Solid State Chemistry in Catalysis 9780841209152, 9780841211100, 0-8412-0915-4

Citation preview

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.fw001

Solid State Chemistry in Catalysis

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.fw001

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

ACS

SYMPOSIUM

SERIES

Solid State Chemistry in Catalysis Robert K. Grasselli, EDITOR Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.fw001

The Standard Oil Company

James F. Brazdil, EDITOR The Standard Oil Company

Based on a symposium sponsored by the Division of Petroleum Chemistry, Inc. at the 186th Meeting of the American Chemical Society, Washington, D.C., August 28-September 2, 1983

American Chemical Society, Washington, D.C. 1985

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

279

Library of Congress Cataloging in Publication Data Solid state chemistry in catalysis. (ACS symposium series, ISSN 0097-6156; 279)

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.fw001

"Based on a symposium sponsored by the Division of Petroleum Chemistry, Inc. at the 186th Meeting of the American Chemical Society, Washington, D.C., August 28-September 2, 1983." Includes bibliographies and indexes. 1. Catalysis—Congresses. 2. Solid state chemistry— Congresses. 1. Grasselli, Robert K., 1930. II. Brazdil, James F., 1953. III. American Chemical Society. Division of Petroleum Chemistry. IV. American Chemical Society. Meeting (186th: 1983: Washington, D.C.) V. Series. QD505.S65 1985 ISBN 0-8412-0915-4

541.3'95

85-9190

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

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

ACS Symposium Series

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.fw001

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

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

Roland F. Hirsch U.S. Department of Energy

James C. Randall Phillips Petroleum Company

Herbert D. Kaesz University of California—Los Angeles

Charles N . Satterfield Massachusetts Institute of Technology

Rudolph J. Marcus Office of Naval Research

W. D. Shults Oak Ridge National Laboratory

Vincent D. McGinniss Battelle Columbus Laboratories

Charles S. Tuesday General Motors Research Laboratory

Donald E . Moreland USDA, Agricultural Research Service

Douglas B. Walters National Institute of Environmental Health

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

C. Grant Willson IBM Research Department

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

FOREWORD

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.fw001

The ACS SYMPOSIUM SERIES was founded in 1974 to provide a

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

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.pr001

PREFACE HETEROGENEOUS CATALYSIS has been synonymous with industrial catalysis and the chemical industry since the time of Berzelius, Sabatier, Ostwald, Haber, Bosch, Mittasch, Fischer, and Huttig, as well as many others. To this day, virtually all chemical and refining processes are based on the use of solid catalysts that effect selective transformations of hydrocarbon molecules to desired products in the vapor phase. Nonetheless, correlations between structural aspects of solid materials and their behavior as catalysts are relatively recent developments. Probably first to recognize the importance of structure in catalysis were researchers in the field of catalytic cracking who investigated the catalytic activity of natural clays and minerals in the early 1930s. The culmination of that work was the discovery of zeolitic cracking catalysts in the early 1960s and the subsequent development of the concept of shape selective catalysis. With the advent of highly sophisticated instrumentation for precise structure determination, key catalytic roles are being recognized for many subtle features of solid state materials, such as point and extended defects, surface structure and surface composition, atomic coordination, phase boundaries, and intergrowths. Today, X-ray structure analysis is routine in heterogeneous catalysis research and has become as common and necessary as BET surface area analysis for characterizing solid catalysts. In addition, high resolution electron microscopy; photoelectron, IR, and Raman spectroscopies; solid state NMR; and even neutron diffraction are assuming increasingly important roles in both applied and fundamental catalysis research. Information about solids on the atomic and molecular level, which these techniques provide when combined with traditional catalytic studies (e.g., reaction kinetics, tracer studies, and molecular probes), gives a better fundamental understanding of complex catalytic phenomena. Correlations between the solid state and catalytic properties assessed through the application of sophisticated instrumentation and classical mechanistic approaches are the central theme of this book. The book comprises 20 chapters that focus on state-of-the-art understanding of solid state mechanisms in heterogeneous catalysis and the relationship between catalytic behavior and solid state structure. The volume contains expanded and updated versions of papers presented on this subject at the ACS symposia in Washington, D C . (1983) and Las Vegas (1982), and of written contributions from invited participants who could not attend these ix In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

meetings. It emphasizes catalysis with oxides, sulfides, and zeolites. Although by no means an exhaustive treatise, we hope that it provides the reader with an understanding of the role the solid state plays in heterogeneous catalysis and gives an appreciation for the contributions solid state chemistry has made to the advancement of catalytic science and technology. We should like to thank all the contributors for their excellent cooperation and patience during the process of editing this book and to the ACS for making this publication possible.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.pr001

ROBERT K . GRASSELLI JAMES F. BRAZDIL

The Standard Oil Company (Ohio) Cleveland, O H 44128 December 7, 1984

x In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

1 Catalysis by Transition Metal Oxides JERZY HABER

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul.Niezapominajek, 30-239 Krakow, Poland

Catalytic oxidation reactions are divided into two groups: electrophilic oxidation proceeding through activation of oxygen and nucleophilic oxidation in which activation of the hydrocarbon molecule is the first step, followed by consecutive hydrogen abstraction and nucleophilic oxygen insertion. Properties of individual cations and their coordination polyhedra determine their behaviour as active centers responsible for activation of hydrocarbon molecules. A facile route for nucleophilic insertion of oxygen into such molecules by group V, VI and VII transition metal oxides is provided by the crystallographic shear mechanism, catalytic properties are thus dependent upon the geometry of the surface. The catalyst surface is in dynamic interaction with the gas phase, and changes of the latter may thus result in surface transformations and appearance of surface phases, which influence the selectivity of catalytic reactions. The vast majority of catalysts used in modern chemical industry are oxides. Because of their a b i l i t y to take part in the exchange of electrons, as well as i n the exchange of protons or oxide ions, oxides are used as catalysts i n both redox and acid-base reactions. They constitute the active phase not only i n oxide catalysts but also i n the case of many metal c a t a l y s t s , which in the conditions of c a t a l y t i c reaction are covered by a surface layer of a reactive oxide. Properties of oxides are also important in the case of preparation of many metal and sulphide c a t a l y s t s , which are obtained from an oxide precursor. Very often, highly dispersed metals are prepared by reduction of an appropriate oxide phase, and sulphide catalysts are formed from the oxide precursor in the course of the hydrodesulphurization by i n t e r a c t i o n with the reaction medium. Finally, oxides play an important role in c a r r i e r s for active metal or oxide phases, very often modifying strongly t h e i r c a t a l y t i c properties. The present paper concerns

0097-6156/85/0279-0003$06.00/0 © 1985 American Chemical Society

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

4

S O L I D STATE C H E M I S T R Y IN CATALYSIS

o n l y one a s p e c t o f the v a s t field of c h e m i s t r y o f o x i d e s , namely the c a t a l y s i s by t r a n s i t i o n m e t a l o x i d e s , w h i c h i s the b a s i s o f the s e l e c t i v e o x i d a t i o n of hydrocarbons. C a t a l y t i c o x i d a t i o n i s one of the most i m p o r t a n t types o f p r o c e s s e s , b o t h from a t h e o r e t i c a l and a p r a c t i c a l p o i n t o f v i e w . As e a r l y as 1918, the p r o d u c t i o n o f p h t h a l i c a n h y d r i d e by o x i d a t i o n o f n a p h t h a l e n e over ^2 5 introduced. The m i l e s t o n e i n the development o f modern p e t r o c h e m i c a l i n d u s t r y was the i n t r o d u c t i o n of the gas phase oxidation of propylene to a c r o l e i n and ammoxidation to a c r y l o n i t r i l e over b i s m u t h molybdate c a t a l y s t s , which p r o v i d e d i n the early s i x t i e s ' , an abundant s u p p l y o f new, inexpensive, and useful chemical intermediates(J_). Today, c a t a l y t i c o x i d a t i o n i s the basis o f the p r o d u c t i o n o f almost a l l monomers used i n the m a n u f a c t u r i n g of synthetic f i b e r s , p l a s t i c s , and many o t h e r p r o d u c t s . W i t h the i n c r e a s i n g c o s t o f energy and s h r i n k i n g s u p p l y o f cheap h y d r o c a r b o n s , much e f f o r t i s now b e i n g expended on the development o f new o x i d a t i o n p r o c e s s e s o f h i g h e r s e l e c t i v i t y and l o w e r energy consumption. S u b s t i t u t i o n o f the d e h y d r o g e n a t i o n by o x i d a t i v e p r o c e s s e s , as i n the p r o d u c t i o n o f s t y r e n e from e t h y l b e n z e n e , may be quoted as an example. Another i n c r e a s i n g l y important field of c a t a l y t i c application is the s e l e c t i v e o x i d a t i o n of p a r a f f i n s .

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

Q

w

a

s

Discussion E l e c t r o p h i l i c and n u c l e o p h i l i c o x i d a t i o n I n every o x i d a t i o n r e a c t i o n two r e a c t a n t s always t a k e p a r t : oxygen and the m o l e c u l e t o be o x i d i z e d . The r e a c t i o n may thus s t a r t e i t h e r by the a c t i v a t i o n o f the d i o x y g e n o r by the a c t i v a t i o n o f the h y d r o c a r b o n m o l e c u l e . At ambient o r moderate t e m p e r a t u r e s , an oxygen m o l e c u l e may be a c t i v a t e d by bonding i n t o an o r g a n o m e t a l l i c complex i n the l i q u i d phase. Depending on the type o f the c e n t r a l m e t a l atom and on the p r o p e r t i e s o f the l i g a n d s , s u p e r o x o - , p e r o x o - o r oxo-complexes may be formed: +M

+0o ->

M0

0

-> M0 M

> 2M0

O

I

ο 0 M

perox

superoxo

M

/

V

> MOM

Μ

μ-peroxo

Μ = 0

oxo

M/

\M

A*-oxo

I n the case o f the s u p e r o x o - c o m p i e x e s , an e l e c t r o p h i l i c a t t a c k o f a t e r m i n a l oxygen atom on the o r g a n i c r e a c t a n t o c c u r s r e s u l t i n g i n the f o r m a t i o n o f a ]U-peroxo-com p i e x , w h i c h decomposes i n t o the oxygenated p r o d u c t ( l o w e r l e f t p a r t o f F i g u r e 1 ) . I n the case o f perox complexes o f group I V , V and V I t r a n s i t i o n m e t a l s , a s t o i c h i ­ o m e t r i c o x i d a t i o n takes p l a c e i f a v a c a n t c o o r d i n a t i o n s i t e e x i s t s a d j a c e n t to s i d e bonded oxygen and i s c a p a b l e o f b e i n g o c c u p i e d by the o r g a n i c r e a c t a n t . Its olefin bond i s then i n s e r t e d i n t o the

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

HABER

5

Catalysis by Transition Metal Oxides

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

1.

F i g u r e 1. Mechanism of the catalytic oxidation c a r b o n s . Reproduced with permission from Ref. 31.

of

hydro-

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

6

S O L I D STATE C H E M I S T R Y IN CATALYSIS

m e t a l - o x y g e n bond forming a p e r o x o m e t a l l o c y c l e w h i c h i s then decomposed i n t o the oxygenated h y d r o c a r b o n m o l e c u l e and oxo m e t a l complex C2)· In o r d e r to t r a n s f o r m the l a t t e r back i n t o the reduced m e t a l w h i c h c o u l d a g a i n form the peroxocomplexes w i t h a new oxygen m o l e c u l e , a c o r e d u c i n g agent i s r e q u i r e d , w h i c h may be a hydrogen donor o r a r e a c t a n t i t s e l f . It s h o u l d be b o r n i n mind that i n l i q u i d phase o x i d a t i o n r e a c t i o n s , the o r i g i n a l oxygen complex may be transformed i n t o o t h e r r e a c t i v e s p e c i e s which p l a y the r o l e o f a c t i v e intermediates. A superoxo-complex may be transformed i n t o an a l k y l p e r o x i d e or p e r a c i d complex, w h i c h i s the oxygen i n s e r t i n g i n t e r m e d i a t e ( 3 - 5 ) . _ At h i g h e r temperatures the p e r o x i d e 0^ and s u p e r o x i d e 0^ s p e c i e s may appear at the surface o f an o x i d e . Under these c o n d i t i o n s , the p e r o x i d e i o n i s _ u n s t a b l e and d i s s o c i a t e s forming the ion radical 0 . Both 0^ and 0 species are strongly e l e c t r o p h i l i c r e a c t a n t s w h i c h a t t a c k the o r g a n i c m o l e c u l e i n the region of i t s highest electron density. At variance w i t h t h e i r b e h a v i o u r i n the liquid phase, the p e r o x y - and epoxy-compiexes formed as the r e s u l t o f an e l e c t r o p h i l i c a t t a c k o f 0 ^οτ 0 species on the o l e f i n m o l e c u l e are intermediates w h i c h l e a d to the d e g r a d a t i o n o f the carbon s k e l e t o n under heterogeneous c a t a l y t i c r e a c t i o n c o n d i t i o n s (6^). Saturated aldehydes a r e formed i n the f i r s t stage (upper l e f t p a r t o f F i g u r e 1 ) . These are u s u a l l y much more r e a c t i v e than u n s a t u r a t e d aldehydes and at h i g h e r temperature undergo rapidly total oxidation. Indeed, experimental data c o l l e c t e d i n r e c e n t y e a r s c l e a r l y show t h a t e l e c t r o p h i l i c oxygen species i n heterogeneous p r o c e s s e s are responsible for total oxidation (7). When hydrocarbon m o l e c u l e s are activated, a v a r i e t y of reaction paths may be i n i t i a t e d , c o n s i s t i n g o f a s e r i e s of c o n s e c u t i v e o x i d a t i v e s t e p s , each o f them r e q u i r i n g a d i f f e r e n t a c t i v e c e n t e r t o be p r e s e n t at the c a t a l y s t s u r f a c e ( 8 - 1 0 ) . It s h o u l d be emphasized at t h i s point t h a t i t i s the c a t i o n s o f the c a t a l y s t w h i c h a c t as o x i d i z i n g agents i n some o f the c o n s e c u t i v e s t e p s o f the r e a c t i o n sequence, forming the a c t i v a t e d hydrocarbon species. These undergo subsequent s t e p s a n u c l e o p h i l i c a t t a c k by l a t t i c e oxygen i o n s 0 , w h i c h are n u c l e o p h i l i c r e a g e n t s w i t h no oxidizing properties. They are inserted i n t o the activated hydrocarbon m o l e c u l e by n u c l e o p h i l i c a d d i t i o n forming an oxygenated product, which a f t e r desorption leaves an oxygen vacancy a t the s u r f a c e o f the catalyst. Such v a c a n c i e s are then f i l l e d w i t h oxygen from the gas phase, s i m u l t a n e o u s l y r e o x i d i z i n g the reduced cations. I t s h o u l d be noted t h a t i n c o r p o r a t i o n o f oxygen from the gas phase i n t o the o x i d e s u r f a c e does not n e c e s s a r i l y take p l a c e a t the same s i t e from where s u r f a c e oxygen i s i n s e r t e d i n t o the h y d r o c a r b o n m o l e c u l e a f t e r b e i n g t r a n s p o r t e d t h r o u g h the l a t t i c e . In the case o f complex hydrocarbon m o l e c u l e s , the n u c l e o p h i l i c a d d i t i o n o f oxygen may take p l a c e a t d i f f e r e n t s i t e s o f the molecule. It will take p l a c e at a site w h i c h i s made most e l e c t r o p o s i t i v e by a p p r o p r i a t e bonding o f the m o l e c u l e at the a c t i v e c e n t e r o f the c a t a l y s t . When a d s o r p t i o n o f the hydrocarbon m o l e c u l e r e s u l t s i n the f o r m a t i o n o f a r a d i c a l , i n t e r a c t i o n between adsorbed m o l e c u l e s i s favoured and d i m e r i z a t i o n o r p o l y m e r i z a t i o n occurs. When the adsorbed species are negatively charged,

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

1.

HABER

1

Catalysis by Transition Metal Oxides

i s o m e r i z a t i o n may be f a v o u r e d . T h i s type o f product o b t a i n e d depends on the type and p r o p o r t i o n o f d i f f e r e n t a c t i v e c e n t e r s at the c a t a l y s t s u r f a c e as w e l l as on the ratio o f the r a t e o f d e s o r p t i o n o f the p a r t i c u l a r intermediate product to the r a t e o f i t s t r a n s f o r m a t i o n i n t o the i n t e r m e d i a t e complex next i n the s e r i e s (upper r i g h t p a r t o f scheme i n F i g u r e 1 ) . These r a t e s may s t r o n g l y depend on the degree o f s u r f a c e r e d u c t i o n a t t a i n e d i n the course o f the r e a c t i o n , as i s the case w i t h the c a r b o x y l a t e complex, w h i c h i s an i n t e r m e d i a t e i n the o x i d a t i o n o f aldehydes t o c a r b o x y l i c a c i d s . On o x i d i z e d s u r f a c e s , t h i s complex d e s o r b s i n the form o f an a c i d , whereas on a reduced surface it undergoes decarboxylation, r e s u l t i n g i n the d e p o s i t i o n o f coke ( 1 1 ) . R e a c t i o n s o f c a t a l y t i c o x i d a t i o n may be thus d i v i d e d i n t o two g r o u p s : e l e c t r o p h i l i c o x i d a t i o n , p r o c e e d i n g t h r o u g h the a c t i v a t i o n of o x y g e n , and n u c l e o p h i l i c o x i d a t i o n , i n w h i c h a c t i v a t i o n o f the hydrocarbon molecule i s the first step, f o l l o w e d by c o n s e c u t i v e s t e p s o f n u c l e o p h i l i c oxygen i n s e r t i o n and hydrogen a b s t r a c t i o n . They may be c o n v e n i e n t l y s y s t e m a t i z e d according to the number o f elementary s t r u c t u r a l transformations i n t r o d u c e d i n t o the r e a c t i n g molecule (Table I ) . An a c t i v e and s e l e c t i v e c a t a l y s t f o r o x i d a t i o n o f hydrocarbons t o oxygenated products with retention o f double bonds or a r o m a t i c i t y s h o u l d thus have the f o l l o w i n g p r o p e r t i e s : - a c t i v a t i o n o f the h y d r o c a r b o n m o l e c u l e by m o d i f y i n g i t s bonds and generating at appropriate sites the electron distribution f a v o u r i n g the n u c l e o p h i l i c a t t a c k o f oxygen; - e f f i c i e n t i n s e r t i o n of the n u c l e o p h i l i c l a t t i c e oxygen i n t o the a c t i v a t e d hydrocarbon molecule; - r a p i d i n t e r a c t i o n w i t h gas phase oxygen to r e p l e n i s h the l a t t i c e oxygen and t r a n s p o r t it through the lattice to a c t i v e s i t e s , where the i n s e r t i o n t a k e s p l a c e ; - s h o u l d not g e n e r a t e e l e c t r o p h i l i c oxygen s p e c i e s . The fundamental q u e s t i o n a r i s e s as to how these p r o p e r t i e s a r e r e l a t e d t o the s o l i d s t a t e c h e m i s t r y o f o x i d e s . A c t i v a t i o n of organic molecule C l a s s i c a l s t u d i e s o f Adams (\2_, JL3) , u s i n g d e u t e r a t e d p r o p y l e n e and C^-Cg o l e f i n s , and of S a c h t l e r and de Boer (14) , w i t h C - l a b e l l e d p r o p y l e n e s , showed t h a t a c t i v a t i o n o f the o l e f i n m o l e c u l e c o n s i s t s o f the a b s t r a c t i o n o f α - h y d r o g e n and the f o r m a t i o n o f a symmetric a l l y l i c intermediate. Conclusions concerning the r o l e o f the c a t i o n i c and a n i o n i c s u b l a t t i c e s o f complex o x i d e c a t a l y s t s h a v i n g an o x y s a l t c h a r a c t e r , such as m o l y b d a t e s , t u n g s t a t e s , e t c . , i n the i n i t i a l α-hydrogen abstraction and the subsequent s t e p s o f the o x i d a t i o n p r o c e s s , were drawn by comparing the b e h a v i o u r o f o^3 and MoO^ f o r the r e a c t i o n of propylene and a l l y l i o d i d e ( 1 5 ) . When a l l y l i o d i d e was passed over Mo0~, p r a c t i c a l l y t o t a l c o n v e r s i o n was observed a l r e a d y at 310°C w i t h 98% s e l e c t i v i t y t o a c r o l e i n . Under the same c o n d i t i o n s , MoO^ was c o m p l e t e l y i n a c t i v e w i t h r e s p e c t t o propylene. On c o n t a c t i n g a l l y l i o d i d e w i t h B i 0 « , t o t a l c o n v e r s i o n a t 310°C was a l s o observed. However, i n t h i s case 70% of the products formed were 1,5-hexadiene w i t h p r a c t i c a l l y no a c r o l e i n being detected. 1,5-hexadiene was also the main product B i

9

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2

2

1. With double bond f i s s i o n 1.1. oxidation of olefins to oxides 1.2. oxyhydratian of olefins to saturated ketones 2. With C-C bond f i s s i o n 2.r. oxidation of olefins to saturated aldehydes 2.2. oxidation of aromatics to anhydrides and acids with ring rupture 3 · Total oxidation to C0 +H 0

E l e c t r o p h i l i c Oxidation Reaction Type

2

5

3

4

2

Co 0 CuCo^ CuCr 0^

2°5 V 0 -Mo0

V

2

3

Sn0 -MoO^

Catalyst 1· Without introduction of heteroatom 1.1. oxidative dehydrogenation of alkanes and alkenes to dienes 1.2. oxidative dehydrodimerization and dehydrocyclization of alkenes 2. With introduction of heteroatom 2.1. introduction of heteroatom into hydrocarbon chain 2.1.1. introduction of oxygen a. oxidation of olefins to unsaturated aldehydes and ketones b. oxidation of alkylaromat i c s to aldehydes 2.1.2. introduction of nitrogen a. ammoxidation of olefins to n i t r i l e s 2.2. introduction of heteroatom into acyl group a. oxidation of~aldehydes to acids b. oxidation of alkylaroma­ t i c s to anhydrides

Nucieophilic Oxidation Reaction Type

Table I. Heterogeneous Oxidation of Hydrocarbons

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

2°5

2

3

2

3

3

2

2

5

v o -Tio

3

Ni0-Mo0

3

2

4

3

uo -sb o

2

Bi 0 -Mo0

2

3

3

Bi 0 -Mo0

3

Mo0 -Al 0

P

3

Bi 0 -Mo0 -

Catalyst

5

%

η

H

π χ m g

5

Ο r

00

1.

HABER

Catalysis by Transition Metal Oxides

i n the r e a c t i o n o f p r o p y l e n e over B i 0 . These r e s u l t s c l e a r l y i n d i c a t e d t h a t a c t i v a t i o n o f the o l e f i n m o l e c u l e , w h i c h c o n s i s t s o f the a b s t r a c t i o n o f α - h y d r o g e n and the f o r m a t i o n o f an a l l y l i c s p e c i e , t a k e s p l a c e on c a t i o n i c a c t i v e c e n t e r s , whereas the MoO^ o r molybdate a n i o n i c s u b l a t t i c e i s responsible f o r the i n s e r t i o n o f oxygen i n t o the h y d r o c a r b o n m o l e c u l e . Indeed, quantum c h e m i c a l c a l c u l a t i o n s o f the s y s t e m , composed of c o b a l t i o n i n o c t a h e d r a l coordination of f i v e oxygen atoms and p r o p y l e n e m o l e c u l e as the s i x t h l i g a n d , have shown (J^o, 17) t h a t on approaching the p r o p y l e n e m o l e c u l e to the p l a n e o f the a c t i v e c e n t e r , the C-H bond i s continuously destabilized with an interaction appearing s i m u l t a n e o u s l y between t h i s hydrogen and the oxygen o f the a c t i v e c e n t e r and i n c r e a s i n g u n t i l the t o t a l energy a t t a i n e d a minimum and an i n t e r m e d i a t e complex i s formed. When the a l l y l specie i s removed, the energy r e q u i r e d f o r t h i s p r o c e s s i s much s m a l l e r than t h a t needed to remove the whole p r o p y l e n e m o l e c u l e . The OH bond i s further strengthened, and its energy attains the value c h a r a c t e r i s t i c o f a normal the OH g r o u p . Thus, i t may be c o n c l u d e d t h a t on c o n t a c t i n g p r o p y l e n e w i t h the s u r f a c e o f the t r a n s i t i t i o n m e t a l o x i d e , r e a c t i v e c h e m i s o r p t i o n takes p l a c e , i n w h i c h the C - H bond i s broken and an a b s o r p t i o n complex w i t h a l l y l s p e c i e s as one o f the l i g a n d s i s formed. C o n s i d e r a b l e charge t r a n s f e r takes p l a c e from the allyl species onto the t r a n s i t i o n metal orbitale, r e n d e r i n g the s p e c i e s p o s i t i v e , the charge d i s t r i b u t i o n depending on the type o f m e t a l , i t s v a l e n c e s t a t e and the l i g a n d f i e l d strength (18). E x p e r i m e n t s , w i t h azopropene and a l l y l a l c o h o l , c a r r i e d out by G r a s s e l l i e t a l . (19-22) , demonstrated t h a t a f t e r the f i r s t hydrogen a b s t r a c t i o n , i n s e r t i o n o f oxygen t a k e s p l a c e , and o n l y then the second hydrogen i s a b s t r a c t e d r e s u l t i n g i n the f o r m a t i o n o f the a c r o l e i n p r e c u r s o r . An i m p o r t a n t q u e s t i o n may be r a i s e d a t t h i s p o i n t as to what i s the s t r u c t u r e o f the c a t i o n i c a c t i v e c e n t e r a c t i v a t i n g the h y d r o c a r b o n m o l e c u l e . Can e v e r y c a t i o n s i t u a t e d a t the s u r f a c e o f the g i v e n o x i d e perform the r o l e o f the a c t i v e c e n t e r , o r must t h i s c a t i o n be l o c a l i z e d a t some s p e c i a l s i t e o f the s u r f a c e , and how does i t s a c t i v i t y depends on t h i s l o c a t i o n ? I n o r d e r to o b t a i n some r e l e v a n t i n f o r m a t i o n about t h i s q u e s t i o n , i s o l a t e d bismuth i o n s were supported at the surface o f Mo0~ ( 2 3 ) . Taking i n t o account the v e r y h i g h e f f i c i e n c y o f MoO^ f o r i n s e r t i n g oxygen i n t o a c t i v a t e d h y d r o c a r b o n m o l e c u l e s , i t might be assumed t h a t e v e r y p r o p y l e n e m o l e c u l e a c t i v a t e d a t the i s o l a t e d b i s m u t h i o n would p i c k up oxygen and be c o n v e r t e d to a c r o l e i n . The number o f a c r o l e i n m o l e c u l e s would thus be a measure o f the number o f p r o p y l e n e molecules activated by the known number o f b i s m u t h ions. Measurements o f the p r o p y l e n e o x i d a t i o n a c t i v i t y as a f u n c t i o n o f the s u r f a c e c o n c e n t r a t i o n o f b i s m u t h i o n s , expressed as t h e i r number per s u r f a c e molybdenum atom, a r e shown i n F i g u r e 2 . When a l l y l i o d i d e was i n t r o d u c e d , the y i e l d o f a c r o l e i n was c o n s t a n t and independent o f the b i s m u t h coverage c o n f i r m i n g the assumption t h a t once a l l y l r a d i c a l s have been g e n e r a t e d they r a p i d l y undergo a n u c l e o p h i l i c a t t a c k by o x i d e i o n s from the l a t t i c e o f MoO^. On i n t r o d u c i n g a m i x t u r e on propylene and oxygen, the a c t i v i t y a t l o w b i s m u t h s u r f a c e coverages increased p r o p o r t i o n a l l y to the s u r f a c e c o n c e n t r a t i o n o f b i s m u t h , the turnover frequency per b i s m u t h i o n 2

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

9

3

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

S O L I D STATE C H E M I S T R Y IN CATALYSIS

10

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

b e i n g thus c o n s t a n t . At h i g h e r b i s m u t h c o v e r a g e s , the a c t i v i t y l e v e l e d o f f because b i s m u t h i o n s became l o c a t e d too c l o s e to each o t h e r to o p e r a t e s i m u l t a n e o u s l y i n the r e a c t i o n . I t i s noteworthy t h a t the y i e l d o f a c r o l e i n observed at the p l a t e a u i s s i m i l a r to t h a t observed i n the case o f the Bi^CMoO^)^ phase. This c l e a r l y demonstrates t h a t the a b i l i t y t o a c t i v a t e the h y d r o c a r b o n m o l e c u l e i s r e l a t e d t o the i n d i v i d u a l bismuth c a t i o n s and t h e i r n e a r e s t neighbors. These a c t i v e c e n t e r s f u n c t i o n independent o f whether they a r e d i s t r i b u t e d randomly as a monolayer at the s u r f a c e of MoO^ o r form the s u r f a c e o f the b i s m u t h molybdate phase w i t h l o n g range order. I t s h o u l d a l s o be noted t h a t the amount o f C0« formed remains c o n s t a n t i n d i c a t i n g t h a t the s i d e r e a c t i o n or total o x i d a t i o n proceeds at some other sites, r e s u l t i n g from the p r o p e r t i e s o f MoO^ i t s e l f . I n s e r t i o n o f Oxygen As a l r e a d y m e n t i o n e d , e x p e r i m e n t s i n w h i c h a l l y l compounds were r e a c t e d w i t h complex o x i d e s , such as molybdatess or t u n g s t a t e s , showed t h a t i t i s MoO^, WO^, o r the corresponding anionic sublattices w h i c h perform the i n s e r t i o n o f oxygen i n t o the h y d r o c a r b o n m o l e c u l e . The q u e s t i o n may t h u s be r a i s e d as to w h i c h p r o p e r t i e s of these oxides are r e s p o n s i b l e f o r the v e r y h i g h a c t i v i t y and s e l e c t i v i t y i n the i n s e r t i o n o f oxygen. One o f the f e a t u r e s common to a l l group V , V I , and V I I t r a n s i t i o n m e t a l o x i d e l a t t i c e s , known t o be good c a t a l y s t s for s e l e c t i v e oxidation of hydrocarbons, i s t h e i r a b i l i t y t o form shear s t r u c t u r e s w h i c h relates to the facile p l a n a r rearrangement of coordination p o l y h e d r a and t h e i r p a r t i c u l a r s p a c i a l arrangement. I n the o c t a h e d r a l c o o r d i n a t i o n o f o x i d e i o n s , 2 3 i n w h i c h d sp h y b r i d i z e d o r b i t a l s a r e used by the m e t a l to form σ - b o n d s , the r e m a i n i n g d ,d and d o r b i t a l s o f group V , V I , and χζ yζ V I I m e t a l s extend f a r enough t o c o n s i d e r a b l y o v e r l a p w i t h π ρ o r b i t a l s o f o x y g e n , and the p o s i t i o n o f t h e i r redox p o t e n t i a l r e l a t i v e to the a n i o n v a l e n c e band edge are f a v o u r a b l e f o r the bond f o r m a t i o n ( 2 4 ) . As a r e s u l t , π - b o n d s w i t h oxygen i o n s a r e formed and the c a t i o n s become d i s p l a c e d from the c e n t e r s o f o c t a h e d r a towards t e r m i n a l oxygen atoms. Large displacement p o l a r i z a b i l i t i e s g i v e r i s e to h i g h r e l a x a t i o n e n e r g y , w h i c h d e c r e a s e s the c a t i o n c a t i o n r e p u l s i o n s opposing the formation of a structure with shorter metal-metal distance (25). Thus, removal of oxygen i o n s from the l a t t i c e o f t h e s e o x i d e s r e s u l t s i n the f o r m a t i o n o f ordered a r r a y s o f oxygen v a c a n c i e s , f o l l o w e d by a v e r y f a c i l e rearrangement o f the l a y e r s o f i n i t i a l l y c o r n e r - l i n k e d m e t a l oxygen o c t a h e d r a i n t o an arrangement of edge-linked octahedra, r e s u l t i n g i n the f o r m a t i o n o f a shear plane ( F i g u r e 3 ) . A h y p o t h e s i s was advanced (26) t h a t the easy e v o l u t i o n o f one oxygen i o n on the t r a n s f o r m a t i o n from c o r n e r - l i n k e d to e d g e - l i n k e d arrangement o f m e t a l - o x y g e n o c t a h e d r a may be one o f the f a c t o r s c r e a t i n g the a b i l i t y of these s t r u c t u r e s to i n s e r t oxygen i n t o the o r g a n i c molecule i n processes of s e l e c t i v e o x i d a t i o n of hydrocarbons. S t u d i e s o f a l l y l i o d i d e a c t i v i t y on d i f f e r e n t t u n g s t e n o x i d e s seem to confirm t h i s hypothesis (27). The experiments were c a r r i e d out w i t h two groups of tungsten oxides: those i n w h i c h shear

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

1.

HABER

11

Catalysis by Transition Metal Oxides

30 * -Ό

I

ι

•

Acrolein from allyl iodide. ° " ι—^-n

•

~ω

2_ ^_2_

π.

20

(

s l

Acrolein from propylene

ko

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

10 Ιο

C0

"Π '111UI

Ηi

—«

0.5

—

2

«

1

1

1.0

1,5

2.0

ι_Κ « 1 20.0 Bi (Mo0 ) 2

4

F i g u r e 2 . Y i e l d of a c r o l e i n and CO i n o x i d a t i o n o f p r o p y l e n e and a l l y l i o d i d e as f u n c t i o n of the coverage o f MoO w i t h bismuth i o n s . (Reproduced w i t h p e r m i s s i o n from Ref. 23.J 3

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

3

SOLID STATE CHEMISTRY IN CATALYSIS

12

a

n

d

s t r u c t u r e s a r e formed on r e d u c t i o n (WO^, ^ 2 0 ° 5 9 ^ those w h i c h do not show t h i s phenomenon (W. o 0 , , W C L ) , as may be seen from t h e i r structures i l l u s t r a t e d i n Figure 4. The s e l e c t i v i t y o f the a l l y l i o d i d e r e a c t i o n to a c r o l e i n as a f u n c t i o n o f the number o f p u l s e s i n t r o d u c e d i n t o the r e a c t o r are seen i n F i g u r e 5 . For comparison, r e s u l t s o b t a i n e d w i t h MoO^ a r e a l s o shown. I n the case o f 20^59 w h i c h , s i m i l a r l y t o MoO^, i s a b l e to g e n e r a t e shear p l a n e s on i n t e r a c t i o n w i t h the r e d u c i n g atmosphere, a c r o l e i n appears i n the p r o d u c t s a f t e r the f i r s t few p u l s e s . On the o t h e r hand, i n the case o f samples o f W . 0 ^ and WO-, w h i c h a r e u n a b l e to form shear s t r u c t u r e s , p r a c t i c a l l y no a c r o l e i n was p r e s e n t i n the p r o d u c t s o f the r e a c t i o n . L e t us now r e t u r n to t h g s u r f a c e o f a molybdate c a t a l y s t . Due to the d i s p l a c e m e n t o f Mo i o n s towards t e r m i n a l oxygens the b r i d g i n g oxygens become more b a s i c and thus more r e a c t i v e i n the nucleophilic attack on the a c t i v a t e d h y d r o c a r b o n m o l e c u l e . On r a i s i n g the temperature, the rearrangement o f the c o r n e r - l i n k e d m e t a l - o x y g e n p o l y h e d r a i n t o edge l i n k e d a r r a y s proceeds more and more r e a d i l y , and p r o v i d e s a f a c i l e and e f f i c i e n t r o u t e f o r the a d d i t i o n on a n u c l e o p h i l i c l a t t i c e oxygen to the h y d r o c a r b o n m o l e c u l e w i t h o u t the g e n e r a t i o n o f p o i n t d e f e c t s , w h i c h c o u l d be i n v o l v e d i n the f o r m a t i o n o f e l e c t r o p h i l i c oxygen s p e c i e s . Strong e v i d e n c e s u p p o r t i n g t h i s model i s p r o v i d e d by the ESR s t u d i e s of Mo0~ i n the c o u r s e o f i t s i n t e r a c t i o n w i t h d i f f e r e n t atmospheres ( 2 8 ) . As an example, F i g u r e 6 shows the ESR s p e c t r a o f MoO^ a f t e r o u t g a s s i n g the 430°C f o r 5 m i n . ( c u r v e A) and f o r 35 m i n . ( c u r v e B ) . A n a l y s i s o f thg v a l u e s o f the g - t e n s o r r e v e a l s the appearance o f two d i f f e r e n t Mo + c e n t e r s : type A , formed at an e a r l y stage of reduction, and c h a r a c t e r i z e d by r h o m b i c a l l y d i s t o r t e d square p y r a m i d a l s u r r o u n d i n g of n o n - a x i a l symmetry a l o n g the - d o u b l e bonded oxygen, and type B , o f d i s t o r t e d o c t a h e d r a l coordination, and appearing in strongly reduced samples. Comparison o f t h e s e r e s u l t s w i t h the s i t u a t i o n a t the s u r f a c e of Mo0« c r y s t a l l i t e s ( F i g u r e 7) l e a d s t o the c o n c l u s i o n t h a t the o n l y s u r f a c e oxygen i o n , w h i c h can be removed l e a v i n g reduced molydenum c a t i o n i n square p y r a m i d a l s u r r o u n d i n g w i t h double-bonded oxygen i n the o p p o s i t e apex, i s the surface oxygen b r i d g i n g two adjacent o c t a h e d r a i n the double s t r i n g o f e d g e - l i n k e d Mo-0 o c t a h e d r a . When c o n c e n t r a t i o n o f v a c a n c i e s i n c r e a s e s , c r y s t a l l o g r a p h i c shear t a k e s p l a c e ( F i g u r e 7 b ) , and Mo c a t i o n s assume the octahedral c o o r d i n a t i o n a l o n g the shear p l a n e s . It i s noteworthy t h a t on e x p o s i n g MoO^ t o a l l y l compounds o n l y the ESR spectrum o f type Β c e n t e r s a p p e a r s . T h i s i n d i c a t e s t h a t i n s e r t i o n o f oxygen i n t o the o r g a n i c m o l e c u l e i s accompanied by a s i m u l t a n e o u s rearrangement o f the c o o r d i n a t i o n o c t a h e d r a at the s u r f a c e o f Μο0~· Mo i o n s r e g i s t e r e d i n the ESR measurement c o n s t i t u t e o n l y ^ i s m a l l f r a c t i o n o f the reduced species, the m a j o r i t y b e i n g Mo i o n s , w h i c h as non-Kramers i o n s are not expected t o g i v e an ESR s i g n a l . As these i o n s a r e l o c a t e d i n the shear planes i n edgel i n k e d o c t a h e d r a , Mo-Mo bonds are formed as r e v e a l e d by the XPS s t u d i e s (_26, 2 9 ) . UV p h o t o e l e c t r o n spectra shown i n F i g u r e 8 indicate t h a t these c l u s t e r s of tetravalent molybdenum i o n s c o n s t i t u t e energy l e v e l s s i t u a t e d i n the f o r b i d d e n energy gap o f the o x i d e (_30). M o 0 , w h i c h has been o x i d i z e d at 4 0 0 ° C , shows the g

W

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

+

3

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

HABER

Catalysis by Transition Metal Oxides II

α

b

χ χ χχ Μ IXχχ χ χ χ χ χ χ χ χ χ χ U χχ χ χχ χ χ χ χ χχ χ χ χχ χ χ χχ χ χχ χχ y χχ

M

χ Χ] IX χ PI χ Χχ χ χ IX χ χ XI Χ Xχ χ χ χ χ χ M y χ Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

IV

F i g u r e 4.

Structures of d i f f e r e n t

tungsten W

oxides:

I - WO , I I - Q ° > H I > " 18°49* P ^p e r m i s s i o n from R e r . 27. C o p y r i g h t 1 9 8 J , Academic P r e s s . W

W O

2

5 8

I

V

R

E

R

O

D

U

C

E

D

T H

2

Figure 5. S e l e c t i v i t y to acrolein on i n t e r a c t i o n of a l l y l i o d i d e w i t h Mo0 and different tungsten oxides (17). Reproduced w i t h p e r m i s s i o n from R e f . 27. C o p y r i g h t 1983, Academic P r e s s . 3

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

S O L I D STATE C H E M I S T R Y IN CATALYSIS

F i g u r e 7. G e n e r a t i o n o f oxygen v a c a n c i e s Mo0 s t r u c t u r e .

and shear p l a n e s i n

o

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

1.

HABER

Catalysis by Transition Metal Oxides

15

v a l e n c e band w i t h two maxima at 5.0 and 7 . 6 eV ( c u r v e I A ) . Its edge i s o b s e r v a b l e a t a b i n d i n g energy o f about 2.8 eV. After o u t g a s s i n g ( c u r v e IB) l o c a l energy l e v e l s appear at about 0.9 eV and 2.0 eV. S i m i l a r l e v e l s are a l s o formed on o u t g a s s i n g the Bi^MoO^, as i n d i c a t e d by c u r v e I I B . These l e v e l s have a donor c h a r a c t e r as the s u b s t i t u t i o n o f h i g h e r v a l e n t i o n s i n the l a t t i c e s i t e s o f an o x i d e by l o w e r v a l e n t i o n s r e s u l t s i n the appearance o f η - t y p e semiconductor. A g e n e r a l c o n c l u s i o n may thus be f o r m u l a t e d t h a t the a b i l i t y o f group V , V I , and V I I t r a n s i t i o n m e t a l o x i d e s to form d i f f e r e n t types o f bonding between c o o r d i n a t i o n p o l y h e d r a p l a y s an i m p o r t a n t role i n determining t h e i r c a t a l y t i c properties by p r o v i d i n g a f a c i l e r o u t e f o r i n s e r t i o n o f oxygen i n t o an o r g a n i c m o l e c u l e . The mechanism of such i n s e r t i o n i s shown i n F i g u r e 9 ( 3 1 ) . Here i n c o n t r a s t to those o x i d e s i n w h i c h the d e s o r p t i o n o f an oxygenated product r e s u l t s i n the g e n e r a t i o n o f an oxygen vacancy at considerable expenditure of energy, the d e s o r p t i o n i s accompanied by the s i m u l t a n e o u s f a c i l e rearrangement o f o c t a h e d r a . A f t e r the g i v e n elementary s t e p o f the c a t a l y t i c r e a c t i o n has taken p l a c e w i t h the p a r t i c i p a t i o n o f the l a t t i c e oxygen and f o r m a t i o n o f a n u c l e u s o f the shear p l a n e , the a c t i v e c e n t e r at the s u r f a c e i s l e f t i n the reduced state. Before such an elementary s t e p can be r e p e a t e d , the a c t i v e c e n t e r must be r e o x i d i z e d . T h i s r e o x i d a t i o n can be r e a l i z e d e i t h e r by i n c o r p o r a t i o n o f oxygen from the gas phase o r by d i f f u s i o n o f oxygen i o n s from the b u l k . Depending on the r a t e o f such r e g e n e r a t i o n o f the a c t i v e c e n t e r , i t s "dead time" may be s h o r t o r l o n g . I n the case o f o x i d e s such as b i s m u t h m o l y b d a t e , the m o b i l i t y o f l a t t i c e oxygen i s h i g h ( 3 2 , 3 3 ) , and r e g e n e r a t i o n by d i f f u s i o n from the b u l k o p e r a t e s v e r y e f f i c i e n t l y because r e o x i d a t i o n o f the l a t t i c e may t a k e p l a c e at c e n t e r s d i f f e r e n t from those p a r t i c i p a t i n g i n the r e a c t i o n . Under such c o n d i t i o n s , the "dead time" of a c t i v e c e n t e r s i s v e r y s h o r t , the t u r n - o v e r frequency v e r y l a r g e , and the c a t a l y s t a c t i v i t y v e r y high. Thus, i t may be concluded t h a t parameters m o d i f y i n g the m o b i l i t y o f o x i d e i o n s o f a s o l i d l a t t i c e may s t r o n g l y i n f l u e n c e the c a t a l y t i c a c t i v i t y . The R o l e o f Surface Geometry As a l r e a d y mentioned i n t r a n s i t i o n m e t a l o x i d e s o f group V , V I , and V I I , the appearance o f a π - b o n d component o f the m e t a l - a n i o n bonding r e s u l t s i n the d i s p l a c e m e n t o f the c a t i o n from the c e n t e r of site symmetry w h i c h s t a b i l i z e s the l a y e r e d arrangement o f c o o r d i n a t i o n p o l y h e d r a and extended d e f e c t s shear and b l o c k structures. As the charge d e n s i t y and hence the acid-base character of oxide ions i s s t r o n g l y i n f l u e n c e d by the m e t a l - o x y g e n s e p a r a t i o n , the d i s p l a c e m e n t causes the d i f f e r e n t i a t i o n o f o x i d e i o n s i n v a r i o u s c r y s t a l l o g r a p h i c p o s i t i o n s i n r e s p e c t to t h e i r acid-base properties. Simultaneously, redox p r o p e r t i e s are m o d i f i e d as m a n i f e s t e d by the dependence o f the work f u n c t i o n on the type o f the c r y s t a l l o g r a p h i c p l a n e . I t may be c o n c l u d e d t h a t i n the case o f such o x i d e s the c a t a l y t i c p r o p e r t i e s w i l l depend on the g e o m e t r i c a l s t r u c t u r e o f the s u r f a c e . Indeed, experimental d a t a c o l l e c t e d i n the l a s t few y e a r s clearly indicate that

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

S O L I D STATE C H E M I S T R Y IN CATALYSIS

and B i M o 0 (II): A - o x i d i z e d at 470RC f o r 1 ~ΊΊΓ at 1 atm of oxygen; Β - o u t g a s s e d f o r 10 hr at 470RC; C - c o n t a c t e d w i t h p r o p y l e n e a t 440RC ( 2 0 ) . Reproduced w i t h p e r m i s s i o n from R e f . 30. C o p y r i g h t 1976, Academic P r e s s . 2

6

,;C=C_H

XDO

+

C H 0 3

4

Figure 9. Mechanism o f the insertion o f oxygen i n t o h y d r o ­ c a r b o n m o l e c u l e on o x i d e c a t a l y s t s w i t h p o i n t d e f e c t s (a) and shear s t r u c t u r e s ( b ) .

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

1.

HABER

17

Catalysis by Transition Metal Oxides

different c r y s t a l m o d i f i c a t i o n s o f o x i d e systems or d i f f e r e n t c r y s t a l planes o f g i v e n o x i d e c r y s t a l l i t e s may d i f f e r c o n s i d e r b l y in their catalytic properties. T a t i b o u e t and Germain ( 3 4 ) , u s i n g c r y s t a l l i t e s o f MoO^, prepared by s u b l i m a t i o n and a p p r o p r i a t e s e i v i n g showed t h a t on the b a s a l (010) p l a n e o f Mo0~, m e t h a n o l , i n presence o f o x y g e n , becomes dehydrogenated to formaldehyde, whereas on the (001) and (101) faces i t i s dehydrated t o d i m e t h y l e t h e r . This suggests that the (010) plane shows more pronounced redox p r o p e r t i e s , whereas the (001) and (101) planes behave as a c i d - b a s e surfaces. T h i s i s i n l i n e w i t h the r e s u l t o f e x t e n s i v e s t u d i e s o f V 0 and V 0 , - - T i 0 c a t a l y s t s (35). Namely, G a s i o r and Grzybowska ( 3 6 ; observed a d r a s t i c d e c r e a s e o f a c i d i t y o f V 0 ^ when i t was s u p p o r t e d on anatase a t s m a l l coverages n o t e x c e e d i n g a m o n o l a y e r . At h i g h e r l o a d i n g s , the a c i d i t y i n c r e a s e d w i t h V 0 ^ c o n t e n t , finally attaining t h a t o f the pure phase. Crystal structure a n a l y s i s o f V«0,- and anatase r e v e a l s a good c r y s t a l l o g r a p h i c f i t between the (001) c l e a v a g e p l a n e o f anatase and the (001) b a s a l plane o f V«0^ c o n t a i n i n g the V=0 groups s t i c k i n g out p e r p e n d i c u l a r t o the s u r f a c e . This structure may be assumed to p r e v a i l i n the monolayer o f V 0 ^ on a n a t a s e . However, such c a t a l y s t s show no acidity. T h e r e f o r e , i t may be concluded t h a t on pure V 0 _ the acid-base properties are located mainly at the s i d e p l a n e s \ 1 1 0 ) and ( 1 0 0 ) . T h i s has a d i r e c t b e a r i n g on the c a t a l y t i c p r o p e r t i e s . Thus, G a s i o r and Machej (37) s t u d i e d the c a t a l y t i c a c t i v i t y o f V 0 , samples o f d i f f e r e n t c r y s t a l h a b i t and found t h a t p l a t e - l i k e c r y s t a l l i t e s e x p o s i n g m a i n l y the b a s a l (001) planes w i t h V=0 groups show v e r y h i g h s e l e c t i v i t y i n the o x i d a t i o n o f o - x y l e n e to p h t h a l i c a n h y d r i d e , whereas i n the c a s e o f n e e d l e - l i k e c r y s t a l l i t e s w i t h predominance o f (110) and (100) side planes mainly t o t a l o x i d a t i o n to C0« i s o b s e r v e d . It is interesting t h a t a t v a r i a n c e w i t h these c o n c l u s i o n s , V o l t a e t . a l . ( 3 8 , 3 9 ) , s t u d y i n g o r i e n t e d samples o f MoO^, o b t a i n e d from i n t e r c a l a t i o n compounds o f MoCl^ - g r a p h i t e , c o n c l u d e d t h a t s e l e c t i v e o x i d a t i o n o f p r o p y l e n e to a c r o l e i n o r i s o b u t e n e t o m e t h a c r o l e i n i s m a i n l y c a t a l y s e d by the (100) s i d e f a c e s , whereas the (010) b a s a l r a c e i s r e s p o n s i b l e f o r t o t a l o x i d a t i o n . I t s h o u l d be remembered, however, t h a t selective oxidation i s a multistep p r o c e s s , c o n s i s t i n g of the c o n s e c u t i v e a b s t r a c t i o n s o f hydrogen atoms and i n s e r t i o n o f oxygen atoms. As a l r e a d y d e s c r i b e d , experiments w i t h a l l y l compounds (15) showed t h a t MoO^ l a t t i c e efficiently inserts n u c l e o p h i l i c l a t t i c e oxygen i o n s i n t o the a c t i v a t e d h y d r o c a r b o n m o l e c u l e but has o n l y a l i m i t e d a b i l i t y to a c t i v a t e the h y d r o c a r b o n , t h i s step being rate determining i n s e l e c t i v e o x i d a t i o n of o l e f i n s . Therefore, determination of c a t a l y t i c a c t i v i t y i n the o x i d a t i o n o f o l e f i n s can o n l y y i e l d l i m i t e d i n f o r m a t i o n about the mechanism o f the r e a c t i o n . Indeed, experiments on the i n t e r a c t i o n of Mo0~ c r y s t a l l i t e o f d i f f e r e n t c r y s t a l h a b i t w i t h a l l y l i o d i d e seems to i n d i c a t e (40) t h a t i t i s the (010) b a s a l plane w h i c h i s r e s p o n s i b l e f o r the i n s e r t i o n o f oxygen i n t o the o r g a n i c m o l e c u l e by the s h e a r i n g mechanism d e s c r i b e d i n the p r e c e d i n g s e c t i o n . 2

2

2

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

2

2

2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

18

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

Dynamics o f the C a t a l y s t

Surface

T r a n s i t i o n metal o x i d e s b e l o n g to n o n - s t o i c h i o r a e t r i c compounds, t h e i r c o m p o s i t i o n depending on the e q u i l i b r i u m between the l a t t i c e and i t s c o n s i t u t e n t s i n the gas phase, i . e . i t i s a f u n c t i o n of oxygen p r e s s u r e . Due to the c o n t r i b u t i o n o f the s u r f a c e f r e e e n e r g y , the compostion o f the surface o f the s o l i d d i f f e r s from t h a t o f the b u l k , and the system i n e q u i l i b r i u m i s composed o f b u l k crystallites, their surface and the gas phase. However, e q u i l i b r a t i o n o f the gas phase w i t h the b u l k o f c r y s t a l l i t e s takes p l a c e o n l y a f t e r a n n e a l i n g at h i g h t e m p e r a t u r e s , when d i f f u s i o n o f l a t t i c e c o n s t i t u t e n t s becomes s u f f i c i e n t l y r a p i d . When an o x i d e , w h i c h has been e q u i l i b r i a t e d at a h i g h temperature i n oxygen at a g i v e n p r e s s u r e , i s then heated under a d i f f e r e n t pressure at a low temperature at w h i c h the d i f f u s i o n o f d e f e c t s i n the l a t t i c e i s s l o w , the new e q u i l i b r i u m comprises o n l y the s u r f a c e l a y e r . When h y d r o c a r b o n m o l e c u l e s , w h i c h have r e d u c i n g p r o p e r t i e s , a r e a l s o p r e s e n t i n the gas phase, a c e r t a i n degree o f r e d u c t i o n o f the s u r f a c e i s reached c o r r e s p o n d i n g t o a s t e a d y s t a t e i n w h i c h the r a t e o f r e d u c t i o n o f the s u r f a c e by h y d r o c a r b o n m o l e c u l e s becomes e q u a l to the r a t e o f i t s r e o x i d a t i o n by gas phase oxygen. When the c o m p o s i t i o n o f the gas phase i s changed, a c o r r e s p o n d i n g change o f the s u r f a c e compostion o c c u r s which i n t u r n may r e s u l t i n changes of c a t a l y t i c a c t i v i t y (40, 41). However, s e v e r a l o t h e r phenomena may a l s o take p l a c e , such as o r d e r i n g of d e f e c t s a t the s u r f a c e , surface transformations, and p r e c i p i t a t i o n o f new b i d i m e n s i o n a l s u r f a c e phases. They may r e s u l t i n the appearance o f new t y p e s o f a c t i v e centers at the surface o f the catalyst, d i r e c t i n g the catalytic reaction along a new pathway and thus profoundly i n f l u e n c i n g the s e l e c t i t y (3J^, 4 1 - 4 4 ) . The c a t a l y s t s u r f a c e i s i n a dynamic i n t e r a c t i o n w i t h the gas phase. Depending on the p r o p e r t i e s o f the m i x t u r e o f r e a c t a n t s o f the c a t a l y t i c r e a c t i o n , d i f f e r e n t surface phases may be formed at the s u r f a c e o f the c a t a l y s t , d i r e c t i n g the r e c t i o n a l o n g d i f f e r e n t reaction paths. Thus, when the s t e a d y s t a t e c o n d i t i o n s o f the r e a c t i o n are changed, the s t r u c t u r e o f the c a t a l y s t s u r f a c e a l s o may change, m o d i f y i n g the a c t i v i t y and s e l e c t i v i t y o f the c a t a l y s t itself. T h i s means t h a t i n the r a t e e q u a t i o n i t i s not o n l y the c o n c e n t r a t i o n term w h i c h depends on the p r e s s u r e o f r e a c t a n t s , but a l s o the r a t e c o n s t a n t . C o n c l u d i n g Remark I n the catalytic r e a c t i o n o f o r g a n i c m o l e c u l e s w i t h gas phase o x i d a n t s ( e . g . oxygen, s u l p h u r , chlorine) either the o x i d a n t i s a c t i v a t e d and performs an e l e c t r o p h i l i c a t t a c k , o r the o r g a n i c m o l e c u l e s are a c t i v a t e d and the r e a c t i o n proceeds i n c o n s e c u t i v e s t e p s o f hydrogen a b s t r a c t i o n and n u c l e o p h i l i c oxygen i n s e r t i o n . R e a c t i o n s o f c a t a l y t i c o x i d a t i o n may be thus d i v i d e d i n t o two groups: a . e l e c t r o p h i l i c o x i d a t i o n , i n w h i c h e p o x i d e s are formed i n case o f l i q u i d phase r e a c t i o n and d e g r a d a t i o n o f the c a r b o n s k e l e t o n takes p l a c e under c o n d i t i o n s o f heterogeneous r e a c t i o n , r e s u l t i n g i n the t o t a l o x i d a t i o n , and b . n u c l e o p h i l i c o x i d a t i o n , i n which products o f the successive n u c l e o p h i l i c i n s e r t i o n of appropriate anionic l a t t i c e constituents i n t o the o r g a n i c m o l e c u l e

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

1.

HABER

Catalysis by Transition Metal Oxides

19

are formed. T h i s i n s e r t i o n t a k e s p l a c e at the site o f the m o l e c u l e , w h i c h by i t s a p p r o p r i a t e bonding at the a c t i v e c e n t e r o f the catalyst, i s made most positive. The s t r u c t u r e o f the i n t e r m e d i a t e complex composed o f the r e a c t i n g m o l e c u l e and the a c t i v e c e n t e r thus d e t e r m i n e s the r e a c t i o n pathway and c o n s e q u e n t l y the s e l e c t i v i t y . The a b i l i t y t o a c t i v a t e the hydrocarbon m o l e c u l e i s r e l a t e d to the p r o p e r t i e s o f i n d i v i d u a l c a t i o n s and t h e i r n e a r e s t n e i g h b o u r s , constituting active centers. When d i s c u s s i n g the b e h a v i o u r o f an i n t e r m e d i a t e complex l o c a t e d a t the s u r f a c e of a s o l i d it i s n e c e s s a r y to take i n t o account the fact t h a t the occupancy o f d i f f e r e n t orbitals is determined by the c h e m i c a l p o t e n t i a l o f e l e c t r o n s i n the s o l i d , g i v e n by the p o s i t i o n o f the Fermi l e v e l . S h i f t i n g of t h i s p o s i t i o n e . g . , by i n t r o d u c t i o n o f a d d i t i v e s changes the o r b i t a l o c c u p a n c y , may i n t u r n change the r e a c t i v i t y o f bonds and modify the a c t i v i t y and s e l e c t i v i t y . T r a n s i t i o n m e t a l o x i d e s a r e n o n s t o i c h i o m e t r i c compounds. The n o n s t o i c h i o m e t r y may be i n t r o d u c e d e i t h e r by the g e n e r a t i o n o f p o i n t d e f e c t s o r by the change o f the mode o f l i n k a g e between the coordination polyhedra, r e s u l t i n g i n the f o r m a t i o n o f extended defects/shear structures. This latter way o f changing the s t o i c h i o m e t r y i s a c h a r a c t e r i s t i c f e a t u r e o f group V , V I , and V I I t r a n s i t i o n m e t a l o x i d e s and i s r e l a t e d to the presence o f the π - o r b i t a l component o f the m e t a l - o x y g e n bonds, r e s u l t i n g i n the d i s p l a c e m e n t o f the c a t i o n s from the c e n t e r o f s i t e symmetry, w h i c h s t a b i l i z e s the l a y e r e d arrangement o f c o o r d i n a t i o n p o l y h e d r a . As the charge d e n s i t y on o x i d e i o n s i s s t r o n g l y i n f l u e n c e d by the m e t a l - o x y g e n s e p a r a t i o n , the d i s p l a c e m e n t causes the d i f f e r e n t a t i o n o f o x i d e i o n s i n v a r i o u s c r y s t a l l o g r a p h i c p o s i t i o n s w i t h r e s p e c t to t h e i r redox and a c i d - b a s e p r o p e r t i e s . As a r e s u l t , the c a t a l y t i c p r o p e r t i e s s t r o n g l y depend on the g e o m e t r i c a l s t r u c t u r e o f the surface. D i f f e r e n t polymorphic m o d i f i c a t i o n s o r d i f f e r e n t c r y s t a l planes may d i f f e r c o n s i d e r a b l y i n t h e i r c a t a l y t i c behaviour. A g e n e r a l c o n c l u s i o n may be f o r m u l a t e d t h a t the a b i l i t y o f group V , V I and V I I t r a n s i t i o n m e t a l o x i d e s to form d i f f e r e n t t y p e s of bonding between c o o r d i n a t i o n p o l y h e d r a p l a y s an i m p o r t a n t r o l e i n determining t h e i r c a t a l y t i c properties by p r o v i d i n g a f a c i l e r o u t e f o r i n s e r t i o n o f oxygen i n t o an o r g a n i c m o l e c u l e . No oxygen v a c a n c i e s are formed and t h e g e n e r a t i o n o f e l e c t r o p h i l i c oxygen, w h i c h c o u l d i n i t i a t e the s i d e r e a c t i o n o f t o t a l o x i d a t i o n , i s thus eliminated. The c a t a l y s t s u r f a c e i s i n dynamic i n t e r a c t i o n w i t h the gas phase. Depending on the p r o p e r t i e s o f the m i x t u r e o f r e a c t a n t s o f the c a t a l y t i c r e a c t i o n , d i f f e r e n t surface phases may be formed at the s u r f a c e o f the c a t a l y s t , d i r e c t i n g the r e a c t i o n a l o n g d i f f e r e n t r e a c t i o n pathways. A change of the steady state-conditions i n f l u e n c e s the c a t a l y t i c r e a c t i o n , not o n l y d i r e c t l y t h r o u g h the c o n c e n t r a t i o n term i n the r a t e e q u a t i o n , but a l s o by m o d i f y i n g the p r o p e r t i e s o f the c a t a l y s t i t s e l f , i . e . the r a t e c o n s t a n t k . Thus, heterogeneous c a t a l y t i c systems s h o u l d not be t r e a t e d as two p h a s e s , but as t h r e e phase systems composed o f the gas phase, the s o l i d , and the surface region. The l a t t e r i s composed o f the s u r f a c e atoms o f the c a t a l y s t l a t t i c e i n t e r a c t i n g w i t h the adsorbed m o l e c u l e s o f the c a t a l y t i c r e a c t i o n .

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

20

SOLID STATE CHEMISTRY IN CATALYSIS

Literature 1.

2. 3. 4. 5.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.

Cited

Callahan, J. L.; Grasselli, R. K.; Milberger, E. C.; Strecker, H. A. Ind. Eng. Chem., Prod. Res. Develop. 1970, 134. Sheldon, R. Α.; Kochi, J. K. Adv. Catal. 1976, 25, 272. Apostol, I.; Haber, J.; Mlodnicka, T.; Poltowicz, J. J. Molecular Catal. 1982, 14, 197. Apostol, I.; Haber, J.; Mlodnicka, T.; Poltowicz, J. J. Molecular Catal. 1984, 26, 239. Apostol, I.; Haber, J.; Mlodnicka, T.; Poltowicz, J. Proc. 8th Intern. Congr. Catalysis, Berlin 1984, p. IV-497. Bielanski, Α.; Haber, J. Catal. Rev. 1979, 19, 1. Libre, J. M.; Barbaux, Y.; Grzybowska, B.; Bonnelle, J. P. React. Kinet. Catal. Lett. 1982, 20, 249. Haber, J. Proc. 5th Intern. Congr. Catalysis, Palm Beach 1972, p. 1006. Haber, J.; Grzybowska, B. J. Catal. 1973, 28, 489. Haber, J.; Pure and Appl. Chem. 1978, 50, 923. Haber, J.; Marczewski, J.; Stoch, J.; Unger, L. Proc. 6th Intern. Congr. Catalysis, London 1976, p. 827. Adams, C. R.; Jennings, I. J. J. Catal. 1963, 2, 63. Adams, C. R. Proc. 3rd Intern. Congr. Catalysis, Amsterdam 1964, p. 240. Sachtler, W. M. H.; de Boer, Ν. H. Proc. 3rd Intern. Congr. Catalysis, Amsterdam 1964, p. 252. Grzybowska, B.; Haber, J.; Janas, J. J. Catal. 1977, 49, 150. Haber, J.; Witko, M. J. Molecular Catal. 1980, 9, 399. Haber, J.; Witko, M. Acc. Chem. Res. 1981, 14, 1. Haber, J.; Sochacka, M.; Grzybowska, B.; Golebiewski, A. J. Molecular Catal. 1975, 1, 35. Burrington, J. D.; Grasselli, R. K. J. Catal. 1979, 59, 79. Grasselli, R. K.; Burrington, J. D.; Brazdil, J. F. Disc. Faraday Soc. 1981, 72, 203. Grasselli, R. K.; Burrington, J. D. Adv. Catal. 1981, 30, 133. Burrington, J. D.; Kartisek, C. T.; Grasselli, R. K. J. Catal. 1983, 81, 489. Bruckman, K.; Haber, J.; Wiltowski, T. in preparation. Goodenough, J. Proc. Solid State Chem. 1971, 5, 145. Catlow, C.A. In "Nonstoichiometric Oxides"; Sorenson O. T., Ed.; Academic Press: New York, 1981, Chap. 2. Haber, J. Proc. 2nd Intern. Conf. Chemistry and Uses of Molybdenum, Oxford 1976, p. 119. Haber, J.; Janas, J.; Schiavello, M.; Tilley, R. J. D. J. Catal. 1983, 82, 395. Haber, J.; Serwicka, E. in preparation. Haber, J.; Marczewski, J.; Stoch, J.; Unger, L. Ber. Bunsenges Phys. Chem. 1975, 79, 970. Grzybowska, B.; Haber, J.; Marczewski, W.; Unger, L. J. Catal. 1976, 42, 327. Haber, J. Proc. 3rd Intern. Conf. Chemistry and Uses of Molybdenum, AnnArbor 1979, p. 114.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

1. HABER 32. 33. 34. 35.

36. 37. 38.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

39. 40. 41. 42. 43. 44.

Catalysis by Transition Metal Oxides

21

Keulks, G. W.; Krenzke, L. D.; Notermann, T. M. Adv. Catal. 1978, 27, 183. Moro-oka, Y.; Veda, W.; Tanaka, S. Proc. 7th Intern. Congr. Catalysis, Tokyo 1980. Tatibouet, J. M.; Germain, J. E. J. Catal. 1981, 72, 375. Gasior, M.; Grzybowska, B.; In "Vanadia Catalysts for Oxidation of Aromatic Hydrocarbons"; Haber, J.; Grzybowska, B.; Ed.; Polish Scientific Publ. Krakow, 1984, p. 133. Gasior, M.; Gasior, I.; Grzybowska, B. Appl. Catal. 1984, 10, 87. Gasior, M.; Machej, T. J. Catal. 1983, 83, 472. Volta, J. C.; Forissier, M.; Theobald, F.; Pham, T. P. Disc. Faraday Soc. 1981, 72, 275. Volta, J. C.; Desquesnes, W.; Moraweck, B.; Tatibouet, J. M. Proc. 7th Intern. Congr. Catalysis, Tokyo 1980, p. 1398. Bruckman, K.; Haber, J.; Wiltowski, T. in preparation. Haber, J.; Stoch, J.; Wiltowski, T. Proc. 7th Intern. Congr. Catalysis., Tokyo 1980. Haber, J.; Stoch, J.; Wiltowski, T. React. Kinet. Catal. Lett. 1980, 13, 161. Haber, J. Proc. 4th Intern. Conf. Chemistry and Uses of Molybdenum, Golden, Colorado, 1982, p. 395. Haber, J. In "Surface Properties and Catalysis by Non-metals"; Bonnelle, J.P.; Delmon, B.; Derouane, E., Ed.; Reidel Publ. Co., Dordrecht, 1983, p. 1.

RECEIVED January 14, 1985

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2 Active Sites on Molybdate Surfaces, Mechanistic Considerations for Selective Oxidation, and Ammoxidation of Propene JANET N. ALLISON and WILLIAM A. GODDARD III

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch002

Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125

Molybdates involving various metal additives play a dominant role in such industrially important catalytic processes as selective oxidation (propene to acrolein) and ammoxidation (propene to acrylonitrile); however, the details of the reaction mechanism and of the surface sites responsible are yet quite uncertain. In order to establish the thermochemistry and detailed mechanistic steps involved with such reactions, we have performed ab initio quantum chemical calculations [generalized valence bond (GVB) and configuration interaction (CI)]. These studies indicate a special importance of multiple surface dioxo Mo sites (possessing two Mo-O double bonds and hence spectator oxo groups) arranged together so as to provide the means for promoting the sequence of transformations. Various catalysts based on molybdates have been used both for selective oxidation of propene to acrolein

and ammoxidation of propene to acrylonitrile,

Numerous experimental studies have provided mechanistic information about these catalytic reactions; however, there are as yet many uncertainties concerning the character of the active site and its relation to the details of the mechanism. In this paper we will use the results of ab initio 0097-6156/85/0279-0023$06.00/0 © 1985 American Chemical Society

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

24

S O L I D STATE C H E M I S T R Y IN CATALYSIS

quantum chemical calculations [generalized valence bond (GVB) and configuration interaction (CI)] to help analyze the details of the reaction mechanisms and the relation of various reaction steps to specific surface sites. In the following sections we discuss the principle of spectator oxo promotion that we find to play a crucial role in promoting particular reaction steps; we then examine the details of selective oxidation; and, finally, we outline preliminary results on ammoxidation . Spectator Oxo Effects

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch002

Molybdates lead to bulk structures involving either octahedral or tetrahedral coordination of oxygens about each Mo. On various surfaces, the most stable configurations for such molybdates are likely to be 0 II

-0

0-

.00 Α Λ

Mo

1

0

1 2 3 where 1 and 3 correspond to bulk octahedral sites and 2 to bulk tetrahedral sites. Here there are four (1) and (3) or two (2) single bonds to oxygen atoms that have a single bond to another Mo center, and one (1) and (3) or two (2) double bonds to oxygens that are not bonded to other Mo atoms. Typically the M-0 single bond lengths are ~1.95 Â and the Mo-0 double bond lengths are 1.67 to 1.73 Â. In addition, the octahedral site 1 would have a sixth oxygen neighbor at 2.2 to 2.4 Â (-L). All three surface structures are formally Mo^ and all involve Mo-0 double bonds. However, we find that these species lead to extremely different chemistry. Thus, in selective oxidation of propene, a critical step is trapping of an allyl radical at an Mo=0 bond. However, we find that only for species 2 is this process strongly exothermic. Δ

Η

(400·Ο

Δ 0

( k c a l / mol)

- Bi^CMoO^)^, Fe^CMoO^)^ and CoMoO^, ( a ) : examples o f o x y s a l t s b e l o n g i n g to trie second c l a s s o f c a t a l y s t s .

F i g u r e 6. C . S . mechanism (vo) P o . 2

2

in

the

transformation

of

y

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

VOPO,-

3.

COURTINE

47

Interfacial Effects in Mild Oxidation Catalysts

This synergetic effect can be e x p l a i n e d by t h e presence o f coherent boundaries whose f i r s t consequence i s t o lower the p o t e n t i a l b a r r i e r f o r e l e c t r o n t r a n s f e r between the n - s e m i c o n d u c t o r T i 0 and V 0 y The r e s u l t i n g e l a s t i c s t r a i n s on b o t h s i d e s o f t h e b o u n d a r i e s a l l o w t h e r e d u c t i o n o f V 0 ^ and the t r a n s m i s s i o n o f t h i s c o o p e r a t i v e t r a n s f o r m a t i o n to anatase w h i c h becomes r u t i l e . When t h i s l a s t transformation i s complete, the i n t e r f a c e v a n i s h e s and the r e a c t i o n s t o p s . b) Under c a t a l y t i c c o n d i t i o n s , t h e same r e d u c t i o n o f V 0 ^ o c c u r s but b e g i n s a t the s u r f a c e w h i l e T i 0 remains anatase s i n c e the temperature does n o t exceed 4 5 0 ° C . The coherent i n t e r f a c e i s not d e s t r o y e d s i n c e the s u c c e s s i v e s u b o x i d e s formed by C . S . mechanism have s i m i l a r c l e a v a g e p a t t e r n s t o V ? ^ 5 ( 7 6 ) . When the s t e a d y s t a t e i s reached under o - x y l e n e and a i r , t h e r e d u c t i o n s t o p s near V ^ O . ^ ( 3 0 ) , c o r r e s p o n d i n g to an o p t i m a l v a l u e o f t h e m i s f i t and o f trie s t r a i n f a c t o r ( 4 5 ) . A way t o v e r i f y this interpretation was t o expect the same phenomena i f T i 0 was r e p l a c e d by another support h a v i n g the same c r y s t a l l o g r a p h i c c o n f i g u r a t i o n . We have shown indeed t h a t the two experiments a) and b) c o u l d be repeated when V 0,- i s supported on A l N b 0 o r GaNbO, (78,79) (Figure 7 ) . The anatase to r u t i l e t r a n s f o r m a t i o n a l s o o c c u r s f o r M o 0 / T i 0 and CoMoO^/TiO- ( 7 9 , 8 0 ) . Other examples found i n the l i t e r a t u r e can be i n t e r p r e t e d i n the same way. I n t h e ammoxidation o f 3 - p i c o l i n e (81) on V 0 ^ / T i 0 ( r u t i l e ) , the maximum a c t i v i t y and s e l e c t i v i t y a r e o b t a i n e d i n n e a r l y the same c o m p o s i t i o n range as f o r o x i d a t i o n o f o - x y l e n e . The c a t a l y s t was prepared by h e a t i n g V 0e * 1150°C w i t h subsequent r e d u c t i o n by H a t 450°C f o r one h o u r . Under these c o n d i t i o n s , a n a t a s e t r a n s f o r m s i n t o r u t i l e and V 0 ^ melts. On c o o l i n g , e p i t a x i a l r e l a t i o n s h i p s a r e formed between (010) V 0 and (110) o f r u t i l e . Since the e l e c t r o n i c properties o f r u t i l e a r e v e r y c l o s e t o a n a t a s e , the same e x p l a n a t i o n s a p p l y . S i m i l a r i n t e r f a c i a l e f f e c t s cannot be expected f o r V ^ ^ - A K O ^ catalysts. The number o f exposed a c t i v e V-0 s p e c i e s d i f f e r depending upon t h e n a t u r e o f the s u p p o r t (70) , and t h e development of the l a y e r e d (010) c l e a v a g e plane o f V ° ^ i s n o t enhanced on Α1 0^ as i t i s by the presence o f coherent b o u n d a r i e s w i t h T i 0 anatase ( F i g u r e 8 ) . Recent s t u d i e s (68-71) have centered on vanadium o x i d e monolayers supported on a n a t a s e . EXAFS and XANES e x p e r i m e n t s have shown t h a t t e t r a h e d r a l vanadium s p e c i e s are anchored w i t h an i n t r i n s i c d i s o r d e r on the s u r f a c e o f a n a t a s e . A l t h o u g h i t seems t h a t s h o r t range matching e x i s t s w i t h some c r y s t a l l i n e faces o f the support ( w h i c h a r e not d e f i n e d ) , the s t r u c t u r a l p a t t e r n o f t h i s monolayer was found t o be d i f f e r e n t from t h e c l e a v a g e p l a n e (010) of c r y s t a l l i z e d V 0 ^ . T h i s r e s u l t c o u l d be expected by t a k i n g i n t o account the l a r g e i n f l u e n c e o f the s u r f a c e p o t e n t i a l o f anatase on the d i s t o r t i o n o f the vanadium e n v i r o n m e n t , w i t h the n o t i o n o f c l e a v a g e plane becoming m e a n i n g l e s s s i n c e i t depends o b v i o u s l y on the e x i s t e n c e o f a b u l k . The monolayer seems t o be a b o r d e r l i n e case as compared t o our experiments where h i g h e r c o n c e n t r a t i o n s o f c r y s t a l l i n e V 0c were u s e d . In order t o d e f i n e more c l o s e l y the c o n f i g u r a t i o n o t the a c t i v e s i t e s i n the i n t e r f a c i a l zone, f u r t h e r 2

2

2

2

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch003

2

2

9

4

3

2

2

a n c

a

n

a

t

2

a

s

e

a

t

2

2

2

2

5

2

2

2

2

2

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

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; Washington, D. C.Society: 20036 ACS Symposium Series; American Chemical Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch003

S O L I D STATE C H E M I S T R Y IN CATALYSIS

F i g u r e 7. C r y s t a l l o g r a p h i c f i t between (010) AINbO and a) ( 0 1 0 ) V O and b) (010) V ^ ^ . Reproduced w i t h p e r m i s s i o n from R e f . 7 8 . C o p y r i g h t 1980, Academic P r e s s , I n c . 2

5

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

3.

COURTINE

Interfacial Effects in Mild Oxidation Catalysts

work i s needed samples.

on

the

preparation

morphology of V 0 , - - T i 0 2

2

r

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch003

. « r red ox a) S i n g l e C a t a l y t i c A c t i v e

and

49

Phases

Except when the r a t e l i m i t i n g s t e p i s the d i f f u s i o n o f oxygen t h r o u g h the b u l k a t l o w e r temperature ( 5 8 , 7 6 , 8 3 , 8 4 ) , the c a t a l y t i c phase i s n e a r l y f u l l y o x i d i z e d . Molybdenum o x i d e c a t a l y s t f o r the dehydrogenation of alcohols (49,50), cobalt molybdate for d e h y d r o g e n a t i o n o f butane ( 2 0 , 2 1 ) and b i s m u t h molybdates b e l o n g to t h i s category. The proposed mechanisms (58) are based on a n i o n i c and c a t i o n i c d e f e c t s which a l l o w b u l k m i g r a t i o n o f oxygen i o n s through the lattice and electronic transfer. Recently, c o n d u c t i v i t y measurements have been performed on a s e r i e s o f catalysts Mo0 CoMoO., F e ( M o 0 ) and B i „ ( M o 0 ) ( 8 5 , 86) f o r w h i c h a compensation e f f e c t i s observed. U s i n g a hopping m o d e l , t h i s e f f e c t i s due to a d i s t r i b u t i o n o f the a c t i v a t i o n e n e r g i e s c o r r e s p o n d i n g to a d i s t r i b u t i o n o f the d i s t a n c e s o f jumps s p e c i f i c to each k i n d o f l a t t i c e . We c a n , t h e r e f o r e , assume t h a t the e l e c t r o n t r a n s f e r proceeds i n these phases by an a c t i v a t e d bound s m a l l p o l a r o n mechanism. There i s no need i n these cases to r e l a t e the s e l e c t i v i t y t o the presence of C . S . p l a n e s , w h i c h are d i f f i c u l t t o c o n c e i v e i n these k i n d s o f i o n o c o v a i e n t s t r u c t u r e s . Recent HREM e x p e r i m e n t s made on (Te-Mo-0) s y s t e m , c a t a l y s t f o r ammoxidation o f p r o p y l e n e , have not r e v e a l e d the presence o f c r y s t a l l o g r a p h i c shear p l a n e s 3>

2

4

3

4

3

(87). b) Supported Multicomponent Molybdates The improvement o f the c a t a l y t i c performances of MCM c a t a l y s t s can o n l y be i n t e r p r e t e d by means o f i n t e r f a c i a l e f f e c t s between t h e i r components. XRD and TEM experiments performed on M Fe B i M o 0 (M - C o , N i , M g ; 0 < χ < 4) (60) b e f o r e and a f t e r n~x χ 12 y ^ ' — r e a c t i o n ( 5 8 - 6 0 , 8 8 ) have shown the presence o f the pure 1 0

i n d i v i d u a l molybdates, which are i n c o r p o r a t e d by a s p e c i a l preparation. These phases can undergo v a r i o u s t r a n s f o r m a t i o n s depending on the gaseous e n v i r o n m e n t . I t has been shown t h a t under reducing c o n d i t i o n s , b i s m u t h molybdates can exchange molybdenum o x i d e by the r e a c t i o n s ( 8 9 - 9 2 ) Bi (Mo0 ) B i M o 0 +2Mo0 + 0 2

4

Bi Mo 0 2

2

whereas a

3

2

—

9

Bi Mo0 2

6

6

2

2

+Mo0 + l / 2 0 2

disproportionation

of

2

Bi Mo 0^ 2

2

is

observed under a i r

(90,93) 2 Bi Mo 0 2

2

9

Bi (Mo0 ) 2

4

3

+ Bi Mo0 2

6

f

below 400°C and above 550°C where b i s n u c l e a t e d i n s t e a d of b . At low temperature the d i s p r o p o r t i o n a t i o n l o o k s l i k e a s p i n o d a l

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

SOLID STATE CHEMISTRY IN CATALYSIS

50

d e c o m p o s i t i o n y i e l d i n g modulated s t r u c t u r e s can be reduced a l s o , i n two steps ( 9 6 ) ; F e ( M o 0 . ) ^ 2 FeMoO, + Mo0 + 1/2 2 4 3 4 3 Mo0 — Mo0 + 1/2 0 o

o

3

o

2

(94,95) .

Fe (Mo0 ) 2

4

3

0 2 o

2

A c c o r d i n g to the model presented ( 6 0 ) , the c a t a l y s t p a r t i c l e s h o u l d c o n t a i n an i n n e r c o r e c o n s i s t i n g of a β-ΜΜοΟ^ s t r u c t u r e , c o v e r e d by F e ( M o 0 ) , w i t h the o u t e r shell o f 100 Â t h i c k n e s s c o n t a i n i n g B i M o 0 ^ and B i ^ M o O ^ ) ^ . I n used c a t a l y s t s , a l l the r e s u l t s c o n f i r m the presence o r 0-FeMoO^. Additional facts lead to the c o n j e c t u r e that coherent i n t e r f a c e s are formed i n such MCM c a t a l y s t s : ( i ) The i s o s t r u c t u r a l jS-MMoO^ molybdates (M = F e , C o , N i , Mg) can form extended s o l i d s o l u t i o n s ( 9 7 ) , o r at l e a s t coherent i n t e r f a c e s depending on the temperature ( 8 6 ) . ( i i ) M i c r o g r a v i m e t r i c experiments performed at 750°C under n i t r o g e n show t h a t the r e d u c t i o n o f F e ^ M o O ^ ) ^ a l o n e l e a d i n g to 0-FeMoO^ i s s l o w e r than when F e ^ M o O ^ ) ^ i s supported by /3-MgMoO^ (85,86)» The s m a l l c r y s t a l l o g r a p h i c m i s f i t s u g g e s t s the p r o b a b l e e x i s t e n c e of coherent boundaries ( F i g . 9 ) . ( i i i ) The same c o n c l u s i o n s can be drawn up f o r the i n t e r f a c e s between B i ^ M o O ^ ) ^ and F e ( M o 0 ) w h i c h have v e r y c l o s e s t r u c t u r e s . T h e r e f o r e , c o h e r e n t b o u n d a r i e s s h o u l d be p r e s e n t between each s h e l l o f t h e c a t a l y s t p a r t i c l e , w i t h the f o l l o w i n g consequences f o r the c a t a l y s i s namely, s e t t i n g up o f a m e t a s t a b l e s t a t e o f the c a t a l y s t , and a r e d u c t i o n i n the energy b a r r i e r f o r e l e c t r o n t r a n s f e r by the redox mechanism proposed f o r MCM c a t a l y s t s ( 9 8 100). 2

4

3

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch003

2

2

c) C o M o 0 / M o 0 / T i 0 4

3

2

4

3

catalyst

S i n c e 0-CoMoO^ i s a catalyst f o r butane o x i d a t i o n to b u t a d i e n e , and M o 0 « / T i 0 anatase f o r b u t a d i e n e - m a l e i c a n h y d r i d e ( 2 8 , 1 0 1 ) , and s i n c e we know t h a t (001) Mo0 f i t s w e l l w i t h (110) /3-CoMo0, , we have recently attempted to use multicomponent CoMo0 /Mo0 ,Ti0 i n the d i r e c t o x i d a t i o n o f butane to m a l e i c anhydride Ç 8 0 ) . We have found t h a t Ί 0 % by weight o f CoMoO, s u p p o r t e d by 90 % of ( O . 3 M o 0 + O.7 T i 0 ) i s e f f e c t i v e i n t h i s reaction, giving s u b s t a n t i a l y i e l d s of maleic anhydride ( F i g u r e 10). The b u t a d i e n e formed does not desorb and r e a c t s a t the s u r f a c e o f the c a t a l y s t a c c o r d i n g to the known mechanism f o r the o x i d a t i o n o f butene t o m a l e i c a n h y d r i d e . The n o t i o n o f i n t e r f a c i a l e f f e c t s has t h e r e f o r e been a p p l i e d , w i t h some s u c c e s s , t o f i n d i n g a new c a t a l y s t . 9

3

4

3

2

3

2

Conclusion To sum up we have shown t h a t : 1. M i l d o x i d a t i o n c a t a l y s t s a r e made up o f phases, the s e l e c t i v i t y of w h i c h depends d i r e c t l y or i n d i r e c t l y on t h e i r thermodynamic and structural properties.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch003

COURTINE

Interfacial Effects in Mild Oxidation Catalysts

0

20

40

60 10 100 V 0 . content mol 7. 9

F i g u r e 8. Promoting e f f e c t o f TiO a n a t a s e on the c a t a l y t i c p r o p e r t i e s o f V ^ O ^ . Reproduced from Ref. 7 0 . Copyright 1979, 1980, 1981 American Chemical Society.

F i g u r e 9. Crystallographic fit between (010) Fe^iMoO^)^ ( l a r g e r c e l l ) and FeMoO^(a) ( s m a l l e r c e l l ) ( o n l y the i o n atoms a r e shown).

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

52

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Selectivity Maleic anh.,mol%

8 . $ , 8

Yield CO + C0 >mol%

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch003

2

F i g u r e 1Θ. C a t a l y t i c r e s u l t s o b t a i n e d w i t h ΰοΜοΟ^/ΜοΟβ, TiÛ2 i n the o x i d a t i o n o f butane t o m a l e i c a n h y d r i d e (2% C ^ H J ^ Q , 1 sec.) Β

2 . T h i s a l l o w s the c l a s s i f i c a t i o n o f t h e s e phases i n t o two g r o u p s : ReO^- l i k e s t r u c t u r e s and o x y s a l t s such as m o l y b d a t e s . The f i r s t one c o n s i s t s o f host m a t r i c e s h a v i n g extended d e f e c t s ; the second one, o f p o i n t d e f e c t s such as a n i o n i c and c a t i o n i c v a c a n c i e s . 3 . Depending on the k i n e t i c c o n d i t i o n s o f the c a t a l y t i c r e a c t i o n , they can a c t either as a s i n g l e phase, or as a b i p h a s i c or m u l t i p h a s i c system. A s i n g l e phase r e q u i r e s s p e c i f i c c o n d i t i o n s to be h i g h l y a c t i v e and s e l e c t i v e . It has to possess a t h e r m o d y n a m i c a l l y p o s s i b l e redox w i t h r e g a r d to the reactants. I t must be a mixed v a l e n c e phase a t the steady state. I t must possess a p a r t i c u l a r morphology e x h i b i t i n g s t a t i s t i c a l l y the s e l e c t i v e l a t t i c e planes a t the surface and must have o p t i m a l s e m i c o n d u c t i v i t y by " s m a l l polarons"· The second c a s e , c o r r e s p o n d i n g to the f i r s t class, results from an e v o l u t i o n o f the c a t a l y s t s t a t e to a b i p h a s i c system t h r o u g h n u c l e a t i o n of C . S . p l a n e s . Coherent i n t e r f a c e s must e x i s t , near w h i c h a c t i v e s i t e s are i n an e x c i t e d s t a t e and i n a m e t a s t a b l e c o o r d i n a t i o n as compared to the normal s i t e s o f the host m a t r i c e s . 4. I t i s t h e r e f o r e not s u r p r i s i n g t h a t supported o r multicomponent c a t a l y s t s w i t h complex formulae e x h i b i t a b e t t e r s e l e c t i v i t y than the a c t i v e phases a l o n e . These phases can undergo s o l i d - s o l i d transformations near the reaction temperature, which are f a c i l i t a t e d by the coherent interfaces. D e s p i t e the l a c k o f q u a n t i t a t i v e knowledge on the v a l u e s o f i n t e r f a c i a l s t r a i n f a c t o r s f o r the f r e e energy o f the s y s t e m , c o m p a r a t i v e s t u d i e s c o u l d be performed u s i n g the p r e s e n t c l a s s i f i c a t i o n . This general rule about s e l e c t i v i t y i n mild oxidation c a t a l y s i s , based on the v e r y numerous e x p e r i m e n t a l d a t a g a t h e r e d i n t h i s f i e l d , s h o u l d be o f s u b s t a n t i a l h e l p f o r the c h a r a c t e r i z a t i o n of selective catalysts and for the improvement o f their preparation. Acknowledgments Thanks a r e due to D r . practical assistance.

E.

Bordes

for

v a l u a b l e d i s c u s s i o n s and

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

3.

COURTINE

Interfacial Effects in Mild Oxidation Catalysts

53

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

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch003

8. 9. 10. 11. 12. 13. 14. 15.

16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.

Courtine, P. ACS Symposium, Las Vegas (1982) Germain, J. E. Intr. Sci. Chem. Reports 1972, 6, 101. Naito, S.; Kondo, T.; Ichikawa, M.;Tamaru, H. J. Phys. Chem. 1972, 76, 2184. Whitney A. G.; Gay, I. D. J. Catal. 1972, 25, 123. Shvets V. Α.; Kasanskii, V. B. J. Catal. 1972, 25, 123. Krylov, O. V. In "Catalysis by Non-Metals", Academic Press, 1970. Krylov, O. V.; Parikkii, G. B.; Spiridonov, Κ. N. J. Catal. 1971, 23, 301. Schuit, G. C. Α.; Chim. Ind. 1969, 51, 1307. Balandine A. A.,et al. Adv. Catal. 1969, 19, 1. Wolkenstein, T. H. In "Theorie electronique de la catalyse", Masson, 1968. Dowden, D. A. Catal. Rev. 1971, 5, 1. Germain, J. E. J. Chim. Phys. 1973, 70, 1048. Ai M.; Suzuki, S. Bull. Chem. Soc. Jap. 1974, 47, 3074. Haber, J. Int. Chem. Eng. 1975, 15, 21. a) Hucknall, D. J. In "Selective Oxidation of Hydrocarbons", Academic Press, 1974. b) Cole, D. J.; Cuius, C. F.; Hucknall, D. J. J. C. S. Faraday I, 1976, 72, 2185. c) Hauffe, K.; Raveling, H. Ber. Bunsenges. Phys. Chem. 1980, 84, 912 Trifiro, F.; Notarbatolo, S.; Pasquon, I. J. Catal. 1971, 22, 324. Mitchell, P. Ch.; Trifiro, F. J. Chem. Soc. 1970, A 3183. Stone, F. S. J. Sol. State Chem. 1975, 12, 271. Cimino, Α.; Indovina, V.; Pepe, F.; Stone, F. S. Gazz. Chim. Ital. 1973, 103, 935. Daumas, J. C. Thesis, Paris, 1970. Courtine, P.; Cord, P. P.; Pannetier, G; Daumas, J. C.; Montarnal, R. Bull. Soc. Chim. Fr. 1968, 4816. Grasselli, R. K.; Suresh, D. D. J. Catal. 1972, 25, 273. Batist, P. Α.; der Kinderen, A. H. W .M.; Uwenburgh, Y. L.; Metz, F. A. M. G.; Schuit, G. C. A. J. Catal. 1968, 12, 45. Bleyenberg, A. C. A. M.; Lippens, B. C.; Schuit, G. C. A. J. Catal. 1965, 4, 581. Eick, F. Α.; Kilhborg, L. Acta. Chem. Scand. 1968, 20. Robb, F. Y.; Glausinger, W. S.; Courtine, P. J. Solid State Chem. 1979, 30, 171-181. Wakabayashi, G. W.; Kamiya, G. W.; Ohta, N. Bull. Chem. Soc., Jap. 1967, 40, 2172. Bordes, E. Thesis, Compiegne, France, 1979. Bordes. E.; Courtine, P. J. Catal. 1979, 57, 236. Van Den Bussche, F.; Jouy, M.; Courtine, P. 6e Colloque Franco-Polonais sur la Catalyse, Compiegne, 1977 a) Grasselli, R. K.; Callahan, J. U. S. Patent 3 414 631, 1968. b) Grasselli, R. K.; Hardman, H. F. U. S. Patent 3 642 930, 1972. c) Standard Oil Company of Ohio, Dutch Patent application 7 006 454, 1970.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

54

S O L I D STATE C H E M I S T R Y IN CATALYSIS

32. 33. 34. 35.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch003

36. 37. 38. 39. 40. 41. 42. 43. 44.

45. 46. 47.

48. 49. 50. 51. 52. 53. 54. 55. 56. 57.

58. 59.

60. 61. 62.

Mars, P.; Van Keverlen, D. W. Chem. Eng. Sci. Suppl. 1954, 3, 41. Boreskov, G. K. 2nd Jap. Sov. Catal. Sem., Tokyo, 1973. Sachtler, W. M. H.; De Boer, Ν. H. Proc. 3rd Int. Cong. Catal. North Holland, Amsterdam, 1965, p. 252. Skalkina, L. V.; Suzdalev, L. P.; Kolchim, J. K.; Margolis, L. Y. Kinet. Katal. 1969, 10, 378. Matsuura, I. 6th Int. Cong. Catal., London B-21, 1976. Moro-oka, Y.; Ozaki, A. J. Catal. 1966, 5, 116. Moro-oka, Y.; Morikawa Y.; Ozaki, A. J. Catal. 1967, 7, 23. Moro-oka, Y.; Ozaki, A. J. Catal. 1960, 12, 361. Boreskov, G. K. Kinet. Catal. 1970, 11, 374. Boreskov, G. K. Discuss. Farad. Soc. 1968, 285. Gaucher, P.; Ernst V.; Courtine, P. J. Solid State Chem. 1983, 47, 47. Ubbelohde, A. R. J. Chim. Phys. 1966, 62, 33. Hyde, B. G.; Bursill, L. A. In "The Chemistry of Extended Defects in Non Metallic Solids"; Leroy-Eyring and O'Keeffe (ed.): North Holland, 1970. Eon, J. G. Thesis, Compiegne, 1984. Andersson, S. Bull. Soc. Chim. Fr. 1965, 1088. a) Bordes, E.; Courtine, P.; Johnson, J. W. J. Sol. State Chem., 1984, 55, 270. b) Bordes, Ε; Raminosona, Α.; Johnson, J. W.; Courtine, P. 10th Int. Symp. Reactivity of Solids, Preprints p. 503, Dijon, 1984. Boudart, M. Adv. Catal. 1966, 26, 153. Volta, J. C.; Desquesnes, W.; Moraweck, B.; Tatibouet, M. J. 7th Int. Congr. Catalysis, 1980, C4, Tokyo. Kozlowski, R.; Ziolkowski, J.; Mokala, K; Haber, J. J. Solid State Chem. 1980, 35, 1. Theobald, F. Thesis, Besancon, 1975. Ziolkowski, J. J. Catal. 1983, 80, 263. Vejux Α.; Courtine, P. J. Solid State Chem. 1978, 23, 93. Anderson, J. S. 7th Int. Symp. Reactivity of Solids, 1972, 1-21, Chapman and Hall. O'Keefe, M. In "Fast Ion Transport in Solids"; North Holland, Amsterdam, 1973, p. 233. Haber, J. Pure and Applied Chemistry, 1978, 50, 923. a) De Rossi, S.; Iguchi, E.; Schiavello, M.; Tilley, R. J. D. Z. Phys. Chem. 1976, 103, 193. b) Barber, S.; Booth, J.; Pyke, D.R.; Reid, R.; Tilley, R. J. D. J. Catal. 1982, 77, 180. Gates, B. C.; Katzer, J. R.; Schuit, G. C. In "Chemistry of Catalytic Processes", 1979. a) Linn, W. J.; Sleight, A. W. Ann. N.Y. Acad. Sci. 1976, 272, 22. b) Sleight, A. W. In "Advanced Materials in Catalysis", Acad. N.Y., 1977. Wolfs, M. W. J.; Batist, P. A. J. Catal. 32, 25. Zeman, J. Heidelberger Beitr. Mineral. Petrogr. 1956, 5, 139. Van Den Elzen, A. F.; Rieck, G. D. a) Acta Crystall. 1973, B29, 2433; B29, 2436 b) Mat. Res. Bull. 1976, 10, 1163.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

3.

63. 64.

65. 66. 67. 68. 69.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch003

70.

71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84.

85. 86. 87. 88. 89. 90. 91. 92.

COURTINE

Interfacial Effects in Mild Oxidation Catalysts

55

Abrahams, S. C.; Reddy, J. M. J. Chem. Phys. 1965, 43, 2533. a) Bart, J. C. J.; Perini, N.; Giordano, Ν. Z. Anorg. Allgem. Chem. 1975, 412; 258. b) Bart. J. C. J.; Giordano, N. Gazz. Chim. Ital. 1979, 109, 73. Middlemiss, N. Ph.D. Thesis, McMaster Univ., Hamilton, Canada, 1979 Gorbunova, Y.; Linde, S. A. Dokl. AKAD. Nauk. SSSR 1979, 245, 5864. Bordes, E.; Courtine, P. Ass. Gen. Soc. Chim. Fr. 1982. Bond, G. C.; Bruckman, K. Faraday Discuss. Chem. Soc. 1982, 72, 235 Bond, G. C.; Sarkany, A. J.; Parfitt, G. D. J. Catal. 1979, 57, 2195. Inomata, M.; Miyamoto Α.; Murakami, J. J. Phys. Chem. 1981, 85, 2372; J. Chem. Soc., Chem. Comm., 1979, 1010; ibid. 1980, 233. Kozlowski,R.; Pittifer, R. F.; Thomas, J. M. J. Chem. Soc., Chem. Comm. 1983, 438 Bond, G. C.; Konig, P. J. Catal. 1982, 77, 309. Vanhove, D.; Blanchard, M. Bull. Soc. Chim. Fr. 1971, 9, 3291. Gasior, M.; Grzybowska, B. Bull. Acad. Pol. Sci. 1979, 27, 835. Courtine, P. unpublished results. Courtine, P.; Vejux, A. C. R. Acad. Sci. Paris 1978, 286 C, 135. Shannon, R. D.; Pask, J. A. Amer. Mineral. 1964, 49, 1707 and J. Appl. Phys. 1964, 35, 3414. Eon, J. G.; Courtine, P. J. Solid State Chem. 1980, 32, 67. Eon, J. G.; Bordes, Ε.; Vejux, Α.; Courtine, P. IXth Int. Symp. on Reactivity of Solids, Cracow, Pologne, 1980. Jung, J. S.; Bordes, E.; Courtine, P. Int. Symp. on Adsorption and Catalysis, Brunel Univ., Uxbridge, U.K., 1984. Anderson, Α.; Lundin, J. J. Catal. 1980, 58, 383.; 65, 9. Anderson, A. Thesis, Lund, Sweden, 1982. Yoshida, S.: Iguchi, T.; Ishada, S.; Karama, K. Bull. Soc. Chim. Jap. 1972, 45, 376. a) Keulks, G. W. J. Catal. 1970, 19, 232. b) Keulks, G. W.; Krenzke, L. D. 6th Int. Congr. Catalysis, B-20, London, 1976. Gaucher, P. Thesis, Compiegne, 1984. Gaucher, P.; Courtine, P. to be published. Gai, P. L.; Boyes, E. D.; Bart, J. C. J. Phil. Mag. 1982, 45, 531. Chaze, A. M.; Courtine, P. J. Chem. Res. (S), 1980, 96; (Μ), 1980, 954-970. Trifiro, F.; Forzatti, P.; Villa, P. L. In "Preparation of Catalysts", Elsevier, 1976. Grzybowska, B.; Haber, J.; Komorek, J. J. Catal. 1972, 25, 25-32. Aykan, K. J. Catal. 1968, 12, 281-90. Batist, Ph. Α.; Van de Moesdijk, G. C. Α.; Matsuura, I.; Schuit, G. C. A. J. Catal. 1971, 20, 40-57.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

56

SOLID STATE CHEMISTRY IN CATALYSIS

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch003

93.

Egashira, M.; Matsuo, K.; Kagawa, S.; Seiyama, T. J. Catal. 58, 409-18. 94. Schultz, Α.; Stubican, V. S. Phil. Mag. 1968, 18 (155), 929-37. 95. Kumar, J.; Ruckenstein, E. J. Solid State Chem. 1980, 31, 41-46. 96. Trifiro, F.; de Vecchi, V.; Pasquon, I. J. Catal. 1969, 15, 8-16. 97. a) Courtine, P.; Daumas, J. C. C. R. Acad. Sci. Paris 1969, 268, b) Cord, P. P.; Courtine, P.; Pannetier, G.; Guillemet, J. Spe ctrochim. Acta 1972, 28A, 1601-13. 98. Lojacono, M.; Noterman, T.; Keulks, C. J. Catal. 1975, 39, 286; 40, 19. 99. Krylov, O. V. Kinet. Katal. 1981, 22, 15. 100. Wolfs, M. W. J.; Van Hoof, J. H. C. In "Preparation of Catalysts"; Delmon, B.; Jocobs, P. Α.; Poncelet, G., Eds.; Elsevier, Amsterdam, 1976. 101. Akimoto, M.; Echigoya, E. Bull. Chem. Soc. Jap. 1075, 48, 3518.

RECEIVED January 14, 1985

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

4 Structural and Thermodynamic Basis for Catalytic Behavior of Bismuth-Cerium Molybdate Selective Oxidation Catalysts 1

1

1

2

J. F. BRAZDIL , R. G. TELLER , R. K. GRASSELLI , and E. KOSTINER 1

Sohio Research Center, The Standard Oil Company (Ohio), Cleveland, OH 44128 Department of Chemistry and Institute of Materials Science, University of Connecticut, Storrs, CT 06268

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch004

2

The Bi Ce Mo O two phase system has been examined for its activity in the catalytic oxidation of propylene to acrylonitrile. The two phases have been characterized as a solid solution of Bi in cerium molybdate and Ce in bismuth molybdate. Results of these oxidation studies have been correlated with structural results on the pure and doped end members. A plot of catalytic activity versus x shows three maxima which coincide with the maximum concentration of Ce in Bi Mo O , Bi in Ce Mo O and equal concentrations of cerium in bismuth molybdate and bismuth in cerium molybdate. These results suggest that selective propylene ammoxidation occurs in a trifunctional matrix which contains metals that; activate propylene to form an a l l y l intermediate (Bi), insert oxygen into the a l l y l i c intermediate, (Mo) and contain a redox couple (Ce). Aspects of phase cooperation in a multiphase catalyst are also discussed. 2-x

x

3

12

2

3

12

2

3

12

Much of the understanding of the s o l i d state mechanism of heterogeneous c a t a l y s i s stems from fundamental studies of single phase model compounds (1-5). In many cases, the r o l e of a metal component i n a c a t a l y t i c process has been discerned through i t s incorporation into s o l i d solutions of r e l a t i v e l y inert host matrices (6). In the case of the selective oxidation and ammoxidation of olefins to unsaturated aldehydes and n i t r i l e s , respectively ( e . g . the ammoxidation of propylene to a c r y l o n i t r i l e ) , such studies have established several important tenets for the process. These include the need for the coexistence of key c a t a l y t i c elements with the proper electronic structure, redox chemistry, and metal-oxygen bond strength (_7). It i s however well recognized that the most effective c a t a l y s t s , be they mixed metals or mixed metal oxides, are usually multiphase in nature (8). Some progress has been made in understanding the source of the synergistic effects observed in these multiphase c a t a l y s t s . For example, S i n f e l t and coworkers 09) have been able to explain the

0097-6156/85/0279-0057$06.00/0 © 1985 American Chemical Society In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch004

58

S O L I D STATE C H E M I S T R Y IN CATALYSIS

c a t a l y t i c b e h a v i o r o f b i m e t a l l i c , b i p h a s i c systems t h r o u g h the use of s t r u c t u r a l c h a r a c t e r i z a t i o n t o o l s such as EXAFS. In oxide c a t a l y s t s for s e l e c t i v e o x i d a t i o n , rigorous studies of multiphase c a t a l y s t s have been l i m i t e d p r i m a r i l y t o vanadium o x i d e s (10-12). I n t h i s c a s e , i t has been shown t h a t the c o e x i s t e n c e o f s t r u c t u r a l l y r e l a t e d o x i d i z e d and reduced vanadium o x i d e phases i s s t a b i l i z e d by the presence of s t r u c t u r a l l y coherent phase boundaries. Recently, significant efforts have been made to d e v e l o p a r a t i o n a l mechanism f o r the c a t a l y t i c b e h a v i o r o f multicomponent bismuth molybdate based selective oxidation c a t a l y s t s ( 1 3 - 1 5 ) . However, the c o m p l e x i t y o f the c a t a l y s t systems i n v e s t i g a t e d has prevented the development o f a r i g o r o u s model f o r c a t a l y t i c behavior. Our r e c e n t work on the b i s m u t h - c e r i u m molybdate c a t a l y s t system has shown t h a t i t can serve as a t r a c t a b l e model f o r the s t u d y o f the s o l i d s t a t e mechanism of s e l e c t i v e o l e f i n o x i d a t i o n by multicomponent molybdate c a t a l y s t s . A l t h o u g h c o m p o s i t i o n a l l y and s t r u c t u r a l l y q u i t e s i m p l e compared to o t h e r m u l t i p h a s e molybdate c a t a l y s t s y s t e m s , b i s m u t h - c e r i u m molybdate c a t a l y s t s are e x t r e m e l y effective for the selective ammoxidation of propylene to a c r y l o n i t r i l e (16). I n p a r t i c u l a r , we have found t h a t the a d d i t i o n of c e r i u m to b i s m u t h molybdate s i g n i f i c a n t l y enhances i t s c a t a l y t i c a c t i v i t y for the selective ammoxidation of propylene to acrylonitrile. Maximum c a t a l y t i c a c t i v i t y was observed for s p e c i f i c c o m p o s i t i o n s i n the s i n g l e phase and two phase r e g i o n s o f the phase diagram ( 1 7 ) . These c h a r a c t e r i s t i c s o f t h i s c a t a l y s t system a f f o r d the o p p o r t u n i t y t o understand the p h y s i c a l b a s i s f o r synergies i n multiphase c a t a l y s t s . I n a d d i t i o n to t h i s p r e v i o u s l y p u b l i s h e d work, we a l s o i n c l u d e some o f our most r e c e n t r e s u l t s on the b i s m u t h - c e r i u m molybdate s y s t e m . As s u c h , the p r e s e n t account r e p r e s e n t s a summary o f our i n t e r p r e t a t i o n s o f the d a t a on t h i s system. Experimental B i s m u t h c e r i u m molybdates were prepared by c o p r e c i p i t a t i o n u s i n g aqueous s o l u t i o n s o f ( N H ^ M o ^ ^ , (NH, K C e d K O , , and B i ( N 0 ~ ) ' 5^O. The c a t a l y s t s were supported on SiO^ (20% by weight) u s i n g an ammonium s t a b i l i z e d s i l i c a s o l . Samples f o r d i f f r a c t i o n a n a l y s i s were u n s u p p o r t e d . Samples were c a l c i n e d i n a i r at 290 and 425°C f o r t h r e e hours each f o l l o w e d by 16 hours a t 500, 550, o r 6 0 0 ° C . X-ray powder p a t t e r n s were o b t a i n e d u s i n g a Rigaku D / M a x - I I A X - r a y diffTactometer u s i n g Cu Κ r a d i a t i o n . Powder n e u t r o n d i f f r a c t i o n d a t a f o r B i ^ gCe^ 2 ^ ° 3 ^ 1 2 c o l l e c t e d at Brookhaven N a t i o n a l L a b o r a t o r y ' s * Higfi F l u x Beam Reactor. D e t a i l s o f the e x p e r i m e n t a l procedure and R i e t v e l d refinement have been r e p o r t e d p r e v i o u s l y ( 1 8 ) . S t a r t i n g parameters f o r Ce doped and pure B i ^ M o ^ O ^ were taken from a s i n g l e c r y s t a l x - r a y d i f f r a c t i o n study o f B i 6 o ^ 0 by Van den E l z e n and R i e c k (19a). Time-of-flight (TOF) Powder n e u t r o n d i f f r a c t i o n d a t a for BiJfo^O.2 c o l l e c t e d at Argonne N a t i o n a l L a b o r a t o r y s I n t e n s e P u l s e d Neutron Source (iPNijO u t i l i z i n g the S p e c i a l Environment Powder D i f f r a c t o m e t e r (SEPD). D e t a i l s o f the i n s t r u m e n t , d a t a c o l l e c t i o n software, and R i e t v e l d a n a l y s i s software have been 3

w

9

1

w

e

r

e

0

5

i

Z

1

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

e

r

e

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch004

4.

BRAZDIL ET AL.

Bi-Ce Molybdate Selective Oxidation Catalysts

previously published(20). TOF d a t a from the b a c k s c a t t e r i n g d a t a banks (20=150°) were used i n the a n a l y s i s and corresponded to TOF(min) of 7500msec (d(min)=.99 Â) and TOF(max) of 23500msec (d(max)=3.10 À ) . In a d d i t i o n to background (_5), h a l f w i d t h ( 3 ) , a b s o r b t i o n , e x t i n c t i o n , l a t t i c e ( 4 ) , and s c a l e p a r a m e t e r s , a l l p o s i t i o n a l and i s o t r o p i c t h e r m a l parameters were v a r i e d i n the l e a s t - s q u a r i n g p r o c e s s , f o r a t o t a l o f 84 v a r i a b l e s . The l e a s t squares p r o c e s s converged to R(p)=O.037, R(wp)=O.056, R(exp)=O.044. Refinements were a l s o c a r r i e d out f o r TOF(min) v a l u e s o f 7000 and 8000 s e c . There was no s i g n i f i c a n t d i f f e r e n c e between the r e s u l t s o f those r e f i n e m e n t s and those presented h e r e . Figure 1 displays the n e u t r o n d i f f r a c t o g r a m of B i ^ M o ^ O ^ S i n g l e c r y s t a l s o f B i doped c e r i u m molybdate were prepared i n s e a l e d tube e x p e r i m e n t s . A quantity of m a t e r i a l of composition B i C e M o ^ O ^ was packed i n t o a welded g o l d c a p s u l e pinched shut a t the_gpen end, p l a c e d i n a . q u a r t z . t u b e .and s e a l e d under vacuum (10 mm H g ) . The tube was h e l d i n a v e r t i c a l p o s i t i o n and kept a t 950°C f o r one week. S i n g l e c r y s t a l s c o r r e s p o n d i n g to two phases were i s o l a t e d from these e x p e r i m e n t s . Red c r y s t a l s h a r v e s t e d possessed c e l l parameters and c o m p o s i t i o n ( v i a EDX a n a l y s i s ) t h a t was c o n s i s t e n t w i t h Ce doped Bi^Mo^O ^ . * s i m i l a r f a s h i o n , the amber c r y s t a l s t h a t were a l s o i s o l a t e d from t h i s procedure were i d e n t i f i e d as h a v i n g the ^ e ^ M o ^ O ^ s t r u c t u r e w i t h some B i p r e s e n t . EDX a n a l y s i s o f t h r e e amber c r y s t a l s gave an average c o m p o s i t i o n o f Ce. g Q i 3 3 ° n Q ( t o 3 Mo atoms, o x y g e n ^ t o i c h i o m e t r y c a l c u l a t e d on t n e ' b a s i s o f charge b a l a n c e assuming Ce ) . S i n g l e c r y s t a l x - r a y d i f f r a c t i o n d a t a were c o l l e c t e d on one of the amber c r y s t a l s w i t h a FACS-I automated P i c k e r d i f f r a c t o m e t e r w i t h Zr f i l t e r e d Mo Κα r a d i a t i o n . L a t t i c e parameters ( a = 1 6 . 8 8 6 ( 5 ) , b = l l . 8 3 9 ( 3 ) , c=15.797(5)A, b = 1 0 8 . 6 4 ( l ) were determined by a l e a s t squares f i t of 24 r e f l e c t i o n s i n the a n g u l a r range 5 1 < 2 0 < 6 1 ° . D e t a i l s o f the i n s t r u m e n t , d a t a c o l l e c t i o n , d a t a r e d u c t i o n , and a n a l y s i s have been p u b l i s h e d ( 2 1 ) . S t a r t i n g parameters f o r the f u l l m a t r i x l e a s t squares refinement were taken from L a ^ M o ^ Q ^ d ^ b ) w i t h 90%Ce and 10%Bi o c c u p a t i o n f o r each L a s i t e . Parameters v a r i e d were: a l l p o s i t i o n a l p a r a m e t e r s , a n i s o t r o p i c temperature f a c t o r s f o r m e t a l atoms, i s o t r o p i c temperature f a c t o r s f o r the oxygen atoms, s c a l e and e x t i n c t i o n f a c t o r s and s c a t t e r i n g f a c t o r s f o r the C e / B i s i t e s . Data i n the a n g u l a r range 201.0 A) and the Β c a t i o n f a i r l y s m a l l (on the o r d e r o f O.6 A ) . The p r o t o t y p e o f t h i s s t r u c t u r e i s the m i n e r a l s c h e e l i t e , CaWO,, w h i c h c r y s t a l l i z e s ( 2 1 ) w i t h four formula u n i t s per u n i t c e l l i n tjjç t e t r a g o n a l space group 1 4 ^ / a (a_ = 5 . 2 4 3 , £ - 11.376 A ) . Each Ca ion is surrounded by eight oxygen atoms from different t e t r a h e d r a l l y coordinated Β i o n s , £our 2.44 A and f o u r at 2.48 A. The n e a r l y r e g u l a r d i s c r e t e WO. t e t r a h e d r a have f o u r e q u a l W-0 d i s t a n c e s ( 1 . 7 8 A ) . The i d e a l s t r u c t u r e i s i l l u s t r a t e d i n F i g u r e 2. One c h a r a c t e r i s t i c of the scheelite structure-type i s the number and e x t e n t o f c a t i o n i c o x i d a t i o n s t a t e s and d e f e c t ( c a t i o n d e f i c i e n t ) s t r u c t u r e s t h a t have been f o u n d . The s i n g l e g u i d e to the f o r m a t i o n o f s c h e e l i t e - t y p e structure seems t o be the a b i l i t y of A c a t i o n s to be e i g h t - c o o r d i n a t e d (i.e., r a t h e r l a r g e ) and Β i o n s ^ t o a t t a i n t e t r a h e d r a l c o o r d i n a t i o n ( n o t e , however, t h a t P0^ or S i O . c o n t a i n i n g s c h e e l i t e s a r e unknown). If we f i r s t c o n s i d e r the s i m p l e r cases o f heterovalent s u b s t i t u t i o n w i t h t h i s s t r u c t u r e - t y p e (22) we f i n d , i n a d d i t i o n to the 2+/6+ (A+/B+) v a l a n c e s typified by CaWO, , the valance c o m b i n a t i o n s 1+/7+ (KRu0 ), 3+/5+ ( G d M o 0 ) , and 4+/4+ ( C e G e 0 ) . E x t e n d i n g t h i s by d o u b l i n g ( o r t r i p l i n g ) the c h e m i c a l f o r m u l a we can o b t a i n , by c o u p l e d s u b s t i t u t i o n , mixed ( o r s u b s t i t u t e d ) i o n s on the A s i t e : ( l + , 3 + ) / 6 + [ N a L a ( M o O , ) ] , ( l + , 4 + ) / 6 + [Na Th(MoO, ) J , and (2+,4+)/5+ [ P b T h C V O ^ J . * S u b s t i t u t i o n on the anion" s i t e (the Β s i t e ) has a l s o been observed. S e v e r a l examples a r e : 3+/(4+,6+) [ L a ( S i O , ) ( W O , ) ] , 3+/(3+,6+) [ B i ( F e 0 ) ( M o 0 ) ] , and 3+/(2+,6+) [ B i ^ Z n O , ) ( M o O j ] . I f two o r more i o n s occupy e i t h e r the A o r the S s i t e , the p o s s i b i l i t y of having e i t h e r an ordered or a d i s o r d e r e d (random) s t r u c t u r e w i l l e x i s t . F o r example, o r d e r on the A s i t e s o c c u r s i n KEu(Mo0 ). The compound B i ^ F e O ^ M o O ^ e x i s t s i n b o t h an o r d e r e d and d i s o r d e r e d f o r m ( 2 3 ) , the ordered form r e s u l t i n g from the o r d e r i n g of the F e 0 and MoO, ( B - i o n ) t e t r a h e d r a . The c r y s t a l c h e m i s t r y o f s c h e e l i t e - t y p e s t r u c t u r e s i s f u r t h e r c o m p l i c a t e d by d e f e c t s t o i c h i o m e t r i e s w i t h extensive vacancies at cation sites. In g e n e r a l , random c a t i o n v a c a n c i e s a r e examples o f point defects. However, at h i g h c o n c e n t r a t i o n s , an o r d e r i n g o f v a c a n c i e s can o c c u r at which time the vacancy can no l o n g e r be c o n s i d e r e d as a p o i n t d e f e c t and a new p e r i o d i c l a t t i c e ( o r u n i t c e l l ) " w i l l have been g e n e r a t e d . As an example o f r a n d o m l y d i s o r d e r e d v a c a n c i e s , the coupled s u b s t i t u t i o n o f two B i i o n s and one vacancy f o r t h r e e Pb ions i n PbMo O. g i v e s r i s e (24) to a s e r i e s of s o l i d solutions

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch004

a

4

t

4

4

9

9

L

2

3

4

4

2

4

4

3+

P b

B i

M o

w

h

e

r

e

(l~3 ) 2x^x^ °4^3' Φ represents a c a t i o n vacancy. Depending on t e m p e r a t u r e , a fairly l a r g e range o f random c a t i o n v a c a n c i e s (up t o 15%) has been o b s e r v e d . At h i g h c o n c e n t r a t i o n o f c a t i o n v a c a n c i e s (Φ = O . 3 3 ) , new o r d e r e d c o m p o s i t i o n s are observed w i t h u n i t c e l l s c o r r e s p o n d i n g to s u p e r c e l l s o f the scheelite structure. The g e n e r a l f o r m u l a f o r

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch004

62

©

A

Οο F i g u r e 2 . A r e p r e s e n t a t i o n o f the s c h e e l i t e (CaWO^) s t r u c t u r e . Reproduced by p e r m i s s i o n from A n n . Ν . Y . A c a d . S c i . , V o l . 272, p . 2 3 , a u t h o r s : A . S l e i g h t and W. L i n n .

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

4.

BRAZDIL ET AL.

63

Bi-Ce Molybdate Selective Oxidation Catalysts

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch004

Β υ

c

a

t

n

these c a t i o n d e f i c i e n t scheelites i s Α^ ^τΦ^ 3 3 4 » ( ~ i° v a c a n c y ) . An e q u i v a l e n t n o t a t i o n ( m u l t i p l y i n g " t h r o u g h by 3) would be A^ φ ( B O ^ ) - o r , more s i m p l y , A ( B O . ) ^ . Three d i f f e r e n t schemes for c a t i o n ordering i n defect s c h e e l i t e have been observed - the Eu^iWO^)^ s t r u c t u r e (25) ( w h i c h i s found f o r s e v e r a l r a r e e a r t h molybdates and t u n g s t a t e s ) , the La^iMoO^)^ s t r u c t u r e (26) (found for larger rare e a r t h molybdates o n l y ) , and B i ^ i M o O ^ ) ^ ( 1 8 ) , a unique s t r u c t u r e . The c a t i o n o r d e r i n g schemes f o r these t h r e e s t r u c t u r e s as d e r i v e d from the i d e a l s c h e e l i t e s t r u c t u r e i s shown i n F i g u r e 3 , w h i c h i s a p r o j e c t i o n o f each o f t h e s e s t r u c t u r e s down t h e i r r e s p e c t i v e m o n o c l i n i c b-axes ( f o r the d e f e c t compounds) w i t h r e f e r e n c e to the c o r r e s p o n d i n g £ - a x i s p r o j e c t i o n o f CaWO^. I t i s evident from t h i s schematic r e p r e s e n t a t i o n t h a t these s t r u c t u r e s not o n l y d i f f e r i n the manner i n w h i c h the v a c a n c i e s o r d e r (and t h e r e f o r e i n t h e i r u n i t c e l l m e t r i c s ) but i n d i s t o r t i o n s of the BO^ p o l y e d r a . Diffraction studies have shown t h a t these d i s t o r t i o n s , w h i c h a r e most pronounced i n the structure of B i ^ i M o O ^ ) ^ ( v i d a i n f r a ) , a l s o a f f e c t the Β i o n c o o r d i n a t i o n . C a t a l y t i c A c t i v i t y and Phase C o m p o s i t i o n All

c a t a l y s t s were t e s t e d

C H 3

6

+

MH

3

+

1.5

f o r p r o p y l e n e ammoxidation a c t i v i t y .

0

—

2

C H N 3

+

3

3^0

The v a r i a t i o n i n a c t i v i t y f o r a c r y l o n i t r i l e f o r m a t i o n as a f u n c t i o n o f c o m p o s i t i o n f o r the *2-x^ x °3^12 * * F i g u r e 4 . Maxima i n a c t i v i t y , as measured by f i r s t o r d e r r a t e constant f o r propylene disappearance, o c c u r at t h r e e c o m p o s i t i o n s . A p a r t i a l phase diagram f o r the s y s t e m , c o n s t r u c t e d w i t h powder x - r a y d i f f r a c t i o n i s d i s p l a y e d i n Figure 5. The phase diagram c o n s i s t s of t h r e e r e g i o n s , two of w h i c h are s i n g l e phase r e g i o n s . On the B i - r i c h s i d e , Ce i s s o l u b l e i n Bi2Mo 0^2« The maximum s o l u b i l i t y of Ce i n b i s m u t h molybdate i s a p p r o x i m a t e l y 10 mole p e r c e n t . The C e - r i c h s i d e c o n s i s t s o f a solid s o l u t i o n of B i i n C e M o 0 . , w h i c h has the La^Mo^^ structure. The maximum s o l u b i l i t y of B i i n t h i s phase Is a p p r o x i m a t e l y 50 mole p e r c e n t . The i n t e r m e d i a t e r e g i o n c o n s i s t s o f a m i x t u r e o f b o t h phases. Comparison o f the v a r i a t i o n i n c a t a l y t i c a c t i v i t y w i t h c o m p o s i t i o n as presented i n F i g u r e 4 w i t h the phase diagram r e v e a l s t h a t maxima i n c a t a l y t i c a c t i v i t y o c c u r s a t the s o l u b i l i t y l i m i t s o f the two s i n g l e phases. An u n d e r s t a n d i n g of the b a s i s f o r t h i s b e h a v i o r r e q u i r e s a b e t t e r d e f i n i t i o n o f the s o l i d s t a t e s t r u c t u r a l a s p e c t s o f the c a t a l y s t s y s t e m . B

e

M

s

v

s

t

e

m

s

s

n

o

w

n

η

3

2

3

2

A n a l y s i s o f the S i n g l e Phase C a t a l y s t s I n an attempt to i d e n t i f y any B i / C e o r d e r i n g o r s i t e p r e f e r e n c e , d i f f r a c t i o n d a t a was c o l l e c t e d on two end members o f the s e r i e s . A powder n e u t r o n d i f f r a c t i o n d a t a set was c o l l e c t e d f o r a sample o f B i . gCe^ 2 ^ ° 3 ^ 1 2 composition. Additionally, since neutron d i f f r a c t i o n d a t a had not been taken on the the end member of the series, Bi Mo^0 , and comparisons to t h i s model were deemed 9

1 9

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Ο

Ο

Ο

Θ

Ο

Θ

Ο

Ο

Ο

o o o o o o o o o

ο ο ο ο

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch004

ο

ο

ο

Ο

Ο

Ο

Θ

Ο

ο

ο

Ο

Ο

Ο Ο

ο ο ο

Θ

Ο

Ο

ο

Ο

ο

Ο

ο

ο

Ο

Ο

ο

Ο

Ο Ο

F i g u r e 3 . I d e a l s c h e e l i t e s t r u c t u r e compared to o r d e r e d d e f e c t s t r u c t u r e s . P r o j e c t i o n i s down the c a x i s w i t h o n l y 1/2 the u n i t c e l l shown i n t h i s d i r e c t i o n . Shaded c i r c l e s and t e t r a h e d r a a r e a t the top o f l e v e l , unshaded 1/4 o f the way down the u n i t c e l l , (a) CaWO . (b) L a ( M o 0 ) . (c) E u ( W 0 ) . (d) B i ( M o 0 ) . Reproduced by p e r m i s s i o n from A n n . Ν. Y . A c a d . S c i . , V o l . 272, p . 2 3 , a u t h o r s : A . S l e i g h t and W. L i n n . 2

4

3

2

4

3

2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

4

4.

BRAZDIL ET AL.

Bi- Ce Molybdate Selective Oxidation Catalysts

65

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch004

• • Ο - 600°C

οI 0

ι

O.2

ι O.4

ι O.6

I

ι

I

1

O.8

1.0

1.2

1.4

χ

F i g u r e 4 . C a t a l y t i c a c t i v i t y as a f u n c t i o n o f c o m p o s i t i o n and r e a c t i o n temperature f o r the B i C e ( M o 0 ) system. Catalytic a c t i v i t y i s e x p r e s s e d as k , the f i r s t o r d e r r a t e c o n s t a n t f o r propylene disappearance. Reproduced from R e f . 17. C o p y r i g h t 1983 American C h e m i c a l S o c i e t y . 2

x

4

3

F i g u r e 5 . P a r t i a l phase diagram f o r the B i _ C e ( M o 0 ) system. Reproduced w i t h p e r m i s s i o n from R e f . 16. C o p y r i g h t 1983, Academic Press, Inc. 2

x

x

4

3

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

66

S O L I D STATE C H E M I S T R Y IN CATALYSIS

s i g n i f i c a n t , powder n e u t r o n d i f f r a c t i o n d a t a were c o l l e c t e d on t h i s m a t e r i a l as w e l l . The structure of Bi^Mo^O (perhaps better written Βί^^Φ^/0M0O, ) , i s based on s c h e e l i t e . As d i s c u s s e d above, t h e r e i s c o n s i d e r a b l e d i s t o r t i o n from the i d e a l s c h e e l i t e s t r u c t u r e , t h i s i s p r o b a b l y due to the B i l o n e p a i r o f e l e c t r o n s ( v i d a i n f r a ) . As a r e s u l t o f t h e s e d i s t o r t i o n s i n the a n i o n p a c k i n g , the Mo atoms are p e n t a - c o o r d i n a t e (as opposed to t e t r a h e d r a l l y c o o r d i n a t e d i n CaWO.) and Mo^Og " d i m e r s " are o b s e r v e d , w i t h M o . . . Mo s e p a r a t i o n s of 3.4 Â and two b r i d g i n g 0 atoms. A l i s t i n g of Mo-0 d i s t a n c e s d e r i v e d from the n e u t r o n d i f f r a c t i o n d a t a f o r B i ^ M o ^ O ^ given i n T a b l e 1. Note t h a t the 0 c o o r d i n a t i o n about each o f the t h r e e c r y s t a l l o g r a p h i c a l l y d i s t i n c t Mo atoms i s similar. Each Mo atom c o n t a i n s one s h o r t ( d o u b l e ) Mo-0 b o n d , one s i n g l y bound 0 atom, two oxygens w i t h i n t e r m e d i a t e bond o r d e r s and one weakly bound oxygen atom. The r e s u l t i s a v e r y asymmetric oxygen c o o r d i n a t i o n sphere about each Mo atom.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch004

i

Table I .

s

A Comparison o f I n t e r a t o m i c Mo-0 D i s t a n c e s i n Bi Mo 0 and Β 1 g C e ^ 2

3

1 2

χ

Bi Mo 0 2

a

3

1 2

#

Bi

K

8

Ce

0

>

2

MD O 3

Mo(l)-0(4) -0(5) -0(2) -0(10) -0(10)'

1.70(1) 1.78(1) 1.80(2) 1.89(1) 2.20(1)

1.78(3) 1.67(2) 1.92(3) 1.85(2) 2.27(2)

Mo(2)-0(l) -0(9) -0(6) -0(3) -0(8)

1.65(2) 1.75(2) 1.85(2) 1.97(2) 2.16(2)

1.63(3) 1.68(3) 1.98(4) 1.90(3) 2.23(3)

Mo(3)-0(12) -0(11) -0(8) -0(7) -0(6)

1.74(2) 1.63(2) 1.87(2) 1.90(1) 2.48(2)

1.72(2) 1.88(3) 1.93(3) 1.83(3) 2.34(3)

1

2

a) Bond d i s t a n c e s a r e g i v e n i n angstroms, w i t h e s t i m a t e d s t a n d a r d d e v i a t i o n o f the l a s t d i g i t g i v e n i n p a r e n t h e s i s .

R e s u l t s ( T a b l e I ) from r e f i n e m e n t o f the powder n e u t r o n d i f f r a c t i o n d a t a f o r the Ce doped m a t e r i a l , B i . g Q 2 3 1 2 ' significant structural alterations have r e s u l t e d from Ce i n c o r ­ p o r a t i o n i n t o the solid. The t h r e e Mo atoms a r e no l o n g e r chemically equivalent. The c o o r d i n a t i o n about M o ( l ) i s unchanged from t h a t o f the parent compound. A second Mo atom c o n t a i n s two s h o r t ( d o u b l e ) bonds t o oxygen atoms. These molybdate d i o x o ( d i m o l y b d e n y l ) groups a r e b e l i e v e d to be an i m p o r t a n t f e a t u r e f o r s e l e c t i v e o x i d a t i o n c a t a l y s i s (_7). The t h i r d Mo c o o r d i n a t i o n sphere c o n t a i n s no Mo-0 b o n d . Bond o r d e r c a l c u l a t i o n s (27) about C e

Μ θ

0

i

n

d

i

c

a

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

t

e

t

n

a

t

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch004

4.

BRAZDIL ET AL.

Bi-Ce Molybdate Selective Oxidation Catalysts

67

t h i s l a t t e r Mo s i t e i n d i c a t e the t o t a l Mo-0 bond o r d e r to be 5 . T h i s apparent r e d u c t i o n of Mo from 6 t o 5 i s most l i k e l y accomp a n i e d by some Ce o x i d a t i o n to 4 a n d / o r oxygen removal from the solid. The d i s t r i b u t i o n o f the Ce dopant i s not u n i f o r m . Of the t h r e e c a t i o n s i t e s i n the parent compound, the f i r s t two are f u l l y o c c u p i e d by B i atoms, w h i l e the t h i r d i s empty. This r e s u l t s i n a sequence o f c a t i o n o c c u p a t i o n on the [010] face as shown below i n A and B . S i t e a i s 92% B i , 4% C e , s i t e b i s 88% B i , 12% C e , and s i t e c , n o r m a l l y v a c a n t , i s 4% Ce o c c u p i e d . E x a m i n a t i o n o f the o x i d e environment about s i t e c i n d i c a t e s t h a t t h e r e i s i n s u f f i c i e n t room f o r a Ce c a t i o n . O c c u p a t i o n o f the s i t e must t h e r e f o r e r e s u l t i n some l o c a l d i s o r d e r , the most l i k e l y m a n i f e s t a t i o n t h e r e o f b e i n g a vacancy i n s i t e a ( t h e d i s t a n c e between s i t e s a and c are too s h o r t to a l l o w s i m u l t a n e o u s o c c u p a t i o n ) . Note a l s o t h a t s i t e a i s not 100% o c c u p i e d . Consequently, about 96% o f the time the d i s t r i b u t i o n of cations (Bi/Ce) in B i , o Q 2 3°12 below ( n o r m a l l y found i n b i s m u t h m o l y b d a t e ; , and 4% o f the time i t i s as i n B . C e

A) B)

M M

M M

• Ce

M •

M M

• •

M M

M o

i

S

a

S

i

n

A

M • M • M=Bi/Ce

The d i s t r i b u t i o n o f c a t i o n s r e p r e s e n t e d i n B i s r e m i n i s c e n t o f t h a t of C e M o 0 on the [010] f a c e . A s t r u c t u r a l study o f a Ce r i c h c a t a l y s t was a l s o u n d e r t a k e n . A s i n g l e c r y s t a l x - r a y d i f f r a c t i o n a n a l y s i s of a c r y s t a l of c o m p o s i t i o n Ce^ 3^°3^12 P ^ > * r e s u l t s (along w i t h comparisons t o I s o s t r u c t u r a l L a ^ o ^ O ^ ) a r e presented i n T a b l e II. The r e s u l t s o f the experiment i n d i c a t e t h a t the B i s u b s t i t u t e s f o r Ce i n the structure. U n l i k e B ^ M o ^ O , the parent compound C e M o 0 - has a r e l a t i v e l y u n d i s t o r t e d s c n e e l i t e - b a s e d s t r u c t u r e w i t h 1/3 c a t i o n v a c a n c i e s . The d i f f e r e n c e between the b i s m u t h and c e r i u m molybdate s t r u c t u r e s l i e s i n the o r d e r i n g o f t h e s e v a c a n c i e s as i n d i c a t e d above. The comparison between L a ^ M o ^ O . ^ , * doped cerium molybdate i s q u i t e good. The Mo atoms are approximately t e t r a h e d r a l l y coordinated to f o u r oxygen atoms, and the C e / B i atoms a r e eight coordinate. Because the gross d i s t o r t i o n s i n b i s m u t h molybdate are a t t r i b u t e d to the B i l o n e pairs, this r e l a t i v e l a c k of scheelite d i s t o r t i o n i n cerium molybdate i s not u n e x p e c t e d . At s i g n i f i c a n t l y lower concentrations of B i atoms l o n e p a i r - l o n e p a i r i n t e r a c t i o n s a r e m i n i m i z e d . As i n the b i s m u t h r i c h structure discussed above, the B i atoms i n B i doped cerium molybdate are not randomly d i s t r i b u t e d on the Ce s i t e s . The c o m p o s i t i o n s o f M ( l ) , M ( 2 ) , and M(3) ( s e e T a b l e I I ) a r e 86%Ce and 14%Bi, 92%Ce and 8 % B i , 73%Ce and 2 7 % B i , respectively. This compositional difference i s r e f l e c t e d i n the M ( 3 ) - 0 d i s t a n c e s as w e l l . Note t h a t the d i f f e r e n c e between the maximum and minimum M ( 3 ) - 0 d i s t a n c e s i s l a r g e r f o r B i doped c e r i u m molybdate than lanthanum m o l y b d a t e . E v i d e n t l y , a 27% o c c u p a t i o n o f B i ( w i t h the a t t e n d e n t l o n e p a i r ) i s s u f f i c i e n t f o r a s m a l l but n o t i c a b l e d i s t o r t i o n i n the average l o c a l 0 environment. Summar i z i n g the structural results on Ce i n c o r p o r a t i o n i n t o b i s m u t h 2

3

1 2

W

a

S

e r

o m e d

a n (

s

o

m

e

2

2

3

2

a n c

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

t

n

e

B

i

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

1 2

1.73 1.75 1.78 1.82 1.75 1.76 1.76 1.82 1.73 1.75 1.79 1.81 1.76 1.76 1.77 1.81 1.75 1.81

3

1.73 1.74 1.82 1.85 1.74 1.77 1.78 1.82 1.75 1.74 1.82 1.79 1.74 1.75 1.77 1.82 1.76 1.80

^i.y^o.a

1M o

3°12

3

La(l)-0(4) -0(12) -0(7) -0(16) -0(5) -0(16) -0(17) -0(10) La(2)-0(14) -0(9) -0(3) -0(6) -0(2) -0(13) -0(18) -0(11) La(3)-0(15) -0(1) -0(2) -0(8) -0(11) -0(8) -0(17) -0(10)

2

La Mo 0 1 2

n

d

2.48 2.48 2.50 2.51 2.52 52 57 57 46 48 52 52 53 2.53 2.54 57 44 46 49 53 55 2.55 2.55 2.59

a

C e

C e

B i

1.7

M

3°12

2.45 2.46 2.51 2.47 2.45 2.48 2.53 2.53 2.44 2.44 2.53 2.47 2.47 2.52 2.52 2.50 2.38 2.38 2.40 2.54 2.48 2.54 2.53 2.62

0.3 °3°12

M o

B i

0.3

M(l)-0(4) -0(12) -0(7) -0(16) -0(5) -0(16) -0(17) -0(10) M(2)0(14) -0(9) -0(3) -0(6) 0(2) -0(13) -0(18) -0(11) M(3)-0(15) -0(1) -0(2) -0(8) -0(11) -0(8) -0(17) -0(10)

l.7

2

3

each Mo-0 d i s t a n c e i s .01. Distances for A) The e s t i m a t e d standard deviation for La Mo 0 a r e t a k e n from Reference 26. A l l bond d i s t a n c e s a r e g i v e n i n a n g s t r o m s .

Mo(l)-0(4) -0(3) -0(1) -0(2) Mo(2)-0(7) -0(5) -0(6) -0(8) Mo(3)-0(9) -0(12) -0(11) -0(10) Mo(4)-0(l4) -0(13) -0(15) -0(16) Mo(5)-O(18) -0(19)

2

1 2

A Comparison o f Mo-0 D i s t a n c e s i n I ^ M o ^ O ^

La Mo 0

Table I I .

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch004

2 n

H

g 53

m

2?

Ο

C/3

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch004

4.

Bi-Ce Molybdate Selective Oxidation Catalysts

BRAZDIL ET AL.

69

molybdate and B i i n c o r p o r a t i o n i n t o c e r i u m molybdate one f i n d s nonrandom dopant s u b s t i t u t i o n . A p p a r e n t l y the symmetry o f the different +3 cat^oji s i t e s , ^ c o u p l e d w i t h the d i f f e r e n t s t e r i c r e q u i r e m e n t s o f Ce and B i i o n s , p l a y s an i m p o r t a n t r o l e i n d e t e r m i n i n g the o c c u p a t i o n s o f the v a r i o u s s i t e s . Additionally, in the case o f b i s m u t h m o l y b d a t e , a r a d i c a l a l t e r a t i o n i n the Mo-0 c o o r d i n a t i o n spheres r e s u l t s upon Ce i n c o r p o r a t i o n . Coupled w i t h t h i s d i s t o r t i o n t h e r e i s an apparent r e d u c t i o n o f one Mo atom, and the c r e a t i o n o f a d i m o l y b d e n y l group on another Mo s i t e . I n comparing the two s t r u c t u r e s , one f a i l s to f i n d a s t r u c t u r e type o r d i s t o r t i o n ( s u c h as M o . . . M o i n t e r a c t i o n s o r g r o s s l y d i s t o r t e d m e t a l environments) common t o b o t h m a t e r i a l s i n the structures described here. Y e t , some commonality i s expected to e x i s t based on the c l o s e correspondence o f the c a t a l y t i c d a t a . There are s e v e r a l p o s s i b l e e x p l a n a t i o n s for t h i s : a) c o n s i d e r a b l y more d i s t o r t i o n i s found i n more B i r i c h c e r i u m molybdate compounds ( e . g . B i g ^Ce i ° 3 ^ i o ^ * analysis of t h i s material i s needeâ, D) l o c a l environments w i t h i n the two structures d e s c r i b e d here are s i m i l a r , but the average s t r u c t u r e s t h a t r e s u l t from d i f f r a c t i o n experiments do not r e v e a l this s i m i l a r i t y or c) i n c o r p o r a t i o n o f a n o t h e r m e t a l more g r e a t l y e f f e c t s the s u r f a c e of each m a t e r i a l not the b u l k , and c a t a l y t i c b e h a v i o r i s a r e f l e c t i o n of surface s t r u c t u r e . I n c r e a s e d p r o p y l e n e ammoxidation a c t i v i t y o f each phase upon a l t e r i o n doping i s due to the j u x t a p o s i t i o n o f a l l n e c e s s a r y elements f o r o x i d a t i o n c a t a l y s i s i n a s i n g l e phase. The r e q u i r e ments o f a good o x i d a t i o n c a t a l y s t are a) a c t i v a t i o n o f the s u b s t r a t e m o l e c u l e , b) o x i d a t i o n a c t i v i t y (oxygen i n s e r t i n g ) and c) f a c i l e redox c a p a b i l i t i e s to ease e l e c t r o n c o n d u c t i o n and s i t e r e c o n s t r u c t i o n . F o r reasons d i s c u s s e d e x t e n s i v e l y i n the l i t e r a t u r e (7), we a s s i g n these r o l e s to B i , Mo, and Ce i o n s i t e s r e s p e c t i v e l y i n the c a t a l y s t s described here. The s o l i d s o l u t i o n f o r m a t i o n observed i n t h e s e m a t e r i a l s e n a b l e s a l l o f these f u n c t i o n s t o be r e p r e s e n t e d i n one phase and on one s u r f a c e o f the c a t a l y s t . M

a n C

a

s

t

r

u

c

t

u

r

e

A n a l y s i s o f the M u l t i p h a s e C a t a l y s t The maximum i n c a t a l y t i c a c t i v i t y observed for the m u l t i p h a s e r e g i o n o f the phase diagram n e c e s s a r i l y a r i s e s from i n t e r a c t i o n s between the s e p a r a t e phases. The b i s m u t h r i c h and c e r i u m r i c h s o l i d s o l u t i o n s can r e a d i l y form coherent i n t e r f a c e s a t the phase b o u n d a r i e s due to the structural s i m i l a r i t i e s between the two phases w h i c h can permit e p i t a x i a l n u c l e a t i o n and g r o w t h . A good l a t t i c e match e x i s t s between the [010] faces o f the compounds, t h i s match i s d i s p l a y e d i n F i g u r e 6. We have a l s o shown t h a t r e g i o n s o f an [010] face o f a Ce doped b i s m u t h molybdate c r y s t a l resembles c e r i u m molybdate c o m p o s i t i o n a l l y . T h i s means t h a t the i n t e r f a c e between the two compounds need not have sharp c o m p o s i t i o n gradients. It i s s t r u c t u r a l l y p o s s i b l e f o r the B i - r i c h phase to possess a m e t a l s t i o c h i o m e t r y at the s u r f a c e t h a t matches t h a t o f the C e - r i c h phase. I n o r d e r to a s s e s s the n a t u r e o f t h i s i n t e r f a c i a l r e g i o n , the thermodynamic treatment of Cahn and H i l l i a r d (28) was employed (17) to d e r i v e the f o l l o w i n g f r e e energy f u n c t i o n :

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

70

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch004

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Bi

Ce

BiCe(Mo0 )

MoO

1.8 0.2(

4)3

4

3

(010) CONTACT PLANE

F i g u r e 6. the

Bi^

C r y s t a I l o g r a p h i c match between t h e [010] f a c e s o f QCCQ

2^°^4^3

Reproduced from R e f .

an(

17.

*

B'CeiMoO^)^ Copyright

solid solutions.

1983 A m e r i c a n C h e m i c a l

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Society.

4.

BRAZDIL ET AL.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch004

Af

= RT X ^ l n

Bi-Ce Molybdate Selective Oxidation Catalysts

[a

B i

/a^]

+ X^ln

71

[ a ^ / a ^ ]

where a and a are the a c t i v i t i e s o f b i s m u t h and c e r i u m , r e s p e c t i v e l y , i n the i n t e r f a c i a l r e g i o n and a and a ^ are the a c t i v i t i e s o f b i s m u t h and c e r i u m , r e s p e c t i v e l y , i n the e q u i l i b r i u m s o l i d s o l u t i o n . The f u n c t i o n can be regarded as the f r e e energy o f an e q u i l i b r i u m m i x t u r e o f the two s o l i d s o l u t i o n phases. The f r e e energy f u n c t i o n A f was c a l c u l a t e d as a f u n c t i o n o f c o m p o s i t i o n i n the two phase r e g i o n o f the Bt^^Ce^HoO^)^ system (Figure 7). Comparison of F i g u r e s 5 and 7 r e v e a l s t h a t energy o f an e q u i l i b r i u m m i x t u r e o f the phases i s m i n i m i z e d at the c o m p o s i t i o n w h i c h g i v e s maximum c a t a l y t i c a c t i v i t y . I t i s apparent t h a t a t c o m p o s i t i o n s where A f i s a minimum, the d i f f e r e n c e i n the c h e m i c a l p o t e n t i a l s o f the components i n the i n t e r f a c i a l r e g i o n and the e q u i l i b r i u m s o l i d s o l u t i o n s i s m i n i m i z e d . Thus, an i n t e r f a c i a l r e g i o n w h i c h i s c h e m i c a l l y s i m i l a r to the s a t u r a t e d s o l i d s o l u t i o n s appears optimum f o r maximum c a t a l y t i c e f f i c i e n c y . P h y s i c a l l y , the r e l a t i o n s h i p between c a t a l y t i c a c t i v i t y and A f can be understood from a s t u d y o f s i n g l e phase b i s m u t h c e r i u m molybdate s o l i d s o l u t i o n s . The r e s u l t s show t h a t maximum a c t i v i t y i s a c h i e v e d when t h e r e e x i s t s a maximum number and o p t i m a l d i s t r i b u t i o n of a l l the key c a t a l y t i c components; bismuth, molybdenum and c e r i u m i n the solid. Therefore, i t reasonably f o l l o w s t h a t the low c a t a l y t i c a c t i v i t y observed f o r the two phase c o m p o s i t i o n s where Af ¥ A f ( m i n ) results from the presence o f interfacial regions i n the catalysts where the compositional u n i f o r m i t y d e v i a t e s s i g n i f i c a n t l y from the e q u i l i b r i u m d i s t r i b u t i o n o f b i s m u t h and cerium c a t i o n s p r e s e n t i n the s o l i d s o l u t i o n s . These c o m p o s i t i o n s may c o n t a i n areas i n the i n t e r f a c i a l r e g i o n w h i c h a r e more b i s m u t h - r i c h o r c e r i u m - r i c h than the s a t u r a t e d s o l i d solutions. C o n v e r s e l y , at A f ( m i n ) , the c a t a l y s t i s s i m i l a r to an i d e a l mixture of the two optimal solid solutions. The c o m p o s i t i o n a l homogeneity o f the i n t e r f a c i a l r e g i o n approaches t h a t of the saturated s o l i d solutions. Therefore, the catalytic b e h a v i o r o f c o m p o s i t i o n s at A f ( m i n ) is s i m i l a r to t h a t o f the saturated s o l i d s o l u t i o n s . Summary and C o n c l u s i o n s S e v e r a l c o n c l u s i o n s can be drawn about the s o l i d s t a t e mechanism o f s e l e c t i v e o l e f i n ammoxidation by b o t h s i n g l e phase and m u l t i p h a s e oxide c a t a l y s t . F i r s t l y , optimum c a t a l y t i c performance i s a c h i e v e d when t h e r e i s maximum i n t e r a c t i o n between key c a t a l y t i c components i n a s o l i d oxide m a t r i x . Maximum i n t e r a c t i o n o c c u r s i n a s i n g l e phase s a t u r a t e d s o l i d solutions s i n c e t h e s e c o n t a i n the maximum number and d i s p e r s i o n o f the c a t a l y t i c a l l y important c o - e x i s t i n g elements. S e c o n d l y , these key c a t a l y t i c components f o r s e l e c t i v e o l e f i n ammoxidation are i d e n t i f i e d a s : an a-H a b s t r a c t i n g element ( B i ) , an o l e f i n c h e m i s o r p t i o n and n i t r o g e n i n s e r t i o n element (Mo) and a m u l t i v a l e n t redox c o u p l e (Ce Ce ) . The m u l t i v a l e n t redox c o u p l e enhances oxygen i o n , e l e c t r o n and a n i o n vacancy t r a n s p o r t i n the s o l i d w h i c h enhances c a t a l y t i c a c t i v i t y by i n c r e a s i n g the r e c o n s t r u c t i o n / r e o x i d a t i o n r a t e o f the a c t i v e B i and Mo c o n t a i n i n g s i t e s . This r e s u l t s i n an e f f e c t i v e i n c r e a s e o f the

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

72

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch004

1500 L-

900 h

O.4 O.6 O.8 X in B i 2 . x C e x ( M o 0 4 ) 3

1.2

Figure 7. Free energy function Af for Β'ο-τχ^χ^ ^^ function of composition. Reproduced from R e f . 1 7 . C o p y r i g h t 1983 American Chemical 0

as

Society.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

a

4.

BRAZDIL ET AL.

73

number of active s i t e s available at the surface at any given time. T h i r d l y , we have found that the chemical and structural nature of the interfacial region between co-existing phases in Bi2_ Ce (MoO, )^ catalysts has a profound effect on c a t a l y t i c behavior. Thermodynamic calculations show that compositions which give maximum c a t a l y t i c a c t i v i t y also give minima in the free energy of mixing of the two phases r e l a t i v e to the saturated s o l i d solutions. This can be explained on the basis that at the free energy minimum the chemical and compositional s i m i l a r i t y between the i n t e r f a c i a l region and the equilibrium s o l i d solutions i s greatest. The simultaneous existence of coherent phase boundaries between the separate phases of multicomponent catalyst i s also an important c r i t e r o n for maximum c a t a l y t i c activity. In bismuthcerium molybdates, close structural s i m i l a r i t y between the two saturated solid solutions permits mutual e p i t a x i a l growth which produces a "pseudo" single phase c a t a l y s t . As a consequence, oxygen i o n , anion vacancy and electron transport between the phases can r e a d i l y occur. In addition, oxygen ion and electron transfer between the individual phases w i l l be f a c i l i t a t e d when the compositional nonuniformity of the region at the interface i s minimized. A compositionally non-uniform region in which the redox couple (cerium in this case) i s not properly distributed w i l l exhibit diminished oxygen ion mobility across the coherent phase boundaries and the overall c a t a l y t i c a c t i v i t y for selective o l e f i n oxidation w i l l be less than optimum. x

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch004

Bi-Ce Molybdate Selective Oxidation Catalysts

x

Acknowledgment s We g r a t e f u l l y acknowledge M. H. Rapposch for c o l l e c t i n g the x-ray data and a s s i s t i n g i n the structure s o l u t i o n , and L . C. Glaeser for catalyst preparation and t e s t i n g . The authors thank the US department of energy for supporting IPNS at Argonne as a national users f a c i l i t y , and the Standard O i l Company (Sohio) for permission to publish this work. Literature 1. 2. 3. 4. 5. 6. 7. 8. 9.

Cited

Grasselli, R. K.; Suresh, D. D. J. Catal., 1972, 25, 273. Bart, J. C. J.; Giordano, N. J. Catal., 1980, 64, 356 Brazdil, J. F.; Suresh, D. D.; Grasselli, R.K. J. Catal., 1980, 66, 347. Aso, I.; Furukawa, S.; Yamazoe, N.; Seiyama, T. 1980, J. Catal., 64, 29. Aso, I.; Amamoto, T.; Yamazoe, N.; Seiyama, T. Chem. Lett.,1980, 365. Stone, F.S. J. Solid State Chem. 1975, 12, 271. Grasselli, R. K.; Burrington, J. D.; Brazdil, J. F. Farad. Disc.,1982, 72, 203 and references therein. Schuit, G. C. Α.; Gates, B. C. Chem. Tech. 1983, November 693. Sinfelt, J. H.; Via, G. H.; Lytle, F. W. J. Chem. Phys., 1980, 72, 4832.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

74

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch004

10.

Vejux, Α.; Courtine, P. J. Solid State Chem.,1978, 23, 93. 11. Bordes, Ε.; Courtine, P. J. Catal.,1979, 57, 236 12. Eon, J. G.; Courtine, P. J. Solid State Chem. 1980, 32, 67. 13. Carson, D.; Coudurier, G.; Forissier, M.; Vedrine, J. C. J. Chem. Soc., Farad Trans. 1, 1983, 79, 1921 1983. 14. Ueda, W.; Moro-oka, Y.; Ikawa, T. J. Chem Soc., Farad, Trans I.,1982, 78, 495. 15. Chaze, A. M.; Courtine, P. J. Chem. Research, 1983, 96. 16. Brazdil, J. F.; Grasselli, R. K., J. Catal. 1983, 79, 104. 17. Brazdil, J. F.; Glaeser, L. C.; Grasselli, R. K. J. Phys. Chem. 1983, 87, 5485. 18. Teller, R. G.; Brazdil, J. F.; Grasselli, R. K.; Thomas, R.; Corliss, L.; Hastings J. J. Solid State Chem., 1984, 52,313. 19a. Van den Elzen, A. F.; Rieck, G. D. Acta. 1973, Cryst., B29,2433. 19b. Zalkin, Α.; Templeton, D. H. J. Chem. Phys. 1964,40, 501. 20. Von Dreele, R. B.; Jorgensen, J. D.; Windsor, C. G. J. Appl. Cryst., 1982, 15, 581 and Jorgensen, J. D.; Faber, J. ICANS-II, proc. VIth Int. Collab. Adv. Neutron Sources, Argonne Natl. Lab., June 28-July 2, 1982, 1983 (ANL-82-80. 21. Rapposch, M. H.; Anderson, J. B.; Kostiner, E. Inorg. Chem., 1980, 19, 3531. 22. Sleight, A. W. In "Advanced Materials in Catalysis"; Ed. Burton and Garten, Academic Press, NY 1977,181. 23. Jeitschko, W.; Sleight, A. W.; McClellan, W. R.; Weiher, J. F. Acta Cryst. 1976, B32, 1163. 24. Sleight, A. W.; Aykan, K.; Rogers, D. B. J. Solid State Chem., 1975, 13, 231. 25. Templeton, D. H.; Zalkin, A. Acta Cryst. 1963, 16, 762. 26. Jeitschko, W. Acta Cryst., 1973, B29, 2074. 27. Brown, I. D.; Wu, Κ. K. Acta. Cryst., 1976, B32, 1957. 28. Cahn, J. W.; Hilliard, J. E. J. Chem. Phys., 1958, 28, 258. RECEIVED

February 26, 1985

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

5 Structure and Activity of Promoted Uranium-Antimony Oxide Catalysts R. A. INNES, A. J. PERROTTA, and H. E. SWIFT

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch005

Gulf Research & Development Company, Pittsburgh, PA 15230

At one time the preferred catalyst for propylene ammoxidation was a uranium-antimony oxide composition whose active phase was USb O . We have found that the partial substitution of certain tetravalent metals for the pentavalent antimony in this phase greatly increases catalytic activity. Catalysts with the empirical formula USb Mo , where M=Ti, Zr, or Sn, were 6, 11, and 13 times as active as the old catalyst, while exhibiting as good or better selectivity to acrylonitrile. The high activity of the modified catalysts is attributed to the generation of oxygen vacancies in the USb O lattice. The stability of these catalysts is enhanced by the addition of small amounts of molybdenum or vanadium which prevent decomposition of the active phase by acting as a catalyst for reoxidation. 3

10

2

3

9-10

10

A c r y l o n i t r i l e i s manufactured by passing propylene, ammonia, and a i r over a mixed-oxide catalyst at 4 0 0 - 5 0 0 ° C . The process i s also a major source of a c e t o n i t r i l e and hydrogen cyanide which are obtained as the result of side reactions. Catalysts used i n this process are generally mixed oxides of bismuth or antimony with other multivalent metals such as molybdenum, i r o n , uranium, and tin. At one time, the preferred catalyst for propylene ammoxidation was a uranium-antimony oxide composition (1-4). This catalyst contained excess Sb20^ and a s i l i c a binder i n combination with the c a t a l y t i c a l l y active phase 1 ^ 0 (3,4). Both uranium and antimony i n the active phase assume the +5 oxidation state. We have found that the p a r t i a l substitution of certain tetravalent metals for pentavalent antimony greatly increases catalytic activity. For example, catalysts with the empirical formula U S b 2 M 0 , where M - T i , Z r , or Sn, were respectively 6, 11, and 13 times as active as the o r i g i n a l uranium-antimony oxide catalyst, while exhibiting as good or better acrylonitrile 3

1 0

9 1 0

0097-6156/85/0279-0075$06.00/0 © 1985 American Chemical Society

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

SOLID STATE CHEMISTRY IN CATALYSIS

76

selectivity. T h i s paper w i l l d i s c u s s how the replacement of antimony by a t e t r a v a l e n t m e t a l a f f e c t s the c r y s t a l l i n e phase d i s t r i b u t i o n , how the r e s u l t i n g d i f f e r e n c e s i n both c o m p o s i t i o n and s t r u c t u r e r e l a t e to the c a t a l y t i c p r o p e r t i e s , and how c a t a l y s t stability is enhanced by the a d d i t i o n of s m a l l amounts of molybdenum o r vanadium.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch005

E x p e r i m e n t a l Methods u n l e s s o t h e r w i s e n o t e d , c a t a l y s t s were prepared by c o p r e c i p i t a t i n g the hydrous o x i d e s of u r a n i u m , antimony, and a t e t r a v a l e n t m e t a l from a h y d r o c h o l o r i c a c i d s o l u t i o n of t h e i r s a l t s by the a d d i t i o n of ammonium h y d r o x i d e . The p r e c i p i t a t e s were washed, oven d r i e d , then c a l c i n e d a t 910°C o v e r n i g h t o r a t 930°C f o r two hours to form c r y s t a l l i n e phases. A t t r i t i o n r e s i s t a n t c a t a l y s t s , c o n t a i n i n g 50% by weight s i l i c a b i n d e r , were prepared by s l u r r y i n g the washed p r e c i p i t a t e w i t h s i l i c a - s o l p r i o r to d r y i n g . I n some c a s e s , s m a l l amounts of molybdenum o r vanadium were added by i m p r e g n a t i n g the oven d r i e d m a t e r i a l w i t h ammonium paramolybdate o r ammonium metavanadate s o l u t i o n . The d e t a i l s of these p r e p a r a t i o n s may be found elsewhere ( 5 - 8 ) . The c r y s t a l l i n e phases present i n each catalyst were determined from X - r a y powder d i f f r a c t i o n p a t t e r n s o b t a i n e d w i t h C u Kct r a d i a t i o n and a n i c k e l f i l t e r . The r e g i o n scanned was 2 θ • 10° to 7 0 ° . I n f r a r e d t r a n s m i s s i o n s p e c t r a from 650 t o 4000 cm*" were o b t a i n e d u s i n g a g r a t i n g 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 demountable c e l l w i t h sodium c h l o r i d e windows, and c a t a l y s t samples prepared as paraffin o i l mulls. M a g n e t i c s u s c e p t i b i l t y measurements were made u s i n g the Faraday method and an apparatus which has been d e s c r i b e d elsewhere ( 9 ) . T h i s apparatus was d e s i g n e d f o r low temperature s t u d i e s so our experiments were l i m i t e d to the 4 - 1 0 5 ° K r a n g e . The magnetic f i e l d s t r e n g t h was 20,369 o e r s t e d . A s t a n d a r d m i c r o a c t i v i t y t e s t was used to determine the e f f e c t of s u b s t i t u t i n g t e t r a v a l e n t metals f o r antimony. A O.5 cm^ sample o f 20-40 mesh c a t a l y s t was weighed and charged to a O.48 cm I . D . tubular stainless steel reactor. The c a t a l y s t was heated to 450°C i n a i r f l o w i n g a t 3 2 . 5 c m (STP) m i n " . The r e a c t i o n was then c a r r i e d out i n c y c l i c f a s h i o n . Ammonia and p r o p y l e n e were added to the air stream a t rates of 3 . 0 and 2 . 5 c m (STP) m i n " , respectively. The furnace temperature was a d j u s t e d so t h a t the reaction temperature was 475°C, as measured by a sheathed thermocouple l o c a t e d w i t h i n the c a t a l y s t b e d . A f t e r 15 minutes on stream, the p r o d u c t stream was sampled and a n a l y z e d by gas chromatography. A f t e r another 15 minutes on s t r e a m , the p r o p y l e n e and ammonia f l o w s were shut o f f and the c a t a l y s t was r e g e n e r a t e d by a l l o w i n g the a i r f l o w to c o n t i n u e . P r o p y l e n e and ammonia f l o w s were then resumed to b e g i n the next c y c l e . T h i s procedure was r e p e a t e d f o r 5 o r 6 c y c l e s and the r e s u l t s a v e r a g e d . Product 1

3

1

3

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

1

5.

INNES ET AL.

Promoted Uranium-Antimony Oxide Catalysts

11

a n a l y s e s were made w i t h a gas chromatograph equipped w i t h a t h e r m a l c o n d u c t i v i t y d e t e c t o r , a 6 χ 1/4" column packed w i t h 5 Â m o l e c u l a r s i e v e s and a 1 5 ' χ 1/8" column packed w i t h 6 ' o f Porapak Τ f o l l o w e d by 9 ' o f Porapak QS. Oxygen-argon, n i t r o g e n , and carbon monoxide were a n a l y z e d on the m o l e c u l a r s i e v e column, w h i l e carbon d i o x i d e and h e a v i e r p r o d u c t s were determined on the Porapak column. A ο m i c r o r e a c t o r h o l d i n g 5 . 0 cm o f c a t a l y s t was used to determine the optiumum a c r y l o n i t r i l e y i e l d and study the e f f e c t of s i l i c a b i n d e r and molybdenum and vanadium a d d i t i o n . f

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch005

A l t e r i n g the A c t i v e Phase S m a l l i n c r e a s e s i n a c t i v i t y may be o b t a i n e d by a d d i n g a v a r i e t y of m u l t i v a l e n t m e t a l o x i d e s to the optimum uranium-antimony o x i d e catalyst (10,11). H e r e t o f o r e , antimony and uranium have been emloyed i n a t l e a s t a four to one atomic r a t i o to ensure t h a t USb30^Q i s the o n l y u r a n i u m - c o n t a i n i n g phase formed. The presence of excess antimony prevents the f o r m a t i o n o f USb0 which i s a l e s s selective catalyst. Our approach was b a s i c a l l y d i f f e r e n t . I n s t e a d of adding s m a l l amounts of promoters to the optimum u r a n i u m antimony o x i d e c o m p o s i t i o n , we attempted to a l t e r the a c t i v e phase (USb30 ) through c r y s t a l l i z a t i o n i n a h y p o t h e t i c a l b i n a r y system 5

10

u s b

U S b

T i 0

T

i

+

4

h

a

s

a

n

i

o

n

i

c

3°10" 2 10* c r y s t a l r a d i u s of O.68 Â, which i s c l o s e to the O.62 Â i o n i c r a d i u s o f S b (12) making i t a l o g i c a l c a n d i d a t e to r e p l a c e Sb i n the USb30iQ s t r u c t u r e . G r a s s e l l i and c o - w o r k e r s ( 3 , 4 ) have determined the c r y s t a l s t r u c t u r e of the 1 ^ 3 0 ^ phase by analogy to the s i n g l e c r y s t a l work of C h e v a l i e r and Gasper i n (13) on UÎN^O^Q. I n another paper ( 1 4 ) , C h e v a l i e r and G a s p e r i n r e p o r t t h a t compounds of the type U ( N b ^ , T i ) N b 2 0 ^ Q have the same s t r u c t u r e . Based on t h i s work, they proposed t h a t the uranium i n Uîtt^O^o i s h e x a v a l e n t and t h a t one atom o f n i o b i u m i s t e t r a v a l e n t . Thus, to compensate f o r the replacement of S b ^ w i t h T i \ i t was expected t h a t uranium would be c o n v e r t e d from the +5 to the +6 o x i d a t i o n s t a t e and t h a t t h i s would have a profound e f f e c t on the c a t a l y t i c p r o p e r t i e s . + 5

x

x

+

+

E f f e c t on C r y s t a l l i n e Phase D i s t r i b u t i o n A series of c a t a l y s t s was prepared to study the e f f e c t of s u b s t i t u t i n g t i t a n i u m , z i r c o n i u m , or t i n f o r antimony i n the USb30^Q l a t t i c e . The c r y s t a l l i n e phases p r e s e n t i n these m a t e r i a l s were determined by X - r a y powder d i f f r a c t i o n . To p r o v i d e a b a s i s f o r comparison w i t h the p r i o r a r t , c a t a l y s t 1 l i s t e d i n T a b l e I was prepared following the p u b l i s h e d r e c i p e (2). This catalyst r e p r e s e n t s the o l d uranium-antimony o x i d e c a t a l y s t w i t h o u t any s i l i c a binder. The c r y s t a l l i n e phases d e t e c t e d i n c a t a l y s t 1 were 3°10 2°4 P c t e d (3,4). u s b

a

n

d

s b

a

s

e x

e

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

S O L I D STATE C H E M I S T R Y IN CATALYSIS

78

Table I .

C r y s t a l l i n e Phases D e t e c t e d By X-Ray Powder D i f f r a c t i o n

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch005

Catalyst

Atomic R a t i o Ti Sb

U

1 2 3 4 5 6 7 8 9

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

4.6 3.0 2.4 2.0 1.5 1.0 O.5

Crystalline

Phases*

I, Sb 0 I , sm-II, sm-Sb^^ I I I , I I , sm-Ti0 I, II, Ti0 I , I I , T i 0 , UTi05 UTi0 , Ti0 I, II, Sb 0 ,sm-Ti0 sm-Sb 0^ I, S b 0 , sm-Ti0 2

O.6 1.0 1.5 2.0 2.5 3.0 O.9

4.0

4

2

2

2

5

2

2

5

2

2

10

1.0

4.0

I = USbo ° i o P p sm = smal 1 amount t v

e

n a s e

>

O.9 1

1

2

5

2

= U S b 0 type phase 5

A s e r i e s of c a t a l y s t s was then prepared h a v i n g the e m p i r i c a l f o r m u l a U S b T i 0 , where χ ranged from 0 t o 3 . T i t a n i u m appeared to s u b s t i t u t e f o r antimony i n the U S b 0 l a t t i c e up to x - 1 , s i n c e o n l y a s i n g l e c r y s t a l l i n e phase c l o s e l y r e s e m b l i n g USb^O^Q was obtained. No peaks were seen c o r r e s p o n d i n g t o T i 0 o r USbO^. The X - r a y d i f f r a c t i o n p a t t e r n s f o r the x=O.6 and x=1.0 c o m p o s i t i o n s are compared i n T a b l e I I w i t h t h a t o f U S b 0 prepared by our method. These p a t t e r n s were almost i d e n t i c a l to p u b l i s h e d p a t t e r n s f o r USb 0 except t h a t the 004 r e f l e c t i o n (3) s h i f t e d from 3.83 to 3.90 Â as χ i n c r e a s e d from 0 t o 1. Attempts to i n c r e a s e t i t a n i u m s u b s t i t u t i o n beyond x=1.0 r e s u l t e d i n the f o r m a t i o n of T i 0 and U S b 0 a t the x=1.5 l e v e l , and e v e n t u a l l y U T i 0 a t h i g h e r l e v e l s . 3 x

x

y

3

1 0

2

3

3

1 0

1 0

2

5

5

Ref. (2,3) USb 0 3

d(A) 3.85 3.18 2.45 1.92 1.83 1.66 1.65 1.59 1.47 1.33

1 0

I/Io 66 100 61 12 29 30 33 14 19 18

Table I I . X-Ray Powder D i f f r a c t i o n P a t t e r n s Catalyst 2 Catalyst 3 Catalyst 4 C a t a l y s t 17 USb 0 3

d(A) 3.83 3.16 2.44 1.91 1.83 1.66 1.64 1.58 1.46 1.33

u s 1 0

I/Io 44 100 53 10 20 35 31 14 17 19

k2.4

d(A)

T i

0.6°y I/Io

USb Ti0 2

d(A)

y

I/Io

USb ZrO 2

d(A)

y

I/Io

3.87 3.18 2.45 1.94 1.83 1.65

52 100 72 15 40 68

3.90 3.18 2.45 1.96 1.83 1.67

55 100 71 12 36 65

3.92 3.22 2.47 1.95 1.84 1.67

67 100 73 16 40 75

1.59 1.46 1.34

15 22 20

1.59 1.47 1.34

17 21 20

1.60 1.48 1.34

46 23 21

T a b u l a t i o n o f peaks e x c e e d i n g I/Io>10

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

5.

INNES E T A L .

79

Promoted Uranium-Antimony Oxide Catalysts

To determine whether t i t a n i u m s u b s t i t u t i o n would occur i n the presence o f excess antimony, c a t a l y s t 9 ( T a b l e I ) was prepared as d e s c r i b e d i n a D i s t i l l e r s patent ( 1 0 ) , w h i l e c a t a l y s t 10 was prepared by our standard c o p r e c i p i t a t i o n method. Small, amounts of TiÛ2 c o u l d be d e t e c t e d i n both c a t a l y s t s . C a t a l y s t 9, which was c a l c i n e d a t a lower temperature than c a t a l y s t 10, c o n t a i n e d USbO^ i n a d d i t i o n to T i 0 * The s h i f t i n d - s p a c i n g f o r the 004 r e f l e c t i o n noted w i t h the t i t a n i u m - s u b s t i t u t e d phases was not seen for c a t a l y s t s 9 and 1 0 . Thus, the presence o f excess antimony appeared to i n h i b i t t i t a n i u m s u b s t i t u t i o n . These c o m p o s i t i o n s were w e l l above (on the excess antimony s i d e ) the b i n a r y j o i n expected to f a c i l i t a t e t i t a n i u m s u b s t i t u t i o n for antimony. Z i r c o n i u m , w i t h a l a r g e r i o n i c r a d i u s (O.79 Â ) , d i d not s u b s t i t u t e as e a s i l y as t i t a n i u m i n t h e U S b 3 0 lattice. In only one c a s e , x = 1 . 0 , was a pure U S b 3 Z r 0 phase o b t a i n e d . The o t h e r c a t a l y s t s c o n t a i n e d USbO^, Sb20^, Sb205, and Z r 0 type phases. The X - r a y d i f f r a c t i o n p a t t e r n of U S b 2 Z r 0 i s compared i n T a b l e I I w i t h the u n s u b s t i t u t e d and t i t a n i u m - s u b s t i t u t e d p h a s e s . As w i t h the titanium catalyst the d-spacing for the 004 r e f l e c t i o n was increased. T i n s u b s t i t u t i o n was a l s o a t t e m p t e d , but the x - r a y d i f f r a c t i o n patterns gave no i n d i c a t i o n of substitution. The peaks c o r r e s p o n d i n g to Sn02 i n c r e a s e d i n d i r e c t p r o p o r t i o n to the amount o f s t a n n i c c h l o r i d e used i n t h e i r p r e p a r a t i o n , and USbO^ and USb30^Q were p r e s e n t i n amounts c o n s i s t e n t w i t h the U/Sb r a t i o . T a b l e I I I shows the X - r a y d i f f r a c t i o n p a t t e r n o b t a i n e d f o r the composition USb2Sn0 . 2

1Q

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch005

x

x

y

2

y

y

Table I I I . d(A)

3.92 3.86 3.36 3.25 3.20 2.64 2.50 2.46 2.37 1.97 1.93 1.87 1.84 1.77 1.69 1.68 1.66 1.59

X-Ray D i f f r a c t i o n P a t t e r n F o r U S b S n O C r y s t a l l i n e Phases . USb0 I/Io Sn0 3°10 2

u s b

48 60 38 80 100 23 33 50 8 6 8 12 19 19 22 22 27 9

5

y

2

X X X X X X X X X X X X X X X X X

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

80

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Effect

on C a t a l y t i c

Properties

F i g u r e s 1 and 2 show how the c a t a l y t i c a c t i v i t y and s e l e c t i v i t y f o r the p r o d u c t i o n of a c r y l o n i t r i l e v a r i e d w i t h t i t a n i u m l e v e l f o r the USb3 Ti 0y series. F o r purposes of comparison we assumed f i r s t o r d e r k i n e t i c s and computed the r e l a t i v e a c t i v i t y per gram u s i n g the f o r m u l a : x

x

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch005

Relative Activity -

045)

( O . 4

(gramfof

catalyst)

where X i s the f r a c t i o n o f p r o p y l e n e c o n v e r t e d . A relative a c t i v i t y o f 1.0 corresponds to the a c t i v i t y of the o l d u r a n i u m antimony o x i d e c a t a l y s t ( C a t a l y s t 1 ) . S e l e c t i v i t y was d e f i n e d on a carbon weight b a s i s . Substituting t i t a n i u m f o r antimony i n the USb30^Q phase dramatically increased c a t a l y t i c a c t i v i t y . The r e l a t i v e a c t i v i t y f o r the U S b 3 T i 0 y s e r i e s peaked at χ - 1 . 5 . The b e s t a c r y l o n i t r i l e s e l e c t i v i t y was o b t a i n e d a t x=O.6 and x=1.O. Reduced a c t i v i t y and s e l e c t i v i t y a t h i g h e r t i t a n i u m l e v e l s corresponded to USbO^ and U T i 0 f o r m a t i o n . The U S b T i O c a t a l y s t seemed to o f f e r the b e s t c o m b i n a t i o n of a c t i v i t y and s e l e c t i v i t y . Under optimum c o n d i t i o n s ( T a b l e I V ) i t y i e l d e d 83-84 mol% a c r y l o n i t r i l e per pass compared to 78% f o r the o l d uranium-antimony o x i d e c a t a l y s t ( 1 , 2 , 4 ) which required six times the contact time to obtain comparable conversions. R e p l a c i n g antimony w i t h z i r c o n i u m i n c r e a s e d c a t a l y t i c a c t i v i t y 11-fold. F i g u r e s 3 and 4 show t h a t a c t i v i t y peaked a t x - 1 . O . The USb2ZrO catalyst was less selective than the corresponding titanium-substituted c a t a l y s t but compared f a v o r a b l y to the o l d uranium-antimony o x i d e c a t a l y s t . x

x

5

2

y

v

Table I V .

Optimum A c r y l o n i t r i l e Y i e l d With U S b T i O 2

R e a c t i o n Temperature, C o n t a c t Time, s C H /Air/NH 3

6

°C

3

% C H Conversion % 0 Conv 3

6

2

475 O.65 1.0/11/1.1

y

Catalyst

475 O.72 1.0/10/1.1

97.6 85.9

98.9 93.9

11.8

13.7 O.4 1.4 84.1 O.4

% Selectivities CO + C 0 HCN Acetonitrile Acrylonitrile Other 2

% Y i e l d Per Pass

O.3 1.3 86.5 O.1 84.4

83.2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Promoted Uranium-Antimony Oxide Catalysts

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch005

INNES E T A L .

15

1 X IN Figure 2.

2

USb Ti OY 3-x

x

3

FORMULA

Relative a c t i v i t y of U S b 3 T i 0 x

x

y

catalysts.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch005

100

X IN Figure 3.

USb Zr O 3x

x

y

FORMULA

E f f e c t of Zr s u b s t i t u t i o n

for Sb.

15 τ

X IN

F i g u r e 4.

USb Zr 0 3x

x

Y

FORMULA

Relative a c t i v i t y of U S b 3 Z r 0 x

x

y

catalysts.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

5.

INNES ET AL.

83

Promoted Uranium-Antimony Oxide Catalysts

A l t h o u g h the X - r a y d i f f r a c t i o n p a t t e r n s i n d i c a t e t h a t v e r y l i t t l e t i n was i n c o r p o r a t e d i n the USb O^Q phase, t i n had the g r e a t e s t e f f e c t on c a t a l y s t a c t i v i t y . A c a t a l y s t c o n s i s t i n g of e q u i m o l a r amounts o f USbO^ and U S b ^ ^ Q w i t h o u t any promoters i s l e s s a c t i v e and l e s s s e l e c t i v e f o r the p r o d u c t i o n of a c r y l o n i t r i l e than t h a n U S b 0 (2). A l s o , S n 0 by i t s e l f i s a poor c a t a l y s t . Yet, i n t i m a t e m i x i n g of these phases produced h i g h l y a c t i v e catalysts. The x=1.0 and x=1.25 c o m p o s i t i o n s had relative a c t i v i t i e s of 1 3 . 0 and 1 3 . 9 , w h i l e e x h i b i t i n g good s e l e c t i v i t y f o r a c y r l o n i t r i l e p r o d u c t i o n ( F i g u r e s 5 and 6 ) . T a b l e V shows the e f f e c t of T i , Z r , and Sn a d d i t i o n when excess antimony was p r e s e n t . A l t h o u g h each i n c r e a s e d c a t a l y s t a c t i v i t y , the e f f e c t was much s m a l l e r than f o r the υδ^β-χΜχΟγ compositions. T i t a n i u m a d d i t i o n about doubled the relative a c t i v i t y compared to the s t a n d a r d uranium-antimony o x i d e c a t a l y s t , w h i l e Z r and Sn a d d i t i o n had a s m a l l e r e f f e c t . The poor s e l e c t i v i t y of the D i s t i l l e r s - t y p e c a t a l y s t , No. 9, i s a t t r i b u t e d to the presence o f USbO^. 3

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch005

3

Table V . Catalyst No.

1 Q

2

Promoting E f f e c t I n Presence Of Excess Antimony Rel. % AN Catalyst % C^H^ Sel. Act. Composition Conv.

1

USb

10 9

U S b

U S b

4.0 4.0

16.0

82.1

1.0

0.9°y 0.9°y

45.7 33.7

84.5 58.4

2.3 1.8

4 > 6

T i

T 1

0x

17

u s b

4.6

Z r

1.0°y

32.2

82.6

1.8

35

U s b

4.6

S n

1.0°y

19.8

80.3

1.2

O x i d a t i o n S t a t e o f Uranium F i g u r e 7 shows a p o r t i o n of the i n f r a r e d t r a n s m i s s i o n spectrum f o r the U S b T i 0 catalysts. I n f r a r e d bands a t 925 and 865 c m " i n USb 0^Q have been a t t r i b u t e d to the U-OJ-J-J- s t r e t c h ( 3 ) , w h i l e a band a t 715 c m i s b e l i e v e d to be an Sb-0 s t r e t c h . As χ was v a r i e d from 0 to 1.5, the i n f r a r e d a d s o r p t i o n bands a t 925 and 865 c m " s h i f t e d t o 950 and 895 c m " , w h i l e the band a t 740 c m " s h i f t e d i n the o p p o s i t e d i r e c t i o n to 715 c m " . As χ was i n c r e a s e d above 1.5, these bands d i s a p p e a r e d . The e f f e c t i v e molar paramagnetic moment of U S b T i O was l e s s than t h a t of the standard uranium-antimony o x i d e c o m p o s i t i o n ( F i g ­ u r e 8) but s t i l l s i g n i f i c a n t . As temperature was i n c r e a s e d from 4 t o 105°K, the e f f e c t i v e magnetic moment o f the o l d uranium-antimony o x i d e c a t a l y s t i n c r e a s e d to a v a l u e c o r r e s p o n d i n g to one u n p a i r e d e l e c t r o n which i s c o n s i s t e n t w i t h U ^ . A t low temperatures the e f f e c t i v e magnetic moment o f U S b T i O was s i g n i f i c a n t l y lower than f o r the o l d uranium-antimony c a t a l y s t , but as the temperature was 1

3 x

x

y

3

- 1

1

1

1

1

2

y

+

2

y

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch005

S O L I D STATE C H E M I S T R Y IN CATALYSIS

X IN F i g u r e 5.

USb Sn O 3x

x

FORMULA

Y

E f f e c t of Sn s u b s t i t u t i o n

for

Sb.

15

X IN USb~ S n O v

F i g u r e 6.

Y

v

FORMULA

Relative a c t i v i t y of U S b 3 S n 0 x

x

catalysts.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Promoted Uranium-Antimony Oxide Catalysts

INNES ET AL.

Λ 0

£

Λ

οοθ η

0

•

1.5

σ Ε

Ώ

..α

ο Xi

ο USb 6°Y

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch005

4 #

• USb TiOY 2

π 1 1 1 1 1 1— 0

20

40

60

80

100

120

TEMPERATURE, °Κ

F i g u r e 7.

I n f r a r e d s p e c t r a of

USb Ti 0 3x

x

y

catalysts.

F i g u r e 8. E f f e c t i v e magnetic moment of uranium i n o r i g i n a l and titanium substituted catalysts.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

86

S O L I D STATE C H E M I S T R Y IN CATALYSIS

i n c r e a s e d to 105°K i t a l s o approached v a l u e s c o r r e s p o n d i n g to one unpaired e l e c t r o n . While t i t a n i u m s u b s t i t u t e d f o r antimony and t h i s had a d r a m a t i c e f f e c t on c a t a l y t i c a c t i v i t y as e x p e c t e d , t h e r e i s a q u e s t i o n as to how much o f the uranium was c o n v e r t e d from the +5 to the +6 o x i d a t i o n s t a t e . The s h i f t s i n the i n f r a r e d bands i n d i c a t e a s h o r t e n i n g of the U-O^-J-J bond d i s t a n c e and a l e n g t h e n i n g of the Sb-0 bond d i s t a n c e w h i c h i s c o n s i s t e n t w i t h an i n c r e a s e in h e x a v a l e n t c h a r a c t e r , but the magnetic measurements show t h a t a s u b s t a n t i a l p o r t i o n of the uranium remained i n +5 s t a t e . I f the v a l e n c e o f uranium i s not changed, then the replacement of S b ^ by T i ^ must g e n e r a t e oxygen v a c a n c i e s i n the USb3Û^Q l a t t i c e . It is t h e s e s i t e s t h a t may be r e s p o n s i b l e f o r the h i g h a c t i v i t y of the promoted c a t a l y s t s . +

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch005

+

Increasing Catalyst S t a b i l i t y The U S b 2 T i 0 g Q c a t a l y s t was made a t t r i t i o n r e s i s t a n t by a d d i n g an e q u a l weight o f s i l i c a b i n d e r p r i o r to c a l c i n a t i o n . X-ray d i f f r a c t i o n p a t t e r n s show t h a t the same U S b 2 T i 0 g ^ Q phase was formed as i n the unsupported c a t a l y s t . Acrylonitrile selectivity was u n a f f e c t e d by the a d d i t i o n of s i l i c a and the a c t i v i t y per gram of a c t i v e phase was as good or b e t t e r than w i t h the unsupported catalyst. Like the original uranium-antimony oxide catalyst, the t i t a n i u m s u b s t i t u t e d c a t a l y s t s were a b l e to o p e r a t e o n l y a s h o r t time w i t h o u t r e g e n e r a t i o n . O t h e r e w i s e , the c a t a l y s t became o v e r reduced, the USb3U^Q type phase decomposed, and s e l e c t i v i t y suffered. The a d d i t i o n o f s m a l l amounts of molybdenum or vanadium prevented o v e r - r e d u c t i o n e n a b l i n g the c a t a l y s t t o o p e r a t e w i t h o u t regeneration. R e p l a c i n g O.10 atom of uranium w i t h molybdenum s t a b i l i z e d the c a t a l y s t w i t h o n l y a s m a l l e f f e c t on a c t i v i t y and a c r y l o n i t r i l e selectivity (Table V I ) . Further replacement of uranium by molybdenum markedly reduced c a t a l y s t a c t i v i t y , so t h a t c o n t a c t time had to be i n c r e a s e d to m a i n t a i n a h i g h c o n v e r s i o n . The a d d i t i o n of molybdenum r e s u l t e d i n a g r e a t e r p r o d u c t i o n o f b y - p r o d u c t HCN and c o r r e s p o n d i n g l y l e s s carbon o x i d e s ( T a b l e V I I ) . A similar effect was o b t a i n e d w i t h vanadium. However, the vanadium seemed to i n c r e a s e a c t i v i t y as w e l l as s t a b i l i z e the c a t a l y s t . 1

Conclusion The c a t a l y t i c a c t i v i t y o f the uranium-antimony o x i d e c a t a l y s t f o r p r o p y l e n e ammoxidation has been i n c r e a s e d an o r d e r o f magnitude by m o d i f y i n g the c a t a l y t i c a l l y a c t i v e phase r a t h e r than by a d d i n g various promoters to the optimum uranium-antimony oxide composition. T h i s m o d i f i c a t i o n was a c c o m p l i s h e d by s u b s t i t u t i n g t i t a n i u m , z i r c o n i u m , or t i n f o r antimony i n c o m p o s i t i o n s w i t h the e m p i r i c a l formula ^3-. Μ 0 . T i t a n i u m and z i r c o n i u m r e p l a c e d ϋ δ

χ

χ

γ

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

5.

Promoted Uranium-Antimony Oxide Catalysts

INNES E T A L .

Table V I .

Molybdenum Or Vanadium A d d i t i o n To Improve C a t a l y s t Stability Hours S t a b l e Contact %C H Acrylonitrile without Time, s Conversion Selectivity Regeneration

X Value Mo V

-

3

-

6

1.7

98

86

O.5

1.7

98

86

O.5

1.7

99

81

O.75

O.10

1.7

96

85

>150.

O.15

3.2

92

83

>150.

6.5

O.025 O.50 Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch005

87

96

84

>150.

O.10

1.1

98

84

>150.

O.05

1.1

99

83

>150.

O.20

O.05 Catalyst

c o m p o s i t i o n 50 wt% U

Contact-time

= 1.7

Temperature C H /air/NH 2

6

0 9

Sb Ti(Mo,V) 0 2

x

/ 5 0 wt% S i 0

9 1 0

• 475°C 3

-

1.0/11/1.1

Table V I I .

E f f e c t Of Molybdenum A d d i t i o n On S e l e c t i v i t i e s

Catalyst

50% U S b T i 0

Composition

50% S i Q

2

2

9 1 0

50% U

0 9

50% S i Q

Sb TiMo 2

98.2

96.3

Selectivities: CO + C 0 HCN Acetonitrile Acrylonitrile

12.0 O.4 1.6 86.0

7.1 6.7 1.7 84.5

2

Contact-time

= 1.7 β

Temperature 3

6

3

0 e l

0

9 1 0

2

% C H Conversion

C H /air/NH

2

s

s

475°C = 1.0/11/1.1

^

^ ^ ^ ^

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

88

S O L I D S T A T E C H E M I S T R Y IN CATALYSIS

antimony i n the USb 0 Q l a t t i c e with only small changes i n the X-ray d i f f r a c t i o n pattern. Tin was not incorporated into the USb30^Q l a t t i c e but s t i l l had a strong effect on c a t a l y t i c activity. Since a s i g n i f i c a n t amount of uranium remains i n the +5 oxidation state, we believe that the replacement of Sb ^ with T i ^ and Z r ^ generates oxygen vacancies i n the c r y s t a l l a t t i c e which enhance c a t a l y t i c a c t i v i t y . The s t a b i l i t y of these catalysts i s increased by the addition of small amounts of molybdenum or vanadium which may catalyze reoxidation of the catalyst preventing over-reduction. 3

1

+

+

+

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch005

Acknowledgments. We thank Professor W. E . Wallace of the University of Pittsburgh and h i s students f o r making the magnetic s u s c e p t i b i l i t y measurements.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

11.

12.

13. 14.

Callahan, J. L.; Gertisser, B. U.S. Patent 3,308,151, issued to The Standard Oil Co. (Ohio), (Aug. 3, 1965). Grasselli, R. K.; Callahan, J. L. J. Catal. 1969, 14, 93-103. Grasselli, R. K.; Suresh, D. D.; Knox K. J. Catal. 1970, 18, 356. Grasselli, R. K.; Suresh, Dev D. J. Catal., 1972, 25, 273-291. Innes, R. Α.; Perrotta, A. J. U.S. Patatent 4,040,983, issued to Gulf Research & Development Company (Aug. 9, 1977). Innes, R. Α.; Perrotta, A. J. U.S. Patatent 4,045,373, issued to Gulf Research & Development Company (Aug. 30, 1977). Innes, R. Α.; Kehl, W. L. U.S. Patent 4,222,899, issued to Gulf Research & Development Co. (Sept. 16, 1980). Innes, R. Α.; Kehl, W. L., U.S. Patent 4,296,046, issued to Gulf Research & Development Co. (Oct. 20, 1980). Butera, R. Α.; Craig R. S.; Cherry, L. V. Rev. Sci. Instr. 1961, 32, 708-711. Ball, W. J.; Barclay, J. L.; Boheman, J.; Gassen, E. J.; Wood, B. Brit. Patent 1,007,929, issued to Distillers Company Limited, (Oct. 22, 1965). Callahan, J. L.; Grasselli, R. K.; Knipple, W. R. U.S. Patent 3,328,315, issued to The Standard Oil Company (Ohio), (June 27, 1967). "Lange's Handbook of Chemistry"; Twelfth Edition, Dean, J. Α., Ed.; Section 3, pp. 120-123, McGraw-Hill Inc., New York, N.Y. (1979). Chevalier, M.; Gasperin, M. C. R. Acad. Sci. 1968, C 267, 481. Chevalier, M.; Gasperin, M. C. R. Acad. Sci. 1969, C 268, 1426.

RECEIVED

October 4, 1984

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

6 Phase Relationships in the Cerium-Molybdenum-Tellurium Oxide System 1

1

J. C. J. BART , N. GIORDANO , and P. FORZATTI

2

1

Instituto di Chimica Industriale, Universita di Messina, Messina, Italy Dipartimento di ChimicaIndustrialeed Ingegneria Chimica del Politecnico, Piazza Leonardo da Vinci 32, 20133 Milano, Italy

2

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch006

The

complex

solid

state

relations

-molybdenum-tellurium oxide determine the boundaries phase

distributions

system

of

of

of

the cerium­

were

studied

to

single phase regions and a

typical multicomponent

ammoxidation catalyst.

Between

the (Ce,Mo,Te)O system

contains the following phases:

CeO ,

MoO ,

2

TeO ,

3

2

3

4

2

2

3

2

2

Ce (TeO ) ,

6

β-Te MoO , 2

7

α-Ce Mo O

12.25

CeTe O ,

15

and

7

Ce Mo O ,

13

β-Ce Mo O , 2

α-Te MoO

2

β-Ce Mo O ,

400° and 600°C in air

2

4

4

a

3

and

15

solid solution

(Ce,Te)O , Ce Mo Te O , Ce Mo Te O , 2

6

10

Ce Mo Te O , 4

11

4

47

2

2

2

13

Ce Mo Te O ,

59

10

2

2

4

Ce Mo Te O ,

17

10

of primary crystallization of in the (Ce,Mo,Te)O system

12

14

fields

79

each of these compounds

are

indicated.

A typical

active (Ce,Mo,Te)O ammoxidation catalyst is composed of

the

binary

α-Ce Mo O 2

4

and

15

phase the

structure

of

a

highly

2

ternary

(eventually together with

The

β-Ce Mo O 3

oxide

Ce Mo Te O 6

active

10

4

47

and/or

13

Ce Mo Te O 4

11

10

59

and MoO ). 3

cerium-molybdenum-tellurium

a c r y l o n i t r i l e c a t a l y s t Q ) has previously been described i n terms of binary

(Ce,Mo)0

and

ternary

(Ce,Mo,Te)0

phasesC2).

concluded that none of the constituent oxides ( C e 0

2>

It

was

Mo0 and Te0 ) 3

0097-6156/85/0279-0089$06.00/0 © 1985 American Chemical Society

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2

90

S O L I D STATE C H E M I S T R Y IN CATALYSIS

o r compounds o f the b i n a r y (Te,Mo)0 o r ( T e , C e ) 0 systems a r e

present

as the a c t i v e phases i n the ammoxidation c a t a l y s t * We have

recently

ternary oxides

identified

(Ce,Mo,Te)0(3),

compositions c a l c i n e d 600°C.

in

after

air

Combined w i t h the

and

at

characterized studying

( C e , T e ) 0 c h e m i s t r y , w h i c h was developed

(Ce,Mo,Te)0

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch006

identification

the

the

between

4 0 0 ° and

The

active

(Ce,Mo,Te)0 a c r y l o n i t r i l e

in

the l a s t decade, i t

is

complex s o l i d - s t a t e r e l a t i o n s o f the

system.

of

over 100 d i f f e r e n t

temperatures

knowledge o f the ( T e , M o ) 0 , ( C e , M o ) 0 , and

now p o s s i b l e t o d e s c r i b e ternary

s e v e r a l new

results

phase

culminated

composition

of

in

a

the

typical

catalyst.

Experimental

P r e p a r a t i v e methods and samples used previous work(3). (CuKA^ the

r a d i a t i o n ) and

following

systems): and

Samples

P-

2

C e

2

M o

(

C e

materials a-Ce Mo 0

7

0

4 i5 I>

2

2

for

t h i s study were those o f

subjected

M o

3

(

(taken

from

4

3

1 3

2

)

3°l2.25 ~ '

3

binary oxide

1 3

Ce (Mo0 ) (6), 2

4

Te0

2

(ASTM 4-593) and Mo0

the

to

and 0 - C e M o O O 5 ) , O J - C e ^ o ^ O ^

C e ( T e 0 ) a ) , (Ce,Te)0 a) together with 2

to x - r a y d i f f r a c t i o n

s p e c t r a were i n t e r p r e t e d w i t h r e f e r e n c e

support

Te Mo0 (4),

were

2

3

CeTe^C^),

(ASTM 1 1 - 6 9 3 ) , C e 0

2

(ASTM 9 - 2 0 9 ) . T e r n a r y o x i d e s (Ce,Mo,Te)0 were

3

i d e n t i f i e d on the b a s i s o f

previous work(3).

T h e - r e l a t i v e amounts

of the phases formed were e s t i m a t e d by comparison o f the h e i g h t s o f the

characteristic

peaks

e s p e c i a l l y T e 0 ~ and 2

after

heating

in

Mo0 ~rich 3

above

550°C,

non-overlapping samples such

positions.

As

are often n o n - c r y s t a l l i n e

preparations

were

calcined

a d d i t i o n a l l y a t 500°C f o r 8 hours f o l l o w e d by s l o w c o o l i n g i n o r d e r to enhance

crystallinity.

R e s u l t s and D i s c u s s i o n

F i g u r e 1 shows ( C e 0 , Mo0 2

3

the

distribution

and T e 0 ) i n 2

temperatures from 4 0 0 ° to exhibits

the

lowest

of

the t h r e e c o n s t i t u e n t o x i d e s

the v a r i o u s t e r n a r y phase c o m p o s i t i o n s at 600°C.

overall

It

is

reactivity.

c l e a r l y seen t h a t C e 0 Noteworthy

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

is

2

the

Cerium-Molybdenum- Tellurium Oxide System

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch006

BART ET AL.

F i g u r e 1. S i n g l e - p h a s e b o u n d a r i e s f o r the component o x i d e s of the (Ce,Mo,Te)0 s y s t e m between 400° and 600° C and r e g i o n s of f o r m a t i o n of n o n - c r y s t a l l i n e r e a c t i o n p r o d u c t s .

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

92

S O L I D STATE C H E M I S T R Y IN CATALYSIS

c o n s i d e r a b l e e x t e n s i o n of the

c o m p a t i b i l i t y range f o r TeO^ between

400° and 4 5 0 ° C , w h i c h c o r r e l a t e s H^TeO^

A l s o of i n t e r e s t

is

w i t h the d e c o m p o s i t i o n process of

the

affinity

of Te0

f o r the

2

other

components ( i n p a r t i c u l a r CeC^) above 5 0 0 ° C . The presence of

-Te^MoOy

(Figure

the phase t r i a n g l e i s expected TeC^-MoO^ s y s t e m ( 4 , 8 ) .

2) i n the CeC^-poor a r e a of

based

Noteworthy

on the known b e h a v i o r of

i s the presence o f

the

considerable

n o n - c r y s t a l l i n e m a t e r i a l i n t h i s p a r t of the d i a g r a m , e s p e c i a l l y a t 550° and

600°C,

which

relates

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch006

^-Te^MoO^ g l a s s ( F i g u r e 1 ) . (4,9)

leads

to

an

partly

to

the

formation of

the

The g l a s s - f o r m i n g tendency of Te Mo0^ 2

underestimate

of

the

extension

of

the

c o m p a t i b i l i t y range of the compound. Among the p r o d u c t s of is extensive

found

phase

a

Ce Mo 0^2 25 2

t

3

abundant

more

o

o

minor

up

to

l o

o

with

more

and

product

under

f o r m a t i o n sequence w i t h the phase

600°C

of

is

reaction

the

500°C

2

2).

an

phase

3

25 *

2

At 600°C,

s

a-

i n comparison to

formed a t about 600°C as a conditions.

in

the

Ce Mo 0^

field

cerium-molybdenum found

occupies

o f oi-Ce^Mo^O^ and

(Figure

phase

c

relations

Above

temperatures.

O. L 4 ID

our

compound

formation

restricted

Brown-red

c

500°C.

complex

e s p e c i a l l y at 550°

J il,ZD .

L

4 5 0 ° C.; the

increasingly higher

C e ^ M o ^ O ^ shows a Ce Mo 0

(Ce,Mo)0 s y s t e m , o n l y y e l l o w - g r e e n

below

field

distribution is

the

the

The

observed

o x i d e phases agrees

(Ce,Mo)0 s y s t e m ( 2 ) ,

even

though the b i n a r y compounds are formed at l o w e r temperatures i n the ternary system.

I n f a c t , whereas

550°C i n ( C e , M o ) 0 ,

this

(Ce,Mo,Te)0

system.

a-Ce Mo 0

and

2

4

1 5

respectively,

as

/3-Ce Mo 0 2

compound

forms

Similarly,

Ce Mo 0 2

3

1 2

opposed

forms by about 650°C i n a i r i n

2

to

temperature i n t h i s s t u d y .

the

i s detected

at

in

at

500°-

a t 400°C i n the

binary

formed

system

550°

the

and

ternary

in air 650°C, system.

t r a n s f o r m a t i o n of c v - C e ^ o ^ O ^

binary The

1 3

already

450°C

polymorphic the

in

are

5

3

system absences

s c h e e l i t e C e ^ M o O ^ ) ^ are not s u r p r i s i n g

at

and at a s l i g h t l y l o w e r of y - C e ^ o ^ O ^ and

the

and are i n accordance w i t h

p r e v i o u s d a t a ( 2^). With

regard

to

the

( F i g u r e 3 ) , we n o t i c e t h a t

phases the

of

the

(Te,Ce)0

solid solutions a -

subsystem

and 0 - ( C e , T e ) O

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2

Cerium-Molybdenum-Tellurium Oxide System

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch006

BART ET AL.

F i g u r e 2 . S i n g l e - p h a s e boundaries for (Te,Mo)0 and (Ce,Mo)0 phases i n the (Ce,Mo,Te)0 system between 400° and 6 0 0 ° C .

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch006

S O L I D STATE C H E M I S T R Y IN CATALYSIS

F i g u r e 3 . S i n g l e - p h a s e boundaries f o r (Ce,Te)0 phases i n the (Ce,Mo,Te)0 system between 400° and 6 0 0 ° C .

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

6.

B A R T ET A L .

Cerium-Molybdenum-Tellurium Oxide System

are both formed i n 450° and 5 0 0 ° C .

rather

These

temperatures(7,10). formed at a 400°C).

r e s t r i c t e d c o m p o s i t i o n a l ranges between

solid

where

stability

are

ELTeO.

range

(up

not s t a b l e at

more

( T e , C e ) 0 system. F o r m a t i o n of

CeTe.O. ΔΟ

the

(Te,Ce)0

broad c o m p a t i b i l i t y range. i n c r e a s i n g temperature,

decomposes to

d i s t r i b u t e d phase i s s l i g h t l y

knowledge of

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch006

solutions

The T e ( V I ) - c o n t a i n i n g s c h e e l i t e

temperature The

95

restricted at

system(7). The

550°C)

Ce^(TeO^)^ to Te0

of

is

(above

o

this

widely

than i n the b i n a r y

550°C conforms w i t h our

T h i s phase a l s o o c c u p i e s a

sequence

namely

higher

from

of phase f o r m a t i o n w i t h

(Ce,Te)02

t o Ce^iTeO^)^ and

CeTe^O^, i s i n good agreement w i t h the b i n a r y s y s t e m Ç O . As

may

be

seen

from

Figure

t e l l u r i u m - r i c h t e r n a r y compounds Ce^Mo^Te^O^y

occupy

4,

important

and

C e

10

M o

is

taken

T e

12 14°79>

Also,

some

t

n

e

minor

equilibria

of

up

by

latter

two

and

c o m p o s i t i o n ranges

The c e n t r a l p o r t i o n of t h i s

with

Ce2Mo Te20 , 2

and

13

broad c o m p a t i b i l i t y r a n g e s .

are

(Ce,Mo,Te)0

and/or

Qe^io^Ze^O^,

Ce^Mo^Te^O^,-,

components

the

molybdenum-

extensive

w i t h i n the (Ce,Mo,Te)0 phase t r i a n g l e . diagram

the

Ce^Mo^Te^O^^,

formed

system,

in

the

namely

solid

state

C e 2 M o T e 2 0 ^ and 3

(Ce Mo Te 0 ). 4

1 3

3

5 1

Under our e x p e r i m e n t a l c o n d i t i o n s , t e r n a r y compounds are formed

already

( C e ^ M o ^ T e ^ O ^ ) , which occupies a not observed at h i g h e r Ce^Mo^Te^O^

is

Ce Mo Te 0

over

a

broad

present

ternary

compound

the f u l l

investigated;

1 3

temperature

range

e x t e n s i v e i n the 4 5 0 ° - 5 0 0 ° C temperature Compounds C e M o T e 0 , 2

a l l formed at about CeyMo^&jQ^ ternary

M

2 °3

T e

4

1 7

450°C

M

the

range.

w h i c h i s s t a b l e over phase f i e l d

i s most

interval.

T e

are

400°-550°C

its

io °12 l4°79

and

range

stable

a

n

d

C e

M

6 °8

T e

6°45

up t o at l e a s t

a

r

e

600°C.

e x h i b i t s the most e x t e n s i v e c o m p a t i b i l i t y range o f a l l

compounds

f i n d s i t s major C e

2

C e

temperature

i n the 4 0 0 ° - 4 5 0 ° C r a n g e .

Ce^Mo^Te^QO^g i s the o n l y

2

Amongst these i s

c o m p a t i b i l i t y a r e a and i s

in

2

aforementioned

The molybdenum-rich compound

stable

2

is

400°C.

small

( 4 0 0 ° - 5 5 0 ° C ) , and i s e x t e n s i v e l y Also,

at

temperatures.

stable

f o u r of the

(in

particular

extension

at

above

600°C.

500°C); Finally,

M

T e

C io °12 14^79 e

the minor phase

2 ° i 6 i s formed at 500°C and i s s t a b l e up t o over 6 0 0 ° C .

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch006

F i g u r e 4 . S i n g l e - p h a s e b o u n d a r i e s f o r (Ce,Mo,Te)0 phases i n the temperature range between 400° and 6 0 0 ° C .

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

6.

BART ET AL. As may

be

seen

from

Figure

c r y s t a l l i n e m a t e r i a l was d e t e c t e d phase

97

Cerium-Molybdenum- Tellurium Oxide System

triangle

at

various

1,

in

x-ray

amorphous o r m i c r o -

f a i r l y e x t e n s i v e areas i n the

temperatures.

c e r i u m - p o o r samples c a l c i n e d

at

600°C

s i n t e r e d a s p e c t and are d a r k

in

colour.

In

show

p a r t i c u l a r , many

a v i t r e o u s or h i g h l y However, o n l y i n a few

cases c o m p l e t e l y amorphous samples were observed but the r e g i o n s of g l a s s f o r m a t i o n o b v i o u s l y s t r o n g l y depend on the c o o l i n g r a t e ; aspect was not f u r t h e r

investigated.

It

appears t h a t

this

considerable

amounts of (Te,Mo)0 g l a s s - l i k e m a t e r i a l ( o f the 3-Te MoO^ type)

are

2

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch006

formed

at

550°

and

s c a t t e r i n g maximum*

600°C. of

max

We

the

c a l c i n e d at 600°C v a r i e s from

Â

in

1 1

1 A

dence t o d=3.34

Â

but

r i c h f r a c t i o n at 500°C but

the

materials with

v a l u e drops to c a . 3.24

Â.

550°C

the

exhibit

7

tellurium-

and

maxima i n c o r r e s p o n -

i n c r e a s i n g cerium concentration

3

conforms

1)

x-ray

samples

this

A l s o , the n o n - c r y s t a l l i n e molybdenum-

molybdenum-poor

temperature ( F i g u r e

the

2

At

c o

molybdenum-rich amorphous

that

f r a c t i o n i n the

s i d e ( c f r . 3.33 À i n 3 - T e M o 0 and

Ce Mo Te O ). 4 11 10 59 /

noticed

d=3.34 Â a t the molybdenum-rich s i d e

t o 3.19 A at the t e l l u r i u m - r i c h 3.29

have

amorphous

microcrystalline is

3

t o g l a s s y (Te,Mo)0 ^ ^ χ * .

different

(d

part =3.15

at

3 3

^) ,

the

A).

same

At lower

max temperatures the presence p a r t l y due starting

to

of

incomplete

products.

non-crystalline material is

decomposition

The e x t e n s i v e

and

probably

interaction

of

the

amorphous phase f o r m a t i o n i n the

c e n t r a l p o r t i o n o f the phase diagram at 450°C may be due t o C e 0 or Ce^Mo^Te^O^ ^ ~ * ^» c e r i u m - p o o r amorphous f r a c t i o n d v a r i e s from c a . 3.36 t o 3.27 Â a t the Mo- and T e - r i c h s i d e s , max 2

d

3

1 5

i

n

t

n

e

m a x

respectively.

At

400°C

respectively.

At

this

these

values

temperature,

are some

3.30

completely

samples were found a t the C e : M o : T e = ( 1 5 - 2 0 ) : 4 5 : ( 3 5 - 4 0 ) D e s p i t e the g r e a t c o m p l e x i t y of phases) a l l x - r a y powder s p e c t r a be a s c e r t a i n e d

from the

c o m p o s i t i o n a l range

w i t h l e s s than about reasonable

5

with at%

and corresponds

to

of

the of the

Â,

amorphous

different

As may e a s i l y

the proposed phase d i s t r i b u t i o n s

p r o p e r l y account f o r the presence full

interpreted.

3.19

ratios.

the system ( w i t h 20

were

figures,

and

each of the c a t i o n s over exception

one

of

the components.

sensitivity

the

of the phase ranges This i s

l i m i t of the

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

x-ray

98

SOLID STATE CHEMISTRY IN CATALYSIS

method.

Only i n the

400°C

s e r i e s , e x t e n s i v e a r e a s i n the c e r i u m -

and t e l l u r i u m - r i c h ranges o f the the presence o f Te o r Mo.

phase t r i a n g l e do not account

T h i s i s , however, a consequence o f

complete d a t a a t t h i s t e m p e r a t u r e . a n a l y t i c d a t a f o r the new the TeO^-Ce^iMoO^)^

for

Based on the r e p o r t e d

less

thermo-

t e r n a r y phases(3) , i t appears t h a t below

binary

juncture

steeply; compositions r i c h i n

the

l i q u i d surface T e

MoO^ and

0

m

e

2

^

t

a

t

descends

considerably

lower t e m p e r a t u r e s . I n some r e a c t a n t m i x t u r e s

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch006

were

detected,

e.g.

Ce:Mo:Te=7:8:5

after

presence o f more

as

as

in

many the

calcination

phases

than

at

as s i x c r y s t a l l i n e phases case

of

the

550°C

for

composition

8

those p e r m i t t e d under

hours.

thermodynamic

e q u i l i b r i u m i s a consequence o f the i n c o m p l e t e n e s s o f the between the components under also noticed that various sensitive

to

the

s

Ce:Mo:Te 5:8:7

Ce^MOgTe^O^,

areas

composed

experimental of

preparative

is

^-Ce^Mo^O^ after

our

the

of

Ce^Mo^Te^O^^

and

c a l c i n a t i o n at 500°C and 550°C

2

It

is

1 7

the composition

C e ^ M o ^ T e ^ O ^ and

f o r 8 hours but c o n s i s t s o f

an

each

conditions.

e.g.

Ce Mo Te^0 , 2

reactions

phase diagram a r e h i g h l y

conditions;

c a l c i n a t i o n at 550°C

The

amorphous for 8 hours.

fraction

after

Another t y p i c a l

example i s g i v e n i n T a b l e I , but v a r i o u s o t h e r such cases were a l s o encountered.

Without v a r i a t i o n s

r e s u l t s are normally (such

as

reduced

o b s e r v e d , as i n d e e d

in

perfectly

the r e a c t i o n p a r a m - e t e r s ,

reproducible.

molybdenum

oxides

and

the

No reduced phases TeMo^O^)

were

ever

expected.

Conclusions

On the b a s i s o f

the

solid-state

s y s t e m , i t i s now p o s s i b l e t y p i c a l unsupported

to

derive

(Ce,Mo,Te)0

composition of R e f . ( 1 1 ) ,

as

r e l a t i o n s h i p s o f the ( C e , M o , T e ) 0 the phase d i s t r i b u t i o n o f a

acrylonitrile

indicated

in

catalyst

Table

I.

with

the

The r e s u l t s

agree w i t h p r e v i o u s c o n c l u s i o n s w i t h r e g a r d to the r o l e o f (Te,Mo)0 and ( T e , C e ) 0 o x i d e s i n t h i s o f the a c t i v e (Ce,Mo)0 XPS r e s u l t s

system and the most l i k e l y c o m p o s i t i o n

phases

( /?-Ce Mo 0 2

3

1 3

and a - C e ^ o ^ O ^ )

( C e ( I I I ) r a t h e r t h a n Ce(IV) i n the c a t a l y s t )

favour

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

02). the

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

3

2

3

+

+ +

a+8

8

600

600

+, p r e s e n t ; - , a b s e n t ; s,

Amorphous f r a c t i o n w i t h d

b

c

r

As p r e c e e d i n g

a

max

=3.33 A .

s m a l l amount.

+

a+8

550

line.

+

+

+

+

a+8

llT

500

4

10

59

Ce Mo e 0

+

15

+

4

8

2

-Ce Mo 0

450

M

2 °3°12.25

Ce

8

13

400

Mo0 -Ce Mo 0

Phase d i s t r i b u t i o n

(Ce,Mo,Te)0

t

(h)

conditions

Phase d i s t r i b u t i o n i n the unsupported

Τ (°C)

Activation

Table I .

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch006

Ce^o^Te^

catalyst.

(c) + (c)

+

+ (c)

amorphous

100

S O L I D STATE C H E M I S T R Y IN CATALYSIS

presence of a - C e ^ M o ^ O ^

and

lead

to

oxygen c o n t e n t of the c a t a l y s t Q J . component

i n the phase diagram

the c a t a l y s t

is

essentially

p r o p y l e n e (J12^.

The

previously

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch006

(Table

In f a c t ,

the

presented

range

of

MoO^

catalyst.

active

ternary

phase(s),

and

the

stoichiometry

(Table m a x

I).

is

our

current

study

The p r e v i o u s l y

the

to

reported

by

catalyst,

x-ray

no t e l l u r i u m -

diffraction

of

the (d

fflax

samples

above 500°C

amorphous

fraction

=3.29 A ) , a compound

n o n - c r y s t a l l i n e form above 5 0 0 ° C ( 3 ) .

unknown

i d e n t i f i e d as C e . M o , . Te. O . 4 11 10 59 a c t i v e phase of the

active

maximum

in

and/or

s t a b l e much above 5 0 0 ° C .

Ce^io^e^O^

obtained

n

being

identification

Ce^Mo^Te^O^

the phase d i s t r i b u t i o n o f

the

detected

= 3 . 3 3 A) p o i n t s

which i s e a s i l y

to

of

However,

namely

not

i s noticed that according

c o n t a i n i n g phase

(d

the

T h i s c o n c l u s i o n agrees

results(2)

C e ^ M o ^ T e ^ O ^ w i t h the l a t t e r

with

in

I).

the

It

ammoxidation

the presence o f t h i s compound as

The new f e a t u r e , d e r i v e d from t h i s work, i s the of

to

i t i s w e l l known

inactive i n selective

compatibility

a s i g n i f i c a n t component of

proposed

of s i g n i f i c a n c e w i t h r e s p e c t

( C e , M o , T e ) 0 system c a s t s doubt on

with

r e v i s i o n of the

c o m p o s i t i o n and a c t i v i t y .

t h a t t h i s phase i s of

a

The absence of Ce^iMoO^)^ as a

ternary Therefore,

cn

(Ce,Mo,Te)0

oxide(2) '

it

has

now been

i s l i k e l y that J

ammoxidation c a t a l y s t

the

c o n s i s t s of

an Q f - C e ^ M o ^ O ^ - r i c h m i x t u r e c o n t a i n i n g C e ^ M o ^ T e ^ O ^ g . I n e v a l u a t i n g our r e s u l t s industrial catalystQ), effect and

of the s i l i c a

yet

is

from

that

to

phase of

In

fresh SiO^-supported a c t i v e

the the

a

also

In of

(Te,Ce)0/Si02 the

fact,

as

here shown

the ( T e , C e ) 0 system system

at

the

same effect

r a t e s and s t a b i l i t y ranges

from x - r a y d i f f r a c t i o n d a t a of a

ternary to

taken o f

d i l u t i o n and i n t e r a c t i o n

formation fact,

be

has not been c o n s i d e r e d role.

distribution

the

the r e s p e c t i v e

of the v a r i o u s phases.

c o n c l u s i o n s i n r e l a t i o n to should

which

play

a c t i v a t i o n temperature due t o which a f f e c t s

and

account

support,

likely

p r e v i o u s l y ( 7 , 1 0 ) , the differs

an

the

phase, results

the f o r m a t i o n o f some

CeO^ i s

inferred,

contrary

system.

For these

reasons, a d d i t i o n a l spectroscopic

of

the

unsupported

and

catalytic

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

6.

BART ET AL.

Cerium-Molybdenum-Tellurium Oxide System

a c t i v i t y studies regard

to

the

are nature

necessary of

i n d u s t r i a l (Ce,Mo,Te)0/Si0

the

to

confirm

active

101

our suggestions with

phases

contained

i n the

catalyst.

Acknowledgment s

One of us ( P . F . ) acknowledges

support from the I t a l i a n Ministry of

Education.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch006

Literature Cited 1. 2. 3.

4. 5. 6. 7. 8. 9. 10. 11. 12.

Caporali, G.; Ferlazzo, N.; Giordano, N. German Patent 1.618.685, Nov. 23, 1972. Bart, J. C. J.; Giordano, N. I.&E.C. Prod. Res. Dev., 1984, 23, 56 Bart, J. C. J.; Forzatti, P.; Garbassi, F.; Cariati, F., Proc. Third Intl. Symp. Ind. Uses Selenium & Tellurium, Stockholm, 1984. Bart, J. C. J.; Petrini, G.; Giordano, Ν. Z. Anorg. Allg. Chem., 1975, 412, 258. Castellan, Α.; Bart, J. C. J.; Bossi, Α.; Perissinoto, P.; Giordano, Ν. Z. Anorg. Allg. Chem., 1976, 422, 155. Bart, J. C. J.; Giordano, N. J. Less Common Metals, 1975, 40, 257. Bart, J. C. J.; Giordano, N.; Gianoglio, C. Z. Anorg. Allg. Chem., 1981, 481, 153. Petrini, G.; Bart, J. C. J. Z. Anorg. Allg. Chem., 1981, 474, 229. Dimitriev, Y.; Bart J. C. J.; Dimitrov, V.; Arnaudov, M. Z. Anorg. Allg. Chem., 1981, 479, 229. Bart, J. C. J.; Giordano, N. J. Catal., 1982, 75, 134. Hucknall, D. J. "Selective Oxidation of Hydrocarbons"; Acad. Press: London, 1974, p. 56. Brazdil, J. F.; Grasselli, R. K. J. Catal., 1983, 79, 104.

RECEIVED March 20, 1985

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

7 Molybdate and Tungstate Catalysts for Methanol Oxidation C. J. MACHIELS, U. CHOWDHRY, W. T. A. HARRISON, and A. W. SLEIGHT

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch007

Central Research & Development Department, Experimental Station, Ε. I. du Pont de Nemours and Company, Wilmington, DE 19898

e ferric, chromium and aluminum molybdates as well as complete series of solid solutions of iron-chromium and iron-aluminum molybdates were synthesized using a solution technique to ensure obtaining pure, single phase, homogeneous powders. The surface area of these molybdates varied from 5 to 15 m /g and homogeneity was confirmed using scanning transmission electron microscopy. The selective oxidation of methanol to formaldehyde was studied over these pure and mixed molybdates. No significant differences were observed between the phases in specific activity, selectivity, and kinetic parameters. 2

The most selective catalysts for the oxidation of methanol to formaldehyde are molybdates. In many commercial processes, a mixture of ferric molybdate and molybdenum trioxide is used. Ferric molybdate has often been reported to be the major catalytically active phase with the excess molybdenum trioxide added to improve the physical properties of the catalyst and to maintain an adequate molybdenum concentration under reactor conditions(1,2). In some cases, a synergistic effect is claimed, with maximum catalytic activity for a mixture with an Fe/Mo ratio of l.T(3). A defect solid solution was also proposed(55) » Aging of a commercial catalyst has been studied using a variety of analytical techniques(h) and i t was concluded that deactivation can largely be accounted for by loss of molybdenum from the catalyst surface. In this laboratory, the mechanism of methanol oxidation over molybdate and tungstate catalysts has been studied using a variety of techniques. Steady state and pulse reactor studies using labeled reactants have established that methanol conversion to formaldehyde is a redox reaction with lattice oxygen being involved(6). A kinetic isotope effect has recently been reported (J}, and i t shows that the rate limiting step in the reaction sequence is removal of a hydrogen from the methyl group. Fourier transform infrared studies have shown that a methoxy, 0097-6156/85/0279-0103$06.00/0 © 1985 American Chemical Society

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch007

104

SOLID STATE CHEMISTRY IN CATALYSIS

CH3O, group i s a surface intermediate during the reaction(53.), and temperature programmed reaction studies have elucidated the nature of surface reactions and have allowed estimation of the number of c a t a l y t i c a l l y a c t i v e s i t e s on powder surfaces(8). In t h i s paper, we w i l l discuss r e s u l t s of the oxidation o f methanol over a series of molybdates including s o l i d solutions of f e r r i c , chromium and aluminum molybdates and also over a new f e r r i c tungstate phase. The mixed molybdates o f iron/chromium, iron/aluminum and chromium/aluminum were made f o r the f i r s t time i n pure well-characterized forms. Results are compared with our e a r l i e r work over commercial mixtures of f e r r i c molybdate and molybdenum t r i o x i d e and a number of pure molybdates{6). There i s a voluminous body of l i t e r a t u r e i n patents(9~19)> papers(20-27) and reports(28-3*0 on the preparation and c a t a l y t i c properties of the methanol oxidation c a t a l y s t , often without d e t a i l e d reference to the chemical composition of the products. Indeed, early investigations(35~3T) suggested that no compound was formed i n the reaction between Fe2Û3 and M 0 O 3 . However, Kozmanov et a l . (38~^) have since prepared the pure i r o n ( i l l ) molybdate, FegiMoOl^, by s o l i d state reaction of a stoichiometric mixture of i r o n and molybdenum oxides a t T00°C. Jager(kO) reported the formation of a bright green compound, while Nassau et a l . ( 4 l ) , i n t h e i r d e t a i l e d study of t r i v a l e n t molybdates, reported a tan compound prepared with c a r e f u l annealing of the oxides at 600°C. X-ray powder measurements by Nassau(4l) and Trunov and Kovba(^3) suggested that Fe2(Mo0l|)3 c r y s t a l l i z e d i n an orthohombic space group, while an early s i n g l e c r y s t a l study by Klevtsov and co-workers(k^-kl) reported the c r y s t a l symmetry t o be monoclinic. Despite contradictory reports(kQ), centric monoclinic i s now the accepted structure of the room temperature phase o f Fe2(Mo0i|)3 as confirmed by Chen(^9) i n t h i s labgratory. The space group i s P2i/a with a=15.707, b=9.231, c=l8.204A, and 3=125.25°. The structure consists of a rather open 3-dimensional network o f corner sharing FeO£ octahedra and MoOlj. tetrahedra, with four c r y s t a l l o g r a p h i c a l l y d i f f e r e n t i r o n s i t e s which lead to some novel low temperature magnetic properties(50~51). Sleight arid Brixner(52) have shown the presence of a f e r r o e l a s t i c phase-transition at 499°C between a low-temperature monoclinic and high-temperature orthohombic phase on the basis o f DSC measurements. Many studies have been made of s o l u t i o n phase preparations of the f e r r i c molybdate system. Aruanno and Wanke(5^) used a preparation from f e r r i c chloride and ammonium heptamolybdate s o l u t i o n s , following the work of Shelton et a l . ( i l ) . This has been developed by I t a l i a n workers under Pernicone(55) into the Montedison process(56-60); they made an i n t e r e s t i n g s t a t i s t i c a l study of the p r e c i p i t a t i o n stage(6l). Pernicone suggests that excess M0O3 may be incorporated into the Fe2(Mo0^)3 lattice(55) modifying the c a t a l y t i c properties; however, not a l l workers are i n agreement with t h i s proposal(6,62). Boreskov et a l . (63,64) used Fe(N03)3 and (ΝΗΐ^ζΜογΟ^ solutions to prepare a p r e c i p i t a t e and a number of other workers have followed t h i s procedure i n d e t a i l . In p a r t i c u l a r , T r i f i r o and co-workers(65) have explored t h i s route. Their method involves

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch007

7.

MACHIELS ETAL.

Molybdate and Tungstate Catalysts

105

a novel aging procedure of recovering the s o l i d i n the mother l i q u o r at 100°C for a few hours(66,67) t o c o n t r o l the Fe:Mo r a t i o i n the f i n a l product. Another s o l u t i o n method prepares an amorphous hydrated molybdate from Na2Mo0i| and Fe( 1^03)3 solutions ( 55 ), following the method of Kerr et al.(68) t o produce a product with a p r e c i s e l y defined i r o n to molybdenum r a t i o . F e r r i c molybdate has been prepared from gels involving f e r r i c n i t r a t e and ammonium heptamolybdate/hydrogen peroxide solutions by Tsigdinos and Swanson(69). A l l of these s o l u t i o n methods involve stages of drying and c a l c i n i n g the i n i t i a l p r e c i p i t a t e t o achieve the f i n a l product; f u l l d e t a i l s are given i n the respective references. Single c r y s t a l s of Fe2(Mo0i|)3 may be prepared hydrothermally following the work of Klevtsov(70) and Marshall(7l). Aluminum molybdate, AlgiMoOii^ was f i r s t prepared by Doyle and Forbes (72) quickly followed by other workers (1+1,73). DSC measurements by Sleight(52) indicate the structure t o be monoclinic below 200°C.; the structure i s isomorphous with chromium molybdate(Ul). There have been few studies on mixed compounds of i r o n , chromium and aluminum molybdates. Abidova and co-workers(7^-75) made a somewhat inconclusive study on the mixed Fe/Al/Mo oxide system. Based on DSC and reflectance measurements on the ' 2-component systems, they concluded that the 3-component mixture would be a complex multiphase system. Another study(76) used c o - p r e c i p i t a t i o n of i r o n and chromium n i t r a t e s and ammonium heptamolybdate. However, t h e i r Mossbauer effect data suggested inhomogeneity i n the f i n a l product. One other notable method has been used i n the preparation of mixed t r a n s i t i o n metal molybdates, amongst many other oxide systems. This novel method(77) involves preparation of the mixed metal oxides v i a an amorphous precursor such as a c i t r a t e s a l t of the appropriate metals, and then thermal decomposition of the complex to y i e l d the r e s u l t i n g mixed oxides. The experimental procedures are described i n four French patents(78-81), giving d e t a i l s of many d i f f e r e n t preparations including a proposed M0O3 r i c h , chromium doped iron molybdate, prepared as a possible s e l e c t i v e oxidation catalyst. Catalyst Preparation For the iron/aluminum s e r i e s , preparations from mixtures of the oxides Fe203, AI2O3 and M0O3 or from the n i t r a t e s Fe(NC>3)3, Al(NC>3)3 and the ammonium molybdate f a i l e d as d i d preparations from mixtures of the end-member molybdates, Fe2(M0O1O3 and A ^ M o O l ^ . A l l the products had very poor homogeneity as determined by semi-quantitative a n a l y t i c a l electron microscopy; s i m i l a r r e s u l t s were experienced over the entire range of composition for x=0-2 i n Fe2- Al (Mo0i )3. Pelleted preparations f i r e d at 700°C for several weeks showed no improvement i n homogeneity with time. Samples f i r e d at up to 1000°C l o s t M0O3 as indicated by the presence of Fe2Û3 x-ray l i n e s i n Guinier photographs, but s t i l l without noticeable improvement i n product homogeneity. A composition range no better than + 20 percent i n χ was the best obtained. x

x

+

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch007

106

S O L I D STATE C H E M I S T R Y IN CATALYSIS

P r e c i p i t a t i o n methods from s o l u t i o n of the t r i v a l e n t metal n i t r a t e s and ammonium heptamolybdate were also attempted. The method was successful f o r f e r r i c molybdate following the aging method of T r i f i r o et al.(66) t o produce a p r e c i p i t a t e of the correct stoichiometry. I t was notable that a f t e r t h i s aging process, but before annealing, the dried p r e c i p i t a t e showed some c r y s t a l l i n i t y , but atomic absorption measurements showed an excess of oxygen, presumably i n d i c a t i n g most of the p r e c i p i t a t e was hydrated. Annealing i n a i r at 400°C just above the temperature at which the l a s t water of hydration i s l o s t f o r 2h hours was found to produce a good Fe2(Mo0i|)3 x-ray pattern without any observable contamination by M0O3 or Fe2Û3. S i m i l a r r e s u l t s were experienced for mixed molybdates doped with small amounts of chromium and aluminum but those preparations containing more than 25$ of dopent showed M0O3 contamination when examined by x-ray d i f f r a c t i o n and the method f a i l e d t o produce the desired r e s u l t f o r chromium or aluminum. I t appears that the i r o n molybdate i s p r e f e r e n t i a l l y formed at low pH values ( t y p i c a l l y 1.5 i n these cases) while the chromium or aluminum i o n remains i n s o l u t i o n . Attempts t o modify the pH of the s o l u t i o n by the addition of NH^OH had l i t t l e e f f e c t on the r e s u l t i n g products which were s t i l l heavily contaminated with M0O3. Even f o r those samples doped with small amounts of A l or Cr, x-ray microanalysis showed poor product homogeneity as with the s o l i d state .preparations. By f a r the most successful method of preparation was v i a an amorphous organic precursor t o the required mixed molybdates, following the method of Delmon et al.(77). The method has proved successful f o r the pure i r o n , chromium and aluminum molybdates, and also f o r the mixed phases. A d e t a i l e d o u t l i n e of the method taking the example of FeCrdyioOi^ i s as follows: i ) ll.UUg ( 2 . 8 3 x l 0 moll of f e r r i c n i t r a t e hydrate, Fe(N03)3«9H20 was dissolved i n lOOcc of d i s t i l l e d water at room temperature r e s u l t i n g i n a yellow s o l u t i o n . i i ) 11.3Ug (2.83xl0" mol) of chromium n i t r a t e hydrate, 0(1103)3.9Η2θ was dissolved i n lOOcc o f d i s t i l l e d water a t room temperature r e s u l t i n g i n a dark blue s o l u t i o n . i i i ) The i r o n and chromium solutions were mixed r e s u l t i n g i n a blue s o l u t i o n , pH ca. 1.5 t o which 20g (9.5xl0~ mol) of c i t r i c a c i d monohydrate Ο^ΗβΟτ-Ι^Ο was added, and s t i r r e d t o d i s s o l v e . i v ) 15g (1.21x10"2 l ) of ammonium heptamolybdate hydrate, (ΝΗΐ|)6Μογθ2ΐ|.^Η2θ was dissolved i n 200cc of pure water r e s u l t i n g i n a clear s o l u t i o n . v) The molybdate s o l u t i o n was added t o the n i t r a t e s o l u t i o n ; no p r e c i p i t a t e formation was observed and the r e s u l t was a blue solution. v i ) This s o l u t i o n was dried on a steambath overnight, t o a blue-green glass and t h i s was transferred t o a vacuum oven at T0°C for 1 hour t o complete the drying of the precursor. The r e s u l t i n g glass i s very hygroscopic gaining a s t i c k y appearance i n only a few minutes. v i i ) The precursor was ground t o a green powder. X-ray d i f f r a c t i o n indicated that i t was t o t a l l y amorphous. v i i i ) The precursor was calcined at 400°C f o r 2k hours i n a i r . -2

2

2

m o

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch007

7.

MACHIELS ET AL.

107

Molybdate and Tungstate Catalysts

X-ray d i f f r a c t i o n indicated a c r y s t a l l i n e molybdate pattern without contamination; the product was a yellowish powder. The procedure i s discussed i n d e t a i l by Delmon et al.(82-8U). The c r u c i a l step appears to be the rapid dehydration of the s t a r t i n g s o l u t i o n before any of the components can c r y s t a l l i z e out of s o l u t i o n separately. Delmon(85) suggests that a rotary vacuum evaporation would be an e f f e c t i v e method of drying the precursor. The a c t u a l structure of the precursor i s not w e l l defined, but appears to require at least one equivalent of c i t r a t e ion per mol of metal ion(83), as presumably the c i t r a t e complexes a l l the metal species i n s o l u t i o n . The r e s u l t i n g powder patterns, a f t e r annealing, indicated no contamination. Delmon(83) suggests that any m u l t i f u n c t i o n a l acid containing at least one carboxyl and one hydroxyl function may be e f f e c t i v e . Experiments with t a r t a r i c a c i d on the iron/chromium system produced r e s u l t s s i m i l a r to c i t r i c a c i d ; a c a l c i n a t i o n temperature of 500°C was necessary before c r y s t a l l i z a t i o n occurred. The preparation of the new f e r r i c tungstate phase has been described p r e v i o u s l y ( j ) . I t i s schematically shown i n Figure 1. Catalyst Characterization A continuous range of s o l i d s o l u t i o n , such as the series Fe2- Cr (Mo0l|)3 provides a good opportunity for the q u a n t i t a t i v e , comparison of two a n a l y t i c a l techniques - " c l a s s i c a l " atomic absorption analysis and x-ray microanalysis. X-ray microanalysis of t h i n samples using scanning transmission electron microscopy has become an e f f e c t i v e quantitative technique i n the l a s t few years(86), as opposed to the well-known electron microprobe analyses of bulk specimens(87). A l l the work described below was c a r r i e d out on a Vacuum Generators HB501 instrument with an a c c e l e r a t i n g voltage of lOOkV, and at a t y p i c a l magnification of 1 m i l l i o n . Powdered samples were dispersed onto carbon coated 3mm copper grids from a suspension i n water, which led to a s a t i s f a c t o r y dispersion over the g r i d . P a r t i c l e s analyzed measured no more than 1000A i n s i z e whenever possible, to minimize absorption e f f e c t s , and those p a r t i c l e s l y i n g near the center of grid-squares were selected to minimize the i n t e n s i t y of the background CuK emission peaks due to the g r i d . The chromium, i r o n and molybdenum K l i n e s were used i n the a n a l y s i s , t h e i r average energies being 5«^3, 6.^3 and 17.50 KeV respectively. The MoL l i n e was not selected for the quantitative analysis due to the high Bremsstrahlung background a t low energy, and hence the d i f f i c u l t y i n estimating an accurate background subtraction. At low energy, there are also absorption effects due to the beryllium detector window, and for t h i s reason, the r e l a t i v e l y feeble Α1Κ peak may give u n r e l i a b l e quantitative r e s u l t s when only a small quantity of A l i s present. Thus, i n t h i s study, the Fe/Al and Cr/Al molybdates were not examined by x-ray microanalysis. For each sample investigated, at least 30 c r y s t a l l i t e s were examined, and the r e s u l t i n g x-ray emission spectra were analyzed using standard Kevex software; background subtractions were made automatically, and peak i n t e n s i t y r a t i o s were calculated. For each sample, a histogram of the Fe:Mo and x

x

a

a

a

α

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

108

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Cr:Mo r a t i o s was p l o t t e d t o ensure that the material was homogeneous and o f s i n g l e phase(86). There was a remarkable lack of impurity phases; i n p a r t i c u l a r , M0O3 was notable by i t s absence. F i n a l l y , C l i f f - L o r i m e r K^y values were determined f o r the pure i r o n and chromium molybdates, and the Fe:Mo and Cr:Mo r a t i o s across the series were obtained. The x-ray microanalysis r e s u l t s are p l o t t e d i n Figure 2. Atomic absorption measurements were c a r r i e d out by Galbraith Laboratories, Inc. These are p l o t t e d i n Figure 3. In general, both methods are i n excellent agreement with the predicted r e s u l t s . The x-ray r e s u l t s show that a s o l i d s o l u t i o n e x i s t s across the whole range o f composition of χ i n Fe2- Cr (Mo0l )3, rather than any mixture o f phases. Independent comparison of both sets of r e s u l t s with the t h e o r e t i c a l values f o r each compound shows the x-ray r e s u l t s t o be the closest t o the predicted values, with a t y p i c a l accuracy of + 1 per cent compared t o an estimated + 3 per cent f o r the atomic absorption measurements. The p a r t i c l e s i z e o f some o f the mixed molybdates produced by the c i t r i c acid technique were determined using a Micromeritics "Sedigraph" instrument. The average p a r t i c l e s i z e i s quite large i n each case, the median p a r t i c l e s i z e i s about 20pm. Scanning electron micrographs of gold-coated samples suggest that t h i s i s a good approximation, many of these larger p a r t i c l e s being agglomerates. Surface areas were recorded f o r the whole series o f each s o l i d s o l u t i o n by the standard N2 B.E.T. method. The r e s u l t s are l i s t e d i n Table I . E s p e c i a l l y notable are the r e l a t i v e l y high surface area o f those compounds r i c h i n aluminum and Al2(Mo0i|)3 i t s e l f . Such values are considerably higher than by previously attempted methods.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch007

x

x

+

Table I gurfaçg Areag o f MoXybdateg 2

n /g

Fe2(MoO] )3 Al2(Mo0lÔ3 Cr (Mo04)3 F e - A 1 ( Μο01+ ) F e - C r ( MoOl! ) 3 Cr2- Al (Mo04)

7 lh Τ 2-17 k-13 8-l6

+

2

2

2

X

X

x

3

x

x

x

3

x=0.0 - 0.2 - O.k

2.0

A l l these molybdates are i s o s t r u c t u r a l with f e r r i c molybdate with an open 3-dimensional network o f MO5 octahedra and MoOi| tetrahedra. A f e r r o e l a s t i c t r a n s i t i o n e x i s t s from the low temperature monoclinic form t o the high temperature orthorhombic form. The t r a n s i t i o n temperature varies from 200 C f o r pure aluminum molybdate t o 385 C f o r pure chromium molybdate and 500 C for pure f e r r i c molybdate. For the mixed molybdates, the t r a n s i t i o n temperature was found t o be a l i n e a r function of composition as i s i l l u s t r a t e d i n Figure k f o r the mixed iron-aluminum molybdates.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

7.

109

Molybdate and Tungstate Catalysts

MACHIELS ET AL.

Na W0 . 2 H 0 AQUEOUS SOLUTION 2

4

Fe (N0 ) . 9 H 0 AQUEOUS SOLUTION

2

2

3

3

2

YELLOW PRECIPITATE I

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch007

Τ

DRY WASH DRY WASH DRY

AMORPHOUS POWDER I Calcination CRYSTALLINE Fe (W0 ) 2

F i g u r e 1.

Preparation

0.7 ρ

1

1

1

4

3

Steps of F e r r i c

1

1

1

1

T u n g s t a t e Phase,

1

r

Mol Percent Cr Figure 2. Samples*

X-ray M i c r o a n a l y s i s f o r Fe/Cr

Molybdate

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch007

20 ι

1

0 F i g u r e 3. Samples.

1

1

20

1

1

1

40 60 Mol Percent Cr

Atomic Absorption

for

1

ι

80

100

Fe/Cr

Molybdate

DSC TRANSITION TEMPERATURES Monoclinic —

Orthorhombic

°C

25 Ft (Mo0 ) 2

F i g u r e 4. Molybdate

4

50

3

Phase T r a n s i t i o n Samples,

76 AI (Mo0 ) 2

Temperature

4

for

3

Fe/Al

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

7.

MACHIELS ET AL.

Molybdate and Tungstate Catalysts

111

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch007

Methanol Oxidation The three pure molybdates, the f e r r i c tungstate, and the mixed molybdates with a 1/1 cation r a t i o were tested as catalysts f o r methanol oxidation i n a continuous flow reactor with external recycle. The equipment and technique were described previously(6); d i f f e r e n t i a l rate and s e l e c t i v i t y data were obtained. The mixed chromium-aluminum sample had very poor mechanical properties ; no recycle could be used as a r e s u l t of excessive pressure drop over the catalyst bed. The f e r r i c tungstate sample showed behavior quite d i f f e r e n t from that of the molybdate, results are shown i n d e t a i l elsewhere(T). The rate of reaction of methanol to dimethylether was the same over both the tungstate and the molybdate phases, but the reaction rate t o formaldehyde was twenty times larger over the molybdate than over the tungstate. As a r e s u l t , the product d i s t r i b u t i o n was d i f f e r e n t f o r the tungstate with dimethylether being the main product. Product d i s t r i b u t i o n s for the three pure molybdates and the mixed molybdates were a l l s i m i l a r to those obtained previously for the commercial methanol oxidation catalyst and various other molybdate phases(6h Figures 5 and 6 i l l u s t r a t e t h i s product d i s t r i b u t i o n for the mixed i r o n aluminum phase, s e l e c t i v i t i e s are plotted over a range of methanol conversion of 20-90%. S e l e c t i v i t y to formaldehyde can be increased to over 90% by running at higher temperature, i n a single pass configuration and by adding water to the feed. Kinetics of the reaction were determined by varying the p a r t i a l pressures of oxygen, water, and methanol as w e l l as the temperature. Other p a r t i a l pressures were kept nearly constant; nitrogen was the diluent. K i n e t i c observations also were s i m i l a r as previously reported(6} as i s i l l u s t r a t e d i n Figures 7, 8 and 9 for d i f f e r e n t phases. The methanol reaction rate was nearly independent of the oxygen p a r t i a l pressure, except at very low oxygen pressures i n the reactor i n which case the catalyst begins to be reduced. I t was shown p r e v i o u s l y ^ ) that a reduced c a t a l y s t i s much less a c t i v e . The reaction rate has a p o s i t i v e dependence on methanol p a r t i a l pressure, but the reaction i s i n h i b i t e d by the addition of water. Water does however increase s e l e c t i v i t y to formaldehyde at the expense of dimethoxymethane, methylformate and dimethylether. The k i n e t i c data are f i t t e d w e l l by a power rate expression, parameters are shown i n Table I I . They are i n the expected ranges with apparent a c t i v a t i o n energies ranging from 18 to 20 kcal/mol. In order to compare the a c t i v i t y of the various phases, turnover numbers were calculated at the following conditions: 250 C, 150 t o r r oxygen and hO t o r r methanol p a r t i a l pressure. These turnover numbers are expressed as molecules of methanol reacting per surface molybdenum atom and per second; they are l i s t e d i n Table I I I . Clearly there i s l i t t l e difference i n a c t i v i t y between the pure and mixed phases studied here and t h e i r a c t i v i t y i s about equal to that of a commercial mixture of f e r r i c molybdate and molybdenum t r i o x i d e . Pure molybdenum t r i o x i d e i s less active by about a factor 3, but we have shown by TPD studies that the predominant (010) phase of molybdenum t r i o x i d e does not chemisorb methanol(Q). In comparison, various bismuth molybdate phases that

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch007

.80 I

1

1

1

1

1

1

1

—τ

r

MeOH Conversion

DMM = dimethoxymethane DME = dimethylether MF = methyl formate

F i g u r e 5&6. P r o d u c t D i s t r i b u t i o n v e r s u s M e t h a n o l C o n v e r s i o n f o r FeAl(Mo04)3«

Fractional

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

M A C H I E L S ET A L .

Molybdate and Tungstate Catalysts

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch007

6.0 ι

0.o « 0

1

1

85

02

1

1

—I 1 170 255 Partial Pressure (torr)

Γ

"

»

340

425 6

F i g u r e 7. R e a c t i o n R a t e o f M e t h a n o l (10"~ m o l e s / s e c , g. c a t a l y s t ) v e r s u s Oxygen P a r t i a l P r e s s u r e f o r Fe2(Mo04)3,

MeOH Partial Pressure (torr)

F i g u r e 8. R e a c t i o n R a t e o f M e t h a n o l v e r s u s M e t h a n o l P a r t i a l Pressure f o r FeAl(Mo04)3*

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

114

S O L I D STATE C H E M I S T R Y IN CATALYSIS

are w e l l known s e l e c t i v e oxidation c a t a l y s t s f o r other reactions are much less a c t i v e f o r methanol oxidation. Table I I .

Power Rate Law Parameters

E

a kcal/mol 19.0 18.U

Fe2(Mo0l|)3 Al (MoOl ) Cr^MoO^h FeAltMoOl^ FeCr(Mo04)3

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch007

2

+

3

Apparent Reaction Orders [0 ]

[MeOH]

0.17

0.58 0.U0

20.3

0.26 0.20 0.16

19.2

0.15

O.hl

17.9

Table I I I .

2

[H 0] 2

0.U7 0Λ9

-0.58 -0.55

Turnover Numbers

Molecules of MeOH Reacted Per Second, Per Surface Molybdenum Atom a t : 250°C 150 t o r r 0 kO t o r r MeOH 2

Fe (Mo0i^)3 Al (Mo0l^)

0.05 0.03 0.08 0.06 0.06

2 2

3

Cr (Mo0lJ3 FeAliMoOi!^ FeCr(Mo0l|)3 2

M0O3 Mo03/Fe (MoOl^3 Bi Mo0£ Bi Mo 09 Bi (Mo04)3 2

2

2

2

2

Bi^(Mo0i ) (Fe0i ) l

2

+

0.02 0.06 0.003 0.009 0.007 0.01

Figure 10 shows a schematic representation o f the r e a c t i o n mechanism over molybdenum t r i o x i d e . There i s competitive d i s s o c i a t i v e adsorption of methanol and water on molybdenum s i t e s . The slow step i n the sequence i s the breaking o f a carbon-hydrogen bond i n the methyl group o f the surface methoxy(5) . Dimethylether when fed over the c a t a l y s t with water does not react either t o methanol or other products up t o 300 C. This implies that the ether does not adsorb d i s s o c i a t i v e l y as methoxy groups. Dimethoxymethane and methyl formate when fed over the c a t a l y s t with water react q u a n t i t a t i v e l y at temperatures as low as 150 C. Dimethoxymethane gives methanol and formaldehyde i n a 2/1 r a t i o with t h i s r a t i o decreasing at higher temperature. Methylformate gives methanol and CO i n a 1/1 r a t i o with the methanol r e a c t i n g further at temperatures above 200 C.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch007

MACHIELS ET AL.

Molybdate and Tungstate Catalysts

F i g u r e 9. R e a c t i o n R a t e o f M e t h a n o l v e r s u s Water P a r t i a l Pressure f o r FeCr^004)3. H 0

H 0

0

•

Il Mo — 0 ~ Mo

I

Mo — 0 — Mo

H0

—

2

CH

iMeOH

0

H 0

1

DME

Mo - 0 — Mo

—

CH„ H 0

H*

CH 0 2

Mo — 0 — Mo — (CH 0)

i » •

CO

C=

I

•

0

I

1

3

CH

2

2

0 0 HC00CH

o

Mo — 0 — Mo HCOOH

F i g u r e 10. S c h e m a t i c R e p r e s e n t a t i o n o f t h e R e a c t i o n Mechanism ,

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

116

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Acknowledgment The authors are thankful for the assistance of C. E. Lyman in analyzing the catalysts "by scanning transmission electron microscopy. Literature Cited 1. 2. 3.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch007

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

"Catalyse de Contact", Le Page, ed., Editions Technip, p. 385 (1978). Allessandrini, G., Cairati, L., Forzatti, P., Villa, P. and Trifiro, F., J. Less-Common Met., 54, 373 (1977). Popov, B., Osipova, K., Malakhov, V. and Kolchin, Α., Kinet. Catal. (Engl. Transl.), 12, 1464 (1971). Burriesci, N., Garbassi, F., Petrera, Μ., Petrini, G. and Pernicone, N., "Catalyst Deactivation", B. Delmon, G. Froment, eds., Elsevier, p. 115 (1980). Machiels, C. J. and Sleight, A. W., J. Catal., 76, 238 (1982). Machiels, C. J. and Sleight, A. W., Proceedings of the 4th Intrntl. Conf. on the Chemistry and Uses of Molybdenum, Golden, Colorado, p. 411 (1982). W. T. A. Harrison, U. Chowdhry, C. J. Machiels, A. W. Sleight, A. K. Cheetham submitted to J. Solid State Chem. F. Ohuchi, U. Chowdhry, Proceedings NATAS Meeting, Williamsburg, VA, Sept. 1983. Anon., British Patent 589, 292, 2 Aug. 1944. F. J. Shelton et al., United States Patent 2,812,309, 22 Aug. 1954. F. J. Shelton et al., United States Patent 2,849,492, 1 Sept. 1957. F. J. Shelton et al., United States Patent 2,849,493, 5 Sept. 1957. V. Langebeck, G. Poblat, and G. G. Reif, U.S.S.R. Patent 116,517, 19 Jan. 1959. Anon., Italian Patent 589,718, 1959. Anon., Austrian Patent 217,444, 10 Oct. 1961. Anon., Austrian Patent 218,539, 11 Dec. 1961. Anon., French Patent 1,310,499, 18 Apr. 1961. Anon., French Patent 1,310,500, 18 Apr. 1961. G. K. Boreskov et al. U.S.S.R. Patent 158,649, 15 Dec. 1964. G. Fagherazzi and N. Pernicone, J. Catal., 1970, 16, 321. G. Alessandrini, L. Cairati, P. Forzatii, P. L. Villa and F. Trifiro, J. Less-Comm. Met., 1977, 54, 373. F. Trifiro, V. de Vecchi and I. Pasquon, J. Catal., 1966, 15, 8. N. Pernicone, J. Less-Comm. Met., 1974, 36, 289. P. L. Villa, A. Sazbo, F. Trifiro and M. Carbucicchio, J. Catal., 1977, 47, 122. F. Figueras, C. Pralus, M. Perrin and A. J. Renouprez, C. R. Acad. Sci., Paris, Ser. C., 1976, 282, 373. P. Courty, H. Ajot and B. Delmon, C. R. Acad. Sci., Paris, Ser. C., 1973, 276, 1147. M. Carbucicchio and F. Trifiro, J. Catal., 1976, 45, 77.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

7.

MACHIELS ET AL.

28. 29. 30. 31. 32. 33.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch007

34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55.

Molybdate and Tungstate Catalysts

117

P. O. Warner and H. F. Barry, Molybdenum Catalyst Biography (1950-1964). Climax Molybdenum Company, Greenwich, CT. Ε. N. Lovey and D. K. Means, Molybdenum Catalyst Biography (1964-1967) Supplement 1. Climax Molybdenum Company, Greenwich, CT. R. Rudolph, Molybdenum Catalyst Biography (1967-1969) Supplement 2. Climax Molybdenum Company, Greenwich, CT. G. A. Tsigdinos, Molybdenum Catalyst Biography (1975-1972) Supplement 3. Climax Molybdenum Company, Greenwich, CT. F. C. Wilhelm, Molybdenum Catalyst Biograph (1973-1976) Supplement 4. Climax Molybdenum Company, Greenwich, CT. W. W. Swanson, Molybdenum Catalyst Biography (1975-1976) Supplement 5. Climax Molybdenum Company, Greenwich, CT. G. A. Tsigdinos, Molybdenum Catalyst Biography (1977-1978) Supplement 6. Climax Molybdenum Company, Greenwich, CT. G. Tamman and F. Westerchold, Zeit. Anorg. Allg. Chem., 1925, 149, 21. A. N. Zelikman and L. V. Delyaevskaya, Zh. Prikl. Khim., 1954, 27, 1155; (Trans.) J. Appl. Chem. (USSR), 1954, 27, 1091. A. N. Zelikman, Zh. Inorg. Khim., 1956, 1, 2778; (Trans.) J. Inorgan. Chem. (USSR) 1956, 1,000. Yu. D. Kozmanov, Zh. Fiz. Khim. 1957, 31, 1861. Yu. D. Kozmanov and T. A. Ugol'nikova, Zh. Inorg. Khim., 1958, 3, 1267: (Trans.) J. Inorg. Chem. (USSR) 1958, 3-V 284. W. Jager, A. Ramel and K. Beker, Arch Eisenhut., 1959, 30, 435. K. Nassau, H. J. Levenstein and G. M. Loiacono, J. Phys. Chem. Sol., 1965, 26, 1805. K. Nassau, J. W. Shiever and E. T. Keve, J. Sol. St. Chem., 1971, 3, 411. V. K. Trunov and L. M. Kovba, Izv. Akad. Nauk SSSR Neorg. Mat., 1966, 2, 151; (Trans.) Onorg. Mat. (USSR) 1966, 2, 127. A. Marcu et al., Rev. Chim. (Bucharest) 1970, 24, 405. P. V. Klevtsova, R. F. Klevtsova, L. M. Kefeli and L. M. Plyasova, Izv. Akad. Nauk SSR Neorg. Mat., 1965, 1, 918; (Trans.) Inorg. Mat. (USSR), 1965, 1, 843. L. M. Plyasova, S. V. Borisov and Ν. V. Belov, Kristallografria, 1967, 12, 33; (Trans.) Sov. Phys. Crystallog., 1967, 12, 25. L. M. Plyasova, R. F. Klevtsova, S. V. Borisov and L. M. Kefeli, Doklad. Akad. Nauk SSR, 1966, 11, 189. M. H. Rapposch, E. Korstiner and J. B. Anderson, Inorg. Chem., 1980, 19, 3531. H. Chen, Mater. Res. Bull., 1979, 14, 1583. P. D. Battle, A. K. Cheetham, G. J. Long and G. Longworth, Inorg. Chem., 1982, 21, 4223. Z. Jirak, R. Salmon, L. Fouirness, F. Menil and F. Hagenmuller, Inorg. Chem., 1982, 21, 4128. A. W. Sleight and L. H. Brixner, J. Sol. St. Chem., 1973, 7, 172. R. P. Groff, J. Catal., 86, 215-218 (1984). S. Aruanno and S. Wanke, Can. J. Chem. Eng. 1975, 53, 301. N. Pernicone, Proc. Climax First International Conference on the Chemistry and Uses of Molybdenum, Reading, England, 1973, p. 155 and references therein.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

118

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Anon., Chem. Proc. Eng. 1970, 4, 100. Anon., Italian Patent 599,419, 1959. Anon., British Patent 909,376, 1962. Anon., German Patent 1,144,252, 1963. Anon., United States Patent 3,152,997, 1967. F. Trania and N. Pernicone, Chem. Ind: (Milan), 1970, 52, 1. V. Massarotti, G. Flor and A. Marini, J. Appl. Crystallogr., 1981, 14, 64. 63. G. D. Kolovertnov, G. B. Boreskov, V. A. Dzis'ko, Β. I. Popov, D. V. Tavasova and G. G. Belugian, Kinet. Katal., 1965, 6, 1052; (Trans.) Kinet. Catal. (USSR), 1965, 6, 950. 64. G. K. Boreskov, G. D. Kolovertnov, L. M. Kefeli, L. M. Plyasova, L. G. Karachia, V. N. Mastikhin, Β. I. Popov, V. A. Dzis'ko and D. V. Taracova, Kinet. Katal, 1966, 7, 144; (Trans.) Kinet. Catal. (USSR), 1966, 7, 125. 65. G. Alessandriani, L. Gairati, F. Forzatti, P. L. Villa and F. Trifiro, Proc. Second Climax International Conference on the Chemistry and Uses of Molybdenum, Oxford, England, 1976, p. 186, and references therein. 66. F. Trifiro, P. Forzatti, and P. L. Villa; in, Preparation of Catalysts, Elsevier, Amsterdam, 1976, p. 143. 67. H. Voge and C. R. Adams, Adv. Catal., 1967, 17, 151. 68. P. F. Kerr, A. W. Thomas and A. M. Langer, Amer. Mineral 1963, 48, 14. 69. G. A. Tsigdinos and W. W. Swanson, Ind. Eng. Chem. Prod. Res. Div. 1978, 17, 210. 70. P. V. Klevtsov, Kristallografia, 1965, 10, 445; (Trans.) Sov. Phys. Crystallog., 1965, 10, 370. 71. D. J. Marshall, J. Mat. Sci., 1967, 2, 294. 72. W. R. Doyle and F. Forbes, J. Inorg. Nucl. Chem., 1965, 27, 1271. 73. V. K. Trunov, V. V. Lutsenko and L. M. Kovaba, Izv. Vysh. Ucheb. Zabed. SSR, Khim, Khim. Tekhnol. 1967, 10, 375. 74. V. N. Vorobev, G. Sh. Talipov and M. F. Abidova, Zh. Obsch. Khim., 1973, 43, 450: (Trans.) J. Gen. Chem. (USSR), 1973, 43, 452. 75. U. B. Khamikov, G. Sh. Talipov, K. A. Samigov, N. Rakhmatulaev and M. F. Abidova, Zh. Obsch. Khim., 1972, 42, 259; (Trans.) J. Gen. Chem. (USSR), 1972, 42, 248. 76. Β. I. Popov, L. L. Sedova, G. N. Kustova, L. M. Plyasova, Yu. V. Maksimov and A. I. Matveev, React. Kin. Catal. Lett., 1976, 5:1, 43. 77. C. Marcilly and B. Delmon, C. R. Acad. Sci., Paris, Ser. C, 1969, 268, 1975. 78. P. Courty, B. Delmon, C. Marcilly and A. Sugier, French Patent, 2,045,612, 9 Jun. 1969. 79. P. Courty, H. Ajot and B. Delmon, French Patent, 2,031,818, 7 Feb. 1969. 80. Anon., French Patent, 1,604,707, 2 Jul. 1968. 81. P. Courty, H. Ajot and B. Delmon, French Patent, 1,600,128, 30 Dec. 1968. Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch007

56. 57. 58. 59. 60. 61. 62.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

7.

MACHIELS ET AL.

82. 83. 84. 85. 86. 87.

Molybdate and Tungstate Catalysts

119

P. Courty and B. Delmon, C. R. Acad. Sci., Paris, Ser. C., 1969, 268, 1874. P. Courty, H. Ajot, C. Marcilly and B. Delmon, Powder Technol., 1973, 7, 21. C. Marcilly, P. Courty and B. Delmon, J. Amer. Ceram. Soc., 1970, 53, 56. B. Delmon, The Catholic University, Louvain, Belgium, Private Communication, 1982. Α. Κ. Cheetham and A. J. Skarnulis, Anal. Chem., 1981, 53, 1060. S. J. B. Reed, Electron Microprobe Analysis, Cambridge University Press, England, 1975. December 17, 1984

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch007

RECEIVED

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

8 Characterization of Vanadium Oxide Catalysts in Relation to Activities and Selectivities for Oxidation and Ammoxidation of Alkylpyridines ARNE ANDERSSON and S. LARS T. ANDERSSON

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch008

Department of Chemical Technology, Chemical Center, Lund Institute of Technology, P.O. Box 124, S-221 00 Lund, Sweden ESCA, XRD, IR, SEM and ESR were used to characterize the composition and structure of V-Ti-O catalysts, i n both precursors and activated forms. Precursors consist at low V/Ti r a t i o s of non-stoichiometric r u t i l e containing Ti and V and with VO on the surface. With increasing V/Ti ratios V i s dissolved up to 6 atom % and V clusters are formed i n the r u t i l e . Excess vanadium forms non-stoichiometric V O crystals on the surface of the 25 µm r u t i l e p a r t i c l e s , which at higher concentrations are completely embedded. Cata­ l y s t s activated by reduction additionally contain non-stoichiometric V O and V O i n amounts increas­ ing with decreasing V/Ti r a t i o s . Isolated V ions i n the vanadium oxides also increase i n concentration. The c a t a l y t i c performance of these catalysts i n oxidation and ammoxidation of some alkylpyridines i s discussed. 3+

4+

2+

4+

2

6

13

2

5

4

4+

The V-Ti-0 system has been extensively studied i n connection with c a t a l y t i c oxidation and ammoxidation reactions of aromatic hydrocar­ bons. Two p r i n c i p a l l y d i f f e r e n t types of catalysts can be d i s ­ tinguished. One type of catalyst i s prepared by impregnation, p r e c i p i t a t i o n or mixing of the vanadium and titanium phases followed by calcination i n a i r below the melting point of Vo°s (1-4). The simultaneous reduction of ν 0 and transformation of AIO^ (anatase) into r u t i l e when heating Below the V 0^ melting point has been demonstrated to be due to topotactic reactions (5). The formation of lower vanadium oxides can be of importance, because i t has been found that reduced phases determine the a c t i v i t y and s e l e c t i v i t y of catalysts (6,7). Another type of V-Ti-0 catalyst i s prepared by mixing V ^ and TiO (anatase) phases, followed by heating the mixture above the melting point of V 0 (8,9). Clauws and Vennik (10) have found a defect, associated with oxygen vacancies, by studying the o p t i c a l absorption of V O. c r y s t a l s . The same defect was found i n TiO^-promoted V 0 crystals (11), but the intensity was greatly enhanced. ?

2

?

2

2

5

0097-6156/85/0279-0121$06.25/0 © 1985 American Chemical Society

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

122

S O L I D STATE C H E M I S T R Y IN CATALYSIS

T h i s shows the importance o f the i n c o r p o r a t i o n o f T i i n creating oxygen v a c a n c i e s . The c a t a l y s t s d e a l t w i t h i n t h i s p r e s e n t a t i o n b e l o n g t o the second type o f c a t a l y s t s d e s c r i b e d above. They have been c h a r a c t e r ­ i z e d by means o f XRD, ESCA, ESR, SEM and IR methods. I t has been d e s c r i b e d i n p a t e n t s t h a t ammoxidation c a t a l y s t s can be a c t i v a t e d by t r e a t m e n t w i t h ammonia a n d / o r hydrogen (12) o r w i t h c a r b o n monoxide ( 1 3 ) . T h e r e f o r e , b o t h p r e c u r s o r s and reduced c a t a l y s t s w i l l be c o n s i d e r e d i n t h i s p r e s e n t a t i o n . I t w i l l be shown t h a t the performance of the c a t a l y s t s a r e r e l a t e d t o t h e i r c h a r a c ­ t e r i s t i c s . The adsorbed s t a t e o f r e a c t a n t s w i l l a l s o be d i s c u s s e d .

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch008

Methods A c t i v i t y measurements. The measurements were performed a t atmos­ p h e r i c p r e s s u r e i n a g l a s s r e a c t o r . A thermocouple was p o s i t i o n e d i n the c e n t e r o f the r e a c t o r . I n the ammoxidation o f 3 - p i c o l i n e , the i n l e t r e a c t i o n m i x t u r e was a d m i t t e d a t a r a t e o f 32 l i t e r s / h r and c o n t a i n e d 232-254 moles o f a i r , 13-14 moles o f ammonia, and 56-62 moles o f water v a p o r f o r each mole o f 3 - p i c o l i n e . The r e a c t i o n was u s u a l l y performed i n the temperature i n t e r v a l 300-400 C . I n the o x i d a t i o n o f MEP ( 2 - m e t h y l - 5 - e t h y l p y r i d i n e ) the m o l a r r a t i o s o f O^/MEP and steam/MEP^ were 75 and 175 r e s p e c t i v e l y , and the space v e l o c i t y was 7000 h . I n ammoxidation s t u d i e s p r e r e d u c e d c a t a l y s t s were used and the measurements e x t r a p o l a t e d i n time t o g i v e d a t a a t the s t a r t o f the r e a c t i o n . I n o x i d a t i o n s t u d i e s unreduced c a t a l y s t s were used and the d a t a were o b t a i n e d at the steady s t a t e . XRD. X-Ray d i f f r a c t i o n a n a l y s e s were c a r r i e d out on c a t a l y s t s by a P h i l i p s X - r a y d i f f r a c t i o n i n s t r u m e n t u s i n g a PW 1310/01/01 g e n e r a t o r and Cu Κα r a d i a t i o n . IR. The i n f r a r e d s p e c t r a were r e c o r d e d on a P e r k i n - E l m e r 580B s p e c t r o p h o t o m e t e r connected t o a d a t a s t a t i o n from the same manufac­ t u r e r . The KBr d i s c method was u s e d . The s p e c t r a were s t o r e d on d i s k s and t r a n s f e r r e d t o a T e k t r o n i x 4051 computer f o r e v a l u a t i o n . ESCA. ESCA measurements were performed on an ΑΕΙ ES 200B e l e c t r o n s p e c t r o m e t e r equipped w i t h an A l - a n o d e (1486.6 e V ) . The f u l l w i d t h a t h a l f maximum (FWHM) o f the Au 4 f y l i n e was 1.8 e V . Sample c h a r g i n g was c o r r e c t e d f o r w i t h the 0 I s l i n e at 529.6 e V , w h i c h has been shown t o be a s u i t a b l e method i n t h i s system ( 1 4 ) . F o r the q u a n t i t a t i v e a n a l y s i s c a l i b r a t e d s e n s i t i v i t y f a c t o r s , o b t a i n e d from pure o x i d e s , were u s e d . These were 0 Is 1, T i 2p = 1.37 and V 2 p = 2.17. 2

s

3 / 2

3 / 2

ESR. A V a r i a n E - 3 s p e c t r o m e t e r was used f o r the ESR s t u d i e s . I n the q u a n t i t a t i v e measurements a c a l i b r a t e d V 0 / T i 0 sample was r u n between each c a t a l y s t . The e r r o r i n t h e s e r e l a t i v e measurements was l e s s t h a n 10 %. F o r the c a l i b r a t e d \/ 2 P P concen­ t r a t i o n was determined t o w i t h i n 3Ό % a c c u r a c y by c a l i b r a t i o n a g a i n s t a CuS0^ 5H 0 s i n g l e c r y s t a l . T h i s measurement, and measure­ ments f o r some or the s a m p l e s , were performed on a V a r i a n E9 equipped w i t h a d u a l c a v i t y . The g - v a l u e s were measured w i t h i n ±O.002. 2

V

(

T i 0

2

s

a

m

l

e

t

n

e

s

i n

2

e

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

8.

ANDERSSON AND ANDERSSON

Characterization of Vanadium Oxide Catalysts 123

SEM. Scanning e l e c t r o n m i c r o s c o p i c i n v e s t i g a t i o n s w i t h a J e o l JSM-U3 o r an I S I - 1 0 0 A i n s t r u m e n t .

were

performed

Catalyst preparation. The c a t a l y s t s were prepared by h e a t i n g V^O^ and T i O ^ (anatase) powders i n a q u a r t z c r u c i b l e i n a h i g h tempera­ t u r e oven f o r 3 h r s at 1 1 5 0 - 1 2 5 0 ° C . The fused c a t a l y s t s were d i v i d e d i n t o s m a l l p a r t i c l e s , and the O.71-1.41 mm f r a c t i o n was used i n the a c t i v i t y measurements. A c t i v a t e d c a t a l y s t s were prepared bv r e ­ d u c t i o n o f the p r e c u r s o r s i n 1 atm o f hydrogen f o r 1 h r at 450 C . Q

R e s u l t s and D i s c u s s i o n

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch008

Catalyst Precursor XRD. X-Ray d i f f r a c t i o n p a t t e r n s o f the p r e c u r s o r s were composed of the p a t t e r n s o f V 0 and T i O ( r u t i l e ) , except f o r the O.5 and 1.0 mole % V^O^ c a t a l y s t s f o r w h i c h o n l y T i O ^ l i n e s were observed ( 1 5 ) . P r e c u r s o r s w i t h more than 50 mole % V ^ O . a l s o c o n t a i n e d v e r y s m a l l amounts o f T i O ( a n a t a s e ) . Scanning o f the l i n e s i n the b a c k - r e f l e c ­ t i o n r e g i o n , ΖΘ = 115-142 d e g r e e s , showed t h a t t h e r e was a s m a l l s h i f t o f the T i O ^ l i n e s . The l a t t i c e c o n s t a n t s o f the r u t i l e phase o f T i O ^ were c a l c u l a t e d . Cohen's l e a s t - s q u a r e s method o f e l i m i n a t i n g e r r o r s was used ( 1 6 ) . The r e s u l t s are g i v e n i n T a b l e I . The u n i t c e l l dimensions o f the c a t a l y s t r u t i l e phase has changed m a i n l y i n the a d i r e c t i o n . The l e n g t h o f the u n i t c e l l i n the c^ d i r e c t i o n was p r a c t i c a l l y the same as t h a t o f pure r u t i l e . Bond and coworkers have o b t a i n e d the same r e s u l t ( 4 ) ^ The c o n t r a c t i o n o f the u n i t c e l l can be due to i n c o r p o r a t i o n o f V i n the T i O ^ phase. No changes o f the l a t t i c e parameters o f the vanadium p e n t o x i d e phase c o u l d be d e ­ t e c t e d , a l t h o u g h i t has been r e p o r t e d t h a t T i can be d i s s o l v e d i n 2°5 g h t be due to d e t e c t a b i l i t y p r o b l e m s . V 0 does not have any l i n e s i n the b a c k - r e f l e c t i o n r e g i o n , where s h i f t s are most e a s i l y s e e n . 2

?

+

(11.).

V

Table I .

T

h

i

s

m i

L a t t i c e constants of T i 0

2

(rutile)

Phase

C e l l dimensions c (Â) a (A)

T i 0 , pure VO , pure Ti6 , catalyst

4.594 4.530 4.583

2

2

2.959 2.869 2.958

Ref. (17) (18) t h i s work

ESCA. I n the ESCA measurements on the fused V ^ / T i O ^ c a t a l y s t s the 0 I s , V 2p«y« and T i 2p~ core l i n e s were o b s e r v e d . The b i n d i n g e n e r g i e s ( B . E . ) indicate'Ibhe presence o f V^O- and T i O ^ f o r a l l samples and the v a l u e s were 5 2 9 . 6 , 516.6 and 4 5 7 . 9 e V , r e s p e c t i v e l y (See T a b l e I I ) . F o r the powder m i x t u r e s (un-fused) the v a l u e s were 5 2 9 . 6 , 516.6 and 458.5 e V , r e s p e c t i v e l y . Thus t h e r e i s a d i f f e r e n c e i n the T i 2p . B . E . w h i c h can be e x p l a i n e d by the f o r m a t i o n of r u t i l e d u r i n g h e a t i n g o f the samples i n a d d i t i o n t o the d o p i n g of the r u t i l e phase w i t h V .

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

124

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Table I I .

B i n d i n g e n e r g i e s and h a l f w i d t h s (eV) o f the V 2 p ^ Ti 2p« core l i n e s

and

2

/ 9

Sample

V

Ti0 ,

anatase

Ti0 , ζ V 0 2 5

rutile

o

o

o

2

5

2

catalyst

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch008

o

5

i

2

P /2 3

516.6 (1.6) 516.6 (1.6) 516.1 (2.8) 516.2 (3.2)

Same a f t e r H S0,+NH„ treatment 2 4 3 V 0 / T i 0 ( 7 0 / 3 0 ) , a f t e r 1100°C i n vacuum f o r 15 h r . 2

T

'3/2

458.5 (1.7) 458.2 (1.9)

c

V 0 /Ti0 ,

2

2

457.9 (1.7) 458.1 (1.9) 457.9 (2.2)

I n an attempt t o study t h i s e f f e c t a V 0 « / T i 0 (70/30) sample w h i c h appears t o be a s o l i d s o l u t i o n from XKD d a t a ( o n l y r u t i l e l i n e s appear i n s p e c t r a ) was measured. The sanje^ T i 2p^# B . E . as f o r the c a t a l y s t s was o b t a i n e d . To measure the V i n the r u t i l e phase some samples were t r e a t e d w i t h s u l p h u r i c a c i d f o l l o w e d by ammonia t o d i s s o l v e the vanadium o x i d e p h a s e . The ESCA a n a l y s i s o f the 10 mole % V 0 c a t a l y s t t r e a t e d i n t h i s manner showed the presence o f 6 atom % V w i t h a B . E . o f 516.1 e V , o b t a i n e d a f t e r s u b t r a c t i o n o f the 0 l s ( K a ^ (i) ^ * f s p e c t r a . No vanadium o x i d e s a r e d e t e c t a b l e by XRD on t n i s sample. T h u s , a p p r o x i m a t e l y 6 atom % V seems t o be d i s s o l v e d i n the r u t i l e phase o f the c a t a l y s t s . The c o m p o s i t i o n V Ti 0 has been suggested i n the l i t e r a t u r e ( 4 ) . That i t i s p r o b a b l y * p r e s e n t as V i s i n d i c a t e d by the lower V 2 p * B . E . and i n correspondence w i t h the V 0 / T i 0 sample. I t i s i n t e r e s t i n g t o n o t e t h a t sample c h a r g i n g phenomena o c c u r f o r samples w i t h low vanadium l o a d i n g . F o r powder m i x t u r e s t h e s e a r e observed a t l e s s t h a n 50 atom % V and f o r fused samples a t l e s s t h a n 6 atom % V . Pure V 0 g i v e s almost no c h a r g i n g whereas pure T i O g i v e s a c h a r g i n g o f %-i V . W i t h enough 2 ^ 5 ^ a t there i s contact between o ° 5 P l t h e r e s h o u l d be a low c h a r g i n g e f f e c t . A t low V 0 ^ l o a d i n g s , however, t h e r e i s a c o n s i d e r a b l y worsened c o n t a c t between the V 0 ^ p a r t i c l e s throughout the b u l k o f the sample, and as f o r pure T i 0 a l a r g e c h a r g i n g e f f e c t a r i s e s . What i s v e r y i n t e r e s t ­ i n g i s t h a t f o r the fused samples t h i s o c c u r s a t a much lower vanadium c o n t e n t . T h i s i n d i c a t e s the e x c e l l e n t coverage o f the V O. on the TiO~ p a r t i c l e s . I t i s f u r t h e r noteworthy t h a t the 50 atom % V powder m i x t u r e and the 6 atom % V fused sample c o n t a i n a p p r o x i m a t e l y the same atom % V as a n a l y s e d by ESCA, k e e p i n g i n mind the h i g h surface s e n s i t i v i t y . I n F i g u r e 1 the V c o n t e n t o f the v a r i o u s V - 0 / T i O - samples as r e v e a l e d by ESCA i s p l o t t e d a g a i n s t the n o m i n a l v - c o n t e n t . P o i n t s for ο ς / 2 l y l s o i n c l u d e d and i t i s e v i d e n t t h a t t h e s e tîénave s i m i l a r l y . The l i n e f o r the powder m i x t u r e s f a l l s c l o s e to the t h e o r e t i c a l l i n e (dashed l i n e ) . An almost p e r f e c t match c o u l d be o b t a i n e d by changing the s e n s i t i v i t y f a c t o r s . However, s i n c e t h i s d e v i a t i o n might be due to p a r t i c l e s i z e e f f e c t s , s e n s i t i v i t y f a c t o r s from pure o x i d e s were used i n s t e a d . F o r the fused samples a much ?

2

n e

0 4

Q

r o m

9 6

3

2

V

V

a

r

c

a

t

i

c

e

2

2

s

o

s

2

2

2

ν

0

δ η 0

t

a

s

t

s

a

r

e

a

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

8.

ANDERSSON AND ANDERSSON

Characterization of Vanadium Oxide Catalysts 125

h i g h e r atom % o f V i s o b t a i n e d by ESCA t h a n was e x p e c t e d . E v i d e n t l y the V^O^ phase c o m p l e t e l y c o v e r s the r u t i l e p a r t i c l e s . T h i s was a l s o i n d i c a t e d by the c h a r g i n g phenomena d i s c u s s e d above. I n c o n c l u s i o n f r o m ^ t h e ESCA r e s u l t s , the fused V 0JTIO^ c a t a l y s t s c o n s i s t of a V doped T i O ( r u t i l e ) phase embedded i n the V 0 phase. +

?

2

5

ESR. I n F i g u r e 2 the ESR s p e c t r a a t R . T . f o r some f r e s h l y p r e p a r e d 2°5^ 2 n « Even a f t e r r e d u c t i o n i n H JL£ 450 C f o r 1 hour pure T i O ^ showed o n l y v e r y weak bands from T i which t j j e j e f o r e does not i n t e r f e r e w i t h the q u a n t i t a t i v e measurements o f V d i s c u s s e d b e l o w . F o r O.05 mole % 2 ° 5 ^ band showing some h y p e r f i n e s p l i t t i n g was o b s e r v e d . T h i s was i n t e r p r e t e d as VO ions on the r u t i l e s u r f a c e i n a c c o r d w i t h the assignments o f some slightly different spectra ( 1 9 , 2 0 ) . These seem t o broaden and d i s a ç j j i e a r on i n c r e a s i n g t h e °5 * b r o a d s i n g l e band due to V i n 2?5 P P « T h u s , almost no f i n e s t r u c t u r e i s seen f o r 10 mole % ^ ^ » ^ d i s c o m p l e t e l y absent f o r the samples 30-100 mole % ^ 2 ^ 5 ' The g - v a l u e f o r the broad resonance was 1.972. The l i n e w i d t h i n c r e a s e s from 100 t o 150 Gauss w i t h i n c r e a s i n g 9 ° 5 from 10 t o 100 %. The 10 % sample showed no s i g n a l a f t e r t h e remova^. of the vanadium o x i d e s . T h i s f a c t s t r o n g l y i n d i c a t e s t h a t t h e V i o n s i n the r u t i l e phase a r e not seen a t R . T . due t o s t r o n g s p i n l a t t i c e coupling>4 and thus do not c o n t r i b u t e t o the q u a n t i t a t i v e measurements o f V I n F i g u r e 3 the ESR s p e c t r a at 7Ί& a r e shown. The reduced T i 0 sample shows a s p e c t r a a s s i g n e d to T i ( 2 1 ) . The i n t e n s i t y o f the Ti s i g n a l i s c o m p l e t e l y n e g l i g i b l e compared t o the i n t e n s i t y o f the o t h e r samples a t 77K. The i n c r e a s e i n i n t e n s i t y w i t h the d e ­ c r e a s e i n temperature ( r e l a t i v e i n t e n s i t y 7 7 K / R . T . ) i s v e r y l a r g e f o r a l l samples except f o r V 0 . Here a 3.6 f o l d i n c r e a s e i s o b ­ s e r v e d , w h i c h corresponds t o tne normal change i n s p i n p o p u l a t i o n w i t h decreased t e m p e r a t u r e . E s t i m a t e s from i n s t r u m e n t g a i n s e t t i n g s p o i n t t o an a p p r o x i m a t e l y 10 -10 f o l d i n c r e a s e f o r the fused V 0 / T i O samples. The i n t e n s i t y at 77K f o r the pure V 0 sample i s n e g l i g i b l e - a few p e r c e n t a t the most - compared w i t h t h a t o f the T i 0 c o n t a i n i n g samples. The most s i g n i f i c a n t e f f e c t i s observed f o r the H-SO^ and NH^ t r e a t e d samples w h i c h show no s i g n a l at a l l at R . T . but one o f s i m i l a r magnitude as f o r a l l V 0 - / T i O « samples a t 77K. E v i d e n t l y the low temperature s i g n a l i s almost e n t i r e l y due t o the r u t i l e p h a s e . I t i s p o s s i b l e t h a t the temperature e f f e c t may J>£ due t o a s p i n - l a t t i c e r e l a x a t i o n e f f e c t whereby s u b s t i t u t i o n a l V would not be o b s e r v a b l e a t R . T . ( 2 2 ) . I t i s known t h a t V - V bonds a r e formed i n t h e ^ V 0 - T i 0 s y s t e m , a p p a r e n t l y by p a i r i n g o f randomly distributed V i o n s at low c o n c e n t r a t i o n s o f V ( 2 3 ) . These would undoubtedly g i v e temperature dependent s p i n - s p i n r e l a x a t i o n e f f e c t s . The c a l c u l a t i o n s o f s p i n c o n c e n t r a t i o n s from measurements a t 77K d o , however, g i v e a b s u r d l y h i g h v a l u e s . I t i s t h e r e f o r e suggested t h a t a l a r g e p a r t o f the i n c r e a s e i n s i g n a l i n t e n s i t y i s due t o a f e r r o m a g ­ n e t i c phase t r a n s i t i o n , ^ t h i g h e r c o n c e n t r a t i o n s i t seems v e r y l i k e l y that c l u s t e r s of V o r V 0 i s l a n d s are formed i n the r u t i l e p h a s e . Anatase doped w i t h up t o 2 atom % V was suggested t o c o n t a i n a l a r g e p r o p o r t i o n o f V 0 i s l a n d s ( 2 4 ) . Pure V 0 does not g i v e any s i g n a l s i n the ESR s p e c t r a . I t i s w e l l known t h a t temperature dependent phase t r a n s i t i o n s do o c c u r i n V 0 and t h a t t h e s e a r e

V

T i 0

c

a

t

a

l

v

s

t

s

a

r

e

s n o w

V

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch008

v

c

o

n

t

e

n

a

t

a n
2 c l u s t e r s g i v e f e r r o m a g n e t i c s p e c i e s a t l o w e r t e m p e r a t u r e s . C o n c e r n i n g the c u r v e forms i t i s seen t h a t f o r the O.05 mole % V«0 sample t h e r e i s ,a h y p e r f i n e s p l i t t i n g i n d i c a t i v e o f a s u b s t i t u ­ t i o n a l s o l u t i o n of V i n r u t i l e ( 2 2 , 2 6 ) . I t i s q u i t e c l e a r t h a t on i n c r e a s i n g the c o n t e n t from O.05 t o O.5 mole %, the ESR s p e c ­ trum i s s t r o n g l y broadened, p r o b a b l y due t o l a r g e s p i n - s p i n r e l a x ­ ation effects for V i n the r u t i l e p h a s e . Any t r a c e s o f h y p e r f i n e s p l i t t i n g have almost d i s a p p e a r e d a t 1 mole % 2 ° 5 * ^ P^f 10 t o 90 mole % showed i d e n t i c a l s i g n a l s , w h i c h was a l s o the case f o r the 10 % H ^ O ^ and NH^ t r e a t e d sample. The g - v a l u e f o r the broad resonance was 1.934 f o r the 30 mole % V«0 sample and i n ­ c r e a s e d c o n t i n u o u s l y t o 1.957 f o r the 90 mole Τ v^O^ sample w h i c h differs from the v a l u e s at R . T . S i m u l t a n e o u s l y , the l i n e w i d t h i n c r e a s e d from 180 to 210 Gauss. A

s a m

e s

r o m

SEM. The p r e c u r s o r s w i t h 50 t o 10 mole % V 0^ t r e a t e d Η SO^ and NH^ were s t u d i e d . F i g u r e s 4(a) and 4(b) show tne T i O ^ p a r t i c l e s i z e i n t h e s e s a m p l e s , w h i c h i s about 25ym i n b o t h c a s e s . F i g u r e 4 ( c ) shows s i n t e r e d T i O ^ . By comparing t h i s w i t h F i g u r e s 4(a) and 4 ( b ) , i t can be c o n c l u d e d t h a t t h é T i O ^ p a r t i c l e s become l a r g e r w h e n ^ i n t e r e d i n the p r e s e n c e o f V ^ O - . The r e a s o n f o r t h i s can be t h a t V is incorp o r a t e d i n t o the T i O ^ l a t t i c e , w h i c h l e a d s t o a d e c r e a s e o f the m e l t i n g p o i n t o f T i O . . T h i s d e c r e a s e i s then r e f l e c t e d i n a g r e a t e r l a t t i c e m o v a b i l i t y . T h e f a c t t h a t the T i 0 « p a r t i c l e s i z e i s the same^ i n p r e c u r s o r s w i t h b o t h 50 and 10 mole % *

O.8 /

s1

.^"ZZ O.6J Pb J ! N

- Pot

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch009

1

χ

-2

-4

I Τ

Amorphous-Crystalline Transition

ι

0 -1

1

O.4 —

O.2

O.6

PbO

++

j ι ι

O.4 O.2

! !

Pb 0^- 2

0

H

ι!

.

ι

1 pH

I

1

1

•

F i g u r e 4 . P o t e n t i a l - p H diagram f o r t h e system l e a d - w a t e r a t 25°C., adapted from P o u r b a i x ( 9 ) .

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

9.

HOROWITZ ET AL.

151

Nonstoichiometric Ruthenate Pyrochlores

a l k a l i n e medium. As w i t h the l e a d r u t h e n a t e s , low l e v e l s of e l e c t r o ­ c h e m i c a l p o t e n t i a l , pH and t e m p e r a t u r e a l l l e a d t o an amorphous product. The s y n t h e s i s of t h e s e m a t e r i a l s i s f u r t h e r c o m p l i c a t e d by t h e e x i s t e n c e o f a c o m p e t i t i v e p h a s e , Bi2RU2O7 33, which has t h e KSb03 s t r u c t u r e ( 1 0 ) . F i g u r e 6 shows t h a t s i n g l e phase b i s m u t h s u b s t i t u t e d b i s m u T F r u t h e n a t e p y r o c h l o r e s can be s y n t h e s i z e d u s i n g the a l k a l i n e s o l u t i o n r o u t e ; however, s i n g l e p h a s e p y r o c h l o r e s a r e r e s t r i c t e d t o a f a i r l y n a r r o w p H / t e m p e r a t u r e window. The bismuth r u t h e n a t e system a l s o d i f f e r s from the l e a d r u t h e n a t e s y s t e m i n one other respect. In t h e l e a d r u t h e n a t e s e r i e s a m i l d heat t r e a t m e n t ( e . g . 4 0 0 C ) i n a i r w i l l c o n v e r t any o f t h e a m o r p h o u s , " i n c i p i e n t p y r o c h l o r e " p r e c i p i t a t e s to pure c r y s t a l l i n e p y r o c h l o r e . In t h e bismuth r u t h e n a t e system on the o t h e r hand, o n l y those amorphous p r e ­ c i p i t a t e s which have been s u b j e c t e d t o a d i g e s t i o n p e r i o d , w i t h i n t h e a l k a l i n e medium, of adequate d u r a t i o n w i l l y i e l d p u r e p y r o c h l o r e upon h e a t t r e a t m e n t . Furthermore, unless the heat treatment i s c a r r i e d o u t i n a low p 0 a t m o s p h e r e ( e . g . f l o w i n g Ar o r He) t h e product w i l l c o n t a i n s u b s t a n t i a l amounts of t h e KSb03 phase. The low temperature of s y n t h e s i s a f f o r d e d by the a l k a l i n e s o l u t i o n r o u t e has r e s u l t e d i n t h e p y r o c h l o r e o x i d e s o f t h i s study having s u r f a c e areas r a n g i n g from 50-200 m /g. Thus, one of the p r i m a r y requirements for m a t e r i a l s being i n v e s t i g a t e d for catalytic a p p l i c a t i o n s has been a c h i e v e d . However, even w i t h t h e s e r e l a t i v e l y h i g h s u r f a c e a r e a m a t e r i a l s , we have o b s e r v e d t h a t t h e s t a t e o f a g g l o m e r a t i o n c a n c r i t i c a l l y a f f e c t the e l e c t r o c a t a l y t i c a c t i v i t y . F i g u r e 7 i l l u s t r a t e s how a s i m p l e v a r i a t i o n i n t h e s y n t h e s i s p r o c e d u r e can d r a s t i c a l l y a f f e c t t h e s t a t e o f a g g l o m e r a t i o n o f powders produced v i a the a l k a l i n e s o l u t i o n r o u t e . The b u l k volume of t h e powder i s seen t o i n c r e a s e a p p r o x i m a t e l y t e n f o l d when i t i s r e c o v e r e d from the a l k a l i n e s y n t h e s i s medium by t h e spray-freeze/ f r e e z e d r y t e c h n i q u e r e l a t i v e t o t h e case where c o n v e n t i o n a l vacuum f i l t r a t i o n i s employed. E l e c t r o d e s f a b r i c a t e d from the high b u l k v o l u m e powder showed a c l e a r improvement i n c a t a l y t i c a c t i v i t y f o r both r e d u c t i o n and e v o l u t i o n of oxygen. β

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch009

e

2

2

Oxygen E l e c t r o c a t a l y t i c P r o p e r t i e s : Oxygen R e d u c t i o n . F i g u r e 8 compares s t e a d y - s t a t e p o l a r i z a t i o n curves f o r t h e e l e c t r o r e d u c t i o n of 0? on a t y p i c a l p y r o c h l o r e c a t a l y s t , P b 2 ( R u i . 4 2 P b o . 5 8 ) ° 6 . 5 » I w/o p l a t i n u m on c a r b o n . The l a t t e r was c o n s i d e r e d r e p r e s e n t a t i v e o f c o n v e n t i o n a l supported noble metal e l e c t r o c a t a l y s t s . The a c t i v i t i e s of both c a t a l y s t s are q u i t e comparable. While the e l e c t r o d e s were not f u r t h e r o p t i m i z e d , t h e i r p e r f o r m a n c e was c l o s e t o the s t a t e of the a r t , c o n s i d e r i n g t h a t c u r r e n t s of 1000 ma/cm c o u l d be r e c o r d e d , at a r e l a t i v e l y moderate temperature (75°C) and a l k a l i c o n c e n t r a t i o n (3M Κ0Η). A l s o , the voltages were not corrected for electrolyte r e s i s t a n c e . The p a r t i c l e s i z e of the p l a t i n u m on t h e c a r b o n s u p p o r t was o f t h e o r d e r o f 2 n a n o m e t e r s , as m e a s u r e d by t r a n s m i s s i o n e l e c t r o n microscopy. F i g u r e 9 i l l u s t r a t e s t h e Ο2 e l e c t r o r e d u c t i o n a c t i v i t y of a number o f l e a d r u t h e n a t e p y r o c h l o r e s , P b 2 ( R u 2 - x P b ) 0 6 5 , where 0 < χ < 1.O. T h e s e d a t a demonstrate t h a t c a t a l y s t s of r o u g h l y e q u i v a l e n t a c t i v i t y can be s y n t h e s i z e d o v e r t h e e n t i r e c o m p o s i t i o n a l r a n g e . In two e x a m p l e s where t h e a c t i v i t y was n o t i c e a b l y lower (x = O.04 and χ = O.98), the s y n t h e s i s c o n d i t i o n s were such t h a t t h e s u r f a c e a r e a s o f a

x

#

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

n

d

5

152

S O L I D STATE C H E M I S T R Y IN CATALYSIS

ι

I

ι

1

1.2

t

1.0

ζtil κ

O.8

>

CRuor;

Ru0 '2H 0 -

1.0

Ru(OH)

O.8

2

-

2

3

O.6

Ru

-

O.6

-PoU

c

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch009

1.2

O.4

- O.4

I

O.2

- O.2

0

Η

.

Ι

ι

Ι

ι

11 -pH -

I

ι

13

2

I

15

Figure 5. P o t e n t i a l -pH diagram f o r the system at 25°C., adapted from P o u r b a i x ( 9 ) .

ruthenium-water

NARROW WINDOW FOR SOLUTION SYNTHESIS OF Bi [Ru„ Bi I O , 2 2-x >c 7-y 0

I

1

\

80

.... j

1

ν

p

+

\

Ρ+

^KSb0 \

Xs

u

3

V

υ

B i

PYROCHLORE^S.

}

B î

2°3

2°3

60

AMORPHOUS

40 —

-

20

1 12

I 10

1 14 —

1

p H —

F i g u r e 6. S y n t h e s i s p r o d u c t s as a f u n c t i o n of temperature and pH f o r t h e B i - R u - 0 s y s t e m i n KOH r e a c t i o n m e d i a . Time o f r e a c t i o n h e l d c o n s t a n t at 7 d a y s . "KSbO^" s i g n i f i e s t h e K S b 0 3 - r e l a t e d phase, Bi2Ru20y 33. "P" s i g n i f i e s t h e p y r o c h l o r e phase, Bi2[Ru2- Bi ]07y. e

x

x

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch009

9.

HOROWITZ ET AL.

Nonstoichiometric Ruthenate Pyrochlores

F i g u r e 7. Equal weights o f Pb2 [Ru1.52Pb.33] 05.5. L e f t - Recovered by c o n v e n t i o n a l vacuum'fi l t r a t i o n "and d r y i n g . R i g h t - Produced by t h e s p r a y - f r e e z e / f r e e z e d r y t e c h n i q u e . Both have s u r f a c e areas of 78 m /g. 2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

153

154

S O L I D STATE C H E M I S T R Y IN CATALYSIS

UJ X 1.0 Û£ (Λ >

O.9 oPb [Ru

"5 O.8 >

2

l t 4 2

Pb

]0

> 5 8

6 > 5

• 1 5 % Pt on Carbon

O.7

I

L

J

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch009

L

100

10

400 1000

mA/cm

F i g u r e 8 . S t e a d y - s t a t e p o l a r i z a t i o n curves f o r O 9 r e d u c t i o n i n 3M Κ 0 Η , 75°C on P b [ R u i 9Pb ç J°6.5* , ^ w/o P t on carbon. Catalyst lx>adin§? areι 6Ό and 50 mg/cms respectively. The surface areas of the pyrochlore and carbon-supported Pt are 67 m /g and 25 m / g , r e s p e c t i v e l y . Reproduced w i t h p e r m i s s i o n from R e f . ^ C o p y r i g h t 1983, The E l e c t r o c h e m i c a l S o c . I n c . 2

2

4

4+

0

an d 9

8

5

2

1.1

1"

1

1

1

.

1.0

—

O.9

X=O.11 X=O.33 X=O.58 X=O.82 X=O.94

O.8

ι

1

( 2 8 rru/g) ( 8 5 rru/g) (67 mVg) (60 mi/g) (81 mVg) — ' J

/

ι 10

1• 100

11 11 I

300

mA/cm

F i g u r e 9 . 0 r e d u c t i o n i n 3M KOH, 75°C on Pb2[Ru2- Pb x]o s a f u n c t i o n o f x . Scans at O.2 mV/sec. C a t a l y s t l o a d i n g i's 60 mg/cm . Reproduced w i t h p e r m i s s i o n from R e f . 1. Copyright 1983, The E l e c t r o c h e m i c a l S o c . I n c . 2

x

4

6

2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

5

a

9.

155

Nonstoichiometric Ruthenate Pyrochlores

HOROWITZ ET AL.

t h e c a t a l y s t s were q u i t e l o w ( R-COO" 4e

2

R-CH^CH " > RCOO" + COg 10e

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch009

2

=

Ο IL 6 . . R-C|CH -R' » R-COO + OOC-R' e

2

F i g u r e 12. T y p i c a l Alkali.

E l e c t r o - O x i d a t i o n s on L e a d R u t h e n a t e

CH =CH(CH ) COO" " > "OOC(CH ) COO" + COg 2

2

8

10e

2

=

8

+

OOC(CH ) COO" 7% 2

7

CH —CH

D—

2

2

CH COO -8e 2

/

- CH-CH.COOH

> HOO(

2

HOOC Figure 13.

O x i d a t i o n of S o l u b i l i z e d U n s a t u r a t e s .

85%

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

SOLID STATE CHEMISTRY IN CATALYSIS

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch009

160

ΙΑ Figure 14. Ruthenate.

Rates of

Oxidation

°κ

of Various

Organics

on

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Lead

9.

HOROWITZ ET AL.

Nonstoichiometric Ruthenate Pyrochlores

161

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch009

Summary and D i s c u s s i o n of R e s u l t s A new f a m i l y o f h i g h c o n d u c t i v i t y , m i x e d m e t a l o x i d e s having t h e p y r o c h l o r e c r y s t a l s t r u c t u r e has been d i s c o v e r e d . T h e s e compounds d i s p l a y a v a r i a b l e c a t i o n s t o i c h i o m e t r y , as g i v e n by E q u a t i o n 1. The a b i l i t y t o s y n t h e s i z e t h e s e m a t e r i a l s i s h i g h l y d e p e n d e n t upon t h e l o w t e m p e r a t u r e , a l k a l i n e s o l u t i o n p r e p a r a t i v e t e c h n i q u e t h a t has been d e s c r i b e d ; the r e l a t i v e l y low thermal s t a b i l i t y o f t h o s e p h a s e s where an a p p r e c i a b l e f r a c t i o n o f t h e B - s i t e s are o c c u p i e d by post t r a n s i t i o n element c a t i o n s p r e c l u d e s t h e i r s y n t h e s i s i n p u r e f o r m by conventional s o l i d state reaction techniques. The d a t a summarized i n t h i s paper have e s t a b l i s h e d t h a t t h e o x i d e p y r o c h l o r e s under d i s c u s s i o n s u b s t a n t i a l l y r e d u c e the a c t i v a t i o n energy overvoltages a s s o c i a t e d w i t h oxygen e l e c t r o c a t a l y s i s . S p e c i f i c a l l y , i t i s found t h a t t h e s e c a t a l y s t s , i n aqueous a l k a l i n e media near ambient t e m p e r a t u r e , a r e s u p e r i o r t o any o t h e r o x y g e n e v o l u t i o n c a t a l y s t and a r e e q u a l i n p e r f o r m a n c e t o the b e s t known oxygen r e d u c t i o n c a t a l y s t s . As b i d i r e c t i o n a l o x y g e n e l e c t r o c a t a l y s t s , t h e y appear t o be unmatched. I t has been demonstrated t h a t t h e s e same m a t e r i a l s a r e a b l e t o p e r f o r m t h e r e l a t i v e l y r a r e r e a c t i o n o f c l e a v a g e o f o l e f i n s and s e c o n d a r y a l c o h o l s , k e t o n e s and g l y c o l s t o t h e c o r r e s p o n d i n g c a r b o x y l a t e s w i t h h i g h y i e l d s and s e l e c t i v i t i e s . There are s e v e r a l p h y s i c a l and chemical c h a r a c t e r i s t i c s o f t h e s e o x i d e p y r o c h l o r e s which may c o n t r i b u t e t o t h e i r h i g h e l e c t r o c a t a l y t i c a c t i v i t y . The p r e v i o u s l y d e s c r i b e d a l k a l i n e s o l u t i o n s y n t h e s i s t e c h n i q u e ( 6 , 7 ) has p r o v i d e d t h e s e m a t e r i a l s w i t h s u r f a c e a r e a s t y p i c a l l y r a n g i n g f r o m 50 t o 200 m / g . T h u s , one o f t h e b a s i c r e q u i r e m e n t s f o r an e f f e c t i v e e l e c t r o c a t a l y s t has been s a t i s f i e d : t h e e l e c t r o c a t a l y t i c a c t i v i t y i s n o t l i m i t e d by the u n a v a i l a b i l i t y of c a t a l y t i c a l l y a c t i v e s u r f a c e s i t e s , as i s so o f t e n t h e c a s e w i t h metal and mixed metal o x i d e s . Since a l l the c a t a l y s t s i n v e s t i g a t e d i n t h i s study d i s p l a y m e t a l l i c or near m e t a l l i c c o n d u c t i v i t y , the a d d i t i o n a l b a s i c r e q u i r e m e n t o f m i n i m i z i n g ohmic l o s s e s w i t h i n t h e c a t a l y s t and between t h e c a t a l y s t and c u r r e n t c o l l e c t o r has a l s o been s a t i s f i e d . W h i l e h i g h s u r f a c e a r e a and m e t a l l i c c o n d u c t i v i t y are b e n e f i c i a l t o e l e c t r o c a t a l y s i s , t h e y do n o t a l o n e e x p l a i n t h e h i g h c a t a l y t i c a c t i v i t y . We s p e c u l a t e t h a t the v a r i a b l e oxygen s t o i c h i o m e t r y o f t h e p y r o c h l o r e l a t t i c e , and the m u l t i p l e v a l e n c e s t a t e s o f t h e c a t i o n s , p a r t i c u l a r l y the r u t h e n i u m , are e s s e n t i a l t o t h e c a t a l y t i c a c t i v i t i e s of these pyrochlores. N o b l e m e t a l p y r o c h l o r e s were o r i g i n a l l y c o n s i d e r e d by the a u t h o r s as prime oxygen e l e c t r o c a t a l y s t c a n d i d a t e s b e c a u s e o f t h e i r a b i l i t y t o accommodate oxygen v a c a n c i e s i n up t o one seventh of the anion s i t e s . Changes i n t h e a v e r a g e v a l e n c e o f t h e r u t h e n i u m can be expected to accommodate any s u c h s t o i c h i o m e t r y changes in Pb2(Ru2- Pb )06 5. The e x i s t e n c e o f h i g h e r and lower v a l e n t o x i d e s ( e i t h e r s u r f a c e or b u l k ) i n the p o t e n t i a l range o f i n t e r e s t a p p e a r s t o be a c h a r a c t e r i s t i c o f many oxygen e l e c t r o c a t a l y s t s such as P t , Ag, Ni ( f o r oxygen e v o l u t i o n ) , A u , e t c . The r e a s o n s f o r t h i s have n e v e r been e x p l a i n e d e x a c t l y , although the a b i l i t y o f the s u r f a c e t o i n t e r a c t w i t h or adsorb the p o t e n t i a l i n t e r m e d i a t e peroxide or h y d r o p e r o x i d e i o n on i t s s u r f a c e i s o f t e n i n v o k e d . 2

x

x

e

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

162

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch009

In t h e c a s e o f l e a d r u t h e n a t e , t h e o x y g e n n o n - s t o i c h i o m e t r y c o n c e p t c a n be d e v e l o p e d f u r t h e r by c o m b i n i n g i t w i t h t h e known r e a c t i o n s of t h e v a r i a b l e v a l e n c e r u t h e n i u m . I t has been shown i n t h i s work t h a t t h e s e same c a t a l y s t s can c l e a v e c a r b o n - c a r b o n double bonds ( 3 ) i n a manner a n a l o g o u s t o t h a t o f osmium and r u t h e n i u m tetroxiïïe (II). I t i s known (12) t h a t 0$04 (and presumably RUO4) c l e a v e o l e f i n s v i a complexes w i t h T h e s t r u c t u r e :

•C

R'

•C

R

1 The f a c t t h a t t h e s e c a t a l y s t s can c a r r y out t h e v e r y same c l e a v a g e s u g g e s t s t h a t t h e y c a n f o r m m o i e t i e s s i m i l a r t o t h e t e t r o x i d e on t h e i r s u r f a c e s . By a n a l o g y , i t i s p o s s i b l e t h a t the r u t h e n i u m atoms at t h e s u r f a c e of the c r y s t a l l a t t i c e can r e a c t w i t h oxygen m o l e c u l e s so as t o form t h e s e same s u r f a c e complexes:

—0, Ru

—0'

h

/

%

T h e s e c a n be r e d u c e d s t e p w i s e t o r e g e n e r a t e t h e s t a r t i n g s t r u c t u r e :

-O.

3

2e+2H?0— —

U

—0

—0

0 \ R^ N

0

V

c

/ 20H-

+20H-

Rtj —0^

—0

.OH y

V

^OH

—0'

T h i s o r a s i m i l a r s e t o f r e a c t i o n s t e p s would a v o i d t h e f o r m a t i o n o f hydrogen p e r o x i d e c o n s i s t e n t w i t h the r e s u l t s of the r o t a t i n g d i s k e l e c t r o d e experiment. W h i l e t h e mechanism above i s s p e c u l a t i v e , we p r e f e r i t t o o t h e r p o s s i b i l i t i e s i n v o l v i n g - R u - O - O - R u - m o i e t i e s , because (1) the e l e c r o o r g a n i c o x i d a t i o n s must i n v o l v e s i n g l e Ru atoms, due t o t h e s t e r i c r e q u i r e m e n t s o f the c l e a v a g e r e a c t i o n ; (2) t h e c a t a l y t i c a c t i v i t y o f n o n - s t o i c h i o m e t r i c l e a d r u t h e n a t e s i s s u r p r i s i n g l y i n s e n s i t i v e to d i l u t i o n o f t h e Ru atoms w i t h Pb, whereas h i g h s e n s i t i v i t y w o u l d be expected i f t h e mechanism i n v o l v e d p a i r s o f n e i g h b o r i n g Ru atoms; and (3) t h e n e a r e s t Ru-Ru d i s t a n c e i n t h e l a t t i c e i s about 3 . 5 A - p r o b a b l y too l o n g f o r a b r i d g i n g p e r o x i d e group.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

9.

HOROWITZ E T A L .

163

Nonstoichiometric Ruthenate Pyrochlores

Acknowledgments The a u t h o r s w o u l d l i k e t o t h a n k Β . B o w l i n g and H. B r a d y f o r a s s i s t a n c e i n the s y n t h e s i s and m a t e r i a l s c h a r a c t e r i z a t i o n o f t h e c a t a l y s t s u s e d i n t h i s s t u d y ; J . Haberman, K. S t r o h m a i e r , and L. Y a c u l l o f o r t h e e l e c t r o c h e m i c a l m e a s u r e m e n t s ; and G. Dupre and K. Rose f o r o r g a n i c product a n a l y s e s .

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch009

Literature Cited

1.

Horowitz, H.S.; Longo, J.M.; Lewandowski, J.T., Mat. Res. Bull. 1981, 16, 489-496.

2.

Horowitz, H.S.; Longo; J.M.; Horowitz, H.H., J. Electrochem. Soc. 1983, 130, 1851-1859.

3.

Horowitz, H.H.; Horowitz, H.S.; Longo, J.M., In "Proceedings of the Symposium on Electrocatalysis" O'Grady, W.E.; Ross, P.N. Jr.; Will, F.G., Eds.; The Electrochemical Society, Inc.: Pennington, N.J., 1982; pp. 285-290.

4.

Sleight, A.W., Inorg. Chem. 1968, 7, 1704.

5.

McCauley, R.A., J. Appl. Phys. 1980, 51, 290.

6.

Horowitz, H.S.; Longo, J.M.; Lewandowski, J.T., 4 129 525, 1978.

7.

Horowitz, H.S.; Longo, J.M.; Lewandowski, J.T., In "Inorganic Syntheses"; Holt, S.L. Ed.; Wiley-Interscience: N.Y., 1983; pp. 69-72.

8.

Beyerlein, R.A.; Horowitz, H.S.; Longo, J.M.; Leonowicz, M.E.; Jorgensen, J.D.; Rotella, F.J., J. Solid State Chem. 1984, 51, 253.

9.

Pourbaix, M.; "Atlas of Electrochemical Equilibria in Aqueous Solutions"; Pergamon Press: New York City, 1966.

U. S. Patent

10.

Abraham, F.; Nowogroki, G.; Thomas, D. C. R. Acad. S. Paris 1974, 278C, 421.

11.

Djerassi, C.; Engle, R.R., J. Am. Chem. Soc. 1953, 75, 3838.

12.

Criegee, R., Annalen 1936, 522, 75.

RECEIVED

December

17, 1984

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

10 Solid State Chemistry of Tungsten Oxide Supported on Alumina S. SOLED, L. L. MURRELL, I. Ε. WACHS, G. B. McVICKER, L. G. SHERMAN, S. CHAN, N. C. DISPENZIERE, and R. T. K. BAKER

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch010

Exxon Research & Engineering Company, Annandale, NJ 08801

The strong interaction between WO and a γ-Αl O support is monitored under high temperature reducing and oxidizing conditions by a combination of physical and spectroscopic techniques. Below monolayer coverage a difficult to reduce highly dispersed surface tungsten oxide complex exists, whereas at higher coverages a more easily reduced bulk like WO species is also present. Dynamic structural changes of the supported phase occur during high temperature treatment. 3

2

3

3

Supports can no longer be considered inert carriers which act solely to disperse a metal or metal oxide and thereby increase effective surface area. In many cases the r e a c t i v i t y and the c a t a l y t i c properties of supported and bulk phases d i f f e r dramatically. A plethora of work has appeared over the last few years describing the strong metal support interaction (SMSI) of Group VIII metals with a t i t a n i a support (I). In the "SMSI" state, metals display a dramatically reduced H and CO chemisorption a b i l i t y . Controversy exists about the basis of SMSI and such diverse explanations as electron transfer and T i 0 migration onto the metal are being argued (1,2). Examples of supports modifying the properties of t r a n s i t i o n metal oxides have also appeared in the l i t e r a t u r e . Recent work points to iron oxide phases as important species in FischerTropsch synthesis (3). Iron oxide supported on S i 0 (4) and T i 0 (5) resist reduction under conditions in which bulk iron oxide easily reduces. Thus supported iron oxide catalysts are potentially interesting Fischer-Tropsch c a t a l y s t s . The extensive studies on ethylene polymerization catalysts suggests that chromium (VI) species exist on a S i 0 surface at temperatures above which bulk chromic anhydride (Cr0 ) decomposes (6). Recent evidence points to a strong interaction between WO3 and γ - Α 1 0 (7-10). The interaction alters the physical and chemical properties of both WO3 and γ - Α Ι ^ . In this review, we 2

2

2

2

3

2

3

0097-6156/85/0279-0165$06.00/0 © 1985 American Chemical Society

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2

166

S O L I D STATE C H E M I S T R Y IN CATALYSIS

d e s c r i b e s t u d i e s of WO3 on γ - Α 1 0 u s i n g such d i v e r s e t e c h n i q u e s as c o n t r o l l e d atmosphere e l e c t r o n microscopy (CAEM) ( 1 1 ) , x - r a y p h o t o e l e c t r o n s p e c t r o s c o p y (XPS or ESCA) ( 1 2 ) , thermaT~gravimetry (TG) ( 1 3 , 1 4 ) , and l a s e r Raman spectroscopy~Tl5) t o examine the n a t u r e of the t u n g s t e n o x i d e - a l u m i n a i n t e r a c t i o n . 2

3

Experimental In t h e s e s t u d i e s , both powder samples and f i l m s were p r e p a r e d . Powder samples of nominal 4 , 6, 10, 25 and 60 wt.% t u n g s t e n o x i d e s on γ - Α 1 0 (Engelhard I n c . , r e f o r m i n g grade, 180 nr/gm, 325 mesh) were prepared by the i n c i p i e n t wetness impregnation method by adding an aqueous s o l u t i o n of ammonium m e t a - t u n g s t a t e t o the alumina powder, d r y i n g at 100°C and c a l c i n i n g i n a i r at 500°C f o r 16 h r s . For the Raman e x p e r i m e n t s , γ - Α ^ Ο ο o b t a i n e d from Harshaw (A1-4104E, 220 m /gm) or E n g e l h a r d , I n c . , ( r e f o r m i n g grade, 180 nr/g) were used as s u p p o r t s . The impact of c a l c i n a t i o n and steaming as a f u n c t i o n of temperatures was s y s t e m a t i c a l l y s t u d i e d . Samples of pure WO3 and Al 0^0^)3 were o b t a i n e d from C e r a c . For the CAEM e x p e r i m e n t s , f i l m specimens of a l u m i n a , a p p r o x i m a t e l y 50 nm i n t h i c k n e s s , were prepared a c c o r d i n g t o the method d e s c r i b e d p r e v i o u s l y ( 1 6 ) . E l e c t r o n d i f f r a c t i o n e x a m i n a t i o n of s e l e c t e d areas of the alumina f i l m showed the predominant phase t o be γ - Α 1 0 ο . Tungsten was i n t r o d u c e d onto t h e alumina as an atomized spray of a O.1% aqueous s o l u t i o n of ammonium m e t a - t u n g s t a t e . The t u n g s t e n l o a d i n g s ranged between 4 20 micromoles/nr (which corresponds t o 10 t o 50 wt.% t u n g s t e n on a γ - Α 1 0 of 100 m / g ) . TG measurements were conducted on a M e t t l e r TA-2000C as d e s c r i b e d elsewhere ( 1 3 ) . For TG r e d u c t i o n s t u d i e s , samples of WOo on γ - Α 1 0 were heated t o 970°C (at 10°/min) i n He and then h e l d i s o t h e r m a l l y u n t i l c o n s t a n t weight was o b t a i n e d . This p r e c a l c i n a t i o n step minimizes o v e r l a p p i n g r e d u c t i o n and d e h y d r o x y l a t i o n weight l o s s e s . A f t e r c o o l i n g t o room t e m p e r a t u r e , H was i n t r o d u c e d , and t h e samples were reheated t o a temperature between 600° and 900°C (at 10°/min) and h e l d i s o t h e r m a l l y f o r two h o u r s . The s e n s i t i v i t y and s t a b i l i t y of the thermobalance (O.05 mg) e s t a b l i s h e s a d e t e c t i o n l i m i t of 1 t o 2% W0j r e d u c t i o n t o W. S l i g h t gray d i s c o l o r a t i o n s i n d i c a t e small amounts of WO3 r e d u c t i o n below the TG d e t e c t i o n l i m i t . In s i t u x - r a y p h o t o e l e c t r o n s p e c t r a (XPS or ESCA) were c o l l e c t e d on a m o d i f i e d Leybold Heraeus LHS-10 e l e c t r o n s p e c t r o m e t e r . A moveable s t a i n l e s s s t e e l block a l l o w e d sample t r a n s f e r i n vacuum from a r e a c t o r chamber t o the ESCA chamber. The i n t e n s i t i e s and b i n d i n g e n e r g i e s of the W 4f 7/0 s i g n a l s ( Α Ι Κ r a d i a t i o n ) were monitored and r e f e r è n d e d t o the A l 2p peak at 74.5 eV. The 10% W0o and 60% WO3 on γ - Α 1 0 powder samples were c a l c i n e d i n a i r at 500°C and at 950°C r e s p e c t i v e l y f o r 16 hrs and then pressed (at 30 Mpa) onto a gold s c r e e n , which i n t u r n was mounted on a moveable s t a i n l e s s s t e e l b l o c k . These samples were c a l c i n e d i n s i t u at 500°C t o c l e a n the s u r f a c e s p r i o r t o a n a l y s i s . For the r e d u c t i o n t r e a t m e n t s , the samples were heated f o r f i v e minutes at the d e s i r e d temperature i n f l o w i n g H (25 c c / m i n . ) , c o o l e d t o 250°C i n

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch010

2

3

2

2

Z

3

2

3

2

5 / 2

α

2

3

9

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

10.

167

Tungsten Oxide Supported on Alumina

SOLED ET AL.

H , evacuated and then t r a n s f e r r e d i n t o the ESCA chamber. Three samples were i n v e s t i g a t e d ; a bulk W0 f o i l , 60 wt.% WO3 on γ A 1 0 , and 10 wt.% WO3 on γ - Α 1 0 . The Raman spectrometer c o n s i s t e d of a t r i p l e monochromator (Instruments SA, Model DL203) equipped w i t h h o l o g r a p h i c g r a t i n g s and F4 o p t i c s . The spectrometer was coupled t o an o p t i c a l m u l t i c h a n n e l a n a l y z e r ( P r i n c e t o n A p p l i e d Reseach, Model 0MA2) equipped w i t h an i n t e n s i f i e d photodiode a r r a y d e t e c t o r c o o l e d t o 15°C. Each spectrum r e p o r t e d here was accumulated f o r about 100 sec or l e s s . The d i g i t a l d i s p l a y of t h e spectrum was c a l i b r a t e d t o g i v e 1.7 cm" /channel w i t h the o v e r a l l s p e c t r a l r e s o l u t i o n at about 6 c m " . An argon i o n l a s e r ( S p e c t r a P h y s i c s , Model 165) was tuned t o the 514.5 nm l i n e f o r e x c i t a t i o n . A p r i s m monochromator (Anaspec Model 300S) w i t h a t y p i c a l band w i d t h o f O.3 nm removed t h e l a s e r plasma l i n e s ( 1 6 ) . A O.2 gm sample was p e l l e t i z e d under 60 MPa p r e s s u r e i n t o a lTlnm diameter wafer f o r mounting on a s p i n n i n g sample h o l d e r . The l a s e r power at the sample l o c a t i o n was set i n the range o f 1-40 mW. The s c a t t e r e d l i g h t was c o l l e c t e d by a l e n s ( F / 1 . 2 , f/55 mm) h e l d at about 45° w i t h r e s p e c t t o the e x c i t a t i o n . 2

3

2

3

2

3

1

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch010

1

Results CAEM. C o n t r o l l e d atmosphere e l e c t r o n microscopy (11) was used t o observe the b e h a v i o r of t u n g s t e n oxide p a r t i c l e s supported on χ. A 1 0 f i l m s when heated at temperatures up t o 1150°C i n O.7 kPa oxygen. The two specimens d e s c r i b e d i n the Experimental s e c t i o n were heated at i n c r e a s i n g temperatures and the specimen changes were recorded i n r e a l time on video tape ( 1 5 ) . The d e t a i l e d o b s e r v a t i o n s of the dynamic b e h a v i o r of the d i f f e r e n t t u n g s t e n o x i d e phases on t h e γ - Α 1 0 f i l m as a f u n c t i o n of temperature and t u n g s t e n o x i d e content w i l l be d e s c r i b e d i n the D i s c u s s i o n section. 2

3

2

3

Thermal g r a v i m e t r y . γ-Α1ο0 on programmed h e a t i n g (10°/min) t o 1100°C i n the presence o f oxygen, c o n t i n u o u s l y l o s t weight as a r e s u l t of d e h y d r o x y l a t i o n : t h e weight l o s t between 200 and 1100°C equaled about 3 . 5 % . In a d d i t i o n , a weak exotherm w i t h an onset near 1050°C o c c u r r e d d u r i n g t h e t r a n s i t i o n of γ-Α1 03 t o or A 1 0 . A 10% W0 on .γ-Α1 0 sample showed d i f f e r e n t b e h a v i o r . When t h i s sample was heated i n an oxygen atmosphere, a l a r g e r exotherm o c c u r r e d at 1050°C as a f r a c t i o n of the A 1 0 support r e a c t e d w i t h W0 t o form A 1 ( W 0 ) . The f o r m a t i o n of A l ( W 0 ) j was confirmed by X - r a y d i f f r a c t i o n measurements. Alumina not u t i l i z e d i n t u n g s t a t e f o r m a t i o n t r a n s f o r m e d predominantly t o θ - Α 1 0 : o n l y a t r a c e o f o r A l 0 was p r o d u c e d . Thus, the presence of t h e t u n g s t e n oxide s u r f a c e phase i n h i b i t s the t r a n s i t i o n of θ - Α 1 0 t o α-Ai 0 . 3

2

2

3

3

2

3

2

3

2

4

3

3

2

4

2

2

3

3

2

2

3

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

3

168

S O L I D STATE C H E M I S T R Y IN CATALYSIS

TG experiments i n d i c a t e t h a t a s u r f a c e t u n g s t e n oxide phase on alumina i s d i f f i c u l t t o r e d u c e . Table I shows the degree o f r e d u c t i o n (expressed as percent WO3 reduced t o W°) as a f u n c t i o n of WO3 l o a d i n g a f t e r two hour r e d u c t i o n s at 600 or 9 0 0 ° C A 10% WO3 on alumina sample was reduced at s e v e r a l i n t e r m e d i a t e temperatures as w e l l . A m b i g u i t i e s r e s u l t i n g from s i m u l t a n e o u s weight l o s s due t o water were minimized by i n i t i a l l y c a l c i n i n g t h e s e samples i n He t o 9 7 0 ° C This p r e - t r e a t m e n t y i e l d s t u n g s t e n o x i d e on a t r a n s i t i o n a l alumina (mostly θ) p o s s e s s i n g a s u r f a c e area o f -30 m /gm. The r e t a r d a t i o n of WO3 r e d u c t i o n depends on l o a d i n g l e v e l s . At low l o a d i n g l e v e l s (

d

c

Propene/Ac >'

(0001) surface 5.63 O.003 14.07 O.015 16.01 O.03 13.81 O.12 O.225 14-93

O.49 O.94 1.03 1.07 1.19

1.47 3.90 4.51 3.67 4.02

3.86(3.02) 3.61(2.82) 3.50(2.72) 3.76(2.93) 3.72(2.88)

O.087(O.52) O.067(O.40) O.065(O.39) O.077(O.47) O.078(O.47)

(5051) surface 9.23 O.005 16.11 O.015 21.23 O.03 O.06 30.24 O.12 31.27 38.84 O.24 43.41 O.54

O.5 O.89 1.31 1.70 1.62 2.31 2.35

2.05 6.45 10.43 17.82 24.15 32.34 44.88

2.88(2.27) 2.50(1.97) 2.04(1.60) 1.70(1.34) 1.31(1.02) 1.20(O.95) O.97(O.76)

O.053(O.32) O.055(O.33) O.062(O.36) O.057(O.33) O.052(O.28) O.060(O.36) O.055(O.32)

4-45 5.93 13.87 17.91 19.27

1.69(1.45) 1.04(O.89) 1.14(O.98) 1.11(O.96) 1.01(O.87)

(0001) surface 7.52 O.0015 O.006 6.14 15.78 O.3 19.96 1.2 3.6 19.51

a

)

e e e e e

e e e e e

Apparent exposure. The a c t u a l exposure was ten to one hundred times higher due to the dosing action.

k)

Ratio of acetone to 2-propanol.

c

)

Ratio of propene to acetone.

d

)

Values i n the brackets are r a t i o s corrected f o r the mass spectrometer s e n s i t i v i t i e s and pumping speeds. They represent the r a t i o s of the molecular f l u x e s f o r desorbing from the surface.

e

)

These values were small. They were not measured because of the high background s i g n a l s i n these experiments.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

211

212

S O L I D STATE C H E M I S T R Y IN CATALYSIS

a t t e m p t was made t o o b t a i n t h e s e r a t i o s u s i n g d e u t e r a t e d 2 - p r o p a n o l because the major c r a c k i n g fragments f o r d e u t e r a t e d acetone and d e u t e r a t e d propene were b o t h a t ra/e=46. Acetone and Propene D e s o r p t i o n . D e s o r p t i o n o f adsorbed acetone y i e l d e d o n l y acetone on a l l t h r e e s u r f a c e s . O t h e r p o s s i b l e p r o d u c t s t h a t were s e a r c h e d f o r but n o t found i n c l u d e d H , H 0 , methane, CO and C 0 . The d e s o r p t i o n p r o f i l e s from t h e s e s u r f a c e s a r e shown i n F i g u r e 3 , and the peak t e m p e r a t u r e s a r e l i s t e d i n T a b l e I . A s i n g l e sharp d e s o r p t i o n peak was o b s e r v e d on the 0 p o l a r s u r f a c e . A b r o a d peak was o b s e r v e d on the Z n - p o l a r s u r f a c e w h i c h might be two o v e r l a p p i n g peaks. Two d i s t i n c t peaks were o b s e r v e d f o r t h e n o n p o l a r (5051) and (1010) s u r f a c e s . The a r e a s o f the two peaks i n c r e a s e d b u t a t d i f f e r e n t r a t e s w i t h i n c r e a s i n g exposure. The a r e a o f the h i g h e r temperature peak i n c r e a s e d more r a p i d l y i n i t i a l l y , b u t appeared t o be s a t u r a t e d a t about O.27 L . T h i s s a t u r a t i o n m i g h t be r e a l , b u t i t m i g h t a l s o be due t o i n c r e a s i n g c o m p e t i t i o n from background w a t e r a d s o r p t i o n w i t h i n c r e a s i n g exposure (see n e x t s e c t i o n ) . The a r e a o f the l o w e r temperature peak i n c r e a s e d more s l o w l y , b u t c o n t i n u e d t o i n c r e a s e u n t i l i t s a r e a was more t h a n f i v e t i m e s the a r e a o f the h i g h e r temperature peak a t the h i g h e s t exposure s t u d i e d (20 L ) . The peak t e m p e r a t u r e s were c o n s t a n t . The dependence on the exposure was n o t s t u d i e d on the o t h e r two s u r f a c e s . The TPD p r o f i l e s o f adsorbed propene a r e shown i n F i g u r e 4 f o r the 0 - p o l a r and t h e n o n p o l a r s u r f a c e . A s i n g l e peak was o b s e r v e d on b o t h s u r f a c e s . The peak temperatures a r e l i s t e d i n T a b l e I . O n l y the d e s o r p t i o n o f propene, and no hydrogen, CO, C0 > o r methane were d e t e c t e d . O n l y one exposure was s t u d i e d . On the Z n p o l a r s u r f a c e , exposure up t o 6 L d i d n o t r e s u l t i n d e t e c t a b l e propene d e s o r p t i o n . H , w a t e r , methane, CO, C 0 , benzene, and c y c l o h e x e n e were s e a r c h e d f o r b u t were n o t found. Thus propene appeared n o t t o adsorb on t h i s s u r f a c e . I n f a c t , e t h y l e n e was a l s o found n o t t o adsorb on the Z n - p o l a r s u r f a c e under t h e s e c o n d i t i o n s . 2

2

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch012

2

2

2

2

Water D e s o r p t i o n . A f t e r each TPD o r h i g h temperature a n n e a l i n g , background w a t e r was adsorbed onto t h e s u r f a c e d u r i n g c o o l i n g o f the sample. The amount o f w a t e r adsorbed i n c r e a s e d w i t h l o n g e r c o o l i n g t i m e . H e a t i n g o f t h e samples a f t e r c o o l i n g i n vacuo a l w a y s showed desorbed w a t e r a t t e m p e r a t u r e s l i s t e d i n J T a b l e I . The e f f e c t o f adsorbed w a t e r was s t u d i e d on t h e (5051) and the (0001) surface. No e f f e c t was found i n the 2 - p r o p a n o l d e c o m p o s i t i o n . However, i n the acetone a d s o r p t i o n on the (5051) s u r f a c e , i t was found t h a t the h i g h e r temperature acetone d e s o r p t i o n peak decreased w i t h i n c r e a s i n g c o o l i n g t i m e , f o r a g i v e n acetone exposure. The l o w e r temperature peak, however, was n o t a f f e c t e d . Discussion The TPD r e s u l t s o f acetone and propene a r e d i s c u s s e d f i r s t , s i n c e t h e y a r e used f o r the d i s c u s s i o n o f 2 - p r o p a n o l d e c o m p o s i t i o n . Propene a d s o r p t i o n on ZnO powder had been s t u d i e d by TPD and by i n f r a r e d s p e c t r o s c o p y (9-12). A r e v e r s i b l y and an i r r e v e r s i b l y adsorbed propene were found. S i n c e the r e v e r s i b l y adsorbed form

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch012

LUI E T A L .

Temperature-Programmed Decomposition of 2-Propanol

—4

1

100

I

r

*

300 TEMPERATURE

500 C

F i g u r e 3. TPD s p e c t r a o f acetone i n acetone d e s o r p t i o n . a) Z n - p o l a r (0001) s u r f a c e , O.6 L exposure; b) n o n p o l a r (5051) s u r f a c e , O.28 L exposure; c) 0 - p o l a r (0001) s u r f a c e , O.03 L exposure.

X 3

0

200 TEMPERATURE

400 C

Figure 4. TPD s p e c t r a o f propene i n propene a d s o r p t i o n . a) n o n p o l a r (5051) s u r f a c e , 14.4 L exposure; b) 0 - p o l a r (0001 ) s u r f a c e , O.72 L exposure.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch012

214

S O L I D STATE C H E M I S T R Y IN CATALYSIS

c o u l d be removed by e v a c u a t i o n a t room temperature (10). i t w o u l d n o t be d e t e c t e d i n our e x p e r i m e n t s . The i r r e v e r s i b l y adsorbed form was found t o e x i s t as a π - a l l y l a n i o n s p e c i e s based on IR s p e c t r o s c o p y (11.12). I t competes w i t h CO f o r a d s o r p t i o n s i t e s (10), i n d i c a t i n g t h a t i t s a d s o r p t i o n i n v o l v e s a Zn i o n . TPD o f t h i s s p e c i e s from powder ZnO showed a peak a t about 100°C (12). T h i s temperature c o r r e s p o n d s w e l l w i t h the d e s o r p t i o n temperature o b s e r v e d i n t h i s s t u d y . T h e r e f o r e the propene d e s o r p t i o n peak o b s e r v e d here must be from the r e c o m b i n a t i o n o f an adsorbed π - a l l y l and hydrogen. D i s s o c i a t i v e a d s o r p t i o n o f propene r e q u i r e s a Zn-0 p a i r s i t e w h i c h i s abundant on the n o n p o l a r s u r f a c e . T h a t the Ο­ ρ ο l a r s u r f a c e , b u t n o t the Z n - p o l a r s u r f a c e a d s o r b s propene i n t h i s manner s u g g e s t s t h a t s u r f a c e d e f e c t s w h i c h expose the n o n p o l a r s u r f a c e s ( o r o t h e r Zn-0 p a i r s ) e x i s t more a b u n d a n t l y on the 0 - p o l a r t h a n on the Z n - p o l a r s u r f a c e . A d s o r p t i o n o f acetone on ZnO was much l e s s s t u d i e d . R e s u l t s from an 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 y showed t h a t acetone i s adsorbed i n the form o f an e n o l a t e (13). A t h i g h c o v e r a g e , a p o l y m e r i c s p e c i e s i s formed. The s i n g l e sharp TPD peak o b s e r v e d f o r the 0 - p o l a r s u r f a c e i n d i c a t e d o n l y one type o f adsorbed acetone. The two d i s t i n c t peaks f o r t h e n o n p o l a r s u r f a c e , and the b r o a d peak f o r the Z n - p o l a r s u r f a c e i n d i c a t e d the presence o f d i f f e r e n t t y p e s o f a d s o r p t i o n on t h e s e s u r f a c e s . S i n c e the c o v e r a g e s were low i n t h i s s t u d y , the f o r m a t i o n o f p o l y m e r i c acetone was n o t l i k e l y . The d i f f e r e n t forms may t h e n be due t o two d i f f e r e n t forms o f adsorbed acetone (such as acetone o r e n o l ) , o r two d i f f e r e n t k i n d s o f s u r f a c e s i t e s . Water a p p a r e n t l y reduced the amount o f h i g h e r temperature form o f acetone. I t might be t h a t a d s o r b e d w a t e r reduces the e n o l f o r m a t i o n (which i s assumed t o r e s u l t i n the h i g h e r temperature acetone peak) i n the former e x p l a n a t i o n , o r i t competes f o r a d s o r p t i o n on the h i g h e r temperature s i t e s i n the l a t t e r e x p l a n a t i o n . More d e f i n i t e statements would r e q u i r e a d d i t i o n a l information. The d e c o m p o s i t i o n o f 2 - p r o p a n o l showed b o t h s i m i l a r i t i e s and d i f f e r e n c e s among the s u r f a c e s . The most n o t a b l e s i m i l a r i t y i s the f a c t t h a t propene and acetone were produced a t about the same r a t i o on a l l s u r f a c e s . D e h y d r o g e n a t i o n t o form acetone was the dominant r e a c t i o n , as has been o b s e r v e d on ZnO powders (7). The d e s o r p t i o n temperatures o f the r e a c t i o n p r o d u c t s , a c e t o n e , propene, and hydrogen were a l w a y s h i g h e r t h a n the temperature o f d e s o r p t i o n o f the adsorbed a c e t o n e , propene, and hydrogen (hydrogen does n o t adsorb on ZnO under our c o n d i t i o n s ) . Thus the e v o l u t i o n o f acetone and propene a r e r e a c t i o n l i m i t e d i n 2 - p r o p a n o l d e c o m p o s i t i o n . These, t o g e t h e r w i t h the o b s e r v a t i o n t h a t acetone and propene were a l w a y s e v o l v e d a t the same temperature suggest t h a t acetone and propene a r e formed from a common i n t e r m e d i a t e on the d i f f e r e n t s u r f a c e s , the f o r m a t i o n o r d e c o m p o s i t i o n o f w h i c h i s the r a t e l i m i t i n g s t e p . The e v o l u t i o n o f w a t e r , however, was a t the same temperature as the d e s o r p t i o n o f adsorbed water. Thus the p r o c e s s i s desorption limited. Three i n t e r e s t i n g d i f f e r e n c e s can be e a s i l y i d e n t i f i e d . The f i r s t i s the d i f f e r e n c e i n the temperature o f d e s o r p t i o n from the d i f f e r e n t s u r f a c e s . I n g e n e r a l , e v o l u t i o n o f the d e c o m p o s i t i o n p r o d u c t s a r e a t the l o w e s t temperature on the 0 - p o l a r s u r f a c e ,

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch012

12.

LUI E T A L .

Temperature-Programmed Decomposition of'2-Propanol

i n t e r m e d i a t e on the n o n p o l a r s u r f a c e , and the h i g h e s t on the Z n p o l a r s u r f a c e . The o r d e r i s i d e n t i c a l t o t h a t o b s e r v e d f o r the d e c o m p o s i t i o n o f methanol on the same t h r e e s u r f a c e s (3-5)* Thus, the common i n t e r m e d i a t e p o s t u l a t e d above i n t e r a c t s most w e a k l y w i t h the 0 - p o l a r s u r f a c e , and most s t r o n g l y w i t h the Z n - p o l a r s u r f a c e . I n a d d i t i o n t o d i f f e r e n t b o n d i n g c h a r a c t e r i s t i c s o f the r e a c t i o n i n t e r m e d i a t e on the d i f f e r e n t s u r f a c e s , a t l e a s t two o t h e r i n t e r a c t i o n s t h a t d i f f e r on the d i f f e r e n t s u r f a c e s can be i d e n t i f i e d . The a t o m i c arrangement o f an i d e a l Z n - p o l a r s u r f a c e i s such t h a t a l a y e r o f Zn i o n s i s more o u t w a r d l y s i t u a t e d t h a n the n e x t l a y e r o f 0 i o n s . Because the exposed i o n s are Zn i o n s w h i c h a r e n o n p o l a r i z a b l e , t h i s s u r f a c e i s a h a r d a c i d (14)* C o n v e r s e l y , an i d e a l 0 - p o l a r s u r f a c e has a l a y e r o f 0 i o n s more o u t w a r d l y s i t u a t e d t h a n the n e x t l a y e r o f Zn i o n s . These exposed 0 i o n s make the s u r f a c e a s o f t base. The i n t e r m e d i a t e i n 2 - p r o p a n o l d e c o m p o s i t i o n i s an e n o l a t e i o n (13)* B e i n g a b a s e , i t s h o u l d i n t e r a c t more s t r o n g l y w i t h a h a r d a c i d t h a n a s o f t base. The second type o f i n t e r a c t i o n i s d i p o l a r i n t e r a c t i o n . The a t o m i c arrangement o f the s u r f a c e s i s such t h a t a s t r o n g d i p o l e p o i n t i n g outward i s p r e s e n t on the Z n - p o l a r s u r f a c e , and a s t r o n g d i p o l e p o i n t i n g i n w a r d i s p r e s e n t on the 0 - p o l a r s u r f a c e . The n o n p o l a r s u r f a c e , r e l a t i v e l y s p e a k i n g , does n o t p o s s e s s a d i p o l e . The o p p o s i t e o r i e n t a t i o n o f the d i p o l e on the two p o l a r s u r f a c e s w o u l d i n t e r a c t w i t h the d i p o l e o f the e n o l a t e i n t e r m e d i a t e i n an o p p o s i t e manner. T h i s s h o u l d c o n t r i b u t e t o the d i f f e r e n t temperature o f e v o l u t i o n o f the p r o d u c t s . The second d i f f e r e n c e among the s u r f a c e s i s the f a c t t h a t , e x c e p t H2O, the o t h e r t h r e e d e c o m p o s i t i o n p r o d u c t s , H , a c e t o n e , and propene, were e v o l v e d a t the same temperature on the two p o l a r s u r f a c e s , b u t H was e v o l v e d a t a l o w e r temperature on the n o n p o l a r surface. I t i s i n t e r e s t i n g t o compare t h e s e r e s u l t s w i t h the o b s e r v a t i o n s by Koga e t a l . who s t u d i e d the d e c o m p o s i t i o n o f 2 p r o p a n o l a t 100°C on ZnO powder (13)* They found t h a t i f the gas phase 2 - p r o p a n o l was s u d d e n l y removed from the gas phase, the e v o l u t i o n o f hydrogen c o n t i n u e d , b u t the e v o l u t i o n o f acetone stopped. The e v o l u t i o n o f acetone resumed a f t e r r e a d m i s s i o n o f 2 p r o p a n o l . T h i s b e h a v i o r can be e x p l a i n e d by the f a c t t h a t the major exposed f a c e o f t h e i r ZnO powder sample was t h e n o n p o l a r p l a n e . I t i s o n l y on t h i s s u r f a c e t h a t H can be e v o l v e d w i t h o u t c o n c u r r e n t e v o l u t i o n o f acetone i n the absence o f gaseous p r o p a n o l . From the temperature a t w h i c h the d e c o m p o s i t i o n p r o d u c t s e v o l v e d , i t w o u l d seem t h a t the 0 - p o l a r s u r f a c e s h o u l d be the most a c t i v e i n 2 - p r o p a n o l d e c o m p o s i t i o n . However, a c l o s e e x a m i n a t i o n o f the t e m p e r a t u r e s i n T a b l e I shows t h a t on the 0 - p o l a r s u r f a c e , the d e s o r p t i o n temperature o f the m i n o r p r o d u c t w a t e r was a c t u a l l y r a t h e r h i g h - h i g h e r t h a n any o f the p r o d u c t s from the n o n p o l a r surface. Thus i n a s t e a d y s t a t e r e a c t i o n a t t e m p e r a t u r e s b e l o w about 100°C., the 0 - p o l a r s u r f a c e c o u l d be e a s i l y p o i s o n e d by adsorbed w a t e r , l e a v i n g o n l y the n o n p o l a r s u r f a c e a c t i v e . T h a t H was e v o l v e d a t a l o w e r temperature t h a n a c e t o n e and propene on the n o n p o l a r s u r f a c e , b u t n o t on the o t h e r s u r f a c e s , was i n t e r e s t i n g . A p o s s i b l e e x p l a n a t i o n i s t h a t the formation of e n o l a t e (13) from 2 - p r o p a n o l t a k e s p l a c e most r e a d i l y on the n o n p o l a r f a c e because o f the a v a i l a b i l i t y o f Zn-0 p a i r s s u c h t h a t 2

2

2

2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

215

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch012

216

S O L I D STATE C H E M I S T R Y IN CATALYSIS

the conversion of i t into propene and acetone was the slow step. On the p o l a r surfaces, i t s formation was the slow step. I t has been reported that the decomposition of 2-propanol was a s t r u c t u r e - i n s e n s i t i v e reaction on ZnO (15). Our r e s u l t s suggest otherwise. Further work being planned to study steady state reactions on these s i n g l e c r y s t a l surfaces w i l l provide the answer to t h i s discrepancy. The t h i r d difference i s the different r e a c t i v i t y of the surfaces. The f r a c t i o n of adsorbed 2-propanol being decomposed i s i l l u s t r a t e d i n Table I as the r a t i o s of acetone/undecomposed 2propanol. These r a t i o s are the lowest on the 0 - p o l a r surface and the highest on the Zn-polar surface. On the nonpolar surface, the r a t i o s changed from a high value close to those on the Zn surface, to a low value close to those on the 0 surface as the coverage of 2-propanol increased. These data can be explained by the presence of two types of s i t e on these surfaces: a reactive s i t e on which 2propanol decomposes, and an unreactive s i t e on which 2-propanol simply adsorbs and desorbs. The s t i c k i n g coefficients of 2propanol on these two s i t e s vary i n the same manner with coverage on the two p o l a r surfaces, but d i f f e r e n t l y on the nonpolar surface, such that the reactive s i t e i s populated more e a s i l y . An alternate explanation i s that there i s only one type of s i t e for each p o l a r surface. But the s i t e s on the two p o l a r surfaces are different, y i e l d i n g d i f f e r e n t acetone to 2-propanol ratios. Both types of s i t e s are present on the nonpolar surface. I f the i n i t i a l s t i c k i n g coefficient of 2-propanol on the s i t e s s i m i l a r to those found on the Zn surface i s higher, the v a r i a t i o n of the acetone to 2propanol r a t i o i s then explained. However, t h i s l a t t e r model does not automatically explain why the products acetone and propene should desorb at one i d e n t i c a l temperature from the nonpolar plane, that was d i f f e r e n t than those on the p o l a r surfaces unless t h i s difference can be accounted f o r by the different d i p o l a r i n t e r a c t i o n between the surface and the adsorbed intermediate. In conclusion, the chemical properties of ZnO depend on the p a r t i c u l a r surface plane that i s exposed. This surface s p e c i f i c i t y has now been demonstrated for the decomposition of 2-propanol, methanol, formaldehyde and formic a c i d , and adsorption and desorption of acetone, propene, water, CO, and C 0 . These data have made possible better understanding of the r e s u l t s using ZnO powder. I t w i l l be i n t e r s t i n g to seo how different are the c a t a l y t i c properties of these surfaces. 2

Acknowledgment Support of t h i s work by the Petroleum Research Fund administered by the American Chemical Society i s g r a t e f u l l y acknowledged. Literature 1. 2. 3.

Cited

V.E. Henrich, Prog. Surface Sci. 1983, 14, 113. W.H. Cheng, and H.H. Kung, Surface Sci. 1982, 122, 21. S. Akhter, W.H. Cheng, K. Lui, and H.H. Kung, J. Catal. 1984,85, 437.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

12. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch012

13. 14. 15.

LUI E T A L .

Temperature-Programmed Decomposition of 2-Propanol

W.H. Cheng, S. Akhter, and H.H. Kung, J. Catal. 1983, 82, 341. S. Akhter, and H.H. Kung, to be published. M. Bowker, H. Houghton, K.C. Waugh, T. Giddings, and M. Green, J. Catal. 1983, 84, 252. O.V. Krylov, "Catalysis by Nonmetals," Academic Press, 1970. K. Lui, MS thesis, Northwestern University, 1984. A.L. Dent, and R.J. Kokes, J. Amer. Chem. Soc. 1970, 92, 6709, 6718. A.A. Efremov, and A.A. Davydov, Kinet. Catal. 1980, 21, 383. R.J. Kokes, Intra-Science Chem. Rept. 1972, 6, 77. A.A. Davydov, A.A. Yefremov, V.G. Mikhalchenko, and V.D. Sokovskii, J. Catal. 1979, 58, 1; R. Spinicci, and A. Tofanari, J. Thermal Analysis, 1982, 23, 45. O. Koga, T. Onishi, and K. Tamaru, J. Chem. Soc. Faraday Trans. I, 1980, 76, 19. R.G. Pearson, editor: "Hard and Soft Acids and Bases," Dowden, Stroudsburg, 1973. G. Djega-Mariadassou, and L. Davignon, J. Chem. Soc. Faraday Trans. I, 1982, 78, 2447.

RECEIVED

October 4, 1984

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

217

13 The Role of Solid State Chemistry in Catalysis by Transition Metal Sulfides R. R. CHIANELLI

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch013

Corporate Research Science Laboratories, Exxon Research & Engineering Company, Annandale, NJ 08801 The Transition Metal Sulfides are a group of solids which form the basis for an extremely useful class of industrial hydrotreating and hydroprocessing catalysts. Solid state chemistry plays an important role in understanding and controlling the catalytic properties of these sulfide catalysts. This report discusses the preparation of sulfide catalysts, the role of disorder and anisotropy in governing catalytic properties, and the role of structure in the promotion of molybdenum disulfide by cobalt. The Transition Metal Sulfides have been widely used i n petroleum upgrading processes for many years, and due to their c a t a l y t i c and structural s t a b i l i t y i n feedstocks containing large amounts of s u l f u r , the demand for better sulfide catalysts w i l l continue as we are forced to upgrade an increasingly heavier feedstock supply (Ο. Although i n d u s t r i a l l y Important for over sixty years, i t has been only recently that progress has been made in forming a basis for a fundamental understanding of how these solids catalyze impor­ tant reactions. An understanding of the solid state chemistry of these solids has played a key role in this recent progress. The c a t a l y t i c a l l y important s o l i d state chemistry of the sulfides includes not only "classical" s o l i d state areas such as the struc­ ture of supported and unsupported catalysts, but also areas which are at the forefront of solid state chemistry i t s e l f . These areas include novel low temperature methods for producing the s o l i d catalysts at low temperature, the study of disorder and i t s effect on the c a t a l y t i c properties of the solids and the importance of c r y s t a l l i n e anisotropy in determining and controlling the r e a c t i v i t y of the s o l i d , both to the c a t a l y t i c environment and to other metals which may cause poisoning or promotion. This report discusses these issues as the basis for understanding the relationship between the properties of the solids and their a b i l i t y to catalyze a reaction. 0097-6156/ 85/ 0279-0221 $06.00/ 0 © 1985 American Chemical Society

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

222

SOLID STATE CHEMISTRY IN CATALYSIS

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch013

B i n a r y T r a n s i t i o n M e t a l S u l f i d e H y d r o d e s u l f u r i z a t l o n (HPS) C a t a l y s t s I n the p e r i o d between WWI and WWII, work p r i m a r i l y i n Germany focused on M0S2 and WS~ as b e i n g the best s u l f i d e s f o r h y d r o g é n a t i o n and heteroatom removal r e a c t i o n s i n the presence of hydrogen and s u l f u r (2). O r i g i n a l l y , these c a t a l y s t s were used i n an unsupported form and w i t h o u t a d d i t i o n a l t r a n s i t i o n metals which serve to promote catalytic activity. R e c e n t l y , the b i n a r y t r a n s i t i o n m e t a l s u l f i d e s ( b i n a r y r e f e r s to the simple t r a n s i t i o n metal s u l f i d e s c o n t a i n i n g one t r a n s i t i o n m e t a l and s u l f u r ) have been i n v e s t i g a t e d f o r t h e i r a c t i v i t y i n a model HDS r e a c t i o n ( 3 ) . The model HDS r e a c t i o n was the d e s u l f u r i z a t i o n o f d i b e n z o t h i o p h e n e (DBT) and a l l the group I V , V , V I , V I I and V I I I t r a n s i t i o n metal s u l f i d e s , w i t h the e x c e p t i o n of T c , were s t u d i e d . The a c t i v e s u l f i d e phases as determined by X - r a y d i f f r a c t i o n a f t e r r e a c t i v i t y measurements, along w i t h the a c t i v i t y of these phases i n the HDS r e a c t i o n are presented i n F i g u r e 1. Most of the phases which were i d e n t i f i e d a f t e r a p p r o x i m a t e l y e i g h t hours under c a t a l y t i c c o n d i t i o n s (A00°C and 1300 kpa) were p o o r l y c r y s t a l l i n e , as determined by broadened Bragg d i f f r a c t i o n peaks. However, i n some cases (Os and I r ) the d i f f r a c t i o n p a t t e r n obtained contained only diffuse scattering and no h i n t of any r e m a i n i n g Bragg p e a k s . The d i s o r d e r ( d i s c u s s e d f u r t h e r below) which appears i n these c a t a l y s t s c o n t r i b u t e s ( i n some c a s e s ) t o an u n c e r t a i n t y r e g a r d i n g the p r e c i s e phase which i s the s t a b l e s t a t e of the a c t i v e s u l f i d e under c a t a l y t i c c o n d i t i o n s . For example, i n the cases o f Os and I r amorphous phases o f the m e t a l s u l f i d e s were obtained. These phases have a p p r o x i m a t e l y a 1:1 metal-to-sulfur s t o i c h i o m e t r y , but c u r r e n t l y t h e i r s t r u c t u r e s are unknown. In the cases of V and Fe the p o o r l y c r y s t a l l i n e s t a t e of the c a t a l y s t s d i d not permit an exact c h o i c e of phase based s o l e l y on X - r a y d i f f r a c t i o n data from among s e v e r a l c l o s e l y r e l a t e d phases ( 4 ) . Nevertheless, i n most cases the c r y s t a l s t r u c t u r e , upon which the c a t a l y s t i s based, can be d i s c e r n e d . On the l e f t of the p e r i o d i c t a b l e i n group I V , V and V I we f i n d s t r u c t u r e s which are l a y e r e d types e i t h e r cadmium i o d i d e or m o l y b d e n i t e l i k e ( T i S , Z r S , NbS^, T a S , M o S , and WS ) or n i c k e l a r s e n i d e r e l a t e d ( V S , C r ^ ) . A l l c o n t a i n o n l y s i x c o o r d i n a t e m e t a l atoms, but as we move f u r t h e r to the r i g h t i n the p e r i o d i c t a b l e we f i n d t h a t the s t r u c t u r e s v a r y to a g r e a t e r e x t e n t . In the f i r s t row we have d i f f e r e n t s t r u c t u r e s s t a r t i n g w i t h MnS through to N i g S , which are dominated by t e t r a h e d r a l c o o r d i n a t i o n as w e l l as v a r i a b l e s t o i c h i o m e t r y . I n the second row we f i n d R u S w i t h a p y r i t e s t r u c t u r e (whereas i n the f i r s t row F e S i s not s t a b l e under c a t a l y t i c c o n d i t i o n s ) , R h S ^ w i t h a nickel arsenide related structure and PdS w i t h a unique structure. I n the t h i r d row we f i n d R e S w i t h a d i s t o r t e d l a y e r e d s t r u c t u r e , Os and I r w i t h undetermined amorphous s t r u c t u r e s , PtS w i t h a unique s t r u c t u r e and Au not forming a s t a b l e s u l f i d e under catalytic conditions. In o t h e r words, we f i n d t h a t as we move a c r o s s the p e r i o d i c t a b l e the s t a t e of the s u l f i d e c a t a l y s t is c o n s t a n t l y changing i n r e g a r d to c r y s t a l s t r u c t u r e , s t o i c h i o m e t r y and degree o f o r d e r . Yet the c a t a l y t i c a c t i v i t y f o r the model r e a c t i o n i s v a r y i n g i n a c o n t i n u o u s f a s h i o n as we move from element to element. F u r t h e r m o r e , the a c t i v i t y i s v a r y i n g i n a way f a m i l i a r 2

2

2

2

1 + X

2

2

2

2

2

2

2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch013

CHIANELLI

Catalysis by Transition Metal Sulfides

PERIODIC POSITION

Figure 1. Periodic trend for the HDS of Dibenzothiophene by Transition Metal Sulfides.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

224

S O L I D STATE C H E M I S T R Y IN CATALYSIS

i n other areas of c a t a l y s i s : the " v o l c a n o " p l o t ( 5 ) . From F i g u r e 1 i t can be seen t h a t the f i r s t row t r a n s i t i o n m e t a l s u l f i d e s a r e i n a c t i v e r e l a t i v e to the second and t h i r d row t r a n s i t i o n m e t a l s u l f i d e s which e x h i b i t maxima i n the group V I I I m e t a l s , at R u S i n the second row and between Os and I r i n the t h i r d row. This b e h a v i o r p o i n t s to the secondary r o l e of c r y s t a l s t r u c t u r e and t o the dominance of the 4 and 5 d e l e c t r o n s i n d e t e r m i n i n g c a t a l y t i c a c t i v i t y i n the t r a n s i t i o n m e t a l s u l f i d e s . A c o n s i d e r a b l e amount o f work has gone i n t o u n d e r s t a n d i n g the o r i g i n of t h i s e f f e c t termed the " e l e c t r o n i c e f f e c t " i n s u l f i d e c a t a l y s i s , but a d e t a i l e d d i s ­ c u s s i o n of t h i s work i s beyond the scope of t h i s paper ( 6 ) . 2

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch013

Preparation of T r a n s i t i o n Metal S u l f i d e C a t a l y s t s The c a t a l y s t s d e s c r i b e d above were prepared v i a low temperature precipitation from non-aqueous solution (7). This technique i n v o l v e s the p r e c i p i t a t i o n of the t r a n s i t i o n m e t a l s u l f i d e from a non-aqueous solvent such as ethyl a c e t a t e by d i s s o l v i n g the a p p r o p r i a t e t r a n s i t i o n m e t a l h a l i d e i n the s o l v e n t and r e a c t i n g i t m e t a t h e t i c a l l y w i t h a s u l f i d i n g agent such as l i t h i u m s u l f i d e to p r e c i p i t a t e the i n s o l u b l e s u l f i d e f o r example: ethyl MoCl

4

+

2 Li S

>

2

MoS

2

Ψ +

4 UCl

(1)

acetate The b l a c k p r e c i p i t a t e i s s e p a r a t e d from the product L i CI by e x t e n ­ s i v e washing w i t h e t h y l a c e t a t e . Because the product i s formed r a p i d l y at room t e m p e r a t u r e , i t i s c o m p l e t e l y amorphous t o X - r a y s . The amorphous MoS w h i c h has i n i t i a l l y low surface a r e a (~ 5m /gm) may then be c o n v e r t e d i n t o h i g h e r s u r f a c e area p o o r l y c r y s t a l l i n e MoS by heat t r e a t i n g i n a f l o w i n g gas of H /15% H S at e l e v a t e d temperature ( 8 ) . For example, i f amorphous M o S , prepared as d e s c r i b e d above, i s t r e a t e d at 400°C or 600°C i n a H /15% H S m i x ­ t u r e , the r e s u l t i n g s u r f a c e a r e a s w i l l be 63 and 44 M /gm, r e s p e c ­ tively. Thus, by c o n t r o l l i n g the temperature of the heat treatment a c o n t i n u o u s s e r i e s o f MoS c a t a l y s t s can be prepared which have v a r i a b l e surface areas. I n a s i m i l a r manner, a l l the t r a n s i t i o n m e t a l s u l f i d e s can be prepared i n an o x i d e free form w i t h s u f f i c i e n t s u r f a c e a r e a f o r c a t a l y t i c measurements. The lower temperature p r e c i p i t a t i o n method o f f e r s an a d d i t i o n a l advantage. In p r e p a r i n g a l l the t r a n s i t i o n m e t a l s u l f i d e s the c h e m i s t r y i s v a r i e d as l i t t l e as p o s s i b l e from one s u l f i d e to the n e x t . No o t h e r preparative method c u r r e n t l y a v a i l a b l e o f f e r s a l l these a d v a n t a g e s . Further­ more, by changing the s o l v e n t to propylene carbonate a homogeneous c o l l o i d a l d i s p e r s i o n o f MoS can be o b t a i n e d which i s s t a b l e over long periods of time. By s l u r r y i n g a support m a t e r i a l such as AUO^, Si0 o r MgO w i t h the c o l l o i d a l M o S , the c a t a l y s t can be s e l e c t i v e l y adsorbed on the support and thus the e f f e c t of s u p p o r t ­ i n g the t r a n s i t i o n m e t a l s u l f i d e s can be s t u d i e d , a g a i n keeping the method o f p r e p a r a t i o n as c l o s e l y r e l a t e d as p o s s i b l e . Another method used i n the p r e p a r a t i o n of the t r a n s i t i o n m e t a l s u l f i d e s i s the thermal d e c o m p o s i t i o n o f a s u i t a b l e p r e c u r s o r i n a 2

2

2

2

2

2

2

2

2

2

2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2

13.

225

Catalysis by Transition Metal Sulfides

CHIANELLI

s u l f i d i n g environment. For example, MoS may be prepared v i a thermal d e c o m p o s i t i o n o f ammonium t h i o m o l y b d a t e w h i c h proceeds through the amorphous i n t e r m e d i a t e M0S3 ( 9 ) : 2

250 °C (NH ) MoS 4

2

>

4

MoS

+

3

H S+ 2

+

2NH^+

(2)

400 °C MoS

>

3

MoS

2

+

S°

(3)

T h i s method has the advantage of b e i n g s i m p l e r than the above method and of p r o v i d i n g c a t a l y s t s w i t h s u r f a c e areas which are g e n e r a l l y h i g h e r than those o b t a i n e d by o t h e r methods (>100 M / g m ) . A third method of p r e p a r a t i o n which i s c o n v e n i e n t i s the d i r e c t s u l f i d a t i o n of the a p p r o p r i a t e ammonium h e x a c h l o r i d e i n H / H S ( 1 0 ) : 2

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch013

2

2

350 °C (NH.).OsCl, 4 2 6

+

2H S 2

>

0

0sS

H /H S 2 2

+

o

2

2NH.C1+ 4

+

4HC1 + K

}m

T h i s method, which has been a p p l i e d to the noble m e t a l s u l f i d e s , has the advantage t h a t a l l b y - p r o d u c t s are e a s i l y removed i n the f l o w i n g gas phase. Both the thermal d e c o m p o s i t i o n method and the d i r e c t s u l f i d a t i o n method can be a p p l i e d to s p e c i f i c s u l f i d e s o n l y i n cases where the p r e c u r s o r m a t e r i a l can be e a s i l y s y n t h e s i z e d . The low temperature p r e c i p i t a t i o n t e c h n i q u e i s the o n l y method which can be a p p l i e d to a l l the t r a n s i t i o n m e t a l s u l f i d e s . A l l the above methods e a s i l y p r o v i d e reasonable q u a n t i t i e s of h i g h s u r f a c e a r e a c a t a l y s t s for further study. The E f f e c t Catalysts

of

Crystal

Structure

in

Transition

Metal

Sulfide

I n the p r e c e d i n g p a r t o f t h i s paper the predominance of the "periodic" effect on HDS by s u l f i d e c a t a l y s t s was d e s c r i b e d . Because p e r i o d i c i t y dominates, c r y s t a l s t r u c t u r e i s of secondary importance. However, i n t h i s s e c t i o n we b r i e f l y examine the e f f e c t of c r y s t a l s t r u c t u r e on the c a t a l y t i c p r o p e r t i e s of the t r a n s i t i o n metal s u l f i d e s . In the case of c a t a l y s t s such as MoS and W S , the most i n d u s t r i a l l y important c a t a l y s t s , the e f f e c t of c r y s t a l s t r u c t u r e i s q u i t e pronounced. An u n d e r s t a n d i n g of the e f f e c t of c r y s t a l s t r u c t u r e i n these c a t a l y s t s i s e s s e n t i a l to o p t i m i z i n g t h e i r c a t a l y t i c properties for a given a p p l i c a t i o n . The e f f e c t of c r y s t a l s t r u c t u r e may be i n v e s t i g a t e d by p r e p a r i n g c a t a l y s t s , as d e s c r i b e d above, at v a r i o u s temperatures which a s s u r e s a set of c a t a l y s t s h a v i n g v a r i a b l e surface a r e a s , pore s i z e d i s t r i b u t i o n s , and c r y s t a l l i n i t y . Measuring the c a t a l y t i c a c t i v i t y as a f u n c t i o n of these p h y s i c a l p r o p e r t i e s w i l l h e l p to d e f i n e the r o l e of c r y s t a l s t r u c t u r e f o r the p a r t i c u l a r t r a n s i t i o n m e t a l sulfide. I n g e n e r a l , the HDS i s p o o r l y c o r r e l a t e d to N BET s u r f a c e area. T h i s n o n - c o r r e l a t i o n can be most e a s i l y seen by p r e p a r i n g a 2

2

2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

226

S O L I D STATE C H E M I S T R Y IN CATALYSIS

s e r i e s of M0S2 c a t a l y s t s by a v a r i e t y of methods and measuring the HDS a c t i v i t y as a f u n c t i o n o f BET s u r f a c e a r e a . There i s v i r t u a l l y no c o r r e l a t i o n between h y d r o d e s u l f u r i z a t i o n and BET s u r f a c e a r e a . There i s , however, a r a t h e r good c o r r e l a t i o n to a s p e c i f i c c h e m i s o r p t i o n t e c h n i q u e , i n t h i s case 0 chemisorption (11). It is b e l i e v e d t h a t t h i s r e s u l t a r i s e s from the a n i s o t r o p y o f the l a y e r e d structure (Figure 2). I n t h i s s t r u c t u r e s i n g l e l a y e r s of t r a n s i t i o n m e t a l s are sandwiched between two l a y e r s of c l o s e - p a c k e d c h a l c o g e n atoms. W i t h i n these l a y e r s the t r a n s i t i o n metal atoms are bound to s i x s u l f u r atoms which are arranged t r i g o n a l p r i s m a t i c a l l y about the metal. Each s u l f u r atom b r i d g e s t h r e e t r a n s i t i o n m e t a l atoms w i t h i n the same l a y e r , forming the o n l y s t r o n g i n t r a l a y e r f o r c e s , and the l a y e r s can be viewed as t w o - d i m e n s i o n a l macromolecules which s t a c k , bound o n l y by van der Waals f o r c e s , to form three d i m e n s i o n a l c r y s ­ t a l s (12). As a r e s u l t of t h i s , the b a s a l planes of M0S2 c a t a l y s t s are e x t r e m e l y i n e r t c o n t r i b u t i n g to the N s u r f a c e area measurements but not to the c a t a l y t i c a c t i v i t y . 0 , on the o t h e r hand, c h e m i sorbs on the MoS edge p l a n e s where the c a t a l y t i c a l l y a c t i v e s i t e s a r e presumed to be l o c a t e d . The i n e r t n e s s of the b a s a l p l a n e s has been demonstrated i n h i g h vacuum s t u d i e s on MoS s i n g l e c r y s t a l s ( 1 3 ) . A d s o r p t i o n and b i n d i n g s t u d i e s of t h i o p h e n e , H S and r e l a t e d m o l e c u l e s were c a r r i e d o u t . These s t u d i e s i n d i c a t e d t h a t o n l y p h y s i c a l a d s o r p t i o n o f t h e s e m o l e c u l e s occur on the b a s a l plane of M o S . The probe m o l e c u l e s desorbed without detectable d e c o m p o s i t i o n i n d i c a t i n g v e r y low c h e m i c a l a c t i v i t y of the b a s a l p l a n e s of M o S . I t was f u r t h e r shown t h a t the b a s a l plane o f MoS was i n e r t to 0 exposure at 520 Κ (14). Only s p u t t e r i n g w i t h He i o n s , which caused the d e s t r u c t i o n of the hexagonal feed p a t t e r n , would induce r e a c t i v i t y toward 0 . The basal plane could be annealed at 100°K and its inertness recovered. From t h i s study i t was concluded t h a t d e f e c t s may be i n t r o d u c e d i n t o the s u r f a c e by s p u t t e r i n g , and t h i s produced a d r a s t i c i n c r e a s e i n the r a t e o f o x i d a t i o n of the s u r f a c e and i n removal of s u l f u r . T h i s i n d i c a t e s t h a t oxygen c h e m i s o r p t i o n (and thus HDS) i s a s s o c i a t e d w i t h d e f e c t s i t e s i n MoS and not o r d e r e d basal planes. I n a w e l l c r y s t a l l i z e d c a t a l y s t these d e f e c t s l i e on the edge p l a n e s of the c r y s t a l l i t e s . Because of the a n i s o t r o p i c n a t u r e of M o S , i t tends to grow i n v e r y t h i n c r y s t a l s which do not permit easy study o f edge p l a n e s . However, Tanuka and Okuhara showed t h a t the edge p l a n e s of MoS were r e a c t i v e f o r c e r t a i n types o f r e a c t i o n s by c u t t i n g s i n g l e c r y s t a l s i n t o p i e c e s and comparing the r a t e s of cut and uncut c r y s t a l s ( 15). F u r t h e r e v i d e n c e f o r 0« i n t e r a c t i o n at the edge planes o f MoS comes from s t u d i e s on s i n g l e c r y s t a l s . When s i n g l e c r y s t a l s of MoS are p l a c e d i n an o x i d i z i n g environment at e l e v a t e d temperatures ( > 4 0 0 ° C ) , o x i d a t i o n of the MoS can be seen to proceed through the edge p l a n e s ( 1 6 ) . F u r t h e r m o r e , oxygen enrichment at edge planes o f MoS s i n g l e c r y s t a l s has been demonstrated by scanning Auger s t u d i e s on s i n g l e c r y s t a l s ( 1 7 ) . R u S , on the o t h e r hand, has a c o m p l e t e l y i s o t r o p i c c u b i c s t r u c t u r e i d e n t i c a l to p y r i t e ( F e S ) . The i s o t r o p i c n a t u r e o f R u S i n the HDS r e a c t i o n can be seen by the l i n e a r c o r r e l a t i o n s between HDS a c t i v i t y and 0 c h e m i s o r p t i o n and HDS a c t i v i t y and BET s u r f a c e

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch013

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

?

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2

CHIANELLI

Catalysis by Transition Metal Sulfides

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch013

13.

Figure 2.

S t r u c t u r e of a s i n g l e l a y e r o f MoS^. Reproduced w i t h

p e r m i s s i o n from R e f . 8.

C o p y r i g h t 1982, T a y l o r & F r a n c i s L t d .

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

228

S O L I D STATE C H E M I S T R Y IN CATALYSIS

area (18). RuS^ and MoS e x h i b i t the two extremes which can occur i n HDS by s u l f i d e s . Presumably, the e f f e c t of c r y s t a l s t r u c t u r e i n a l l o t h e r t r a n s i t i o n m e t a l s u l f i d e s l i e s somewhere i n between. 2

The R o l e Catalysts

of

Edge

Planes

in

Transition

Metal

Sulfide

The importance of edge p l a n e s a l s o a r i s e s i n the i n d u s t r i a l l y i m p o r t a n t promoted t r a n s i t i o n m e t a l s u l f i d e c a t a l y s t systems. I t has been known f o r many years t h a t the presence of a second m e t a l such as Co o r N i to a MoS o r WS c a t a l y s t l e a d s to promotion (an i n c r e a s e i n a c t i v i t y f o r HDS or h y d r o g é n a t i o n i n excess of the a c t i v i t y of the i n d i v i d u a l components) ( 2 ) . Promotion e f f e c t s can e a s i l y be observed i n supported o r unsupported c a t a l y s t s . The supported c a t a l y s t s are c u r r e n t l y the most i m p o r t a n t i n d u s t r i a l c a t a l y s t s , but the unsupported c a t a l y s t s are e a s i e r to c h a r a c t e r i z e and s t u d y . Unsupported, promoted c a t a l y s t s have been prepared by many d i f f e r e n t methods, but one c o n v e n i e n t way of p r e p a r i n g these catalysts is by a p p l y i n g the nonaqueous p r e c i p i t a t i o n method d e s c r i b e d above. F o r example, f o r Co/Mo, a p p r o p r i a t e m i x t u r e s of C o C l and MoCl^ are r e a c t e d w i t h L i S i n e t h y l a c e t a t e : 2

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch013

Promoted

2

2

2

ethyl xCoCl

2

+ yMoCl

4

+ (x + 2 y ) L i S

>

2

a c e t a t e 25°C x"CoS"(amorphous) + yMoS + 2(x + 2 y ) L i C l

(5)

2

The amorphous p r o d u c t s are then heat t r e a t e d i n 15% H / H S at the d e s i r e d t e m p e r a t u r e , as d e s c r i b e d above f o r the b i n a r y systems. By u s i n g t h i s t e c h n i q u e , the r a t i o of Co/Mo i n the c a t a l y s t can e a s i l y be c o n t r o l l e d . I t has been w e l l e s t a b l i s h e d t h a t somewhere i n the r e g i o n of a Co/Mo r a t i o of O.2 to O.3 a maximum i n c a t a l y t i c a c t i v i t y w i l l occur ( 2 ) . The enhancement i n a c t i v i t y can be as much as an order of magnitude above the unpromoted systems. The p r e c i s e shape of the a c t i v i t y v s . promotion c u r v e s depends g r e a t l y on the method of p r e p a r a t i o n of the c a t a l y s t . The unsupported c a t a l y s t , a f t e r heat t r e a t m e n t , c o n t a i n s b o t h Co

t o the d e n s i t y o f Mo atoms i n an edge p l a n e .

c o n c e n t r a t i o n was below the l i m i t o f d e t e c t i o n . T h i s a l l o w s one make s e v e r a l c o n c l u s i o n s c o n c e r n i n g the l o c a t i o n o f the Co atoms

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

to in

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch014

14.

TOPS0E ET AL.

Promoter Atoms in Co-Mo and Ni-Mo Catalysts

239

Co-Mo-S. F i r s t o f a l l , i n agreement w i t h e a r l i e r r e s u l t s we can e x c l u d e the presence o f Co i n b u l k i n t e r c a l a t i o n ( p o s i t i o n (a) i n F i g . 1) and b u l k s u b s t i t u t i o n a l p o s i t i o n s ( p o s i t i o n ( c ) ) . W i t h the p r e s ent l i m i t o f d e t e c t i o n , the maximum amount o f Co t h a t c o u l d be p r e s ent i n t h e s e p o s i t i o n s c o r r e s p o n d s t o O.1 a t ? Co. I t can a l s o be c o n c l u d e d from t h e s e measurements t h a t no s i g n i f i c a n t amount o f Co i s p r e s e n t a t the b a s a l p l a n e s u r f a c e s ( p o s i t i o n ( e ) ) (the d e t e c t i o n l i m i t c o r r e s p o n d s t o about 5% coverage o f the b a s a l p l a n e s ) . I n view o f the above r e s u l t s , the l a r g e Co s i g n a l measured w i t h the e l e c t r o n beam p r o b i n g the edge r e g i o n s must be e x c l u s i v e l y caused by Co atoms l o c a t e d a t the edge s u r f a c e s . T h i s i s p r o b a b l y the f i r s t d i r e c t e v i d e n c e f o r the edge l o c a t i o n o f the Co atoms i n Co-Mo-S. The AEM r e s u l t s a l o n e do not a l l o w one t o c o n c l u d e whether the edge Co atoms a r e l o c a t e d a t edge s u b s t i t u t i o n a l ( p o s i t i o n ( d ) ) or edge i n t e r c a l a t i o n p o s i t i o n s ( p o s i t i o n ( b ) ) . However, i n view o f the e a r l i e r r e s u l t s d i s c u s s e d above, the edge i n t e r c a l a t i o n p o s i t i o n i s not l i k e l y . The Co coverage a t the edges g i v e n i n T a b l e I p r o b a b l y c o r r e sponds c l o s e t o the maximum a c h i e v a b l e s i n c e f u r t h e r s u l f i d i n g a t h i g h temperatures l e a d s t o p a r t i a l s e g r e g a t i o n o f Co from Co-Mo-S t o form CogS8* AEM s t u d i e s o f unsupported Ni-Mo c a t a l y s t s w i t h l a r g e c r y s t a l s suggest t h a t the N i atoms a r e a l s o l o c a t e d a t the M0S2 edges but the r e s u l t s a r e more q u a l i t a t i v e than i n the case o f the Co-Mo c a t a lysts. P r e v i o u s s t u d i e s o f M0S2 s i n g l e c r y s t a l s (27) have shown t h a t the NO a d s o r p t i o n o c c u r s a t the edge p l a n e s . Thus, a d s o r p t i o n s t u d i e s u s i n g NO s h o u l d p r o v i d e the p o s s i b i l i t y o f f u r t h e r e l u c i d a t i n g the p r o p e r t i e s o f Mo c a t a l y s t s promoted by e i t h e r Co o r N i . I n the p r e s e n t NO a d s o r p t i o n s t u d i e s i n f r a r e d s p e c t r o s c o p y was used s i n c e t h i s a l l o w e d d i s t i n c t i o n between the a d s o r p t i o n o c c u r r i n g on m o l y b denum and promoter atoms (see (20) f o r d e t a i l s ) . The Ni-Mo and Co-Mo c a t a l y s t s s t u d i e d were p r e p a r e d by a d d i n g d i f f e r e n t amounts o f p r o moter atoms t o the same M0/AI2O3 c a t a l y s t . EXAFS s t u d i e s on the s u l f i d e d c a t a l y s t s i n d i c a t e t h a t the s i z e o f the s m a l l M0S2 domains i s about the same f o r a l l the c a t a l y s t s ( 1 4 , 1 6 ) . Thus, the t o t a l number o f Mo edge s i t e s i s a l s o e x p e c t e d t o be the same. N e v e r t h e l e s s , w i t h i n c r e a s i n g Co l o a d i n g i n c r e a s i n g amount o f promoter atoms a d s o r b i n g NO i s s e e n , whereas the number o f Mo edge atoms a v a i l a b l e f o r a d s o r p t i o n d e c r e a s e s ( F i g . 2a and c ) . T h i s i n d i c a t e s t h a t the Co o r N i promoter atoms a t the M0S2 edges p a r t i a l l y c o v e r t h e Mo atoms. Comp a r i s o n w i t h MES r e s u l t s (12) shows t h a t t h e s e Co atoms form Co-Mo-S s t r u c t u r e s . From the NO a d s o r p t i o n ( F i g . 2a and c) and a c t i v i t y d a t a ( F i g . 2b and d ) , i t i s c l e a r t h a t f o r the promoted c a t a l y s t s the HDS and h y d r o g é n a t i o n a c t i v i t i e s a r e m a i n l y a s s o c i a t e d w i t h the edge promoter atoms w h i l e the edge Mo s i t e s a r e l e s s i m p o r t a n t . The a c t i v i t y r e s u l t s a l s o show t h a t the h y d r o g é n a t i o n i s l e s s promoted than the HDS r e a c t i o n . T h i s i s i n agreement w i t h p r e v i o u s r e s u l t s i n the l i t e r a t u r e (30-32). By c o m b i n i n g the IR and AEM r e s u l t s i t f o l l o w s t h a t the n a t u r e o f the a c t i v e phases i s s i m i l a r i n s u p p o r t e d and unsupported HDS c a t a l y s t s . T h i s i s i n accordance w i t h MES r e s u l t s ( 6 , 9 , 11) w h i c h show t h a t s i m i l a r Co-Mo-S type s t r u c t u r e s e x i s t i n both s y s t e m s . From t h e s e r e s u l t s one may u n d e r s t a n d why Voorhoeve and S t u i v e r (33)

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch014

240

F i g u r e 2. IR absorbances o f the NO a b s o r p t i o n bands and a c t i v i t y d a t a f o r s e r i e s o f s u l f i d e d C0-M0/AI2O3 ( ( a ) and ( b ) ) and Ni-Mo/AI2O3 c a t a l y s t s ( ( c ) and ( d ) ) . (Due t o opagueness o f the h i g h N i l o a d i n g s a m p l e s , the IR d a t a are o n l y shown f o r c a t a l y s t s w i t h Ni/Mo r a t i o s l e s s than O.3). F i g u r e adapted from R e f s . (20, 32).

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch014

14.

TOPS0E ET AL.

Promoter Atoms in Co-Mo and Ni-Mo Catalysts

241

and several l a t e r authors (see e.g., (3, 4, 13, 17, 34)) have ob­ served many c a t a l y t i c s i m i l a r i t i e s between unsupported and supported catalysts. Magnetic s u s c e p t i b i l i t y has previously been used i n many i n ­ stances to study Co-Mo HDS catalysts (35-42). Most of the studies have been devoted to the calcined state of the catalysts and only i n few cases to the sulfided state (39-42). In the f i r s t studies of sulfided Co-Mo/AI2O3 c a t a l y s t s , the magnetic moments were above 4yg and were quite close to the values found f o r the catalysts before s u l f i d i n g (39, 41). This was taken as an i n d i c a t i o n that the Co a t ­ oms are not sulfided or only sulfided to a very small extent. In a more recent study (42), the magnetic moment was observed to decrease a f t e r s u l f i d i n g to a value of about 3.2 μβ. This decrease indicates that a sulfided Co surface phase was formed and on the basis of var­ ious assumptions concerning the Co phase d i s t r i b u t i o n , a magnetic moment of about 1.73 (i«e. Co(II) low-spin) was estimated for the surface Co phase. In view of the recent Môssbauer investigation, the differences i n magnetic moments observed i n previous studies are probably r e lated to differences i n the Co phase d i s t r i b u t i o n which, i n fact, has been found to be sensitive to the choice of preparation parameters. To avoid such ambiguities, i t i s thus necessary to measure the magnetic s u s c e p t i b i l i t y on samples where the Co phase d i s t r i b u tion i s known. This w i l l also allow one to obtain magnetic propert i e s for the c a t a l y t i c a l l y active Co-Mo-S species. In order to avoid the contribution from Co i n the alumina, we have studied unsupported Co-Mo catalysts for which a l l the Co atoms were found by MES to be present as Co-Mo-S. Measurements of the magnetic s u s c e p t i b i l i t y vs. temperature show (see F i g . 3) that the e f f e c t i v e moment per cobalt atom varies smoothly from O.39 UB 5.1 Κ to O.73 μβ at 275 K. The very low magnetic moment indicates that Co i s i n a s u l f i d e environment. However, the r e s u l t s are not compatible with Co i n any magnetically isolated configuration ( i . e . high or low spin Co(II) or Co(III)) independent of the geometry and nature of surrounding non-magnetic l i g a t i n g atoms. Rather the result suggests that extensive electron d e r e a l i z a t i o n i s occurring due to the i n ­ teractions with the neighboring non-magnetic atoms. The reduced low temperature moment indicates some degree of antiferromagnetic ordering of the spin l a t t i c e . The antiferromagnetic interactions are expected to take place v i a s u l f u r atoms along the long chains of nearest neighbor Co centers at the edges of the M0S2 p a r t i c l e s . In view of the above findings, the neighboring Mo atoms may also be involved i n the exchange paths. The determination of the magnetic moment of Co i n Co-Mo-S f o r the more important alumina supported catalysts i s more d i f f i c u l t since the Co atoms may be present i n several d i f f e r e n t phases (e.g., Co-Mo-S, Co i n alumina, and CogSg). Therefore, i n order to calculate the magnetic moment of Co-Mo-S i t i s necessary to know both the phase composition as well as the magnetic moments of C09S8 and Co i n alumina. Consequently, magnetic measurements were carried out on ca­ t a l y s t s f o r which the Co phase composition previously has been de­ termined by use of MES (12). The magnetic s u s c e p t i b i l i t y of C09S8 has been found by several investigators (see e.g., (43, 44)) to be very low and i s not expected to give any s i g n i f i c a n t contribution to a

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

t

242

S O L I D STATE C H E M I S T R Y I N C A T A L Y S I S

the magnetic moments. A l t h o u g h the Co atoms p r e s e n t i n the a l u m i n a are found i n both o c t a h e d r a l and t e t r a h e d r a l c o o r d i n a t i o n i n c a l ­ c i n e d Co-Mo c a t a l y s t s ( 4 5 , 4 6 ) , d e t a i l e d MES s t u d i e s (45) i n d i c a t e t h a t o n l y the t e t r a h e d r a l l y c o o r d i n a t e d Co atoms remain a f t e r s u l f i d a t i o n . The magnetic moment o f such Co s p e c i e s i s expected t o be around 4.1 ]ΐβ ( 4 2 ) . Thus on b a s i s o f the Co phase c o m p o s i t i o n , a moment o f 4.1 UR f o r Co i n the a l u m i n a and n e g l e c t i n g the c o n t r i b u ­ t i o n from C 0 9 S 8 , the magnetic moment o f Co i n Co-Mo-S can be e s t i ­ mated by use o f the e x p r e s s i o n : V

= Σ a. μ?

(1)

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch014

i where i r e p r e s e n t s Co i n a l l d i f f e r e n t c o n f i g u r a t i o n s . I n F i g . 4 we have shown the i n v e r s e s u s c e p t i b i l i t i e s o f c a t a l y s t s w i t h d i f f e r e n t Co/Mo r a t i o s and i n T a b l e I I the r e s u l t i n g average magnetic moments of the c a t a l y s t s are g i v e n t o g e t h e r w i t h the c a l c u l a t e d moments f o r Co-Mo-S. Table I I . lysts Catalyst Co/Mo

Magnetic moments a t room temperature

Co phase d i s t r i b u t i o n ( a t %) Co-Mo-S Co:Al203 C09S8

o f Co-Mo/AI2O3 C a t a -

Magnetic moments u

total

μ

(PR)

Co-Mo-S

O.09

11+4

0+4

89+4

2.05

1.41-1.80

O.27

15+5

0+4

85+5

1.95

O.74-1.54

8+4

73+4

20+4

1.52

1.35-2.61

1.19 2.09

1.12

-

For each Co:Mo r a t i o , an i n t e r v a l i s g i v e n f o r the d e r i v e d mo­ ment o f the Co atoms i n Co-Mo-S, as c a l c u l a t e d from the e s t i m a t e d u n c e r t a i n t i e s o f the Co phase d i s t r i b u t i o n s . The i n t e r v a l s are wide but o v e r l a p p i n g w i t h an average v a l u e o f around 1.4 μ g per Co atom. I t i s seen t h a t a l t h o u g h the c o n t e n t o f Co i n the a l u m i n a f o r a l l the c a t a l y s t s i s v e r y s m a l l i t g i v e s r i s e to the l a r g e s t c o n t r i b u ­ t i o n to the observed moment. T h e r e f o r e , the u n c e r t a i n t i e s i n the magnetic moment o f Co i n Co-Mo-S (see T a b l e I I ) are m a i n l y d e t e r ­ mined by the u n c e r t a i n t y i n the d e t e r m i n a t i o n o f the c o n t e n t o f Co in alumina. The magnetic moment o f the unsupported c a t a l y s t (pure Co-Mo-S phase) i s temperature dependent, and i t s room temperature v a l u e i s s u b s t a n t i a l l y s m a l l e r than t h a t found i n the supported c a t a l y s t s . These r e s u l t s suggest t h a t the e l e c t r o n d e r e a l i z a t i o n and exchange e f f e c t s are more i m p o r t a n t f o r Co-Mo-S i n the unsupported c a t a l y s t s than i n the s u p p o r t e d c a t a l y s t s . A p o s s i b l e e x p l a n a t i o n f o r t h i s b e ­ h a v i o r i s the presence o f support i n t e r a c t i o n s . Such i n t e r a c t i o n s may i n v o l v e oxygen b r i d g e s between the Mo atoms i n Co-Mo-S and the a l u m i n a . The presence o f such oxygen l i g a n d s i n the M 0 S 2 s t r u c t u r e ,

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

14. TOPS0E ET AL.

Promoter Atoms in Co-Mo and Ni-Mo Catalysts

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch014

0

100

200

243

300 Κ

Figure 3. Magnetic s u s c e p t i b i l i t y ( l e f t scale; cgs units per gram atom Co) and e f f e c t i v e moment (right scale; Bohr magnetons) versus temperature f o r an unsupported Co-Mo HDS catalyst exhibiting Co-Mo-S as the only Co phase (295 data points are shown). io /% 3

A

2 u/

/u

/

0

/

/

/ X 0 0

1 100

Co:Mo

Meff

•

2.09

1.12

0

1.19

1.52

ο O.27

1.95

•

2.05

O.09

I

I

200

300

Κ

Figure 4. Inverse magnetic s u s c e p t i b i l i t y (cgs units gram atom Co) versus temperature f o r a series of sulfided C0-M0/AI2O3 catalysts with d i f f e r e n t Co/Mo r a t i o s .

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

244

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch014

which i s a serai-conductor, may very well a l t e r i t s electronic and magnetic properties i n a way that i t becomes less conductive. This may r e s u l t i n less electron d e r e a l i z a t i o n and thus i n a higher moment of the Co atoms present i n the alumina supported Co-Mo-S phase. It i s also possible that i n the alumina supported catalysts, which have the highest M0S2 dispersion, Mo3+ or Mo^+ species may contribute to the observed moment. Exposure of the catalysts to even small traces of oxygen leads to large increases i n the observed magnetic moment. In agreement with previous MES (9, 11) and EXAFS studies (15, 16), t h i s shows that the Co edge atoms e a s i l y coordinate to oxygen. This r e s u l t also implies that i t i s important to carry out i n s i t u studies of Co-Mo-S.

Acknowledgments The authors are g r a t e f u l to K. Reiter, J . Refslund Andersen, J.W. 0rnbo, and F. Rasmussen for technical assistance. One of us (Erik Pedersen) acknowledges support by the Danish Natural Science Research Council through grant numbers 511-742, 511-3993 and 51110516. Literature Cited

1.

de Beer, V.H.J.; Schuit, G.C.A. B. Delmon, P.A. Jacobs, and Amsterdam, 1976; p. 343.

2.

Massoth, F.E. In "Advances in Catalysis"; D.D. Eley, H. Pines, and P.B. Weiz, Eds.; Academic Press, New York, 1978; Vol. 27, p. 265. Delmon, B. In "Proceedings of the Climax Third Intern. Conf. on Chemistry and Uses of Molybdenum"; H.F. Barry and P.C.H. Mitchell, Eds.; Climax Molybdenum Co., Ann Arbor: Michigan, 1979; p. 73. Grange, P. Catal. Rev.-Sci. Eng. 1980, 21, 135. Ratnasamy, P.; Sivasanker, S. Catal. Rev.-Sci. Eng. 1980, 22, 371. Topsøe, H.; Clausen, B.S.; Candia, R.; Wivel, C.; Mørup, S. Bull. Soc. Chim. Belg. 1981, 90, 1189. Mitchell, P.C.H. In "Catalysis"; C. Kemball and D.A. Dowden, Eds.; Specialist Periodica Report, Royal Society of Chemistry: London, 1980; Vol. 4, p. 175. Topsøe, H. In "Surface Properties and Catalysis by Non-Metals"; J.P. Bonnelle et al., Eds.; 1983, p. 329. Clausen, B.S.; Mørup, S.; Topsøe, H.; Candia, R. J. Phys. Colloq. 1976, C6, p. C6-249. Topsøe, H; Clausen, B.S.; Burriesci, N.; Candia, R.; Mørup, S. In "Preparation of Catalysts II"; B. Delmon, Grange, P., Jacobs, P.Α., Eds.; Elsevier Scientific Publishing Company: Amsterdam, 1979; p. 479. Topsøe, H.; Clausen, B.S.; Candia, R.; Wivel, C.; Mørup, S. J. Catal. 1981, 68, 435.

3.

4. 5. 6. 7.

8. 9. 10.

11.

In "Preparation of G. Poncelet, Eds.;

Catalysts"; Elsevier:

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

14. TOPSØE ET AL. 12. 13. 14.

15. 16.

17.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch014

18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

31. 32.

33. 34. 35. 36. 37. 38. 39. 40. 41. 42.

Promoter Atoms in Co-Mo and Ni-Mo Catalysts

245

Wivel, C.; Candia, R.; Clausen, B.S.; Mørup, S., Topsøe, H. J. Catal. 1981, 68, 453. Candia, R.; Clausen, B.S.; Topsøe, H. J. Catal. 1982, 77, 564. Clausen, B.S.; Topsøe, H.; Candia, R., Villadsen, J.; Lengeler, B.; Als-Nielsen, J.; Christensen, F. J. Phys. Chem. 1981, 85, 3868. Clausen, B.S.; Lengeler, B.; Candia, R., Als-Nielsen, J.; Topsøe, H. Bull. Soc. Chim. Belg. 1981, 90, 1249. Clausen, B.S.; Topsøe, H.; Candia, R.; Lengeler, B. In "Catalytic Materials: Relationship Between Structure and Reactivity"; American Chemical Society: Washington, D.C. 1984; p. 71. Candia, R.; Clausen, B.S.; Topsøe, H. Bull. Soc. Chim. Belg. 1981, 90, 1225. Sørensen, O.; Clausen, B.S.; Candia, R.; Topsøe, H. Appl. Catal. in press. Topsøe, N.-Y.; Topsøe, H. J. Catal. 1982, 75, 354. Topsøe, N.-Y.; Topsøe, H. J. Catal. 1983, 84, 386. Michelsen, K.; Pedersen, E. Acta Chem. Scand. A32,847 and references therein (1978) Johansson, T.; Nielsen, K.G. J. Phys. E: Sci. Instrum. 1976, 9, 852. Topsøe, N.-Y. J. Catal. 1980, 64, 235. Topsøe, N.-Y.; Topsøe, H. Bull. Soc. Chim. Belg. 1981, 90, 1311. Groszek, A.J.; Witheridge, R.E. Powder Metall. 1972, 15, 115. Tanake, K.; Okuhare, T. Catal. Rev. Sci. Eng. 1982, 15, 249. Suzuki, K.; Soma, M.; Onishi, T.; Tamaru, K. J. Electron Spectrosc. Relat. Phenom. 1981, 24, 283. Stevens, G.C.; Edmonds, T. J. Catal. 1975, 37, 544. Salmeron, M.; Somorjai, G.A.; Wold, Α.; Chianelli, R.; Liang, K.S. Chem. Phys. Lettr. 1982, 90, 105. Hargreaves, A.E.; Ross, J.R.H. In "Proc. 6th Int. Congr. Catal". G.V. Bond; P.B. Wells; F.C. Tompkins, Eds. Chem. Soc.: London, 2, 1977, p. 937. Massoth, F.E.; Chung, K.S. In "Proc. 7th Int. Congr. Catal.", T. Seiyama; K. Tanabe, Eds. Elsevier: New York, 1980, p. 629. Candia, R.; Clausen, B.S.; Bartholdy, J.; Topsøe, N.-Y.; Lengeler, B.; Topsøe, H. In "Proc. 8th Int. Congr. Catal.", Verlag Chemie: Weinheim, 1984, Vol. II, p. 375. Voorhoeve, R.J.H.; Stuiver, J.C. M. J. Catal. 1971, 23, 228. Furimsky, E.; Amberg, C.H. Can. J. Chem. 1975, 53, 2542. Richardson, J.T.; Ind. Eng. Chem. Fundam. 1964, 3, 154. Aschley, J.H.; Mitchell, P.C.H. J. Chem. Soc. A 1968, 2821. Lipsch, J.M.J.G.; Schuit, G.C.A. J. Catal. 1969, 15, 163. Lo Jacono, M.; Cimino, Α.; Schuit, G.C.A. Gass. Chim. Ital. 1973, 103, 1281. Mitchell, P.C.H.; Trifiro, F. J. Catal. 1974, 33, 350. Perrichon, V.; Vialle, J.; Turlier, P.; Delvaux, G.; Grange, P.; Delmon, B. Comptes rendus, 1976, 282 série C, 85. Ramaswamy, A.V.; Sivasanker, S.; Ratnasamy, P. J. Catal. 1976, 42, 107. Chiplunker, P.; Martinez, N.P.; Mitchell, P.C.H. Bull. Soc. Chim. Belg. 1981, 90, 1319.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

246 43. 44. 45. 46.

S O L I D STATE C H E M I S T R Y I N C A T A L Y S I S

Townsend, M.G.; Horwood, J.L.; Tremblay, R.J.; Ripley, L.G.; Phys. Stat. Soc. (a) 1972, Κ 137. Knop, O.; Huang, C.-Y.; Reid, K.I.G.; Carlow, J.S.; Woodhams, F.W.D. J. Solid State Chem. 1976, 16, 97. Wivel, C.; Clausen, B.S.; Candia, R.; Mørup, S.; Topsøe, H. J. Catal. 1984, 87, 497. Candia, R.; Topsøe, N.-Y.; Clausen, Β . S . ; Wivel, C.; Nevald, R.; Mørup, S.; Topsøe, H. In "Proceedings of the Climax Fourth Intern. Conf. on Chemistry, and uses of Molybdenum", Ann Arbor: Michigan, 1982; p. 374.

October 4, 1984

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch014

RECEIVED

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

15 Preparation and Properties of Cobalt Sulfide, Nickel Sulfide, and Iron Sulfide 1

D. M. PASQUARIELLO, R. KERSHAW, J. D. PASSARETTI , K. DWIGHT, and A. WOLD

2

Department of Chemistry, Brown University, Providence, RI 02912

Co S , Ni S , and Fe S were prepared as single-phase polycrystalline materials by heating the appropriate metal sulfates in a controlled mixture of H and H S at low temperature. The products were characterized by x-ray diffraction, thermogravimetric analysis, and magnetic susceptibility measurement. The x-ray diffraction pattern and field dependent magnetic susceptibility of Fe S were affected by the thermal history of the sample. The observed differences can be related to the vacancy ordering associated with ferrimagnetic F e S .

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch015

9

8

3

2

7

8

2

7

7

2

8

8

The t r a n s i t i o n metal sulfides CogSg, N 1 3 S 2 and FeySg have been i d e n t i f i e d as possible promotors i n hydrodesulfurization c a t a l y s t s . However, the actual catalysts are amorphous, and discrete sulfide phases have never been observed i n γ - Α ΐ 2 θ 3 supported systems within the composition range of commercial catalysts. Since the preparation of CogSg, Ni3S2,and FeySg i s d i f f i c u l t to achieve by direct combination of the elements, a low temperature synthesis involving the treatment of anhydrous sulfates i n a controlled H 2 / H 2 S atmosphere was developed at Brown University (1). Since magnetic s u s c e p t i b i l i t y appears to be capable of distinguishing the various members of the Fe-S, Co-S and Ni-S systems (2-4), i t was decided to characterize the low temperature single-phase products by magnetic s u s c e p t i b i l i t y measurements as well as x-ray d i f f r a c t i o n analysis. Delafosse, et a l . (S), have shown that sulfides of nickel and cobalt can be prepared by heating their anhydrous sulfates i n a stream of H 2 / H 2 S at low temperatures. However, the experimental conditions f o r obtaining pure N 1 3 S 2 and CogSg were not specified. It has been shown (2), (6) that both CogSg and N 1 3 S 2 permit l i t t l e variation from ideal stoichiometry. For both compounds, there i s no observable v a r i a t i o n i n the l a t t i c e parameter as determined from x-ray analyses, and magnetic measurements of CogSg have confirmed i t s narrow homogeneity range. 1

Current address: Exxon Research & Engineering Company, Annandale, ΝJ 08801 Author to whom correspondence should be directed.

2

0097-6156/85/0279-0247$06.00/0 © American Chemical Society

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

248

S O L I D STATE C H E M I S T R Y IN CATALYSIS

S y n t h e t i c samples o f the low temperature phase o f FeySg have been p r e p a r e d by L o t g e r i n g (4) and magnetic measurements c o n f i r m e d the work o f o t h e r i n v e s t i g a t o r s (7-9) t h a t the spontaneous magnetism of FeySg r e p r e s e n t s a f e r r i m a g n e t i c s t r u c t u r e which i s based upon an o r d e r i n g o f i r o n v a c a n c i e s . T h i s can be r e p r e s e n t e d by the f o r m u l a Fe4[Fe3 Q ] S g . I f t h i s model i s c o r r e c t , then r a n d o m i z a t i o n o f the v a c a n c i e s s h o u l d a f f e c t markedly the observed magnetic b e h a v i o r .

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch015

Experimental Section P r e p a r a t i o n o f Samples. The s u l f i d e s CogSg, N13S2, and FeySg were p r e p a r e d by t r e a t i n g p r e - d r i e d s u l f a t e s a l t s o f c o b a l t , n i c k e l , and i r o n w i t h a m i x t u r e H2 and H2S i n a v e r t i c a l r e a c t o r ( F i g u r e 1) a t 325°C f o r FeySg, and 525°C f o r CogSg and N13S2. C o b a l t and n i c k e l s u l f a t e s were d r i e d i n i t i a l l y a t 135°C.; p r e l i m i n a r y d r y i n g o f f e r r i c s u l f a t e was u n n e c e s s a r y . A f t e r placement o f the s u l f a t e i n the r e a c t o r t u b e , the system was purged w i t h n i t r o g e n , and a d r y i n g s t e p f o l l o w e d . A f t e r one hour o f d r y i n g under a n i t r o g e n f l o w , the d e s i r e d f l o w r a t e s f o r H2 and H2S were s e l e c t e d and a l l o w e d t o e q u i l i b r a t e . A t t h i s p o i n t , the temperature was e l e v a t e d t o ensure complete r e a c t i o n . For b o t h CogSg and N13S2, the r e a c t o r tube was removed from the furnace a t the end o f the r e a c t i o n and a i r quenched t o room t e m p e r a t u r e . The quenched samples o f FeySg were p r e p a r e d i n a s i l i c a r e a c t o r tube ( f i t t e d w i t h a V y c o r f r i t ) which was c o o l e d r a p i d l y w i t h i c e water a t the end o f the r e a c t i o n . An annealed sample o f FeySg was p r e p a r e d by h e a t i n g the quenched p r o d u c t i n a s e a l e d evacuated s i l i c a tube f o r two weeks a t 3 0 0 ° C . The tube was a l l o w e d t o r e a c h room temperature o v e r n i g h t . S l o w - c o o l e d samples o f FeySg were p r e p a r e d by l o w e r i n g the temperature o f the r e a c t o r from 325°C t o 175°C a t a r a t e o f l ° C / m i n . The r e a c t o r tube was then removed from the furnace and a l l o w e d t o r e a c h room t e m p e r a t u r e . For a l l the s y n t h e s e s , once the r e a c t o r tube reached room t e m p e r a t u r e , the system was purged w i t h n i t r o g e n b e f o r e the samples were removed. The e x p e r i m e n t a l c o n d i t i o n s f o r the p r e p a r a t i o n o f CogSg, N13S2, and FeySg are g i v e n i n T a b l e I . C h a r a c t e r i z a t i o n o f Samples. Powder d i f f r a c t i o n p a t t e r n s o f the samples were o b t a i n e d w i t h a P h i l i p s d i f f r a c t o m e t e r u s i n g monochromated h i g h - i n t e n s i t y CuKai r a d i a t i o n (λ = 1.5405A). F o r q u a l i t a t i v e i d e n t i f i c a t i o n o f the phases p r e s e n t , the p a t t e r n s were t a k e n from 12° < 2Θ 8 O.29x0.57 : ί

Cu/ZnO c a t a l y s t :

^

CsOH/Cu/ZnO c a t a l y s t :

3c

njco

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch018

O.45

=

0

> α

.0-Ο Ο

> > i

c-0

>ot

>

> a

o

> > > : ΐ

> > > i

, α

Ό-Η 0'Ύα

~

0

> a

0

O-H C>YC ~

O.89

Cu/ZnO c a t a l y s t :

>>>:i

a

ic-0 O ^C -0-H> C>'Yc

CsOH/Cu/ZnO c a t a l y s t :

i

c

Q

^ ^

> α

Ο

> > > : ί

Ο-Η

, α

0

~

0

, γ

0

0 "

I t i s e v i d e n t t h a t the i n c r e a s e d H 2 / C O r a t i o s u p p r e s s e s the 3c a d d i ­ t i o n w h i l e the CsOH promoter enhances i t and t h a t the 3c a d d i t i o n r e q u i r e s a s t r o n g e r base than the ao a d d i t i o n o r the l i n e a r growth The p r e s e n t f i n d i n g t h a t the a d d i t i o n i s n e g l i g i b l e i s appar­ e n t , f o r example, from the absence o f i s o p r o p a n o l i n our p r o d u c t s . E v i d e n t l y the CLQ attachment i s o v e r r i d d e n by the O I Q , 3 C ic-0 P ~ c e s s e s under our e x p e r i m e n t a l c o n d i t i o n s . The l a c k o f addition i s i n disagreement w i t h e a r l i e r r e p o r t s on the p r o d u c t c o m p o s i t i o n i n low a l c o h o l s y n t h e s e s ( 2 - 4 ) . I t i s suggested t h a t the e f f e c t i v e ac attachment may have been r e a l i z e d i n t h i s e a r l i e r work by an i s o m e r i ­ z a t i o n o f p r i m a r y a l c o h o l s v i a a d e h y d r a t i o n - r e h y d r a t i o n mechanism i n v o l v i n g a c i d c e n t e r s w h i c h a r e absent i n our c a t a l y s t s . Each o f the growth s t e p s can m a t e r i a l i z e by s e v e r a l s p e c i f i c mech­ anisms w h i c h have not been r e s o l v e d i n d e t a i l . We s h a l l g i v e exam­ p l e s o f p l a u s i b l e mechanisms f o r the t h r e e dominant p r o c e s s e s 3c > o and i c - o f ° the dependence o f the p r o d u c t c o m p o s i t i o n on the H 2 / C O r a t i o and on the b a s i c i t y o f the a l k a l i h y d r o x i d e p r o m o t e r . The 3Q a t t a c k dominates a t low H 2 / C O r a t i o s and i n the p r e s e n c e o f CsOH, i n d i c a t i n g t h a t a l d o l s y n t h e s i s o f aldehyde p r e c u r s o r s o r p r o ­ ducts of a l c o h o l dehydrogenation i s i n v o l v e d . As shown b e l o w , o n l y p r i m a r y o r secondary but not t e r t i a r y 3-carbon i s a t t a c k e d by the 3c p r o c e s s , a f e a t u r e c h a r a c t e r i s t i c of a l d o l c o n d e n s a t i o n . a n Q l

R O

a

t

0

a

c

c

o

u

n

t

r

RCH CH CH OH 2

2

RCH CH CHO + H

2

2

2

2

Θ

RCH CH CHO + CsOH Ξ = Ξ RCH CHCHO + H 0 2

2

2

H H

2

+ CO — *

\

Η

Q

// C

Η

3 = ^

2

Ο®

\ C. /

Η

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

18.

VEDAGE ET AL.

309

Alkali-Promoted Cu-ZnO Catalysts

H Θ

CT

\ ® /

RCH CHCHO + C

- RCH CH-CHO

2

2

m

a

- R C H C H - C H O H + CsOH 2

2

v

The i n s e r t i o n i p r o c e e d by the mechanism proposed by N a t t a and coworkers (3) i . e . , by the r e a c t i o n of s u r f a c e a l k o x i d e s w i t h CO as shown b e l o w : c

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch018

i

0

R C H d % CO

*RCH CO(f

2

C-0

RCH COO°+ 2H 2

2

*RCH CH OH + ΟΗ Θ

2

2

2

Since t h i s r e a c t i o n r e q u i r e s hydrogen, i t w i l l occur at h i g h e r r a t e s when the H^/CO r a t i o i s i n c r e a s e d , as o b s e r v e d . The m e t h y l e s t e r - f o r m i n g a t t a c k a c o u l d be due t o the C a n i z z a r o type c o n d e n s a t i o n of two a l d e h y d e s , to the r e a c t i o n o f s u r f a c e carboxyl a t e s w i t h f o r m y l o r formaldehyde, o r t o the r e a c t i o n o f s u r f a c e metho x i d e w i t h an aldehyde w i t h a h y d r i d e e l i m i n a t i o n . To be c o n s i s t e n t w i t h the e x p e r i m e n t a l o b s e r v a t i o n s , we s h a l l g i v e an example f o r the l a s t mechanism because i t r e q u i r e s the fewest number o f s t e p s , u t i l i ­ zes methoxide w h i c h may be f a v o r e d i n hydrogen r i c h s y n t h e s i s gas and does not r e q u i r e a s t r o n g base c a t a l y s t . Q

a

f

0

CH cF+,C® H R 3

Ο > CH3-O-C-R H θ

Ο > CH3OCR + H®

M e t h y l formate i s the p r o d u c t o f t h i s r e a c t i o n when R = H . I f m e t h y l formate were the r e s u l t o f the i n s e r t i o n Î Q J J , one would expect t h i s r e a c t i o n to produce a l s o h i g h e r formate e s t e r s , c o n t r a r y t o the f i n d i n g t h a t o n l y m e t h y l e s t e r s were p r o d u c e d . The p r i n c i p a l d i f f e r e n c e between the a l d o l a d d i t i o n 3^, the i n s e r tion i o > 0 e d i t i o n i s t h a t the f i r s t o f these t h r e e r e a c t i o n types r e q u i r e s a s t r o n g base c a t a l y s t w h i l e the r e m a i n i n g two u t i l i z e a l k o x i d e s which are merely h e t e r o l y t i c a l l y d i s s o c i a t e d a l c o h o l s and a r e expected to be common s u r f a c e i n t e r m e d i a t e s i n a wide range of H /C0 r a t i o s and o f s u r f a c e b a s i c i t y . I t i s t h e r e f o r e e v i dent t h a t the s u r f a c e c o n c e n t r a t i o n s of the a l k a l i promoters and the H2/CO r a t i o can be used as n e a r l y independent v a r i a b l e s t o a c h i e v e s e l e c t i v i t i e s g i v e n i n F i g u r e s 4 and 5. The s e l e c t i v i t y p a t t e r n c o n t r o l l e d by the p r o c e s s e s 3^, i-c-0 * CXQ has an i n h e r e n t l i m i t a t i o n , however, w h i c h i s apparent from the f o l l o w i n g c o n s i d e r a t i o n . I f one w i s h e s to suppress the f o r m a t i o n o f h i g h e r l i n e a r a l c o h o l s , the i n s e r t i o n i ^ - o roust be m i n i m i z e d but as a consequence a l s o the f i r s t C-C bond f o r m i n g r e a c t i o n from to C w i l l be suppressed s i n c e n e i t h e r 3ς; n o r OLQ can be i n v o l v e d i n t h i s a

n

d

t

n

e

a

c

2

a n c

2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

310

S O L I D STATE C H E M I S T R Y IN CATALYSIS

reaction. Hence the s u p p r e s s i o n o f i ç o r e s u l t s i n s e l e c t i v e methanol s y n t h e s i s no m a t t e r how e f f e c t i v e the c a t a l y s t may be f o r the 3ç a d d i tion. On the o t h e r hand, when the i ^ Q growth mechanism does o p e r a t e , the &Q a d d i t i o n must be s i g n i f i c a n t l y f a s t e r i n o r d e r t h a t the a l c o h o l s y n t h e s i s may be k i n e t i c a l l y r e s t r i c t e d so t h a t o n l y low a l c o h o l s a r e formed. I f one t a k e s the r a t i o o f ( l - b u t a n o l : l - p r o p a n o l ) i n the p r o duct as a crude measure of Î Q - Q * i ° °f (2-methyl-l-propanol: 1-propanol) as a measure o f 3 ç , then i t appears from F i g u r e s 4 and 5 t h a t the e f f e c t o f a l k a l i , p a r t i c u l a r l y CsOH, has been to i n c r e a s e the &C *C-0 ^ ° * comparison t o the undoped Cu/ZnO c a t a l y s t . a n c

:

r a t

t

n

e

r

a

t

n

Appendix

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch018

C a l c u l a t i o n s o f the s u r f a c e c o n c e n t r a t i o n s XPS i n t e n s i t i e s

o f a l k a l i and barium from

The model f o r the c a l c u l a t i o n of s u r f a c e c o n c e n t r a t i o n s here i s t h a t o f D r e i l i n g ( 1 1 ) .

The measured p h o t o e l e c t r o n

utilized intensity

1^ due t o s p e c i e s i t h a t i s e v e n l y d i s t r i b u t e d over a specimen o f t h i c k n e s s t^ i s g i v e n by τ

±

=

^ X^gX (l-exp[-t /gX ])/E i

i

i

i

(A-l)

i

where Κ i s an i n s t r u m e n t a l c o n s t a n t ,

i s the p h o t o i o n i z a t i o n c r o s s

s e c t i o n , X? i s the atomic volume c o n c e n t r a t i o n o f element i , g i s escape a n g u l a r f a c t o r ,

and λ . i s the escape depth o f

of k i n e t i c energy E^ i n the specimen.

the

photoelectrons

The i n t e n s i t y 1^ may be a t t e n ­

uated by a s u r f a c e o v e r l a y e r o f t h i c k n e s s t.. c o n t a i n i n g s p e c i e s j by a f a c t o r e x p ( - t ^ / g X ) , w h i c h f o r v e r y t h i n l a y e r s approximates i

as

D e f i n e the a t o m i c s u r f a c e c o n c e n t r a t i o n X? = X ? t ? where t ? i s t h i c k n e s s o f a m o n o l a y e r , and take e f f e c t i v e t^. = t ^ X ^ / X ^ .

the

Then the

a t t e n t u a t e d p h o t o e l e c t r o n s i g n a l o f s p e c i e s i p a s s i n g through an o v e r l a y e r of t h i c k n e s s t.. i s g i v e n by 1

±

= Ka X^gX (l-t?xJ/(gX.X^))/E i

i

where we have t a k e n i n t o account l a r g e t ^ / g X ^ . photoelectrons Ij - K a

j X

from Jt°/

E j

(A-2)

i

The i n t e n s i t y I j o f

s p e c i e s j i n the t h i n o v e r l a y e r i s g i v e n by - Κσ.Χ*/Ε.

(A-3)

w h i c h f o l l o w s from ( A - l ) f o r s m a l l t ° / g X . . . The r e l a t i v e i n t e n s i t i e s o f the o v e r l a y e r s p e c i e s j and the b u l k s p e c i e s i a r e ,

from (A-2) and

(A-3),

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

18.

I

σ

311

Alkali-Promoted Cu-ZnO Catalysts

VEDAGEETAL.

X?

Ε.

t?

4

^*

*r

/

(

i

t

x

- ;

/

'

(

x

i

g

x

i

)

)

(

a

4

-

)

where we used X ^ t ° = X ^ . The

r a t i o o f s u r f a c e c o n c e n t r a t i o n s f o l l o w s from S

X.

I.

gX,

X? i

T

t? i

i

σ.Ε, a

E

i j

J

t?

I.

t° ι

\

(A-4).

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch018

E q u a t i o n (A-5) p e r m i t s the i n t e n s i t y 1^ o f the b u l k s p e c i e s t o be used as an i n t e r n a l r e f e r e n c e

f o r the d e t e r m i n a t i o n o f the s u r f a c e c o n c e n ­

t r a t i o n o f the s p e c i e s j t h a t i s p r e s e n t i n the o v e r l a y e r o n l y . In

the p r e s e n t work i n t e r e s t

c e n t e r s upon o v e r l a y e r s o f a l k a l i

compounds on the Cu/ZnO c a t a l y s t .

The Zn 2p^ p h o t o e l e c t r o n i n t e n s i t y

is

s i g n a l anâ hence the s p e c i e s i

t a k e n as the i n t e r n a l r e f e r e n c e

Zn and s p e c i e s j i s an a l k a l i i o n .

is

The escape depth X ^ was e s t i m a t e d n

from a r e l a t i o n g i v e n by Chang ( 1 2 ) ,

λ

Ζη

" ° .

where t £

n

2

( A

^ Ζ η

= O.2824 nm was t a k e n t o be the s p a c i n g between the

p l a n e s o f ZnO (13) and E

Z

= 232.1 e V , y i e l d i n g X

n

l o w i n g D r e i l i n g ( 1 1 ) , g = O.75 was u s e d .

Z

n

"

6 )

(10Ϊ0)

« O.86 nm.

Other d a t a used i n the

Fol­ cal­

c u l a t i o n s o f the a l k a l i s u r f a c e c o n c e n t r a t i o n s a r e g i v e n i n T a b l e A - I . Table A - I XPS

Data f o r P h o t o e l e c t r o n s o f Elements A n a l y z e d i n the P r e s e n t Work

Element i

Photoelectron

o

O.0593

±

Li

Is

1198.1

Na

Is

182.2

Κ

2p£

Rb

2

3d

>

3

2

Cs 2

Ba Zn

.

2

2

P1

a

E (eV)

±

7.99

960.7

2.67

1143.1

4.44

529.6

22.93

473.6

24.75

232.1

18.01

b

From r e f e r e n c e (14). 'Sum o f p h o t o i o n i z a t i o n c r o s s s e c t i o n s f o r the Rb 3 d | and 3 d | photoelectrons. The e x p e r i m e n t a l i n t e n s i t y ( c f . T a b l e I I ) i s the i n t e g r a t e d i n t e n s i t y o f t h e s e two p h o t o e l e c t r o n e m i s s i o n s . In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

312

SOLID STATE CHEMISTRY IN CATALYSIS

The a l k a l i monolayer thickness t ° was taken equal to the sum of the diameters of the OH +

group, O.272 nm, and of the a l k a l i i o n , O.12 +

+

+

+

nm for L i , O.19 for N a , O.266 for K , O.296 for R b , O.338 for C s , 44

and O.27 for Ba *, y i e l d i n g t ° t°

b

t

>

A

= O.568, t ° a n (

^ ' ^Zn

(I^/l )

s

±

= O.610 and t °

* *"Zn

a

S

n o t e c

= O.392, t °

a

« O.542 nm.

= O.462, t £ « O.538, Using these values of

* above, the measured i n t e n s i t y r a t i o s

shown i n column 2 of Table I I » as well as the parameters S S

Z n

x

X

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch018

l i s t e d i n Table A - I , the surface concentration r a t i o s ( ^ Z n ^

w

e

r

e

calculated from equation (A-5) and summarized i n column 3 of T a b l e I I . Acknowledgments This work was supported by the U.S. Department of Energy (Subcon­ tract No. XX-2-02173 under prime contract No. EG-77-C-01-4092). References Cited 1. Morgan, G. T., Hardy, D. V. N. and Procter, R. H. J., Soc. Chem. Ind. Trans. and Comm. 51, 1T (1932). 2. Graves, G. D., Ind. and Eng. Chem. 1381 (1931). 3. Natta, G., Colombo, U. and Pasquon, I., "Catalysis," Reinhold, New York, NY, 5, p 131 (1957). 4. Smith, K. J., and Anderson, R. B., "The Higher Alcohol Synthesis over Promoted Methanol Catalysts," presented at the 8th Can. Symp. on Catalysis, May 26-29, 1982, U. of Waterloo, Ont., Canada. 5. Herman, R. G., Klier, Κ., Simmons, G. W., Finn, B. P., Bulko, J. B. and Kobylinski, T. P., J. Catal. 56, 407 (1979). 6. Mehta, S., Simmons, G. W., Klier, K. and Herman, R. G., J. Catal. 57 (1979). 7. Buch, Von P., Bärnighausen, H., Acta Cryst., B24, 1705 (1968). 8. The systematic errors are likely to originate, in the order of decreasing magnitude, from (a) the uncertainties in λ (if Seah's (9) λ = 1.4 nm were used in the present calculations, there would ensue unrealistic surface alkali concentrations higher by relative 30% than the total amount of the dopant alkali introduced into the system; (b) the values of S ; (c) the average escape angular factor g; (d) the estimated thickness tÅ, and (e) the possibly uneven distribution of the alkali between the surface and the bulk. 9. Seah, M. P., Dench, W. Α., Surf. Interface Analysis 1(1), 2(1979). 10. Klier, Κ., Chatikavanij, V., Herman, R. G., and Simmons, G. W., J. Catal. 74, 343 (1982). 11. Dreiling, M. J., Surface Sci. 71, 231-246 (1978). 12. Chang, C. C., Surface Sci. 48, 9 (1975). 13. Bulko, J. B., Thesis, Lehigh University, 1980. 14. Scofield, S. Η., J. Electron Spectroscopy 8, 129 (1976). Zn

Zn

Zn

RECEIVED

October 26, 1984

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

19 Water Gas Shift over Magnetite-Based Catalysts Nature of Active Sites for Adsorption and Catalysis 1

2

3

CARL R. F. LUND , JOSEPH E. KUBSH , and J. A. DUMESIC 1

Exxon Research & Engineering Company, Annandale, ΝJ 08801 Davison Chemical Division-Research, W. R. Grace & Company, Columbia, MD 21044 Department of Chemical Engineering, University of Wisconsin, Madison, WI 53706

2

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

3

Recent studies of the adsorptive and catalytic proper­ ties of magnetite (Fe O ) are discussed with respect to the water-gas shift reaction (WGS) in this review arti­ cle. Proposed mechanisms are examined, and the proper­ ties of the active sites required for these mechanisms are considered. Kinetic relaxation measurements of the rates of surface oxidation and reduction by WGS reactants and products suggest that a primary WGS pathway involves successive oxidation and reduction of the mag­ netite surface. This regenerative mechanism takes place over a small fraction (ca. 10%) of the catalyst surface. Surface coverages by CO and CO were determined volumetrically and gravimetrically at WGS temperatures (e.g., 660 K) through the use of CO/CO gas mixtures. These results suggested that coordinatively unsaturated iron cations were the sites for CO adsorption, while adsorption of CO was associated with surface oxygen species. This was confirmed directly by temperature programmed desorption studies of magnetite surfaces under ultra-high vacuum conditions. Studies of a series of silica-supported magnetite catalysts suggested that the total extent of adsorption from CO/CO gas mixtures was proportional to the number of active sites for WGS. In contrast, the nitric oxide uptake at room temperature was found to be equal to the BET monolayer uptake of magnetite. Thus, the ratio of the CO/CO uptake to the NO uptake provides a measure of the fraction of the magnetite surface which is active for WGS. Finally, catalytic effects of solid-state substitutions in magne­ tite are discussed with respect to the geometric proper­ ties of the WGS sites. It is suggested that octahedrally-coordinated iron cations are important for the WGS reaction. 3

4

2

2

2

2

2

0097-6156/85/0279-0313$07.50/0 © 1985 American Chemical Society

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

314

SOLID STATE CHEMISTRY IN CATALYSIS

M a g n e t i t e , Ρ β β Ο , , i s the major c o n s t i t u e n t and t h e a c t i v e component i n i n d u s t r i a l , nigh temperature ( c a . 650 K) w a t e r - g a s s h i f t (WGS) c a t a l y s t s ( 1 ) , The k i n e t i c s of the WGS r e a c t i o n , CO + H 0

j

2

C0 + H 2

(1)

2

have been s t u d i e d e x t e n s i v e l y , and s e v e r a l m e c h a n i s t i c i n t e r p r e t a ­ t i o n s have been developed ( 1 , 2 ) , C o n s i d e r a b l y l e s s i n f o r m a t i o n i s a v a i l a b l e c o n c e r n i n g the nature of the a c t i v e s i t e f o r the r e a c t i o n . However, recent r e s e a r c h i n the area has focused upon i d e n t i f y i n g the s u r f a c e f e a t u r e s of F e ^ which are r e s p o n s i b l e f o r i t s a c t i v i t y in this reaction. The s o l i d s t a t e s t r u c t u r e of m a g n e t i t e , a s p i n e l (j$), c o n t a i n s i r o n c a t i o n s i n two d i f f e r e n t o x i d a t i o n s t a t e s ( F e and F e ) and i n two l a t t i c e s i t e s of d i f f e r e n t c o o r d i n a t i o n ( o c t a h e d r a l and t e t r a h e d r a l ) ; t h e r e f o r e , the c a t a l y t i c s u r f a c e of t h i s m a t e r i a l may be expected t o p r o v i d e a v a r i e t y of p o s s i b l e s i t e s c a p a b l e of a c t i n g as a d s o r p t i o n or r e a c t i o n c e n t e r s . A l s o , i t has been demonstrated t h a t s u b s t i t u t i o n of o t h e r c a t i o n s f o r i r o n can s i g n i f i c a n t l y a l t e r the c a t a l y t i c a c t i v i t y f o r WGS ( 4 , 5 ) . The nature of the s u r f a c e s i t e s f o r WGS on F e 0 i s the s u b j e c t of t h i s short review. To b e g i n , the s o l i d s t a t e p r o p e r t i e s o f mag­ n e t i t e w i l l be b r i e f l y d e t a i l e d a l o n g w i t h proposed r e a c t i o n p a t h ­ ways f o r WGS on m a g n e t i t e - b a s e d c a t a l y s t s . These pathways are d i s ­ cussed w i t h r e s p e c t t o both t h e i r k i n e t i c i m p l i c a t i o n s and the c a t a l y s t s u r f a c e c h a r a c t e r i s t i c s which may f a c i l i t a t e these proposed mechanisms. A more d e t a i l e d i n v e s t i g a t i o n o f the oxygen t r a n s f e r c h a r a c t e r i s t i c s of a m a g n e t i t e - b a s e d c a t a l y s t i s then presented and d i s c u s s e d w i t h r e s p e c t t o proposed o x i d a t i o n / r e d u c t i o n pathways f o r WGS. C h e m i s o r p t i o n experiments are then d e t a i l e d which p r o v i d e i n ­ f o r m a t i o n about the t o t a l a v a i l a b l e magnetite s u r f a c e area and the d e n s i t y of s i t e s i n v o l v e d i n the redox r e a c t i o n pathway. Finally, s o l i d s t a t e s u b s t i t u t i o n s i n the magnetite l a t t i c e are r e l a t e d t o t h e i r e f f e c t on WGS a c t i v i t y .

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

z +

3

M a g n e t i t e S t r u c t u r a l and C a t a l y t i c

J +

4

Properties

M a g n e t i t e e x i s t s i n the s p i n e l s t r u c t u r e which can be r e p r e s e n t e d by the formula (Fe^ )[Fe ,Fe ]0 , where t h e parentheses denote cations in tetrahedral l a t t i c e s i t e s , and t h e b r a c k e t s denote cations in octahedral l a t t i c e s i t e s (3). Figure 1 i s a representa­ t i o n of the i d e a l i z e d s p i n e l s t r u c t u r e (note t h a t the s t r u c t u r e has been extended i n the [001] d i r e c t i o n f o r c l a r i t y ) . The oxygen anions form a c u b i c c l o s e - p a c k e d framework i n which t h e r e are 2 t e t r a h e d r a l v a c a n c i e s and 1 o c t a h e d r a l vacancy per oxygen a n i o n . From the above f o r m u l a , i t can be seen t h a t o n e - e i g h t h of the t e t r a ­ hedral s i t e s and o n e - h a l f o f the o c t a h e d r a l s i t e s are o c c u p i e d by iron cations. The ordered o c c u p a t i o n of o c t a h e d r a l s i t e s shown i n F i g u r e 1 f a c i l i t a t e s e l e c t r o n hopping between f e r r o u s and f e r r i c c a t i o n s at temperatures above 119 As a r e s u l t , the o x i d a t i o n s t a t e of t h e s e o c t a h e d r a l c a t i o n s can be c o n s i d e r e d t o be +2.5. +

2 +

3 +

4

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

19.

LUND ET AL.

315

Water Gas Shift over Magnetite-Based Catalysts

U n f o r t u n a t e l y , a t p r e s e n t , w h i l e t h e bulk s t r u c t u r e o f magnetite i s w e l l u n d e r s t o o d , l i t t l e i s known about t h e s u r f a c e s t r u c t u r e o f magnetite-based c a t a l y s t s . In g e n e r a l , t h e s u r f a c e may be n o n s t o i c h i o m e t r i c ( F e ^ O ^ ) w i t h i r o n c a t i o n s i n both o c t a h e d r a l and t e t r a hedral s i t e s ( b ) . The l i t e r a t u r e p e r t a i n i n g t o t h e c a t a l y t i c p r o p e r t i e s o f magnet i t e focuses p r i m a r i l y on t h e water-gas s h i f t r e a c t i o n . A number o f r e a c t i o n k i n e t i c s s t u d i e s have been r e p o r t e d i n which WGS r e a c t i o n pathways have been proposed ( 1 , 2 , 7 - 1 8 ) , In s h o r t , two types o f mechanisms have been put f o r w a r d , these being t h e a d s o r p t i v e and regenerative mechanisms. In t h e a d s o r p t i v e pathway, reactants adsorb on t h e s u r f a c e where they r e a c t t o form s u r f a c e i n t e r m e d i a t e s , f o l l o w e d by decomposition t o products and d e s o r p t i o n from t h e s u r f a c e ( 1 2 - 1 8 ) , Support f o r t h i s a d s o r p t i v e mechanism has been p r o v i d e d by t r a c e r s t u d i e s and apparent s t o i c h i o m e t r i c number a n a l y s e s . Two such a d s o r p t i v e mechanisms c o n s i s t e n t w i t h e x p e r i m e n t a l o b s e r v a t i o n s a r e shown below.

c o

(ads)

2°(g)

2 H

(ads)

°(ads)

c 0

2(ads)

c o

H

c 0

(ads)

+

c 0

2(ads)

2 H

(ads)

c o

(g)

H

c o

(ads)

+

(g)

C0 H

2°(g)

0 H

(ads)

c 0

2(ads)

2 H

(ads)

2a

°(ads)

+

2b

2 c

)

(2d)

2(g)

< > 2e

c o

(ads)

0 H

(ads)

C 0

< ) (

)

HC00

(ads)

H C 0 0

2 ( g

< )

< > 3a

+

H

(ads)

3b

(3c)

( a d s )

2(ads)

< )

+

H

(ads)

< > 3d

C0 (g)

(3e)

H

(3f)

2

2 ( g )

General statements c o n c e r n i n g t h e n a t u r e o f t h e a d s o r p t i o n s i t e s r e q u i r e d by such a d s o r p t i v e mechanisms can be suggested based upon t h e chemical nature o f CO, C 0 , H 0 , and Ho and t h e manner i n which these s p e c i e s have been shown t o i n t e r a c t w i t h metal o x i d e surfaces. Carbon monoxide, a s o f t b a s e , i s expected t o i n t e r a c t w i t h a s o f t a c i d i c s u r f a c e s i t e ( 1 9 ) » The o c t a h e d r a l i r o n c a t i o n s (+2.5 average o x i d a t i o n s t a t e ) a r e t h e s o f t e r o f t h e a c i d s i t e s on magnet i t e and may be expected t o p r o v i d e CO a d s o r p t i o n s i t e s . The i n i t i a l i n t e r a c t i o n should r e s u l t i n a c a r b o n y l s u r f a c e s p e c i e s , and such s p e c i e s have been observed by i n f r a r e d s p e c t r o s c o p y (20-22)< 2

2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

316

S O L I D STATE C H E M I S T R Y IN CATALYSIS

This a d s o r p t i o n mode may undergo an a c t i v a t e d t r a n s f o r m a t i o n , i n t e r a c t i n g w i t h s u r f a c e oxygen t o form a b i d e n t a t e c a r b o n a t e . In c o n t r a s t , steam i s a hard base and p r e f e r s t o adsorb on hard a c i d sites. C o o r d i n a t i v e l y unsaturated F e c a t i o n s should p r o v i d e such hard a c i d s i t e s . The r e s u l t i n g i n t e r a c t i o n may i n i t i a l l y i n v o l v e a s i n g l e c o o r d i n a t i v e bond between the oxygen of water and the c a t i o n , but depending on temperature and s u r f a c e hydroxyl c o n c e n t r a t i o n , d i s s o c i a t i o n of HoO i n t o s u r f a c e h y d r o x y l s may o c c u r . T h i s d i s s o c i ­ a t i o n r e q u i r e s a p a i r - s i t e c o n s i s t i n g of a hard c a t i o n and an a d j a c e n t oxygen a n i o n . Carbon d i o x i d e i s a hard a c i d , and as s u c h , i t i s expected t o i n i t i a l l y adsorb on hard base s i t e s , presumably oxygen anions and s u r f a c e hydroxyl groups. However, the adsorbed carbon d i o x i d e can be as c a r b o x y l a t e or carbonate s p e c i e s , and the carbonate may be monodentate or b i d e n t a t e : Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

3 +

0

v/c

0

0

\ c/

0

0

I c

I Μ-0-Μ-0-Μ

I O-M-O-M-O

/ \ Μ-0-Μ-0-Μ

Carboxylate

Monodentate Carbonate

Bi dentate Carbonate

The e x i s t e n c e of such s p e c i e s has been confirmed by IR s p e c t r o s c o p y f o r C0 i n t e r a c t i o n s with several metal o x i d e surfaces.(23-26) F i n a l l y , hydrogen can adsorb e i t h e r h e t e r o l y t i c a l l y (H+ on an a n i o n and H- on a c a t i o n ) or r e d u c t i v e l y ( f o r m i n g two hydroxyl groups) on o x i d e s (23). Depending upon r e l a t i v e s u r f a c e p o p u l a t i o n s , d e s o r p t i o n i s p o s s i b l e i n each of these f a s h i o n s . For h e t e r o l y t i c a d s o r p t i o n , p a i r - s i t e s are again i n d i c a t e d . The e x i s t e n c e o f s u r f a c e i n t e r m e d i a t e s r e s u l t i n g from the i n t e r a c t i o n of two a d s o r b a t e s , as p o s t u l a t e d , f o r example, i n Step 3c above, has a l s o been c o n f i r m e d on s e v e r a l metal o x i d e sur­ f a c e s (22,29-30). Formate s p e c i e s have been observed by i n f r a r e d s p e c t r o s c o p y f o l l o w i n g i n t e r a c t i o n of CO w i t h a h y d r o x y l a t e d o x i d e surface. The decomposition of f o r m i c a c i d on a v a r i e t y of o x i d e s , i n c l u d i n g i r o n o x i d e , was a l s o shown t o depend on s u r f a c e a c i d i t y and b a s i c i t y , w i t h a c i d i c s u r f a c e s forming CO and HoO (WGS r e a c t a n t s ) and b a s i c s u r f a c e s forming C0 and H (WGS p r o d u c t s ) (31-36), It i s c l e a r t h a t a d s o r p t i v e mechanisms i n v o l v i n g formate s p e c i e s are dependent on the acid/base c h a r a c t e r of the magnetite s u r f a c e . In g e n e r a l , i t appears t h a t both a c i d i c and b a s i c p r o p e r t i e s are d e s i r e d t o make formate s p e c i e s from CO and HoO and then t o decom­ pose t h e s e s p e c i e s t o C0 and Ho. 2

2

2

2

In the second c l a s s of WGS mechanisms ( i . e . , r e g e n e r a t i v e p r o ­ c e s s e s ) , the magnetite c a t a l y s t serves as an oxygen t r a n s f e r a g e n t . In such r e g e n e r a t i v e mechanisms, oxygen i n the c a t a l y s t s u r f a c e and perhaps even the c a t a l y s t bulk p a r t i c i p a t e s i n the r e a c t i o n . T h i s can be r e p r e s e n t e d by the f o l l o w i n g e q u a t i o n s :

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

19.

Water Gas Shift over Magnetite-Based Catalysts

LUND ET AL.

CO + * - 0

C0

o

(4)

+ *

(5)

HoO + * >

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

317

where * denotes a s u r f a c e s i t e . In t h i s s i m p l e form, the r e g e n e r a ­ t i v e mechanism merely r e q u i r e s a c a t i o n s i t e which i s capable o f reversible oxidation/reduction. I t i s a l s o p o s s i b l e t h a t the two steps above are s i m p l i f i c a t i o n s of the a c t u a l p r o c e s s e s . For exam­ p l e , weakly adsorbed s p e c i e s may a l t e r n a t e l y o x i d i z e and reduce a n i o n - c a t i o n p a i r - s i t e s , and the t r a n s f o r m a t i o n t o an a d s o r b e d , a c ­ t i v a t e d i n t e r m e d i a t e may be r a t e - l i m i t i n g . In t h i s c a s e , the r e ­ quirements upon the s i t e s are s t r i c t e r and s i m i l a r t o those d e s c r i b e d above f o r the a d s o r p t i o n of CO, COo, H 0 and Ho. The regenerative mechanism fits well with tne observed c a t a l y t i c p r o p e r t i e s of F e 0 . The o c t a h e d r a l i r o n c a t i o n s , which undergo r a p i d e l e c t r o n h o p p i n g , would appear t o be the n a t u r a l c a t i o n s i t e s f o r the r e v e r s i b l e o x i d a t i o n / r e d u c t i o n r e q u i r e d by the mechanism, as w i l l be d i s c u s s e d l a t e r . Indeed, assuming the s i t e s t o be e x p o n e n t i a l l y d i s t r i b u t e d w i t h r e s p e c t t o oxygen a f f i n i t y , a r a t e e x p r e s s i o n can be d e r i v e d which f i t s measured k i n e t i c d a t a w e l l ( 7 J . A d d i t i o n a l support f o r t h i s mechanism comes from the work of Boreskov et a l . ( 9 , 1 0 ) who showed t h a t the r a t e of WGS corresponds t o the r a t e at which HoO o x i d i z e s and CO reduces the s u r f a c e of magnetite. T h i s c o m p a r i s o n , however, was not c a r r i e d out over a range of r e a c t a n t and product p a r t i a l p r e s s u r e s . 2

3

4

Oxygen T r a n s f e r P r o p e r t i e s of M a g n e t i t e R e c e n t l y , the s i m p l i f i e d r e g e n e r a t i v e mechanism has been t e s t e d i n a more r i g o r o u s manner ( 3 7 ) « K i n e t i c r e l a x a t i o n measurements were c a r ­ r i e d out f o r each of the two r e a c t i o n s of the r e g e n e r a t i v e mecha­ n i s m , e q u a t i o n s 4 and 5, u s i n g an i n s i t u g r a v i m e t r i c t e c h n i q u e . A chromia-promoted magnetite c a t a l y s t was e q u i l i b r a t e d i n either CO/COo or H 0/H at 637 K. E i t h e r the gas phase c o m p o s i t i o n or the temperature was then p e r t u r b e d , and the k i n e t i c s of r e l a x a t i o n t o the new e q u i l i b r i u m s t a t e were m o n i t o r e d . I t can be shown t h a t under these c o n d i t i o n s , i n t e g r a t i o n of the k i n e t i c e x p r e s s i o n g i v e s the f o l l o w i n g r e l a t i o n s h i p : 2

2

(6) η - η

Λ

*

e

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

318

S O L I D STATE C H E M I S T R Y IN CATALYSIS

where η i s the number of s i t e s which c o n t a i n oxygen at a g i v e n t i m e , the s u b s c r i p t e denotes the value measured a f t e r e q u i l i b r i u m i s e s t a b l i s h e d , the s u p e r s c r i p t 0 denotes an a r b i t r a r y r e f e r e n c e s t a t e , S i s the c a t a l y s t s u r f a c e a r e a , r i s the e q u i l i b r i u m exchange r a t e , and t i s the t i m e . The v a r i a b l e s a ^ and a^ are the thermodynamic a c t i v i t i e s of s u r f a c e oxygen and anion v a c a n c i e s , r e s p e c t i v e l y . The e x t e n t of s u r f a c e oxygen removal, x , i s d e f i n e d by the e q u a t i o n : Q

Q

(7) A r e l a t e d q u a n t i t y , θ , i s the value of χ n o r m a l i z e d t o the BET monolayer. The chemical a c t i v i t i e s of s u r f a c e o x i d i z e d s i t e s , a and s u r f a c e reduced s i t e s , a^ are d e f i n e d at e q u i l i b r i u m by the equation:

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

Q

P c 0

V* a

LU

*

K

2

P h

2 UU

C0

2

W

b

5

K

x

2°

H

2

here i s the e q u i l i b r i u m c o n s t a n t f o r e i t h e r r e a c t i o n 4 or r e a c ­ t i o n 5 above, w r i t t e n w i t h s p e c i e s i as the r e a c t a n t . F i g u r e 2 shows the e x p e r i m e n t a l l y determined e x t e n t of oxygen removal as a f u n c t i o n of the a c t i v i t y r a t i o , measured v i a p e r t u r b a ­ t i o n s of e i t h e r CO or C0 i n t h e i r m i x t u r e s , or Ho i n H /Ho0 m i x ­ tures. I t should be noted t h a t both CO and C0 aasorb a p p r e c i a b l y on the c a t a l y s t under the c o n d i t i o n s of these e x p e r i m e n t s , and c o r ­ responding c o r r e c t i o n s t o observed weight changes were made i n d e t e r m i n i n g x. (A f u r t h e r d i s c u s s i o n of CO and C0 a d s o r p t i o n on m a g n e t i t e - b a s e d c a t a l y s t s i s presented i n the f o l l o w i n g s e c t i o n . ) From t h i s p l o t , the d e r i v a t i v e i n e q u a t i o n 6 can be e v a l u a t e d and thus r can be measured from k i n e t i c r e l a x a t i o n e x p e r i m e n t s . The value of r measured at a number of p r e s s u r e r a t i o s (both C0/C0 and H /Hp0) can then be used t o determine the forward and r e v e r s e r a t e c o n s t a n t s of r e a c t i o n s 4 and 5. Rate c o n s t a n t s f o r these r e a c t i o n s were determined assuming t h a t e q u a t i o n s 4 and 5 are m e c h a n i s t i c s t e p s . This a n a l y s i s a l s o assumes t h a t the s u r f a c e i s u n i f o r m . With these a s s u m p t i o n s , r i s r e l a t e d t o θ a c c o r d i n g t o the e x p r e s s i o n : 2

2

2

2

e

e

2

2

Q

χ

k

ïïf= i

( οί- °χ> ""Λ θ

θ

(i

=C0orH ) 2

f o r the forward r a t e of r e a c t i o n 4 and the r e v e r s e r a t e o f 5, and a c c o r d i n g t o ^

= k.e° + k.e x

x

(i = C0

2

o r H 0) 2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

(9) reaction

(10)

19.

LUND ET AL.

Water Gas Shift over Magnetite-Based Catalysts

319

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

Ο Ο

ν Ο . C) Ο.Ο φ.ΟνΟ Ο ι CM

φ»Ο»Q Ο.Ο ν Ο . φ Ον Ο •Ο f

•[010]

Ρν\

0 839nm

\0° (b)

(α)

F i g u r e 1. The c r y s t a l s t r u c t u r e o f m a g n e t i t e : (a) expanded i n [001] d i r e c t i o n (b) overview o f i n d i v i d u a l anion l a y e r s . Large open c i r c l e s r e p r e s e n t oxygen a n i o n s ; small open c i r c l e s , t e t r a ­ hedral c a t i o n s ; small f i l l e d c i r c l e s , o c t a h e d r a l c a t i o n s ; and v ' s , vacant o c t a h e d r a l s i t e s .

^50h * ο σ

ο 2ol ϋ ce ΙΟ

c& 0

5 5| L

Δ

-L

-O.02 -O.01O.0O.01O.02O.03 OXYGEN REMOVED, 0 , χ

BET

MONOLAYERS

F i g u r e 2. S u r f a c e oxygen a c t i v i t y r a t i o as a f u n c t i o n o f s u r f a c e oxygen removal measured v i a gas phase p e r t u r b a t i o n s o f (0) CO, ( • ) COo and (Δ) HO p r e s s u r e . Reproduced w i t h p e r m i s s i o n from R e f . 3 7 . C o p y r i g h t 1982, AIChE.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

320

S O L I D STATE C H E M I S T R Y IN CATALYSIS

f o r the remaining two r a t e s . Here r e f e r s t o the r a t e c o n s t a n t s of the m e c h a n i s t i c steps 4 and 5 and θ ° i s the v a l u e of θ at t h e reference s t a t e . F i g u r e s 3 and 4 show a p p r o p r i a t e p l o t s of equa­ t i o n s 9 and 10 from which the r a t e c o n s t a n t s can be d e t e r m i n e d . Table I p r e s e n t s the r a t e c o n s t a n t s so determined and the values o f θ . In t h i s t a b l e , the s u b s c r i p t s r e f e r to the s p e c i e s which i s the r e a c t a n t f o r a p a r t i c u l a r r a t e c o n s t a n t , "sat" r e f e r s t o t h e c o n d i t i o n where the a c t i v e s i t e s are a l l i n the o x i d i z e d form. The v a l i d i t y of the measured values of the f o u r r a t e c o n s t a n t s was checked i n two ways. F i r s t the f o l l o w i n g r e l a t i o n should be obeyed: χ

ϋ

χ

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

k

C0

#

k

H 0 2

k ΓΤ C0 H K

K

2

WGS *

=

K

W b

2

Using the e x p e r i m e n t a l l y determined v a l u e s of the r a t e c o n s t a n t s , WGS .4 e q u a t i o n 1 1 . The a c t u a l v a l u e of the e q u i l i b r i u m c o n s t a n t at 637 Κ i s 1 7 . 6 , and t h i s agreement i s acceptable. The second check was performed by p r e d i c t i n g both the r a t e and the k i n e t i c r a t e e x p r e s s i o n f o r WGS and comparing t h e s e p r e d i c t i o n s to reported values. For a u n i f o r m s u r f a c e , the r e g e n e r a t i v e mechanism p r e d i c t s the following k i n e t i c expression f o r the forward r a t e of the WGS reaction: K

w

a

s

c

a

1

c

u

l

a

t

e

c

l

t

0

b

e

2 5

k

W

b

b

K

C0 C0 K

K

C0 C0 ox C0 H 0 k

2

H 0 H 0 2

K

2

e

t p

K

P

C0

2

K

2

C0

2

K

H

R e a c t i o n r a t e s p r e d i c t e d by e q u a t i o n 12 were then r a t e s given by the e m p i r i c a l l y determined e q u a t i o n :

C

r

W Q S

= 2.0 χ 10

15

K

H

2

compared t o

the

O.25

O.9 CO

2

r

H 0 2

(13) r

C0

o

T h i s e q u a t i o n was determined u s i n g a c a t a l y s t s i m i l a r t o t h a t used i n the g r a v i m e t r i c study ( l ) The p r e d i c t e d r a t e of 2 . 1 χ 1 0 m'^ s " ( f o r the case i n which the p a r t i a l p r e s s u r e s of both r e a c t a n t s and products are set equal t o 10 kPa) agreed w e l l w i t h the e x p e r i ­ mental v a l u e of 7.3 χ 1 0 nf^ s . Furthermore, F i g u r e 5 compares the e x p e r i m e n t a l l y determined e f f e c t s of changing each of the c o n ­ s t i t u e n t p a r t i a l p r e s s u r e s from e q u a t i o n 13 t o the e f f e c t s p r e d i c t e d t

1

1 5

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

i b

19.

321

Water Gas Shift over Magnetite-Based Catalysts

LUND ET AL.

r /R

.lo'W'kPa-'

e

2.5-

\

2.0-

•

) 1.5-

i.o\o Ο Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

O.5O.0

,

1 -O.01 OXYGEN

1 O.01

REMOVED, 0 , χ

BET

. ^

1

O.02

MONOLAYERS

Figure 3. versus e x t e n t of s u r f a c e oxygen removal for i = CO (0); P = 19.4 kPa; P varied for 1 = CO (Δ); P = 1.99 kPa; P varied, and for i = H (•); P = 2.18 kPa; P CQo

CQ

COo

varied. AIChE.

at 637 Κ

CO

HQ

2

Ho

Reproduced w i t h p e r m i s s i o n from R e f . 3 7 .

r /F> , I0

14

e

C o p y r i g h t 1982,

m-V'kPa"

1

6.05.0-4.0 O.22-

o.ie-O.14

1

-O.01 OXYGEN

ι

O.0

R E M O V E Ο, 0 , χ

I O.01 BET

ι

1

O.02

MONOLAYERS

F i g u r e 4. r / P i versus e x t e n t of s u r f a c e oxygen removal at 637 Κ f o r i = COo ( 0 ) ; P = 19.4 kPa; P varied, for i = C0 ( Δ ); C Q o

P

c o

= 1.99

kPa, Ρ

kPa; ?

C Q

^ varied,

C Q

and f o r

varied. Reproduced w i t h 2 r i g h t 1982, AIChE. μ

2

i

= HoO ( • ) ;

P ^ Q = 2.18

p e r m i s s i o n from R e f . 37. Copy-

M

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2

Pulses

—

5.2 χ Ι Ο "

8

8

8

9

9

2.6 χ . Ι Ο "

—

2.1 χ 10~

3.2 χ Ι Ο "

9

8

8

f^u

—

—

(m/s)

7.0 χ Ι Ο "

7.0 χ Ι Ο "

Parameters*

8

8

O.087

O.086

O.092

O.082

Ο*

O.052

O.050

O.058

O.049

C o p y r i g h t 1982, A m e r i c a n I n s t i t u t e o f C h e m i c a l E n g i n e e r s .

5.5 χ Ι Ο "

5.5 χ Ι Ο "

—

—

"2 (m/s)

CO£ (m/s)

S o u r c e : Reproduced w i t h p e r m i s s i o n from R e f . 3 7 .

Average

Η

4.7 χ Ι Ο "

Pulses

C0

2

5.6 χ Ι Ο "

CO P u l s e s

CO (m/s)

Summary of K i n e t i c

Table I

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

19.

LUND ET AL.

Water Gas Shift over Magnetite-Based Catalysts

323

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

by e q u a t i o n 12. Q u a l i t a t i v e l y , the p r e d i c t i o n s are i n agreement w i t h the observed b e h a v i o r . These r e s u l t s p r o v i d e c o n v i n c i n g evidence t h a t the r e g e n e r a t i v e mechanism i s an important pathway f o r WGS over m a g n e t i t e . The d i f ­ f e r e n c e s between the p r e d i c t e d k i n e t i c b e h a v i o r and the observed k i n e t i c s may be due t o any of s e v e r a l p o s s i b i l i t i e s , i n c l u d i n g a c o n t r i b u t i o n from an a d s o r p t i v e mechanism or n o n u n i f o r m i t y of the catalyst surface. T h i s study a l s o i n d i c a t e s t h a t the a c t i v e p o r t i o n o f the c a t a l y s t s u r f a c e f o r the r e g e n e r a t i v e pathway may be q u i t e small ( e . g . , c a . 10% of the BET m o n o l a y e r ) . S i n c e t h i s study i n d i ­ c a t e d t h a t the r e a c t a n t gases chemisorb t o a s i g n i f i c a n t e x t e n t on magnetite s u r f a c e s , the a d s o r p t i o n p r o p e r t i e s of Fe^O^ may p r o v i d e f u r t h e r i n s i g h t i n t o the d e t a i l s of the a c t i v e s i t e s , as d i s c u s s e d below. The T i t r a t i o n

of M a g n e t i t e S u r f a c e S i t e s

The p r e v i o u s d i s c u s s i o n s have suggested t h a t a n i o n - c a t i o n p a i r - s i t e s are important a c t o r s i n both the a d s o r p t i v e and c a t a l y t i c c h a r a c t e r ­ i s t i c s of m a g n e t i t e . These p a i r - s i t e s can serve as s i t e s f o r C 0 , H , or H 0 a d s o r p t i o n , and may p a r t i c i p a t e i n a r e g e n e r a t i v e pathway f o r WGS. We c o n s i d e r f i r s t the t i t r a t i o n of c o o r d i n a t i v e l y u n s a t u ­ r a t e d i r o n c a t i o n s , these c a t i o n s being p o t e n t i a l members of a n i o n cation p a i r - s i t e s . N i t r i c oxide has been r e p o r t e d t o adsorb at room temperature on Fe^O^ i n o n e - t o - o n e correspondence w i t h the s u r f a c e i r o n c a t i o n s , ( 3 8 ) and thus i s u s e f u l i n t h i s r e s p e c t . A l t h o u g h the a d s o r p t i o n of W on unsupported Fe^O^ has been shown t o be dependent upon sample p r e t r e a t m e n t , a s t a n d a r d t i t r a t i o n procedure has been i d e n t i f i e d ( 3 9 ) , S p e c i f i c a l l y , a f t e r m a g n e t i t e samples were reduced i n a C0/C0 m i x t u r e which was chosen on thermodynamic grounds t o ensure t h a t Fe^O^ was the i r o n phase, the sample was evacuated a t 650 Κ f o r 1 h Defore c o o l i n g t o 273 Κ and i n i t i a t i n g NO c h e m i s o r p t i o n . ( I t was shown t h a t prolonged e v a c u a t i o n s at 650 Κ r e s u l t e d i n apparent NO uptakes which exceeded as much as 3 BET m o n o l a y e r s . ) An NO a d s o r p t i o n i s o t h e r m was then c o l l e c t e d at 273 K. The sample was s u b s e q u e n t l y evacuated f o r O.5 h at 273 K, a f t e r which a second i s o ­ therm was c o l l e c t e d . E x t r a p o l a t i o n of the high p r e s s u r e p o r t i o n s of t h e two isotherms t o zero p r e s s u r e and s u b t r a c t i o n y i e l d e d on NO uptake equal t o the No uptake i n the BET monolayer. The t i t r a t i o n o f FeoO,, s u r f a c e s w i t h NO i s e s p e c i a l l y u s e f u l f o r supported m a g n e t i t e ( 4 0 j A s e r i e s of s i l i c a - s u p p o r t e d m a g n e t i t e c a t a l y s t s were prepared w i t h v a r i o u s ΕββΟ^ l o a d i n g s . The standard NO t i t r a t i o n scheme presented above was used t o measure s e l e c t i v e l y t h e magnetite s u r f a c e a r e a s , from which i t was p o s s i b l e t o c a l c u l a t e the average Fe^O^ p a r t i c l e s i z e f o r each c a t a l y s t p r e p a r a t i o n . The p a r t i c l e s i z e was then measured by X - r a y d i f f r a c t i o n l i n e broadening and by low and high f i e l d m a g n e t i z a t i o n methods. Agreement between a l l the methods was s a t i s f a c t o r y , as i n d i c a t e d i n Table I I . While the a d s o r p t i o n of n i t r i c o x i d e i s very u s e f u l f o r t h e measurement of magnetite s u r f a c e a r e a s , i t i s not n e c e s s a r i l y t r u e t h a t t h i s m o l e c u l e t i t r a t e s the a c t i v e s i t e s f o r the w a t e r - g a s s h i f t reaction. ( T h i s p o i n t w i l l be d i s c u s s e d l a t e r i n t h i s p a p e r . ) For t h i s r e a s o n , recent s t u d i e s have focused on the a d s o r p t i v e p r o p e r ­ t i e s of magnetite f o r o t h e r m o l e c u l e s , and i n p a r t i c u l a r , on the 2

2

2

2

t

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

7.4

6.6

18.3%

Reproduced w i t h p e r m i s s i o n from R e f . 4 0 .

6.5

7.7

7.4

7.3

7.1

6.2

6.2

High Field

Magnetization

2

9.4

9.6

9.1

X-ray line-broadening

Samples

C o p y r i g h t 1981, R e a c t i o n K i n e t i c s and C a t a l y s i s L e t t e r s .

6.1

for Silica-Supported

(nm)

Low Field

P a r t i c l e Size

Gravimetric

NO a d s o r p t i o n

Volumetric

9.2%

4

4.5%

3

Sample wt.% F e 0

II

Comparison o f M a g n e t i t e P a r t i c l e S i z e D e t e r m i n a t i o n s

Table

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

19.

LUND ET AL.

325

Water Gas Shift over Magnetite-Based Catalysts

a d s o r p t i o n of w a t e r - g a s s h i f t r e a c t a n t s and products at r e a c t i o n temperatures. The most d e t a i l e d study t o date has i n v o l v e d the a d s o r p t i o n of CO and C 0 from C0/C0 gas m i x t u r e s ( 4 1 ) , the c o m p o s i t i o n of these m i x t u r e s chosen such t h a t magnetite was the thermodyn a m i c a l l y s t a b l e phase of i r o n . Indeed, i t i s important t o note t h a t magnetite would be reduced t o m e t a l l i c i r o n i n pure CO, w h i l e i t would be o x i d i z e d t o F e Û 3 ™ P ^ 0 at w a t e r - g a s shift temperatures. Three d i f f e r e n t C0/C0 r a t i o s were s t u d i e d at t h r e e d i f f e r e n t temperatures over chromia-promoted m a g n e t i t e . Figure 6 presents Langmuir isotherms f o r the d i f f e r e n t gas r a t i o s at 637 K. From the small changes i n c o m p o s i t i o n of the gas phase upon a d s o r p t i o n , t h e i n d i v i d u a l amounts of CO and C 0 adsorbed were measured i n d e p e n d e n t ly. F i g u r e s 7 and 8 show Langmuir isotherms f o r CO and C 0 i n d i v i d u a l l y measured i n t h i s manner at 637 Κ i n the t h r e e C0/C0 gas phase c o m p o s i t i o n s . The a d s o r p t i o n b e h a v i o r shown i n t h e s e f i g u r e s was demonstrated t o be r e v e r s i b l e . In a d d i t i o n t o the o b s e r v a t i o n t h a t the t o t a l uptake was o n l y about 20% of the BET monolayer, i n agreement w i t h the p r e v i o u s l y d i s c u s s e d g r a v i m e t r i c s t u d i e s , the data showed t h a t when the C 0 p r e s s u r e was i n c r e a s e d at c o n s t a n t CO p r e s s u r e , the amount of a d ­ sorbed CO d e c r e a s e d . S i m i l a r l y , i n c r e a s i n g the p r e s s u r e of CO d e ­ c r e a s e d the amount of adsorbed C 0 . These r e s u l t s are c o n s i s t e n t w i t h a d s o r p t i o n on a n i o n - c a t i o n p a i r - s i t e s , where CO adsorbs on a c a t i o n and i n t e r a c t s w i t h a n e i g h b o r i n g a n i o n , and where C 0 adsorbs as a b i d e n t a t e carbonate s p e c i e s . For c o m p e t i t i v e a d s o r p t i o n on a f i x e d number of s u r f a c e s i t e s , t h e coverages are given by the following expressions: 2

2

u r e

2

2

2

2

2

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

2

2

2

2

co co T Ρ + Κ Ρ co co co ( K

ft

co" 1 + K

P

6

r

2

Κ Θ

C Q

and K ^

o

i

=

2 where K

Ρ„ sat C0 θ Λ

C0

CO CO

a

o

τ

LA C0

M5) o

C0

o

are e q u i l i b r i u m a d s o r p t i o n c o n s t a n t s and 9 j

co

s a t

the f r a c t i o n of the BET monolayer composed of a d s o r p t i o n s i t e s . t h e case where Κ « Κ , e x p r e s s i o n s 14 and 15 may be reduced yield: 2 c

o

c o

co 9

co

CO,

C0

„sat o

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

(16)

is In to

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

326

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Δ Ρ,

kPa

F i g u r e 5. Measured ( s o l i d l i n e s ) e f f e c t of p r e s s u r e changes on WGS r a t e compared t o e f f e c t p r e d i c t e d by oxygen t r a n s f e r ( r e g e n e r ­ a t i v e ) mechanism (dashed l i n e s ) . (ΔΡ = 0 corresponds t o a l l = 10 k P a ) . Reproduced w i t h p e r m i s s i o n from Ref. 3 7 . Copyright 1982, AIChE.

0

10

20 P , T

30

40

50

k Pa

F i g u r e 6. Langmuir isotherms f o r t o t a l C0 /C0 a d s o r p t i o n at (0) 613 K, ( • ) - 637 K, and ( Δ ) - 663 Κ. Reproduced w i t h p e r m i s s i o n from R e f . 4 1 . C o p y r i g h t 1981, Academic P r e s s . 2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

19. LUND ET AL.

Water Gas Shift over Magnetite-Based Catalysts

0

2

6

4 P

c o

327

8

; k Pa

F i g u r e 7. Langmuir isotherms f o r CO a d s o r p t i o n i n C0 /C0 = 5.45 m i x t u r e at (0) - 613 K, (•) - 637 K, and (Δ) - 663 Κ. Reproduced w i t h p e r m i s s i o n from Ref. 4 1 . C o p y r i g h t 1981, Academic P r e s s . 2

Figure 8. Langmuir i s o t h e r m f o r C 0 a d s o r p t i o n i n C0 /C0 = 5.45 m i x t u r e at (0) - 613 K, (•) - 637 K, and (Δ) - 663 Κ. Reproduced w i t h p e r m i s s i o n from Ref. 4 1 . C o p y r i g h t 1981, Academic P r e s s . 2

2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

328

S O L I D STATE C H E M I S T R Y IN CATALYSIS

F i g u r e 9 shows t h a t when p l o t t e d a p p r o p r i a t e l y , the isotherms f o r CO and COo measured w i t h d i f f e r e n t feed r a t i o s superimpose, c o n s i s t e n t w i t h tfîe c o m p e t i t i v e model d e s c r i b e d by i s o t h e r m , e q u a t i o n 16. The idea of c o m p e t i t i v e a d s o r p t i o n on p a i r - s i t e s has a l s o been used t o d e s c r i b e the i n t e r a c t i o n of Ho and HoO w i t h magnetite (42). When isotherms were c o l l e c t e d u s i n g Ήο/ΗοΟ m i x t u r e s f o l l o w i n g the same approach as d i s c u s s e d above, i t was f o u n d t h a t the data f i t a model where H adsorbed d i s s o c i a t i v e l y and HoO adsorbed a s s o c i a t i v e l y , w i t h both s p e c i e s competing f o r p a i r - s i t e s . These s t u d i e s were conducted at w a t e r - g a s s h i f t r e a c t i o n temperatures ( e . g . , 650 K) and as f o r the a d s o r p t i o n of CO and C 0 , o n l y a f r a c t i o n of the magne­ t i t e s u r f a c e was capable of a d s o r b i n g H and H 0. While the c o m p e t i t i v e model adequately d e s c r i b e s the C0 /C0 a d s o r p t i o n d a t a , i t i s p o s s i b l e t h a t the r e l a t i o n s h i p between CO and C0 coverages observed f o r the m a g n e t i t e - b a s e d c a t a l y s t may be due t o v a r i a t i o n s i n the o x i d a t i o n s t a t e o f the s u r f a c e . In t h e p r e v i o u s d i s c u s s i o n of the oxygen t r a n s f e r c h a r a c t e r i s t i c s of t h i s same c a t a l y s t , i t was shown t h a t the s u r f a c e oxygen content was dependent on the C0 /C0 gas phase r a t i o ( F i g u r e 2). The f r a c t i o n of the s u r f a c e which p a r t i c i p a t e d i n redox r e a c t i o n s was about 10% of the BET monolayer. In view of the aforementioned acid/base p r o p e r ­ t i e s of C0 and C O , i t i s reasonab1e(41) t o c o r r e l a t e 6 with θ ^ (a surface oxygen anion) and e with (a coordinatively unsaturated c a t i o n ) . Table III summarizes t h i s a c i d / b a s e - t y p e c o r ­ relation. S a t u r a t i o n coverages by CO do, i n f a c t , correspond d i r e c t l y t o θ^, w h i l e changes i n C0 s a t u r a t i o n coverage c o r r e l a t e w i t h changes i n θ ^ f o r the t h r e e P Q O ^ C O examined. A twos i t e a d s o r p t i o n model was then developed i n which the coverages o f CO and COo are d e s c r i b e d by the f o l l o w i n g i s o t h e r m e x p r e s s i o n s : 2

2

2

2

2

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

2

2

2

C 0

0

c Q

2

r

P

0

K

1

0

S

P

K

co

co

2

+ φ

"θ 0-*

parameter

t

co co 1 + co co

CO

The

a

2

Φ was ο

co

2

2

(18) 1

+

ο

included

(17)

P

to

K

co

2

P

co

2

account

for

values

of

e ^ c o

l a r g e r than B ^ , and t h e r e f o r e , r e p r e s e n t s s u r f a c e oxygen s p e c i e s c a p a b l e of a d s o r b i n g C0 but not capable of p a r t i c i p a t i n g i n the oxygen t r a n s f e r p r o c e s s . In view of the above d i s c u s s i o n , the f o l l o w i n g q u a l i t a t i v e model may be suggested t o d e s c r i b e the r e g e n e r a t i v e mechanism over magnetite-based c a t a l y s t s : Q

2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

19.

LUND ET AL.

Water Gas Shift over Magnetite-Based Catalysts

P f T

329

k Pa

Figure 9. Langmuir isotherms f o r c o m p e t i t i v e a d s o r p t i o n at 637 Κ f o r C0 /C0 m i x t u r e s of (0) - 5 . 4 5 , ( Δ ) - 2.37 and (•) - 1 . 5 6 . Reproduced w i t h p e r m i s s i o n from Ref. 4 1 . C o p y r i g h t 1981, Academic Press. 2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

III

O.095

O.080

O.061

Θ at co Saturation

1.10

1.10

1.02

O.027 O.001

O.014

2

O.121

O.139

O.156

Saturation

Θ at co^

1.38

1.31

co

ΔΘ;

2

sat

M a g n e t i t e i n C0 /C0 Gas M i x t u r e s at 637 K

C o p y r i g h t 1981, Academic P r e s s I n c .

©*

S i t e D e n s i t i e s on Chromia-Promoted

S o u r c e : Reproduced w i t h p e r m i s s i o n from R e f . 4 1 .

O.086

1.56

co

O.060

/Ρ

O.073

2

2.37

co

5.45

Ρ

Summary o f A d s o r p t i o n

Table

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

1

19.

LUND ET AL.

331

Water Gas Shift over Magnetite-Based Catalysts

0 C CO + [0 - M]

j [0 - M]

0 C

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

[0 - M]

t

C0 + 2

[•-

H H0 +

[•-

2

H

M]

M]

H

+ [0 - M]

H

[0 - M]

t

H

2

+ [0 - M]

where [0 - M] i s an a n i o n - c a t i o n p a i r - s i t e and • r e p r e s e n t s an anion vacancy. In the f i r s t s t e p , CO adsorbs weakly on a c o o r d i n a t i v e l y u n s a t u r a t e d metal c a t i o n of an a n i o n - c a t i o n p a i r - s i t e . T h i s weakly adsorbed m o l e c u l e then r e a c t s w i t h s u r f a c e oxygen t o form C 0 and an anion vacancy. In the t h i r d s t e p , the oxygen atom of water f i l l s t h e anion vacancy v i a d i s s o c i a t i v e a d s o r p t i o n of H 0 . Finally, d e s o r p t i o n o f H t a k e s p l a c e i n the f o u r t h s t e p , r e g e n e r a t i n g an anion-cation p a i r - s i t e . In view of the arguments presented by Kubsh and Dumesic ( 3 7 ) , i t i s suggested t h a t the above r e a c t i o n s i n v o l v i n g s u r f a c e oxygen and anion v a c a n c i e s t a k e p l a c e w i t h weakly adsorbed w a t e r - g a s s h i f t r e a c t a n t s and p r o d u c t s , such t h a t these r e a c t i o n s can be d e s c r i b e d by E l e y - R i d e a l k i n e t i c s i n s t e a d of LangmuirHinshelwood k i n e t i c s . It i s p o s s i b l e , however, t h a t a v a i l a b l e p a i r - s i t e s may be b l o c k e d by the a d s o r p t i o n of C 0 or H 0 . For example, C 0 may adsorb on these sites, forming bidentate carbonate species. Although such adsorbed s p e c i e s may suppress the r e g e n e r a t i v e mechanism over a n i o n - c a t i o n p a i r - s i t e s , t h e s e s p e c i e s may be important f o r a d s o r p t i v e WGS mechanisms. The general c o n c l u s i o n , however, i s t h a t the a d s o r p t i o n isotherms f o r CO and CO? and f o r H and H 0 are u s e f u l f o r p r o b i n g the p a i r - s i t e s necessary f o r WGS. The above i d e a s t h a t a n i o n - c a t i o n p a i r s i t e s are the s u r f a c e s i t e s f o r CO and C 0 a d s o r p t i o n on magnetite was v e r i f i e d d i r e c t l y by Udovic and Dumesic ( 4 3 ) , These authors prepared f i l m s of magnet i t e on p o l y c r y s t a l l i n e i r o n f o i l s and v a r i e d the o x i d a t i o n s t a t e of t h e s u r f a c e by vacuum-annealing at d i f f e r e n t t e m p e r a t u r e s . In s h o r t , i t was shown by Auger e l e c t r o n s p e c t r o s c o p y and X - r a y p h o t o 2

2

2

2

2

2

2

2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2

332

S O L I D STATE C H E M I S T R Y IN CATALYSIS

e l e c t r o n s p e c t r o s c o p y t h a t the s u r f a c e became more reduced ( i . e . , Fe/0 and Fe /Fe atomic r a t i o s both i n c r e a s e d ) as the vacuuma n n e a l i n g temperature was i n c r e a s e d . Temperature programmed d e s o r p t i o n experiments, under u l t r a - h i g h vacuum c o n d i t i o n s , were then performed. The s a t u r a t i o n coverage by CO i n c r e a s e d as the s u r f a c e became more reduced, i n d i c a t i n g t h a t adsorbed CO was i n t e r a c t i n g w i t h c o o r d i n a t i v e l y u n s a t u r a t e d c a t i o n s , as suggested above. In c o n t r a s t , the s a t u r a t i o n coverage by CO? decreased as the s u r f a c e became more reduced. T h i s r e s u l t woula imply t h a t the number of p a i r - s i t e s was l i m i t e d not by the number of s u r f a c e i r o n c a t i o n s , but by the number of r e a c t i v e oxygen^species on the s u r f a c e . The o b s e r v a t i o n of i s o t o p i c exchange o f 0 between C 0 and the s u r f a c e c o n f i r m e d t h a t the C 0 was present as b i d e n t a t e carbonate s p e c i e s . F i n a l l y , the s a t u r a t i o n coverages by C 0 and CO were s i g n i f i c a n t l y l e s s than one monolayer. T h i s i s c o n s i s t e n t w i t h the above f i n d i n g s t h a t o n l y a small f r a c t i o n of the s u r f a c e i s capable of a d s o r b i n g these s p e c i e s . A d d i t i o n a l i n s i g h t i n t o the nature of the a d s o r p t i o n s i t e s f o r CO? was o b t a i n e d by c a r r y i n g out temperature programmed d e s o r p t i o n a f t e r c o a d s o r p t i o n of C 0 and NO. S p e c i f i c a l l y , i t has been noted above t h a t NO adsorbs on i r o n c a t i o n s , and t h i s was f u r t h e r c o n f i r m e d by n o t i n g t h a t the s a t u r a t i o n coverage by NO i n c r e a s e d as the s u r f a c e of magnetite became more reduced. I m p o r t a n t l y , i t was then observed t h a t exposure of the magnetite s u r f a c e t o NO caused a decrease i n the amount of C 0 which c o u l d be subsequently adsorbed on the s u r f a c e . Thus, i r o n c a t i o n s must be a s s o c i a t e d w i t h the s i t e s for C0 adsorption. S i n c e i t had a l s o been shown t h a t CO? a d s o r p t i o n i s a s s o c i a t e d w i t h s u r f a c e oxygen, i t was concluded t h a t C 0 a d s o r p t i o n t a k e s p l a c e on a n i o n - c a t i o n p a i r - s i t e s . (This would, i n f a c t , be expected f o r f o r m a t i o n of b i d e n t a t e carbonate s p e c i e s ) . It has r e c e n t l y been proposed by T i n k l e and Dumesic (44) t h a t t h i s mode of a d s o r p t i o n may not f o l l o w the Langmuir i s o t h e r m . 1 8

2

2

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

2

2

2

2

2

S o l i d S t a t e Probes of the A c t i v e WGS S i t e s The e f f e c t s of s o l i d s t a t e a l t e r a t i o n s of the magnetite s t r u c t u r e on the c a t a l y t i c a c t i v i t y f o r WGS p r o v i d e a d d i t i o n a l i n s i g h t i n t o the nature of the a c t i v e s i t e s . While g r a v i m e t r i c and c h e m i s o r p t i v e s t u d i e s p r o v i d e d a chemical p i c t u r e of the a c t i v e s i t e s , a geometric or c r y s t a l l o g r a p h i c d e s c r i p t i o n was l a c k i n g . S o l i d s t a t e probes of t h e a c t i v e s i t e s have s u p p l i e d i n f o r m a t i o n on t h i s aspect of the mechanism. I t has been shown t h a t the a d d i t i o n of l e a d t o a c h r o m i a promoted magnetite WGS c a t a l y s t enhances the a c t i v i t y f o r WGS ( 4 ) , A study of the s o l i d s t a t e changes which occur upon t h i s s u b s t i t u t i o n was made t o probe the a c t i v e s i t e s ..of the c a t a l y s t . Through a comb i n a t i o n of o x i d a t i o n s t u d i e s , Mossbauer s p e c t r o s c o p y , and X - r a y d i f f r a c t i o n l i n e b r o a d e n i n g , a model f o r the^ c a t a l y s t was d e v e l o p e d . I t was concluded t h a t Pb was present as P b at t e t r a h e d r a l s i t e s . The Pb s u b s t i t u t i o n r e s u l t e d i n the expansion of the t e t r a h e d r a l s i t e s , c o n t r a c t i o n of the o c t a h e d r a l s i t e s , and the o x i d a t i o n of some Fe^ t o Fe . The r e s u l t i n g o c t a h e d r a l c a t i o n s became more c o v a l e n t i n n a t u r e , and s i n c e the o c t a h e d r a l c a t i o n s have been r e p o r t e d t o be the a c t i v e s i t e s f o r CO o x i d a t i o n over f e r r i t e s 4 +

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

19.

LUND ET AL.

(45-48), i t was s p e c u l a t e d t h a t t h i s i n c r e a s e d c o v a l e n c y was r e s p o n s i b l e f o r the enhanced a c t i v i t y . In another s t u d y , the e f f e c t of s i l i c a i n c o r p o r a t i o n i n t o the F e 0 l a t t i c e was s t u d i e d ( 5 , 4 9 , 5 0 ) * A 20% Fe^O^ on s i l i c a c a t a l y s t was prepared u s i n g c o n v e n t i o n a l t e c h n i q u e s . It was found t h a t w h i l e d i r e c t o x i d a t i o n of the c a t a l y s t at 800 Κ produced the expected α F e 2 Û 3 , i f the c a t a l y s t was p r e v i o u s l y reduced i n CO/CO2 t o produce m a g n e t i t e , then subsequent o x i d a t i o n r e s u l t e d i n the f o r m a t i o n of γ Fe^. F i g u r e 10 shows MCfssbauer s p e c t r a of t h i s c a t a l y s t a f t e r v a r i o u s thermal t r e a t m e n t s . In these s p e c t r a , the c e n t r a l d o u b l e t s were demonstrated t o be a r e s u l t of small i r o n oxide p a r t i c l e s which were superparamagnetic at the c o n d i t i o n s where the spectrum was r e ­ corded. The s u p p r e s s i o n of the γ -FeoOg t o α - F e ^ t r a n s i t i o n i s c h a r a c t e r i s t i c of the s u b s t i t u t i o n of f o r e i g n c a t i o n s i n t o the mag­ n e t i t e l a t t i c e (4,51)« When a s e r i e s of silica supported-magnetite catalysts of v a r y i n g i r o n oxide p a r t i c l e s i z e were i n v e s t i g a t e d , i t was d e t e r ­ mined t h a t Si s u b s t i t u t e s i n t o the magnetite l a t t i c e a c c o r d i n g t o the f o l l o w i n g r e a c t i o n ( 4 9 ) : 3

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

333

Water Gas Shift over Magnetite-Based Catalysts

4

2Si0

2

+ 2(Fe

3 +

)[Fe

2 +

,Fe

3 +

]0

4

+

(Si fe)[Fe 2

2

2 +

,Fe

4

3 +

]0

1 2

< > 19

A d d i t i o n a l l y , i t was proposed t h a t r e a c t i o n 19 o c c u r r e d only i n the outermost 3-4 atomic l a y e r s of the magnetite c r y s t a l l i t e s . The MOssbauer spectrum of the c a t a l y s t i n the reduced form agreed w i t h this substitution. The s p e c t r a l parameters of the t e t r a h e d r a l c a t i o n s were u n a f f e c t e d by the s u b s t i t u t i o n , whereas the isomer shift and magnetic h y p e r f i n e f i e l d of the o c t a h e d r a l cations decreased. A l s o , the l i n e width of the o c t a h e d r a l c a t i o n s i n c r e a s e d r e l a t i v e t o an u n s u b s t i t u t e d c a t a l y s t . F i n a l l y , the s p e c t r a l area r a t i o of the i r o n c a t i o n s i n the t e t r a h e d r a l t o o c t a h e d r a l s u b l a t t i c e s decreased. The e f f e c t of Si s u b s t i t u t i o n on the t u r n o v e r frequency f o r WGS i s shown i n F i g u r e 1 1 . The t u r n o v e r f r e q u e n c i e s p l o t t e d i n t h i s f i g u r e were based on the magnetite s u r f a c e area as determined by the NO c h e m i s o r p t i o n t e c h n i q u e . The t u r n o v e r f r e q u e n c i e s shown f o r u n ­ supported Fe^O^ i n d i c a t e t h a t the f a c t o r of 10 d e c l i n e i n a c t i v i t y f o r the s i l i c a - s u p p o r t e d c a t a l y s t s i s not a p a r t i c l e s i z e e f f e c t , but i n s t e a d i s a consequence of the s u b s t i t u t i o n of Si i n t o the l a t t i c e . However, when the a d s o r p t i o n of CO/C0o at 663 Κ was used t o t i t r a t e the s u r f a c e s i t e s i n s t e a d of NO, the r e s u l t i n g t u r n o v e r frequencies were e s s e n t i a l l y c o n s t a n t as shown i n F i g u r e 1 2 . A c c o r d i n g l y , the C0/C0 m i x t u r e a p p a r e n t l y t i t r a t e s the s i t e s a c t i v e f o r WGS. C l e a r l y , the number of a c t i v e s i t e s i s decreased markedly as the p a r t i c l e s i z e decreases i n the s i l i c a - s u b s t i t u t e d magnetite catalysts. The s u b s t i t u t i o n of Si i n t o the magnetite l a t t i c e near the s u r ­ face causes the e f f e c t i v e charge of the o c t a h e d r a l c a t i o n s t o i n ­ c r e a s e from i t s nominal value of 2.5+. F u r t h e r m o r e , because the s u b s t i t u t i o n appears t o occur i n the outermost s u r f a c e l a y e r s o n l y , the p e r t u r b a t i o n i s g r e a t e s t f o r the s m a l l e s t p a r t i c l e s , because i n 2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

334

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

S O L I D STATE C H E M I S T R Y IN CATALYSIS

-10

-5

0

VELOCITY

5

(mm/s)

F i g u r e 10. Room temperature MGssbauer s p e c t r a of 20% F e 3 0 / S i 0 p c a t a l y s t (a) u n t r e a t e d , (b) o x i d i z e d ( 7 7 3 K ) , (c) reduced i n C0 /C0 ( 6 5 8 K ) , (d) r e o x i d i z e d ( 7 7 3 K ) , and (e) rereduced (663K) ( f ) unsupp o r t e d F e ^ 0 . (g) m e t a l l i c F e . Reproduced from R e f . 4 9 . C o p y r i g h t 1981, ACS. 4

2

4

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

19.

LUND ET AL.

Water Gas Shift over Magnetite-Based Catalysts

335

MAGNETITE PARTICLE SIZE (nm)

Figure 11. Turnover frequency versus p a r t i c l e s i z e f o r unsup­ ported (open) and S i 0 - s u p p o r t e d ( s o l i d ) Fe^O^. Reproduced w i t h p e r m i s s i o n from Ref. 5. C o p y r i g h t 1982, Academic P r e s s . 2

F i g u r e 12. Comparison o f t u r n o v e r f r e q u e n c i e s based upon NO ( Δ ) and CO/COo (0) a d s o r p t i o n . (•) - r a t i o of NO t o C0/C0 u p t a k e s . Reproduced w i t h p e r m i s s i o n from Ref. 5. C o p y r i g h t 1982, Academic Press. 2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

336

S O L I D STATE C H E M I S T R Y IN CATALYSIS

t h e s e p a r t i c l e s the s u b s t i t u t e d l a y e r s comprise a more s i g n i f i c a n t f r a c t i o n of the t o t a l p a r t i c l e . Because the t e t r a h e d r a l s i t e s i n S i - s u b s t i t u t e d magnetite do not c o n t a i n Fe c a t i o n s and y e t the c a t a l y t i c a c t i v i t y i s m a i n t a i n e d f o r the l a r g e r magnetite p a r t i c l e s , i t can be suggested t h a t the t e t r a h e d r a l s i t e s are not r e s p o n s i b l e f o r the WGS r e a c t i o n . S i n c e a c c o r d i n g t o the r e g e n e r a t i v e mechan i s m , the a c t i v e s i t e s must be c a p a b l e of both r e c e i v i n g and d o n a t i n g oxygen, the o c t a h e d r a l c a t i o n s are the n a t u r a l s i t e s f o r t h i s r e a c t i o n , as d i s c u s s e d p r e v i o u s l y . Upon s u b s t i t u t i o n by S i , these c a t i o n s become more f e r r i c i n n a t u r e , thereby p e r t u r b i n g the balance between F e ^ and F e responsible for c a t a l y t i c a c t i v i t y . In c l o s i n g , i t i s important t o note t h a t the 00/00 a d s o r p t i o n t e c h n i q u e e f f e c t i v e l y t i t r a t e s the a c t i v e s i t e s f o r WGS on magnetite c a t a l y s t s which d i f f e r i n a c t i v i t y by over an order of magnitude. N i t r i c o x i d e on the o t h e r hand t i t r a t e s a l l of the s u r f a c e c a t i o n s i t e s and i s u n a f f e c t e d by S i - s u b s t i t u t i o n . Indeed, NO i s known t o chemisorb s t r o n g l y on i r o n oxides and may even be a b l e t o r e c o n s t r u c t the s u r f a c e . Thus, the combined use of NO and C0/C0 a d s o r p t i o n p r o v i d e s i n f o r m a t i o n about the t o t a l magnetite s u r f a c e area and f r a c t i o n of the magnetite s u r f a c e which i s a c t i v e f o r the WGS reaction. +

3 +

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

2

2

Conclusions T h i s review has focused on recent r e s e a r c h d i r e c t e d toward c h a r a c t e r i z a t i o n of the a c t i v e s i t e s f o r water-gas s h i f t over m a g n e t i t e based c a t a l y s t s . The r e a c t i o n can be d e s c r i b e d by a r e g e n e r a t i v e mechanism wherein gas phase or weakly adsorbed CO reduces anion s i t e s and steam o x i d i z e s the r e s u l t a n t s u r f a c e oxygen v a c a n c i e s . K i n e t i c r e l a x a t i o n t e c h n i q u e s i n d i c a t e t h i s t o be a primary pathway. The s i t e s which p a r t i c i p a t e i n t h i s r e a c t i o n comprise only about 10% of the BET monolayer, and these s i t e s can be t i t r a t e d u s i n g C0/C0 a d s o r p t i o n at 663 K. In c o n t r a s t , the t o t a l c a t i o n s i t e d e n s i t y i s e f f e c t i v e l y t i t r a t e d w i t h NO at 273 K. In f a c t , the r a t i o of the e x t e n t of C0/C0 a d s o r p t i o n t o the e x t e n t of NO a d s o r p t i o n p r o v i d e s a measure of the f r a c t i o n of the magnetite s u r f a c e which i s a c t i v e f o r water-gas s h i f t . S o l i d s t a t e s u b s t i t u t i o n of S i i n the magnetite structure o c c u r s only near the s u r f a c e and at the t e t r a h e d r a l s i t e s . On l a r g e c r y s t a l l i t e s t h i s s u b s t i t u t i o n a l t e r s the c a t a l y t i c a c t i v i t y s l i g h t l y , i n d i c a t i n g t h a t the t e t r a h e d r a l s i t e s are not c a t a l y t i c a l l y s i g nificant. As the c r y s t a l l i t e s i z e d e c r e a s e s , the a c t i v i t y d e c l i n e s i n S i - s u b s t i t u t e d m a g n e t i t e , and c o n c o m i t a n t l y the f e r r i c c h a r a c t e r of the o c t a h e d r a l c a t i o n s i n c r e a s e s . Thus, the r e g e n e r a t i v e mechanism appears t o occur at s i t e s w i t h o c t a h e d r a l i r o n c a t i o n s which have an e f f e c t i v e 2.5+ c h a r g e . 2

2

Acknowledgment The authors thank Martha T i n k l e f o r c r i t i c a l s c r i p t and h e l p f u l d i s c u s s i o n and comments.

reading

of

the

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

manu-

19.

LUND ET AL.

Water Gas Shift over Magnetite-Based Catalysts

337

Literature Cited 1.

2. 3. 4. 5. 6. 7.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.

Bohlbro, H., "An Investigation on the Kinetics of the Conver­ sion of Carbon Monoxide with Water Vapour Over Iron Oxide Based Catalysts," (2nd ed). Gjellerup, Copenhagen, 1969. Happel, J., Catal. Rev. 1972, 6, 221. Verwey, E. J. W., and Heilmann, E. L., J. Chem. Phys. 1947, 15, 174. Topsøe, H., and Boudart, M., J. Catal. 1973, 31, 346. Lund, C. R. F., and Dumesic, J. Α., J. Catal 1982, 76, 93. Udovic, T. J., Ph.D Dissertation, Univ. of Wisconsin 1982. Shchbrya, G. G., Morozov, Ν. M., and Temkin, M. I., Kinet. Catal. 1965, 6, 955. Wagner, C., Adv. Catal. 1970, 20, 323. Boreskov, G. K., Yur'eva, T. M., and Sergeeva, A. S., Kinet. Catal. 1970, 11, 1230. Boreskov, G. K., Kinet. Catal. 1980, 11, 312. Stotz, S., Ber. Bunsenges. Phys. Chem. 1966, 70, 37. Kaneko, Y., and Oki, S., J. Res. Inst. Catal. Hokkaido Univ. 1965, 13, 55. Kaneko, Y., and Oki, S., J. Res. Inst. Catal. Hokkaido Univ., 1965, 13, 169. Kaneko, Y., and Oki, S., J. Res. Inst. Catal. Hokkaido Univ. 1967, 15, 185. Oki, S., Happel, J. Hnatow, Μ., and Kaneko, Y., Proc. Int. Congr. Catal, 5th, 1, (J. Hightower, editor), 173. Oki, S., and Mezaki, R., J. Phys. Chem. 1973, 77, 447. Oki, S., and Mezaki, R., J. Phys. Chem. 1973, 77, 1601. Mezaki, R., and Oki, S., J. Catal. 1973, 30, 488. Jensen, W. B., Chem. Rev. 1978, 78, 1. Kasatkina, L. Α., Nekipelov, V. N. and Zhivotenki, Ν. N., Kinet. Katal. 1973, 14, 304. Kozub, G. M., Voroshilov, I. G., Roev, L. M., and Rusov, M. T., Kinet. Katal. 1976, 17, 903. Rubene, Ν. Α., Davydov, Α. Α., Kravtsov, Α. V., Usheva, Ν. V., and Smol'yaninov, S. I. Kinet. Katal. 1976, 17, 400. Burwell, R. L., Jr., NBS Special Publication 455, 1970, 155. Knözinger H., Adv. Catal. 1976, 25, 184. Ai, M., J. Catal. 1978, 54, 223. Morterra, C., Ghiotti, G., Boccuzzi, F. and Coluccia, S. J. Catal. 1978, 51, 299. Mars, P., Z. Physik. Chem. (Frankfurt) 1959, 22, 309. Ueno, Α., Onishi, T. and Tamaru, K., Trans. Faraday Soc. 1970, 66, 756. Amenomiya, Y., Applied Spectroscopy 1978, 32, 484. Deluzarche, Α., Kieffer, R., and Papadopoulos, M., C.R. Hebd. Seances Acad. Sci., Ser. C 1978, 287, 25. Ai, M., J. Catal. 1977, 49, 305. Ai, M., J. Catal. 1977, 49, 313. Ai, M., J. Catal. 1978, 50, 291. Ai, M., J. Catal. 1978, 52, 16. Ai, M., J. Catal. 1978, 54, 223. Ai, M., J. Catal. 1978, 54, 426. Kubsh, J. E., and Dumesic, J. Α., AICHE J. 1982, 28, 793.

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

S O L I D STATE C H E M I S T R Y I N C A T A L Y S I S

338

38. 39. 40. 41. 42. 43. 44. 45.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch019

46.

47. 48. 49. 50. 51.

Otto, K. and Shelef, M. J. Catal. 1970, 18, 184. Lund, C. R. F., Schorfheide, J. J., and Dumesic, J. Α., J. Catal. 1979, 57, 105. Kubsh, J. E., Lund, C. R. F., Chen. Y., and Dumesic, J. Α., React. Kinet. Catal. Lett. 1981, 17, 115. Kubsh, J. E., Chen, Y., and Dumesic, J. Α., J. Catal. 1981, 71, 192. Tinkle, M., and Dumesic, J. Α., J. Phys. Chem., in press. Udovic, T. J. and Dumesic, J. Α., J. Catal. in press. Tinkle, M., and Dumesic, J. Α., J. Phys. Chem. 1983, 87, 3557. Krishnamurthy, T. R., Viswanathan, B., and Sastri, M.V.C. J. Res. Inst. Catal. Hokkaido Univ. 1977, 24, 219. Schwab, G.-M., Roth, E., Grintzos, C., and Mavrakis, N. "Structure and Properties of Solid Surfaces." University of Chicago Press, Chicago 1953. Popovskii, V. V., Boreskov, G. Κ., Dzevetski, Z., Muzykantov, V. S. and Shul'meister, T. T. Kinet. Katal. 1971, 12, 979. Wolski, W. Roczniki Chemii. 1960, 34, 309. Lund, C. R. F., and Dumesic, J. Α., J. Phys. Chem. 1981, 85, 3175. Lund, C. R. F., and Dumesic, J. Α., J. Phys. Chem. 1982, 86, 130. DeBoer, F. E., and Selwood, P. W., J. Amer. Chem. Soc. 1954, 76, 3365.

RECEIVED

October 10,

1984

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

20 Secondary Ion Mass Spectrometry Studies of the Structure and Reactivity of Carbon on Ruthenium(001) 1

L. L. LAUDERBACK and W. N. DELGASS

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch020

School of Chemical Engineering, Purdue University, West Lafayette, IN 47907 Most of the ethylene that interacts with an Ru(001) sur­ face at 323 Κ produces a nondesorbable carbon layer. Thermal desorption of CO, Auger electron spectroscopy, and temperature programmed oxidation a l l show that the carbon layer 1) is immobile below 550 Κ 2) forms a more densely packed surface phase at temperatures of 5501150 Κ and 3) dissolves into the bulk at 1350 K. SIMS measurements of isotope mixing in the C-2 ions confirm formation of dense-phase (graphitic) islands after heating the carbon layer to 923 K. SIMS spectra also demonstrate that at 520 K, CO dissociates on Ru(001). The oxygen­ -free carbon layer that forms behaves similarly to the carbon from ethylene. Both SIMS and thermal desorption results show no positive interaction between adsorbed CO and D but significant attraction between D and C formed by CO dissociation. 2

2

In a recent investigation (1) we showed, using secondary ion mass spectrometry (SIMS) and thermal desorption spectroscopy (TDS), that the interaction of ethylene with Ru(001) at 323 Κ i s accompanied by substantial dissociation and subsequent desorption of hydrogen giving r i s e to an adlayer consisting mainly of non-desorbable carbon along with small amounts of dissociated hydrogen and various hydrocarbon species. Most importantly, i t was shown that the hydrocarboncontaining secondary ions seen i n SIMS could be d i r e c t l y related to hydrocarbon species on the surface and that the high temperature desorption peak for C^H^ results from associative desorption of C^H^. In this paper we extend our previous study by investigating i n more d e t a i l the nature of the non-desorbable carbon adlayer formed by C H^ or CO interaction with Ru(001). SIMS, CO thermal desorption spectroscopy, temperature programmed oxidation (TPO) and Auger electon spectroscopy (AES) measurements are used to characterize the structure and r e a c t i v i t y of the carbon adlayer and to develop SIMS as a t o o l for analysis of the chemistry and structure of surface layers. 2

1

Current address: Department of Chemical Engineering, University of Colorado, Boulder, CO 80309 0097-6156/85/0279-0339$06.00/0 © 1985 American Chemical Society

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

S O L I D STATE C H E M I S T R Y IN CATALYSIS

340

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch020

Experimental A l l experiments were c a r r i e d out i n an ionjgumped, s t a i n l e s s s t e e l UHV b e l l j a r w i t h ^ base p r e s s u r e o f 1x10 Torr. I n SIMS e x p e r ­ i m e n t s , p r i m a r y A r i o n s were generated by a R i b e r C I 50 i o n gun, and secondary i o n s were d e t e c t e d w i t h a R i b e r Q156 quadrupole mass s p e c t r o m e t e r equipped w i t h a 4 5 ° energy p r e f i l t e r . The mass s p e c t r o ­ meter i s a l s o equipped w i t h an i o n i z a t i o n f i l a m e n t f o r r e s i d u a l gas a n a l y s i s and t h e r m a l d e s o r p t i o n measurements. A l l experiments were performed w i t h 5 keV A r i o n s i m p i n g i n g on the sample s u r f a c e a t a 4 5 ° p o l a r a n g l e m e a s u r e d g r o m the s u r f a c e n o r m a l . The p r i m a r y i o n c u r r e n t d e n s i t y was 5x10 amps/cm2. Auger e l e c t r o n s p e c t r o s c o p y measurements were c a r r i e d out u s i n g a P H I , s i n g l e - p a s s , c y l i n d e r i c a l m i r r o r a n a l y z e r and an o f f - a x i s e l e c t r o n gun. The p r i m a r y e l e c t r o n energy was 3.5 keV and the p r i ­ mary e l e c t r o n c u r r e n t was 9μΑ i n a l l e x p e r i m e n t s . The Ru s i n g l e c r y s t a l was o r i e n t e d by Laue x - r a y b a c k - s c a t t e r i n g t o w i t h i n 1° o f the Ru(001) p l a n e , c u t by a diamond saw and mechan­ i c a l l y p o l i s h e d . A f t e r b e i n g etched i n h o t a q u a - r e g i a f o r about 15 m i n . , the c r y s t a l was spot welded to two t a n t a l u m h e a t i n g w i r e s w h i c h were connected to two s t a i n l e s s s t e e l e l e c t r o d e s on a sample m a n i p ­ ulator. The temperature was m o n i t o r e d by a Pt/Pt-10%Rh thermocouple w h i c h was s p o t welded to the back o f the c r y s t a l . In t h i s configura­ t i o n temperatures up t o 1700 Κ c o u l d be r o u t i n e l y a c h i e v e d . The s u r ­ f a c e c l e a n i n g p r o c e d u r e , w h i c h was s i m i l a r to t h a t used b y Madey e t a l . ^ ( 2 ) , i n v o l v e d many h e a t i n g and c o o l i n g c y c l e s up to 1600 Κ i n 5x 10 T o r r o f oxygen, f o l l o w e d by h e a t i n g i n vacuum 2-5 times to 1700 Κ t o remove s u r f a c e oxygen. S u r f a c e c l e a n l i n e s s was v e r i f i e d by SIMS and A E S . R e s u l t s and D i s c u s s i o n Carbon L a y e r s From E t h y l e n e . Carbon coverage as a f u n c t i o n o f e t h y l ­ ene exposure a t 323 Κ has been determined p r e v i o u s l y b y r e c o r d i n g the amount of CO desorbed d u r i n g TPO ( 1 ) . H a l f monolayer ( M . L . ) c o v e r ­ age by carbon r e q u i r e s about 2 Langmuirs (L) of C H ^ , w h i l e 1.1 monolayer coverage c o r r e s p o n d s t o 15 L . M o l e c u l a r s p e c i e s removed from the s u r f a c e by TDS a f t e r 15 L exposure t o C^H, amount t o o n l y 1% of a m o n o l a y e r . We d e s c r i b e below s t u d i e s o f the r e s i d u a l c a r b o n l a y e r b o t h i n i t s i n i t i a l s t a t e and a f t e r h e a t i n g to e l e v a t e d temper­ atures. CO Thermal D e s o r p t i o n . On a c l e a n Ru(001) s u r f a c e , 9 L o f CO i n d u c e s s a t u r a t i o n coverage by m o l e c u l a r CO. When t h i s dose o f CO i s a p p l i e d to a s u r f a c e preexposed to C^H^ a t 323 K , the CO uptake i s d i m i n i s h e d b u t n o t c o m p l e t e l y b l o c k e d by the c a r b o n l a y e r . The CO uptake i s s t i l l 90% o f the s a t u r a t i o n v a l u e when the c a r b o n coverage i s 1/4 M . L . and f a l l s t o 1/4 o f the s a t u r a t i o n v a l u e a f t e r 15 L p r e ­ exposure to e t h y l e n e . I n o r d e r to probe the e f f e c t of temperature on the s t r u c t u r e o f t h e c a r b o n l a y e r , c a r b o n was f i r s t d e p o s i t e d by exposure to C«H^. The sample was then h e a t e d to the d e s i r e d temperature a t a r a t e o f 6 K / s e c and then c o o l e d t o 323 Κ b e f o r e exposure t o 9 . 0 L o f CO. F i g u r e 1 shows the CO TDS s p e c t r a f o r a 15 L C«H^ p r e e x p o s u r e . The TDS h e a t i n g r a t e was 6 K / s e c . The 1145 Κ spectrum i n t h i s f i g u r e i s 2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch020

20.

LAUDERBACK AND DELGASS

Structure and Reactivity of C on Ru(001)

341

i d e n t i c a l i n shape to t h a t from the c l e a n s u r f a c e , b u t the CO c o v ­ erage i s about 1/2 the s a t u r a t i o n v a l u e . A s i g n i f i c a n t i n c r e a s e i n CO coverage o c c u r s a f t e r a n n e a l i n g to 763 Κ and above. A n a l y s i s o f t h e CO d e s o r p t i o n s p e c t r a , and o t h e r s o b t a i n e d a t l o w e r c a r b o n c o v e r a g e , i n terms of r e s u l t s f o r CO a d s o r p t i o n on the c l e a n s u r f a c e ( 2 - 5 ) , l e a d s to s e v e r a l c o n c l u s i o n s . A t low carbon coverages the e f f e c t o f the c a r b o n i s s l i g h t . T h i s corresponds t o a CO TPD spectrum s i m i l a r t o t h a t found f o r a c l e a n s u r f a c e but d e c r e a s e d i n i n t e n s i t y and w i t h a broadened h i g h temperature peak. H i g h carbon coverage b l o c k s some s i t e s and weakens the r e m a i n d e r . T h i s e f f e c t s h i f t s the CO TPD spectrum to lower d e s o r p t i o n tempera­ tures. The e f f e c t s o f a n n e a l i n g the carbon l a y e r s a r e f i r s t seen a t 763 K , where carbon m o b i l i t y a l l o w s a rearrangement o f the carbon l a y e r to accommodate more CO, as shown i n F i g u r e 1. The 1145 Κ s p e c ­ trum s u g g e s t s t h a t the c a r b o n l a y e r rearrangement has produced some c l e a n Ru(001) w h i c h desorbs CO i n i t s c h a r a c t e r i s t i c f a s h i o n . Since TPO shows t h a t the amount o f c a r b o n r e m a i n i n g on the s u r f a c e i s a l ­ most unchanged a f t e r a n n e a l i n g a t 980 Κ and i s s t i l l 80% o f the o r i g ­ i n a l v a l u e a t 1145 K , the c a r b o n l a y e r appears to r e a r r a n g e i n t o a more dense phase a t temperatures above 763 K . Temperature Programmed O x i d a t i o n . These measurements c h a r ­ a c t e r i z e b o t h the amount and c h e m i c a l n a t u r e o f the c a r b o n on the surface. A f t e r a s u r f a c e i s exposed to e t h y l e n e and p r e t r e a t e d as d e s i r e d , i t r e c e i v e s a 6 L dose o f 0^ a t 323 K . The TPO spectrum i s the CO d e s o r p t i o n s i g n a l a t a 6 K / s e c programming r a t e . C 0 accounts f o r l e s s than 1% o f the o x i d a t i o n , so the CO s i g n a l accounts f o r e s s e n t i a l l y a l l o f the carbon removed. 0 d o s i n g i s repeated u n t i l no f u r t h e r CO i s e v o l v e d d u r i n g h e a t i n g . SIMS r e s u l t s show t h a t a l l carbon has been removed from the s u r f a c e a t the TPO end p o i n t . TPO s p e c t r a as a f u n c t i o n of a n n e a l i n g temperature a r e shown i n F i g u r e 2 f o r a c a r b o n coverage of about O.25 m o n o l a y e r s , c o r r e s p o n d ­ i n g to an e t h y l e n e exposure o f 1 L . One TPO c y c l e removes a l l the c a r b o n a t t h i s c o v e r a g e . A q u a l i t a t i v e e x a m i n a t i o n o f these s p e c t r a r e v e a l s t h a t the low temperature peak, A , does not change p o s i t i o n w i t h c a r b o n coverage and i s c o n s i s t e n t w i t h the f i r s t o r d e r e v o l u t i o n of CO from immobile C - 0 n e i g h b o r s . The sharp peak, B , o c c u r s i n a temperature r e g i o n where c a r b o n t r a n s f o r m a t i o n t o the dense phase and oxygen d i s o r d e r i n g (6) o c c u r . Most i n t e r e s t i n g i s t h e e v o l u t i o n o f a h i g h temperature peak b e g i n n i n g a t an a n n e a l i n g temperature o f 663 K . The g r a d u a l d e c l i n e o f the low temperature peak and growth o f the h i g h temperature peak as the a n n e a l i n g temperature i n c r e a s e s from 663 Κ to 1145 Κ c o r r e l a t e s w e l l w i t h the c o n v e r s i o n of the low d e n s i t y carbon phase to the h i g h d e n s i t y c a r b o n phase seen i n t h i s r e g i o n i n the CO TDS s p e c t r a . The l o s s o f carbon a f t e r 1323 Κ and 1573 Κ a n n e a l i n g ( F i g u r e 2) i n d i c a t e s d i s s o l u t i o n of carbon i n t o the bulk. Auger E l e c t r o n S p e c t r o s c o p y . The dominant Ru MNN and c a r b o n KLL peaks o v e r l a p s t r o n g l y . A carbon f i n e s t r u c t u r e peak a t 249 eV i s p a r t i a l l y r e s o l v e d , however. Up t o an a n n e a l i n g temperature o f about 600 K , t h i s peak p o s i t i o n from 1.1 monolayers of carbon s t a y s c o n s t a n t , but i t f a l l s t o 245.5 eV a f t e r a n n e a l i n g a t 600 to 900 K . T h i s change a g a i n corresponds to the temperature r e g i o n i n w h i c h t r a n s f o r m a t i o n to the dense c a r b o n phase o c c u r s . The s h i f t i s c o n s i s t e n t w i t h assignment o f the dense phase t o a g r a p h i t i c s t r u c ­ t u r e (7) . 2

2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

SOLID STATE CHEMISTRY IN CATALYSIS

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch020

342

380

387.5

475

562.5

650

TEMPERATURE (K) Figure 1. Thermal desorption of CO following a 15 L C^H^ exposure at 323 K, heating to the temperature indicated, cooling to 323 K, and exposing to 9.0 L CO.

A Β

M573 Κ 900

525

750

875

1200

TEMPERATURE (Κ) Figure 2 . Temperature programmed oxidation spectra (CO signal) following a 1.0 L o 4 dose, heating to the temperatures indicated, cooling to 323 K, and exposing to 6.0 L 0 . C

H

?

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

20.

Structure and Reactivity of C on Ru(001)

L A U D E R B A C K A N D DELGASS

Secondary I o n Mass S p e c t r o m e t r y . The s u r f a c e s t r u c t u r a l changes i m p l i e d by the d a t a d i s c u s s e d so f a r i n f l u e n c e a v a r i e t y o f SIMS i o n s . We d i s c u s s h e r e a SIMS experiment designed s p e c i f i c a l l y to examine e f f e c t s o f temperature on c a r b o n m o b i l i t y and the f o r m a t i o n of dense phase carbon i s l a n d s on the s u r f a c e by m o n i t o r i n g carbon i s o t o p e ^ mixing. The loY^mass n e g a t i v e i o n spectrum of O.5 monolayers of C d e p o s i t e d from C H , a t 435 Κ shows peaks a t 1 2 , 1 6 , and 24 amu w i t h o n l y minor c o n t r i b u t i o n s a t 1 3 , 25, and 26. The h i g h e r exposure temperature was used i n these experiments t o e l i m i n a t e the m o l e c u l a r s p e c i e s on the s u r f a c e s i n c e fragments from those s p e c i e s c o m p l i c a t e i n t e r p r é t â t j g n of the^spectra. The s t r o n g peaks a t 12 and 24 c o r respond to 2 2. H i m p u r i t i e s a t 25 and 26 are C^H and ^2' ° ~ P k t 16 i s s t r o n g even a t the v e r y low oxygen coverages p r e s e n t because o f the h i g h SIMS s e n s i t i v i t y f o r oxygen. Most i m p o r t a n t l y , these d a t a show t h a t the background due to h y d r o c a r b o n c o n t a m i n a t i o n i s low i n the 25, 26 amu r e g i o n . The i s o t o p e m i x i n g experiments were conducted as f o l l o w s . I n Case A, 2 L of C H and 3 L o f C H , were adsorbed s e q u e n t i a l l y a t 4 3 5 ° Κ to g i v e e q u a l coverage o f carbon from e a c h . The s u r f a c e was then a n n e a l ­ ed by h e a t i n g t o a d e s i r e d temperature a t 6 K / s e j a n d c o o l i n g to 435 K . I n Case B , the sample was exposed t o 2 L of O A f 5 ^ 5 Κ, annealed a t 923 K , c o o l e d to 435 K , exposed t o 3 L o f 2 A' annealed a g a i n to the temperature c o r r e s p o n d i n g t o t h a t f o r Case A . S i n c e p r o x i m i t y i s e s s e n t i a l f o r c l u s t e r i o n f o r m a t i o n i n SIMS ( 8 , 9 ) , the m o b i l i t y and s t r u c t u r e of the c a r b o n l a y e r i s r e f l e c t e d i n t h e ^ i s o t o n i c c o m p o s i t i o n of the C i o n s . W i t h the e q u a l coverages of C and C used i n t h e s e e x p e r i m e n t s , a random d i s t r i b u t i o n o f carbon would y i e l d r e l a t i v e i n t e n s i t y o f 1:2:1 f o r masses 2 4 : 2 5 : 2 6 . Complete i s o l a t i o n would y i e l d 2 : 0 : 2 f o r the c o r r e s p o n d i n g i n t e n ­ sities. F i g u r e 3 d i s p l a y s the 2 5 ^ 2 4 26^ r a t i o s as a f u n c t i o n o f a n n e a l i n g temperature f o r both Cases A and B . Examples of the C d a t a a r e shown i n the i n s e t . For Case A, the peak a r e a r a t i o o f 25 t o (24 + 26) goes from O.75 a t 435 Κ t o n e a r l y 1 a t 818 K . The s u b s t a n t i a l i s o t o p e m i x i n g a t low temperature shows t h a t m o l e c u l a r e m i s s i o n of i n t a c t C from the o r i g i n a l e t h y l e n e m o l e c u l e does not o c c u r . I t i s i n t e r e s t i n g , however, t h a t the i s o t o p e m i x i n g i s not complete a t t h e l o w e r a n n e a l ­ i n g t e m p e r a t u r e s . T h i s s u g g e s t s t h a t p r o x i m i t y of the p a r e n t carbons i s p r e s e r v e d a t l o w temperature and s u p p o r t s the c o n c l u s i o n t h a t the c a r b o n l a y e r i s immobile a t temperatures below about 550 K . The much lower e x t e n t o f atomic m i x i n g a t 435 Κ f o r c a s e Β r e l a t i v e to case A c l e a r l y demonstrates t h a t a n n e a l i n g to 923 Κ a l t e r s the d i s t r i b u t i o n o f c a r b o n on the s u r f a c e to prevent c l o s e p r o x i m i t y of much o f the annealed carbon w i t h the a d d i t i o n a l c a r b o n subsequently deposited. This i s consistent with previously described r e s u l t s i n d i c a t i n g the f o r m a t i o n of h i g h d e n s i t y carbon i s l a n d s upon a n n e a l i n g above 763 Κ s i n c e i n t h a t c a s e t h e annealed carbon can be i n c l o s e p r o x i m i t y to the carbon d e p o s i t e d subsequent to a n n e a l i n g o n l y a l o n g i s l a n d edges. The i n c r e a s e i n the peak a r e a r a t i o of 25 to (24 + 26) from O.4 a t 435 Κ t o a p l a t e a u of O.6 a t 670 to 923 Κ c o r r e s p o n d s t o the c a r b o n phase t r a n s i t i o n p r e v i o u s l y d i s c u s s e d . The r a p i d i n c r e a s e i n the e x t e n t o f m i x i n g to the s t a t i s t i c a l l y complete l e v e l as the a n n e a l i n g temperature i n c r e a s e s from 1000 Κ t o about 1450 Κ c o r r e s p o n d s to the temperature r e g i o n where c a r b o n d i f f u s e s i n t o the b u l k . This suggests that during annealing at these 2

a

c

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch020

343

2

4

n

d

C

T

T l i e

h

e

s

e a

m

a

a

2

2

c

H

C

H

A

2

I

I

+

I

i

n

t

e

n

s

i

t

v

2

2

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

N

D

344

S O L I D STATE C H E M I S T R Y IN CATALYSIS

h i g h temperatures the h i g h d e n s i t y carbon phase becomes u n s t a b l e , a l l o w i n g i n d i v i d u a l c a r b o n atoms to m i g r a t e i n d e p e n d e n t l y over the s u r f a c e as w e l l as to d i f f u s e i n t o the b u l k t h e r e b y i n c r e a s i n g the e x t e n t o f i s o t o p i c r a n d o m i z a t i o n i n b o t h the low and h i g h d e n s i t y carbon phases. Carbon L a y e r s from CO CO D i s s o c i a t i o n . I t i s w e l l e s t a b l i s h e d i n the l i t e r a t u r e and i m p l i c i t i n the d a t a a l r e a d y d i s c u s s e d t h a t CO adsorbs m o l e c u l a r l y on Ru(OOl) a t temperatures below 350 K . D i s s o c i a t i o n o f CO i s to be expected a t h i g h e r t e m p e r a t u r e s , however, and i s thought t o be a key s t e p i n c a t a l y t i c methanation ( 1 0 - 1 2 ) . F i g u r e 4 shows d i r e c t SIMS e v i d e n c e f o r CO d i s s o c i a t i o n a t 520 K . S p e c t r u m A c o r r e s p o n d s to m o l e c u l a r a d s o r p t i o n and shows the RuCO and R u C 0 i o n s c h a r a c t e r i s ­ t i c o f m o l e c u l a r CO on the s u r f a c e ( 1 3 ) . Spectrum Β shows t h a £ a l o n g C 0 exposure a t 520 Κ d e p o s i t s c a r b o n , as i n d i c a t e d by RuC and R u ^ C . , b u t l e a v e s e s s e n t i a l l y no m o l e c u l a r CO s i n c e the RuCO s i g n a l i s n e g l i g i b l e . The absence of m o l e c u l a r C 0 a t t h i s temperature i s as expected from F i g u r e 1. The absence of RuO i n spectrum Β shows t h a t oxygen does not accumulate i n the s u r f a c e l a y e r and s u g g e s t s t h a t i t i s removed e i t h e r by r e a c t i o n w i t h CO to form C0~ or by d i f f u s i o n i n t o the b u l k . R e a c t i o n p r o b a b i l i t i e s f o r f o r m a t i o n of c a r b o n from CO were found to be 4.9x10 , 3.7x10 and 2.1x10 a t 435 K , 520 Κ and 815 Κ r e s p e c t i v e l y . A coverage o f n e a r l y 1/2 monolayer was found a f t e r 480 L exposure to CO a t 435 K . Temperature programmed o x i d a t i o n experiments showed t h i s c a r b o n l a y e r to be s i m i l a r t o t h a t formed from C^H^ a t e q u i v a l e n t c o v e r a g e s . A n n e a l i n g t o 663 Κ produced the h i g h d e n s i t y carbon phase ( 1 4 ) . I n t e r a c t i o n w i t h D . Thermal d e s o r p t i o n s p e c t r a f o r coadsorbed CO and c o n f i r m e d l i t e r a t u r e f i n d i n g s (15,16) t h a t CO d i s p l a c e s D from the s u r f a c e and TDS peak p o s i t i o n s do not s h i f t when b o t h gases a r e p r e s e n t on the s u r f a c e . SIMS s p e c t r a a l s o show no D/CO combina­ t i o n i o n s under t h e s e c o n d i t i o n s . As shown i n F i g u r e 5 , however, the D TDS spectrum i s c l e a r l y s h i f t e d to h i g h e r temperature by the p r e s ­ ence o f s u r f a c e c a r b o n from d i s s o c i a t e d CO. The CO p r e a d s o r p t i o n was done a t 425 Κ to e l i m i n a t e the presence o f m o l e c u l a r CO on the surface. The h e a t i n g r a t e f o r TDS o f D was 65 K / s e c . These r e s u l t s i n d i c a t e t h a t the presence o f s u r f a c e carbon c r e a t e s a new d e u t e r i u m a d s o r p t i o n s t a t e h a v i n g a s i g n i f i c a n t l y h i g h e r b i n d i n g energy than t h a t f o r the c l e a n s u r f a c e . The r e l ­ a t i v e l y narrow and symmetric d e s o r p t i o n peaks and the i n v a r i a n c e o f the peak maximum w i t h carbon c o v e r a g e , f u r t h e r m o r e , i n d i c a t e t h a t o n l y a s i n g l e new s t a t e i s c r e a t e d by the presence of s u r f a c e c a r b o n , as opposed to a d i s t r i b u t i o n o f a d s o r p t i o n s t a t e s o f d i f f e r e n t b i n d ­ ing energies. T h i s s u g g e s t s the p o s s i b i l i t y t h a t the new s t a t e may be due to the f o r m a t i o n of a s p e c i f i c d e u t e r o - c a r b o n complex h a v i n g a w e l l d e f i n e d C-D b i n d i n g e n e r g y . T h i s seems p l a u s i b l e s i n c e i f the new a d s o r p t i o n s t a t e were c r e a t e d by m o d i f i c a t i o n of m e t a l a d s o r p t i o n s i t e s by a t h r o u g h - m e t a l type e l e c t r o n i c i n t e r a c t i o n , then one would g e n e r a l l y expect a d i s t r i b u t i o n o f b i n d i n g s t a t e s due t o a n o n ­ u n i f o r m l o c a l d i s t r i b u t i o n o f atoms w h i c h would change w i t h c a r b o n coverage. The e x i s t e n c e o f s t r o n g C-D i n t e r a c t i o n i s supported by SIMS +

+

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch020

2

+

+

?

2

2

2

+

d a t a w h i c h show the p r e s e n c e of RuCD and the l o s s o f t h i s i o n i n c o n c e r t w i t h the t h e r m a l d e s o r p t i o n o f D ( 1 4 ) . ?

In Solid State Chemistry in Catalysis; Grasselli, R., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch020

20.

Structure and Reactivity of C on Ru(001)

LAUDERBACK AND DELGASS

300

600

900

1200

TEMPERATURE

1500

(K)

F i g u r e 3 . I n t e n s i t y of the 25 amu peak d i v i d e d by the sum o f the i n t e n s i t i e s o f the 24 and 26 amu peaks, i n d i c a t i n g the degree o f i s o t o p e m i x i n g i n SIMS o f the i o n as a f u n c t i o n o f ^ n n e a l i n g f o r Case A , s e q u e n t i a l a d s o r p t i o n o f e q u a l amounts o f C«H, and 24 Î3 ' 24 f o l l o w e d by a d d i t i o n of ^2^4* l i n d i c a t e s n e g a t i v e i o n SIMS s p e c t r a f o r Case A (upper) and Case Β (lower) a f t e r a n n e a l i n g a t 663 K . C

H

d

Case

B

C

n

s

e

H

annealed

t0

923

K

t

Ru2

Ru2C13 O»100>

RuC23

ω Lu

RuCieO ^ n >

RoC C»1BB> Rol80

ft*