ι Ι Ι spectra with ) at 773 Κ and 90 K.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CATALYST CHARACTERIZATION SCIENCE

288

A

L

< III> -

A(L ) n

C

V

= Cw

T

T T

Κ

3/2

fR^l/2)

Κ/

2 +

(h \

h

3/2V

(4)

15

{2)13

0/n

where A(L) i s the area of the L-edge resonance; h ^

(5)

2

3/2

a r e

t

*

i e

number of 5d-electron holes of indicated symmetry; and the factor (2) i s due to the double degeneracy of the L J J J t r a n s i t i o n compared to and must be included when both edges are normalized to unit edge jump as i s done here.

The projected density of states curve f o r Pt

i n reference 19 shows tha curves are v i r t u a l l y i d e n t i c a l with (J = 5/2) « 3X (J = 3/2). i s the reason f o r the greater strength of the

This

edge resonance.

As shown by Horsley (16) the t r a n s i t i o n to the d-band, the edge resonance, may be convolved with an arctangent to produce the kind of absorption curves observed i n Pt L ^

In the Pt density of

states curves the Fermi surface at E„ cuts the curves i n a region of F maximum slope.

This would make the L ^

t r a n s i t i o n s very s e n s i ­

t i v e to changes i n E^,, i . e . , electron flow to or from the c a t a l y s t clusters. ec

^11 I I I * 8

By t h i s discussion we have attempted to show that the Pt e

r e s o n a n c e s

m

a y be considered as images of t h e i r respec­

t i v e d-bands (broadened by the appropriate l i f e t i m e and r e s o l u t i o n functions) and w i l l discuss the c a t a l y s t data i n terms of a d-band model. The differences between the Pt

XANES of the c a t a l y s t i n

H^ and He and bulk Pt as a function of temperature i s shown i n figure 4.

(There i s no measurable difference i n Pt metal over t h i s

temperature range.)

There i s a continuous, r e v e r s i b l e change f o r the

catalyst XANES shown i n A or C (773 K) to that shown i n Β or D (90 K) as shown i n Figure 6. The data was taken as follows:

The catalyst was reduced i n flowing

H^ at atmospheric pressure to a maximum temperature of 773 Κ and then cooled i n H , taking data at ^ 100 degree i n t e r v a l s to a low tempera­ 2

ture of 90 K.

I t was then heated back up to 773 Κ i n Η · 2

At 773 Κ

a p u r i f i e d He flow at atmospheric pressure was i n i t i a t e d and repeated data scans were taken at 773 Κ u n t i l the sample e q u i l i b r a t e d as shown by the data.

The sample was then cooled i n flowing He taking data as

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

24. LYTLE ET AL.

before.

289

Structure of Platinum Catalysts

Under these conditions we believe the c a t a l y s t to be below

s i n t e r i n g temperature (this was proved by c y c l i n g the c a t a l y s t ) and to consist of naked Pt c l u s t e r s i n He or with chemisorbed H when i n H^ atmosphere over most of the temperature range with some e q u i l i b rium f o r Pt-H bonds at the temperatures where H desorbs. In comparison to Pt metal there are s i g n i f i c a n t differences i n the u n f i l l e d c a t a l y s t d-states at both temperatures.

At high tempera-

ture the d-band i s s l i g h t l y narrower than bulk P t , at low temperature i t i s much broader.

The extra x-ray absorption i n the 5-15 eV i n -

t e r v a l shown i n figure 4 D) was the basis f o r the e a r l i e r conclusion (11-13) that P t / S i 0 i 2

electro

deficient

Although th 1*

d

L J J J edges appear to b the r e s u l t of mixing the d i f f e r e n t L ^ and L-^jj edge resonance i n t e n s i t i e s with the onset (the arctangent curve) of the absorption edge.

The difference spectra i n figure 5, where the appropriate Pt

metal edge has been subtracted, show that the changes r e l a t i v e to Pt for both edges are very s i m i l a r . p l i e r i n the

Allowing f o r the factor of 2 m u l t i -

edge the r e l a t i v e difference amplitudes are of the

expected magnitude, i . e . , L-j-^ > L ^ .

We can conclude that the

J » 5/2 and J - 3/2 states i n the c a t a l y s t are s i m i l a r to each other as they are i n Pt metal but they are s i g n i f i c a n t l y d i f f e r e n t from Pt metal. What are the e l e c t r o n i c and s t r u c t u r a l implications of that conclusion?

We have considered the f o l l o w i n g p o s s i b i l i t i e s to explain

the data: 1.

A change i n the Fermi energy due to electrons flowing to and from the Pt c l u s t e r s would correspondingly

change that part of

the d-band a v a i l a b l e f o r the edge resonance.

The edge narrowing

with increasing temperature implies electron flow to the Pt clusters as Pt-0 bonds to the support are broken. cannot explain the extra absorption at 5-15 eV with

This e f f e c t decreasing

temperature i f the d-band of the c l u s t e r i s s i m i l a r to bulk P t . There are no sharp peaks i n the density of states located at appropriate 2.

energies.

Could a change i n shape of the small Pt c l u s t e r s produce suff i c i e n t l y large changes i n the density of states to cause the effect?

One of us (Horsley) has made preliminary multiple

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CATALYST CHARACTERIZATION SCIENCE

290

A)

Pt L „ AT 500 C

B)

Pt Lit AT 90 Κ

1 Pt L

D)

Pt L,rr AT 90 Κ

e

o.o Ο

1

m

1 Ι­ AT 500 C e

t 0.0 -I

1

2

Pt L „ Pt L , „

4.0

0

10

20

30

Ε, eV Figure 5. Difference spectra of the data i n figure 4 where Pt metal L u m has been subtracted appropriately from the c a t a l y s t data. In*E) the difference areas Cat. (He, 773 K) - Cat. (He, 90 K) are shown.

200

400

600

800

DEGREES KELVIN

Figure 6. Area of the f i r s t peak i n difference curve of the Pt L edge versus absolute temperature f o r 1% Pt on Cabosil i n H . The high temperature measurement (773 K) was subtracted from each of the other spectra. I I : t

2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

24.

LYTLE ET AL.

291

Structure of Platinum Catalysts

scattering Χα c a l c u l a t i o n s to investigate t h i s question.

For Pt

c l u s t e r s corresponding to 13 atom spheres, two-layer, and mono­ layer r a f t s no changes were observed i n the d-states that would cause the e f f e c t . 3.

The low temperature EXAFS data showed Pt-0 bonds to the support. I f those bonds broke with increasing temperature would that cause the effect?

(The thermal smearing of the EXAFS at high tempera­

ture makes i t impossible to investigate the bonds d i r e c t l y . )

One

of us (Horsley) has made preliminary c a l c u l a t i o n s which show that the Pt 5d-orbitals i n t e r a c t with the 0 2p-orbitals to create a hybrid o r b i t a l with above the Pt d-band at low temperature i s due to t r a n s i t i o n s to the empty a n t i bonding o r b i t a l e .

At high temperatures the Pt-0 bonds to the

support begin to be broken because of the increased thermal motion, hence the o r b i t a l s disappear. Presumably, at higher temperatures a l l the bonds would break and the clusters would be­ come mobile, thereby providing a model f o r agglomeration and sintering. I t has been shown by electron microscopy (21) and EXAFS (22) that highly dispersed supported metal c a t a l y s t s consist of a mix of sphere-and-raft-like shapes.

We believe that as Pt-0 bonds break

(or form) the r a f t - l i k e c l u s t e r s c u r l up (or f l a t t e n out) on the support.

The bonds to the support are the d r i v i n g force f o r r a f t ­

l i k e dispersion.

The high temperature difference between

and He

may be due to a s i m i l a r anti-bonding resonance between the Pt(5d) and H(Is) o r b i t a l s , the width of the resonance a r i s i n g from an i n t e r ­ action with the Pt 5-P band.(23)

In a UPS study of highly dispersed

P t / S i 0 Ross, et a l . (24) investigated the f i l l e d bonding o r b i t a l s 2

below the Pt d-band and found a large increase i n the density of states.

This would confirm, i n part, our explanation.

The narrower

edge resonance i n He at high temperature i s not explained by these bonding arguments.

The e f f e c t i s experimentally s i g n i f i c a n t and may

be a true density of states e f f e c t caused by the small c l u s t e r s i z e .

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

292

CATALYST CHARACTERIZATION SCIENCE

Conclusions The new results of this study include a clear demonstration of Pt-0 bonds to the support at low temperature in either He or H^.

As the

temperature is raised above 600 Κ the Pt-0 bonds break and the Pt raft-like clusters curl up to be more sphere-like.

Concurrently with

this bond-breaking and change in shape, electrons flow to the Pt d-band.

At temperatures above 600 Κ there is a d-electron surplus

in the Pt clusters compared to bulk Pt, i . e . , they are more noble than Pt.

This may be a significant result since practical applica­

tions of Pt-containing catalysts are in the temperature range where these changes occur. Acknowledgment The work at Boeing and WSU was supported by NSF grant CHE 8219605. The x-ray measurements were made at the Stanford Synchrotron Radia­ tion Laboratory which is supported by DOE. Literature Cited 1.

Lee, P. Α . , Citrin, P. Η., and Kincaid, Β. Μ., Rev. Mod. Phys.

2.

Lytle, F. W., Via, G. Η., and Sinfelt, J . Η., "X-ray Absorption

53 769 (1981). Spectroscopy:

Catalyst Applications" in Synchrotron Radiation

Research, H. Winick and S. Doniach Eds., Plenum (1980), p. 401. 3.

Sinfelt, J . H . , Via, G. Η., Lytle, F. W., Catal. Rev. 26, 81

4.

Via, G. Η., Sinfelt, J . Η., and Lytle, F. W., J . Chem. Phys. 71

5.

Jaklevic, J . , Kirby, J . Α . , Klein, M. P . , Robertson, A. S.,

(1984). 690 (1979). Brown, G. S., and Eisenberger, P . , Sol. St. Comm. 23, 679 (1977). 6.

Bearden, J . A. and Burr, A. F . , Rev. Mod. Phys. 39, 125 (1967).

7.

Marques, E. C . , Sandstrom, D. R., Lytle, F. W., and Greegor, R. B., J . Chem. Phys., July 15 (1982).

8.

Lytle, F. W., Greegor, R. B . , Marques, E. C . , Sandstrom, D. R., Via, G. Η., and Sinfelt, J . Η., "Structural Genesis of Pt on SiO : 2

Determination by X-ray Absorption Spectroscopy,"

presented at Advances in Catalytic Chemistry II, Salt Lake City, Utah, May 1982.

J . Chem. Phys. (submitted).

The procedure

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

24.

293

Structure of Platinum Catalysts

LYTLE ET AL.

is as follows:

Apply the Pt-Pt phase shift (derived from Pt

metal) to the 1st forward transform of PtO .

This will partially

2

smear the Pt-O peak. smeared Pt-O peak.

Then take the back transform of this Extract a new phase shift from this back

transform using the known Pt-O distance, 2.07 Å. This phase shift can now be used on the catalysts to focus the Pt-O peak region. 9.

Lyon, H. B. and Somorjai, G. Α . , J . Chem. Phys. 44, 3707 (1966).

10.

Lytle, F. W., J . Catal. 43, 376 (1976).

11.

Lytle, F. W., Wei, P. S. P . , Greegor, R. B . , Via, G. Η., and

12.

Gallezot,

Sinfelt, J . H . , J . P . , Weber

Ζ für Naturfor 34A, 40 (1979). 13.

Mansour, A. N . , Cook, J . W., Sayers, D. E . , Emrich, R. J . , and Katzer, J . R., J . Catal. (submitted).

14.

Short, D. R., Mansour, A. N . , Cook, J . W., Sayers, D. Ε . , and Katzer, J . R., J . Catal. (submitted).

15.

Tauster, J . J . , Fung, S. C . , and Garten, R. I . , JACS 100, 170

16.

Horsley, J . Α . , J . Chem. Phys. 76, 1451 (1982).

17.

Mott, N. F . , Proc. R. Soc. Lond. 62, 416 (1949).

18.

Brown, M., Pierls, R. E . , and Stern, Ε. Α . , Phys. Rev. B15,

(1978).

738 (1977). 19.

Mattheiss, L . F . , and Dietz, R. Ε . , Phys. Rev. B22, 1663 (1980).

20.

Mansour, A. N . , Cook, J . W., and Sayers, D. E . , "Quantitative Technique for Determination of the Number of Unoccupied d Electron States in a Pt Catalyst from the

L

ΙΙ,III

X-ray Ab­

sorption Edge Spectra," J . Chem. Phys. (to be published). 21.

Prestridge, Ε. B . , Via, G. Η., and Sinfelt, J . H . , J . Catal.

22.

Greegor, R. B . , and Lytle, F. W., J . Catal. 63, 476 (1980).

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Louie, S. G . , Phys. Rev. Lett. 42, 476 (1979).

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In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

25 The Effect of Support-Metal Precursor Interactions on the Surface Composition of Supported Bimetallic Clusters 1

H. Miura, S. S. Feng, R. Saymeh, and R. D. Gonzalez

Department of Chemistry, University of Rhode Island, Kingston, RI 02881

The effect of precursor-suppor the surface composition of supported bimetallic clusters has been studied. In contrast to Pt-Ru bimetallic clusters, silica-supported Ru-Rh and Ru-Ir bimetallic clusters showed no surface enrichment in either metal. Metal particle nucleation in the case of the Pt-Ru bimetallic clusters is suggested to occur by a mechanism in which the relatively mobile Pt phase is deposited atop a Ru core during reduction. On the other hand, Ru and Rh, which exhibit rather similar precursor support interactions, have similar surface mobilities and do not, therefore, nucleate preferentially in a cherry model configuration. The existence of true bimetallic clusters having mixed metal surface sites is verified using the formation of methane as a catalytic probe. An ensemble requirement of four adjacent Ru surface sites is suggested. I t has g e n e r a l l y been assumed t h a t t h e most i m p o r t a n t c o n s i d e r a t i o n i n t h e s u r f a c e e n r i c h m e n t o f one m e t a l i n p r e f e r e n c e t o a n o t h e r i n a s u p p o r t e d b i m e t a l l i c c l u s t e r i s based on d i f f e r e n c e s i n t h e e n t h a l p i e s o f s u b l i m a t i o n o f t h e m e t a l s w h i c h comprise t h e c l u s t e r . In most c a s e s , t h e s u r f a c e c o m p o s i t i o n i s e n r i c h e d i n t h e m e t a l h a v i n g t h e l o w e r e n t h a l p y o f s u b l i m a t i o n (.1). The r o l e p l a y e d by t h e s u p p o r t o f i n f l u e n c i n g t h e s u r f a c e comp o s i t i o n o f s u p p o r t e d b i m e t a l l i c c l u s t e r s has o n l y r e c e n t l y begun t o r e c e i v e some a t t e n t i o n . M i u r a , e t a l (2^) have shown t h a t t h e n a t u r e o f t h e s u p p o r t can p l a y an i m p o r t a n t r o l e i n d e t e r m i n i n g n o t o n l y the surface composition o f the supported b i m e t a l l i c c l u s t e r s but a l s o t h e morphology o f t h e p a r t i c l e s . For silica-supported Pt-Ru

1

Author to whom correspondence should be directed.

0097-6156/85/0288-0294$06.00/0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

25.

MIURA ET AL.

Supported Bimetallic Clusters

295

b i m e t a l l i c c l u s t e r s , a cherry model structure having an inner core consisting p r i m a r i l y of Ru (85% Ru) was observed. The composition of t h i s inner core d i d not depend s i g n i f i c a n t l y on the o v e r a l l Ru composi­ t i o n of the b i m e t a l l i c c a t a l y s t . When Pt-Ru b i m e t a l l i c c l u s t e r s were supported on alumina, the Ru content of the inner core was observed to increase monatomically with the o v e r a l l concentration of Ru i n the c a t a l y s t . In another study involving porous supports, H a l l e r , et a l (3) have observed marked differences i n the a c t i v i t y of ethane hydro­ genolysis over a series of Cu-Ru b i m e t a l l i c c a t a l y s t s . These differences could be related to changes i n the texture of the support. The p o s s i b i l i t y that metal precursor-support i n t e r a c t i o n s could have a strong influence on the dynamics of the b i m e t a l l i c nucleation process cannot be ruled out and has already been alluded to i n pre­ vious work performed i n t h i s laboratory (1-2). Because H2PtCl «6H20 i s much more weakly adsorbed on the surface of s i l i c a than RuCl3*3H20, bimetallic clusters wit nucleation of the mobil mina, differences i n the r e l a t i v e a d s o r p t i v i t i e s of the metal pre cursor phases are less pronounced (4). This r e s u l t s i n b i m e t a l l i c c l u s t e r s which have a more homogeneous i n t e r n a l structure as opposed to the cherry model configuration observed on s i l i c a (2). These r e ­ s u l t s suggest that the dynamics of the metal nucleation process are an important v a r i a b l e which may override thermodynamic e f f e c t s based on enthalpies of sublimation. In order to pursue these ideas i n more d e t a i l , metal precursors which would be expected to i n t e r a c t v i a an i o n exchange mechanism with the hydroxyl protons of s i l i c a were used to prepare the support­ ed b i m e t a l l i c c l u s t e r s . We therefore report on the preparation of silica-supported Ru-Rh and Ru-Ir b i m e t a l l i c c l u s t e r s using RuCl3«3H20, RhCl3»3H20 and IrCl3«3H20 as metal precursors. In order to v e r i f y the presence of b i m e t a l l i c p a r t i c l e s having mixed metal surface s i t e s ( i . e . , true b i m e t a l l i c c l u s t e r s ) , the methanation reaction was used as a surface probe. Because Ru i s an excellent methanation c a t a l y s t i n comparison to P t , I r or Rh, the incorporation of mixed metal surface s i t e s into the structure of a supported Ru c a t a l y s t should have the effect of d r a s t i c a l l y reducing the methanation a c t i v i t y . This observation has been a t t r i b u t e d to an ensemble e f f e c t and has been previously reported f o r a series of silica-supported Pt-Ru b i m e t a l l i c c l u s t e r s (5). 6

Experimental Procedures Apparatus and Procedure. The apparatus and procedure were i d e n t i c a l to those outlined i n r e f . 2. Surface composition measurements were based on an O2-CO t i t r a t i o n technique described by Miura and Gonzalez (5-6). The r a t i o of surface metal/02/CO was 1/1/1 on R u - s i l i c a , 1/0.5/1.75 on R h - s i l i c a , 1/0.5/2.0 on P t - s i l i c a and 1/0.5/1.6 on I r s i l i c a . These t i t r a t i o n r a t i o s were found to be independent of sur­ face composition. Surface compositions determined by the O2-CO t i t r a ­ t i o n method have been v e r i f i e d using a v a r i e t y of experimental tech­ niques (2,5-6). Metal dispersions were obtained by the dynamic pulse method us­ ing e i t h e r H 2 , CO or O2 chemisorption at 298 Κ ( 7 ) . Catalyst Preparation.

The silica-supported samples used i n t h i s

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CATALYST CHARACTERIZATION SCIENCE

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study were prepared by impregnation. I n i t i a l l y , the appropriate weight of RuCl3*3H 0, RhCl *3H20 and IrCl «3H20 (Strem Chemical) was dissolved i n an amount of doubly deionized water s u f f i c i e n t to ensure the complete wetting of the support. The solutions were mixed w i t h Cab-O-Sil (grade M-5, Cabot Corp., Boston, MA), or Alon-C (Cabot Corp.) u n t i l a s l u r r y having the consistency of a t h i n paste was formed. The surface area and the average pore s i z e of Cab-O-Sil, as reported by the manufacturer, are 200 m*/g and 14.0 nm, respectively. The s l u r r y was dried i n a vacuum desiccator at room temperature f o r one or two days and s t i r r e d r e g u l a r l y during the drying process to r e t a i n uniformity. The dried catalyst was then ground into a f i n e powder before use. Total metal loadings were 0.3 mmoles of metal/g of c a t a l y s t f o r both the monometallic and the b i m e t a l l i c c a t a l y s t s . 2

3

3

Determination of Metal Precursor M o b i l i t i e s During Pretreatment. Relative precursor m o b i l i t i e or alumina-supported meta grade M-5, Cabot Corp.) or pure alumina (Alon C, Cabot Corp.) i n a 1:2 r a t i o p r i o r to pretreatment. The c a t a l y s t and s i l i c a were ground together using a mortar and pestle f o r at least 0.5 hr. be­ fore they were placed i n the Pyrex microreactor f o r pretreatment. The procedure was s i m i l a r to that used by Sarkany and Gonzalez (8). A large increase i n dispersion following pretreatment was explained by considering the migration of the metal precursor from the c a t a l y s t to the a d d i t i o n a l s i l i c a support during pretreatment. Pretreatment was as follows: The temperature was increased from 298 to 493 Κ at 10K/min i n flowing He (25 ml/min); the c a r r i e r gas was switched to H2 and the temperature was increased from 493 to 673K at 10K/min; reduction f o r 2 hr. i n flowing H2 was followed by f l u s h i n g i n He and cooling to room temperature. Oxygen impurities i n the He c a r r i e r gas were reduced to the p.p.b. range through the use of a Supelco oxygen gas p u r i f i e r backed by a 13-X molecular sieve main­ tained at 160K. Further p u r i f i c a t i o n was obtained by i n s e r t i n g an MnO trap i n the l i n e . Methanation Studies. Turnover frequencies f o r methane formation were measured at e i t h e r 493 or 498 Κ by a procedure which was i d e n t i c a l to that i n r e f . 5. Hydrogen and CO were premixed to a H2/CO r a t i o of 3. Results Surface Composition Measurements. The surface composition and metal dispersion f o r a series of s i l i c a (Cab-O-Sil) supported Ru-Rh b i ­ m e t a l l i c c l u s t e r s are summarized i n Table I . Surface enrichment i n Rh, the element w i t h the lower heat of sublimation, was not observed over the e n t i r e b i m e t a l l i c composition range. In f a c t , to w i t h i n the experimental l i m i t of error of the measurements, surface compositions and c a t a l y s t compositions were nearly equal. A small l o c a l maximum i n the dispersion was observed f o r the c a t a l y s t having a surface composition of 50% Rh.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

25.

MIURA ET AL.

297

Supported Bimetallic Clusters

Table I . Catalyst C o m p o s i t i o n ^ and Surface Composition f o r Silica-Supported Rh-Ru B i m e t a l l i c Catalysts Surface Catalyst (b) Composition Dp Composition Rh/Ru % Rh/Ru Catalyst Rh/Si0

100/0

25

100/0

2

Rh-Ru/Si0

75/25

21

69/31

2

Rh-Ru/Si0

50/50

23

51/49

2

Rh-Ru/Si0

27/75

19

18/82

2

Ru/Si0

0/100

11

0/100

2

(a) metal loading « 0.3 (b) measured using CO chemisorptio The corresponding data f o r a s i m i l a r series of silica-supported RuI r c a t a l y s t s are shown i n Table I I . Table I I . Catalyst Composition and Surface Composition f o r Silica-Supported Ru-Ir B i m e t a l l i c Catalysts

Catalyst Ir/Si0

Catalyst Composition Ir/Ru

(%)

100/0

61

2

Surface Composition 100/0

Ir-Ru/Si0

2

75/25

28

86/14

Ir-Ru/Si0

2

40/60

14

45/55

Ir-Ru/Si0

2

25/75

15

27/73

10/90

12

5/95

11

0/100

Ir-Ru/Si0 Ru-Si0

2

0/100

2

(a) measured using both 0

2

and CO chemisorption.

As was the case f o r the silica-supported Ru-Rh b i m e t a l l i c c a t a l y s t s , there was no s i g n i f i c a n t surface enrichment i n either metal over the e n t i r e range of b i m e t a l l i c c a t a l y s t compositions. Metal dispersions were observed to decrease as the concentration of Ru was increased. This same trend was observed f o r the Ru-Rh c a t a l y s t s and was i n marked contrast to observations on s i l i c a supported Pt-Ru c a t a l y s t s (2). In t h i s case a large increase i n dispersion was obtained as a r e s u l t of b i m e t a l l i c c l u s t e r i n g i n the cherry model configuration. A word should be said regarding the use of 0 chemisorption t o measure Ru-Ir metal dispersions. The stoichiometry of the CO adsorpt i o n on I r (CO/Ir j) was taken from the l i t e r a t u r e t o be 0.5 (9-10). The measured CO/O2 chemisorption r a t i o on I r was determined using the dynamic pulse method and found t o be 1.56. These r e s u l t s give 2

is

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CATALYST CHARACTERIZATION SCIENCE

298

an 0 2 / I r ( ) c h e m i s o r p t i o n r a t i o o f 0 . 3 2 , i n e x c e l l e n t agreement w i t h an 0 / I r ( ) r a t i o o f 0.33 r e p o r t e d by F a l c o n e r , e t a l ( 9 ) . The s t o i c h i o m e t r y o f t h e 0 2 / R u ( ) c h e m i s o r p t i o n r a t i o has been r e a s o n a b l y w e l l e s t a b l i s h e d and i s c l o s e t o one ( 5 ) . On t h e assumption t h a t these s t o i c h i o m e t r i e s are preserved i n the I r - R u b i m e t a l l i c c l u s t e r s , m e t a l d i s p e r s i o n s can r e a d i l y be o b t a i n e d from a knowledge o f t h e surface composition. The s u r f a c e - c a t a l y s t c o m p o s i t i o n d a t a f o r t h e s i l i c a - s u p p o r t e d Ru-Rh and R u - I r c a t a l y s t a r e shown i n F i g u r e 1. A s i m i l a r p l o t f o r t h e s e r i e s o f s i l i c a - s u p p o r t e d P t - R u b i m e t a l l i c c a t a l y s t s t a k e n from r e f . (2) i s i n c l u d e d f o r comparison p u r p o s e s . Enthalpies of s u b l i m a t i o n f o r P t , R u , Rh and I r a r e 552, 6 2 7 , 5 4 3 , and 648 K J / m o l e . D i f f e r e n c e s i n e n t h a l p i e s o f s u b l i m a t i o n (^75 K J / m o l e ) between P t and Ru and between Rh and Ru a r e v i r t u a l l y i d e n t i c a l , w i t h P t and Rh h a v i n g the lower e n t h a l p i e s o f s u b l i m a t i o n . For t h i s reason surface enrichment i n P t f o r the cas cannot be a t t r i b u t e d s o l e l Other p o s s i b i l i t i e s must a l s o be c o n s i d e r e d . s

2

s

s

P r e c u r s o r M o b i l i t y E x p e r i m e n t s . The m o b i l i t y o f the m e t a l p r e c u r s o r d u r i n g c a t a l y s t pretreatment i s , of course, a strong f u n c t i o n of the i n t e r a c t i o n between t h e m e t a l p r e c u r s o r and t h e s u p p o r t . Because H P t C l i s adsorbed as t h e P t C l ^ " a n i o n , i t i n t e r a c t s o n l y w e a k l y w i t h t h e hydroxy1 groups o f s i l i c a . R u , R h , and I r , on t h e o t h e r hand, a r e adsorbed as c a t i o n s and r e a d i l y exchange w i t h t h e a c i d i c h y d r o x y l groups on s i l i c a . The r e l a t i v e s u r f a c e m o b i l i t i e s o f t h e m e t a l p r e c u r s o r s d u r i n g p r e t r e a t m e n t can r e a d i l y be s t u d i e d by u s i n g the d i l u t i o n technique d e s c r i b e d i n the experimental s e c t i o n . A l a r g e i n c r e a s e i n d i s p e r s i o n f o l l o w i n g p r e t r e a t m e n t would be i n d i c a t i v e o f weak m e t a l p r e c u r s o r - s u p p o r t i n t e r a c t i o n s . Small increases i n d i s p e r s i o n , on t h e o t h e r h a n d , s u g g e s t s t r o n g m e t a l p r e c u r s o r support i n t e r a c t i o n s . The r e s u l t s f o r t h e c a t a l y s t s s t u d i e d a r e shown i n T a b l e I I I . 2

2

6

Table I I I .

Catalyst

E f f e c t o f D i l u t i o n on the D i s p e r s i o n o f Supported M e t a l C a t a l y s t s Dispersion %

6% P t / S i 0 6% P t / S i 0 : S i 0 (1:2) 6% P t / A l 0 6% P t / A l o 0 (1:2) Ru/Si0 (&) Ru/Si0 :Si0 (1:2) Rh/Si0 Rh/Si0 :Si0 (1:2) Ir/Si0 Ir/Si0 :Si0 (1:2) 2

2

2

2

3

3

2

2

2

2

2

2

2

2

2

28 43 57 79 11 14 24 30 61 78

Increase i n % Dispersion 54 38 27 25 27

(a) measured u s i n g H2 a d s o r p t i o n . (b) measured u s i n g CO a d s o r p t i o n . (c) measured u s i n g CO and O2 a d s o r p t i o n •

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

MIURA ET AL.

Supported Bimetallic Clusters

1001 8 2 ° 3 * TiO^)· These supported c a t a l y s t s are conventionally obtained by coimpregnation or sequential impregnation of the support with a solu­ t i o n containing the active phase or the promoter s a l t followed by drying, c a l c i n a t i o n , and reduction. Recently a new class of "sup­ ported" catalysts has been developed at the University of Pittsburgh (1-4). They are formed by reaction of synthesis gas or 0^ with a A 1

2

binary i n t e r m e t a l l i c compound (ex: Α Β • , — > A/B0 , A = N i , Co, χ y LU/h^ * Fe, and Β = S i , T i , Th, and Ce). The transformed materials e x h i b i t high s p e c i f i c a c t i v i t y i n methanation, ammonia synthesis, and e t h y l ­ ene hydrogénation reactions. The s i m i l a r i t y between the i n d u s t r i a l methanation c a t a l y s t s and catalysts obtained by decomposition of various i n t e r m e t a l l i c s i s s t r i k i n g . Most catalysts obtained by decomposition of a binary a l l o y involve an associative combination of 0

0097-*156/85/0288-Ό305$06.00/0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

306

CATALYST CHARACTERIZATION SCIENCE N i

T i

o r

t h e

an a c t i v e phase ( N i , Co) with the c a r r i e r (ex. N i S i , ) promoter (ex. N i Th , N i La , N i Ce , Co Th ). The purpose o f the present work i s £wo¥old: fïrst, to outlîne the r e s u l t s of our recent study of the surface structure of binary i n t e r m e t a l l i c s , representat i v e of the two types of a l l o y s described above (e.g., N i S i and N i Th ) and second, to extend t h i s study to ternary a l l o y s . The ternary system considered here i s derived from a given i n t e r m e t a l l i c highly active i n synthesis gas conversion (e.g., ThNi^) by p a r t i a l s u b s t i t u t i o n of N i by Fe. Ternary systems are of i n t e r e s t because 1) they provide a means to t a i l o r the s e l e c t i v i t y of the f i n a l catal y s t f o r a given product. For example, i n the CeNi^_ Cu system N i promotes methanation, whereas Cu i s known to catalyze the formation of oxygen-containing products (e.g., CH 0H) and 2) i n ternary i n t e r m e t a l l i c s such as T h N i F e _ , the r a t i o (Ni+Fe)/Th i s maintained constant. With binary systems such as N i Th the wide range of Ni/Th r a t i o s involved leads t the decomposed i n t e r m e t a l l i c s of spectroscopic and a c t i v i t y Surface and bulk characterization were c a r r i e d out using electron spectroscopy f o r chemical analysis (ESCA or XPS) and x-ray d i f f r a c t i o n (XRD). The r e s u l t s w i l l be discussed i n r e l a t i o n to methanation a c t i v i t y . x

x

X

y

v

y

x

y

x

x

f

x

5

x

Materials Binary and ternary i n t e r m e t a l l i c s were prepared by induction melting of the component metals i n a water cooled copper boat under a flow of p u r i f i e d argon. The systems considered here are the following: N i S i ( N i S i , N i S i , N i S i , and N i S i ) , N i T h (Ni Th, Ni Th, x

y

5

2

2

3

2

2

NiTh, and N i T h ) , and T h N i F e ^ 3

y

x

x

x

y

5

2

(χ = 0, 1, 2, 3, 4, and 5). A l l

were obtained i n s i n g l e phase form, as evidenced by x-ray d i f f r a c t i o n r e s u l t s . Two types of materials w i l l be examined i n t h i s study: un­ treated, noted as (Ni Th ) , (Ni S i ) , or (ThNi F e ) , refer to χ y'u χ y'u χ 5-x u the a l l o y s ground i n a i r and sieved through 45 mesh; and oxidized, abbreviated hereafter as (Ni Th ) , (Ni S i ) , or (ThNi Fe ) , χ y'o* χ y'o* χ 5-x ο' designate the a l l o y s which were treated with oxygen for 20-24 hours at r e l a t i v e l y high temperatures [450°C f o r (Ni S i ) systems and 350°C f o r the remaining systems]. x y ο c

c

v

v

Physico-Chemical Characterization X-ray d i f f r a c t i o n (XRD) measurements were carried out using a Diano XRD-6 powderdiffractometer with CuKa r a d i a t i o n . ESCA measurements were performed with an ΑΕΙ ES200 spectrometer equipped with an A l anode (1486.6_gV) and operated at 12 kV and 22 mA with a base pressure of 4 χ 10 t o r r . The N i 2p~, , Ni 3ρ, 2

2

9

T

f

9

Fe ϊ> /2 3 / 2 ' ^ 7/2 ° ' 8* scanned. More experimental d e t a i l s are given elsewhere (5). In the case of a homogeneous binary a l l o y , A B (A:Ni, Β:Si, the ESCA i n t e n s i t y r a t i o of two peaks ( / ) reïated to the atomic r a t i o n(A)/n(B) i n the a l l o y , as shown i n the following Equation (6): F e

2ρ

S i

2 p >

l s

& n d

C

l s

r e

o n s

w e r e

1

I

I

A

i s

B

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Th)

307

Catalysts Derivedfrom Intermetallics

26. H O U A L L A ET AL.

h. I

B

η (A) * σ(Α) - λ(A) - D(A) " n(B) · σ(Β) · λ(Β) · D(B)

where σ(Α,Β) represents the photoelectron cross sections of A and Β l e v e l s taken from S c o f i e l d (7); λ(Α,Β) i s the escape depth of A or Β photoelectrons; D(A,B) stands f o r the detector e f f i c i e n c i e s for the A and Β photoelectrons, which according to the design of the ΑΕΙ i n ­ strument v a r i e s l i n e a r l y with k i n e t i c energy (8). Methanation

Activity

CO conversion data r e l a t i v e to (Ni S i ) and (ThNi F e ) series χ ν ο χ 5—χ ο were taken from r e f . (3) and (9), r e s p e c t i v e l y . C a t a l y t i c measure­ ments were obtained f o i intermetallics Prio to each run, a sample mixtur was reduced i n H a t 275° c a r r i e d out at 2/5°C using H /C0 r a t i o * 9. More experimental det a i l s are given elsewhere (10). c

2

2

Surface and Bulk Characterization of Binary A l l o y s (Ni S i and N i Th ) Untreated A l l o y s . As previously noted, the untreated a l l o y s were ground i n a i r p r i o r to any surface or bulk a n a l y s i s . Examination of the ESCA spectra r e l a t i v e to N i 2p^ S i 2p, and Th indicates that the surface N i i s e s s e n t i a l l y In the m e t a l l i c state TBE = 852.5 eV) i n a l l (Ni S i ) a l l o y s and i n a mixed state ( N i , NiO) i n (Ni Th ) samples, δι 2s p a r t i a l l y oxidized. The extent of oxida­ t i o n siens to increase with increasing N i content. Conversely, Th i s present e x c l u s i v e l y as Th0 . Ni surface concentrations determined from ESCA are plotted as a function of bulk N i content i n Figures 1 and 2. In the case of homo­ geneous a l l o y s the points should f a l l on the 45° diagonal l i n e . I t can be seen that i n both (Ni S i ) and (Ni Th ) series the surfaces χ y u χ y il of the a l l o y s are nickel-poor, as compared to'the bulk. Similar ob­ servations have been made i n the case of N i A l (11,12) and Co Th (13) a l l o y s . Surface enrichment i n S i or τδ is* to be expected*be^ cause of the higher heats of formation of S i 0 and Th0 compared to NiO (-210, -292, and -58.4 kcal/mol, r e s p e c t i v e l y ) . Tnis would lead to a higher chemical a f f i n i t y of S i and Th toward the ambient gas and consequently an increased d r i v i n g force of S i and Th f o r segregation. l

2

2

2

Oxidized A l l o y s . The most s t r i k i n g difference between the (Ni S i ) and (Ni Th ) a l l o y s can be r e a d i l y seen i n Tables I and I I whïch show the nature of phases present, as i d e n t i f i e d by XRD, following oxygen treatment at 450°C and 350°C, r e s p e c t i v e l y . Thus, whereas (Ni Th ) i n t e r m e t a l l i c s are extensively transformed to N i , NiO, and ThO* u$on o x i d a t i o n , ( i ) a l l o y s are l i t t l e affected by oxygen treatment. Only i n the c a s l of Ni,-Si was unequivocal evidence found for the formation of a separate N i phase. Similar behavior was observed for N i A l and Co S i a l l o y s 03)· S i m i l a r l y , the r e l a t i v e ease of decomposition of*Ni Th a l l o y s i n 0 atmosphere i s equally observed i n the a l l o y s whicîi consist of a chemical union of a Group V I I I metal with rare earth or a c t i n i d e elements known as y

N i

s

x

2

y

0

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

U

308

CATALYST CHARACTERIZATION SCIENCE

Bulk

Ni / NI+SI

F i g u r e 1. V a r i a t i o n o f the s u r f a c e composition o f N i S i a l l o y s as a f u n c t i o n o f bulk Ni content ( r e l a t i v e standard d e r i v a t i o n o f x

i

10%).

(O): (·):

untreated a l l o y s oxidized a l l o y s

Reproduced from Ref. 5. Copyright 1983, American Chemical S o c i e t y .

Bulk N i / N i + T h

F i g u r e 2. V a r i a t i o n o f the surface composition o f N i T h a l l o y s as a f u n c t i o n o f bulk Ni content ( r e l a t i v e standard d e r i v a t i o n o f + 10%). x

(•): (•):

v

untreated a l l o y s oxidized a l l o y s

Reproduced with permission from Ref. 10. Copyright 1984, Academic Press. In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

HOUALLA ET AL.

26.

Catalysis Derivedfrom intermetallics

309

promoters of Fisher-Tropsch reactions (e.g., N i La , N i Ce , Co Th , Fe Th , etc.) (14). x y x y — y

7

Table I . Nature of Phases Present i n (Ni S i ) as I d e n t i f i e d by X-ray Diffractïon y

(Ni S i ) x y ο

(NiSi ) 2

Crystalline phases present Table I I .

NiSi

(Ni Si )

Q

3

2

Ni Si

2

3

Q

Series

0

(Ni Si) 2

(Ni Si ) 5

o

Ni Si

Ni Si

5

2

2

Crystalline phases present

N i , NiO + Th0

3

?

(NiTh)

o

Q

N i , NiO + Th0

2

2

(Ni Th) 2

+

Series

Nature of Phases Present i n (Ni Th ) as I d e n t i f i e (Ni Th )

2

o

NiO

modified

(Ni Th ) x y ο

2

(Ni Th) 5

Q

N i , NiO + Th0 2

o

N i , NiO + Th0 2

Surface characterization of (Ni S i ) and (Ni Th ) systems further i l l u s t r a t e s the difference i n tneîr behavior. ESCA spectra of N i 2p~* l e v e l s i n d i c a t e that i n the case of s i l i c o n r i c h a l l o y s ( N i S i ^ , N i L s i ^ the surface N i remains i n the m e t a l l i c state upon oxidation \5). Substantial surface oxidation occurs, however, at high N i content ( N i S i , N i S i ) (5). Conversely, ESCA analyses show that the surface N i of (Ni Th ) samples consists exclusively of NiO. The evolution of the surface Compositions of (Ni S i ) and (Ni Th ) a l l o y s upon oxidation i s equally d i f f e r e n t . I t ïs c l e a r from Figures 1 and 2 which compare the surface composition of (Ni S i ) and (Ni Th ) samples determined from ESCA data to those of e'unx y ο treated a l l o y s that oxidation induces surface N i depletion i n the N i S i system and surface N i enrichment i n the case of N i Th a l l o y s . In prïnciple, because of the higher heat of formation of §10? and Th0 compared to NiO, oxidation of (Ni S i ) and (Ν1 Τη ) a l l o y s should lead, i n both cases, to S i and $h Segregation? ¥h¥s has been indeed the case f o r N i S i a l l o y s . Their observed behavior i s i n accordance with previous Studies of the oxidation of metal s u i c i d e s (15,16) which show that S i i s p r e f e r e n t i a l l y oxidized and migrates to the surface to form a passivating S i 0 l a y e r , thus i n h i b i t i n g further oxidation of the i n t e r m e t a l l i c s . The opposite trend observed f o r the N i Th system can be, t e n t a t i v e l y , ascribed to t h e i r extensive transformation under oxidation conditions. Indeed, i n accordance with the data r e l a t i v e to N i S i system, mild oxidation of N i T h (100°C, 1 h) brings about a s i g n i f i c a n t surface enrichment i n Th i l ­ l u s t r a t e d by a decrease i n n(Ni)/n(Th) atomic r a t i o from 2 to 0.97 (Table I I I ) . More d r a s t i c conditions (200°C, 20 minutes) caused the bulk i n t e r m e t a l l i c to decompose i n t o N i and Th0 as observed by x-ray d i f f r a c t i o n ; under these conditions the atomic Ni/Th r a t i o y

2

2

5

2

X

X

2

χ

2

5

2>

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

y

U

CATALYST CHARACTERIZATION SCIENCE

310

began to increase (Ni/Th = 2.9). Higher oxidation temperature (350°C, 8 hours) resulted i n further decomposition of the i n t e r m e t a l l i c , as w e l l as a further increase i n the atomic r a t i o (n(Ni)/n(Th) = 13.2). Table I I I .

V a r i a t i o n of the Surface and Bulk Composition of N i T h A l l o y as a Function of Oxidation Conditions

Treatment

Untreated

5

(Surface) n(Ni)/n(Th) from ESCA data Phases present from XRD data

In A i r at 100°C for 60 min.

2.0

In A i r at 200°C f o r 20 min.

In A i r at 350 C f o r 60 min.

2.9

6.0

0.97

Ni Th

Ni,.Th, N i , Th0

Ni Th

5

5

2

e

NiJTh, N i , Th0

In A i r at 350°C f o r 480 min. 13.2

N i , Th0

2

2

Surface and Bulk Characterization of Ternary A l l o y s (ThNi Fe.

)

Untreated A l l o y s . ΤηΝ1 Γβ _ a l l o y s (x = 0, 1, 2, 3, 4, and 5) are a l l s t r u c t u r a l l y isomorphous* They a l l have the hexagonal CaCu^ structure with P^/MMM symmetry. As observed i n the case of (Ni Th ) a l l o y s , ESCA spectra r e l a ­ t i v e to N i 2 p y , Fe 2 p * , and Th $ f ^ l e v e l s indicate the presence of ( N i , NiO), \?e, F e ^ ) , and Th0 as'major surface species. The percent of surface NiO Increased with increasing bulk N i content, whereas the f r a c t i o n of Fe-^O^ i n the t o t a l surface Fe did not vary s i g n i f i c a n t l y with bulk Fe concentration (85%). A p l o t of the sur­ face atomic r a t i o s n(M)/n(Th) (M = Fe, Ni) calculated from ESCA data versus bulk Ni/Th content (Figure 3) shows that the surface of (ThNi^Fe,.^ )^ a l l o y s i s poor i n N i and Fe, as compared to the bulk ( 1 7 ) . P r e f e r e n t i a l segregation of Fe, as compared to N i , can be c l e a r l y seen i n Figure 4 which shows the v a r i a t i o n of the surface r a t i o n(Ni)/n(Ni)+n(Fe) versus the bulk Ni/Ni+Fe content. The ob­ served surface composition of the untreated a l l o y s i s i n accordance with the r e l a t i v e heat of formation of ThO^, F e O and NiO (-292, -100, and -58 k c a l / a t g). χ

5

χ

U

3

2

3

2

7

2

2

X

?

v

J

Oxidized A l l o y s . Ternary i n t e r m e t a l l i c s undergo extensive trans­ formation when they are treated i n a i r f o r 24 hours at 350°C. XRD data indicate the presence of NiO, Fe 0~, and Τη0 · The presence of Ni-Fe a l l o y could not be confirmed by XRD because of the small d i f ­ ference i n the various structures involved (Fe, N i , NiFe). However, evidence f o r the formation of Ni-Fe a l l o y has been obtained from the observed values of Curie temperatures determined from thermomagnetic analysis performed on these i n t e r m e t a l l i c s (9). 2

2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

26.

HOUALLA ET AL.

311

Catalysts Derivedfrom Intermetallics

0 ™»5

1

2

3 X : Ni/Th

ThNi

5

Figure 3. V a r i a t i o n o_f the surface composition o f ThNi'Fec a l l o y s as a f u n c t i o n o f bulk Ni/Th r a t i o (relativèstandârd d e v i a t i o n ^ i 10%). Reproduced from Ref. 17. Copyright 1984, Amer. Chem. S o c i e t y . r

ι Ά η

(0): (·): (•): (•):

r\f

ΚιιΊ U

M4 /TU

n(Ni)/n(Th); n(Ni)/n(Th); n(Fe)/n(Th); n(Fe)/n(Th);

~»4--' ^

/

untreated oxidized untreated oxidized

0

fi

-χ -·..-

v

-j.

ι

ι

.

Λ ·

? Τ

Λ

„

J

alloys alloys alloys alloys

Q2 0.4 0.6 Bulk N i / N i +Fe

0.8

1

Figure 4. V a r i a t i o n o f the surface r a t i o n(Ni)/n(Ni)+N(Fe) as a f u n c t i o n o f bulk Ni/Ni+Fe content i n T h N i F e 5 - a l l o y s . x

(•): (•):

x

untreated a l l o y s oxidized a l l o y s

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CATALYST CHARACTERIZATION SCIENCE

312

The s a l i e n t features of surface analysis by ESCA of the oxidized ternary a l l o y s are i n general agreement with the r e s u l t s i n the case of the binary Ν1 Τη system. NiO, ï ^ O y 2 P " nant species ρresen?. Surface enrichment i n N i and Fe were observed upon oxidation. P r e f e r e n t i a l segregation of Fe, as compared to N i , i s conspicuously shown i n Figure 4. a n d

T n 0

a r e

t l i e

r e d o m i

χ

C o r r e l a t i o n Between Surface Structure and CO Conversion A c t i v i t y Before any attempt to e s t a b l i s h a c o r r e l a t i o n between the surface structure of the oxidized a l l o y s and t h e i r CO conversion a c t i v i t y one must stress that the surface composition of the samples under reaction conditions may not necessarily be i d e n t i c a l to that deter­ mined from ESCA data. Moreover, surface n i c k e l content estimates from ESCA r e l a t i v e i n t e n s i t y measurements are at best semi-quantita­ t i v e . This can be r e a d i l t i o n ESCA f i n i t e escape ^i^Si Th^ , areas 01 N i ana SiO^ (or ThO^) and on the l o c a t i o n of N i with respect to S i 0 (or Th0 ). F i n a l l y , assuming an i d e a l s i t u a t i o n where the v a r i a t i o n of ESCA i n t e n s i t y r a t i o Ι^Τ-α* indeed r e f l e c t s the changes i n surface N i content, a l i n e a r c o r r e l a t i o n between rate and surface Ni concentration i s not necessarily obtained. This can be e a s i l y v i s u a l i z e d i f one takes i n t o account that most often the a c t i v e s i t e s are only a small f r a c t i o n of the a c t i v e phase exposed and that some reactions are strongly affected by the s i z e of the a c t i v e metal par­ ticle. o r

2

I

o n

t h e

b u l

2

Binary A l l o y s . The reported methanation a c t i v i t y (3) of the oxidized a l l o y s , expressed as the amount of CO consumed/gs, i s p l o t t e d i n Figure 5B as a function of t h e i r n i c k e l content. The a c t i v i t y of ( N i S i ) was not measured i n r e f . (3)· However, because of the s t a ­ b i l i t y of the untreated a l l o y , i t s reported a c t i v i t y can be con­ sidered as representative of that r e l a t i v e to the oxidized form. One can r e a d i l y note the close c o r r e l a t i o n between the observed v a r i ­ ations of the c a t a l y t i c a c t i v i t y and the evolution of surface n i c k e l concentration (Figure 5A). However, the dramatic difference between the a c t i v i t y of n i c k e l r i c h a l l o y s [ ( N i ^ O and ( N i S i ) ] and s i l i c o n r i c h i n t e r m e t a l l i c s [ ( N i ^ S i ^ and ( 8 i S i ) ] t a r exceeds that expected, s o l e l y on the basis of the observed v a r i a t i o n i n sur­ face n i c k e l content estimated from ESCA data. This can be p a r t i a l l y ascribed, as noted above, to the dependence of ESCA i n t e n s i t y r a t i o L . / I g . on the r e l a t i v e BET surface areas of N i and S i 0 and on the l o c a t i o n of N i with respect to S i 0 . There i s also another charac­ t e r i s t i c feature of the surface of n i c k e l r i c h a l l o y s [ ( N i e S i ) , ( N i S i ) ] reportedly a c t i v e i n methanation when compared with those of the i n a c t i v e N i S i i n t e r m e t a l l i c s [ ( N i S i > , ( N i S i J J . Upon oxidaj^on only the ac?ive a l l o y s present ESCA spectra c h a r a c t e r i s t i c of N i i n an o x i d i c environment, thus i n d i c a t i n g the p a r t i a l decom­ p o s i t i o n of the i n t e r m e t a l l i c . Comparison of the surface analysis and methanation a c t i v i t y of (Ni Th ) a l l o y s (Figure 6) shows that there i s some interdependence betweeX ?he surface concentration of N i and a c t i v i t y ; high N i surface concentration generally r e s u l t s i n greater methanation a c t i v i t y . 2

o

2

2

Q

Q

2

2

2

2

3

2

u

X

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

H O U A L L A ET AL.

Catalysts Derived from Intermetallics

0

0.5

1

Bulk N i / N i * Si

Figure 5:

C o r r e l a t i o n between surface composition and CO con­ version a c t i v i t y of oxidized N i S i a l l o y s (A) : V a r i a t i o n of the surface n i c k e l abundance n(Ni)/n(Ni)+n(Si) as a function of bulk n i c k e l content i n oxidized N i S i a l l o y s . (B) : V a r i a t i o n of CO conversion a c t i v i t y of oxidized N i S i a l l o y s as a function of n i c k e l content, χ y J

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

313

314

CATALYST CHARACTERIZATION SCIENCE

Figure 6:

Correlation between surface composition and CO conversion a c t i v i t y of oxidized N i Th a l l o y s (A) : V a r i a t i o n of surface n i c k e l abundance n(Ni)/n(Ni)+n(Th) as a function of bulk n i c k e l content. (B) : V a r i a t i o n of CO conversion a c t i v i t y (•) and CO sorption capacities (•) as a function of bulk n i c k e l content.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

26.

HOUALLA ET AL.

Catalysts Derived from Intermetallics

315

However, Figure 6 i n d i c a t e s that a s i g n i f i c a n t l y better c o r r e l a t i o n e x i s t s between the CO conversion rate and the CO sorption data. The lack of c o r r e l a t i o n between N i surface concentration, as determined from ESCA data and CO chemisorption measurements, can t e n t a t i v e l y be interpreted by considering that, as a r e s u l t of oxygen treatment, a large f r a c t i o n of N i i n Th-rich catalysts i s encapsulated w i t h i n the porosity of Th0 p a r t i c l e s and cannot be detected by ESCA. The texture of ThO^ i s probably spongy and can be penetrated by hydrogen as w e l l as CO. Thus, N i under t h i s permeable Th0 layer i s r e a d i l y attainable and can chemisorb CO. 2

2

Ternary A l l o y s . The v a r i a t i o n s of CO conversion rate as a function of N i content i n ( T h N i F e ^ _ ) catalysts are compared i n Figure 7 to N i surface concentration as determined from ESCA data. I t i s evident x

x

Q

X: Ni/Th

Figure 7:

C o r r e l a t i o n between surface n i c k e l abundance n(Ni)/n(Ni)+n(Fe)+n(Th) and CO conversion a c t i v i t y of oxidized ThNi Fe,-_ a l l o y s . (A) : V a r i a t i o n of surface n i c k e l abundance n(Ni)/n(Ni)+n(Fe)+n(Th) as a function of bulk Ni/Th content. (B) : V a r i a t i o n of CO conversion a c t i v i t y as a function of bulk n i c k e l content. x

x

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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CATALYST CHARACTERIZATION SCIENCE

that CO conversion rate follows closely the changes in surface Ni content. Such a simple correlation is presumably due to the ex­ tremely low activity of the Fe phase and the constancy of the ratio Fe+Ni/Th in the various samples which limits the variation of the BET surface areas of the treated materials and, consequently, en­ hances the reliability of surface composition measurements from ESCA intensities data. Acknowledgments This work was supported by the National Science Foundation under Grant CHE-8020001. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Wallace, W. E . , Chemtech Imamura, H. and Wallace Imamura, H. and Wallace, W. E . , J . Phys. Chem. 1979, 83, 2009-2012. Chin, R. L . , Elattar, Α . , Wallace, W. E . , and Hercules, D. Μ., J . Phys. Chem. 1980, 84, 2895-2898. Houalla, M . , Kibby, C. L . , Petrakis, L . , and Hercules, D. Μ., J . Phys. Chem. 1983, 87, 3689-3693. Penn, D. R., J . Electron Spectrosc. Relat. Phenom. 1976, 9, 29-40. Scofield, J . H . , J . Electron. Spectrosc. Relat. Phenom. 1976, 8, 129-137. Barrie, A. in "Handbook of X-ray and Ultra-Violet Photoelectron Spectroscopy", Briggs, D . , Ed.; Heyden: London, p. 116. France, J . , Ph.D. Thesis, University of Pittsburgh, Pittsburgh, 1982. Dang, Τ. Α . , Petrakis, L . , Kibby, C. L . , and Hercules, D. M . , J . Catal. 1984, 88, 26-36. Storp, S., Berresheim, Κ., and Wilmers, M . , Surface and Inter­ face Analysis 1979, 1, 96. Klein, J . and Hercules, D. M. Anal. Chem. 1981, 53, 754-758. Houalla, Μ., Dang, Τ. Α . , Kibby, C. L . , Petrakis, L . , and Hercules, D. Μ., to be published. Imamura, H. and Wallace, W. E. J . Phys. Chem. 1980, 84, 3145-3147. Grunthaner, P. J . , Grunthaner, F. J . , Scott, D. Η., Nicolet, Μ. Α . , and Mayer, J . W. J . Vac. S c i . , Technol. 1981, 19, 641-648. Abbati, I . , Rossi, G . , Galliari, L . , Braicovich, L . , Lindau, I . , and Spicer, W. E. J . Vac. Sci. Technol. 1982, 21, 409-412. Dang, Τ. Α . , Petrakis, L . , and Hercules, D. M. J . Phys. Chem. 1984, 88, 3209-3215.

RECEIVED March 5, 1985

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

27 Secondary Ion Mass Spectroscopic Studies of Adsorption and Reaction at Metal Surfaces Correlations with Other Surface-Sensitive Techniques C. R. Brundle, R. J. Behm, P. Alnot, J. Grimblot, G. Polzonetti, H. Hopster, and K. Wandelt IBM Research Laboratory, San

The use and limitations of SIMS ion intensity distributions to provide quantitative and chemical state information for adsorption and reaction of small molecules at metal surfaces is discussed. We concentrate on well-defined surfaces where there is sufficient information on the adsorption system from other surface sensitive techniques to test the information content of SIMS. Several years ago, Secondary Ion M a s s Spectroscopy ( S I M S ) seemed to be showing considerable promise for studying adsorption at metal surfaces (1). T h o u g h static S I M S is now widely used for studying organic films (2-5) and angular-resolved static S I M S (6,7) is being used i n successful but l i m i t e d studies for the determination of adsorption geometries on well-defined surfaces, the general promise of the technique i n using cluster i o n intensities for chemical and bonding analysis i n adsorption studies has not been fully born out. T w o factors were most important i n the early promise of the technique. They were the direct chemical specificity of the mass spectrometric analysis and the often extreme surface sensitivity of the technique. F o r the latter, i t is possible i n favorable cases to detect m u c h less than 1% overlayer concentrations. F o r the former, i t is sometimes possible to make straightforward statements about the chemical identity of the adsorbate and its manner of bonding to the substrate. The problems that have led to the technique being less than generally applicable are strongly correlated w i t h the above t w o advantages. T h o u g h S I M S can be extremely surface sensitive, i t is hard t o make i t a quantitative analysis technique because the sensitivity, or secondary i o n yields, vary enormously w i t h the changing chemical nature of the surface species. The reason is that most species ejected from the surface during the S I M S process are neutrals, but only that small fraction w h i c h is positive or negative ions is detected. The fractions w h i c h escape as ions depend strongly on the charge distributions i n the bonds being broken and the w o r k functions at the surface. T h u s the yield of N i species from a N i surface can increase four orders of magnitude i n the presence of adsorbed +

0097-6156/85/0288-0317S06.00/0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CATALYST CHARACTERIZATION SCIENCE

318

oxygen atoms (8). The yield o i Ο containing cluster ions can vary at least t w o orders of magnitude depending on the chemical nature of the oxygen at the surface (chemisorbed atomic, oxide, physisorbed or H 0 , etc.). The question of determining the chemical nature of the adsorbate and its bonding 2

characteristics to the surface i n v o l v e s somehow r e l a t i n g the d i s t r i b u t i o n of molecular cluster ions removed from the surface by sputtering to the original bonding situation before the i o n impact. Often a r i c h variety of cluster ions are observed. F o r instance, for C O adsorbed on N i ( 1 0 0 ) , N i , Ni£, N i C O + , N i C O + and N i C O + can a l l be observed (8). The temptation is to draw conclusions concerning the adsorption situations based on the "dominant" cluster ion(s). In the N i / C O situation, for instance, increased N i C O / N i C O ratios under different C O adsorption situations have been over-simplistically taken as an i n d i c a t i o n that C O was moving from an ontop to bridge-bonded situatio (9,10) U n f o r t u n a t e l y th S I M S is far too complex and the great for such simple correlations to have any general v a l i d i t y , though i n specific cases, they may be correct. The more fundamental approach of calculating ejected cluster distributions for different adsorbate sites by classical trajectory c o l l i s i o n methods (6,7) has more v a l i d i t y but is partly flawed by the fact that i t related to the neutrals not the ions and also involves some empirical pair-interactions. In addition these studies suggest that the ejected molecular ions consisting of combinations of substrate atoms and adsorbate atoms/molecules, M A d s * , are always formed by recombination of neutral and ionized particles just above the surface. There is experimental evidence, however, that this is not always the case and that some such species are formed by direct emission of intact units (11,12). In any case, the trajectory calculation approach seems to have been used m u c h more successfully i n determining adsorbate geometries by comparison to angle-resolved experimental S I M S than to experimental cluster i o n distributions. +

2

+

3

+

2

x

W e have adopted the approach that because of the large and p o o r l y understood variations i n i o n yields and cluster i o n distributions w i t h v a r i a t i o n i n chemistry, i t is necessary to empirically characterize these effects by using well-defined adsorbate situations w h i c h are simultaneously monitored by other, better understood, surface sensitive techniques such as L E E D , X P S and thermal desorption. W e report here some of the progress made to this end. M o s t of the w o r k discussed is our o w n , but i n some cases, we re-interpret other authors' S I M S results i n the light of a better understanding of the adsorption situations to w h i c h those results refer. The object of this paper is to illustrate what correlations can and cannot be safely made between observed S I M S behavior and the nature of the adsorbate/substrate interaction. In doing this, we use a variety of adsorbate/substrate situations. One major class is the dissociative adsorption of oxygen followed b y the onset of oxidation. W o r k on N i ( 1 0 0 ) and W ( 1 0 0 ) is discussed. A second class is t o follow the changes i n S I M S on increasing coverage and, therefore, changing geometric and bonding conditions for C O adsorption on N i ( 1 0 0 ) . A t h i r d class is adsorption at alloy surfaces. The final class is l o w temperature molecular adsorption, followed by reactions a n d / o r desorptions on raising the temperature.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

27. BRUNDLE ET AL.

319

SIMS Studies of Metal Surfaces

Experimental O u r S I M S data is taken using a V G Q 8 quadrupole w i t h unit mass resolution between 0-300 amu and a differentially pumped argon i o n gun used i n a defocused mode. A current of 1-2 χ 1 0 " A i n a ~0.5 c m spot area is used. The S I M S system is mounted on a U H V spectrometer w h i c h also has X P S , U P S , L E E D and thermal desorption capabilities (13). H e a t i n g is achieved by electron bombardment from a filament mounted on the manipulator behind the sample. C o o l i n g is achieved by circulating l i q u i d N or H e . Temperatures of 2 5 K can be reached. The samples used, N i ( 1 0 0 ) , C u ( 1 7 % ) N i ( 8 3 % ) (100) and (111) and A g ( l l l ) were oriented w i t h i n 1° and cleaned in situ by standard heating and A r i o n sputtering procedures. 9

2

2

+

S I M S Cluster Ion Characterizatio D u r i n O x y g e A d s o r p t i o d Oxidation F o r heavy o x i d a t i o n , that the metal can be determine positiv negativ y distributions (1). T h o u g h similar attempts have been made to characterize the nature of the surface during the early stages of oxygen interactions (14,15), we now k n o w from the extensive information available from other techniques that such interpretations are incorrect. W e use the by now well-characterized W ( 1 0 0 ) / O and N i ( 1 0 0 ) / O systems as examples. F o r W ( 1 0 0 ) / O , Benninghoven et al. (14) made some conclusions concerning the different stages of the reaction based on the behavior of the 0 , W , W O + , O " , W O j and W O 3 S I M S intensities as a function of exposure. They concluded that a t < l L exposure, only dissociative adsorption occurred and was characterized by W and O " emission. Between 1 and 1 0 L , W O J emission was observed and considered to be representative of a "monomolecular W - O structure" on the surface. A b o v e 1 0 L , W O emission was observed and i t was suggested that 3 D o x i d a t i o n was occurring. The above suggested sequence of reaction stages is now k n o w n not to represent the reaction stages for the majority oxygen species. There is a great deal of evidence (16) to show that, over the whole range of exposures at 3 0 0 K up to saturation under U H V conditions, the majority species adsorption products are overlayer atomic oxygen only. Y u (IT) repeated the S I M S measurements, but w i t h the additional important factor, the determination of t o t a l Ο coverage b y A E S . O n l y the 0 S I M S signal was found to be linear w i t h coverage over the complete adsorption range. W e have replotted his data for the other i o n intensities as a function of coverage i n F i g u r e 1. The O" and W O j intensities are shown before and after annealing to 1 3 0 0 K . T h i s process is now k n o w n to reconstruct the surface 0 6 ) . The following facts can be deduced from Y u ' s w o r k . F i r s t , since the relationship of 0 S I M S intensity to t o t a l Ο coverage is independent of annealing, the 0 secondary i o n yield is clearly independent of the drastic geometric and electronic structure effects brought about b y reconstruction. O n the other hand, the O " and WO" plots have obvious linear segments w i t h breaks at discrete coverages and are strongly affected by the reconstruction process. C l e a r l y , the bonding a n d / o r geometric environment is important i n determining the O " and W O j yields. Second, W , W O and W O j and W 0 are observed i n small quantities at l o w coverages, but there is no linear relationship w i t h coverage. The signals, however, increase very r a p i d l y near saturation. Indeed, they continue +

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Θ (mL; 1 mL defined as sat. AES intensity ) Figure 1. (a) S I M S intensities versus θ for W ( 1 0 0 ) / O derived from Ref. 17. C u r v e s ending i n an arrow signify that the signals are s t i l l rising rapidly even though 0, as determined by A E S , has saturated; (b) reconstruction of W ( 1 0 0 ) / O surface occurring above ~ 6 5 0 K . 2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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increasing rapidly w i t h exposure even when the Ο A E S signal has ceased to increase (particularly W O j ). There are t w o possible explanations for this behavior: (1) the signals come from the oxygen of minority species, such as incipient oxide nuclei, w h i c h continue to grow w i t h exposure even though total coverage barely changes; (2) the high yields occur above a c r i t i c a l local Ο coverage. T h i s c o u l d include case (1) but is not restricted to m i n o r i t y species. F o r example, a patch w i t h 1 M L Ο coverage might have zero W , W O and W O 3 intensities but a slight increase i n coverage may change the bonding and geometric relationship such that all the Ο atoms i n the patch became " a c t i v e " for W , W O and W O 3 emissions. In the W ( 1 0 0 ) case, we cannot really distinguish between possibilities (1) and (2). In the N i ( 1 0 0 ) case discussed below, we have clear evidence that i t i s , indeed, oxide species w h i c h lead to the high yields of certain cluster ions. H o p s t e r and Brundle N i ( 1 0 0 ) / O interactions w i t ( L E E D ) and the onset of oxidation ( X P S of the N i 2p levels). The data are replotted i n Figure 2. The downward arrows indicate the k n o w n coverage at w h i c h N i O nucleation begins as documented by a large variety of other techniques (8). Several points become clear from these correlations. F i r s t , the positive i o n intensities are clearly proportional to the chemisorbed Ο coverage and oxide oxygen does not contribute over the early exposure range. Then the signals reach a m a x i m u m at the end of the chemisorption stage. Second, there is no dependency of these positive i o n yields on the ordering of the chemisorbed Ο (i.e., no break i n the curve associated w i t h switching from p ( 2 x 2 ) to c ( 2 x 2 ) 0 , as was suggested by F l e i s c h et al (1$) i n a study w h i c h did not have the benefit of L E E D measurements). T h i r d , the negative i o n signals are proportional to the amount of o x i d a t i o n and have very l i t t l e or zero c o n t r i b u t i o n from chemisorbed O . T h i s general behavior is also found for N i ( l l O ) and N i ( l l l ) (19), w i t h the additional factor that after saturation coverage was reached ( A E S determined), the N i and N i signals started to increase again w i t h exposure. T h i s further increase c o u l d represent elimination of defects i n the N i O bilayer (19), or the uptake of small amounts of Ο on this bilayer. W h a t i t does not represent is oxide thickening as was originally concluded from comparable S I M S w o r k on polycrystalline N i w h i c h d i d not u t i l i z e an independent coverage measurement (20). O u r conclusion then for the oxygen interactions w i t h metals is that because of the specific association of cluster ion intensities w i t h particular types of oxygen rather than t o t a l coverage, the technique is not suitable for monitoring coverages or kinetics i n an independent manner. Once it is established which type of oxygen a particular cluster i o n is representative of, then that i o n may, i n favorable circumstances, be used for quantification. In the case of N i , i t seems that the negative ions are very sensitive to the i n i t i a t i o n of oxide nucleation. In the case of W ( 1 0 0 ) , the W O 3 , W O + and W 0 ions may f i l l a similar role. +

+

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Coverage and Secondary Ion Y i e l d Relationship for N i ( 1 0 0 ) / C O . W e showed above the enormous variation i n yields that occurred on going from adsorption to oxide nucleation. In the case of N i ( 1 0 0 ) / C O , one can perform more subtle bonding changes by changing the C O coverage. B e l o w θ~0Α M L , no ordered L E E D structure is formed, and v i b r a t i o n a l spectroscopy ( H R E L S ) indicates In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CATALYST CHARACTERIZATION SCIENCE

Figure 2. (a) S I M S intensities versus Θ for for N i ( 1 0 0 ) / O , Ref. 8; (b) schematic of the different interaction stages of N i ( 1 0 0 ) / O . 2

2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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27. BRUNDLE ET AL.

that both bridge-bonded and linear-bonded C O are present (21). Between 0.4 and 0.5 M L , a c ( 2 x 2 ) C O overlayer is formed w h i c h is k n o w n (22) to have linearly bonded C O . A t saturation coverage (0.68 M L , 7 7 K ) , a hexagonal L E E D pattern is formed. B o t h the L E E D and the v i b r a t i o n a l spectra are consistent w i t h this being an out-of-registry compression of the basic c ( 2 x 2 ) structure, such that a range of C O bonding sites between linear and bridge bonding are formed. In the table, we show the positive S I M S intensities for the above three situations, w h i c h were also monitored by X P S for C O coverage and L E E D for structural condition. In a d d i t i o n , we also show the intensities for the 0.5 M L situation where the C O molecule has been deliberately dissociated to C and Ο atoms by an electron beam. T h i s process was also monitored by X P S , w h i c h confirmed that the C O had, indeed, dissociated (from C ( l s ) and O ( l s ) chemical shifts) and that the total coverage was now ~0.25 M L ( C ( l s ) and O ( l s ) intensities). Severa (1) The observation o signify the presence or absence of molecular C O on the surface. N o t e that after dissociation of the C O , but w i t h atomic C and Ο s t i l l on the surface, the intensity of C O containing cluster goes to zero. (2) The yields vary considerably w i t h coverage; that is, the S I M S intensities are not proportional to coverage. In particular, the increase i n coverage from 0.5 to 0.68 M L ( 3 6 % increase) causes an approximately fourfold increase i n intensities. One might at first suspect that this is associated directly w i t h the change i n bonding geometry accompanying the coverage change (linearly bonded C O converting to a range of bonding situations between linear and bridge-bonded). L o o k i n g at the yields for the low-coverage (~0.25 M L ) C O situations, however, this seems u n l i k e l y , since there are bridge-bonded CO*s present at l o w coverage, yet the secondary i o n yields are weak. The effect then appears to be a local coverage one, rather than being directly traceable to geometry changes. O f course, an increase i n the local coverage and therefore C O / N i ratio must i m p l y some electronic structure/bonding changes, and i n fact we k n o w that the C O i n the out-of-registry 0.68 M L hexagonal structure has a lower heat of adsorption than does the 0.5 M L registered c ( 2 x 2 ) structure. It is possible then that there is a correlation between the S I M S intensities and bonding, even though there is no direct correlation to geometric structure. In early S I M S w o r k on the N i / C O system, i t was suggested that the Ni CO /NiCO ratio was related to the relative amount of bridge and linearly bonded C O present (9,10). F r o m our above discussion, we w o u l d anticipate that this is a very unlikely suggestion and the table bears this out. A g a i n , the N i C O / N i C O ratio appears to be a function of coverage, but shows no correlation w i t h the k n o w n linear and bridge-bonding behavior of CO. B r o w n and V i c k e r m a n n (23) have recently revisited the N i ( 1 0 0 ) / C O data. They relate the ratio N i C O / ( N i C O + N i C O ) to the relative amounts of linearly bonded C O and bridge bonded C O on the e m p i r i c a l basis that C O i n a linear M - C O site w i l l give 9 / 1 0 t h M C O + and l / 1 0 t h M C O + and that C O i n a bridge-bonded site w i l l give 5 / 1 0 t h M C O + and 5 / 1 0 t h M C O * r O v e r their l o w to high exposure adsorption range (they have no w a y of knowing actual L E E D structures since L E E D was not monitored) the above ratio changes from 0.78 to 0.91. In other data by Fleisch et al. (24), i t varies +

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In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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from 0.84 to 0.94, and i n our data (Table) at 0.2 M L coverage where both the bridge-bonded and linear- bonded species are present (210 the ratio is 0.86, and at a θ of 0.5 M L where only linear-bonded C O is present i t is 0.91. H o w e v e r , at a Θ of 0 . 6 8 L , at w h i c h coverage a range of structures between linear and bridge is present the ratio does not decrease but stays constant or aven increases again slightly to 0.92. C l e a r l y i n our data, w h i c h is directly correlated against L E E D structure and coverage, the small variations i n M C O / ( M C O + M C O ) do not correlate w i t h structural change. A l s o i t seems to us to be a l i t t l e unreasonable to t r y and make something of such a small relative change when the absolute intensities, w h i c h vary by a factor of 10 over this coverage range, are ignored. O u r conclusion then, from this w o r k , is that though the chemical speciation capability of S I M S is quite clear, quantification is s t i l l not easy, even for a system where the bonding changes w i t h coverage are much more subtle than those encountere a d d i t i o n , except when angle-resolve no direct relationship between the C O bonding site and the N i C O / N i C O S I M S intensity ratio, or the N i C O / N i C O + N i C O ratio. B r o w n and V i c k e r m a n (23) also present new data on R u ( 0 0 1 ) / C O and N i ( l l l ) / C O i n w h i c h coverage has been calibrated using T D S . In the N i ( l l l ) / C O data they find a m u c h stronger v a r i a t i o n of M C O / ( M C O + + M C O + ) , 0.5 to 0.8, w i t h increasing Θ w h i c h appears to correlate w i t h conversion of bridge to linear C O species, as determined from literature H R E L S data over the same exposure range. T h i s data looks m u c h more convincing than the N i ( 1 0 0 ) / C O data but again i t is difficult to see h o w effects on the M C O / ( M C O + M C O ) ratio of changes i n coverage, heat of adsorption, and changes i n dipole or w o r k function can be separated from geometric effects. +

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R e a c t i o n at C u / N i A l l o y Surfaces. The C u / N i alloy system is one w h i c h involves strong surface segregation of C u . T h u s , C u ( 1 7 % ) / N i ( 8 3 % ) has 7 0 % C u i n the top ~ 4 À at a clean (100) surface and 6 5 % C u at a clean (111) surface, as determined by l o w angle X P S (25). C O adsorbs only at N i sites at 3 0 0 K , as determined by U P S . T h i s preferential adsorption results i n some back-segregation of N i to the surface (25). F o r oxygen reaction, there also is preferential o x i d a t i o n of N i and back-segregation to the surface, but the system is more complex than for C O because adsorption w i l l take place at C u sites also and eventually the C u component w i l l oxidize. W e have monitored the changes i n S I M S cluster i o n intensities for these systems while m o n i t o r i n g C O and Ο coverage and C u and N i surface concentrations by X P S . The strong, but well-characterized, chemical specificity of the systems offers an ideal case to test the useful information content of S I M S cluster i o n intensities. W e refer the readers to the original paper for the results and just list the main conclusions here: (1) S I M S intensities from the " c l e a n " C u / N i surfaces cannot be used to determine C u / N i surface concentrations, or relative change i n concentration from one surface to another. T h i s is because trace impurities (of very l o w but unknown concentration) preferentially bond to N i sites and therefore the N i containing S I M S cluster ions are preferentially enhanced, leading to an erroneously h i g h determination of N i concentration.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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(2) Since C O at 3 0 0 K bonds only t o N i sites, preferentially enhancing N i containing S I M S cluster yields, a comparison of intensities from one composition surface to another d u r i n g C O adsorption does provide a measure of the relative N i concentrations i n the t w o surfaces, e.g., the S I M S cluster i o n intensities do correctly indicate that the (111) surface of the b u l k 1 7 / 8 3 C u / N i crystal has more N i at the surface than the (100) surface. A n advantage of S I M S measurements of this k i n d is that the information refers s t r i c t l y to the top layer, whereas the X P S i n f o r m a t i o n even at grazing angle looks several layers deep. (3) F o r oxygen interactions, similar interpretations as for C O can be offered i n the chemisorption stage, but the onset of oxidation complicates the S I M S data at high exposures. A t very high exposures, i t is possible to detect the onset of C u oxidation by a sudden rise i n the, by then, very l o w C u / N i ratio. +

+

Coadsorption and Decompositio cases where there are coad&orbed species present, w h i c h may react, and cases where molecular adsorption converts to dissociative (or associative) products. W e have already discussed an example of the last case, the dissociation of C O on N i ( 1 0 0 ) . It was mentioned i n the context of demonstrating that C O containing S I M S clusters signified the presence of molecular C O on the surface. A s can be seen from the table, however, the yields of the C containing and Ο containing clusters after dissociation of C O are l o w compared to those of the C O containing cluster, and on the basis of the S I M S data above, one might have been led to believe desorption rather than dissociation of C O had occurred. T h o u g h the secondary i o n yields for the C O containing clusters is m u c h higher than for the C and Ο containing clusters, the y i e l d for H 0 containing clusters following H 0 adsorption is even higher. T h u s , a 1:1 m i x t u r e of H 0 and C O adsorbed on the N i ( 1 0 0 ) surface at 7 7 K ( X P S determined) gives a N i ( H 0 ) / N i ( C O ) ratio of 30 (8). In fact, small traces of H 0 are always detectable by S I M S for the adsorption at l o w temperature of H , 0 , C O and C 0 on N i ( 1 0 0 ) (8) and for N O , N 0 and N on A g ( l l l ) (26) even when the quantity there is below the detection l i m i t s of X P S . C o n v e r s i o n of H 0 t o O H by reaction w i t h O * is easily observed by S I M S . In the case of N i ( 1 0 0 ) / O , this removed a point of contention concerning the assignment of a second O ( l s ) X P S peak of ~1 1/2 e V higher B E than the main peak. Suggestions that i t represented molecular 0 , N i O as opposed to O , or one geometric arrangement of O as opposed to another, etc., had a l l been made. S I M S unambiguously showed (27) that the h i g h B E O ( l s ) was representative of O H by correlating O H " S I M S signal intensities w i t h i t . The reaction between residual H 0 and O to give O H was the cause. The A g ( l l l ) / N O system turns out to be rather complex for adsorption at 2 0 K despite the fact that at 3 0 0 K v i r t u a l l y no adsorption occurs, and one might therefore expect that at l o w temperature only physisorption and condensation w o u l d occur. In fact, condensed N O exists as ( N O ) dimers (28), and a complex set of reactions leading to O , N O , N 0 and N 0 species takes place when the temperature is raised as determined by combined X P S and T P D measurements (29). F o l l o w i n g the S I M S cluster behavior during the reactions shows that several of the reaction species can be identified from the S I M S molecular clusters. 2

2

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Rabalais and coworkers (30) have reported on the S I M S of N O on N i ( 1 0 0 ) as a function of temperature. T h e y were not able to go to l o w enough temperature to observe ( N O ) condensation, but they d i d observe that the decomposition of N O to Ν and Ο fragments w i t h temperature increase was accompanied by a decrease of N O containing clusters and an increase i n Ν and Ο containing clusters. T h i s result i s , therefore, rather similar to that mentioned i n this paper earlier for C O dissociation on N i ( 1 0 0 ) (8). 2

Conclusions B y combining measurements on S I M S cluster i o n intensities w i t h parallel X P S , L E E D and T P D studies for well-defined adsorption systems, we have been able to provide some guidance concerning i n w h i c h areas the S I M S i o n intensities can give useful quantitative and chemical information. T h o u g h there are cases where the cluster i o n intensitie easy to k n o w for w h i c h syste productive to use static S I M S as a means for t r a c k i n g m i n o r i t y species w h i c h cannot be monitored by conventional means, or to distinguish chemical species, such as O H from O , where other techniques fail. It also seems very promising for adsorption at binary alloy surfaces where preferential reactions w i t h one component may be occurring. Table. Intensities (Arbitrary Units) of SIMS Clusters for the Ni(100)/CO System +

+

Surface Condition

Ni

+

NiCO

+

Ni CO 2

+

Ni C 2

+

Ni 0 2

+

NiCO Ni CO 2

-

3.1

0.5

-

-

6.2

0.86

40.0

20.0

2.0

-

-

10.0

0.91

230.0

110.0

10.0

-

-

11.0

0.92

3.0

0.6

0.2

0.1

-

-

0.2 M L , 300K

9.3

-

+

2

-

-

0.25 M L of C and 0.25 M L of Ο ~0.05 CO

+

-

0.3

Hexagonal overlayer, 77K, 0.68 M L

NiCO NiCO +Ni CO

-

"Clean"

c(2x2) overlayer, 0.5 M L

+

-

Literature Cited 1. 2. 3. 4. 5. 6. 7.

Benninghoven, A. In "Chemistry and Physics of Surfaces"; Vanselow, R.; Tong, S. Y.; Eds., C. R. C. Press: Cleveland, Ohio, 1977; p. 207. Benninghoven, Α.; Ed. "Ion Formation from Organic Solids," Vol. 25, Springer Series in Chemical Physics; Springer-Verlag: Berlin, 1983. Day, R. J.; Unger, S. E.; Cooks, R. G. Anal. Chem. 1980, 52, 557A. Cotton, R. J. J. Vac. Sci. Tech. 1981, 18, 737. Busch, K. L.; Cooks, R. G. Science 1982, 218, 805. Winograd, N.; Garrison, B. J. Acc. Chem. Res. 1980, 13, 400. Garrison, B. J.; Winograd, N. Science 1982, 216, 805. In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

27. BRUNDLE ET AL. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

28. 29. 30.

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Hopster, H.; Brundle, C. R. J. Vac. Sci. Tech. 1979, 16, 548. Barber, M.; Vickerman, J. C.; Wolstenholme, J. J. Chem. Soc. Farad. Trans. I 1976, 72, 40. Barber, M.; Bordoli, R.; Wolstenholme, J.; Vickerman, J. C. Proc. Seventh Int. Vac. Congr., Vienna, 1977, p. 933. Yu, M. L. Phys. Rev. B. 1981, 24, 5625. Yu, M. L. Appl. Surf. Sci. 1982, 11/12, 196. Brundle, C. R. IBM J. Res. 1978, 22, 235. Benninghoven, A.; Loebach, E.; Plog, C.; Trietz, N. Surf. Sci. 1973, 39, 397. Müller, K. H.; Beckmann, P.; Schemner, M.; Benninghoven, A. Surf. Sci. 1979, 80, 325. Brundle, C. R.; Broughton, J. Q. In Vol. 3B of "The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis," King, D. Α.; Woodruff, P.; Eds., t Yu, M. Surf. Sci. 1978 Fleisch, T.; Winograd, W.; Delgass, W. N. Surf. Sci. 1978, 78, 161. Rieder, Κ. H. Appl. of Surf. Sci. 1978, 2, 76. Müller, Α.; Benninghoven, A. Surf. Sci. 1974, 41, 493; Müller, Κ. H.; Beckmann, P.; Schemmer, M.; Benninghoven, A. Surf. Sci. 1979, 80, 325. Andersson, S. Proc. Seventh Int. Vac. Congr., Vienna, 1977, p. 1019. Andersson, S.; Pendry, J. B. Phys. Rev. Lett. 1979, 43, 363. Brown, Α.; Vickerman, J. C. Surf. Sci. 1982, 117, 154. Fleisch, T.; Ott, G. L.; Delgass, W. N.; Winograd, N. Surf. Sci. 1979, 81, 1. Wandelt, K.; Brundle, C. R. Phys. Rev. Lett. 1981, 46, 1529. Brundle, C. R.; Behm, R. J.; Grimblot, J.; Polzonetti, G.; Alnot, P., to be published. Brundle, C. R.; Hopster, H. In "SIMS II," Benninghoven, Α.; Evans, C. Α.; Powell, R. Α.; Shimuzu, R.; Storms, Η. Α.; Eds., Vol. 9, Springer Series in Chemical Physics; Springer Velag: Berlin, 1979. Nelin, C. J.; Bagus, P. S.; Behm, J.; Brundle, C. R. Chem. Phys. Lett. 1984, 105, 58. Behm, R. J.; Brundle, C. R.; Grimblot, J.; Polzonetti, G., to be published. Rabalais, J. W. Nucl. Instr. Methods 1981, 191, 323.

RECEIVED June 21, 1985

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

28 Electron Microscopy and Diffraction Techniques for the Study of Small Particles J. M. Cowley Department of Physics, Arizona State University, Tempe, AZ 85287 Recent advances i ments having a resolution of 2Å or better provide the possibility of atomic-scale imaging of small particles and, in favorable cases, atom positions can be deter­ mined with an accuracy approaching 0.1Å. The scanning transmission electron microscope provides complemen­ tary information through the use of special detector configurations and the possibilities for obtaining microdiffraction patterns and microanalysis signals from very small specimen regions, 10Å or less in diameter. Examples are given of the analysis of supported catalyst systems using electron beams of about 10Å in diameter to obtain diffraction patterns from individual metal particles of comparable diameter. Advances i n the design of transmission electron microscopes, combined with the use of accelerating voltages higher than the 100keV of the older high r e s o l u t i o n instruments, have provided the very important improvements of the r e s o l u t i o n l i m i t which allow the atom positions i n many inorganic s o l i d s to be distinguished. Point-to-point resolutions of 28 or better approached by some of the older m i l l i o n v o l t microscopes and achieved by some of the newer instruments should allow meaningful images of the atom configurations i n small regions of t h i n specimens to be interpreted q u a n t i t a t i v e l y and r e l i a b l y . Developments of the s p e c i a l detector configurations for scanning transmission electron microscopy (STEM) have made i t possible to perform s e l e c t i v e imaging making use of known c h a r a c t e r i s t i c s of the specimen, such as composition or c r y s t a l l i n i t y , to answer more s p e c i f i c questions. The techniques of m i c r o d i f f r a c t i o n have advanced to the stage that d i f f r a c t i o n patterns from regions 108 or l e s s diameter can be obtained r e a d i l y . This can provide information on the structures of i n d i v i d u a l small p a r t i c l e s or regions within small p a r t i c l e s , thus complementing i n an important way the information from the selected area electron d i f f r a c t i o n and X-ray d i f f r a c t i o n methods which refer to averages over very large numbers of individual particles. 0097^6156/85/0288-0329$06.00/0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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In a d d i t i o n , use of the scanning p r i n c i p l e allows microanalysis of very small specimen regions to be performed by detection of e i t h e r the c h a r a c t e r i s t i c X-rays emitted or the c h a r a c t e r i s t i c energy l o s s peaks i n the energy spectrum of transmitted electrons. In t h i s review an attempt w i l l be made to assess the current c a p a b i l i t i e s of these techniques i n t h e i r a p p l i c a t i o n to the study of small metal and oxide p a r t i c l e s which are of i n t e r e s t i n r e l a t i o n to c a t a l y s i s . Some examples w i l l be given of recent a p p l i c a t i o n s and some suggestions w i l l be made concerning probable d i r e c t i o n s for future developments. Transmission electron microscopy

(TEM)

For many years i t has been possible to detect s i n g l e heavy atoms as small black (or white) spot possible to detect d e t a i scale of 1A or better even with electron microscopes having a nominal "point-to-point" r e s o l u t i o n of 3-^8 Π,2). However with such microscopes the i n t e r p r e t a t i o n of image d e t a i l on t h i s scale i n terms of structure i s possible only for very s p e c i a l cases of extended, perfect t h i n c r y s t a l s of very simple structure and i s not possible for small c r y s t a l s or c r y s t a l s with defects. The p r a c t i c a l use of electron microscopes as a means for d e r i v i n g the atom arrangements i n small p a r t i c l e s or other t h i n specimens had to wait for the development of electron microscopes having a point-to-point resolution around or better since the interatomic distances i n projections of the structures of metals, semiconductors, oxides and other materials tend to be 1.5-28 for even the most favorable o r i e n t a t i o n s . The required r e s o l u t i o n has been attained by use of microscopes having higher accelerating voltages than the 100keV which has been conventional i n the past. Interpretation of the images i s s t i l l not straightforward even when there seems to be a simple one-to-one correspondence between black (or white) dots i n the image and atom p o s i t i o n s . E s p e c i a l l y when q u a n t i t a t i v e data on interatomic distances i s to be derived, detailed c a l c u l a t i o n s based on many-beam dynamical theory (3) must be applied to derive calculated images for comparison with experiment. For t h i s purpose the experimental parameters describing the imaging conditions and the specimen thickness and o r i e n t a t i o n must be known with high accuracy. A recent example of a successful analysis comes from the studies of small gold p a r t i c l e by Marks and Smith (4,5) using the 600keV Cambridge microscope, (see also t h e i r a r t i c l e i n t h i s volume). With the incident beam p a r a l l e l to the (110) face of a gold c r y s t a l , i n [100] d i r e c t i o n , the configuration of atom rows extending about 5θ8 i n the beam d i r e c t i o n could be seen c l e a r l y , showing the 2x1 surface reconstruction, which had previously been postulated from LEED r e s u l t s . Displacements of the gold surface atoms from the bulk l a t t i c e p o s i t i o n s could be determined with an accuracy of about 0.l8. These displacements, derived by comparison with calculated images

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were d i s t i n c t l y d i f f e r e n t from those suggested by the positions of the black dots associated with the atom rows i n the images. An extreme case of the apparent d i s t o r t i o n of structures due to the complications of the imaging process i s shown i n figure 1, an image of the corner of a cubic c r y s t a l of MgO smoke viewed along the [ 0 0 1 ] d i r e c t i o n , p a r a l l e l to the edge of the cube. The image was obtained with JEM 2 0 0 C X microscope by Dr. T. Tanji i n our laboratory. The 2 A square g r i d corresponding to the 2 0 0 and 0 2 0 l a t t i c e p e r i o d i c i t i e s i s v i s i b l e i n the bulk of the c r y s t a l . In the small c r y s t a l projecting from the top of the large c r y s t a l and at the corner of the large c r y s t a l the l a t t i c e planes appear to be bent, curving away from the c r y s t a l face by 1 or 28. There may, of course, be some s l i g h t displacements of the atoms present, but these large apparent displacements are almost c e r t a i n l y the r e s u l t of an a r t i f a c t produced by dynamical d i f f r a c t i o Determinations of projected atom positions are much more d i f f i c u l t for atoms i n the i n t e r i o r of the p a r t i c l e i f the atoms are not conveniently aligned i n s t r a i g h t rows i n the d i r e c t i o n of the incident electron beam. For the immediate future only the most favorable cases w i l l be studied but with the a p p l i c a t i o n of a n t i c i ­ pated improvements of r e s o l u t i o n to the 1.58 l e v e l or better and the means for more accurate and automated measurement of the necessary instrumental parameters, the d e t a i l e d study of configurations of atoms i n small p a r t i c l e s should become generally f e a s i b l e . In the meantime a great deal of more q u a l i t a t i v e but h i g h l y s i g n i f i c a n t information on small p a r t i c l e s should flow from the high resolution instruments now a v a i l a b l e . Scanning transmission electron microscopy (STEM) While STEM instruments have not shown the same r e s o l u t i o n or picture q u a l i t y as the fixed beam TEM instruments f o r b r i g h t - f i e l d imaging, they have important advantages derived from the f l e x i b i l i t y with which d i f f e r e n t detector systems may be arranged to obtain a v a r i e t y of image s i g n a l s . Also the fact that multiple images from d i f f e r e n t detectors can be produced as p a r a l l e l e l e c t r o n i c s i g n a l s i n s e r i a l form allows great f l e x i b i l i t y i n on-line image processing. Early work by Crewe and associates (6,7) established the benefits of STEM for d a r k - f i e l d imaging and for images using combinations of s i g n a l s from i n e l a s t i c and e l a s t i c s c a t t e r i n g . These, and other means involving s p e c i a l detector configurations, have increasingly been applied to the s p e c i a l problems of imaging small heavy-atom p a r t i c l e s supported on, or embedded i n , light-atom m a t e r i a l . The Ζ contrast method, i n v o l v i n g signals from i n e l a s t i c a l l y and e l a s t i c a l l y scattered electrons, has been shown to be e f f e c t i v e for the study of supported c a t a l y s t p a r t i c l e s ( 8 ) . Later, advantage was taken of the fact that heavy atoms scatter more strongly to higher angles than l i g h t atoms and i t was shown that heavy atom p a r t i c l e s could be revealed more r e a d i l y i f the images were obtained only with electrons scattered to high angles, of the order of 1 0 ~ radians ( 9 ) . unless the s c a t t e r i n g angle i s s u f f i c i e n t l y l a r g e , the remaining

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signal s t i l l shows some contrast due to c r y s t a l l i n i t y of the l i g h t atom m a t e r i a l , with maxima or minima of i n t e n s i t y from small regions giving strong d i f f r a c t i o n e f f e c t s , A further refinement of the method has been made i n which only those electrons were used which were scattered to higher angle regions of the d i f f r a c t i o n pattern where c r y s t a l l i n e r e f l e c t i o n s were weak or absent. Then a difference s i g n a l was obtained from electrons scattered to very high angles and those scattered to high angles (10). The s i g n a l l e v e l s from such a scheme are low but the discrimination between heavy and l i g h t atoms can be very good i f the specimen i s not too t h i c k . For most studies of c a t a l y s t p a r t i c l e s the electron microscopy i s done on very t h i n specimens and the use of high angle scattering for dark f i e l d imaging i use of t h i c k specimens thes then the choice of s p e c i f i c detector configurations can enhance the contrast of small p a r t i c l e s . I t has been shown, for example, that the v i s i b i l i t y of small p a r t i c l e s on t h i c k supports can be improved considerably by using a detector displaced from the normal bright f i e l d imaging p o s i t i o n so that i t l i e s on the edge of the c e n t r a l spot (the d i r e c t l y transmitted beam) i n the detector plane ( 1 1 ) . Figure 2 shows two STEM images of small gold p a r t i c l e s on a c r y s t a l of MgO. For the image on the l e f t , the detector was c e n t r a l in the beam spot containing the d i r e c t l y transmitted electrons, as for normal bright f i e l d imaging. The other image was obtained with the detector displaced so that i t was j u s t at the edge of the c e n t r a l beam spot, giving an image produced p a r t l y by the d i r e c t l y transmitted electrons and p a r t l y by electrons deflected by e l a s t i c and i n e l a s t i c s c a t t e r i n g processes. In t h i s , the small gold p a r t i c l e s are seen much more c l e a r l y . Microanalysis When the fine electron beam of a STEM instrument passes through a specimen, i t generates secondary r a d i a t i o n through i n e l a s t i c scattering processes. When inner s h e l l electrons of the atoms are excited, the secondary r a d i a t i o n signals may be c h a r a c t e r i s t i c of the elements present and so provide a basis for the microanalysis of the small specimen regions which are i r r a d i a t e d . F i r s t l y , the energy losses of the incident electrons which produce the inner s h e l l e x c i t a t i o n s may be detected as peaks i n electron energy l o s s spectroscopy (EELS). The elecrons transmitted by the specimen are dispersed i n a magnetic f i e l d spectrometer and the peaks, due to K, L and other s h e l l e x c i t a t i o n s giving energy losses i n the range of 0-2000eV, may be detected and measured. Secondly, the c h a r a c t e r i s t i c X-rays, emitted as the electrons displaced from the inner s h e l l s of the atoms are replaced, can be detected by use of an energy-sensitive detector placed close to the specimen. An account of the a p p l i c a t i o n of both the energy dispersive spectroscopy (EDS) of the emitted X-rays and EELS to the

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Figure 1. High r e s o l u t i o n Electron Micrograph of a cubic MgO c r y s t a l viewed i n [100] d i r e c t i o n showing square net of 2A fringes and apparent bending of atom planes at edges. Courtesy of Dr. T. T a n j i .

Figure 2. (a) Bright f i e l d STEM image of small gold c r y s t a l s on a large MgO smoke c r y s t a l . Marker indicates 100A. (b) As f o r (a) but with a displaced detector.

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study of the composition of small p a r t i c l e s of i n t e r e s t i n c a t a l y s i s i s given by the a r t i c l e of Lyman i n t h i s volume. In each case the analysis may be done of very small specimen regions of diameter l e s s than 100A. The l i m i t a t i o n s on s i z e of p a r t i c l e which may be analysed, or on the percentage of a p a r t i c u l a r element present i n any sample area, are determined i n each case by the s i g n a l strength. The relevant parameters include the i n t e n s i t y of the incident beam, the scattering cross section for inner s h e l l e x c i t a t i o n s , the detection e f f i c i e n c y and the r a t i o of signal to background. In general the detection e f f i c i e n c y i s high for EELS but the background l e v e l s of the s i g n a l s are also high. For X-ray EDS the detection e f f i c i e n c y i s not so good but the signals show lower background l e v e l s . Each technique has i t s own p a r t i c u l a r areas of strength and weakness. A t h i r d s i g n a l , dependent on the nature of the elements present, i s given by the Auger electrons when the energy of an inne the emitted electrons are of r e l a t i v e l y low energy (0-2000eV) and so have only l i m i t e d penetration through s o l i d s , Auger electron spectroscopy (AES) and the corresponding scanning imaging technique (SAM: scanning Auger Microscopy) have been thought of as surface a n a l y s i s techniques to be applied to bulk samples. Currently, however, instruments are being b u i l t to combine AES with STEM imaging i n the transmission or r e f l e c t i o n mode. With a projected s p a t i a l r e s o l u t i o n of 508 or l e s s , AES and SAM may w e l l take t h e i r place as major t o o l s for the i n v e s t i g a t i o n of the composition and surface modifications of small p a r t i c l e s . P a r t i c u l a r l y for l i g h t elements, the cross sections for the production of Auger electrons may be greater than for X-rays and the c o l l e c t i o n e f f i c i e n c i e s may also be greater. REM electron microscopy

(REM)

In recent years the technique f o r imaging with d i f f r a c t e d beams, obtained i n the RHEED mode with an incident beam at grazing incidence to the f l a t surface, has been shown to be e f f e c t i v e as a means for studying surface structure and surface reactions 02,J3.). While i t i s desirable to use a microscope having an u l t r a - h i g h vacuum specimen environment i f surface reactions are to be studied, some valuable determinatons of structure can be made with a conventional i n s t r u ment. In each case s i n g l e atom steps on the surface give good contrast, d i s l o c a t i o n s i n t e r s e c t i n g the surface are c l e a r l y v i s i b l e and a number of other i n t e r e s t i n g surface features have been seen and explored. In the case of the regular arrays of steps seen on v i c i n a l surfaces of gold c r y s t a l s , a r e s o l u t i o n of better than 10A has been demonstrated (JJO. This technique has been applied most e f f e c t i v e l y for the study of extended surfaces of bulk samples and the i m p l i c a tions for the knowledge of surfaces of small p a r t i c l e s are, at best, i n d i r e c t . The equivalent type of imaging using the scanning mode, i s more d i r e c t l y relevant. Scanning r e f l e c t i o n electron microscopy (SREM) By use of a scanning transmission electron microscope, with the incident beam grazing the c r y s t a l surface, the s t r u c t u r a l features on surfaces have also been revealed with a r e s o l u t i o n of 10A or better

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(15). This technique has been applied e f f e c t i v e l y to examine d e t a i l s of surfaces of p a r t i c l e s 1ym or l e s s i n diameter. I t has been used, for example, t o detect the ordering i n l i n e a r arrays of small gold p a r t i c l e , 2θ8 i n diameter, on the surfaces of MgO c r y s t a l s (16). The extraordinary r e s u l t i s that the gold p a r t i c l e ( s ) apparently are aligned on surface steps which are i n c l i n e d to each other, and to the [100] c r y s t a l edge d i r e c t i o n s , at angles of only 2 or 3 degrees. As i n the case of STEM, the main benefit a r i s i n g from the use of the scanning mode i s that the incident electron probe can be stopped or c o n t r o l l e d i n i t s motion and a v a r i e t y of detector types and configurations can be used to obtain p a r t i c u l a r s i g n a l s , g i v i n g information beyond that obtained i n the normal imaging modes. When the scan of the incident beam i s stopped at a chosen part of the image, a d i f f r a c t i o region which may have a 10A or l e s s . Also electron energy loss analysis of the scattered electrons may allow deductions concerning the energy states of very small surface regions. The most s t r i k i n g r e s u l t s obtained i n t h i s way come from experiments i n which an electron beam of 1θ8 diameter i s made to traverse the vacuum j u s t outside the face of a small c r y s t a l (17-19). In t h i s way the surface e x c i t a t i o n s can be examined with no complication from s c a t t e r i n g or e x c i t a t i o n s of the bulk c r y s t a l . The main features of the energy l o s s spectra have been shown to be i n e s s e n t i a l agreement with the deductions from the known d i e l e c t r i c constant of MgO, but there are i n d i c a t i o n s of e f f e c t s due to surface states appearing w i t h i n the band gap of the bulk c r y s t a l and to surface channelling phenomena (V7). Experiments have also been conducted to investigate the form of the p o t e n t i a l f i e l d extending from the c r y s t a l i n t o the surrounding vacuum by detection of the d e f l e c t i o n of electrons passing close to the c r y s t a l surface (20) .

M i c r o d i f f r a c t i o n i n a STEM instrument The d i f f r a c t i o n pattern obtained i n the detector plane when the beam scan i n a STEM instrument i s stopped at a chosen point of the image comes from the illuminated area of the specimen which may be as small as 38 i n diameter. In order to form a probe of t h i s diameter i t i s necessary to i l l u m i n a t e the specimen with a convergent beam. The pattern obtained i s then a convergent beam electron d i f f r a c t i o n (CBED) pattern i n which the c e n t r a l spot and a l l d i f f r a c t i o n spots from a t h i n c r y s t a l are large discs rather than sharp maxima. Such patterns can normally be interpreted only by comparison with patterns calculated for p a r t i c u l a r postulated d i s t r i b u t i o n s of atoms. This has been attempted, as yet, for only a few cases such as on the d i f f r a c t i o n study of the planar, nitrogen-rich defects i n diamonds (21) . D i f f r a c t i o n patterns having r e l a t i v e l y well-defined sharp spots can be obtained from small u n i t - c e l l c r y s t a l s with an incident beam of diameter 10-158. Such patterns have been used i n the study of the structures of small metal p a r t i c l e s (22). For p a r t i c l e s 10-20A diameter the electron beam can i l l u m i n a t e the whole of the p a r t i c l e

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so that any s t r u c t u r a l features such as twin or f a u l t s can be revealed. For larger p a r t i c l e s , i n the 20-508 s i z e range, the d i f f r a c t i o n pattern may be seen to change as the beam i s moved across the p a r t i c l e . For the smaller p a r t i c l e s which include only a few tens or hundreds of atoms, any twinning or f a u l t i n g reduces the range of ordering to the extent that the pattern can not be interpreted i n the same way as a pattern from an extended c r y s t a l . The i n d i v i d u a l s i n g l e - c r y s t a l regions may contain only two or three atomic planes. Interpretation can be made only by c a l c u l a t i o n of patterns from postulated models for the configurations of atoms (22). This technique has been applied, for example, to test the t h e o r e t i c a l p r e d i c t i o n that for small p a r t i c l e s of face-centered cubic metals the equilibriu with preference for configuration t e t r a h e d r a l l y shaped s i n g l e c r y s t a l regions are r e l a t e d by twining on (111) planes (23)· For p a r t i c l e s of gold i n a polyester f i l m , formed by co-sputtering (24), i t was shown that i n the s i z e range of 30-50A approximately h a l f were m u l t i p l y twinned but i n the size range of 15-20A a much smaller proportion of the p a r t i c l e s could be i d e n t i f i e d as such. Most were s i n g l e c r y s t a l s or had at most one or two twin planes. I t i s not necessarily to be concluded that, i n general, the proportion of small metal p a r t i c l e s having the m u l t i p l i c i t y twinned form decreases as the s i z e i s decreased. The evidence concerning p a r t i c l e s formed i n other ways shows a great deal of v a r i a b i l i t y . For example 20A gold p a r t i c l e s epitaxed on MgO (100) faces are almost i n v a r i a b l y s i n g l e c r y s t a l s when formed by i n d i r e c t evaporation on faces not exposed to the d i r e c t f l u x of incident gold atoms (16), although gold p a r t i c l e s formed on MgO (100) faces by d i r e c t deposit i o n from the source are i n random o r i e n t a t i o n , u s u a l l y with (111) planes p a r a l l e l to the surface, and are frequently twinned or multiply twinned. P a r t i c l e s of Pd on s i n g l e - c r y s t a l ot-Al-O- faces were sometimes twinned and sometimes not for d i f f e r e n t regions of the same specimen. In agglomerates of pure Pt p a r t i c l e s , i n d i v i d u a l p a r t i c l e s i n the 100A s i z e range showed a r e l a t i v e l y high incidence of twinning and multiple twinning but Pt p a r t i c l e s i n the s i z e range of 15-30A, supported on alumina or s i l i c a substrates gave mostly s i n g l e - c r y s t a l patterns. The extent to which small p a r t i c l e s of Pd and Pt show evidence of oxidation a f t e r exposure to a i r i s also highly v a r i a b l e . I t i s d i f f i c u l t to confirm the evidence of X-ray d i f f r a c t i o n and EXAFS (25) that most p a r t i c l e s i n the 15-20A s i z e range consist e n t i r e l y of oxide. We have found that such p a r t i c l e s usually give s i n g l e c r y s t a l patterns a t t r i b u t a b l e to the metals. There i s , however, considerable evidence t h a t , i n the case of Pt on alumina, the Pt c r y s t a l s have a well-defined e p i t a x i a l r e l a t i o n s h i p with the c r y s t a l l i t e s (20-50A diameter) of the nominally "amorphous" alumina substrate. Similar observations of e p i t a x i a l r e l a t i o n s h i p s have been observed for small c r y s t a l s of Ru and Au on MgO (26). Figure 3(a)

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for example, shows the pattern of the MgO c r y s t a l substrate i n [111] o r i e n t a t i o n , obtained with an incident beam of diameter approximately 10A. The a d d i t i o n a l hexagonal array of spots of figure 3(b) comes from a c r y s t a l of Ru about 158 i n diameter, aligned i n a p a r a l l e l o r i e n t a t i o n . Figure 3(c) shows the spots from an Au c r y s t a l , about 20A i n diameter, seen i n an approximate [110] d i r e c t i o n with one set of (111) planes almost p a r a l l e l to MgO (220) planes, with some i n d i c a t i o n that i t i s accompanied by a small Ru c r y s t a l aligned as for Figure 3(b). The STEM images obtained when the incident beam, used to obtain m i c r o d i f f r a c t i o n patterns such as i n Figure 3f i s scanned over the specimen w i l l have a r e s o l u t i o n no better than the beam diameter of about 10A, as i n Figure 4(a). This i s usually s u f f i c i e n t to allow the p a r t i c l e s i n question to be located and i d e n t i f i e d i n images subsequently obtained wit aperture, such as Figur i s currently l i m i t e d to about 38 but t h i s i s s u f f i c i e n t to provide considerable information on the p a r t i c l e morphology and to allow some c o r r e l a t i o n with more d e t a i l e d images now possible with the best TEM instruments. S t a t i s t i c a l information from s i n g l e c r y s t a l patterns The p o s s i b i l i t y of obtaining s i n g l e c r y s t a l d i f f r a c t i o n patterns from regions of very small diameter can obviously be an important addition to the means for i n v e s t i g a t i n g the structures of c a t a l y t i c materials. The d i f f i c u l t y a r i s e s that data on i n d i v i d u a l small p a r t i c l e s i s usually, at best, merely suggestive and at worst, completely meaningl e s s . What i s normally required i s s t a t i s t i c a l data on the r e l a t i v e frequencies of occurrence of the various s t r u c t u r a l features. For adequate s t a t i s t i c s , i t would be necessary to record and analyse very large numbers of d i f f r a c t i o n patterns. The powder patterns obtained by X-ray d i f f r a c t i o n and selected area electron d i f f r a c t i o n do represent averages over very large numbers of p a r t i c l e s but the averaging over s i z e , o r i e n t a t i o n and imperfection of c r y s t a l s removes much of the important information, e s p e c i a l l y that on the c o r r e l a t i o n s of properties,e.g. the o r i e n t a t i o n a l r e l a t i o n s h i p of adjacent c r y s t a l regions or the dependence of twinning on s i z e . In order to take advantage of the c a p a b i l i t i e s of the microd i f f r a c t i o n method i t thus seems necessary to find some a l t e r n a t i v e to the laborious compilation of vast numbers of analyses of i n d i v i d u a l r e s u l t s . One a l t e r n a t i v e which we have explored i s to use our automatic d i g i t a l data c o l l e c t i o n equipment (25) i n combination with a pattern recognition device (26). In our system the small electron probe of the STEM instrument i s scanned over a chosen area of a specimen and the m i c r o d i f f r a c t i o n patterns from each successive probe p o s i t i o n are viewed by a low l i g h t - l e v e l TV camera and d i s played on a video screen. A set o f detectors i s arranged such that when a d i f f r a c t i o n pattern which includes a p a r t i c u l a r array of spots appears on the screen, a s i g n a l i s generated to stop the scan and record the d i f f r a c t i o n pattern i n d i g i t a l form i n the computer

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Figure 3. M i c r o d i f f r a c t i o n patterns obtained with an electron beam of diameter about 1o8 from p a r t i c l e s of Ru and Au on a MgO support, (a) MgO c r y s t a l , (b) Ru c r y s t a l , 15& i n diameter, on MgO. (c) Au c r y s t a l , 20& i n diameter, on MgO.

Figure 4. STEM images of Au p a r t i c l e s on a MgO support, (a) Image taken with the small objective aperture used f o r m i c r o d i f f r a c t i o n ; (b) Image obtained with larger objective aperture showing better r e s o l u t i o n .

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memory. I t i s then possible to perform d i g i t a l c o r r e l a t i o n analysis of a l l such patterns recorded and derive answers to s p e c i f i c questions such as, f o r example, i f a small metal p a r t i c l e has a p a r t i c u l a r o r i e n t a t i o n , i s there evidence that neighboring parts of the metal p a r t i c l e or of the supporting material have a tendency to occur i n a s i m i l a r or related orientation? There are many variants of t h i s system which can be envisaged as means by which the current p o s s i b i l i t i e s for automation i n data c o l l e c t i o n can be applied for s p e c i f i c purposes. There are consider­ able dangers i n t h i s approach i n that i t may be a l l too easy to b u i l d i n r e s t r i c t i o n s which predetermine the r e s u l t s . These dangers, however, are not l i k e l y to be worse than those normally encountered i n electron microscopy or single c r y s t a l d i f f r a c t i o n where the one p a r t i c u l a r l y "good-looking" picture i s taken as being " t y p i c a l " of a sample. I t i s f e l t that the use of electron microbeam methods o f f e r s the basis for a revolutionary new approach to the study of c a t a l y s t p a r t i c l e s . Some r e s u l t s can be obtained immediately but to r e a l i s e the f u l l p o t e n t i a l of the method a considerable amount of further exploration of data c o l l e c t i o n and data analysis methods w i l l be needed. Acknowledgment The author wishes to thank Dr. J.B. Cohen for supplying samples of Pt and Pd on alumina and s i l i c a and Drs. J . Schwank and A.K. Dayte for samples of Ru and Au on magnesia and s i l i c a . This work was supported by the US Department of Energy under Contract DMR-76ER02995 and has make use of the resources of the ASU F a c i l t i t y for High Resolution Electron Microscopy, supported by NSF grant DMR 8306501. Literature Cited 1. Hashimoto, H.; Endoh, H.; Tanji, T.; Ono, Α.; Watanabe, E.; J. Phys. Soc. Japan 1977, 42, 1073. 2. Izui, K.; Furuno, S.; Ono, Α.; J. Electron Microscopy 1977, 26, 129. 3. Cowley, J.M.; Diffraction Physics, 2nd Edit. North Holland Publ. Co., 1981. 4. Marks, L.D.; Surface Sci. 1984, 139, 281-98. 5. Marks, L.D.; D.J. Smith; Ultramicroscopy (1984) In Press. 6. Crewe, A.V. in "Electron Microscopy in Material Science;" U. Valdre, Ed.; Academic Press, New York, 1971, p. 62. 7. Langmore, J.P.; Wall, J.; Isaacson M.; Optik 1973, 38, 335. 8. Brown, L.M. in Developments in Electron Microscopy and Analysis 1978; D.L. Misell Ed.; Institute of Physics, Bristol, England 1977 p. 14. 9. Treacy, M.M.J.; Howie, Α.; Pennycook, S.J. in Electron Microscopy and Analysis, 1979, (T. Mulvey, Ed.) Institute of Physics, Bristol, England 1980, p. 261. 10. Butler, J.H.; Turner, P.S.; Cowley, J.M. 1984 In preparation. 11. Cowley, J.M.; J. Electron Microscopy Techniques 1984, 1, 83.

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340 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

CATALYST CHARACTERIZATION SCIENCE Osakabe, N.; Tanishiro, Y . ; Yagi, K.; Honjo, G . ; Surface Sci. 1981, 109, 353. Hsu, Tung; Cowley, J.M.; Ultramicroscopy 1983, 11, 239. Hsu, Tung; Ultramicroscopy 1983, 11, 167. Cowley, J.M. in Microbeam Analysis 1980, D.B., Wittry, Ed., San Francisco Press, San Francisco 1980, p. 33. Cowley, J.M.; Neumann, K.D.; Surface Sci. 1984, In press. Cowley, J . M . , Surface Sci. 1982, 114, 587. Marks, L.D. in Electron Microscopy and Analysis, 1981, M.J. Goringe, Ed.; Institute of Physics, Bristol, England 1981, p. 259. Batson, P . E . ; Ultramicroscopy 1983, 11, 299. Tan, C.S.; Cowley, J.M.; Ultramicroscopy 1983-4, 12, 333-44. Cowley, J.M.; Osman, Μ.Α.; and Humble, P.; Ultramicroscopy 1984, In press. Monosmith, W.B.; Cowley Allpress, J . A . ; Sanders Roy, R.A.; Messier, R.; Cowley, J.M.; Thin Solid Films 1979, 79, 207. Cohen, J . B . , These proceedings. Datye, A.K.; Schwank, J . in Proceedings of 8th International congress on catalysis, Berlin, 1984. In press. Strahm, M.; Butler, J . H . ; Rev. Sci. Instr. 1981, 52 840. Monosmith, W.B.; Cowley, J.M.; Ultramicroscopy 1983, 12, 51.

RECEIVED March 11, 1985

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

29 Atomic Imaging of Particle Surfaces 1 , 3

2 , 4

L. D. Marks and David J. Smith 1

Department of Physics, Arizona State University, Tempe, AZ 85287 High Resolution Electron Microscope, Department of Metallurgy, University of Cambridge, Cambridge CB2 3RQ, England

2

High resolution electro demonstrated the atomic structure of surfaces on small particles and thin films. In this paper we briefly review experimental observations for gold (110) and (111) surfaces, and analyse how these results when combined with theoretical and experimental morphological studies, influence the interpretation of geometrical catalytic effects and the transfer of bulk surface experimental data to heterogeneous catalysts. During the l a s t twenty years, small metal p a r t i c l e systems, often model c a t a l y s t s or commercial heterogeneous c a t a l y s t s , have been extensively studied by electron microscopy. The primary objective has been t o characterise t h e i r chemical and s t r u c t u r a l nature, with the intention of eventually understanding the nucleation and growth of small c l u s t e r s as well as heterogeneous c a t a l y s i s . In the process, e s s e n t i a l l y the whole range o f electron microscope imaging techniques have been used. Conventional bright f i e l d and dark f i e l d techniques are i l l u s t r a t e d by the c l a s s i c work o f Ino ( 1_), and Ino and Ogawa (2). More sophisticated dark f i e l d techniques have also been developed which give improved p a r t i c l e contrast, such as selected zone dark f i e l d ( 3 ) , annular dark f i e l d 0 0 , and weak beam dark f i e l d C5). Other approaches, p r i m a r i l y using a scanning trans­ mission instrument, have also been developed which produce a n a l y t i c a l information (e.g. 6-8). Some recent reviews o f these techniques can be found elsewhere (9-12) and reference should be made t o the a r t i c l e by J.M. Cowley i n these proceedings. A l l o f these methods suffer from one serious shortcoming - the s p a t i a l resolution i s r e l a t i v e l y poor (~1θ8) and information about the c a t a l y t i c a l l y i n t e r e s t i n g region of the p a r t i c l e , namely the surface structure, i s then not a v a i l a b l e . One p a r t i c u l a r technique, high resolution electron microscopy, has f o r many years been slowly 3

Current address: Department of Materials Science and Ipatieff Laboratory, Northwestern University, Evanston, IL 60201 Current address: Center for Solid State Science, Arizona State University, Tempe, AZ 85287

4

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progressing towards t h i s goal. Although useful r e s u l t s were obtained using the technique to unravel complicated p a r t i c l e structures (13-17), surface information remained unavailable except for a few unusually favorable circumstances (e.g. 18). As a r e s u l t of t e c h n i c a l improvements (1£) to the Cambridge High Resolution Electron Microscope (20), we have recently succeeded i n d i r e c t l y resolving the atomic structure of surfaces on small p a r t i c l e s and t h i n f i l m s (21-27)* This has included the f i r s t d i r e c t observation of a surface reconstruction, the so-called missing row model (28) of the 2x1 (110) gold surface (see Figure 1 and 21,22), e f f e c t s due to e l a s t i c and p l a s t i c deformations at surfaces (see Figures 2 and 3 and 23,26) and d e t a i l s of surface steps and facetting (see Figures 4,6 and 21,27). In t h i s paper we b r i e f l y describe the p r i n c i p l e of the technique, review these observations, and consider t h e i r implications with respect to geometrical e f f e c t s , l i n k i n g the experimental data with t h e o r e t i c a Basis of the Technique The technique employs a beam of s w i f t (~500kV) electrons grazing the surface of i n t e r e s t . Provided that the beam i s accurately aligned along a c r y s t a l zone a x i s , and that the e l e c t r o n - o p t i c a l imaging system i s adequate, then images are obtained which appear to show the atomic surface structure i n p r o f i l e (see Figure 1). Interpretation of these images i s both complicated and simple. With any electron microscope technique, the f i n a l image i s the r e s u l t of a complicated d i f f r a c t i o n and lens aberration process and i t i s necessary to avoid the trap of naive i n t e r p r e t a t i o n , that seeing i s b e l i e v i n g . I t i s generally accepted that detailed c a l c u l a t i o n s are required to confirm image i n t e r p r e t a t i o n s , p a r t i c u l a r l y for high resolution imaging, but also f o r other techniques. Fortunately, i n most cases, high r e s o l u t i o n images are f a i t h f u l representations of the surface structure. The reasons for t h i s are discussed i n d e t a i l i n 25,29,30, and can be summarised thus. With s w i f t electrons and a heavy element, the electron waves channel (e.g. 30-32) down the atomic columns (for a c r y s t a l zone-axis orientation) with minimal cross-talk between adjacent columns. With optimal imaging conditions, p r i m a r i l y depending on the objective lens defocus, the spherical aberration and the damping e f f e c t s of the microscope i n s t a b i l i t i e s balance each other out (25,29). The f i n a l image i s then an accurate l o c a l representation of the object, and i t i s correct to believe what i s seen. Monolayer s e n s i t i v i t y and, with some care, l i m i t e d s e n s i t i v i t y to chemical impurities can be achieved (25). When these conditions are not met an averaged (over the object) image i s obtained rather than a l o c a l one and measurement of, for instance, surface relaxations i s w e l l nigh impossible. Results Gold (110). The gold (110) surface has been observed to undergo a 2x1 reconstruction, with every other surface column missing, as shown i n Figure 1. This p a r t i c u l a r image i s from a p a r t i c l e of approxi­ mately 30o8 i n radius, and elements of the reconstruction were also observed on smaller p a r t i c l e s (~10θ8 i n r a d i u s ) . One i n t e r e s t i n g feature of the reconstruction i s a 20 + 5% expansion at the top of

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the corrugated structure which becomes apparent i n c a r e f u l , d i g i t a l comparisons of experimental and calculated images (22). This expansion has recently been confirmed by X-ray grazing incidence d i f f r a c t i o n and ion scattering experiments (33,34). Gold (111). The main feature of the gold (111) surface i s that the surface mesh expands r e l a t i v e to the bulk - there i s a tangential surface pressure (23,26,35). This behaviour manifested i t s e l f during an electron beam induced etching of contaminant carbon layers (by water vapour) as a " h i l l and v a l l e y " reconstruction, as shown i n Figure 2. This i s a roughening mechanism which provides space for an expansion to occur which i s eventually accommodated by surface d i s ­ l o c a t i o n s , e s s e n t i a l l y m i s f i t d i s l o c a t i o n s to accommodate the surface pressure (see Figure 3). I t i s important to recognise here that the nature of the specimen used for electron microscopy d i f f e r s from the bulk surfaces studied by might expect some manifestatio h i l l and v a l l e y roughening. It i s i n t e r e s t i n g to consider t h i s surface as i f i t were an e p i t a x i a l system, that i s a monolayer of gold epitaxed on a gold (111) surface. With any e p i t a x i a l system, provided that the m i s f i t between the adsorbate and the substrate i s small, the adatoms are e l a s t i c a l l y strained to the substrate surface mesh y i e l d i n g pseudomorphic growth ( f o r a review see, for instance, 36). For an i n f i n i t e surface the s t r a i n i s purely homogeneous, but with a f i n i t e adsorbed layer there i s some buckling due to the i m p l i c i t boundary condition of no t r a c t i o n s at the edges of the layer (37,38). This buckling also appears i n some of the e a r l i e r analyses of surfaces using simple Morse p o t e n t i a l s (e.g. 39) since these have an i n b u i l t expansive surface pressure. I f the m i s f i t between the adsorbate and the substrate i s s u f f i c i e n t l y l a r g e , i t becomes e n e r g e t i c a l l y favorable to nucleate m i s f i t d i s l o c a t i o n s to r e l i e v e the s t r a i n s . Numerical c a l c u l a t i o n s by Snyman and Snyman (40) for the case of a (111) layer on a (111) substrate show that Shockley p a r t i a l dislocations and stacking f a u l t s are a low energy mechanism for t h i s s t r a i n r e l i e f , c o r r e l a t i n g with the case of s i l v e r epitaxed on gold (111) (40). These c a l c u l a t i o n s are i n excellent agreement with our results. Benzene on Gold (111). One chemically i n t e r e s t i n g event seen on the gold (111) surface was the formation of a benzene monolayer during the etching of the i n i t i a l carbon contaminant layer by water vapour (25,26). This i s a f l a t π-bonded l a y e r , with a benzene to benzene spacing of 7.3 0+0.2)8, and i s shown i n Figure 4. I t i s i n t e r e s t i n g to connect t h i s observation with the mechanism of the etching process, which i s probably s i m i l a r to that of the water-gas r e a c t i o n . We would hypothesise that the carbon acts as a temporary sink for the hydrogen during the etching with the i n i t i a l reaction products being hydrocarbons and carbon monoxide. With various cracking reactions taking place under the electron beam, benzene can be one of many reaction products. Since benzene i s probably r e l a t i v e l y r a d i a t i o n r e s i s t a n t , p a r t i c u l a r l y when π-bonded to a metal which can act as an energy sink, i t could be a favored metastable product. There could also be some c a t a l y t i c effect from the gold substrate.

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Figure 1. Area of roug numerical c a l c u l a t i o black.

Figure 2. H i l l and v a l l e y roughening of a gold (111) surface i n a) before roughening (carbon covered) and b) following carbon removal.

Figure 3. Area of clean gold (111) surface showing a surface Shockley p a r t i a l d i s l o c a t i o n (arrowed) - see 26. Atomic columns are black.

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Geometric E f f e c t s There are a number of concepts concerning the structure of small p a r t i c l e s which have a bearing upon geometrical c a t a l y t i c e f f e c t s (e.g. 4 1 - 4 3 ) . These follow both from the surface imaging r e s u l t s , and a d e t a i l e d experimental (13-15) and t h e o r e t i c a l ( 4 4 - 4 7 ) study of p a r t i c l e morphologies. F i n i t e s i z e e f f e c t s . I t i s w e l l known t h a t , as the s i z e of an atomic c l u s t e r drops, the r e l a t i v e proportion of edge and surface atoms increases. However, c a l c u l a t i o n s of the r e l a t i v e f r a c t i o n of these d i f f e r e n t surface s i t e s have to date made one important assumption, namely that the external morphology of the p a r t i c l e remains constant. This assumption i s not i n f a c t v a l i d ; edge atoms for example have a higher i n t r i n s i c energy than surface atoms, so i t i s possible that a morphological change coul edge atoms. Detailed c a l c u l a t i o n s ( 4 7 ) show that e f f e c t s from the edge atoms are present and that there i s also a stronger e f f e c t which can be linked to sphere packing. The number of atoms on a p a r t i c u l a r face, or i n the c l u s t e r , deviates s u b s t a n t i a l l y from the continuum value (parameter!sed i n terms of the surface area and c l u s t e r volume respectively) when the p a r t i c l e s are small. This introduces further large edge-like terms i n the t o t a l p a r t i c l e energy which w i l l drive s u b s t a n t i a l morphological changes. For instance, for a simple broken bond model of an fee p a r t i c l e r e s t r i c t e d to having only (111) and (100) faces, the f r a c t i o n of (100) surface drops markedly as the c l u s t e r s i z e decreases as shown i n Figure 5. This e f f e c t only occurs when the c l u s t e r energy i s minimised with respect to i t s morphology. I t i s also possible to have d i s c r e t e microfacetting i n small p a r t i c l e s . For a large ( e s s e n t i a l l y continuum) c r y s t a l , v i c i n a l surfaces are important, t h e i r r o l e being w e l l understood through the Wulff construction (e.g. 48-50). However, the u n i t c e l l of a v i c i n a l surface i s l a r g e , and there may be i n s u f f i c i e n t space on a small p a r t i c l e . Only small u n i t mesh surfaces can be accommodated, and t h i s can lead to microfacetting, a possible example being shown i n Figure 6. We note that there i s a l i k e l y c a t a l y t i c p a r t i c l e s i z e e f f e c t here i n the disappearance of v i c i n a l surfaces. Boundary Conditions. The i m p l i c i t boundary condition of small surface area can a f f e c t surface reconstructions and chemisorption. Surface steps, for example, are important f o r reconstructions (e.g. 51)t and can determine the p a r t i c u l a r domain that occurs (e.g. 52). An example of a s t r u c t u r a l e f f e c t i s on the gold (111) surface where there i s an in-plane tangential surface pressure ( 2 3 , 2 6 , 3 5 ) . On extended surfaces, a h i l l - a n d - v a l l e y roughening occurs to accommodate the expansion, as described e a r l i e r . In contrast, small p a r t i c l e s accommodate the pressure by a surface buckle (26). We would expect s i m i l a r behaviour when there i s chemisorption i n v o l v i n g i n t e r a c t i o n s between the adsorbed molecules. Gas-Particle e f f e c t s . The gas environment and chemical i m p u r i t i e s , such as promotors or poisons, can strongly influence the t o t a l p a r t i c l e morphology ( 4 9 1 , 5 0 , 5 3 , 5 4 ) . E f f e c t s can occur v i a morphological changes which may eliminate or promote c e r t a i n

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Figure 4. Area of benzene covered gold (111) .surface, for two d i f f e r e n t objective lens defoci as required for unique image interpretation (see 24, ?5). Tn a) the gold atomic columns are black, i n b) white. Moire f r i n g e s , rather than any true s t r u c t u r a l image, r e s u l t from the benzene monolayer. Simulations ( r i g h t ) have benzene overlay on top surface only.

Surface

Figure 5. Relative f r a c t i o n of the d i f f e r e n t surface atoms as a function of Log M, where Ν i s the number of atoms, for a (111) and (100) facetted f.c.c. c r y s t a l when the surface morphology i s e q u i l i b r i a t e d . F r e f e r s to the edges between the (100) and (111) faces, Ε to the edges between two (111) faces. The (111) curve i s drawn using the axes to the r i g h t . 1

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( c a t a l y t i c a l l y important) facets. (For a review of e f f e c t s on large surfaces see _12,49 and 50). This i s equivalent t o the blocking of an enzyme v i a a conformational change, rather than a d i r e c t attack upon the active s i t e . One example of t h i s type of process a r i s e s i n gold p a r t i c l e s . When grown i n UHV, the stable structures are i n the form of non-crystallographic p a r t i c l e s c a l l e d multiply twinned p a r t i c l e s or MTPs (l,2,J^,_l^,44-46,55 and see Figure 7)· I n - s i t u experiments by Yagi et a l (56), who observed the formation of MTPs both during growth and following coalescence, demonstrated that these p a r t i c l e s are thermodynamically preferred when small (see also 44). However, specimen c a t a l y s t s produced by Avery and Sanders (56) did not show s i g n i f i c a n t concentrations of MTPs, which would appear to contradict the s t a b i l i t y r e s u l t s of Yagi et a l (56). The difference can probably be a t t r i b u t e d to the e f f e c t s of trace water vapour, which experimentally i n h i b i t s gold MTP formation (26 and Figure 8 ) , and whic preparation of Avery an through the change i n surface free energy and tangential surface pressure with contaminants, since both a f f e c t the energy balance between MTPs and single c r y s t a l s ( 4 £ ) . The surface pressure term i s probably dominant, with the surface expansion upon the clean gold

Figure 6. Microfacetted region of a gold c r y s t a l , with the facet indexing marked. The atomic columns are black.

Figure 7. A decahedral HTΡ o f gold showing white atomic columns, supported on an amorphous carbon f i l m - see 25. An explanation o f t h ^ g ^ a ^ ^ t i j ^ T P s i s given i n

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Figure 8. Image and d i f f r a c t i o specimen of gold prepare deposition onto KC1 and then transfer onto amorphous carbon. Here water vapour was the dominant residual gas (determined by mass spectrometry). The p a r t i c l e s are square pyramidal single c r y s t a l s .

(111) surface (23,26) presumably being suppressed by water adsorption, thus favoring single c r y s t a l formation. There are good t h e o r e t i c a l reasons (35) for believing that s i m i l a r e f f e c t s can occur i n other systems. Conclusions We have discussed here, very b r i e f l y , some recent observations of small p a r t i c l e surfaces and how these r e l a t e to geometrical c a t a l y t i c e f f e c t s . These demonstrate the general conclusion that high r e s o l u t i o n imaging can provide a d i r e c t , s t r u c t u r a l l i n k between bulk LEED analysis and small p a r t i c l e surfaces. Apart from applications to conventional surface science, where the s e n s i t i v i t y of the technique to surface inhomogenieties has already yielded r e s u l t s , there should be many useful a p p l i c a t i o n s i n c a t a l y s i s . A useful approach would be to combine the experimental data with surface thermodynamic and morphological analyses as we have attempted herein. There seems no fundamental reason why r e s u l t s comparable to those described cannot be obtained from commercial c a t a l y s t systems. Acknowledgment s The authors would l i k e to thank Drs. A. Howie, V. Heine, E. Yoffe, R.F. W i l l i s , J.M. Cowley and J.A. Venables for advice and comments during the course of t h i s work. We acknowledge f i n a n c i a l support from the SERC, U.K. and L.D. Marks also acknowledges support on Department of Energy (USA) Grant No. DE-AC02-76ER02995.

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Ino, S.; J . Phys. Soc. Japan 1966, 21, 346. Ino, S.; Ogawa, T . ; J . Phys. Soc. Japan 1967, 22, 1369. Heinemann, K.; Poppa, H . ; Appl. Phys. Letts. 1972, 20, 122. Freeman, L . A . ; Howie, Α.; Treacy, M.M.J.; J . Microsc. 1977, 111, 165. Yacaman, M . J . ; Ocana, T . ; Phys. Stat. Sol. (a) 1977, 42, 571. Treacy, M.M.J.; Howie, Α.; Wilson, C . J . ; Phil. Mag. A 1978, 38, 569. Pennycook, S . J . ; J . Microsc. 1981, 124, 15. Pennycook, S . J . ; Howie, Α.; Shannon, M.D.; Whyman, R.; J . Molecular Catalysis 1983, 20, 345. Howie, Α.; Marks, L . D . ; Pennycook, S . J . ; Ultramicroscopy 1982, 8, 163. Lyman, C.E. In "Catalyti Structure and Reactivity" D.C., 1983, p. 311. Treacy, M.M.J. i b i d . , p. 367. Baird, T. In "Catalysis"; The Royal Society of Chemistry: London, 1982; Vol 5, p. 172. Marks, L . D . ; Smith, D . J . ; J . Crystal Growth 1981, 54, 425. Smith, D . J . ; Marks, L . D . ; J . Crystal Growth 1981, 54, 433. Marks, L . D . ; Smith, D . J . ; J . Microscopy 1983, 130, 249. White, D.; Baird, T . ; Fryer, J . R . ; Freeman, L . A . ; Smith D . J . ; Day, M.; J . Catal. 1983, 81, 119. Smith, D . J . ; White, D.; Baird, T . ; Fryer, J . R . ; J . Catal. 1983, 81, 107. Sanders, J . V . ; Chemica Scripta 1978/79, 14, 141. Smith, D . J . ; Camps, R.A.; Freeman, L . A . ; H i l l , R.; Nixon, W.C.; Smith, K.C.A.; J . Microscopy 1983, 130, 127. Smith, D . J . ; Camps, R.A.; Cosslett, V . E . ; Freeman, L . A . ; Saxton, W.O.; Nixon, W.C.; Ahmed, H . ; Catto. C.J.D.; Cleaver, J.R.A.; Smith, K.C.A.; Timbs, A . E . ; Ultramicroscopy 1982, 9, 203. Marks, L . D . ; Smith, D . J . ; Nature 1983, 303, 316. Marks, L . D . ; Phys. Rev. Letts. 1983, 51, 1000. Marks, L . D . ; Heine, V . ; Smith, D . J . ; Phys. Rev. Letts. 1983, 52, 656. Smith, D . J . ; Marks, L . D . ; Proc. 7th Int. Conf. High Voltage Electron Microscopy, 1983, p. 53. Marks, L . D . ; Surf. S c i . 1984, 139, 281. Marks, L . D . ; Smith, D . J . ; Surf. Sci. 1984, 143, 587. Smith, D . J . ; Marks, L . D . ; submitted to Ultramicroscopy. Noonan, J.R.; Davis, H . L . ; J . Vac. Sci. Tech. 1979, 16, 587. Marks, L . D . ; Ultramicroscopy 1983/84, 12, 237. Marks, L . D . ; Submitted to Acta Cryst. Berry, M.V.; J . Phys. C 1971, 4, 697. Cowley, J . M . ; "Diffraction Physics", 2nd edition, North-Holland: Amsterdam, 1981. Robinson, I . K . ; Kuk, Y . ; Feldman, L . C . ; Phys Rev. Β 1984, 29, 4762. R.J. Culbertson, private communication. Heine, V . ; Marks, L . D . ; in preparation

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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CATALYST CHARACTERIZATION SCIENCE van der Merwe, J . H . ; Ball, C.A.B. In "Epitaxial Growth"; Matthews, J.W., Ed.; Academic Press: New York, 1975; Part B, p. 493 (see also other articles in both Parts A and B). Neidermeyer, R.; Thin Films 1968, 1, 25. Snyman, J . A . ; van der Merwe, J . H . ; Surf. Sci. 1974, 42, 190. Wynblatt, P.; in Interatomic Potentials and Simulation of Lattice Defects, (Ed Gehlen, Beeler and Jaffee, Plenum Press, New York, 1972) pp. 633. Snyman, J . A . ; Snyman, H.C.; Surf. Sci. 1981, 105, 357. van Hardeveld, R.; Montfoort, V . ; Surf. Sci. 1966, 4, 396. Burton, J . J . ; Catal. Rev. Sci. Eng. 1974, 9, 209. Andersson, J.R. "Structure of Metallic Catalysts": Academic Press: London, 1975. Marks, L . D . ; J . Crystal Growth 1983, 61, 556. Marks, L . D . ; Phil Mag A 1984, 49, 81. Howie, Α.; Marks, L . D . Marks, L . D . ; Submitte Linford, R.G.; In "Solid State Surface Science"; Green, M., Ed; Marcel Dekker: New York, 1973; Vol. 2, p.1. Flytzani-Stephanopoulos, M.; Schmidt L . D . ; Prog. Surf. Sci. 1979, 9, 83. Drechler, M.; In "Surface Mobilities on Solid Materials", Binh, V . T . , Ed; Plenum Press: New York and London, 1983; p. 405. Chabai, Y . J . ; Rowe, J . E . ; Christman, S.B.; Phys. Rev. B. 1981, 24, 3303. Krueger, S.; Monch, W.; Surf. Sci 1980, 99, 157. Lacmann, R.; Springer Tracts in Modern Physics 1968, 44, 1. Metois, J . J . ; Spiller, G.D.T.; Venables, J . A . ; Phil Mag A 1982, 46, 1015. Buttet, J . ; Borel, J . P . ; Helv. Phys. Acta, 1983, 56, 541. Yagi, K.; Takayanagi, K.; Kobayashi, K.; Honjo, G . ; J . Crystal Growth, 1975, 28, 117. Avery, N.R.; Sanders, J . V . ; J Catal. 1970, 18, 129.

RECEIVED March 20, 1985

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

30 Microanalysis of a Copper-Zinc Oxide Methanol Synthesis Catalyst Precursor 1

1

1

2

P. B. Himelfarb , G. W. Simmons , Κ. Klier , and M. José-Yacamán 1

Center for Surface and Coatings Research and Department of Chemistry, Lehigh University, Bethlehem, PA 18015 Instituto de Fisica Universidad Nacional Autónoma de México, Apdo. Postal 20-264, México 20 DF, México

2

The naturally occurring mineral aurichalcite has been used as a model for the methanol synthesis catalyst precursor that is formed by coprecipitation from aqueous copper and zinc nitrate solution by the addition of alkali carbonate. The chemical and morphological transformations that occur in both materials upon calcination and subsequent reduction have been monitored by transmission electron microscopy, selected area electron diffraction, and X-ray powder diffraction. The treatments did not change the platelet morphology of these samples, but produced platelets that were porous and polycrystalline, in contrast to the original single crystal materials. Calcination of the mineral and synthetic samples yielded ZnO crystallites in crystallographic registry, oriented with major zone axes of [1010] and [3031]. The preferred orientations of ZnO were in epitaxial registry to the original aurichalcite orientation having a [101] zone axis, such that, the aurichalcite [040] and [202] axes were aligned with the ZnO [1210] and [0002] axes, respectively. In the reduced materials, the Cu(211) planes were parallel to the ZnO(1010) planes such that the Cu[111] axis was aligned with the ZnO[0002] axis. The coprecipitated precursor of the most active binary Cu/ZnO methanol synthesis c a t a l y s t (1) has recently been shown to be a s i n g l e phase hydroxy carbonate, ( C u q . Z n -7)5(003)2(0H) , a u r i c h a l c i t e (2) . In the present i n v e s t i g a t i o n , the natural a u r i c h a l c i t e mineral of composition (Cu ^Ζη ) (0Ο ) (ΟΗ) , which consisted of large, t h i n p l a t e l e t s having dimensions on the order of micrometers, was used as a model compound f o r following the chemical, s t r u c t u r a l , and morpho­ l o g i c changes during c a t a l y s t preparation. The phase d i s t r i b u t i o n s and morphology of the synthetic and mineral samples were compared 3

Q

0β6

5

3

2

0

6

6

0097-6156/ 85/0288-0351 $06.00/0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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throughout the preparation procedures. Although the p r e c i p i t a t e d precursors and the n a t u r a l mineral gave r i s e to e s s e n t i a l l y i n d e n t i c a l c a t a l y s t s , the l a r g e r p l a t e l e t dimensions of the natural mineral provided an i d e a l morphology f o r studying the genesis of the f i n a l Cu/ZnO c a t a l y s t by transmission electron microscopy (TEM). Techni­ ques of x-ray d i f f r a c t i o n (XRD), selected area electron d i f f r a c t i o n (SAD), and dark f i e l d and b r i g h t f i e l d imaging i n the TEM were used to characterize the mineral during and a f t e r c a l c i n a t i o n and a f t e r reduction. Experimental Copper ore containing a deposit of a u r i c h a l c i t e was obtained from Wards Natural Science Establishment. The mineral a u r i c h a l c i t e crys­ t a l l i t e s were gently scraped from the ore and rinsed i n ethanol p r i o r to use. The synthetic precurso a mixture of 1M Cu and 1 mole r a t i o of 30/70 was prepared, y dropwis at 90°C u n t i l the pH increased from approximately 3 to 7. C a l c i n a ­ t i o n and reduction of the mineral were performed as i n standard cata­ l y s t preparation procedures, which have been described i n d e t a i l e a r l i e r (1). A P h i l i p s EM 400T transmission electron microscope which included a scanning transmission mode was used i n the electron microscopic characterization studies. Samples were prepared by dispersing the a u r i c h a l c i t e mineral i n ethanol and p l a c i n g a drop of the dispersion on a carbon-coated titanium or copper g r i d . For reduced specimens exposure to a i r was minimized by preparing and transporting samples i n a nitrogen f i l l e d glove bag. Energy dispersive X-ray analysis (EDS) f o r the i d e n t i f i c a t i o n of elements and q u a n t i t a t i v e analysis was c a r r i e d out i n the manner described i n reference (3). Powder d i f f r a c t i o n patterns were obtained with a P h i l i p s XRG 3100 X-ray generator coupled with an APO 3600 c o n t r o l u n i t using CuK^ r a d i a t i o n . Scans were conducted with a step s i z e of 0.01° i n 2Θ and a counting time of 1.2 sec. Results Representative TEM micrographs and SAD patterns of the mineral and synthetic a u r i c h a l c i t e are given i n Figures 1 and 2, r e s p e c t i v e l y . The SAD patterns were indexed to a [101] zone a x i s , as described i n reference (2). The u n i t c e l l parameters of the mineral and synthetic a u r i c h a l c i t e are given i n Table I together with the Cu/Zn r a t i o s . The XRD data and Zn/Cu r a t i o are also given f o r a reference a u r i c h a l ­ c i t e specimen reported i n the l i t e r a t u r e (4). A l l d-spacings i n the mineral and synthetic a u r i c h a l c i t e matched the l i t e r a t u r e values w i t h i n the l a t t i c e volume changes ( , a process which is complete at - 8 0 percent metal exposed. There is l i t t l e contact area between the metal and support ( 1 0 ) . Reduction by hydrogen c o m p l e t e l y alters the c h e m i c a l r e a c t i v i t y and its variation w i t h size reduced. The mean-squar A study of the area of the Lftj absorption edge resonance shows a c o r r e l a t i o n w i t h c h e m i c a l a c t i v i t y . This implies a c o r r e l a t i o n w i t h the number of d-band vacancies. This occurred when Pt oxide was reduced by hydrogen or when the p a r t i c l e size was decreased ( 1 3 ) . 4

Pd Catalysts (8,14) T h e c a t a l y t i c a c t i v i t y of P d / S i 0 of low percentage metal exposed (for methylcyclopropane (MCP) hydrogenolysis at 0 ° C ) i s less for catalyst cooled from 723°K i n H , than for the same m a t e r i a l cooled in H e . We have shown that this is due to hydride formation (when cooling in H ). T h e ease of hydride formation decreases w i t h decreasing p a r t i c l e size up to ~ 30 percent metal exposed. Exposure to hydrogen results i n nearly complete conversion to hydride, but purging w i t h H e (even at 0 ° C ) r e converts the hydride to metal, although there is some induction period. Passing hydrogen plus M C P over the catalyst results i n the conversion into hydride of a substantial portion of the catalyst originally present as palladium metal. The l a t t i c e parameter is the same as for bulk Pd, at least for particles of 45 A or larger. No hydride forms for very small particles. Y e t when catalysts w i t h such a small metal p a r t i c l e size are stored i n air, they are converted to (crystalline) PdO. R e d u c t i o n of this oxide w i t h hydrogen produces Pd metal, not hydride. 2

2

2

A l l o y Catalysts (9) C o - R h catalysts were prepared on s i l i c a gel by impregnation w i t h a pentane solution of Rhj C o . (COX . A f t e r reduction at 823°K for 20 hours, there were diffraction peaks due to the C o - R h alloy, but also peaks from the metals themselves. 2

Acknowledgments Our research in this area was supported by D O E (Grant 7 7 E R 0 4 2 5 4 ) . The X - r a y measurements were performed CHESS synchrotron f a c i l i t y , Cornell University, or i n the F a c i l i t y of Northwestern U n i v e r s i t y s M a t e r i a l s R e s e a r c h in part by N S F under Grant N o . D M R - M R L - 7 6 - 8 0 8 9 7 . 1

No. D E - A C 0 2 either at the X - r a y Diffraction Center, supported

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Pielaszek, J.; Cohen, J.B.; Burwell, Jr. R . L . ; Butt, J.B. Jrnl. of Catalysis. 1983, 89, 479-481. Schlosberg, W.H.; Cohen, J.B. Jrnl. Appl. Crystallogr. 1983, 16, 304308. Nandi, R.K.; Kuo, H.K.; Schlosberg, W.; Wissler, G.; Cohen, J.B.; Crist, Jr., B. Jrnl. Appl. Crystallogr. 1983, 17, 22-76. Goodisman, J.; Βrumberger, H; Cupelo, R. Jrnl. Appl. Crystallogr. 1981, 14, 305-308. Georgopoulos, P.; Cohen, J.B. Submitted for publication. Amelse, J.A.; Arcuri, K.B.; Butt, J.B.; Matyi, R.J.; Schwartz, L . H . ; Shaprio, A . Jrnl. Phys. Chem.1981, 85, 708-711 Nandi, R.K.; Molinaro, F . ; Tang, C.; Cohen, J.B.; Butt, J.B.; Burwell, Jr., R . L . Jrnl. Catal. 1982, 78, 289-305. Nandi, R.K.; Pitchai Butt, J.B. Jrnl. Catal Pielaszek, J.; Cohen, J.B. Adv. in X-ray Analysis, In Press Uchijiima, T.; Herrmann, J.M.; Inouye, J.Y.; Burwell, Jr., R . L . ; Butt, J.B.; Cohen, J.B.; J. Catal. 1977, 50, 464-478. Sashital, S.R.; Cohen, J.B.; Burwell, Jr., R . L . ; Butt, J.B. Jrnl. Catal. 1977, 50, 479-493. Otero-Schipper, P.H.; Wachter, W.A.; Butt, J.B.; Burwell, Jr., R . L . Jrnl. Catal.1977, 50, 494-507. Rorris, E . Ph.D. Dissertation, Northwestern University, Evanston, IL June 1983. Nandi, R.K.; Georgopoulos, P.; Cohen, J.B.; Butt, J.B.; Burwell, Jr., R.L. Jrnl. Catal. 1982, 77, 421-431.

R E C E I V E D March 21, 1985

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

34 Vibrational Analysis of Adsorbed Molecules H. Ibach and J. E. Müller Institut für Grenzflächenforschung und Vakuumphysik and Institut für Festkörperphysik, Kernforschungsanlage Jülich, D-5170 Jülich, Federal Republic of Germany

The principles o v i b r a t i o n spectru fingerprint r i c h enough i n d e t a i l s to reveal the essentials of the structure and bonding of the species containing up to about 20 atoms. The analysis usually involves a comparison to spectra of free molecules or model compounds. Mode assignment i s performed through the use of isotopes. Selection rules can be used to determine the point group of a surface species. Surface decomposition reactions are easily studied using the technique of v i b r a t i o n analysis. Recently, r e l i a b l e calculations of the electronic structure of adsorbed species have become available. Calculated equilibrium geometries and frequencies provide an important connection between theory and experiment. Results of such calculations for H2O andNH3are presented and compared to experimental data. Our understanding of the chemistry of molecules adsorbed on metals has greatly improved i n recent years. Much of our present knowledge i s owed to the advent of r e l i a b l e tools to study the v i b r a t i o n spectrum of adsorbed species (1,2). Though v i b r a t i o n spectroscopy i s not an exact method and the methodology does not follow a p a r t i c u l a r routine pattern, i t yields important qualitative information on the structure, the adsorption sites and reaction paths of surface species, as we s h a l l see. The purpose of this paper i s to summarize the important principles i n the analysis of v i b r a t i o n spectra from surfaces and to demonstrate the type of information which i s obtained. Regarding the experimental technique we s h a l l refer to electron energy loss spectroscopy (ELS) only i n this paper. The reason i s that t h i s method provides for the largest amount of information on the entire v i b r a t i o n spectrum. It i s suited for adsorption studies on single c r y s t a l surfaces as well as on real surfaces, as long as they are stable i n vacuum. Other techniques, i n particular infrared r e f l e c t i o n absorption spectroscopy have a higher resolution i n a l i mited spectral range. They frequently serve as a source of a d d i t i onal information on the d e t a i l s of the structure. 0097-6156/85/0288-0392$06.00/0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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On s i n g l e c r y s t a l s u r f a c e s v i b r a t i o n a l modes a r e i n p r i n c i p l e w a v e l i k e e x c i t a t i o n s w i t h the wave v e c t o r p a r a l l e l t o the s u r f a c e . For most m o l e c u l e s d i s p e r s i o n e f f e c t s are s m a l l , however, and s h a l l be d i s r e g a r d e d i n t h i s paper which i s more o r i e n t e d towards the chem i c a l a s p e c t s . However, d i s p e r s i o n o f adsorbate modes has been measured r e c e n t l y ( 3 ) . The second p a r t o f the paper i s devoted t o some a s p e c t s o f the c u r r e n t s t a t u s o f the t h e o r y o f c h e m i s o r p t i o n . There, i n p a r t i c u l a r , the a d s o r p t i o n o f water and ammonia w i l l be s t u d i e d and d i s c u s s e d . Through the t h e o r y a q u a l i t a t i v e understanding o f the bonding o f these m o l e c u l e s t o s u r f a c e s i s a c h i e v e d which i s i n agreement w i t h the e x p e r i m e n t a l o b s e r v a t i o n s . P r i n c i p l e s o f the V i b r a t i o n a l A n a l y s i s The V i b r a t i o n Spectrum a t i o n a l a n a l y s i s comprise cases a w e l l e s t a b l i s h e d path o f l o g i c a l r o u t i n e i s f o l l o w e d . I n o t h e r cases the a n a l y s i s works on a n a l o g i e s to a l a r g e group o f known compounds and t h e i r v i b r a t i o n spectrum. We s h a l l d i s c u s s the l a t t e r aspect f i r s t . F i n g e r p r i n t i n g works w i t h the n o t i o n t h a t , b y and l a r g e , the v i b r a t i o n a l mode i s a s s o c i a t e d w i t h a p a r t i c u l a r bond w i t h i n a m o l e c u l e , be i t i n gas phase o r adsorbed on the s u r f a c e . T h i s i s , o f c o u r s e , a v e r y approximate concept o n l y , a s i n p r i n c i p l e the v i b r a t i o n a l mode i s a p r o p e r t y o f the e n t i r e system and not a p r o p e r t y o f a n i n d i v i d u a l bond. Y e t , the comparison o f v i b r a t i o n a l s p e c t r a o f a l a r g e number o f molecules as w e l l as dynamical c a l c u l a t i o n s shows t h a t , t o a l a r g e r o r l e s s e r degree, the concept o f a v i b r a t i o n a l mode c h a r a c t e r i z i n g a bond can be used. T y p i c a l examples are the hydrogen s t r e t c h i n g v i b r a t i o n s . Hydrogen has a s i g n i f i c a n t l y s m a l l e r mass than the atom w h i c h i t bonds t o ( c a r b o n , s i l i c o n , oxygen) · Consequently i t s v i b r a t i o n a l modes a r e r a t h e r l o c a l i z e d t o the hydrogen bond and t h e i r f r e q u e n c i e s a r e l i t t l e a f f e c t e d by the r e s t o f the m o l e c u l e . The f r e q u e n c i e s a r e s e n s i t i v e , however, t o the o r b i t a l s t r u c t u r e o f the atom which hydrogen bonds t o . Thus one c a n d i s t i n g u i s h between sp, sp and sp h y b r i d i z e d carbon atoms. T h i s p r o v i d e s a sound b a s i s f o r d i s c r i m i n a t i n g s a t u r a t e d and u n s a t u r a t e d h y d r o c a r b o n s even on s u r f a c e s . V i b r a t i o n a l a n a l y s i s o f a l a r g e number o f m o l e c u l e s i n s u r f a c e compounds has a l s o shown t h a t the c a r bon-carbon s t r e t c h i n g v i b r a t i o n i s c h a r a c t e r i s t i c o f the bond and s i n g l e , double and t r i p l e bonds can be d i s t i n g u i s h e d . When a m o l e c u l e i s adsorbed on a s u r f a c e and a v i b r a t i o n a l anal y s i s i s attempted one u s u a l l y t r i e s t o r a t i o n a l i z e the v i b r a t i o n spectrum i n analogy t o the w e l l documented v i b r a t i o n s p e c t r a o f f r e e m o l e c u l e s . T h i s renders a f i r s t guess. F r e q u e n t l y , a l s o , the answer i s p r o v i d e d as t o whether the molecule decomposes upon a d s o r p t i o n o r not. The s i m p l e s t case t h e r e i s the a d s o r p t i o n o f d i a t o m i c m o l e c u l e s where the absence o r presence o f the c h a r a c t e r i s t i c s t r e t c h i n g v i b r a t i o n d e c i d e s upon the q u e s t i o n o f d i s s o c i a t i o n . For l a r g e r molec u l e s and a more d e t a i l e d a n a l y s i s o f the v i b r a t i o n spectrum, t h e comparison t o the s p e c t r a o f f r e e m o l e c u l e may not s u f f i c e . Here the comparison t o o r g a n o m e t a l l i c compounds of known s t r u c t u r e and v i b r a t i o n spectrum can be most h e l p f u l . A good example i s p r o v i d e d w i t h the a n a l y s i s o f the s p e c i e s formed from e t h y l e n e adsorbed on P t ( l l l )

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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(4) and P d ( l l l ) (5_) a t room temperature. While i t became c l e a r v e r y q u i c k l y t h a t the s p e c i e s on the s u r f a c e c o u l d not be a c e t y l e n e as was assumed i n the o l d e r l i t e r a t u r e , the p r e c i s e n a t u r e of the spec i e s was debated f o r a w h i l e . The f i n a l answer came when the model compound C C l ^ i ^ i C C O g , named t r i c o b a l t - e t h y l i d e n e - n o n a c a r b o n y l , was prepared and the v i b r a t i o n spectrum was s t u d i e d ( 6 ) . F i g u r e 1 shows t h a t the v i b r a t i o n spectrum of the s u r f a c e s p e c i e s c o r r e l a t e s v e r y w e l l w i t h the spectrum of the model compound. This c o r r e l a t i o n a l s o i n c l u d e s the symmetry assignment of the modes f o r which we s h a l l d e s c r i b e the methodology s h o r t l y . Isotopes. As we have seen, the f r e q u e n c y of modes c o n t a i n s important i n f o r m a t i o n on the nature of the bonding. While the i n f o r m a t i o n i s q u a l i t a t i v e i t s t i l l serves f o r an educated f i r s t guess on the s t r u c t u r e of the m o l e c u l e . For the i d e n t i f i c a t i o n of the s p e c i e s i n F i g u r e 1, e.g., the assignmen the C-C s t r e t c h i n g v i b r a t i o n range of C-C s i n g l e bon g frequencies, , low the range of the t y p i c a l C-C double bond f r e q u e n c i e s (1500-1800 cm )· Thus from the assignment i t i s c l e a r t h a t the s p e c i e s has the bonding c h a r a c t e r i s t i c s of a s a t u r a t e d hydrocarbon. The q u e s t i o n i s , of c o u r s e , how the c o r r e c t assignment i s a c h i e v e d . Here s t u d i e s o f i s o t o p e s are i n d i s p e n s a b l e . Upon t o t a l or p a r t i a l d e u t e r a t i o n one n o t i c e s the CH v i b r a t i o n s t o s h i f t downwards by a f a c t o r of ~ 1.35 w h i l e the C-C v i b r a t i o n changes v e r y l i t t l e . In a few cases , where a s t r o n g c o u p l i n g between the C-C s t r e t c h i n g v i b r a t i o n and the CH s t r e t c h i n g v i b r a t i o n s of the same symmetry e x i s t s , l a r g e r s h i f t s of the "CC" v i b r a t i o n can o c c u r . Such s h i f t s a r e , however, not complet e l y a r b i t r a r y . The f r e q u e n c i e s o f modes w i t h i n the same r e p r e s e n t a t i o n of the p o i n t group of the s u r f a c e s p e c i e s f o l l o w the T e l l e r R e d l i c h r u l e when atoms are d i s p l a c e d by i s o t o p e s . The T e l l e r - R e d l i c h r u l e a l s o serves a u s e f u l purpose to check whether a proposed symmetry and the assignment of the mode a c c o r d i n g to the proposed symmetry i s c o r r e c t . An example i s p r o v i d e d w i t h the modes of C2H2 and C D on Fe(110) (7_). There i t was shown by the T e l l e r - R e d l i c h a n a l y s i s t h a t the o n l y symmetry of the s p e c i e s c o n s i s t e n t w i t h the s p e c t r a i s C^, t h a t i s , the s t r u c t u r e i s such t h a t no element o f symmetry e x i s t s . While t h i s i s somewhat d i s a p p o i n t i n g t o the t h e o r i s t as i t c o m p l i c a t e s the t h e o r e t i c a l a n a l y s i s of the bonding, the low symmetry f i n d s a n a t u r a l e x p l a n a t i o n i n the s t r u c t u r e of the lowest unoccupied o r b i t a l of a c e t y l e n e . 2

2

S u r f a c e P o i n t Groups and S e l e c t i o n R u l e s . The most important p o i n t groups of s u r f a c e s p e c i e s are C j , C , C , C , C 3 , C/ . Except f o r C , examples a r e known f o r each of the p o i n t groups. The a n a l y s i s of the p o i n t group of the s u r f a c e s p e c i e s makes use of the f a c t t h a t the s p e c t r o s c o p y i m p l i e s p a r t i c u l a r s e l e c t i o n r u l e s . Here a g a i n the advantage of ELS over o t h e r v i b r a t i o n s p e c t r o s c o p i e s f o r s t r u c t u r e a n a l y s i s becomes o b v i o u s . ELS has the unique c a p a b i l i t y t o employ d i f f e r e n t s c a t t e r i n g mechanisms merely by o b s e r v i n g under d i f f e r e n t s c a t t e r i n g c o n d i t i o n s . When s p e c t r a are observed i n the d i r e c t i o n s of s p e c u l a r r e f l e c t i o n ((OO)-beam) d i p o l e s c a t t e r i n g i s employed w h i c h means t h a t o n l y the t o t a l l y symmetric v i b r a t i o n s are e x c i t e d . Impact s c a t t e r i n g i s used when the spectrum i s observed o f f the speg

2

2y

V

2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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c u l a r d i r e c t i o n . Then, a l l even modes w i t h r e s p e c t t o the s c a t t e r i n g plane c o n t r i b u t e to the spectrum p r o v i d e d the s c a t t e r i n g plane i s a l i g n e d a l o n g a plane o f symmetry. In a g e n e r a l o r i e n t a t i o n o f the s c a t t e r i n g p l a n e , a l l modes a r e e x c i t e d . These s e l e c t i o n r u l e s a l l o w the d e t e r m i n a t i o n of the p o i n t group of the s u r f a c e s p e c i e s i n most cases. A famous, y e t s i m p l e example i s CO. CO tends t o adsorb i n h i g h ­ l y symmetric p o s i t i o n s on low index s u r f a c e s , so t h a t the p o i n t groups are C , C^ and C^ . The t o t a l l y symmetric v i b r a t i o n s then are the CO s t r e t c h i n g and the h i n d e r e d t r a n s l a t i o n of the m o l e c u l e v e r t i c a l to the s u r f a c e (metal-carbon stretch)· C o n s e q u e n t l y , t h e s e two modes are seen i n d i p o l e s c a t t e r i n g . Another example i s p r o v i d e d w i t h ammonia F e ( l l O ) ( F i g u r e 2 ) , when the coverage i s low ( 8 ) . Again, o n l y the t o t a l l y symmetric v i b r a t i o n s appear, under the p o i n t group C g . I n t e r e s t i n g l y here, the p o i n t group C^ i s i n c o n s i s t e n t w i t h the symmetry of the s u b s t r a t C χ c o n t a i n s o n l y th r e l e v a n t p o i n t group of ammoni , , y modes would appear. One must, t h e r e f o r e , conclude t h a t the b r e a k i n g of the m o l e c u l a r symetry through the s u r f a c e i s n e g l i g i b l e . T h i s i s b e s t r a t i o n a l i z e d when the NHo-molecule i s adsorbed w i t h the n i t r o ­ gen atom bonded head on a s u r f a c e atom ( i n s t e a d of b r i d g i n g c o n f i g u ­ r a t i o n s ) s i n c e then the symmetry o f the adsorbed ammonia i n c l u d i n g the n e a r e s t neighbor s u r f a c e atom i s 0^ · We s h a l l see l a t e r i n t h e t h e o r e t i c a l s e c t i o n t h a t the proposed s t r u c t u r e i s i n accordance w i t h t o t a l energy c a l c u l a t i o n s . 2y

y

y

v

v

2

g

ν

C o n t r o l l e d Reactions. V i b r a t i o n a l a n a l y s i s of surface chemical r e ­ a c t i o n s , decomposition r e a c t i o n s i n p a r t i c u l a r , i s a l s o f e a s i b l e . A number of d e c o m p o s i t i o n r e a c t i o n s have been s t u d i e d a l r e a d y . The s i m p l e s t of these i n v o l v e the d i s s o c i a t i o n of d i a t o m i c m o l e c u l e s such as CO, 0 , H , and NO. Y e t , a l s o the d e c o m p o s i t i o n of more com­ p l e x o r g a n i c m o l e c u l e s , w h i c h proceed v i a s e v e r a l s t e p s w i t h metas t a b l e i n t e r m e d i a t e compounds, has been i n v e s t i g a t e d . T y p i c a l l y such an experiment i s performed w i t h the molecule adsorbed a t low tempe­ r a t u r e s . The temperature i s then r a i s e d up t o a p a r t i c u l a r v a l u e f o r a p a r t i c u l a r t i m e . S p e c t r a are recorded a t t h a t temperature or w i t h the sample c o o l e d down a g a i n . The e t h y l i d e n e s p e c i e s from F i g u r e 1 r e p r e s e n t s such an i n t e r m e d i a t e . We have seen t h a t the a n a l y s i s of t h a t s p e c i e s was f i n a l l y confirmed through comparison t o an appro­ p r i a t e o r g a n o m e t a l l i c compound. T h i s , however, was the l a s t s t e p un­ d e r t a k e n i n o r d e r to c o n f i r m an a l r e a d y e x i s t i n g p r o p o s i t i o n . To a r ­ r i v e a t a r e a s o n a b l e c o n j e c t u r e about the n a t u r e of an u n i d e n t i f i e d o b j e c t on the s u r f a c e , the method of p r e p a r a t i o n p r o v i d e s e q u a l l y v a l u a b l e h i n t s . For the example d i s c u s s e d here,e.g., i t was i m p o r t a n t t o l e a r n t h a t the s p e c i e s c o u l d be grown from adsorbed e t h y l e n e , but a l s o from coadsorbed a c e t y l e n e and hydrogen. The s p e c i e s , t h e r e f o r e , had to have more than two hydrogen atoms, y e t no more than f o u r . An­ o t h e r example where the " h i s t o r y " was important i s the methoxy s p e ­ c i e s formed from the d e c o m p o s i t i o n of methanol on many m e t a l s u r ­ f a c e s . There, t h e s i m i l a r i t y o f the spectrum of methoxy t o m e t h a n o l , except f o r the removal of the OH s t r e t c h i n g v i b r a t i o n from the spec­ trum upon warming the sample t o h i g h e r temperature, was the i m p o r t a n t clue (9> 2

2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CATALYST C H A R A C T E R I Z A T I O N S C I E N C E

396

Pti111)+C H 2

4

exposed to 92 Κ annealed to 415 Κ v-CM(s) v-CC 6-CH(as) 6-CCM v-CM(as) p-CH 6-CH(s)

v-CH(asl v-ch3(s)

3

3

3

3

>< en I— GD on
°

\ -JS-

2 =i

-0.5

Ζ M0M(

>ID

H

\

[Q.U.]

•

•—

-1.0

ÛL Ο

ν =290 cm-1 Μ 2 =0.33χ10" 3 •

3

•

H

30 60 90 120 TILT ANGLE a t [degree]

6

5 4 DISTANCE z 0 [Bohr] c)

ci 2 =L g-0.5 ^

^=11x10'^ " - ^ ( 3 . 5 χΙΟ'3)

5 -0.1 CD 60

μ (Gas)

v=1480 cm 1 (1560)

90 120 150 180 BOND ANGLE a b [degree]

Figure 4·

T o t a l energy

o Σ 1 Ui —I ο Q_

210

1.6

1.8 2.0 2.2 BOND LENGTH R 0 H lBohr]

c u r v e s and d i p o l e moments f o r Ην,Ο.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

399

CATALYST C H A R A C T E R I Z A T I O N

400

F i g u r e 5.

SCIENCE

T o t a l energy c u r v e s and d i p o l e moments f o r NH^.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

34.

IBACH A N D

MULLER

Vibrational Analysis of Adsorbed Molecules

401

v e r y enharmonic energy v a r i a t i o n . The mode has a v e r y l a r g e z e r o p o i n t m o t i o n , which t o g e t h e r w i t h the l a r g e dynamic d i p o l e g i v e s r i s e t o a s t r o n g EELS s i g n a l ( 1 4 ) . The d y n a m i c a l d i p o l e i s a p p r o x i ­ mately g i v e n by άμ/άα^ = 2μ s i n a , where μ - 0.7 a.u. (1 a.u. - 2 . 5 Debye) i s the permanent d i p o l e of H 0 and the f a c t o r 2 ( i n our c a l c u l a t i o n o n l y 1.4) i s the enhancement due to the image. I t has been observed t h a t w i t h the f o r m a t i o n H 0 l a y e r s t h i s mode s t i f f e n s and becomes even s t r o n g e r ( 1 5 ) . For ΝΗβ the o r i e n t a t i o n a l energy i s q u i t e l a r g e . However, s i n c e i n the ground s t a t e the permanent d i p o l e i s p e r p e n d i c u l a r to the s u r f a c e , the d y n a m i c a l d i p o l e f o r t h i s mode i s z e r o , so t h a t the mode i s EELS-inactive. I n t e r n a l Bend ( F i g u r e s 4c and 5 b ) . The i n t e r n a l modes a r e used to i d e n t i f y the adsorbed s p e c i e s , and a comparison w i t h the i s o l a t e d m o l e c u l e v a l u e s i n d i c a t e s t o what e x t e n t the m o l e c u l e i s a f f e c t e d by the s u r f a c e . I n the " s c i s s o r s mode s u r f a c e and the o s c i l l a t i o n frequency i s almost u n a f f e c t e d . For NH3, on the o t h e r hand, the so c a l l e d " u m b r e l l a mode" i s d r a s t i c a l l y s t i f f e n e d by the s u r f a c e because the p r o t o n s move a g a i n s t the s u r ­ f a c e and s u f f e r a s t r o n g Coulomb r e p u l s i o n . Here a g a i n the d y n a m i c a l d i p o l e moment i s dμ/dα^ « 2 μ^. s i n d a ^ . For NH3, where μ - 0.53 a.u., the enhancement of the dynamic d i p o l e by the s u r f a c e i s com­ pensated by the s m a l l e r o s c i l l a t i o n a m p l i t u d e . I n t e r n a l S t r e t c h ( F i g u r e s 4d and 5 c ) . I n s p i t e o f an enhance­ ment of the m a t r i x elements by the s u r f a c e , t h i s mode shows a r a t h e r s m a l l i n t e n s i t y and e s s e n t i a l l y no change i n f r e q u e n c y . The o c c u r ­ rence o f a l a r g e peak s h i f t e d t o lower f r e q u e n c i e s must be i n t e r ­ p r e t e d i n terms of hydrogen bonded networks ( 1 5 ) . Μ

t

Μ

2

2

Μ

Comparison Between Theory and

Experiment 9

In Table I we compare c a l c u l a t e d f r e q u e n c i e s and m a t r i x elements M ( i n p a r e n t h e s i s ) f o r H 0 and NHo on A l ( 1 0 0 ) w i t h e x p e r i m e n t a l v a l u e s f o r the NH -Fe(110) and H 0-Cu(100) systems, the data t a k e n from (8) and ( 1 4 ) . Only r e l a t i v e v a l u e s of M a r e presented because o f d i f f i ­ c u l t i e s i n d e t e r m i n i n g the s u r f a c e coverage. Even though we a r e com­ p a r i n g d i f f e r e n t s y s t e m s j the good agreement found f o r the i n t e r n a l modes i s an i n d i c a t i o n t h a t these modes are r a t h e r independent o f the s u b s t r a t e and t h e r e f o r e can be used t o c h a r a c t e r i z e the adsorbed s p e c i e s . The h i n d e r e d modes, on the o t h e r hand are much more sub­ s t r a t e s p e c i f i c . For i n s t a n c e , NH3 binds more s t r o n g l y t o Fe than t o A l , and H 0 more weakly t o Cu than t o A l . Note t h a t the t i l t e d ground s t a t e geometry reduces the i n t e n s i t y of the i n t e r n a l H 0 modes by a f a c t o r cos α = 0.25, the i n t e r n a l s t r e t c h a p p a r e n t l y below the l i m i t of d e t e c t i o n . The h i n d e r e d r o t a t i o n , on the o t h e r hand, i s o n l y ob­ s e r v a b l e i n the t i l t e d geometry. As mentioned above, t h i s mode i s f o r b i d d e n by symmetry f o r NH3, but o b s e r v a b l e w i t h l a r g e i n t e n s i t y f o r H 0. F i n a l l y , the h i n d e r e d t r a n s l a t i o n e x h i b i t s a s m a l l but f i n i t e a m p l i t u d e b o t h f o r NH3 and H 0, i n d i c a t i n g charge t r a n s f e r , i n t h i s case from the m o l e c u l e to the m e t a l . 2

3

2

2

2

2

2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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SCIENCE

Table I. Comparison o f c a l c u l a t e d f r e q u e n c i e s and m a t r i x elements M ( i n p a r e n t h e s e s ) f o r H 0 and N H on A l ( 1 0 0 ) w i t h e x p e r i m e n t a l v a l u e s f o r t h e NH -Fe(1107 and H 0-Cu(100) s y s t e m s . 2

2

3

3

2

H 0-Cu

H 0-A1

NH -Fe

NH ~A1

(exp)

(theory)

(exp)

(theory)

Hindered translation

121 cm". (0,.60)

290 cm" (0.33)

1

Hindered rotation

230 cm" (26.0)

400 cm" (24.0)

1

Internal bend

1590 cm".. (2,• 7)

1480 cm" (2.7)

1

2

2

1

3

3

429 cm" (0,• 09)

370 cm" (0,• 05)

1170 cm" (15.0)

1260 cm" (15.0)

1

1

1

1

Internal stretch

Next we d i s c u s s t h e e f f e c t o f d e u t e r a t i o n on low frequency modes i n ­ v o l v i n g the p r o t o n s . Because o f t h e anharmonic v a r i a t i o n o f t h e e n ­ ergy as a f u n c t i o n o f t i l t a n g l e α ( F i g . 4b), t h e h i n d e r e d r o t a t i o n s o f H 0 and D 0 t u r n out t o be q u a l i t a t i v e l y d i f f e r e n t . The f i r s t v i b r a t i o n a l e x c i t e d s t a t e o f H 0 i s l e s s l o c a l i z e d than t h a t o f D 0 , because o f i t s l a r g e r e f f e c t i v e mass. The o s c i l l a t i o n frequency o f the mode decreases by a f a c t o r 1.19 and t h e m a t r i x elements by a f a c t o r 1.51 upon d e u t e r a t i o n . T h e r e f o r e , t h e harmonic a p p r o x i m a t i o n , which y i e l d s an i s o t o p i c f a c t o r 1.4 f o r both the frequency and t h e i n t e n s i t y , i s q u i t e i n a p p r o p r i a t e f o r t h i s mode. In Table I I we compare the c a l c u l a t e d frequency and m a t r i x e l e ­ ment M ( i n p a r e n t h e s i s ) f o r t h e DoO-AKlOO) h i n d e r e d r o t a t i o n w i t h e x p e r i m e n t a l r e s u l t s f o r D 0-Cu(100) ( 1 4 ) . The e x p e r i m e n t a l i s o t o p i c f a c t o r i s 1.16 f o r t h e frequency and - 1.6 f o r t h e m a t r i x e l e m e n t s , i n good agreement w i t h our c a l c u l a t i o n . I n Ref. 14, u s i n g t h e h a r ­ monic a p p r o x i m a t i o n , t h e anomalous i s o t o p i c f a c t o r f o r t h e f r e q u e n c y was i n t e r p r e t e d as due t o m i x i n g w i t h t h e h i n d e r e d t r a n s l a t i o n . How­ e v e r , as we have shown, t h e harmonic a p p r o x i m a t i o n i s i n a p p r o p r i a t e i n t h i s case. 2

2

2

2

2

T a b l e I I . Comparison o f c a l c u l a t e d f r e q u e n c y and m a t r i x element M (τη p a r e n t h e s e s ) f o r t h e D 0-A1(100) h i n d e r e d r o t a t i o n w i t h e x p e r i m e n t a l r e s u l t s f o r D^O-CudOO). 2

D 0-Cu

D 0-A1

(exp)

(theory)

2

Hindered

198 cm"

rotation

(16.0)

2

1

337 cm"

1

(15.9)

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2

34.

IBACH A N D M U L L E R

Vibrational Analysis of Adsorbed Molecules

403

In concluding, we point out an essential role of v i b r a t i o n a l spectra in theoretical studies. Total energy calculations y i e l d quantities of much interest, l i k e equilibrium geometries and binding energies, which are not accessible i n a direct experimental way. Only the v i ­ brational quantities can be meaningfully compared with experiment and provide a way to assess the adequacy of these calculations.

Literature Cited 1.

2. 3. 4. 5. 6. 7. 8. 9.

10. 11. 12.

13. 14. 15.

For a general review see: Ibach H.; M i l l s , D.L. "Electron Energy Loss Spectroscopy and Surface Vibrations", Academic Press: New York, 1982. Brundle, C.R.; Morawitz, M.; Eds. "Vibrations at Surfaces" Elsevier: Amsterdam, 1983. Szeftel, J.M.; Lehwald J.E.; M i l l s , D.L. Steininger, H.; Ibach, H.; Lehwald, S. Surf. S c i . (1982) 117, 685. Kesmodel, L.L.; Gates, J.A. Surf. S c i . (1981) 111, L747. and i n (2) p. 307. Shinner, P.; Howard, M.W.; Oxton, L.A.; Kettle, S.F.A.; Powell, D.B.; Sheppard, N. J . Chem. Faraday Trans. 2 (1981) 77, 397. Erley, W.; Baro, A.M.; Ibach, H. Surf. Sci. (1982) 120, 273. Erley, W.; Ibach, H. i n (2) p. 263 and Erley W., private communication. Sexton, B.A. Surf. S c i . (1979) 88, 299. Demuth, J.E.; Ibach, H. Chem. Phys. Lett. (1979) 60, 395. Sexton, B.A. Surf. S c i . (1981) 102, 271. See, for instance, Port, D.; Baerends, E.J. Surf. S c i . (1981) 109, 167. See, for instance, Herman, K.; Bagus, P.S.; Brundle C.R.; Menz e l , D. Phys. Rev. Β (1981) 24, 7025. See, for instance, Wimmer, E.; Freeman, A.J.; Hiskes J.R.; Karo, A.M. Phys. Rev. Β (1983) 28, 3074 and Feibelman P.J.; Hamann, D.R. Phys. Rev. Lett. (1984) 52, 61. Müller, J.E.; Jones, R.O.; Harris, J. J. Chem. Phys. (1983) 79, 1874. Andersson, S.; Nyberg, C.; Tengstål, C.G. Chem. Phys. Lett. (1984) 104, 305. T h i e l , P.Α.; Hoffmann, F.M.; Weinberg, W.H. J. Chem. Phys. (1981) 75, 5556.

R E C E I V E D March 28,

1985

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

35 IR Spectroscopic Characterization of Adsorbed Species and Processes on Surfaces John T. Yates, Jr., PatrickGelin1,and Thomas Beebe Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260

The use of infrared spectroscop cal phenomena on surfaces extends back to the early work of Terenin in Russia(1) who first employed near-IR spectroscopy as a tool to observe surface OH groups on SiO2, working in the region of the second harmonic OH stretching mode. This work was extended dramatically in the 1950's by R. P. Eischens and coworkers, who first applied transmission IR spectroscopy to the study of species chemisorbed on supported metals of catalytic interest(2). Eischens found that the use of group vibrational frequency assignments was a powerful method for deducing general structural information about adsorbed surface species, building on spectra of compounds of known structure. Results of this body of work are summarized in three monographs (3-5). The infrared method is widely applied for surface studies in industry and academia today, a testament to the wide ranging utility of the method even after almost 30 years of use. In addition, a wide range of other types of surface vibrational spectroscopic methods have now been developed and are widely employed. IR spectroscopy has several distinct advantages as a probe of surface species character, as listed below: • Ability to work under high gas densities to study catalytic surfaces under working conditions. • Ability to use high resolution to accurately characterize small shifts in oscillator frequencies, as well as to perform lineshape analysis. • High sensitivity of species. vibrational frequency to bonding modes at surfaces. • Strong correlation of the spectra of surface species with vibrational spectroscopy of molecules of known structure. The first two advantages listed above allow an optical method like transmission or reflection IR spectroscopy to be used for studies which would be impossible for a widely used competitive technique, electron energy loss spectroscopy (EELS). EELS must 1

Current address: CNRS Laboratories, Lyon, France 0097-6156/85/0288-O404$06.00/0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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405

o p e r a t e under u l t r a h i g h vacuum c o n d i t i o n s and does not possess the r e s o l u t i o n i n h e r e n t i n the o p t i c a l methods(6). Thus the examples s e l e c t e d f o r p r e s e n t a t i o n i n t h i s paper c o u l d not have been done u s i n g the EELS t e c h n i q u e . Two examples of the a p p l i c a t i o n of t r a n s m i s s i o n IR methods w i l l be p r e s e n t e d . The f i r s t , d e a l i n g w i t h the c h e m i s o r p t i o n of CO on a Pd/Si02 c a t a l y s t s u r f a c e , i l l u s t r a t e s the f i r s t o b s e r v a t i o n of a l o c a l s t o i c h i o m e t r i c s u r f a c e s p e c i e s i n t e r c o n v e r s i o n process w h i c h o c c u r s among chemisorbed CO s p e c i e s at h i g h CO coverages. Evidence f o r the o p e r a t i o n o f the l o c a l s t o i c h i o m e t r i c process has been o b t a i n e d on 75Â Pd p a r t i c l e s . These p a r t i c l e s seem t o show c l o s e s i m i l a r i t i e s as w e l l as to d i f f e r i n some r e s p e c t s from a P d ( l l l ) s i n g l e c r y s t a l s u r f a c e i n s o f a r as t h e i r i n t e r a c t i o n w i t h CO i s concerned. The second example d e a l s w i t h the use of IR s p e c t r o s c o p y t o s t u d y bonding d e t a i l s w i t h i S i 0 2 s u r f a c e . Here s p e c i f i groups i s observed; i n a d d i t i o n , CO m o l e c u l e s h a v i n g r o t a t i o n a l freedom are produced a t h i g h e r CO coverages, and the degree of r o t a ­ t i o n a l freedom a l l o w e d seems to be determined by the magnitude of s h i e l d i n g of the p o l a r c e n t e r s on the Si02 s u r f a c e by CO s p e c i e s w h i c h bond to SiOH, and a l s o p a r t i c i p a t e as a d i e l e c t r i c s c r e e n i n g medium. Experimental

Methods

Commercial IR Spectrometer Developments. For the study of the IR spectrum of s p e c i e s p r e s e n t on h i g h a r e a s u r f a c e s , both g r a t i n g and F o u r i e r t r a n s f o r m i n s t r u m e n t s are commonly employed. B o t h types of i n s t r u m e n t s now f e a t u r e computer d a t a a c q u i s i t i o n t e c h n i q u e s w h i c h p e r m i t enhancement of s i g n a l / n o i s e r a t i o s by m u l t i p l e scan a v e r a g i n g methods. T h i s f e a t u r e , coupled w i t h smoothing r o u t i n e s and background f i t t i n g p r o c e d u r e s , has l e d to a s i g n i f i c a n t enhancement o f the q u a n t i t a t i v e a s p e c t s of IR s u r f a c e s p e c t r o s c o p y . The d a t a shown i n t h i s paper have been o b t a i n e d a t a r e s o l u t i o n of 3-4 cm"l w i t h a d a t a a c q u i s i t i o n time of from 0.4 - 1 sec/cm""!. Under these c o n d i t i o n s , a background n o i s e l e v e l o f about 0.001 absorbance u n i t s i s r e a d i l y a c h i e v e d , p e r m i t t i n g the o b s e r v a t i o n of weak a b s o r p t i o n bands due t o s u r f a c e s p e c i e s . IR C e l l Developments. The d e s i g n of c e l l s f o r IR s p e c t r o s c o p y has taken many c o u r s e s , depending upon the o b j e c t i v e s of the r e s e a r c h e n v i s i o n e d . For example, i n the case of combining IR s p e c t r o s c o p y w i t h s u r f a c e k i n e t i c s measurements, the d e s i g n of V a n n i c e ( 7 ) i s i d e a l ; here, r e a c t a n t gases f l o w through the porous pressed d i s k sample d u r i n g IR measurements. S i m i l a r arrangements are employed by B e l l u s i n g FTIR methods under r e a c t i o n c o n d i t i o n s ( 8 ) . Another v a r i a t i o n i n IR c e l l d e s i g n w h i c h we have developed i s shown i n F i g u r e 1. I n t h i s c e l l , t h r e e s u r f a c e measurement o b j e c ­ t i v e s are r e a d i l y a c h i e v e d : • S t a b i l i z a t i o n of s u r f a c e s p e c i e s and r e a c t i o n i n t e r m e d i a t e s at c r y o g e n i c temperatures. • Simultaneous a d s o r p t i o n s t u d i e s on supported m e t a l s and on t h e i r s u p p o r t , where both have been t r e a t e d w i t h r e a c t a n t s i n e x a c t l y the same manner.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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• Measurement o f s u r f a c e s p e c i e s under h i g h gas p r e s s u r e s , by means o f complete c a n c e l l a t i o n of i n t e r f e r i n g gas phase spectra. The u l t r a h i g h - v a c u u m i n f r a r e d c e l l , F i g u r e 1, c o n s i s t s of a s t a i n l e s s s t e e l body made from a d o u b l e - s i d e d confiât f l a n g e , 2-3/4" d i a m e t e r , s e a l e d by two CaF2 windows i n confiât f l a n g e s ( a v a i l a b l e from Harshaw C h e m i c a l Co., C r y s t a l and E l e c t r o n i c P r o d u c t s Dept., 6801 S o l o n , Ohio, 44139). T h i s c e l l may be c o n v e n i e n t l y heated t o 500K by b e i n g p l a c e d i n s i d e a s m a l l oven. Other c e l l s , f e a t u r i n g i n t e r n a l e l e c t r i c a l h e a t i n g , may be o p e r a t e d a t h i g h e r temperatures(9)· The c e n t r a l body o f the c e l l i n F i g u r e 1 c o n t a i n s a Cu support r i n g w h i c h may be c o o l e d by c i r c u l a t i o n o f l i q u i d N2 t h r o u g h t u b i n g b r a z e d to the copper. The temperature o f the support r i n g i s monitored by means o f a 0.003" diameter chrome1-alumel t h e r mocouple. The temperature o f the sample below room temperature may be c o n t r o l l e d to ± IK b t h r o u g h the a p p a r a t u s u s i n an e l e c t r o n i c s e r v o s y s t e m experimentally a d s o r b a n t sample, supported on a c i r c u l a r CaF2 p l a t e c l i p p e d i n s i d e the Cu r i n g , r e a c h e s the temperature of the r i n g a t a l l p o i n t s to w i t h i n about 1K(10). Samples of h i g h a r e a powders and o f supported m e t a l s may be a p p l i e d t o the CaF2 support p l a t e by a s p r a y i n g t e c h n i q u e , p r e v i o u s l y described i n d e t a i l ( l l ) . In F i g u r e 1, we show a " h a l f p l a t e " d e s i g n i n w h i c h a supported m e t a l d e p o s i t , produced by H2 r e d u c t i o n of m e t a l i o n s h e l d on the s u p p o r t , o c c u p i e s one h a l f o f the p l a t e w h i l e the pure support o c c u p i e s the o t h e r h a l f . T r a n s l a t i o n o f the c e l l l e f t and r i g h t p e r m i t s the achievement of each of the t h r e e o b j e c t i v e s l i s t e d above, u s i n g a p p r o p r i a t e d a t a s u b t r a c t i o n p r o c e d u r e s t o remove c o n t r i b u t i o n s from gas phase spec i e s i f p r e s e n t d u r i n g measurements. The c a n c e l l a t i o n of gas phase s p e c t r a l f e a t u r e s u s i n g the " h a l f p l a t e " d e s i g n i s f a r s u p e r i o r t o methods i n v o l v i n g a second gas c e l l p l a c e d i n the r e f e r e n c e beam. T h i s i s because the gas d e n s i t y and i t s r o t a t i o n a l s t a t e p o p u l a t i o n w i l l d i f f e r i n the two c e l l s f o r d i f f e r e n t sample (and t h e r e f o r e gas) t e m p e r a t u r e s . For h i g h sens i t i v i t y measurements, these e f f e c t s can be d i f f i c u l t t o handle u s i n g two c e l l s . Experimental

Results

The C h e m i s o r p t i o n o f CO on Pd/Si02 - O b s e r v a t i o n of S t o i c h i o m e t r i c S p e c i e s I n t e r c o n v e r s i o n E f f e c t s i n the Chemisorbed CO Layer a t H i g h Coverages ( 1 2 , 1 3 ) . I t has l o n g been r e c o g n i z e d t h a t b r i d g i n g - C O r e a d i l y forms upon c h e m i s o r p t i o n on b o t h supported Pd(14-16) and on many Pd s i n g l e c r y s t a l s u r f a c e s ( 1 7 - 2 1 ) . At temperatures below 300K, o r a t h i g h CO p r e s s u r e s , a t e r m i n a l form of chemisorbed CO may a l s o be p o p u l a t e d , e x h i b i t i n g a c a r b o n y l s t r e t c h i n g f r e q u e n c y near 2100 cm".. T h i s i s shown i n F i g u r e 2, where spectrum a^ r e p r e s e n t s the s t a b l e IR s p e c i e s w h i c h remain on Pd/Si02 f o l l o w i n g CO s a t u r a t i o n and e v a c u a t i o n a t 300K. We see t h a t s e v e r a l bridged-CO s p e c i e s e x i s t t o g e t h e r i n the 1700 c m - 2000 cm". r e g i o n i n c l u d i n g a sharp h i g h frequency b r i d g e d c a r b o n y l band a t 1979 cm" (B^) w h i c h i s most p r o m i n e n t . Upon c o o l i n g t h i s s u r f a c e to 80K and adding s m a l l quan-1

A

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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F i g u r e 2.

Additional

1900

1800

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AL.

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t i t l e s of CO ( s p e c t r a b-Ji) a t e r m i n a l CO s p e c i e s (L) a t 2103 cm"" d e v e l o p s ; as the t e r m i n a l 2103 cm" CO species develops, systematic changes o c c u r i n the 1979-1995 cm" r e g i o n . In a d d i t i o n , a s m a l l band a t 1883 cm" i s e v i d e n t at the h i g h e s t CO c o v e r a g e s . A l t h o u g h the s p e c t r a l developments shown i n F i g u r e 2 have been o b t a i n e d by c o o l i n g the Pd/Si02 s u r f a c e to 80K, we have shown t h a t s p e c i e s L may a l s o be formed a t 300K a t CO p r e s s u r e s of s e v e r a l hundred T o r r . T h i s b e h a v i o r i s c o n s i s t e n t w i t h s p e c i e s L having a f a i r l y low heat of a d s o r p t i o n . S u b t r a c t i o n o f the s p e c t r a i n F i g . 2 y i e l d s a s e t of d i f f e r e n c e s p e c t r a shown i n F i g . 3. I t may be seen t h a t as the 2103 cm" band d e v e l o p s a t 80K, t h e r e i s a l o s s of i n t e n s i t y at 1979 cm" ( B j ) and a concomitant g a i n i n i n t e n s i t y a t 1995 cm" ( B 2 ) . These changes, as judged by the peak a b s o r b a n c e s , are a c c u r a t e l y l i n e a r f u n c t i o n s of each o t h e r ( F i g . 4 ) , s u g g e s t i n g t h a t a s i m p l e s t o i c h i o m e t r i c p r o ­ cess occurs. I t i s als s i o n i n v o l v e s an i s o s b e s t i s i m p l e s t o i c h i o m e t r i c p r o c e s s i s i n v o l v e d , r a t h e r than some s o r t of f r e q u e n c y s h i f t as a consequence of i n c r e a s i n g CO coverage. F u r t h e r i n s i g h t i n t o the nature of the s t o i c h i o m e t r i c p r o c e s s has been o b t a i n e d u s i n g i s o t o p i c a l l y l a b e l e d ^CO ( d e s i g n a t e d .) as the s p e c i e s used t o p o p u l a t e L. - CO a t 80K on top of a C 0 l a y e r produced a t 300K. The r e s u l t s of t h i s experiment are shown i n F i g . 6, and a s u r p r i s i n g f e a t u r e i s n o t e d . The a d s o r p t i o n of L. - CO i n d u c e s the f o r m a t i o n of L - CO. The C 0 - L s p e c i e s produced by a d s o r p t i o n can o n l y a r i s e from preadsorbed C 0 - B j s p e c i e s . D e t a i l e d i n v e s t i g a t i o n of t h i s B^ L conversion suggests that about 2Bi -»· 2L o c c u r f o r each L. - CO adsorbed. The s t o i c h i o m e t r i c r e l a t i o n s h i p induced by L - CO a d s o r p t i o n c a u s i n g 2Βχ .• 2L i s shown i n F i g u r e 7, where i s o t o p i c a l l y l a b e l e d CO has been employed i n s e p a r a t e e x p e r i m e n t s as B^ or as L. A f t e r a s m a l l c o r r e c t i o n i s made f o r the e x p e r i m e n t a l l y determined d i f f e r e n c e i n e x t i n c t i o n c o e f f i c i e n t f o r C 0 ( a d s ) compared to C 0 ( a d s ) , i t may be seen t h a t the s t o i c h i o m e t r y 2Β^ + L ->· 3L i s c l o s e l y v e r i f i e d . A s c h e m a t i c o n e - d i m e n s i o n a l model of the p r o c e s s i s i l l u s t r a t e d i n F i g . 8. Here i t i s e n v i s i o n e d t h a t the p r o c e s s 4Bi+L > 3L+2B2 i s o c c u r r i n g . An e l e c t r o n i c model suggests t h a t t h i s s t o i c h i o m e t r i c s p e c i e s c o n v e r s i o n p r o c e s s may be due t o the f a c t t h a t the e l e c t r o n a c c e p t o r c a p a c i t y of an ensemble of 2Bj + L exceeds the donor capa­ c i t y of the 3 Pd s i t e s ; a c o n v e r s i o n to 3L reduces the t o t a l a c c e p ­ t o r c a p a c i t y of the CO ensemble to a p o i n t where the donor c a p a c i t y o f the 3 Pd s i t e s i s not exceeded. These p r o c e s s e s are e n t i r e l y r e v e r s i b l e upon warming the Pd s u r f a c e to 300K. The average Pd c r y s t a l l i t e s i z e , measured by CO c h e m i s o r p t i o n uptake i s about 75Â. The s t o i c h i o m e t r i c e f f e c t seen here i s s u r p r i s i n g f o r c r y s t a l s of t h i s s i z e , and must i n d i c a t e t h a t a l o c a l p i c t u r e of CO c h e m i s o r p t i o n a p p l i e s to these s m a l l Pd c r y s t a l s , i n agreement w i t h m e t a l ensemble i d e a s of S a c h t l e r ( 2 2 ) . I t i s very i n s t r u c t i v e to compare the r e s u l t s shown above f o r Pd/Si02 s u r f a c e s w i t h s i m i l a r measurements made on P d ( l l l ) u s i n g r e f l e c t i o n IR methods(20). As shown i n F i g . 9, a c l o s e s i m i l a r i t y of CO s p e c i e s development o c c u r s i n comparing r e s u l t s w i t h F i g . 2 f o r Pd/Si02« In b o t h c a s e s , f o u r c a r b o n y l s t r e t c h i n g bands are seen a t s a t u r a t i o n c o v e r a g e s , d i f f e r i n g by o n l y a few cm" i n frequency i n comparing 1

1

1

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0.2h

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2000

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F i g u r e 3. Difference low t e m p e r a t u r e .

1800

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s p e c t r a f o r CO c h e m i s o r p t i o n on P d / S ^

0.05 BRIDGED

(.ΔΑ F i g u r e 4. Evidence f o r adsorbed CO s p e c i e s .

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Figure 6. D i r e c t evidence f o r b r i d g e d - t o - l i n e a r i s o t o p i c CO e x p e r i m e n t s .

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a f a

(transmission)

Porous m a t e r i a l s , such as s i l i c a and a l u m i n a , have thermal d i f f u s i o n l e n g t h s o f a p p r o x i m a t e l y 1 0 ~ m, which i s much l e s s than t h e t y p i c a l t h i c k n e s s o f pressed d i s c s . The small thermal d i f f u s i o n l e n g t h g i v e s p h o t o a c o u s t i c s p e c t r o s c o p y a l a r g e r dynamic range than t r a n s m i s s i o n methods when a p p l i e d t o powdered samples. An a d d i ­ t i o n a l advantage i s t h e ease o f sample p r e p a r a t i o n , s i n c e pho­ t o a c o u s t i c s p e c t r o s c o p y uses powdered samples w i t h no s p e c i a l preparation required. Transmission s p e c t r o s c o p y i s f u r t h e r c o m p l i c a t e d by s c a t t e r i n g and r e f l e c t i o n o f t h e i n f r a r e d r a d i a t i o n . Even w i t h d i s c s 0.1 mm t h i c k t h e o p t i c a l t h r o u g h p u t i s t y p i c a l l y l e s s than 5% making i t d i f f i c u l t t o o b t a i n good s i g n a l t o n o i s e . Furthermore, l i g h t s c a t ­ t e r i n g can cause s p e c t r a l d i s t o r t i o n . With p h o t o a c o u s t i c s p e c t r o s c o p y l i g h t s c a t t e r i n g by micron s i z e p a r t i c l e s i s o f secon­ dary importance. I t r e d i s t r i b u t e s t h e photon energy r e s u l t i n g i n a s l i g h t enhancement o f t h e p h o t o a c o u s t i c s i g n a l ( 5 , 6 ) , but has a n e g l i g i b l e e f f e c t on t h e s p e c t r a as a p h o t o a c o u s t i c s i g n a l cannot be generated w i t h o u t t r u e a b s o r p t i o n . T r a n s m i s s i o n s p e c t r o s c o p y o f f e r s two s i g n i f i c a n t advantages over p h o t o a c o u s t i c s p e c t r o s c o p y o f powders. F i r s t , t r a n s m i s s i o n s p e c t r o s c o p y i s not s u s c e p t i b l e t o e x t e r n a l a c o u s t i c d i s t u r b a n c e s . Commercial s p e c t r o m e t e r s must be m o d i f i e d f o r v i b r a t i o n a l i s o l a t i o n i n o r d e r t o o b t a i n good p h o t o a c o u s t i c s p e c t r a . Secondly, t r a n s m i s s i o n s p e c t r o s c o p y can use s o l i d s t a t e d e t e c t o r s w i t h v e r y f a s t response t i m e s , whereas p h o t o a c o u s t i c s p e c t r o s c o p y i s much s l o w e r , w i t h s p e c t r a t a k i n g a few minutes t o c o l l e c t as compared t o a few seconds f o r t r a n s m i s s i o n s p e c t r a when both are taken w i t h an FTIR. The main advantages and disadvantages o f t r a n s m i s s i o n and pho­ t o a c o u s t i c s p e c t r o s c o p i e s are summarized i n Table I . The advantages 5

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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451

o f t h e p h o t o a c o u s t i c method suggest i t c o u l d be a u s e f u l a n a l y t i c a l t e c h n i q u e f o r c a t a l y s t samples. TABLE I .

routine

A Comparison o f Transmission and P h o t o a c o u s t i c Spectroscopies Photoacoustic

Transmission Detected Signal

I

Sample Preparation

Pressed Disc o f M u l l

Loose Powder

Effect of Scattering

Can Cause S p e c t r a l Distortio

Negligible

Spectrum Acquisition Time

Seconds

Minutes

Spectral Region Probed

L i m i t e d t o 4000-1200 c m " from A b s o r p t i o n by L a t t i c e Modes o f S o l i d

4000-400 c m " Only S p e c t r a l L i m i t a t i o n s are Imposed by Spectrometer

Special Problems

R e f l e c t i o n Losses R e s u l t i n Low O p t i c a l Throughput L i m i t i n g S i g n a l t o Noise

H i g h l y Suscepti b l e to Airborne Noise and B u i l d ing V i b r a t i o n s

T

= I exp[-(a 0

a

+ a )i]-lR s

PA - I < . a («s) 1 « f(a ) « 4 f

0

s

1

1

Experimental Experiments were c a r r i e d out i n a Bomem DA3.002 FTIR u s i n g a spec i a l l y b u i l t p h o t o a c o u s t i c c e l l d e s c r i b e d elsewhere ( 7 ) . The c e l l was equipped t o a l l o w jm s i t u sample h e a t i n g i n vacuum o r under a c o n t r o l l e d atmosphere a t temperatures up t o 400 C. The s p e c t r a shown were o b t a i n e d a t room t e m p e r a t u r e i n f l o w i n g hydrogen. A m i r r o r v e l o c i t y o f 0.05 cm/s was used w i t h 200 scans coadded f o r each s p e c t r u m . The s p e c t r a were t r a n s f o r m e d a t 8 c m " r e s o l u t i o n u s i n g r a i s e d c o s i n e a p o d i z a t i o n . The i n t e r f e r o g r a m was phase c o r r e c t e d and n u m e r i c a l l y f i l t e r e d by t h e c o n v o l u t i o n method w i t h 256 c o e f f i c i e n t s . The s p e c t r a were n o r m a l i z e d a g a i n s t a g r a p h i t e r e f e r e n c e spectrum o b t a i n e d i n t h e same c e l l . The t o t a l d a t a a c q u i s i t i o n t i m e f o r each spectrum was a p p r o x i m a t e l y 15 m i n . The i n t e r f e r o m e t e r assembly has been m o d i f i e d t o p r o v i d e a c o u s t i c i s o l a t i o n from b o t h b u i l d i n g v i b r a t i o n s and a i r b o r n e n o i s e (7). These improvements have g r e a t l y enhanced t h e s i g n a l t o n o i s e . The s p e c t r a p r e s e n t e d here show a s i g n a l t o n o i s e r a t i o i n excess o f 500 f o r s i l i c a samples and i n excess o f 100 f o r t h e alumina samples; t h e d i f f e r e n c e s are due t o d i f f e r e n t sample p o r o s i t i e s . No smoothing o f t h e s p e c t r a has been p e r f o r m e d , and a l l s p e c t r a r e p o r t e d a r e d i r e c t r e p r o d u c t i o n s o f t h e p l o t t e r o u t p u t from t h e spectrometer. 1

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Results S i l i c a - P h o t o a c o u s t i c s p e c t r a o f a v a r i e t y o f s i l i c a samples are shown i n F i g u r e 1 . F i g u r e 1a i s o f an a e r o s i l sample (ALFA Chemicals #89376) t h a t had been d r i e d a t 350 C f o r 4 hr and c o o l e d t o room t e m p e r a t u r e i n f l o w i n g hydrogen. T h i s sample i s comprised o f 60 A p a r t i c l e s o f s i l i c a w i t h a nominal s u r f a c e area o f 400 m / g . The l a r g e a b s o r p t i o n f e a t u r e between 1000 and 1200 cm"" i s due t o t h e t r a n s v e r s e and l o n g i t u d i n a l l a t t i c e v i b r a t i o n s ( 8 ) . These f e a t u r e s are broadened due t o t h e amorphous s t r u c t u r e o f t h e a e r o s i l , and t h e small p a r t i c l e s i z e ( 9 ) . Other l a t t i c e v i b r a t i o n s occur a t 812 cm"" and 468 c m " . Overtones o f t h e fundamental l a t t i c e v i b r a t i o n s occur a t 1625, 1860 and 2004 c m " . The r e m a i n i n g f e a t u r e s i n t h e spectrum are due t o s u r f a c e g r o u p s . Surface h y d r o x y l groups g i v e r i s e t o an OH s t r e t c h i n g band a t 3744 c m " . The long t a i l on t h e OH s t r e t c h i n g band t h a t extends from 3700 t o 3400 c m " i s due t o a s m a l surface. Hydrogen bondin The spectrum o f t h e same a e r o s i l sample b e f o r e d r y i n g i s shown i n F i g u r e 1b. The o n l y n o t i c a b l e d i f f e r e n c e from t h e d r i e d sample i s a broader band due t o adsorbed w a t e r . The changes due t o t h e adsorbed water are more c l e a r l y seen i n t h e d i f f e r e n c e spectrum shown i n F i g u r e 2 a . I n F i g u r e 2 , p o s i t i v e f e a t u r e s i n d i c a t e a b s o r p t i o n s by t h e sample w i t h adsorbed water t h a t are not p r e s e n t i n the d r i e d sample and n e g a t i v e f e a t u r e s i n d i c a t e a b s o r p t i o n by t h e d r i e d sample t h a t are absent i n t h e wet sample. I n t h e OH s t r e t c h i n g r e g i o n a broad peak due t o hydrogen bonded water i s c e n t e r e d a t 3400 c m " . The adsorbed water i n f l u e n c e s t h e h y d r o x y l groups c a u s i n g a s l i g h t b l u e s h i f t , which r e s u l t s i n t h e p o s i t i v e peak a t 3756 c m " and a n e g a t i v e peak a t 3740 c m " . The OH bending mode o f t h e adsorbed water i s seen a t 1626 c m " . There are t h r e e h i n d e r e d r o t a t i o n s o f adsorbed water t h a t g i v e r i s e t o t h e t h r e e bands a t 600, 538 and 468 c m " . The a b s o r p t i o n band a t 764 c m " appears t o be due t o t h e OH bending mode o f adsorbed h y d r o x y l groups ( 1 0 ) . L a s t l y , t h e r e are changes i n t h e s i l i c a phonon s p e c t r a due t o water a d s o r p t i o n t h a t g i v e r i s e t o b o t h p o s i t i v e and n e g a t i v e f e a t u r e s i n t h e d i f f e r e n c e spectrum around 1000 c m " . The n e g a t i v e f e a t u r e a t 1008 c m " appears t o be due t o an Si-OH s t r e t c h i n g mode. T h i s f e a t u r e i s red s h i f t e d t o 926 c m " when water i s adsorbed. The band between 1050 and 1200 c m " o v e r l a p s w i t h t h e s i l i c a phonon band and may be due t o a change i n t h e b u l k phonon spectrum due t o s u r f a c e e f f e c t s ( H ) . F i g u r e 1c shows t h e spectrum o f a e r o s i l t h a t has been s l u r r i e d i n water and then d r i e d a t 100°C. T h i s t r e a t m e n t i n i t i a t e s g e l f o r m a t i o n , so t h a t t h e sample i s no l o n g e r a c h a i n o f s i l i c a p a r t i c l e s h e l d t o g e t h e r by e l e c t r o s t a t i c f o r c e s , but a porous network h e l d t o g e t h e r by s i l o x a n e l i n k a g e s . The most o b v i o u s f e a t u r e s i n t h i s spectrum are an i n c r e a s e i n t h e water a d s o r p t i o n f e a t u r e s a t 3400 cm" and 1632 c m " . I n a d d i t i o n , a band a t 976 c m " i s e v i d e n t , t h a t was much l e s s obvious i n t h e s p e c t r a o f t h e o t h e r two s i l i c a samples. T h i s f e a t u r e i s due t o s i l o x a n e b r i d g e s formed d u r i n g g e l formation (10,12). The spectrum i n F i g u r e 1d i s f o r a c r y s t a l l i n e form o f s i l i c a , s i l i c a l i t e (Union Carbide S-115, see r e f . 1 3 ) . The s t r u c t u r e i s comprised o f t w e l v e s i l i c a t e t r a h e d r a l i n k e d i n t o f i v e p e n t a s i l groups and one h e x a s i l g r o u p . T h i s b u i l d i n g b l o c k i s repeated 2

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In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

38.

BENZIGER ET A L .

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IR Photoacoustic Spectroscopy

,

Wavenumber

Fig.

1.

(d)

(cm ) H

I n f r a r e d spectra of s i l i c a s , a) A e r o s i l d r i e d a t 3 5 0 C , b) A e r o s i l as r e c e i v e d , c ) A e r o s i l s l u r r i e d i n water and d r i e d a t 1 0 0 C , d) S i l i c a l i t e as r e c e i v e d . e

e

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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CATALYST CHARACTERIZATION SCIENCE

Wavenumber

Fig.

2.

a) D i f f e r e n c e s p e c t r a between 1b - 1a (as r e c e i v e d a e r o s i l - d r i e d a e r o s i l ) , b) D i f f e r e n c e s p e c t r a f o r m e t h o x y l a t e d s i l i c a (methoxylated s i l i c a - d r i e d s i l i c a ) .

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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f o r m i n g a z i g - z a g channel s t r u c t u r e . T h i s m a t e r i a l i s d i s t i n c t from t h e o t h e r s i l i c a samples as t h e c r y s t a l l i n e s t r u c t u r e does not have s u r f a c e s t h a t are t e r m i n a t e d w i t h h y d r o x y l g r o u p s . T h i s i s e v i d e n t i n t h e i n f r a r e d spectrum which shows a broad band due t o adsorbed w a t e r , but no s i g n i f i c a n t f e a t u r e a t 3750 c m due t o s u r f a c e h y d r o x y l g r o u p s . The remainder o f t h e spectrum a l s o shows some significant differences. Being a c r y s t a l l i n e m a t e r i a l , t h e l o n g i t u ­ d i n a l and t r a n s v e r s e o p t i c a l modes are seen as w e l l d e f i n e d peaks a t 1228 and 1072 cm"" r e s p e c t i v e l y . The broad f e a t u r e between t h e two peaks appears t o be due t o t h e small p a r t i c l e s i z e which g i v e s r i s e t o a d i s t r i b u t i o n o f modes. The o t h e r two b u l k phonon modes a t 800 and 460 c m " are s l i g h t l y red s h i f t e d r e l a t i v e t o t h e amorphous silica. There i s a l s o an a b s o r p t i o n f e a t u r e a t 560 c m " , not observed w i t h t h e o t h e r samples, t h a t i s due t o t h e r i n g s t r u c t u r e formed by s i l i c a t e t r a h e d r a . T h i s i s a c h a r a c t e r i s t i c band observed i n z e o l i t e s where s i m i l a r s t r u c t u r e s e x i s t ( 1 4 ) . - 1

1

1

1

An a e r o s i l sample wa e f f e c t o f s u r f a c e c o m p o s i t i o n on t h e i n f r a r e d s p e c t r u m . The d i f ­ f e r e n c e spectrum between t h e m e t h o x y l a t e d s i l i c a and t h e d r i e d s i l i c a i s shown i n F i g u r e 2b. Comparing t h i s w i t h t h e d i f f e r e n c e spectrum f o r h y d r o x y l a t e d s i l i c a (2a) s e v e r a l changes are a p p a r e n t . F i r s t , t h e band due t o t h e h y d r o x y l s t r e t c h e s a t 3744 c m " i s d i m i ­ n i s h e d and r e p l a c e d by bands a t 2958 and 2856 c m " due t o t h e asym­ m e t r i c and symmetric CH s t r e t c h i n g modes o f t h e adsorbed methoxy. The CH bending modes are a l s o e v i d e n t a t 1464 and 1404 c m " , as i s t h e l o s s o f t h e OH bending mode a t 760 c m " . A band a t 1112 c m " superimposed on t h e changes o f t h e phonon band i s due t o t h e CO s t r e t c h o f t h e adsorbed methoxy, and a band a t 852 c m " appears t o be t h e r e s u l t o f t h e S i - 0 C H s t r e t c h . The r e m a i n i n g f e a t u r e s are due t o t h e adsorbed w a t e r , a broad OH s t r e t c h a t 3400 c m " , t h e OH bending mode a t 1626 c m " , and t h e t h r e e water l i b r a t i o n s a t 597, 546, and 478 c m " . As t h e replacement o f t h e h y d r o x y l groups was not q u a n t i t a t i v e , t h e l o s s f e a t u r e s a t 1008 c m " ' and t h e p o s i t i v e d e v i a t i o n s a t 926 c m " due t o t h e Si-OH s t r e t c h e s are s t i l l e v i ­ dent. There i s an a d d i t i o n a l l o s s a t 980 c m " , which i s due t o t h e Si-OH groups t h a t have been r e p l a c e d by s u r f a c e methoxys. The n a t u r e o f t h e two d i f f e r e n t f r e q u e n c i e s f o r Si-OH s t r e t c h i n g w i l l be addressed f u r t h e r below. L a s t l y , t h e changes i n t h e phonon band due t o adsorbed water remain almost the same between m e t h o x y l a t e d and hydroxylated s i l i c a . Alumina - Alumina forms a v a r i e t y o f o x i d e s and h y d r o x i d e s whose s t r u c t u r e s have been c h a r a c t e r i z e d by X - r a y d i f f r a c t i o n ( 1 6 ) . From t h e c a t a l y t i c v i e w p o i n t γ - a l u m i n a i s t h e most i m p o r t a n t . This i s a m e t a s t a b l e phase t h a t i s produced from s u c c e s s i v e d e h y d r a t i o n o f aluminum t r i h y d r o x i d e ( g i b b s i t e ) t o aluminum o x i d e h y d r o x i d e (boehmite) t o γ - a l u m i n a , or from d e h y d r a t i o n o f boehmite formed hydrothermally. γ - a l u m i n a i s c o n v e r t e d i n t o α - a l u m i n a (corundum) a t t e m p e r a t u r e s around 1000°C. The i n f r a r e d s p e c t r a f o r v a r i o u s aluminum o x i d e s and h y d r o x i d e s a r e shown i n F i g u r e 3. F i g u r e 3a i s α - a l u m i n a (Harshaw A13980), ground t o a f i n e powder w i t h a s u r f a c e area o f 4 m^/g. The a b s o r p ­ t i o n between 550 and 900 c m " i s due t o two o v e r l a p p i n g l a t t i c e modes, and t h e low f r e q u e n c y band a t 400 c m " i s due t o another s e t of l a t t i c e v i b r a t i o n s . These r e s u l t s are s i m i l a r t o those o b t a i n e d by r e f l e c t i o n measurements, except t h a t t h e powder does not show as 1

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Gibbsite

40O~êû

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Wavenumber Fig.

3.

I n f r a r e d s p e c t r a o f aluminum o x i d e s and h y d r o x i d e s . a) α - A 1 0 ; 4 m / g , b) γ - A 1 0 ; 234 » V g , c ) ΑΊΟΟΗ ( B o e h m i t e ) ; 325 m / g , d) A 1 ( 0 H ) (gibbsite). 2

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much band s p l i t t i n g as was observed by r e f l e c t i o n from s i n g l e c r y s t a l s (17). There i s a l s o a v e r y weak and broad band a t 3400 cm"" due t o water adsorbed on t h e alumina s u r f a c e . As t h e s u r f a c e area i s low t h i s band i s not expected t o be v e r y s t r o n g . F i g u r e 3d i s t h e i n f r a r e d spectrum o f g i b b s i t e ( A l d r i c h 2 3 , 9 1 8 - 6 ) , which i s t h e p r e c u r s o r o f most aluminum o x i d e s . The spectrum shows a broad band due t o OH s t r e t c h e s a t 3400 c m " , a f e a t u r e c e n t e r e d around 1510 c m " due t o c a r b o n a t e , and a s h o u l d e r a t 1632 c m " due t o OH bending o f w a t e r . The A1-0H o r A l - 0 s t r e t c h i n g modes g i v e r i s e t o a broad band t h a t begins t o absorb s t r o n g l y a t 1250 c m " and extends t o below 400 c m " . There are s m a l l e r a b s o r p t i o n bands a t 668, 634, and 556 c m " superimposed on the broad A l - 0 band which appear t o be due t o bending modes o f c a r ­ b o n a t e s . Since g i b b s i t e i s produced by p r e c i p i t a t i o n from a b a s i c s o l u t i o n b u f f e r e d w i t h sodium c a r b o n a t e (18) t h e h i g h water and c a r ­ bonate c o n t e n t s observe Dehydration of g i b b s i t boehmite (aluminum o x i d e m o n o - h y d r a t e ) . An i n f r a r e d spectrum o f boehmite ( K a i s e r s u b s t r a t e grade alumina) i s shown i n F i g u r e 3c. When t h e g i b b s i t e i s dehydrated a s t r u c t u r a l c o l l a p s e occurs w i t h a l a r g e i n c r e a s e i n s u r f a c e a r e a . The boehmite sample has a nominal s u r f a c e area o f 325 m / g . The i n f r a r e d spectrum o f t h e boehmite shows d i s t i n c t s t r u c t u r e i n t h e OH s t r e t c h i n g r e g i o n , w i t h two peaks l o c a t e d a t 3090 and 3320 c m " . There are t h r e e f e a t u r e s a t 1648, 1516 and 1392 c m " t h a t a r e due t o adsorbed water and c a r b o n a t e , which are removed upon h e a t i n g t h e boehmite t o 350°C i n hydrogen. The l a t t i c e v i b r a t i o n s b e g i n t o absorb s t r o n g l y below 1200 c m " . An a d d i t i o n a l f e a t u r e a t 1072 c m " , c h a r a c t e r i s t i c o f boehmite, i s t h e r e s u l t o f t h e A1-0H s t r e t c h . Both t h e OH s t r e t c h e s and t h e A1-0H s t r e t c h have been p r e v i o u s l y i d e n t i f i e d by t r a n s m i s s i o n s t u d i e s o f boehmite s i n g l e c r y s t a l s (1?). F u r t h e r d e h y d r a t i o n o f boehmite a t 600 C produces γ - a l u m i n a , whose spectrum i s shown i n F i g u r e 3b. There i s a l o s s i n s u r f a c e area i n g o i n g from boehmite t o γ - a l u m i n a . The sample shown here has a s u r f a c e area o f 234 m / g ( t h i s sample was o b t a i n e d from Harshaw #A23945; t h e c a l c i n e d K a i s e r s u b s t r a t e gave an i d e n t i c a l i n f r a r e d s p e c t r u m ) . The γ - a l u m i n a sample shows two major d i f f e r e n c e s from a a l u m i n a . F i r s t , t h e r e i s a more i n t e n s e broad a b s o r p t i o n band a t 3400 c m ' due t o adsorbed water on t h e γ - a l u m i n a . Second, t h e γ alumina does not show s p l i t t i n g o f t h e phonon bands between 400 and 500 c m " as was observed f o r α - a l u m i n a . The γ - a l u m i n a i s a more amorphous s t r u c t u r e and has much s m a l l e r c r y s t a l l i t e s so t h e phonon band i s b r o a d e r . The γ - a l u m i n a a l s o shows t h r e e f e a t u r e s a t 1648, 1516 and 1392 c m " due t o adsorbed water and c a r b o n a t e . The f e a t u r e s due t o adsorbed water and c a r b o n a t e s observed on t h e boehmite and γ - a l u m i n a deserve f u r t h e r a t t e n t i o n as t h e y d i f f e r from r e s u l t s p u b l i s h e d by p r e v i o u s i n v e s t i g a t o r s . F i g u r e 4 shows a s e r i e s o f d i f f e r e n c e s p e c t r a f o r a d s o r p t i o n on γ - a l u m i n a . Spectra were taken a f t e r d r y i n g t h e γ - a l u m i n a a t 350°C, c o o l i n g t o room tem­ p e r a t u r e and c a r r y i n g out room t e m p e r a t u r e a d s o r p t i o n . The s p e c t r a a r e t h e d i f f e r e n c e o f t h e sample b e f o r e and a f t e r a d s o r p t i o n . Spectrum 4e i s t h e spectrum f o r t h e as r e c e i v e d alumina d i f f e r e n c e d w i t h t h e d r i e d a l u m i n a . The p o s i t i v e band a t 3400 c m " i s due t o adsorbed w a t e r , and t h e s m a l l n e g a t i v e f e a t u r e a t 3740 c m " i s due t o i s o l a t e d h y d r o x y l s on t h e d r i e d s u r f a c e . Besides t h e t h r e e 1

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D i f f e r e n c e s p e c t r a o f adsorbed s p e c i e s on γ - a l u m i n a ( a i r e x p o s u r e ) , a) C 0 ; b) H 0 ; c ) H 0 f o l l o w e d by C 0 ; d ) Methanol r e a c t e d w i t h t h e alumina a t 350°C.; e) As r e c e i v e d γ - a l u m i n a ( a i r e x p o s u r e ) . 2

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f e a t u r e s p r e v i o u s l y noted a t 1644, 1516 and 1392 c m " , t h e r e are f e a t u r e s a t 1074 c m " due t o an A1-0H2 s t r e t c h o f c o o r d i n a t i v e l y bound w a t e r , and s m a l l f e a t u r e s a t 748 and 628 c m " . Spectrum 4a i s f o r CO2 a d s o r p t i o n . The gas phase 0 0 shows bands a t 2346 and 668 c m " ; bands a t 1654, 1436 and 1228 c m " are due t o adsorbed b i c a r ­ bonate s p e c i e s ( 2 0 ) . There i s a weak band a t 3610 c m " due t o t h e OH s t r e t c h i n t h e b i c a r b o n a t e t h a t i s accompanied by s m a l l n e g a t i v e f e a t u r e s a t 3740 c m " c o r r e s p o n d i n g t o t h e h y d r o x y l t h a t r e a c t e d t o form t h e b i c a r b o n a t e . A weak band a t 1050 c m " i s due t o t h e C-0 s t r e t c h i n the b i c a r b o n a t e . The b i c a r b o n a t e bands disappear when t h e sample i s exposed t o w a t e r . A f t e r exposure o f t h e γ - a l u m i n a sample t o water s e v e r a l f e a t u r e s are apparent i n t h e i n f r a r e d spectrum ( F i g u r e 4b) t h a t were a l s o apparent i n the spectrum o f t h e as r e c e i v e d sample. An OH bending mode f o r adsorbed water appears a t 1644 c m " , as w e l l as an A1-0H2 s t r e t c h i n g mode a t 1058 c m " and a n o t h e r f e a t u r e a t 628 c m " t h a t i s p r o b a b l y due t o a f r u s t r a t e d r o t a t i o n o f t h e adsorbe water i s exposed t o CO2 a d d i t i o n a l f e a t u r e s appear a t 1536 and 1384 c m " , as shown i n F i g u r e 4c. A small f e a t u r e a l s o s t a r t s t o grow a t 748 c m " . These r e s u l t s suggest t h a t a monodentate c a r b o n a t e l i g a n d i s formed when CO2 adsorbs on the alumina s u r f a c e w i t h water adsorbed. The f e a t u r e s a t 1536 and 1384 c m " are due t o t h e asym­ m e t r i c and symmetric s t r e t c h e s r e s p e c t i v e l y . The C-0 s t r e t c h i n g mode t h a t occurs around 1050 c m " c o i n c i d e s w i t h t h e A1-0H2 s t r e t c h . The f e a t u r e a t 748 c m " corresponds t o t h e o u t - o f - p l a n e d e f o r m a t i o n o f t h e c a r b o n a t e ( 2 ) . A f e a t u r e due t o t h e p l a n a r d e f o r m a t i o n appears as a h i g h frequency s h o u l d e r on t h e water l i b r a t i o n a t 628 cm" . A s h o u l d e r a l s o forms on t h e water band a t 3620 c m " , which i s a l s o v i s i b l e i n t h e as r e c e i v e d a l u m i n a . T h i s i s t h e p o s i t i o n f o r t h e OH s t r e t c h i n a b i c a r b o n a t e . However t h e r e i s no evidence f o r t h e OH bending mode a t 1230 c m " , s u g g e s t i n g t h a t t h e c a r b o n a t e i n t e r a c t s w i t h adsorbed w a t e r , but does not r e a c t t o form a b i c a r ­ bonate s p e c i e s . 1

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The c a r b o n a t e can a l s o be compared w i t h adsorbed f o r m a t e spe­ c i e s prepared by r e a c t i n g methanol w i t h t h e alumina s u r f a c e a t 350°C (22). The spectrum f o r adsorbed f o r m a t e , F i g u r e 4 d , shows t h e asym­ m e t r i c c a r b o x y l a t e s t r e t c h e s a t 1565 and 1440 c m " r e s p e c t i v e l y , t h e CH s t r e t c h a t 2832 c m " , and the CH bending mode a t 1505 c m " . The A1-0C s t r e t c h i n g mode i s seen a t 1060 c m " , and t h e o u t - o f - p l a n e d e f o r m a t i o n a t 750 c m " . The s i g n a l t o n o i s e r a t i o i n t h e low f r e ­ quency end o f t h e spectrum i s i n s u f f i c i e n t t o see t h e p l a n a r d e f o r ­ m a t i o n , which s h o u l d occur around 630 c m " . I t s h o u l d be noted t h a t t h e c a r b o n a t e and f o r m a t e s p e c i e s are v e r y s i m i l a r , t h e main d i s t i n c t i o n b e i n g t h e v i b r a t i o n s a s s o c i a t e d w i t h t h e CH bond. 1

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Discussion The i n f r a r e d p h o t o a c o u s t i c s p e c t r a p r e s e n t e d here complement and extend p r e v i o u s r e s u l t s from t r a n s m i s s i o n i n f r a r e d s t u d i e s . As an extension of previous studies of s i l i c a the photoacoustic r e s u l t s p r e s e n t e d here have i d e n t i f i e d f e a t u r e s i n t h e i n f r a r e d s p e c t r a t h a t c o i n c i d e w i t h b u l k phonon modes between 1000 and 1200 c m " and below 500 c m " . The p h o t o a c o u s t i c s p e c t r a o f water adsorbed on a e r o s i l 1

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w i t h h y d r o x y l a t e d and methoxylated s u r f a c e s shown i n F i g u r e 2 i n d i ­ c a t e water a d s o r p t i o n can p e r t u r b t h e phonon spectrum o f t h e s o l i d , and t h a t t h e r e a r e d i f f e r e n c e s i n t h e s u r f a c e h y d r o x y l g r o u p s . Adsorbed water causes a red s h i f t i n t h e Si-OH s t r e t c h from 1008 t o 926 c m " . The spectrum f o r methoxylated s i l i c a i n d i c a t e s t h a t r e p l a ­ cement o f h y d r o x y l by methoxy1 groups causes a decrease i n i n t e n s i t y a t 980 c m " . These r e s u l t s suggest t h e r e may be two d i f f e r e n t s u r ­ face h y d r o x y l g r o u p s . An a l t e r n a t i v e e x p l a n a t i o n i s t h a t t h e s u r f a c e groups a r e p e r ­ t u r b e d by p a r t i c l e - p a r t i c l e i n t e r a c t i o n s . The p a r t i c l e s are h e l d t o g e t h e r by e l e c t r o s t a t i c f o r c e s which g i v e r i s e t o s i g n i f i c a n t e l e c t r i c f i e l d s a t t h e p o i n t s o f c o n t a c t ( 9 , 2 3 ) . The e l e c t r i c f i e l d s can p e r t u r b t h e s u r f a c e bonds and hence t h e v i b r a t i o n a l f r e q u e n c i e s . Water a d s o r p t i o n p r o b a b l y occurs by condensation near t h e p o i n t s o f c o n t a c t , whereas m e t h o x y l a t i o n w i l l occur more u n i f o r m l y over t h e s u r f a c e . Removal o f wate Si-OH s t r e t c h a t 1008 c m " p e r t u r b e d h y d r o x y l s w i t h an Si-OH s t r e t c h a t 976 c m " . The adsorbed water i n t e r a c t s w i t h t h e h y d r o x y l groups on t h e s i l i c a s u r f a c e r e s u l t i n g i n a red s h i f t i n t h e Si-OH band t o 926 c m " . Besides a f f e c t i n g t h e s u r f a c e p r o p e r t i e s o f t h e s i l i c a t h e adsorbed water caused changes i n t h e b u l k phonon modes. These changes are p r o b a b l y due t o a s h i e l d i n g e f f e c t o f t h e e l e c t r o s t a t i c f o r c e s between p a r ­ ticles. For small p a r t i c l e s t h i s s h i e l d i n g i s expected t o produce significant effects. A n o t e w o r t h y f e a t u r e o f t h e p h o t o a c o u s t i c s p e c t r a shown i n F i g u r e 2 i s t h e presence o f water l i b r a t i o n s . These are f r u s t r a t e d r o t a t i o n s and have been observed f o r i c e (24) by i n f r a r e d s p e c t r o s c o p y , as w e l l as f o r water adsorbed on Pt and Ag s u r f a c e s by e l e c t r o n energy l o s s s p e c t r o s c o p y ( 2 5 - 2 7 ) . The t h r e e l i b r a t i o n modes have been a s s o c i a t e d w i t h t h e bands a t 600, 538 and 468 c m " ' t h i s s e t o f peaks occurs f o r water adsorbed on b o t h t h e h y d r o x y l a t e d and methoxylated s i l i c a . The r e s u l t s f o r the m e t h o x y l a t i o n o f s i l i c a are a u s e f u l e x t e n ­ s i o n o f t h e s t u d i e s c a r r i e d o u t by Morrow (15,28) i n which t h e CH s t r e t c h i n g and bending modes had been o b s e r v e d . The s p e c t r a recorded here are a t h i g h e r r e s o l u t i o n and the band s p l i t t i n g o f t h e asymmetric s t r e t c h ( i n d i c a t i n g t h a t i t has C symmetry) i s more p r o ­ nounced. The spectrum a l s o shows t h e C-0 s t r e t c h a t 1112 c m " and t h e S i - 0 s t r e t c h a t 852 c m " t h a t had not p r e v i o u s l y been o b s e r v e d . The C-0 s t r e t c h i n t e n s i t y i s d i m i n i s h e d r e l a t i v e t o t h e C-H f e a t u r e s because i t i s superimposed on t h e phonon band where t h e r e i s a change i n t h e dominant mode o f s i g n a l g e n e r a t i o n ( 6 ) . The M-0 band f o r t h e methoxy bond t o t h e s u r f a c e i s a t a h i g h e r frequency than has been observed on t r a n s i t i o n metals such as Ni and Cu ( 2 9 , 3 0 ) . T h i s i s expected as t h e S i - 0 bond i s a much s t r o n g e r bond than t h a t found on t r a n s i t i o n m e t a l s . The r e s u l t s o b t a i n e d f o r t h e v a r i o u s aluminum o x i d e s and h y d r o x i d e s i n d i c a t e t h a t i n f r a r e d p h o t o a c o u s t i c spectroscopy may be u s e f u l i n c h a r a c t e r i z i n g s t r u c t u r a l t r a n s f o r m a t i o n s i n these spe­ c i e s . Very c l e a r d i f f e r e n c e s between α - a l u m i n a and γ - a l u m i n a were noted i n t h e r e g i o n o f t h e l a t t i c e v i b r a t i o n s . The monohydrate, boehmite, showed a v e r y d i s t i n c t A1-0H s t r e t c h i n g f e a t u r e a t 1070 cm" as w e l l as c h a r a c t e r i s t i c s t r u c t u r e i n t h e OH s t r e t c h i n g 1

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r e g i o n . The g i b b s i t e sample examined showed v e r y l i t t l e s t r u c t u r e i n t h e r e g i o n above 3000 c m " , i n c o n t r a s t t o r e s u l t s o b t a i n e d by F r e d r i c k s o n (19), though i t was e a s i l y d i s t i n g u i s h e d from t h e o t h e r samples examined. T h i s d i f f e r e n c e i s p r o b a b l y t h e r e s u l t o f sample p r e p a r a t i o n and i m p u r i t i e s . F r e d r i c k s o n prepared l a r g e pure c r y s t a l s of g i b b s i t e . The commercial sample used i n t h e p r e s e n t s t u d y was p r o b a b l y prepared by the Bayer process (18) and c o n t a i n e d excess water and c a r b o n a t e . A number o f i n v e s t i g a t o r s have examined C0 a d s o r p t i o n on a l u mina (20,31-37). R e s u l t s presented here agree w i t h t h e f i n d i n g s o f Baumgarten and Zachos who c l e a r l y showed b i c a r b o n a t e f o r m a t i o n on an alumina sample d r i e d a t 400 C ( 2 0 ) . Exposure t o water causes t h e b i c a r b o n a t e t o decompose w i t h t h e C0 b e i n g r e p l a c e d q u a n t i t a t i v e l y with water. However, carbonate f o r m a t i o n was observed on t h e s u r f a c e w i t h adsorbed w a t e r . The band p o s i t i o n s suggest t h a t t h e C0 adsorbs as a monodentat duces c o o r d i n a t i v e l y u n s a t u r a t e t u r a t e d a n i o n s . Depending on t h e d e h y d r a t i o n t e m p e r a t u r e t h e r e are v a r y i n g c o n c e n t r a t i o n s o f c o o r d i n a t i v e l y u n s a t u r a t e d ions and h y d r o x y l groups ( 3 8 ) . The u n s a t u r a t e d c a t i o n s behave as Lewis a c i d s and adsorb e l e c t r o n d o n o r s , such as C0 and H 0 . A f t e r C0 a d s o r p t i o n a t an Al c a t i o n the carbon can undergo n u c l e o p h i l i c a t t a c k by e i t h e r an oxygen anion or a h y d r o x y l a n i o n . At h i g h degrees o f d e h y d r a t i o n t h e a t t a c k by the oxygen anion i s the o n l y p o s s i b l e r e a c t i o n . T h i s produces b r i d g e d c a r b o n a t e s p e c i e s , w i t h bands at 1800 and 1200 c m " . T h i s has been observed by P e r i (37) and by Parkyns ( 3 1 ) . When t h e r e are h y d r o x y l groups on t h e s u r f a c e t h e p r e f e r r e d r e a c t i o n p a t h appears t o be t h e f o r m a t i o n o f a b i c a r bonate. The d i f f e r e n c e s p e c t r a do not show any w a t e r , but t h e s p e c t r a f o r t h e alumina d r i e d a t 350°C shows t h a t a p p r o x i m a t e l y 45 p e r c e n t o f t h e i n t e n s i t y i n t h e OH band a t 3400 c m " remains. 1

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Conclusion I n f r a r e d p h o t o a c o u s t i c s p e c t r o s c o p y has s i g n i f i c a n t p o t e n t i a l t o be u s e f u l as an a n a l y t i c t e c h n i q u e f o r c h a r a c t e r i z i n g c a t a l y s t samples. I t o f f e r s ease o f sample p r e p a r a t i o n and has a h i g h dynamic range due t o t h e small thermal d i f f u s i o n l e n g t h s o f h i g h s u r f a c e area materials. Commerical i n s t r u m e n t s need t o be a c o u s t i c a l l y m o d i f i e d t o o b t a i n h i g h q u a l i t y i n f r a r e d p h o t o a c o u s t i c s p e c t r a . These modif i c a t i o n s are not d i f f i c u l t and new s p e c t r o m e t e r s w i l l become a v a i l a b l e t h a t w i l l be good f o r p h o t o a c o u s t i c s p e c t r o s c o p y . The r e s u l t s presented here f o r s i l i c a s and aluminas i l l u s t r a t e t h a t there i s a wealth of s t r u c t u r a l information i n the i n f r a r e d s p e c t r a t h a t has not p r e v i o u s l y been r e c o g n i z e d . I n p a r t i c u l a r , i t was found t h a t adsorbed water a f f e c t s t h e l a t t i c e v i b r a t i o n s o f s i l i c a , and t h a t p a r t i c l e - p a r t i c l e i n t e r a c t i o n s a f f e c t t h e v i b r a t i o n s o f s u r f a c e s p e c i e s . I n t h e case o f a l u m i n a , i t was found that a l u m i num o x i d e s and h y d r o x i d e s c o u l d be d i s t i n g u i s h e d by t h e i r i n f r a r e d s p e c t r a . The absence o f s p e c t r a l windows f o r p h o t o a c o u s t i c s p e c t r o s c o p y a l l o w e d more complete band i d e n t i f i c a t i o n o f adsorbed s u r f a c e s p e c i e s , making d i s t i n c t i o n s between d i f f e r e n t s t r u c t u r e s easier. The a b i l i t y t o p e r f o r m s t r u c t u r a l analyses by i n f r a r e d spectroscopy c l e a r l y i n d i c a t e s the u t i l i t y of photoacoustic spectroscopy.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Ac k η οw1edgmeηts The authors thank the National Science Foundation (CPE-8217364) f o r support of t h i s work. One of us (SJM) thanks Mobil Research and Development Corp. for their f i n a n c i a l support. Literature Cited 1.

V.

Hair, M. L. In "Vibrational Spectroscopies for Adsorbed Species"; B e l l , A. T.; Hair, M. L., Eds.; American Chemical Society: Washington, D.C., 1980; pp. 1-11. 2. L i t t l e , L. H. "Infrared Spectra of Adsorbed Species"; Academic: New York, 1966. 3. Knözinger, H. In "The Hydrogen Bond"; Schuster, P.; Zundel, G.; Sandorfy, C.; Eds.; North Holland: New York, 1976; pp. 1263-1364. 4. Rosencwaig, Α.; Gersho 5. Yasa, Ζ. Α.; Jackson, W. B.; Amer, N. M. Appl. Optics 1982, 21, 21. 6. McGovern, S. J.; Royce, B. S. H.; Benziger, J . B. J . Appl. Phys. 1985, 57, 1710. 7. McGovern, S. J . ; Royce, B. S. H.; Benziger, J . B., Applications of Surface Science 1984, 18, 401. 8. Scott, J . F.; Porto, S. P. S. Phys. Rev. 1967, 161, 903. 9. Clippe, P.; Evrand, R.; Lucas, A. A. Phys. Rev. Β 1976, 14, 1715. 10. Boccuzzi, F.; Coluccia, S.; G h i o t t i , G.; Morterra, C.; Zecchina, A. J. Phys. Chem. 1978, 82, 1298. 11. Zhizhin, G. N.; Vinogradov, Ε. Α.; Maskalova, Μ. Α.; Yakovlev, A. Appl. Spectrosc. Rev. 1982, 18, 171. 12. Kinney, J . B.; Staley, R. H. J. Phys. Chem. 1983, 87, 3735. 13. Flanigen, Ε. M.; Bennett, J . M.; Grose, R. W.; Cohen, J . P.; Patton, R. L.; Kirchner, R. M.; Smith, J . V. Nature 1978, 271, 512. 14. Flanigen, Ε. M. In "Zeolite Chemistry and Catalysis"; Rabo, J . Α.; Ed.; American Chemical Society: Washington, D.C. 1976; pp. 80-117. 15. Morrow, B. A. J. Chem. Soc. Faraday I. 1974, 70, 1527. 16. Wefers, K.; B e l l , G. M. "Oxides and Hydroxides of Aluminum"; Technical Paper No. 19; Alcoa Research Laboratories: East St. Louis, 1972. 17. Baker, A. S. Phys. Rev. 1963, 132, 1474. 18. MacZura, G.; Goodboy, K. P.; Koenig, J . J . In "Encyclopedia of Chemical Technology"; Wiley: New York, 1978; V o l . 2, pp. 218-244. 19. Fredrickson, L. D. J r . Analytical Chemistry 1954, 26, 1883. 20. Baumgarten, E.; Zachos, A. Spectrochim. Acta 1981, 37A, 93. 21. Borello, E.; Dalla Gatta, G.; Fubini, B.; Morterra, C.; Venturello, G. J . Catalysis 1974, 35, 1. 22. Greenler, R. G. J. Chem. Phys. 1962, 37, 2094. 23. U l r i c h , G. D. Chem. Eng. News 1984, 62(32), 22. 24. Bertie, J . E.; Whalley, E. J . Chem. Phys. 1964, 40, 1637. 25. Sexton, B. A. Surface Science 1980, 94, 435.

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26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38.

B E N Z I G E R E T AL.

IR Photoacoustic Spectroscopy

463

Ibach, H.; Lehwald, S. Surface Science 1980, 91, 187. Stuve, E. M.; Madix, R. J . ; Sexton, B. A. Surface Science 1981, 111, 11. Morrow, Β. Α.; Thomson, L. W.; Wetmore, R. W. J . Catalysis 1973, 28, 332. Demuth, J . E.; Ibach, H. Chem. Phys. Lett. 1979, 60, 395. Sexton, B. A. Surface Science 1980, 88, 299. Parkyns, N. D. J. Chem. Soc. (A) 1969, 1910. Gregg, S. J . ; Ramsay, J . D. F. J. Phys. Chem. 1969, 73, 1243. Morterra, C.; Coluccia, S.; G h i o t t i , G.; Zecchina, A. Z. Phys. Chem. 1977, 104, 275. P e r i , J . B. J. Phys. Chem. 1966, 70, 3168. L i t t l e , L. H.; Amberg, C. H. Canad. J . Chem. 1962, 40, 1997. Amenomiya, Y.; Morikawa, Y.; P l e i z i e r , G. J . Catalysis 1977, 46, 431. P e r i , J . B. J . Phys P e r i , J . B. J . Phys

RECEIVED April 24,1985

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

39 Carbon Monoxide Oxidation on Platinum: Coverage Dependence of the Product Internal Energy D. A.Mantell1,K. Kunimori2, S. B. Ryali3, and Gary L. Haller Department of Chemical Engineering Time resolved FTIR emission spectroscopy is used to detect vibrationally excited gas phase CO from catalyzed CO oxidation on a Pt f o i l . A continuous O free jet and a pulsed CO jet (= 200 μsec FWHM) supply the reactants to the surface. The infrared emission of the CO product is analyzed with 30 μ s e c time resolution using the time multiplexing capabilities of a commercial Fourier trans­ form spectrometer. At low CO pressures the total signal parallels the time dependent flux to the surface with only minimal changes in the infrared spectra. At high CO pressures the reaction can be shut off as the oxygen on the surface is depleted. These IR spectra show large changes in internal energy of the product CO . 2

2

2

2

The o x i d a t i o n of CO on group V I I I metals i s one of the most studied metal catalyzed reactions and many aspects of i t are w e l l under stood 0 ^ 2 ) . Reaction occurs by a Langmuir-Hinshelwood mechan­ ism between chemisorbed m o l e c u l a r CO and atomic oxygen. However, the k i n e t i c s are complex because high coverage of CO i n h i b i t s oxygen d i s s o c i a t i o n but CO chemisorbs on top of an oxygen covered surface. At low surface concentration of both species, the reaction i s l i m ­ ited by oxygen chemisorption and the rate i s d i r e c t l y proportional to oxygen p a r t i a l pressure. With high coverage of adsorbed oxygen atoms and low CO coverage, the reaction i s d i r e c t l y proportional to CO p a r t i a l pressure. At r e l a t i v e l y high coverage of both species, the case for most p r a c t i c a l applications of c a t a l y t i c oxidation of CO, the r a t e i s about f i r s t order i n oxygen p r e s s u r e and i n v e r s e f i r s t order i n CO. 1Current address: Physics, B-268, National Bureau of Standards, U.S. Department of Commerce, Washington, DC 20234 2Current address: Institute of Materials Science, University of Tsukuba, Sakura-mura, Ibaraki 305, Japan 3Current address: Aerodyne Research, Inc., Billerica,MA01821

0097-6156/85/0288-0464$06.00/0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

39.

M A N T E L L ET A L .

Carbon Monoxide Oxidation on Platinum

465

In the case of CO o x i d a t i o n on P t , there have been performed several experiments which e l u c i d a t e the dynamics of the r e a c t i o n . Because the reaction between adsorbed CO and oxygen atoms is a c t i ­ vated and CO2 has only a van der Waals i n t e r a c t i o n with the surface, a part of the energy acquired to form the activated complex may be d i s t r i b u t e d among degrees of freedom of the product m o l e c u l e . In the l i m i t i n g case where no energy i s exchanged w i t h the s u r f a c e d u r i n g the d e s o r p t i o n event and a complete a n a l y s i s of i t s p a r t i ­ t i o n i n g between t r a n s l a t i o n , r o t a t i o n , v i b r a t i o n and e l e c t r o n i c energies in the product is a v a i l a b l e , i t is in p r i n c i p l e possible to construct a p o t e n t i a l energy surface which would describe the molec­ u l a r s t r u c t u r e of the a c t i v a t e d complex and the dynamics of the d e s o r p t i o n e v e n t . While t h i s i d e a l i s not yet p r a c t i c a l , we have made s i g n i f i c a n t steps toward gaining t h i s information for CO oxida­ t i o n on Pt. Several investigations have found that COJ2 molecules l e a v e the surface wit peaked i n the d i r e c t i o t r a n s i at ional energy.(3-5). Time-of-f 1 ight analysis of CO oxidation on a p o l y c r y s t a l 1 ine Pt f o i l a l s o demonstrates t h a t the p:>T,I?W 1 leaves the surface with k i n e t i c energy in excess of that expected :'f the m o l e c u l e were i n e q u i l i b r i u m w i t h the surface(6,). Infrared emission experiments show that the CO2 product of CO oxidation i s , moreover, v i b r a t i o n a l l y ( 7 , 8 ) and r o t a t i o n a l ly(7.) h o t t e r than the s u r f a c e . In v e r y recent experiments i t has f u r t h e r been observed that the angular d i s t r i b u t i o n s (transi at ional energy) (9) and v i b r a ­ t i o n a l energy of the product CO2 (15) are s t r o n g f u n c t i o n s of s u r ­ face coverage. We w i l l c o n f i r m here t h a t t h i s i s the case f o r r o t a t i o n a l and v i b r a t i o n a l d i s t r i b u t i o n s . Because the a c t i v a t i o n energy f o r the surface r e a c t i o n between adsorbed CO and adsorbed oxygen atoms depends on the coverage(4), t h i s i s not unexpected. Q u a l i t a t i v e l y one might anticipate that the product C0|2 would have less excess energy at high oxygen coverage because there is l e s s of a b a r r i e r to reaction and therefore the activated complex would have less energy to dissipate. However, the matter is somewhat compli­ cated by the f a c t that as the coverage i n c r e a s e s , the heat of a d s o r p t i o n of both CO and ©2 d e c r e a s e ( 4 , 1 0 ) , but s u f f i c e i t to say that at low coverage the activated complex is about 1 0 0 kJ above COj i n the gas phase and that t h i s v a l u e i n c r e a s e s as coverage increases. In experiments using a steady-state mixed CO-O^ molecular beam reacting on a Pt f o i l , we observed excess energy in a l l vibrations and rotation and large changes i n the amount of excess energy i n the symmetric stretch of desorbed (X>2 as a function of surface tempera­ ture ( . 1 1 . 1 2 ) . There was a strong s u g g e s t i o n that the l a t t e r was p r i m a r i l y due to changes i n surface coverage which accompany a change i n surface temperature rather than a d i r e c t coupling between the i n t e r n a l energy states and the surface temperature. In order to test this hypothesis, we i n i t a t e d pulsed molecular beam experiments with time r e s o l v e d - i n f r a r e d emission s p e c t r o s c o p y of the desorbed product. A steady-state beam of Οχ was incident on a Pt f o i l and a second pulsed beam was simultaneously directed onto the f o i l . The CC p u l s e s were 2 0 0 με halfwidth at h a l f maximum height with a 2 0 0 0 με pause between p u l s e s ( 1 2 ) . The 2 0 0 0 μβ pause is long enough such

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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CATALYST C H A R A C T E R I Z A T I O N SCIENCE

that the surface coverage of oxygen c o u l d r e c o v e r to i t s steadys t a t e v a l u e at the g i v e n r e a c t i o n temperature and the p a r t i c u l a r f l u x used i n the oxygen beam. The number of CO m o l e c u l e s i n the pulse could be e a s i l y varyed by changing the stagnation pressure i n the pulsed nozzle source. At low pressures the surface coverage of oxygen did not vary much from i t s steady state value while at high pressures we could e f f e c t i v e l y perform a t i t r a t i o n of the surface oxygen as the p u l s e passed o v e r the s u r f a c e . The l e a d i n g edge of the pulse reacted at the r e l a t i v e l y high oxygen coverage but, since r e a c t i o n removed oxygen f a s t e r than i t c o u l d be r e p l e n i s h e d by a d s o r p t i o n ( i n p a r t due to CO i n h i b i t i o n ) , toward the end of the p u l s e r e a c t i o n o c c u r r e d at v e r y low oxygen coverage. The p u l s e d experiment allowed us to vary the coverage isothermal l y over a much larger range than would have been p r a c t i c a l by varying the f l u x of a steady-state source. Time-resolved infrare COg pulse e f f e c t i v e l y provides a coverage-resolved picture of the v i b r a t i o n a l and r o t a t i o n a l energy d i s t r i b u t i o n in the product CO^. The time-resolved spectroscopy is accomplied using a Fourier trans­ form spectrometer ar>.f! \cF

I I t per domain

FERROMAGNETISM (Q)

per cluster

SUPERPARAMAGNETISM

Ο.Θ

Η

/LL«IO per Ion PARAMAGNETISM

40 H/T

60

80

100

120

140

(OERST./DEG.)

F i g u r e 2. P l o t o f r e l a t i v e m a g n e t i z a t i o n , σ/σ , a s a f u n c t i o n of H/T. (a) A paramagnetic system i s c h a r a c t e r i z e d by an e f f e c t i v e magnetic moment, w i t h a Bohr Magneton number M.0 p e r i o n , and by t h e absence o f h y s t e r e s i s . Paramagnetic s a t u r a t i o n o c c u r s a t v e r y h i g h "H/T" ^ 10^ Oe K" . (b) L a n g e v i n c u r v e f o r S.P. c l u s t e r s , (c) Part of a ferromagnetic h y s t e r e s i s curve. 3

1

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

42.

MULAY AND PANNAPARAYIL

Magnetic and Môssbauer Characterization

paramagnetism and f e r r o m a g n e t i s m . F o r pure w i t h o u t m e c h a n i c a l s t r a i n , t h e H can be as Hence, one c a n a p p l y t h e c l a s s i c a l L a n g e v i n f o r t h e n o n - i n t e r a c t i n g paramagnetic s p i n s , ( i d e a l ) S.P. c l u s t e r s . Thus,

ferromagnetic m a t e r i a l s , s m a l l as 0.5 O e r s t e d s . function, f i r s t derived t o the non-interacting

c

a/a

s

-

501

Coth (μ H/kT) - (kT/μ (H) c c

Here, y s t a n d s f o r t h e ( g i a n t ) magnetic moment o f t h e c l u s t e r which r e p l a c e s t h e "μ" f o r s i n g l e , i s o l a t e d paramagnetic s p i n s ; k i s t h e Boltzmann c o n s t a n t . In t h e above e q u a t i o n , a l l q u a n t i t i e s except Pc & n be measured. The μ can be d e r i v e d f o r ( i d e a l ) S.P. c l u s t e r s by a c u r v e - f i t t i n g p r o c e d u r e (Cf r e f . 3 ) . The g i s t o f o b t a i n i n g t h e average volume ν o f a S.P. c l u s t e r l i e s i n t h e b a s i c p h y s i c s d e f i n i t i o n of o , which μ /ν. From t h e l o w - f i e l [4,5] o f t h e L a n g e v i n f u n c t i o n s t a t e d below, one can c a l c u l a t e t h e V"LP f o r l a r g e c l u s t e r s (which m a g n e t i c a l l y s a t u r a t e e a s i l y a t low v a l u e s o f H/T) and v^p f o r s m a l l c l u s t e r s (which s a t u r a t e w i t h d i f f i c u l t y a t h i g h v a l u e s o f H/T). From t h e V~LF and V"HF> t h e average volume ν o f c l u s t e r s can be e s t i m a t e d by t a k i n g t h e a r i t h m e t i c mean. c

c

α

s

0

s 4 F

-

e -

(

1

-

O/ c o _

ΟΙΟ

60

—

40 20

0 · 14 J

ι 0

50

I ΙΟΟ

Ι50

200

B o r o n Doping

250

(ppm)

F i g u r e 10. The v a r i a t i o n i n t h e F e r m i l e v e l Ep [ l e f t hand s c a l e ] and t h e TOF f o r Nco and N c i [ r i g h t hand s c a l e ( x l O ^ s - i F e s " " ) ] , b o t h as a f u n c t i o n o f boron d o p i n g . 1

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

42.

MULAY AND PANNAPARAYIL

Magnetic and Môssbauer Characterization

513

i n t e r a c t i o n i n t h e e a r l y phases o f t h e work, w h i c h showed t h a t t h e TOF v a l u e s a l m o s t doubled w i t h i n c r e a s i n g boron c o n t e n t , a c a r e f u l a n a l y s i s of t h e o v e r a l l r e s u l t s d i d n o t suggest a carbon-Fe interaction. Fe and Fe2Ru on Carbons U s i n g F e ^ ( C O ) i ? and

Fe2Ru(C0)l2

H i g h s u r f a c e a r e a carbon (CSX-203 d e s u l f u r i z e d ) was impregnated w i t h F e 3 ( C 0 ) i 2 and s u b s e q u e n t l y reduced a t 723 Κ f o r 16 h i n f l o w i n g H2 (30 c c / m i n ) . The magnetic and Môssbauer c h a r a c t e r i z a t i o n was c a r r i e d out on t h e d e c a r b o n y l a t e d ( f r e s h ) and reduced p r o d u c t s . T y p i c a l r e s u l t s f o r a sample w i t h a l o a d i n g of 6.4 wt.% Fe a r e shown i n F i g s . 11 and 12. σ v s H/T p l o t f o r t h e f r e s h sample i s shown i n F i g s . 11 and 12, r e s p e c t i v e l y . A v e r y good s u p e r p o s i t i o n o f d a t a p o i n t s i n F i g . 11 r e v e a l e d t h a t t h e m e t a l s p e c i e s i n t h e f r e s h sample was s u p e r p a r a m a g n e t i c t h e average p a r t i c l e s i z 59.5 emu/g Fe i n d i c a t e specie sample was y-Fe203. The room temperature Môssbauer spectrum f o r t h e f r e s h sample ( F i g . 13A) c o n s i s t e d of a w e l l - r e s o l v e d asymmetric quadrupole d o u b l e t (δ = 0.3631 mm/s, A E Q = 0.7966 mm/s). The δ-value i s c h a r a c t e r i s t i c o f h i g h s p i n Fe3+. The r e l a t i v e l y l a r g e v a l u e f o r A E Q f o r an Fe^+ i o n ("S5/2 s t a t e ) i s a g a i n due t o t h e v e r y s m a l l p a r t i c l e s i z e of t h e m e t a l l i c s p e c i e s . The i n t e n s i t y asymmetry o f t h e d o u b l e t c o u l d be a t t r i b u t e d t o t h e asymmetry i n bonding a t t h e s u r f a c e Fe^+ i o n s . As can be seen from F i g . 12, t h e r e i s a s u b s t a n t i a l f e r r o m a g n e t i c c o n t r i b u t i o n t o t h e m a g n e t i z a t i o n o f t h e reduced sample. The room temperature Môssbauer spectrum o f t h e reduced sample ( F i g . 13B,C) r e v e a l e d a broad c e n t r a l l i n e and f o u r v e r y weak o u t e r l i n e s . The o u t e r l i n e s appeared a t t h e c o r r e s p o n d i n g l i n e p o s i t i o n s i n t h e m a g n e t i c a l l y s p l i t s i x - l i n e spectrum o f b u l k i r o n ( F e ) . The broad c e n t r a l l i n e appeared t o be s u p e r p o s i t i o n of f o u r l i n e s , a quadrupole d o u b l e t and two innermost l i n e s o f a magnetic s i x - l i n e spectrum. Q u a l i t a t i v e l y , t h i s spectrum i n d i c a t e d t h e p r e s e n c e o f m e t a l l i c i r o n i n t h e superparamagnetic ( c o r r e s p o n d i n g t o t h e quadrupole d o u b l e t ) and f e r r o m a g n e t i c ( c o r r e s p o n d i n g t o t h e s e x t e t ) f o r m s . However, t h e Môssbauer spectrum o f t h e reduced sample r e c o r d e d a t 4.2 Κ ( F i g . 13D) was a w e l l - d e f i n e d s e x t e t c o r r e s p o n d i n g t o a slow r e l a x a t i o n of S.P. p a r t i c l e s . The room temperature Môssbauer spectrum o f t h e f r e s h sample Fe2Ru(CO)l2/CSX-203 (Fe « 0.7 wt.%) e x h i b i t e d a s i n g l e q u a d r u p o l e d o u b l e t (δ - 0.3240 mm/s, A E Q = 0.7334 mm/s). From t h e δ - v a l u e t h e F e - s p e c i e s p r e s e n t was i d e n t i f i e d t o be ( s u p e r p a r a m a g n e t i c ) Y-Fe203« The above Fe2Ru/C system when reduced i n hydrogen showed o n l y a s i n g l e l i n e i n t h e room temperature Môssbauer spectrum w i t h δ - 0.1372 (mm/s) w h i c h was a s c r i b e d t o S.P. F e . A v e r y low v a l u e o f ^3 emu/g.Fe was observed f o r t h e s a t u r a t i o n m a g n e t i z a t i o n (σ) even a t 80 K. T h i s low v a l u e f o r σ and t h e non-zero v a l u e f o r δ ( w i t h r e s p e c t t o b u l k m e t a l l i c i r o n ) suggest t h e p o s s i b i l i t i e s t h a t (a) e l e c t r o n s from t h e P a u l i paramagnetic Ru and/or (b) e l e c t r o n s from t h e c a r b o n s u b s t r a t e e n t e r e d t h e d-band o f Fe. #

e

e

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

514

CATALYST CHARACTERIZATION SCIENCE

Fe (C0) 3

1 2

/ C S X - 2 0 3 - D S (Fe = 6 - 4 w t . % )

Decarbonylated

80

at 4 7 3 Κ

70 g

60

f

50

296 Κ

| 4 0 b

3

89

Κ

4-0lnm

0

3-68nm

20 10 0

10

F i g u r e 1 1 . σ v s H / T p l o t f o r t h e f r e s h F e 3 ( C 0 ) i 2 / C S X - 2 0 3 sample Fe = 6.4 w t . % ) .

Fe (C0) /CSX-203-DS 3

l 2

(Fe = 6 - 4 w t . % ) R e d u c e d (at 7 2 3 K f o r I 6 h )

220 200 180 160 ,140

296 Κ 89

120

Κ

100 80 60 40 20 0

F i g u r e 12. sample.

0

J L 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 180 H / T (Oe/K)

σ v s H/T p l o t f o r t h e reduced

Fe3(CO)i2/CSX-203

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

42.

MULAY AND PANNAPARAYIL

Magnetic and Môssbauer Characterization

F e ^ C 0 ) , / C S X - 2 0 3 - D S (Fe = 6 - 4 wt.%) 2

Decorbonyloted (at 4 7 3 K) -

100

. H r w 4 , .

T = 300K m

, α . . . ( A )

90 80 70 60 50

Γ

Reduce

S

Reduced (at 7 2 3 K )

i

Reduced ( a t 7 2 3 K )

100

g

99

I

9 8

5

(B)

.97

ω ζ α: 1 0 0 h90

ω ρ

80

κ

- ι l 07 00

T = 4 2Κ m

LU ' 90

80

70

-4

0 V

4

(mm/s)

F i g u r e 13. Môssbauer s p e c t r a f o r Fe3(C0)12/CSX-203 (a) i n t h e d e c a r b o n y l a t e d ( f r e s h ) s t a t e a t room t e m p e r a t u r e , (b) i n r e d u c e d s t a t e a t room t e m p e r a t u r e , ( c ) same a t 80 K, and (d) same a t ^4.2 K.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

515

CATALYST CHARACTERIZATION SCIENCE

516

Conclusions We have shown t h a t t h e Fe o r F e - C o / Z e o l i t e systems w i t h σ v s H / T t y p e r e s u l t s p r o v i d e a u n i q u e probe f o r o b t a i n i n g p a r t i c l e s i z e s (ïï) o f t h e m e t a l c l u s t e r s , and c a n be extended f o r computing a p a r t i c l e - s i z e d i s t r i b u t i o n . I n a d d i t i o n , e l e c t r o n i c i n t e r a c t i o n s between t h e B r o n s t e d a c i d s i t e s and t h e Fe s p e c i e s c a n be e l u c i d a t e d . Môssbauer s p e c t r o s c o p y has been shown t o be a u s e f u l a u x i l i a r y t e c h n i q u e f o r i d e n t i f y i n g v a r i o u s o x i d e s and c a r b i d e s o f Fe and t o d i s c e r n w h i c h c a r b i d e and w h i c h t y p e o f b i m e t a l l i c c l u s t e r s a r e r e s p o n s i b l e f o r t h e enhancement o f t h e g a s o l i n e range hydrocarbons i n t h e F-T r e a c t i o n . The c a r b o n y l s o f Fe e t c . gave a f i n e r d i s p e r s i o n of p a r t i c l e s and v e r y steady c a t a l y t i c a c t i v i t y and l o w f r a c t i o n s o f a r o m a t i c s . W i t h t h e n i t r a t e i m p r e g n a t i o n , t h e a c t i v i t y was found t o f l u c t u a t e w i t h an i n i t i a l h i g h f r a c t i o n o f a r o m a t i c s . The magnetic s u s c e p t i b i l i t y techniqu d t o b t a i band s t r u c t u r parameters such as t h e i n g changes i n Ep, f o r b l e d w i t h B-doping. P r e l i m i n a r y r e s u l t s on t h e Fe/carbon and Fe2Ru/ carbon showed t h e n a t u r e o f e l e c t r o n i c i n t e r a c t i o n between Fe and Ru. Acknowledgments The a u t h o r s s i n c e r e l y a p p r e c i a t e t h e r e s e a r c h c a r r i e d o u t by former graduate s t u d e n t s (now) D r s . C. L o , M. O s k o o i e - T a b r i z i , H . J . J u n g , and A.V. P r a s a d Rao, under t h e d i r e c t i o n o f one o f us (LNM), d u r i n g t h e p a s t f i v e y e a r s . We a r e g r a t e f u l t o v a r i o u s r e s e a r c h a s s o c i a t e s / c o l l a b o r a t o r s , namely D r s . K.R.P.M. Rao (Bhabha Atomic Research C e n t e r , Bombay, I n d i a ) and t o D r s . Β. B e r n s t e i n , R. S c h e h l , R. D i f f e n b a c h , and V.U.S. Rao ( P i t t s b u r g h Energy Technology C e n t e r , P i t t s b u r g h , P A ) . We w i s h t o thank o u r c o l l e a g u e s P r o f s . Μ.Α. V a n n i c e and P.L. Walker J r . f o r t h e i r a s s i s t a n c e i n CO-hydrogenation w i t h t h e v a r i o u s Fe/carbon c a t a l y s t s and many h e l p f u l d i s c u s s i o n s . The r e s e a r c h on t h e F e , F e - C o / Z e o l i t e s was supported by t h e U.S. Department o f Energy and t h e work on t h e Fe/carbon c a t a l y s t s was supported by t h e N a t i o n a l S c i e n c e F o u n d a t i o n , USA. The Government of I n d i a g e n e r o u s l y gave a f e l l o w s h i p t o A.V. P r a s a d Rao. Literature Cited 1.

2a 2b

3.

L.N. Mulay, Magnetic Susceptibility, Wiley-Interscience, New York, NY (1966); Reprint Monograph, R.E. Krieger Publ. Co., Melbourne, FL (1980); see also L.N. Mulay, "Techniques for Magnetic Susceptibility," Ch. 7 in Physical Methods of Chemistry, A. Weissberger and W. Rossiter, eds., Wiley Interscience, New York (1972). L.N. Mulay and E.A. Boudreaux, eds., "Theory and Applications of Molecular Diamagnetism," and "Theory and Applications of Molecular Paramagnetism," Wiley-Interscience, New York, NY (1976). [These give a detailed discussion and factors for converting the classical Gaussian cgs-emu parameters to the mksA S.I. units.] Note: 2a i s now available from Krieger Publishers; see ref. 1. D.W. Collins and L.N. Mulay, IEEE-Mag-Trans 4, 470 (1968); see also L.N. Mulay (review) on "Mössbauer Spectroscopy and

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

42.

4. 5. 6. 7. 8. 9.

MULAY AND PANNAPARAYIL

Magnetic and Môssbauer

Characterization

517

Superparamagnetism," p. 103 and "Philosophy of Research-MössbauerSpectroscopy . . . etc.," p. 79 in "Mössbauer Methodology," Vol. 3, I.J. Gruverman, ed., Plenum Press, New York (1967); see also a paper by L.N. Mulay et a l . ; this describes "Superparamagnetism" in Proc. Am. Inst. Chem. Engrs. Conf. (Philadelphia, 1978), "Microfische No. 60" available from Am. Inst. Chem. Engrs., New York, NY. H. Yamamura and L.N. Mulay, J. Appl. Phys. 50, 7795 (1979). P.W. Selwood, Chemisorption and Magnetization, Academic Press, New York (1975). M. Boudart, A. Delbouille, J.A. Dumesic, S. Khamamouna, and H. Topsøe, J. Catal. 37, 486 (1975). W.N. Delgass, L.Y. Cheng, and G. Vogel, Rev. Sci. Instrum. 47, 968 (1976). A.V. Prasad Rao, Ph.D. Thesis in Solid State Science, Aug. 1983, The Pennsylvania Stat H.J. Jung, M.A. Vannice Delgass, J. Catal. 76, 208 (1982).

Supplementary Literature Cited 1 2 3 4 5 6 7 8

V.U.S. Rao, R.J. Gormley, L.C. Schneider, and R. Obermyer, Preprints Div. of Fuel Chemistry, ACS 25, 119 (1980). V.U.S. Rao and R.J. Gormley, Hydrocarbon Processing 59(11), 139 (1980). G.T. Kokotailo, S.L. Lawton, D.H. Olson, and W.M. Meier, Nature 272, 437 (1978). E.M. Flanigen, J.M. Bennett, R.W. Grose, J.P. Cohen, R.L. Patton, R.M. Kirchner, and J.V. Smith, Nature 271, 512 (1978). F. Van der Woude and G.A. Sawatsky, Physics Letters 12C, 335 (1974). W. Kundig, K.J. Ando, R.H. Lindquist, and G. Constabaris, Czech. J. Phys. 17, 467 (1967). C. Kittel, "Introduction to Solid State Physics," Fifth Ed., John Wiley and Sons Inc., New York, NY (1980). K.M. Sancier, W.E. Isakson, and H. Wise, Preprints, Div. of Petroleum Chemistry, ACS 23(2), 545 (1978).

RECEIVED March 28, 1985

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

43 Characterization of Supported Iron Oxide Particles Using Mössbauer Spectroscopy and Magnetic Susceptibility 2

J. Phillips1, Y. Chen , and J. A. Dumesic3 1Departmentof Chemical Engineering, The Pennsylvania State University, University Park, PA 16802 2Department of Chemistry, Nanjin 3Department of Chemical Engineering, University of Wisconsin, Madison,WI53706 The size and structure of iron oxide particles supported on Grafoil and used as water-gas s h i f t catalysts were studied using magnetic s u s c e p t i b i l i t y and Mössbauer spectroscopy. The use of a Mössbauer spectra modeling program which accounts for magnetic relaxation effects (both superparamagnetic and collective excitation) aided in the i d e n t i f i c a t i o n of the i r o n phase under reaction conditions (magnetite) and permitted a quantitative determination of p a r t i c l e s i z e . The particle size determined using Mössbauer spectroscopy was in good agreement with that obtained using the well established magnetic susceptibility technique. It was also shown that the Grafoil supported particles sintered slowly under water-gas shift reaction conditions. During recent decades, while significant advances have been made in understanding t h e b e h a v i o r o f s u p p o r t e d m e t a l c a t a l y s t s , r e l a t i v e l y l i t t l e a t t e n t i o n has been g i v e n t o s u p p o r t e d metal-oxide c a t a l y s t s . Yet, supported oxide c a t a l y s t s are p o t e n t i a l l y o f g r e a t i n d u s t r i a l s i g n i f i c a n c e , and work needs t o be done i n t h i s a r e a . The f i r s t r e q u i r e m e n t f o r t h e s t u d y o f s u p p o r t e d o x i d e c a t a l y s t s i s t h e development o f t e c h n i q u e s f o r measuring s u p p o r t e d m e t a l - o x i d e p a r t i c l e s i z e s and d i s t r i b u t i o n s . I n t h i s paper t h e a p p l i c a t i o n s o f Môssbauer s p e c t r o s c o p y and magnetic s u s c e p t i b i l i t y t o t h e measurement o f s u p p o r t e d i r o n - o x i d e p a r t i c l e s i z e s a r e d i s c u s s e d . I t i s demonstrated t h a t b o t h methods g i v e i m p o r t a n t p a r t i c l e s i z e i n f o r m a t i o n . Theory Môssbauer S p e c t r o s c o p y . S m a l l , s i n g l e domain, f e r r o - o r f e r r i magnetic p a r t i c l e s can show b o t h c o l l e c t i v e magnetic e x c i t a t i o n ( p r e c e s s i o n o f the magnetic moment) and superparamagnetic ( r e l a x a 0097-6156/85/0288-O518$06.00/0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

43.

PHILLIPS ET AL.

Supported Iron Oxide Particles

519

t i o n o f t h e magnetic moment) b e h a v i o r . These m o d i f i c a t i o n s i n t h e magnetic b e h a v i o r o f s m a l l p a r t i c l e s produce s i g n i f i c a n t changes i n t h e r e s u l t a n t Môssbauer s p e c t r a . Thus, i n f o r m a t i o n about p a r t i c l e s i z e (and shape) i s c o n t a i n e d i n t h e Môssbauer s p e c t r a o f s m a l l p a r t i c l e s . T h i s f a c t has been r e c o g n i z e d f o r some t i m e and many workers have a t t e m p t e d t o a n a l y z e Môssbauer s p e c t r a t o o b t a i n i n f o r m a t i o n about p a r t i c l e s i z e . The e a r l y e f f o r t s were based e x c l u s i v e l y on t h e a n a l y s i s o f superparamagnetic e f f e c t s . I n a s e r i e s o f papers Brown (1,2) and A h a r o n i (3» .) d e v e l o p e d t h e t h e o r y f o r t h e r e l a x a t i o n o f magnetic moments i n s i n g l e domain magnetic systems c o n t a i n i n g s e v e r a l e q u i v a l e n t low energy d i r e c t i o n s . They showed t h a t i n such systems there i s a f i n i t e p r o b a b i l i t y that the magnetization vector w i l l s p o n t a n e o u s l y change d i r e c t i o n s . The energy b a r r i e r f o r t h i s p r o c e s s i s dependent on t h e o r i g i n o f t h e magnetic a n i s o t r o p y ( e . g . , magneto c r y s t a l l i n anisotropy, surface anisotropy) barriers are a function of p a r t i c l e s i z e . (The o r i g i n s and magni tudes o f v a r i o u s a n i s o t r o p i e s a r e d i s c u s s e d a t l e n g t h i n r e f e r e n c e 5.) The average l i f e t i m e TR o f a g i v e n s t a t e can be w r i t t e n : 1

TR = ( l / 2 f ) e x p ( < v / k T ) 1

(1)

1 1

where f " i s o f t h e o r d e r o f 10~9 - i o " sec, κ i s the anisotropy energy c o n s t a n t (ergs/cm3) and ν i s t h e volume (cm3) o f t h e magnetic p a r t i c l e . Brown showed t h a t t h e p r e e x p o n e n t i a l f a c t o r i s a l s o a f u n c t i o n o f t h e a n i s o t r o p y energy c o n s t a n t s and t e m p e r a t u r e . I n t h e l i m i t o f l a r g e a n i s o t r o p y b a r r i e r s ( « x

we f i n d t h a t / = 1.96 f3, where o=1.25. D i v i d i n g t h e measured v a l u e o f v / v by 1.96 and t a k i n g t h e cube r o o t , we f i n d t h a t t h e measured v a l u e o f t h e mean r a d i u s i s 89^. The agreement between t h e r e s u l t s o f magnetic s u s c e p t i b i l i t y and Môssbauer s p e c t r o s c o p y i s v e r y good ( T a b l e I I I ) . A s t u d y o f t h e r a t e o f s u p p o r t e d p a r t i c l e growth was a l s o conducted u s i n g l o w - f i e l d magnetic s u s c e p t i b i l i t y . The average r a d i u s o f p a r t i c l e s i n sample 3 ( s e e T a b l e I ) was measured a f t e r v a r i o u s l e n g t h s o f time i n C0/C0 (15:85) a t 663 K. The f i r s t measurement was made a f t e r t h e sample had been h e a t e d f o r a t o t a l of 8 1/2 hours a t 663 K. The second measurement was made a f t e r the sample had been t r e a t e d f o r a t o t a l o f 52 hours a t 663 K. The average p a r t i c l e r a d i u s i n c r e a s e d by o n l y 15 p e r c e n t d u r i n g t h e a d d i t i o n a l 44 hours o f h i g h temperature t r e a t m e n t (see T a b l e I V ) . The t h i r d measurement was made a f t e r t h e sample had been t r e a t e d f o r a t o t a l o f 146 hours a t 663 K. The a d d i t i o n a l 94 hours o f h i g h temperature treatment r e s u l t e d i n an average i n c r e a s e i n t h e p a r t i c l e r a d i u s o f l e s s t h a n 15 p e r c e n t . From t h e s e experiments we f i n d t h a t m a g n e t i t e s u p p o r t e d on G r a f o i l s i n t e r s s l o w l y b u t s t e a d i l y a t 663 K. F o l l o w i n g t h e c o m p l e t i o n o f t h e s i n t e r i n g s t u d y , a s t u d y was conducted t o demonstrate t h a t 663 K, t h e temperature a t w h i c h a l l magnetic s u s c e p t i b i l i t y measurements were t a k e n , was i n d e e d greater than T . As d i s c u s s e d i n t h e t h e o r y s e c t i o n , measurements were made a t s e v e r a l temperatures u n t i l i t was determined t h a t p l o t s o f M/M v e r s u s H/T c o l l a p s e d onto a s i n g l e c u r v e . I t i s c l e a r from F i g u r e 4 t h a t T must be l e s s t h a n 450 K. That i s , f o r any temperature above 450 Κ t h e average p a r t i c l e s i z e was measured t o be n e a r l y 200 Â i n d i a m e t e r . For s m a l l e r particles, T w i l l o f c o u r s e be an even lower t e m p e r a t u r e . T h i s proves t h a t a l l measurements made a t 663 Κ were indeed a c c u r a t e . 6

2

2

c m

s

c m

c m

Summary I n t h i s paper t h e use o f Môssbauer s p e c t r o s c o p y and magnetic s u s c e p t i b i l i t y t o measure t h e s i z e o f s u p p o r t e d o x i d e c a t a l y s t

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

43.

Table I I I .

Sample

1

P a r t i c l e S i z e Determined U s i n g B o t h Môssbauer S p e c t r o s c o p y and M a g n e t i c S u s c e p t i b i l i t y Methods

Avg. P a r t i c l e Radius (Hemispheres)

Technique

3

85A,105AC 95A,115A

5

Reduced S p l i t t i n g S i m u l a t i o n o f Môssbauer S p e c t r a

1A

d

95A,115A

S i m u l a t i o n o f Môssbauer S p e c t r a

1

IB

1

1C

1

ID

2

a

Figure

529

Supported Iron Oxide Particles

PHILLIPS ET AL.

100A,12of

Reduced S p l i t t i n g S i m u l a t i o n o f Môssbauer S p e c t r a

2A

120A,l40A 65A,75A

S i m u l a t i o n o f Môssbauer S p e c t r a Magnetic S u s c e p t i b i l i t y

d

d

90Â

S e e f i g u r e c a p t i o n s f o r d e s c r i p t i o n o f sample t r e a t m e n t .

^Average s i z e determined from t h e r e d u c t i o n i n t h e h y p e r f i n e f i e l d according t o the formula: measured s p l i t t i n g . kT bulk s p l i t t i n g 2KV w

C

A sites, Β sites

d

, l/3

h

e

p

e

κ

m

8

x

1

q

5

/

c

m

3

TABLE IV. P a r t i c l e s i z e v e r s u s Time i n C0/C0 a t 663 Κ as Measured U s i n g M a g n e t i c S u s c e p t i b i l i t y 2

T o t a l Time a t 663 Κ ( h r s )

8

1/2

d

Average P a r t i c l e R a d i u s

90Â

52

100A

146

11 OA

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

530

CATALYST CHARACTERIZATION SCIENCE

0

50

100

150

200

250

300

350

0

50

100

150

200

250

300

350

RADIUS

(A)

F i g u r e 3. Computer S i m u l a t i o n s o f t h e Môssbauer S p e c t r a o f F i g u r e 1. A) S i m u l a t i o n o f F i g u r e ΙΑ; B) S i m u l a t i o n o f F i g u r e I B ; C) D i s t r i b u t i o n o f p a r t i c l e r a d i i ; D) R e l a t i v e volume f r a c t i o n s as a f u n c t i o n of r a d i u s . For these s i m u l a t i o n s , the f o l l o w i n g parameters were used: o«1.25, mean r a d i u s « 95A, κ«8 χ 1θ5 e r g s / c m 3 . The Klebsch-Gordon c o e f f i c i e n t s used were 3:3:1. In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

PHILLIPS ET AL.

Supported Iron Oxide Particles

0

0.5

1.0

1.5

2.0

2.5

H/T F i g u r e 4. P l o t s o f M/M temperatures.

s

- Measurements atmosphere. - Measurements - Measurements - Measurements - Measurements

v e r s u s H/T f o r Sample 3 a t v a r i o u s

made w h i l e sample was a t 660 Κ i n a CO/CO2 made made made made

while while while while

sample sample sample sample

was was was was

at at at at

570 447 600 295

Κ k Κ Κ

in in in in

vacuum. vacuum. vacuum. vacuum.

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

532

CATALYST CHARACTERIZATION SCIENCE

p a r t i c l e s i s d i s c u s s e d i n t h e o r y and demonstrated i n p r a c t i c e . Môssbauer s p e c t r o s c o p y i s shown t o be a p a r t i c u l a r l y p o w e r f u l t e c h n i q u e because w i t h t h e c o r r e c t m o d e l i n g e q u a t i o n s , i t can be used t o measure t h e s i z e and phase o f c a t a l y s t p a r t i c l e s under r e a c t i o n c o n d i t i o n s . I t was a l s o shown t h a t t h e p a r t i c l e s i z e determined u s i n g Môssbauer s p e c t r o s c o p y i s i n v e r y good agreement w i t h t h a t o b t a i n e d u s i n g t h e w e l l e s t a b l i s h e d magnetic suscept i b i l i t y technique. Môssbauer s p e c t r o s c o p y and magnetic s u s c e p t i b i l i t y were used t o demonstrate t h a t m a g n e t i t e p a r t i c l e s s u p p o r t e d on a G r a f o i l s u b s t r a t e s i n t e r v e r y s l o w l y under water-gas s h i f t r e a c tion conditions.

Literature Cited 1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

W. F. Brown, Jr. W. F. Brown, Jr. A. Aharoni, J. Appl. Phys. 33, 1324 (1962). A. Aharoni, Phys. Rev. 135A, 447 (1964). S. Mørup, J. A. Dumesic and H. Topsøe, in "Appl. of Mössbauer Spectroscopy," (R. L. Cohen, ed.) Vol. 2, p. 1, Academic Press, N.Y. (1980). H. H. Wickman in "Mössbauer Effect Methodology" 2, 39 (1966). S. Mørup, H. Topsøe and J. S. Lipka, Jnl. de Phys. 37, C6-287 (1976). S. Mørup and H.Topsøe, Appl. Phys. 11, 63 (1976). A. M. Van der Kraan, Phys. Stat. Sol. 18A, 215 (1973). T. Shinjo, T. Matsuzawa, T. Takada, S. Nasa and Y. Murakami, J. Phys. Soc. Japan 35, 1032 (1973). L. Gerward, S. Mørup and H.Topsøe, J. Appl. Phys. 47, 822 (1976). T. K. McNab, R. A. Fox and J. F. Boyle, J. Appl. Phys. 39, 5703 (1968). W. Kündig, H. Bömmel, G. Constabaris and R. H. Lindquist, Phys. Rev. 142, 327 (1966). W. Kündig, K. J. Ando, R. H. Lindquist and G. Constabaris, Czech. J. Phys. B17, 467 (1967). W. Kündig and R. S. Hargrove, Sol. State Comm. 7, 223 (1969). R. S. Hargrove and W. Kündig, Sol. State Comm. 8, 803 (1970). P. Roggwiller and W. Kündig, Sol. State Comm. 12, 901 (1973). H. Topsøe, J. A. Dumesic and M. Boudart, Jnl. de Phys. 35, C6-411 (1974). S. Mørup, B. S. Clausen and H. Topsøe, J. Phys. Colloq. Paris 40, C2-78 (1979). P. W. Selwood, "Chemisorption and Magnetization", Acad. Press NY (1975). J. W. Cahn, Trans. AIME 209, 1309 (1959). R. E. Dietz and P. W. Selwood, J. Chem. Phys. 35, 270 (1961). R. Pauthenet, Ann. Phys. 7, 710 (1952).

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C. P. Bean and J. D. Livingston, J. Appl. Phys. 30, 120S (1959). J. Phillips, B. Clausen, J. A. Dumesic, J. Phys. Chem. 84, 1814 (1980). R. H. Bartholomew and M. Boudart, J. Cat. 26, 173 (1972). J. Phillips, Ph.D. Thesis, University of Wisconsin-Madison (1981). A. Muan and E. F. Osborn, "Phase Equilibrium Among Oxides in Steelmaking", Addison-Wesley Pub. Co., Reading, MA (1965). C. G. Granqvist and R. A. Burhman, J. Appl. Phys. 47, 2220 (1976). C. G. Granqvist and R. A. Burhman, Appl. Phys. Lett. 27, 693 (1976). C. G. Granqvist and R. A. Burhman, J. Appl. Phys. 47, 2200 (1976). C. G. Granqvist (1976). C. G. Granqvist and R. A. Burhman, J. Cat. 42, 477 (1976). G. A. Sawatsky, F. van der Woude and A. H. Morrish, Phys. Rev. 183, (1969). F. van der Woude, G. A. Sawatsky and A. H. Morrish, Phys. Rev. 167, 533 (1968). T. Riste and L. Tanzer, J. Phys. Chem. Solids 19, 117 (1961).

RECEIVED March 28, 1985

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

44 In Situ Spectroscopic Studies of Oxygen Electrocatalysts Involving Transition Metal Macrocycles Ernest Yeager, Daniel A. Scherson, and Cristian A. Fierro Case Center for Electrochemical Sciences and The Chemistry Department, Case Western Reserve University, Cleveland, OH 4410

Some of the transition metal macrocycles adsorbed on electrode surfaces are of special interest because of their high catalytic activity for dioxygen reduction. The interaction of the adsorbed macrocycles with the substrate and their orientation are of importance in understanding the factors controlling their catalytic activity. In situ spectroscopic techniques which have been used to examine these electrocatalytic layers include v i s i b l e reflectance spectroscopy; surface enhanced and resonant Raman; and Mossbauer effect spectroscopy. This paper i s focused principally on the cobalt and iron phthalocyanines on s i l v e r and carbon electrode substrates. Of special interest as O2 reduction electrocatalysts are the transition metal macrocycles in the form of layers adsorptively attached, chemically bonded or simply physically deposited on an electrode substrate 0^-5)· Some of these complexes catalyze the 4-electron reduction of 0 to H 0 or 0H~ while others catalyze principally the 2-electron reduction to the peroxide and/or the peroxide elimination reactions. Various in situ spectroscopic techniques have been used to examine the state of these transition metal macrocycle layers on carbon, graphite and metal substrates under various electrochemical conditions. These techniques have included (a) visible reflectance spectroscopy; (b) laser Raman spectroscopy, u t i l i z i n g surface enhanced Raman scattering and resonant Raman; and (c) Môssbauer spectroscopy. This paper will focus on principally the cobalt and iron phthalocyanines and porphyrins. Before considering these catalysts, some background on the 0 reduction reaction w i l l be reviewed (6). The reduction of Oo i s considered to proceed by two p a r a l l e l pathways, namely, (A) a z-electron process which generates peroxide and (B) a 4-electron pathway i n which the presence of H0 ~ in the 2

2

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b u l k o f t h e s o l u t i o n p h a s e c a n n o t be d e t e c t e d . In the f i r s t case t h e r e a c t i o n may be f o l l o w e d by t h e f u r t h e r r e d u c t i o n o f H0 "~, o r the p e r o x i d e can undergo d e c o m p o s i t i o n by a d i s m u t a t i o n p r o c e s s t o g i v e OH" and 0 . The p e r o x i d e p a t h w a y (A) has b e e n f o u n d t o be t h e predominant mechanism on g r a p h i t e , g o l d , most t r a n s i t i o n m e t a l ox­ i d e s (e.g., NiO o r s p i n e l s ) and c e r t a i n t r a n s i t i o n m e t a l m a c r o c y c l e s such as c o b a l t t e t r a s u l f o n a t e d p h t h a l o c y a n i n e , Co-TsPc. The r e d u c t i o n o f 0 on p l a t i n u m , s i l v e r , p a l l a d i u m and a f e w o t h e r m a t e r i a l s , i n c l u d i n g some t r a n s i t i o n m e t a l m a c r o c y c l e s appears t o f o l l o w a d i r e c t 4 - e l e c t r o n pathway. The d i a g n o s t i c c r i t e r i o n as t o w h i c h p a t h w a y s a r e i n v o l v e d w i t h a g i v e n c a t a l y t i c s u r f a c e has i n v o l v e d a l m o s t i n v a r i a b l y the r o t a t i n g r i n g - d i s k e l e c t r o d e t e c h ­ nique. The r e a c t i o n m e c h a n i s m s a s s o c i a t e d w i t h p a t h w a y s (A) and (B) may share i n common such adsorbed i n t e r m e d i a t e s as p e r o x i d e , super­ o x i d e and t h e i r c o r r e s p o n d i n between pathways (A) an e r an i n t e r m e d i a t e s u c peroxide(H0 significan q u a n t i t i e s o r not; f o r example, 2

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An u n d e r s t a n d i n g o f the 0 r e d u c t i o n mechanisms i s c r i t i c a l l y depen­ d e n t on t h e i d e n t i f i c a t i o n o f s u c h a d s o r b e d i n t e r m e d i a t e s and t h e understanding of t h e i r i n t e r a c t i o n w i t h v a r i o u s types of a d s o r p t i o n s i t e s . A c h i e v i n g such i n f o r m a t i o n has been s e v e r e l y hampered by the s p a r c i t y of i n s i t u spectroscopic techniques w i t h s u f f i c i e n t s e n s i ­ t i v i t y t o d e t e c t s u c h a d s o r b e d i n t e r m e d i a t e s on m o s t e l e c t r o d e s u r f a c e s . An a t t e m p t has been made t o g a i n some i n s i g h t as to how 0 i n t e r a c t s w i t h s u c h s u r f a c e s a s p l a t i n u m f r o m ex situ s p e c t r o s c o p i c s t u d i e s u s i n g u l t r a h i g h vacuum t e c h n i q u e s ( 7 ) b u t e x t r a p o l a t i o n from the vacuum t o t h e e l e c t r o c h e m i c a l e n v i r o n m e n t s u f f e r s from c o n s i d e r a b l e u n c e r t a i n t y . Consequently the models p r o p o s e d f o r 0 r e d u c t i o n on m e t a l s and m e t a l o x i d e s m u s t be r e ­ garded as r a t h e r s p e c u l a t i v e . D u r i n g t h e l a s t two d e c a d e s a v a r i e t y o f t r a n s i t i o n m e t a l m a c r o c y c l e s have been shown to e x h i b i t a c t i v i t y f o r 0 r e d u c t i o n i n a l k a l i n e and a c i d media when adsorbed, c h e m i c a l l y anchored o r p h y s i ­ c a l l y d i s p e r s e d on e l e c t r o d e s u r f a c e (1_-5)· T h i s c l a s s o f compounds p r o v i d e s a u n i q u e o p p o r t u n i t y t o e x a m i n e i n d e t a i l some o f t h e f a c t o r s i n v o l v e d i n t h e a c t i v a t i o n and f u r t h e r r e d u c t i o n o f 0· These would i n c l u d e the e f f e c t s a s s o c i a t e d w i t h a x i a l , p e r i p h e r a l 2

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and r i n g s u b s t i t u t i o n s , t h e n a t u r e o f t h e m e t a l c e n t e r , and t h e degree o f c o n j u g a t i o n o f t h e m a c r o c y c l e r i n g . F u r t h e r m o r e , i t has been p o s s i b l e under c e r t a i n c i r c u m s t a n c e s t o s y n t h e s i z e s t a b l e 0 m a c r o c y c l e a d d u c t s (8) o r t o t r a p such s p e c i e s i n i n e r t gas m a t r i c e s (9). T h i s has f a c i l i t a t e d the a c q u i s i t i o n o f s t r u c t u r a l and s p e c t r o s c o p i c i n f o r m a t i o n w h i c h may be d i r e c t l y r e l e v a n t t o e l e c t r o c a t a l y s i s a s t h e a d d u c t s may be r e g a r d e d a s m o d e l i n t e r m e d i a t e s i n t h e r e d u c t i o n of 0 . Of a number o f t r a n s i t i o n m e t a l m a c r o c y c l e s i n v e s t i g a t e d , t h e d i m e t a l Co-Co-4 f a c e - t o - f a c e p o r p h y r i n ( 1 0 ) and t h e d i c o b a l t b i p y r i d a l complex (11) have been found t o c a t a l y z e the 4 - e l e c t r o n r e d u c t i o n o f On t o H 0. These complexes can form d i o x y g e n b r i d g e s w h i c h can f a c i l i t a t e the c l e a v a g e o f t h e 0-0 bond. The t e t r a s u l f o n a t e d i r o n p h t h a l o c y a n i n e (Fe-TsPc) adsorbed on g r a p h i t e s u r f a c e s has a l s o b e e n f o u n d t o c a t a l y z e t h e 4-e r e d u c t i o n (4^5)· T h i s compound i s w a t e r s o l u b l e and p r o p o s e aqueous s o l u t i o n s . Th b r i d g e d s p e c i e s may a l s o be found i n the adsorbed l a y e r . C o n s i d e r a b l e p r o g r e s s has been made r e c e n t l y i n the development °^ IR . t s p e c t r o s c o p i c techniques a p p l i c a b l e to the study of t r a n s i t i o n m e t a l m a c r o c y c l e s adsorbed a t submonolayer c o v e r a g e s onto electrode surfaces. These have been aimed a t g a i n i n g i n s i g h t i n t o t h e n a t u r e o f t h e i n t e r a c t i o n s o f t h e s e compounds w i t h the s u r f a c e and w i t h 0 · Most o f the a t t e n t i o n i n the a u t h o r s ' l a b o r a t o r y has been f o c u s e d on Fe- and Co-TsPc, a l t h o u g h some p r e l i m i n a r y r e s u l t s have a l r e a d y been o b t a i n e d f o r some i r o n and c o b a l t p o r p h y r i n s . The m a i n c o n c l u s i o n s o b t a i n e d from these i n v e s t i g a t i o n s w i l l be o u t l i n e d i n the f o l l o w i n g s e c t i o n s . 2

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U V - V i s i b l e R e f l e c t a n c e Spectroscopy The m o l e c u l a r e x t i n c t i o n c o e f f i c i e n t f o r the prominent Q a b s o r p t i o n bands i n the U V - v i s i b l e s p e c t r a o f most m a c r o c y c l e s can r e a c h v a l u e s o f 10^-10^. Hence, t h e s e compounds a r e e x c e l l e n t c a n d i d a t e s f o r the s t u d y o f t h e o p t i c a l p r o p e r t i e s o f m o l e c u l e s a t submonolayer c o v e r ages. F i g . 1 g i v e s the U V - v i s i b l e r e f l e c t a n c e s p e c t r a of Fe-TsPc adsorbed onto a h i g h l y o r i e n t e d p y r o l y t i c g r a p h i t e (H0PG) and a P t electrode i n alkaline m e d i a e The c i r c l e s and t r i a n g l e s c o r r e s p o n d t o measurements t a k e n i n the absence and presence o f 0 , r e s p e c t i v e l y , w i t h o u t the macrocycles i n s o l u t i o n . (Other c o n d i t i o n s are s p e c i f i e d i n t h e f i g u r e c a p t i o n . ) Even though t h e s e compounds a r e h i g h l y s o l u b l e i n w a t e r , t h e amount o f m a t e r i a l w h i c h d e s o r b s from t h e s u r f a c e d u r i n g the e x p e r i m e n t s i s n e g l i g i b l e . The s p e c t r a f o r t h e s e compounds d i s s o l v e d i n t h e same e l e c t r o l y t e were found t o be q u i t e s i m i l a r t o t h o s e s h o w n i n F i g . 1. T h i s s u g g e s t s t h a t t h e bonding to the s u b s t r a t e i n v o l v e s o r b i t a l e with l i t t l e ring-chara c t e r and t h a t the m a c r o c y c l e i s most l i k e l y adsorbed i n an edge-on f a s h i o n on the s u r f a c e . The changes i n t h e s p e c t r a l f e a t u r e s i n t r o d u c e d by t h e p r e s e n c e o f 0 were s i m i l a r to t h o s e found i n t h e s o l u t i o n phase. T h i s p r o v i d e s e v i d e n c e t h a t the p h y s i c o c h e m i c a l c h a r a c t e r i s t i c s o f t h e a d s o r b e d l a y e r , s u c h a s a g g r e g a t i o n and r e a c t i v i t y towards 0 , may n o t d i f f e r much f r o m those o f t h e s p e c i e s i n the s o l u t i o n . 2

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In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

I 600

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F i g u r e 1. R e f l e c t a n c e s p e c t r a o f F e - T s P c i n 0.1 M NaOH a d s o r b e d on t h e b a s a l p l a n e o f a h i g h l y o r d e r e d p y r o l y t i c g r a p h i t e (HOPG) e l e c t r o d e a t 0.90 V v s . α-Pd a n d o n a P t e l e c t r o d e a t 0.70 V w i t h Ar (0) and 0 ( A ) s a t u r a t e d s o l u t i o n s . Reproduced w i t h p e r m i s s i o n from Ref. 12. C o p y r i g h t 1979, E l s e v i e r .

560

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S u r f a c e Enhanced Raman S p e c t r o s c o p y (SERS) S i n c e i t s d i s c o v e r y , SERS has r e c e i v e d a t t e n t i o n b o t h from t h e o r e t i c a l and e x p e r i m e n t a l v i e w p o i n t s (13). The l a r g e enhancement has been o b s e r v e d o n l y f o r c e r t a i n m e t a l s s u c h a s Ag, Cu a n d Au. The enhancement mechanism i s n o t q u a n t i t a t i v e l y u n d e r s t o o d . Even s o , v e r y u s e f u l i n f o r m a t i o n c o n c e r n i n g a d s o r b e d s p e c i e s o f some e l e c t r o d e s u r f a c e s c a n be o b t a i n e d i n - s i t u w i t h t h i s e f f e c t . With p r o p e r c h o i c e o f l a s e r r a d i a t i o n w a v e l e n g t h , advantage c a n be t a k e n o f t h e resonance Raman e f f e c t t o f u r t h e r enhance t h e s i g n a l s f o r t h e a d s o r b e d t r a n s i t i o n m e t a l . As a n i l l u s t r a t i o n , F i g . 2 s h o w s t h e Raman s p e c t r a o f adsorbed H - and Fe-TsPc on s i l v e r i n a c i d media i n the absence o f 0 a t d i f f e r e n t p o t e n t i a l s (14). H -TsPc e x h i b i t s v e r y s t r o n g f l u o r e s c e n c e when i n t h e s o l u t i o n p h a s e . This i s q u e n c h e d , h o w e v e r , when t h e m o l e c u l e s a r e a d s o r b e d o n t h i s s u b s t r a t e , through a r a d i a t i o n l e s number o f a v a i l a b l e s t a t e i n the r e l a t i v e i n t e n s i t y as w e l l as i n the p o s i t i o n o f few o f the b a n d s w e r e o b s e r v e d a s a f u n c t i o n o f t h e a p p l i e d p o t e n t i a l . A. b e t t e r d e s c r i p t i o n o f t h e s e phenomena i s g i v e n i n F i g s . 3 a n d 4. The f i r s t o f t h e s e f i g u r e s s h o w s t h e f r e q u e n c y s h i f t o f t h e Raman band a t 612 c n T . ( a s s o c i a t e d w i t h a d e f o r m a t i o n mode o f t h e i n n e r r i n g ) f o r a d s o r b e d Fe-TsPc on s i l v e r (14). A c o m p a r i s o n o f t h i s c u r v e w i t h t h e c o r r e s p o n d i n g c y c l i c voltammogram g i v e n i n t h e same f i g u r e suggests t h a t t h e s h i f t s c o r r e l a t e w i t h the peaks i n t h e voltammetry. An i l l u s t r a t i o n o f t h e second e f f e c t mentioned above i s g i v e n i n F i g . 4 w h i c h shows t h e v a r i a t i o n s i n i n t e n s i t y o f t h e bands a t 1346 and 699 cm".. f o r Fe-TsPc as a f u n c t i o n o f t h e a p p l i e d p o t e n t i a l a t d i f f e r e n t pH v a l u e s . A f u l l q u a n t i t a t i v e u n d e r s t a n d i n g o f t h e s e phenomena w i l l most c e r t a i n l y r e q u i r e r e f i n e d t h e o r e t i c a l t r e a t m e n t s w h i c h would i n c l u d e t h e e f f e c t s o f a d s o r p t i o n on t h e s e l e c t i o n r u l e s as w e l l as account f o r p o s s i b l e d i s t o r t i o n s i n d u c e d by t h e e l e c t r i c f i e l d on t h e m e t a l s u r f a c e . R e c e n t l y , t h e i n s i t u Raman s c a t t e r i n g f r o m F e - T s P c a d s o r b e d onto t h e l o w i n d e x c r y s t a l l o g r a p h i c f a c e s o f Ag was examined and t h e r e s u l t s o b t a i n e d a r e s h o w n i n F i g . 5 ( 1 5 ) . On t h e b a s i s o f t h e s i m i l a r i t i e s o f t h e s e s p e c t r a w i t h those o b t a i n e d f o r t h e m a c r o c y c l e i n s o l u t i o n phase, a s w e l l as t h e p o l a r i z a t i o n b e h a v i o r c h a r a c t e r i s t i c s , i t has been c o n c l u d e d t h a t t h e most l i k e l y c o n f i g u r a t i o n i s t h a t w i t h t h e m a c r o c y c l e edge-on w i t h r e s p e c t t o t h e s u r f a c e . T h i s i s i n agreement w i t h c o n c l u s i o n s reached from t h e U V - v i s i b l e r e f l e c t a n c e s p e c t r a . The p r e f e r r e d c o n f i g u r a t i o n , however, may depend on the p a r t i c u l a r m a c r o c y c l e , as w e l l a s on t h e n a t u r e o f t h e a d s o r p tion site. 2

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Môssbauer E f f e c t S p e c t r o s c o p y (MES) Môssbauer e f f e c t s p e c t r o s c o p y , MES, i s b a s e d o n t h e a b i l i t y o f c e r t a i n n u c l e i t o undergo r e c o l l l e s s e m i s s i o n and a b s o r p t i o n o f γr a y s ( 1 6 ) . The e n e r g y a n d m u l t i p l i c i t y o f t h e g r o u n d and e x c i t e d s t a t e s o f a g i v e n n u c l e u s a r e m o d i f i e d by t h e c h e m i c a l environment. I t i s thus most o f t e n n e c e s s a r y t o compensate f o r t h e d i f f e r e n c e s i n

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F i g u r e 2. Surface-enhanced Raman s p e c t r a o f Fe-TsPc (A) and H TsPc (B) adsorbed on a s i l v e r e l e c t r o d e a t v a r i o u s p o t e n t i a l s v s . SCE i n 0.05 M H2SO4. L a s e r e x c i t a t i o n l i n e : 632.8 nm; o u t p u t power: 20 mW; r e s o l u t i o n : 5 cm" ; s c a n n i n g t i m e : 10 min/spec­ trum. Ρ and S r e f e r t o d e p o l a r i z e d and p o l a r i z e d l i g h t , r e s p e c ­ tively. Reproduced w i t h p e r m i s s i o n from Ref. 14. C o p y r i g h t 1983, Elsevier. 2

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In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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F i g u r e 3· Frequency s h i f t o f the Raman band a t 612 cm" f o r FeTsPc adsorbed on a s i l v e r e l e c t r o d e as a f u n c t i o n o f the a p p l i e d p o t e n t i a l v s . SCE i n 0.05 M H S 0 ^ . L a s e r e x c i t a t i o n l i n e : 514.5 nm; p o t e n t i a l s w e e p r a t e : 10 mV s ; e l e c t r o d e a r e a : 0.27 cm4 See c a p t i o n F i g . 2. 2

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F i g u r e 4. I n t e n s i t y as a f u n c t i o n o f p o t e n t i a l v s . SCE f o r two o f t h e Raman b a n d s (1346 c m " a n d 699 c m " ) o f F e - T s P c a d s o r b e d on a s i l v e r e l e c t r o d e a t d i f f e r e n t pH v a l u e s . These measurements w e r e o b t a i n e d a t a p o t e n t i a l s c a n r a t e o f 10 mV s " . See c a p t i o n F i g . 2. 1

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F i g u r e 5. I n s i t u Raman s p e c t r a f o r F e - T s P c a d s o r b e d o n A g ( 1 0 0 ) , A g ( l l l ) and A g ( l l O ) a t 0.2 V v s . SCE i n 0.1 M H C 1 0 A r s a t u r a t e d aqueous s o l u t i o n s (15). 4

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

44.

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543

t r a n s i t i o n e n e r g y b e t w e e n e m i t t e r and a b s o r b e r i n o r d e r t o b r i n g about resonance. T h i s i s u s u a l l y a c c o m p l i s h e d by v a r y i n g t h e v e l o c i t y o f e i t h e r the e m i t t e r o r the a b s o r b e r , a c h o i c e d i c t a t e d m a i n l y by the s p e c i f i c s y s t e m under study. The resonance c o n d i t i o n can be m o n i t o r e d by t h e use o f d e t e c t o r s s e n s i t i v e t o Y - r a y s , X - r a y s o r e l e c t r o n s w h i c h r e s u l t f r o m the d e - e x c i t a t i o n o f t h e n u c l e u s . The r e c o i l l e s s f r a c t i o n , t h a t i s , t h e r e l a t i v e number o f e v e n t s i n w h i c h no exchange o f momentum o c c u r s between the n u c l e u s and i t s e n v i r o n m e n t , i s d e t e r m i n e d p r i m a r i l y by the quantum m e c h a n i c a l and p h y s i c a l s t r u c t u r e of the s u r r o u n d i n g media. I t i s thus not poss i b l e t o o b s e r v e a Môssbauer e f f e c t o f a n a c t i v e n u c l e u s i n a l i q u i d , s u c h a s an i o n o r a m o l e c u l e i n solution. This repres e n t s a s e r i o u s l i m i t a t i o n t o t h e s t u d y o f c e r t a i n phenomena; i t a l l o w s , however, the i n v e s t i g a t i o n o f f i l m s o r adsorbed m o l e c u l e s on s o l i d s u r f a c e s without i n t e r f e r e n c e from other species i n s o l u t i o n . This factor i n conjunctio t h i n layers of l i q u i d s spectroscopy p a r t i c u l a r l y a t t r a c t i v e f o r i n s i t u studies of a v a r i e t y o f e l e c t r o c h e m i c a l systems. These advantages, however, have n o t a p p a r e n t l y been f u l l y r e a l i z e d , as evidenced by the r e l a t i v e l y s m a l l number o f r e p o r t s i n the l i t e r a t u r e (17). The f i r s t i n s i t u MES I n v e s t i g a t i o n o f m o l e c u l e s a d s o r b e d on e l e c t r o d e s u r f a c e s was aimed p r i m a r i l y a t a s s e s s i n g the f e a s i b i l i t y of such measurements i n systems of i n t e r e s t to e l e c t r o c a t a l y s i s ( 1 8 ) . I r o n p h t h a l o c y a n i n e , F e P c , was c h o s e n as a m o d e l s y s t e m because o f the a v a i l a b i l i t y o f p r e v i o u s ex s i t u Môssbauer s t u d i e s and i t s i m p o r t a n c e as a c a t a l y s t f o r 0 r e d u c t i o n . The r e s u l t s o b t a i n e d have p r o v i d e d c o n s i d e r a b l e i n s i g h t i n t o some o f the f a c t o r s which c o n t r o l t h e a c t i v i t y o f F e P c and p e r h a p s o t h e r t r a n s i t i o n m e t a l m a c r o c y c l e s f o r 0 r e d u c t i o n . These can be summarized as f o l l o w s : 2

2

(1) The Môssbauer s p e c t r a o f F e P c p r e a d s o r b e d on V u l c a n XC-72 c a r b o n a t l o a d i n g s r a n g i n g f r o m 3.5 up t o 10% w/w e x h i b i t o n l y one d o u b l e t w i t h a quadrupole s p l i t t i n g much s m a l l e r than t h a t f o r b u l k FePc. T h i s d o u b l e t has been denoted as 2 and t h a t c o r r e s p o n d i n g t o t h e b u l k F e P c a s 1^ i n t h e r e f e r e n c e d c o m m u n i c a t i o n s ( 1 8 ) ( s e e F i g . 6 ) . The d i s p e r s i o n m e t h o d c o n s i s t e d i n d i s s o l v i n g t h e F e P c in p y r i d i n e to a c o n c e n t r a t i o n o f 10""^ M f o l l o w e d by the a d d i t i o n o f the h i g h s u r f a c e a r e a c a r b o n i n s m a l l q u a n t i t i e s under u l t r a s o n i c a g i t a t i o n i n an amount s u f f i c i e n t t o o b t a i n the d e s i r e d c a t a l y s t t o c a r b o n f r a c t i o n i n the f i n a l product. Most o f the s o l v e n t was t h e n evaporated under vacuum w i t h a w a t e r a s p i r a t o r and l a t e r heated i n f l o w i n g h e l i u m a t 300 C, a t e m p e r a t u r e b e l i e v e d t o be s u f f i c i e n t f o r the r e m o v a l o f p y r i d i n e e i t h e r o c c l u d e d i n the sample o r o t h e r w i s e b o u n d t o F e P c . A t h i g h e r l o a d i n g s (> 15% w/w) and u s i n g t h e same p r e a d s o r p t i o n p r o c e d u r e , d o u b l e t s ± and 2_ a r e observed i n d i c a t i n g a s a t u r a t i o n o f s u r f a c e s i t e s l e a d i n g to the f o r m a t i o n o f FePc crystals. (2) S i g n i f i c a n t l y d i f f e r e n t r a t i o s f o r the r e s o n a n t a b s o r p t i o n a r e a , A, o f the two d o u b l e t s have been o b t a i n e d f o r FePc preadsorbed a t h i g h l o a d i n g s (> 15%) upon changing the a v a i l a b l e s u r f a c e a r e a o f the carbon. S p e c i f i c a l l y , a h i g h e r v a l u e o f A /A^ was found 2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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F i g u r e 6. Ex s i t u Môssbauer s p e c t r a o f 30% w/w FePc d i s p e r s e d on h i g h s u r f a c e a r e a c a r b o n s . S p e c i m e n s p r e p a r e d b y m i x i n g t h e c a r b o n w i t h an FePc s o l u t i o n i n p y r i d i n e and s u b s e q u e n t l y removi n g t h e s o l v e n t b y e v a p o r a t i o n u n d e r vacuum. The s a m p l e s w e r e then heat t r e a t e d a t 420° C i n a f l o w i n g He:H (4:1) a t m o s p h e r e . ( A ) RB c a r b o n (1200 m g ) a n d ( B) V u l c a n XC-72 (250 m V ^ U i ) . 2

2

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In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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f o r t h e same l o a d i n g o f FePc on RB c a r b o n (1200 m g" ) than f o r XC-72 (250 m g"" )(17^) ( s e e F i g . 6 ) . These o b s e r v a t i o n s were r e g a r d e d as p r o v i d i n g e v i d e n c e t h a t d o u b l e t 1_ c o r r e s p o n d s t o FePc preadsorbed a t monolayer c o v e r a g e s on the c a r b o n s u r f a c e . I n an e f f o r t t o g a i n f u r t h e r i n s i g h t i n t o t h e s t r u c t u r a l and e l e c t r o n i c m o d i f i c a t i o n s introduced by t h e presumed a d s o r p t i o n o f FePc onto h i g h s u r f a c e a r e a c a r b o n , c y c l i c v o l t a m m e t r y measurements were conducted w i t h the m a t e r i a l on XC-72 c a r b o n e x h i b i t i n g o n l y t h e i n n e r d o u b l e t i n t h e Môssbauer s p e c t r u m . T h e s e e x p e r i m e n t s w e r e p e r f o r m e d w i t h t h e c a t a l y s t - c a r b o n sample i n t h e form o f a T e f l o n b o n d e d p o r o u s c o a t i n g s p r e a d i n t o a v e r y s h a l l o w (~0.1 mm deep) c y l i n d r i c a l c a v i t y o f an o r d i n a r y p y r o l y t i c g r a p h i t e r o t a t i n g d i s k e l e c t r o d e . The c y c l i c v o l t a m m e t r y o b t a i n e d i s shown i n F i g . 7. The Môssbauer s p e c t r u m o f t h e m a t e r i a l i n d r y f o r m i s g i v e n i n F i g . 8. The voltammogram o b t a i n e adsorbed from aqueous s o l u t i o same c a r b o n ( s e e F i g . 9 ) . The redox p r o c e s s e s a s s o c i a t e d w i t h t h e v o l t a m m e t r y f o r what was b e l i e v e d t o be FePc adsorbed have been r e c e n t l y examined w i t h i n s i t u MES i n 1 M NaOH (19) Two s t r i k i n g l y d i f f e r e n t s p e c t r a were o b t a i n e d a s a f u n c t i o n o f t h e a p p l i e d p o t e n t i a l , as shown i n F i g . 10. The Môssbauer p a r a m e t e r s a s s o c i a t e d w i t h t h e s e s p e c t r a l f e a t u r e s were found t o be v e r y s i m i l a r t o those f o r a f e r r i c ( c u r v e A) and a f e r r o u s ( c u r v e B) h y d r o x i d e s p e c i e s (20). F u r t h e r m o r e , t h e l i t e r a t u r e (21) i n d i c a t e s t h a t an i r o n m e t a l e l e c t r o d e d e v e l o p s , upon r e p e a t e d c y c l i n g i n 1 M NaOH s o l u t i o n , a c y c l i c voltammogram w i t h p e a k p o s i t i o n a l m o s t i d e n t i c a l t o t h a t s h o w n i n F i g . 7. I t t h u s appears t h a t t h e manner i n w h i c h t h e samples were p r e p a r e d l e a d s t o a p a r t i a l or t o t a l degradation of the macrocycle involving the d e m e t a l l a t i o n a n d f u r t h e r f o r m a t i o n o f an i r o n h y d r o x i d e s p e c i e s . T h i s i s a r a t h e r s u r p r i s i n g f i n d i n g as b u l k FePc c a n be s u b l i m e d a t t e m p e r a t u r e s a s h i g h a s those employed d u r i n g t h e r e m o v a l o f p y r i d i n e from t h e sample w i t h o u t undergoing d e c o m p o s i t i o n . I t s h o u l d be mentioned t h a t i n a r e c e n t r e p o r t (22) i t has been c l a i m e d t h a t a μoxo f o r m o f FePc c a n be f o r m e d u n d e r c e r t a i n c i r c u m s t a n c e s w h i c h becomes t h e r m a l l y u n s t a b l e a t ~340 C r e s u l t i n g i n the d e s t r u c t i o n o f t h e m a c r o c y c l e . I t i s t h u s p o s s i b l e t h a t t h e unexpected phenomenon found i n t h e p r e s e n t s t u d y may be r e l a t e d t o t h e f o r m a t i o n o f t h i s l>-oxo compound d u r i n g t h e p r e p a r a t i o n o f t h e s a m p l e s p r i o r t o o r d u r i n g t h e heat t r e a t m e n t . A l t e r n a t i v e l y , t h e m a c r o c y c l i c degrada­ t i o n may be a s s o c i a t e d w i t h t h e presence o f a d v e n t i t i o u s t r a c e 0 i n the h e a t i n g system. A f u l l i n v e s t i g a t i o n o f such p o s s i b i l i t i e s i s c u r r e n t l y under way i n t h i s l a b o r a t o r y . T h i s s t u d y i l l u s t r a t e s t h e use o f JLn s i t u MES as a p p l i e d t o t h e i n v e s t i g a t i o n o f s p e c i e s i n v o l v e d i n redox processes i n porous electrodes. I t i s expected that a systematic u t i l i z a t i o n of t h i s t e c h n i q u e may e n a b l e t h e a c q u i s i t i o n o f m i c r o s c o p i c l e v e l i n f o r ­ m a t i o n o f d i f f i c u l t a c c e s s i b i l i t y w i t h o t h e r s p e c t r o s c o p i c methods, a l t h o u g h l i m i t e d t o o n l y Môssbauer a c t i v e n u c l e u s . 2

1

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Acknowledgments

Support f o r t h i s work has been p r o v i d e d by t h e Department o f Energy, the O f f i c e o f N a v a l R e s e a r c h , t h e Diamond Shamrock C o r p o r a t i o n and

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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F i g u r e 7. C y c l i c voltammogram o f 7% w/w FePc d i s p e r s e d on V u l c a n XC-72 c a r b o n . The s p e c i m e n was p r e p a r e d by m i x i n g t h e c a r b o n w i t h an FePc s o l u t i o n i n p y r i d i n e and s u b s e q u e n t l y removing t h e s o l v e n t b y b o i l i n g i t o f f . The s a m p l e was t h e n h e a t t r e a t e d a t 300°C i n f l o w i n g He t o remove c o o r d i n a t e d p y r i d i n e . The c y c l i c voltammogram was o b t a i n e d w i t h t h e m a t e r i a l i n t h e form o f a t h i n porous c o a t i n g i n 1 M NaOH a t 25°C. Sweep r a t e : 5 mV s " ( 1 9 ) . 1

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YEAGER ET AL.

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F i g u r e 9. C y c l i c voltammogram o f Fe-TsPc adsorbed on V u l c a n XC72 a t monolayer coverages. T h i s measurement was o b t a i n e d w i t h the m a t e r i a l i n t h e form o f a t h i n porous c o a t i n g . Scan r a t e : 50 mV s · Other c o n d i t i o n s a r e t h e same a s those i n c a p t i o n o f F i g . 6.

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1155 16th St.,Deviney, N.W. M., et al.; In Catalyst Characterization Science; ACS Symposium Series; American Chemical Society: Washington, DC, 1985. Washington, D.C 20036

547

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F i g u r e 10. I n s i t u M o s s b a u e r s p e c t r a f o r a V u l c a n XC-72 c a r b o n e l e c t r o d e c o n t a i n i n g F e P c o b t a i n e d a t 0·0 V v s . Hg/HgO,OH~, ( F i g u r e A) and a t -1.05 V v s . Hg/HgO,OH~, ( F i g u r e B ) ( 1 9 ) . In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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the I n t e r n a t i o n a l B u s i n e s s M a c h i n e s C o r p o r a t i o n t h r o u g h a F a c u l t y D e v e l o p m e n t A w a r d t o one o f t h e a u t h o r s (D.S.). The Raman d a t a i n t h i s p a p e r h a v e b e e n o b t a i n e d b y Dr. B. S i m i c - G l a v a s k i , S. Z e c e v i c and R. A d z i c . The a u t h o r s e x p r e s s a p p r e c i a t i o n t o them f o r t h e i r important contribution.

Literature Cited 1. Shigehara K.; Anson, F.C. J. Phys. Chem. 1982, 86, 2776. 2. Liu H.Y.; Weaver, M.J.; Wang, C.B.; and Chang, C.K. J. Electroanal. Chem. 1983, 145, 439. 3. Chang, C.K.; L i u , H.Y.; and Abdalmuhdi, I. J. Am. Chem. Soc. 1984, 106, 2725. 4. Zagal, J.; Sen, R.K.; and Yeager, E.B. Electroanal. Chem. 1977, 83, 207. 5. Zagal, J.; Bindra, 1980, 127, 1506. 6. Tarasevich, M.; Sadkowski Α.; Yeager, E.B. In "Comprehensive Treatise of Electrochemistry"; Conway, Β. E.; Bockris, J. O'M.; Yeager, E.B.; Khan, S.U.M.; White, R.E., Eds.; Plenum Press: New York, 1983; Vol. VII, p. 301. 7. Gland, J. L.; Sexton Β. Α.; and Fisher, G.B. Surf. Sci. 1980, 95, 587. 8. Collman, J. P.; Gagne, R. R.; Reed, C. Α.; Robinson, W. T.; and Rudley, G. A. Proc. Nat. Acad. Sci. 1974, 71, 1326. 9. Watanabe, T.; Ama, T.; and Nakamoto, K. J. Phys. Chem. 1984, 88, 440. 10. Collman, J.P.; Denisevich, P.; Konai, P.; Marrocco, M.; Koval, C.; and Anson, F. C. J. Am. Chem. Soc. 1980, 102, 6027. 11. Sarangapani, S.; Urbach, F.; and Yeager, Ε. B. i n preparation. 12. Nikolic, Β. Z.; Adzic, R. R.; and Yeager, E. B. J. Electroanal. Chem. 1979, 103, 281. 13. "Surface Enhanced Raman Spectroscopy"; Chang, R.K.; Furtak, T. E., Eds.; Plenum Press: New York, 1982. 14. Simic-Glavaski, B.; Zecevic, S.; and Yeager, E. B. J. Electro­ anal. Chem. 1983, 150, 469. 15. S i m i c - G l a v a s k i , B.; Adzic, R. R.; and Yeager, Ε. B. i n preparation. 16. "Applications of Mossbauer Spectroscopy"; Cohen, R.L. Ed.; Academic Press: New York, 1980. 17. Scherson, D.; Yao, S. B.; Yeager, E. B.; Eldridge, J.; Kordesch, M. E.; and Hoffman, R. W. J. Electroanal. Chem. 1983, 157, 129. 18. Scherson, D.; Yao, S. B.; Yeager, Ε. B.; Eldridge, J.; Kordesch, M. E.; and Hoffman, R. W. J. Phys. Chem. 1983, 87, 932. 19. Scherson, D.; Fierro, C.; Gupta, S. L.; Tryk, D.; Yeager, Ε. B.; Eldridge, J.; and Hoffman, R.W. submitted to J. Electroanal. Chem. 20. O'Grady, W. E. J. Electrochem. Soc. 1980, 127, 555, and refer­ ences therein. 21. Burke, L. D.; and Murphy, O. J. J. Electroanal. Chem. 1980, 109, 379. 22. Ercolani,C.;Gardini, M.; Monacelli, F.; Pennesi G.; and Rossi, G. Inorg. Chem. 1983, 22, 2584. RECEIVED June 21, 1985 In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

45 IR Spectroscopy as an In Situ Probe for Molecular Structure in Electrocatalytic and Related Reactions Alan Bewick and Maher Kalaji Department of Chemistry, University of Southampton, Southampton, England

Techniques are describe spectra of species, either adsorbed or free in the electrode/electrolyte solution interphase. Applications slanted towards topics relevant to electrocatalytic processes are discussed to illustrate the capabilities of the methods in probing molecular structure, orientation and interactions. Data obtained using spectroscopic methods have had a profound influence on the development of many areas of chemistry. In most cases the value of these methods stems from their high sensitivity and molecular specificity which can lead directly to a quantitative assessment and a structural characterisation of the species present in a chemical system and possibly also to information on their interactions and reactions. High vacuum surface science provides a particularly spectacular example of the value of spectroscopic methods and the rapid progress that can be engendered by their intelligent deployment; thus the use of electron diffraction techniques to determine the long range atomic or molecular order on solid surfaces and adlayers in conjunction with UPS, XPS, EELS and IR reflection spectroscopy to characterise molecular structure and orientation have produced rapid advances in the understanding of surface chemistry and surface physics at the fundamental level. There has also been a direct technological benefit in the use of this information to improve catalytic processes and to develop new catalysts. It is a relatively simple step to remove the gas in a solid surface/gas phase system without seriously violating the structural integrity of the interface which can then be characterised using the high vacuum techniques. It would be expected, however, that only in very few cases will i t be possible to remove the electrolyte from the electrode/electrolyte solution interface without producing major changes; thus the high vacuum methods have only limited applicability as ex-situ probes for the structure of the interface in electrochemical systems. Fortunately, the ease with which the electrode potential can be used to control and change, (i) the electrochemical potential of species in the electrical double layer, (11) the free energy of adsorption/desorption processes and ( i l l ) the rate constants 0097-6156/ 85/0288-0550S06.00/0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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for electron transfer processes, allows the small number of species i n the electrode/electrolyte solution interphase to be distinguished from the overwhelming number i n the bulk e l e c t r o l y t e i n a manner which has enabled i n - s i t u spectroscopic methods of investigation to be developed. These methods now include ESR spectroscopy (1), u v / v i s i b l e r e f l e c tion (2), transmission (3) and Raman scattering spectroscopies (4) and IR r e f l e c t i o n spectroscopy ( 5 , 6 , 7 , 8 , 9 ) . This paper w i l l be r e s t r i c t e d to a discussion of the l a t t e r . Experimental techniques There are two major problems to be overcome i n developing i n - s i t u , external reflectance methods with submonolayer s e n s i t i v i t y towards adsorbates on smooth electrode surfaces. The f i r s t i s to design a c e l l allowing s u f f i c i e n t radiation to emerge from i t after passing twice through the e l e c t r o l y t aqueous s o l u t i o n . The absorbance change, usually i n the range 10" to 10" , caused by the adsorbate i n the presence of the overwhelming quantity of absorbing species i n the t o t a l o p t i c a l path. Use of a t h i n layer c e l l overcomes the f i r s t problem and adequate s e n s i t i v i t y i s achieved by the marriage of electrochemical and spectroscopic techniques. The basic method has developed i n two d i s t i n c t d i r e c t i o n s : the f i r s t employs a grating spectrometer and i s c a l l e d electrochemically modulated i n f r a red spectroscopy (EMIRS) (5); the second i s subtractively normalized i n t e r f a c i a l Fourier transform infrared spectroscopy (SNIFTIRS) (6). Both methods obtain the necessary s e n s i t i v i t y by modulating the e l e c trode potential between two values which define two d i s t i n c t states of the electrode surface; thus the chemistry to be observed i s d i r e c t l y modulated and may be detected with great s e n s i t i v i t y by an appropriate form of synchronous detection. In the case of EMIRS, the modulation frequency i s made s u f f i c i e n t l y high compared to the wavelength scanning rate to enable a phase sensitive detection system to be used whereas, for SNIFTIRS, the electrode potential i s held for a s u f f i c i e n t period at each potential to accumulate data from several i n t e r f e r o metric scans and, after an adequate number, the two sets of data are ratioed. C e l l design Figure 1 i l l u s t r a t e s a t y p i c a l c e l l design containing the three e l e c trodes required for control of the electrode potential and i n which the t h i n layer of e l e c t r o l y t e , t y p i c a l l y 1 urn to 50 urn, i s produced by pushing the working electrode surface up to the surface of the IR window into the c e l l . The window can be a disc with p a r a l l e l working surfaces or i t can be prismatic to allow normal incidence of the r a d i a t i o n at the surfaces; the l a t t e r design minimises r e f l e c t i o n losses at the air/window interfaces but i t requires a longer pathlength i n the window. The t h i n layer c e l l thus formed necessarily has a large value of uncompensated resistance due to the e l e c t r o l y t e l a y e r . T y p i c a l l y , t h i s leads to a time constant of the order of 10" s for the charging of the double layer i n response to a potential step. This factor together with the potential drop across the uncompensated resistance due to faradaic currents needs to be taken into account when designing experiments. 2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Reflection/absorption at a metal surface For a vibrating molecule to absorb radiation from an incident IR beam at the frequency of a p a r t i c u l a r normal mode i t must be situated at a position of f i n i t e intensity and with an orientation such that there i s a f i n i t e component of the dipole derivative du^/dQ^ i n the d i r e c t i o n of the e l e c t r i c vector of the r a d i a t i o n f i e l d , where d u i s the change of dipole for the change of normal mode coordinate dQ^. At a metal surface there i s always a zero intensity for radiation having the e l e c t r i c vector p a r a l l e l to the surface. As a result s-polarised radiation cannot interact with a molecule on a metal surface; ppolarised radiation has a f i n i t e intensity and can interact but the intensity of the resultant absorption band w i l l depend upon the o r i e n t a t i o n of the molecule with respect to the surface, i . e . t h e r e i s an apparent selection r u l e , the surface selection rule (10), which operates i n addition to th l selectio rule fo IR absorption In EMIRS and SNIFTIRS measurement t i o n i s prevented from reaching i t i e s of the v i b r a t i o n a l bands observed i n the spectra from the remaining p-polarised radiation are used to deduce the orientation of adsorbed molecules. It should be pointed out, however, that v i b r a t i o n a l coupling to adsorbate/adsorbent charge transfer (11) and also an electrochemically activated Stark effect (7,12,13) can lead to apparent v i o l a t i o n s of the surface selection rule which can invalidate simple deductions of o r i e n t a t i o n . The surface active/surface inactive difference between p - p o l a r i s e d / s-polarised r a d i a t i o n has enabled an alternative modulation technique, p o l a r i s a t i o n modulation, to be developed (15,16). In electrochemical applications, i t allows surface s p e c i f i c i t y to be achieved whilst working at fixed potential and without electrochemical modulation of the interface. It can be implemented either on EMIRS or on SNIFTIRS spectrometers and can be very valuable i n dealing with electrochemically i r r e v e r s i b l e systems; however, the achievable s e n s i t i v i t y f a l l s well short of that obtained with electrochemical modulation. It should also be noted that i t s "surface s p e c i f i c i t y " i s not t r u l y surface but extends out into the e l e c t r o l y t e with decreasing s p e c i f i c i t y to about half a wavelength. i

Information accessible The EMIRS and SNIFTIRS methods provide the IR v i b r a t i o n a l spectra ( r e a l l y the difference spectra - see later) of a l l species whose population changes either on the electrode surface or i n the e l e c t r i c a l double layer or i n the diffusion layer i n response to changing the electrode p o t e n t i a l . Spectra w i l l also be obtained for adsorbed species whose population does not change but which undergo a change i n orientation or for which the electrode potential a l t e r s the i n t e n s i t y , the p o s i t i o n or shape of IR absorption bands. Shifts i n band maxima with potential at constant coverage ( < ^ / 9 ) 0 very common for adsorbed species and they provide valuable information on the nature of adsorbate/absorbent bonding and hence also additional data on adsorbate o r i e n t a t i o n . In p r i n c i p l e , therefore, these valuable techniques can provide a l l of the information needed to specify the molecular structure of the electrode/electrolyte solution interphase, the dynamics of adsorption/ E

a

r

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desorption processes and the nature of reaction intermediates. In many cases, i t w i l l not be easy unambiguously to deduce quantitative molecular populations; t h i s aspect has been neglected to date but w i l l certainly command increasing attention i n future. It w i l l be clear that EMIRS and SNIFTIRS spectra are difference spectra and can be somewhat complex (5). T y p i c a l l y they w i l l contain positive absorption bands from species present i n excess at potential Ei compared to potential E and negative absorption bands from species whose polulation changes oppositely with p o t e n t i a l . In addition, bands which s h i f t with potential w i l l appear as a single bipolar band either with one lobe of each sign, figure 2, (or even more complex structures with three lobes). 2

Electrocatalytic

reactions

E l e c t r o c a t a l y s i s necessaril and the characterisation t i a l for an adequate understanding of the reaction pathways. Many of these reactions are important technologically; both the anodic and the cathodic reactions i n fuel c e l l s are good examples i n t h i s category. Hydrogen evolution and hydrogen oxidation are both of interest i n t h i s context. The exact characterisation of the adsorbed hydrogen atom involved i n these reactions i s not l i k e l y to stimulate technological progress but EMIRS studies have yielded information of considerable interest (16,17). Hydrogen adsorption On platinum group metals, hydrogen adsorption/desorption i s reversible and thus readily lends i t s e l f to investigation by EMIRS. The two kinds of adsorbed hydrogen on p o l y c r y s t a l l i n e platinum, strongly adsorbed hydrogen (H ) formed at less negative potentials and weakly adsorbed hydrogen (H^) produced nearer the reversible potential for hydrogen evolution, are c l e a r l y distinguishable spectroscopically either using u v - v i s i b l e (18) or infrared radiation (16). In the i n f r a r e d , the formation of H substantially increases the r e f l e c t i v i t y of the electrode and t h i s i s observed as a featureless negative absorption over a wide range of wavelength. When the electrode i s covered with both H and H^, absorption bands are observed superimposed on this baseline s h i f t but, over the range 4000 cm" to 1250 cm" , these are a l l at the v i b r a t i o n a l frequencies of hydrogen bonded water; no bands corresponding to Pt-H vibrations are observed. The bands a l l had the same sign: increased absorption when the metal was covered by H . Their assignment to v i b r a t i o n a l modes of water associated with the adsorbed were confirmed from isotopic s h i f t s using H 2 O / D 2 O mixtures; examples are i l l u s t r a t e d i n figure 3. The d i f f i c u l t task of seeing t h i s surface water through the large amount i n the ambient e l e c t r o l y t e was f a c i l i t a t e d by using the H 2 O / D 2 O mixtures to maximise energy throughput over the spectral region. Nine bands were c l e a r l y i d e n t i f i e d and measured from the range: V i , V 2 , V 3 , (V2 + V 3 ) , 2V3,(2V2 + V 3 ) for H 2 O , HDO and D2O. The r e l a t i v e i n t e n s i t i e s of these bands indicated a p a r t i c u l a r orientation for the water structure interacting with the adsorbed H atoms. Application of the surface selection rule leads to the model shown i n figure 4(a) i n which oriented water dimer units are hydrogen bonded to on one side g

s

g

1

w

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Reference electrode

Figure 1.

The IR spectroelectrochemical c e l l .

Ε 1

Wavenumber

Figure 2.

Wavenumber

Origin of a bipolar difference band.

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Figure 4 . The structure and orientation of: (a) weakly bound hydrogen and i t s associated water on a Pt or Rh electrode; (b) water on the electrode surface at potentials i n the double layer region.

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and, on the other, to water molecules further out from the surface. Figure 4(b) shows the average water orientation assumed for a bare Pt surface at potentials i n the double layer region; one lone pair o r b i t a l i s assumed to be roughly normal to the surface. This o r i e n t a t i o n i s i n agreement with other electrochemical data and was used as the reference state for the EMIRS difference spectra. E l e c t r o c a t a l y t i c oxidations Interest i n fuel c e l l s has stimulated many investigations into the detailed mechanisms of the e l e c t r o c a t a l y t i c oxidation of small organic molecules such as methanol, formaldehyde, formic a c i d , e t c . The major problem using platinum group metals i s the rapid b u i l d up of a strongly adsorbed species which e f f i c i e n t l y poisons the electrodes. Thus a substantial effort has been made by electrochemists to identify this species; use of non-spectroscopi technique t evaluat th number of surface s i t e s number of electrons require census view that i t was :COH. One of the f i r s t successful applications of EMIRS was an attempt to corroborate t h i s conclusion (19); however, the spectroscopic data c l e a r l y i d e n t i f i e d CO as the poison and no evidence for COH could be obtained. Further detailed spectroscopic measurements using a variety of organic fuels ( C H 3 O H , HCHO, HCOOH and CH2OH.CH2OH) and electrode materials (Pt, Rh, Pd, Au) substantiated t h i s finding as a general result i n such systems (20,21,22). Examples of EMIRS spectra obtained from f u l l y poisoned electrodes are shown i n figures 5, 6 and 7. In every case, absorption bands are seen from one or more adsorbed CO species, the types and t h e i r exact v i b r a t i o n frequencies depending upon the nature of the metal. Thus, for example, Pt supports both l i n e a r l y adsorbed CO, the band near 2070 cm" , and also CO s i t t i n g i n a higher coordination s i t e with a v i b r a t i o n frequency near 1850 cm" . On Pd bridge-bonded and more highly coordinated CO can be i d e n t i f i e d . These assignments have been made on the basis of the c l a s s i f i c a t i o n which has developed from gas phase adsorption of CO followed by the use of high vacuum spectroscopic techniques (23). There i s a very close agreement between these data and the EMIRS spectra. Direct adsorption of CO on the electrodes from gas dissolved i n the e l e c t r o l y t e gave almost i d e n t i c a l spectra, at high coverage, to those obtained after organic oxidation. The same adsorbed CO species were obtained on Pt by reduction of C0 (24). This was observed to take place only i n the range of potentials where adsorbed hydrogen i s formed, i n d i c a t i n g that the reduction takes place i n d i r e c t l y v i a the adsorbed atomic hydrogen. In t h i s case, i t was noticed that the more highly coordinated adsorbed CO, seen near 1850 cm"" , was formed very r e a d i l y , whereas i t i s formed d i r e c t l y from dissolved CO gas only after potential c y c l i n g of the electrode. There i s evidence that t h i s species i s usually formed by reduction of C 0 ; i t should be noted that, at non-poisoned electrodes, some CO2 i s produced from the organic substrates at quite low potentials and many of these also form adsorbed hydrogen during t h e i r dissociative e l e c t r o sorption. 1

1

2

1

2

Adzic et a l (25) have shown that p a r t i a l coverage of the e l e c trode by adsorbed Pb can substantially reduce the effects of poisoning, presumably by blocking the surface s i t e s required by the adsorbed CO. This i s nicely confirmed by spectroscopic measurements. Figure 8

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