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Catalysis Under Transient Conditions
 9780841206885, 9780841208704, 0-8412-0688-0

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 Understanding Heterogeneous Catalysis Through the Transient Method......Page 8
Some Experimental Methods......Page 9
Modes of Operation......Page 16
The Oxidation of Carbon Monoxide on Platinum......Page 25
The Reaction of Hydrogen and Carbon Monoxide over Iron......Page 31
Literature Cited......Page 36
2 Transient Low-Pressure Studies of Catalytic Carbon Monoxide Oxidation......Page 40
Transients Induced by Temperature Changes......Page 41
Transients Induced by Pressure Changes......Page 48
Acknowledgement......Page 63
Literature Cited......Page 64
3 The Dynamic Behavior of Three-Way Automotive Catalysts......Page 66
Time-Averaged Conversion Measurements......Page 67
Dynamic Gas-Phase Measurements......Page 69
Measurement of Catalyst Oxygen Content......Page 79
Future Studies......Page 81
Literature Cited......Page 84
4 Dynamics of High-Temperature Carbon Monoxide Chemisorption on Platinum-Alumina by Fast-Response IR Spectroscopy......Page 86
Fast-Response Infrared Reactor System......Page 87
Theory of Transient, Diffusion-Coupled CO Chemisorption......Page 91
Results and Discussion......Page 98
Concluding Remarks......Page 104
Nomenclature......Page 106
Greek Letters......Page 107
Literature Cited......Page 108
5 Nitric Oxide Reduction by Hydrogen over Rhodium Using Transient Response Techniques......Page 111
Experimental......Page 112
Results......Page 115
Discussion......Page 122
Nomenclature......Page 145
Literature Cited......Page 146
Introduction......Page 148
Experimental......Page 149
NO, CO And HC Conversions......Page 151
Sulfur Dioxide and Propane Oxidation......Page 154
Temperature Programmed Reduction (TPR)......Page 157
Infrared (IR) Study......Page 162
Conclusion......Page 164
Practical Considerations......Page 165
Literature Cited......Page 166
7 Decomposition of Nitrous Oxide on Magnesium Oxide Compared to That on Cupric Oxide......Page 168
Experimental......Page 169
Results......Page 170
Discussion......Page 180
Literature Cited......Page 184
8 Transient and Steady-State Vapor Phase Electrocatalytic Ethylene Epoxidation Voltage, Electrode Surface Area, and Temperature Effects......Page 185
Experimental Methods......Page 188
Results......Page 189
Discussion......Page 203
Conclusions......Page 208
List of symbols......Page 209
Literature Cited......Page 211
9 Characterization of the Adsorbed Layer of a Silver Catalyst in the Oxidation of Ethylene from Its Transient Adsorption Behavior......Page 213
Experimental Method......Page 215
Transient Behavior of Oxygen......Page 216
Adsorption Behavior of C2H4......Page 224
Transient Behavior of CO2......Page 231
Literature Cited......Page 241
10 Development of a Pulse Reactor with On-Line MS Analysis to Study the Oxidation of Methanol......Page 242
Methanol Oxidation......Page 244
Experimental......Page 245
Results and Discussion......Page 247
Literature Cited......Page 254
11 Square Concentration Pulses in Oxidation Catalysis over Perovskite-type Oxides of Manganese and Iron......Page 255
Micro Flow Unit and Square Pulse Unit......Page 257
Results......Page 259
Discussion......Page 262
Literature Cited......Page 267
Abstract......Page 269
Results and Discussion......Page 271
Acknowledgements......Page 276
Literature Cited......Page 277
Abstract......Page 278
Basic equations......Page 280
Propagation speed......Page 282
Application of wavefront analysis to watergas shift reaction......Page 283
Conclusions......Page 297
List of Symbols......Page 300
Literature Cited......Page 301
C......Page 303
G......Page 304
Ν......Page 305
P......Page 306
T......Page 307
Z......Page 308

Citation preview

Catalysis Under Transient Conditions

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Catalysis Under Transient Conditions Alexis T . Bell, EDITOR University of

California—Berkeley

L . Louis Hegedus, EDITOR W.

Based on a symposium sponsored by the Division of Colloid and Surface Chemistry at the Second Chemical Congress of the North American Continent (180th A C S National Meeting), Las Vegas, Nevada, August 27-28,

ACS

1980.

SYMPOSIUM

AMERICAN WASHINGTON,

SERIES

CHEMICAL D.

SOCIETY C.

1982

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

178

Library of Congress CIP Data Catalysis under transient conditions. (ACS symposium series, ISSN 0097-6156; 178) Includes bibliographies and index. 1. Catalysis—Congresses. I. Bell, Alexis T., 1942. II. Hegedus, L. Louis, 1941. III. American Chemical Society. Division of Colloid and Surface Chemistry. IV. Chemical Congress of the North American Continent (2nd: 1980: Las Vegas, Nevada). V . Series. QD505.C385 541.3'95 81-20639 ISBN 0-8412-0688-0 AACR2 ACSMC8 178 1-308 1982

Copyright © 1982 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each article in this volume indicates the copyright owner's consent that reprographic copies of the article 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. 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 new collective work, for resale, or for information storage and retrieval systems. 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.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

ACS Symposium Series M . Joan Comstock,

Advisory

Series

Editor

Board

David L. Allara

Marvin

Robert Baker

Robert O r y

Donald D . Dollberg

Leon Petrakis

Robert E . Feeney

Theodore Provder

Brian

Charles

W.

M.

Harney

Jeffrey

Howe

Margoshes

N.

Satterfield

Dennis Schuetzle

James D . Idol, J r .

Davis L . T e m p l e , J r .

Herbert D .

Gunter Z w e i g

Kaesz

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

FOREWORD The A C S

S Y M P O S I U M

S E R I E S

was founded in 1 9 7 4 to provide

a medium for publishing symposia quickly in book form. T h e format of the Series parallels that of the continuing EST

C H E M I S T R Y

S E R I E S

A D V A N C E S

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 since symposia may embrace both types of presentation.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

PREFACE The

interest in the dynami

is experiencing a renaissance. Attention to this area has been motivated by several factors: the availability of experimental techniques for monitoring species concentrations both in the gas phase and at the catalyst surface with a temporal resolution and sensitivity not previously possible, the development of efficient numerical methods for predicting the dynamics of complex reaction systems, and the recognition that in selected instances operation of a catalytic reactor under dynamic conditions can yield a better performance than operation under steady-state conditions. The papers in this book were selected so that both the chemical and the engineering aspects of catalyst dynamics are represented; in addition, an attempt was made to draw a balance between theory and experiment. The editors thank all the contributors to this volume and express their hope that the material will stimulate further interest in the dynamic operation of catalysts, both for its diagnostic powers and for its potential in improving reactor performance.

ALEXIS T. BELL University of California Berkeley, California L.LOUISHEGEDUS W. R. Grace & Company Columbia, Maryland July 27, 1981

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

1

Understanding Heterogeneous Catalysis T h r o u g h the T r a n s i e n t M e t h o d C. O. BENNETT University of Connecticut, Department of Chemical Engineering, Storrs,CT06268 Although catalysis can be understood through steady-state ex­ periments, it seems clear that transient experiments will usually furnish much additiona be explained by a numbe transient experiments are usually so rich that only a detailed, complex model will come close to explaining the results. These ideas have long been applied in other fields, but in heterogeneous catalysis they have come into acceptance only during the last 15 years or so. Pioneering experiments were done by Wagner (1) in 1938, and Tamaru (2) became an advocate of the method in 1963, em­ phasizing the measurement of adsorption during catalysis, the es­ sence of the transient method. A quantitative framework was laid out in 1967 (3), and its implementation was carried out a few years later by Kobayashi and Kobayashi (4) and by Yang et al. (5). Effects due to transport resistances must be carefully avoid­ ed in work at atmospheric pressure, so that chemical effects hav­ ing response times less than about a second cannot be followed. These limitations are avoided at low pressure, and some of the first transient results under vacuum conditions have been reported by Bonzel and Ku (6) and by Jones et al. (7). Progress in tran­ sient studies up until about 1975 has been reviewed (8,9). The transient processes under discussion here must be de­ scribed in terms of elementary steps, and the relative rates of the steps will change as conditions change during transients. This point of view has been advocated by Temkin (10), and it is central to a recent discussion by Krylov (11). The adsorptive capacity of the catalyst influences the transients—one has ad­ sorption during reaction. Although modeling in terms of elemen­ tary steps is central to understanding catalysis and to modeling reactors during transients, chemical engineers did not take this idea into account until recently. From the first cycling studies of Dorawala and Douglas (12) through the time of the review of Schmitz in 19 75 (13), reactor models were based on the use of a steady-state rate expression in the component balances. This state of affairs has now changed, and the importance of elementary step models is generally acknowledged (14). 0097-6156/82/0178-0001$08.00/0 © 1982 American Chemical Society In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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The bulk of t h i s review concerns t r a n s i e n t experiments on heterogeneous c a t a l y s i s at atmospheric pressure. A f t e r some comments on current methods f o r doing the experiments, the a p p l i c a t i o n of the method w i l l be i l l u s t r a t e d by two examples: the o x i d a t i o n of CO over P t , and the hydrogénation of CO over Fe. Some Experimental Methods Gas-phase K i n e t i c s . A b e t t e r a p p r e c i a t i o n of the e x p e r i ments to be discussed l a t e r w i l l be obtained a f t e r a review of some experimental aspects of the t r a n s i e n t method. Here we deal w i t h experiments at atmospheric pressure. A flow sheet f o r k i n e t i c measurements i s given i n F i g . 1, a descendant of that f i r s t given by Bennett et a l . (15). Chemical a n a l y s i s of the gases during t r a n s i e n t s i s i d e a l l y done by a mass spectrometer, although Kobayashi and Kobayash i n order to get sample Step f u n c t i o n s , p u l s e s , and square waves can be generated w i t h a low volume, chromatographic-type 4-way v a l v e . We have found that the d e s i r e d two gas mixtures are best made up and s t o r e d i n c y l i n d e r s r a t h e r than made continuously by b l e n d i n g two streams. At the time of the s w i t c h , there i s a momentary stopping of the flow, and t h i s u s u a l l y r e s u l t s i n a change i n composition i f the mixture i s made by the continuous blending of two streams. By t h i s method one or more spurious peaks are added to the d e s i r e d step f u n c t i o n . N a t u r a l l y these are t r i v i a l f o r slow responses, but important f o r f a s t ones. The needle valves of F i g . 1 are operated choked, so that there i s a constant flow independent of downstream r e s i s t a n c e s . However, the v a l v e shown upstream of the bubble meter i s very imp o r t a n t . I t i s adjusted so that the pressures at the s w i t c h i n g v a l v e and upstream are h a r d l y perturbed by a s w i t c h . The flows of both mixtures are adjusted to be i d e n t i c a l by the needle valves and the bubble meter. When a l l i s p r o p e r l y r e g u l a t e d , the b a l l i n each rotameter makes a s h o r t , quick (~ls) e x c u r s i o n at the moment of the s w i t c h , and i d e n t i c a l l y f o r the r e t u r n to the f i r s t p o s i t i o n of the v a l v e . I f the flow of each mixture to the s w i t c h i n g v a l v e i s not i d e n t i c a l , w i t h minimal pressure disturbance at the s w i t c h , spurious peaks are also introduced. For instance i n an experiment at steady s t a t e w i t h H /C0 (9/1) f l o w i n g to a d i f f e r e n t i a l r e a c t o.r, a common t r a n s i e n t experiment i s a switch to H alone, so that the hydrogen can react w i t h a carbonaceous s u r f a c e i n t e r mediate, produce methane, and e f f e c t i v e l y t i t r a t e the s t e a d y - s t a t e s u r f a c e . I f t h i s experiment i s not done by a s w i t c h from H /C0 to H , but r a t h e r by suddenly c l o s i n g the CO supply v a l v e i n a system using continuous gas b l e n d i n g , a peak of methane i s formed which has nothing to do w i t h t i t r a t i n g the s u r f a c e . While H /CO i s s t i l l i n the l i n e from the 4-way v a l v e to the r e a c t o r ( t h i s 2

2

2

2

2

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

®-

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Figure 1.

QUANTASORB ROTAMETER

1

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REACTOR

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QS

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Flow sheet jor transient experiments.

He

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CALIBRATION OR PULSE GAS

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C A T A L Y S I S

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T R A N S I E N T

C O N D I T I O N S

length should be minimized), i t s flow r a t e i s reduced when the CO i s cut o f f , conversion to CH^ i s i n c r e a s e d , and there i s a f a l s e peak at the beginning of the CH^ t r a n s i e n t caused by the r e a c t i o n of H w i t h surface carbon. The same e f f e c t i s obtained by a s w i t c h from H /C0 to helium f l o w i n g at a lower r a t e . There i s a methane peak, l e a d i n g to the erroneous idea that methane has been desorbed. With p r o p e r l y ad­ j u s t e d i d e n t i c a l flows the CH^ f a l l s q u i c k l y to zero w i t h i n the response time of the system - t y p i c a l l y ~ l s . The four-way v a l v e at the o u t l e t of the r e a c t o r permits send­ i n g known gases to the mass spectrometer f o r c a l i b r a t i o n . The flow to the r e a c t o r need not be d i s t u r b e d . This arrangement i s p a r t i c u ­ l a r l y u s e f u l f o r measuring the water peak. I t i s w e l l known that water adsorbs on the i n s i d e of the instrument. A f t e r a few days of pumping, p r e f e r a b l w i t h bakeout s t a b l wate background peak can be obtained. measure the peak cause y g throug i n l e t . The response i s slower than f o r other gases, but reprodu­ c i b l e over s e v e r a l r e p i t i t i o n s a few minutes apart. However, i f a r e a c t o r product stream c o n t a i n i n g water i s sampled c o n t i n u o u s l y , the background water peak g r a d u a l l y i n c r e a s e s . Good a n a l y s i s f o r water can be made by r e c o r d i n g the 18-peak a f t e r a switch to dry helium v i a the r e a c t o r o u t l e t 4-way v a l v e . By thus keeping t r a c k of the v a r y i n g background water peak we can make a good oxygen balance f o r the Fischer-Tropsch r e a c t i o n at any i n s t a n t ; without the c o r r e c t background c o r r e c t i o n t h i s i s not p o s s i b l e . I t i s c l e a r that the i l l u s t r a t e d flowsheet i s q u i t e f l e x i b l e . As shown, pulses can be produced by a 6-way v a l v e . The valves are e l e c t r i c a l l y actuated so a c y c l e d feed can e a s i l y be produced. A separate mixture p r e p a r a t i o n apparatus has been b u i l t , so that pre­ p a r i n g the v a r i o u s mixtures needed i s q u i c k and s i m p l e . The 12" magnetic-sector mass spectrometer i s computer-con­ t r o l l e d . A t y p i c a l arrangement i s to jump the a c c e l e r a t i n g v o l t ­ age over the d e s i r e d mass numbers at a r a t e of about 5 numbers per second. The r e s u l t i n g v o l t a g e s are a p p r o p r i a t e l y converted to d i g i t a l form, s t o r e d and used to give an output of mole f r a c t i o n versus time f o r each component. O r d i n a r i l y hydrogen and helium cannot be obtained i n such a moderately f a s t scan; f o r t h i s the magnetic f i e l d would need to be changed. However, we have a sep­ arate s i d e tube of s m a l l (4") r a d i u s , w i t h a separate d e t e c t o r . With t h i s arrangement a f a s t scan from 2 to about 100 i s f e a s i b l e . E i t h e r Faraday cup o r e l e c t r o n m u l t i p l i e r detectors are used; the l a t t e r has the h i g h e r s e n s i t i v i t y but i s subject to s a t u r a t i o n effects. A c l a s s i c a l i n l e t system i s used to o b t a i n a f a s t response and to avoid mass f r a c t i o n a t i o n . The gas to be sampled impinges on a c a p i l l a r y (150 μπι I.D.) d i s c h a r g i n g i n t o a s m a l l chamber (lmL) h e l d at 1 0 Pa by an a u x i l i a r y pump. The v i s c o u s flow through the c a p i l l a r y i s of the order of 5mL/mn. From the chamber a s m a l l sample of gas passes through ca 5-ym holes i n a gold f o i l 2

2

2

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

1.

B E N N E T T

Catalysis

Through

Transient

Method

5

i n Knudsen flow and through a tube l e a d i n g t o the spectrometer source. I n f r a r e d Measurements. Elementary-step models can be f i t to t r a n s i e n t gas-phase data (5) which are obtained by the methods described above. The models w i l l then p r e d i c t the surface i n t e r mediate c o n c e n t r a t i o n during t r a n s i e n t s and at steady s t a t e . I t i s c l e a r that i t i s a l s o important t o observe these surface species e x p e r i m e n t a l l y , during t r a n s i e n t s as w e l l as at steady s t a t e . I n f r a r e d spectroscopy can be used during c a t a l y s i s i n the presence of the gas phase, so i t plays an important r o l e i n t r a n sient studies. Some of the f i r s t t r a n s i e n t i n f r a r e d data on heterogeneous c a t a l y s i s were gathered by Heyne and Tomkins i n 1966 (16). The method has been advocated d d b Tamaru wh ha r e c e n t l reviewed h i s work s i n c used the t r a n s i e n t metho (18) the method has been a p p l i e d t o k i n e t i c s t u d i e s i n p a r t i c u l a r by Ueno et a l . (19, 20) and most r e c e n t l y by Savatsky and B e l l (21). As already explained ( 9 ) , the k i n e t i c measurements are e a s i e r to i n t e r p r e t q u a n t i t a t i v e l y i f done e i t h e r i n a r e c i r c u l a t i n g r e a c t o r (CSTR) or i n a d i f f e r e n t i a l r e a c t o r but w i t h measurable conversion. I n the t y p i c a l i n f r a r e d c e l l we s t r i v e f o r a c l o s e approximation t o a d i f f e r e n t i a l r e a c t o r . The conversion i s u s u a l l y too s m a l l t o measure. Dwyer (22) and e a r l i e r Ueno et a l . (19, 20) have used the o p t i c a l system of F i g . 2. The c e l l of F i g . 3 permits c a n c e l l i n g gas phase adsorption bands. By operating the c e l l i n the r e f l e c t i o n mode as shown, p o s i t i v e c o n t r o l of the temperature of the c a t a l y s t d i s k i s achieved, w i t h a very low c e l l volume (~2 mL) and the p o s s i b i l i t y of c o o l i n g the c e l l window, as u s u a l l y r e q u i r e d by window m a t e r i a l which i s transparent over a wide range of frequencies. The temperature i s measured i n the metal block j u s t behind the m i r r o r . The thermal c o n d u c t i v i t y of the s o l i d between the thermocouple and the s u r f a c e i s high compared to the e f f e c t i v e c o n d u c t i v i t y of the gas which t r a n s f e r s heat by convection from the surface t o the c o o l e r window. Thus the s u r f a c e temperature i s assumed t o be close t o that measured by the thermocouple. Operation at 2 mL/s gives a residence time of I s , and at t h i s flow r a t e the 5 mg c a t a l y s t d i s k gives a n e g l i g i b l e conversion f o r most systems. Hegedus (23) , f o r the study of adsorbed CO over a l i m i t e d range of f r e q u e n c i e s , was able t o use sapphire windows which could be operated at 450°C. H i s r e a c t o r c e l l thus r e q u i r e s no c o o l i n g ; i t operates i n t r a n s m i s s i o n . The c e l l volume i s about 22 mL. A flow r a t e of 250 mL/s was used so that the residence time was only 0.09s. Since the flow system can be made w i t h such a f a s t response, i t becomes important to improve the response of the i n f r a r e d det e c t o r . Use of a l i q u i d - n i t r o g e n - c o o l e d Hg-CdTe d e t e c t o r , w i t h 200 t o 1000 Hz chopping and a l o c k - i n a m p l i f i e r , permits the IR

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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TO MONOCHROMATOR

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Journal of Catalysis Figure 2.

IR optical system (19).

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Figure 3.

IR cell reactor.

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In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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spectrometer to keep up w i t h the imposed s i g n a l changes (23). With a d i s p e r s i o n instrument the experiment can be run at s e v e r a l constant wave numbers i n repeated experiments. At the s i g n a l l e v e l s o f t e n obtained i n s t u d i e s of the bands of adsorbed s p e c i e s , a F o u r i e r transform instrument may r e q u i r e about 20 scans of Is each to o b t a i n an adequate i n t e r f e r o g r a m , which of course repre­ sents only an average spectrum (21) . Software has been developed to r e s o l v e t h i s problem through m u l t i p l e x i n g i n time and m i r r o r p o s i t i o n (55, 56). However, t h i s approach only works f o r a r e p e t i t i v e t r a n s i e n t , such as might r e s u l t from a c y c l e d feed. During each scan the s i g n a l , m i r r o r p o s i t i o n , and time are s t o r e d . The beginning of each scan i s o f f s e t w i t h reference to the b e g i n ing of the t r a n s i e n t by a time i n t e r v a l that may be as low as 50ys (55). Then the program s o r t s the data accumulated over enough repeated c y c l e s f o r goo signal-averaged i n t e r f e r o g r a i n t e r v a l s during the t r a n s i e n t . This technique has been a p p l i e d to gas-phase t r a n s i e n t s (55, 56) but apparently not to s u r f a c e transients. With the d i s p e r s i v e IR instrument, i t i s important to use computer c o n t r o l and data a c q u i s i t i o n . For conventional e x p e r i ­ ments a l s o , the a t t r a c t i v e n e s s of such a system has been p o i n t e d out by P e r i (24). Measurements by E l e c t r o n Spectroscopies. The v a r i o u s e l e c ­ t r o n s p e c t r o s c o p i e s permit the determination of the s u r f a c e s t a t e of a s o l i d because emitted e l e c t r o n s a few l a y e r s beneath the s u r f a c e do not escape. For example, emitted e l e c t r o n s may be e x c i t e d by a beam of i n c i d e n t e l e c t r o n s (Auger e l e c t r o n s p e c t r o s copy-AES) or by a beam of X-rays (X-ray p h o t o e l e c t r o n s p e c t r o s copy-XPS). The spectrum of i n t e n s i t y of the produced e l e c t r o n s i s recorded as a f u n c t i o n of the energy of the e l e c t r o n s , l e a d i n g to a good estimate of surface composition. By XPS even the valence s t a t e of a given element can o f t e n be determined. U s u a l l y XPS i s used i n connection w i t h t r a n s i e n t s t u d i e s only to measure the s u r ­ face c o n c e n t r a t i o n of n o n - v o l a t i l e intermediates l i k e carbon (25); the measurement must be made a f t e r the gas phase i s removed. How­ ever, Matsushima et a l . (26) have been able to f o l l o w by AES the t r a n s i e n t s u r f a c e c o n c e n t r a t i o n of oxygen on platinum as the oxygen r e a c t s w i t h carbon monoxide from the gas phase present at working spectrometer p r e s s u r e s , 1.86 χ 10~ to 4.8 χ 10~ p a s c a l s . The l i m i t a t i o n of pressures to t h i s range l i m i t s the number of systems which can be observed during t r a n s i e n t s caused by changes i n the pressure (concentration) of the gas phase i n contact w i t h the s o l i d c a t a l y s t . Some progress has r e c e n t l y been made towards adopting XPS or UPS to i n s i t u measurements during t r a n s i e n t s . R u b l o f f (27) has s t u d i e d the i n t e r a c t i o n of CO w i t h N i ( l l l ) by d i r e c t i n g a pulse of CO (200-300 ms). to the n i c k e l s u r f a c e w h i l e observing the 5σ/1π peak by UPS. The chamber pressure i s l i t t l e a f f e c t e d by 7

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the p u l s e , but Rubloff estimates that the gas pressure near the sample i s about 150 times the system pressure during the pulse (27). The pulse i s achieved by a f a s t a c t i n g d i f f e r e n t i a l l y pumped dosing v a l v e . A r e s u l t i s shown i n F i g . 4. Joyner and Roberts (28) have gone f u r t h e r by c o n s t r u c t i n g a sample c e l l , d i f f e r e n t i a l l y pumped, w i t h i n the spectrometer. Gas i s leaked at a constant r a t e i n t o the sample r e g i o n . There i s a s m a l l hole about 2 mm above the s u r f a c e of the sample through which the e l e c t r o n s e x c i t e d from the sample pass i n t o the s p e c t r o ­ meter. X-rays are d i r e c t e d at the s u r f a c e through an aluminum window; a second s m a l l opening i s provided i f e x i t a t i o n i s by UV r a d i a t i o n . In t h i s way the system pressure can be kept at 10"" Pa w h i l e there i s a steady pressure of 100 Pa above the sample. There i s of course a t t e n u a t i o n of the s i g n a l , as shown i n F i g . 5, taken from Joyner and Robert a l s o be obtained, but t h i the s i g n a l of the s o l i d . This sample c e l l arrangement thus per­ mits the study of the s t a t i o n a r y - s t a t e surface during c a t a l y s i s and a l s o i t s e v o l u t i o n i n response to pulses and step f u n c t i o n s i n the gas composition. The temperature of the sample should be con­ t r o l l e d so that the surface can be s t u d i e d during temperature-pro­ grammed desorption and r e a c t i o n . 3

Modes of

Operation

Steps and P u l s e s . Supposing that the proper equipment can be devised f o r measuring gas-phase a n d — i n some c a s e s — s u r f a c e t r a n ­ s i e n t s , the question a r i s e s as to what k i n d of f o r c i n g f u n c t i o n should be used. An important c l a s s of experiments i n v o l v e s tem­ perature as the p e r t u r b i n g v a r i a b l e . Temperature programmed de­ s o r p t i o n or r e a c t i o n and f l a s h desorption are w i d e l y used. Follow­ ing t r a n s i e n t s induced by temperature change by mass s p e c t r o m e t r i c a n a l y s i s of the gas phase has been h i g h l y developed by Madix and h i s students (29). In the present review we s h a l l l i m i t our d i s ­ cussion to concentration f o r c i n g f u n c t i o n s . Step f u n c t i o n s up and down as p e r t u r b a t i o n s have been r e l a t e d to many common one- and two-step processes by Kobayashi and Kobayashi (8). Figure 6, from our previous review (9) i l l u s t r a t e s the parametric s e n s i t i v i t y of t h i s v e r s i o n of the t r a n s i e n t method A switch i s made from a flow of argon to N 0/Ar over a N i 0 / S i 0 c a t a l y s t at 350°C, and curve (1) of F i g . 6 shows the N response. This curve has been computer-simulated, by a model which r e q u i r e s a. steady-state oxygen surface coverage θ of 0.58. Other values of θ can be used i n the s i m u l a t i o n , and they can l e a d to the c o r r e c t steady-state r a t e , but as shown, the t r a n s i e n t responses would be unambiguously a l t e r e d from that a c t u a l l y observed. Note that the e q u i l i b r i u m oxygen surface coverage at the steady-state gas-phase oxygen concentration would be lower than 0.58 (5). A l l these experiments i n v o l v e a multicomponent response, i d e a l l y of both the gas phase and the s u r f a c e . The number of 2

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0

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Figure 4. Adsorption-desorption of CO on Ni(III) at 175°C due to 200-ms doses (curve a) starting after 100-ms delays (from t — 0) and 300-ms doses (curve b) after similar delays (21).

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Surface Science Figure 5.

The attenuation of A g 3d signal by Ar. Key: O, experimental points; and —, theory (28J. 5/2

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Catalysis Review Figure 6. Decomposition of N 0 over NiO in a differential reactor. The steadystate surface oxygen coverage θ was 0.58 (curve 1), 0.69 (curve 2), and 0.28 (curve 3) (9). 2

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responses to f i t — a n d thus the r e l i a b i l i t y of the r e s u l t — c a n be i n c r e a s e d by using s t a b l e i s o t o p e s . The q u a n t i t a t i v e manipulation of such a method has been most h i g h l y developed by Happel and co­ workers (30). In t h i s paper i s o t o p i c t r a c i n g i s used to make i t p o s s i b l e to i d e n t i f y the r a t e parameters of v a r i o u s steps i n a sequence by a l i n e a r a n a l y s i s . Methanation over a n i c k e l c a t a l y s t has been s t u d i e d . I n the gas f l o w i n g to a g r a d i e n t l e s s r e a c t o r at steady s t a t e a step change i s made from H / C 0 to H / C 0 . The response i s f o l l o w e d by mass spectrometry. A l l the chemistry, such as surface r a t e s and coverages, i s assumed to be u n a l t e r e d by the change, so the r e s u l t of the p e r t u r b a t i o n i s that of a s e r i e s of f i r s t - o r d e r processes, each one r e p r e s e n t i n g the accumulation of C i n a p a r t i c u l a r intermediate or gas component. We can speak of pools f o r the accumulation of C without knowing the com­ p l e t e i d e n t i t y of each t r a t i o n s of t r a c e r s ca a n a l y s i s . The mathematics of a s i m i l a r s u p e r p o s i t i o n method has a l s o been s t u d i e d by Le C a r d i n a l et a l . (31). He and Happel have exchanged polemics on the matter (32, 33). A disadvantage of t r a c e r methods i s the frequent presence of exchange processes which are d i f f i c u l t to account f o r . This ham­ pered Happel s work w i t h 0 (30) and complicated the i n t e r p r e t a ­ t i o n of the work of Conner and Bennett (34), who used pulses of t r a c e r s and a q u a l i t a t i v e i n t e r p r e t a t i o n of t h e i r data on CO o x i d a ­ t i o n over NiO. 12

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Cycled Feed. The q u a l i t a t i v e i n t e r p r e t a t i o n of responses to steps and pulses i s o f t e n p o s s i b l e , but the q u a n t i t a t i v e e x p l o i t a ­ t i o n of the data r e q u i r e s the numerical i n t e g r a t i o n of n o n l i n e a r d i f f e r e n t i a l equations i n c o r p o r a t e d i n t o a program f o r the search f o r the best parameters. A s i n u s o i d a l v a r i a t i o n of a feed com­ ponent c o n c e n t r a t i o n around a steady s t a t e value can be analyzed by the w e l l developed methods of l i n e a r a n a l y s i s i f the r e l a t i v e amplitudes of the responses are under about 0.1. The a p p l i c a t i o n of these ideas to a modulated molecular beam was developed by Jones et a l . (7) i n 19 72. A number of simple sequences of l i n e a r steps produces frequency responses shown i n F i g . 7 (J). Here ε i s the r a t i o of product to reactant amplitude, η i s the s t i c k i n g prob­ a b i l i t y , ω i s the f o r c i n g frequency, and k^ i s the desorption r a t e constant f o r the product. For the s e r i e s process k-^ i s the r a t e constant of the s u r f a c e r e a c t i o n , and f o r the branched process Ρ i s the f r a c t i o n r e a c t i n g through path 1 and desorbing w i t h a r a t e constant k ^ . This method has r e c e n t l y been a p p l i e d to the decom­ p o s i t i o n of hydrazine on I r ( l l l ) by M e r r i l l and Sawin (35). Whether to use as p e r t u r b i n g f u n c t i o n a s t e p , p u l s e , or c y c l e d feed depends on the p a r t i c u l a r system under study. For ex­ pensive t r a c e r s , a pulse i s o f t e n mandatory. However, simple t e x t ­ book r e l a t i o n s based on a D i r a c f u n c t i o n do not u s u a l l y apply, f o r a r e l a t i v e l y long pulse may be r e q u i r e d to get a good s i g n a l . A long enough pulse becomes two step f u n c t i o n s , and as already men-

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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277 Journal of Vacuum Science and Technology Figure 7. Polar plots of the reaction product vector for three surface mechanisms. Key: a, simple adsorption-desorption; b, series process, kt — k^ — k; and c, branch process, Ρ = 0.5 (1).

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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t i o n e d , there i s a convenient d i r e c t o r y of common responses ( 8 ) . For a c y c l e d feed (a chain of i n t e r f e r i n g pulses) the i n t e r p r e t a t i o n r e q u i r e s some mathematical t r a n s f o r m a t i o n s , but r e s u l t s such as F i g . 7 can a l s o be used as q u a l i t a t i v e i n d i c a t o r s of c e r t a i n processes (35). The complete computer s i m u l a t i o n i s q u i t e tedious and has so f a r been done f o r r e l a t i v e l y few r e a c t i o n s ; thus the p a r t i a l e x p l o i t a t i o n of the data i n the s p i r i t of preceding work i s o f t e n a l l that i s attempted i n p r a c t i c e . S t r o z i e r (36) has a l s o used a c y c l e d (pulsed) feed at TJHV. As i n the molecular beam experiment, the r e a c t o r volume, pumping speed, and r a t e of i n t r o d u c t i o n of reactants have values which lead to a f l u x of reactants w e l l d e f i n e d i n time. S t r o z i e r , however, simply doses gas i n t o the vacuum system ( r e a c t o r ) r a t h e r than using a molecular beam. He s t u d i e d CO o x i d a t i o n , which has n o n l i n e a r i t i e s i n the surface r a t a n a l y t i c s o l u t i o n s are constant frequency and v a r y i n g temperature as shown i n F i g . 8, which i s a computer s i m u l a t i o n (37). Cycled-feed experiments are more complicated to analyze i f done at atmospheric pressure and at r e l a t i v e l y high conversion. Now the s u r f a c e r e a c t i o n rates a l t e r the gas-phase reactant conc e n t r a t i o n s . C u t l i p (38) has s t u d i e d CO o x i d a t i o n over P t / A l 0 3 i n a g r a d i e n t l e s s r e a c t o r under c o n d i t i o n s o f t e n l e a d i n g to comp l e t e conversion. The feed gas a l t e r n a t e d between 2% CO and 3% 0 i n argon. Figure 9 shows some t y p i c a l r e s u l t s . C l e a r l y there i s no hope of s i m u l a t i n g such data by anthing but a complicated computer model. Figure 10 (38) shows that under some circumstances the r a t e i s g r e a t l y enhanced ( 20 times) by a c y c l e d feed. At the c o n d i t i o n s shown there are no m u l t i p l e steady s t a t e s , the only steady s t a t e corresponds to a very high CO surface coverage. By a l t e r n a t e l y s w i t c h i n g to pure oxygen, C0 i s generated at a high r a t e from the adsorbed CO. The net r e s u l t i s as i f the s u r f a c e were more e q u a l l y covered w i t h oxygen and carbon monoxide w i t h a Langmuir-Hinschelwood r a t e equation: r = k (CO d )(°ads)" Figure 11 shows the e f f e c t on CO conversion ( o x i d a t i o n ) over two Rh/Al Û3 c a t a l y s t s of c y c l i n g at 550°C as s t u d i e d by S c h l a t t e r and M i t c h e l l (39). Steady s t a t e o p e r a t i o n (high frequency) gives complete conversion of a s t o i c h i o m e t r i c reactant mixture, and c y c l i n g around t h i s steady s t a t e r a t i o must give a lower average conversion. Since the c o n t r o l o p e r a t i o n to maintain an average s t o i c h i o m e t r i c r a t i o i n the r e a c t o r feed leads to c y c l e s of f r e quency about 1 Hz (39) , i t i s of p r a c t i c a l i n t e r e s t to understand these e f f e c t s . Cerium has been added to the c a t a l y s t to t r y to r e t a i n oxygen and keep the r a t e up during the p a r t of the c y c l e during which there i s i n s u f f i c i e n t gaseous oxygen f o r complete conversion. As shown, S c h l a t t e r and M i t c h e l l (39) found the Cec o n t a i n i n g c a t a l y s t s u p e r i o r , but the e x p l a n a t i o n f o r i t s behavior i s more complicated than simple oxygen storage. The p o s s i b l e importance of o b t a i n i n g improved y i e l d i n a 2

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In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Responses averaged in time to steady and pulsed feeds for CO oxidation over Pt (37). Figure 8.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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Figure 9. Reactor outlet concentrations during a 3-min period at 60°C (38). Feed is cycled from 2% CO at 80 s to 3% 0 at 100s over a Pt/Al 0 catalyst at 60°C. 2

MAXIMUM RATE

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AlChE Journal

Rate comparison of periodic to steady-state operation for stoichio­ metric feed of CO/0 to Pt/Al O catalyst (3$). 2

2

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In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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Figure 11. Effect of Ce on the cycled performance of a Rh/Al O catalyst fed C0/0 mixtures at 550°C and a space velocity of 104,000 h' (39J. 2

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In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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multiproduct c a t a l y t i c process by feed concentration c y c l i n g has already been discussed i n our previous review (9). B i l i m o r i a and B a i l e y (40) have s t u d i e d the hydrogénation of acetylene t o ethylene and ethane, and ΑΙ-Taie and Kerschenbaum (41) have s t u d i e d the hy­ drogénation of butadiene to butylène and to butane. In the l a t t e r work the y i e l d of butylène was improved by c y c l i n g f o r c e r t a i n r e a c t o r c o n d i t i o n s . No mechanistic a n a l y s i s i s done i n these papers from the Chemical Reaction Engineering Meeting i n Houston beyond acknowledging that c o n s i d e r a t i o n o f surface rates and capacities i s essential. S e l f - s u s t a i n e d O s c i l l a t i o n s . Under c e r t a i n c o n d i t i o n s , i s o thermal l i m i t cycles i n gaseous concentrations over c a t a l y s t s are observed. These are probably caused by i n t e r a c t i o n of steps on the s u r f a c e . Sometimes heat and mass t r a n s f e r e f f e c t s i n t e r v e n e , l e a d i n g to temperature o s c i l l a t i o n a l s o Sinc t h i subject ha r e c e n t l y been reviewed mentioned here. C u t l i p and Kenney (44) have observed isothermal l i m i t c y c l e s i n the o x i d a t i o n of CO over 0.5% P t / A l 0 3 i n a g r a d i e n t l e s s r e a c t o r only i n the presence of added 1-butene. Without butene there were no o s c i l l a t i o n s although regions of m u l t i p l e steady s t a t e s e x i s t . Dwyer (22) has f o l l o w e d the surface CO i n f r a r e d adsorption band and found that i t was i n phase w i t h the gas-phase concentration. Kurtanjek e t a l . (45) have s t u d i e d hydrogen o x i d a t i o n over N i and have a l s o taken the l o g i c a l step of f o l l o w i n g the surface concent r a t i o n . Contact p o t e n t i a l d i f f e r e n c e was used to f o l l o w the o x i d a t i o n s t a t e of the n i c k e l surface. Under some c o n d i t i o n s , o s c i l l a t i o n s were observed on the surface when none were detected i n the gas phase. Recently, Sheintuch (46) has made a d d i t i o n a l s t u d i e s of CO o x i d a t i o n over P t f o i l . The N 0 decomposition, CO o x i d a t i o n , and H o x i d a t i o n react i o n s are known to e x h i b i t concentration o s c i l l a t i o n s over noble metal c a t a l y s t s . Flytzani-Stephanopoulos et a l . (47) have observed o s c i l l a t i o n s f o r the o x i d a t i o n of NH3 over P t . The e f f e c t s are dramatic and l e a d to l a r g e temperature c y c l e s f o r the c a t a l y s t w i r e . Heat and mass t r a n s f e r e f f e c t s are important. 2

2

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The Oxidation of Carbon Monoxide on P l a t i n u m This subject has been thoroughly reviewed by Engel and E r t l (48) and most r e c e n t l y by White (49). Here the i n t e n t i o n i s to describe some c o n t r i b u t i o n s of the t r a n s i e n t method to our understanding of t h i s c l a s s i c a l problem. Only the temperature range below 450K i s discussed here. The question of the r o l e o f the E l e y - R i d e a l (ER) mechanism i n the r e a c t i o n i s u s u a l l y i n v e s t i g a t e d by the r e a c t i o n of preadsorbed oxygen w i t h CO from the gas phase. The w e l l known r e s u l t s of Bonzel and Ku (6) are shown i n F i g . 12. C0 i s immediately produced, lending some support t o the existance of an ER step. However, Matsushima et a l . (26) have used Auger e l e c t r o n s p e c t r o s 2

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copy to f o l l o w the surface c o n c e n t r a t i o n of oxygen ( d i s s o c i a t e d i n t o adsorbed atoms) during the same process. Although the r a t e i s about f i r s t - o r d e r i n gas-phase CO c o n c e n t r a t i o n , i t i s zeroorder i n surface 0, as shown i n F i g . 13. The order does i n c r e a s e as the surface oxygen i s depleted. The r e s u l t i s i n t e r p r e t e d as showing that CO does not r e a c t by impact on oxygen atoms but must adsorb i n a mobil precursor s t a t e . In a l a t e r a r t i c l e , Matsushima (50) has used a t r a n s i e n t t r a c e r method at 301X to study the ER process f u r t h e r . The surface of the Pt was i n i t i a l l y exposed to a pulse of a mixture of C0/ C0 so that the CO s u r f a c e coverage was 0.23 and the r a t i o of C 0 ( a ) to C 0 ( a ) was 1.6. Then a pulse of C 0 + 0 was admitted, and the f i r s t C0 to be produced had the same C / C r a t i o as the preadsorbed CO. This r e s u l t i s i n t e r p r e t e d as meaning that the surface oxygen react the gaseous CO; the ER This process has been s t u d i e d by i n f r a r e d spectroscopy by Dwyer (22), using 9% P t / S i 0 and the c e l l used by Ueno et a l . (19). The bands a r i s i n g from gaseous C0 and surface CO were f o l l o w e d a f t e r a switch i n the gas phase composition from 3kPa 0 i n helium, to pure helium f o r a few seconds and then to 2kPa CO i n helium. This i s b a s i c a l l y the same sequence as that of F i g . 12. The r e s u l t s are shown i n F i g . 14. The C0(a) band grows immediately and at the same time C0 i s produced. Taken w i t h the r e s u l t s of Matsushima et a l . (26, 50) i t seems reasonable that the ER step p l a y s a minor r o l e i n the r e a c t i o n between CO and 0 ( a ) . The process 0 + C0(a) i s a l s o i n t e r e s t i n g and t h i s can be s t u d i e d by a gas phase s w i t c h from CO to 0 . As mentioned above a short exposure to helium between the CO and 0 i s used. This procedure e l i m i n a t e s the i n t e r a c t i o n of CO and 0 during the mixing time (ca 2 s) f o r the c e l l . Oxygen does not desorb i n helium (T He -> 0 experiments. The desorption of F i g . 15 starts from a surface coverage of CO near u n i t y . From the i n i t i a l r a t e s an a c t i v a t i o n energy of 8 kcal/mole i s obtained. This may be compared w i t h 9 kcal/mole reported by Nashayima and Wise (51). F i g ure 16 (22) shows that the s l i g h t desorption of l o o s e l y bound CO has a remarkably l a r g e e f f e c t on the CO(a) l e f t on the s u r f a c e . The r e s u l t s of the CO •> He 0 experiment (22) are shown i n Fig. 17. There i s an i n d u c t i o n time between the admission of 0 and s t a r t of the r e a c t i o n of C0(a). Oxygen cannot r e a c t from the gas phase, and i t s adsorption must await the desorption of a t r a c e of C0(a). Bonzel and Ku _0 found a s i m i l a r r e s u l t , as shown i n F i g . 18. T h e i r i n d u c t i o n times are much longer than those of Dwyer (22), perhaps because of the r e g u l a r i t y of t h e i r (110) surface. From these examples i t i s c l e a r that the t r a n s i e n t r e s u l t s add important understanding to our ideas about the o x i d a t i o n of CO over P t . 13

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Reaction between CO and preadsorbed oxygen on Pt (110) at 218°C (6).

100 150 DECAY TIME ( sec) Figure 13. Reaction of surface Ο with CO for several fixed CO pressures over Pt measured by Auger electron spectroscopy. Key: Fixed CO pressures (Pa) are 1.86 Χ 10 (1), 6.27 χ W (2), 1.67 χ 10~ (3), 2.8 χ 10 (4), and 4.8 X 10* (5). The light and dark symbols denote separate experiments (26). 7

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Reaction between CO and 0 preadsorbed on Pt/Si0 andflowrate of 9.5 mL/60 s. 2

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Variation of IR frequency during CO desorption from Pt/Si0 . 100°C; •, 120°C; Φ, 140°C;and 0 , 1 6 0 ° C (22). 2

Key:

TIME ( sec ) Figure 17.

Reaction between 0 and CO preadsorbed on Pt/Si0 andflowrate of 9.5 mL/60 s. 2

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In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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t(sec) Surface Science Figure 18. Rate of C0 formation on Pt (110). Initially, the Pt (110) surface was exposed to a feed mixture of CO and oxygen. Then att — O, the partial pressure of CO was suddenly reduced and the subsequent rate of CO formation was measured (6). 2

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The Reaction of Hydrogen and Carbon Monoxide over Iron The t r a n s i e n t method has c o n t r i b u t e d t o our understanding of H /CO r e a c t i o n s , and a few r e s u l t s of s t u d i e s over i r o n w i l l show the power of the method. Matsumoto and Bennett (52) worked w i t h a CCI fused i r o n c a t a l y s t (ca 96% i r o n ) which had been reduced i n f l o w i n g H at 450°C f o r a t l e a s t 15 h. Upon s w i t c h i n g to a feed of 10% CO i n H , the data of F i g . 19 were obtained. Large rates of production o f H 0 and C 0 are observed as CO i s d i s s o c i a t e d on the f r e s h c a t a l y s t . The production of methane and higher hydro­ carbons s t a r t s at zero and g r a d u a l l y i n c r e a s e s . This r e s u l t was i n t e r p r e t e d as evidence that s u r f a c e carbon reacted p r e f e r e n t i a l l y w i t h i r o n t o form a carbide. As t h i s r e a c t i o n slowed w i t h time, s u r f a c e carbon b u i l t up and l e d t o the production of methane and lower production r a t e s f o C 0 dH 0 Figur 20 fro Reymond et a l . (25) shows the e v o l u t i o over a longer p e r i o d . A pur produced a peak of methane as shown (250°C). This i n c r e a s e i n CH^ production r a t e shows that H must be adsorbed (probably d i s s o c i ­ ated as H) i n order to r e a c t , and a l s o that the s t e a d y - s t a t e s u r ­ face must be l a r g e l y covered w i t h carbonaceous intermediate. Methane production seems t o be governed by the equation r = k C(a) H(a), w i t h C(a) > H(a) so that an i n c r e a s e i n H(a) leads t o a higher r a t e . The second peak i n F i g . 20 represents methane gener­ ated by the d e c a r b u r i z a t i o n of the b u l k of the i r o n . 2

2

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When the f r e s h l y reduced c a t a l y s t i s suddenly exposed t o 10% ΰ Η i n H the r e s u l t i s shown i n F i g . 21 (25). Chain growth occurs as w i t h C0/H , but the r e a c t i o n r a t e decreases from an i n i t i a l maximum as the s u r f a c e becomes covered w i t h i n t e r m e d i a t e s . A switch t o pure H produces only a t r a c e of methane, even on h e a t i n g t o 500°C, so no s u r f a c e carbon o r b u l k carbide has been formed. Probably the sequence of steps passes through a CH i n t e r ­ mediate, whereas C i s needed f o r c a r b u r i z i n g the i r o n . The r a t e of r e a c t i o n of C H /H i s higher than that of C0/H , l e n d i n g weight to the concept of the hydrogénation of C(a) as the rate-determining step f o r the l a t t e r . Some of these same experiments have been done using 10% Fe/Al Û3 r a t h e r than the fused i r o n c a t a l y s t (53). Figure 22 shows the r e s u l t of a s w i t c h from H to 10% CO i n H over a f r e s h l y r e duced c a t a l y s t . Here a l a r g e i n i t i a l r a t e of methane formation i s observed and water does not appear u n t i l most of the i n i t i a l peak has passed. The probable e x p l a n a t i o n f o r the presence of the CH^ peak i s that water produced by methanation i s adsorbed on the i n i t i a l l y dry γ-Α1 03 support (100 m /g). Thus the i r o n remains b r i e f l y i n a r e l a t i v e l y reduced s t a t e . For the CCI c a t a l y s t the A 1 0 promoter i s not s u f f i c i e n t t o prevent the water from r i s i n g q u i c k l y as shown i n F i g . 19. The H/0 r a t i o on the surface i s r e ­ duced, and c a r b u r i z a t i o n occurs more r a p i d l y than methanation, as f o r the unsupported c a t a l y s t . When the 10% F e / A l 0 c a t a l y s t i s p r e t r e a t e d w i t h water vapor, 2

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Figure 19. Formation of major components as a function of exposure time for the reaction of 10% CO/H at 250°C over CCI fused iron catalyst (reduced at 450°C) (52).

Ut

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HR Journal of Catalysis Figure 20. Hydrogénation of surface carbon over catalyst initially reduced for 15 h at 500°C and then cooled to 250°C. At t — 0 flow is switched to H /CO = 9, 250°C. At 20 h feed is switched to pure H , 250°C; at 24 h temperature is raised to 500°C (25). 2

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SECO Ν DS Figure 22.

Initial production rates of principal products from a 10% catalyst (53). Τ

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Figure 23. Response of the 10% Ni/Si0 catalyst to a step increase in P (54). Conditions: 190°C; P 1 atm; t < 0, P = 0; t > 0, P — 0.44 atm; and flow rate = 100 mL/60 s. 2

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the methane p r o d u c t i o n behaves as f o r the CCI c a t a l y s t — i t r i s e s from zero as i n F i g . 19. Van Ho and H a r r i o t t (54) have done s i m i l a r t r a n s i e n t e x p e r i ments on H2/CO over 10% N i / S i 0 2 , and a r e s u l t i s shown i n F i g . 23. Even though the b u l k of the n i c k e l i s not c a r b i d e d , the methane p r o d u c t i o n rate r i s e s from z e r o , as shown. The complete d i s c u s s i o n of these experiments i s beyond the scope of t h i s review. By showing these few r e s u l t s some idea of the r i c h n e s s i n i n f o r m a t i o n of the t r a n s i e n t data i s conveyed. Any model which can e x p l a i n a l l such complicated r e s u l t s i s bound to be nearer the t r u t h than one based on steady s t a t e data alone. An understanding improves, i t i s p e r m i t t e d to hope that t h i s w i l l e v e n t u a l l y lead to the development of b e t t e r c a t a l y s t s . Summary From the examples discussed i n t h i s review i t i s c l e a r that a proper understanding of heterogeneous c a t a l y s i s i s l i n k e d t o understanding t r a n s i e n t e f f e c t s . The i n t e r p r e t a t i o n requires a model based on elementary s t e p s , and v a r i o u s steps may be r a t e - d e t e r m i n ing as the conversion and temperature v a r y . Complicated programs f o r f i t t i n g a s t e a d y - s t a t e equation to data are not s u f f i c i e n t i f the equation i t s e l f changes w i t h c o n v e r s i o n , f o r example. Any mathematical c o n t r o l o r design c a l c u l a t i o n s i n r e a c t o r design must take i n t o account the elementary steps and the storage capacity of the c a t a l y s t s u r f a c e . Understanding c a t a l y s i s has been emphasized h e r e , but the p o s s i b l e p r a c t i c a l e f f e c t s of d e l i b e r a t e c y c l i n g of a r e a c t o r feed to improve y i e l d must a l s o be kept i n mind. A l s o , undesired feed o s c i l l a t i o n s must be taken i n t o account i n the design of a reactor. Design f o r such a s i t u a t i o n i s not p o s s i b l e based on steady s t a t e l a b o r a t o r y data alone. Acknowledgement This work was supported by NSF grants //ENG-7921890, and //CPE7901018 A01. Literature Cited 1. Wagner, Carl, and Hauffe, Karl. Untersuchungen über den stationären Zustand von Katalysatoren bei heterogenen Reaktionen. I: Ztschr. Elektrochem. 1938, 44 (3) 172; II: ibid. 1939, 45 (5) 409. 2. Tamaru, Kenzi. Adsorption measurements during surface cataly­ sis. Adv. Catal. 1964, 15, 65. 3. Bennett, C. O. A dynamic method for the study of heterogene­ ous catalytic kinetics. A.I.Ch.E. J. 1967, 13, 890. 4. Kobayashi, Μ., and Kobayashi H. Application of transient re­ sponse method to the study of heterogeneous catalysis. J. Catal. 1972, 27, 100, 108, 114.

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Yang, C. C., Cutlip, Μ. Β., and Bennett, C. O. A study of nitrous oxide decomposition on nickel oxide by a dynamic method. Proc. of the Vth Congr. on Catal., Palm Beach, 1973. 6. Bonzel, Η. P., and Ku, R. Mechanisms of the catalytic carbon monoxide oxidation on Pt(110). Surface Sci. 1972, 33, 91. 7. Jones, R. H., Olander, D. R., Siekhaus, W. J., and Schwarz, J.A. Investigation of gas-solid reactions by modulated molecular beam mass spectrometry. J. Vac. Sci. Technol. 1972, 9, 1429. 8. Kobayashi, Μ., and Kobayashi, M. Transient response method in heterogeneous catalysis. Catal. Rev.-Sci. Eng. 1974, 10 (2), 139. 9. Bennett, C. O. The transient method and elementary steps in heterogeneous catalysis. Catal. Rev.-Sci. Eng. 1976, 13 (2), 121. 10. Temkin, M. I. Relaxatio tion. Kin. and Catal. 1976, 17 (5), 1095. 11. Krylov, O. V. Elementary processes determining the mechanisms of a catalytic reaction. Kin. and Catal. 1980, 21 (1), 79. 12. Dorawala, T. G., and Douglas, J. M. Complex reactions in oscillating reactors. A.I.Ch.E. J.. 1971, 17 (4), 974. 13· Schmitz, Roger A. Multiplicity, stability, and sensitivity of states in chemically reacting systems—A review. Adv. Chem. Sci. 1975, 148, 154. 14. Sheintuch, Μ., and Schmitz, R. A. Kinetic modeling of oscillatory catalytic reactions. ACS Symp. Ser. 1978, 65, 487. 15. Bennett, C. O., Cutlip, Μ. Β., and Yang, C. C. Gradientless reactors and transient methods in heterogeneous catalysis. Chem. Eng. Sci. 1972, 27, 2255. 16. Heyne, Η., and Tomkins, F. C. Application of infrared spec­ troscopy and surface potential measurements in a study of the oxidation of carbon monoxide on platinum. Proc. Roy. Soc., Ser. A. 1966. 292, 460. 17. Tamaru, K. Recent progress in elucidating the mechanism of heterogeneous catalysis. Pure and Appl. Chem. 1980. 52, 2067. 18. Dent, A. L., and Kokes, R. J. Intermediates in ethylene hy­ drogenation over zinc oxide. J. Phy. Chem. 1970. 74, 3653. 19. Ueno, Α., Hochmuth, J. Κ., and Bennett, C. O. Interaction of CO2, CO, and NiO studied by infrared spectroscopy. J. Catal. 1977. 49, 225. 20. Ueno, Α., and Bennett, C. O. Infrared study of CO2 adsorption on SiO2, J. Catal. 1978. 54, 31. 21. Savatsky, B. J., and Bell, A. T. Studies of NO reduction by H2 using transient response techniques. This volume. 22. Dwyer, S. M. Transient infrared studies of carbon monoxide oxidation on a supported platinum catalyst. M.S. Thesis, University of Connecticut, 1980. 23. Hegedus, L. L. Infrared spectroscopic reactor system to study the transient operation of heterogeneous catalysts.

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32. 33. 34. 35. 36. 37. 38. 39. 40. 41.

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Peri, J. B. Characterization of catalyst surfaces by com­ puterized infrared spectroscopy. Prepr., Div. Pet. Chem., Am. Chem. Soc. 1978. 23 (4), 1281. Reymond, J. P., Mériaudeau, P., Pommier, Β., and Bennett, C. O. Further results on the reaction of H2/CO on fused iron by the transient method. J. Catal. 1980. 64, 163. Matsushima, T., Almy, D. Β., and White, J. M. The reactivity and Auger chemical shift of oxygen adsorbed on platinum. Surface Sci. 1977. 67, 89-108. Rubloff, G. W. Photoemission studies of time-resolved sur­ face reactions: Isothermal desorption of CO from Ni (111). Surface Sci. 1979. 89, 566. Joyner, R. W., and Roberts, M. W. A high-pressure electron spectrometer for surface studies. Surface Sci. 1979. 87, 501. Johnson, S. W., an and desorption of CO and H 2 and surface reactions of alcohols by sulfur on Ni (100). ACS Symp. Ser. 1981; Madix, R. J. Adv. in Catal. 1980. 29. Happel, J., Suzuki, I., Kokayeff, P., and Fthenakis, V. Multiple isotope tracing of methanation over nickel catalyst. J. Catal. 1980. 65, 59. Le Cardinal, G., Walter, Ε . , Bertrand, P., Zoulalian, A. and Gelas, M. A new operating method for the kinetic study of open systems by means of tracer elements. Chem. Eng. Sci. 1977. 32, 733. Happel, J. Transient tracing. Chem. Eng. Sci. 1978. 33, 1567. Le Cardinal, G. Chem. Eng. Sci. 1978. 33, 1568. Conner, W. C., and Bennett, C. O. Carbon monoxide oxidation on nickel oxide. J. Catal. 1976. 41, 30. Merrill, R. P., and Sawin, H. H. Decomposition of hydrazine on Ir (111). This volume. Strozier, J. Α., Jr. Oxidation of CO on Pt by an a.c. puls­ ing technique. II. Experiment. Surface Sci. 1979. 87, 161. Strozier, J. Α., Jr. Cosgrove, G. J., and Fischer, D. A. Oxidation of CO on Pt and Pd. I. Theory. Surface Sci. 1979. 82, 481. Cutlip, M. B. Concentration forcing of catalytic surface rate processes. A.I.Ch.E. J. 1979. 25 (3) 502. Schlatter, J. C., and Mitchell, P. J. Three-way catalyst response to transients. Ind. Eng. Chem. Prod. Res. Dev. 1980. 19, 288. Bilimoria, M. R., and Bailey, J. E. Dynamic studies of acetylene hydrogenation on nickel catalysts. ACS Symp. Ser. 1978. 65, 43. Al-Tai, A. S., and Kerschenbaum, L. S. Effect of periodic operation on the selectivity of catalytic reactions. ACS Symp. Ser. 1978. 65, 42.

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45. kinetic 46. 47. 48.

49. 50. 51. 52. 53. 54. 55. 56.

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Sheintuch, Μ., and Schmitz, R. A. Oscillations in catalytic reactions. Catal. Rev. 1977. 15 (1) 107. Slin'ko, M. G., and Slin'ko, M. M. Self-oscillations of heterogeneous catalytic reaction rates. Catal. Rev.-Sci. Eng. 1978. 17 (1) 119. Cutlip, M. B., and Kenney, C. N. Limit cycle phenomena during catalytic oxidation reactions over a supported platinum cata­ ACS Symp. Ser. 1978. 65, 39. Kurtanjek, Α., Sheintuch, Μ., and Luss, D. Surface state and oscillations in the oxidation of hydrogen on nickel. J. Catal. 1980. 66, 11. Sheintuch, M. Oscillatory state in the oxidation of carbon monoxide on platinum. A.I.Ch.E. J. 1981. 27, 20. Flytzani-Stephanopoulos, Μ., Schmidt, L. D., and Caretta, R. Steady state and transient oscillations in NH oxidation on Pt. J. Catal. 1980 Engle, T., and Ertl, G. Elementary steps in the catalytic oxidation of carbon monoxide on platinum metals. Advances in Catalysis. 28. New York: Academic Press, Inc., New York. 1979. White, J. M. Transient low-pressure studies of catalytic carbon monoxide oxidation. This volume. Matsushima, T. Kinetic studies on the CO oxidation over platinum by means of carbon 13 tracer. Surface Sci. 1979. 79, 63. Nishiyama, Y., and Wise, H. Surface interactions between chemisorbed species on platinum: Carbon monoxide, hydrogen, oxygen, and methanol. J. Catal. 1974. 32, 50. Matsumoto, Η., and Bennett, C. O. The transient method applied to the methanation and Fischer-Tropsch reactions over a fused iron catalyst. J. Catal. 1978. 53,331. Teule-Gay, F. Transient studies of the CO/H2 reaction on various supported iron catalysts. M. S. Thesis, University of Connecticut, 1981. Van Ho, S., and Harriott, P. The kinetics of methanation on nickel catalysts. J. Catal. 1980. 64, 272. Murphy, R. Ε . , Cook, F. Η., and Saki, H. J. Opt. Soc. Am. 1975. 65, 600. Mantz, A. W. Appl. Spectroscopy. 1976. 30, 459.

Received July 28, 1981.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

2 Transient Low-Pressure Studies of Catalytic Carbon Monoxide Oxidation J. R. C R E I G H T O N and J . M . W H I T E University of Texas, Department of Chemistry, Austin, TX 78712

The purpose of this article is to review the results of transient low pressure studies of carbon monoxide oxidation Particular emphasi electron spectroscopy, flash desorption, modulated beam and titration techniques. The strengths and weaknesses of these w i l l be assessed with regard to kinetic insight and quantification. An attempt w i l l be made to identify questions that are ripe for investigation. Although not limited to i t , the presentation emphasizes our own work. A very recent review of the carbon monoxide oxidation reaction (1) will be useful to readers who are interested in a more comprehensive view. The carbon monoxide oxidation reaction has been widely studied over the course of many years. The pioneering work of Langmuir (2) set the tone for much of the subsequent work and we continue to refine many of his ideas. Catalyzed carbon monoxide oxidation is perhaps better understood than any other heterogeneous reaction and, because i t shows l i t t l e variation with crystal plane (3), there is a good correlation of low area single crystal results with high area supported catalyst results. This is particularly true under conditions where CO accumulates and inhibits the rate of CO production (4,5). In the realm of applications, CO oxidation is a significant consideration in systems designed to limit undesirable emissions from combustion processes. At the outset, i t is worth noting how transient data fit into the overall picture of this reaction and what we expect to learn by doing such experiments. With the exception of oscillations (6), transients are induced by rapidly changing one or more of the independent variables of the system. For the case 2

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T R A N S I E N T

C O N D I T I O N S

o f CO o x i d a t i o n , c h a n g i n g t h e s u b s t r a t e t e m p e r a t u r e o r t h e p a r t i a l p r e s s u r e s o f CO o r O2 i n d u c e s s u c h transients. The i n f o r m a t i o n g e n e r a t e d comes f r o m measurement o f t h e time e v o l u t i o n o f s u r f a c e c o v e r a g e s and gas p h a s e c o m p o s i t i o n . The f o r m e r a r e , o f c o u r s e , c e n t r a l t o any k i n e t i c d e s c r i p t i o n i n v o l v i n g a d s o r b e d s p e c i e s ; measurements under w o r k i n g c o n d i t i o n s g i v e the g r e a t e s t i n s i g h t because they a r e d i r e c t l y r e l a t e d t o t h e CO2 p r o d u c t i o n r a t e s . The l a t t e r p r o v i d e p r o d u c t f o r m a t i o n and a d s o r p t i o n / d e s o r p t i o n r a t e s d u r i n g t r a n s i e n t s . I n a d d i t i o n , when u s e d i n c o n j u n c t i o n w i t h a m o d e l , c o v e r a g e s c a n be c a l c u l a t e d from p r e s s u r e versus time data. Taken t o g e t h e r t h e s e measurements i n p r i n c i p l e p r o v i d e t h e r a t e s and coverages needed f o p r a c t i c e , there ar the measurements. I n p a r t i c u l a r t h e dynamic range o v e r w h i c h s u r f a c e c o n c e n t r a t i o n s c a n be r e l i a b l y m e a s u r e d i s 0 . 0 1 t o 1 m o n o l a y e r ; t h e l o w e r end o f t h i s range i s p a r t i c u l a r l y troublesome. Moreover, because adsorbed s p e c i e s o f t e n occupy s i t e s of v a r i a b l e a c t i v i t y on a s u r f a c e , m e a s u r e m e n t o f t h e t o t a l c o n c e n t r a t i o n o f an a d s o r b e d s p e c i e s may be insufficient. I m p r o v e d r e s o l u t i o n and m e a s u r e m e n t of s m a l l c o n c e n t r a t i o n s of r e l a t i v e l y l a b i l e s u r f a c e species l i e a t the heart of experimental progress i n the s u r f a c e chemistry of heterogeneous c a t a l y s i s . S u c c i n c t l y p u t , the o l d c a t a l y t i c chemists' proverb, " i f you can see a c e r t a i n s u r f a c e s p e c i e s , i t i s n o t k i n e t i c a l l y i m p o r t a n t " , must be t a k e n s e r i o u s l y . H o w e v e r , i n t h e c a s e o f CO o x i d a t i o n , t h e k i n e t i c a l l y a c t i v e a d s o r b e d oxygen and c a r b o n monoxide s p e c i e s c a n i n f a c t be m e a s u r e d u n d e r many, b u t n o t a l l , conditions. Transients

I n d u c e d by T e m p e r a t u r e

Changes

One o f t h e s t a n d a r d s u r f a c e s c i e n c e m e t h o d s f o r a s s e s s i n g t h e c o n c e n t r a t i o n and s t a b i l i t y o f a chemisorbed species i s thermal desorption spectroscopy (TDS). An e a r l y p a p e r by R e d h e a d ( 7 ) d e v e l o p e d t h e c o n c e p t u a l framework f o r c e r t a i n cases. Many p a p e r s s i n c e then have expanded the a p p l i c a b i l i t y of t h i s method. Recent work of Madix ( 8 ) , Weinberg (9) Schmidt (10) i s p a r t i c u l a r l y noteworthy. Most o f t h i s w o r k f o c u s e s on t h e d e s o r p t i o n o f a s i n g l e m o l e c u l a r s p e c i e s a n d n o t on r e a c t i o n s i n d e s o r b i n g systems. However, q u a l i t a t i v e f e a t u r e s of t h e t e m p e r a t u r e d e p e n d e n c e o f r e a c t i o n s c a n be a s s e s s e d u s i n g t h i s method. F i g u r e s 1 and 2 t a k e n from t h e a

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

n

d

2.

CREiGHTON

A N D

1

-

Catalytic Carbon

W H I T E

1

1

1

Monoxide

1

1

χ

< «ΓΙ0.0

g

I

X

\

1x1

1\

ce

4.0

< g

4 1

\

I '

\'

or 6jo ο

2

.0

- 1173

/

h-8.0

I

35

Oxidation

'

\/

1073^

/

973

//

»

\J

A

/

v

/

' \

^

Θ73

\ \ \ \ \ \

M

^

\

773

ω

673

^

573 f?I

/ ^

- ' V/



m

373 00

/

ι 40

ι 80

ι 120

TIME (s)

1 160

1 200

1 240 Journal of Catalysis

Figure 1. Variation of C0 pressure with time during the flash of a Pd substrate preexposed to oxygen at high temperatures. The total pressure was about 10' Pa and Ο /CO ~ 1.5. Key: , temperature variation; and , C0 pressure (11). 2

A

2

2

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

36

C A T A L Y S I S

10

20

30 T I M E

U N D E R

40

T R A N S I E N T

C O N D I T I O N S

50

(s) Journal of Catalysis

Figure 2. Non-steady state variation of 0 pressure with time during flashing of a stable poly crystalline Pd substrate. Initial Ο J CO = 1.5. The origin of the ordinate is defined as the 0 pressure at 300 K. Key: O, observed 0 ; V, calculated on the basis of C0 production; , difference between calculated and observed; and , temperature (11). 2

2

2

2

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

2.

CREiGHTON

A N D

W H I T E

Catalytic

Carbon

Monoxide

37

Oxidation

w o r k o f J o h n C l o s e (11) i n o u r l a b o r a t o r y adequately i l l u s t r a t e the p o i n t . F i g u r e 1 shows t h e v a r i a t i o n of t h e CO2 pressure d u r i n g t h e f l a s h and s u b s e q u e n t c o o l down o f a p o l y c r y s t a l l i n e Pd f o i l . The s a m p l e , m o u n t e d w i t h i n a c o n t i n u o u s l y pumped UHV s y s t e m , was e x p o s e d t o 10~^ t o r r o f a m i x t u r e c h a r a c t e r i z e d by C^/CO = 1.5. After s t a b i l i z a t i o n a t 300K. t h e s a m p l e was h e a t e d r e s i s t i v e l y a t a r a t e o f 12KS"" w h i l e t h e CO2 p a r t i a l p r e s s u r e was m o n i t o r e d mass s p e c t r o m e t r i c a l l y . The CO2 signal, w h i c h i s a m e a s u r e o f t h e CO2 p r o d u c t i o n r a t e , i s s t r o n g l y p e a k e d on b o t h t h e h e a t i n g and c o o l i n g c y c l e s . S i n c e t h e m a x i m a b o t h o c c u r n e a r 573K, we e x p e c t t o f i n d maxima i n t h e s t e a d y - s t a t e CO2 p r o d u c t i o n r a t e s n e a r t h e same t e m p e r a t u r e q u a l i t a t i v e guide only d i f f e r e n c e s i n the r a t e s are the a m p l i t u d e , w h i c h i s h i g h e r d u r i n g c o o l - d o w n , and t h e p e a k t e m p e r a t u r e , w h i c h i s s l i g h t l y l o w e r d u r i n g c o o l down. These are a s c r i b e d t o two d i f f e r e n c e s : (1) t h e a b s o l u t e r a t e o f t e m p e r a t u r e c h a n g e and (2) t h e a m o u n t s o f a d s o r b e d CO and o x y g e n w h i c h a r e p r e s e n t d u r i n g t h e t r a n s i e n t s . F i g u r e 2 shows d a t a f r o m t h e v a r i a t i o n o f P t a k e n u n d e r c o n d i t i o n s s i m i l a r t o t h o s e o f F i g u r e 1. The o r i g i n o f t h e o r d i n a t e i s d e f i n e d as t h e s t e a d y s t a t e O2 p r e s s u r e m e a s u r e d a t 300K. As t h e s a m p l e i s h e a t e d t h e O2 p r e s s u r e d r o p s ( o p e n c i r c l e s ) g o e s t h r o u g h a l o c a l minimum and a maximum b e t w e e n 15 and 40s (500 and 8 0 0 K ) . The t r i a n g l e s show a p r e d i c t e d O2 p r e s s u r e c u r v e b a s e d on CO2 p r e s s u r e . Although one m i g h t a r g u e t h a t i n a c c u r a t e r e l a t i v e s e n s i t i v i t i e s c a n a c c o u n t f o r some o f t h e d i f f e r e n c e i n t h e s e two c u r v e s , i t i s c l e a r t h a t t h e l o c a l maximum i n t h e e x p e r i m e n t a l O2 c o n s u m p t i o n i s n o t r e f l e c t e d i n t h e c a l c u l a t e d c u r v e b a s e d on CO2 p r o d u c t i o n . As i n d i c a t e d by t h e d i f f e r e n c e s p e c t r u m ( s o l i d l i n e i n t h e u p p e r p o r t i o n o f F i g u r e 2) t h e r e i s n o t a s i n g l e s c a l i n g f a c t o r w h i c h w i l l b r i n g any s i g n i f i c a n t p o r t i o n o f t h e s e two c u r v e s i n t o c o i n c i d e n c e . From t h e s e r e s u l t s we c o n c l u d e t h a t 0 p e n e t r a t i o n i n t o t h e s u b s u r f a c e r e g i o n i s an i m p o r t a n t process, p a r t i c u l a r l y at h i g h t e m p e r a t u r e s . The s i g n i f i c a n c e o f t h i s p r o c e s s has b e e n c o n f i r m e d by w o r k i n o u r own (12) and o t h e r l a b o r a t o r i e s ( 1 3 ) . A c e n t r a l p o i n t t o be made i n c o n n e c t i o n w i t h F i g u r e s 1 and 2 i s t h a t a g r e a t d e a l o f q u a l i t a t i v e i n s i g h t c a n be g a i n e d f r o m r e l a t i v e l y s i m p l e e x p e r i ments. The r e s u l t s g u i d e t h e s e l e c t i o n o f t e m p e r a t u r e s f o r s t e a d y - s t a t e and t r a n s i e n t e x p e r i m e n t s w h i c h c a n be a n a l y z e d more q u a n t i t a t i v e l y . 1

Q

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

38

C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

A n o t h e r k i n d of t h e r m a l l y i n d u c e d t r a n s i e n t i s i l l u s t r a t e d i n F i g u r e 3. T h i s d a t a , taken from work o f C h a r l e s C a m p b e l l and S h e i - K u n g S h i Ç14), i n v o l v e s t h e t r a n s i e n t p r o d u c t i o n o f CO2 w h i c h o c c u r s when a c o a d s o r b e d m i x t u r e o f CO m o l e c u l e s and 0 atoms on a Rh s u r f a c e i s h e a t e d . The v a r i o u s s p e c t r a a r e f o r d i f f e r e n t t i t r a t i o n t i m e s i n d i c a t e d on t h e f i g u r e ( 0 - 2 0 m i n ) ; a p o l y c r y s t a l l i n e Rh s u r f a c e p r e s a t u r a t e d w i t h o x y g e n was t i t r a t e d a t 360K f o r χ m i n . by 9.6 χ 10~ Pa (133 Pa = 1 T o r r ) o f CO a n d , a f t e r e v a c u a t i o n , was h e a t e d t o 750K o r h i g h e r . With t i t r a t i o n time ( i . e . p r i o r t o f l a s h i n g ) CO2 i s p r o d u c e d . This lowers t h e amount o f c h e m i s o r b e d o x y g e n and a l l o w s i n c r e a s i n g a m o u n t s o f CO t o a d s o r b . Thus, the areas beneath the curves i n Figure 3 r e f l e c t q u a l i t a t i v e l th mathe m a t i c a l p r o d u c t of coverage of oxygen p r e s e n g cycle. The f o r m e r i s s m a l l a t s h o r t t i t r a t i o n t i m e s w h i l e the l a t t e r i s s m a l l at long t i t r a t i o n times. I t i s i m p o r t a n t t o n o t e t h e s p e c t r a shown i n F i g u r e 3 w e r e g e n e r a t e d by h e a t i n g t h e s u r f a c e i n v a c u u m . The a r e a b e n e a t h t h e CO2 p r e s s u r e t r a n s i e n t v a r i e s s t r o n g ­ l y w i t h t i t r a t i o n t i m e i n d i c a t i n g t h a t CO2 i s r e a d i l y p r o d u c e d by r e a c t i o n o f c o a d s o r b e d CO and 0 . In f a c t , c o m p a n i o n CO s p e c t r a show t h a t a l l t h e c h e m i s o r b e d CO r e a c t s t o make CO2 so l o n g as t h e r e i s e x c e s s oxygen. By a n a l y z i n g C 0 , CO and O2 t r a n s i e n t s l i k e t h o s e o f F i g u r e 3, t h e t e m p e r a t u r e d e p e n d e n c e o f t h e r a t e o f CO2 p r o d u c t i o n c a n be e v a l u a t e d f o r a r e l a t i v e l y w i d e r a n g e o f CO and 0 c o v e r a g e s ( 6 and 0 ). T h i s a n a l y s i s shows t h a t t h e r a t e i s n o t d e s c r i b a b l e by a s i m p l e LH e x p r e s s i o n o f t h e f o r m 7

2

c Q

Q

R

co

=

2

A

e x

P

E

{"

R T }

( L / H

Θ

Ο co 6

d>

E i t h e r coverage dependent terms i n the r a t e c o e f f i ­ c i e n t s or f r a c t i o n a l exponents i n the coverage terms m u s t be i n c o r p o r a t e d . Table I summarizes our a n a l y s i s of s e v e r a l cases u s i n g the e q u a t i o n R

=

co

2

A

LH

exp

* α

{ CO

θ

co

(E „ + α θ + LH 00 T

)/RT}

e

m

ο

θ

n

co

A l t h o u g h we c a n n o t d e m o n s t r a t e u n a m b i g u o u s l y w h i c h of t h e s e , i f any, i s p h y s i c a l l y most s a t i s f a c t o r y , we p r e f e r m o d e l I V b e c a u s e i t p r e s e r v e s t h e e x p o n e n t s o f u n i t y on t h e c o v e r a g e s , as an e l e m e n t a r y equation m u s t , b e c a u s e i t g i v e s a good f i t to t h e d e t a i l e d r e a c t i o n r a t e v e r s u s t i t r a t i o n t i m e a t 360K ( c a l c u l a -

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

CREiGHTON

A N D

W H I T E

Catalytic

Carbon

Monoxide

Oxidation

TIMEAsec.) 15

T/(IO · K ) Applications of Surface Science Figure 3. C0 pressure vs. time during heating a polycrystalline Rh sample cov­ ered with CO and O. These species were deposited at 360 Κ in varying amounts by predosing oxygen (15.9 L) and then titrating with CO(g) at 7 X 10 ton. The heating was then done in vacuo. Titration times are marked in minutes. The small high temperature peak is due to CO interactions with the walls (14). 2

9

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

Table I Kineti R

C0

=

A

e X

2

[

P "

( E

LH

Case #

m = order in θ

I II III IV V

1 1.6 1 1 1

η = order in Θ Γ Π

1 1 0.66 1 1

ot ot 0 CO k c a l mole'^ k c a l mole"

E

n

^ molec. 41.1 48.5 41.0 47.6 42.1

Scaled

± ± ± ± ±

cm~^s"^ 1.02 0.56 0.40 0.40 0.29

according

LH k c a l mole""- 1

8.97 13.31 8.64 14.30 8.60

to Ε „ γ

± ± ± ± ±

0 0 0 0 2.1

0 0 0 -4.4 0

0.86 0.48 0.34 0.34 0.24

for direct

Standard deviation α 0.299 0.114 0.121 0.073 0.087 a

a

a

a

comparison with I .

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

2.

C R E I G H T O N

A N D

Catalytic

W H I T E

Carbon

Monoxide

Oxidation

41

ted from a p l o t of Q v e r s u s t i t r a t i o n t i m e ) , and because i t g i v e s the b e s t s t a t i s t i c a l f i t to the d a t a . P e r h a p s some c o m b i n a t i o n o f m o d e l s I V and V w o u l d be e v e n b e t t e r b u t o u r d a t a c e r t a i n l y do n o t r e q u i r e i t . I n any c a s e , m o d e l V a l o n e d o e s n o t g i v e a l a r g e e n o u g h CO^ p r o d u c t i o n r a t e when t h e o x y g e n c o v e r a g e i s h i g h , so i t c a n n o t be u s e d a l o n e . The i n t e r p r e t a t i o n o f t h e s e coverage-dependent e f f e c t s i n v o l v e s s u c h i d e a s as i s l a n d f o r m a t i o n , m i x e d d o m a i n s o f CO and 0, t h e m o b i l i t y o f CO and 0 and t h e a d s o r p t i o n o f CO on o x y g e n - c o v e r e d r e g i o n s ( 1 , 1 8 , 2 6 , 27). A deeper u n d e r s t a n d i n g of the r o l e s of these p r o c e s s e s comes f r o m i s o t h e r m a l e x p e r i m e n t s i n v o l v i n g pressure transients. A l l of the r e s u l t s u r f a c e coverages fro ments. Although these i n f e r e n c e s are soundly based i n t h e c a s e o f CO o x i d a t i o n , we w o u l d be much more c o m f o r t a b l e w i t h d i r e c t measurement of the s u r f a c e coverage. q

Transients

Induced

by

Pressure

Changes

The m o s t s o p h i s t i c a t e d and i n c i s i v e t r a n s i e n t experiments are those d e r i v e d from modulated m o l e c u l a r beam r e a c t i v e s c a t t e r i n g e x p e r i m e n t s . R e s u l t s f r o m s u c h e x p e r i m e n t s w i l l be d i s c u s s e d below a f t e r p r e s e n t a t i o n of data from s i m p l e r e x p e r i m e n t s w h i c h n o t o n l y a r e c o n s i s t e n t w i t h t h e beam r e s u l t s but a l s o extend the range of t e m p e r a t u r e c o v e r a g e c o n d i t i o n s a n d , i n some c a s e s , d i r e c t l y assess the coverage. F i g u r e 4 shows t h e r e s u l t s o f a v e r y s i m p l e e x p e r i m e n t i n v o l v i n g t r a n s i e n t CO2 p r o d u c t i o n on a p o l y c r y s t a l l i n e P t f o i l a t 540K ( 1 5 ) . The t r a n s i e n t was s e t up by e s t a b l i s h i n g a s t e a d y - s t a t e r e a c t i o n w i t h 0 / C O - 1 0 and t h e n r a p i d l y c l o s i n g (-3 s e c ) t h e 0 l e a k v a l v e i n t o the c o n t i n u o u s l y pumped r e a c t i o n c h a m b e r . In the absence of r e a c t i o n , t h e CO l e a k r a t e g a v e a p a r t i a l p r e s s u r e o f 2 x 1 0 " t o r r . The e n s u i n g t r a n s i e n t CO2 comes f r o m t h e r e m o v a l o f c h e m i s o r b e d o x y g e n atoms p r e s e n t a t i t s o r i g i n . From F i g u r e 4 i t i s i m m e d i a t e l y c l e a r t h a t t h e CO2/CO r a t i o , and t h e r e f o r e t h e C 0 p r o d u c t i o n rate per u n i t CO p r e s s u r e , r e m a i n s n e a r l y c o n s t a n t o v e r t h e f i r s t 2 min of the t i t r a t i o n w h i l e the oxygen c o v e r a g e d r o p s o f f more t h a n a f a c t o r o f 2. This r e s u l t c l e a r l y e l i m i n a t e s the p o s s i b i l i t y of r e a c t i o n through a s i m p l e ER p r o c e s s o f t h e f o r m 2

2

2

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

42

CATALYSIS

CO(g) for

which

c o



TRANSIENT

CONDITIONS

+ 0 ( a ) -»• C 0 ( g ) 2

the rate of C0 R

UNDER

= k „

Ρ

p

p r o d u c t i o n w o u l d be

2

θ .

(3)

A c c o r d i n g t o t h i s e x p r e s s i o n t h e r a t e p e r u n i t CO p r e s s u r e s h o u l d be p r o p o r t i o n a l t o t h e c o v e r a g e o f o x y g e n w h i c h i t i s n o t . O r i g i n a l l y (15) t h i s d a t a was i n t e r p r e t e d i n t e r m s o f a p h y s i s o r b e d p r e c u r s o r s t a t e l a r g e l y b e c a u s e no s i g n i f i c a n t accumulation o f c h e m i s o r b e d CO i s p o s s i b l e a t t h i s temperature and i t was f e l t t h a t t h e CO r e s i d e n c e t i m e was probably very shor (), w h e r e φ i s t h e p h a s e l a g , as a f u n c t i o n o f Τ can be i n t e r p r e t e d i n t e r m s o f an a c t i v a t i o n e n e r g y d i f f e r e n c e ( E - E^) b e t w e e n r e a c t i o n and d e s o r p t i o n . The r e s u l t f o r P t ( l l l ) i s -10.8 k c a l m o l e " as shown i n F i g u r e 12 ( 1 6 b ) . F o r an E l e y - R i d e a l p a t h w a y i n w h i c h a gas p h a s e CO m o l e c u l e makes a d i r e c t o r i m p a c t a t t a c k on an o x y g e n a d a t o m , E^ = 0 and we r e q u i r e r

1

r

2

2

2

2

2

r

1

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

52

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Journal of Chemical Physics Figure 12. Logarithmic plot of the tangent of the phase lag as a function of tem­ perature for CO2 reactively scattered from Pt (111). The experiment was run with a background CO pressure of 10' ton and a modulated 0 beam (10 Hz) which had an equivalent pressure of 6.3 χ 10 ton. Key: Φ, data; and , model, E * = 24.1 kcal/mol, v = 0.11 cm /particles s (16b). 7

2

7

LH

h

2

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

2.

CREiGHTON

A N D

Catalytic

W H I T E

Carbon

Monoxide

53

Oxidation

E < 0 which i s unreasonable. I f a p h y s i s o r b e d CO i s i n v o l v e d , i t s b i n d i n g energy would have to exceed 10 k c a l m o l e i n order f o r E t o become p o s i t i v e and i t i s not r e a s o n a b l e to expect a b i n d i n g energy of t h i s magnitude. We a r e t h u s l e d t o t h e c o n c l u s i o n t h a t t h e r e a c t i o n o c c u r s t h r o u g h an LH p a t h w a y . The same c o n c l u s i o n s h o l d f o r PdÇlll)Çl6a) w h e r e , on t h e b a s i s o f p h a s e s h i f t m e a s u r e m e n t s , E - E^ = -9.5 k c a l mole"" . I n t h i s same v e r y i n t e r e s t i n g w o r k , t r a n s i e n t p r e s s u r e jump e x p e r i m e n t s w i t h e x c e l l e n t time r e s o l u t i o n were p e r f o r m e d . An e x a m p l e i s shown i n F i g u r e 13 i n w h i c h a CO beam, w i t h a f l u x e q u i v a l e n t to P = 6x10 t o r r i m p i n g e s on a P d ( H l ) s u b s t r a t e immersed i n a c o n s t a n t ambient P 1x10" t o r r . The i m p o r t a n t p o i n t a t 375K and t h e t r a n s i e n t s a t 375 and 450K. S i n c e t h e beam c a n be t u r n e d on i n a b o u t 1 msec t h e r e s p o n s e shown i n F i g u r e 13 i s n o t an a r t i f a c t o f t h e p r o c e d u r e and we m u s t c o n c l u d e t h a t more t h a n ER k i n e t i c s i s i n v o l v e d e v e n u n d e r t h e s e c o n d i t i o n s o f h i g h o x y g e n c o v e r a g e and r e l a t i v e l y l o w t e m p e r a t u r e s ; an ER p a t h w o u l d show no i n d u c t i o n time. The maxima o b s e r v e d i n F i g u r e 13 a r e r e l a t e d t o t h e v a r i a t i o n of t h e C 0 p r o d u c t i o n r a t e w i t h c o v e r a g e s o f CO and 0. At s h o r t times 0 i s low w h i l e 0 i s h i g h and t h e r e a c t i o n r a t e g r o w s as CO a c c u m u l a t e s . W i t h t h e p a s s a g e of t i m e , 9 d e c l i n e s and 9 continues t o grow. The r e s u l t i n g p r o d u c t Q ^ c o i i when θ = θ = 0.5. A t l o n g e r t i m e s and l o w e r t e m p e r a t u r e s 6 i n h i b i t s d i s s o c i a t i v e o x y g e n a d s o r p t i o n and t h e r a t e d r o p s t o some s t e a d y - s t a t e v a l u e . A t h i g h temp­ eratures, 6 cannot r i s e to i n h i b i t 0 adsorption. C o n s e q u e n t l y , t h e r a t e r i s e s u p o n a d m i s s i o n o f CO b u t t h e r e i s no p r o n o u n c e d maximum. F i n a l l y , we n o t e a v e r y c l e v e r i s o t o p e e x p e r i m e n t d o n e by T a t s u o M a t s u s h i m a Ç24). The r e s u l t s a r e s u m m a r i z e d i n F i g u r e 14 and show t h a t when CO i s p r e a d s o r b e d on P t and t h e n a m i x t u r e o f l^CO j Q i s i n t r o d u c e d i n t o t h e gas p h a s e , t h e p r o d u c t C 0 m o l e c u l e s i n i t i a l l y formed a l l a r e l a b e l l e d w i t h ^C. I n f a c t t h e e x p e r i m e n t i s n o t q u i t e so s i m p l e . An i n i t i a l ^ C 0 / ^ C 0 r a t i o i s measured ( u n i t y i n F i g u r e 14) w h i c h i s e q u a l t o t h e C O / C O r a t i o i n t h e p r e a d s o r b e d CO. The c o n c l u s i o n i s t h a t C 0 i s p r o d u c e d by t h e r e a c t i o n o f a d s o r b e d , r a t h e r t h a n g a s e o u s , CO and a d s o r b e d 0 a t o m s - i . e . t h e L a n g m u i r H i n s h e l w o o d p r o c e s s . T h i s c o n c l u s i o n has d o m i n a t e d a l l o f t h e r e s u l t s p r e s e n t e d h e r e and a p p e a r s t o be u b i q u i t o u s f o r a l l t r a n s i t i o n m e t a l s under a l l c o n d i t i o n s t h a t have been s t u d i e d t o d a t e . r

- 1

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r

1

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2

c O

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2

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1 2

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In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

T I M E (sec) Journal of Chemical Physics Figure 13. Transient C0 production rates as a function of time for the titration of preadsorbed oxygen on Pd (111) at 375, 450, and 500 K. The experiment was run in a background of 10' torr of 0 , and the CO pressure 6 X 10 torr was introduced at the point indicated (16a,). 2

7

2

8

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

2.

CREiGHTON

A N D

Catalytic Carbon

W H I T E

Monoxide

55

Oxidation

2.5 (α)

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0co 11). We have measured c o n t a i n i n g three-way c a t a l y s (18). The apparatus that was used i n our study i s shown i n F i g u r e 10. A f t e r the d e s i r e d exposure of the 5 g c a t a l y s t bed to exhaust, the r e a c t o r was f l u s h e d w i t h N f o r 10 min, and then the oxygen content of the c a t a l y s t bed was determined i n the f o l l o w i n g procedure. F i r s t , both s o l e n o i d v a l v e s a t the i n l e t of the r e a c t o r were closed and a s m a l l flow of N was i n j e c t e d i n t o the r e a c t o r through a 4-port s w i t c h i n g v a l v e . The s w i t c h i n g v a l v e was then turned to i n j e c t a stream of 2.3% 0 /0.1% Ar/He i n t o the r e a c t o r as the f l o w from the r e a c t o r was analyzed c o n t i n u o u s l y w i t h the mass spectrometer. A r was monitored to determine the e l u t i o n curve o f an i n e r t s p e c i e s , and i t a l s o served as an i n t e r n a l c o n c e n t r a t i o n standard i n the C 0 measurement. The e l u t i o n curve of 0 r e l a t i v e t o t h a t of Ar was used to determine the amount o f 0 that reacted w i t h the c a t a l y s t . A s m a l l amount of 0 reacted w i t h carbon on the c a t a l y s t to form C 0 , which was a l s o measured (H«0 formation was n e g l i g i b l e ) . The oxygen content of the c a t a l y s t a f t e r exposure t o exhaust was c a l c u l a t e d by s u b t r a c t i n g the net oxygen uptake (measured uptake l e s s the amount reacted t o form C0 ) from the maximum oxygen content, o r oxygen c a p a c i t y of the c a t a l y s t bed. The oxygen c a p a c i t y of the c a t a l y s t bed i s d e f i n e d as the maximum amount of oxygen that was r e t a i n e d by the c a t a l y s t a f t e r treatment w i t h 0 and that could be removed by r e a c t i o n w i t h CO. The r a t e o f i n c r e a s e o f oxygen content a f t e r a r i c h - t o - l e a n step change i n a i r - f u e l r a t i o was s t u d i e d using the f o l l o w i n g procedure. The r e a c t o r flow was switched to N a f t e r s t a b i l i z a t i o n of the c a t a l y s t i n exhaust a t an a i r - f u e l r a t i o of 14.1, a c a t a l y s t i n l e t temperature of 680 K, and a space v e l o c i t y of 110,000 h (STP). Next, the a i r - f u e l r a t i o was changed t o a s e t t i n g o f 15.1 w h i l e the exhaust bypassed the r e a c t o r . Then the r e a c t o r was given a p u l s e of l e a n exhaust by s w i t c h i n g from N« t o exhaust f o r a s p e c i f i e d p e r i o d , and then back to N«. 2

2

2

2

2

2

2

2

2

2

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In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Three-Way

H E R Z

Automotive

Catalysts

• ENGINE EXHAUST

N

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INJECTION PORT

REACTOR

Vl Xsj

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l&EC Product Research and Development Figure 10. Schematic diagram of the apparatus used to measure changes in the oxygen content of a catalyst with changes in exhaust air-fuel ratio. The 2.54-cm i.d. tubular reactor contained 5 g of catalyst and was heated by an electric furnace (18j.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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A f t e r t h i s exposure, the oxygen content of the c a t a l y s t was measured. F i g u r e 11 shows the r e s u l t s of these experiments f o r the same p e l l e t e d Pt/Pd/Rh/Ce/Al^O^ c a t a l y s t used f o r the data i n F i g u r e 9. The oxygen content of the c a t a l y s t bed increased l i n e a r l y w i t h pulse d u r a t i o n and reached the s t e a d y - s t a t e l e a n l e v e l w i t h i n 0.5 s. The oxygen c a p a c i t y of the c a t a l y s t was a s s o c i a t e d p r i m a r i l y w i t h o x i d a t i o n of the 190 ymol of Ce cont a i n e d i n each gram of c a t a l y s t ; the c a t a l y s t contained only 8 pmol of precious metal per gram. A comparison of the oxygen c a p a c i t y to the amount of Ce i n the c a t a l y s t suggests that about 76% of the Ce could change between the +3 and +4 o x i d a t i o n s t a t e s (the most common o x i d a t i o n s t a t e s of Ce). The i d e n t i t i e s of the Ce compounds which undergo o x i d a t i o n and r e d u c t i o n i n exhaust are not known hydroxides are l i k e l y candidate The r a t e of decrease of oxygen content of the c a t a l y s t a f t e r a l e a n - t o - r i c h step change was s t u d i e d using the procedure described above w i t h the l e a n and r i c h exposures reversed. F i g u r e 12 presents the r e s u l t s of these experiments. After exposure to r i c h exhaust f o r 1.0 s, the oxygen content of the c a t a l y s t bed decreased 36% of the way from i t s l e a n s t e a d y - s t a t e l e v e l to i t s r i c h s t e a d y - s t a t e l e v e l . This change i n oxygen content could correspond to a conversion of 58% of the CO and H i n the 1.0 s pulse of r i c h exhaust (hydrocarbons are l e s s r e a c t i v e w i t h the oxygen held by the c a t a l y s t than CO and H ) . The d i f f e r e n c e i n c a t a l y s t oxygen content between the r i c h and l e a n s t e a d y - s t a t e s i s s u f f i c i e n t l y l a r g e to account f o r the e x t r a conversion obtained i n the CO step response experiment shown i n F i g u r e 9B. In a d d i t i o n , the r a t e s of change of c a t a l y s t oxygen content shown i n Figures 11 and 12 are s u f f i c i e n t l y f a s t to a f f e c t the performance of the c a t a l y s t when the a i r - f u e l r a t i o i s c y c l e d about the s t o i c h i o m e t r i c p o i n t at frequencies on the order of 1 Hz. 2

2

Future

Studies

The s t u d i e s described i n the preceding two s e c t i o n s have i d e n t i f i e d s e v e r a l processes that a f f e c t the dynamic behavior of three-way c a t a l y s t s . Further s t u d i e s are r e q u i r e d to i d e n t i f y a l l of the chemical and p h y s i c a l processes that i n f l u e n c e the behavior of these c a t a l y s t s under c y c l e d a i r - f u e l r a t i o c o n d i t i o n s . The approaches used i n f u t u r e s t u d i e s should i n c l u d e (1) d i r e c t measurement of dynamic responses, (2) mathematical a n a l y s i s of experimental data, and (3) f o r m u l a t i o n and v a l i d a t i o n of mathematical models of dynamic converter o p e r a t i o n . Such models must be used i n c o n j u n c t i o n w i t h a model of convert e r warmup (1) and a model of c a t a l y s t d e a c t i v a t i o n (6) when d e s i g n i n g the d i s t r i b u t i o n of a c t i v e components i n a c a t a l y s t and converter and the engine c o n t r o l s t r a t e g y .

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982. 1

I&EC Product Research and Development 2

s

Figure 11. Rate of increase in the oxygen content of a pelleted Pt/Pd/Rh/Ce/Al O catalyst in lean exhaust at a space velocity of 110,000 h' (STP) and a catalyst inlet temperature of 680 Κ as;.

3

5

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982. 1

2

3

Figure 12. Rate of decrease in the oxygen content of a pelleted Pt/Pd/Rh/Ce/Al 0 catalyst in rich exhaust at a space velocity of 110,000 h' (STP) and a catalyst inlet temperature of 680 Κ

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Three-Way

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Catalysts

77

Literature Cited 1.

Oh, S. H . ; Cavendish, J. C.; Hegedus, L . L . "Mathematical Modeling of Catalytic Converter Lightoff: Single P e l l e t Studies"; AIChE J., 1980, 26, 935.

2.

Canale, R. P . ; Winegarden, S. R.; Carlson, C. R.; Miles, D. L., SAE Paper No. 780205, SAE Trans. 1978, 87, 843.

3.

Seiter, R. E.; Clark, R. J., SAE Paper No. 780203, SAE Trans. 1978, 87, 828.

4.

Hegedus, L . L.; Gumbleton, J. J. CHEMTECH 1980, 10, 630.

5.

Gandhi, H. S.; Piken Paper No. 760201, SA

6.

Hegedus, L . L.; Summers, J. C.; Schlatter, J. C . ; Baron, K. J. Catal. 1979, 56, 321.

7.

Falk, C. B . ; Mooney, J. J. "Three-Way Conversion Catalysts: Effect of Closed-Loop Feed-Back Control and Other Parameters on Catalyst Efficiency"; SAE Paper No. 800462, 1980.

8.

Schlatter, J. C.; Sinkevitch, R. M . ; M i t c h e l l , P. J. "A Laboratory Reactor System for Three-Way Catalyst Evaluation"; GM Research Publication GMR-2911; presented at 6th N. Amer. Mtg. Catal. Soc., Chicago, IL, March 1979.

9.

Schlatter, J . C . ; M i t c h e l l , P. J. Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 288.

10.

Joy, G. C . ; Molinaro, F. S.; Lester, G. R. "Water-Gas Shift and Steam-Reforming Ability of Group VIII Metals i n Simulated Automotive Exhaust"; UOP Inc. report presented at 6th N. Am. Mtg. Catal. Soc., Chicago, I L , March 1979.

11.

Cooper, B. J.; Keck, L . "NiO Incorporation i n Three Way Catalyst Systems"; SAE Paper No. 800461, 1980.

12.

Meitzler, A. H. "Application of Exhaust-Gas-Oxygen Sensors to the Study of Storage Effects i n Automotive Three-Way Catalysts"; SAE Paper No. 800019, 1980.

13.

Hill, J. C . ; Majkowski, R. F. "Time-Resolved Measurement of Vehicle Sulfate and Methane Emissions with Tunable Diode Lasers"; SAE Paper No. 800510, 1980.

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14.

S e l l , J. Α.; Herz, R. K . ; Monroe, D. R. "Dynamic Measure­ ment of Carbon Monoxide Concentrations i n Automotive Exhaust Using Infrared Diode Laser Spectroscopy"; SAE Paper No. 800463, 1980.

15.

S e l l , J. Α.; Herz, R. K . ; Perry, E. C. "Time-Resolved Measurement of Carbon Monoxide i n the Exhaust of a Computer Command Controlled Engine"; SAE Paper No. 810276, 1981.

16.

Yao, H. C . ; Shelef, M. J. Catal. 1976, 44, 392.

17.

Kaneko, Y.; Kobayashi, H . ; Komagome, R.; Hirako, O.; Nakayama, O., SAE Paper No. 780607, SAE Trans. 1978, 87, 2225. Herz, R. K. "The Dynami I. Catalyst Oxidatio Prod. Res. Dev. 1981, i n press.

18.

RECEIVED

July 28, 1981.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

4 Dynamics of High-Temperature Carbon Monoxide Chemisorption on Platinum-Alumina by FastResponse IR Spectroscopy S. H. OH and L. L. HEGEDUS

1

General Motors Research Laboratories, Warren,MI48090 Since the early work of Langmuir (1), the chemisorption of carbon monoxide on platinum surfaces has been the subject of numerous investigations understanding of CO chemisorptio p r a c t i c a l importance; for example, the c a t a l y t i c reaction of CO over noble metals (such as Pt) i s an essential part of automobile emission control. There is a wealth of information available on CO chemisorption over s i n g l e - c r y s t a l and polycrystalline platinum surfaces under ultrahigh-vacuum conditions; research efforts i n this area have gained a significant momentum with the advent of various surface analysis techniques (e.g., 2-8). In contrast, CO chemisorption on supported platinum catalysts (e.g., 9, 10, 11) i s less well understood, due primarily to the i n a p p l i c a b i l i t y of most surface-sensitive techniques and to the difficulties involved i n characterizing supported metal surfaces. In part i c u l a r , the effects of transport resistances on the rates of adsorption and desorption over supported catalysts have rarely been studied. Transmission i n f r a r e d spectroscopy i s one o f the few t e c h niques a p p l i c a b l e to the i n s i t u study of supported c a t a l y s t systems (e.g., 12). Much of the evidence concerning the nature of the adsorbed s t a t e s of CO over supported P t c a t a l y s t s has been obtained from i n f r a r e d s p e c t r o s c o p i c s t u d i e s . F o r example, i t has been shown (13, 14) t h a t CO can chemisorb on supported P t c a t a l y s t s e i t h e r i n a l i n e a r o r i n a bridged form. A t h i g h temperatures, however, the a b s o r p t i o n band due t o the b r i d g e bonded CO disappears (e.g., 15), and thus the l i n e a r form o f adsorbed CO r e p r e s e n t s the dominant s t a t e , g i v i n g r i s e t o a strong,_Yell-defined i n f r a r e d band a t a frequency o f about 2070 cm . T h i s provides a convenient means o f m o n i t o r i n g t h e s u r f a c e c o n c e n t r a t i o n of CO under r e a l i s t i c o p e r a t i n g c o n d i t i o n s by observing the i n t e n s i t y o f the a s s o c i a t e d i n f r a r e d band. 1

Current address: W. R. Grace & Company, Washington Research Center, Columbia, MD 21044. 0097-6156/82/0178-0079$06.25/0 © 1982 A m e r i c a n Chemical Society

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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T R A N S I E N T

C O N D I T I O N S

Recently there has been a growing emphasis on the use of t r a n s i e n t methods to study the mechanism and k i n e t i c s of c a t a ­ l y t i c r e a c t i o n s (16, 17, 18). These t r a n s i e n t s t u d i e s gained new impetus w i t h the i n t r o d u c t i o n of computer-controlled c a t a ­ l y t i c converters f o r automobile emission c o n t r o l (19); i n t h i s l a r g e - s c a l e c a t a l y t i c process the composition of the feedstream i s o s c i l l a t e d as a r e s u l t of a feedback c o n t r o l scheme, and the frequency response c h a r a c t e r i s t i c s of the c a t a l y s t appear to p l a y an important r o l e (20). P r e l i m i n a r y s t u d i e s (e.g., 15) i n d i c a t e t h a t the t r a n s i e n t response of these c a t a l y s t s i s dominated by the r e l a x a t i o n of s u r f a c e events, and thus i t i s necessary to use f a s t - r e s p o n s e , s u r f a c e - s e n s i t i v e techniques i n order to understand the c a t a l y s t ' s behavior under t r a n s i e n t conditions. In t h i s paper we w i l f r a r e d r e a c t o r system whic temperatures and pressures. We w i l l d i s c u s s the r e a c t o r c e l l , the feed system which a l l o w s c o n c e n t r a t i o n step changes or c y c l i n g , and the m o d i f i c a t i o n s necessary f o r c o n v e r t i n g a commercial i n f r a r e d spectrophotometer to a high-speed i n s t r u ­ ment. This m o d i f i e d i n f r a r e d s p e c t r o s c o p i c r e a c t o r system was then used to study the dynamics of CO a d s o r p t i o n and d e s o r p t i o n over a Pt-alumina c a t a l y s t a t 723 Κ (450°C). The measured step responses were analyzed using a t r a n s i e n t model which accounts f o r the k i n e t i c s of CO a d s o r p t i o n and d e s o r p t i o n , e x t r a - and i n t r a p e l l e t d i f f u s i o n r e s i s t a n c e s , s u r f a c e accumulation of CO, and the dynamics of the i n f r a r e d c e l l . F i n a l l y , we w i l l b r i e f l y d i s c u s s some of the t r a n s i e n t response ( i . e . , step and cycled) c h a r a c t e r i s t i c s of the 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 ( i . e . , CO + o ) . 2

Fast-Response I n f r a r e d Reactor System a. Reactor c e l l . The o b j e c t i v e was to c o n s t r u c t a rugged i n f r a r e d c e l l which can be operated a t h i g h temperatures (about 500°C), which has a s m a l l enough volume t o a l l o w f a s t response c h a r a c t e r i s t i c s , and which behaves as a well-mixed r e a c t o r (CSTR). The r e a c t o r body was manufactured of a p a i r of V a r i a n n o n r o t a t a b l e blank Confiât f l a n g e s , according to a suggestion of B e l l (21). These f l a n g e s were machined such that a s m a l l c a v i t y was created f o r placement of a t h i n c a t a l y s t d i s c . A t h i n thermocouple was placed i n d i r e c t contact w i t h the d i s c . The d i s c holder was machined to f i t i n t o the s t a i n l e s s s t e e l f l a n g e i n such a way t h a t i t d i r e c t s the gas to consecu t i v e l y sweep both faces of the c a t a l y s t d i s c , w i t h an expansion volume in-between. T h i s c o n f i g u r a t i o n provided good gas-phase mixing i n the c e l l , thus a l l o w i n g the r e a c t o r to be c h a r a c t e r i z e d as a CSTR. T h i s mode o f i n t e r n a l mixing e l i m i n a t e s the need f o r i n t e r n a l moving p a r t s or e x t e r n a l r e c y c l e loops and pumps.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

4.

O H

A N D

H E G E D U S

Carbon

Monoxide

Chemisorption

81

The r e a c t o r assembly was heated by e l e c t r i c h e a t e r s . The maximum o p e r a t i n g temperature i s determined by the window con­ s t r u c t i o n . Sapphire windows (from EIMAC), brazed i n t o Kovar s l e e v e s , were used; the sleeves were then welded d i r e c t l y i n t o the s t a i n l e s s s t e e l r e a c t o r housing. We found t h a t the c e l l so constructed was capable of t r o u b l e - f r e e , continuous o p e r a t i o n a t 450°C; operations a t somewhat higher temperatures are probably s t i l l p o s s i b l e but were not explored. Sapphire was chosen as a window m a t e r i a l because i t i s i n s e n s i t i v e to water vapor and i s transparent i n tljie wave number range of our i n t e r e s t (about 2400 cm to 2000 cm i n these experiments). Moreover, the thermal expansion c h a r a c t e r i s t i c s of the r e a c t o r were found to match w e l l w i t h those of the window f i x t u r e . The c a t a l y s t was prepared by impregnating γ-alumina (Alon) to i n c i p i e n t wetness u s i n A f t e r impregnation, th 773 Κ (500°C) f o r 2 h. The i n f r a r e d d i s c was prepared by com p r e s s i n g 0.08 g of the c a t a l y s t powder a t 58 840 N. The prop­ e r t i e s of the c a t a l y s t d i s c are l i s t e d i n Table I . b. Feed system. F i g u r e 1 shows a f l o w diagram of the feed system. Two feedstreams of d i f f e r i n g compositions were employed; these were a l t e r n a t i v e l y interchanged by f o u r , e l e c t r i c a l l y operated high-speed s o l e n o i d s w i t c h i n g v a l v e s (Automatic Switch Co.). I n c y c l e d experiments, the amplitude i s determined by the c o n c e n t r a t i o n d i f f e r e n c e between the two feedstreams, and the frequency by the v a l v e s w i t c h i n g frequency. An e l e c t r o n i c c l o c k was used to v a r y the s w i t c h i n g frequency i n the range of i n t e r e s t (ranging from s i n g l e steps to about 10 Hz) . The r e a c t o r was preceded by a preheater which had a s u f ­ f i c i e n t l y s m a l l mixing volume so that i t d i d not c o n t r i b u t e s i g n i f i c a n t l y to the d i s p e r s i o n of the feedstream p u l s e s . Steady streams of ^ 0 vapor or SO2 could be introduced before the preheater to simulate automobile exhaust. When ^ 0 was added to the feedstream (we used a l i q u i d chromatographic pump), i t was condensed out before the gas entered the vent l i n e . However, no water- or SO^-containing feedstreams were used i n the examples shown i n t h i s paper. The gas-phase composition before or a f t e r the r e a c t o r was monitored by mass spectroscopy; although no such data a r e shown here, experience i n d i c a t e d the n e c e s s i t y of a f a s t - r e s p o n s e i n l e t system. c. Fast-response IR spectrophotometer. A Perkin-Elmer Model 180 i n f r a r e d spectrophotometer was m o d i f i e d f o r these experiments t o provide the necessary time response. The modi­ f i c a t i o n s i n c l u d e d the replacement of the o r i g i n a l chopper by a high-speed v a r i e t y (Laser P r e c i s i o n C o r p o r a t i o n , 15-1000 H z ) , and r e d i r e c t i n g the i n f r a r e d beam so t h a t i t was focused onto a

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

0.017

Thickness (cm)

130

2

(cm /s)**

^Determined by a CO f l o w - c h e m i s o r p t i o n technique.

. (A) micro

err

3

(g/cm )

0.586

0.0246

1.52

**Computed from the random pore model of Wakao and Smith (22) and g i v e n here a t 101.3 kPa and 723 Κ.

r

2.0

(A)

r

3

211 150

ε Ρ Diameter (cm)

0.346

oo, D - » oo. m

2

eff

2

m

m

eff

eff

1.0 r 0.8 -

,l

ι

ι

»

0

5

10

15

Time(s) Figure 10. Effects of internal and external transport resistances on the computed step-response of CO desorption. Curve A corresponds to our experimental condi­ tions. Key: A,k — 60 cm/s, D — 0.0246 cm /s; B, k - » oo, D = 0.0246 cm /s; and C, k -> oo, D -> oo. m

2

eff

2

m

m

eff

eff

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

96

C A T A L Y S I S

U N D E R

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C O N D I T I O N S

where (36/3 ) represents the slope of the a d s o r p t i o n isotherm. The q u a n t i t y l N a (30/3 )] i n Equation (15) accounts f o r the c a p a c i t y of the c a t a l y t i c surface f o r the chemisorption of CO. For porous c a t a l y s t p e l l e t s w i t h p r a c t i c a l l o a d i n g s , t h i s quan­ t i t y i s t y p i c a l l y much l a r g e r than the p e l l e t v o i d f r a c t i o n , i n d i c a t i n g t h a t the dynamic behavior of supported c a t a l y s t s i l dominated by the r e l a x a t i o n of s u r f a c e phenomena (e.g., 36). T h i s i m p l i e s that a q u a s i - s t a t i c approximation f o r Equation (1) (i.e., ΟΥ = 0) can o f t e n be s a f e l y invoked i n the t r a n s i e n t modeling of porous c a t a l y s t p e l l e t s . The c a l c u l a t i o n s showed t h a t the q u a s i - s t a t i c approximation i s indeed v a l i d i n our case; the model p r e d i c t e d v i r t u a l l y the same step responses, even when the v a l u e of was reduced by a f a c t o r of 10. I t f o l l o w s d i r e c t l y from Equation (15) that the c h a r a c t e r i s ­ t i c response time f o r during the a d s o r p t i o n - d e s o r p t i o [ ( L / D ) N a (30/3c)]. (Note that t h i s c h a r a c t e r i s t i c r e s ­ ponse time represents the product of the c h a r a c t e r i s t i c time f o r i n t r a p e l l e t d i f f u s i o n , L / D f f , and the s u r f a c e c a p a c i t y f o r CO chemisorption, Ν a (30/3c).) The e s t i m a t i o n of the response time i s complicated by the f a c t that the slope of the a d s o r p t i o n isotherm (30/3c) v a r i e s w i t h gas-phase CO c o n c e n t r a t i o n ; how­ ever, when i t s average v a l u e over the CO c o n c e n t r a t i o n range of 0 to 1 v o l % was taken to be 5x10° cm /mol (= 1 ( v o l see F i g u r e 4a), the c h a r a c t e r i s t i c response time was c a l c u l a t e d to be 0.75 s. I n s p e c t i o n of the time v a r i a t i o n of the i n t r a p e l l e t gasphase concentrations shown i n F i g u r e s 7 and 8 ( s o l i d l i n e s ) lends support to our estimated response time; that i s , the i n t r a p e l l e t gas-phase c o n c e n t r a t i o n r e l a x e s on the time s c a l e of 1 s during both the a d s o r p t i o n and d e s o r p t i o n processes. In c o n t r a s t to the i n t r a p e l l e t gas-phase c o n c e n t r a t i o n , the s u r f a c e c o n c e n t r a t i o n of CO i n the p e l l e t r e l a x e s at 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 e s , depending on the d i r e c t i o n of the step changes. As shown e a r l i e r , the surface c o n c e n t r a t i o n of CO decays much more s l o w l y during CO d e s o r p t i o n than i t i n ­ creases during CO a d s o r p t i o n (compare F i g u r e s 5 and 6 ) . T h i s i s , however, merely due to the f a c t that c l o s e to a d s o r p t i o n e q u i l i b r i u m , the s u r f a c e coverage of CO can be high even at low gas-phase CO c o n c e n t r a t i o n s , as shown i n F i g u r e 4a. c

g

c

ε

2

e f f

g

e

b. R e a c t i v e experiments. In a d d i t i o n to the chemisorp­ t i o n s t u d i e s described above, we a l s o conducted t r a n s i e n t i n f r a r e d experiments under r e a c t i o n c o n d i t i o n s ( i . e . , CO + 0^ over Ρt-alumina). The r e s u l t s of these r e a c t i v e experiments, though not yet analyzed i n q u a n t i t a t i v e d e t a i l , w i l l be never­ t h e l e s s shown here ( i n the transmittance mode) because they i l l u s t r a t e some i n t e r e s t i n g f e a t u r e s of the t r a n s i e n t response of the C0/0 /Pt system. 9

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

4.

O H

A N D

Carbon

H E G E D U S

Monoxide

Chemisorption

97

F i g u r e 11 shows how 0 pretreatment of the c a t a l y s t i n f l u ­ ences the step response during CO a d s o r p t i o n . P r i o r to t h i s experiment, the c a t a l y s t was f i r s t p r e t r e a t e d i n 0.7 v o l % 0 ( i n N ) f o r 5 min and then f l u s h e d w i t h N f o r 5 min ( a l l at 723 K j . A f t e r t h i s N f l u s h , the c a t a l y s t was suddenly exposed to a step flow of 1 v o l % CO ( i n N ) . As shown i n F i g u r e 11, the growth of the 2070 cm Pt-CO band i s s i g n i f i c a n t l y delayed a f t e r 0 pretreatment (Curve Β ) , i n d i c a t i n g t h a t a t the e a r l y stages of the step change, the CO on the P t surface i s r a p i d l y consumed by the s t r o n g l y chemisorbed oxygen. The r a t e of CO removal from the Pt surface i s a l s o a f f e c t e d by the presence of 0 i n the gas phase, as demonstrated i n F i g u r e 12. In t h i s experiment the c a t a l y s t , i n i t i a l l y i n e q u i l i b r i u m w i t h 1 v o l % CO ( i n N~), was suddenly exposed to a feedstream of 0.7 v o l % 0 ( i n N ;. I t can be seen from F i g ­ ure 12 that the Pt-CO ban than i n N (Curve A f o surface r e a c t i o n between CO and oxygen i s f a s t e r than the r a t e of CO d e s o r p t i o n . F i g u r e 13 shows the time v a r i a t i o n of the s u r f a c e concen­ t r a t i o n of CO (on the transmittance s c a l e ) during feedstream composition c y c l i n g . C y c l i n g between two feedstreams, one o x i d i z i n g (1 v o l % CO, 1.03 v o l % 0 ) and the other reducing (1 v o l % CO, 0.23 v o l % 0 ) , was accomplished by f o u r f a s t - a c t i n g s o l e n o i d v a l v e s at a s w i t c h i n g frequency of 1 Hz. In the o x i ­ d i z i n g feedstream, the P t s u r f a c e was found to be e s s e n t i a l l y f r e e of CO, w h i l e a s i g n i f i c a n t l y higher CO coverage was ob­ served over Pt i n e q u i l i b r i u m w i t h the reducing feedstream. I t i s i n t e r e s t i n g to note t h a t , as F i g u r e 13 shows, the s u r f a c e c o n c e n t r a t i o n of CO assumed the same s t a t i o n a r y p a t t e r n a f t e r s e v e r a l t r a n s i t o r y c y c l e s , r e g a r d l e s s of whether the c a t a l y s t was i n i t i a l l y s t a b i l i z e d i n an o x i d i z i n g or i n a reducing feedstream. 2

2

2

2

2

2

-1

2

2

2

2

2

Concluding Remarks A fast-response i n f r a r e d s p e c t r o s c o p i c r e a c t o r system has been described which i s capable of o p e r a t i n g a t high temperatures (e.g., 450-500°C). The i n f r a r e d r e a c t o r system was s u c c e s s f u l l y used to monitor the response of the surface c o n c e n t r a t i o n of CO to step changes or o s c i l l a t i o n s i n the feedstream composition, under both r e a c t i v e and nonreactive c o n d i t i o n s . The dynamics of high-temperature CO a d s o r p t i o n and desorp­ t i o n over Pt-alumina was analyzed i n d e t a i l using a t r a n s i e n t mathematical model. The model combined the mechanism of CO a d s o r p t i o n and d e s o r p t i o n ( e s t a b l i s h e d from ultrahigh-vacuum s t u d i e s over s i n g l e - c r y s t a l or p o l y c r y s t a l l i n e Pt s u r f a c e s ) w i t h e x t r a - and i n t r a p e l l e t t r a n s p o r t r e s i s t a n c e s . The numerical v a l u e s of the parameters which c h a r a c t e r i z e the s u r f a c e pro­ cesses were taken from the l i t e r a t u r e of c l e a n s u r f a c e s t u d i e s ;

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

98

C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

CLEAN

Time Figure 11. Effect of 0 pretreatment on the rate of CO adsorption (transmittance mode) at 723 K, 257 cm /s, and 2,070 cm' . Key: A, prereduced Pt; and B, Pt preexposed 2

CLEAN [Pt-CO]

EQUILIBRIUM

Time

^

Figure 12. Effect of O on the rate of CO removal (transmittance mode) at 723 K, 257 cm Is, and 2,070 cm . Key: A, CO removal by N ; and B, CO removal by 1% 0 . z

3

1

2

2

Time Figure 13. Surface transients over Pt-alumina, starting with an oxidizing feedstream (A) or with a reducing feedstream (B) (transmittance mode) at 723 K, 257 cm /s, 2,070 cm' , and 1 Hz switching frequency. 3

1

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

4.

O H

A N D

H E G E D U S

Carbon

Monoxide

Chemisorption

99

we found good agreement between the measured and c a l c u l a t e d step responses, i n d i c a t i n g t h a t , a t l e a s t i n t h i s case, i t i s p o s s i ble to apply s u r f a c e chemistry i n f o r m a t i o n to supported c a t a l y s t s o p e r a t i n g a t high temperature and pressure ( 4 5 0 ° C , s l i g h t l y above atmospheric p r e s s u r e ) . Parametric s e n s i t i v i t y a n a l y s i s showed that f o r nonreactive systems, the a d s o r p t i o n e q u i l i b r i u m assumption can be s a f e l y i n voked f o r t r a n s i e n t C O a d s o r p t i o n and d e s o r p t i o n , and that i n t r a p e l l e t d i f f u s i o n r e s i s t a n c e s have a strong i n f l u e n c e on the time s c a l e of the t r a n s i e n t s (they tend to slow down the responses). The l a t t e r o b s e r v a t i o n has important i m p l i c a t i o n s i n the a n a l y s i s of t r a n s i e n t a d s o r p t i o n and d e s o r p t i o n over supported c a t a l y s t s ; that i s , the r e s u l t s of t r a n s i e n t chemisorption s t u d i e s should be viewed w i t h c a u t i o n , i f the e f f e c t s o f i n t r a p e l l e t d i f f u s i o n r e s i s t a n c e s are not p r o p e r l Nomenclature 2

3

a

=

l o c a l P t s u r f a c e area, cm

Pt/cm

pellet

A

=

t o t a l e x t e r n a l s u r f a c e area of the c a t a l y s t p e l l e t , cm 2

A Q C

A

G

A

T

c

=

absorbance f o r adsorbed C O

=

absorbance corresponding a complete monolayer coverage of C O

=

C O c o n c e n t r a t i o n i n the i n t r a p e l l e t gas phase, mol/cm^ o r v o l %

C

Q

=

i n i t i a l C O c o n c e n t r a t i o n i n the i n t r a p e l l e t gas phase, mol/cm

c

g

=

s u r f a c e c o n c e n t r a t i o n of C O , mol/cm

c

œ

=

C O c o n c e n t r a t i o n i n the b u l k gas phase of the r e a c t o r c e l l , mol/cm^

c

=

i n i t i a l C O c o n c e n t r a t i o n i n the r e a c t o r c e l l , mol/cm

2

c

C O c o n c e n t r a t i o n a t the i n l e t of the r e a c t o r . 5 c e l l , mol/cm

œ,xn

t D ££ e

Pt

-

f r a c t i o n a l conversion of C O

=

e f f e c t i v e d i f f u s i v i t y of C O i n the c a t a l y s t p e l l e t , cm^/s

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

100

C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

=

a c t i v a t i o n energy f o r d e s o r p t i o n , kJ/mol

=

a c t i v a t i o n energy f o r d e s o r p t i o n e x t r a p o l a t e d to zero s u r f a c e coverage of CO, kJ/mol

F

=

f l u x of CO molecules s t r i k i n g the s u r f a c e , mol/ (s-an P t )

km

=

e x t e r n a l mass t r a n s f e r c o e f f i c i e n t of CO,

L

=

h a l f - t h i c k n e s s of the c a t a l y s t p e l l e t , cm

M

=

molecular weight of CO, g/mol

=

s a t u r a t i o n CO c o n c e n t r a t i o n over the a c t i v e s i t e s mol/c

Q

=

gas v o l u m e t r i c f l o w r a t e ,

R

=

E^

N

o

g

cm/s

3

cm/s

7 2 2 gas constant, 8.3144x10 g cm / ( s -mol-K) or 8.32x10 kJ/(mol.K) 2 r a t e of CO a d s o r p t i o n , mol/(s*cm P t ) 2 r a t e of CO d e s o r p t i o n , mol/(s*cm P t ) J

R

=

R^

=

ο

r macro or = micro

i n t e g r a l - a v e r aog e rd pore r a d i i>, A

S

=

s t i c k i n g p r o b a b i l i t y f o r CO

=

i n i t i a l s t i c k i n g p r o b a b i l i t y f o r CO

Τ

=

temperature, Κ or °C

TÇQ

=

transmittance f o r adsorbed CO

t

=

time, s

V

=

volume of the r e a c t o r c e l l ,

=

p e l l e t macro- or micropore volumes, cm /g

=

d i s t a n c e from the p e l l e t c e n t e r , cm

S

Q

0

3

V

macro or micro

χ

cm 3

Greek L e t t e r s α

=

parameter d e s c r i b i n g the v a r i a t i o n of the desorp­ t i o n energy w i t h f r a c t i o n a l s u r f a c e coverage (see Equation 11), kJ/mol

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

4.

O H

A N D H E G E D U S

Carbon

Monoxide

Chemisorption

ε Ρ

=

pellet void

Θ

=

f r a c t i o n a l s u r f a c e c o v e r a g e o f CO

Θ

=

integral-averaged CO i n t h e p e l l e t

Q

=

i n i t i a l CO c o v e r a g e

ν

=

pre-exponential t i o n , s""^-

pp

=

3 p e l l e t d e n s i t y , g/cm p e l l e t

p

=

pellet soli

Q

g

fraction

f r a c t i o n a l surface coverage o f

factor

f o r the r a t e o f desorp-

Acknowledgments E. J . S h i n o u s k i s b u i l t t h e i n f r a r e d r e a c t o r c e l l a n d c o n ­ d u c t e d t h e i n f r a r e d r e a c t o r e x p e r i m e n t s . The a u t h o r s a r e i n ­ d e b t e d t o J . A. S e l l f o r t h e m o d i f i c a t i o n s o f t h e i n f r a r e d s p e c t r o p h o t o m e t e r , a n d t o P r o f e s s o r A. T. B e l l f o r s h a r i n g h i s e x p e r i e n c e w i t h i n f r a r e d r e a c t o r c o n s t r u c t i o n . The CO c o n v e r ­ s i o n d a t a o f F i g u r e 3 w e r e o b t a i n e d b y J . E. C a r p e n t e r .

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Morgan, A. E.; Somorjai, G. A. J. Chem. Phys. 1969, 51, 3309.

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Nishiyama, Y.; Wise, H. J. Catal. 1974, 32, 50.

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Collins, D. M.; Lee, J. B.; Spicer, W. E. Surface Sci. 1976, 55, 389.

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McCabe, R. W.; Schmidt, L. D. Surface Sci. 1977, 65, 189.

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U N D E R TRANSIENT

CONDITIONS

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Bain, F. T.; Jackson, S. D.; Thomson, S. J.; Webb, G.; Willocks, E. J. Chem. Soc. Faraday Trans. I 1976, 72, 2516.

11.

Foger, K.; Anderson, J. R. Appl. Surface Sci. 1979, 2, 335.

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Delgass, W. N.; Haller, G. L.; Kellerman, R.; Lunsford, J. H. "Spectroscopy in Heterogeneous Catalysis," Academic Press: New York, 1979.

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Eischens, R. P.; Francis, S. A . ; Pliskin, W. A. J. Phys. Chem. 1956, 60, 194.

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Heyne, H.; Tompkins, F. C. Proc. Roy. Soc. (London) 1966, A292, 460.

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Kobayashi, H.; Kobayashi, M. Catal. Rev.-Sci. Eng. 1974, 10, 139.

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Bennett, C. O. Catal. Rev.-Sci. Eng. 1976, 13, 121.

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Tamaru, Κ. "Dynamic Heterogeneous Catalysis," Academic Press: New York, 1978.

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Hegedus, L. L.; Gumbleton, J. J. ChemTech 1980, 10, 630.

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Hegedus, L. L.; Summers, J. C.; Schlatter, J. C.; Baron, K. J. Catal. 1979, 56, 321.

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Bell, A. T., personal communications (1980).

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Smith, J. M. "Chemical Engineering Kinetics," McGraw-Hill: New York, 1970.

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Somorjai, G. A. "Principles of Surface Chemistry," PrenticeHall: Englewood Cliffs, New Jersey, 1972.

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McCabe, R. W.; Schmidt, L. D. Surface Sci. 1977, 66, 101.

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Weinberg, W. H.; Comrie, C. M. J. Catal. 1976, 41, 489.

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Menzel, D. "Desorption Phenomena," in "Topics in Applied Physics," Vol. 4; Gomer R., Ed.; Springer-Verlag: New York, 1975.

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H E G E D U S

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Monoxide

Chemisorption

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27.

Baetzold, R. C.; Somorjai, G. A. J . Catal. 1976, 45, 94.

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Falconer, J . L.; Madix, R. J . J . Catal. 1977, 48, 262.

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Taylor, J . L.; Weinberg, W. H. Surface Sci. 1978, 78, 259.

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Bonzel, H. P.; Burton, J . J . Surface Sci. 1975, 52, 223.

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Lambert, R. M.; Comrie, C. M. Surface Sci. 1974, 46, 61.

32. 33.

Donnelly, R. G.; Modell, M.; Baddour, R. F. J . Catal. 1978, 52, 239. Madsen, Ν. K.; Sincovec, R F "PDEPACK: Partial Differen­ tial Equations Services, Livermore

34.

Hoffmann, F. Μ.,; Bradshaw, A. M. J . Catal. 1976, 44, 328.

35.

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Received July 28, 1981.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

5 Nitric Oxide Reduction by Hydrogen over Rhodium Using Transient Response Techniques BRUCE J . SAVATSKY and ALEXIS T. B E L L University of California, Department of Chemical Engineering, Berkeley, C A 94720

The catalyzed reduction of nitric oxide over rhodium is of considerable interest in view of extensive research (1-7) showing that catalytic convertors containing rhodium are particularly effective for controlling the emission of nitric oxide from auto­ mobiles. While it has hydrogen are the primary agents participating in NO reduction, little is known thus far concerning the mechanism and kinetics of the reduction process. Studies by Kobylinski and Taylor (1) with Rh/Al2O3 have shown that H2 is a more effective reducing agent than CO, as evidenced by the fact that the temperature required to achieve a given degree of conversion is lower using H2 rather than CO as the reducing agent. It was also demonstrated that even when H2 and CO are combined, H2 is still the preferred reducing agent. A more detailed investigation of NO reduction by H2 has been reported by Yao et al. (6). These authors noted that the global activation energy for NO reduction decreased from 14.7 to 8.8 kcal/ mole as the Rh loading increased from 0.34 to 12.4%. Nitrogen and N2O were observed as the major products with N2O being the dominant product under all conditions studied. It was also noted that the ratio of N2/N2O was not very much affected by either the reactant concentrations or the catalyst temperature. Although not observed directly, the authors proposed that a sur­ face complex containing two NO molecules and two H atoms could be an important reaction intermediate. More recently, Myers and Bell (8) have investigated the interaction of NO with H2 over Rh/SiO2 using temperature programmed reaction (TPR). Their results suggest that NO dissociation to form Ν and O atoms is the first step of the reaction sequence. Ammonia and H2O are believed to be formed by the stepwise hydrogenation of the atomic species and the small amount of N2 observed is formed by the recombination of Ν atoms. The present work was undertaken to investigate the applicability of transient response techniques [e.g., ref. (9-11)] for characterizing the mechanism and kinetics of NO reduction. Mass spectrometry was used to trace the dynamics of product formation and in situ infrared spectroscopy was used to 0097-6156/82/0178-0105$09.25/0 © 1982 American Chemical Society In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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observe the s t a t e of adsorbed Ν0· Two types of experiment were performed. The f i r s t i n v o l v e d the r e d u c t i o n of preadsorbed NO i n a constant flow of H2. The second type of experiment i n v o l v e d the s u b s t i t u t i o n of ^NO f o r ^NO, e i t h e r during s t e a d y - s t a t e r e d u c t i o n or j u s t p r i o r to the a d d i t i o n of H2 to a f l o w i n g stream of NO. I t w i l l be shown that the data obtained from these experiments can be i n t e r p r e t e d i n terms of a mechanism of NO deduced from evidence presented i n the recent l i t e r a t u r e . Experimental C a t a l y s t . A 4.4% Rh/Si02 c a t a l y s t was used f o r a l l of the work presented here. The c a t a l y s t was prepared by impregnation of Davison 70 s i l i c a g e l w i t h an aqueous s o l u t i o n of RhCl3. The f r e s h l y prepared m a t e r i a l was d r i e d and then reduced i n f l o w i n g H2 at 673K f o r 2 h r . Th mined to be 25% by H2 chemisorption Apparatus. F i g u r e 1 shows a schematic of the r e a c t o r . The r e a c t o r body was made of s t a i n l e s s s t e e l f l a n g e s . To e l i m i n a t e the c a t a l y t i c a c t i v i t y of the r e a c t o r , the i n t e r i o r w a l l s were coated w i t h aluminum and then o x i d i z e d to produce an alumina s u r f a c e . Calcium f l u o r i d e windows were mounted i n each h a l f of the body so that an i n f r a r e d beam can be passed through the r e a c t o r . These windows were sealed to the s t a i n l e s s s t e e l flanges by compression between two G r a p h o i l gaskets. The s e a l between the two halves of the r e a c t o r was made through a copper gasket, gold p l a t e d to e l i m i n a t e i t s c a t a l y t i c a c t i v i t y . The c a t a l y s t , a 15 mm diameter d i s k , weighing 51 mg, was held i n place between two aluminum r i n g s . The assembled r e a c t o r i s compact and has a dead volume of only 1.6 cm^. The r e a c t o r was heated by two 200W d i s k heaters (Thermal C i r c u i t s , I n c . ) , placed over the c i r c u l a r f a c e s , and the c a t a l y s t temperature was measured w i t h a 1/16" s t a i n l e s s - s t e e l sheathed thermocouple, fed through a port (not shown) i n the r e a c t o r body. The r e a c t o r was connected to the balance of the apparatus as shown i n F i g . 2. One of two premixed gas streams could be fed to the r e a c t o r . By s w i t c h i n g the i n d i c a t e d v a l v e , a step change i n e i t h e r r e a c t a n t or isotope c o n c e n t r a t i o n could be achieved. The e f f l u e n t from the r e a c t o r was sampled through a d i f f e r e n t i a l l y pumped i n l e t system i n t o an EAI 250B quadrupole mass spectrometer. The s i g n a l from the mass spectrometer was then fed to a micro­ processor-based data a c q u i s i t i o n system. This u n i t was designed to monitor as many as ten p r e s e l e c t e d mass peaks, as w e l l as the c a t a l y s t temperature and the gas f l o w r a t e . The time r e q u i r e d f o r the c o l l e c t i o n of one data set depended on the number of mass peaks tracked and the d w e l l time on each peak. The s h o r t e s t c y c l e time used i n the present work was 0.5 s, d u r i n g which the i n t e n s i t i e s of four mass peaks were read, together w i t h the c a t a l y s t tempera­ ture and the gas f l o w r a t e .

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

S A V A T S K Y

A N D

B E L L

Figure 1.

Nitric Oxide

Reduction

Schematic of the reactor — IR cell.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Mini

Figure 2.

Vent

Quadrupole M.S.

S. Inlet

System

Data Acquisition

Vacuum System

Schematic of the experimental apparatus.

Computer

Fourier-Transform Infrared Spectrometer

ν Reactor

Vent

Gas Feed System

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Transmission i n f r a r e d s p e c t r a of species adsorbed on the c a t a l y s t were taken w i t h a D i g i l a b FTS-10M F o u r i e r - t r a n s f o r m i n f r a r e d spectrometer, u s i n g a r e s o l u t i o n of 4 cm~l. To improve the s i g n a l - t o - n o i s e r a t i o , between 10 and 100 i n t e r f e r o g r a m s were co-added. Spectra of the c a t a l y s t taken f o l l o w i n g r e d u c t i o n i n H2 were s u b t r a c t e d from s p e c t r a taken i n the presence of NO to e l i m i n a t e the spectrum of the support. Because of the very short o p t i c a l path through the gas i n the r e a c t o r and the low NO p a r t i a l pressures used i n these s t u d i e s , the spectrum of gasphase NO was extremely weak and d i d not i n t e r f e r e w i t h the o b s e r v a t i o n of the spectrum of adsorbed s p e c i e s . Results Reaction of Preadsorbe out u s i n g the f o l l o w i n g procedure. P r i o r to each experiment, the c a t a l y s t was reduced i n H2 f o r 5 min a t 423 K. The c a t a l y s t was then exposed t o a 0.28% NO/Ar mixture f o r 5 to 60 s. I t was noted that NO a d s o r p t i o n was accompanied by a p a r t i a l r e d u c t i o n of the adsorbing NO w i t h adsorbed H2 l e f t on the c a t a l y s t s u r f a c e from the p e r i o d of r e d u c t i o n . I n f r a r e d s p e c t r a taken a t the end of the NO a d s o r p t i o n p e r i o d showed an i n t e n s e band at 1660-1680 cm""l, the p o s i t i o n of the band s h i f t i n g upscale w i t h the d u r a t i o n of adsorpt i o n , and a much weaker band a t 1830 cm*~l. Based on previous s t u d i e s of NO a d s o r p t i o n on supported Rh c a t a l y s t s (12-14) and analogy w i t h the s p e c t r a of Rh n i t r o s y l s (15-17), the band a t 1660-1680 cm"^ can be assigned to a N0 ^~ s t r u c t u r e and the band at 1830 cm"^ t o a N0 s t r u c t u r e . The r e d u c t i o n of adsorbed NO was i n i t i a t e d by s u b s t i t u t i n g a 10% H^/Ar mixture f o r the NO/ Ar mixture. While the formation of products began immediately, no change i n the c a t a l y s t temperature was observed during the course of the r e a c t i o n . Figure 3 i l l u s t r a t e s a t y p i c a l s e r i e s of product responses. N i t r o g e n and N2O were formed immediately upon contact of the c a t a l y s t w i t h H2 and the maximum i n the s i g n a l s f o r these products was observed i n about 1.2 s. The hydrogen-containing products, NH3 and H2O, a l s o appeared as sharp peaks, but the maximum i n the s i g n a l s f o r these products occurred a t 2.5 s. Changes i n the experimental c o n d i t i o n s d i d not a l t e r the q u a l i t a t i v e f e a t u r e s of the responses. I n a l l cases the peaks f o r N2 and N2O occurred simultaneously and preceded the peaks f o r NH3 and H2O. I t was f u r t h e r observed that the r e a c t i o n temperature, d u r a t i o n of NO exposure, and the H2 p a r t i a l pressure during NO r e d u c t i o n a f f e c t e d the i n t e n s i t i e s of the product peaks and the time delay between the N2/N2O peaks and the NH3/H2O peaks. To a much l e s s e r degree, the r e a c t i o n c o n d i t i o n s a l s o a f f e c t e d the time a t which the N2 and N2O s i g n a l s reached a maximum. a

a

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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Figure 3. The transient responses for N , N 0, H O and NH , during the reduc­ tion of preadsorbed NO. Before reaction, NO was adsorbed for 15 s: P = 1.0 X 10' atm, P = 2.8 χ 10 atm, and Τ = 423 Κ. 2

2

2

f

S

H2

1

N0

3

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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F i g u r e 4 shows t h e e f f e c t s o f NO e x p o s u r e t i m e on t h e a b s o r b a n c e o f t h e band a t 1660-1680 c m ~ l , o b s e r v e d p r i o r t o r e a c ­ t i o n , and t h e maximum i n t e n s i t i e s o f t h e NH3, N 2 , N2O, a n d H2O s i g n a l s , observed during r e a c t i o n . The a b s o r b a n c e o f t h e i n f r a r e d band p r o v i d e s a measure o f t h e q u a n t i t y o f NO a d s o r b e d on t h e Rh s u r f a c e . As may be s e e n , t h e c o v e r a g e by a d s o r b e d NO i n c r e a s e s r a p i d l y f o r NO e x p o s u r e s up t o 15 s and t h e n l e v e l s o f f a t a p l a t e a u . The p r o d u c t maxima show a s i m i l a r t r e n d , b u t f o r NO e x p o s u r e s g r e a t e r t h a n 20 s, t h e s i g n a l s f o r H2O, NH3, and N2 show a s l i g h t d e c l i n e . A s s u m i n g t h a t t h e maximum s i g n a l i n t e n s i t y f o r e a c h p r o d u c t i s p r o p o r t i o n a l t o t h e t o t a l amount o f p r o d u c t f o r m e d , t h e r e s u l t s p r e s e n t e d i n F i g . 4 i n d i c a t e t h a t f o r NO e x p o s u r e t i m e s g r e a t e r t h a n 20 s t h e amount o f a d s o r b e d NO u n d e r ­ g o i n g r e d u c t i o n d e c r e a s e s s l i g h t l y w i t h i n c r e a s i n g NO c o v e r a g e . Such a t r e n d s u g g e s t s t h a t h e a d s o r b e d NO d e s o r b The t i m e o f e x p o s u r e t o NO h a d no e f f e c t on t h e t i m e r e q u i r e d f o r t h e N2 and N2O s i g n a l s t o r e a c h a maximum d u r i n g NO r e d u c t i o n . However, t h e NO e x p o s u r e t i m e d i d a f f e c t t h e t i m e a t w h i c h t h e NH3 and H2O s i g n a l s a t t a i n e d a maximum. As shown i n F i g . 5, t h e d e l a y b e t w e e n t h e maximum f o r t h e N2 peak a n d t h e maximum f o r t h e NH3 peak was t h e same a s t h e d e l a y b e t w e e n t h e N2 peak and t h e H2O peak. I n b o t h c a s e s , as t h e exposure time i n c r e a s e s , t h e d e l a y i n c r e a s e s f r o m 0 t o 2.5 s and t h e n l e v e l s o f f f o r NO e x p o s u r e s o f more t h a n 20 s. R e s u l t s s i m i l a r t o t h o s e shown i n F i g s . 4 and 5 were a l s o o b t a i n e d a t o t h e r temperatures. I n c r e a s i n g t h e temperature from 398 t o 473 Κ h a d two p r i n c i p a l e f f e c t s . The f i r s t was t o p r o d u c e n a r r o w e r b u t more i n t e n s e p r o d u c t p e a k s . The s e c o n d e f f e c t was t o r e d u c e t h e d e l a y between t h e N2 peak and e i t h e r t h e NH3 o r H2O p e a k . Thus, f o r e x a m p l e , f o r NO e x p o s u r e t i m e s o f g r e a t e r t h a n 20 s, t h e d e l a y d e c r e a s e d f r o m 7.5 s a t 398 Κ t o 0.5 s a t 473 K. The e f f e c t s o f t h e H2 p a r t i a l p r e s s u r e , u s e d t o r e d u c e t h e a d s o r b e d NO, on t h e maximum p r o d u c t s i g n a l i n t e n s i t i e s and t h e t i m e s a t w h i c h t h e p r o d u c t p e a k s a p p e a r e d was e x p l o r e d f o r a c o n s t a n t NO e x p o s u r e t i m e o f 35 s and a t e m p e r a t u r e o f 423 K. The r e s u l t s i n F i g . 6 show t h a t t h e maximum i n t e n s i t y o f e a c h p r o d u c t s i g n a l i n c r e a s e s s l i g h t l y a s t h e p a r t i a l p r e s s u r e o f H2 i s i n c r e a s e d . B o t h t h e t i m e a t w h i c h N2 and N2O s i g n a l s r e a c h a maximum and t h e d e l a y o f t h e NH3 and H2O s i g n a l s a r e a f f e c t e d b y t h e H2 p a r t i a l p r e s s u r e . F i g u r e 7 show t h a t t h e t i m e f o r maximum p r o d u c t i o n o f N2 and N2O d e c r e a s e s f r o m 3 t o 1.5 s a s t h e H2 p a r t i a l p r e s s u r e i n c r e a s e s f r o m 0.01 t o 0.1 atm a n d F i g . 8 shows t h a t t h e t i m e d e l a y f o r t h e maximum p r o d u c t i o n o f NH3 and H2O d e c r e a s e s f r o m 5.5 t o 3.0 s. I s o t o p i c T r a c i n g w i t h ^NO. To t r a c e o u t t h e d y n a m i c s o f n i t r o g e n i n c o r p o r a t i o n i n t o t h e p r o d u c t s u n d e r c o n d i t i o n s where t h e s u r f a c e c o v e r a g e by a d s o r b e d NO r e m a i n s c o n s t a n t , e x p e r i m e n t s

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CD

ο

10

15 20 25 30 NO Exposure Time (s)

35

Figure 4. The maximum intensity of the reaction products and the absorbance of the IR band at 1680 cm' , as a function of NO exposure time during the reduction of preadsorbed N0 : P = 1.0 χ 10' atm, P = 2.8 X 10~ atm, and Τ = 423 Κ. 1

2

1

H

3

N0

σ CVJ

X

ο ΙΟ χ

ο CVJ

χ σ _Ε Ο CNJ Χ

Figure 5. The effect of NO exposure time on the time delay in the maximum production of H 0 and NH relative to N during the reduction of preadsorbed NO: P =ζ10χ 10 atm, P = 2.8 χ 10' atm, and Τ = 423 Κ. 2

Hz

3

1

2t

N0

3

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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2

N0

Figure 7. The effect of H partial pressure on the time delay in the maximum production of H 0 and NH relative to N , during the reduction of preadsorbed NO: P = 2.8 χ 10~ atm, Τ = 423 Κ, and t = 30 s.

0.01

0.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

2

2

N0

3

N0

2

Figure 8. The effect of H partial pressure on the time for the maximum production of N and N 0, during the reduction of preadsorbed NO: P = 2.8 χ 10' atm, Τ = 423 Κ, and t ~ 30 s.

LU

H—^ H-*

S" s

R

s:

X

S* Ο

r r

W W

Ο

oo

H

> < >

116

C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

were performed i n which N 0 was s u b s t i t u t e d f o r N 0 . Because of problems i n r e s o l v i n g products w i t h very n e a r l y i d e n t i c a l molecu­ l a r weights, only the responses f o r the isotopes of N2 were followed. 1 5

1 4

Two experiments were performed. I n the f i r s t , the c a t a l y s t was i n i t i a l l y exposed t o a ^NO/Ar mixture f o r 60 s a f t e r which t h i s mixture was replaced by a ^NO/I^/Ar m i x t u r e . The responses for N2 and Ν Ν were recorded as f u n c t i o n s of time, but the responses f o r ^2 could not be r e s o l v e d from that f o r N 0 , s i n c e the two s p e c i e s have n e a r l y the same value of m/e. To e s t a b l i s h the N2 response, the experiment was performed a second time but now r e v e r s i n g the order i n which ^ N 0 and ^ N 0 were i n t r o d u c e d . The response f o r the N2 s i g n a l i n t h i s case i s i d e n t i c a l to that f o 9 i l l u s t r a t e s the p a r t i a N observed f o l l o w i n g i n t r o d u c t i o n of the I ^ - c o n t a i n i n g stream. The N2 response begins immediately but then passes through a maximum as the adsorbed ^ N 0 i s consumed. The l ^ N ^ N response e x h i b i t s an i n d u c t i o n p e r i o d of about 2.5 s, a f t e r which i t a l s o r i s e s r a p i d l y and then passes through a maximum. The N2 response begins a f t e r a 2 s delay and r i s e s m o n o t o n i c a l l y t o a maximum v a l u e i n about 9 s. The second experiment was performed t o determine the responses of N , N N , and ^2 N0/H /Ar mixture was r e p l a c e d by a ^NO/I^/Ar m i x t u r e . This experiment was i n i t i a t e d by exposing the c a t a l y s t to a ^ N 0 / A r mixture f o r 60 s and then r e p l a c i n g t h i s mixture by a ^NO/I^/Ar m i x t u r e . A f t e r 2 min of r e a c t i o n , the NO/H /Ar mixture was r e p l a c e d by a NO/H /Ar m i x t u r e , and the responses f o r N2 and N^^N were f o l l o w e d as f u n c t i o n s of time. For the same reason d i s c u s s e d e a r l i e r , the response of N could not be r e s o l v e d from that f o r N 0 . To de­ termine the ^2 response, the experiment j u s t d e s c r i b e d was repeated, but the sequence i n which ^NO and ^NO were i n t r o d u c e d was reversed. The N2 response obtained i n t h i s case was used to represent the response f o r the f i r s t case. F i g u r e 10 i l l u s t r a t e s the responses f o r N^N, and following the s u b s t i t u t i o n of N 0 f o r N 0 . The N s i g n a l i s un­ a f f e c t e d d u r i n g the f i r s t second a f t e r the i s o t o p i c s u b s t i t u t i o n but then d e c l i n e s m o n o t o n i c a l l y to z e r o . I t i s s i g n i f i c a n t t o observe that the shape of the N2 response f o r t > 2 s and the shapes of the N^N and N responses are v i r t u a l l y i d e n t i c a l to those observed i n F i g . 9 . 1 4

2

1 4

1 5

a

f

t

e

r

a

2

2

14

15

2

2

1 5

1 4

2

1 5

1 4

1 4

2

2

Discussion Two g e n e r a l f e a t u r e s of the r e a c t i o n mechanism can be deduced from the o b s e r v a t i o n s made d u r i n g the r e d u c t i o n of adsorbed NO by H 2 . F i r s t , the c l o s e c o i n c i d e n c e of the N2

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

S A V A T S K Y

A N D

B E L L

Nitric Oxide

Reduction

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

118

C A T A L Y S I S

U N D E R

T R A N S I E N T

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

C O N D I T I O N S

5.

S A V A T S K Y

A N D

B E L L

Nitric

Oxide

Reduction

119

and N 0 peaks suggests that these products share a common r a t e l i m i t i n g s t e p , such as, f o r example, the d i s s o c i a t i o n of adsorbed NO. Second, the delay i n the appearance of the NH3 and H2O peaks r e l a t i v e to the N and N 0 peaks suggests that formation of the former p a i r of products i n v o l v e s adsorbed r a t h e r than gaseous hydrogen. This deduction i s f u r t h e r supported by the manner i n which changes i n NO exposure time, H p a r t i a l pressure, and r e a c t i o n temperature a f f e c t the magnitude of the delay. F i g u r e 11 i l l u s t r a t e s a p o s s i b l e mechanism f o r NO r e d u c t i o n , which i s c o n s i s t e n t w i t h the q u a l i t a t i v e f e a t u r e s of the present r e s u l t s and i n f o r m a t i o n a v a i l a b l e i n the l i t e r a t u r e . A d i s c u s s i o n of the j u s t i f i c a t i o n f o r i n c l u d i n g s p e c i f i c elementary steps w i l l be presented next. The a s s o c i a t i v e a d s o r p t i o n of NO i s supported by s e v e r a l i n f r a r e d s t u d i e s (12-14) show an i n t e n s e band a cm . These f e a t u r e s are a s s o c i a t e d w i t h NO adsorbed as N0° and N0 , r e s p e c t i v e l y . I f the s u r f a c e i s p a r t i a l l y o x i d i z e d , a t h i r d band i s observed at 1910 cm" , a s s o c i a t e d w i t h N0^ . During steady-state r e d u c t i o n of NO by H , only the N0_""form of adsorbed NO i s observed (10). The r e v e r s i b i l i t y of NO a d s o r b t i o n i n t o t h i s s t a t e has been examined by Savatsky and B e l l (14)· Their work shows that at temperatures above 293 K, e q u i l i b r i u m desorpt i o n occurs when a stream of argon i s passed over a R h / S i 0 c a t a l y s t b e a r i n g preadsorbed NO. Evidence f o r d i s s o c i a t i v e chemisorption comes from s e v e r a l sources (8,19-21). TPD s t u d i e s conducted w i t h Rh s i n g l e c r y s t a l s (19-21) suggest that a p o r t i o n of the NO adsorbed at ambient temperatures occurs d i s s o c i a t i v e l y . F u r t h e r d i s s o c i a t i o n i s presumed to occur at e l e v a t e d temperatures s i n c e N and N 0 are observed during TPD at temperatures s l i g h t l y above the t h r e s h o l d f o r NO d e s o r p t i o n . A s i m i l a r behavior has a l s o been observed i n TPD s t u d i e s conducted w i t h R h / S i 0 ( 8 ) . While the exact mechanism of d i s s o c i a t i o n i s not e s t a b l i s h e d by these i n v e s t i g a t i o n s , i t seems p l a u s i b l e to propose that d i s s o c i a t i o n proceeds as i n d i c a t e d by r e a c t i o n 2 i n F i g . 11. Studies by a number of authors (8*22-27) have shown that H2 adsorbs d i s s o c i a t i v e l y on Rh and that t h i s process i s r e v e r s i b l e at the temperatures used i n the present s t u d i e s . As noted e a r l i e r , the atomic hydrogen formed by t h i s means i s b e l i e v e d to be r e s p o n s i b l e f o r the formation of NH3 and H 0. Consequently, these products are assumed to be formed by a sequence of LangmuirHinshelwood steps. While there i s no independent evidence to support t h i s hypothesis f o r the s y n t h e s i s of NH3, recent r e s u l t s reported by T h i e l et a l . (27) i n d i c a t e that the formation of H 0 from H and adsorbed 0-atoms does proceed v i a a two step sequence such as that represented by r e a c t i o n s 6 and 7 i n F i g . 11. 2

2

2

2

a

1

+

2

2

2

2

2

2

2

2

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

120

C A T A L Y S I S

(1)

U N D E R

NO + S

^-

N0

rr-

^ 2H

T R A N S I E N T

C O N D I T I O N S

Q

(2) (3)

H

2

+ 2S

(4)

N0

+ N

a

(5)

2N

(6) (7) (8)

0a

0H

N

(9)

N H

(10)

NH

Figure 11.

+ H

a

a 2

+

a

H

— •

—— a

a

+H

2

a

a

+ H

a

N0



—— • · N

Q

H

+

Q

n a



——



+ 2S + 2S

2

0H

a

+ S

H 0 + 2S 2

NH

-— ——

a

Q

+ S

NHo

+ S

NH

+ 2S

3

The reaction mechanism proposed for the interpretation of transient response experiments.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

5.

S A V A T S K Y

A N D

Nitric

B E L L

Oxide

Reduction

111

The s p e c i f i c a t i o n of r e a c t i o n s 4 and 5 f o r the formation of N 0 and N i s based on the TPD s t u d i e s reported by Castner et a l . (19), Campbell and White (20), B a i r d et a l . (21) and Myers and B e l l ( 8 ) . The work of Myers and B e l l has shown s p e c i f i c a l l y that the formation of N 0 does not occur v i a a R i d e a l - E l e y process. I t appears more l i k e l y that t h i s product i s formed v i a a LangmuirHinshelwood process. To determine whether the mechanism i n F i g . 11 could e x p l a i n the experimental o b s e r v a t i o n reported here, a model of the r e d u c t i o n process was developed and examined. The k i n e t i c s of NO adsorption and r e d u c t i o n of the adsorbed NO by H can be described by eqns. 1 and 2. 2

2

2

2

dP dt

,

_Q_ V

=

£

(Pj-P*) +

p RTM c

(1-e) I v ^ r . ε j

m

r

dt

(1)

j

D e f i n i t i o n s f o r the v a r i a b l e s and constants appearing i n eqns. 1 and 2 are given i n the nomenclature s e c t i o n at the end of t h i s paper. The f i r s t of these equations represents a mass balance around the r e a c t o r , assuming that i t operates i n a d i f f e r e n t i a l manner. The second equation i s a species balance w r i t t e n f o r the c a t a l y s t s u r f a c e . The r a t e of elementary r e a c t i o n j i s rep­ resented by r j , and v ^ j i s the s t o i c h i o m e t r i c c o e f f i c i e n t f o r com­ ponent i i n r e a c t i o n j . The r e l a t i o n s h i p of r j to the reactant p a r t i a l pressures and surface species coverages are given by expressions of the form - kjP*^

(3)

and r

j

=

k

4

jWn

f o r R i d e a l - E l e y and Langmuir-Hinshelwood steps, r e s p e c t i v e l y . Equations 1 and 2 can be solved n u m e r i c a l l y using an a l g o r i t h m which handles s t i f f d i f f e r e n t i a l equations (28). Two sets of boundary c o n d i t i o n s are r e q u i r e d . For 0 t^o> corresponding value f P£ d P^ 2

(7)

Table I l i s t s the values of the r a t e c o e f f i c i e n t s used to simulate the t r a n s i e n t response experiments shown i n F i g s . 3 through 8. These values were obtained i n the f o l l o w i n g manner (29). S t a r t i n g from a set of i n i t i a l guesses, the values of k were v a r i e d s y s t e m a t i c a l l y t o o b t a i n a f i t between the p r e d i c t e d product responses and those obtained from experiments i n which H was added suddenly t o a flow of NO. These experiments w h i l e not described here were i d e n t i c a l t o that presented i n F i g . 9, w i t h the exception that only 1%0 was used. Because of the l a r g e number of parameters i n the model, only a rough agreement could be achieved between experiment and theory even a f t e r 500 i t e r a ­ t i o n s of the o p t i m i z a t i o n r o u t i n e (30). The parameter values obtained a t t h i s p o i n t were now used to c a l c u l a t e the responses expected during the r e d u c t i o n of adsorbed NO. These computations produced responses s i m i l a r to those observed e x p e r i m e n t a l l y ( i . e . , F i g . 3) but the appearance of the product peaks i n time d i d not c o i n c i d e w i t h those observed. To c o r r e c t f o r t h i s , the values of k^, ky, and kg were adjusted i n an e m p i r i c a l manner. The c r i t e r i a used at t h i s p o i n t were that the p r e d i c t e d depen­ dences of the delays between the appearance of the NH3 and H 0 peaks and the N and N 0 peaks on the d u r a t i o n of NO a d s o r p t i o n and the H p a r t i a l pressure during r e d u c t i o n agree as c l o s e l y as p o s s i b l e w i t h the dependences found e x p e r i m e n t a l l y and shown i n F i g s . 5 and 8. The r a t e parameters presented i n Table I were used together w i t h the parameter values l i s t e d i n Table I I t o p r e d i c t the product responses during the a d s o r p t i o n of NO on a hydrogen covered Rh surface and the subsequent r e d u c t i o n of the adsorbed 2

2

2

2

2

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

5.

S A V A T S K Y

A N D

B E L L

Nitric Oxide

Reduction

123

Table I . Rate C o e f f i c i e n t s Used to Simulate the T r a n s i e n t Response Experiments

Step //

1. -1.

k

4

1

2.8 χ 1 0 " s " atm" 1.6 s-1 1

1



9.3 χ 1 0 "

s""

3.

2.8 χ 1 0 s " l atm" 2

-3.

1.9 x 1 0

4.

3.7 x Ι Ο

5.

9.9 x 10-1 - l

6.

6.8 s-1

7.

1.1 χ 1 0 l s-1

8.

6.0 s-1

9.

1.4 χ 1 0 s " l

10.

1.8 χ 1 0 s-1

_ 1

- 1

s"

1

1

1

s"" s

3

2

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

124

C A T A L Y S I S

Table I I

V

T R A N S I E N T

C O N D I T I O N S

Parameters and V a r i a b l e s Used to Simulate the T r a n s i e n t Pvesponse Experiments

Q =

9.55 cm

ε

=

0.96

r =

1.6

cm

3

(STP)/s

J

Pc "

0.8 gm/cm^

\

10"

=

U N D E R

5

mol Rh sites/gm c a t a l y s t

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

5.

S A V A T S K Y

A N D

B E L L

Nitric

Oxide

125

Reduction

NO. The same parameters were then used to p r e d i c t the responses for the i s o t o p i c t r a c e r experiments reported i n F i g s . 9 and 10. The r e s u l t s of these c a l c u l a t i o n s are presented below. Figure 12 i s r e p r e s e n t a t i v e of the responses p r e d i c t e d f o r the product p a r t i a l pressures both during the p e r i o d of NO a d s o r p t i o n and the subsequent r e d u c t i o n of adsorbed NO. P r i o r to the a d d i t i o n of NO, the surface i s considered to be at e q u i l i ­ brium w i t h the p a r t i a l pressure of H used to reduce the c a t a l y s t . At time zero the H flow i s terminated and NO i s added to the Ar c a r r i e r . Reaction products appear immediately due to the r e d u c t i o n of NO by adsorbed Η-atoms. A f t e r 30 s of NO exposure, the flow of NO i s terminated and the flow of H i s r e s t o r e d . The i n t r o d u c t i o n of H causes a sudden i n c r e a s e i n the product p a r t i a l pressures, which then pass through a maximum as the adsorbed NO i s consumed. I t i s note o b s e r v a t i o n ( i . e . , see F i g s that NH3 i s the dominant n i t r o g e n - c o n t a i n i n g product and that N and N2O are formed i n s u c c e s s i v e l y lower c o n c e n t r a t i o n s . The curves i n F i g . 12 show f u r t h e r that the production of N 0 and N a t t a i n maxima w i t h i n about 1 to 1.5 s a f t e r the i n t r o d u c t i o n of H but that the production of NH3 and H 0 occurs about 2 s l a t e r . Both of these f e a t u r e s are i n good agreement w i t h the behavior of the experimental data.. _ The Responses f o r θ^, θ^ο> ν shown i n F i g . 12. The value of 6g, which i s i n i t i a l l y 0.925, decreased during the p e r i o d of NO exposure at the same time that the value of 9^Q increases r a p i d l y from zero. I t should be noted that the p r e d i c t e d r a t e of accumulation of adsorbed NO i s q u a l i t a t i v e l y c o n s i s t e n t w i t h the dynamics of the band appearing at 1680 cm""l, a s s o c i a t e d w i t h No *~ shown i n F i g . 4. I t i s seen, though, that w h i l e the experimental r e s u l t s e x h i b i t a short i n d u c t i o n p e r i o d f o l l o w e d by a r a p i d r i s e i n the absorbance of the 1680 cm"" 1 band to a s a t u r a t i o n l e v e l , the p r e d i c t e d curve shows a smooth monotonie i n c r e a s e i n θ^ο· The vacancy coverage, θ , which i s i n i t i a l l y equal to 0.075, r a p i d l y decreases during the i n i t i a l p e r i o d of NO exposure but then very s l o w l y i n c r e a s e s . This behavior__can be a t t r i b u t e d to the f o l l o w i n g f a c t o r s . The f i r s t i s that θ i n e q u i l i b r i u m w i t h 0.10 atm of H i s l a r g e r than θ i n e q u i l i b r i u m w i t h 0.0028 atm of NO. C a l c u l a t i n g the e q u i l i b r i u m constants f o r H and NO ad­ s o r p t i o n and desorption of these gases, given i n Table I , one 2

2

2

2

2

2

2

2

2

a n d

θ

a r e

a

ν

ν

2

ν

2

2

Ν0

concludes that θ"^ = 0.075 and θ" = 0.2. It i s this difference i n the s t r e n g t h of a d s o r p t i o n which d r i v e s the decrease i n _θ . Figure 13 shows, though, that during the a d s o r p t i o n of NO, θ f a l l s below 0.02 and then g r a d u a l l y climbs back up. This i n ­ crease i s due to formation of a v a r i e t y of surface intermediates formed during the i n i t i a l moments of NO a d s o r p t i o n , which are not r e l e a s e d i n s t a n t a n e o u s l y as r e a c t i o n products. The surface ν

ν

ν

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

oo

1

Ο H

Π Ο

w

H

&o

w

H

σ

d

C/5

κ! GO

>

> Η

G\

Ο

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

to

ι—^

--Ι

as ο ο*

ft

χ

Β*

ο

w

r r

Ο

w

>

GO

> < >

GO

H

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

128

C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

coverage by these species reduces the vacancy f r a c t i o n . As time progresses the i n t e r m e d i a t e s a r e consumed, and the surface g r a d u a l l y exposes a g r e a t e r number of vacant s i t e s . Upon i n t r o d u c t i o n of H 2 , f o l l o w i n g NO a d s o r p t i o n , the magnin i t u d e of immediately i n c r e a s e s w h i l e that of θ^ο decreases. The f i r s t of these changes r e f l e c t s the d i f f e r e n c e between the r a t e s of H2 a d s o r p t i o n and consumption i n the r e d u c t i o n of NO. The decrease i n θ^ο occurs f o r two reasons: a) i n h i b i t i o n of the r e a d s o r p t i o n of desorbing NO as a consequence of H2 a d s o r p t i o n ; and b) the consumption of adsorbed NO by r e d u c t i o n . I t should be noted that the d i s s o c i a t i o n of NO, and hence the r a t e of NO r e d u c t i o n i s a c c e l e r a t e d by the c r e a t i o n of vacant s i t e s . The i n c r e a s e i n θ , seen i n F i g . 13, can be a s c r i b e d t o a consumption ν

of chemisorbed NO and th as was d i s c u s s e d e a r l i e r Figure 14 shows the computed responses f o r θ^, θο> and 9QH» These coverages i n c r e a s e s l o w l y d u r i n g NO a d s o r p t i o n but r i s e r a p i d l y and pass through a__maximum when H2 i s added t o the f l o w . The responses f o r Θ^Η Θ^Η s i m i l a r i n shape t o the response f o r 9 but are s i g n i f i c a n t l y s m a l l e r i n magnitue and, hence, have not been shown in__Fig. 14. I t i s s i g n i f i c a n t to note that the maxima i n 0Q and 6fj occur a t times which c o i n c i d e w i t h the time a t which the r a t e s of N * 2 ° ^ach a maximum. S i m i l a r l y , the maxima i n 9QH d Θ^Η P p e a r a t times n e a r l y i d e n t i c a l t o the times a t which the r a t e s o f 1^0 and NH3 formation reach a maximum. C a l c u l a t i o n s s i m i l a r t o those j u s t d i s c u s s e d were a l s o c a r r i e d out f o r NO exposure times between 5 and 30 s. F i g u r e 15 i l l u s t r a t e s the e f f e c t s of NO exposure time on the p r e d i c t e d maximum i n t e n s i t y of each product peak. The curves appearing i n t h i s f i g u r e may be compared w i t h the experimental r e s u l t s shown i n F i g . 4. I t i s noted that w h i l e the d i s t r i b u t i o n of products p r e d i c t e d f o r each NO exposure time i s q u a l i t a t i v e l y c o n s i s t e n t w i t h that observed e x p e r i m e n t a l l y , the shape of the product peak i n t e n s i t y curves i s not. The experimental data show a r a p i d i n i t i a l i n c r e a s e which i s f o l l o w e d by the attainment of a broad maximum. By c o n t r a s t , the p r e d i c t e d curves show a slow monotonie increase. The e f f e c t of NO exposure time on the time a t which the N2 and N2O s i g n a l s a t t a i n a maximum i s shown i n F i g . 16. I t i s seen that the model of NO r e d u c t i o n p r e d i c t s that N2 formation peaks about 0.5 s a f t e r the peak i n the N2O formation and that the peak times f o r both products d e c l i n e by about 0.5 s as the NO exposure time i s i n c r e a s e d from 5 to 30 s. These trends are i n good agreement w i t h the data. I t should be noted that s i n c e a product a n a l y s i s c o u l d be taken only once every 0.5 s, i t was not p o s s i b l e to determine product peak p o s i t i o n s w i t h an accuracy of b e t t e r than 0.5 s. Consequently, both the p r e d i c t e d d i f f e r e n c e between A N D

a r e

Q H

anc

N

r

2

a n

a

2

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Η*

to

a

«·* δ*

Ci

Ο

w

r r

Ο

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In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

132

C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

tfj and t N o d the change i n these times w i t h the d u r a t i o n of NO a d s o r p t i o n l i e w i t h i n the l i m i t s of u n c e r t a i n t y f o r the experimental data. The p r e d i c t e d e f f e c t s of NO a d s o r p t i o n time on the time delay between the maxima i n NH3 and N production and the maxima i n H 0 and N production are shown i n F i g . 17. I t i s seen t h a t the p r e d i c t i o n s bound the experimental observations and show the the proper trend w i t h i n c r e a s i n g NO exposure. C a l c u l a t i o n s were a l s o performed to assess the a b i l i t y of the model to p r o p e r l y p r e d i c t the e f f e c t s of H p a r t i a l pressure on the product peak i n t e n s i t i e s and the time at which each peak occurs. Figure 18 shows the p r e d i c t e d e f f e c t of H partial pressure on the product peak i n t e n s i t i e s . In c o n t r a s t to the a n

9

2

2

2

2

2

2

trend observed e x p e r i m e n t a l l model p r e d i c t s that i n t e n s i t i e pressure. N e v e r t h e l e s s , the model does show that the changes are s m a l l , and i t provides a q u a l i t a t i v e l y c o r r e c t p i c t u r e of the product d i s t r i b u t i o n . F i g u r e 19 shows the p r e d i c t e d and e x p e r i m e n t a l l y observed time at which the formation of N reaches a maximum. As may be seen, the model provides a reasonably good d e s c r i p t i o n of the e x p e r i m e n t a l l y observed t r e n d . The e f f e c t of H2 p a r t i a l pressure on the time delay between the maximum i n the production of H 0 or NH3, and the maximum p r o d u c t i o n of N i s i l l u s t r a t e d i n F i g . 20. Here too, the c a l c u l a t e d curves are i n reasonable agreement w i t h the experimental data and i t i s noted that the model p r e d i c t s the same time delay f o r both NH3 and H 0 under most circumstances. To f u r t h e r t e s t the model, c a l c u l a t i o n s were performed to simulate the i s o t o p i c t r a c e r experiments presented i n F i g s . 9 and 10. I t should be noted that w h i l e the t r a c e r experiments were performed at 438K, the r a t e c o e f f i c i e n t s used i n the model were chosen to f i t the experiments i n which chemisorbed NO was reduced at 423 K. Figures 21 and 22 i l l u s t r a t e the n i t r o g e n p a r t i a l pressure and surface coverage responses p r e d i c t e d f o r an e x p e r i ment i n which 15^0 i s s u b s t i t u t e d f o r ^NO at the same time that H i s added to the NO f l o w . S i m i l a r p l o t s are shown i n F i g s . 23 and 24 f o r an experiment i n which ^NO i s s u b s t i t u t e d f o r ^NO during s t e a d y - s t a t e r e d u c t i o n . Comparison of F i g s . 21 and 23 w i t h F i g s . 9 and 10 shows that the p r e d i c t e d p a r t i a l pressure responses f o r the N isotopes are i n f a i r agreement w i t h those observed e x p e r i m e n t a l l y . The p r i n c i p a l d i f f e r e n c e s are that the experimental ^ N ^ N response i s s i g n i f i c a n t l y more asymmetric than the corresponding theor e t i c a l response and that a l l of the experimental t r a n s i e n t s occur over a s h o r t e r time frame than would be deduced from the t h e o r e t i c a l r e s u l t s . The s t r o n g asymmetry of the e x p e r i m e n t a l l y observed 2

2

2

2

2

2

14 15 N

N

response may

be due to the f a c t that ^ N

and 15

c o n t a i n i n g adspecies

are not randomly mixed but, r a t h e r , e x i s t

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

N

S A V A T S K Y

A N D

Nitric

B E L L

Oxide

Reduction

Figure 18. The predicted effects of P °n the intensities of H O NH , N , and N 0 at maximum production, during the reduction of preadsorbed NO: P o == 2.8 χ 10 atm; Τ = 423 Κ; and t = 30 s. H2

2

f

S

2

2

N

s

N0

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

*S

6-

M

\

\

\ \

E

"c7 5

(t NH, •t ) predicted



\

• Exptl.

ι

σ

± 4

ro X

Ε

-F 2 1

I

χ Ο

O.OI

0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 Η.

(atm)

Figure 20. Comparison of the predicted and the observed effects of P on the time delay in the maximum production of H 0 and NH relative to N , during the reduction of preadsorbed NO: P = 2.8 χ 10' atm, Τ — 423 Κ, and t = 30 s. H

2

N0

3

3

2

N0

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

5.

S A V A T S K Y

A N D

B E L L

Nitric

Oxide

135

Reduction

Figure 21. Predictions of the partial pressure responses for N , N , and NN following substitution of a feedstream containing NO/Ar by a stream containing NO/H /Ar: P = 8.0 χ W atm, P = 8.0 χ 10 atm, and Τ = 423 Κ. 14

2

15

2

15

14

2

H

2

N0

3

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

14

15

136

C A T A L Y S I S

P° (atm) P,° A Β

0.080

NO

U N D E R

T R A N S I E N T

C O N D I T I O N S

(atm) P ° (atm) NO 0.992 0.008 0.912

(atm) P,°

A

5

0.008

r

1.0

5.0

0.8-

4.0

0.6-

3

0.4

2.0

0.2

1.0

0

δ

20 t (s) Figure 22. Predictions of the surface coverage responses for ïï , ÏÏ , $ , and ïï following substitution of a feedstream containing NO/Ar by a stream contain1ItN0

15N0

llfN

15

1!tN

:

7 4 λΤS~\

ι τι

/

A

r>

ο η

κ y

m-2

~*

r>

ο η

^ s

ιn-3

ι τ

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Ο

ν

5.

S A V A T S K Y

A N D

B E L L

Nitric

Oxide

PR (atm) P°

Reduction

A Β

r

N0

0.912 0.912

0.008 0.008

••-A—·+. 120s

2.0

ε

0.080 0.080

(atm) P ° ( a t m )

(atm) P ° l 5

137

1.5

1.0

0.5

0 20 Figure 23. Predictions of the partial pressure responses for N , N , and NN following substitution of a feedstream containing NO/H /Ar by a feedstream con­ taining NO/H /Ar: P = 8.0 χ 10' atm, P = 8.0 χ 10~ atm, and Τ = 423 Κ. 14

14

15

2

H2

2

N0

2

15

2

2

3

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

14

15

138

C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

Τ

PSH A Β

(atm) P £ ( a t m ) (atm) P° NO N0 0.912 0.008 0.912 0.008

(atm) P.° 2

0.080 0.080

r

,5

h—A—Η

Β

t (s) Figure 24. Predictions of the responses for Θ , Ô , Θ , and J following substitution of a feedstream containing NO/H /Ar by a feedstream containing NO/H /Ar: P = 8.0 χ 10' atm, P = 8.0 χ 10 atm, and Τ = 423 Κ. 15Ν0

14

15

2

H2

2

1]tN0

15Ν

1JfN

2

N0

3

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

5.

S A V A T S K Y

A N D

Nitric

B E L L

Oxide

139

Reduction

i n s m a l l patches. Such non-uniform mixing would impede the i n i t i a l formation of the i s o t o p i c a l l y mixed N . The s h o r t e r d u r a t i o n of the e x p e r i m e n t a l l y observed resonses i s not s u r ­ p r i s i n g i n view of the f a c t that the experiments were performed at 438 Κ whereas the c a l c u l a t i o n s were done f o r a temperature of 423 K. 2

Conclusions Transient response experiments have revealed that the forma­ t i o n of N and N 0 during NO r e d u c t i o n by H over Rh proceeds without the i n t e r v e n t i o n of H . By c o n t r a s t , the formation of NH3 and H 0 i n v o l v e s the r e a c t i o n s of d i s s o c i a t i v e l y chemisorbed H w i t h Ν and 0 atoms, r e s p e c t i v e l y The r e s u l t s obtained from experiments i n v o l v i n g th s u b s t i t u t i o n of ^NO f o the r e a c t i o n mechanism presented i n F i g . 11. Key elements of t h i s mechanism are that NO i s adsorbed r e v e r s i b l y i n t o a molecu­ l a r s t a t e , that r e d u c t i o n i s i n i t i a t e d by the d i s s o c i a t i o n of m o l e c u l a r l y adsorbed NO, and that a l l products are formed v i a Langmuir-Hinshelwood process. 2

2

2

2

2

2

Acknowledgment This work was supported by a grant from the NSF (CPE7826352). Nomenclature kj ^

-

P?

-

)?l

-

p

i

Rate c o e f f i c i e n t f o r r e a c t i o n j . Moles of Rh surface s i t e s per gram of c a t a l y s t [mol/gm]. P a r t i a l pressure of component i a t the r e a c t o r i n l e t [ atm ]. P a r t i a l pressure of component i a t the r e a c t o r o u t l e t [atm]. Average p a r t i a l pressure of component i i n the r e a c t o r , P

Q rj R t Τ V _ε 0 v-^j P r

m

c

Ξ

(

P

+

P

)

/

2

[ a t m ]

i i i - T o t a l v o l u m e t r i c flow r a t e [cm^/s]. - Rate of r e a c t i o n j [ s " ] . - Gas constant [atm · cm^/K · m o l ] . - Time [ s ] . - Temperature [ K ] . - Reactor volume [cm^]. - Reactor v o i d f r a c t i o n . - Surface coverage. - S t o i c h i o m e t r i c c o e f f i c i e n t f o r component i i n r e a c t i o n j . - C a t a l y s t d e n s i t y [gm/cm ]. 1

J

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

140

C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

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. 24. 25. 26. 27.

Kobylinski, T. P. and Taylor, B. W., J. Catal. 33, 376 (1974). Klimisch, R. J and Larson, J. G., Eds., "The Catalytic Chemistry of Nitrogen Oxides", Plenum Press, New York, 1975. Shelef, Μ., Cat. Rev.-Sci. Eng. 11, 1 (1975). Schlatter, J. C. and Taylor, K. C., J. Catal. 49, 42 (1977). Hegedus, L. L., Summers, J. C., Schlatter, J. C., and Baron, K., J. Catal. 56, 321 (1979). Yao, H. C., Yu Yao, Y. F., and Otto, Κ., J. Catal. 56, 21 (1979). Summers, J. C. and Baron, Κ., J. Catal. 57, 380 (1979). Myers, E. C. and Bell, A. T., subbmitted to J. Catal. Kobayashi, H. and Kobayashi, Μ., Catal. Rv.-Sci. Eng., 10, 139 (1974). Bennett, C. O., Catal. Rev.-Sci. Eng. 13, 121 (1976). Tamaru, K., "Dynamic Heterogeneous Catalysis", Academic Press, New York, 1978. Arai, H. and Tominaga, H., J. Catal. 43, 131 (1976). Solymosi, F. and Sarkany, J., Appl. Surface Sci. 3, 68 (1979). Savatsky, B. J. and Bell, A. T., submitted to J. Catal. Connely, N. G., Inorg. Chim. Acta Rev., 47 (1972). Nappier, Jr., T. E., Meek, D. W., Kirchner, R. H. and Ibers, J. Α., J. Am. Chem. Soc. 95, 4194 (1973). Goldberg, S. Z., Kubiak, C., Meyer, C. D. and Eisenberg, R., Inorg. Chem. 14, 1650 (1975). Hecker, W. C. and Bell, A. T., unpublished results. Castner, D. G., Sexton, B. A. and Somorjai, G. Α., Surface Sci. 71, 519 (1978). Campbell, C. T. and White, J. Μ., Appl. Surface Sci. 1, 347 (1978). Baird, R. J., Ku, R. C. and Wynblatt, P., Surface Sci. 97, 346 (1980). Mimeault, V. J. and Hansen, R. S., J. Phys. Chem. 45, 2240 (1966). Zakumbaeva, G. D. and Omaskev, Kh. G., Kin. Katal. 18, 450 (1977). Edwards, S. M., Gasser, R. P. Η., Green, D. P., Hawkins, D. S. and Stevens, A. J., Surface Sci. 72, 213 (1978). Kawasaki, Κ., Shibata, Μ., Miki, Η., and Kioka, T., Surface Sci. 81, 370 (1979). Yates, Jr., J. T., Thiel, P. A. and Weinberg, W. Η., Surface Sci. 84, 427 (1979). Thiel, P. A. Yates, Jr., J. T., and Weinberg, W Η., Surface Sci. 90, 121 (1979).

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

5.

S A V A T S K Y

A N D B E L L

Nitric Oxide

Reduction

28.

141

Hindmarsh, Α., "Gear: Ordinary Differential Equation Solver", UDID-3001, Rev. 1, August 20, 1972, Computer Center Library, University of California, Berkeley, CA. 29. Savatsky, B. J., Ph.D. Thesis, Department of Chemical Engineering, University of California, Berkeley, CA, 1981. 30. Nelder, J. A. and Mead, R., Computer J. 7, 308 (1964).

Received August 6, 1981.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

6 The Use of Molybdenum in Automotive Three­ -Way Catalysts

H . S. G A N D H I , H . C. Y A O , and H. K . S T E P I E N Ford Motor Company, Dearborn, M I 48121

Abstract Recent developments in three-way catalyst (TWC) technology may have potential to decrease the amount of rhodium needed in TWC formulations. Current TWC formulations contain platinum (Pt) and rhodium (Rh). The amount of Rh used in the TWCs, that have desired durability, is considerably higher than the mine ratio of Pt/Rh of 17-19. For large scale vehicle application, it is necessary to find ways to minimize the use of this scarce material. Recent findings show that improved net ΝOx activity, with minimum NH formation, is accomplished by the 3

incorporation of molybdenum oxide in Pt and Pt-Rh catalysts. Tempera­ ture programmed reduction and IR studies were carried out to investigate the interaction of MoO with P t O and with the γ-Αl Ο support. The 3

2

2

3

results are used to explain the improved selectivity for the NO to N reaction and the decreased CO poisoning of molybdenum-containing Pt catalysts when compared to pure Pt catalysts. 2

Introduction Three-way catalysts (TWC), or equilibrium catalysts are designed for the simultaneous control of three automotive pollutants: NO, C O , and hydrocarbons (HC). Since rhodium has the desired selectivity for the reduction of NO to N with minimum NH^ formation and is also selective 2

in reducing NO near stoichiometry or under slightly oxidizing (1 2 3) c o n d i t i o n s - , it is one of the active metal components used in TWC formulations.^ The amount of rhodium used in the TWCs having the desired durability is considerably higher than the mine ratio of Pt/Rh of 1719. Therefore, for large-scale vehicle application it is necessary to find ways to minimize the use of this scarce material. Ruthenium, which has

0097-6156/82/0178-0143$05.00/0 ©

1982 A m e r i c a n Chemical Society

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

144

C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

the desired selectivity for the NO to N reaction, has not found use in TWC formulations because of the potential loss of Ru as RuOy The attempts to 2

stabilize Ru against volatilization have been partially successful/-' However, the uncertainty of being able to achieve virtually zero ruthenium emissions, under varying driving conditions, has discouraged the use of Ru in TWC. This paper describes studies on the use of molybdenum in TWC formulations as a substitute for part of the amount of Rh needed.

Experimental Catalysts: Catalysts used in the flow reactor study for activity and selectivity measurements are described in Table I. They were monolithic catalysts with a fresh BE Catalysts used in the temperature programmed reduction (TPR) study were granular and are described in Table II. These catalysts were made by 2 agglomerating Dispal-M γ-Α^Ο^ powder (180 m /g BET area, Continental Oil Co.) and subsequently wetting, drying and calcining at 600°C for 5 h. The resulting solid mass was crushed and sieved and the 0.5 - 1.0 mm diam. fraction was impregnated with an (NH^) Mo 0 solution of desired 2

2

7

concentration, dried and calcined at 500°C in air for 2 h to obtain the ΜοΟ^/γ-Α1 0^ sample. Part of this sample was impregnated again with a 2

H P t C l ^ solution of desired concentration, dried and calcined at 500°C in 2

air for 2 h to get the Ρ Κ > - Μ ο 0 / γ - Α 1 0 2

3

catalysts, the granular γ - Α 1 0 2

3

2

3

sample.

For supported Pt

support was impregnated with a H P t C l 2

6

solution, dried and calcined at 500°C in air for 5 h to obtain the P t 0 / 2

A

*2°3

s a m

Pl * e

Y

The pure P t 0 powder (40 mesh) was a commercial sample 2

obtained from Alpha Chemicals, Ventron Corporation. Apparatus and Procedure; Flow Reactor for Activity and Selectivity Measurements; The experimental apparatus, testing procedures and methods for analyzing the gas streams were identical to those used in ref. (fj). The activity and selectivity of the catalysts were measured with a simulated exhaust gas mixture containing NO, CO, 0 , H , C^H^, C^Hg, 2

2

C 0 , H 0 , and Ν · The C O / H ratio was kept at 3. Propylene and propane 2

2

2

2

were used to represent fast-burning and slow-burning hydrocarbons, respectively. The C^H^/C^Hg * i o was 2. The synthetic exhaust gas ra

composition and a schematic diagram of the flow-reactor used are also given in ref. (4). In addition, the gas mixture also contained 20 ppm of S 0 . 2

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

6.

G A N D H I

E T

Automotive

A L .

Three-Way

145

Catalysts

TABLE I DESCRIPTION OF MONOLITHIC CATALYSTS* USED IN FLOW REACTOR STUDY Catalyst

Composition wt % Pt

Pt

Pd

Rh

Mo

0.18

Mo0

-

-

2

0.25

-

2

-

2

3

Pt-Mo0

3

Pd-Mo0

0.2

3

Pt-Rh-Mo0

0.2

3

Pt-Rh

0.12

0.012

TABLE Π DESCRIPTION OF THE GRANULAR CATALYST USED IN TPR STUDY Catalyst

Composition, wt % Pt

Ρΐ/γ-Α1 0 (Α)

13.8

Ρτ/γ-Α1 0 (Β)

1.6

3

2

2

3

Μο/γ-Α1 0 (Α)

10.1

Μο/γ-Α1 0 (Β)



2

2

3

3

Ρί-Μο/γ-Α1 0 (Α) 2

3

1.6



Pt-Mo/y-Al 0 (B)

3.2



Pt-Mo/y-Al 0 (C)

13.4



2

2

Catalysts

Mo

3

3

were prepared

on Corning monoliths

cells/in . The fresh BET area of catalyst was 2

10 wt % of the uncoated monolith).

with

15 m / g . 2

300

square

(γ-Α1 0 = 2

3

Active metal composition as wt

% of the total monolith weight including washcoal.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

146

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U N D E R

T R A N S I E N T

C O N D I T I O N S

The redox ratio R, used to express the oxidizing or reducing characteristics of the exhaust gas mixture, is defined as follows: p

co

^

+

PH.

_

+ 3np

2

c H, N

+ ( 3 n + 1)p N

P

c H N

η 2n NO

+

2 p

0

?N

4

,

η 2n + Ζ 2

where ρ is the partial pressure of the indicated species. Therefore, R > 1 represents an overall reducing gas mixture and R = 1 represents a stoichiometric gas mixture. The activity and the selectivity measurements were made in a flow reactor with the synthetic gas mixture at 1022°F (550°C) and a space-velocity of 60,000 h~unless otherwise noted. Temperature Programmed Reduction (TPR):

The apparatus and

procedure used in the TPR were reported in a previous paper/-^ A small amount of sample (0.02 t mixture of 15% H and 85% Ar was passed over the catalyst at a rate of 25 2

cc/mia

The H uptake during the reduction was measured by a thermo2

conductivity detector. During the reduction the programmed temperature rise of the sample was controlled at a constant rate of 10°C/min. Infrared (IR) Study: The apparatus and procedure used in the IR study "

(o)

were reported in a previous paper. - The sample wafers were prepared by pressing about 200 mg of the P t / M o / y - A l 0 catalyst (3.2 wt % Pt and 3Λ wt % Mo) in a 2.86 cm diam. metal die and cutting to a size of 6 mm χ 20 mm. 2

3

Results and Discussion NO, CO And HC Conversions The steady-state activity and selectivity characteristics of a monolithic MoO^ (2 wt % Mo) catalyst are shown in Figure 1A. Percent conversions of CO, NO, HC and ammonia formation are shown as a function of redox ratio, R. Clearly molybdenum oxide itself does not have any activity for NO reduction over the entire range of R values from R = 0.8 to R = 2. The CO and HC conversions are - 15% over this range. Platinum and palladium catalysts are known to have poor selectivity for the NO to N reaction under reducing conditions. Under these 2

conditions a large fraction of NO is converted to N H ^ / — ^ Figure IB shows the characteristic high NH^ formation over a fresh monolithic 0.18 wt % Pt catalyst. For example, at R = 1.6 gross NO conversion is 58%, however, the net NO conversion is ~1%.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Automotive

(%)

Three-Way

Catalysts

N0ISM3AN00

American Chemical Society Library 1155 16* st N. w. In Catalysis Under Transient Conditions; Bell, A., et al.;

ACS Symposium Series; American Chemical Washington, D. C. Society: 20086Washington, DC, 1982.

147

148

C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

Figure 1C shows the activity and selectivity characteristics of a monolithic catalyst containing 2 wt% Mo in the form of MoO^ and 0.25 wt% Pt.

The addition of MoO^ to the Pt catalyst significantly improves

the net NO conversion over the entire range of R values. For example, at R = 1.6, 86% net NO conversion is achieved, compared to zero and 1% net NO conversion for MoO^ only, and Pt only catalysts respectively. The PtMoO^ catalyst also maintains good CO and HC activities. Similarly the incorporation of 2% Mo in the form of MoO^ in a 0.2 wt% Pd catalyst significantly improves the net ΝΟ conversion in the rich χ

region. For example, at R = 1.6, 92% net NO conversion is achieved and the NH^ formation is only 2% of the converted NO. The addition of molybdenum oxide to Pt and Pd changes selectivity characteristic

the

the rich region, Pt-MoO^ and Pd-MoO^ catalysts suppress N H formation 3

almost completely and thereby yield over 80% net ΝΟ conversion for R χ

values as high as 1.9 over the Pd-MoO^ catalyst and as high as 2.3 over the Pt-MoO^ catalyst. In the lean region (R < 1), the Pt-MoO^ catalyst is able to reduce NO by CO (and H ) in the presence of excess 0 · 2

2

For example,

at R = 0.9 the Pt-Mo0 catalyst converts 50% of the NO compared to no conversion for MoO^ and - 5% conversion for Pt catalysts, respectively. 3

At R = 0.9 the Pd-Mo0 catalyst exhibits 70% NO conversion. Thus, PtM o 0 and Pd-Mo0 catalysts minimize N H formation under reducing conditions and improve NO conversion Under somewhat oxidizing conditions. 3

3

3

3

The addition of Rh to Pt-Mo0 further improves the NO conversion throughout the range of R values. At R = 1 peak NO conversion for the PtRh-Mo0 catalyst is 98% (Figure ID) compared to 90% for the Pt-Mo0 3

3

3

catalyst (Figure 1C). In addition, the incorporation of M o 0

3

substantially decreases N H

3

formation over Pt-Rh catalysts. For example, a catalyst containing 0.19% Pt and 0.017% Rh shows a decrease in N H formation, at R = 1.8, from 20% 3

to ~ 2% by the incorporation of 2% Mo as MoO^ However, when the amount of Mo is decreased from 2 to 0.3 wt % (i.e. to Mo/Pt atomic ratio of 3) this improvement in the activity or selectivity behavior is lost. This effect suggests the necessity for the Pt and Mo ions to be present in close proximity. By having a large excess of Mo with respect to Pt, the statistical probability for Pt to be located near Mo ions is greatly increased.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

6.

G A N D H I

E T

Automotive

A L .

Three-Way

Catalysts

149

Additional experiments were performed to quantify the effect of varying the Mo/Pt ratio on the selectivity for reduction of NO to N (or 2

N^O). The Pt concentration on the monolithic support was maintained at 0.2 wt % and Mo concentration was varied from 0 to 3Λ wt %. The NOH - C O reaction was done over these catalysts at 60,000 h " space-velocity. 1

2

The selectivity is defined as: S

2 * 2° 2N + 2N 0 + N H 2 N

2 Ν

2

2

x 3

"

1 0 0 x

1 U U

Net NO Conversion Gross NO Conversion

{ Q Q x

A U U

The results for NO to N conversion as a function of varying Mo/Pt atomic 2

ratio at 500°C (Figure 2) show the increase of selectivity with Mo concentration, leveling off at Mo/Pt atomic ratio of 15. Model Reactions Study over Pt and Pt-MoO^ Catalysts: In order to further understand the activity and selectivity behavior of Mo-containing Pt catalysts, N O - H , N O - C O - H , and NO-CO model reactions were studied 2

2

over Pt (0.25 wt %) and Pt-Mo0

3

(Pt = 0.25 wt % and Mo = 2 wt %)

catalysts. The study was carried out over a temperature range of 25°C to 600°C at a space-velocity of 60,000 h~*. The relative catalyst activity as expressed by the temperature required for 50% and 80% NO conversion is given in Table III. For both Pt and Pt-Mo0 catalysts, the presence of CO 3

in the feedgas poisons the Pt sites, as evidenced by an increase in the 50% and 80% NO conversion temperatures. However, the CO poisoning effect is much stronger for pure Pt catalysts than for a mixed Pt-Mo0 catalyst. 3

For example, for the NO-CO reaction, 50% NO conversion can not be reached over the Pt catalyst, while 345°C and 360°C temperatures are required for 50% and 80% NO conversions, respectively, over the Pt-Mo0 catalyst. 3

Table IV shows the striking difference in selectivity behavior between Pt and Pt-Mo0 catalysts for N O - H and N O - H and N O - H - C O reactions. For the N O - H - C O reaction at 600°C,only 12% of the reacted NO is 3

2

2

2

2

converted to N over the Pt catalyst compared to 100% for the Pt-Mo0 catalyst. 2

3

Sulfur Dioxide and Propane Oxidation Platinum catalysts are known for their good activity for S O ^ — ^ and C H g ^ — ^ oxidation. Rhodium on the other hand is a poor catalyst for these reactions. Therefore, these reactions were studied on Pt and Pt3

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

150

C A T A L Y S I S

Ο

U N D E R

T R A N S I E N T

C O N D I T I O N S

M

Figure 2.

Selectivity as a function of Mo to Pt ratio (Pt, 0.2 wt %; R, 1.6) at 500°C.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

6.

G A N D H I

E T

Automotive

A L .

Three-Way

151

Catalysts

TABLE ΠΙ TEMPERATURE (°C) FOR 50% AND 80% GROSS NO CONVERSION IN N O - H , N O - H - C O AND NO-CO REACTIONS 2

2

NO-H Catalysts

T

50%

3

80%

T

50%

T

80%

60

100

360

500

175

185

250

350

Pt(0.25%) Pt-Mo0

T

NO-H^-CO

2

NO-CO T

50%

T

80% NR

NR

360

3*5

(Pi:=0.25%, Mo=2%) Feedgas

NO = 0.1%

NO = 0.1%

NO = 0.1%

CO = 0

CO = 0.75%

CO = 1%

H

H

H

2

=

1%

2

=

0.25%

2

= 0

TABLE IV SELECTIVITY (%) FOR NO TO N (or N 0 ) REACTION IN N O - H 2

2

N O - H - C O REACTIONS AS A FUNCTION OF TEMPERATURE °C 2

T°C

NO-H

2

Pt NO-H -CO 2

Pt-MoO. NO-I^ NO-H -CO 2

250

0

0

0

30

300

0

0

0

43

350

0

0

0

53

W0

0

0

12

60

450

0

0

22

67

500

0

2

30

73

550

0

8

38

80

600

0

12

46

100

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

2

152

C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

MoO^ catalysts to determine whether the Pt-MoO^ catalyst exhibits Ptlike or Rh-like behavior. The results for S 0

2

oxidation over supported Pt

and Pt-MoO^ catalysts are shown in Table V. The Rh-like behavior, i.e., poor S O oxidation activity, is evident. For example, at 500°C and 60,000 -1 h space-velocity, S 0 conversion is 45% for the Pt catalyst compared to ?

2

11% for the Pt-MoO^ catalyst.

Table VI shows similarly the extreme

difference in propane oxidation activity for these catalysts.

Effect o i Oxygen Pulse on Steam-Reforming Activity The effect of an oxygen pulse on the steam-reforming activity of fresh Pt, Rh, Pt-Rh, and Pt-Mo0 catalysts was studied both in the 3

presence and absence of 2 steam-reforming activity of Pt, Rh, and Pt-Rh catalysts is severely reduced by the addition of 20 ppm of S 0 in the feed-gas. The 2

incorporation of MoO^ significantly suppresses the steam-reforming activity of Pt catalysts even in the absence of the S 0 in the feed-gas. In this case, the Pt-MoO^ catalyst performance is similar neither to Pt nor Rh 2

catalysts. Upon adding a 30 sec 1% 0

2

pulse, the HC conversion improves

for all the catalysts. The average values of percent HC converted during five consecutive 30 sec 0 pulses, both in the presence and absence of S 0 in the feed-gas, also are given in Table VII. The improved HC conversion lasts for several minutes after the 30 sec 0 pulse is terminated. The HC 2

2

2

conversion values at t = 10, 15, 20 and 30 sec, after the 0

2

pulse was

ended, are shown in Table VIII. A comparison of HC conversion values shows that the Pt-MoO^ catalyst performance is somewhat inferior to that of the Pt catalyst under these cyclic conditions. These results show that incorporation of MoO^ into a Pt catalyst changes the basic characteristics of the Pt catalyst and suggest the possibility of an interaction between Pt and Mo oxides on the surface of the support. This possibility was investigated by a TPR study with a Pt-MoO^ catalyst.

Temperature Programmed Reduction (TPR) The rates

of

H

2

uptake (in arbitrary units) as a function of

temperature for the γ - Α 1 0 2

supported P t 0 , MoO^, and P t 0 - M o 0

3

2

2

mixtures of various concentrations are plotted in Fig. 3. TPR of pure P t 0 (15 16)

3

9

and M o 0 supported on γ - Α 1 0 have been reported. — C u r v e A is the 3

2

3

TPR of the P t O sample of high concentration (13.8 wt % Pt) in which two ?

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

G A N D H I

E T

Automotive

A L .

Three-Way

Catalysts

TABLE V S0

2

OXIDATION OVER Pt AND P t - M o 0

3

CATALYSTS

% SO- Conversion

Catalyst

400°C

500°C

600°C

10

45

59

3

11

16

Pt (0.2 wt %) Pt-Mo0

3

(Pt = 0.2 wt % (Mo = 2 a

wt %)

Feedgas:

S 0 = 0.02% 2

o = 1% 2

S.V.

=

60,000 h "

1

TABLE VI PROPANE OXIDATION OVER Pt AND Pt-MoO, CATALYSTS

Catalyst

T

25%

T

50%

T

75%

Pt (0.2%)

230

267

298

Pt-•MoO,

450

640

N.R.

(Pt = 0.2%, Mo = 2%) Feedgas:

C H 3

0

= 0.05% = 2.25%

2

S0 N

g

2

= 20 ppm = Balance

2

S. V. = 60,000 h"

1

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

154

C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

TABLE ΥΠ EFFECT OF OXYGEN PULSE ON STEAM REFORMING ACTIVITY % HC Conversion

n

Steady-State 0

20

Oxygen Pulse 20 0

Pt (0.2%)

70

20

90

73

Rh (0.02%)

80

8

93

72

12

4

57

57

Catalyst

ppm S 0 : 2

Pt (0.2%), Rh (0.02% Pt (0.2%), Mo (2%)

A

Feedgas:

C H 3

6

= 0.036%

C H

g

=

0.018%

=

0.5%

so

8%

N

3

H

2

C0 Τ

= 600°C,

2

=

S.V.

H 0

= 60,000 h "

= 8%

2

2

2

=

0 or 0.002%

=

Balance

1

Average % HC converted during 5 consecutive 30 sec 1% Ο pulses. 2

rate maxima are observed. The one at 10°C has been assigned to the reduction of PtO~ in the particulate phase and the one near 90 C to the (15) reduction of P t O in the saturated dispersed phase.— ?

Curve Β is TPR of

the P t 0 sample of low concentration (1.6 wt % Pt or 0.48 ymol/m BET), 2

which is far below the saturation concentration of the dispersed phase (2.2 unol/m

BET). The rate maximum in Curve Β has shifted to a higher

temperature, ~ 250°C, indicating a stronger Ρ Κ > - γ - Α 1 0 2

2

3

interaction in

this sample. Curve C is the TPR of the M o 0 sample of high concentration 3

(10.1 wt % Mo). The surface concentration of MoO- in this sample is 8.2 2 (16) umol/nrr (BET), of which about 50% is present in the particulate phase.— The TPR of this sample shows three rate maxima, indicating the difference in the ease of removal for the different oxygen atoms in M o 0 by Η · The first rate maximum at 450°C has been assigned^—^ to the removal of only 3

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

2

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Pt + 0.02%

Pt + 2% Mo

0.2%

Rh

Pt

0.2%

0.02%

0.2%

2

Pt + 2% Mo

0.2%

With 20 ppm S 0

Pt + 0.02%

Rh

Pt

w/o so2

Catalyst

0.2%

0.02%

0.2%

1

Rh

Rh

1620

1500

1600

1500

1620

1500

1600

1500

Feedgas HC (ppm)

85

12

4

3

8 72

82

86

77

98

89

21

95

2

20 sec pulse ( 0

61

70

78

68

44

97

94

43

62

69

61

17

97

93

HC Conv.(%)

10 sec 15 sec after (first) 30 sec 0

81

70

HC Conv. % 2

32

55

56

54

9

97

92

30 sec = 1%)

STEAM REFORMING WITH EFFECT OF OXYGEN PULSE (T = 600°C, S.V. = 60,000 h" )

TABLE Vm

156

C A T A L Y S I S

I

U N D E R

T R A N S I E N T

Α.

l3.8wt%Pt

Β.

l.6wt%Pt

C O N D I T I O N S

0

Α C.

^\

10.1 wt%Mo

>

to ο ο

8.

Electwcdtalytic

STOUKiDES A N D V A Y E N A S

201

Epoxidation

ature f o r the e x i s t e n c e of s u r f a c e s i l v e r oxides i n c l u d i n g the work of Seo and Sato (13) who observed a continuous e x o e l e c t r o n emission from s i l v e r c a t a l y s t s during ethylene e p o x i d a t i o n which was p r o p o r t i o n a l to the r a t e of ethylene oxide formation. They ex­ p l a i n e d t h e i r r e s u l t s i n terms of formation of s u r f a c e s i l v e r oxide Ag 0 w i t h molecular oxygen i o n 0 adsorbed on i t (13). This p i c t u r e seems to be i n e x c e l l e n t agreement w i t h the present r e ­ s u l t s , i . e . w i t h equation (5) 2

2

Δ Γ . = k. ·α.·Δν·Ρ (5) ι ίο 1 o i f the change i n the amount of s u r f a c e s i l v e r oxide i s p r o p o r t i o n ­ a l to Δ ν and the coverage of molecular oxygen adsorbed on the oxide i s p r o p o r t i o n a l to P , or at l e a s t i f the r a t e s of e p o x i ­ d a t i o n and combustion oxygen. Equations ( 2 ) , (3) and (5) which d e s c r i b e the dependence of Δ Γ . on the overvoltage are extremely simple but deserve some speci a l a t t e n t i o n . They a l l i n d i c a t e t h a t Δ Γ . i s p r o p o r t i o n a l to Δ ν but independent of the s u r f a c e area of s i l v e r e l e c t r o d e . This i s why the r e l a t i v e i n c r e a s e i n the r a t e s Δ Γ . / Γ . i s highest f o r r e ­ a c t o r s w i t h s m a l l s u r f a c e area ( F i g . 11) and Becomes i n s i g n i f i c a n t f o r r e a c t o r s w i t h l a r g e s u r f a c e area. I t thus appears that a l ­ though Δ ν i s an i n t e n s i v e v a r i a b l e i t i s n e v e r t h e l e s s a measure of the chang S'hc i n the mole number of s i l v e r oxide and not of the c o n c e n t r a t i o n change Ac. This i s because a l l the r e a c t o r s used had the same e l e c t r o l y t e area A = 2cm as e x p l a i n e d i n d e t a i l be­ low: N e g l e c t i n g the double l a y e r capacitance of the e l e c t r o d e e l e c t r o l y t e i n t e r f a c e the constant c u r r e n t i imposed d u r i n g the t r a n s i e n t can be s p l i t i n two p a r t s (25): 2

Q

1

2

1

ad

dt

+

X

CTR

U

)

where 1 > i s the c u r r e n t corresponding to the charge t r a n s f e r r e actions, i . e . ητΓτ

2

0" 2

0~->

+ C H *Products + 2e~ 2

lr

1/2 0 ( g ) + 2e" 2

(8) (9)

and C , i s the a d s o r p t i o n pseudo capacitance of the oxide which by d e f i n i t i o n equals (25):

where q i s the charge s t o r e d i n the oxide and λ i s a constant. I f one assumes that Sc v a r i e s l i n e a r l y w i t h V, i . e . t h a t the oxide pseudo capacitance i s constant one o b t a i n s c

=x- A-

K 2

2

S

P

' ' ET'

C

v

( 1 6 )

( 1 7 )

i . e . the model assumed t h a t oxide can form only on s i t e s covered o r i g i n a l l y w i t h molecular O 2 . According to t h i s equation the r e ­ l a x a t i o n time constant of the system τ = τ i s given by 1

4 F S C

M

K 2 P

1

ET °

°

~c ~~ ~ ^ ν * 7 ° c O2 O2 O2 O2 Equation (18) i s i n e x c e l l e n t agreement w i t h f i g u r e 10 which shows l/τ f o r 5 d i f f e r e n t r e a c t o r s to be a l i n e a r i n c r e a s i n g func­ t i o n of w i t h a p o s i t i v e i n t e r c e p t . One of the i m p l i c a t i o n s i s that the amount of s u r f a c e oxide at f u l l coverage Sc^ i s comp­ a r a b l e t o the r e a c t i v e oxygen uptake Q of the c a t a l y s t . Temperature e f f e c t : There are s e v e r a l ways to account f o r the observed maximum i n Ar. at constant gas composition and imposed current i ( F i g . 12). Although the behavior shown i n the f i g u r e i s q u i t e r e p r o d u c i b l e , f u r t h e r experimental work i s r e q u i r e d to examine the e f f e c t of gas phase composition and i on the temper­ ature dependence of Ar.. One p o s s i b l e e x p l a n a t i o n f o r the ob­ served maximum can be obtained by c l o s e r examination of equation (7). The charge t r a n s f e r c u r r e n t i can be w r i t t e n as i^r™ ^+ i"CTR ^ where the former term corresponds to the charge t r a n s f e r r e a c t i o n (8) and the l a t t e r t o the charge t r a n s f e r r e a c t i o n ( 9 ) . Using the low f i e l d law (25) one o b t a i n s (

1

+

^CTR,A W

)

V

= i

Ï

(

4

T

F

0 )

B'


/ R T

( 1 + K

2v

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

( 1 8 )

( 1 9 >

( 2 0 )

204

C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

The exchange current d e n s i t i e s i ^ ^ and i ^ a r e temperature dependent and can be expressed as β

9

VA



VB

=

K

E

O , A

VB

X

P ( "

e x p (

9