Advances in Catalysis, Volume 26

ADVANCES IN CATALYSIS VOLUME 26

Advisory Board

G. K. BORESKOV Noaosihirsk, U.S.S.R.

M . BOUDART StanJord, Cali/imiu

P. H. EMMETT Baltimore, Maryland

G. NATTA Milan, Italy

M . CALVIN Berkeley, Cal~fimtia

W. JOST

J. HORIUTI Sapporo, Japan

Gottingen, Gcwnunj

P. W. SELWOOD Santa Barbara, California

ADVANCES IN CATALYSIS VOLUME 26

Edited by

D. D. ELEY The University Notringham, England

HERMANPINES

PAULB. WEISZ

Northwestern Vnioersity Euanston , Iliino is

Mobil Research and Development Corporaiion Princeton, N e n Jersey

1977

ACADEMIC PRESS NEW YORK

SAN FRANCISCO LONDON

A Subsidiary o f Harcourt Brace Jooanooich, Publishers

COPYRIGHT 0 1977, BY ACADEMIC PRESS, INC. ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM O R BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRLTING FROM THE PUBLISHER.

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(Inired Kingdom Edition published by ACADEMIC PRESS, INC. (LONDON) LTD. 24/28 Oval Road. London NWI

LIBRARY OF CONGRESS CATALOG CARD NUMBER:49-7755 ISBN 0-12-007826-0 PRINTED IN THE UNITED STATES OF AMERICA

Contents CONTRIBUTORS .................................................. PREFACE........................................................ S I R ERICK . RlDEAL (1890-1974) ................................... MICHAELPOLANY1 (1 891 1976) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

........

IX

........

X

. . .. . . . . . . .

xiii

~

. . . . . . . . rvii

Active Sites in Heterogeneous Catalysis G. A. SOMORJAI

I. 11. 111.

IV. V. VI.

........ Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Atomic Structure of Surfaces. Structures of Low and High Miller Index Crystal Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Techniques to Study the Relationship between Reactivity and the Structure and Composition of Surfaces in the Atomic Scale ............................. Chemisorption of Hydrocarbons on Low and High Miller Index Surfaces of Platinum, Iridium, and Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Reactions on Platinum Crystal Surfaces ..................... Active Sites for C-H, H- H, and C-C Bond Breaking on Platinum Crys

................................................... VII.

VIII.

IX. X. XI. XII.

e Carbonaceous Overlayer in Hydrocarbon Reactions on Platinum Surfaces . . . . . . .......... The Mechanism of the Dehydrogenation of Cyclohexane and Cyclohexene : Expanded Classification of Reactions According to Their Structure Sensitivity . . A Descriptive Model of Hydrocarbon Catalysis on Platinum Surfaces . . . . . . . . Theory of Low Coordination Number Active Sites on Surfaces . . . . . . . . . . . . . . Aspects of Enzyme Catalysis on Metal Surfaces ........................... Possible Correlations between Homogeneous and Heterogeneous Catalysis . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 5

16

28 39 53 54 56 58 60 64 65 66

Surface Composition and Selectivity of Alloy Catalysts

w.M. H. SACHTLER A N D R. A . V A N SANTI:N I. 11. 111. IV. V.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface Composition of Equilibrated Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Selectivity of Alloys in Hydrocarbon Reactions . . . . . . . . . . . . . . . . Ensemble and Ligand Etrects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....... V

69 71 100 115

vi

CONTENTS

Mossbauer Spectroscopy Applications to Heterogeneous Catalysis JAMES A. DUMESIC AND HENRIK TOPS~E 1. 11.

111. IV.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . Applications to Heter Concluding Remark Appendix I: Nuclear Data for Mossbauer Isotopes ........................ Appendix 11: Mossbauer Isotope Feasibility for Catalyst Studies . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

122

230 236 239

Compensation Effect in Heterogeneous Catalysis A. K . GALWW

I. I1 111. IV

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 Compensation Behav Compensation Behav Conclusions . . . . . . . . . . . . . . . . 307 Appendix I : Compen 31 1 Variations in Concentrations of Surface Reactants. ........................ Appendix I1 : Statistical Formulas Used in Linear Regression (Least Squares) .................................. 314 List ofSymbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 .................................. 315

Transition Metal-Catalyzed Reactions of Organic Halides with CO, Olefins, and Acetylenes R. F. HECK

I. 11. 111. IV. V.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbonylation Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Olefinic Substitution Reactions . ...................... Substitution Reactions of Termin cetylenes . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..............

Manual of Symbols and Terminology for Physicochemical Quantities and Units-Appendix Part II : Heterogeneous Catalysis

323 324

347 348

II

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 1 . Definitions and Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . 1 Catalysis and Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

351 353 353

CONTENTS

1.2 Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Composition, Structure and Texture of Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Catalytic Reactors . . ..................................... 1.5 Kinetics of Heterogeneous Catalytic Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I .6 Transport Phenomena in Heterogeneous Catalysis ............................ 1.7 Loss of Catalytic Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8 Mechanism of Catalytic Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... 1.9 Nomenclature of Catalytic Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 2. List of Symbols and Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3. Alphabetical Index . . . . . . . . . . . . . . . . . . . . . . . . . . ............. AUTHOR

INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.......................................................... CONTENTS OF PREVIOUS VOLUMES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SUBJECT INDEX

vi i 355 366 369 371 376 377 379 383 384 386 393 408 414

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Contributors Numbers in parentheses indicate the pages on which the authors’ contributions begin

ROBERTL. BURWELL,JR., Department of Chemistry, Northwestern University, Evanston, Illinois (351) JAMES A. DUMESIC,* StaufSer Laboratories of Chemistry and Chemical Engineering, Stanford University, Stanford, California (121) A. K. GALWEY, Department of Chemistry, Queen’s University, Belfast, Northern Ireland (247) R. F. HECK,Department of Chemistry, University of Delaware, Newark, Delaware (323) W. M. H. SACHTLER, KoninklijkelShetl-Laboratorium,Amsterdam (Shell Research B. V . ) , The Netherlands (69) G. A. SOMORJAI, Materials and Molecular Research Division, Lawrence Berkeley Laboratory and Department of Chemistry, University of Calijornia, Berkeley, Caltfornia (1) HENRIK TOPS~E Haldor , Topsije Research Laboratories, Lyngby, Denmark (121)

R. A. VAN SANTEN,KoninklijkelShell-Laboratorium,Amsterdam (Shell Research B. v.),The Netherlands (69)

* Present address : Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706. ix

ACTIVESITESIN HETEROGENEOUS CATALYSIS If we survey the recent history of heterogeneous catalysis on metals we recognize a period (1915-1925) when Langmuir laid stress on the role of the regular lattice, and another (1925-1935) when H. S. Taylor was vigorously propagating the concept of “active centers.” A third era (1935 onward) dates from the influence of E. K . Rideal (whose obituary we publish here), J. K. Roberts, and other members of the Cambridge School, which returned the emphasis to interpretations based on the surface lattice, and has continued right through into the modern surface physics era. Now a proponent of the latter, G. A. Somorjai, has provided definite evidence for the role of steps on surfaces as centers for chemisorption and catalysis at w r y /OM’ pressures. This important development is outlined in our first chapter, and the correlation of these results with those at high pressures, where the role of active sites is less apparent, is an important area of research. Similarly, going down from the atomic to the electronic level, we detect a dialectic between electronic energy band structure and localized surface bonding (or perhaps between physicist and chemist!), and W. M. H. Sachtler and R. A. van Santen present in their article a powerful analysis of alloy catalysis at this level. In their emphasis on surface ensembles, we can see a return to the idea that motivated Rienacker’s early studies on the effect of order-disorder changes on alloy catalyst activity. While our first two chapters lean heavily on work in UHV studies on single crystals, the third chapter by J. A. Dumesic and H. Topsde gives a review of the Mossbauer technique, which finds application to supported fine particles of a number of elements of interest in “catalysis,” and is therefore a modern technique (ESCA is another) that may be applied to industrial catalysts. The comprehensive character of this chapter will surely stimulate more laboratories to employ this technique. The compensation effect in heterogeneous catalysis is the subject of a timely review by A. K. Galwey. In his day M. Polanyi (whose obituary we also publish) had studied these relations with M. G. Evans and brought out their general nature, appearing as they do in both equilibrium and rate data. Exner in particular has focused attention on the possible effects of experimental error, and Galwey sets out the necessary tests for statistical significance of the selection that need to be proved in each case. The compensation effect is something of a trial to physical chemists-life would be much simpler without it, but it will not go away. X

PREFACE

xi

To remind ourselves that the proper objects of all catalyst research are more powerful and selective syntheses, we have R. F. Heck’s chapter describing a wide range of new organic halide reaction catalyses by metal carbonyls and related catalysts. Physics may be fun but chemistry is our bread and butter, and homogeneous catalysis is an area in which we must expect to give increasing space in our Advances in Catalysis in the future. Finally, we are happy to print the recent IUPAC Recommendations on Symbols and Terminology in Heterogeneous Catalysis, prepared for publication by Robert L. Burwell, Jr. Everybody writing papers in this area will want to consult this document, the first major move toward uniformity of presentation in our field. D. D. ELEY

SIRERICK. RIDEAL (1 890- 1974)

Sir Eric K. Rideal (1 890-1 974) Eric Keightley Rideal was born on April 11, 1890 at Sydenham, near London. His father was Samuel Rideal, D. Sc. Lond., a Fellow of University College, London, a leading consulting chemist of the day, and an authority on water supplies, whose name lives on in the Rideal-Walker test for disinfectants. Eric Rideal went to Oundle School and after a successful scholastic career entered Trinity Hall, Cambridge in 1907 with an Open Scholarship. At Cambridge he came under the influence of W. B. Hardy, at that time lecturer in physiology, who kindled his lifelong interest in surface chemistry. Graduating in 1910 with First Class Honours, Rideal went on to electrochemical studies on uranium, first at Aachen, then at Bonn, where he submitted his Ph.D. thesis in 1912 and graduated in 1913. His first job was to collaborate with his Cambridge friend, Dr. U. R. Evans, working in his father’s laboratory at 8 Victoria St., Westminster ; the two young men produced a neat electrochemical cell for estimation of ozone or chlorine in water and also conducted a survey of the fuel cell problem, which was published in 1921. At the outbreak of war, Rideal was in Ecuador working on their water supplies, and returning to England he joined the Artists’ Rifles, but was transferred to the Royal Engineers. In 1916 he was serving with the Australians at the Somme supervising their water supplies, but a severe attack of dysentery led to his being invalided out and attached to the Munitions Inventions Board, working in Donnan’s laboratory in University College, London. Here he started up on catalytic work, first on ammonia synthesis with Greenwood, Maxted, and Partington, and later collaborating with Hugh S. Taylor, recently returned from Princeton, on a catalyst (a mixture of iron and chromium oxides) to selectively oxidize carbon monoxide in the presence of hydrogen. In solitary nights in a gasworks in Wapping, Rideal and Taylor planned their major work “Catalysis in Theory and Practice,” which appeared in 1919. In 1919 on the advice of Professor James Kendall, Rideal was appointed Visiting Professor at the University of Illinois, returning in 1920 to the Humphrey Owen Jones Lectureship in physical chemistry at Cambridge and a Fellowship at his old College, Trinity Hall. On the boat he met the charming American lady, Peggy, whom he married in 1921. At this time he started research in Lowry’s laboratory, his first student being R. G. W. ...

XI11

XlV

SIR ERIC K . RIDEAL

Norrish. During the period to 1930 he supervised numerous research students in the areas of catalysis, photochemistry, homogeneous kinetics, electrochemistry, colloid science, and spectroscopy. From this period one might single out Bowden’s overvoltage studies and his Langmuir trough studies with Cary, Schofield, Schulman, and others for special mention. In the area of heterogeneous catalysis he attempted with Wansbrough-Jones to link the oxidation of platinum with its work function, an early effort in the “electron-factor’’ area of heterogeneous catalysis, unfortunately not followed up. In 1930 Rideal was elected Humphrey Owen Jones Professor of Colloid Science, and moved his research to a new laboratory in Free School Lane, which became famous as one of the world centers of activity in its area. Here he was joined by J. K. Roberts, a pioneer of chemisorption studies on clean metals, whose influence, exerted through his questions at the weekly colloquia, influenced the studies of Rideal’s students, such as Bosworth, Twigg, Orr, Craxford, Herington, Barrer, and numerous others, in the adsorption and catalysis area. For a time A. and L. Farkas worked with Rideal, introducing parahydrogen and deuterium exchange techniques, which Rideal employed to good purpose. At this time Rideal put forward the idea of reaction occurring between chemisorbed atoms and physically adsorbed molecules (the Rideal mechanism). Rideal, a disciple of Langmuir, may be regarded as one of the pioneers of surface science in the United Kingdom. At the same time he was carrying out numerous investigations in the colloid area and stimulating polymer studies under H. W. Melville. With the outbreak of war the Colloid Science Department largely went over to war work, and after the war, in 1946, Rideal left for London, to become Fullerian Professor and Director of the Davy Faraday Laboratory at the Royal Institution. Trapnell’s chemisorption studies date from this period. In 1950 he moved on to a Chair at King’s College, London, where he was joined by A. J. B. Robertson and J. T. Davies. In 1955 he retired, joining Imperial College as a Senior Research Fellow in his old pupil’s, R. M. Barrer, department. It was here in 1968 that he published his book “Concepts in Catalysis,” fifty years after his book with H. S. Taylor. Rideal was a great scientific leader who through his own activities and those of his numerous successful pupils has exerted a profound influence on world science. His father undoubtedly influenced the breadth of his interests and his enthusiasm for applied sciences, and W. B. Hardy helped to focus his research interests and to shape his scientific attitudes. Rideal’s characteristics were friendliness and helpfulness, and an enthusiasm for colloid science research which was infectious in the extreme. He worked hard and served the Chemical Society, the Society of Chemical Industry, and the Faraday Society as President, in each case a fruitful period of office. For his service to the Government he was made M.B.E.

SIR ERIC K . RIDEAL

xv

in 1919, and knighted in 1951. He valued his contacts with scientists in the United States and visited that country most years of his working life. He was born and died in the same year as Sir Hugh S. Taylor. The two lived and worked in friendly rivalry, the one favoring catalytic activity occurring on homogeneous lattice planes, the other on active sites, both hypotheses set up by Irving Langmuir, and both still attracting animated debate. In 1949 Rideal joined Komarewsky and Frankenburg as the founding Editors of Aduanres in Catalysis. There is something peculiarly satisfying in the thought of a sapper of World War I joining in with a Russian cavalry officer and a holder of the Iron Cross, First Class, after World War I1 in what has proved to be an enduring scientific project. D. D. ELEY

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Michael Polanyi (1 89 1-1 976) Michael Polanyi was born in Hungary and qualified as a medical doctor, coming into physical chemistry via a pre- 1914 interest in biochemistry. While serving in the Austro-Hungarian army in the First World War he commenced publication of his “Potential Theory of Adsorption of Vapours on Solids,” and developed his interest in quantum theory through a correspondence with Einstein. The potential theory was overshadowed by the Langmuir monolayer theory and its BET developments, but Polanyi’s theory was restored to an important place in treating physical adsorption by Dubinin (1960) and at present is being actively applied to adsorption from solution by two groups of workers. In 1920 Polanyi joined Herzog’s Institute of Fiber Chemistry in Berlin, and in three short years he and his group (which included H. Mark, E. Schmid, and K. Weissenberg) invented the rotating-crystal X-ray diffraction method, elucidated the fiber diagram of cellulose, and proposed the concept of “dislocations” to explain slip in single crystals of metal. In 1923 Haber admitted him as a Member of the Kaiser Wilhelm Institute for Physical Chemistry thus allowing Polanyi to return to his major interest of kinetics. Here he developed quantitative methods for studying the sodium vapor reactions recently examined by Haber and Zisch, turning the area into a keystone of chemical kinetics. He had already correctly analyzed the kinetics of the hydrogen-bromine reaction by the stationary-state method (1919), and published the square-term formula (1920) for activation of molecules. Now he established structure-rate relationships for the reaction of sodium with organic halides, leading to an explanation of the Walden inversion by anions in solution in terms of his negative mechanism (equivalent to the S N mechanism). ~ On the theoretical side there was the PolanyiWigner formulation of unimolecular decomposition rates, the PolanyiEyring semiempirical potential energy surface, the transition state theory with M. G. Evans (1935), and with R. A. Ogg, Jr. (1934, 1935) the electronswitch theory of ionogenic reactions, applied with Juro Horiuti (1936) to hydrogen overvoltage and developed along general lines with M. G. Evans (1938). These considerations provided a basis in terms of which Polanyi considered heterogeneous catalysis in terms of dissociation of molecules on surface free valencies. His conclusion that there was an optimum adsorption xvii

xviii

MICHAEL POLANYI

energy for maximum rate later became familiar through Balandin’s writings on the “volcano-shaped curve.” On arriving at Manchester in 1933, Polanyi and Horiuti discovered the exchange of deuterium with water catalyzed by metals and enzymes, and went on to investigate deuterium-benzene on supported nickel, inventing the mechanism of the “half-hydrogenated state.” With A. Szabo he made the first H,OI8 tracer experiment, establishing the mechanism of ester hydrolysis. He also set up early work on radioactive I in alkyl iodide reactions, but the war in 1939 interrupted this, his assistant, J. L. Tuck leaving for work with Lindemann, subsequently going on to Los Alamos. During the 1939-1945 war, Polanyi, with A. G. Evans, H. A. Skinner, and others studied the isobutene polymerization catalyzed by boron trifluoride, establishing the initiation process as a proton transfer involving the “cocatalyst” water. At the same time Polanyi was developing his earlier studies on the pyrolytic method for carbon bond energies (later continued by M. Swarcz). In 1948 Polanyi relinquished his Chemistry Chair to become Professor of Social Sciences at Manchester. The background to this move came in the 193Os, when Polanyi became preoccupied with the problems of high unemployment leading to totalitarian political regimes. In 1936 Keynes published his ideas about controlled degree of inflation (budgetary deficit) being used to secure full employment. Polanyi adopted these ideas enthusiastically and engaged in their public exposition, first by a film with Mary Field “Unemployment and Money,” and after the war with his book “Full Employment and Free Trade.” Later on he moved into philosophy with his magnum opus “Personal Knowledge.” It is a matter of interest that his elder brother Karl (1886-1964), the economic historian, was as convinced a critic of the free market as Michael was its supporter. Paul Ignotus, comparing the two, called Michael the most moderate of radicals, and Karl the most radical of radicals. Polanyi’s outstanding success in the field of homogeneous kinetics was based on his powerful grasp of the concepts of wave mechanics and statistical thermodynamics, in which he predated most contemporary physical chemists. The most modest of men, he once commented that he might have done more if he had known more mathematics, but it is difficult to see this, his achievements were so great. Coming to surface catalysis late on in life, he showed less interest than heretofore in experimental detail, and this undoubtedly limited his achievements in this area. Thus in 1937-1939 he had good ideas about phthalocyanine crystals and metal oxides, but the experimental side got bogged down and finally stopped with the onset of the war. At that time his philosophy was that a proper test of a theory of catalysis was that it should predict a new catalyst. But in the case of phthalocyanine the first positive results were not reproducible, and only after an interval

MICHAEL POLANYI

xix

of some thirty years have we learned that Polanyi’s thoughts about phthalocyanine were basically correct, if we allow for surface polymer being present and more active than bulk monomer. Polanyi was awarded many distinctions and on retirement in 1958 became a Senior Research Fellow of Merton College, Oxford. Here he continued his social researches and writings, active until the last few months of his life. Reasons of space have precluded mentioning more than a few of his collaborators and pupils by name. He had a profound effect on the thinking of these pupils, and his influence will continue to be felt through his own and their writings for many years to come. One and all they remain grateful for their contact at a formative age with Polanyi’s outstanding intellect. Michael Polanyi is survived by his wife, Magda, and by their son, John, Professor of Physical Chemistry at Toronto University, who is also well known for his work in reaction kinetics. D. D. ELEY

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Active Sites in Heterogeneous

Catalysis* G. A. SOMORJAI Materials and Molecular Research Division Lawrence Berkeley Laboratory and Department of Chemistry University of California Berkeley, California

I. Introduction . . . . . . . . . . . . . . . . . . . . . . .

.

.

.

.

2

11. The Atomic Structure of Surfaces. Structures of Low and High Miller Index

Crystal Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . A. The Atomic Surface Structure of the Clean (1 1 1) Platinum Crystal Face . B. The Atomic Surface Structure of the Clean (100) Platinum Crystal Face . C. The Atomic Surface Structure of the Clean (I 10) Platinum Crystal Face . D. The Atomic Surface Structure of High Miller Index Surfaces. . . . . 111. Techniques to Study the Relationship between Reactivity and the Structure and Composition of Surfaces in the Atomic Scale . . . . . . . . . . . A. Static Techniques . . . . . . . . . . . . . . . . . . . . . . . B. Transport Techniques . . . . . . . . . . . . . . . . . . . . . C. Cleaning and Preparation of Single-Crystal Surfaces . . . . . . . . IV. Chemisorption of Hydrocarbons on Low and High Miller Index Surfaces of Platinum, Iridium, and Gold . . . . . . . . . . . . . . . . . . . . A. Chemisorption of Hydrocarbons on the Pt(ll1) and Pt(100) Crystal Faces . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Summary of Experimental Findings . . . . . . . . . . . . . . . C. Hydrocarbon Chemisorption on High Miller Index (Stepped) Platinum Surfaces . . . . . . . . . . . . . , . . . . . . . . . . . . . D. The Chemisorption of Hydrocarbons on Gold and Iridium Crystal Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Chemical Reactions on Platinum Crystal Surfaces . . . . . . . . . . . A. The H,-D, Exchange on Platinum Crystal Surfaces at Low Pressures . . B. Dehydrogenation and Hydrogenolysis of Cyclohexane on Platinum Crystal Surfaces at Low Pressures . . , . . . . . . . . . . . . . C. Dehydrogenation and Hydrogenolysis of Cyclohexene on Platinum Crystal Surfaces at Low Pressures . . . . . . . . . . . . . . . . D. Hydrocarbon Reactions on Platinum Crystal Surfaces at High Pressures (1 -lo3 Tom). Cyclopropane, Cyclohexane, and n-Heptane . . . . . .

5 8 9 11 12 16 16 25 27

28 28 29 35

31 39 39 43 49

51

* This work was supported by the U.S. Energy Research and Development Administration. 1

2

G. A. SOMORJAI

VI. Active Sites for C- H, H-H, and C-C Bond Breaking on Platinum Crystal Surfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII. The Role of the Carbonaceous Overlayer in Hydrocarbon Reactions on Platinum Surfaces. . . . . . . . . . . . . . . . . . . . . . . . . VIII. The Mechanism of the Dehydrogenation of Cyclohexane and Cyclohexene: Expanded Classification of Reactions According to Their Structure Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1X.A Descriptive Model of Hydrocarbon Catalysis on Platinum Surfaces . . X. Theory of Low Coordination Number Active Sites on Surfaces . . . . . Active Sites on Nonmetallic Surfaces . . . . . . . . . . . . . . . . XI. Aspects of Enzyme Catalysis on Metal Surfaces . . . . . . . . . . . . XII. Possible Correlations between Homogeneous and Heterogeneous Catalysis . References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

53

54

56 58 60 63 64 65 66

1. Introduction

The catalytic function of surfaces is exhibited in two ways. The catalyst reduces the potential energy barrier (activation energy) in the path of the chemical reaction, which is otherwise thermodynamically feasible, by temporarily forming chemical bonds with the adsorbing molecules. The ability of the surface to break some of the strong chemical bonds of the reactant molecules (for example, H-H, C-H, C-C, C=O, N=N bonds), bind them with strong enough surface bonds so that the residence time of the adsorbate is sufficiently long for the necessary chemical rearrangement to occur, and then permit the release of the product molecules to make the various active surface sites available for new reactions is one of the essential features of heterogeneous catalysis. It is well known that too strong chemical bonds between the surface atom and the reaction intermediate lead to permanent blocking of the catalyst surface, i.e., poisoning. If the chemical bonds between the reactant molecules and the surface are too weak, either the crucial bond breaking processes will not be permitted to occur or the adsorbate residence time becomes too short for the necessary and sometimes complex chemical rearrangements to take place. There is another equally important function of a good catalyst surface that leads to selectivity. A proper catalyst will facilitate the formation of only one out of many possible reaction products. There may be many thermodynamically possible paths that could yield a wide variety of product molecules. However, the proper catalyst may produce only one product, selectively. This enzymelike characteristic of heterogeneous catalysis has not been receiving as wide attention as the ability of the catalyst to lower the activation energy of the chemical reaction by forming temporary chemical

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bonds with the reactants. Both of these features are the properties of most technologically important working catalysts, especially those that exhibit ability to catalyze the production of structurally complex molecules or catalyze complex molecular rearrangement. Identification and study of the “active sites” where chemical bond scission or rearrangement occurs, so crucial to the working of a catalyst, requires that we investigate the structure and the chemical composition of the working catalyst on the atomic scale. Ideally, we would like to inspect each nonidentical surface site and determine its structure and chemical composition while the chemical reaction is taking place. Over the past 10 years a multitude of new techniques has been developed to permit characterization of catalyst surfaces on the atomic scale. Lowenergy electron diffraction (LEED) can determine the atomic surface structure of the topmost layer of the clean catalyst or of the adsorbed intermediate ( I ) . Auger electron spectroscopy (2) (AES) and other electron spectroscopy techniques (X-ray photoelectron, ultraviolet photoelectron, electron loss spectroscopies, etc.) can be used to determine the chemical composition of the surface with the sensitivity of 1% of a monolayer (approximately l O I 3 atoms/cm2). In addition to qualitative and quantitative chemical analysis of the surface layer, electron spectroscopy can also be utilized to determine the valency of surface atoms and the nature of the surface chemical bond. These are static techniques, but by using a suitable apparatus, which will be described later, one can monitor the atomic structure and composition during catalytic reactions at low pressures ( < Torr). As a result, we can determine reaction rates and product distributions in catalytic surface reactions as a function of’surface structure and surface chemical composition. These relations permit the exploration of the mechanistic details of catalysis on the molecular level to optimize catalyst preparation and to build new catalyst systems by employing the knowledge gained. Ideally, we would like to study the structure and composition of supported, dispersed catalyst particles in the same configuration used in the chemical technology. However, the determination of the atomic surface structure of the catalyst particle that is situated inside the pores of the high-surface-area support by LEED, for example, is not possible. This technique requires the presence of ordered domains 200 8, or larger to obtain the sharp diffraction features necessary to define the surface structure. Even Auger electron emission, which is the property of individual atoms and can even be obtained from liquid surfaces, can only be employed for studies of supported catalyst surfaces with difficulty. Identification of the active sites does require the determination of the structure and composition of the catalyst surface, however. To avoid the difficulties of carrying out these experiments on supported

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catalyst surfaces, we have adopted the strategy in our studies of catalyst surfaces that is used successfully in synthetic organic chemistry and many other fields of chemistry as well. We shall prepare the various building blocks of the catalyst surface and study them separately. Then we put the parts together and the resultant structure should have all of the properties of the working catalyst particle. Just as in the case of synthetic insulin or the B I 2 molecule, the proof that the synthesis was successful is in the identical performance of the synthesized and “natural” products. Our building blocks are crystal surfaces with wellcharacterized atomic surface structure and composition. Cutting these crystals in various directions permits us to vary their surface structure systematically and to study the chemical reactivity associated with each surface structure. If we do it properly, all of the surface sites and microstructures with unique chemical activity can be identified this way. Then, by preparing a surface where all of these sites are simultaneously present in the correct configurations and concentrations the chemical behavior of the catalyst particle can be reproduced. The real value of this synthetic approach is that ultimately one should be able to synthesize a catalyst that is much more selective since we build into it only the desirable active sites in a controlled manner. In our modeling approach to heterogeneous catalysis we carry out studies on well-characterized crystal surfaces first in the following sequence: structure of crystal surfaces and of adsorbed gases

tl surface reactions on crystals at low pressures (I

Torr)

tJ surface reactions on crystals at high pressures (1 03- lo5 Torr)

L t reactions on dispersed catalysts

First we study the surface structure and chemisorption characteristics of crystals cut along different crystallographic orientations. Then a wellchosen chemical reaction is studied at low pressure to establish correlations between reactivity and surface structure and composition. Below Torr the surface can be monitored continuously during the reaction with various electron spectroscopy techniques. Then the same catalytic reaction is studied at high pressures (1 -100 atm) and the pressure dependence of the reaction rate is determined using the same sample over the nine orders of magnitude range. Finally, the rates and product distributions that were determined at

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high pressures on single-crystal surfaces are compared with the reactivity of polydispersed small-particle catalyst systems. At low pressures a quadrupole mass spectrometer is used as a detector of both the chemical reaction rates and the product distributions (3).At high pressures a gas chromatograph is employed, which is as sensitive as a mass spectrometer that is used at low pressures (4). Our experiments indicate that small-surface-area (approximately 1 cm2) single-crystal catalyst samples can readily be used in studies product molecules/surface as long as the reaction rate is greater than atom/sec. The rate so defined is commonly called “turnover number” in the field of catalysis. Most of the important catalytic reactions of hydrocarbonshydrogenation, dehydrogenation, oxidation, isomerization, dehydrocyclization, hydrogenolysis-have rates usually greater than the detection limit, even at low pressures. Using this approach to study heterogeneous catalysis on the atomic scale, we have investigated the mechanism of hydrocarbon catalysis by platinum surfaces. We shall describe in detail the results of these studies, which are pertinent in determining the nature of the active sites on the surface of this metal. We shall show how the results obtained for platinum may be extrapolated to other catalyst systems. Finally, we shall present a model of metal catalysis that has been emerging from our studies of platinum surfaces.

II. The Atomic Structure of Surfaces. Structures of Low and High Miller Index Crystal Surfaces Figure 1 shows the schematic diagram of a solid surface. The surface is clearly heterogeneous on an atomic scale. There are atoms in various positions that are distinguishable by their number of nearest neighbors:

TERRACE NATOMIC STEP

VACANCY

FIG. 1. Model of a heterogeneous solid surface depicting different surface sites. These sites are distinguishable by their number of nearest neighbors.

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atoms in steps and in kinks, adatoms, and point defects, vacancies in the surface. Experimental evidence for the existence of all of these surface species has come mostly from LEED and field ion microscopy studies. Of the surface sites shown schematically in Fig. 1, the terrace sites, the kink, and the step sites are perhaps the most important for purposes of heterogeneous catalysis. The concentration of these sites can be large, from 5 to over 50% of a monolayer -10" sites/cm2), while the concentration of adatoms and vacancies is very small, < lo-'%, even at the melting point of most metals. By cutting single crystals in various crystallographic directions, we can change the relative concentrations of atoms in terraces and steps and kinks. Figure 2 shows a stereographic triangle of an fcc metal. At the corners the (1 1 l), (loo), and (1 10) crystal faces are shown. These are the lowest surfacefree energy, highest atomic-density crystal orientations. When crystals are cut to these low Miller index orientations, most of the surface atoms will be in terrace positions. The surface will be relatively smooth on the atomic scale and most of the surface atoms have the highest possible coordination number or number of nearest neighbors. One of these surfaces, the (111) face of platinum, is shown schematically in Fig. 3a. O n cutting high Miller index surfaces at some angle with respect to the low Miller index surface, the

FIG. 2. Stereographic triangle indicating various crystallographic orientations of fcc solid surfaces using Miller index notations.

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FIG.3 . LEED patterns and schematic representations of the surface configurations of platinum single-crystal surfaces. (a) Pt( 11 I ) containing less than 1 O I 2 defects/cm2, (b) Pt(557) face containing 2.5 x loL4step atoms/cm2 with an average spacing between steps of 6 atoms, and (c) Pt(679) containing 2.3 x IOl4 step atoms/cm2 and 7 x loi4 kink atoms/cm2 with an average spacing between steps of 7 atoms and between kinks of 3 atoms.

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atomic surface structure changes completely. A (557) surface, for example, exhibits periodic steps of monatomic height separated by terraces that are 6 atoms wide. This is shown schematically in Fig. 3b. The terraces have (1 1 1) orientations since they are cut closest to the (111) crystal face, while the steps have (100) orientations since the high Miller index surface is in the direction of the (100) crystal face. The high and low Miller index surfaces, their atomic structure, and their chemistry will be discussed in some detail in this section. On cutting a crystal surface in the middle of the stereographic triangle, a surface structure that exhibits a large density of kinks in the steps will be produced. One of these high-kink-density surfaces is shown schematically in Fig. 3c. Platinum crystal surfaces that were prepared in the zones indicated by the arrows at the sides of the triangle are thermally unstable. These surfaces, on heating, will rearrange to yield the two surfaces that appear at the end of the arrows. There is reason to believe that the thermal stability exhibited by various low and high Miller index platinum surfaces is the same for other fcc metals. There are, of course, differences expected for surfaces of bcc solids or for surfaces of solids with other crystal structures. We have found that the chemical reactivity of low Miller index surfaces of platinum is very different from that of high Miller index stepped or kinked surfaces, and that the reactivities of surfaces with steps and with kinks in the steps are very different from each other (5). Surface irregularities, i.e., atomic positions of the surface with different numbers of nearest neighbors, exhibit different chemical activities (chemical bond breaking or rearrangement abilities) depending on their configuration. Thus, it is appropriate to discuss these various surfaces separately. First we shall discuss the atomic structure of low Miller index surfaces, and then the atomic surface structures of high Miller index stepped and kinked surfaces.

A. THEATOMIC SURFACE STRUCTURE OF THE CLEAN (1 1 1 ) PLATINUM CRYSTAL FACE LEED studies have revealed that the atoms in this platinum surface are in the positions expected from the projection of the X-ray unit cell to the surface (5).The diffraction pattern that is exhibited (Fig. 4)clearly indicates a sixfold rotational symmetry that is expected in such a surface. Calculations of surface structure from LEED beam intensities indicate that atoms are in those positions in the surface layer (with respect to the second layer) as indicated by the X-ray unit cell within 5% of the interlayer distance (6,7).

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FIG.4. Diffraction pattern and schematic representationof the Pt(ll1) crystal face.

B. THEATOMIC SURFACE STRUCTURE OF THE CLEAN (100) PLATINUM CRYSTAL FACE Figure 5a shows the diffraction pattern associated with the clean (100) platinum surface. There are extra diffraction features in addition to those expected for this surface structure from the X-ray unit cell. This surface exhibits a so-called (5 x 1) surface structure (8).There are two perpendicular domains of this structure and there are f , +,and 4 order spots between the (00) and (10) diffraction beams. The surface structure is not quite as simple as the shorthand notation indicates, as shown by the splitting of the fractional order beams. The surface structure appears to be stable at all temperatures

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FIG.5. (a) Diffraction pattern from the Pt(100)-(5 x 1 ) structure. (h) Schematic representation of the (100) surface with hexagonal overlayer. (c) Diffraction pattern from the Pt(t00)-(1 x 1) structure. (d) Schematic representation of the (100) surface.

from 25°C to the melting point, although at elevated temperatures carbon can diffuse to the surface and cause transformation of the structure to the impurity-stabilized (1 x 1) surface structure. The same structure is observed for other 5d transition metals that are neighbors of platinum in the periodic table, such as gold and iridium. The diffraction beam intensities of the (5 x 1) surface structure are under

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close investigation in many laboratories. Preliminary calculations favor a model in which the surface atoms assume a distorted, hexagonal configuration by out-of-plane buckling. The apparent (5 x 1 ) unit cell is the result of coincidence of the atomic positions of atoms in the surface with atoms of the undistorted second layer below. It has been suggested that the surface reconstruction arises from the high polarizability of these metal atoms, which intensifies the driving force toward reconstruction under action of the surface electric field (9u). We call this Pt(100)surface reconstructed. Surface reconstruction is defined as the state of the clean surface when its LEED pattern indicates the presence of a surface unit mesh different from the bulklike (1 x 1) unit mesh that is expected from the projection of the bulk X-ray unit cell. Conversely, an unreconstructed surface has a surface structure and a so-called (1 x 1) diffraction pattern that is expected from the projection of the X-ray unit cell for that particular surface. Such a definition of surface reconstruction does not tell us anything about possible changes in the interlayer distances between the first and the second layers of atoms at the surface. Contraction or expansion in the direction perpendicular to the surface can take place without changing the (1 x 1) two-dimensional surface unit cell size or orientation. Indeed, several low Miller index surfaces of clean monatomic and diatomic solids exhibit unreconstructed surfaces, but the surface structure also exhibits contraction or expansion perpendicular to the surface plane in the first layer of atoms (9h).

c. THEATOMICSURFACE STRUCTURE OF THE CLEAN (1 10) PLATINUM CRYSTAL FACE The ( 1 10) crystal face, just like the (100) crystal face, is reconstructed (IOU). The surface unit cell is an apparent (1 x 2) unit mesh, indicating that the lattice unit cell vector is twice as large in one direction but the same in the other direction as expected from the projection of the bulk X-ray unit cell to this surface. Thus, the rectangular surface unit mesh that would be expected from the projection of the X-ray unit cell is elongated in one direction while remaining unchanged in the other direction. This surface has not been investigated to such an extent as the (1 11) and (100) crystal faces of platinum. The chemisorption characteristics of various adsorbates are certainly less explored than those of the other two low Miller index surfaces. However, adsorbates that have been investigated, CO and 0, ( I O U , IOc), are more strongly bound than on other low index surfaces; thus the valley and ridge structure make this surface adsorb similarly to high Miller index step surfaces.

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D. THEATOMIC SURFACE STRUCTURE OF HIGHMILLER INDEX SURFACES Studies of surfaces of high Miller index and higher surface-free energies are important in their own right. It is important to elucidate their atomic structure and stability under a variety of experimental conditions in the presence of reactive and inert gases and in vacuum. Recent LEED investigations of copper (1I),germanium (12),gallium arsenide (12),and platinum (13) surfaces indicate that the surfaces of crystals characterized by high Miller index consist of terraces of low index planes separated by steps often one atom in height. The ordered stepped surfaces display varying degrees of thermal stability. Figure 6 shows a stereographic triangle of an fcc crystal depicting the various high Miller index surfaces of platinum that were studied. The diffraction pattern from a high Miller index surface exhibits diffraction beam doublets that appear at well-defined electron beam energies. Some of the diffraction patterns that are obtained from the high Miller index surfaces and the surface structures that can be derived from these diffraction patterns are indicated in Fig. 7. The terrace widths are calculated from the doublet

Fic. 6. A stereographic triangle of a platinum crystal depicting the various high Miller index surfaces of platinum that were studied.

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separation. The step height is obtained from the variation of the intensity maximum of the doublet diffraction beam features with electron energy. The detailed analysis of the surface structure based on these diffraction patterns is described elsewhere (13). The terrace width does not have to be uniform to obtain satisfactory diffraction patterns. Houston and Park (14) in a theoretical study have shown that although there may be a great deal of variation in the step width about an average value, one still obtains a diffraction pattern of satisfactory quality. That is, if the diffraction pattern indicates that the terrace width is six atoms, that does not rule out the presence of a large number of terraces of five or seven atoms. Since the rearrangement of high Miller index surfaces to ordered low index terraces separated by periodic steps takes place regardless of the chemical bonding in the crystal, it may be regarded as a general structural property of high index surfaces. It is therefore of value to have a standardized nomenclature to identify stepped surface structures. Stepped surfaces are indicated by the postscript S, so that Pt(S) indicates a stepped platinum crystal surface. The ordered step array can then be completely designated by the widths and orientations of the terraces and the height and the orientations of the steps. The stepped surface may be designated Pt(S)-[M( 11 1) x N ( loo)], where M ( 1l l ) designates a terrace of (1l l ) orientation M atomic rows in width, and N(100) indicates a stepped (100) orientation N atomic layers high. Pt(S)-[M(lll) x (loo)] indicates the structure of various high Miller index platinum stepped surfaces having step heights of one atomic layer. (The number one is not shown in front of the step orientation.) A more detailed description of the nomenclature of more complex stepped structures is given elsewhere (13).In Fig. 6 the stereographic triangle indicates both the high Miller index notation as well as the step notation, which is more descriptive of the real atomic structure of high Miller index surfaces. The thermal stability of the steps is of great interest. We have found that for platinum many high Miller index surfaces show extraordinary thermal stability away from the arrows indicated in Fig. 6. These surfaces may be heated above 1200"C, where they may disorder. However, on cooling to 800°C or in that temperature range the ordered step structure is reestablished. Because of the high thermal stability of these surfaces they must play important roles in catalytic surface reactions that take place at temperatures appreciably below the temperature at which the surface structure orders by annealing. However, high Miller index surfaces in the range of the arrows indicated in Fig. 6 facet to crystal surfaces at the end of the arrows. Thus, on heating a (510) surface will restructure into a (100) and (210) surface. This faceting is easily detectable and monitored by LEED.

i-------1

PERIODIC IT Y

(b) FIG.7a and b. See facing page for legend

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fl 1

PERIODICITY (C)

FIG.7. Diffraction patterns and schematic representations of additional platinum stepped surfaces uscd in these studies: (a) Pt(S)-[9(111)x (IOO)] at 84 V, (b) Pt(S)-[4(11I) x (IOO)] at 34 V, and (c) Pt(S)-[7(1I 1) x (310)] at 49 V.

Perhaps the most significant property of stepped platinum surfaces is their great reactivity compared to low index crystal surfaces. The chemisorption of hydrogen, oxygen, and carbon monoxide was studied by LEED on ordered stepped surfaces of platinum (15). The stepped surfaces behave very differently during chemisorption from those of low index platinum surfaces, and the various stepped surfaces also behave differently from each other (15). Hydrogen and oxygen, which do not chemisorb easily on the (1 11) and (100) crystal faces of platinum, chemisorb readily at relatively low temperatures on the stepped platinum surfaces. All in all, these surfaces play important roles in breaking large binding energy chemical bonds (H-H, C-H, C-C, etc.), which would not break readily on low Miller index surfaces (16). It appears that steps and kinks are active sites and their chemical properties play an important role in catalytic surface reactions. Much of our discussions of chemisorption and reactivity associated with catalyst surfaces are centered on discussions of properties of atomic sites (steps and kinks) of low coordination number. These properties will be discussed shortly.

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111. Techniques to Study the Relationship between Reactivity and the Structure and Composition of Surfaces in the Atomic Scale

A. STATICTECHNIQUES 1. LEED

A typical LEED experiment consists of a monoenergetic beam of electrons, 10-500 eV in energy, incident on one face on a single crystal (Fig. 8). Roughly 5% of the electrons are elastically back-scattered and this fraction is allowed to impinge on a fluorescent screen. If the crystal surface is well ordered, a diffraction pattern consisting of bright, well-defined spots will be displayed on the screen. The sharpness and overall intensity of the spots are related to the degree of order on the surface ( I ) . Although the surface may be irregular on a microscopic and submicroscopic scale, the presence of sharp diffraction features indicates that the surface is ordered on an atomic scale, the atoms lying in a plane parallel to the surface, characterized by a twodimensional lattice structure. The size of these ordered domains determines the quality of the diffraction pattern (17).Because of experimental limitations on the coherence width of the electron beam, ordered domains larger than approximately 500 8 in diameter are not distinguishable from smaller ones. However, if the ordered domains become significantly smaller than 500 8, diffraction spots broaden and become less intense. The presence of sharp

TWO-DIMENSIONAL CRYSTAL LATTICE” (MAGNIFIED)

FIG.8. Scheme of the LEED technique.

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FIG.9. Typical diffraction patterns from the ( I 1 I ) face of a platinum single crystal at four different incident electron beam energies: (a) 51 eV, (b) 63.5 eV, (c) 160 eV, and (d) 181 eV.

diffraction features in LEED (Fig. 9) establishes that the surfaces are ordered on the atomic scale. In addition, the positions and symmetry of the diffraction spots can be used to determine the two-dimensional periodicity of the surface structure. We can imagine for the moment that the surface structure will be rather like the determination of the bulk structure along the crystal plane, although there may be a rearrangement or reconstruction of the surface atoms from the bulk structure. The presence of the surface destroys the bulk translational periodicity in the direction normal to the presumed planar surface, while the translational periodicity of the solid parallel to the surface is retained. The diffraction pattern gives a representation of the surface reciprocal lattice, and the unit cell vectors may be determined from measurement of the beam angles. The basic complication of surface structure analysis by LEED comes from the fact that observation of the diffraction pattern geometry serves only to determine the size and shape of the two-dimensional unit cell, which

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characterizes the translational periodicity parallel to the surface ( I ). Critical information relating to structural variation in the direction normal to the surface must be extracted from the analysis of the intensity of the diffracted beams. Such an intensity analysis is in principle required, for example, to determine the packing sequence and interlayer spacing of the top few atomic layers of a single-crystal surface. LEED studies of clean surfaces have revealed that most of these surfaces, if prepared under proper conditions, are ordered on an atomic scale and exhibit sharp diffraction beams and high diffraction beam intensities. Metal, semiconductor, alkali halide, inert gas, and organic crystal surfaces have been studied this way, and all exhibit ordered surface structures. One of the most exciting observations of LEED studies of adsorbed monolayers on low Miller index crystal surfaces is the predominance of ordering within these layers (18).These studies have detected a large number of surface structures formed upon adsorption of different atoms and molecules on a variety of solid surfaces. Conditions range from low temperature, inert gas physisorption to the chemisorption of reactive diatomic gas molecules and hydrocarbons at room temperature and above. A listing of over 200 adsorbed surface structures, mostly of small molecules, adsorbed on low Miller index surfaces can be found in a recent review (1). There are two systems to denote the unit mesh of ordered monolayer structures formed upon adsorption, The first system, originally proposed by Wood (19a), is probably the most commonly used and can be applied to systems in which the angle between the vectors a and b is the same for the adsorbed structure as for the substrate. The structure is labeled by the general form p(n x rn)Rb" or c(n x m)R+", depending on whether the unit mesh is primitive or centered. For example, in Fig. 10 the diffraction pattern of a clean Pt(l11) surface and a pattern with adsorbed acetylene (C,H,) on the (1 11) crystal face are shown. The structure deduced from this figure is thus labeled p(2 x 2), having unit cell vectors twice as large as the unit cell of the platinum substrate and pointing in the same direction. The total system is then referred to as Pt(ll1)-(2 x 2)-C,H2. For cases in which the angle between the unit mesh vectors of the ordered substrate and the ordered adsorbate is different, a matrix notation is generally used (19b). The unit mesh vectors or the adsorbed structure are related to substrate mesh vectors by the transformation a' = m l l a

+ mlzb,

+ mz2b

b' = m Z 1 a

These equations define the transformation matrix

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(0)

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(b)

FIG.10. Dilrraction patterns of (a) clean Pt(l1l) surface and (b) surface with adsorbed ordered acetylene cxhibiting a (2 x 2) structure.

that is used to characterize the structure. For the structure illustrated in Fig. 10 the transformation matrix is M = (i:). Using this notation the reciprocal lattice transformation matrix and thus the diffraction pattern can be obtained by taking the inverse transpose of M , M* = A-', and this equation can obviously also be used in the reverse direction to obtain the real space unit mesh from the diffraction pattern. Over the past several years, LEED theory has been developed that allows us to compute from the diffraction beam intensities the precise locations of atoms or molecules on surfaces. The basic experimental data are the measured intensities of the diffraction beams as a function of electron energy, and the only adjustable parameters are the surface atomic geometry itself. Once the intensity versus voltage curve (I versus V )is computed, assuming a certain atomic location in the surface, the results are compared with experiments. The computation is repeated using various locations for surface atoms until the best agreement between experiment and theory is obtained. Figure 1 1 shows the computed and experimental intensities of diffraction beams from a Pt( 11 1) clean surface where best agreement between experiment and theory has been obtained. For this surface the atoms appears to be positioned according to the predictable projection of the X-ray unit cell to that particular surface.

FIG. 1 1 , Comparisons of theory and experiment for I - - V profiles from Pt(l11) at room temperature for (a) the (00)beam and (b) the (TO) beam at three angles of incidence. The vertical scales are of relative intensity in arbitrary units and are not necessarily compatible from one curve to the next. The theoretical results were calculaied on the assumption of the bulk interplanar spacing for all atomic layers parallel to the surface.

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There are two major features of the electron-solid interaction evidenced in the I-I/ profiles and in other scattering data in LEED electron diffraction that the theory must provide for: (1) In contrast to the case of X-ray scattering, cross sections for lowenergy electrons from atoms are large (on the order of 10 A2/atom). (2) The incident electrons interact strongly with the valence electrons in the solids, resulting in a high probability of inelastic scattering.

These two features, taken together with the wavelike behavior ofthe electrons, make LEED a sensitive probe of the surface atomic structure. Feature (l), however, renders the use of the simple kinematical scattering theory that is used so successfully in X-ray diffraction inadequate in LEED and necessitates the use of multiple scattering or so-called dynamical theories. Feature (2), on the other hand, means that the electrons are removed from the elastic electron beam due to inelastic collision damping with a characteristic mean free path of 3 to 10 A. The inelastic collision damping tends to reduce, though by no means eliminate, the effect of multiple scattering. The presence of multiple scattering introduces secondary maxima in the I-I/ profiles in addition to the Bragg peaks that are also observed in X-ray diffraction and anticipated from kinematical theory. Over the past several years the surface structures of several clean monatomic solid surfaces and a variety of adsorbed atoms on solid surfaces have been determined by LEED (I). This field of study is now called surface crystallography and is one of the most rapidly growing fields of surface science. By studying the atomic surface structure of clean surfaces and adsorbed molecules, the nature of the surface chemical bond can be explored in a systematic manner.

2. AES If a high-energy electron beam ( 103-105 eV) or high-energy electromagnetic radiation (X rays) is allowed to strike a solid surface, in addition to electron emission from the valence band, electrons are also excited from inner electron shells. The two primary electron shell excitation processes that lead to the production of a free electron that can be collected by a suitable detector is illustrated in Fig. 12. The notation we have adopted to designate the electron energy levels in the atoms is that most commonly used in atomic spectroscopy. The K, L, and M shells refer to those with principal quantum numbers 1, 2, and 3, respectively, and L,, L,, and L3 indicate the multiplicity J, which is a vector sum of the angular momentum L and the spin quantum number S, J = L S.

+

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7

Free electron Conduction bond EF

Valence bond

Lm Ln

Lm Ln

LI

LI

K

(a 1

lnsulotor

K

(b)

FIG.12. Energy level diagram representation of (a) photoelectron cmission and (b) X-ray absorption.

The electron, upon excitation, is ejected from an inner shell into vacuum and the energy of the free electron is then measured. This technique is called X-ray photoelectron spectroscopy. If the electron is ejected from the valence band by ultraviolet radiation, the technique is called ultraviolet photoelectron spectroscopy. Excitation energies not greater than those provided by ultraviolet radiation are necessary for electron excitation from the valence band or for electrons from the valence shell ofadsorbed molecules. Let us turn our attention to the dominant recombination or deexcitation processes that follow the excitation of electrons from the inner shell or from the valence shell (Fig. 13). The first mode of deexcitation is the Auger process, which leads to further electron emission. The second mode of deexcitation may result in the emission of electromagnetic radiation and is commonly called X-ray fluorescence. In the Auger transition, the electron vacancy in an inner shell is filled by an electron from an outer band. The energy released by this transition is transferred to another electron in any

I

FIG.13. Energy level diagram representation of the excitations by (a) Auger electron emission and (b) X-ray fluorescence.

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of the electron levels, which is then ejected. Energy analysis of the emitted electrons will give differences in binding energy between electronic bands participating in the Auger process, which are characteristic of a given element. Analysis of the X-ray fluorescence spectra gives similar information. It has been found, however, that for light elements (with the exception of hydrogen and helium which cannot be detected since these elements have no inner electronic shells) the probability of Auger transitions is much greater than that of X-ray fluorescence. In recent years, AES (2),ultraviolet photoelectron spectroscopy (20),and X-ray photoelectron spectroscopy (2Za) have come to play prominent roles in studies analyzing the composition and bonding at surfaces. These techniques can conveniently be used to determine nondestructively the composition of the surface and changes of the surface composition under a variety of experimental conditions. Since the Auger transition probabilities are large, especially for elements of low atomic number, surface impurities in quantities as little as 1% of a monolayer ( - l O I 3 atoms/cmz) may be detected. The experimental apparatus to detect Auger electron emission that is frequently used at present utilizes the geometry of the LEED apparatus. Thus, both AES and LEED studies can be carried out on the same crystal surface by using the same electron optics in two different modes alternately in the same experimental system. In the Auger mode, however, we analyze the energy distribution of the inelastically scattered electrons. Separation of the Auger peaks from the background of secondary electrons is carried out by superimposing a small ac signal on a retarding dc potential (2). Suitable detection allows the monitoring of the first and second derivatives of the electric current as a function of the retarding potential, dZ/dV and d12/dV2. In this way, the Auger peaks or other characteristic energy loss peaks can easily be distinguished from the background of other electron emission processes. The energy at which the Auger peak is detected in such a spectrum, Eobs,is actually the binding energy difference of the electronic shells that participate in the process (Fig. 14). Since the electronic binding energies are tabulated, in most cases inspection of these tables allows one to determine the element responsible for the energy loss and the particular electronic transitions that took place. By suitable calibration with known standards, the intensities of the peaks can be used for quantitative as well as qualitative surface analysis (21b). A typical Auger spectrum from platinum surfaces is shown in Fig. 15. The presence of small concentrations of carbon, the most common impurities on surfaces, are easily discernible. Since both Auger electron and photoelectron emission are atomic properties, these techniques can be applied to studies of solid surfaces with various degrees of crystallinity (foil, crystal, dispersed particles, etc.) and to studies of liquid surfaces as well.

ANALYZER 7

CRYSTAL

7 EKIN

EkIN

VACUUM LEVEL

FIG. 14. AES, experimental configuration and energy diagram.

1

0.I

x0.7 I

0

100

I

I

I

I

I

200

303

400

500

600

volts

FIG. 15. Auger spectra of platinum (a) in the presence of carbon and carbon monoxide and (b) in the clean state.

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

25

B . TRANSPORT TECHNIQUES 1. Studies of Surface Reaction Rates at Low (10-7-10-4 Torr) and at High (1 O3 - 1O5 Torr) Pressirres

During the past five years, new instruments have been developed in our laboratory to permit in situ studies of the reactivity of crystal surfaces at both low and high pressures (22a).In all of these experiments, small-surfacearea (approximately 1 cm’), single-crystal or polycrystalline catalyst samples can readily be used as long as the reaction rate is greater than product molecules/surface atom/sec. The scheme of one of these apparatuses is shown in Fig. 16. At low pressures (10-7-10-4 Torr) the reaction rate and product distributions are monitored by quadrupole mass spectrometer, while the surface structure and composition are determined by LEED and AES, respectively, during the surface reaction if desired. Then a small cup (total volume approximately 10 cm3) can be placed around the crystal sample to isolate it from the rest of the chamber. The chamber can be pressurized to over 100 atm, if desired, during the mixing of gaseous reactants. The highpressure reaction chamber is connected to a gas chromatograph, which serves to monitor both rate and product distribution in this circumstance. The structure and composition can be determined in situ by LEED and AES before and after the high-pressure experiment once the cup is removed. Crystal samples may be heated during both low- and high-pressure experiments and a vacuum of Torr can be maintained outside the pressurized @Pressure

gauge

(a) S.S. welded bellows n

Sampling volve

n

)

Gas \ chromatograph

II

To mechanicol pump

To gas monifold

Gas intr0duction-J needle

Welded metal be1lows pump

FIG. 16. Schematic of the experimental apparatus to carry out catalytic reaction rate studies on single-crystal surfaces at low and high pressures in the range 10-’-lO4 Torr.

26

G . A. SOMOFUAI

cup in the reaction chamber. The effect of adding an impurity or a second constituent (alloying) to the surface on the reactivity can also be studied in this system. The second constituent may be vaporized at low ambient pressure onto the surface of the crystalline sample from an external vapor source until the desired surface composition is obtained. The crystal surface can be cleaned by ion bombardment, which is also available as an attachment on the reaction chamber. This and similar instruments ( 3 , 4 ) that allow one to study reaction rates and product distributions on small-area crystal and catalyst surfaces have been used in our studies of the mechanism of heterogeneous catalysis and the nature of active sites. These studies, which concentrated primarily on hydrocarbon reaction as catalyzed by platinum crystal surfaces, will be reviewed in the next section. 2. Molecular-Beam Surface Scattering Another apparatus that is very useful in studies of the mechanism of catalytic surface reactions is shown in Fig. 17. This is used in a molecularbeam surface scattering experiment (22%)in which a well-collimated beam of the reactant gas or gas mixture is scattered from a crystal surface and the products that are desorbed after a single scattering at a given solid angle

MANIPULATOR

OAS INLET

+. GAS INLET

- ION PUMP

TO PUMPS

TO WMPS

MASS SPECTROMETER

ION PUMP *-ROTATABLE CHAMBER

TO PUMPS

FIG. 17. Schematic of the UHV molecular-beam surface scattering apparatus

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

27

are detected by mass spectrometry. By rotation of the mass spectrometer around the sample, the angular distribution of the scattered products can be determined. If the incident molecular beam is chopped at well-defined frequencies, the time of flight of the incident molecules between the chopper and the detector is determined by phase shift measurements (23). This information yields the residence time of molecules on the surface. Chopping the product molecules that desorb from the surface permits determination of their velocity. The experimental variables of this system are the temperature, atomic structure, and composition of the surface, and the velocity and the angle of incidence of the molecular beam. In reactive scattering experiments the mass spectrometer detects the product distribution and rates of formation of product molecules (reaction probabilities on single scattering) as a function of the system variables. From the dependence of the reaction rate on the incident beam velocity (or “beam temperature”) the activation energy for adsorption, if any, is determined. From the surface temperature dependence ofthe rate, the activation energy ofthe surface reaction is obtained. The surface residence time of the molecules, the kinetic energy, and angular distribution of the products reveal the nature of energy transfer during the gas-surface interactions (23). A detailed description of molecular-beam surface scattering experiments and the results of these studies are given elsewhere (22b,23). Here we shall discuss only those studies that are important in verifying the nature of active sites in heterogeneous catalysis. C. CLEANING AND PREPARATION OF SINGLE-CRYSTAL SURFACES

The catalyst crystal samples are generally cut from single-crystal rods that are electron beam zone-refined to obtain low impurity concentrations (in the 10-ppm range). The common impurities in platinum samples are carbon, calcium, phosphorus, and sulfur ; these must be removed before beginning the studies of catalytic reactions. The catalyst samples were prepared by orienting with a LauC back-reflection X-ray technique, spark cutting an approximately l-mm thick slice with the proper crystallographic orientation exposed, polishing both sides, and etching. The carbon, phosphorus, and sulfur impurities can be removed by oxidation in 5 x Torr oxygen at 1000 K. The adsorbed oxygen is removed by heating the sample to 1300 K in vacuum. The high concentration of calcium impurity, which possibly remained in the sample from the reduction of the platinum ore, could only be removed by extensive oxidative heat treatments. The sample was oxidized at 1500 K in 10- Torr oxygen for 24 to 48 hours. This treatment fixes calcium on the surface in the form of a stable oxide, which will decompose with calcium vaporization from the surface upon heating

-

28

G. A. SOMORJAl

to 1800 K. A small amount of calcium impurity may also be removed by argon ion bombardment at 1100 K. The clean platinum surface structure can be identified by both LEED pattern and the LauC X-ray diffraction pattern. The cleaning of the various catalyst samples has to be scrutinized for each material studied. For iron for example, the major impurity is sulfur, and its removal must be carried out outside the vacuum system in a furnace in a constant hydrogen flow for a long period of time (days). Trace metallic impurities or nonmetallic impurities may be removed either by argon ion bombardment in the vacuum chamber or by chemical treatment using gas-surface interactions of different types. We shall restrict most of our discussion to studies of platinum surfaces, which will serve as a model of surface studies of other catalysts.

IV. Chemisorption of Hydrocarbons on Low and High Miller Index Surfaces of Platinum, Iridium, and Gold

A.

CHEMlSORPTION OF

HYDROCARBONS ON THE Pt(111) CRYSTAL FACES

AND

Pt(100)

The adsorption and ordering characteristics of a large group of organic compounds has been studied on the Pt( 100) and Pt( 1 1 1) single-crystal surfaces (24).LEED has been used to determine the surface structures. Work function change measurements have been made to determine the charge redistribution that occurs on adsorption. The molecules that have been studied are acetylene, aniline, benzene, biphenyl, n-butybenzene, t-butylbenzene, cyanobenzene, 1,3-~yclohexadiene,cyclohexane, cyclohexene, cyclopentane, ethylene, n-hexane, mesitylene, 2-methylnaphthalene, napthalene, nitrobenzene, propylene, pyridine, toluene, and rn-xylene. The shape and the bonding characteristics of the organic molecules have been varied systematically so that correlations can be made between these properties and their interaction with the metal surface. The two platinum crystal faces, (1 11) and (lo), that were used as substrates in this study have six- and fourfold rotational symmetry, respectively. Thus, we can find out how the atomic surface structure of the metal influences the nature of chemisorption of the various organic molecules. The adsorption of molecules with molecular dimensions smaller than substrate interatomic distances usually gives rise to the formation of ordered adsorbed structures, with the rotational symmetry of the substrate such that the unit vectors of the overlayer are closely related to the substrate unit cell vectors (25). Thus in most cases local interactions

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

29

between substrate and adsorbate seem to play a dominant role in determining their adsorption characteristics. However, as the surface density of small molecules is increased, adsorbate-adsorbate interactions often become increasingly important, as evidenced by continuous two-dimensional compressions in the unit cell size for some of the adsorbates (26). Studies of the adsorption of large molecules where the molecular size is larger than the interatomic distances in the substrate are especially interesting because of the possibility that localized surface atom-adsorbed molecule interaction may not play a dominant role in the interaction between the substrate and the adsorbate. Large molecules may interact simultaneously with several surface atoms upon adsorption, so that.the characteristics of the adsorbed layers may be less controlled by the local substrate bond while the adsorbate-adsorbate interaction becomes more predominant. In the extreme, the interaction of these large molecules with metal substrates may be similar to the interaction of large polarizable rare gas atoms, such as xenon with metal substrates (27,28). The surface structure of adsorbed xenon at high coverage is independent of the atomic structure of the substrate. We have found that most of the monolayers of organic molecules that were studied did not undergo chemical change on these low Miller index platinum surfaces during the adsorption studies, which were carried out at low pressures ( 10-9-10-6 Torr) and in the temperature range 300-500 K, but remained intact so that their ordering characteristics and surface structure could readily be studied. B. SUMMARY OF EXPERIMENTAL FINDINGS

All the organic molecules studied adsorb on both the Pt(ll1) and Pt( 100)(5 x 1) surface. The results of adsorption experiments are shown in Table I. Ordering in the adsorbed layer was more pronounced on the Pt(ll1) surface than on the Pt(l00)-(5 x 1) surface. One of these ordered surface structures, that of the adsorbed monolayer of benzene on the Pt( 1 1 1) face, is shown in Fig. 18. In general, the adsorbed layer is more ordered and causes a larger work function change (A4) on adsorption if the incident flux is lower. The work function decreases with adsorption for all the organic molecules studied. This implies that the adsorbed molecules are acting as electron donors to the metal surface. This might be expected, since the metal has a high work function ( - 5.7 V) and all of the molecules studied are polarizable. The magnitude of the work function change associated with the adsorption of unsaturated hydrocarbons where n-electrons make major contributions to the bonding is in the range - 1.3-2.0 V. Saturated hydrocarbons that were studied produce much smaller work function changes, in the range

TABLE I Work Funcrion Changes und Structuml Ii!formutinri ,firAdsorprion of Organic Cornpourids on rhr Pt(l 11) ~ n Pt(1001-(5 d x II S I I ~ ~ & W

Pt(ll11 Work function change

Work function changc

---

Adsorbate Acetylene

Adsorbarc

Tcrnp (T)

Pressure (Torr)

WF(:

20 20

1 x IWS 1 x I O - ~

-1.5 -1.65

(? x 2)

Disordered Streaksat4order diffuse (3 0)features Poorly ordered

diffraction features or surfacc struciure

PI

Substrate

WFC (V)

structure after adsorption

Adsorbate diffraction features or surface structures

4 x lo-'

-1.65

(1 x I )

(v:? x $)R45'

4 x lo-' 1 x lo-'

-1.7 -1.75

(1 x 1) (1 x 1)

(vj2 x q'!T)R45''

3 x 10-7

-1.6

(1 x 1)

Diffuse ringlike order streak

3 x lo-' ( 2 hr) ?xIO-' 8 x lo^ 5x10-*

- 1.3

(1 x 1)

-1.8 - 1.5 -1.75 -1.5

(1x1)

Diffuse f order streak Disordered Disordered Disordered Disordered

Pressure (Tvrr)

Disordered

(10 min)

w

c

4 x lo-'

Aniline

150 20

1 x lo-*

-1.8 -1.8

Benzene

20

4 x lo-'

- I .8

20

4 x 10-7 (5 min) 4 x 10-l (40min) 2 x

-1.4

20 Biphenyl o-Bulylhensene r-Butylbenzene Cyanobenzenc

20 20

20 20

X x 5 x lo'-' 1 x

-0.7 - 1.85

-1.5

-1.7 - 1.6

'

Very poorly ordered Disordered Disordorcd Diffuse (i0)features

-

1 x

1U-*

(1 x I )

(1x1)

Faint ( 5 x 1)

Disordered

20

2 x 10-8

- I .7s

Poorly ordered

2 x lo-*

- 1.7

20

2 x (1 hr) 3 x 10-7 (5 hr) 6 x 10-9

-1.3

I-: I: I-: I:

2 x 10-8 (1 hr) 2 x ( 5 hr)

- 1.6

( I x 1) low background

6 x 1 0 '

4 x 10'

- 0.7

Very poorly ordered

4

10-7

-0.75 - 0.4

4

10-7

- 1.2

20

Cyclohcxane

Cyclohexene

20 20

-

1.2

150

4 x lo-'

- 1.1

Apparent (2

300 20

4

lo-'

- 1.4

Disordered

6 x

- " ~

1so

6 x lo-'

--

20 20

7 x 10-9 4 x 10-7

-0.95 - 0.7

w Cyclopentane

'

- 0.8

1.7

-

1.6

Cyclopentene

20

2,6-Dimethylpyridine

20

4 x

- 1.6

3,5-Dimethylpyridinc Ethylene

20

6 x loF8 I x 10-8 1 x 10-8

-1.5

Graphitic ovcrlaycr

20 250 950

x

2)

4 x 10-7

:1 I-:

- 1.7 - 1.1

.-

1.5

10-7

- 1.6

lW7

- 1.5

Apparcnt (2 x 2)

6

(1 x 1) low background Disordered

7 x 10-9

- 0.4

4 x 10-7

-0.3

2 x 10-7

- 1.4

4

- 1.5

~

- 2.3

- 1.4

Diffuse 1/3.2,2/3.2 order streaks Diffuse order streak Diffuse ( $ 0features ) Disordered Ringlike diffraction featurcs

x

x

10-8

h x 10-8 I x 10-8 1 x 10'

--

Diffuse order streak Diffuse f order streak Diffuse f order streak Luw background Llifrusc streaked (2 x I ) pattcrn Streaked(2 x 1) pattern Disordered Diffusc (i0) Ceaturcs Slreaked(2 x 1 ) pattern Low background Diffuse features at order Diffuse streaked ($0) features Disordered

+

2.2

-- 1.2

- 1.5

-- 1.0 -

(continued)

TABLE I (continued)

Pt(100)-(5 x 1)

Pt(l1l)

Work function change

h) w

Adsorbate n-Hexatic

Isoquinoline Mesitylene

2-Mcthylnaphthalenc

Temp ("C1

Pressure (Torr)

20 20

250 20

5 x 1o'-u 5 x ( 5 hr) 5 x lo-' 6 x lo-'

-1.5 -1.9

20

4 x lo-'

-1.7

20

4 x lo-'

20

h x 1 W 8

-1.35 -2.0

WFC (V) -1.1

-0.9

Work function change

Adsorbate diffraction features or surface structure Uisordercd Disordered Disordered Diffuse (4 0)and (5 0 ) features Streaks at 113.4 order diffuse (2/'3.40) features Disordered Vcry poorly ordered

Pressure (Torr) 5 x 5 x (5 5 x 6 x

10 - u

lo-* hr)

WFC (V)

Substrate structure after adsorption

Adsorbate diffraction reatures or surfau: structures

(I x 1) Faint

Disordered Disordered

-

-0.8 -0.6 -1.2

lo-'

-2.1

4 x lo-'

-1.7

4 x 10-7 4 10-9

-1.2 -1.6

(5 x 1)

20 150 20

9 x 10-9 9 x 10-9 9 x 10-9

- 1.95 - 2.0 - 1.5

Pipcridine

20

8 x 1 0 n

-2.1

Apparent (3 x 1) (6 x 6 ) Diffuse (f 0)features (pattern electron beam sensitive) Disordered

Propylene

20

2 x 10-8

- 1.3

i 2 x 2) (pattern electron

Naphthalene Nitrobenzene

9

10-9

- 1.7 - 1.65 - 1.4

x

x lo-"

- 2.05

9

10-9

9 x 10-9

2 x 10-8

- 1.2

1 x 10-8 1 x 10-8

- 2.4

6 x lo-@

- 1.6

3 x 10-8 16 min) 3 x 10-8 (14 min) 6 x lo-* I x 10-9 1 x 10-9

-

beam sensitivcl

20 250

I x 10-8 I x 10-.8

- 2.7

Pyrrole

20

6 x lo-@

- I .45

Quinoline

20

3 x 10-8

- 1.45

Pyridine

W

w

Styrene Toluene n-X ylene

20 20 150 20

6 x I 10-9 I x 10-9 1 x 10-8

-1.7

- 1.7

- 1.7

- 1.65 - 1.8

Difhise (; 0)features Well-dcfincd streaks at +, 4 , ; order Diffuse (f 0)features (pattern electron beam sensitive) Diffuse f order streaks

+

Streaks at order Streaks at order I4 x 2) Streaks at 112.6 order

I x 10-8

-

- 1.7 - 1.65 - 1.55 .'-

1.5 1.65

Disordered Disordered Disordered

Disordered order streaks (pattern clectron beam sensitive) Disurdered > ;,( x J2)R45" Diffuse (i0) features Diffuse order streaks Disordered Very poorly ordered Streaks at f order Disordered Strcaks at order

4

34

G . A. SOMORJAI

k

T 9'60 "

13.85

-I

.* . . . .T .T . .

7.36

FIG.18. Diffraction patterns for the benzene surface structure of the P t ( l l 1 ) crystal face taken at several voltages and a schematic diagram of the unit cell. The benzene is shown in two orientations. All dimensions are in angstroms.

-0.9-1.2 V. The largest work function change was observed during the adsorption of pyridine ( - 2.7 V) and reflects the large contribution of the nitrogen lone electron pair and/or the permanent dipole moment to the charge transfer. The work function change on adsorption for most of the molecules studied varies approximately inversely with the first ionization potential of the adsorbate (24).The data are scattered, for while many types of molecules are represented, some in fact have sizable permanent dipole moments. Several compounds undergo pressure-dependent transformations (usually > Torr adsorbate pressure) on the platinum surfaces studied; in fact, the transformations occur over unexpectedly long time periods. For instance, at a surface pressure of Torr, typical transformation times involve several thousand seconds of exposure. The compounds studied that undergo

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

35

transition at 2 0 ° C as indicated by changes in A 4 and diffraction information are benzene, 1,3-cyclohexadiene (dehydrated to benzene on the surface), cyclohexane, n-hexane, cyclopentane, and mesitylene. These transitions are changes in the chemistry of the adsorbate-surface interaction since they occur with only a few of the molecules studied. The adsorption and ordering characteristics of the various hydrocarbon molecules on the low Miller index platinum surfaces are discussed in great detail elsewhere. These two surfaces appear to be excellent substrates for ordered chemisorption of hydrocarbons, which permit one to study the surface crystallography of these important organic molecules. The conspicuous absence of C-H and C-C bond breaking during the chemisorption of hydrocarbons below 500 K and at low adsorbate pressures (10-9-10-6 Torr) clearly indicates that these crystal faces are poor catalysts and lack the active sites that can break the important C-C and C-H chemical bonds with near zero activation energy. Upon heating the adsorbed organic layers above 550 K, partial desorption and partial thermal decomposition of the molecules take place. Thus C-H and C-C bond breaking on the terrace sites require considerable activation energy, which can be overcome at higher surface temperature or by the application of higher reactant pressures. Heating the surface above 900 K results in the formation of a graphitic overlayer that exhibits a diffraction pattern characteristic of the basal plane of graphite.

C. HYDROCARBON CHEMISORPTION ON HIGHMILLER INDEX (STEPPED) PLATINUM SURFACES The chemisorption of over 25 hydrocarbons has been studied by LEED on four different stepped-crystal faces of platinum (4,the Pt(S)-[9(111) x (loo)], Pt(S)-[6(111) x (loo)], Pt(S)-[7(111) x (310)], and Pt(S)-[4(111 x (loo)] structures. These surface structures are shown in Fig. 7. The chemisorption of hydrocarbons produces carbonaceous deposits with characteristics that depend on the substrate structure, the type of hydrocarbon chemisorbed, the rate of adsorption, and the surface temperature. Thus, in contrast with the chemisorption behavior on low Miller index surfaces, breaking of C-H and C-C bonds can readily take place at stepped surfaces of platinum even at 300 K and at low adsorbate pressures (10-9-10-6 Torr). Hydrocarbons on the [9(100) x (loo)] and [6(111) x (loo)] crystal faces form mostly ordered, partially dehydrogenated carbonaceous deposits, while disordered carbonaceous layers are formed on the [7(111) x (310)l surface, which has a high concentration of kinks in the steps. The distinctly different chemisorption characteristics of these stepped-platinum surfaces can be explained by

36

G . A. SOMORJAI

considering the interplay of four competing processes : (1) the nucleation and growth of ordered carbonaceous surface structures, (2) dehydrogenation, i.e., breaking of C-H bonds in the adsorbed organic molecules, (3) decomposition of the organic molecules, i.e., breaking of both C-H and C-C bonds at steps, and finally, (4)rearrangement of the substrate by faceting. On the [9(111) x (loo)] and [6(111) x (lOO)] crystal [;ices, processes (1) and (2) predominate. On the [7(111) x (310)l face, process (3) predominates, while process (4) is the most important on the (4(1 1 I 1 i loo)] face. The lack of reactivity of low Miller index surfaces in hydroc i P I I reactions indicates the importance of steps in breaking C-H and C- honds so important in various surface reactions of hydrocarbons. Atomic steps and kinks, i.e., low coordination number sites, are responsible for decomposition via dehydrogenation and C-C bond breaking of hydrocarbon molecules, which can take place at these sites with near zero activation energy. In the absence of a large concentration of the low coordination number sites, the hydrocarbon molecules remain intact below 450 K and Torr) and their surface crystallography may be at low pressures (readily studied. However, in the presence of atomic steps and kinks only carbonaceous residues remain on the surface, which are the products of decomposition of the various hydrocarbon molecules that participate in chemisorption or in surface chemical reactions. The properties of this carbonaceous residue are also important in heterogeneous catalysis as will be shown below. Platinum displays a unique surface chemistry in that low coordination number sites are predominantly responsible for the bondbreaking processes. In the absence of these sites, the low Miller index surfaces do not exhibit bond breaking at low temperatures ( - 450 K) and pressures (< Torr). Such a marked change in the chemical activity from surface site to surface site is one of the major attributes ofplatinum that is responsible for its unique catalytic activity. On iridium surfaces (and as will be discussed later, even on low Miller index surfaces) partial decomposition of hydrocarbons may occur even at low temperatures and pressures due to the stronger adsorbate-substrate, hydrocarbon-metal bonds. Even though for other transition metals the chemistry of low coordination number surface sites is likely to be different from the terrace atom sites that are predominant on low Miller index surfaces, the hydrocarbon molecules may not remain intact on either high or low Miller index crystal faces. Platinum and perhaps palladium and nickel are the elements to show this drastic variation of reactivity when one compares low and high Miller index crystal faces at low temperatures and reactant pressures. \'

I

I

-

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

37

D. THECHEMISORPTION OF HYDROCARBONS ON GOLDAND IRIDIUM CRYSTAL SURFACES The chemisorption of hydrocarbons, ethylene, cyclohexene, n-heptane, benzene and naphthalene at room temperature and above were studied on both the Au(l11) and Au[6(11 I ) x (loo)] stepped surfaces (29). The difference in the adsorption characteristics of hydrocarbons on gold surfaces and on platinum surfaces is striking. The various light hydrocarbons studied (ethylene, cyclohexene, n-heptane, and benzene) chemisorb readily on the Pt(l11) surface. These molecules, on the other hand, do not adsorb on the Au(l11) surface under identical experimental conditions as far as can be judged by changes that occur in the Auger spectra. Naphthalene, which forms an ordered surface structure on the Pt(lI1) face, forms a disordered layer on adsorption on the Au(l11) surface. The stepped Pt [6( 1 11) x (loo)] face reacts readily with all of the adsorbed hydrocarbons and certainly with those that are listed here. The partially dehydrogenated carbonaceous layers that form as a result of dissociated hydrocarbon chemisorption are largely disordered. In contrast, stepped gold surfaces of the same atomic structure remain inert to adsorption of the light hydrocarbon moleculesjust as the Au( 111) crystal face and the chemisorption behavior of the two types of gold surfaces (those with low and high Miller index surfaces) are indistinguishable. Naphthalene, however, adsorbs on both gold surfaces, and the adsorption behavior indicates dissociative chemisorption. The hydrocarbon fragments that form are strongly bound. These results indicate that while chemisorption of hydrocarbons on platinum surfaces requires little or no activation energy, chemisorption on gold has large enough activation energy for most hydrocarbons to prevent adsorption at the low pressures ( l o p 6Torr) and temperatures ( <550°C) studied. While the activation energy for surface reactions such as the rupture of C-H and C-C bonds is greatly reduced at atomic steps on the platinum surface, this effect is not at all apparent on gold surfaces. The chemisorption of acetylene, ethylene, benzene, and cyclohexane were also studied on the Ir(ll1) and stepped Ir[6(111) x (loo)] crystal surfaces (30). Chemisorption characteristics of the Ir(l11) and Pt(ll1) surface are markedly different. Also, the chemisorption characteristics of the low Miller index Ir(lI1)surface andthesteppedIr[6(111) x (loo)] surfacearemarkedly different for each of the molecules studied. The hydrocarbon molecules form only poorly ordered surface structures on either the Ir(l11) or stepped iridium surfaces. Acetylene and ethylene (C2H2 and C2H,) form surface structures that are somewhat better ordered on the stepped iridium than on the low Miller index Ir(ll1) metal surface. The lack of ordering on iridium surfaces as compared to the excellent ordering characteristics of these molecules on

38

G . A . SOMOKJAI

the Pt(l11) surface indicates either the lack ofmobility of hydrocarbon molecules necessary for ordering at these temperatures or a chemical reaction, i.e., decomposition. The observation that C2H2,C2H,, and CbHl2all yield the same diffraction pattern on the stepped iridium surface regardless of molecular size would suggest that decomposition occurs on iridium substrates even at 300 K on stepped iridium surfaces. The degree of decomposition appears to be different on the two crystal faces at room temperature, being higher on stepped iridium surfaces than on the Ir(1 1 1) surface, as the differences in surface structure and flash desorption studies indicate (30). The poorly ordered (2 x 2) structure that has been observed on Ir[6(111) x (loo)] surfaces after adsorption of C,H2 and C2H4 at room temperature is not found on the Ir( 1 11) surface below 500 K. This could indicate a higher degree of dehydrogenation on the stepped surface than on the ( 1 1 1) surfaces. The difference between the Ir(ll1) and Pt(1I I ) surfaces in their reactivity to C-H bond breaking as indicated by flash desorption spectra is striking. From the Pt(l1 I j crystal face, ethylene, acetylene, and benzene can all be desorbed in large quantities upon heating. On the Ir(ll1) surface, however, benzene is the only adsorbate that can be desorbed upon flash desorption. Ethylene remains largely on the surface, with only a few percent removed by heating, and acetylene cannot be desorbed at all. Only hydrogen evolution is observed under conditions of flash desorption. The differences between the ordering characteristics of the Pt(l1 I ) and Ir( 1 1 1j surfaces after heating to high temperature ( >800°C) following hydrocarbon adsorption, are marked. Ir(ll1) yields an ordered (9 x 9) coincidence carbon structure, which can be attributed to hexagonal overlayers of carbon similar to that of the basal plane of graphite or benzene, deposited on the (1 1 1 ) surface. A similar structure was found on the Pt(S)-[6(111) x (loo)] surface when this surface was heated to high temperature in the presence of various hydrocarbons. However, on the Pt( I 1 1) surface under similar experimental conditions, one observes a ringlike diffraction feature that is characteristic of a graphite overlayer with rotationally disordered domains. It appears that the stronger metal-carbon interaction on iridium surfaces imposes the periodicity on the carbon atoms in the overlayer, while the structure of the graphite overlayer on the Pt(ll1) face is independent of the substrate periodicity and rotational symmetry. Ordering of the dehydrogenated carbonaceous residue on the stepped iridium surface is absent when the surface is heated to above 1100 K. Atomic steps of(100)orientation appear to prevent the formation of ordered domains that are predominant on the Ir(l11) crystal face. The reasons for this are not clear. Perhaps the rate of C-C bond breaking on account of the steps is too rapid to allow nucleation and growth of the ordered overlayer. On the (1 1 1 ) face, the slower dehydro-

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

39

genation allows ordering as observed. It is tempting to list the stepped and ( I 1 1) iridium and platinum surfaces according to their ability of breaking C-H and C-C bonds as Ir(S)-[6( 11 1) x (loo)] > Ir(ll1) z Pt(S)-[6(111) x (loo)] > Pt(l11). The surfaces at the two ends of this series are not likely to be versatile catalysts in reactions where C-H and C-C bond dissociations are necessary. The stepped iridium surface would decompose the reactants too rapidly and the residue that forms would block the surface rather well to further chemical reaction. The Pt(ll1) surface interacts with the reactants too weakly and would not efficiently break the chemical bonds. The surfaces in the middle of the series would likely be very versatile catalysts. This contention is, of course, subject to experimental scrutiny at the present.

V. Chemical Reactions on Platinum Crystal Surfaces

A. THE H,-D, EXCHANGE ON PLATINUM CRYSTAL SURFACES AT LOW PRESSURES One of the fundamental questions of heterogeneous catalysis is how surfaces lower the activation energy for simple reactions on an atomic scale so that they proceed readily on the surface while the same reaction in the gas phase is improbable. The reaction of hydrogen and deuterium molecules to form hydrogen deuteride is one of the simple reactions that takes place readily on metal surfaces even at temperatures below 100 K. The same reaction is completely inhibited in the gas phase by the large dissociation energy of H, or D, (103 kcal/mole). Once the H2 molecule is dissociated, the successive atom-molecule reaction (H + D, HD + D) in the gas phase still has a potential energy barrier of roughly 10 kcal/mole. The H,-D2 exchange reaction was studied by Bernasek and Somorjai (31) using platinum single-crystal surfaces of low and high Miller index. Under conditions of the experiments, which put strict limitations on the residence time of the detected molecules, the reaction product HD could not be detected from the (111) crystal face. However, the reaction product was readily detectable from the high Miller index stepped surface. The integrated reaction probability (defined as total desorbed H D flux divided by H, flux incident on the surface) is approximately lo-', while H D formation was below the limit of detectability on the Pt(1 1 1) surface (reaction probability < lo-'). TThus, atomic steps ut the plutinum surfuce must plajl controlling roles in dissociuting the diatomic molecules. Figure 19 shows the scattering distributions from both the Pt( 111) and the stepped platinum surfaces. Varying the chopping frequency of the incident molecular beam has yielded HD residence times of about 25 msec --f

40

G . A. SOMORJAI

Periodicity

H,

Dz INTENSITY

-: I -

H p , DZ

, HD

INTENSITY

"5

P

ANGLE FROM SURFACE NORMAL

> 0.35

t z

0.30

z

c

5

0.25

9

F 0.20 k

5 + z 9

015

0.10

K

c

-u 8, ANGLE FROM SURFACE NORMAL

0.05 0

0

10

e,

20 30 40 5 0 60 70 8 0 ANGLE FROM SURFACE NORMAL

FIG. 19. Scattering distribution of H,,D,, and HD formed at the surface from P t ( l l 1 ) and platinum single-crystal surfaces. O n single scattering, HD signal from the Pt( I 11) surface is not observable.

and longer on a stepped platinum surface at 700 K surface temperature. Such long residence time should result in complete thermal equilibration between the surface and the reaction products. Indeed, it was found by experiments that the desorbing HD beam exhibits cosine angular distribution, as seen in Fig. 19. The pressure dependence of the exchange reaction indicates that an atommolecule reaction or possibly an atom-atom reaction on the surface is the

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

41

rate-limiting step. The absence of beam kinetic energy dependence of the rate indicates that the adsorption of hydrogen does not require activation energy. The surface is able to store a sufficiently large concentration of atoms, which react with the molecules by a two-branch mechanism. The rate constants for the H2-D, reaction were also determined under conditions of constant hydrogen atom coverage. At lower temperatures (<600 K) the rate constant for this exchange is k , = 2 x lo5 exp(-4.5 kcal/RT) sec-'. The rate-determining step appears to be a Langmuir-Hinschelwood type reaction between the diffusing D, molecules or D atoms on the surface to a step site where the hydrogen atom is located, where HD is formed by a three- or a two-center reaction. At higher temperatures ( >600 K), the reaction between an adsorbed hydrogen atom and an incident D, gas molecule competes with the low-temperature branch. This reaction thus appears to follow an Eley-Rideal mechanism. The rate constant for this branch is k , = 10, x exp( -0.6 kcal/RT) sec-'. The catalyst action of the platinum surface for the exchange reaction is due to its ability to adsorb and dissociate hydrogen molecules with near zero activation energy and to store atomic hydrogen on the surface, thereby converting the gas phase molecule-molecule reaction to an atom-molecule or an atom-atom reaction of low activation energy. The detailed mechanism of H2-D, exchange on platinum crystal faces is described elsewhere (31).Similar techniques are being used to study deuterium exchange with methane and other hydrocarbon molecules to test the C-H bond-breaking process on a variety of metal crystal surfaces. The H,-D, exchange reaction was also studied by Palmer et al. (32) on platinum and nickel surfaces that were prepared as evaporated thin films. They observed angular distributions that are peaked near the surface normal following a relationship, where is the angle of desorption from the surface relative to the surface normal. The value of n varies from 2.5 to 4.0 on the different surfaces studied. This angular distribution, instead of cos 8, indicates perhaps incomplete accommodation of the reaction product with the surface. The peaked angular distribution could be related to the presence of impurities such as carbon and sulfur contamination on the metal surfaces. There is evidence from the experiments of Stickney et al. (33) that as the surface is cleaned of sulfur, oxygen, or carbon, n approaches unity. Copper, on the other hand, shows noncosine angular distribution for scattered H D even from a clean surface. Unlike for platinum, the adsorption of H, or D, is activated on a copper surface. The activation energy of adsorption is about 5 kcal/mole, as determined by Balooch and Stickney ( 3 4 4 from the beam temperature dependence of the reaction probability. It would be important to measure the velocity of the scattered products in addition to their angular

42

G . A. SOMORJAI

distribution in order to determine the nature of energy transfer between the HD product molecules and the surface prior to desorption. These studies are in progress in several laboratories. Lu and Rye (34b)have also studied the H2-D, exchange using various platinum crystals under similar conditions of temperatures and pressures as those used by Bernasek and Somorjai (31).They found only a factor of twenty difference in the exchange rates of the various crystal faces instead of the orders of magnitude differences found in the previous work. The major difference in the H2-D2 exchange studies reported by these two groups is that one used a chopped molecular beam (31)that detects only short-lived (25 msec or shorter surface residence time) HD product molecules while the other used steady state reactant flow conditions that detects all (HD molecules that have short and long surface residence times) products that desorb from the crystal surface. In fact the results of the two sets of experiments converge as the chopping frequency in the molecular beam experiment is reduced to 1 cps so that HD molecules that desorb after surface residence times of one second or less are all collected. Christmann et al. (3%) have studied H,-D, exchange on the Pt(ll1) surface by flash desorption after adsorption of hydrogen at 150 K. These experiments employed much higher surface coverages (about a monolayer) than the high temperature experiments discussed previously. The investigation revealed high rate of exchange even on the Pt(ll1) crystal face that was not active under conditions of the molecular beam experiments, while some degree of structure sensitivity for this reaction was still discernible. Of course H,-D, exchange when carried out on platinum surfaces of any atomic structure near one atmosphere pressure even at low temperatures would lead to rapid equilibration between reactants and the product. The HD product concentration, in this circumstance, is determined by the thermodynamics of this system and structure sensitivity could not at all be observed. These studies carried out in different pressure and temperature ranges that markedly influence surface coverage and residence times clearly indicate that elementary steps of surface reactions must be studied as far from reaction equilibrium as possible. At high pressures (under conditions similar to those utilized in the chemical technology) the surface coverage is near unity and the various surface reaction steps are equilibrated. In this circumstance either gas phase diffusion or desorption that is the most endothermic surface reaction step, in general, controls the reaction rate and the structure sensitivity of the reaction can be masked. As the pressure and thus the coverage is reduced the surface site sensitivity of the catalytic reaction becomes detectable since various surface sites have different heats of adsorption, reactant populations, and reaction rates. The surface structure sensitivity is revealed using crystal surfaces and especially in molecular beam studies that probe the

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

43

reactivity upon single scattering and can separate the products according to their surface residence times. For a catalyzed surface reaction like the exchange of H, with D, we cannot talk about a single mechanism for the reaction. We must specify the experimental conditions (pressure, surface coverage, temperature, and surface structure) as the reaction mechanism is likely to change with changing conditions of the experiments. Also, since there are several reaction paths available at the various surface sites, even under specified experimental conditions it is likely that the experimental technique utilized to monitor the reaction rate and product distribution may not detect products that form along the various reaction branches with equal probability. Thus, a combination of techniques that are employed over a wide range of experimental variables is necessary to reveal the nature of the complex catalytic process.

B. DEHYDROGENATION AND HYDROGENOLYSIS OF CYCLOHEXANE ON PLATINUM CRYSTAL SURFACES AT Low PRESSURES (< TORR) In a series or studies, the variation of the turnover number for the dehydrogenation reaction (the number of product molecules/platinum surface atoms/second) with the hydrogen to hydrocarbon ratio at a constant hydroTorr was determined. The results are shown in carbon pressure of 4 x Fig. 20 for the several stepped surfaces studied. The reaction rates increase with increasing hydrogen to hydrocarbon ratio. If no hydrogen is introduced into the reaction chamber, the catalyst behaves very differently. No benzene

Z

1

0

100

I

1

200

300

Hydrogen /Hydrocarbon

FIG.20. The steady-state rate of production of benzene and cyclohexene from cyclohexane as a function of hydrogen to hydrocarbon ratio. The reaction conditions are 4 x lo-* Torr ofcyclohexane and 423 K catalyst temperature. A,Pt(S)-[7(1OO) x (1 1 l)]; I?,Pt(S)-[3(111) x (loo)]: 0,Pt(S)-[6(111)x (loo)] @, Pt(S)-[6(111) x (710)]; .,Pt(S)-[7(111) x (310)l.

44

G. A. SOMORJAI

is produced and cyclohexene production is reduced greatly. There is also a higher than normal amount of carbon residue on the surface, approximately one monolayer. Pretreating the catalyst in hydrogen and then removing it prior to hydrocarbon introduction does not increase the activity for dehydrogenation or hydrogenolysis. We shall present the results ofthe reaction rate studies for dehydrogenation and hydrogenolysis that were obtained on stepped platinum surfaces first (35). Then we shall present the same rate data obtained for stepped surfaces with a large concentration of kinks in the step. In Fig. 21a the turnover number for dehydrogenation to benzene and hydrogenolysis to n-hexane is shown as a function of step density at 423 K. The dehydrogenation rate is independent of step density, while the hydrogenolysis rate increases with increased step density. The hydrogenolysis rate that was measured via the rate of formation of n-hexane, one of the hydrogenolysis products, was lower than that of dehydrogenation to benzene. The molar hydrogenolysis product distribution (saturated aliphatic hydrocarbons only), appears to be a C , : C , : C , = 1:1:4. I

I

0-@ 0--

150'

C

4 x Io-' torr reactant 1.5x10-s~

I.oxIo-5

2

1 . 5 X d - Dehydrogenation I.0X Io-J'L

0.5XIO-'

0

,A,'

1

,+

-,'

'

1

T

,

-

' 0

(b)

* ,*\Hydrogenolysis

-

For 2.5x 10I4step atoms/cm' I

I

FIG.21. (a) Cyciohexane dehydrogenation to benzene (0) and hydrogenolysis to n-hexane a function of step density. (b) Cyclohexane dehydrogenation to benzene and hydrogenolysis to n-hexane as a function of kink density at a constant step density of 2.0 x 10'4/cm2.

(A) as

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

45

Even though n-hexane is a minority hydrogenolysis product, it is a reliable measure of the degree of hydrogenolysis because of its ease of mass spectrometric detection and it is not formed in a background reaction with the walls of the reaction chamber. Besides the saturated hydrogenolysis products and benzene, we found the olefinic products cyclohexene, ethylene, and propylene. Cyclohexene is an intermediate in the dehydrogenation to benzene and its various reactions will be discussed separately in the next section. The olefinic product distribution of ethylene: propylene: cyclohexene: benzene is 10: 1 :0.5:1. The turnover numbers of dehydrogenation and hydrogenolysis on kinked surfaces are shown in Fig. 21 b. The kink density is defined as the number of kink sites per square centimeter (the total number of atoms on the surface is approximately 1.5 x 10'5/cm2).For example, on the Pt(S)-[7(111) x (310)l surface every third atom along the step should, on the average, be in a kink position. Therefore, for this surface the step density is 2.0 x 1014/cmZand the kink density is approximately 7 x lo13 / ~ m zBy . comparing the turnover numbers with those obtained from stepped surfaces that were shown in Fig. 21a, it appears that the rate of hydrogenolysis is markedly higher in the presence of kinks. The dehydrogenation rate is approximately constant and remains unaffected by variation of kink density, while the hydrogen01ysis rate increases by an order of magnitude from a surface that is almost free of steps, Pt(l11). The kinks in the stepped surface appear to be very effective in breaking C-C bonds, leading to much enhanced hydrogenolysis rates. The hydrogenolysis product distributions do not change appreciably with step or kink density; only the rate increases. The independence of the dehydrogenation rate from the step and kink density shows that this reaction is indeed structure insensitive. The hydrogenolysis rate increases with kink density just as with increasing step density ; thus hydrogenolysis appears to be structure sensitive. There was always an induction period of 10 to 20 min before the benzene product reached its steady-state rate of production as detected by the mass spectrometer after the introduction of cyclohexane onto the crystal surface. This is shown in Fig. 22 for several catalyst temperatures. The catalyst was initially at 300 K. When steady-state reaction rates were obtained, the catalyst temperature was rapidly increased (in approximately 30 sec) to 423 K and the reaction rate monitored. This was repeated with heating to 573 and 723 K. The benzene desorbed during rapid heating of the catalyst surface is approximately 1 x 1013 molecules or less and represents only a small fraction of the carbon on the surface. The steady-state reaction rates at a given temperature are the same whether the catalyst was initially at that temperature or another. This induction period coincides with a higher than steady-state uptake of cyclohexane. A mass balance calculation on carbon, utilizing the known

46

G. A . SOMORJAI 4x10-~

I

I

I

~ x I O - ~

~ X I O - ~

z-

1x10-5

0

-I

.-----\,----

L

a,

n

2

0 ~ x I O - ~

C 0 L

s

C, HI2 uptake

3x10-'

~ X I O - ~

Ix 1 0 - 4

0

0

30

60

90

0

Time (minutes)

FIG.22. Induction period for the production of benzene () and cyclohexene (-- ) from cyclohcxane. Hydrogen to cyclohexane ratio 20: 1 ; cyclohexane pressure 4 x 10 Torr. ~

~

adsorption and desorption rates of reactants and products during the induction period, indicated that carbon was deposited on the surface. The amount calculated agreed reasonably well with that determined by the Auger electron spectra taken after the reaction mixture was pumped from the chamber, since the electron beam may induce polymerization of hydrocarbons and further carbon deposition. The formation ofthe adsorbed carbon layer always precedes the desorption of benzene and olefinic products. However, the amount of adsorbate changes as a function of temperature. This is shown in Fig. 23. A 4: 1 ratio of the carbon 274-eV Auger peak to the platinum 238-eV Auger peak corresponds to a complete monolayer of carbon by calibration with acetylene. The carbon coverage ranges from 0.1 monolayer at 300 K to almost 1.0 monolayer at 723 K. The line has a slope of 2 k 0.2

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

47

Fiti. 23. The amount of carbon o n the catalyst surface at steady-state reaction under standard conditions. An Augcr peak height tatio of 4.0 corresponds to approximately 1.0 monolayer of carbon. Line through points at a 2-kcal/mole slope.

kcal/mole. During and after the reaction, this carbon deposit was always present on the surface, not only at our low pressure reaction conditions, but also after reactions that were carried out in another apparatus at higher pressures (approximately 200 Torr total pressure). The temperature dependence of the dehydrogenation and hydrogenolysis rates for the various crystal faces at a fixed hydrogen to hydrocarbon ratio 20: 1 is shown in Fig. 24. The dehydrogenation rate to benzene decreases slightly at 723 K. The rate of formation of olefinic products has a similar temperature dependence as that of benzene. The hydrogenolysis rate to saturated products increases with increasing temperature, and an Arrhenius plot gives an activation energy of 3 k 0.3 kcal/mole, which is the same for all of the crystal faces within our experimental accuracy. We have found that the dehydrogenation reaction of cyclohexane to form benzene was sensitive to the ordering of the carbonaceous overlayer, as shown in Fig. 25. Initially, the overlayer was ordered on all of the stepped surfaces that were studied, and dehydrogenation yielded more benzene than cyclohexene. The LEED pattern from the carbon deposit formed on stepped surfaces in 20: I hydrogen to hydrocarbon reaction mixture at 423 K and above has a hexagonal unit cell approximately 5.1 A on a side. This is about 5% larger than the next nearest neighbor distance of platinum and considerably smaller than the van der Waal's radius of either benzene (7.3 A) or cyclohexane (7.6 A), indicating that the adsorbed layer is at least partially dehydrogenated and the diffraction pattern is certainly not due to the intact reactant or product molecules. Complete dehydrogenation, which occurs on

48

G. A. SOMORJAI

FIG.24. Temperature dependence of dehydrogenation of cyclohexane to benzene (0) and hydrogenolysis (A). The overall activation energy for hydrogenolysis is 3 5 0.5 kcal/mole. Standard reaction conditions, data for Pt(S)-[6(111) x (lOO)].

--

-i

Ordered overlayer

1 1 .. 5 5~ ~1 10 0~ ~ --

.....* ...*

Disordered overlayer

*......

___------o--

...-

&.---

-

E

-.\

I

0

I

3 Time ( h o u r s )

2

I

4

5

FIG.2 5 . Inhibition of benzene (0) from cyclohexane and increasing cyclohexene formation x (IOO)] surface. All catalysts with ( 1 1 1 ) orientation terraces behave similarly. T, 150°C;4 x Torr reactant; H,:HC, 20:l.

(0) with time on Pt(S)-[6( 1 1 I )

heating the adsorbed layer to above lo00 K, yields graphitic deposits characterized by ringlike diffraction features of 2.46 A unit cell size. After several hours of reaction time, the carbonaceous overlayer slowly disorders. Simultaneously, the rate of production of cyclohexene increases, while the rate of benzene formation decreases until the product becomes predominantly cyclohexene. As shown in Fig. 25 for the Pt(S)-[6(111) x (loo)] surface at 423 K, the initial 2 : l benzene to cyclohexene product ratio typical for de-

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

49

hydrogenation on ordered carbonaceous overlayers becomes 1 :3 on a disordered overlayer. Thus, for all practical purposes, the dehydrogenation on disordered overlayers produces cyclohexene as further dehydrogenation to benzene is poisoned. A small amount of oxygen on a stepped surface is an effective poison for dehydrogenation. If the catalyst sample was not vacuum reduced at 1375 K after oxygen cleaning, approximately 0.1 of a monolayer of oxygen (by AES) would be left on the catalyst. This was enough to stop completely the production of benzene and decrease the cyclohexene production by 50% at 423 K on the Pt(S)-[6(111) x (loo)], The 0.1 monolayer coverage would be less than one oxygen atom/step atom if all the oxygen was adsorbed at the steps. The oxygen was still present on the surface after 1 hour of reaction at 423 K and standard pressure conditions.

c. DEHYDROGENATION AND HYDROGENOLYSIS OF CYCLOHEXENE ON PLATINUM CRYSTAL SURFACES AT Low PRESSURES (35) The turnover number for the dehydrogenation of cyclohexene to benzene is about two orders of magnitude greater than for the dehydrogenation of cyclohexane. In Fig. 26a we plot the dehydrogenation rate as a function of step density. The turnover number increases rapidly with step density, indicating that unlike the slower dehydrogenation reaction of cyclohexane, this reaction is structure sensitive. In Fig. 26b the turnover number is plotted as a function of kink density. Although there is a small increase in the dehydrogenation rate, it may be considered insignificant compared to the marked change of rate with step density. Unlike the dehydrogenation of cyclohexane, the cyclohexene dehydrogenation reaction poisons rapidly on many catalyst surfaces. Using a hydrogen to cyclohexene mixture of 20: 1, the rate of dehydrogenation reaches a maximum, then decreasing rapidly as poisoning occurs, the catalysts losing approximately one-half their activity in 10-12 min. Figure 27 shows a representative plot of the turnover number as a function of time. On many catalyst surfaces, particularly on those with (111) orientation terraces, a disordered carbonaceous overlayer forms, which poisons further dehydrogenation of cyclohexene. The poisoning is greatly decreased, however, if the carbonaceous overlayer is ordered. The overlayer is disordered on (1 11) orientation terraced stepped surfaces, while the overlayer orders on surfaces with (I 00)orientation terraces upon cyclohexene-hydrogen adsorption at 423 K. With an ordered overlayer, the rate of dehydrogenation remains high for hours and there is only slow deactivation of these catalysts. On both types of catalyst surfaces the coverage is approximately 1.0 monolayer of carbon after the induction period during the chemical reactions.

50

G . A. SOMORJAI 1

'

1

'

1

ZXIO-~-

-

- (a) Totol otorns

I

,

I

,

I

#

w

Y

t

1

Step density (atoms/cm2

E L

E

,

0 0

5 ~ 1 0 ' ~I O X I O ' ~ Kink density (kink atoms/cm2

1

FIG. 26. Cyclohexene dehydrogenation to benzene as a function of (a) step density and (b) kink density. Standard reaction conditions.

Io x 10-3

i '7(lOO)x(lll) ordered overlayer 13(111)x(310)disordered overlayer

"0

5

10

15

20

25

30

35

40

Time (minutes)

FIG.27. Inhibition of benzene formation from cyclohexene on disordered carbonaceous overlayers( --), Pt(S)-[6(111j x (lOOj] and Pt(S)-[13(111j x (IOOj]; and lack ofinhibition of ordered carbonaceous overlayers (- -), Pt(Sj-[7(100) x ( 1 I lj]. Standard reaction conditions. ~

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

51

D. HYDROCARBON REACTIONS ON PLATINUM CRYSTAL SURFACES HIGHPRESSURES ( 1 - 1 O3 TORR). CYCLOPKOPANE, CYCLOHEXANE, AND II-HEPTANE

AT

Perhaps the most significant step in bridging the gap between catalytic reaction studies on crystal surfaces at low pressures, on the one hand, and dispersed metal particles at high pressures, on the other, is represented by high-pressure studies of chemical reactions on crystal surfaces. A series of experimental apparatuses have been developed that permit us to study catalytic reactions on crystal surfaces at high pressures after suitable cleaning by ultrahigh vacuum (UHV) techniques and analyzing the surface structure and surface composition by LEED and AES. Detailed descriptions of these equipments are given elsewhere (22a) and a brief description of the most versatile apparatus, which allows the study of the reactivity of the crystal surface at both low and high pressures in situ, is in the experimental part of this paper. Using these various apparatuses for high-pressure studies on crystal surfaces, the turnover numbers and product distributions have been determined for the hydrogenolysis of cyclopropane and cyclohexane, the dehydrogenation of cyclohexane, and the dehydrocyclization of n-heptane (36h).The purpose of these investigations is to determine the rate equations that govern these reactions at high pressures and compare them to the rate equations that were determined at much lower pressures on the same crystal surface. In this way, the mechanisms of the same reaction at low and high pressures are compared and attempts made to analyze the chemical changes that occur over nine orders of magnitude reactant pressure range. These studies are beginning to produce important kinetic information only in recent years. We shall summarize the experimental information available from studies ofthese reactions on the stepped [6(111) x (loo)] and low Miller index (1 11) crystal surfaces and compare the low- and highpressure rates when these data are available. The hydrogenolysis of cyclopropane was studied at 1 atm total pressure on the stepped single-crystal surface of platinum (4). The hydrogenolysis of cyclopropane was chosen as the test reaction because of the considerable amount of data and experience that has been collected in studies of this reaction in various laboratories. The rate is relatively high even at room temperature on supported platinum catalysts and only one product, propane, is formed below 150"C, thereby simplifying the analysis of the results. Table I1 summarizes the results that were obtained and compares our results on stepped single-crystal surfaces at atmospheric pressure with those of others obtained using supported platinum catalysts. It appears that at 1 atm pressure the platinum stepped single crystal behaves very much like a highly dispersed supported platinum catalyst for the cyclopropane hydrogenolysis.

52

G. A. SOMORJAI

TABLE 11 Compurison of Initial Specific Rare Data for the Cyclopropune Hydrogenolysis on Platinum Cutulysrs Calculated specific reaction rate at Pcp = 135 Torr and T = 75°C ~

Dala source

Type of catalyst

Present study

Run 10A Run 12A Run 15 Run 16 Average 0.04 wt Pt on

Hegedus (36")

Boudart et ul. (3W Dougharty (361)

441203

0.3% and 2.0% Pt on q-Al,03 0.3% and 0.6% Pt on y-AI,O,

(moles C,H,/min cmz Pt)

~~

(molecules C,H,/min Pt site)

2.1 x 1.8 x 1.8 x 2.1 x 10-6 1.95 x 10-7

312 410

based on 100% Pt dispersion 8.9 x 1 0 - 7

480

1.1

I340

2.5 x

In addition, the same studies that were carried out on the Pt(ll1) crystal face result in reaction rates identical to those found on stepped crystal surfaces of platinum. These observations support the contention that well-defined crystal surfaces can be excellent models for polycrystalline supported metal catalysts. It also tends to verify Boudart's hypothesis that cyclopropane hydrogenolysis is an example of a structure-insensitive reaction. The initial specific reaction rates, which were reproducible. within lo%, are within a factor oftwo identical to published values for this reaction on highly dispersed platinum catalysts. The activation energies that were observed for this reaction, in addition to the turnover number, are similar enough on the various platinum surfaces so that we may call the agreement excellent. In a series of studies, the dehydrogenation and hydrogenolysis of cyclohexane was studied on both the stepped and low Miller index (1 11) crystal faces of platinum at a surface temperature of 300°C and a hydrogen to cyclohexane ratio of 20: 1. While the rates on the stepped and low Miller index surfaces were not very different for the formation of benzene and hexane, the formation of cyclohexene was very structure sensitive, its rate being 100 times greater on the stepped surface than on the (111) crystal face. In Table 111 t-nmnare the initial turnover numbers - for the various reactions at low -1'-

__

-----

_I...

53

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

TABLE 111 I t i i l i u l T u r n o w Numbers o t i Phtiririni ('rysrril Siirjires ,for Vnrious /Iyrlroc~irbotiReortiom (11 H i y h t i t i d Idow Pressures"

High pressure total pressure 21 5 'l'orr

React ion

C,,Hi, C,?Ifil

+

+

CbH, C'i,Hi,i

C',,Hio C,,H,, /i-Heptanc .--t Loluene 11-Hcptanc --t methyl cyclohcxanc Cyclopropanc + propane -

"

+

reactant pressure I S Torr

3 ' X I 0

100 10

in

I

150

Low pressurc total pressure 8 x 10

'

Torr reactant pressure 4 x lo-* Torr 6.3 10-5 2.0 x 1 0 - 5 4 x 10 ~

10

A t 300 c': i n s c c - ' ,

the rate cquation, both at low and high pressures, and to determine the rate constants under these two widely different experimental conditions. These studies should lead to a complete picture of the mechanism of hydrocarbon reactions on platinum surfaces at both low and high pressures. VI. Active Sites for C-H, H-H, and C-C Bond Breaking on Platinum Crystal Surfaces

Dehydrogenation of cyclohcxanc and cyclohexene to benzene occurs readily at low pressures ( < 1 V"'Torr) on stepped platinum catalyst surfaces (35).This is in contrast with the very slow or negligible dehydrogenation rate of these molccules, a t low pressures on the Pt ( 1 11) catalyst surface (36c). Th~is,C H bond breaking takes place at atomic steps, the same stcps that are etkctivc in breaking H-H bonds as revealed by studies in this laboratory of the H - D 2 exchange reaction at low pressures, using molecular-beam scattering tccliniques (31). Atomic steps on platinum surfaces appear to be the active sites for C-H and 1-1 H bond scissions. We have been able to identify another active site by studying the ratio of the dehydrogenation rate to hydrogenolysis rate of cyclohexane to benzene and rz-hexane, respectively (36tr). While the bcnzene: n-hexane ratio is 3: I on ii stepped surface (with roughly 177<, ofthe surface atoms in stcp positions), the ratio decreases rapidly with increasing kink density (Fig. 21b). Using a set of catalyst surfaccs that were cut to maintain the same terrace width (step density equal to 2.5 x 10'"/cm2), but with variable kink density in the steps, wc have found that thc hydrogenolysis rate increases linearly with kink

54

G . A. SOMORJAI

density, while the dehydrogenation rate remains unaffected. On a Pt(S)[7(111) x (310)l catalyst surface, approximately 30% of the atoms in the step are in kink positions (in addition to the thermally generated kinks). For this surface the benzene to n-hexane ratio has reached unity. Thus, the microstructure of kinks in the steps is effective in breaking C-C bonds in addition to C-H and H-H bonds. The selectivity of these bond-breaking processes at different atomic surface sites on platinum is certainly significant in that the atomic surface structure of platinum may be properly tailored to provide selectivity in chemical reactions where C-H and C-C bond-breaking processes are to be separated. VII. The Role of the Carbonaceous Overlayer in Hydrocarbon Reactions on Platinum Surfaces During dehydrogenation of cyclohexane and cyclohexene, the platinum crystal surfaces are always covered with a carbonaceous deposit of 0.1 to 1.O monolayer judged by the carbon:platinum Auger peak intensity ratio. The coverage appears to increase with increasing reaction temperature but is rather independent of pressure, as indicated by recent high-pressure studies on the Pt(S)-[6( 1 11) x (loo)] catalyst surfaces in this laboratory. The overlayer coverage also depends on the particular surface reaction, higher molecular weight reactants and products (cyclohexene, benzene, n-heptane, toluene) yielding greater coverage than low molecular weight reactants and products (cyclopropane, propane, etc.). Low molecular weight hydrocarbons (cyclopropane, ethane) that do not form carbonaceous overlayers do not readily react on platinum surfaces at low pressures. The buildup of adsorbates during the induction period for cyclohexane and cyclohexene dehydrogenation to benzene indicates the need for the formation of carbonaceous overlaycr to obtain the products. This is not a buildup ofthe product benzene, since it will desorb at a rate two orders of magnitude higher as evidenced by the rate of cyclohexene dehydrogenation. During the dehydrogenation of cyclohexane the carbonaceous overlayer is ordered initially. After a few hours of reaction 423 K, however, the overlayer becomes successively more disordered as judged by its LEED pattern. The amount of carbon in the overlayer, however, remains constant at approximately 0.3 monolayer as determined by AES. Simultaneously, the product distribution in the dehydrogenation reaction changes as well. While benzene is the dominant product in the presence of the ordered overlayer, cyclohexene becomes the major product of the dehydrogenation reaction in the presence of the disordered overlayer. This is shown in Fig. 25. Thus, the disordering of the carbonaceous overlayer poisons the formation of benzene, i.e., the dehydrogenation of cyclohexene, and under the reaction conditions

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

55

the cyclohexene intermediate becomes the final product. It should be noted that the turnover number for the cyclohexeneebenzene reaction is two orders of magnitude higher (approximately 10- 3/se~)than for the cyclohexanebenzene reaction (approximately 10- '/set). Thus, the presence of the disordered overlayers poisons the fast second step, but not the first slow step, in the dehydrogenation of cyclohexane to benzene. The marked effect of the ordering characteristics of the carbonaceous deposit on the reaction rate is also clearly displayed during our studies of the dehydrogenation of cyclohexene. As shown in Fig. 27, there is rapid poisoning of the dehydrogenation rate within minutes as the disordered carbonaceous overlayer forms. However, when the overlayer is ordered [on (100) orientation terraced surfaces], the catalytic activity decreases much more slowly. Again, the poisoning of benzene production is prevented by the formation of an ordered overlayer. Since the platinum catalyst surface is covered with a carbonaceous layer at low as well as at high pressures, we must consider this layer an important part of the surface reaction. Carbonaceous overlayers can have an important effect in both the catalytic activity and selectivity of a metal surface. Weinberg et al. (37)postulated that the carbonaceous overlayer is the catalytic site for the hydrogenation of ethylene on the Pt(l11) surface. Gardner and Hansen (38) reported similar results for tungsten stepped surfaces. Yasumori et al. (39) found that preadsorbing acetylene prevents poisoning or restores the activity of a palladium film for the hydrogenation of ethylene. In all three cases, the structure of the carbonaceous overlayer has a marked effect on the catalytic activity in a manner that is not simple site blockage poisoning. Holbrook and Wise (40) found a specific pretreatment of their palladium catalyst that involved oxygen activation, and hydrocarbon preadsorption could markedly affect the selectivity of an isomerization reaction. The rate of dehydrocyclization of nheptane, as well as the selectivity to isomerization and hydrogenolysis, was observed in this laboratory (41) to be dependent on the ordering of the carbonaceous overlayer. These observations, in addition to the data presented in this paper indicate that the formation of the carbonaceous overlayer on the catalyst surface can affect the selectivity as well as activity of a catalytic reaction. The presence of these effects at both atmospheric and low pressures and on a variety of metals indicates the importance of the carbonaceous overlayers and the need for their further characterization. It is likely that the carbon atoms in the overlayer participate in the carbon-metal bonds similar to those found in many chemically active homogeneous metalorganic systems. Exploration of the nature of chemical bonding between the transition metal (platinum in this case) and the carbonaceous deposit is an area of promising research for the future. We are led to the conclusion that not all carbon on a heterogeneous catalyst surface is deleterious and only amorphous forms cause site blockage poisoning.

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G . A . SOMOKJAI

VIII. The Mechanism of the Dehydrogenation of Cyclohexane and Cyclohexene: Expanded Classification of Reactions According to Their Structure Sensitivity

In dispersed-metal catalysts, the metal is dispersed into small particles, on the order of 5 to 500 A in diameter, which are generally located in the micropores (20- 1000 .$) of a high surface area support. This provides a large metal surface area per gram for high, easily measurable reaction rates, but hides much of the structural surface chemistry of the catalytic reaction. The surface structure of the small particles is unknown; only their mean diameter can be measured and the pore structure could hide reactive intermediates from characterization. Some of the same difficulties also hold for thin films. However, we can accurately characterize and vary the surface structure of our single-crystal catalysts, and in our reactor the surface composition can also be readily measured; both are prerequisites for the mechanistic study of the catalysis on the atomic scale. We have been able to identify two types of structural features of platinum surfaces that influence the catalytic surface reactions: (a) atomic steps and kinks, i.e., sites of low metal coordination number, and (b)carbonaceous overlayers, ordered or disordered. The surface reaction may be sensitive to both or just one of these structural features or it may be totally insensitive to the surface structure. The dehydrogenation of cyclohexane to cyclohexene appears to be a structure-insensitive reaction. It takes place even on the Pt(ll1) crystal face, which has a very low density of steps, and proceeds even in the presence of a disordered overlayer. The dehydrogenation of cyclohexene to benzene is very structure sensitive. It requires the presence of atomic steps [i.e., does not occur on the Pt( 1 11) crystal face] and an ordered overlayer (it is poisoned by disorder). Others have found the dehydrogenation of cyclohexane to benzene to be structure insensitive (42, 43) on dispersed-metal catalysts. On our catalyst, surfaces that contain steps, this is also true, but on the Pt(l1I ) catalyst surface, benzene formation is much slower. Dispersed particles of any size will always contain many steplike atoms of low coordination, and therefore the reaction will display structure insensitivity. Based on our findings, we may write a mechanism for these reactions by identifying the sequence of reaction steps: CsH ,,(gas1

I

C6H,&ods)

C6H,,(gas) ordered or disordered over layer 'low

It

C6Hlo(ads)

c6 H6(gOS)

step ordered over lo y er

I

C6H6(add

57

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

The slow step in the dehydrogenation of cyclohexane to benzene is the production of the cyclohexene intermediate at these low pressures on stepped surfaces. Cyclohexene dehydrogenates very rapidly at a step to form benzene; approximately one in every three collisions of a cyclohexene molecule with an unpoisoned step results in the formation ofa benzene molecule. However, on the Pt(l1 I) surface, which is practically free of steps, the rate of dehydrogenation of cyclohexene had become slow enough to be rate-limiting (36c). Sinfelt et a/. ( 4 4 ) concluded the dehydrogenation of methylcyclohexane to toluene, a very similar reaction to cyclohexane dehydrogenation to benzene, was rate limited by the desorption to toluene. Their arguments are equally valid if the slow step was the desorption of methylcyclohexene, followed by its very rapid dehydrogenation to toluene, which would be hidden by the pore structure. Maatman et ul. (43) postulated the slow step, in agreement with out results, as the formation of an intermediate species. Haensel et ut. ( 4 5 ) have observed the intermediate cyclohexene species at very high [approximately 30,000 LHSV (liquid hourly space velocity)] space velocities. This indicates that the intermediate is also found at atmospheric pressure reaction conditions and is very reactive at the step and edge atoms that must exist on the dispersed-metal particles. In addition to dehydrogenation reactions, hydrogenolysis is also taking place on the platinum surfaces. By monitoring the benzene to n-hexane ratio on the various catalysts as a function of surface structure, we have identified steps as primarily responsible for C-H and H-H bond breaking and kinks for C-C bond breaking, in addition to C-H and H-H bond scissions. Thus, hydrogenolysis is initiated at kinks in the atomic steps. Since we need specific surface sites for hydrogenolysis to occur, this is also a structure-sensitive reaction. However, hydrogenolysis is insensitive to the state of ordering of the carbonaceous overlayer. It proceeds whether the carbonaceous overlayer is ordered or disordered. It appears that the classification of structure-sensitive reactions should be expanded to separate those reactions that exhibit step (or kink) sensitivity into one group, and those that are also sensitive to the structure of the overlayer into another group. This expanded classification is shown in Table IV. TABLE IV Classificution o / Recri’tion~by Step Densif!: imtl Carhonuceous Overlayer Dependence ~~

Step sensitive Overlayer sensitive Cyclohexene --t benzene ti-Heptane --* toluene

Overlayer insensitive Cyclohexane

~~

Step insensitive

+

hexanc

Overlayer sensitive

Overlayer insensitive Cyclohexane ---t cyclohexene Cyclopropane -+ propane

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G. A. SOMORJAI

In addition to the dehydrogenation and hydrogenolysis reactions described in this paper, we have included two other reactions that were studied recently. It would be ofgreat value to include in this classification several other hydrocarbon reactions (isomerization, hydrogenation, exchange). More reactions are presently being studied to expand these results on characterized surfaces. Manogue and Katzer (46)have proposed a subdivision of structure-sensitive (demanding) reactions along very similar lines. “Primary structure-sensitivity” is the effect of changing particle size or step and kink density. Their “secondary structure-sensitivity” includes effects ofself-poisoning and oxygen impurity on reaction rate. The self-poisoning phenomenon is, for hydrocarbon reactions on platinum, at least at low pressure, the sensitivity of a reaction to the order in the carbonaceous overlayer. However, caution must be exercised in studies of structure sensitivity as the reaction mechanism or the surface structure may change markedly with pressure, temperature, and reactant ratio. Most of the surface structure sensitivity of various catalytic reactions was derived from the particle size dependence of the reaction rate on polydispersed-metal catalyst systems. Although there is excellent agreement between the classifications of the various reactions based on studies using supported metal catalysts with variable particle size and our studies using various single-crystal surfaces, this may not be the case for all reactions. Perhaps the step density or the kink density is proportional to particle size, while the ordering characteristics of the carbonaceous overlayer may or may not be affected by changes of particle size. In addition, studies similar to those reported on platinum must be carried out using crystal surfaces of other transition metals to ascertain that these arguments are more broadly applicable to describe the catalytic chemistry of transition elements. There is evidence that the heat of adsorption of hydrogen on palladium and iridium crystal surfaces varies markedly with step density (30,47),while gold crystal surfaces exhibit chemisorption behavior independent of step density (29).

IX. A Descriptive Model of Hydrocarbon Catalysis on Platinum Surfaces

Studies to correlate the reactivity and the surface structure and composition of platinum surfaces indicate that the active platinum crystal surface must be heterogeneous. The heterogeneity involves the presence of various atomic sites that are distinguishable by their number of nearest neighbors (atoms in terraces, steps, and kinks), and also variation in surface chemical composition. A model that depicts the active platinum surface is shown schematically in Fig. 28. Part of the surface is covered with a partially de-

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

59

Disordered Overlayer

FIG.28. Schematic representation of a platinum catalyst with a monolayer of carbonaceous overlayer showing the exposed platinum clusters.

hydrogenated carbonaceous overlayer, ordered or disordered, from which islands of platinum clusters protrude. These are the platinum atoms in steps and at kinks that are active in various C-C, C-H, and H-H bond-breaking activity. Perhaps because of the ease of dissociation and higher binding energy of hydrogen at the steps, these sites and their vicinity remain clean (as long as there is excess hydrogen) and represent areas of high turnover number. The species that form as a result of bond scission at these clusters may rearrange and then diffuse away onto the terrace, which is covered with the overlayer, where desorption takes place. Alternately, rearrangement takes place on the ordered carbonaceous overlayer prior to desorption. The heat of desorption may be lower on the portion of the surface that is covered with the overlayer than at an exposed step. It should be noted that the presence ofexcess hydrogen is always necessary during hydrocarbon reactions, even in those circumstances when the reaction is hydrogen producing (dehydrogenation, dehydrocyclization, etc.). I t appears that the main role of excess hydrogen is to keep the step and kink sites

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clean. The reduction of the hydrogen pressure can lead to irnmcdiate deactivation as it is frequently experienced in reactor studies. The discovery that kink sites in steps are effective in breaking C -C bonds in addition to C-H and H-H bonds, thereby initiating hydrogenolysis reactions, may also explain the effect of trace impurities or second component metals that introduce selectivity. Since these kink sites have fewer nearest neighbors than step or terrace sites, they are likely to bind impurities or other metal atoms with stronger chemical bonds. Thus, these sites are readily blocked by impurities. As a result selective poisoning of hydrogenolysis may be obtained by minute concentrations o vell-chosen impurities or another metal component.

X. Theory of Low Coordination Number Active Sites on Surfaces

The large difference in the bond-breaking ability between various surface sites that are distinguishable by the number of nearest neighbors must be the result of their unique local structural environment and charge density. The charge density at a corner on the surface has been calculated by Kesmodcl and Falicov (48) employing the configuration dcpicted in Fig. 19. Their calculations utilized the jellium model for metal, which permits computations of the charge density at the surface self-consistently, and also utili7cd the free electron gas properties of tungsten. A t the corner site there is enhanced amplitude of the charged density fluctuation (Friedel oscillation) that leads to an increased potential energy A 4 for electrons on the corner atom. The magnitude of this potential energy difference for free electrons between and away from the corner sites depends on the local atomic structure at the surface irregularity. As a result, some of the free electrons are displaced away from the corner site, leaving behind a net positive charge. The number of electrons AN that are removed from the corner sites is proportional to -A4D(Ef), where D(E,) is the density ofstates at the Fermi level, while the magnitude of A 4 is determined by the local step structure. Thus, there is a large local electric field present at the corner sites of the order of 0.38 V/& which should help further to polarize the incoming molecules that have well-defined polarizabilities and break them apart. The higher the density of states at the Fermi level, the larger is the positive charge at the corner site. For transition metals, the density of states at the Fermi level is very large indeed, while for nontransition metals D(E,) can be quite small. Some of the values of D(E,)(in electron/V/atom) that havc been determined are Pt: 2.1,

Ni:

1.1,

W : 0.7,

Cu: 0.4,

Au: 0.3

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

61

These values yield AN of - 0.6, - 0.3, and - 0.2 for Pt, Ni, and W, respectively, taking A$ = 0.3 V. Thus, for metals with large values of D(E,) there are large variations of charge density at surface irregularities (i.e., low coordination number sites). For gold, on the other hand, A N is about - 0.09. Thus, surface irregularities do not show much charge density variation with respect to atoms at surface sites away from steps. Therefore the surface is likely to present uniform charge density to the incoming reactants and is homogeneous regardless of variations of surface sites. These conclusions are certainly supported by our experiments of chemisorption and chemical reactions on platinum, gold, and iridium surfaces. While surface irregularities like atomic steps have different chemistry for platinum and iridium, both metals with high density of states, gold surfaces show the same chemical behavior regardless of surface atomic structure. For platinum, C-H, H-H, and C-C bond breaking occurs predominantly at low coordination number sites (steps and kinks) at low pressures and temperatures, while atoms in terraces are relatively inactive under these experimental conditions. Thus, the bond scission activities for these three bond-breaking processes could be identified by experiments on low and high Miller index surfaces with relative ease, due to the marked change in reactivity with step and kink density. For iridium and, for that matter, most transition metals to the left of platinum in the periodic table, C-H, H-H, and C-C bonds may be broken on atomic terraces as well, due to the increased M-C and M-H bond energies. Thus, distinguishing between the chemical activities of surface sites of different coordination number may be somewhat more difficult. Detailed experimental analysis of the chemical behavior of different surface sites on cobalt, iron, rhodium, ruthenium, etc., surfaces is yet to be carried out. It is likely that larger binding energy diatomic molecules, CO and N,, should perhaps be more sensitive probes of the bond-breaking abilities associated with different low coordination number sites of these elements than hydrocarbons with weaker chemical bonds. While low coordination number sites, steps, and kinks, arr the active sites for bond breaking in platinum. the atomic terrace sites with larger coordination numbers may also become active sites with unique chemistry for other elements. It will perhaps become possible to identify the bond-breaking ability of various coordination number sites of a given metal in breaking H-H, C--H, C--C, C=0, N r N , etc., chemical bonds. By varying the atomic surface structure, which would change the relative concentrations of the different coordination number surface sites, the product distribution in surface chemical reactions may be markedly varied. Tsang and Falicov (4Y) have calculated the charge density distribution at corner sites in ionic and rare gas crystal surfaces. For ionic solids, low coordination number surface sites should have large charge density variations

62

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and even changes in interionic distances at steps when compared to sites away from steps. On rare gas crystal surfaces, the charge density appears to be uniform, independent of atomic irregularities. These calculations will no doubt be subjected to experimental scrutiny in the not too distant future. Results similar to those obtained by Falicov et a!. for corners on metal surfaces were obtained by Johnson using Xa computational methods. Calculations are also being carried out to determine the lattice vibrational (phonon) spectrum and the mean square displacement associated with surface atoms in steps and kinks (52b). One would expect that the local field experienced by ;-I transition metal ion deriving from (1) the self-consistent field of the s-electrons and (2) the crystal field from the lattice would be different, depending on the ion's position-in the bulk, on the plane surface, or near a step corner. This local field has two major effects: ( 1 ) causing a sizable electron transfer from the stepped corner to the bulk or vice versa, (2) producing different d-orbital level splittings and different d-orbital occupations depending on the position of the metal ion. Tsang and Falicov (50) have calculated the energy levels of d-electrons at different corner sites when the crystal field is turned on. These calculations were made across the periodic table for ions with different number of delectrons, namely, V 2 + , C r 2 + ,Fe2+,Co2+,Ni2+,and Cu2+.The d-electron wave function is a product of the radial and the angular part. Assuming that the radial part is constant, they display the resultant constant contour of the angular part for these various ions. These contours clearly indicate that the stereochemistry of' the comer utom ran he quite itiflerent,fvom thut qf u sur{iire atom; therefore ( I difliwiit kind oj' hoririiriy i s 5rvorcd at diflkrent sites. The results of their calculations have been applied to explain the large differences between the catalytic activity of platinum and gold at atomic steps. Studies of the charge density, the energy, and the spatial arrangement of localized electronic orbitals at low coordination number surface sites (i.e., steps and kinks) appear to be an important and challenging area oftheoretical surface chemistry. The effect of surface irregularities on orbital symmetry considerations, so important in chemical reactions, may further elucidate the unique chemical activity of these asymmetric sites. Although potential energy surface calculations either by trajectory or by transition-state methods for surface reactions are in their infancy, these computations should be of great value in obtaining theoretical insight into the dynamics of surface reactions. Experimental determination of surface reaction rate constants, activation energies, and preexponential factors are slow in coming; however, there are enough data available at present to provide tests for surface reaction theories. This is certainly the case for the H,-D, exchange reaction on various surfaces, as the data for this and other reactions have been reviewed and tabulated recently. It i s hoped that the theoretical models that will be developed will

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

63

take into account the heterogeneity of the surface and the unique chemical activity associated with low coordination number surface sites of various local atomic structure and symmetry. Perhaps one of the important conclusions of these studies that points to the unique chemistry of surface irregularities, steps, and kinks, which appear to be active sites, is the controlling influence of the iocal atomic structure, local surjiace composition, and local bonding between adsorbates and surface sites. The microstructure of the metal surface controls bond scission and thus the rate and path of chemical reactions. Calculations taking into account this local bonding picture should help to unravel the elementary bond-breaking steps in catalytic surface reactions.

ACTIVE SITESON NONMETALLIC SURFACES On transition metal surfaces, much of the chemical activity that results in the breaking of large binding energy chemical bonds (C-H, C-C, etc.) is likely to be associated with surface sites of low coordination number (steps and kinks). The question arises to what extent these low coordination number sites are responsible for bond scission in nonmetallic catalyst surfaces such as oxides and various other semiconductors. Experimental studies over the past several years indicate that low coordination number surface sites are also chemically active in nonmetallic surfaces. Stone et al. (5Za) have studied the decomposition ofmethanol and formic acid on doped MgO surfaces. Doping with cobalt and other metals changes the surface composition and introduces excess vacancies at tetrahedral sites. The decomposition of hydrocarbons has been markedly enhanced with increasing concentrations of these low coordination number defect sites. Cimino and lndovina (5Zb)in a similar study found doping MgO with manganese increased the concentration of surface defect sites and increased the rate of N 2 0 decomposition and CO oxidation. Boudart et al. ( S l c )found that H2-D2 exchange occurs at specially prepared defect sites on an MgO surface. Thus defect sites on oxide surfaces, similar to steps on metal surfaces, have greater catalytic activity. Studies of the sticking probability of oxygen on silicon surfaces revealed a change from to 1 with increasing density of surface steps (52a).Crystals with a large density of steps may be prepared by cleavage, and their chemistry can be readily studied. Ibach has associated the increased activity of dissociatively chemisorbed oxygen to the presence of electron orbitals that became available on silicon atoms at low coordination number step sites on surfaces. Even in the absence of a free electron gas that causes charge redistribution at low coordination number sites on metal surfaces there are strong chemical effects associated with atoms in surface irregularities. The rehybridization of localized electron orbitals should have a marked effect on the chemical

64

G . A . SOMOKJAI

bonding at these sites. These are indicated by experiments on MgO and silicon surfaces as well as by calculations of crystal field splitting of metal ions in steps and charge distribution at corners or ionic-crystal surfaces. It is likely that low coordination number sites are active sites for breaking of chemical bonds of various strength, as well as types of nonmetallic surfaces. Clearly, studics of the surface chemistry associated with low coordination number sites in nonmetallic surfaces is an important area of exploration of catalysis science. XI. Aspects of Enzyme Catalysis on Metal Surfaces

Our studies of hydrocarbon reactions on platinum surfaces when coupled with AES determinations of the composition of the reactive surface indicate that the catalyst surface is covered with a carbonaceous deposit during the surface reaction either at low or at high pressure. Thus the reaction intermediates and products form in the presence of, or perhaps with the participation of, these carbonaceous residues. Studies by Hansen et ul. (53) of ammonia decomposition on tungsten surfaces also indicate that the reactive metal catalyst surface is covered with nitrogen or nitrogen-tungsten compounds during the surface reaction. During the oxidation of ammonia on platinum surfaces there is evidence by AES that tenaciously held chemisorbed nitrogen or nitride is present on the surface during the chemical reaction (54). It is not too difficult to rationalize the necessary presence of such an overlayer by surface thermodynamic arguments. The total surface free energy of metal surfaces would be lowered by the presence of carbon or nitrogen contamination or other deposits. The surface segregation of impurities such as calcium, carbon, or sulfur is well demonstrated by electron spectroscopy studies. The driving force for surfacc segregation of these impurities on metal surfaces comes largcly from the lowering ofthe surface frce energy ofthe metal catalyst system due to their presence. Thus, the catalyst activity is associated not with the metal surface but with the metal-carbon or metal-nitrogen, i.e., the metal--adsorbate system. It is perhaps misleading to consider the metal alone as providing the catalytic surface, as one ought to scrutinize the surface properties of the catalyst in the presence of the reaction mixture. In this circumstance, the surface carbonaceous overlayer or other deposit attributable to the reactant is likely to be an active participant in creating the active catalyst surface. More important, the carbonaceous deposit on the platinum catalyst surfaces was often ordered, and ordering imparted to it unique properties. The presence of an ordered overlayer eliminated the poisoning of dehydrogenation reactions (C6H,, to C6Hb).The dehydrocyclization of n-heptane to

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

65

toluene would only occur in the presence of the ordered carbonaceous overlayer that must form during the course of the reaction. On platinum crystal surfaces where ordering of the hydrocarbon residue was inhibited by either the atomic surface structure of platinum or by the reaction conditions, dehydrocyclization could not occur at all in competition with the much more facile hydrogenolysis or isomerization reactions. It appears that complex molecular rearrangements on the surface require the presence of an ordered overlayer or template to compete successfully with other simpler reactions that take place at an appreciably faster rate. The rates of hydrogenolysis or isomerization reactions are about one order of magnitude faster than the dehydrocyclization rate of n-heptane to toluene at low pressures and also markedly more facile at high pressures. It is not unreasonable to suggest that the ordered overlayer provides the template for slower, more complex chemical rearrangements during the course of the surface reactions by providing sites at proper distances and symmetry commensurate with the molecular dimensions and structure of the complex product to be formed. Future studies must explore the precise experimental conditions for ordering of the carbonaceous overlayer, its composition, and the unique relationship between its structure and that of the product molecules. It is likely that this simple mechanism for product selectivity is also the property of other catalyst surfaces that are employed in hydrocarbon reactions or in reactions of nitrogen- and oxygen-containing compounds both in reducing and oxidizing environments. Perhaps these overlayer structures provide the bridge between heterogeneous catalysis and enzyme catalysis. XII. Possible Correlations between Homogeneous and Heterogeneous Catalysis

Heterogeneous catalyst surfaces with their multitude ofirregularities, steps, and kinks of various configurations provide ready sites for bond-breaking processes of many types, which may occur simultaneously. Homogeneous catalysts that consist of a single metal atom surrounded by ligands cannot easily match the varied reactivity of the heterogeneous surface, especially in breaking simultaneously strong chemical bonds of different types. However, there are similarities between the homogeneous and heterogeneous systems that are already apparent. Internal rearrangement of the molecule, which results in exchange among the surrounding ligands, is much faster and takes place on a time scale much shorter than the exchange of the ligand with external molecules. Likewise, rearrangement of adsorbed molecules takes place much faster than desorption of the final product on heterogeneous surfaces. Thus, rearrangement on the surface of both heterogeneous

66

G. A . SOMORJAI Enzyme Cotolysis

Heterogeneous Cotolysis

r' CI

f

ters

Homogeneous C a t a l y s i s

FIG.29 Relationship between homogeneous, heterogeneous, and enzyme catalysis as inferred from the experimental studies of hydrocarbon catalysis on platinum surfaces.

and homogeneous systems is fast compared to external chemical exchange. We should increase the surface area of the homogeneous system and provide metal-metal bonds to increase the similarity further. Metal clusters, i.e., molecules with several metal atoms joined together and then stabilized by suitable ligands, are likely to approximate the catalyst activity of the heterogeneous systems. Clusters of this type, even bimetallic clusters, have already been synthesized. It appears that the asymmetric structure of a step or kink is important in providing the charge density distribution and orbital configuration necessary to break strong chemical bonds. Asymmetric clusters might be able to exhibit bond-breaking activity similar to that ofsteps and kinks on metal surfaces. It is also likely that the metal-carbon bonds, which form between the metal substrate and the carbonaceous overlayer, are similar to metal-carbon bonds found in many organometallic clusters, thus leading to similar chemistry for the two systems. We may then view the relationship between homogeneous, heterogeneous, and enzyme catalysis as depicted in Fig. 29. The two dominant features of heterogeneous metal catalysis, the importance of low coordination number sites to break chemical bonds and the structural properties of overlayers that control the path of more complex surface reactions, are the bridges between these fields. Future studies will verify how well these views are justified. RI!kERENCES

1. 2. 3. 4. 5.

Kesmodel, L. L., and Somorjai, G. A., M T P I n i . Reu. Sci. 7, 1 (1975). Szalkowski, F. J . , and Somorjai, G. A,, Adz,. Hiyl7 Temp. Chem. 4, 137 (1971) Lang, B., Joyner, R. W., and Somorjai, G. A,, J . Curd. 27. 405 (1972). Kahn, I).R.. Petersen, E. E., and Somorjai, G. A , , J . Cuial. 34,294 (1974). Baron, K., Blakely, D. W., and Somorjai, G. A,, Surf. Sci. 41, 45 (1974).

ACTIVE SITES IN HETEROGENEOUS CATALYSIS

6. Kesmodel, L. L., and Somorjai, G. A,, Phys. Reu. B I t , 630 (1975). 7. Stair, P. C., Kaminska, T. J., Kesmodel, L. L., and Somorjai, G. A,, Phys. Rev. B 11, 623 (1975). 8. Morgan, A. E., and Somorjai, G. A., Sur/: Sci. 12, 405 (1968). 9a. Fedak, D. G . , and Gjostein, N . A., Surf: Sci. 8, 77 (1967) and Ignatiev, A,, Jones, A. V., and Rhodin, T. N., Surf. Sci. 30, 573 (1972). Yb. Martin, M. R., and Somorjai, G. A,, Phys. Reo. B 7, 3607 (1973). I O U . Tucker, C. W., J . Am. Phys. 35, 1897 (1964). IOb. Ellis, W. P., and Schwoebel, R. L., Surf. Sci. 11, 82 (1968); Surf. Sci. 30. 573 (1972). 1Oc. Bonzel, H . , and Ku, R., Surf. Sci. 33, 91 (1972). f1. Perdereau, J., and Rhead, G . E., Surf Sci. 24, 555 (1971). 12. Henzler, M., Surf. Sci. 19. 159 (1970); 22, 12 (1970). 13. Lang, B., Joyner, R. W., and Somorjai, G . A., Surf: Sci. 30,440 (1972). 14. Houston, J. E., and Park, R. L., Surf. Sci. 26, 269 (1971). 15. Lang, B., Joyner, R. W., and Somorjai, G. A,, Surf Sci. 30,454 (1972). 16. Somorjai, G . A,, in “Proceedings of the Battelle Congress Heterogeneous Catalysis, 1975” (E. Dranglis, and R. 1. Jaffee, eds.), pp. 395-414. Plenum, New York, 1976. 17. Farrell, H . H., and Somorjai, G. A., Ado. Chem. Phys. 20, 215 (1971). 18. Somorjai, G. A., Surf: Sci. 34, 156 (1973). IYa. Woad, E. A,, J . Appl. Phys. 35, 1306 (1964). 1%. Park, R. L., and Madden, H. H., Surf Sci. 11, 188 (1968). 20. Eastman, D. E., Phys. Rev. B 3, 1769 (1971). 2la. Siegbahn, K., e t a / . , “Electron Spectroscopy for Chemical Analysis” Almquist & Wiksells, Uppsala, 1967. ?Ih. Palmberg P., et al., “Handbook of Auger Electron Spectra.” Phys. Electron. lnd., Edna, Minnesota, 1972. 22n. Blakely, D. W., Sexton, B. A., Kozak, E., and Somorjai, G. A,, J . Vac. Sci. Technol. to be published (1976). 22h. Brumbach, S . B.. and Somorjai, G. A., Crit. Rev. Solid Srate Sci. 4,429 (1974). 23. Bernasek, S. L., and Somorjai, G. A,, Prog. Surf: Sci. 5, 377 (1975). 24. Gland, J. L., and Somorjai, G. A,, A h . Colloid Inzerface Sci. 5, 203 (1976). 25. Szalkowski, F. J., and Somorjai, G . A,, J . Chem. Phys. 54, 389 (1971). 26. Tracy, J. C., and Palmberg, P. W., J . Chem. Phys. 51,4852 (1969). 27. Chesters, M. A., and Pritchard, J., Surf. Sci. 28,460 (1971). 28. Ignatiev, A., Jones, A. V., and Rhodin, T. N., Surf Sci. 30, 573 (1972). 2Y. Chesters, M. A., and Somorjai, G. A., Surf: Sci. 52, 21 (1975). 30. Nieuwenhuys, B. E., Rovida, G. F., and Somorjai, G. A., Surf Sci. to be published (1976). 31. Bernasek, S. L., and Somorjai, G. A,, J . Chem. Phys. 62, 3149 (1975). 32. Palmer, R. L., Smith, J. N., Saltsburg, H., and O’Keefe, D. R., J . Chem. Phy.7. 53, 1666 ( 1970). 33. Bradley, T. L., Dabiri, A. E., and Stickney, R. E., Surf Sci. 29, 590 (1972). 34a. Balooch, M., and Stickney, R. E., Surf: Sci. 44, 310 (1974). 34h. Lu, K. E., and Rye, R. R., Surf Sci. 45, 677 (1974). 34c. Christmann, K., Ertl, G., and Pignet. T., Surf: Sci. 54, 365 (1976). 35. Blakely, D. W., and Somorjai, G . A,, J. Catul. to be published (1976). 36a. Blakely, D. W., and Somorjai, G. A,, Nature (London)258, 580 (1975). 36b. Herz, R., Blakely, D. W . , and Somorjai, G . A,, J . Catd. to be published (1976). 36c. Gland, J. L., Baron, K., and Somorjai, G. A,, J . Caral. 36,305 (1975). 36d. Hegedus, L. L., and Petersen, E. E., J . Curd. 28, 150 (1973). 36e. Boudart, M., Aldag, A., Benson, J. E., Dougharty, N. A,, and Harkins, C. G., J. Catal. 6, 92 (1966).

67

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36J Dougharty, N . A , , Ph.D. Thesis, University of California, Berkeley, 1964. 37. Weinberg, W. H., Deans, H . A,, and Merrill, R. P., Surf. Sci. 41, 312 (1974). 38. Gardner, N. C., and Hansen, R. S., J . Phys. C‘henz. 74, 3298 (1970). 3Y. Yasumori, I . , Shinohara. H., and Inoue, Y . , in “Catalysis” (J. W . Hightower, ed.), p. 771. North-Holland Publ., Amstcrdam, 1972. 40. Holbrook, C. M.. and Wise, H., to be published. 41. Lang, B., Joyner, R. W., and Somorjai, G. A,, Proc. Roy. Soc., London, Ser. A 331, 335 (1972). 42. Poltorak, 0. M . , and Boronin, V. S., Zh. Fiz. Khirn. 40, 2671 (1966); Mitrofanova, A. N., Boronin, V . S., and Poltorak, 0. M. ihitl, 46, 32 (1972). 43. Maatman, R. W., Mahaffy, P., Hoekstra, P., and Addink, C., J . Cu/ul. 23, 105 (1971); Cusamano, J. A,, Dembinski, G. W., and Sinfelt, J . H., &id. 5, 471 (1966); Kraft, M., and Spindler. H., Pruc. trit, C‘ongr. Cutcil., 41h, 1970 Vol. 2, p. 286, 1971. 44. Sinfelt, J . H., Hurwitz, H., and Shulman, R. A,, J . Ph.ys. Chrm. 64, 1559 (1960). 45. Haensel, V., Donaldson. G . R., and Riedl, F. J., Proc. In!. Conyr. Cuzal., 3rd, 1964, Vol. I , p. 294 (1965). 46. Matioguc, W. H , , and Katzer, J . R., J . Caral. 32, 166(1974). 47. Conrad, H., Ertl, G., and Latta, E. E., Sirr$ Sl,i. 40, 435 (1974). 48. Kesmodcl, L. L., and Falicov, L. M.. Solid State Comrnun. 16, 1201 (1975). 4Y. Tsang, Y . W., and Falicov, L. M., Phjs. R w . 12, 2441 (1975). 50. ?’sang, Y . W., and Falicov, L. M., J . P I i j s . C’. 9, Sl(1976). 5la. Stone, F. S . . private communication. Slh. Ciinino%A., and Indovina, V., J . C‘ural. 33,493 (1974). S I C . Boudart, M., Delbouille, A , , Derouane, E. G . ,Indovina, V., and Walters, A. B., J . Am. Chrm. Soc. 94, 6622 (1972). 524. Ibach, H., Horn, K., Dorn, R., and Luth, H., Surf: Sci. 38,433 (1973). 52h. Maradudin, A,, and Wallis, R., private communication. 53. Hanscn. R . S., private communication. 54. Schmidt, L. D., and Luss, D., J . C‘ural. 22, 269 (1971).

Surface Composition and Selectivity of Alloy Catalysts W. M. H. SACHTLER

AND

R. A. VAN SANTEN

KoninklijkelShell-Lahoratorium Amsterdam (Shell Research B. V . ) , The Netherlands

I. Introduction. . . . . . . . . . . . . . . . . . 11. Surface Composition of Equilibrated Alloys . . . A. Biphasic Alloys. . . . . . . . . . . . . . . B. Monophasic Ordered Alloys . . . . . . . . C . SolidSolutions . . . . . . . . . . . . . . . 111. Selectivity of Alloys in Hydrocarbon Reactions . . IV. Ensemble and Ligand Effects. . . . . . . . . . . A. The Ligand Effect . . . . . . . . . . . . . . B. Heat of Adsorption on Alloys. . . . . . . . V. Conclusions . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . .

. . . . . . . . . . .

. . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

69 71 75 78 82 87 100 103 106 114 1 15

1. Introduction

Interest in heterogeneous catalysis by alloys was very strong in the heydays of the “electronic factor” concept in the 1950s, when it was hoped that the rigid-band model of binary alloys would describe and predict their catalytic behavior. In this model it was assumed that in an alloy the valence electrons of both constituents form common bands consisting of one d-band and one s,p-band overlapping the former. As the holes in the d-band of transition metals were assumed to be responsible for adsorption and catalysis by these metals, a filling of these holes by alloying would drastically influence the adsorption capacity of the alloy. No adsorption of, e.g., hydrogen would be possible for alloys with a filled d-band, unless specific excitation processes would again create the required d-band holes. An increasing number of results, either contradictory to this model or requiring additional ad hoc assumptions, weakened the faith in such a simple model for the prediction of heterogeneous catalysis, with, as a result, a decrease in research effort devoted to this subject. 69

70

W. M . H . SACHI’LER A N D R. A. V A N SANTEN

Interest in alloy catalysis was, however, revived in the late 1960s and the early 1970s. A number of causes can be cited for this renewed interest, which include the following: (1) The discovery that some bimetal systems exhibited a higher catalytic activity than either of their constituents. It is not absolutely clear, however, whether this enhanced activity applied to the initial state of the virgin catalyst or to its steady state, where a lower degree of poisoning by strongly adsorbed by-products would create a higher apparent activity. In this case the lower poisoning of an alloy as compared to its pure components would be due either to a lowcr rate of the reaction creating this poisoning by-product or to an enhanced relative rate for its removal. In either case the rate of the main reaction per unit of unpoisoned surface area might be equal to, or smaller than, that for the unalloyed metal; the apparently enhanced activity would, in other words, result from a change in selectivity. (2) The finding that the selectivity of alloys often differs very strongly from that of their components or any mechanical mixture of them. In particular, the reactions involving C-C fissions are often affected much more severely by alloying than reactions that leave the carbon number of the organic molecule unchanged. (3) The important discovery that supported bimetal and multimetal catalysts when used under severe industrial conditions often display a stability distinctly superior to that of their monometal counterparts. (4)The ascertaining by classical and modern methods that the surface composition of alloys can strongly deviate from the composition of the bulk. These methods have opened up the possibility of correlating catalytic phenomena with catalyst surface composition, thus removing a major drawback of older studies with alloy catalysts. (5) The discovery by ultraviolet photoelectron spectroscopy that the rigidband model is not even applicable to the bulk of alloys such as Ni-Cu and Pd-Ag. The d-electrons of copper or silver in such alloys experience a potential that is predominantly the same as in the metals before alloying. For instance, upon alloying the d-bands of copper and nickel remain discernible in the alloy and no common d-band is formed, as is supposed by the rigidband theory.

The coherent potential approximation for a disordered alloy ( I , 2) provides a satisfactory framework for &scribing the effect of alloying within two extremes: on the one hand, the rigid-band approximation, which supposes that band shapes do not alter upon alloying, and on the other hand, the minimum polarity model, which supposes the electron distribution of the elements forming an alloy to be similar to that in free atoms.

SELECTIVITY OF ALLOY CATALYSTS

71

Even before the results discussed later were known, chemisorption data had shown the inadequacy of the rigid-band model in describing the chemistry ofsurface reactions. Highly diluted alloys ofnickel or platinum in metals such as copper, silver, or gold proved capable of chemisorbing gases such as hydrogen and CO in amounts that were roughly proportional to the concentration of the transition metal element in the alloy surface. These results led to the “individual surface atom” approach of chemisorption (3), where every atom is assumed to retain its chemical identity although the quantitative parameters describing bonds with this atom may be somewhat modified by its near neighbors in and below the surface. For alloy systems where this modification is weak, the individual surface atom approach provides the basis for chemisorptive titration, i.e., the transition metal atoms in an alloy with a nonadsorbing element can be counted by measuring the number of CO molecules or hydrogen atoms chemisorbed by the alloy. The individual surface atom approach parallels the minimum polarity model developed in the electron theory of alloys. We stress that this point of view implies that it is mainly the immediate environment of the atoms, active in the particular catalytic reaction under study, that determines their chemical properties. It is, therefore, the surface composition of an alloy that determines its catalytic properties. We have chosen to present a review ofexperiments and the theory ofsurface enrichment before dealing with the catalytic properties of alloys and discussing the existing evidence for the relevance of changes in the electron distribution in catalytically active atoms.

11. Surface Composition of Equilibrated Alloys Ever since it was demonstrated, both experimentally and theoretically, that in equilibrated Ni-Cu films the surface composition can differ substantially from that of the bulk (#a-#d)-and this was also found for alloy powders (5)-it has been generally accepted that a catalytic study of alloys should be accompanied by a study of the surface concentration of the alloys. Equilibrium in the surface layer will in practice often be reached, because it has been shown (6) that the surface of thin metal films can be equilibrated by heating for about 30 min at a temperature of 0.3Tm,where T , is the melting point (K). Of course, researchers had wondered about the surface composition before. Takeuchi et ul. ( 7 4 76) were the first to provide experimental evidence of the surface composition being different from the bulk composition in Ni-Cu catalysts by reaction with HCl. In a later paper (7c) they concluded that the

72

W. M. H. SACHTLER AND R. A. VAN SANTEN

amount of hydrogen adsorbed, the heat of adsorption, and the catalytic activity of granular Ni-Cu alloys are related to the nickel concentration at the surface. A difference in composition between surface and bulk had been suggested by Tuul and Farnsworth (8) in order to explain the decreases in activity of Ni-Cu alloys upon heat treatments at high temperatures (500°C).However, the possibility cannot be ruled out that these,results were due to impurities in the surface (9). Recent developments in the use of Auger electron spectroscopy (AES) have improved the possibilities for a quantitative study of the surface composition (IOU-1Ud). By employing an internal standard (11) many of the ambiguities of previous applications have been eliminated. The application of AES has greatly improved the reliability of surface composition determinations compared with previous results. A drawback of the AES method (12) is that it samples a weighted average over several outer layers of the sample, depending on the escape depth of the measured electrons. A comparison of results of this technique and chemisorptive titration (13,14), in which chemisorption takes place selectively on one of the constituent atoms, has been made for the Pt-Sn system. This has provided information on the concentration profiles near the surface. It should be realized that selective chemisorption titrates only atoms on the surface, but that the surface composition can be changed upon adsorption (15).A difficulty in the interpretation of chemisorptive titration is the possible presence of electronic effects, which can alter the chemical nature of the titrated atoms. Adsorbates can also show strong interactions with each other on pure metals, and decreases in this interaction can cause an increase in the number of molecules adsorbed per active metal atom exposed.The surface composition of active metal atoms deduced from these measurements will then be too high. It is obvious that chemisorptive titration is most ideally applied under such conditions of pressure and temperature that only adsorbates bonded to one surface atom exist. If, in addition, there are surface species bonded to several surface atoms, selective chemisorption applies if

where f3is the surface coverage of adsorbate defined as the amount of molecules adsorbed divided by the total number of surface atoms present, x the surface concentration of the atoms that are titrated by selective chemisorption, and AHadsthe heat of adsorption. In Section IV this condition will be discussed in more detail.

SELECTIVITY OF ALLOY CATALYSTS

73

Changes in the work function can also be used as a sensitive probe for differences in composition of the outer layers (4). If used with care, it is a valuable tool in the study of surface composition, as is shown in the work on Ni-Cu alloys. It has been demonstrated (25,16)that in general there is no linear relationship between surface composition and work function @. Data on work functions can, however, be useful to draw the following conclusions: (1) Different @s after equal sintering treatments point to different surface compositions. (2) Equal 0's mean equal surface compositions. ( 3 ) A change in 0 upon chemisorption is a qualitative measure of the extent of adsorption. Mossbauer spectroscopy (I 7), too, has been used in combination with chemisorption in order to determine surface enrichment on supported alloys. The phenomenon of surface enrichment in liquid solutions has been known for a long time. For instance, in a mixture of soap and water, the concentration of the soap will be higher at the surface than in the middle of the liquid. In a two-component system, the difference rl between surface and bulk composition of component 1 is given by Gibbs' rule (18):

where y is the surface tension, nl and n2 the concentrations of components 1 and 2, respectively, p1 the chemical potential of component 1, and A , the surface area. For a critical discussion of the concepts of surface and surface tension we refer to the work of Linford (19). From Gibbs' rule we read that if the surface tension decreases, rl will increase with increasing concentration n , because

'0.

(dPll~nl)T.V+AE,n2

Assuming an ideal solution and a high temperature, the following equations are valid: p1 = pl0 RT In xl, (14

+

Y

= XlYl +

XZY2,

(Ib)

where x1 and x 2 are the partial concentrations of components 1 and 2, and y1 and y 2 their surface tensions, respectively. One then finds

74

W. M. H. SACHTLER AND R. A. VAN S A N l E N

where ci is the average surface area of the components and N Avogadro’s number. Equation (2) is the mathematical expression of the intuitive rule that enrichment occurs in that component which has the lower surface tension. A great deal of work has been done to improve on approximations (1). For a review of this work we refer to Defay et al. (20)and Ono et al. (21). Most interesting for the purpose of discussing surface enrichment in alloys is the so-called cel model of a liquid. In this model the liquid is assumed to be a close-packed crystal. The surface is found by cutting the crystal and it is characterized by the number of neighbors of the surface atoms. If one uses only nearest-neighbor interactions and a binary solution, the energy depends on only three parameters: E l , and E,,, the bond energies of the equal components, and El,, the bond energy between two different components. Another assumption is that the mixture is homogeneous and disordered. This assumption is reasonable for liquids but is not always acceptable for alloys. The solution is considered to be ideal if M is zero. M is defined by:

is proportional to the heat of formation. If M is fully taken into account in the computation of enthalpy changes, but is neglected in the calculation of the entropy of mixing, the computational model is called a regular solution. The discussion of surface enrichment in alloys presented here is an extension of this model for the alloy. Surface enrichment is dependent on the phase diagrams of the alloys. If M < 0, alloy formation is an exothermic process and there is a critical temperature T , below which the alloy is ordered and above which it is disordered. The standard example is the p-phase of the Cu-Zn alloy, with T , = 470°C. A system ofcatalytic interest that belongs to this category is the Pt-Sn system (14). For a particular stoichiometry it forms ordered compounds (22). The critical temperatures for disordering of these compounds are higher than their melting points. If M > 0, alloy formation is endothermic. In this case there is a concentration range for which a critical temperature exists below which demixing occurs and above which the alloy forms a solid solution. Although it has been thought for a long time that the Ni-Cu system does not have a miscibility gap, which was one of the reasons of the popularity of the Ni-Cu system for catalytic studies, closer inspection of the surface composition of these alloys (4) has shown that the Ni-Cu system does have a miscibility gap at low temperatures. This is in line with the endothermicity of this alloy system ( 2 3 ~23h). . Moreover, the clustering observed upon neutron bombardment of a solid Ni-Cu solution shows that the latter is metastable at room temperature (24).

B

SELECTIVITY OF ALLOY CATALYSTS

75

The precise temperature T , at which the miscibility gap closes is still a question ofsome dispute. The value 322°C measured by the National Physical Laboratory in Teddington (25)is often quoted. Disagreements on the surface enrichments can often be traced to (a) a temperature of annealing high above the miscibility gap (fob, 27a27c), (b) a selective removal of one of the alloy components by cleaning procedures, such as argon ion sputtering ( f o b ,27a-27c), (c) chemisorption-induced surface enrichment (7~8,15), if no ultrahigh vacuum is used, (d) the presence of impurity-stabilized surface complexes (28). Ni-Au (ZY, 30) and Pt-Au ( 3 / , 3 2 )belong to the same category as Ni-Cu. We will restrict our discussion to the case where effects related to the particle size of the alloy can be neglected, but it should be remarked that the phase diagram of an alloy can change considerably if the particle size decreases. Although the phenomenon of surface enrichment in the Cu-Ni alloy is explained from its phase diagram, surface enrichment in small particles can differ widely from that in large particles. Anderson et al. (33)found that the surface composition can be a function of particle size in supported Pt-Cu and Rh-Ag alloys. Bartholomew and Boudart (17) did not find enrichment in highly dispersed Pt-Fe catalysts. Hoffman (34)argued that if there is a significant difference in atomic size, phase separation will not occur in very small particles. Sinfelt (35)showed that metals that do not form bulk alloys form metal clusters when they are finely dispersed on a support. As long as the temperature of equilibration is higher than the temperature of disordering or demixing, the alloys will form solid solutions both if ci < 0 and if a > 0. Ag-Au (36,37)and Pd-Ag alloys (6) are examples of catalytic interest that form solid solutions. A. BIPHASICALLOYS We will only briefly discuss the surface enrichment model proposed for the Ni-Cu alloy, since it has been reviewed several times elsewhere (38, 39). The phase diagram of this alloy has a miscibility gap. Figures 1 and 2 show the results of two experiments, which demonstrate composition differences between bulk and surface in Ni-Cu alloys (4u, 46). The alloys are films deposited under ultrahigh vacuum. After sintering the binary systems all have nearly the same work function, despite the fact that the overall Cu:Ni ratio in the copper-rich system is about four times that of the metal-rich system.

76

W. M. H. SACHTLER AND R. A. VAN SANTEN

- 054 Ni

NI-RICH

SO/sO

CIrRlCH

- 084 Cu

NI

M-RICH

som

cu-RlcH

cu

FIG. I . (kji).Work functions of films prepared by evaporation of nickel on top of a copper film, followed by sintering and admission of carbon monoxide. 0, Fresh; 0, after sintering (200 C); A, after admission of CO (3 x Torr). From Sachtler and Dorgelo (46). FIG.2. (right). Work functions of films prepared by evaporation of nickel on top of a copper film, followed by sintering and admission of carbon monoxide. 0, Fresh; 0, after sintering (200°C); A,after admission of CO (5 x Torr). From Sachtler and Dorgelo (46).

X-ray diffraction work showed the existence of two phases. The work function data suggest that the copper-rich alloy in the two-phase system is located at the surface and the nickel-rich phase below the surface. To check this, CO was admitted at a pressure of lo-* Torr. The gas is strongly adsorbed on nickel, but not on copper at such low pressures. The work function of copper was not altered. The binary alloys showed a constant increase in work function between 0.04 and 0.1 1 eV. Therefore, the adsorbing surface belonged to the copper-rich phase. Chemisorption of H, on Ni-Cu films (40) leads to essentially the same conclusions. At temperatures below the miscibility gap, several classes of alloy systems characterized by their concentration ranges can be distinguished ( 4 4 , as illustrated in Fig. 3: Runge I : x2 I x 5 1. Equilibrium is established after all the nickel has been dissolved. A homogeneous alloy, rich in copper, is formed and the concentration of either metal at the surface is equal to the surface enrichment proper to the concentration of the metal in the segment (see Section 11, B).

Range 2 : x1 < x < x,. Equilibrium is established after the copper has been consumed, resulting in a two-phase system. Each crystallite consists of a kernel of almost pure nickel (x = xJ, enveloped in a skin of alloy with

SELECTIVITY OF ALLOY CATALYSTS

e

77

x2

FIG. 3. Location of phases in Ni-Cu alloys. a,, atomic fraction of copper in alloy; x,, x2, phase concentrations. From Sachtler and Jongepier (4c).

x = x2.[This has been called the cherry model (44.1 Which of the two phases

will form the outer envelope is determined by the surface energy of that phase. The final surface composition will be determined by surface enrichments occurring in this outer layer, according to the model we will discuss later for a solid solution. The concentration of either metal in the surface phase will always be the same, irrespective of the overall composition. Range 2a: x1 < x < (xl + Ax). Small patches of alloy with x = x2 cover crystallites of almost pure nickel (x = xl). The alloy skin does not completely surround the nickel crystallites. Range 3 : 0 I x I xl. A homogeneous alloy is formed, containing more than 95% nickel.

Analogous results have been found for Pt-Au (42)and Pt-Ru (43)films, which also have a miscibility gap. Each time, enrichment takes place because that phase which yields the lower surface energy will form the outer layer. Therefore, enrichment occurs in the component having the lower sublimation energy. Recently Franken and Ponec (44) have published photoelectric work functions of Ni-A1 alloy films. Since the phase diagram contains many intermetallic compounds, the surface composition will now be determined by that compound which yields the lower surface energy, in this case the compound having the largest amount of aluminum. Indeed after an initial steep decrease in work function over a limited concentration range, a plateau of constant work function is found.

78

W . M . H. SACHTLER A N D K. A. VAN SANTEN

B. MONOPHASIC ORDERED ALLOYS

An illustrative model of a monophasic ordered system is the Pt,Sn compound (45). This alloy remains ordered up to its melting point at 1365'C. Experimental evidence of surface enrichment in Pt,Sn stems from AES (11) and selective chemisorption (14). Both techniques indicate surface enrichment of tin, the element with the lower heat of sublimation. Table I shows that AES yields lower values of surface enrichment than surface titration. This is not surprising because AES scans not only the atoms of the surface layer, but also those of lower-lying layers. However, a quantitative comparison of AES and chemisorption data (14) shows that the results can only be matched if enrichment occurs by inversion of the outer layers, i.e., if depletion of tin atoms occurs in layers next to the outer layer enriched in tin atoms. TABLE I Compurison

of Surfgee

Compositions us Meustired h j

A ES qtid C'hmisorpriott"

Pt atoms ( x 10" m-') Alloy

Theory

AES

Chemisorption

Pt,Sn PtSn

49.1

68.3 32.6

44. I

a

-

16.7

From Verbeek and Sachtler (14).

The physical cause of this phenomenon (45) can be most easily explained by means of a one-dimensional chain with equal concentrations of the elements, as an analog to the three-dimensional alloy. The theoretical results can be extended to a three-dimensional system. They are only applicable to a true alloy if lattice strain effects are negligible. The surface energy in a one-dimensional chain can be considered to be haif of the difference in energy between a closed and an open chain. This energy difference is one bond energy and, hence, the surface energy is half of it. If the chain is built up of equal numbers of A and B atoms, then in the ordered alloy there are only bonds between atoms A and B. Therefore, formation of the open chain gives the same result, regardless of where the closed chain is broken. In the open chain, one end atom is an A atom and the other a B atom. In this model surface enrichment takes place if the end atoms become equal. We will now compute the energy differences between these situations.

SELECTIVITY OF ALLOY CATALYSTS

79

Let us consider a row with an equal number of A and B atoms, assuming < 0 and T < T,. The system should then be ordered and can be represented by A B A B...B Model la:

a

Surface enrichment occurs in this model if one of the end atoms, say A, is replaced by an atom B. This can take place in two ways. First, end atom A can interchange with its neighbor B (process l), giving rise to the situation Model l b :

B A A B . . .B

The energy cost of this process is

Second, interchange can occur between an A atom and a B atom from the bulk (process 2), giving rise to the situation Model lc:

B B A BA A...B

The energy cost of this process is

A peculiarity of one-dimensional models is that they may involve so-called Umklapp processes (46)annihilating, for instance, the situation of model lb. However, as there are more than two neighboring atoms in three dimensions, we assume that in our example such a process will not occur. If no enrichment takes place, the surface energy is

This is, of course, positive because EAB< 0. If enrichment occurs, the surface energy becomes y1 = - 3 E A B AE,, (44 or y 2 = - 3 E A B 4- AE2, (4b)

+

depending on which of the two processes takes place. Hence, there is surface enrichment if AE < 0, where AE can be either AEl or AE2. We note that

80

W. M. H. SACHTLER AND R. A. VAN SANTEN

Since a < 0, process 2 requires - 2a more energy than process 1. This is the energy necessary to replace two AB bonds by one AA and one BB bond. Therefore, in an ordered system only process 1 will occur. Physically, this means that if surface enrichment takes place, it will be by interchange of atoms in a surface layer with atoms from the first monolayer under it. It then follows from Eq. (4) that surface enrichment takes place if AEl < 0. This, together with Eq. (3), gives or We are dealing with the case where a < 0. In consequence, there is a possibility of surface enrichment if the bond strength of a BB pair is smaller than that of an AA pair. Here we touch upon the rule that surface enrichment occurs in the component with the lower heat of sublimation. This effect can, however, be counterbalanced if a is highly negative. If Eq. (5) is not satisfied, there will be no surface enrichment, even if T = 0. For another explanation we revert to the closed chain. This chain can only be broken such that the open chain represents the situation depicted in model l b if two AB pairs are replaced by an AA and a BB pair. Hence, some disordering has to take place in the closed chain. As a result, the energy in the closed chain is raised by -2a. If less energy is required for breaking the closed chain between a BB pair than between an AB pair, we have a decrease of the difference in energy between the surface-enriched system and the one where no surface enrichment took place. The condition under which this will occur is given by Eq. (5). For the actual Pt,Sn compound, the surface enrichment of (111) and (100) planes has been computed (45) by assuming that it only takes place by inversion between surface atoms and neighboring atoms. Since in this calculation only interaction between nearest neighbors was taken into account, this assumption was justified. However, if interactions are of longer range, the layers that interchange atoms with atoms from the surface can extend deeper into the solid. The degree of surface enrichment can be derived by subtracting from the change in enthalpy the change in entropy of demixing and by minimizing this expression. This is basically the procedure that has been followed. There is, however, an essential difference between the entropy expression to be used here and that used in the case of a solid solution, to be dealt with below. This is because in the case of an ordered compound, the entropy of the surface layer is not independent of the entropy in the layer next to it. As we have seen, surface enrichment takes place by interchange between nearest neighbors, a restriction that is not valid for solid solutions.

81

SELECTIVITY OF ALLOY CATALYSTS

The expression for surface enrichment in the (1 11) plane is 1 d($ - x,) = exp ( - A E I k T ) ,

where x, is the change in the percentage composition of those atoms which have the lower concentration, and

Values for E and a have been derived from the cohesive energies of platinum and tin and the estimated heat of formation of Pt,Sn. The final result is that in Pt,Sn at 500°C a complete inversion of the outer layers is predicted, as illustrated in Fig. 4(45). This is in accord with the experimental findings quoted in Table I. As can be seen in Table 11,measurements of the adsorption of D2 on PtSn and Pt,Sn (14) show that far less D2 is adsorbed than can be expected on the basis of the adsorption found for CO. Since for the same surface concentration H2 has been found to adsorb well on Ni-Cu (40) and Pt-Au (31)alloys, this could be due to the occurrence of a d-band filling effect in PtSn, as will be discussed in Section IV. It is also possible that the large decrease in H2 adsorption compared with that of CO is related to the very large decrease in the number of platinum atom pairs found on the surface, because of the large heat of formation of Pt,Sn and PtSn. Indeed, owing to short-range ordering (45) it is expected that for equal surface concentrations Ni-Cu and Pt-Sn the number of nickel atom pairs at the surface of the Ni-Cu alloy is much larger than that the number of platinum atom pairs expected at the Sn

Pt

FIG.4. Composition of (I 1 1) layers in Pt,Sn ( T

c<

T,). From Van Santen and Sachtler (45).

82

W . M. H, SACHTLEK A N D R . A. VAN SANTEN

TABLE 11

Maximum Coverage on Platinttm and Some of Its Alloys by CO, C2H4,and D, Number of molecules ( x 10'' m-') ~~

Alloy

Adsorbate

Adsorbed

~

Desorbed

Nondcsorbable

Pt

co

90.8

57.0

33.8

co Pt,Sn

C2H,"

44.1 15.7 8.1

28.3 15.7 <8

15.8 2

16.7 3.5

11.8 3.5 i 1.9

-

D,

co PtSn

C2H4a DZ

1.9

4.9

2

co PtSn,

CzH4 D2

In contrast with what was found with platinum itself, no hydrogen, methane, or ethane was produced.

Pt-Sn surface. Even if hydrogen needs more than one transition metal atom in order to dissociate, this would only explain the large decrease in hydrogen adsorption if the possibility of spillover is largely reduced on the alloys. The ligand effect, to be discussed in Section IV, would have to be present for this assumption. It has also been found (14) that ethylene does not dissociate on Pt,Sn and PtSn, whereas it does on platinum. It may be of interest to note that as far back as 1934 Rienacker (47)reported that the decomposition of formic acid over Au-Cu alloys is influenced by long-range ordering of the alloys. C. SOLIDSOLUTIONS A large number of alloys will form a solid solution when heated above a critical temperature if their melting point is not too low. It has been shown that for a liquid alloy of silver and gold near its melting point, the regular solution model gives surface tensions that correspond well with measured values (48).To calculate the surface concentration and profile ofa solid alloy, this model has been applied to the Fe-Cu system (49),account being taken of enrichment in several outer layers.

SELECTIVITY OF ALLOY CATALYSTS

83

Recently, a detailed disscussion has been given (SO) of surface profiles and their dependence on the heat of formation. If the heat of formation is zero, only enrichment in the surface layer will be found and the surface composition y can be computed from the bulk composition x using (20)

It is of interest that McLean (51) employs a similar equation to discuss equilibrium grain boundary segregation. However, he ascribes the driving forces entirely to releases in strain energy. In that case, surface enrichment should depend on the difference in size and compressibility of the atoms in the lattice. In an investigation of Au-Cu alloys it has been shown that these effects become significant (52) only if the radii of the atoms differ by 10% or more. Equation (6) has been used by Berglund and Somorjai (28) in a study of the liquid P b I n alloy system. The computed enrichment in lead proved to be too large. One can improve Eq. (6) by again restricting surface enrichment to the outer layer, but taking into account the heat of formation. One finds (2O,S3)

kT In

{(v-) I-y 1-x (-?)-I}

+ a,y, - a,y2 +-I + m

+

(21 m)x - 21y -

(7) where 1 is the number of nearest-neighbor bonds per atom parallel to the surface plane, m the number of nearest-neighbor bonds per atom in the bulk outside the plane parallel to the surface plane, and i2 the heat of formation of the alloy. The conditions under which Eq. (7) is valid have been discussed in detail by Van Santen and Boersma (36). Using the broken-bond model of surface enrichment one derives (36)

The first term in Eq. (8) is due to the difference in entropy between surface layer and bulk. The decrease in entropy on enrichment is balanced by the gain in enthalpy, determined by the difference between the numbers of bonds broken in the surface and bulk on enrichment. E~ and e l are the bond energies and can be computed from the heat of sublimation. In an extensive review, Overbury et ul. (12) have shown that the surface energy of a metal is proportional to the heat of sublimation of the metals.

84

W. M . H. SACHTLER A N D R . A. VAN SANTEN

From Eq. (7) it follows that enrichment in the component with the lower heat of sublimation increases with decreasing coordination of the surface atoms. Enrichment decreases with temperature. One can correct (52) for the delocalization of the electrons in the metal by using for ei the values derived from the surface energy according to Skapski (55) instead of the sublimation energy. The resultant decrease in surface enrichment can become quite important. Defay and Prigogine (56) pointed out that Gibbs’ rule, the exact thermodynamic law that determines surface enrichment, can be satisfied only if changes in the two outer layers are taken into account. If one takes into account two layers instead of one, the surface layer composition is given by Eq. (9a) instead of Eq. (8):

The concentration y’ in the second layer is given by kT In

{(L)(L)-’} - a{ -(21+ m)x + 21y’ + m y ) = 0. 1-y’ 1-x

(9b)

Equations for more than two layers are easily found from Eqs. (9a) and (9b) (49,50). It is clearly seen from Eq. (9b) that an increase or decrease of the concentration in the layers next to the surface layers is determined by the sign of a, i.e., it depends on whether alloy formation is endothermic or exothermic. As long as the temperature is high compared with the critical temperature of demixing, the deviations in layers other than the outer layer are found to be small. They will be large if the temperature is near the temperature of demixing or disordering. If alloying is endothermic (a > 0) enrichment in the second layer is also found. However, if alloying is an exothermic process ( a < 0)depletion in the second layer by the component that becomes enriched in the surface layer would occur. Taking into account changes in concentration of four layers, Williams and Mason (50) have shown that if a > 0, enrichment in the component with the lower heat of sublimation is enhanced compared to that found for ideal solutions. Near the critical temperature of demixing, the dependence of surface concentration on bulk would be highly reminiscent of this dependence for temperatures lower than the critical temperature. If a < 0, the enrichment is less than computed according to the ideal solution model (45, 50). For temperatures not too high compared with the

85

SELECTIVITY OF ALLOY CATALYSTS

temperature of disordering it has also been found (50)that there is depletion in the layer next to the surface. This situation resembles the enrichment by inversion (45), as found for the ordered system below the temperature of disordering. In the hypothetical case that 211x1 is larger than the difference lea - ell according to Eq. (94, enrichment can occur in both components. Which component will be enriched then depends only on the relative concentration and the sign of a.For instance, if x > 3, a > 0, and e l = e 2 , the surface concentration in x will decrease, but if x < 3its surface concentration increases. However, for the alloys discussed in this paper it is always found that Ie2 - ell > 21~x1.Hence, at equilibrium no reversal in surface enrichment can occur as a function of alloy concentration. Such an inversion, as reported by Takasu and Shimizu (57) for Ni-Cu alloys, has to be related to the particular conditions under which their experiment was performed. It is possible that their results were influenced by the presence of oxygen or carbon impurities in their alloy samples, which would cause enrichment in nickel. There is very little experimental evidence pointing to enrichment in solid solutions. Bouwman et a/. (15) studied the surface composition of Ag-Pd alloys by measuring the changes in work function of the alloys upon adsorption of CO. The result they obtained after brief exposition to CO at room temperature is shown in Fig. 5. Since the dependence on alloy concentration is nonlinear in the silver concentration, enrichment in silver is concluded, this enrichment being a function of the concentration of the alloys. After 16 hours’ exposure to a CO atmosphere Bouwman et al. found an additional increase in work function, which they ascribed to chemisorption-induced enrichment

0

20

-

90 60 X (at.% Pd)

80

100

FIG.5. Change of work functions of Ag-Pd alloy films caused by CO chemisorption as a function of overall composition. From Bouwman et a/. (IS).

86

W. M. H. SACHTLER AND R. A . VAN SANTEN

of the surface with palladium, as this element forms strong bonds with CO molecules. Chemisorption-induced enrichment has also been invoked by Moss and Thomas (54,58)to explain enrichment with silver of Pd-Ag surfaces in contact with gaseous oxygen. Christmann and Ertl (59) found increases in work function of the Pd-Ag alloys upon CO adsorption similar to those found by Bouwman et al. (15) after a short time. However, they also performed Auger electron spectroscopy measurements, from which they concluded that no enrichment occurs in equilibrated Pd-Ag alloys. This seems to be inconsistent with ascribing the increase in work function upon chemisorption to surface enrichment. AES, however, measures a concentration averaged over several outer layers and since enrichment should occur by inversion between the outer layers, enrichments based on AES data are a lower limit to the actual surface composition. Moreover, another point of dispute might be the surface equilibration, which could be an extremely slow process for a low-index plane. Williams and Boudart (29) measured the surface composition of Ni-Au alloys that had been preequilibrated above the miscibility gap and subsequently generated. They found large enrichments in gold. The surface composition measurements fit the surface enrichment calculated according to Eq. (8) rather well. They report enhancements of the nickel concentration by treatment with oxygen. No enrichment for Ni-Cu and Pd-Ag is reported by Hardy and Linnett (26). Their Ni-Cu alloys were equilibrated at temperatures high above the miscibility gap of this system. Neither do Ertl and Kuppers ( I O U ) report enrichment for Ni-Cu alloys equilibrated at high temperatures, where phase separation is not expected. At temperatures near the temperature of demixing, the catalytic activity of Ni-Cu alloys has been found to vary little with bulk composition over a large range (60).The trend of increasing enrichment on lowering the temperature is in accord with expectations derived from Eq. (6). The conversion of hexane proved to be relatively insensitive to the bulk composition, in contrast to the selectivities, which showed a clear dependence on the bulk composition. This may be due to the dependence of the probability of finding particular clusters of nickel at the surface on the composition of the alloy layers next to the surface layer o r on enrichment in nickel due to the gas mixture. Fain and McDavid (16)measured the surface composition of Ag-Au alloys with low-energy Auger electrons. The surface concentration proved to be linear and the work function nonlinear in the bulk concentration. This clearly shows that a deviation from linearity of the work function is in itself no proof of surface enrichment.

SELECTIVITY OF ALLOY CATALYSTS

87

Assuming only enrichment in the outer layer, some enrichment in silver has been predicted for Ag-Au alloys on the basis of the regular solution model (36).The origin of this discrepancy is not very clear, especially since above the melting point the experimental surface tensions (48) agree well with that computed according to the regular solution model.

111. Selectivity of Alloys in Hydrocarbon Reactions

The catalytic activity of group IB metals in hydrocarbon catalysis is known to be greatly inferior to that of the group VIII metals. Following the previous review articles on catalysis by alloys written by Allison and Bond (61) and Moss and Whalley (39),results have been published showing that alloying a group VIII metal with a group IB metal affects hydrogenolytic reactions much more severely than reactions involving C-H bond rupture or formation (62, 63). It is obvious that these drastic changes in selectivity upon alloying cannot simply be accounted for by a decrease in the number of surface atoms exposed to the reactants, and that changes in geometry of the reactive sites and changes in intrinsic activity of the metal surface atoms have to be considered. These selectivity changes with respect to destructive and nondestructive reactions will be discussed in this section. Changes in selectivity have also been reported for a different class of alloys, viz., those where both metals are either active or inactive for the studied reaction (64a-64g). Since for these alloys a rationalization of the scarce data is hardly possible, we shall not consider them here and confine ourselves to those binary alloys in which one constituent is active and the other is virtually inert with respect to the reaction considered under the conditions of pressure and temperature under which the alloy is tested. It has been found that some reactions are sensitive to particle size while others are not, and the particle size dependence has tentatively been interpreted as a structure sensitivity ( 6 5 ~ - 6 5 c )It. will be shown that there is apparently a relationship between structure-sensitive reactions and reactions that are drastically reduced upon alloying (66). The comparison of the catalytic performances of metals and their alloys is sometimes hampered by the different degree of deactivation by carbonaceous residues (107,67). Therefore, it seems appropriate to start with a discussion of the exchange reactions of the hydrogen isotopes protium and deuterium on platinum and Pt-Au films (31). A comparison of this reaction on platinum and its alloy shows that of the two reaction paths possible on platinum in the temperature region studied, one remains unchanged on the alloy but the other, which prevails on platinum except at very low temperatures, seems

88

W. M. H. SACHTLER AND R. A. VAN SANTEN

+

to be completely absent on the alloy. Although the H2 D, equilibration reaction does not reveal which fraction of the catalyst surface is active at the low temperatures where the reaction is not diffusion controlled, the number of participating sites is measured when the exchange of, e.g., gaseous protium with adsorbed deuterium is studied. This has been done on platinum and Pt-Au films prepared under ultrahigh vacuum in the temperature range 78-300 K. Pt-Au alloys are known to have a miscibility gap (68).The measured ratio a of adsorbed hydrogen/adsorbed xenon is shown in Fig. 6 as a function of bulk concentration of the equilibrated films. The equality of the surface composition with the overall composition within the miscibility gap supports the cherry model discussed in the previous section. If one assumes that every surface platinum atom adsorbs a hydrogen atom, the surface composition of these films is 15 & 5% Pt and 85 f 5% A q in fair agreement with the composition of the gold-rich phase of this two-phase system. The chemisorptive titration result is confirmed by catalytic data, showing the same activity per unit surface area for all Pt-Au alloys with compositions within the miscibility gap. The films were first saturated with adsorbed deuterium, then cooled to 78 K, and evacuated. The exchange of gaseous protium molecules with this adsorbate was then studied. For platinum films at 78 K a very fast exchange was found to take place over the first 15 sec; in this exchange the R” reaction

(69) Hzgas

+ 2Dads =

+ 2Hads

is significant. A similar behavior was found by Eley and Norton (70) with nickel. After 15 sec a slower exchange yielding HD and obeying first-order kinetics prevails (71). The rate constant and the fraction of adsorbed ex-

0

2

0

4

0

6

0

8

0

9

at.%

0

Au

FIG.6. a as a function of alloy composition. From Kuyers et a / .(31).

89

SELECTIVITY OF ALLOY CATALYSTS

V

1.0

-

-1.s

-

C

FIG.7. Arrhenius plot for exchange reaction on platinum (curve I) and Pt-Au (curve 2). Hz is the hydrogen absorbing surface, C, is the concentration of HD at r = t,. From Kuyers et al. (31).

changeable deuterium both increase with temperature. The Arrhenius plots of the H/D exchange are shown in Fig. 7 and are linear for the alloys but show a distinct break for platinum at 110 K. The apparent activation energy and reaction order are identical on the alloys and for the low-temperature reaction on platium. For T < 110 K, the apparent activation energy is found to be 0.07 kcal/mole; for T > 110 K the value is 0.6 kcal/mole. Breakspere et al. (72) in a study of hydrogen chemisorption and exchange on polycrystalline platinum wires observed a similar change in reactivity of platinum at T = 110 K. Both for platinum and the alloys the pressure dependence of the amount that is ultimately exchanged at a given temperature can be described by c, = a&,. For alloy films rn is virtually independent of the temperature, whereas for platinum m decreasesfrom 0.9 at T = 78 K to - 2 at T > 200 K. It is seen that by alloying platinum with gold the fraction of adsorbed deuterium that can ultimately be exchanged at 78 K is increased. This higher reactivity suggests that the heat of adsorption of deuterium and hydrogen is lower on these alloys than on unalloyed platinum. This seems to be a general phenomenon in alloying a group VIII metal with a group IB metal (7c, 73,74). It has also been observed in the systems Pd-Ag (754 and Ni-Cu (7%) with CO. The sharp break in the Arrhenius plot for platinum occurs around the temperature at which extensive surface migration sets in (76).

-

-

90

W. M. H. SACHTLEK AND R. A. VAN SANTEN

Three phenomenological criteria can be identified for the exchange reaction on the alloys: (1) initial occurrence of D, in excess over equilibrium values, i.e., an R" reaction ; (2) apparent activation energy E 0; (3) pressure coefficient m 1.

-

-

Although the equality of the three experimental criteria might be purely incidental, it appears more probable that the same molecular mechanism that prevails on platinum only at T < 110 K remains predominant on Pt-Au alloys up to the temperature where rates become immeasurable with the apparatus used. This reaction appears to include an appreciable contribution of a Bonhoeffer-Farkas exchange (77). These results suggest that on platinum weakly bonded hydrogen atoms form a minority group in the population of adsorbed hydrogen; only at very low temperatures does this minority group, which exchanges with the lower activation energy, dominate the observed kinetics. O n the Pt-Au alloys, however, the weakly bonded hydrogen prevails and consequently dominates the overall kinetics over the entire temperature range of the experiments. This conclusion is consistent with an earlier result found by Takeuchi er a/. ( 7 4 on Cu-Ni alloy catalysts. They observed an average decrease in heat of adsorption of hydrogen upon alloying nickel with copper. It is important to observe the qualitative difference upon alloying between the change in the hydrogen isotope exchange reaction (i.e., H, with Dads)and the ortho-para-hydrogen equilibration reactions (both molecules in the gas phase in quantities far in excess over the adsorbed hydrogen atoms). In the classic work of Couper and Eley (78) with ortho-para H2 on wires of Pd-Au alloys, a sudden increase in activation energy was observed at 60% Au; later Couper and Metcalfe (79) observed a more gradual increase in activation energy on Pd-Ag alloys. These results were often cited (80) to illustrate the occurrence of d-band filling. Although the detailed dependence on bulk concentration differs for the three alloys under discussion, they share the feature that the heat of adsorption of hydrogen decreases. The increase in activation energy of the ortho-para equilibration reaction found by Couper and Eley could be due to an increase in activation energy of dissociation. This implies that the rate of adsorption is rate determining, which is only possible if the surface is sparsely covered or if equilibration occurs by a Rideal mechanism (70). A logarithmic decrease in the rate of H2-D, equilibration has been observed by Takasu and Yamashima as a function of nickel surface composition in a Ni-Cu alloy at 273 K. After heating at 800°C for 8 hours in uucuo these

SELECTIVITY OF ALLOY CATALYSTS

91

alloys were cleaned by argon ion bombardment to remove impurities from the uppermost surface layers. The alloy composition of the clean surface was determined by means of AES. The catalytic activity was observed to decrease appreciably upon annealing at 300"C, which is to be ascribed to enrichment in copper. The same authors (81) also measured the work functions of the same alloys before annealing and found a decrease linear in the copper surface concentration. These results clearly illustrate the dependence of H2-D, equilibration and work function on surface composition. The observation of a surface composition greatly dependent on the bulk concentration is only apparently in conflict with the cherry model discussed in Section 11, since the surfaces studied by Takasu and Yamashima had not been equilibrated and their composition was entirely determined by the way in which they had been prepared. The exchange reaction of methane with deuterium has been found to show a decrease in specific activity upon alloying of palladium with gold (82), reflecting the decrease in number of reactive surface metal atoms upon alloying. The hydrogenations of acetylene over Pd-Au (83) and Ni-Cu (644, and of methylacetylene ( M a ) and 2-butyne over Ni-Cu ( 8 4 ) and Pd-Au (85) have also been studied. The selectivity for consecutive hydrogenation was found to be little influenced by alloying (647). Rushford and Whan (85) did not find any correlation of their data with d-band filling. They postulated that catalysis in their system is associated with palladium centers in the alloys, with gold acting merely as an almost inert diluent. They found a linear correlation between the frequency factor for hydrogenation and the probability of finding clusters of four atoms. So their work strongly suggests that the ensemble effect, to be discussed in Section IV, determined changes in selectivity upon alloying. Whereas in one of the first papers on the hydrogenation of ethylene over nickel and Ni-Cu alloys (86) the reaction had been reported to be relatively insensitive to alloying up to a copper concentration of SO%, 15 years later Best and Russell (87) reported large increases in activity. Some of the contradictory evidence has been clarified by Takeuchi et af. (@), whose discussion has been extended recently by Takusu and Himiru (89), who showed that the catalytic activity may be very sensitive to the ways of preparing the alloys. Takeuchi et a!. prepared films by the evaporation of copper and nickel metals or their alloys on a substrate cooled by liquid oxygen. Prior to use, the film was treated in vacuum at 30 or 250°C. The catalytic activity was tested by the hydrogenation reaction of ethylene. Their result is illustrated in Fig. 8. Essentially similar results have been reported by Vblter and Alsdorf

92

W. M. H . SACHTLER AND R. A. VAN SANTEN

400

-

x Y

0

2 0 4 0 6 0

Ni (AT.)

8oxx) O/O

FIG.8. Catalytic activities of hydrogenation reaction of ethylene at 30°C on films treated Films treated at 30°C; 0,films treated at 250°C. From Takeuchi et al. (88). at 30 and 250°C. 0,

(90).It is seen that the film evaporated on the low-temperature substrate shows a strong dependence on composition of the Ni-Cu concentration and a large increase in activity compared with pure nickel. However, the films treated at 250°C show a gradual decrease upon alloying. The treatment at 250°C apparently equilibrates the alloy, and the behavior can be understood on the basis of the cherry model disscussed in the previous section. A similar behavior independent of bulk composition has been found by Campbell and Emmett (91) over Ni-Au films. These alloys also have a miscibility gap. Takasu et al. suggest that the large increase in activity of the lowtemperature films is due to an abundance of lattice imperfections. The absence of impurities can also be a reason, because this has been shown to increase activities (92a-92c). Furthermore, in some previous experiments (87,91,93a,93b) large amounts of hydrogen may have been adsorbed by the catalysts, because they were cooled in the presence of hydrogen. This might cause an enrichment of the surface with nickel (94). Another possibility that explains increases in activity upon alloying is a reduction in the degree of poisoning. If this were true in this case, however, it is not obvious why the equilibrated alloys do not also show strong enhancements. Recently (64a,67) new light has been shed on the problem of poisoning. Van Barneveld and Ponec (67) studied the hydrogenation of benzene on nickel catalysts with 5% Cu and 10% Cu, where the copper is fully soluble in nickel at all temperatures. At low temperatures (20-150°C) the alloys were

93

SELECTIVITY OF ALLOY CATALYSTS

less active (per meter squared total surface area) than nickel, whereas at temperatures at which cracking also occurs (above 220°C) the alloys are more active than nickel. Apparently at high temperatures, alloying results in a decrease in self-poisoningdue to coke formation, whereas at low temperatures no such effect occurs. The hydrogenolysis of ethane has been demonstrated to be structure sensitive (95a, 95b). Figure 9 shows that alloying of nickel with 10%Cu decreases the specific activity for the hydrogenolysis of ethane by a factor of loo0 (63).Use was made of Ni-Cu alloys prepared by coprecipitation of the metals as carbonates, followed by calcination and reduction of the coprecipitate. The strong initial decrease in ethane hydrogenolysis could not be explained by the decrease in number of active sites as measured by the volume of strongly adsorbed €-I2.One of the explanations given is the extensive dissociation of carbon-hydrogen bonds, leading to a highly unsaturated dicarbon surface residue as the reaction intermediate (96).It seems likely s-

40'

40'

-

I

I

I

p--- B

i

I

(b)

i,

a

10

-

'! \

1-

toa -

-

1

-

-

ad-,

40'-

10a

I

-

1 I

I

I

I

AT. '10 COPPER

I

1

.,

FIG.9. Specific activities of Ni-Cu alloy catalysts for hydrogenolysis of (a) ethane to methane and (b) dehydrogenation of cyclohexane to benzene at 316°C. 0, Ethane hydrogenolysis at ethane and hydrogen partial pressures of 0.030 and 0.20 atm, respectively; cyclohexane dehydrogenation at cyclohexane and hydrogen pressures of 0.17 and 0.83 atm, respectively. From Sinfelt er a!. (63).

94

W. M. H. SACHTLEK A N D K. A. VAN SANTEN

that such an intermediate would form a multiple bond with the surface metal atoms. The amount of intermediate is probably very sensitive to restrictions in the number of multiple nickel atoms available. This explanation is supported by the fact that a sharp decrease in preexponential factor is found, while the activation energy remains relatively constant. Figure 9 also shows the activity of cyclohexane dehydrogenation. Remarkable is the dependence on copper concentration. Initially, a small enhancement in activity is found and then a sharp drop in activity when the copper concentration becomes 80%. This increase in specific activity could be due to a low steady-state concentration of carbonaceous residues on the surface of Ni-Cu alloys as compared to nickel. Except for the small initial enhancement, a similar behavior had been reported in 1939by Rienacker and Bommel (86)for the hydrogenation ofethylene. The sharp decrease at 80% Cu reported by these authors and by Sinfelt for the cyclohexane dehydrogenation coincides with the boundary of the miscibility gap of the Ni-Cu system. Sinfelt, et al. (63a)ascribe the initial enhancement to a decrease in heat of adsorption of benzene. However, results have been reported that contradict the hypothesis that desorption of benzene is the rate-limiting step. According to Paal and TetCnyi (97)a decrease in H, concentration on the surface increases the yield of benzene. The rapid decline at SO% Cu has to be ascribed to a change in the rate-limiting step involving more than one nickel atom. Sinfelt (98) reports similar results for alloy clusters of Ru-Cu and 0s-Cu on a support. The study of the H2--D2exchange with propane (107) over nickel and Ni-Cu alloys has led to the conclusion that the enhanced activity due to alloying was actually a decrease in self-poisoning. At -40°C the addition of copper to nickel resulted in a decrease in the activity from about 1OI3 to 1OI2 moles/cm2 sec for alloys containing up to 40% copper. The rate on copper was less than 10’O moles/cm2 sec, and distributions obtained from the alloy experiment were indistinguishable from those observed on nickel at -40°C. The situation was entirely different at 5 0 T : at this temperature the nickel films were rapidly inactivated by self-poisoning, so that hardly any exchange was found to take place, whereas the rate on copper was higher than that on nickel. The initial rate at 50°C proved to be higher for some alloys than for copper and nickel. This suggests that maxima and increases in the activity pattern as discussed earlier (e.g., cyclohexane dehydrogenation) can be caused by self-poisoning, i.e., are expected to be absent if no self-poisoning occurs. The observation was made that the maximum in the “activity pattern” was accompanied by a minimum in multiplicity. The results at 50°C seem to indicate that alloying with copper decreases the tendency of nickel to self-poisoning and multiple exchange. The cyclopropane molecule offers a possibility to study the changes in selectivity upon alloying with respect to two parallel reactions (99a-99d).

95

SELECTIVITY OF ALLOY CATALYSTS

At higher temperatures cyclopropane decomposes to methane by the overall reaction cyclo-C,H,

+

3HZ (111)

--t

3CH4

Whereas the hydrogenolysis of hexane and other alkanes (60,62, 100, 101) can be observed only at temperatures slightly above the temperatures closing the miscibility gap in Ni-Cu alloys, reaction (11) occurs at substantially lower temperature, where separated phases (if present) are thermodynamically stable. Reactions (I) and (11) were studied (102) on alloys prepared by thermal decomposition (at 400°C)in air of coprecipitated Ni-Cu carbonates, followed by reduction with hydrogen (at 300 to 400°C). The results, presented in Fig. 10,are very similar to those already discussed for ethylene and ethane. Reaction (I),the hydrogenation of cyclopropane, has been shown earlier to be structure insensitive (103~1,103b). The activity pattern of this reaction is reminiscent of cyclohexane dehydrogenation (63). Initially, a small increase in activity is found, followed at 80% Cu by a rapid decline. These results show that reaction (11) is of the hydrogenolysis type and that reaction (I) is hydrogenation of an unsaturated bond.

-0

!so

I00

at.%Cu

at. o/o Cu FIG. 10. Activity parameters A , and A, as a function of alloy composition (at. Cu). Upper curves A , and A , are for cyclopropane at 90°C; lower curves A , and A, are for propane at 320°C. A, = Iog(u/w); A, = log(u/sw9; u is conversion, s specific surface area of catalyst, w weight of catalyst. From Beelen r f al. (102).

96

W. M. H. SACHTLER AND R. A. VAN SANTEN

The feature that a reaction needing multiple bonding is more influenced by alloying than a reaction needing less bonds with the surface atoms has also been found in a study of the hydrogenation of 1,3-butadiene on Ni-Cu alloys (104). The rate of hydrogenation as a function of alloy composition proved to be two orders of magnitude lower than that for pure nickel. The 1-butene/2-butene ratio tended to increase with increasing copper content. This agrees well with the structure sensitivity of these reactions reported by Oliver and Wells (105). Earlier (106) it had been shown that the product distribution of 1,3-butadiene hydrogenated over palladium and Pd-Au alloys is relatively insensitive to alloying. This is considered proof of the fact that in this case hydrogenation occurs via a n-allylic complex. In a series of papers, studies have been reported of the exchange of deuterium in, and the deuteration of, benzene (101,40), the exchange of deuterium in cyclopentane (62), and the reactivity of hexane (60) and methylcyclopentane (100) over nickel and Ni-Cu alloys. The constancy of the activity for benzene hydrogenation over a wide range of overall concentrations of copper and nickel was shown to be indeed due to a surface phase of constant concentration, in accord with the cherry model. Initial rates on films were also measured. After an initial drop, these rates remained constant as a function ofalloy composition, as illustrated in Fig. 11. a,

a u

P

< 15 ,.

I

I

m

I I

0

3

I

a,

I I

0

5 10E

I

I

1

I

0

0’

x

v

a,

c

e

---0

5-

C

.-0 4-

0

0 0 0

0

AV

A 8 A

1

1

8;

ob” 1

1

1

1

1

1

1

1

FIG. 11. Activity pattern for benzene hydrogenation at 150°C. pH2= 322 Torr; pbenz= 5.8 Torr. 0, Nickel deposited on top of copper, films sintered at 200°C for 18 to 20 hours; A, copper deposited on top of nickel, films sintered at 200°C for 18 Lo 20 hours; V, copper deposited on top ofnickel, films sintered at 300°C for 14 hours. From van der Plank and Sachtler (101).

SELECTIVITY OF ALLOY CATALYSTS

97

The activation energy is roughly twice as large for the alloys as for pure nickel. Lyubarski (108) was the first to show that the specific activity of benzene hydrogenation decreases upon alloying of nickel with copper. He found, however, an initial rise in activity on plotting the activity as a function of weight of the catalysts. The H/D exchange between Dz and benzene was found to have a rate exceeding that of benzene deuteration by several orders of magnitude. This result shows that exchange and hydrogenation reactions follow different reaction paths. The exchange parameters were also found to be independent of the overall alloy composition. Cadenhead and Masse (109) report similar results for the benzene hydrogenation. They stress the importance of measuring specific activities because plots of the surface areas versus alloy composition show a maximum (108). For Pd-Cu and Pd-Au samples it is concluded (109) that the catalytic behavior found indicates the formation of ternary transition metal-group IB metal-hydrogen systems. Cinneide and Clarke (110) have studied the activity of Pd-Au films for the deuteration and exchange of benzene and the hydrogenation of p-xylene. The authors report that the activity for the exchange reaction between benzene and deuterium persists to the palladium-lean compositions, which is in agreement with results obtained by Honex et al. (111) in a study of the exchange of toluene over alloys of the same kind. The rates are much reduced (by 10’ to lo3)compared to those found with palladium-rich films. The hydrogenation of benzene over supported Pd-Au catalysts initially exhibits a rise in activity as gold is added to the catalyst, but further addition brings about a pronounced activity decrease (112). The same authors find a marked increase in catalytic hydrogenation activity for Pd-Au alloy microspheres containing up to 60 at.% gold as compared with that measured for palladium. Cyclopentane-deuterium exchange has been followed on nickel and some Ni-Cu alloy films in the temperature range 200-430 K (62).Over the entire ranges, the reaction is accompanied by self-poisoning, and on Ni at 340 to 430 K also by hydrogenolysis.The catalytic effect of alloying has been found to be most pronounced on hydrogenolysis and self-poisoning, but is rather small with respect to multiple exchange. In having a low activity with respect to C-C bond fission and in promoting isomerization, the Ni-Cu alloys are more reminiscent of platinum than nickel. The explanation given is similar to that proposed for the suppression of ethane hydrogenolysis. Hydrogenolysis requires multicenter adsorption and is therefore more sensitive to alloying than reactions needing fewer centers. This was examined in detail by Ponec et al. (60)in a study of the

98

W . M. H. SACHTLER A N D R. A. VAN SANTEN

P

I I I

hS

.

n\T-

I

J

40

20

at.

60

O/O

A*

80

1

100

Cu

FIG. 12. Reaction parameters of n-hexane conversion by nickel and Ni-Cu alloys. A , = log rw at 330' C , A , = log ry at 330°C. activation energy of the overall reaction E,,,, fission parameter M , selectivity parameter S ; all as a function of alloy composition (in at. Cu). rs is rate per cm2,r, rate per gram catalyst. From Ponec and Sachtler (14).

reactivity by n-hexane of nickel and Ni-Cu powders. The results are presented in Fig. 12. It is worth noting that: (1) Between 0 and 23% Cu the activity parameters A , and A 2 decrease sharply, and this decrease is accompanied by an increase of the selectivity parameter S to values previously found for extremely thin nickel films (113); S is the selectivity in producing other C,-hydrocarbons: /

6

where the subscript k denotes the hydrocarbon in the feed. (2) A much more pronounced increase in S and the fission parameter M is observed at 40 to 73% Cu, where S reaches values common for platinum; in this region the A parameters change only little. The fission parameter M is defined as 5

M

=

1

j i=2

(6

- i)C!"

I

(Cl)meas

SELECTIVITY OF ALLOY CATALYSTS

99

The temperatures adopted during catalyst preparation and subsequent reaction were chosen above the temperatures closing the miscibility gap of Ni-Cu alloys (25). These results confirm the different influences of alloying upon destructive and nondestructive reactions. The reaction rate per surface nickel atom remains essentially the same on alloying with copper. In alloys with 0-23% Cu the activation energy of the total conversion of n-hexane is only marginally influenced and the observed effects are consequently connected with the preexponential factors. Since the selectivity of nickel diluted with copper is near the value found by Anderson et al. (113) for highly dispersed films, considering a common cause is suggested (60). Anderson assumes that with a large fraction of surface atoms in very small crystals the “isolated” corner atoms favor the formation of carbocyclic intermediates of isomerization, whereas hydrogenolysis requires two or more adjacent platinum atoms in a crystal plane. An effect other than this ensemble effect has to be invoked in order to explain the increase in M and S for the alloys with 40-70% Cu. Although the origin of this apparent energetic effect is not clear, the effect could be due to differences in size of the nickel clusters in the surface caused by differences in bulk concentration of the alloy, or to adsorption-induced enrichments of nickel in the surface. The latter can also depend on the bulk concentration of the alloys. The temperature at which the reactivity of n-hexane has been studied is very near the temperature of demixing of the Ni-Cu alloys. Therefore, the surface phase will be reminiscent of the phase expected for lower temperatures, except that the transition to the solid solution has already started, which can explain the slight dependence on bulk composition. Reman et ul. (114) found essentially the same change in product distribution with alloying, when investigating the conversion of n-hexane on alloys. Methylcyclopentane (MCP)(100)shows much stronger self-poisoning than n-hexane or cyclopentane. The activity and the selectivity pattern is essentially the same as that already discussed for the other reactants. Alloying nickel films with copper lowers the activity for the overall conversion of MCP and leads to a higher selectivity for C,-hydrocarbons (slowing down hydrogenolytic cracking reactions) and to a higher activation energy. Simultaneously, the 2-methyl/3-methylpentaneratio and the contribution of random splitting show an increase. This pattern resembles that of platinum. It has been reported (115) that n-heptane and n-octane dehydrocyclize upon alloying of palladium with silver. The dehydrocyclization products are to a considerable degree dealkylated. The most thoroughly studied alloy system is Ni-Cu, because of the original suggestion by Dowden (116u) and Reynolds (116b) that d-band

100

W. M . H. SACHTLER AND R. A. VAN SANTEN

vacancies are essential for alloys to be active as catalysts and the ignorance of the existence of a miscibility gap in these alloys. From the present discussion it is seen that surface enrichment dominates the catalytic behavior of Ni-Cu alloys. Pd-Ag and Pd-Au alloys have also been the subject of many catalytic studies (61). The interpretation of the experiments is, however, hampered by the dissolution of H, in palladium. d-Band filling (78,80) has been thought to be responsible for the characteristic catalytic behavior of these alloys. In the next section experimental and theoretical approaches will be discussed to settle the dispute about the relevance of the electronic factor (117) to catalysis.

IV. Ensemble and Ligand Effects

Crucial to the understanding of the selectivity patterns discussed in the previous section is the concept of an ensemble of surface metal atoms. Inasmuch as parallel catalytic reactions differ in the number of adjacent surface atoms of the active metal that are required for forming the respective chemisorption complexes, it is clear that the reaction requiring the largest ensemble of these atoms will be the most sensitive to alloying with a second metal unable to form such chemisorption bonds (62,63,85)/as illustrated by the following considerations. In every catalytic reaction involving a hydrocarbon molecule, an important intermediate is the monoadsorbed complex, e.g., for n-hexane on a surface containing platinum atoms, the complex

I

Pt

will be formed. This intermediate can either be desorbed or undergo a chemical reaction, e.g., a dehydrogenation or a H/D exchange. A second possibility is that it becomes diadsorbed, e.g.,

In practice this will occur at high temperature, resulting in a complex that according to Anderson (118) is a necessary prerequisite for skeletal isomerization.

SELECTIVITY OF ALLOY CATALYSTS

101

Another possibility is to become triadsorbed, e.g.,

From this state, nondestructive desorption might be difficult, so that hydrogenolysis (metal cracking) might become the preferred way to regenerate the free sites of the catalyst. From this example it is clear that the selectivity for (a) dehydrogenation, (b) isomerization, and (c) cracking is likely to be related to the relative concentrations of mono-, di-, and tri-adsorbed complexes, etc. More generally, the selectivity of a catalytic reaction will depend on the relative chance for a molecule adsorbed on n-surface atoms either to desorb or become adsorbed on (n + 1) surface atoms. This idea easily permits us to understand that dilution of an element A, capable of forming chemisorption bonds with a given molecule, with an inert element B will lower the ratio of poly- to monoadsorbed molecules and have an effecton catalytic selectivity. We will call this concept the primary ensemble eflect. Simple examples where this geometric effect might be operating are the oxidation of ethylene to ethylene oxide and of cumene to cumene hydroperoxide (119) on Ag-Au alloys. In these processes the monoadsorbed 0,ion is decisive for the selectivity (120). While on silver these ions can further dissociate to form 0’- ions, this dissociation is less important on Ag-Au surfaces with, as a result, an enhanced selectivity. However, for possible reasons to reject the ensembleeffect as the decisive cause of the selectivity pattern of these examples, the reader is referred to Van Santen and Boersma (36). The geometry of the ensembles of A atoms in the surface can, however, influence the adsorption complexes not only by changing the number of single bonds to different atoms of the adsorbed molecule, as illustrated above: a second type of ensemble effect can be visualized for any given atom of the adsorbate. Taking the adsorption complexes of CO on a transition metal as an example, we can discern “linear,” “bridged,” and “multisite” complexes:

Again, it is predictable that with increasing dilution of the A atoms the concentration of the multisite complexes will decrease more strongly than that of the single-site complex and, again, the different complexes can be defined by the ensembles A, they require. Dilution of the surface with an

102

W. M. H . SACHTIXK AND R . A. VAN SANTEN

inert element will thus change not only the probability of formation of adsorbates in which more than one atom is connected with the surface, but also the average coordination of the adsorbate atoms with surface atoms. In this review we shall call the latter effect of alloying the secondary msernhle effect. It is a possible cause of changes in heat of chemisorption. Alloying A with B involves, however, not only changes in geometry, i.e., the concentration of the A, ensembles in the surface. Even if a well-defined bond of an atom X in the adsorbed molecule and, e.g., one A atom of the surface is considered, the X-A bond strength may be expected to depend on the nature of the electronic interaction of the A atom with its neighbors in and below the surhce; it is therefore changed if these neighbors are changed by alloying. This effect of alloying on the adsorption is called the ligand eflect. This term has been chosen (41) because in complex chemistry it is a wellestablished fact that the strength of the bond between a metal atom Me and another atom X depends on all other ligands L of the same metal atom:

Replacing some ofthe ligands L by other ligands L' will influence the strength of the Me-X bond and, e.g., the position of the 1R band caused by the Me-X vibration. In the same manner, it is of great importance for the adsorption bond between, e.g., an organic molecule and a platinum atom whether the atoms adjacent to that atom are also platinum atoms, as in the case of a platinum crystal, or, e.g., tin atoms, as in the surface of a Pt-Sn alloy. Figure 13 shows temperature-programmed desorption spectra for the desorption of chemisorbed CO from two different Pt-Sn alloys, Pt,Sn and PtSn (14). The spectra clearly indicate that a higher temperature is required to remove CO from Pt,Sn than from PtSn. As the adsorption of CO is known to be nonactivated for either alloy, it is safe to conclude that the heat of adsorption for CO is larger on Pt,Sn than on PtSn. Changes in specific catalytic activity (i.e., per A atom exposed) and changes in selectivity due to alloying can thus be caused by the chemical interaction of the alloy partners, resulting in, e g , an increase in the number of d-electrons on the atoms active in the catalytic reaction considered. It is clear that the ensemble effects and the ligand effect will often occur together and concomitantly contribute to the changes in the nature and the concentration of the adsorbed complexes caused by alloying, and, hence, to the changes in catalytic performance. If the ligand effect and the secondary ensemble effect were truly independent of each other, the change in bond strength would be the product of the changes caused by each effect separately.

103

SELECTIVITY O F ALLOY CATALYSTS

I I II

1/;

I

\

\

/

I

\ \

I /

\

\ \

I

-200

\

,

/’

-100

I

0

100

I

‘.

200

1

I

300

400

500

T DESORP (“C) FIG 13 Temperature-programmed desorption spectra for CO on two Pt-Sn alloys Only gas desorbed below 500 C I S recorded Adsorption temperature, 28 C. cooling temperature, -78 C , heating rate, 145 C/min From Verbeek dnd Sdchtler (14)

It would be of considerable interest to demonstrate the importance of the ligand eflect in a catalytic reaction. Unfortunately, the experimental conditions have to be rather severe in order to ensure that no artifacts are introduced by changes in impurity level or poisoning. The ideal experiment to establish a pure ligand effect unequivocally would involve a comparison of alloys having the same group VIII metal but containing second metals of different electronegativity, which are inactive to the gases used in the experiment. If such alloys with identical surface composition and structure show a different catalytic behavior under equal conditions these differences would characterize a pure ligand effect. A. THELIGANDEFFECT Whereas in the older literature (78,80,116)no clear distinction was made between surface properties and bulk properties, it is now generally accepted that the catalytic properties of the surface atoms are primarily determined by their immediate environment ( 3 ) . The term “ligand effect” (41) stresses that the influence on an adsorbing atom by its neighbors in and below the surface decreases steeply with their distance, the greatest contribution coming from the direct neighbors of the metal atom considered.

104

W. M. H. SACHTLER AND R. A. VAN SANTEN

Our view that in chemisorption and catalysis one has to look at the properties of the individual atoms at the surface rather than at parameters of the continuum (3) has received decisive support from recent advances in solid state physics. Experimental results from ultraviolet photoelectron spectroscopy (121~121d, 122) and from ESCA (123) clearly show that the shape of the d-bands of metals of group VIII and group IB is greatly changed upon alloying. This demonstrates that the rigid-band model (124) or the virtual-crystal approximation (125), which assumes that band shapes do not alter upon alloying, is not applicable to the alloys of interest here. This necessitates a thorough revision of all those conclusions which had been based in the past on the assumption that the rigid-band approximation is a permissible basis for discussing catalysis by alloys (80).The other extreme is the minimum polarity model (126),which has been applied successfullyto the Ni-Cu alloy. According to this approximation the electronic configuration of each component in its pure state is carried over into the alloy. In other words, the individual properties of the atoms are retained in the alloy. This is in accord with the point of view taken in van der Plank and Sachtler (101, 40).The ensemble effect interprets the catalytic properties of the alloy solely in terms of the minimum polarity model. If the active metal becomes highly diluted the minimum polarity model leads to the virtual bound-state model (127, 128, 129). This model has also been applied to highly diluted Ni-Cu alloys ( I214. The nickel d-states are then found to form not a common band with the copper d-states but narrow virtual levels between the copper d-states and the Fermi level. The levels are in resonance with the s,p-band of the metal. The coherent potential approximation (1, 2) is a consistent theoretical frame, which unifies the different alloy models. In order to account for changes in the electronic nature of the atoms, the coherent potential approximation for a disordered alloy appears at present to be the best. It has been applied to single- and two-band systems (130a- 130c). Two parameters of interest on a site active in chemisorption and thus in catalysis are (1) local energy density of states, and ( 2 ) local electron density of states. The energy density of states determines among other things the energies involved in the transfer of an electron between adsorbate and substrate; the electron density in a particular orbital then contributes to the probability of transfers back and forth between substrate and metal. Theoretically and experimentally, large decreases in the d-band widths have been observed. For instance, a decrease by a factor of 5 in the d-band width of Pd-Ag has been reported (121b, 122). One also expects changes in the relative positions of the bands, but these are found to be small, at least in the Ni-Cu and Ag-Pd alloys (122).

SELECTIVITY OF ALLOY CATALYSTS

I05

This implies that the interaction of the d-electrons with their electronic environment is in general found to decrease. In highly diluted alloys the interaction with other d-electrons is nearly absent and broadening of the levels is then caused by interaction with the s,p-band. It should be noted that according to modern metal theory (131) in the pure metals this interaction is very small compared with d-d interactions. In contrast to earlier ideas (132~7,132b) hybridization is only of secondary importance in these metals. The valence electrons are considered to fill a narrow d-band overlapped by a broad s,p-band, which weakly interact with each other. The percentage of d-character of the metallic bond, although giving some correlation with catalytic activity (63, 132a, 132b), is not based on a correct theory of metals. Therefore, we will use a theory of chemisorption based on modern concepts of transition metals. Before the advent of ultraviolet photoelectron spectroscopy and ESCA, experimental evidence on the energy density was mainly available from static magnetic susceptibility and specific heat measurements (134). These provide information on the density of states at the Fermi level and it is impossible to base any conclusions on such experimental information with regard to the shapes of the d-bands in the alloys. It is currently believed that there is very little transfer of d-electrons between the atoms. If an increase in the number of d-electrons on a particular atom does occur, it is due to transfer of electrons from the s,p-band to the lower d-band. This is, of course, related to the difference in electronegativity of the alloying components (135~7,135b). There is very little conclusive evidence regarding d-band filling. According to Seib and Spicer (121~7)no appreciable electron transfer takes place in Ni-Cu alloys. Gelatt and Ehrenreich (130b)estimate a transfer from copper to nickel that is less than 0.1 electron per atom for dilute nickel in copper. At present there are no experimental techniques available to measure electron transfers with such precision. Norris and Myers (121b) propose that the number of d-band holes of palladium in Pd-Ag alloys is zero at a silver concentration of 60%. However, if the concentration of silver increases, the number of d-band holes in palladium increases again; at very high dilutions it is similar to that in pure palladium. Hardy and Linnett (26) have compared the recombination of hydrogen atoms on Pd-Ag and Ni-Cu alloys under conditions where the surface composition is nearly equal to that of the bulk. In both cases they observe a maximum in activity, as shown in Fig. 14. The maximum is at 40% Ag in the Ag-Pd alloy, but at 60% Cu in the Ni-Cu alloy. Since the final decrease in activity is probably the result of a decrease in rate of adsorption, the rapid decline in the Pd-Ag alloys compared with that in the Ni-Cu alloys could be due to the loss of holes at 60% Ag. Another contribution to the lower rate can be the solution of hydrogen or deuterium in the Pd-Ag alloy.

106

W. M. H . SACHTLER AND K. A. VAN SANTEN

I

h

0 v

-2.2

0

A

-2.4 -26 20

60 80 at. o/o Ag

40

20

-0

loo

40

60 80 at. 0% cu

FIG.14. Dependence of log,, y,, on bulk composition for Pd-Ag and Ni-Cu alloys. yk, is the recombination coefficient of hydrogen atoms. From Hardy and Linnett (26).

B. HEATOF ADSORPTION ON ALLOYS This section deals with changes in the heat of chemisorption caused by the secondary ensemble effect and the ligand effect, which are used to determine the factors governing the changes in bond strength upon alloying. Recently, a simple theory dealing with the ligand effect has been developed (136). Differences in chemisorption on transition metals are thought to be mainly due to changes in the d-band widths and electron occupation in different metals. Assuming that the distance between adsorbate and metal atom is the same for single bonding as for different kinds of multiple bonding, changes in heat of chemisorption have been computed for a hydrogen atom using simple models. The results are shown in Table 111. The heat of adsorption of complexes with Z coordinating atoms compared with the single bonded case is given as a function of the parameter p , which is a measure of the bond strength and is the quotient of the exchange integral between one metal atom and a hydrogen atom and the exchange integral between the metal atoms. The latter is proportional to the d-band width of the transiTABLE 111 T h e Ensemble Egect: Cornprison of Relutiue Adsorption Energies fi

2

0.5

0.6

0.7

0.8

0.9

1

2

3

4

5

6

7

8

9

10

6 5 4 3

5.62 4.73 3.82 2.90

5.56 4.69 3.80 2.88

5.48 4.63 3.16 2.86

5.40 4.56 3.71 2.83

5.33 4.52 3.67 2.81

5.27 4.46 3.64 2.78

4.74 4.06 3.34 2.58

4.31 3.71 3.07 2.39

3.93 3.41 2.84 2.22

3.64 3.16 2.65 2.09

3.41 2.98 2.51 1.99

3.24 2.84 2.40 1.91

3.12 2.73 2.32 1.85

3.02 2.65 2.25 1.81

2.94 2.59 2.20 1.77

SELECTIVITY OF ALLOY CATALYSTS

107

tion metal. The larger p, the stronger the interaction will be between metal atom and adsorbate. If adsorption is strong, changes in heat of chemisorption due to the secondary ensemble effect will be proportional to the square root of the number of coordinating atoms 2. If adsorption becomes weaker it will change in proportion to the number of coordinating atoms. The fact that the dependence of the heat of chemisorption on the number of coordinating atoms is weaker for strong adsorbates than for weak adsorbates is due to the stronger competition between the coordinating metal atoms to bind the hydrogen electron if adsorption is strong. Weak adsorption is understood to be chemisorption simply determined by charge transfer, in Mulliken's sense (137); for instance, adsorption of xenon to transition metals (138)as compared to physical adsorption determined by dispersion forces. In terms of this secondary ensemble effect the influence of alloying is smaller for strong adsorbates than for weak adsorbates, if adsorption on the same sites is considered. Since these results have been derived for initial heats of adsorption, extension of these results to the case with a finite surface coverage should be done with care. If the interaction between adsorbates were important, one could imagine that the increase in average distance of the adsorbates because of surface dilution would lead to a decrease in interaction energy at continuing coverage of the surface. If the interaction were 'repulsive, the decrease in heat of adsorption upon adsorption would be smaller on an alloy than on a nonalloyed metal. Interaction between adsorbates is expected to be larger for multiply bonded species than for singly bonded species, because in the latter case adsorbates do not have to compete with each other for binding to the same surface atom. Christmann and Ertl (59) have measured the heats of CO adsorption on the (100) planes of palladium and a few of its alloys with silver (Fig. 15). Increasing the surface coverage results in a decline of the heat of adsorption,

0 0

0.2

0.4

0.6 0.8

A9 [VI FIG. 15. Heats of CO adsorption on different (100)planes as a function of Ad. 0, Compact 62:; Pd. From Christmann and Ertl(59). crystal (139);0,lOOT::, Pd; A,82:,, Pd; 0,

108

W. M. H. SACHTLER AND R. A . VAN SANTEN

which is steeper according as the concentration of silver is higher. Therefore, the differencebetween the curves cannot be ascribed to a difference between the interactions of the adsorbates. Figure 16 shows some curves of the calculated heats of chemisorption for nonlocalized strong adsorption at different degrees of alloying. In the case of the pure metal A the adsorbates are assumed to be coordinated with four A atoms. Dowden (140) has published analogous results assuming proportionality of the heat of adsorption with the number of coordinating atoms. Interaction between the adsorbates is neglected. Also the alloying metal atoms are supposed not to contribute to the bonding of the adsorbates to the substrate. 8 is the number of adsorbates divided by the total number of metal atoms present. The temperature is assumed to be so low and the pressure so high that even at very high dilutions complete coverage is possible. The curves in Fig. 16 for low coverage show a striking similarity with the

FIG.16. Heat of adsorption as a function of surface coverage 0 for a particular x (concentration of inert atoms) on a (100) face of an fcc crystal for nonlocalized adsorption.

I09

SELECTIVITY OF ALLOY CATALYSTS

curves found by Christmann and Ertl (Fig. 15). AI$ is a measure of the coverage. However, the dependence on the silver concentration found is much stronger than that computed. Two explanations are possible: one is enrichment of the surface in silver, and the other is that filling of the palladium d-band by electrons out of the s,p-band upon alloying with silver causes an additional decrease in heat of chemisorption. Observe, however, that the secondary ensemble effect already goes a long way toward accounting for the decrease in heat of adsorption. These results clarify the conditions of applicability of chemisorptive titration. It is clear that if a large contribution to chemisorption stems from multiply adsorbed species, chemisorptive titration cannot be used to determine the surface concentration because the surface coverage of adsorbate will be much less influenced than the surface concentration of active metal atoms. From these considerations, the conclusion already mentioned in Section I1 can be derived, that chemisorptive titration can be employed if

-0

if # > x.

We have assumed in this section that multiple bonding gives a larger binding energy than single bonding (136, 141). This assumption will be valid only if the distance between the metal atoms coordinated with the hydrogen atom is so small that the increase in coordination is not canceled by a much larger increase in repulsive forces. Indeed, Doyen and Ertl (142) have published calculations, accounting for the Pauli repulsion, that show that the heat of adsorption of multiply adsorbed species is comparable to that of singly bonded species. The ligand effect can also be discussed (136) using current theoretical concepts on chemisorption (1434 143b). The effects expected will be illustrated with some results of cluster calculations. Table IV gives for different TABLE IV The Ligand Eflect: Comptrrison of Relrrtive Adsorption Energies

Cluster

5

B C

1.12 0.878

D

0.858

6

7

1.11

1.10 0.703

0.791 0.760

0.701

8

9

10

1.09

1.08

1.07

0.660 0.654

0.627 0.620

0.591

0.587

110

W. M. H. SACHTLEK AND K. A . VAN SANTEN

clusters (see Fig. 17) the calculated heat of adsorption relative to the heat of adsorption of a chosen standard cluster A. In cluster A a hydrogen atom is supposed to be bound to one metal atom, which in its turn has 6 neighbors. The assumptions involved in the quantum-chemical calculations on these . these clusters are similar to those made in the Anderson model ( 1 2 8 ~ )Within assumptions, the heat of chemisorption can be calculated exactly. Each metal atom is represented by one orbital, characterized by the valence state ionization potential Q. The interaction between the metals is represented by the exchange integral fl. The hydrogen atom has an interaction p’ with the metal atom to which it is bound. One electron on the hydrogen atom has the energy xH, and two electrons on the hydrogen atom have energy 2a, + 2, where i,is the repulsion integral between two electrons. The calculations have been performed for fixed aH, a,, p’, and A, but also with varying values of the parameter p , which is a measure of the bonding strength and is represented by

For cluster A, where each metal atom is supposed to contain one electron, the heat of adsorption as a function of p is given in Fig. 18. The values calculated are found to be less than 1.33p’, which is the heat of formation of the MeH molecule. The largest decrease is found for the lowest values of p . The large a

/\

x

FIG.17. Metal clustcrs.

X

O +Singly occupied orbital; 0,doubly occupied orbital.

111

SELECTIVITY OF ALLOY CATALYSTS

0.1

15

1

CL-

FIG. 18. Heat of adsorption E,,, as a function of p,

difference in heat of chemisorption as a function of p explains why it is found (144) that the bond strength of a CuH molecule is larger than that of NiH, whereas the heat of dissociative adsorption of hydrogen is less on copper than on nickel (145). The effect of alloying is studied in the model clusters (see Fig. 17) by letting the metal orbitals simulate a d-band in a true transition metal cluster. As already discussed, alloying narrows the d-band of transition metal atoms. In our model this can be represented by a decrease of the number of neighbors of the metal atom active in bonding. As illustrated in Fig. 17 in cluster B this number is decreased from six to four. As with the secondary ensemble effect, the largest contribution is found for the lowest values of p, the reason being that for lower values of p the metal-metal interaction becomes more important than the Me-H interaction. It is actually a ligand effect because the extent of narrowing will depend on the metal used for alloying. In cluster C the number of neighbors of the bonding metal is again six, but now the metal orbitals are doubly filled. The largest decrease is now found for the highest values of p. The decrease in heat of chemisorption becomes smaller for lower values of p and the heat of chemisorption can even be found to increase for low values of p. The behavior of the heat of chemisorption as a function of p is opposite to that found for cluster A. The negative ion of the MeH molecule gives here the best zero-order approximation to the binding energy, even for low values of p, because the localization energy for two electrons on a metal atom is zero in this case. The stronger the metal-metal interaction, the more energy is gained by rebonding. The large

112

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M. H. SACHTLER AND R. A . VAN SANTEN

increase for low values of p is due to this increasing probability of the formation of negative ions. Prior to adsorption, charge transfer is already favorable and chemisorption becomes ionosorption. If the number of neighbors decreases from six to four as in cluster D, the heat of adsorption is found to decrease rather than to increase, as it does in the case where the metal atom orbitals are half-filled. It is seen that a decrease in heat of chemisorption due to filling of the band is counteracted by the changes in bandwidth. From calculations on quasi-infinite lattices it was found that there is a distinct difference between initial changes in heat of chemisorption due to the presence of a surface layer of inactive atoms and changes induced by alloying in several outer layers (136). In the first case changes in metal-adsorbate bond strength are very small; in the second case the effect of atom isolation is found to be comparable to band filling. Only with very large filling of the band does the heat of chemisorption decrease. If one compares adsorbates on similar sites, our results allow of two generalizations : ( 1 ) Ifthe electron band contains enough holes, weak adsorption complexes are more affected than strong adsorption complexes. ( 2 ) If the metal electron band is completely filled, the strength of the stronger chemical bond is more affected than that of the weaker bond.

If condition ( 1 ) is satisfied, the ligand effect is smaller than the secondary ensemble effect. Under condition (2) the two effects can become comparable. According to the model the heat of adsorption is related to the cohesive energy. The latter is proportional to /?.The cohesive energy of most of group VIII metals decreases with increasing number of band electrons, paralleling the behavior for the heats of adsorption. Measurements of the infrared spectra of carbon monoxide on supported palladium and Pd-Ag atoms (7.54 shed light on the relative importance of the ensemble and ligand effects. Three CO absorption bands were observed on palladium and its alloys at -2060, 1960, and 1920 cm-'. As shown in Fig. 19, the most marked result is that the band frequency remains almost constant for palladium and Pd-Ag alloys, but the relative intensities change in a dramatic manner. The 2060 cm-' band, which is ascribed to the linear CO complexes and is rather weak for CO on palladium, becomes the most important feature of the spectrum of CO on Pd-Ag alloys, where the bands characteristic of multicenter-adsorbed CO are very faint. Measurement on Cu-Ni alloys (731) show an almost similar behavior. Hence, it is found that the stronger adsorbates are more influenced by alloying than the weaker adsorbates. According to the theory just discussed

SELECTIVITY OF ALLOY CATALYSTS

FIG.19. Spectra of CO adsorbed on Pd-Ag alloys. Torr. From Soma-Nota and Sachtler (7%).

~

113

; pco = 0.01 Torr, ---, pco = 0.5

this is only possible if adsorption takes place on different sites, characterized by different ensembles. Thermal desorption of hydrogen from platinum and Pt-Au films (146) results in a similar conclusion for these alloys. On average, hydrogen is more loosely bound to the alloys than to pure platinum. About 50% of the adsorbate is desorbed by pumping at 78 K from the alloys, while only a very small percentage is desorbed from platinum at this temperature.

I14

W. M. H . SACHTLER AND R. A. VAN SANTEN

After maximum coverage of platinum films upon hydrogen adsorption, three desorption peaks have been observed. The same peaks have been found for the alloys, but the relative populations of the various adsorption types were different. Here again, the peak corresponding to the larger heat of adsorption is most influenced, leading to the conclusion that it corresponds to hydrogen atoms bonded to several metal atoms.

V. Conclusions Alloying of a catalytically active metal with an inert component changes the selectivity in hydrocarbon reactions such that C-C bond fission is suppressed compared with C-H bond breaking. Upon alloying, therefore, the selectivity for cracking reactions will decrease. In this review, we have shown that the surface composition of macroscopic alloys will often differ from the bulk composition and that the laws governing the actual surface composition are now reasonably well understood. We have further stressed that selectivity changes can often be understood in terms of the primary ensemble effect, i.e., dilution of the metal surface with inactive atoms diminishes the probability for metal-adsorbate complexes containing several neighboring metal atoms as required for cracking. The concept of an ensemble of metal atoms, crucial to the understanding of selectivity patterns on alloys, implies that the metal atoms in the surface of the alloy keep their individuality and are only influenced by their immediate environment. Indeed, the evidence from solid state physics that the rigid-band theory of an alloy, implying loss of individuality of the atoms, is not valid for alloys of group VIII metals with catalytically inactive metals is overwhelming. In this paper we have introduced the secondary ensemble effect, which ascribes changes in heat of chemisorption of multiply bonded atoms to a decrease in the coordination of these atoms to the surface metal atoms. This effect will in general lead to a decrease in heat of adsorption upon alloying. The heat of adsorption can, in principle, increase because of the ligand effect, which ascribes changes in adsorbate-metal bond strength to differences in electronic properties of the binding atoms induced by alloying. Although the ensemble effects primarily determine selectivity changes, they cannot explain all changes in reactivity induced by alloying. The ligand effect must then be invoked and can become important. The ligand effect can influence the heat of adsorption in two ways. First, it can decrease the localization energy of the electrons needed for the formation of a chemical bond with the adsorbate. This would lead to an increase in heat of adsorption. Second, the heat of adsorption can decrease, if there is an increase of electrons in the d-band because of electron transfer from the

SELECTIVITY OF ALLOY CATALYSTS

115

alloying component to the transition metal. Theory shows that both effects can become of the same order of magnitude. Because adsorbates of different bond strengths are influenced in different ways by the ligand effect, this will influence selectivities. Since the specific activity depends primarily on the electronic properties of the active metal atoms, the ligand effect can become of great importance in the search for more stable catalysts. REFEKENCES Soven, P., Phys. Rer. 156, 809 (1967). Velicky, B., Kirhpatrick, S., and Ehrenreich, H., Phys. Rev. 175, 747 (1968). Sachtler, W . M. H., and van der Plank. P., Surf Sci. 18, 62 (1969). Sachtler, W. M . H., and Dorgelo, G . J. H., J . Catal. 4, 100 (1965). Sachtler, W. M . H., and Dorgelo, G. J. H., J. Catal. 4, 654 (1965). 4c. Sachtler, W . M. H., and Jongepier, R., J . Card. 4, 665 (1965). 4rl. Sachtler, W. M . H., Dorgelo, G. J. H., and Jongepier, R., Proc. Inr. Symp. Basic Problems Thin Film Phys. 1Y64 p. 218 (1965). 5. Cadenhead, D. A., and Wagner, N. J., J . Curd. 27,475 (1972). 6. Bouwman, R., and Sachtler, W. M. H., Surf: Sci. 24, 350 (1971). 7u. Takeuchi, T., Shibata, F., and Sakaguchi, M., Z . Phys. Chem. [NF] 14, 339 (1958). 76. Takeuchi, T., and Sakaguchi, M., Bull. Chem. Sac. Jpn. 29, 117 (1957). 7c. Takeuchi, T., Sakaguchi, M., Miyoshi, I . , and Takabatake, T., Bull. Chem. Soc. Jpn. 35, 1390 (1962). 8. Tuul, J., and Farnsworth, M . E., J . A m . Chem. SOC.83,2247 (1961). Y. Takasu, Y . , and Yamashina, T., J . Catul. 28, 174 (1973). IOU. Ertl, G., and Kuppers, Surf. Sci. 24. 104 (1971). 106. Quinto, D. T., Sundaram, V. S., and Robertson, W . D., SurL Sci. 28, 504 (1971). IOc. Ferrante, J., Acta Metall. 19, 743 (1971). IOd. Bonzel, H. P., and Aaron, H. B., Scr. Metall. 5, 1057 (1971). 11. Bouwman, R., Toneman, L. H., and Holscher, A . A,, Surf: Sci. 35, 8 (1973). 12. See, e.g., Overbury, S. H., Bertrand, P. A,. and Somojai, G . A,, Chem. Rev. 75, 547 (1975). 13. Sachtler, W. M. H., J . Vac. Sei. Twhnol. 9,828 (1971). 14. Verbeek. H., and Sachtler, W . M. H., J . Catul. 42, 257 (1976). 15. Bouwman, R., Lippits, G. J . M.. and Sachtler, W. M. H., J. Cafa!. 25, 350 (1972). 16. Fain, S. C., and McDavid, J. M., Phys. Rev. B 9, 5099 (1974). 17. Bartholomew, C. H.. and Boudart, M., J . Card. 29,278 (1973). 18. See, e.g., Lewis, G. N., and Randall, M., “Thermodynamics,” Mc-Graw-Hill, New York, 1961. I Y . Linford, R. G., Chem. Soc. Reu. I, 445 (1972). 20. Defay, R., Prigogine, I., Belleman, A., and Everett, D. H., “Surface Tension and Adsorption.” Longmans, London, 1966. 21. Ono, S., and Kondo, S., in “Handbuch der Physik” (S. Fliigge, ed.), Vol. X, p. 134. Springer-Verlag, Berlin and New York, 1960. 22. Hansen, M., “Constitution of Binary Alloys.” McGraw-Hill, New York, 1965. 23a. Rapp, R. A,, and Maak, F., Acta Metall. 10, 62 (1962). 23b. Vecher, A. A , , and Gerasimov, Ya. I., Russ. J . Phys. Chem. 37, 254 (1963). 24. Tr$nsdal, G. D., and S@rum,H., Phys. Status Sofidi4. 493 (1964). 25. Elford, L., Miiller, F., and Kubaschewski. O., Ber. Bunsenges. Phys. Chem. 73,601 (1969). I. 2. 3. 4a. 4h.

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Mossbauer Spectroscopy Applications to Heterogeneous Catalysis JAMES A . DUMESIC* Stauffer Laborarories of' Chemistry and Chemical Engineering Stanford University Stanford. Caiifbrnia AND

HENRIK TOPSQE Haldor Top.nje Research Laboratories Lynghy. Denmark

1.Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . A . General Remarks about Mossbauer Spectroscopy . . . . . . . . . . . 1. Advantages of the Technique . . . . . . . . . . . . . . . . . . 2 . The Literature . . . . . . . . . . . . . . . . . . . . . . . . . 3 . Objective of This Article . . . . . . . . . . . . . . . . . . . . 4 . A Brief Introduction to Mossbauer Spectroscopy . . . . . . . . . . B. Mossbauer Spectroscopy: Resonant Absorption and Perturbations of Nuclear Levels . . . . . . . . . . . . . . . . . . . . . . . . . 1. Occurrence of the Effect . . . . . . . . . . . . . . . . . . . . 2 . Chemical Perturbations of Nuclear Levels . . . . . . . . . . . . . C . Mossbauer Spectroscopy: Derived Catalytic Information . . . . . . . 1. Recoil-Free Fraction . . . . . . . . . . . . . . . . . . . . . . 2 . Isomer Shift . . . . . . . . . . . . . . . . . . . . . . . . . . 3 . Quadrupole Splitting . . . . . . . . . . . . . . . . . . . . . . 4 . Magnetic Hyperfine Splitting and Superparamagnetism . . . . . . . 5 . Line Intensities and Shapes . . . . . . . . . . . . . . . . . . . I1. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . A . Mossbauer Isotopes and Feasibility for Study. . . . . . . . . . . . . 1. Source of Radiation . . . . . . . . . . . . . . . . . . . . . . 2 . Chemical Information . . . . . . . . . . . . . . . . . . . . . B . Spectrometer. . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Velocity Modulation and Calibration . . . . . . . . . . . . . . . 2 . Detectors and Nuclear Counting System . . . . . . . . . . . . . 3 . Geometry. . . . . . . . . . . . . . . . . . . . . . . . . . .

122 122 122 123 124 124 126 126 130 136

136 138

140 142 147 151 151 151 153 157 157 160 162

* Present address: Department of Chemical Engineering. University of Wisconsin. Madison. Wisconsin 53706 . 121

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JAMES A . DUMESIC AND HENRIK

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C . Sample Preparation and Study . . . . . . . . . . . . . . . . . . 1. Mossbauer Spectroscopy Cells and Sample Mounting . . . . . . . 2 . Sample “Thickness” and Source-Detector Distance . . . . . . . 3 . Data Processing . . . . . . . . . . . . . . . . . . . . . . 111. Applications to Heterogcneous Catalysis . . . . . . . . . . . . . . A . Catalyst Preparation, Genesis, and Characterization . . . . . . . . 1 . Surface and Bulk Mobility . . . . . . . . . . . . . . . . . 2 . Textural and Chemical Promoters . . . . . . . . . . . . . . 3 . Particle Size and Size Distribution . . . . . . . . . . . . . . . 4. Interaction with and Location on the Support of Supported-Metal Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . B . Surface Properties of Catalysts . . . . . . . . . . . . . . . . . I . The Surface Chemical State . . . . . . . . . . . . . . . . . . a . Measurement. . . . . . . . . . . . . . . . . . . . . . . b . Correlation with Catalytic Properties . . . . . . . . . . . . 2 . The Surface Structure . . . . . . . . . . . . . . . . . . . a . General Remarks . . . . . . . . . . . . . . . . . . . . b . Surface Structure Measurement . . . . . . . . . . . . . . . C . Chemisorption and Reaction . . . . . . . . . . . . . . . . . . I . Interaction of Surface Sites with Gases . . . . . . . . . . . . 2 . Kinetics of Slow Processes . . . . . . . . . . . . . . . . . 3. Stationary-State Effects . . . . . . . . . . . . . . . . . . 4 . The Mossbauer Isotope as a Chemical Probe . . . . . . . . . IV . Concluding Remark . . . . . . . . . . . . . . . . . . . . . . . Appendix I: Nuclear Data for Mossbaucr Isotopes . . . . . . . . . . Appendix 11: Mossbauer Isotope Feasibility for Catalyst Studies . . . . Rel‘erences . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

i63

. 163 . 167 168 169 169 169 173 . 179

. . .

.

.

.

186 193 193 193 198 201 201 203 209 209 213 221 226 229 230 236 239

.

1 Introduction

A . GENERAL REMARKS ABOUT MOSSBAUER SPECTROSCOPY 1 . Advantages of the Technique

The purpose of the present article is to deal with the use of Mossbauer spectroscopy’ for the study of problems in heterogeneous catalysis. In recent years through a number of spectroscopic and other techniques. great progress has been made in the understanding of catalytic phenomena . However. many of the new spectroscopic techniques are somewhat limited in value because of the necessity to work under conditions very far from the actual conditions of the catalytic process . That is. many of these techniques are used under ultrahigh vacuum (UHV). and the samples are best studied in the form of films. ribbons. or single crystals. Such samples are very different This technique is sometimes called. especially in the Russian literature. y-ray resonance spectroscopy .

MOSSBAUER SPECTROSCOPY APPLICATIONS

123

from the small particles of which actual catalysts usually consist. Therefore, researchers have been searching for new spectroscopic techniques that permit such actual catalysts to be studied and which do not necessitate UHV but can be carried out under typical reaction conditions. That is, what is sought is a spectroscopic technique that allows in situ studies of actual catalyst samples. These “criteria” for an ideal spectroscopic technique have been met, to a large extent, by Mossbauer spectroscopy. U H V conditions are not required since one utilizes y rays with energies in the keV range, and the technique lends itself easily to in situ studies. Furthermore, the technique is ideally suited for studies of small particle systems, and in several instances information about particle size can also be obtained. The unique feature of Miissbauer spectroscopy is the extremely high energy sensitivity of the technique. This allows detailed chemical, structural, and magnetic information to be obtained about atoms on the surface or in the bulk phase. There has been to some degree the belief that Mossbauer spectroscopy, although in principle an ideal technique for catalyst studies, for practical purposes can only be applied to problems in catalysis if the catalyst contains either iron or tin. Therefore, one of the main purposes of this review is to show how Mossbauer spectroscopy can be directly extended to many additional “Mossbauer atoms or isotopes” (such as antimony, europium, nickel, ruthenium, gold, and tungsten) and, perhaps more importantly, how the technique can be extended to obtain information about systems that do not contain a “Mossbauer atom.” 2. The Literature Mossbauer spectroscopy started mainly as a tool for the physicists. Quite early however, chemists realized its potential and today Mossbauer spectroscopy is widely used throughout such fields as nuclear and solid state physics, chemistry, biology, metallurgy, ceramics, archaeology, and even the fine arts. The extensive literature on Mossbauer spectroscopy is thus written by scientists from many different fields and is therefore somewhat difficult to read. We have tried t o write this review in a quite complete and self-contained manner so that studies of the referenced literature need not be made. We have, however, also endeavored to include sufficient and up-to-date references to literature for more detailed information, particularly related to examples of applications of the technique. For those who want to follow Mossbauer spectroscopy literature regularly, we refer to Stevens and Stevens, who annually catalog papers dealing with Mossbauer spectroscopy in the Miissbauer Ejfect Data Index ( I -7b). In addition, a series of comprehensive reviews of the Mossbauer spectroscopy annual

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JAMES A. DUMESIC AND HENRIK TOPS@E

literature dealing with inorganic and organometallic compounds is available (8- 13). The developments in Mossbauer spectroscopy methodology and applications that continually take place are periodically presented in Miissbauer Efect Methodology (14), and in a biennial series appearing in Analytical Chemistry (15-18). MZissbauer spectroscopy conferences are held annually and the proceedings of these are widely available (19-24). Several books on Mossbauer spectroscopy have appeared (25-33) and reviews of interest for catalytic studies have also been published (34-48).

3. Objective of’ This Article In this article, various chemical phenomena that are pertinent to catalysis and can be studied through Mossbauer spectroscopy will be discussed. To do this, the physical basis for Mossbauer spectroscopy and the resulting Mossbauer parameters will first be briefly introduced (Section I, A, 4). In Section I, €3, a more detailed discussion of the conditions necessary for observation of the effect and the origin of the Mossbauer parameters will be presented. While this section provides the basis necessary to utilize Mossbauer spectroscopy more completely, the general features of this article may still be extracted without a thorough reading of this section. After this rather mathematical treatment, the more chemical aspects of Mossbauer spectroscopy will be discussed in Section I, C . Special emphasis is here on information pertinent to catalysis. Because the greatest limitation for the general use of Mossbauer spectroscopy is that it cannot be observed for all elements, those criteria which restrict the application of Mossbauer spectroscopy to certain isotopes will be discussed in detail in Section 11, A. Here, it will be shown which of these Mossbauer isotopes may be used to obtain chemical information, and how these isotopes may also be used to obtain in an indirect manner similar information about elements for which there exists no Mossbauer effect. Finally, in Section 111, the utility of Mossbauer spectroscopy in different types of catalytic studies is demonstrated using some illustrative examples from the literature. 4. A Brief lntroduction to Miissbauer Spectroscopy One can describe a Mossbauer spectroscopy experiment as follows (and as shown in Fig. 1): (1) A y ray is emitted by an atom (Mijssbauer isotope) in a source by transition from a nuclear excited state to a nuclear ground state. (2) The pray energy is modulated by a small varying amount 6E.

MGSSBAUER SPECTROSCOPY APPLICATIONS

125

Nuclear

excited state

El

I

Nuclear

gmund state

1 Source +,-,Absorber

Doppler velocity V,

-

0

6E=(V/c)El

+

Doppler velocity (energy)

FIG.1. A Massbauer spectroscopy experiment.

(3) Resonant absorption of the modulated y ray takes place by an atom of the same Mossbauer isotope located in an absorber. (4) Detection of the y rays transmitted through the absorber versus the modulation energy produces the Mossbauer spectrum.

In order to observe the resonant absorption (step 3), the emitted y ray (in step 1 ) must possess the full transition energy E , between the ground and excited states; that is, the nuclear transitions must occur in a “recoilless” manner with a collection of atoms sharing the recoil momentum. The fraction of transitions so occurring is given by the recoil-free fraction, and thus the Mossbauer effect is normally observed only for atoms bound in or on solids. Chemical perturbations of the nuclear levels are extremely small and, therefore, only minute energy modulations need be supplied to the primary pray energy to measure these effects. This energy modulation can be conveniently produced using the Doppler effect by imparting small relative velocities V between source and absorber. After the pray detection (step 4), the Mossbauer spectrum is recorded as a plot that shows the intensity of the transmitted y rays (transmission) versus the Doppler velocity. If the Mossbauer isotope in the source and absorber is situated in identical chemical surroundings, the nuclear energy levels in both will be identical and the observed resonance will occur at zero relative velocity (6E = 0). Different chemical environments, however, result in different perturbations

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JAMES A. DUMESIC AND HENKIK

TOPSQ~E

of the nuclear levels, and shifts and/or splittings in the Mbssbauer resonance are observed. One may group these perturbations into three classes: (1) the isomer shfr (which is also referred to by many as the chemicul shgr) 6, which arises from the Coulomb interaction between the nuclear charge of finite volume and the electron density present in this volume, (2) the quadrupole splitting AEQ, which originates from the interaction between the nuclear quadrupole moment and the electric field gradient at the nucleus, and (3) the magnetic hyperJne interaction, which stems from the interaction between the nuclear magnetic dipole moment and the magnetic field present at the nucleus. These three interactions give rise to different types of Mossbauer spectra. The Coulomb interactions leading to the isomer shift result in energy level displacements that differ for the ground and the excited nuclear states. Therefore, for a given source, absorbers with different electronic structures (e.g., different valences) will be characterized by different isomer shifts, that is, different shifts of the resonance peaks with respect to zero relative velocity. The quadrupole interaction partially lifts the degeneracy of the nuclear levels, producing several possible resonances. For example, for '7Fe, two different transitions are produced, and the difference in energy between these transitions, or the splitting AEQ, is a measure of the symmetry of the electron distribution about the nucleus, with larger deviations from cubic symmetry giving rise to larger values Of AEQ. In general, this quadrupole splitting takes place about the isomer shift, and the observed Mossbauer spectrum is a multipeak pattern displaced 6 from zero velocity. The magnetic hyperfine interaction completely lifts the nuclear degeneracies, which for the case of "Fe results in six allowed transitions. From the observed splittings, the magnetic field present at the nucleus can be determined, and from the line positions, information concerning the isomer shift and quadrupole splitting is obtained.

B. M~SSBAUER SPECTROSCOPY ; RESONANT ABSORPTION AND PERTURBATIONS OF NUCLEAR LEVELS 1. Occurrence of the Efect

The criteria for observation of the Mossbauer effect are qualitatively deduced through the systematic statement of energy and momentum conservation (29, 30,32) (Fig. 2). A y ray emitted in the x direction by a nucleus undergoing a transition from an excited state with energy E, to the ground state with energy E , has an associated momentum E/c, where E is the y-ray

127

MOSSBAUER SPECTROSCOPY APPLICATIONS BEFORE EM I SSlON

MOMENTUM CONSERVATION

AFTER EMISSION

0

P=+

P;_Mv;



PsMV;

Pa$

y-ray

VELOCITY CHANGES

Qf

EKINETIC’ M v; $1’

ENERGY CONSERVATION E

~ E,+~uv;* ~ ~

~

=

ETorAL:E . E ~.+M

I -

4

I vt -6)1

FIG.2 . Conservation and velocity relationships for the y emission.

energy and c the speed of light. In general, E may be different from E, - E, ( - E , ) , as seen below. For a collection of atoms with total mass M , and velocity V,, momentum conservation requires a decrease in V,, (the component of V, in the x direction) by E / M , c upon emission, and energy conservation requires E to be less than E, by E, - ED, where E , is the recoil energy E 2 / 2 M , c 2 ,and EDis the Doppler shift (EVl,./c) caused by the motion of the emitting nucleus. For resonant absorption of the y ray by a chemically identical nucleus in a collection of atoms with total mass M2 and velocity V,, momentum conservation requires an increase in V2, by E / M , c and, to conserve energy, E must be greater than E , by ER + EV2Jc. I f , for example, each collection of atoms undergoes random motion, then the different ED terms broaden the energy distributions for y-ray emission and absorption, but the ER terms lead to displacements in energy of these two distributions as shown in Fig. 2. For thermal motion, ( V x 2 ) = k T / M (where k is the Boltzmann constant) making (ED) equal to (2E,kT)*’,. A typical value of E , for an isolated atom ERois of the same order of magnitude as the value of kT at room temperature eV), making ER and (ED) of the same order, i.e., the displacement in energy of the emission and absorption distributions is of the order of their widths. Thus, in the gas phase, some resonant absorption may be observed corresponding to the overlap region of the respective energy distributions. However, “chemical information” is obtained, as will be seen later, only through resolution of the y-ray energy to within lo-’ eV, thereby making this gas phase resonance of little use in the study of chemistry and catalysis. Reducing, and ultimately eliminating, the effects of (ED) broadening reduces the problem to an examination of recoil energy losses.

-

I28

JAMES A. DUMESIC AND HENRIK

TOPSGE

E, Energy

FIG.3. Photon energy distributions for emission and absorption with recoil. N ( E ) d E = number of photons with energy between E and E + d E .

The finite mean life z, (I,, = In 2 x z l i z , where zIi2 is the half-life) of the nuclear excited state gives rise to a characteristic width, the natural lirzewidth r,, of the pray energy distributions as given by the uncertainty principle r,zn= h, where h is Planck’s constant divided by 2n. A typical value of T,, is lo-’ sec, corresponding to a natural linewidth of the order of lo-* eV. Indeed, this width is small enough that chemical information may be obtained. However, this value is six orders of magnitude smaller than the free-atom recoil energy ERO,and therefore the utility of the y-ray resonance in the study of chemistry and catalysis can be realized only through elimination of this large recoil effect (Fig. 3). This criterion is met for an atom bound (with a binding energy E , greater than ERo)in a collection of atoms, all of which share the required recoil momentum. Therefore, it becomes possible to observe the resonance in the solid state or viscous liquids where E, is of the order of 1 eV. The effective value of M is thereby increased, resulting in a decrease in ER. For example, chemical information may be obtained for atoms in an isolated particle several tens of nanometers in size, and for smaller particles where interactions with other particles or the catalyst support make the effective mass of the particle larger. The above condition, as will be shown below, is not sufficient for observation of the Mijssbauer effect. If the free-atom recoil energy is much greater than the characteristic energy for phonon excitation h a , , where wI is the associated lattice vibration frequency, then phonon creation represents another mode of energy loss, which destroys resonance (29, 30, 32). For E,* less than or of the order of hwl, a significant fraction of the nuclear events (emission and absorption)

MOSSBAUER SPECTROSCOPY APPLICATIONS

129

occurs without phonon excitation, and the y-ray resonance is preserved. In general, the fraction of recoil-free or zero phonon events f is given by

where 3, is the y-ray wavelength and (x2) the mean square vibrational amplitude of the emitting or absorbing nucleus in the direction of the y ray. The value ( x 2 ) increases with temperature, as can be empirically approximated by using, for example, the Debye model for the phonon spectrum, making f temperature sensitive. In the preceding discussions, several characteristic energies were shown to be of importance for observation of the Mossbauer effect. These necessary conditions can be expressed by the following inequalities :

ERo< E, E,'

< ha,

If the above three conditions are satisfied, the Mossbauer effect is observed, and the cross section for resonant absorption a(E) (the probability for resonant absorption per unit flux of impinging y photons with energy E ) is given by 0 = ao(~r,)Z/[(E - E,)~ (5)

+

where a. is the cross section at E = E,. The value of o0 is given by

where I , and I, are the nuclear spin quantum numbers of the excited and ground states, respectively, and a is the internal conversion coefficient, defined as the ratio of the probabilities for electron ejection (internal conversion electrons) to pray emission accompanying the nuclear decay. The preceding discussions form the basis for deciding under what conditions the Mossbauer effect will be observed. However, observation of resonance between the nuclear levels of a source and absorber is just the first step in the application of this effect to chemical and catalytic phenomena. The perturbation of the nuclear levels by the chemical environment must now be considered, since it is in the latter that information pertinent t o catalysis is found.

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JAMES A . DUMESIC AND HENRIK TOPSGE

2, Chemical Perturbations of Nuclear Levels The chemical perturbations of the nuclear levels can be divided into two classes. One effect is the electrostatic interaction between the nuclear charge and the surrounding electric charges, and the other effect is the magnetic interaction between the nuclear magnetic moment and the surrounding electron spin density. The former of these interactions will be discussed next. The electrostatic energy of interaction EE between the nuclear charge density pN(ijk) at point (ijk) and the surrounding electric charge is given by (28,491 E , = pN(ijk)U(ijk) dl/, (7) where U(ijk) is the electrostatic potential at point (ijk) produced by the electron distribution, and VN is the nuclear volume. Near the nucleus (i = j = k = 0), U(ijk) can be expanded in a Taylor series as follows:

where U = U(OOO), Ul = (dV(ijk)/allat (OW), U l l = la2V(ijk)/d121at (OOO), and the coordinate axes i, j, and k are chosen such that U,,lm = 0. Thus, E , can be written

where eZ is the total nuclear charge spN(ijk) dVN. The energy of the emitted y ray is determined by the energy difference between the nuclear excited and ground states. This energy difference is not altered by the first term, e Z U , since this term equally affects the nuclear excited and ground states in both the source and absorber. In addition, atomic nuclei do not possess electric dipole moments, and therefore the second term in Eq. (9) must be equal to zero. Thus, when considering the Mossbauer effect, E , can be written in the form

where r2 = i2

+ j 2 + k2. Using Poisson’s equation,

MOSSBAUER SPECTROSCOPY APPLICATIONS

131

where l$(0)l2 is the probability per unit volume of finding an electron at (000),the expression for EE’ can be rewritten in the form

The first term in this equation, EE1,describes the interaction between the nuclear charge and the electronic charge surrounding it, giving rise to the isomer shift 6, and the second term, EE2, is the interaction between the nuclear quadrupole moment and the surrounding electric field gradient, giving rise to the quudrupole splitting AEQ. Now consider the first term, EE1,which can be written

E,, = #nezZl$(0)12(r2)

(13)

where

is the average value of the squared radius of the nuclear charge distribution. The emitted (or absorbed) y-ray energy ET is thus

where Ee,o and E,,o are the unperturbed energies of the excited and ground states, respectively; and Eg,E1are the perturbations of these levels by the EEl term; and E,,o and E,,,, are the associated transition energies, equal to Ee,o - E,,o and Ee,EI- E,,,,, respectively. Making use of Eq. (13), the expression for ET,E1can be written

In general, the electron density at the nucleus, 11)(0)1~, will be different for the source and absorber, giving rise to a displacement in energy, 6, between the respective pray distribution maxima (Fig. 4) with 6 given by

where the subscripts A and S refer to the absorber and source, respectively. The terms in front of the electron density difference are nuclear parameters,

132

JAMBS A. DUMESICAND HENRIK T O P S ~ E

Source. S

Absorber. A

Velocity FIG.4. The isomer shift 6.

constant for each MiSssbauer isotope. A measurement of 6, the chemical or isomer shift, is then a measure of the electron density at the nucleus in the absorber (the catalyst) with respect to that in the source (which can be chosen as a standard). The symmetry of the electron distribution around the nucleus is contained in the second term of EE‘.This term, EE2,vanishes for an electron distribution with spherical or cubic symmetry, i.e., U i i = U j j = ukk, as follows:

and thus it is only for axial (Uii # U , = U k k ) or lower symmetries that the E,, term shifts the nuclear energy levels. The three components of the electric field gradient tensor are related by Poisson’s equation, as shown earlier. However, the electrons that have a finite probability density at the nucleus, the s and pi,, electrons, have a spherically symmetric distribution around the nucleus and as such do not contribute to EE2.Thus, in the computation of E,,, the U,, can be related by

MOSSBAUER SPECTROSCOPY APPLICATIONS

133

and for axial symmetry the following expression can now be written:

where i = x,j = y, k = z, and U,, # U,, be evaluated (28,49) as

=

U y y .The nuclear integral can

where m is the projection of the nuclear spin I onto the z axis, and QI the corresponding nuclear quadrupole moment (Q, can be nonzero only for I > 4).Substitution of Eq. (20) into (19) gives the final expression for EE2:

Nuclear energy states are specified by angular momentum quantum numbers I, each of these states composed of substates specified by magnetic quantum numbers m, which correspond to the projection of I onto the z axis. The E,, term (giving rise to the isomer shift) was independent of m, i.e., the substates of each nuclear state were degenerate; however, the E,, term partially removes this degeneracy. That is, while EEI shifts the energy levels, E,, can give rise to a splitting (the quadrupole splitting AEQ) in energy of these levels, as shown in Fig. 5 for I , = 3 and I, = 3 (57Fe,for example). When the symmetry of the electric potential becomes less than axial, a simple expression for E,, cannot be written, and the complete expression

must be used in calculating the effect of this term on the ground and excited states. An important special case, for which the integral can be evaluated in closed form, is I = +,in which case Eq. (22) becomes

+ +

3m2 - I ( I I) EE2= $UzzeQ, _ _ -~ 31, - 1(1 1)

~

where the coordinate system is chosen such that lUzz[> lUyyl > lUxxl and q = ( U y y- U x x ) / U z zand , is called the asymmetry parameter.

134

JAMES A. DUMESIC AND HENRIK T O P S ~ E

SourceS

0

Velocity

FIG.5. The quadrupole splitting AE,. For ~llustration,UzzQ z 0.

Besides the above interactions between the nuclear and electronic charge distributions, the nucleus also interacts with magnetic fields. In the presence of such a field at the nucleus, different orientations of the nuclear magnetic moment p with respect to the field direction will have different energies E M (e.g., 2Y, 30,32): EM = - p * H = - y H m / l (24) where H is the magnitude of the magnetic field H, y the magnitude of p, and m the projection of the nuclear spin I on the direction of the magnetic field. As was the case for the quadrupole splitting (the E,, term), the magnetic interaction lifts the degeneracy of the m-substates for a particular value of I , this removal of degeneracy being complete for the present case. Thus, the magnetic interaction produces a splitting of the nuclear energy levels (Fig. 6), and the transitions, allowed by the selection rules (see Section 1, C, 5), between the split excited and ground states provide a measure ofthe magnetic field at the nucleus. This magnetic field may be applied externally, or it may result from the electronic structure about the nucleus. In general, an expression for H may be written as follows (29,30):

MOSSBAUERSPECTROSCOPY APPLICATIONS

/

135

I

/---

1 22

Source, S

Absorber,A

I

I

Magnetic Magnetic + interaction quadrupole interaction

I

0

Velocity FIG.6. The magnetic hyperfine splitting and the combined quadrupole interaction. For illustration, lU,,eQ/pHI << 1 ; pe < 0, pg > 0; U,,Q(3 cosz 0 - 1) > 0.

where Ho is the externally applied field, HM the field resulting from the magnetization of the particle within which the absorbing (or emitting) nucleus is located (depolarization and Lorentz fields), and Hs, HL, and HD effective magnetic fields due to the electronic structure about the nucleus. Specifically, H, is the Fermi contact interaction resulting from a net spin density difference between spin-up and spin-down electrons at the nucleus; HLrepresents the interaction of the nuclear moment with the orbital angular momentum of the electron distribution; and H, is the result of a dipolar interaction between the total electronic spin and the nuclear moment. It is in these latter three magnetic terms that valuable chemical information can be found, as will be discussed later. In addition, through the time dependence of these terms (magnetic relaxation), effects of catalyst particle size, shape, and surface structure can be deduced. In the previous discussions, electrostatic (EE)and magnetic (EM)effects were considered separately. However, the simultaneous presence of these two effects presents a more complex case, the solution to which cannot

136

JAMES A . DUMESIC A N D HENKIK TOPS@E

generally be expressed in a simple form. For I = 3, however, two important special cases can be considered. For an axially symmetric electric field gradient tensor with IU,,eQl/yH << 1, the energies for the different values of 3 m (3 ~ 1, z -, 71, -z) are

E

= (-pHm/l)

+ (-

l)lm1+112 $eU,,Q(+ cos’ # -

f)

(26)

where 0 is the angle between the magnetic axis and the principal component of the electric field gradient tensor U z z .This situation is also shown in Fig. 6, where E is defined as E = ~eU,,Q(~cos 82- 2 ) (27) Energy levels for arbitrary 1 U,,eQ(/pH and arbitrary electric field gradient symmetry, but with fl = 0, are given by the following expressions:

C.

MOSSBAUER

SPECTROSCOPY ; DERIVED CATALYTIC INFORMATION

1. Recoil-Free Fraction

The recoil-free fraction f, while strictly speaking not the result of a chemical interaction, can indirectly provide useful chemical, as well as structural, information. As shown earlier, ,f is related to (x2), the mean square vibrational amplitude of the resonant atom in the direction of the y ray. The temperature dependence of (x2) is often approximated using the Debye model (29, SO),

J’

E

exp[ - 6ERoT/kOD2]

for

T > +OD

(31)

where OD is the Debye temperature, T the absolute temperature, and k Boltzmann’s constant. The Debye temperature is a measure of the “stiffness” of the lattice in which the resonant atom is vibrating, a large value of 0, corresponding to a small value of (2). A measure ofthe absolute magnitude off and/or its temperature dependence is thus a measure of the strength of binding between the resonant atom and its surroundings.

MOSSBAUER SPECTROSCOPY APPLICATIONS

137

For example, a resonant atom contained in a surface-adsorbed species and a resonant atom exchanged onto a supporting material are cases where a measure off is a probe of the respective chemisorptive and exchange bond strengths. One can consider an experiment in which the Mbssbauer spectral area is measured as a function of the surface coverage by an adsorbate containing the Mbssbauer isotope. The shape of this curve would then contain information about the coverage dependence of the chemisorption bond strength, reflecting surface nonuniformity and/or interactions between adsorbed species. These two effects may in turn be distinguished by observing the effect of isotopic enrichment on the shape of this curve. To illustrate this latter point, in the case of an iron-containing adsorbate, this would be accomplished by first dosing the adsorbent to a certain coverage with the 57Fecontaining species, followed by further dosing with the 6Fe-containing analog species (since only the isotope 57Fe shows a Mossbauer effect). An analogous series of experiments would also give information about the exchange bond strengths for atoms exchanged onto a support. The magnitude of (x2) for a “surface atom” is expected to be different (probably larger) than that for a bulk atom, and thus these different types of atoms may in principle be distinguished through their respective f temperature dependences. In addition, the mean square vibrational amplitude for a surface atom may be quite anisotropic, thereby affecting the relative yray intensities for the different nuclear transitions (the Gol’danskii-Karyagin effect, which will be discussed in Section 1, C, 5). Thus, a careful study of the relative y-ray intensities may provide information about the vibrational anisotropy of the atoms at or near the surface and changes thereof upon chemisorption. It should be noted that this vibrational anisotropy is also expected for a Mossbauer isotope-containing species adsorbed on a surface, and a measure of the relative y-ray intensities may allow the location of the resonant atoms to be assigned as being “on” ( < t Z )> < a 2 ) )o r “in” ((n2) > (t’)) the surface, where ( n 2 ) and (t2) are the mean square vibrational amplitudes normal and tangent to the surface, respectively (50). It was seen earlier that small particles (- 10 nm) must be bound to a support (or to other particles) in order for one to observe a recoil-free event and, furthermore, the phonon spectrum of the material may be particle size dependent. The former effect can give rise to either an increase or a decrease in f with decreasing particle size, depending on the strength of the support interaction, while the latter effect may result in an increase in f with decreasing particle size (due to a “cut-off” of the low-frequency phonons). Both of these effects have to be taken into account, in addition to that resulting from an increase in the surface to volume ratio with decreasing particle size, since as mentioned above the value o f f for a surface atom may be different from that for a bulk atom. Thus, the dependence o f f on

138

JAMES A. DUMESICAND HENRIK TOPS@E

particle size may be rather complex, but through a careful study of its magnitude and anisotropy, and the effects of chemisorption thereon, information concerning the catalyst surface and support interactions may be inferred. 2. Isomer Sh$ As shown in Section 1, B, 2, the isomer shift 6 is a measure of the electron density at the nucleus of an atom in the absorber (catalyst) relative to that at the nucleus of an atom in the source. The only electrons with a nonzero probability for being located at the nucleus are those in the s and pliz orbitals. The relative importance of electrons in the nth pli2 and nth s orbitals is given by (49)

111/,,,,,(0))2/l~,,,(O)1z z (e’Z/hc)’

=

5.3 x 10-5Zz

for n 2 2 (32)

Thus, the effect of the P , , ~orbitals is often negligible and Eq. (16) for 6 is writ ten (33) 6 = 5neZmZ)I(r2)e - (r2>,I[l$,(O)(i - l$s(o)l;l where S ( Z ) reflects the effect of the pljZ electrons, i.e., S(2) -, 1 as 2 decreases. While 6 is proportional to the s-electron density at the nucleus, valuable information about the d (and other) electrons can still be obtained from this parameter due to screening effects. That is, the addition of a d electron reduces, via screening, the effective nuclear charge felt by the s electrons thereby leading to an expansion of the s-electron cloud and a decrease in the electron density at the nucleus. A measurement of 6 thus reflects to some extent the entire electron distribution surrounding the nucleus, giving information about both the atom and its bonding characteristics. In the expression for the isomer shift, the term [ ( r 2 ) , - (r2>,] can be considered a known nuclear constant, which has been determined either by direct nuclear measurement or by the measurement of the isomer shift for compounds with known electronic structures (51). Ideally then, a measure of 6 provides a determination of lI//q(0)12, the latter related to the electronic structure as expressed, for example, by the occupation numbers of the various orbitals, e.g., 3d74s’ for metallic iron (Fig. 7) (52). In this manner, it is often possible to identify the oxidation state of the Msssbauer atom and to deduce information concerning the bonding of this atom to its surroundings. In some cases, different electronic structures may have similar values of l1/1,(0)(~ (the low-spin Fe2+ and Fe3+ pair is an example), and electronic structure information from Mossbauer spectroscopy is most

MOSSBAUER SPECTROSCOPY APPLICATIONS

0.8

I-

139

I

0.4

- 0.8 -1.2

1

,

I

I

,

1

0

20

40

60

80

100

x = 4s electron contribution 1%)

FIG.7. Isomer shift versus electron density for 57Fe.6,, is the isomer shift with respect to that for metallic iron. Figure according to Danon (52).

effectively obtained through the combined considerations of the isomer shift, quadrupole splitting, and the magnetic interaction. While a determination of the electronic structure from the magnitude of 6 alone is sometimes ambiguous, changes in 6 can be of particular interest in catalytic studies. For example, if the electronic properties of a small catalyst particle are different from those of larger particles, due either to a support interaction or an intrinsic effect, then these differences may be manifested in a particle size-dependent isomer shift. The isomer shift may also be a sensitive probe of differences between the electronic structure of surface and bulk atoms in a small catalyst particle, and effects of chemisorption on the electronic properties of these small particles are amenable to study using the isomer shift. The study of alloy catalysts is ideally suited for Mossbauer spectroscopy, since a composition-dependent isomer shift may provide evidence for the presence (or absence) of alloying in small catalyst particles, or clusters. It should be stressed that in addition to large particle systems, Mossbauer spectroscopy can give information about small-particle and amorphous systems, which are often difficult to study by techniques such as conventional X-ray diffraction. In addition to the behavior of the isomer shift at a given temperature, its temperature dependence is also of catalytic interest. It was shown earlier

140

JAMES A. DUMESIC AND HENRIK TOPS$E

that the resonant pray energy is shifted by the velocity of the emitting or absorbing nucleus due to the Doppler effect, EV/c. An atom vibrating in a lattice site (the characteristic time for this vibration being lo-', sec) gives rise to an average velocity equal to zero (( V ) = 0) for the time scale of the nuclear decay from the excited state, but a second-order term proportional to ( V 2 ) does not so average to zero. This so-called second-order Doppler rflect, EZD, changes the energy of the emitted y ray by

E2D

= -(

V2)ET/2c2

(34)

E,, is temperature dependent due to the temperature dependence of ( V ' ) . The temperature dependence of E,D for a one-component system is given by (29,30) (8E2o/JT), = - C,E,/2M,c2 (35)

where C , is the molar heat capacity at constant pressure and M , is the atomic mass. Thus, the second-order Doppler shift supplies chemical information analogous to that provided by the recoil-free fraction. That is, a weakly bound atom (corresponding to a large value of (x') and a small recoil-free fraction) will have a large ( V 2 ) and hence a large second-order Doppler shift. 3. Quadrupole Splitting

The symmetry of the electron distribution about the nucleus, as reflected in the quadrupole splitting, can be divided into two parts (32, 53,54). First, the atomic electrons about the central nucleus may fill orbitals in such a manner that the resulting electron cloud produces an electric field gradient at the nucleus. Second, the electric charges external to the central atom from the neighboring atoms, ligands, or ions must also be considered in a calculation of the field gradient at the central atom nucleus. These two effects are expressed as : (36) u,, = (1 - y,juk + (1 - Rju;;' where Uyi' and U & are the electric field gradient tensors resulting from the atomic (or valence) electrons and the surrounding (or lattice) charges, respectively, and y m and R are Sternheimer antishielding factors. The latter reflect the deformation of the atomic core electron orbitals by the valence or lattice electric field gradients. Consider first the effect of the atomic electrons. A filled or half-filled electron shell has a spherically symmetric electron distribution, and as such gives rise to no electric field gradient (except through external deformation, i.e., Sternheimer antishielding). Thus, of all the atomic electrons, only the

MOSSBAUER SPECTROSCOPY APPLICATIONS

141

valence electrons give rise to a nonzero field gradient, as illustrated by the following examples (54):

+ )(Npx + NPy)]

P electrons:

U;t'/e

=

$(r-3)p[-Np,

d electrons:

U;;'/e

=

$ ( r - 3 ) d [ - Ndz2+ N d x 2 - y Z

(37)

where ( r - 3 ) i is the expectation value of F 3 for the i orbital, and N iis the occupation number (0 = empty; 2 = full) for the i orbital. For convenience, the values of N ifor a particular atom can be considered t o be dependent on two effects: a crystal field and a molecular orbital effect. The former is due to the crystal field splitting of the central atomic orbitals by the neighboring electron distribution, resulting in preferential occupation of certain atomic orbitals; and the latter effect is due to the overlap between the central atomic orbitals and neighboring orbitals, resulting in charge transfer to or from the central atom. While bond lengths and thus the molecular orbital effect do not vary greatly with temperature, the crystal field effect is quite temperature dependent. This stems from the temperature dependence of the occupation numbers, as given by appropriate Boltzmann exponentials for the populations of the crysial field split energy states. For cases where the crystal field splitting does not result in an electric field gradient, e.g., t$g or e;, the field gradient at the central atom resulting from the electric charges on the neighboring lattice sites must be considered. Unlike the crystal field splitting, this lattice effect is not very sensitive to temperature. Thus, it can be seen that the electric field gradient at the nucleus of a resonant atom and its temperature dependence reflect the symmetry of that atom through effects due to crystal field splitting, charge transfer between the central atom and its neighbors, and the distribution of electric charges over the lattice sites. It is from this symmetry information that the location of the resonant atom in or on the catalytic material may be established. For example, a large, temperature-dependent quadrupole splitting for an Fe2+ (3d6)ion in the bulk is evidence that the ion is in the high-spin state. In addition, the observed quadrupole splitting may serve to identify that site in the structure in which the Mossbauer atom is located, while at the same time serving as a probe of that site. Yet, because the effects that give rise to the electric field gradient may be quite long-range, the quadrupole splitting of the Mossbauer atom may also be sensitive to the neighboring sites in the structure, thereby providing information about the whole structure. In addition, the lower symmetry of a surface atom (or an atom near the surface) compared to that of a bulk atom may be reflected in a larger quadrupole splitting for the former, thereby allowing these different types ofatoms to be distinguished. As the catalyst particle size is decreased and the surface to volume ratio

142

JAMES A. DUMESIC‘A N D HENRIK TOPS~JE

increases. the “effective” quadrupole splitting (the statistically weighted summation of the contributions from the surface and bulk atoms) may increase, allowing this splitting to be used for particle size measurement. In addition, for a multicomponenl system one may be able to determine the surface concentration of the resonant atom, and its dependence on chemisorption, by measurement of the spectral areas under the “surface” and “bulk” quadrupole split peaks. Since the quadrupole splitting of the surface atoms is related to their respective symmetries, a measure of this splitting may provide information about the corresponding surface structure. This measure may not uniquely specify the surface structure, but changes in the latter as a result of surface reconstruction may be amenable to study. The presence of a surface chemisorbed species may change the electric field gradient, depending on the strength of the chemisorptive bond and the location of the chemisorbed species with respect to the surface atoms. Therefore, a measure of the change in the surface quadrupole splitting accompanying the chemisorption of known quantities of adsorbate may provide information about the possibility of surface nonuniformity, induced and/or ( I priori. In addition to the above examples where a significant fraction of the resonant atoms were on the surface, the quadrupole splitting of bulk atoms may, of course, also be of catalytic significance. A resonant atom located on a site with octahedral or tetrahedral symmetry will have no lattice contribution to the electric field gradient, and if the resonant atom has valence shells that are filled or half-filled, then there will be no quadrupole splitting. However, a distortion of the lattice will give rise to a lattice contribution to the field gradient. Such a distortion, which may extend to the surface, can be evidenced through thc quadrupole splitting. Also, the distribution of the resonant atoms throughout the bulk, e.g., uniform or clustered, or the distribution of other species about the resonant atom is reflected in the quadrupole splitting. 4. Magnetit Hyperfine Splitting and Suprrpuramagnetism It was shown earlier that the presence of a magnetic field at the nucleus removes the degeneracy of the substates for each angular momentum nuclear state, producing a splitting of the pray energy distribution. The value of this splitting is directly proportional to the magnitude of the internal magnetic field. Magnetically ordered compounds are therefore easily examined and since the resulting Mossbauer spectrum is a summation of the individual “atom spectra,” detailed information is obtained not only about simple ferro- and ferrimagnetic materials (which possess a net moment, and therefore are easily examined by conventional magnetic methods), but also for antiferromagnetic materials, mixed magnetic phases, and compounds where a

M ~ S S B A U E RSPECTROSCOPY APPLICATIONS

143

distribution of magnetic fields is present. In addition, effects of chemisorption on the electronic spin state of the resonant atom are readily measured. One may imagine two such effects: a localized surface spin cancellation (or creation) and a collective change in the spin state of the entire catalyst particle. The first effect would be evidenced by a change in the spectral area of the magnetic hyperfine split peaks, whereas the second effect would result in a change in the magnitude of the observed internal magnetic field for the entire particle. Also, differences in the magnitude of the hyperfine field for surface and bulk atoms may be detected using the magnetic splitting, and collective changes with particle size in the magnetic moment per unit volume will be reflected in a change in the hyperfine splitting. Qualitatively, there are two conditions that must be met in order to observe a distinct magnetic hyperfine splitting. First, the magnetic splitting must be larger than the linewidth rnof the y-ray energy distribution, i.e., PHI1

k

which can be rewritten zn

k

r,

(39) (40)

ZL

where z, is the lifetime of the nuclear excited state (=ti/r,) and zL the nuclear Larmor precession time ( = I h / p H ) . If the internal magnetic field fluctuates (relaxes) with a characteristic time zH that is much smaller than T ~ then , the magnitude of the magnetic field at the stationary nucleus will effectively average to zero ( 5 9 , i.e., the characteristic “response time” of the magnetic hyperfine interaction is long compared to the relaxation time for the origin of the effect (the magnetic field). This second condition for observation of the magnetic hyperfine splitting can thus be written TH 2 T L (41) and T L can be thought of as an experimental observation time zc, for study of magnetic effects using Mossbauer spectroscopy. The above criterion and, in general, the effects of magnetic relaxation ( t J on the Mossbauer spectrum, are shown nicely in the theoretically calculated spectra of Wickman et al. (56) (Fig. 8). In addition, a recent review of this subject has been written by Wegener (57),in which the effects of magnetic relaxation on the Mossbauer spectrum have been described and discussed in detail. For 57Fe,zc (=zL) is IO-’sec, to be contrasted with that for magnetic susceptibility. The latter method is well suited for the study of magnetic moments that are free to align with an applied field, and the experimental observation time zc’ for this type of measurement is 10’ sec (58), the time scale for application of the external magnetic field. Thus, it can be seen that

-

-

144

JAMES A . DUMESIC AND HENRIK TOPS@E

Velocity (crn sec-'1

FIG.8. Mossbauer spectra lor various spin relaxation times. Figure according to Wickman yr

a/. (56).

for rH > T,-, the electron magnetic moments give rise to a hyperfine splitting in the Mossbauer spectrum, and for zH > zc' the spins appear fixed in magnetic susceptibility. In discussing characteristic times for magnetic interactions, it seems appropriate also to compare Mossbauer spectroscopy with electron spin resonance (ESR) and nuclear magnetic resonance (NMR) (59). For ESR, the dominant interaction is that between the electron spin and an applied magnetic field H,, and the time scale for this interaction (the Larmor precession time) is 10- sec for H o = 5 kG. The nuclear moment may also precess about the applied field, and the interaction between the electron spin and the nuclear magnetic moment (the magnetic hyperfine interaction discussed with reference to the Mossbauer effect) creates a splitting of the ESR resonance. However, since the nuclear magnetic moment is much smaller ( lo00 times) than the electron magnetic moment, the characteristic time scale for alignment of the nuclear spin with a magnetic field ( - lo-' sec for H , = 5 kG) is much longer than that for the electron spin. Thus, a rapidly flipping electron spin (zH sec) and its interaction with the nuclear moment can be studied using ESR, whereas for the Mossbauer effect and NMR, the magnetic field produced by the electron spin averages to zero. It is only for slower electronic relaxation times that Mossbauer spectroscopy and NMR can be used to study the magnetic hyperfine interaction.

-

-

-

MGSSBAUER SPECTROSCOPY APPLICATIONS

145

For paramagnetic spin systems, there are two major processes of relaxation (55). One relaxation mode involves spin-flipping accompanied by lattice

phonon creation and/or annihilation (spin-lattice relaxation), and the other mode is due to the mutual flipping of neighboring spins such that equilibrium between the spins is maintained (spin-spin relaxation). For the former mode of relaxation, zH decreases with increasing temperature, and the latter relaxation mode, while in certain cases temperature dependent, becomes more important (tHdecreases) as the concentration of spins increases. When neighboring spins are strongly ferromagnetically or antiferromagnetically coupled, the flipping of individual spins is energetically unfavorable compared to the collective sharing of the spin-flip by the exchange coupled spins (spin waves) (60).Thus, relaxation of the magnetic field at the nucleus must be accompanied by the collective fluctuation of the entire spin system. This phenomenon is commonly called superpuramagnetism. In this case, a single temperature-dependent magnetization vector M can be associated to each spin system. (For catalysts consisting of fairly small particles, several tens of nanometers or smaller, each particle can often be considered to be a separate spin system, as will be seen later in this paper.) In general, the magnetic energy is not isotropic with respect to the magnetization orientation. That is, there are certain low-energy directions along which M tends to lie, and the flipping of M from one low-energy direction to another is accompained by the crossing of magnetic anisotropy energy burriers. Comparing the magnitude of the average magnetic anisotropy energy barrier EA to the thermal energy kT allows an expression, first derived by NCel, to be written for the magnetization relaxation time tH(61-63):

where t ois a proportionality parameter. In general, it has been shown that tHis a more complex function of E,/kT than the above simple exponential ( 6 4 6 5 ) .Specifically, for uniaxial anisotropy, i.e., two low-energy directions displaced by rt rad, the above expression was found (65)to be quite good for E,/kT 2 0.5, while for cubic anisotropy Eq. (42) was found valid only for larger values of E,/kT ( 24). With Mossbauer spectroscopy, superparamagnetism is often studied at values of E,/kT 2 1.5, and thus under these conditions Eq. (42) can be used to estimate the magnitude of the anisotropy energy barrier. Finally, it should be noted that if the sample is studied under an applied magnetic field, then tH is dependent on both the anisotropy energy barrier and the strength of the external magnetic field (66).Then, by observing the Mossbauer spectra at different applied field strengths, additional information about E, can be obtained. As will be shown presently, it is in E A , its magnitude, and origin that interesting catalytic information can be found. One

146

JAMES A. DUMESIC A N D HENKIK TOPSGE

origin of magnetic anisotropy is that of magnetocrystalline anisotropy, i.e., the magnetic energy is not isotropic with respect to the crystallographic axes. For example, in metallic iron the lowest energy direction of M is the [ 1001 direction (67), and M must pass through high-energy directions in order to move from one low-energy direction to another symmetry-related direction, as shown in Fig. 9. All of the atoms in the spin system contribute to this magnetic anisotropy, and thus the magnitude of the energy barrier is proportional to the volume of the spin system. The proportionality parameter is obtained from single-crystal measurements. For small catalyst particles below the single magnetic domain size, the spin system is the entire particle, and a measure of the magnetic anisotropy energy barrier becomes a particle size determination. Due to demagnetization effects, the overall shape of the particle can also produce a magnetic anisotropic effect (68).Low-energy directions for M are now dictated by the particle shape and correspond to the elongated particle directions. The magnetic anisotropy energy barrier for magnetization relaxation is again proportional to the particle volume, the proportionality parameter calculable from the particle shape and the square of the magnetization. Thus, for materials with a large magnetization magnitude, magnetic

f

Direction

FIG.9. Magnetocrystallinc anisotropy. K , V and K,V are the energy barriers [or two dilkrent directions of rclaxation. Figure according to Boudart tv crl. (21.7).

MGSSBAUEK SPECTROSCOPY APPLICATIONS

147

shape anisotropy may well overshadow magnetocrystalline anisotropy even for moderate shape elongations. In these cases, a measure of the magnetic anisotropy energy barrier provides information about the shape of the spin system, uhich for small particles is equivalent to the particle shape. In contrast to the above two volumetric effects, Ntel has proposed a phenomenological theory of “magnetosurface anisotropy” (69, 70),according to which the lower symmetry of the surface atoms compared to that of the bulk atoms gives rise to a surface-sensitive magnetic anisotropy. The magnetic anisotropy energy for a surface atom depends on the orientation of the magnetization with respect to that atom (as will be discussed in greater detail later), thus making this interaction sensitive to the surface structure. Summing this interaction over all surface atoms gives the total magnetosurface anisotropy energy E,, which is proportional to the number of surface atoms. The prop01tionality parameter contains information about the surface structure, thereby making magnetosurface anisotropy of great interest for the study of small catalyst particles. Finally, it should be noted that there are a number of other magnetic anisotropic effects (e.g., stress, impurity, and exchange anisotropies) that may for a particular system be important. Detailed discussions of these effects, and of superparamagnetism in general, can be found in the literature (71-74).

5. Line Intensities und Shapes In the previous sections, chemical perturbations that split the nuclear energy states were discussed. These splitting result in multipeak Massbauer spectra, and as such provide valuable catalytic information. However, from the shape and relative intensities of the various resonance peaks, additional information can be obtained, as will be discussed presently. The probability of pray emission or absorption by a nucleus is proportional to the square of the matrix element ($f/cY?l$i) between the final, $f, and initial, qi, nuclear states, where X is the Hamiltonian describing the interaction between the nucleus and the photon. A property of the nuclear state $ j is its parity P, defined as the change in sign of $ j upon inversion of the wave function through the origin. Similarly, a parity can be associated to i@. Since the matrix element is an integral over all space, it vanishes if the product of t,hf, 2’,and $i has odd parity ( P = - 1). In this case y-ray emission or absorption cannot occur (75).Earlier it was shown that the electrostatic interaction between the nucleus and the surrounding electron distribution could be expanded in a series (multipole expansion), and it is similarly advantageous to expand the expression for 2. Thus, the electric and magnetic fields generated by the photon are expressed by a series of dipoles (first order), quadrupoles (second order), octupoles (third order), etc., and the matrix element ($flXl$i)can be expressed by a sum of corresponding

148

JAMES A. DIJMESICAND HENRIK TOPS@E

terms. In general, the electric field components of even order have even parity ( P = I), while those of odd order have odd parity. The opposite is true for the magnetic field components. Thus, if I)f and I)i are of the same parity, then only the terms E2, E4, E6,. . . and MI, M3, M5,. . . will contribute to pray emission or absorption, where E and M refer to electric and magnetic field components, respectively, and the number associated with each gives the order of that component (75). The y-ray emission or absorption must also conserve angular momentum. This condition results in a requirement for Am (the change in rn) going from the initial to the final state. For a transition oforder i, Am canequalo, k l , . . . , + ( i - l), i i . Generally, the electric field components of X are much larger than the magnetic components of the same order (or multipolarity), and the magnitude of these decreases greatly with increasing order (30, 49, 75). Thus, for 57Fe (I, = P = - 1 ; I , = i, P = -1) and l19Sn (I, = +, P = 1. Ig = 1 23 P = 1 ) the M1 transition is expected to be the largest contributing factor to the pray resonance. From the parity and spin of the ground and excited nuclear states, along with the multipolarity of the transition between the states found as described above, the relative intensities of the allowed transitions can then be determined. In general, these relative intensities depend on the angle 0 between the y-ray direction and the principal axis of the magnetic field or the electric field gradient tensor, and they are given by the product of an angular-independent and dependent term. In Table I, these two terms and their product for two values of 0 are given for an M l (I = 3,f) transition for different values of rn in the substates (nzl, mz).The corresponding intensities for other transi-

3,

9

TABLE I Relafile Intensirie.;for (4 M1, I =

Magnctic hyperfine Eplit spectrum

mz

“1

+t

+t +$ ++ ++

+t

-; -1 ti

+i

1

-+

-;I

3 2 1

0 I

-i

-I

+t

Iti+j

1 1

3

k;

Arbitrarily normalized.

-7

1

Angulardependent term” 1

+

COS’

0

2 sinZ 6 1

+ cosz 0

0

2 3

-2

Quadrupole split spectrum

Angularindependent term’

2,j Trmsirion

0 0

1 + cos2 O 2 sin’ 6 1 + COSZO

2 3 sin’ O 3(1 4- cos’ 0)

+

Relative intensity

0

=

90“

0 = 0”

3

6

4 1 0 0

0

1

2 0 0 2

4

0

3

6

5 3

2 6

MOSSBAUER SPECTROSCOPY APPLICATIONS

149

tions are summarized elsewhere (30). The orientation of these axes with respect to the crystallographic axes can therefore be determined from the Mossbauer spectra in cases where the orientation of the sample is known, e.g., single crystals. For inany catalytic materials, however, the orientation of the crystallographic axes is not known, e.g., powder and small-particle systems, but in these cases the angular dependence of the relative intensity pattern may still be of importance, when coupled with an anisotropic recoil-free fraction (30,49,76,77). This is the so-called Gol’danskii-Karyagin effect, and it has been recently discussed by these authors (78). The relative intensity R i j of peak i to peak j is given for a collection of randomly oriented systems by

where Ai is the angular dependence of the nuclear transition giving rise to peak i (for example, see Table I); 0,4 are polar angles measured with respect to a z axis coincident with the principal axis of the magnetic field or the electric field gradient tensor; and f ( 0 , 4) is the recoil-free fraction, which may in general depend on 6 and 4. R,, may depend on f ( 0 , q5), thereby providing chemical and structural information. The linewidth (corrected for instrumental effects) may also provide important chemical information of several types. For example, if the chemical environment of a resonant atom is not the same for all of the atoms in the sample, then a broadening of the observed resonance is expected. That is, the observed resonance is a sum of the contributions from each atom, the latter not all having the same Mossbauer parameters. Thus for a small catalyst particle, interesting particle size information might be contained in the linewidth due to the contribution from the “surface” atoms to the Mossbauer spectrum. The distribution (clustered o r uniform) of resonant atoms throughout a multicomponent catalyst particle may also be reflected in the linewidth. Apparent broadening of the y-ray resonance can also arise from the presence of an unresolved quadrupole or magnetic hyperfine splitting. In this case, the magnitude of the respective splitting is not large enough to result in a resolved multipeak pattern, but instead the various peaks of the resonance overlap to produce a broadened envelope. A detailed study of the observed line shape, however, allows the magnitude of the appropriate splitting to be estimated. In contrast to the above origins of linewidth broadening, which were time independent, the fluctuation of the internal magnetic field direction (magnetic relaxation) may also broaden the y-ray resonance. It was shown

150

JAMES A. DUMESIC AND HENRIK TOPSQE

earlier that for fast relaxation rates (zH << zc) the magnetic hyperfine interaction vanishes, while for slow relaxation rates (zH >> zC) a distinct and unbroadened hyperfine splitting results; however, for intermediate rclaxation rates (zH T ~ a) broadened hyperfine splitting is observed (55). As zH is decreased (approaching zC), the hyperfine split peaks closest to thc center of the spectrum are the ones that first begin to broaden and collapse toward the zero field positions. Similarly, as zH is increased (approaching T~.), the various quadrupole split peaks will also broaden at diflerent rates. Thus, for conditions where zH T ~ the , linewidth and line shape may provide information about the magnetic relaxation rate. Finally, another mode of line broadening is due to the motion of the nucleus, reflecting the mobility (or diffusivity) of the resonant atom. That is, if the nucleus emits or absorbs a y ray while the nucleus is undergoing a movement from site A to site B, then a broadening of the y-ray distribution results if the time scale for this motion is of the order of the nuclear decay time (79).The time scale for the y-ray emission or absorption process is the sec); the “resolution” of the y ray life time of the excited state, T , ( is its wavelength 2 (-0.1 nm); thus, effective diffusivities of order Izz/z, cm2 sec-’) can be studied using the Mossbauer effect (80). An order of magnitude estimate for the broadening of the y-ray resonance, AT by random diffusion can be made in the following manner. By analogy with the expression for the natural linewidth rn(r, = h/z,), Ar is written

-

-

AT =

h/TL,

(44)

where z, is the time for diffusion of the resonant atom by one wavelength. An expression for T D can be written zD

where D is the diffusivity, and

=

L2/D

(45)

Ar becomes

Ar = hD/Iz2 = hK2D/(2n)’

(46)

where K = 2n/L Detailed calculations (79)and experimental work (81)show that the dependence of Ar on D is

Ar

=

2hK2D

(47)

consistent with the order of magnitude estimate. The above calculations, indicating proportionality between Al- and D,are not applicable when AT is very much greater than r, (the natural linewidth) (49).In the latter case, the diffusivity is so large as to impart a net velocity to the nucleus during the mean lifetime of the excited state, and Doppler

MOSSBAUER

SPECTROSCOPY APPLICATIONS

151

broadening ( E D ) results ( ( E D ) = E( V‘2)112/cas shown in Section I, B,l). Thus Al- is now proportional to D”’, since the latter is in turn proportional to the mean distance traveled by the diffusing atom per unit time. It also seems appropriate to discuss briefly the “jump model” of diffusion and its effect on linewidth. In this model, the resonant atoms are described as jumping from one lattice site to another by a function h(r),where this function (the correlation function) is the probability of finding the atom at r after a jump from the origin. The average residence time on each site is T ~ Since the distance between lattice sites is of the order of the y-ray wavelength, an estimate for the linewidth broadening is

AT = hi?,

(48)

and this estimate is consistent with the result of more detailed calculation (79),

where K = P/h and P is the pray momentum (IKI = K = 27c/L). A possible modification of this expression is presented elsewhere (82).The value of z,can be related to a diffusion coefficient (e.g., zJ = E2/6D, where I is the jump distance), thereby making the Ar expressions qualitatively similar for continuous and jump diffusion. A point of major contrast, however, is the inclusion of anisotropic effects in the jump diffusion model (83). That is, jumps perpendicular to the y-ray direction do not broaden the y-ray resonance. This diffusive anisotropy will be reflected in the Mossbauer effect in a manner analogous to that for the anisotropic recoil-free fraction, i.e., for single-crystal systems and for randomly oriented samples through the angular dependence of the nuclear transition probabilities (78). In this case, the various components of the Mossbauer spectrum are broadened to different extents, while for an anisotropic recoil-free fraction the relative intensities of these peaks were affected. II. Experimental

A.

MOSSBAUER

ISOTOPESAND FEASIBILITY FOR STUDY

1. Source of’fiadiation

In the previous sections it was shown that chemical perturbations of the nuclear energy states provide information useful in catalytic studies. To obtain this information, however, a source of y radiation with variable energy

.

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JAMES A. DUMESIC AND HENKIK

TOPSGE

is required so that a measure of y-ray transmission (or absorption) versus energy will allow the appropriate resonances to be located. The source of y rays is conveniently generated by means of a parent nuclide that decays to the excited state of the Mossbauer isotope under study; the energy modulation can be provided by imparting relative motion between the source and the absorber (the Doppler effect). An important consideration for practical observation of the Mossbauer effect for a particular isotope is the half-life of the decaying parent nuclide. Nuclear decay processes that are often used to populate Mossbauer isotope excited states are (30) electron capture (electron proton 3 neutron), /3 decay (neutron -+ proton + electron), and isomeric transition ( a long halflife nuclear excited state decays to the Mossbauer excited state). In addition, several of the parent nuclides of the heavy isotopes can be populated by aparticle emission. Parent nuclides produced by the processes mentioned above can all be used for several half-lives. In contrast, one can also populate the Mossbauer excited state directly via Coulomb excitation (84).In this technique, a beam of high-energy ( - 10 MeV) charged particles (e.g., 04+, C17+)is directed onto the Mossbauer isotope and the electromagnetic field generated by these particles induces nuclear transitions, which can include transitions to the Mossbauer excited state. Subsequent decay to the nuclear ground state then provides the appropriate y radiation. The half-life of a source created in this manner is the half-life of the Mossbauer excited state (e.g., several nanoseconds), and thus Coulomb excitation is necessarily an in situ technique, i.e., the Mbssbauer effect experiment must be performed at the location of the charged particle beam. The feasibility of using the Mossbauer effect also depends on the pray energy. If this energy is too large (greater than 150 keV), the recoil-free fraction will be too small for observation of the Mossbauer effect, even at low temperatures. Conversely, too small a y-ray energy (- 10 keV) will lead to significant nonresonant attenuation of the radiation intensity by the source and absorber. In addition, the internal conversion coefficient M should be small (of the order of 100 or less) so that the transition from the excited to ground nuclear state has a significant probability [l/(l + a)] of emitting a y ray. The cross section gofor resonant absorption ofthe emitted y ray should be large (of the order of cm2 or greater), and indeed, a large value of co is favored by a small y-ray energy and a small value of a. To date, 79 isotopes (Appendix I) have been found for which the Mossbauer effect can be observed (7,30).It should be noted that the lightest isotope in this collection is 40K, since the nuclear energy spacings in lighter elements are usually quite large and the corresponding y-ray emission (or absorption) is too energetic to take place without recoil.

+

-

153

MOSSBAUER SPECTROSCOPY APPLICATIONS

We propose to divide the 79 Mossbauer isotopes into three classes according to the half-life of the parent nuclides since this determines the ease with which experiments can be carried out: about 30 days (class A), 1-30 days (class B), less than 1 day (class C). Whereas experiments with class A isotopes can be carried out in most laboratories, experiments with class C isotopes require access to a nuclear facility or a charged particle beam.

Class A . 57Fe,s3Kr, lolRu, Il9Sn, IZ1Sb,I 2 T e , Iz7I, Iz9I, Iz9Xe,133Cs, 141pr, 145Nd, 149Sm, 151EU, 152Sm, 153EU, 154Gd 155Gd, 159Tb, l60DY, ~ 168&, 169Tm, 170yb 171yb 174yb, 175Lu, 181Ta, 182W 1 8 3 1~ 8 4 19spt > , > 232Th,236U,238U,239Pu,237Np(36 isotopes). ClassB. 67Zn, 99Ru, 99Tc, 131Xe, '33Ba, 147Pm,147Sm, 156Gd,I6'D Y, 166Er, 172yb 1 7 7 ~ f1 7 8 ~ f1, 8 6 ~1 8 6 0 s , 1 8 7 ~1 ~8,9oS, , 190os, 1 911r, 1931~, "'Hg, 231Pa(22 isotopes). Cluss C. 40K, 61Ni, 73Ge, lo7Ag, 154Sm,157Gd,ls8Gd, 16'Gd 162DY, 1 6 4y,~ 164Er,16'Ho, lh7Er, 17'Er, 176Hf,176Yd , ""Hf, 'OW, lssOi, 1 9 7 A ~ , 243Am(21 isotopes). 9

9

3

9

3

Not all ofthe 79 isotopes for which the Mossbauer effect has been observed, however, are sensitive to chemical effects, as will be discussed subsequently. 2. Chemical Information

In order to examine in which cases the different Mossbauer parameters can be observed, we will in this section make use of parameters Ri(which will be defined below) where i = 1,2, and 3 refers to the isomer shift, quadrupole splitting, and magnetic hyperfine splitting, respectively. In a Mossbauer spectrum, the resolution of overlapping peaks (which are often present in the spectra of catalytic materials) is not easily achieved when the difference in energy between the peaks is less than the width of the individual peaks. This is true even though the position of a single, isolated peak can be determined with an uncertainty that is smaller than the peak width. Thus, to obtain chemical information using Mossbauer spectroscopy, the chemical perturbations of the nuclear levels must result in resonance shifts or splittings (in energy) that are of the order of or greater than the because both the source and absorber observed width of the resonance (2r,,, contribute r, to the width of the observed resonance). For purpose of analysis, an equivalent criterion is that the ratio Ridefined as the ratio of the respective energy displacement to the resonance width should be of the order of or greater than unity. Numerical estimates of Ri will be made below for the Mossbauer isotopes, using known nuclear parameters and order of magnitude values for the electron density, electric field gradient, and spin at the nucleus. These order of magnitude estimates will be called

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JAMES A. DUMESIC AND HENRIK TOPS~IE

Ri’. The value of Ri‘will then be compared to the experimentally determined value of Ri for the s7Fe resonance. The result of this comparison taken together with other information will lead to better estimates of Ri,which will be called Ri.Finally, the values of R, (our best estimate of Ri)for a particular isotope will be used to decide if that isotope may be of interest in the studies of catalytic and chemical phenomena. It should be noted here that the analysis of the Mossbauer isotopes in terms of the ratio Ri provides a simple “physical feeling” for the associated nuclear parameters. The treatment in this section is based on nuclear parameters available from a variety of sources (1- 7,30,85).However, these parameters are not available in a form readily usable for chemists. Consider first the energy shifts due to the electron density at the nucleus (the isomer shift). These shifts can be measured if [see Eq. (16)]

where 1$(0)l2: 1 = I$(O)l: - l+(O)[l. The first term is nuclear in nature, and serves as a scaling factor for the chemical term, l$(O)Iz . : 1 An order of magnitude estimate for 1$(0)12: 1 is ~ m - and ~ , with this value R,‘ is equal to R , ’ = 0.0452 Z A ( r 2 > / E , r ,

(51)

where A(r2) = (r’), - ( r ’ ) , in fm2, E , is the y-ray energy in keV, and r,,is in mm sec-’. In Appendix I are collected values of R,‘ for those MBssbauer isotopes where estimates of A(r2) could be found (85).For s7Fe (14.41 keV), R,‘ is equal to 20; experimentally the range of isomer shifts for this isotope is approximately 2 mm sec- and the observed width of the resonance is equal to 0.19 mm sec-’. Thus, the “experimental” value of R , is of the order of 10, consistent with the above estimate of R,‘. For the heavier isotopes the previous estimate of J$(0)12: 1 is about an order of magnitude too small (85) and, therefore, the value of R,’ should be increased by a factor of ten for these isotopes. Summarizing, for Z 5 36 = 5R,‘ for Z 2 43

’

it“’

Our estimates 1?, for all Mossbauer isotopes are given in Appendix 11. The electric field gradient (axially symmetric for convenience) at the nucleus can be measured using the Mossbauer effect, if

R 2 = (e23Q(ml2 - n1,~)/81(21- l)T,).(q; >, 1

(53)

155

MoSSBAUER SPECTROSCOPY APPLICATIONS

where q = U J e , and the quadrupole splitting is measured between the m, and m2 states. Again, the first term contains nuclear parameters and scales the symmetry information contained in q. The value of q is typically of the order of loz5~ m - and ~ , with this value Rz' is given by

R2' = 216Q/E,T,

(54)

where Q is in barns ( = cm'), 3(m12 - m22)/41(21- 1 ) is of the order of unity, and r, and E , are in the same units as used in the estimate of R,'. For s7Fe (14.41 keV) the range of quadrupole splittings is approximately 2 mm sec- ',corresponding to an "experimental" value of R , 9; the value of R,' for this isotope is 29. Thus, if y remains of the same order for the different Miissbauer isotopes, the following order of magnitude estimate for R can be made : gZ= 0.3R2' (55)

-

,

and the respective values of R,' and R", for the various isotopes are collected in Appendixes I and 11, A magnetic field at the nucleus lifts the degeneracy of the m sublevels, thereby creating a splitting of the y-ray resonance of order pH. This interaction can be observed with the Mossbauer effect if

Typically, the value of the magnetic field H at the nucleus generated by the surrounding electrons is of the order of 100 kOe, and with this value R3' becomes R3' = 47.3p/E,Tn (57) where p is in nuclear magnetons ( = 5.05 x 10- 2 7 A m2 = 5.05 x ergOe-') and E , and rnare in the same units used in the estimates of R,' and R2'. The above estimate of H is sufficiently accurate so that the value of r?, can be set equal to R3'. It will now be used as a first-order estimate that a Mossbauer isotope may be a sensitive probe of its chemical environment if both 8 , and r?, are of the order of unity or greater, and if d , is also of this order then additional information may be obtained. For more detailed examinations, the values of d imust be analyzed (using the previous formulas) more carefully, since a particular Mossbauer isotope may have a value of 1$(0)12 q, or H greater than that used in the above treatment. Of the class A isotopes, 57Fe,83Kr, l19Sn, lZ1Sb, l Z 7 J, lZ9I,I4'Sm, ls1Eu, 15,Eu, "'Gd, "'Yb, and 237Npmay be of chemical importance, with

, :I

156

JAMES A. DUMESIC AND HENRIK TOPS$E

1 0 I R u , 145Nd, 152Sm, 154Gd, 1 6 0y,~ 168Er,181Ta, and 195Ptas possible additions. In class B, 99Ru, '"Gd, I6lDy, l9'1r, and 1931rappear to have nuclear properties compatible with chemical sensitivity, with b7Zn, 133Ba, and 147Pmas possible additions. Finally, in class C, 61Ni, 157Gd,Ig7Au, and 243Ammay be of significance in catalytic studies, with the possible addition of 73Ge,"'Ag, lG2Dy,164Er,17'Er, and 176Yb. In the above lists of Mossbauer isotopes with possible catalytic uses, the following have large recoil energies (requiring observation of the effect at very low temperature), and as such their use may be somewhat restricted: 67Zn, lolRu, 1s2Sm,lS3Eu, 154Gd,and ls5Pt. Thus, in classes A-C there are 11, 5, and 4 isotopes, respectively (20 total), that may be particularly useful in catalytic studies, with 4,2, and 6 possible additions to these classes. With only two exceptions Mossbauer isotopes with a significant recent literature (more than about five articles in the combined 1971 and 1972 literature) are found in the above collection of 20 isotopes (5,6).The exceptions are Iz5Te, which typically has large values of q (the "experimental" value of R 2 for this isotope is 1.4) (30),and 18'W for which isomer shifts are just barely greater than the linewidth (30). It should be added here that, for each Mossbauer isotope, the y-ray resonance can also be studied with the source as the sample (or catalyst) and the absorber as a reference standard. Due to effects caused by the nuclear decay in the sample, these so-called source experiments may be difficult to perform and interpret. Several papers dealing with these ctTects can be found (23).In principle, however, the applicability of Mossbauer spectroscopy to catalytic studies can be extended to include both the Mossbauer isotopes and the corresponding parent nuclides. We therefore list below the Mossbauer isotopes and corresponding parent nuclides that may be of greatest use in catalytic studies, as deduced from their nuclear properties.

Class A . 57Fe-"Co, R3Kr-83Rb, l19Sn-' 19"1Sn, 121Sb-121mSn,125Te1251, 1271-1 2 7 m ~ 1291-129m~~ ~ , 149Sm-149EU, 1 5 lEu-l 51Gd, 155Gd-155 Eu, 170yb- 1 7 0 ~182~-182T ~, a, z37Np241Am, (145Nd-14sPm), ( I 6oDy- 60Tb), ( 168Er-'68Tm), ( 181Ta-181W1. C l a s B. " R L I - ~ ~ R156Gd-'5hE~, ~, '61Dy-161Tb, 1911r-191Pt(or "'Os), 1 93Ir-'930s, ( 1 33B~i-133mBa), ( 147pm-147Nd). C[ab,s C. hlNi-hlCo, ls7C;d- l S 7 E ~197A~-19~pt, , 243Am-24"u, (73Ge73Ga),('07Ag- Io7Cd),(16'Dy-*), (164Er-164Ho), (l7OEr-*), (176Yb-*). 9

In the above pairs of elements the first is the Mossbauer isotope and the second the parent nuclide, the pairs in parentheses are possible additions to the classes, and * represents y-ray production using nuclear reaction or coulombic excitation. (See Appendix I for alternative sources.)

M ~ S S B A U E RSPECTROSCOPY APPLICATIONS

157

The ultimate choice of a particular Mossbauer isotope depends not only on the nuclear parameters of that isotope, but also on its chemical or catalytic interest. Certainly, in cases where an isotope has both appropriate nuclear parameters and is of catalytic interest (e.g., 57Fe,61Ni, 99Ru, I19Sn, 15’Eu, IB2W, 19’Au) the choice is straightforward. Tn addition, for samples containing a Mossbauer isotope, the Mossbauer effect can provide information not only about the chemical state of that isotope but also information about the neighboring elements in the sample. Finally, for samples that do not normally contain a Mossbauer isotope, a small amount (up to a few percent) of a Mossbauer isotope can be added to the sample, and in this manner that isotope can serve as a “probe” of its environment. In this case, the choice of isotope depends on its compatibility with the structure that it must probe. The important concept of using a Mossbauer isotope to obtain information about its surrounding structure will be discussed in detail in Section 111.

B. SPECTROMETER 1 . Velocity Modulation and Culibmtion

As mentioned earlier, the Mossbauer effect is observed by recording the absorption of y radiation versus its energy. Energy modulation is produced by the Doppler effect, created by an accurately known relative velocity between the source and absorber, which may be produced purely mechanically or electromechanically. The experimental method using mechanical devices (e.g., lathe, spinning disk, pendulum, cam) has several advantages (30, 32): it can provide an accurately known absolute velocity, a velocity interval offset from zero can be readily scanned, and mechanical devices are usually quite sturdy. However, there are several important disadvantages in using mechanical velocity modulation. Extraneous vibrations (which result in broadening of the y-ray resonance) may be difficult to eliminate; mechanical wear may present a problem; the Mossbauer spectrum must be constructed from individual determinations of the y-ray absorbance at a particular velocity (constant velocity mode), and thus drifts in the electronic counting equipment and decay in the strength of the y-ray source must be taken into account; finally, the working range of velocities is usually restricted from -0.1 to 10 mm sec-’. The disadvantages associated with mechanical velocity modulation appear to outweigh the advantages, and most of the recent Mossbauer spectroscopy studies have utilized electromechanical devices (e.g., loudspeaker coil, vibrator). A typical electromechanical device for velocity modulation consists of two coils (30,32,86-88): a drive coil across which a voltage related to the

158

JAMES A. INMESIC AND HENRIK TOPS$E

desired velocity is applied, and a velocity-sensing coil, which supplies feedback information to the driving voltage thereby ensuring that the velocity imparted to a moving drive shaft (which carries the y-ray source) closely follows a reference waveform. It is with this reference waveform that different modes of velocity modulation are generated (Fig. 10). For example, if the reference voltage is a constant, then the velocity of the drive shaft will be constant, within its specified distance displacement range. At the displacement limit, a “flyback” signal is sent to the drive coil to return the shaft to its original position (e.g., Fig. 1Oa). Alternatively, if a ramp function is used as the reference signal, the y-ray source (or absorber) will move in a constantacceleration mode. The latter, however, can be divided into two modes, depending on the manner in which the ramp functions are repeated in time. In one case (Fig. lob), each ramp function is followed by another ramp of opposite slope (symmetric ramp mode), and the Mossbauer spectra obtained during the two ramp functions are mirror images. In the second case, the first ramp is followed by a “flyback” signal, which quickly returns the drive

Velocity

1

lb)

Velocity

(C)

’ I I time

FIG. 10. Doppler velocity modes.

MOSSBAUER SPECTROSCOPY APPLICATIONS

159

shaft to its original position (Fig. lOc), and in this manner a single Mossbauer spectrum is collected (flyback mode). Combination of the constant-velocity and constant-acceleration modes produces the velocity-offset mode. In this case, a velocity interval offset from zero velocity (and the negative of this interval) can be scanned, and the corresponding reference voltage is shown in Fig. 10d. The choice of velocity mode depends on the application of the Mossbauer effect (30, 87, 88). For example, if only a small velocity range that is not centered at zero velocity is of interest, then the constant-velocity or the velocity-offset modes will give the highest rate of data collection in the region of interest. For the continuous monitoring of spectral changes with time, the constant-velocity mode is very convenient; if, however, the peak width and/or position changes with time, then the velocity-offset mode may well be the best suited for study of this type of phenomenon. For studying weak absorbance peaks, the possible baseline drift inherent in the constant-velocity mode, due to drifts in the electronic counting equipment or decay of the source strength, requires appropriate corrections. In this case, the velocityoffset mode (absorbance peak displaced from zero velocity) or the constantacceleration mode (absorbance peak near zero velocity) should be used. The constant-velocity mode may also suffer from technical difficulties at zero velocity. For scanning large velocity ranges (greater than -20 mm sec-’) the constant-velocity mode also becomes inappropriate, since the period of data collection becomes comparable to the flyback time. Finally, Mossbauer effect studies using a radioactive source whose half-life is comparable to the time of the data collection may not be convenient with the constant-velocity mode due to baseline changes. The absolute velocity imparted to the drive shaft can be determined either directly or indirectly (30, 32,87,88). In the latter technique, the spectrum of a compound with “well-established’’ Mossbauer parameters is collected, and to the positions in the spectrum where resonances appear, specific absolute velocities can be assigned. The velocities at other positions in the spectrum are then inferred by interpolation between these known velocities. This indirect calibration is then used in the interpretation of other spectra obtained with the same drive unit. Unfortunately, compounds with well-established Mossbauer parameters may not be available for the Mossbauer isotope of interest. For 57Fe, however, this is not a problem, and metallic iron foils and sodium nitroprusside are often used for calibration purposes. Thus, the 57Fe resonance may be used to calibrate the drive unit, and this unit can then be used to study other Mossbauer isotopes if the drive unit is operated under identical conditions. There are a number of cases, however, for which the above indirect velocity calibration is not well suited. The relationship between velocity and position

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JAMES A. DUMESIC AND HENRIK TOPSI$E

may not be easily interpolated; the peaks of the 57Feresonance, which are in the range of 2 10 nim sec-’ may not span the velocity range of the Miissbauer isotope under study; and also the drive unit may operate differently if the source for the 57Feresonance and that for the isotope under study are not the same size and mass. It is especially in these cases that a method of direct velocity calibration is required. An excellent method of direct calibration is provided by means of a laser interferometer (32),whereby a laser beam (with a known wavelength 2, e.g., 6328.1983 A for He-Ne) is directed to a beam splitter, which divides the beam into two fractions. One fraction is directed to a stationary mirror and the other fraction to a mirror mounted on the end of the drive shaft opposite to the y-radiation source, and after reflection, these beams are combined by the beam splitter and sent to a photodiode. Motion of the drive shaft by i./2 (change in path length of the corresponding reflected beam by L)produces alternately an intensity maximum and minimum of the combined laser beam detected by the photodiode (resulting in a “fringe” corresponding to two “counts” by the photodiode). Thus, for a He-Ne laser, 1 mm motion of the drive shaft produces 6320.9144 counts. At various positions in the Miissbauer spectrum, the photodiode counts are recorded for a specific length of time (measured by a crystal oscillator, e.g., 50 kHz), and the velocity of the drive shaft corresponding to that position in the spectrum is thereby directly determined. A similar absolute calibration method uses a diffraction grating on the end of the drive shaft opposite to the y-ray Source (30).In either case, however, these methods of absolute velocity calibration provide a determination of the drive shaft motion during collection of all Miissbauer spectra, and as such offer an important advantage over indirect methods of velocity calibration.

2. Detectors and Nuclear Countirzg System Emission of the y radiation by the parent nuclide is followed by the detection of the transmitted or scattered photons (depending on the geometry, to be discussed later). The photon energy range of interest in the Mossbauer effect is from 5 t o 150 keV, and essentially three types of detectors have bcen used to measure this radiation (87,88):scintillation counters (NaI/Tl), proportional counters, and solid state diodes (lithium-drifted silicon or germanium). For photons with energy greater than ‘c.20 keV, scintillation counters mounted on photomultiplier tubes provide a reasonable resolution (a spread in the magnitude of the electronic signal of about 20%, full-width at half-maximum), and they are nearly 100% efficient. By suitable choice of the crystal thickness, the detector can be made inefficient for detection of

-

MOSSBAUER SPECTROSCOPY APPLICATIONS

161

photons with energy higher than that under study. Detection of radiation with energy less than -20 keV is often more suitably accomplished with a proportional counter. The latter provides better resolution (approximately a 10% spread in the electronic signal) than does the scintillation counter, while still operating at nearly 100% efficiency. When the y radiation of interest strongly overlaps (in energy) other radiations, the solid state diode detectors are often used in studying the Mossbauer effect, since these detectors offer excellent resolution (only a few percent spread in the produced signal). That these detectors are not used for all Mossbauer isotopes is due to their high price (32) (approximately ten times more expensive than proportional or scintillation counters) and the necessity of their operation at liquid nitrogen temperature. It should be mentioned that for all three of the above detector types the ratio of the signal resulting from detection of the desired radiation to that resulting from other radiations can often be improved by suitable shielding and filtering of the source radiation, as described elsewhere (87). The pulses produced via photon detection are then electronically reshaped and amplified so that their characteristics are suitable for single-channel analysis (87,88).The single-channel analyzer (SCA) in turn accepts only those reshaped pulses whose energy is greater than E s C A and less than E,,, + AE,,,, where EsCA and A&,, are set by the experimenter and serve as a “window” (in energy) around the y radiation of interest. When the input pulse falls within the energy window, the single-channel analyzer generates a logic pulse, which is sent to a scaler-timer (an electronic counter and a precision “clock,” e.g., crystal oscillator) in the case of constant-velocity operation, or to a multichannel analyzer (described presently) when constant-acceleration operation is used. In both cases, the number of logic pulses collected per unit time is proportional to the radiation intensity at the Doppler velocity in question. A multichannel analyzer (30, 32, 87,88) provides a collection of memory channels (e.g., 512) into which the pulses from the single-channel analyzer are stored as the drive shaft executes its Doppler velocity cycle. A square wave is generated within the multichannel analyzer, which causes the latter to step progressively through all of its channels. This square wave is also the origin of the reference voltage for the Doppler velocity. Thus, as the drive shaft sweeps through its Doppler velocity cycle, the multichannel analyzer sweeps through all of its channels, thereby allocating a specific velocity increment to each channel. Thus, during one cycle of the square wave, the multichannel analyzer spends a well-defined time (-100 psec) in each channel, storing logic pulses at the Doppler velocity assigned to the channel, and a display of counts versus channel number gives the Mossbauer spectrum, i.e., the y-radiation transmission (absorbance) versus Doppler velocity (Fig. 11).

162

JAMES A. DUMESIC AND HENRIK TOPS@E

.-C .-

I

0.94

a

.

8

s

..

1

$

.

.;...

z I-

*

0.&3}

.* ... . .... ? .’ I

FIG. 1 1 . Mossbauer spectrum in the constant-acceleratioii mode. Zero velocity is with respect to “Co in copper source. Figure according to Boiidart et t i / . (21.5).

3. Grometry Most often the transmission mode is found to be the most convenient in Mossbauer spectroscopy, i.e., the y radiation passes from the source through the absorber, and the attenuation of the primary beam is measured at the various Doppler velocities. However, there are a number of cases where a “scattering geometry” may be advantageous (30).The basis for this geometry lies in those processes that take place after resonant absorption ofy radiation by the Mossbauer isotope. Specifically, after excitation the Mossbauer isotope may reemit the y ray, or it may decay by emission of internal conversion electrons and X rays [with the probability of internal conversion equal to u/(l + a)]. Consider first a Mossbauer isotope with a large associated resonant yradiation energy. The large recoil energy thus results in a small recoil-free fraction, and in the transmission mode the Miissbauer effect is only observed by measuring a small change in the primary beam intensity. The radiation reemitted as a result of these recoil-free events may, however, be superimposed on a weak background if observed for a direction differcnt from the primary beam (30).Thus, measurement of this radiation intensity versus

MOSSBAUER SPECTROSCOPY APPLICATIONS

163

Doppler velocity may provide a Mossbauer spectrum with a large increase in the signal to noise ratio compared to that obtained in the transmission mode. The type of radiation used to generate the scattered Mossbauer spectrum depends on the internal conversion coefficient a ; a large value of a, which favors the emission of X rays by the Miissbauer isotope, makes X-ray detection appropriate, while a small value of LY favors y-ray detection. The study of “surface layers” is also facilitated by means of the scattering geometry, the energy of the scattered radiation determining the thickness of the surface layer studied. The penetration depth of y and X rays in solids is large( -0.1 mm), making the scattered Mossbauer spectrum from these radiations sensitive to essentially bulk phenomena. For very thick samples, however, in which case the primary beam cannot pass through the sample making the transmission mode impossible, the scattering geometry (using y or X rays) provides a nondestructive mode for study ofthe Mossbauer effect. In contrast to the y and X ray, the penetration depth of the internal conversion electrons is quite small ( - 100 nm), and under ideal conditions surface layers of thickness 1 nm can be studied through detection of these scattered electrons (89). Measurements of energy distribution of the scattered conversion electrons by means of an electron spectrometer may be a method of obtaining Mossbauer spectra from different depths in the sample (90,91). However, related to the small penetration depth, the use of conversion electron scattering for in situ studies may present a problem. That is, unlike y and X rays that can pass through the gaseous environment surrounding the surface layer thereby presenting no problem for in situ studies, conversion electron scattering studies of surface layers have been conducted with the sample inside the detector (a flow-proportional counter) (89, 92, 93). If the surface is inert to the fill-gas (e.g., He-CH, mixture for the 7-keV electrons for the 57Feresonance) this may not present a problem, since the sample can be pretreated, exposed to the flowing fill-gas, and a Mossbauer spectrum taken. Ifthis is not the case, however, the proportional counter and a chamber containing the sample can be constructed to share a common “window” transparent to the conversion electrons (e.g., differential pumping of the two chambers), with the pressure of the desired gaseous environment over the sample sufficiently reduced to allow a significant flux of electrons to the detector.

-

C. SAMPLE PREPARATION AND STUDY 1. Miissbaurr Spectroscopy Cells and Sample Mounting

Simply stated, the problem in the design of Mtissbauer spectroscopy cells for catalytic studies is the following: to design an in situ cell that has “windows” transparent to the y radiation and can operate at temperatures of interest in

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catalytic studies (often this means high temperatures, e.g., up to 700 K) and of interest for observation of the Mossbauer effect (often this means low temperatures, e.g., 77 K or lower). For example, the Massbauer effect for isotopes with large associated recoil energies may be observable only at low temperatures, and for all isotopes the effect of temperature on the Mossbauer parameters may provide valuable chemical information. In addition, the sample may have to be treated in a certain gaseous atmosphere at a given temperature before it is of interest for study; after this treatmcnt it may also have to be brought in situ to the conditions suitable for study ofthe Mossbauer effect. If the isotope ofinterest shows an appreciable Mijssbauer effect above 77 K (with the source at room temperature), then the design o f a corresponding cell is straightforward. The measurement and control of temperature have been discussed elsewhere (86-88). For operation at temperatures from room temperature to 800 K, in situ Mossbauer cells consist of ii controlled atmosphere space containing, or enclosed by, an electrical heating filament ( 9 4 4 7 ) . The temperature range from 77 to 800 K can be studied (86-88, 98) by mounting the sample (as described later) in a copper block, the latter containing a resistance heater and thermally connected to a cryogenic fluid reservoir (Fig. 12). A controlled-atmosphere space surrounds the reservoir and the sample (an additional chamber can be used to separate the atmospheres around the reservoir and sample as seen below). Temperatures above room temperature are obtained and controlled with the heater; specific temperatures below room temperature are reached using suitable cryogenic fluids, e.g., dry ice-acetone (195 K), liquid nitrogen (77 K); and temperatures between these specific temperatures are obtained by varying the thermal contact between the copper block and the low-temperature reservoir, e g , using a heater or by changing the gas pressure in an “exchange tube” (87,98). Above room temperature, in sifu gas treatments and Mossbauer effect studies are conducted with the appropriate gas in the controlled-atmosphere space. However, to obtain progressively lower temperatures with cryogenic fluids, the pressure in the controlled-atmosphere space surrounding the reservoir must be correspondingly decreased: if this presents a problem to in situ studies, a separate chamber must be used to enclose the sample and separately regulate its environment. The requisite windows transparent to the y radiation are often of beryllium or Mylar, and during high-temperature treatments they can be cooled by flowing water through surrounding metal tubing. Mossbauer effect cells capable of operation below 77 K normally use liquid hydrogen (20.4 K ) or helium (4.2 K), and as such must be carefully designed (86,87,92,98).The use of hydrogen as a coolant possesses the possibility of explosion, while helium has a small heat of vaporization, requiring the minimization of “heat leaks” to the sample. After taking these considerations into account, and in addition surrounding the liquid hydrogen or helium reservoir

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Fic. 12. Metal dewar liquid nitrogen cell. (A) Nitrogen fill, (B) nitrogen vent, (C) liquid nitrogen, (D) main pump out, (E) detachable tail section, (F) sample, ( G )aluminum foils, (H) Mylar window, and (I) thermocouple. Reproduced from Herber and Hazony (87) with permission.

with a liquid nitrogen reservoir, the remaining features of these cells are similar to those of the previously described liquid nitrogen cells. For in situ treatments and studies, a controlled atmosphere for the sample separate from that surrounding the low-temperature reservoirs is advisable (99).Ultralow temperatures (of the order of 0.03 K) can be attained by using 3He-4He dilution refrigerators or by utilizing adiabatic demagnetization techniques, and Mossbauer cells encorporating these features have been discussed by Cohen and Wertheim (88). While a Mossbauer effect cell with an operating range from 77 to 800 K is routinely used in catalytic studies, temperatures lower than 77 K may not be as commonly encountered. In this case, it may be advantageous to pretreat the sample and then seal it between pray transparent disks (e.g., Mylar) in

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a controlled atmosphere (e.g., inside a “dry-box”) for the few times when low-temperature studies need to be made. In this manner, the sealed sample can be mounted in a fairly simple and inexpensive cell (ZOO) that does not need the controlled-atmosphere capability. A high degree of sophistication is required when both source and absorber need to be at low temperature for observation of the Mossbauer effect, i.e., the imparting ofthe Doppler velocity at low temperature. There are a number of Mossbauer effect cells, however, for which this constraint has been overcome, using either separate dewars for the source and absorber, or a single dewar containing both. Excellent discussions of the design considerations for these cells have been presented elsewhere (86-88, 98, IOZa). Finally, Mossbauer cells that allow the sample to be studied in an applied magnetic field have also been discussed in detail in the literature (88, IOIh). It should be noted here that in addition to collecting in situ Mossbauer spectra (as described above), it may be advantageous to perform “dynamic” experiments in the Mossbauer spectroscopy cell, i.e., the simultaneous collection of the Mossbauer spectrum and the measurement of the catalytic reaction rate over the sample. This point has recently been discussed by Dumesic et al. (ZO20), and simple cells for this purpose have been described elsewhere (1020,IOZh). In the transmission mode, samples are prepared for Mossbauer spectroscopy by forming disks. In order to have appreciable transmission of y rays, a thickness less than 1 mm is needed when using low-energy transitions ( 530 keV), while for higher energy transitions the sample thickness is not critical. For single-crystal studies, large crystals can be cleaved and used directly, while smaller crystals can be oriented on an inert supporting matrix. Powdered samples can be compressed into a self-supporting wafer either from the sample material alone or from a mixture of the sample with an appropriate inert diluent. Alternatively, powdered samples can be “sandwiched” between two y-ray transparent disks, and then placed in the optical path of the spectrometer. It should be noted that during the preparation of powdered samples, preferential orientation may well be produced thereby affecting the Mossbauer spectrum (103). One method of eliminating this effect is first to embed the powder in a textureless matrix (e.g., in a glue) and then to grind this matrix into powdered form. This resulting powder can then be formed into a disk without orientation of the original sample. Foils can be used directly, and liquids can be frozen in thin cells with y-ray transparent windows for Mossbauer effect study. In the scattering geometry, modifications of the above sample preparation procedures can be used depending on the detailed geometry (30). In this case the sample thickness can be indefinitely large.

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2. Sample “Thickness” and Source-Detector Distance In the literature, the term sample “thickness” applies most often not to the overall path length through the sample that the y radiation must pass, t (transmission mode), as discussed above, but rather it is used to describe the number of Mossbauer resonant atoms per unit area in the sample. As mentioned earlier, t must be small enough to allow transmission of the y radiation of interest, while at the same time providing enough resonant atoms for observation of the Mossbauer effect. In cases where this constraint poses a problem and when the element under study has a number of stable isotopes (such as iron), it may be advantageous to prepare samples from materials in which the Mossbauer element has been enriched with the resonant isotope (e.g., 57Fe).However, while on one hand a finite number of resonant atoms are necessary in the sample, too great a concentration (cm-’) results in calculable “distortions” of the spectrum (104-113). For example, the observed linewidth is given by (32,87)

where rexp is the experimentally observed linewidth, l-, the natural linewidth, and T , (the so-called sample thickness) =f,n,aa,t. In this expression f,is the recoil-free fraction in the absorber, nA the number of atoms of the Mossbauer element per cm3 of absorber, and a the fractional abundance of the Mossbauer isotope. The ultimate choice of the value for T , depends on the nature of the investigation. For example, a detailed study of the shapes of the resonance peaks must be made at small values of T A , while in cases where the Mossbauer spectrum must be obtained in a short period of time, a larger T , value is favored. In general, however, values of T , 5 1 are often used in Mijssbauer effect studies. A recent review of the literature and a detailed analysis of finite thickness (T,) effects has been given by Shenoy et ul. (112). They showed that for well-resolved spectral peaks the effects of thickness are not too critical, while for partially resolved peaks the thickness corrections become quite important. This latter result was also found by M$rup and Both ( I 13). The solid angle R of y radiation seen by the detector (which is a function of the soure-detector distance L) is also subject to optimization. On the one hand, R must be large enough to obtain a reasonable count rate at the detector, since the degree of detail in the Mossbauer spectrum is limited by the standard deviation in the number of y-ray events N (counts) collected at each velocity increment (channel) of the spectrum. This standard deviation is equal to N 1 / 2(30,32), and it becomes advantageous to operate at a high

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count rate to obtain the best resolution for a given counting time period. The maximum usable count rate is determined by the source strength and the counting speed of the spectrometer electronics. On the other hand, increasing 2 ! has the disadvantage of broadening and shifting the resonance peaks as shown below. Photons emitted at an angle 0 with respect to the direction of the Doppler velocity, will leave the source with an energy E,given by (30,32,114)

E = E(0) + (V/c)Ecos 0

(59)

where E(0) is the corresponding energy when the Doppler velocity V is equal to zero. As R increases, the Doppler velocity (which is used to shift the pray energy uniformly) thus produces a greater distortion of the natural resonance line shape. In general, if R,/L(where R , is the radius of the source or the detector window, whichever is larger) is less than -0.1, then this “cosine broadening” can usually be neglected (32, 114). Thus, the choice of 2 ! involves a trade-off between count rate and cosine broadening effects.

3. Duta Processing The output from the spectrometer is the number of “counts” for each velocity increment of the spectrum. For graphical display and analysis, these data are then recorded on an X - Y recorder ( X , velocity; Y, counts) either directly in digital form or after conversion to analog form. In addition, a permanent record of the data is provided by transfer to a teletypewriter, a serial printer, a paper tape, and/or a magnetic tape recorder. In order to obtain accurate values for the M6ssbauer parameters, and in order to analyze complex spectra, it may be necessary to use a digital computer and employ an appropriate fitting procedure. A number of computer programs are currently available (32, 87, 92). For computer analysis, the paper or magnetic tapes serve as the input to the computer either directly or after conversion to punched cards. Alternatively, a printed record of the data (e.g., from a teletypewriter) may be transferred onto punched cards and then used as the computer input; it has also become possible to transmit the data from the MCA directly into the computer memory using, for example, a RS-232-C signal. Finally, the rationale for fitting Mijssbauer spectra will be discussed, by means of examples, in the remaining sections of this paper. An excellent discussion of computational methods in Mossbauer spectroscopy has been presented elsewhere (32),and criteria for judging the “goodness of the fit” have been discussed by Ruby (114).

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111. Applications to Heterogeneous Catalysis

In Section I, C the different Mossbauer parameters were individually discussed with reference to possible catalytic applications. The purpose of that discussion was to provide a “physical feeling” for the parameters and an appreciation of their possible uses in catalysis. In general, however, for the study of a particular catalytic phenomenon the decision whether also to employ Mossbauer spectroscopy is not based only on the consideration of a single Mossbauer parameter. Thus, in the next sections we discuss, based on a number of examples, the manner in which various catalytic phenomena can be investigated through the systematic employment of the Mossbauer parameters. A. CATALYST PREPARATION, GENESIS, AND CHARACTERIZATION 1. Surface and Bulk Mobility In the preparation and stablization of small, supported-catalyst particles, the consideration of surface mobility is essential. If the active component is in a high state of dispersion, conditions under which high mobility is attained must be avoided, since these conditions lead to particle size growth. On the other hand, a poorly dispersed component may be partially redispersed by treatment in a more highly mobile state. In supported catalyst systems, the interaction between the dispersed species (the active component) and the support is always of important concern, and a measure of the mobility of the active component is an indirect measure of this important interaction. For unsupported catalysts, where particle sizes are typically an order of magnitude larger than those for supported catalysts, the mobility of various species in the bulk structure may be of interest when considering how the bulk structure and composition are reflected in the surface properties of the particle. In addition, bulk mobility is an important consideration in the understanding of solid state reactions and phenomena such as sintering. The manifestations of mobility in the Mossbauer parameters are perhaps best introduced with reference to a series ofpapers by the Russian Laboratory at the Institute of Chemical Physics in Moscow (125-120).Several examples from this series will demonstrate the underlying principles. One system studied was tin (using the l19Sn, 23.9-keV transition) on 300 m2gm-’ silica gel. The Mossbauer spectra of this sample are characteristic of both SnO and SnO,.nH,O components being present in approximately equal proportions (215), and it is from the temperature dependences of the respective spectral areas that information about the mobilities of these two species

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JAMES A. DUMESIC AND HENRIK TOPS@E

r l

73

123

173

223

273

T (K) FIG. 13. Temperature dependence of spectral area for tin supported on silica. (a) SnO,.nH,O; (b) SnO. Figure according to Suzdalev ~ ‘ d. t ( I /S).

can be obtained. As seen in Fig. 13, the spectral area (related to the recoilfree fraction) for the SnO,.nH,O decreases with increasing temperature much more rapidly than that for the SnO component or for bulk SnO,.nH,O. This indicates a weaker support interaction for the SnO,.nH,O than for the SnO species. That is, a weaker interaction of the resonant tin atom with the support results in a larger value of (x’) (the mean square vibrational amplitude), and this is reflected in a recoil-free fraction that has a greater temperature dependence. Physically, the parameter (x2) was considered to have two contributions: a term due to vibrations of the tin atom within the tin compound and a term arising from vibration of the tin compound on the silica gel support. For the weakly held SnO,xH,O component the temperature dependence of (x2) is shown in Fig. 14. At low temperature, intramolecular vibrations provide the dominant contribution to (x2), while above 230 K the bonds between the SnO,.nH,O and the support become sufficiently weak such that their vibrational amplitudes increase dramatically. Finally, at 260 K, the SnO,.nH,O species reached a state of high mobility on the surface. In a subsequent publication (118), the SnO and S n 0 2 . n H 2 0interactions with the silica support were studied on samples with different pore diameters ranging from 0.5 to 27 nm, and on a synthetic mordenite of 0.6-nm pore diameter. From the temperature dependence of the respective spectral areas, it was concluded that both the SnO and the SnO2viH2Ocomponents were more strongly bonded to supports with smaller pore diameters. In addition to the spectral area, however, the spectral width is also expected to reflect

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T (K) FIG. 14. Mean square vibrational amplitude of SnO,.nH,O on silica. Figure according to Suzdalev er ul. (115).

changes in tin mobility with temperature and pore diameter. In Fig. 15, the dependence of this parameter on these two variables is shown. Indeed, the SnO species (strongly bonded to the surface) does not show any significant line broadening, reflecting its localized state on the support. The SnO,.nH,O species on the 0.5-nm pore diameter samples, however, shows a broadened

a

-

A H

e x

I

100

I

I

200 T (Kl

I

I

300

FIG.15. Temperature dependence of spectral width for tin on silicas for different pore diameters. (a) SnO2viH20;(b) SnO. x , Mordenite (-0.5 nm); A,small-pore silica (-0.5 nm); 0,14-nrn pore silica; 0 . 27-nm pore silica. Figure according to Suzdalev (118).

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JAMES A. DUMESIC AND HENRIK TOPS$E

spectral peak at temperatures higher than 250 K, while on the large-pore supports the Sn0,.nH20 spectral component already broadens at temperatures in excess of 150 K. For the latter case, the temperature dependence of the line broadening allows a value of 12 kJ for the activation energy of Sn02.nH,0 diffusion to be calculated, assuming that the diffusion takes place via a jump mechanism. The above study is an excellent demonstration of how studies of the temperature dependences of the spectral area (recoilfree fraction) and spectral linewidth can lead to information about surface mobility and support interaction. Another similar study is that of Plachinda et aI. (IZO),who investigated iron ( 57Fe,14.4-keV transition) exchanged into a sulfonic acid resin. Again, through the recoil-free fraction and the spectral width, it was concluded that the strength of the iron exchange bond decreases as the degree of hydration of the resin increases. When the water concentration in the resin became greater than approximately four molecules per iron atom, the exchange bond became sufficiently weak to allow diffusion of the iron ions throughout the resin, and the broadening of the spectral peak allowed the diffusion coefficient to be determined. An example of diffusion in the bulk phase can be taken from the very detailed studies of Greenwood and Howe (121).Using the broadening of the y-ray resonance, these authors investigated the diffusion of defect clusters in Fe, -,O at temperatures between 1074 and 1173 K for x 0.1. From the diffusion coefficient and its temperature dependence so determined, mechanistic considerations about the diffusion process could be made. For example, one possible diffusion mechanism considers that the overall process is the result of the rapid diffusion of a small fraction of the iron ions; if this were the case, then the observed Mossbauer spectrum would consist of both a broad component (due to the rapidly diffusing ions) and a narrow spectral component (due to the slowly diffusing ions). A single broad spectral component, however, was observed, ruling out the above possible mechanism. The above discussions of mobility involved the motion of the Mossbauer isotope from one “site” to another, in the case of all sites being equivalent with respect to their Mossbauer parameters, or when the relative amount of the Mossbauer isotope in the different chemical states remained time invariant. An example of a study where the movement of the ions is accompanied by a net change in the chemical state can be found in the work of Duncan et al. (122).These investigators studied the reaction

-

ZnO

+ Fe,OJ

--t

ZnFe,O,

(60)

by observing the room-temperature 57FeMBssbauer spectrum of an equimolar mixture of ZnO and Fe,O, after various times of heating at elevated temperatures (e.g., 1030 K) (Fig. 16).The spectra could be decomposed into 57Fesignals from Fe,O,, ZnFe,O,, and perhaps also an intermediate phase

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0 Velocity

(cm sec-'1

FIG.16. Mossbauer spectra after different reaction times between ZnO and Fe,03. (A) 3 min. (B) 6 min, (C) 9 min. Zero velocity is with respect to metallic iron. Figure according to Duncan et al. (122).

present to a small extent. The corresponding spectral areas allowed the ZnFe,O, conversion to be determined. During the early stages of reaction, the 57Feions were found present in a number of chemically different sites, as evidenced by initially large linewidths; however, as the reaction approached completion, the linewidth approached its natural value. In addition, as the reaction approached completion, the ZnFe,O, conversion obtained from Mossbauer spectroscopy equaled the value determined from X-ray diffraction, while at low conversions the latter value exceeded the former. Thus, it was concluded that the rates at which the oxygen and zinc ions attained their final states (these ions accounted for much of the X-ray scattering) were faster than that for the iron atoms, thereby providing information on the relative mobilities of these ions during the solid state reaction.

2. Textural and Chemical Promoters The production and stabilization of high catalyst surface areas under reaction conditions is an important problem in catalytic studies and practice. Typically for metals, this consideration has led to the use of supported

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JAMES A. DUMESIC AND HENKIK T O P S ~ E

catalysts and texturally promoted catalytic materials, i.e., the addition of several percent of a foreign component (the promoter) to the active material. The study of supported catalysts using the Mossbauer effect will be discussed in Sections 111, A, 3 and 111, A, 4,while the location and role of promoters, as evidenced by Mossbauer spectroscopy, will be discussed presently. An example of a texturally promoted catalyst is that of iron for the synthesis of ammonia. Traditionally, several percent of A1,0, is melted together with magnetite (Fe,O,), which upon reduction yields stable metallic iron particles in the 30-nm range. Emmett and Brunauer in their classical physical and chemisorption studies (123,124)found that a significant fraction of the iron surface ( - 5073 is covered by the promoter. Later, Solbakken et u1. (125) found that this “pronioter skin” is only a single layer thick. A mass balance then shows that much (-SO;,) of the aluminum in some chemical form must be inside the iron particle, perhaps contributing to the promoting effect. An iron synthetic ammonia catalyst containing 3 x A120, was studied by Hosemann rt a/. (126-129) using X-ray diffraction; it was concluded that the aluminum inside the iron particle was present as FeAI,O, groups randomly distributed throughout the particle, creating strain in the a-Fe lattice. The same catalyst, however, was also studied using the Mnssbauer effect by Tops6e rt uf.(99, and the Mbsbauer spectra of the reduced catalyst showed no peaks attributable to bulk FeAI,O,. From the statistics of the spectra it could be concluded that less than 15% of the aluminum can be in this form. In addition, the room-temperature internal magnetic field at the metallic iron nucleus is 330.6 0.1 kOe, within the experimental uncertainty equal to that for pure metallic iron (NBS foil). For dilute Fe-AI alloys, Mossbauer spectroscopy has shown that the internal magnetic field decreases by approximately 2 kOe per atomic percent of aluminum in the alloy (130), in which the aluminum atoms act as “holes” in the magnetic structure. Since it is likely, to a first approximation, that FeAl,O, molecules will also act as holes in the magnetic structure of the iron particles, FeA1,0,, if indeed present, must exist as clusters of about 100 molecules ( - 3 nm) for consistency with the internal magnetic field data. Because of their small size, these clusters might not give rise to a well-defined FeAI20, Miissbauer spectrum due, for example, to variations in quadrupole interactions. That is, the spectral component due to these clusters would be sufficiently broadened so as to become part of the “background.” The computer-fitted backgrounds of the spectra indced showed that progressive reduction of the catalyst results in the conversion of a broadened spectral component to metallic iron. For the first two reductions, the broadened spectral component is due to the surface oxide passivation layer (- 10 monolayers thick), while the last reduction corresponds to a spectral area change equivalent to

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FeAI,O, reduction to metallic iron. The background of the fully reduced catalyst indicated the absence of FeAI,O, or any other paramagnetic iron phase. From these results, it is concluded that, in a fully reduced catalyst, FeAl,O, is not present; furthermore, the aluminum inside the iron particle is present as a phase that does not contain iron (e.g., A1,0,), and this phase must be clustered as inclusions 3 nm in size. These inclusions may well account for the strain observed by Hosemann et a!. From the Mossbauer effect investigation then, the process schematically shown in Fig. 17 was suggested for the reduction of a singly promoted iron synthetic ammonia catalyst. Finally, these inclusions and their associated strain fields provide another mechanism for textural promoting (131). The rationale of the above study is quite general, as will be shown below, in determining the location of promoters in catalytic materials. The Mossbauer

-

FIG.17. Picture of the reduction process of a singly promoted iron catalyst. (a) Unreduced large catalyst particle with the promoter distributed homogeneously. (b) Catalyst after short reduction. Aluminum-rich regions appear. (c) Catalyst after further reduction consists of a-Fe and FeAI,O, inclusions. (d) Fully reduced catalyst consists of small a-Fe particles with A1,03 inclusions. Figure according to Tops@ert a/. (95).

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JAMES A . DUMESIC AND HENRIK T O P S ~ E

isotope-containing phases are first identified from the appropriate spectral peaks. However, the absence of peaks characteristic of a certain phase does not imply the absence of that phase, due to the possible broadening and loss into the background of the resonance. This difficulty can be handled by a careful analysis of the background and/or absolute spectral area calculations (to be discussed later). Finally, information about the non-Mossbauer isotope components is obtained by studying their effect on the Mossbauer parameters of the neighboring resonant isotopes. In the above study, the magnetic interaction proved to be the most sensitive to the promoter location. However, as shown below, the other parameters may be equally revealing in other cases. While the purpose of textural promoters is to stabilize a high catalyst surface area, compounds that when added change the catalytic activity per unit surface area are termed chemical promoters. Such a promoter is PbO, which when added to chromium-containing magnetite catalysts increases the catalytic activity for the CO shift reaction (i.e., CO + H 2 0 H2 CO,). A study by Tops$e and Boudart (96) of this system using Mossbauer spectroscopy revealed the location of the lead in the catalyst and gave indications of the nature of its promoting effect. The general cation distribution in the spinel structure of the iron oxidechromium oxide catalyst (denoted Cr-Fe,O, subsequently) is shown below: --f

+

where cations in round and square brackets refer to the tetrahedral A and octahedral B cation sites, respectively, in the fcc close-packed oxygen ion lattice, 6 is the degree of inversion (6 = 1 for a “normal” spinel and 6 = 0 for an “inverse” spinel), and y is the degree of chromium substitution. The possibilities of substitution of lead into this structure are not a priori known. In Figs. 18 and 19 are shown Mossbauer spectra of a Cr-Fe,O, catalyst (5 cation Cr) and a PbCr-Fe304 catalyst ( 5 cation % Cr and 5 cation % Pb), respectively, after various treatments. The dominant feature of these spectra is the appearance of sets of “doublets” resulting from a superposition of two six-peak patterns. The spectrum with the larger magnetic field is due to Fe3+ in the tetrahedral sites, while the other component arises from both the octahedral Fe2 and Fe3 ions due to the rapid electronic exchange(hopping) between these ions (132).The tetrahedral Fez spectral component may well be lost in the background (133). In these spectra, a parameter sensitive to chromium and lead substitution into Fe,O, is the area ratio S, defined as the area of the octahedral iron component divided by the corresponding peak for the tetrahedral component. For Fe,04 (6 = 0), S should equal approximately 2, as experimentally observed; however, for the PbCr-Fe,O, catalyst the value of S is about +

+

+

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177

Velocity (mm set')

FIG. 18. Mossbauer spectra of Cr-Fe,O, catalyst after room temperature exposure t o air and C 0 2 / C 0treatment at 703 K. (a) Spectrum in air at 296 K after sample has been stored in air. (b) Sample from (a) reduced in a CO,/CO = 4 mixture at 703 K for 10 hr. Spectrum obtained in reaction mixture at 703 K. (c) After cooling (b) to 483 K. (d) After cooling (c) to 296 K. (e) Spectrum of a 0.001-in. Fe NBS standard foil at 296 K. Zero velocity is with respect t o a s'Co in copper source. Reproduced from Tops4e and Boudart (96) with permission.

twice as large. This change in S is much greater than that due to the substitution of iron ions alone (tetrahedral and/or octahedral) by lead, thus indicating that the lead causes a change in the catalyst structure, i.e., the degree of inversion (the structure becomes more normal with 6 approaching 0.5). This is proof that lead does indeed enter the spinel structure, and the linewidths and internal magnetic fields show that this ion enters the tetrahedral site. That is, the linewidth for the tetrahedral component of the PbCr-Fe,O, catalyst is small and temperature independent, as it is in the Fe,04 spectrum.

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JAMES A . DUMESIC A N D EIENRIK TOPSOE

- 10

0 Velocity (mm sef')

FIG. 19. Mossbauer spectra of PbCr~Fe,O, catalyst after room temperature exposure to air and CO2/C0treatment at 703 K. (a) Spectrum in air at 296 K after sample has been stored in air. (b) Sample from (a) exposed to a CO,/CO = 4 mixture at 703 K for 12 hr. Spectrum obtained in reaction mixture at 707 K. (c) After cooling (b) to 483 K. (d) After cooling (c) to 296 K. (e) After exposing (d) to a CO,/CO = 4 mixture for 14 hr. Spectrum obtained in reaction mixture at 296 K . ( f ) After heating (g) t o 208 K. (g) Cooling (e) to 119 K in helium (1 Torr). Zcro velocity is with respect t o B " C o in copper source. Reproduccd from Tops@ and Boudart (Y6) with permission.

This indicates a similar chemical environment for the A site in these two compounds. On the other hand, the B-site linewidth in the PbCr-Fe,O, catalyst is significantly larger than that for Fe,O,, pointing to a distribution in chemical environments of this site for the former compound. Both of these observations indicate the presence of lead in the A sites. Similarly the internal magnetic field at the nuclei of iron ions in A sites is the same for the PbCr-Fe304 and supported Fe,O, samples (for which the particle sizc is

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179

the same); however, for the B site this magnetic field is smaller for the PbCrFe304 catalyst. This points to nonmagnetic neighbors of the B site iron ions (i.e., the presence of lead in the A sites). The isomer shift also adds information about the effect of lead on the catalyst structure. The isomer shift at and below 300 K for the A-site iron ions is larger for the PbCr-Fe,O, sample than for Fe304. This indicates that the electron density at the A-site iron nuclei is smaller (A(r2) is negative for the ”Fe 14.4-keV transition) in the PbCr-Fe,O, sample, due perhaps to an expansion of this site. Indeed, the temperature derivative of the isomer shift (resulting from the second-order Doppler effect)for the A-site iron ions is greater for the PbCr-Fe,O, catalyst than for the Fe30, sample, pointing again to an expansion of this site for the former sample. Turning to the B-site, the temperature dependence of the isomer shift for iron nuclei in this site is smaller for the PbCr-Fe,O, catalyst than for Fe,O,, indicating a contraction of this site for the promoted catalyst. In addition, this contraction is reflected in an increase in the recoil-free fraction of the B-site iron atoms in the promoted catalyst compared to the corresponding ions in Fe304. Finally, the B-site contraction results in a lowering of the symmetry of this site. For this reason, the B-site quadrupole interaction should be different for the PbCr-Fe,O, catalyst and Fe304,as experimentally observed. Thus, using Mossbauer spectroscopy it was shown that lead enters the catalyst structure in the tetrahedral sites, and as such results in an expansion of all tetrahedral sites and a contraction of the octahedral sites. Accompanying this change, the degree of inversion increases. The chemical promoter, lead, is thereby shown to change the electronic (isomer shifts) and geometric (expansion and contraction of A and B sites, respectively) structure of the catalyst, the knowledge of these changes being necessary before the promoting mechanism can be understood (as discussed later). The above discussion exemplifies how a study of the different Mossbauer parameters and their temperature dependences can give detailed information about the location of a non-Mossbauer isotope, lead, in its surrounding structure. It should perhaps for comparison be mentioned that the conventional technique of structure analysis, X-ray diffraction, did not enable the above information to be obtained, again showing the advantage of Mossbauer spectroscopy in the study of catalyst systems, which often may show X-ray “amorphous” features. 3. Purticle Size und Size Distribution As will be shown presently, there are a number of reasons to expect that the Mtjssbauer parameters should be particle size dependent, thereby opening the possibility of using the Mossbauer effect for particle size measurement.

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JAMES A . DUMESIC A N D HENRIK TOPS$E

The advantage of the technique is that the particle size may be determined with the sample in a controlled atmosphere and at a temperature different from -300 K, i.e., in situ particle size measurement, and measurement of changes in particle size may be possible. The problem, however, is that the quantitative relation between the Mijssbauer parameters and particle size is rather complex and in some cases not theoretically available. Therefore, the application of the Mossbauer effect to particle size measurement is often facilitated through an experimental “calibration” of the Mossbauer parameters to particle size for the particular catalyst system of interest, i.e., the measurement of the parameters for a set of samples of known particle size as determined by other experimental methods. This point will become clearer below, as the effects of particle size on the Mossbauer parameters are discussed. The recoil-free fraction f is one parameter that is often found to be particle size dependent (134-139). For a given catalyst system, however, this dependence is not a priori known. On the one hand, the finite size of the particle may lead to an increase in ,f with decreasing particle size due to the change with particle size of the phonon spectrum (135).There are at present, however, some contradictions in the literature concerning this effect (135,139).On the other hand, a decrease in particle mass, which is accompanied by an increase in the particle’s recoil energy upon y-ray emission or absorption, may cause f to decrease. The latter possibility, of course, is dependent on the interaction between the small particles and the supporting matrix (140).The mean square vibrational amplitude (x’) of the surface atoms may be different from that of bulk atoms, due, for example, to chemisorption, or different force constants and number of nearest neighbors for surface and bulk atoms. This results not only in different .f values for surface and bulk atoms, but this change in (x2) may be felt throughout the particle (141). Thus, for the purpose of particle size measurement the use of the recoil-free fraction, for which the relation to particle size is complicated, appears very uncertain at the present and should be coupled to experimental calibration. Furthermore, absolute values of the recoil-free fraction are often difficult to calculate and measure, requiring additional calibration. An example of such a determination is that made by Meisel (142) for iron, iron oxides, and iron hydroxides. A similar situation exists for the isomer shift, and to some extent for the quadrupole interaction. For both interactions, the problem is theoretical in nature; that is, for a particular catalytic system, it is not a priori known whether the particle size-dependent isomer shift or quadrupole interaction should be interpreted in terms of a “shell model” or an “internal pressure” effect. In the shell model, the Mossbauer parameters of the surface atoms are considered to be different from the corresponding parameters of the bulk atoms, and thus the total spectrum is a sum of these two contributions

181

MOSSBAUEK SPECTROSCOPY APPLICATIONS

weighted by the particle dispersion (143,144), i.e., the fraction of the resonant atoms that are in the surface. It is expected that the quadrupole interaction will be different for surface and bulk atoms due to the lower symmetry around the former, and this difference can be theoretically estimated (145). In addition, the effect of the surface on the electric field gradient of atoms below the surface layer can also be calculated (146). Thus, the shell model for the quadrupole interaction may provide a semiquantitative estimate of particle size. To the contrary, however, any difference between the isomer shift for surface and bulk atoms is not readily calculable, and the shell model for this parameter should be coupled closely to experimental calibration. For both parameters, however, the dependence of the surface Mossbauer parameters on chemisorption must be considered in a meaningful determination of particle size using the Mossbauer effect (145). In connection with the shell model, the novel experiments of van der Kraan (147-149) need be mentioned. Small particles ( - 10 nm) of cr-Fe,O, were first prepared using an aqueous solution of ferric nitrate and ammonia, followed by exposure to a solution containing ferric nitrate enriched with 57Fe. In this manner, the “surface shell” can be made to have a much higher concentration of 57Fe atoms than does the bulk, and the ratio of the spectroscopic signal arising from the surface to that arising from the bulk can be greatly increased for a given particle size. It was thereby shown that the quadrupole splitting was larger and the recoil-free fraction smaller for the surface atoms than for the bulk atoms on small iron particles ( - 7 nm), while no such effect was observed for larger particles (- 50 nm in size). Although the actual location of the 57Fe atoms in the “surface shell” for these samples has been a subject ofcontroversy (150), the idea of enriching the surface region with 57Fe(or resonant atoms in general) remains very important for the use of Mossbauer spectroscopy in surface and catalytic problems. For example, Lauer et ul. (151) have recently applied this idea to thin films of metallic iron to study the magnetic properties of the surface layers. While it may be reasonable (at least to a first approximation) to distinguish between resonant atoms in a surface shell and those in the bulk, the application of this model to particle size determination is also complicated by the possible presence of lattice modifications (e.g., expansions or contractions) for small particles. This point has been emphasized by Schroeer (152) and interpreted in terms of an “internal pressure.” For example, Kundig et al. (143) found a nearly linear dependence with dispersion of the quadrupole splitting E for a-Fe,O, microcrystals. As noted above, this was explained in terms of the shell model. However, if the lattice parameter a of a 5-nm particle is increased over the bulk value by 2% ( I S ) , then the increase in the quadrupole splitting AC with decrease in particle size may be related to a corresponding increase in the lattice parameter Au by (Ac/c)/(Au/a) 65

-

182

JAMES A. DUMESIC AND HENKIK TOPSgE

(154). That this value is reasonable is shown by comparison with the magnitude increase of E with thermal expansion of the cr-Fe20, lattice, i.e., (AE/c)/(Aa/u) 80 (154). Finally, Vaughan and Drickamer (155) found the magnitude of E to decrease by a factor of two by increasing the external pressure to 200 kbar. Thus, the increased value of E (and also the lattice expansion) of 5-nm r-Fe,O, particles may be represented in terms of an internal pressure of -200 kbar, as an alternative to that interpretation in terms of the shell model. This internal pressure effect may actually be quite general in Mossbauer cffect studies of small particles, as discussed by Schroeer ct al. for the recoilfree fraction (156)and the isomer shift (157). In addition, Schroeer (152) has summarized a number of origins for Mossbauer parameters being particle size dependent. Thus, from the above discussion, it seems apparent that LI priori particle size determination using the recoil-free fraction, quadrupole splitting, or isomer shift is not possible for an arbitrary catalytic system. However, the “experimental calibration” of these parameters, which not only facilitates particle size measurement, may also provide valuable information about the chemical state (e.g., electronic, dcfect, stress) of the small particles. This point will be illustrated later. For magnetic materials, the relaxation of the magnetization (superparamagnetism) is sensitivc to the size of the spin system, thereby providing another and perhaps more direct possibility for particle size measurement using the Miissbauer effect (158).As mentioned earlier, when the relaxation time T~ is much longer than the Larmor precession time zL of the nuclear moment about the internal magnetic field, a magnetically split spectrum is observed (six peaks for the 57Feresonance); on the other hand, when T ~ is, much smaller than T ~ the , magnetic interaction averages to zero (a single- or two-peak spectrum is observed for the 57Fe resonance corresponding to a zcro or nonzero quadrupole interaction, respectively). As might be expected, 5,- corresponds to a complex Mossbauer spectrum resulting the case of T,, from the partial collapse ofthe magnetic hyperfine pattern (Fig. 8); however, it is also clear that it is in this time scale region that the Mossbauer effect is most sensitive to relaxation phenomena, and hence to particle size measurement. To extract the desired relaxation time information from the Mossbauer spectrum, there are essentially two approaches. The first requires a theoretical calculation of the Mossbauer spectra for various relaxation times spanning the range from T,JTI, << 1 to s ~ / >> T ~ 1 (e.g., 56, 159, 160). As shown earlier [Eq. (42)], for a given particle of size do, the relaxation time can be estimated by rH = T~ expi Kf‘(d,)lkT (62)

-

--

where K is the anisotropy energy constant and J ( d ) a function of the particle size, e.g., f ( d ) = d 3 .Thus, if T~ can be deduced from the Mossbauer spectrum

MOSSBAUER SPECTROSCOPY APPLICATIONS

183

(by comparison of the observed spectrum with the theoretical spectra for different values of zH), then do can be calculated provided K and f'(dj are known. In general, however, the presence of a particle size distribution complicates the analysis, requiring that the observed Mossbauer spectrum be deconvoluted into its contributions from the different particles in the size distribution. This is accomplished using a computer, by which the theoretical spectra for various values of T~ are superimposed and compared to the observed spectrum to yield the desired particle size distribution (161). If the particle size distribution is sufficiently broad, the distribution in t H for the sample (due to the particle size distribution) will be much larger than the change in zH needed to change a magnetic hyperfine split spectrum to a paramagnetic spectrum for any given particle. In this case, the above method of particle size measurement can be simplified, as proposed by Kundig et al. (162) and later verified by McNab (161). In this approach, particles for which sH> sL are assumed to contribute a completely magnetically split component, and particles for which sH< zL are assumed to contribute a paramagnetic component to the total observed Mossbauer spectrum. Thus, the spectral areas A , and A 2 of these two respective components are measured, and at the temperature T , for which A , = A , , the value of T~ for the average particle size ( d ) is equal to T ! ~ .Then ( d ) is calculated from T , , = T ~ ) exp{Kf((d))/kT, j . In addition, the temperature dependence of A , / A 2 over the temperature interval approximately centered at T , yields the particle size distribution (Fig. 20). That is, if A 1/(,41 + A 2 ) = 0.3 at temperature T,', then 30% of the Mossbauer atoms are in particles of size greater than that d', calculated by T L = t()exp{Kf'(d'j/kT,') (63) Below the superparamagnetic transition temperature T,, a hyperfine split pattern will occiir, as discussed above. However, the hyperfine fields are often observed to be smaller than the corresponding values for large crystals. This makes the analysis of complicated spectra involving small particles difficult, and a satisfactory understanding of the origin of the smaller fields has been looked for. A simple model explaining this effect has recently been proposed by M4rup and Tops$e (163). Instead of assuming that the magnetization vector is fixed at an anisotropy energy minimum at temperatures below T,, small collective thermal excitations of the magnetization were taken into account. For a particle with uniaxial anisotropy the average magnetization is thereby reduced by a factor of I -kT/2KV, where the symbols have the same meaning as in Eq. (42). Experimental results on small particles of Fe,04 and sr-Fe,03 showed qualitatively this temperature and particle size dependence of the hyperfine fields (Fig. 21). Furthermore, the value of K V determined in this way agrees well with that measured by conventional superparamagnetic relaxation.

184

JAMES A. DUMESIC AND HENRIK T O P S ~ E

Particle size (nm)

Temperature ( K 1 FIG.20. Temperature and particle size dependence of the superparamagnetic fraction for cc-Fe,O,. The histogram is obtained from these data and the calculated anisotropy constant. Figure according to KUndig e t a / . (162).

The above method, however, has several features that make it especially interesting. First, the measurements are carried out at much lower temperatures than those used in conventional superparamagnetic relaxation studies; therefore, such effects as sintering or modification of the catalyst structure may be minimized. Second, samples with large values of K f ( d ) may be studied. Finally, combination of the above two methods (and possibly including studies with applied magnetic fields) may give especially detailed information about phenomena such as particle size distribution and dominant anisotropies. These authors also showed that as long as the lines remain quite narrow, the line shifts arising from relaxation effects can be neglected in comparison with those originating from collective excitations. Thus in this case, the latter can be unambiguously measured and used in catalytic studies. A problem common to all the above approaches for particle size determination is the estimation of the anisotropy energy parameter K. The determination of particle size using magnetic relaxation is thus coupled to a knowledge of K ; this is reminiscent of the way that selective chemisorption techniques of surface area measurement are related to the chemisorption stoichiometry. That is, the particle size can be estimated without an accurate knowledge of K since the magnitude of K can be estimated by invoking physical arguments;

185

MOSSBAUER SPECTROSCOPY APPLICATIONS

I

I

0

100

1

200 TEMPERATURE ( K )

1

300

FIG.21. Relative change in the observed hyperfine field in microcrystals due to collective thermal excitations. h(V. T ) = H ( V , T ) / H ( V= c,T ) . H is the hyperfine field and V the 12 nm: 0. 10 nm; 0 , 6 nm. Reproduced from MCrup and Topsge particle volume. F e 3 0 4 :0. (163) with permission.

relative changes in the particle size can be readily measured without the value of K ; and the value of K for a given catalytic system can be determined by experimental calibration, perhaps providing information in addition to that dealing with particle size. For example, cr-Fe,O, is antiferromagnetic, and the value of K can be estimated mainly from consideration of magnetocrystalline anisotropy effects. Indeed, the particle size determined from the magnetic relaxation using this value of K agrees well with the particle size determined by other physical and chemical methods (97, 162,164). On the other hand, the large net magnetization of metallic iron suggests that shape anisotropy may provide the dominant contribution to K ; using a value of K typical for shape anisotropy, the agreement is good between the particle size so determined and that measured by other methods (97).Although an accurate value for K is not known, magnetic relaxation effects on the Mossbauer spectrum revealed that small metallic iron particles in the range down to 2

186

JAMES A . DUMESIC AN11 HENKIK TOPSC/jE

nm and supportcd on MgO do not sinter under conditions of atmospheric ammonia synthesis at 670 K (165). Finally, experimental calibration of the value of K for the iron particles supported on MgO provided evidence for the presence of magnetic surface anisotropy, which, as discussed later, led to the measurement of surface structures on these small particles (165). It IS important at this point, however, to note that the analogy between particle size determination using magnetic relaxation (or the Mossbauer effect in general) and selective chemisorption is not strictly speaking complete, because while the former method may provide the particle size distribution, the latter is a determination of a surface average characteristic dimension. That is, the Mossbauer spectrum is not an average of the contributions from the different particle sizes but results from the volume weighted sum of these components. This definitely may be an advantage for the application of Mossbauer spectroscopy to small particle systems. For example, in a single 15 nm in size Miissbauer spectrum of a-F'e,O,, particles greater than appear magnetically split at 300 K, while particles smaller than this size appear paramagnctic (162). Diffcrcnces in the chemical and catalytic propcrties between the different particles of the particle size distribution thus become distinguishable in the Mossbauer spectrum.

-

For supported-metal catalysts, the questions of interaction with and location of the metal on the support are of important concern, since these factors may be instrumental in determining, for example, the metal particle size and size distribution, thc particle size stability to thermal and chemical treatments, and the accessibility of the metal to the reactants of the catalytic process. That these questions are amenable to study using the Mossbauer effect is the topic of this section. For the production and stabilization of small metal particles, the interaction of the metal with the support may have an optimum strength: too weak an interaction between support and metal may lead to sintering of the metal particles at high temperatures; on the other hand, a strong support interaction may stabilize an unwanted oxidation state of the metal and prevent the reduction to the zero valence state. For example, the interaction of iron on carbon is sufficiently small for the reduction to metallic iron, but the resulting particles are quite large ( - 10 nm) (98). On the other hand, iron oxidc supported on Si0,-due to the rather strong support interaction-is difiicult to reduce beyond the Fez+ state. Thus at low iron loadings ( - 3%) hydrogen reduction at 720 K did not lead to formation of metallic iron (166). For higher metal loadings on SO,, hydrogen reduction does result in metallic iron, but the formed particles are no longer small ( - 10 nm)

MOSSBAUER SPECTROSCOPY APPLICATIONS

187

(167, 168). The 3(MgC0,)(Mg(OH),).3H2O/MgO system, with hybrid properties between the above two supports, provides a convenient support for small, and stable, metallic iron particles (97).In this system, approximately one-halfthe iron present on the support is metallic, with the other halfpresent as Fe2+-richclusters. The above studies, all of which employed the Mossbauer effect, reflect an important generalization: an advantage for use of the Mossbauer effect in the study of support interactions is the possibility to determine simultaneously the particle size and the chemical state of the metal component. While for some catalytic systems this information may be obtainable using a combination of other conventional techniques (e.g., electron microscopy, X-ray diffraction), the information deduced from the Mossbauer spectrum may be unique. For example, in the genesis of supported-gold catalysts, a gold species on yAI,O, was observed when using Mossbauer spectroscopy, but escaped detection by conventional techniques normally applied to supported catalysts (169).This species was not observed for gold supported on MgO, pointing to the dependence of the species on a special support interaction with yAI,O,. In the preparation of supported catalysts, the metal salt used in the impregnation or exchange may also affect the ultimate catalyst structure, perhaps through a support interaction. In this respect, it was found that F e 2 0 3particles supported on SO2 via Fe(NO,), or Fe(CO), impregnation gave rise to a Mossbauer spectral doublet at 300 K, indicating superparamagnetic behavior and thus a particle size less than 10 nm (170). On the other hand, FeCI, impregnation resulted in larger Fe,O, particles as evidenced by a six-peak Mossbauer spectrum at 300 K. For this system, the results were interpreted in terms of the decomposition temperatures of the salts [370,380, and 770 K, respectively, for Fe(NO,),, Fe(CO),, and FeCl,]. The advantage of applying this technique to the study of support interactions is apparent. In addition to the particle size effect described above, the support interaction may also affect the surface chemistry of the supported-metal particles. Such an effect for europium supported on v-Al,03 and Cab-0-Sil was studied using the Mossbauer effect by Ross and Delgass (171,172). A strong interaction between metal and support was evidenced by the dependence on metal loading (wt. Eu) of the europium Debye temperature and reduction characteristics. Specifically, supported europium (present in the ELI,' state) is strongly bonded to the oxide support, since the Debye temperature of the former was found to be considerably higher than that of pure E u , 0 3 (172). As seen in Fig. 22, the fraction of the Eu3+ that is converted into Eu2+ (the reducibility) by H, and CO between 670 and 770 K is not constant with metal loading, again pointing to the effect of the support (174. Indeed, it was found that these supported europium catalysts behave quite differently than

-

188

JAMES A. DUMESIC AND HENRIK TOPS@E

0

I

I

I

I

I

I

L

8

12

16

20

2L

Europium (%)

FIG.22. Dependence of europium reducibility on the metal loading. MBssbauer spectral area ratios taken alter 6 h r reduction in H 2 at 770 K, for europium supported on alumina. Figure according to Ross and Dclgass (172).

Eu,O, with respect to both CO, adsorption and the kinetics of the reverse water-gas shift reaction (i.e., H, + CO, -+ CO + H,O) (172). If this reaction takes place via a regenerative sequence involving the lattice oxygen of the catalyst, then the number of active sites on a given supported catalyst may be expected to correlate with the product of the reducibility and the metal loading, as experimentally observed. Thus, the results of the Mossbauer spectroscopic investigation provide clear evidence for a support interaction in supported-europium catalysts, changing the surface chemistry of the latter from that of hulk Eu,O,. In addition, these results are strongly suggestive of a regenerative sequence to describe the reverse water-gas shift kinetics over these catalysts. As stated earlier, the location of the supported metal on the carrier may also be deduced from the Mossbauer effect, and this is illustrated in the study of iron-exchanged zeolites (94, 173-182). The most recent article by Garten et ul. (178) will be discussed here. Approximately 53% of the sodium ions in the zeolite mordenite in this study were exchanged by Fez+, the resulting sample designated by FeZf-M. In Fig. 23, the room temperature Mossbauer spectra ofthis sample are shown after the initial preparation and subsequent dehydration experiments. Two important effects are therein observed: increasing the degree of dehydration results in an increased spectral area; the maximum effect is at one-half complete dehydration, and subsequent dehydration produces new spectral components. In view of the arguments presented in Section 111, A, 1, the first effect is due to an increase upon dehy-

MOSSBAUER SPECTROSCOPY APPLICATIONS 1.01,

,

1

,

,

,

,

189

,

-2.w -100 0 0 0 1.00 2.00 3.00 4.00 5.00

Velocity ( m ms e t ' )

FIG.23. Effect of dehydration temperature on the Mossbauer spectrum of FeZ+-M. (a) 15 Torr of H,O, 273 K ; (b) evacuated 24 hr, 273 K ; (c) evacuatcd 3 hr, 517 K ; (d) evacuated 8 hr, 800 K ; (e) 15 Torr of H,O, 273 K. All spectra at room temperature and on the same sample. Zero velocily is with respect to a 57Coin chromium source. Figure according to Garten et ul. (I78).

dration of the bond strength between the Fe2+ species and the crystalline lattice. Indeed, as water molecules are removed from the coordination sphere of Fez+ and the interaction of this ion with the zeolite lattice increases, the corresponding quadrupole splitting is expected to increase, as experimentally observed. Further dehydration (i.e., removal of the remaining water molecules from the Fez coordination sphere) then produces new spectral components, which have been interpreted through a computer analysis of spectra taken at different temperatures for the dehydrated Fe '+-M sample. The results of this analysis are shown in Table 11, after it was found that four peaks provided the most meaningful fit of the data (low X-squared with physically realistic Miissbauer parameters). The outer two peaks form a doublet whose isomer shift is that expected for high-spin Fe", and whose quadrupole splitting is temperature dependent, reflecting the change with +

190

JAMES A . DUMESIC AN11 HENRIK TOPSdE

Peak position'

'rlK 78 298 517 732 "

Inner peaks

Outer peaks

I

2

3

4

ISh

QS

ISb

QS

0.19 0.14 0.1 1 0.32

0.74 0.60 0.50 0.44

1.66 1.65 1.46 1.29

2.64 2.33 2.00 1.66

1.07 1.11

0.98 1.05 0.96 0.84

1.42 1.24

2.46 2.19 1.88 1.34

0.98 037

1.0.5

0.99

According lo Garten r / rrl. ( I 78). With respect to " C o i n Cr 50urcc.

temperature in the relative occupations of the crystal field split d orbitals by the sixth d electron (the first five d electrons form a half-filled and thus spherically symmetric shell). The inner two peaks form a doublet whose isomer shift is also characteristic of high-spin Fe", but the quadrupole splitting is surprisingly temperature independent. These peaks were formed and increased in intensity, however, with successive dehydration, suggesting that the coordination number of the corresponding Fe2+ ions is lower than that for the Fe2+ ions giving rise to the outer peaks. The tcmperature independence of the inner doublet quadrupole splitting reflects the above expectation that a low coordination number corresponds to a large crystal field splitting. That is, the excited crystal field split electronic states are not thermally populated over the temperature range studied, corresponding to a temperature-independent quadrupole splitting. It should be noted that the low coordination number of the Fe2+ ions corrcsponding to the inner doublet is quite consistent with the small quadrupole splitting observed. This results from the fact that the electric field gradient at the Fe2+ nucleus produced by the surrounding lattice ions, (I"'', may be opposite in sign to that created by the sixth d-electron, 4"a' (183, 184, and thus a decrease in Fe2+ coordination number may increase yidt and at the same time decrease the observed quadrupole splitting. The effects of chemisorption on the two spectral doublets further elucidate thc nature of Fez+ in the zeolite framework. Ammonia, which is small enough to enter both the main channels and the side "pockets" of the zeolite, is expected to affect both spectral doublets, as experimentally observed. Of the molecules methylamine, dimethylamine, trimethylamine, and piperidine, only the first has any effect on the Mossbauer spectrum after room temperature adsorption (resulting in decreased spectral area for both doublets). At 340 K, however, dimethylamine adsorption resulted in a spectral area de-

MQSSBAUER SPECTROSCOPY APPLICATIONS

191

crease for the inner doublet, and at 518 K piperidine adsorption increased the quadrupole splitting of both spectral doublets while decreasing the total spectral area by 40%. Of the above four molecules, methylamine should be able to enter both the side pockets and the main channels, while the other three molecules should be able to enter only the main channels. The hightemperature results of dimethylamine and piperidine adsorption thus clearly point to the location of the inner doublet F e 2 + ions in the main channels; and since these ions were formed by the outer doublet Fez+ dehydration, the latter are probably also in the main channels. The room temperature adsorption results must then point to an effective decrease in the diameter of the main channels due to Fe2 exchange. That is, dimethylamine and piperidine, which are small enough to freely enter the main channels of a zeolite containing no Fe2+,are found to affect the Fe2+ spectrum of Fe2+-M only at high temperatures, indicating activated diffusion of these molecules in the main channels. This result is also consistent with the location of Fez+ in the main channels, as opposed to the side pockets. An iron-exchanged mordenite was also studied by Meisel et u1. (182),who incorporated Fe3 into the zeolite structure. Upon calcination at temperatures greater than 500 K the appearance of Fe2+ was noted in the Mossbauer spectrum, and for calcination temperatures higher than 770 K the formation of !x-Fe20, was observed to take place inside the mordenite. For the iron-mordenite system, it can now be seen that the Mossbauer effect provides information about the chemical state, symmetry, interaction strength with the support, and location on the support of the resonant iron ions. This information enhances the understanding of the catalytic activity of this and other zeolites ( I 78). In a series of studies of ferrous-exchanged A zeolites, Dickson and Rees (179-181) studied in detail the location ofthe ferrous ion in this structure and the effects of hydration and dehydration ( I 79). Mossbauer spectra were also observed after dosing the samples with different amounts of ethanol (181).By following the resulting spectral area changes, it was concluded that two molecules of adsorbate coordinate to each ferrous ion, and that even after the highest doses 20% of the ferrous ions remained uncoordinated, probably due to steric effects. Very interesting results about the mobility of ethylene in the zeolite structure were also found (180). Upon cooling a sample with adsorbed ethylene, the spectra changed from a single quadrupole split doublet into a spectrum with two doublets. This was taken as evidence that at high temperatures the ethylene moves freely in the zeolite structure, whereas at lower temperatures the ethylene “residence time” on the ferrous ions becomes of the same order of magnitude as the mean lifetime of the nuclear excited state. The possibility of using the magnetic hyperfine interaction to elucidate support interactions is clearly illustrated in the study by Suzdalev et al.

-

+

+

-

192

JAMES A. DUMESIC AND HENRIK TOPSI$E

(185,186) and Gol'danskii rt ul. (187) of 57Fe in ion exchange resins. For example, a sulforesin exchanged with from 1.6 to 3 wt. "/, Fe3+ shows virtually no hyperfine splitting at 90 K ; after water adsorption corresponding to approximately 6 molecules per Fe3 ion, however, a fully developed magnetic hyperfine pattern is observed at 90 K (185).Thus, for the dehydrated resin, the electronic spin relaxation time zH is small compared to the precession time zL of the nuclear magnetic moment about the hyperfine magnetic field, and water adsorption increased zH to a value greater than zI,. For the low loading of iron in the resin, spin-spin relaxation effects are expected to be negligible, and the independence of sH on the metal loading from 1.6 to 3 wt. y/o is consistent with this expectation. Spin-lattice relaxation is thus responsible for the small value of TH before hydration. The etfect of water adsorption is then due to a weakening ofthe interaction between the Fe3+ion and the resin lattice, resulting in an increased spin-lattice relaxation time and the appearance of a magnetic hyperfine pattern. One must, however, determine whether the increase in zH with water adsorption is due to thc formation of an ice lattice inside the resin or instead results only from the interactions ofFe3+with the water molecules within its coordination sphere (187). A study of partially hydrated resins provides the answer, the results of which are shown in Fig. 24. The final form of the +

V e l o c i t y (mm sec-9

FIG.24. Etkct or degree of hydration on the Miissbauer spectrum o f an ion exchangc resin containing iron. Water concentration: ( I ) 0, (2) 0.32, ( 3 ) 1.24. (4) 2.93, (5)4.5, (6)6 H,O molecules per sullonic acid group. Zero velocity is with rcspecl to a "Co in chromium source. Figure according to Gol'danskii et ul. (187).

193

MOSSBAUER SPECTROSCOPY APPLICATIONS

magnetic hyperfine splitting is achieved at water concentrations of about 6 molecules per Fe3+ ion, and further increase to 20 molecules per Fe3 has no significant effect on the Mossbauer spectrum. In addition, the spectra of the partially hydrated samples cannot be generated by superposition of the spectra corresponding to complete and zero hydration. That is, in the partially hydrated resin, the Fe3+ ions are coordinated to less than 6 water molecules, and these ions do indeed give rise to a magnetic hyperfine splitting. Thus, the appearance of the magnetic splitting is not related to the formation of the ice lattice inside the resin, but provides information about the interaction of the Mossbauer ion (Fe3+)with its support (the resin) and adsorbed species (water). In general, it may be expected that the sites for Fez+ and Fe3+ in these noncrystalline ion exchange resins will have a large distribution of chemical environments. This expectation should be reflected as a significant broadening of the Mossbauer resonance, as experimentally observed by Johansson (188).In addition, this broadening should result in a non-Lorenzian spectral line shape. Indeed, a computer analysis of the spectra showed that Gaussian peaks provided a better fit of the data than did Lorenzian peaks. In this case then, the linewidth and peak shape provide information about the distribution of support interactions for the various resonant atoms in the sample. +

B. SURFACE PROPERTIES OF CATALYSTS 1. The Surjuce Chemicul State

a. Measurement. It is important to remember that the sensitivity of Mossbauer spectroscopy permits one to resolve spectra only if -5"/;; or more of the Mossbauer atoms present are in a certain chemical state. For catalysts with dispersions of 52, or more-corresponding to particle sizes of -20 nm or less-Mossbauer spectroscopy therefore becomes a true surface investigation technique capable of yielding chemical information about the outmost layer, and as discussed such information can be obtained under reaction conditions. In certain instances, one is interested in information about not just the outmost surface layer but a larger surface region. In such cases the backscattering geometry may be important, and for particles larger than 100 nm (corresponding to the escape depth of the electrons) the backscattering geometry becomes more advantageous for surface studies. An interesting point must be made now with respect to the possibility of determining the chemical state of the catalyst as a function of the depth into the catalyst from the surface. This technique involves the use ofthe conversion electrons, the principle for which is shown in Fig. 25. The conversion electrons emitted bv an excited 57Fe atom in the sample lose energy as they pass

194

JAMES A. DUMESIC AND HENRIK TOPS@E

0.5

1.0

15

2a

2.5

1 (pm)

Fie. 25. Dependence of the conversion electron energy E , on the escape depth. Figure according to Bonchev et ul. (191).

through the solid toward the surface (189, 190);thus a determination of the Mossbauer spectrum for different conversion electron energies yields the desired depth profile. This method has been used by Bonchev et al. (lY1,192) for studying the interaction of metallic tin with bromine, employing a spectrometer to select the conversion electron detection energy. After exposure of a metallic tin plate to bromine vapor for 10 sec, the Mossbauer spectrum showed only /I-Sn when low-energy conversion electrons were detected. At increasingly higher energies, however, SnBr, and subsequently SnBr, were detected, indicating surface layers of these compounds on the bulk /I-Sn film. The above example also illustrates the use of Mossbauer spectroscopy as related to an important question often asked in catalytic studies: does the exposure of the catalyst to the reactants and/or products of the catalytic process change the chemical state of the catalyst'? Other examples of the utility of Mossbauer spectroscopy for study of reactions with solids and solid surfaces are found with reference to the interaction of oxygen with iron (193, 194).That is. Meisel (193)used the backscattering geometry to show that the room temperature oxidation of iron in the presence of water vapor produces y-Fe,O, and y-FeOOH; whereas using the transmission geometry and thin films ( 30 nm) Keune and Gonser (194)found that films produced by slow vacuum deposition (50 nm hr-') interacted with the residual gases of the Pa) to form y-FeOOH at room temperature. vacuum (at

-

MOSSBAUERSPECTROSCOPY

APPLICATIONS

195

For multicomponent systems (e.g., alloys), a change in the surface composition can occur accompanying the exposure to various gases of the catalyst surface. In this case, also, Mossbauer spectroscopy may be valuable in understanding the phenomenon, as illustrated in the work of Bartholomew and Boudart (195). These authors studied Pt-Fe alloys supported on carbon, with alloy particle sizes ranging from 1.5 to 13 nm for total metal loadings from 1 to 12.1 wt. O0, respectively. The room temperature Mossbauer spectra of these Pt-Fe/C samples were characterized by a broad spectral doublet, which was best fit using four peaks, representing two spectral doublets as shown in Fig. 26. Indeed, the isomer shifts of these doublets for the various catalysts agreed well with those reported for bulk Pt-Fe alloys of the same overall alloy composition. Due to their small size, the alloy particles appeared superparamagnetic at room temperature, as evidenced by the aforementioned spectral doublets; however, spectra at 77 K showed magnetic hyperfine splitting for the larger particles (greater than - 3 nm) while the smaller particles gave rise to a magnetic hyperfine splitting at 10 K. In all cases, the observed magnetic field was significantly smaller than that expected for metallic iron, and instead its value agreed well with that expected for a Pt-Fe alloy of the same overall composition. Thus, for these small supported particles, Mossbauer spectroscopy provides clear evidence for alloying between platinum and iron.

0.0..

s

-

1.0..

c

-2a 2.0 .. 0 n Y)

Q

3.0

1.

V e l o c i t y Imm sei')

FIG.26. Computer-fitted MBssbauer spectrum for small-particle PtGFe alloy. Peaks (1) and (4) form the outer surface doublet. Peaks (2) and (3) form the inner doublet. Zero velocity is with respect to a "Co in copper source. Reproduced from Bartholomew and Boudart (195) with permission.

196

JAMES A. DUMESIC A N D HENRIK TOPSOE

Considcr for a moment the physical significance of the two spectral doublets used to fit the Pt-Fe/C M6ssbauer spectra. Due to the lower symmetry of a “surface” iron atom than that of an iron atom inside the alloy particle, the electric field gradient at the former (and thus the quadrupole splitting) is expected to be larger than that for the latter. This is, in fact, the shell model discussed in Section 111, A, 3. An ambiguity, of course, arises in distinguishing between “surface” and “bulk” atoms for these small particles, but as a first-order approximation the underlying physical interpretation seems quite reasonable. The inner doublet is thus attributed to iron atoms inside the particle, while the outer doublet is assigned to iron atoms in or near the surface. Mossbauer spectra were taken after ( 1 ) hydrogen reduction at 670 K, (2) exposure to air at 300 K, (3) exposure to hydrogen at 300 K, and (4) treatment for 0.1 hr in oxygen at 570 K. Of interest in the present discussion is the dispersion parameter DFe%,defined as the percentage ratio of the spectral area for the outer doublet to the total spectral area. [A slight complication arises for the spectra after treatment (3) due to the presence of extra peaks. These new peaks, however, are attributable to surface atoms, and their spectral area is added to that of the outer doublet.] An analysis of these spectra showed that 570 K oxygen treatment significantly increased the value of D, ,”,. This treatmcnt is necessary to remove carbon contamination from the surface (196)before standard chemisorption titrations can be conducted. Combining then the results of H,-0, titrations (assuming plausible surface stoichiometries for the interactions of these gases with platinum and iron) with the OF,‘;/, value after the 570 K oxygen treatment provides a calculation of the alloy composition at the surface. In Table 111, the results of such calculations are shown, where X and X , are the atomic fractions of iron inside the

1 .0 1 .x I .0 9.4

After exposure 62 61 64 40

to 0, at 570 K, 10 min 79 45 0.51 68 57 0.34 85 57 0.25 72 36 0.101

0.65 0.38 0.33 0.182

After exposure of reduced catalyst to air at 300 K 62 57 68 0.51 0.47 61 56 63 0.34 0.31 64 65 64 0.25 0.25 40 53 38 0.101 0.135

I .0 1 .x I .o 9.4

MbSSBAUER SPECTROSCOPY APPLICATIONS

197

particle and on its surface. In addition, if the 570 K oxygen treatment does not change the alloy particle size, the measured DFe% values before this treatment can be used to calculate the corresponding values of X , (Table 111). Thus, the results of Mossbauer spectroscopy and H 2 - 0 2 chemisorption measurements seem to indicate that a treatment of the Pt-Fe/C alloys in oxygen at 570 K changes the surface composition in the direction of increasing iron concentration. In view of the fact that the heat of chemisorption of oxygen on iron is larger (by a factor of nearly two) than that of oxygen on platinum, this result is indeed expected. For small-particle catalyst systems, another possible modification of the surface chemical state is due to the actual size of the particle: as the particle size is decreased, the electronic properties of the particle may deviate from those of the bulk metal due to the finite number of atoms in the particle and/or an interaction with the support. If present, these deviations should be evidenced in the Mossbauer parameters. Changes in electronic structure due to lattice modifications were discussed in Section 111, A, 3. For example, the electronic properties of small, isolated metallic iron particles can be modeled by the study of iron atoms at low concentration in a frozen argon matrix at 4.2 K (197). For an Ar-Fe number ratio of 50, approximately 23% of the iron atoms will have another iron atom in their first coordination sphere, producing an iron dimer. In the Mossbauer spectrum of this material, a spectral component with a large quadrupole splitting (4.06 mm sec- 'j was observed, and tentatively assigned to the dimer structure, since the overlap along the Fe-Fe bond direction is expected to produce a large electric field gradient (and thus a large quadrupole splitting). The isomer shift of the spectral component was less than that of bulk metallic iron (considering the second-order Doppler shift), indicating an increase in the electron density at the nucleus with decreasing particle size. Because these dimers are surrounded by an argon matrix, this change in electronic structure may well be due to the decrease in particle size itself, as opposed to a support interaction. It is interesting to note that Mossbauer spectroscopic studies of 1.5-nm metallic iron particles (97) and Pt-Fe alloy particles (195) showed electronic properties characteristic of those for the respective bulk materials, thus indicating that significant electronic deviations for these materials begin below this particle dimension. In contrast to the above systems, where electronic support interactions were negligible, the work of Bowles and Cranshaw (198) points to the presence of such effects for tin on platinum. Tin was deposited on platinum electrodes at a potential high enough to ensure a fractional tin monolayer. Subsequent '"Sn Mossbauer spectroscopy showed a tin isomer shift characteristic of a Pt-Sn alloy, thereby evidencing the electronic interaction between these two metals. The possible modification by the support of the

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JAMES A. DUMESIC AND HENRIK TOPSOE

electric field gradient at the nucleus of a supported atom is illustrated in the study of thin metallic iron films on glass substrates (199,200). Metallic films with an average thickness of 1 nm showed an apparent quadrupole splitting at 300 K of 0.6 mm sec-’. At fractional monolayer coverages of the support, however, a large quadrupole splitting at 300 K was found (2.2 mm secC’), reflecting the effect of the support on the electronic environment of the supported metal atoms and demonstrating its measurement using Mossbauer spectroscopy. With the advent of new counting systems and more efficient detectors and detection modes (201), thin-film systems may have very interesting applications as model catalyst systems. Previously, in studies of films one was required to make “sandwich films” in order to have cnough resonant atoms in the pray beam, and the “surface” atoms of these films were thus covered by the substrate and not available for chemisorption or reaction. With the recent advances in methodology it should be possible to study a single thin film with a surface accessible to gases. Such studies may have the advantage over the method of internal conversion electron backscattering from ‘‘bulk‘’ samples in that the fraction of the resonant atoms that contribute to the Mossbaucr spectrum and are located on the surface is large and can approach 1 OOY!.

b. Corrrlution with Cu~ulyticPro2rrtit.s. As a step toward the ultimate goal of understanding catalytic processes in terms of the catalyst surface properties and structure, a correlation betwecn catalytic properties and Mossbauer parameters is sought. Let us first mention a few examples of such correlations. As noted earlier, Delgass et a!. (169) detected an electron-deficient gold species (compared to metallic gold) in the Mossbauer spectrum of a 640 K treated HAuCI, on q-A120, catalyst. No such species was observed for a similarly treated HAuCI, on MgO sample. Indeed, the former catalyst was more active for the oxidation of C O to CO, by N 2 0 at 520 K. The implication, then, is that this electron-deficient species may itself be catalytically active or serve as a precursor to an active species for the CO ox idation. The catalytic properties of iron-exchanged zeolites also appear to be correlated with information obtainable from the Mossbauer effect. While iron exchanged into Y-zeolites, Fe--Y,showed little or no catalytic activity for the reverse water-gas shift reaction at 770 K (H, + CO, -+ H,O + CO), the iron-exchanged mordenite, Fe-M, was far more active (178). In addition, Fe M was much more active for propylene ammoxidation than Fe-Y, and the former showed a greater selectivity toward acrylonitrile. Mossbauer spectroscopy of Fe Y (Y4, 176,177) showed that 70% of the exchanged iron atoms were present in the hexagonal prisms or sodalite cages of the zeolite (with 0.22-nm openings to these cages), thereby making these ions inaccessible

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199

for such molecules as CO, and propylene. O n the other hand, the iron atoms in Fe-M were located, as evidenced by the Mossbauer effect, in the main channels of the mordenite, readily accessible for the reactants CO, and propylene (Z 78).Thus, the catalytic properties ofthese iron exchanged zeolites can be related to steric factors, as deduced via the Mossbauer effect. Garten e t al. (178) also noted that the quadrupole splitting of Fe3+ ions exchanged in mordenite is significantly greater than that for Fe3+ ions in Y-zeolites. And this can perhaps be related to the different selectivities toward acrylonitrile. This observation is in agreement with the results of Skalkina et a/. (202),who established a correlation (Fig. 27) between the selectivity toward acrylonitrile production and the quadrupole splitting for a series of mixed iron oxide catalysts for the propylene ammoxidation. In this latter work the quadrupole splitting of the bulk atoms was used in the correlation (due to the low dispersions of the catalysts), and a direct comparison with the results of Garten et a/. (178) cannot be made. The suggestion, however, is that the I

I

I

I

Velocity (mm sec-'1

FIG.27. Correlation between quadrupole splitting and selectivity t o acrylonitrile in the ammoxidation of propylene over mixed iron oxides. Filled symbols, CO,: open symbols, CH,CHCN. Figure according to Skalkina et C J ~ (202). .

200

JAMES A. DUMESICA N D HENRIK T O P S ~ E

propylene ammoxidation is sensitive to the symmetry of the Fe3+ ions in the zeolitic materials Fe-Y and Fe-M, as well as in the mixed iron oxides studied by Skalkina et al. The success of the correlation of catalytic behavior with bulk Mossbauer parameters by Skalkina et ul. is also reflected in the work of Tops$e and Boudart (96). As discussed earlier, these authors found a decrease in the isomer shift of the octahedral iron ions in a lead-promoted Cr-Fe,O, carbon monoxide shift catalyst, indicative of an increased covalency of these ions. Schwab et al. (203)have proposed a correlation of the activity for CO oxidation by ferrites with the octahedral ions in these materials, and the electron transfer required for this catalytic process may be facilitated by an increased covalency of the metal ions (204). In view of these suggestions, the leadpromoted catalyst is expected to possess a higher catalytic activity for the CO shift reaction than an unpromoted catalyst, as evidenced by the Mossbauer parameters of these two samples. This has in fact been shown experimentally to be the case (96).For the reverse CO shift reaction over supported europium (176),the success of the correlation between catalytic activity and the Mossbauer parameters (in this case the reducibility) has already been noted in Section 111, A, 4. The reducibility was also used by Hobson and Gager (205) to correlate the catalytic activity of supported-iron catalysts for the hydrogenation of 1-butene at 300 K. The iron was supported on AlzOJ and S O z , and after reduction by H, for various times at temperatures between 770 and 870 K, Mossbauer spectra characteristic of both Fe2+ and metallic iron were observed for these samples, Thus, for a particular reduction treatment, the fraction of the iron in the metallic state can be determined from the associated Mossbauer spectrum, and the 300-K hydrogenation of I-butene can be measured. Indeed, the hydrogenation rates so determined when normalized by the metallic-iron surface area (determined from the fraction of the iron in the metallic state, assuming that the metallic-iron particle size was the same as that for the oxide from which it was produced) were constant for different samples with the same support. The indication, then, is that the metallic iron is the active iron phase for the 1-butene hydrogenation. The dependence of the normalized reaction rate on the type of the support, however, also suggested a bifunctional nature of this reaction. Iron-supported-on-MgO catalysts behave in some ways differently from the above catalyst systems. That is, while the catalytic activity of these metallic-iron particles for the atmospheric-pressure ammonia synthesis depends markedly on particle size in the range 1.5-10 nm (206),the Mossbauer parameters (isomer shift, quadrupole splitting, and magnetic hyperfine splitting) are independent of iron particle size in this range (97). This thus rules out an “electronic effect” in the interpretation of the effect of particle

M ~ S S B A U E RSPECTROSCOPY APPLICATIONS

20 1

size on the catalytic activity and indicates that the surface structure of these particles changes with decreasing particle size in the region below 10 nm. This surface structure change would then be related to the observed catalytic activity changes. Indeed, as discussed in Section 111, B, 2, a study of these catalysts using the Mossbauer effect and traditional gas chemisorption methods verifies this latter interpretation (165). 2. The Surface Structure

a. General Remarks. One must of course expect that catalytic activity can depend on the detailed geometrical arrangement of the surface atoms-or the surface structure (207). Thus variations in catalytic activity are often related to relative numbers of face, edge, and corner atoms. These relative numbers can depend on particle size, support interaction, etc. In this section we are interested in relations between surface structure and the Mossbauer parameters. While these parameters of surface atoms may differ from those of atoms in the bulk, the influence of surface structure on such differences is clearly an effect of second order and often not too well understood. For example, a theory for quantitative prediction of the change in isomer shift or magnetic hyperfine splitting for different surface structures is not yet available. At present, those Mossbauer parameters which depend largely on the symmetry (which is o priori known for a given structure) of the surface appear to be most suitable for use in surface structure determinations. The first of these parameters is the recoil-free fraction, and its anisotropy. For surface atoms, the mean square vibration amplitude perpendicular to the surface is not expected to equal that parallel to the surface, giving rise to different recoil-free fractions for resonant absorption in the two respective directions. If the orientation of the surface with respect to the y-ray direction is known, these two recoil-free fractions can be measured directly (208).Thus, for iron on a tungsten ribbon, the recoil-free fraction was determined with the y-ray beam perpendicular to the surface and then at an angle of 60" from the surface normal (50). The Debye temperature along the surface normal was deduced to be 350 K, while that parallel to the surface was 250 K, indicating that the mean square vibration amplitude parallel to the surface was greater than that perpendicular to the surface. This then suggests that the iron atoms are bound on the tungsten surface, as opposed to being in the surface (for which case the amplitude perpendicular to the surface would be expected to be the larger). Of importance in catalytic studies, is that the anisotropic nature of the recoil-free fraction can be deduced also for samples where the orientation of the surface is not known (e.g., powdered samples),as noted earlier (Section I, C, 5). In this case, the recoil-free fraction is not measured directly, but its

202

JAMES A . DUMESIC AND HENRIK T O P S ~ E

anisotropy is deduced through the spectral areas of the various Mossbauer transitions (209, 210). For a $, f (total spin quantum numbers I ) nuclear transition (as in 57Fe)in the absence of magnetic hyperfine splitting, the dependence of the relative spectral area pattern on the surface vibrational anisotropy is shown in Fig. 28. In the spectral area analysis, however, it is necessary to know the sign of the electric field gradient, to determine which + l+f(magnetic $ quantum numbers of the two peaks corresponds to the rn) transition, i.e., whether ZJlo equals 2 or 0.5, for example. Physical arguments may aid in this determination. The electric field gradient at metal ions in oxide surfaces, for example, may indeed be positive due to the removal of the oxygen ions along the surface normal as a result of the surface formation (208,210).It should also be noted that an observed spectral area pattern characteristic of an anisotropic recoil-free fraction may instead be due to partial orientation of the resonant particles in the sample, as seen in Section I, C, 5, and the difficulty in distinguishing between this effect and that due to an anisotropic recoil-free fraction has been demonstrated by Pfannes and Gonser (103) and Kreber and Gonser (211). Conclusions concerning the anisotropy of the recoil-free fraction should thus be checked by observing the

3.0

I

I

‘

1.1

T

-

1.0

200s-

I

1

-10

I

I

I

I

-8

-6

-2

-2

0

I

I

I

I

2

2

6

8

10

€2

FIG.28. Asymmetry of quadrupole split peaks due to the Gol’danskii ~Karyagineffect. + +f transition; 0 = +f 4 transition; E = ( 2 7 ~ / 1 . ) ~ ( ( 2-~ )(x’)). z and x are parallel and pcrpcndicular, respectively to the surface normal. Figure according to Suzdalev =

*+

and Makarov (208).

*+

203

MGSSBAUER SPECTROSCOPY APPLICATIONS

Having observed an anisotropic surface recoil-free fraction, there remains the problem of relating this to the surface structure. While this relation is generally complex [see Section 111, A, 3 and (208,209,2l3,2l4)],a simplification may be appropriate (209).A qualitative theory for the recoil-free fraction anisotropy may be sufficient for understanding surface structure changes accompanying various chemical treatments and particle size changes. Specifically, the mean square vibrational amplitude in a particular direction of a surface atom may be obtained by counting the number of bonds (and projections of bonds) in that direction between the central atom and its neighbors (209).In this way, the anisotropy of the recoil-free fraction for a surface atom is related to its symmetry (and thus its “local structure”). The measured anisotropic recoil-free fraction and the desired surface structure are then composed of sums of the contributions from the individual surface atoms. In general, there may not be a unique surface structure for an observed anisotropic recoil-free fraction; nevertheless, the information obtained from Mossbauer spectroscopy, when combined with that obtained from other techniques, may be invaluable in eventual determination of surface structure, as seen in the next section. The quadrupole splitting (as shown in Section I, C , 3) and the relaxation of the magnetic hyperfine interaction (as will be seen presently) are also related closely to the symmetry, and thus the structure of the surface. Determination of the surface structure using these parameters then follows the pattern outlined above for the recoil-free fraction. For clarification, this pattern will be illustrated in the following section for the magnetic relaxation. b. Surjuce Structure Measurement. Metallic iron, while providing an excellent Mossbauer isorope, is also of catalytic interest for the synthesis of ammonia. Dumesic et a/. (206) studied the dependence of this catalytic reaction on the metal surface structure by observing the rate of synthesis as a function of the iron particle size and, in addition, collecting Mossbauer spectra of the catalysts under reaction conditions (165).They found that the reaction rate per metallic iron surface area decreases by a factor of 30 for decreasing particle size from 30 to 1.5 nm (206). As mentioned earlier, the isomer shift and internal magnetic field of the metallic iron particles are independent of particle size, ruling out particle size-dependent electronic properties of these metallic-iron particles. The Mossbauer spectra do, however, indicate that the anisotropy energy J (215), barrier for the particle magnetization flipping is quite large, for the 1.5-nm iron particles. As discussed in Section 111, A, 3, this barrier was estimated from Mossbauer spectra at various temperatures by measuring the fraction of the spectral area that appears paramagnetic. At the temperature for which this ratio is 0.5, the relaxation time can be estimated

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204

JAMES A. DUMESIC AND HENKIK TOPS@E

for those particles with size equal to the average of the particle size distribution; from the relaxation time, the anisotropy energy barrier is then deduced. This measured anisotropy energy barrier for the 1.5-nm particles is several orders of magnitude greater than that expected for magnetocrystalline anisotropy, and is instead comparable to the energy barriers expected for magnetosurface or shape anisotropy (215) as discussed in Section I, C, 4. In Fig. 29, however, it is seen that for iron particles less than - 6 nm in size, magnetosurface anisotropy may well dominate the effects of shape anisotropy. The possible presence of magnetosurface anisotropy is indeed interesting, since the associated energy barrier can be related to the surface structure through direct symmetry arguments (216). This energy barrier should be sensitive to surface phenomena, such as chemisorption, thereby providing a means for establishing the presence of magnetosurface anisotropic effects. 1000

600 -

400 -

200 -

R

x W

60-

1

2

4

8

14

d (nm)

FIG.29. Magnetic anisotropy energy barriers for small particles. K,V and K,S are the barriers for shape and magnetosurface anisotropy, respectively, for metallic iron. Figure according to Boudart el a/. (215).

MdSSBAUER SPECTROSCOPY APPLICATIONS

205

For this reason, Mossbauer spectra were obtained for the 1.5-nm iron particles at 670 K, with the sample first under hydrogen and then under helium. It was found that chemisorbed hydrogen increased the fraction of the metallic iron in the superparamagnetic state by -20% (216).The effect of hydrogen chemisorption did not vary in a linear fashion with metalliciron dispersion, suggesting that the observed effect is not simply due to a cancellation of the surface magnetic field by chemisorbed hydrogen. This conclusion was also reached by Dumesic et a/. (217) using magnetic susceptibility to study the effect of hydrogen chemisorption on the magnetic properties of these small metallic-iron particles. Thus, hydrogen chemisorption decreases the magnetic anisotropy energy barrier, pointing to the presence of a surface-sensitive anisotropy for the small metallic-iron particles. It was mentioned with respect to the recoil-free fraction that it may be more advantageous to study changes in the surface structure using Mossbauer spectroscopy than to determine the surface structure itself of a given catalyst from Mossbauer spectra. This situation also exists for determinations using the magnetic hyperfine interaction, because the theory of magnetosurface anisotropy is not sufficiently developed for quantitative prediction of the associated energy barrier (218), and the transformation from anisotropy energy barrier to surface structure is not unique, as will be seen later. In this respect, chemical treatments, which change the activity of a catalyst, may be elucidating. It was shown by these authors that the amount of nitrogen present during pretreatment of a catalyst affects the ultimate activity for ammonia synthesis (206).Specifically, it was found that treating H,-reduced small particles with ammonia at 670 K, followed by re-reduction of the catalyst with a H, :N, gas mixture, gave rise to an increase in the catalytic activity compared to the activity measured after H2 reduction alone. However, when the catalyst in this high-activity state was further treated with H, alone at 670 K, the catalytic activity was found to decrease to that value observed before the above “ammonia treatment.” Subsequent ammonia treatment returned the catalyst to its high-activity state. No such effects were observed for metalliciron particles greater than 10 nm in size. Mossbauer spectra were then taken of the small iron particles after various pretreatments, with the catalyst under reaction conditions (165). For increased sensitivity the velocity-offset mode was used (Section 11, B, l), and the magnetically split spectral area versus temperature curves after the various pretreatments are shown in Fig. 30. It is therein seen that the ammonia treatment, which increases the catalytic activity, decreases the magnetically split spectral area at a given temperature; this is the result of a decrease in the magnetosurface anisotropy energy barrier. While the effects of these pretreatments are in themselves interesting, the important point for surface

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JAMES A. DUMESIC AND HENRIK TOPSC~E

FIG.30. Effect of sequential pretreatment on magnetically split spectral area versus temperature 013% Fe/MgO; M6ssbauer spectra in H,:N,. Pretreatment sequence: 0,H, reduction; 0, NH,; A, H,; 0, NH3.Figure according to Dumesic et a!. (165).

structure measurement is that increased catalytic activity for ammonia synthesis is associated with a decreased magnetosurface anisotropy of the surface. In relating the observed magnetosurface anisotropy energy change to the associated surface structure change, the description of the latter in terms of exposed crystallographic planes seems awkward, in view of the many highindex planes (of very small extent) that are undoubtedly present. Instead, it seems more natural to specify the concentrations of the various surface sites that are present on the small particle surface, where a surface site Ci is defined as a surface atom with i nearest neighbors. Examples of these sites on the low-index planes of metallic iron are given in Fig. 31. NCel’s phenomenological theory of magnetosurface anisotropy (69, 70) is then used to calculate the associated anisotropy energy barriers for these sites (216).The surface sites can thereby be arranged in order of decreasing magnetosurface anisotropy : C6, C5 C4 Thus, the effect of the ammonia treatment, which increases the catalytic activity, is to convert sites from the left to the right in the above sequence.

M ~ S S B A U E RSPECTROSCOPY APPLICATIONS

207

(100) PLANE C4

StTE

0

(110) PLANE C 6 SITE

0

(11 1 I PLANE CLSITE

0

C7SlTE C7SITE

@

FIG.31. Surface sites on bcc surface planes. Figure according to Dumesic et al. (165).

Clearly, this sequence does not provide a unique identification of the surface site(s) that is associated with the increased catalytic activity. Carbon monoxide chemisorption provides the additional information required for this determination. Based on well-established steric considerations (224),the surface sites can also be arranged in an order of decreasing CO chemisorption ability (165):

c,,

c5,

C67

c 7

Experimentally, it was found that the ammonia treatment, which increases the catalytic activity, decreases the CO uptake of the metallic-iron surface without sintering of the particles (165).Thus, an increased catalytic activity is accompanied by a conversion of sites on the left toward sites on the right

208

JAMES A. DUMESIC AND HENRIK T O P S ~ E

in the above sequence of sites. Comparison of the results obtained from Mossbauer spectroscopy with those of CO chemisorption implies that the increased catalytic activity for the ammonia synthesis accompanying the ammonia treatment is associated with an increase in the number of C7 sites on the metallic-iron surface. Returning now to the observed effect of particle size on catalytic activity, van Hardeveld and Hartog (219)have calculated that the relative concentration of C , sites on octahedral iron crystallites decreases with decreasing particle size and that, in general, the C, site is not a small-particle surface site. The above correlation of increased catalytic activity with increased C7 site surface concentration thus also explains the observed “structure sensitivity” (particle size dependence) for this reaction. Finally, this correlation is consistent with results obtained from field electron microscopy of iron (220), single crystal reaction studies on tungsten (also a bcc metal) (224, and symmetry considerations (222). The utility, and also the limitations, of Mossbauer spectroscopy in surface structure measurement can now be seen. While this determination using the Mossbauer effect alone may be possible, it is often difficult. The ultimate determination of surface structure and changes thereof, however, can be deduced through combined studies using the Mossbauer effect and other physical methods. In agreement with the work of Dumesic et af.(165,216,217),evidence for the presence of a surface-sensitive magnetic anisotropy was also recently found by Berkowitz et al. (223). These authors observed strong pinning of the surface spins of 8-nm NiFe20, particles when these particles were coated with organic molecules, such as oleic acid. Particles coated in such a way reached at 4.2 K only about 75% of saturation magnetization in an applied field of 200 kOe. For samples in alcohol, no such decrease from saturation magnetization was found. Mijssbauer spectra at 25 K were recorded for bulk NiFe20, and for small particles coated with oleic acid and in alcohol. The spectra were taken in zero applied field and in a 68.5-kOe field applied parallel to the direction of the y-ray emission. The spectra of the first two samples are shown in Fig. 32. For the bulk sample, it is seen that the Am = 0 transition vanishes in the applied field, while this is not the case for the small particles. As seen from Table I, the Am = 0 transition should disappear for a hypcrfine field collinear with the applied field. For the samples covered with oleic acid, the presence of the Am = 0 transition was thus taken as evidence for a strong pinning of the surface spins, preventing alignment with the applied field. The Am = 0 transition disappeared for the sample in alcohol. The results of this study and the work on the small iron particles discussed above show the existence of large magnetic surface anisotropies (surface spin pinning) and that these anisotropies are sensitive to the presence of chemisorbed species.

MOSSBAUER SPECTROSCOPY APPLICATIONS

209

I00

0.95

I

'.

BULK

1.30 K, H ' O C O e

COATED

7.23 K , H=ohoe

COATED

1.23 K, H ~ 6 8 5 k O e

I

1.00

c

0 ._ ._

;

0.95

CI c L

I

I

1.00 095

090

.I0

-5

0

-5

-10

Velocity (mm s e t ' ) FIG. 32. Mossbauer spectra of bulk NiFe,O, and organic coated N i F e 2 0 4 particles with and without a magnetic field applied collinear with y-ray direction. Zero velocity is with respect t o metallic iron. Reproduced from Berkowitz et ul. (223) with permission.

C. CHEMISORPTION AND REACTION 1. Interaction of Surface Sites with Gases It is through the changes in the electronic structure of the chemisorbed species or the surface that Mossbauer spectroscopy can be used in the study of surface interactions between gases and surface sites. Because a Mossbauer spectrum represents a sum of the contributions from the various interactions present, in contrast to an average value, information may also be deduced about the nonuniformity of the surface for the studied chemisorption or catalytic process. In such studies, the Mossbauer isotope may be part of the catalytic surface and/or present in the chemisorbed species, as illustrated in the following examples.

210

JAMES A. DUMESIC AND HENRIK TOPS$IE

The chemisorption of ammonia and methanol on silica-gel-supported iron was studied by Hobson and Gager (224, 225). The room temperature Mossbauer spectrum of a sample containing 1.5 wt. Fe consists of three resolved peaks after reduction in hydrogen at 770 K (Fig. 33).The Mossbauer parameters of the two outermost peaks, taken as a doublet, are consistent with those of Fe2+,as expected in view of the high-temperature hydrogen treatment of the initially present Fe3+. The high state of dispersion of the iron on the support (greater than about 20%) seems to rule out the assignment of the center Mossbauer peak to a spectral singlet (due to the large expected quadrupole splitting). In addition, the observation that the spectral area under the leftmost peak remains nearly constant with chemisorption while the respective areas under the other two peaks change dramatically

I

I

-2

-1

I

0

I

I

I

*1

*2

i 3

velocity (mm sec-’1

FIG 33. Ammonia adsorption elfect on the Mijssbauer spectrum of silica-supported iron. Ammonia added: A-I. 0.5 x 10 mmoles NH,; A-2, 1.64 x lo-’ mmoles NH3; A-5, 2.99 x l o - ’ mmoles NH,; A-8. 4.31 x lo-’ mmoles NH,. Zero velocity is with respect to SNP. Figure according to Hobson and Gager (225).

~’

MOSSBAUER SPECTROSCOPY APPLICATIONS

21 1

suggests that the center peak is part of a spectral doublet, the other peak forming part of the leftmost resolvable peak. The isomer shift of this doublet then falls between the values expected for Fez+ and Fe3+, the significance of which will be discussed later. For convenience, the spectral doublet with the smaller quadrupole splitting and isomer shift will be considered as arising from “A sites” and the other spectral doublet from “B sites,” as proposed by Hobson and Gager (225). Upon chemisorption of ammonia, it is seen in Fig. 33 that the area of the rightmost peak grows at the expense of the central peak area, evidencing a conversion of A sites into B sites. A plot of the central peak area versus the amount of NH3 chemisorbed by the catalyst is S-shaped, suggesting that the initial ammonia uptake is the result of chemisorption on the support. In general, it is clear that the combined measurement of total chemisorption uptake with the in situ recording of Mossbauer spectra provides information as to the specificity for chemisorption on surface sites that do or do not contain Mossbauer isotopes, respectively. The differences between sites, both of which contain Mossbauer isotopes, are deduced through further study of the Mbssbauer spectrum. For the case in question, the spectral area of the A sites increases at the expense of B site area with increasing temperature of reduction. This observation, coupled with the fact that the A sites are sensitive to chemisorption, rules out the possibility of assigning one of these sites to surface atoms and the other to bulk atoms. It thus appears that both the A and B sites are on the surface. As mentioned earlier, the isomer shift of the A sites is smaller than that for the B sites, suggesting that the number of nearest-neighbor oxide ions around the central Fez+ ion is smaller for the former site (226). [Arguments postulating that the A-site iron is formed by rapid electron exchange between Fe2+ and Fe3+ do not lead to physically reasonable conclusions (225).] The A sites would thus be readily accessible to ammonia; and since the process of chemisorption increases the number of nearest neighbors around this site, the resulting Mossbauer spectrum would be shifted toward that of the B sites. In addition, increasing the temperature of reduction, which is expected to decrease the average number of nearest-neighbor oxygen ions surrounding the iron ions, should favor the formation of A sites relative to B sites, as experimentally observed. Thus, in the work of Hobson and Gager two different types of “surface sites” were observed and their interactions with ammonia studied. More recently, Gager et a/. (227) have also studied the adsorption of H2S and H 2 0 on silica gel-supported iron. The concepts involved in reaching the conclusion that these molecules are dissociatively adsorbed are similar to those described above. Finally, combining Mossbauer spectroscopy with IR spectroscopy, Hobert and Arnold (228) studied the interaction of amines with the surface of MFe,O, supported on silica gel. Changes in both the Mossbauer and IR

212

JAMES A. DUMESIC AND HENRIK TOPS$E

spectra upon exposure of the sample to amine indicated adsorption on the iron, and based mainly on the observation of a change in the iron isomer shift upon this amine adsorption, an electron transfer from the amine to the iron was shown. Surface site interactions, as measured with the Mossbauer isotope present in the chemisorbed species, are illustrated in the work of Karasev et al. (229, 230). In Fig. 34, the Mossbauer spectra of Sn(CH3), before and after chcmisorption on y-Al,03 are shown at room temperature. Upon hydration of the alumina at 770 K and subsequent chemisorption, the peak at -2.5 mm sec-' nearly disappears, leaving only the peak centered at the zero of velocity. Thus, the spectrum (Fig. 34) corresponding to chemisorbed Sn(CH3), is not a spectral doublet, but is instead composed of two spectral singlets. This is suggestive of two different adsorption sites. The peak centered at zero velocity is suggestive of a SnOJike species, which could be formed

I

I

I

I

I

\

z

b

_A-

1.08

106 1.04 1.02 1.o

-1

0

1

2

3

4

Velocity (mrn sec-'I

FIG.34. Miissbauer spectrum of tetramethyl tin before and after chemisorption on y-AlzO,. N is the count rate of the y quanta at the given value of V ; N o is the count rate of the y quanta when the absorption agent is stationary. [a) Tetramethyl tin, (b) tetramethyl tin adsorbed on y-Al,03. Zero velocity is with respect to SnO,. Figure according to Karasev et al. (230).

MOSSBAUER SPECTROSCOPY APPLICATIONS

213

only through alkyl radical detachment and interaction of the tin species with the oxygen ions of the y-Al,O,. Indeed, upon chemisorption of the Sn(CH,),, methane is detected in the gas phase. Since this site is not destroyed by hydration, the presence of hydroxyl groups in the site is suggested; the formation of CH3D upon Sn(CH,), chemisorption on deuterated y-AI,O, is consistent with this suggestion. The nature of the second site, which is destroyed by hydration of the y-AIz03, and its interaction with Sn(CH,), is not as clear. The authors postulate, however, that the corresponding spectral peak is the result of dissociation of the C-H bond and chemisorption of (CH3),SnCH2 on an aluminum site. Arnold and Hobert (231) studied the chemisorption of ferrocene, (C5H.J2Fe, on a silica surface from an alcohol solution. Ferrocene itself shows a symmetric quadrupole splitting. After chemisorption this doublet is no longer symmetric, and the authors explain this in terms of a Gol’danskiiKaryagin effect, where the iron atoms in the adsorbed state have a larger mean square displacement perpendicular to the silica surface than parallel to it. 2. Kinetics of Slow Processes Since the Mossbauer effect provides information about the chemical state of the resonant atoms in the sample under study, changes with respect to time of the latter can be studied with Mossbauer spectroscopy, if the time scale for these changes is greater than the time required to collect the Mossbauer spectrum. Clearly, this latter time is dependent on the precision with which the Mossbauer parameters must be determined in order to infer the desired chemical information; however, an order of magnitude estimate (at least for 57Fe)for this time is 0.1 hr. When this time scale criterion is met, the repetitive collection of Mossbauer spectra with time provides the rate and path of the observed chemical changes, as shown by the example below. It should be noted that this characteristic time can be effectively reduced in some cases by “quenching” the sample after various times of reaction and recording the Mossbauer spectrum at a temperature for which the reaction rate is negligible. Using this method, characteristic times of the order of 10 sec can be achieved, as seen later. A review of Mossbauer spectroscopy applied to the study of chemical reactions has recently been published by Vertes (232),in which much of the extensive work in this area from his group is mentioned. There are many examples on hydration and decomposition reactions, and also examples of reactions in solution. The latter are obtained by studying quickly frozen solutions and serve as demonstrations of how Mossbauer spectroscopy may be used in homogeneous catalysis.

FIG.35. Mossbauer spectra of an iron foil after various oxidation times. (a) 3.2, (b) 15.5, (c) 30.9 hr. Reproduced from Channing and Graham (233) with permission from The Electrochemical Society.

MOSSBAUER SPECTROSCOPY APPLICATIONS

215

FIG.36. Conversion electron spectrum of an iron film after treatment in oxygen at 620 K for 5 min. Figure according to Simmons et ul. (235).

The oxidation of iron is a chemical process, the time scale of which is sufficiently long for study using Mossbauer spectroscopy. Thus, Channing et a!. (233, 234) studied, using transmission Mossbauer spectroscopy, the oxidation of 9.5-pm iron foils at temperatures greater than 750 K in oxygen at atmospheric pressure, as shown, for example, in Fig. 35. For oxidation times less than 31 hr, Fe,O, is the major product, but significant amounts of a-Fe,O, can also be detected in the spectra. After 71 hr, however, the metal is completely consumed, and oxidation of the Fe,O, to a-Fe,O, becomes the major reaction path. After a total of -395 hr, the oxidation is virtually complete, with a-Fe,O, as the final reaction product. The oxidation of iron at temperatures between 500 and 700 K was also studied by means of backscattered conversion electron spectroscopy by Simmons et ul. (235).An example of these typical “upside-down” backscatter spectra is shown in Fig. 36. The authors found that the kinetics for oxide formation followed a parabolic rate law and that the resulting oxide formed at these low temperatures was nonstoichiometric Fe,O,. The kinetics of a similar oxidation reaction was studied by Pritchard and Dobson (236). These authors studied the oxidation between 450 and 560 K of a metallic-iron foil (0.02 mm thick electroplated with 1 mg cm-’ 57Fe) by deoxygenated water. The resulting Mossbauer spectra (at room temperature) showed F e 3 0 4 to be the only detectable reaction product, and from the ratio of the Fe,O, spectral area to that of metallic iron, the magnetite film thickness y can be calculated. Assuming that the rate law is of the form

216

JAMES A. DUMESIC AND HENRIK

TOPSGE

Mijssbauer spectra (providing the value of y ) taken during successive time intervals allow the value of n and the activation energy of K to be determined. Specifically, n was found to equal 3, and the activation energy of K was calculated to be 60 kJ. The value of n suggests that the reaction is diffusion controlled, and the measured activation energy indicates that a surface reaction involving the decomposition of ferrous hydroxide (the expected activation energy for this reaction lying in the interval from 40 to 120 kJ) may also be an important step in the overall oxidation process. In general, as discussed earlier, the chemical properties of small particles may be different from the properties of the corresponding bulk samples. An investigation of this effect in oxidation reactions was made by Topsq5e et ul. (237) in the study of the room temperature oxidation of Fe,O, particles 40 nm in size. The Mossbauer spectra of several partially oxidized samples and that of magnetite are shown in Fig. 37, in which it can be seen that oxidation is reflected in the ratio S of octahedral to tetrahedral spectral areas (see Section 111, A, 2). Specifically, the value of S for stoichiometric magnetite

-

I I . 1

-10

-5

0

5

10

Velocity (mm sed')

FIG.37. Mossbauer spectra of small Fe,O, particles exposed to air at room temperature and after CO,/CO treatment at 700 K. (a) and (b) are room temperature Mossbauer spectra of nonsupported magnetites exposed t o air for 80 days; (c) is sample (b) treated at 700 K with a CO,/CO mixture. Spectrum at room temperature. Zero velocity is with respect to a "Co in copper source. Figure according to Tops$c rl d.(237).

M~SSBAUERSPECTROSCOPY APPLICATIONS

217

is 2, while for the oxidized samples S is significantly smaller. This is the expected result if the magnetite is oxidized to y-Fe203,producing either a two-phase system or a single-phase solid solution. The distinction between these two possibilities is found in the kinetics of the oxidation. Both Fe30, and y-Fe,O, have the spinel structure, and the oxidation process results in the creation of cation vacancies, the concentration of which is given by (2 - S)/(5S 6). Mossbauer spectra taken after various oxidation times t therefore allow this process to be studied, and as seen in Fig. 38, a plot of the vacancy concentration versus t’” yields a straight line. This suggests that diffusion through an oxidized shell (y-Fe,O,) is the rate-determining step in the oxidation process, and thus that the oxidized sample is a twophase system. It is interesting to note that stoichiometric magnetite is found in nature, illustrating the change in oxidation behavior as the particle size is reduced to 40 nm. The oxidation of small particles of tin was studied by Suzdalev et al. (238). The procedure of study for this reaction was to heat the small particles, 30 nm in size, in air for 3 hr at increasingly higher temperatures spanning the range 290-770 K. They found that the resulting spectra could be reproduced by superimposing the spectra of SnO,, SnO, and p-Sn in various relative proportions depending on the oxidation temperature. The results of this analysis are shown in Fig. 39. While the formation of SnO, is observed over the entire temperature range, its presence is detected by X-ray diffraction only for oxidation temperatures higher than 670 K, indicating the

+

-

(daysn)

FIG.38. Dependence of vacancy concentration on room temperature oxidation time for small Fe304particles. Vacancy concentration = (2 - S ) / ( 5 S + 6). Figure according to Tops$e et al. (237).

218

JAMES A. DUMESIC AND HENRIK TOPSGE

A'

8-

6-

c

U

al

2 -

r

W

0

'

20

I

80

I

130

I

200

I

I

300 320

I

LO 0

500

T ("C) FIG.39. Spectral area changes for the Oxidation of tin small particles. Relation between the Mtissbauer areas and the oxidation temperature for the S n 0 2 peaks ( I , 1') and the SnO /I-% peak ( 2 . 2 ) . (1, 2) measured at 77 K ; (1'. 2 ) measured at room temperature. Flgure according to Suzdalev ct a/. (238).

+

amorphous nature of this oxide at lower temperatures. Above the melting point of tin ( 500 K ) but below 670 K, extensive SnO formation takes place, while above 670 K the major oxidation product is S n 0 2 . The presence of metallic tin, however, is observed at temperatures up to 670 K. This is in marked contrast to oxidation studies of metallic-tin foils at 470 K, in which an oxide film of thickness 17 nm was observed, i.e., the 30-nm particles should have been completely oxidized at 470 K. An effect of particle size in oxidation, and its study with the Miissbauer effect, is again demonstrated. For highly dispersed systems, slow chemisorption processes and surface compound formation can be studied using Mossbauer spectroscopy. Suzdalev et a / . (239) investigated the chemisorption of acrolein on iron molybdate samples (50 wt. '%; iron molybdate on aerosil; total surface area 76 m2 gm-'), as shown in Fig. 40. The spectrum (characteristic of trivalent iron) of the catalyst before chemisorption is only slightly altered by acrolein exposure at 0.6 kPa for 2 hr at 550 K. After 8 hr, however, new peaks in the Mossbauer spectrum appear, and exposure to acrolein for 40 hr at 620 K completely removes the initial peak characteristic of the starting material. Treatment in oxygen at 670 K does not restore the initial spectrum and, indeed, X-ray diffraction indicates that the original iron molybdate structure is destroyed with the formation of a new compound. The corresponding Mossbauer spectrum is most consistently interpreted as two doublets (peaks

-

M ~ S S B A U E RSPECTROSCOPY APPLICATIONS

219

110 108 -

106101102 100-

92

ln + 3 c

0 0 *0

jf

50 10

16

u

42

10

96 -1

-2

0

2

4

Velocity (mm set?)

FIG.40. Effect of acrolein chernisorption on the Miissbauer spectrum of a FeeMo catalyst. (a) Initial; (b) after adsorption of acrolein at 550 K for 2 hr; (c) adsorption for 8 hr at the same temperature; (d) adsorption for 40 hr at 620 K (pressure of acrolein during adsorption 5 Torr in all experiments). Zero velocity is with respect to SNP. Figure according to Suzdalev et a / . (239).

1-3 and 2-4), corresponding to trivalent and divalent iron, respectively. The former was suggested as arising from Fe3+ in an imperfect F e 2 0 3 lattice, while the latter may be due to Fez+ with a bond of the type Fe-0-C. A similar study of the reaction of acetylene with iron supported on quartz was made by Maksimov rt ul. (240).The Mossbauer spectrum before reaction with acetylene was a spectral doublet characteristic of iron silicate. After reaction at 1270 K for 50 sec the sample was quenched to room temperature, and in the subsequent Mossbauer spectrum a new peak was noted. The intensity of this peak increased with increasing reaction time up to 0.1 hr, after which time the intensity remained constant. In this case, it was only possible to study the rate of this surface reaction using a series of lowtemperature “quenches,” since the characteristic reaction time was the order of time required to obtain the Mossbauer spectrum.

220

JAMES A. DUMESIC AND HENRIK TOPS@E

Another class of chemical processes, the rate of which may be sufficiently slow for measurement using the Mossbauer effect, is that of decomposition reactions. One such example is the decomposition of FeC,04, as studied by Halsey and Pritchard (241).The room temperature Mossbauer spectrum of this compound is a symmetric doublet, which was not altered by subsequent heating to temperatures between 440 and 590 K (followed by quenching and collection of the spectrum). At this latter temperature, however, a slow decomposition reaction began. Specifically, after 2.2 hr at 590 K (followed by room temperature quenching) the decomposition was 50% complete, and after 4.2 hr at this temperature the FeC20, doublet was no longer visible. In this type of reaction, however, the decomposition product (as well as the decomposition rate) is of importance and amenable to study with Mossbauer spectroscopy. Halsey and Pritchard found Fe30, to be the decomposition product for the above study. Also, Suzdalev et al. (242)and Zhabrova et al. (243)have studied the products of the decomposition of the iron oxalates FeC,0,.2H20 and Fe,(CZ04)3.5H20.The interesting feature of these two investigations is the combined use of differential thermal analysis (DTA) and gravimetry (DTG) with Mossbauer spectroscopy. Peaks in the DTA and DTG curves at specific temperatures (indicating changes in enthalpy and weight, respectively, of the sample) bracket temperature intervals in which certain decomposition products are stable. A Mossbauer spectrum then recorded with the sample in one of these temperature intervals (or after room temperature quenching) serves to identify the corresponding decomposition product. For example, in the DTA and DTG curves of Fe2(Cz04)35Hz0 there are peaks at 470,530, and 665 K. Between 470 and 530 K the Mossbauer effect shows both FeC,04 and Fe,(C,0J3; at 530 K the latter decomposes to Fe,O, and Fe; and at 665 K the remaining FeC,O, also decomposes to Fe304and Fe. As dealt with in Section 111, A, 3, the Mossbauer effect is also sensitive to particle size effects, providing another advantage in its use for studying decomposition reactions. Suzdalev et al. (242) found that whereas the decomposition of FeC204 to Fe,O, produced large particles of Fe304, the Fe2(C,0,), decomposition produced Fe30, particles of size less than 10 nm. Clearly, if the particle size is time dependent and the rate of particle size change is sufficiently slow, this phenomenon can also be studied with the Mossbauer effect (244). When Fez(C204)35H20is heated in the presence of oxygen at 510 K, small particles of Fe,Oj are produced, the Mossbauer spectrum of which shows superparamagnetic behavior at 77 and 300 K. The fraction of the spectral area that is magnetically split then provides a measure of the iron oxide particle size (Section 111, A, 3), which was shown to be time dependent. The Fe20, particle size increased with time, reaching a constant value of 10 nm (162)after 1.5 hr of treatment at 510 K.

-

-

M~SSBAUERSPECTROSCOPY APPLICATIONS

22 1

3. Stationary-State Effects Knowledge of the chemical state of the catalyst (and its surface) under reaction conditions or after chemisorption is of importance in obtaining a detailed understanding of the associated catalytic processes. This chemical state may be different before, during, and after reaction and/or chemisorption. Here Mossbauer spectroscopy may be useful, as seen in the work of Firsova et al. (245).These authors studied the surface compound formation due to the chemisorption at 470 K of propylene and acrolein on a Sn-Mo catalyst (Sn: Mo = 2: 1 supported on aerosil). The Mossbauer spectrum, at 300 K, of the sample before chemisorption shows a single peak (Fig. 41) characteristic of SnO,; and exposure of the catalyst to propylene at 470 K does not change this spectrum. However, propylene chemisorption at 670 K, or exposure of an oxygen-pretreated sample to propylene at 470 K results in surface compound formation, as evidenced in the Mossbauer spectrum (Fig. 41). In the former case, propylene may react with the lattice oxygen of

F

0

178

L

n

s

z

I\

7

1

17L 17L 170

-

166

-

162 -

b

1

- 4 - 2

0

I

I

I

2

4

6

V e l o c i t y (mm sec-')

FIG.41. Oxygen-propylene interaction with a Sn-Mo catalyst as observed using Mossbauer spectroscopy. (a) Original; (b) after adsorption of oxygen and propylene. Zero velocity is with respect to SnO,. Figure according to Firsova et al. (245).

222

JAMES A . DUMESIC AND HENRIK TOPS@E

the catalyst, while in the latter case propylene reacts with the preadsorbed oxygen. During this surface reaction the tin is partially reduced from Sn4+ to Sn2+, as detected by the isomer shift of the surface compound, and importantly, no such surface reaction is observed for supported-tin (SnO,) samples. Thus, although molybdenum does not possess a Mossbauer isotope, its effect in the catalyst is seen through study of the Sn resonance. The spectrum of Fig. 41 is similar to that for partially oxidized H,C,-C-

0- Sn 0

/I

0

C--C,H,

ll

0

and it was thus suggested by the authors that the surface compound formed during propylene chemisorption involves bonding of the organic molecule through oxygen to tin. In contrast to the above catalytic system, the study of Sb Sn and Sb-Fe samples provides the possibility of Mossbauer effect observation for both components in the respective samples ('19Sn, "Fe, and I2lSb). Thus, Suzdalev et al. (246)investigated the chemisorption of propylene and acrolein on both a solid solution of stannic oxide in antimony oxide (Sb:Sn = 2: 1 ; 38 m2 gm-') and on FeSbO, (40 m2 gm-'). Before chemisorption, the Mossbauer spectra (antimony resonance) of these samples show peaks corresponding to both SbSf and Sb3+. From the respective spectral areas, the ratio n of the number of Sb5+ to Sb3+ ions present can be estimated (assuming equal recoil-free fractions at 77 K); for Sb-Sn, n = 2.3, and for Sb-Fe, n = 4.3. The effect of chemisorption of propylene, acrolein, and mixtures thereof with oxygen at 470 K is to decrease the value of n to 1.3 for Sb-Sn and to 1.9 for Sb-Fe, reflecting a reduction in the antimony oxidation state for these two catalysts. Simultaneously, however, the lI9Sn resonance shows that no change in the chemical state of tin takes place with chemisorption (unlike the Sn-Mo sample discussed previously), and "Fe Mossbauer spectroscopy evidences that only a small fraction of the iron ions are affected by the chemisorption (reduced to the divalent state) compared to the effect observed for a Fe-Mo sample. Thus, for these Sb-Sn and Sb-Fe samples the chemisorption of propylene and acrolein occurs mainly on the antimony ions, and the role of the tin and iron components is to displace the initial antimony oxidation state toward Sb5+, and to stabilize a value of n approximately equal to unity under reaction conditions. The interaction of CO and C 0 2 with the surface of Fe,03 (for -8-nm particles) was also studied by Suzdalev et ul. (247) using the Mtjssbauer effect. Due to the small size of these particles, more than 50% of the iron atoms appear superparamagnetic in the Mossbauer spectrum at room temperature, and the effects of chemisorption were investigated through analysis

MOSSBAUER SPECTROSCOPY APPLICATIONS

223

of this latter component. Specifically, after the catalytic oxidation of CO at 370 K over this sample and freezing out the product C 0 2 , the Mossbauer spectrum was unchanged; however, if the C 0 2 was not frozen out, two new peaks appeared in the spectrum. These same peaks, in fact, appeared in the study of C 0 2 chemisorption on these samples, and their presence may be interpreted in terms of surface carbonate formation. Unlike the CO oxidation results, the reaction of CO at 370 K with the Fe20, surface and freezing out the C 0 2 produced leads to the appearance of two new peaks in the Mossbauer spectrum (not the same peaks as those formed in the CO oxidation without freezing out CO,). The isomer shift of this new spectral doublet is indicative of Fez+, suggesting a surface reduction. Indeed, there was a one-to-one correspondence between the amount of CO reacted and the amount of C 0 2 formed (and subsequently frozen out); therefore, carbon is not present in the surface compound that gives rise to the new spectral doublet. Thus, the reaction of CO is with the oxygen ions of the catalyst, resulting in C 0 2 formation and reduction of the Fe3+ to Fe2+, the latter perhaps present in a compound of the FeO type. As expected, in view of this interpretation, the subsequent oxygen treatment of the CO-reacted sample results in a return of the Mossbauer spectrum to its original shape, i.e., the Fe2+ is oxidized back to Fe,O,. In the above studies, important information was obtained from the Mossbauer spectrum, even though the latter was not obtained with the sample under reaction conditions; however, for certain catalytic studies in situ Mossbauer spectroscopy may be advantageous, as illustrated previously and in the work of Maksimov et a / . (248).To study the propylene oxidation at 580 K over 57Fe-substituted cobalt molybdate, the Mossbauer effect cell was made part of a recirculation apparatus [described recently by Dumesic et al. (102a)I. A propylene-oxygen (1 : 10) mixture was circulated over the catalyst at 580 K, the produced acrolein was frozen out in a trap near the cell, and the remaining gas mixture was analyzed chromatographically. The spectrum of the sample at room temperature before reaction is a broad singlet, characteristic of Fe3+ substituted for cobalt in the cobalt molybdate solid solution. Heating the sample to 580 K, however, resulted in the formation of an additional spectral doublet, the area of which was -4% of that for the spectral singlet. When the proplyene oxidation reaction was started at this same temperature, the intensity of the spectral doublet increased by a factor of nearly two, and after the reaction was stopped by evacuation of the apparatus, the intensity of the spectral doublet decreased to its original value (-4%). The isomer shift ofthis spectral doublet was more positive than that of the original singlet, indicating a partial reduction of the iron at high temperature and under reaction conditions; as postulated by the authors, the high-temperature effect may be the result of an activated electron transfer

224

JAMES A. DUMESIC AND HENRIK TOPS@E

process between the cobalt molybdate and the iron, and the effect observed under reaction conditions may be due to electronic interactions between the chemisorbed species and the molybdate structure. For the present discussions, however, the important point is that the observed spectral changes could only have been detected with the catalyst under reaction conditions. For supported catalysts, the interaction between the catalytic material and its support was discussed in Section 111, A, 4. This interaction, however, may be dependent on the chemisorption processes and thc reactions that take place on the catalyst. In this respect, Maksimov et al. (249) investigated the reactions of C,H, and H, with supported (on quartz) and unsupported Fe,O, by use of Mossbauer spectroscopy. For unsupported Fe,O,, the reaction with C,H2 at 1270 K leads to changes in the Mossbauer spectrum (after quenching to room temperature, Fig. 42), which are indicative of mainly wustite formation (Fe, -xO) after 30 sec, an increased amount of u-Fe and corresponding decrease in the amount of wustite after 60 sec, and for longer times a decrease in the amounts of a-Fe and wustite at the expense

0.96

1

\I

-8

-6

-4 - 2

0

2

4

6

8

Velocity (mm se?)

FIG.42. MBsshauer spectra of Fe,O, before and after reaction with C,H, at 1270 K. Berore the reaction (a) and after the reaction with CzH2 for periods of (b) 0.5, (c) 1, (d) 3, and (e) 6 min; spectra (a)-(e) were obtained at 300 K. Zero velocity is with respect to SNP. Figure according to Maksirnov er a/. (249).

225

M~SSBAUERSPECTROSCOPY APPLICATIONS

A/----

A

-

0 -1

x -2

A -3 0

-4 X,

20

-

I

4

6

10

Reaction time (min)

FIG.43. Iron phases observed during the reaction of C,H, with Fe,O, at 1270 K. ( 1 ) Fe, -xO, (2) a-Fe, ( 3 ) Fe,C, (4) paramagnetic component. Figure according to Maksimov et nl. (249).

of carbide formation (Fe,C). From the Mossbauer spectral areas, the respective amounts of these various iron phases can be measured after specific times of reaction, and the results of such an analysis are shown in Fig. 43 (assuming equal recoil-free fractions for all phases). It is therein seen that a stationary-state phase composition is reached after 0.1 h r of reaction, corresponding to a-Fe, Fe,C, and a small amount of an iron species that gives rise to a spectral singlet near the zero of velocity (Fig. 42). In contrast to this behavior, the reaction of C2H2 with the supported Fe203at 1270 K reaches stationary state after lo2 sec, and the only reaction product is that species which corresponds to the spectral singlet near zero velocity. The reduction of Fe203 by H2 at 1270 K adds to the understanding of this support effect. Here, reduction of unsupported Fe20, led only to a-Fe; reduction of supported F e 2 0 3(which had previously reacted with C2H2,as described above, followed by oxidation of the iron to Fe,O,) led to formation of both a-Fe and the Mossbauer singlet species, and reduction of supported Fe,O, (which had not previously reacted with C2H2)led again only to a-Fe. Thus, it was concluded that the reaction of supported Fe203 with C,H2 leads to the formation of carbonaceous compounds on or in the support (quartz), and that these compounds interact with the iron in a reducing atmosphere (H2or C2H2)to form the Mossbauer singlet species. For further reference, much of the Russian literature pertaining to this and to Sections 111, C, 2 and 111, C, 3 has been reviewed by Suzdalev (250).

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-

226

JAMES A. DUMESIC AND HENRIK TOPS$E

4. T h e Miissbauer Isotope as a Chemical Probe Because all of the elements do not possess convenient Mossbauer isotopes (Section 11, A), the concept of the Miissbauer isotope as a “probe” of its chemical surrounding becomes important for the more general application of the Miissbauer effect to heterogeneous catalysis. Several examples of this concept have already been mentioned in this paper. In the study of Pt-Fe alloys by Bartholomew and Boudart (195) (Section 111, B, 1, a), 57Fecan be considered a Mijssbauer probe of the surrounding platinum structure; H2 and O2 chemisorptions and treatments, as typically conducted in a titration of supported-platinum catalysts, were thereby studied with the Mossbauer effect using the 57Feresonance. The study of Sn-Mo catalysts by Firsova et al. (245) (Section 111, C, 3) may also be considered an example of how the chemistry of molybdenum is reflected in the ”Sn resonance; the chemisorption of propylene at 670 K on this catalyst led to a partial reduction of the tin from Sn4+ to Sn2+,while on SnO, samples no such effect was observed. The studies of Tops$e er al. (95) and Tops$e and Boudart (96) (Section 111, A, 2) also illustrate how information about elements that do not possess Mossbauer isotopes (aluminum and lead, respectively, for these two studies) can be obtained through the Massbauer effect of neighboring resonant isotopes (57Fefor these two examples); however, the Fe concentration was high in these two studies (90 wt. % or more), and strictly speaking the iron was not a “probe” of the chemical structure. In a recent study, Tops$e and M$rup (25Ia) doped a Co-Mo-Al,O, hydrodesulfurization catalyst with 57Fe(FeS7/Co<< 1) in order to obtain structural information about this industrially very important catalyst system. They studied the spectra after calcination and after different stages of sulfidation with mixtures of H,/H2S. A large fraction of the iron was found to be located close to the alumina surface in the calcined sample, and after H2/H2S treatment about 40% of the iron was sulfided (even at room temperature) and was present in tetrahedral coordination. These results indicated that the “monolayer model” rather than the “intercalation model” [see Schuit and Gates (252)]is valid for the sulfided catalyst. However, in a later study (25Ib) where 57Co-Mo-A120, was used as a source it was found that 57Fedoes not always, even in low concentration, occupy the same sites as Co. This illustrates some of the limitations of the use of Mossbauer isotopes as probes of their surroundings. Another catalytically interesting system is that of supported-nickel particles. In the work of Lindquist et al. (253),57C0was incorporated into small nickel particles and used to study their magnetic properties by means of the 7Fe resonance. The use of a Mossbauer isotope in probing its surroundings is also clearly illustrated in the work of Garten and Ollis (254).These authors investigated

M6SSBAUER SPECTROSCOPY APPLICATIONS

227

the reduction and chemisorption characteristics of four catalysts; catalyst A was 0.05 wt. % Fe on q-A1203, catalyst B was 2.2 wt. % Pd on q-A1203, catalyst C was 0.05 wt. % Fe and 2.2 wt. % Pd on q-A1203 prepared by impregnation of catalyst B with iron, and catalyst D was a physical mixture of catalyst A with a catalyst of 2.37 wt. % Pd on q-AI2O3. The Mossbauer spectrum of catalyst A before reduction (Fig. 44a) is a doublet characteristic of Fe3+,and room temperature evacuation followed by H, exposure at this temperature did not significantly change the Mossbauer spectrum (Fig. 44b). Reduction to Fez+ was, however, achieved by H2 treatment at 770 K (Fig. 44c); when this sample was then cooled to room temperature, at which temperature evacuation and exposure to 0, were performed, the Fez+ was not converted back into Fe3+ (Fig. 44d). Finally, a more severe reduction

I

-2.0

0

2.0

l

l

4.0

Velocity (mm sec-'1

FIG.44. Mossbauer spectra of iron supported on A1,03 catalyst A (0.05 wt. % Fe/q-AI,O,). (a) Catalyst as initially prepared. (b) Exposed to H,, 273 K. (c) Evacuated 1 hr, 770 K, reduced in H,. 4 hr, 770 K. (d) Exposed to Oz, 273 K. (e) Reduced 2 hr, H,, 970 K. Zero velocity is with respect to a 57C0in chromium source. Figure according to Garten and Ollis (254).

228

JAMES A. DUMESIC AND HENRIK TOPS$E

of the catalyst (H, treatment at 970 K) did not change the Mossbauer spectrum; in no case was metallic iron observed. The Miissbauer spectra of Fig. 44 thus serve to “characterize” the FeA1,03 system for the above H, and O2 treatments; for catalyst C , deviations from these characteristic spectra then indicate Fe-Pd interactions. The Mossbauer spectrum of catalyst C before reduction (Fig. 45a) is similar to that for catalyst A, but exposure of catalyst C to H2at room temperature led to Fez+ formation (Fig. 45b), in marked contrast to the behavior for catalyst A. Evacuation and reduction in Hzat 770 K (followed by cooling to room temperature) produced the spectrum shown in Fig. 4%. In addition

FIG.45. Mossbauer spectra of Fe-Pd supported on A1,0, catalyst C (0.05 wt. % Fe, 2.2 wt. y:, Pd/q-AI,03). (a) Catalyst as initially prepared. (b) Exposed to H1,273 K. (c) Evacuated 1 hr, 770 K, reduced H2, 4 hr, 770 K. (d) Exposed to O , , 273 K. (e) Exposed to H,, 273 K. Zero

velocity is with respect to a 57C0in chromium source. Figure according to Garten and Ollis (254).

MOSSBAUERSPECTROSCOPY APPLICATIONS

229

to Fe2+ (which was the only species present for a similar treatment of catalyst A), an intense spectral peak at -0.3 mm sec-' is also evident. Subsequently, evacuation and O2 treatment at room temperature led to additional spectral changes (Fig. 45d), which may be interpreted in terms of Fe3+ formation. Again, for catalyst A no such effect was observed. Finally, evacuation and H 2 treatment at room temperature produced a Mossbauer spectrum (Fig. 45e) which was similar to that of Fig. 45c, illustrating the reversibility of these room temperature chemisorption effects. The behavior of catalyst D with respect to the above treatments was intermediate between catalysts A and C. That is, the Mossbauer spectral changes followed qualitatively those observed for catalyst C, but much more cf the iron was in the Fez+ state after treatments (c)-(e). The effects of palladium on the 57Feresonance are thus quite apparent. Perhaps the most dramatic effect is the initial room temperature reduction from Fe3+ to Fe2+.This is a direct consequence of the ability of palladium to chemisorb hydrogen dissociatively, which subsequently migrates to the Fe3+ followed by reduction of Fe3+.The study of catalyst D shows that the migration is not mainly over the A1203 surface, but takes place between palladium and iron oxide particles that are in direct contact. The hightemperature treatment of catalyst C then results in Fe-Pd alloy formation (the intense peak at -0.3 mm sec-'). In these alloy particles, the effect of palladium on the 57Feresonance is again apparent. In these particles, iron undergoes reversible oxidation and reduction at room temperature, quite unlike the behavior for iron alone. Similar behavior also seems to exist for the Sn-Pt system in which reduction of the tin (from Sn4+ to a Pt-Sn alloy) by hydrogen was observed using Mossbauer spectroscopy with the "'Sn resonance, to be facilitated by the presence of platinum (255). These results thus illustrate the feasibility of using a Mossbauer isotope as a chemical probe, thereby extending the application range of the Mossbauer effect in the study of catalysis. IV. Concluding Remark

Mossbauer spectroscopy is a technique that spans many different disciplines. We hope that this has been reflected in the present paper, in which we have discussed catalytic problems in terms of the physical principles that form the basis for Mossbauer spectroscopy. Certainly, this technique has great potential as a tool in catalytic research. However, in order to take full advantage of this potential, strong ties between researchers in catalysis on one hand and physicists and theoretical chemists on the other hand are necessary, and this will undoubtedly lead to further Advances in Catalysis.

Appendix I: Nuclear Data for Messbauer Isotopes

Q and j~ data from (/-7, 30); A ( r 2 ) data from ( 1 - 7 . 85);G refers to ground state; M refers to excited state; CE refers to Coulomb excitation; a source enclosed in parentheses refers to a possible y-ray source, but one that has not been referred to widely in the literature.

Y3.1

83.9

4.34

I S6

6.6 x 10 1.39

'

0.344

17.34

1.41

1.87

0.0386

12.29

0.M I

2.10

0.29

81.00 86.79 25.65 43.84

2.65

-

7 jj

0.219 U.641

0.540

-

0.0539

0.102

4.9

0.380 6 78

0 294

0.953 0.283

G M

-

M

0 1.76 2.36 2.37 2.36

74.51

1.86

1.10

0.067I

2.36 I .30

80.7

2.16

1.54

0.250

0 ~

1.76

15.5

0.227

G

- 0.0035 G

-

73.39

G M

0.594 0 0

~

1.59

G M

0 - 1.12

--"0.114 G Irl 1.511 G

0.145 0.416 -~

G M G M

0.888

0.454

2.58 2.44 O 0.714 -0.472 0.558 -0.472

G

M

-

M Tr M

-0.472 -0.400 O 0.74 O 0.84

3.10

-

ti M

18.2

G

M

M G M G M G G M G M G M

-

-

-

M

G

-

7.76

M G

G M G M

0.035 ~

M

G

1.770 0 0

-

0 5.510 105. 105. 3.430 -.

0.988 -

__

12.4 6-85

M G M G M G M G M G M G M Ci M

17.0 x 1 0 ~ G M 1.280 ti M 00950 G 212 M G -

5.57 1.42 0 0.488 457 540 0150 -

0.543

'"€'u,

4.98 hr

"'Pt,

18 hr

'""%a, 39 hr

M

c

'"Ba, 1.2 years

M G M G

-6"rb, 72.1 days

"'Tb, 6.9 days

M

c Irl G

0.w

M

0

G

0

G

-

M G M

0.562 0 0.069

h.l G M

0 0.430

'"'C'd. 6.5 h r

CE I1"'Ho. 2.5 hr) '"Tb. 6.9 days Dy(n,;)

"'Ho,

37 min

91.5

2.74

1.97

0.487

-

0 -

80.56

2.10

I .87

0.236

- u.4

0

- 2.0 79.32

2.02

33.5

0.0772

2.83

~

~

79.8

2.04

1.79

0.45

-

0 ~

79.3

1.99

I .80

0 243

0

~

G M G M G M

G M G

M 21.64 83.37

0.166 2-44

1.30

4.69

0.142 0.0670

30 - 4.2

114 1.50 2.Y0 _ ..

97.43

3.33

13.4

0. I82

- 120

2.90 -.

103.2 14.41 136.3

3 ?'l O.lY6

17.5

0.698 0.194 0.230

0.0544 2.57 0.0430

- 13i1

-25 -

5.29

1.88

0.38

2@

I .25

19.0

0.0999

105.3

n8.w 54.5

64.0

2.60

0 500

0.344

- 10

3.84

2.24

0.249

-

2.72

1-02 1.40

1.MI

26.8 0.00929

0.30

36

0.0907 0.448

21

0 0.66 0 0.62. 346 2.58 1-53 1.80 1.53 3.21 1.53 204

G M G

G M ti

M G M G M

c;

0.0904 C

M G M G M G M G M G

-0.155 M 0.0904 G 0.88 hi 0 G 0.73 M -0.258 G M -0.258 G -0.55 M -0.258 G

0

1.59 0.35 1.59 1.45

M

M

n

ci

1.73 2.04

M ci

-

M

2.04 3.63

G

M

0.13 0

0.60 -0.336 -0.336 -0.51

M G M

0

G M G M G M

5.74 0.460 ~

-

0061

0

G

M

0.41

17.5 21.0 3-20

20.8

0.960

G M

174 3.14 0

G

28.9

M

0

G M G M G M

0.500

O ~

0.600 -

-

0.16X

15.9 3.49 2.01 2.66 0 6.00

-

204.

c;

-

-

1.34

M

M

~

-

C

. ..

0.522 10.4

-

-

~

6.06

0 3.67 0 0.396 0.020 0 0.436 0

0 ~

M

ci

M

-

M

G

G

0.016

M ti

O -

~

M

0.187 0

-

86.54

-

0

1.59

~

0.632 -0.565

c;

-

60.00

M ti M G M

0

G %I G hl

2.90 0.524

__ 123.1

G

0 0.70

O.6W

1480. 2640.

M ci

G M G M G M

G M G M

11.6 8.66 0.369 0.435 0.1 11 0.232 2.01 2.67 3.05 5.23 0.272 2.65 0 0.298 0.0210

-

G

'@"Ho,37 min

M b " H ~26.9 , hr

G M G M G M G

CE ['"Ho, 3.1 hr) '&'Trn, 85 days

G M

15'Gd. 120 days ["'Srn. 93 years)

G

"'Sn. 47 hr (153Gd, 242 days) 'j3tid3 242 days

M

G

I

CE

M

G M G M G M G

"Sin. 47 hr (153Gd. 242 days) "Co, 270 days "Co, 270 days ""Eu,

Ibyears

M G

'"Eu, 1.81 years

M

0.563 1.19 0.103 0.0520

G M G M

0

G

0.455 M 0.0220 G M 53.3 G 81.0 M

'"Eu, 1.81 years '55Eu, 1.81 years r5%y. I S days I5'Eu, 15.2 hr

"'Eu. 15.2 hr

(continued)

Appendix I-continued

Isotope

E, (keV) 79.51 75.3 13.3

ER x 10' (eV)

2.15 1.91

0.129

2r. ( m msec-l)

u x 10"

1.50

0.278

1.45 0.005 16

@m2)

A(?>

x

(lrn') 1 .o

0.4

0.21 I o.o(l41 7

-_

IO'

Q x loz4 @m*) 0 1.5 0 1.6 -0.18 _.

67.03

3.48

2.14

0.229

33

-0.18 -

88.36 1 13.0

2.38

3.86

2.22 4.84

0.253 0.075

1.6

-_

0 -

93.17 93.33 32.19 57.60 27.75 82.m

129.40

2.62 2.60 0.277 1.40

0.321 1.91

4.71

1.96 1.96 42.5 2.50 0.587 0.874

23.8

0.252

0.7

0.8

0.255 0.0094 0.214

0.404

-

-11

14

0.0154

5.6 1 .z

0.0569

0 - 1.95 0 - 1.94 0.50 0 -0.78 -0.71 -0.55 - 0.68 I50 1S O -

13.03 138.9 29.4

1.4R

5.37 1.14

0.595 24.6 2.1 8

46

0.0306 0.0584 0.290

- 0.5

-1

I .5 0 1.5

-

- 0.066 -.

P (nuclear magnetonsl

G M G

O 0.77 O

M

0.61

G

-0.879

M G M G

M G M G M G M G M

G M G M G

M G M G

M G M G M

G M G M G

First estimates of Ri R,'

0.0484

0.61

1.0 O 0.55 O 0.70 -0.557 -

2.81 2.02 2.62 2.80 0.145 0.542 0.145 0.58 0.159 0.470 0.159 0.70

0

5.43 0.02 12 -

M -0.879 O

R,'

Ci M

0.m

G M G

0.053 .I

0

6.33 1130.

C; M

0.0250

G M

0.0284

G M G M

G M G

-

4.10 0.540

0.540

G

-

0

M G

-

-

M G

O 4.61 O 4.63 0.180 2.34 2.13 14.6 18.0 900

0.0028

G M G

n.736

0.001

M

- 1.30

G

-

M

0.0268

0.1 1

CE

'%a. 5 hr

M G CE M ('%a, 5 hr) ci 1'6"Lu, 3.7 hr M G ''7Lu. 6.7 days

n. I 73 0

G

'"W,21.5 days

0.284 0 0.361 0.0380

M G

leomHf,5.5 hr

G M

G M G M

G M G

G M

14.9

G

0 -

M G M

0.44s

G

-

M

0.190

0

M ci

CE

G M

-

0.210

0.578

G M G

M

M

M

I2 10.

-

M

G

G

0 0.609 0 0.527

M

-

0.366

G M G M

-

U

Source

R3'

-

M

M G M

20'Ti 73 hr

G "'Te, 109 days M G lx9T'e, 33 days M G I "Pt, 3 days M (1910s, IS days) , days O.Oo40 G 191oS15

1.84 1.32 15.2 16.2 0.190 0.71U

0.018 M 0.345 G M 1.02 O.Oo40 G 0.0190 M 1.92 G I

M

ly3Os,31 hr

'930s, 31 hr 99K(n.~)

83Kr 16'Ho

'"Lu 14'Nd '45Nd O'Ni

2J'Np lg60s '"0s

'ayOs

9.40 94.70 113.8 67.25 72.50 67.40 59.54 137.2 155.0

36.30

0.0572

2.92 3.97

I .68 1.95 4.00

0.803 5.43 6.W

0.374

0.198

130. 24.0 0.138 5.24 0.781 0.0689 2.37 2.54 15.1

1.172 0.0355 0.07 I 5 0.038 I 0.0592 0.721 0 306

0.284 0.280 0.0115

12.5

-

1890~

lP0Os 23'Pa 14'Prn ld

Pr

195Pt

195Pt

239Pu

69.59 95.3 186.7 84.2 91.03 145.2 9R.84 129.8 57.3

1.38 2.58 9.87 1.65 3.03

8.03 2 69 4.64

0.737

2.40 9.57 3.1 I 8.33 1.17 1.02

16.3 3.40 47.3

0.0842 0.0058 0.336 0.370 0.0692

3.2

-S

- 27 -

-2.14 -2.10

-0.12 -

0.0742 0.0999

15.68

-

-0.254

G M G

M G

M G

M

M

4.1

G

2.75

4.1

M G

M

1.45 0 0.562

G

0

M ci

0.58 0.656 0.226 0.656 0.977 0.656

0.162

G

O -1.5 0 - 1.36 0.91

O

M

0.91 -0.72 0.91

G M

-

G

M

-

0 0.63

0

G

M

-

0.70 6. -0.0589

G

-

8 -11

-18

-

-

-0.20

-

- 1.18 -

-

-0.967 -0.939 4.09 4.05 2.22 1.90 -0.654 -0.654 -0.319 -0.749 0.43

G M

-0.254

-3.95

0.107 0.061 1

-

-

'4 hr

1s90s

0.260 0.459 2.7

'3 M G .M G

M G

G M G

G M

G

2.18

M

II -

G M

G

0

G

G

0.606 -0.62 0.606 0.90 0.20

M

-

M

-

55.2

0.03Y0 0.0264 0.0050 -

0.466

M G

M G

0.090

0.900

-

0.28R 0.0480

-

M G

M G M

2.90

G

-

M

1.33

G

1.W 432. 432. O 1.99

M

0

1.49 0.720 0 2.35 1.86 0.430 0

G M G M G

M C

M G

M G

2.84 24.3 0.170

0

O

-

49.0 47.6 0.0310 0.0310 0.0770 0.066 6.65

-

0.163 0.0790 1.34 0.770 63.3 33.4 0 0.163 0 0.139 0.113 0.0390 0.371 0.552 0.068

G "3r. 2.4 hr M ("'Rb, 83 days) G laSDy,140 min

M CE 1 i75Hf,70 years) M (*'?'Yb. 101 hr) G '"Pm, 17.7 years G

M G

ti G

L86Re,90 hr

ci

''$Re, 16.7 hr

M G

M

' %. 13.3 days

M G

I "It,

G

G

M

2.29 3.14

G

2.M

G

M

1.77 M 0.0355 G 0.0363 M 0.130 G 0.193 M 0.0070 G

G M

-

' %, 13.3 days

G

0 0.102

M

Am. 458 yearb

G M

G M

G M G

t

M

M

-

h'Co, 99 mln

M

-

G

'"'Prn, 17.7 years

M

M

-

-

0.288

G

-

11.8

0.880

M G

M

-

M

M

-

-

0.240

G M G

G

-

ci M

-

M

-

0.0456

M G M G M

60.3 107.

-

G M

G

-

-

M

2.58 3.54 4.14

M

21.8

13.3 days

1901r, 12 days

M 131Th, 26 hr '"'Nd,

1 1 . 1 days

M

M

141Ce,33 days l9'"Au, 183 days

"'Au.

113 days

"'Cm, 30 years "'Np, 2.4 days

(continued)

Appendix I-continued ~

~~~~~~

~

~

Isotope '"Re

E, Ea (keV) 134.2

x

2rn

10'

5.17

(mm sec-l) 204.

u x (cm2) 0.0540

A(?)

x

(fm*i

lo3

Q x loz4 (cm2) 2.6

99Ru '"'Ru

I2'Sb

"'Sm rosSrn

", '5'Sm

89.30 127.2 37.15 122.1 22.5 121.8

4.33

3.m 0.6 I 2 5.45

o.Jn? 5,27

0.1 49 0.0037 I 2.10 2.80

1.71 1.61

0.143 o.i)8m 0.197 0.06 IS

0.07I I (1.75

P

3.20

0.05 0.15

G

..

Ci

-0.623 -2.28 - 0.68

-

M

-.

ll9Sn

81.99 23.83

2.?4 0.256

1.1 1

0.647

0.301 1.41

'"Ta

6.23 136.3

0.0115

5.51

0.00646 36.5

1.68 0.0659

-0.18

Cj

-031

M

-0.31

0.060 0.50

G

0

G M G M G M G M G M

-0.670 -0.622 O 0.832 O 0.778

0

-

0

2.70 2.9 2 70 -

'"Th 9'Tc

5R.0 140.5

1.14

10.7

36.3 10.1

0.0983

0.0877

232Th

35.46 49.75

0.540

0.573

4.94 15.9

8.42

0.022 5

8.33

G M

0.34

G M G M

o.2trn

0

0.0164

-0.19 0

0.212

M

-

-

169Tm

M

2.0 -

I2'Te

0.207 3.36

G

- 0.06 Is1Ta

M

-~

- 0.26 -0.35

-

'"Sm

(nuclear magnetonsi

G M

-

0 - 1.1

G M C

M

2.48 0,807

- 1.046 0.685 2.35 5.22 2.35 1.22

~

~

~~~~~~

First estimates of R ,

P

G Ir2 G M

Ci

-

8.06

320. I.YO

ci

.-

0.192

M

G M

0.570

G M G M G

0.0616 1.46 -

1.44 1.94 0.230 0.390 0.670 5.61 0

c

-

M G M G M

31130. -

0.230

-

M __

1.9 1.5

G M

5.68 3.2 -0.887 0.60 O -0.231 0320

G

0.104

M

-

G M G

-

0

0.470 0

-

?;I

G tvl

0.410 -

0.1 S8

0.0444

10.2

M

O 1.68 28980

0 6.78

'"'Rh. 3 years

M G

O

'aR h. 16 days

M G

-

-

M G M

Ci M

G

G M G M

G M G M G

M G M G

M

I87

0.01 10 G h.l 4.42 G

M G

-

-

M

G

1.62 4.87

Source

R3

00410 G .-

M G >I h4 Ci

R''

R,'

136. 41.4

4.07 3.00 0.223 0.123 I .64 1.53

0 0.401 0 0.807 6.40 4.19 5510 I2240 0.0450 0.0230 0.085 0.067 0.378 0.113 0.478 0.323

W. 14 h r

M ci M G

M G

I1'"Sn. 76 years ,I:

Ell. 22 days

"'Eu. Ill6 days

M

G

' "E u, I2 years

tvl

G

CE

M G

''9mSn, 250 days

M G

'"W. 140 days

31

G ??I

''lH', 140 days

G

'"Gd, 18 hr M ("'Dy, 144days) ti '"Mn. 67 hr M

G M

>'
0

G

(33611. 2 x 10' years)

-

M

0.311 0.700

G M

CE ' "'fr, 9.4 days ~ ' " ' Y b ,32 days1

45.3

0.466

25.7

0.00983

.

0

G M G

-3.10

M

0

~~

- 2.95

44.7 103.

0.451 3.17

24.9 2.21

0.009I I 0.262

0.3 -

-

- 1.87

100.1

2.96

2.00

0.252

0

-1

- 1.87

46.48

0.634

12.2

n

0.552

G M

G M G

M

o m

2-88

2.83

0.0837

0 -

I 11.1 122.5

3.61 3.33

1.95 2.21

0.261 0.314

39.S8

Oh52

6.84

0.233

80.16

2.63

6.82

0.0718

84.26 66.74

2-24 I-40

2.03

4.71

0.239 0.0900

78.67 76.5 R2.1 93.31

1.81 1.93 1-81

3.06

6.98

2.12 1.93

2.03 I .67

o.ono315

0.131 0.208 0.207 0.202 0.173

M

11.16

0

G

M

0.35

1.71 0 0.74

G

0

G

1.5 -

2 1.8

-0.41 -0.12

0.7 0.7 0.6 -

-

M M

0 0

G M G

-2.14

M

0

G

O 0.526

G ht G M G M G hi

O

0.59 O 0.634 -0.777 O.68 0.691 -

G

M 0 M G

-

0.0334

O

G

M G M G

M

1.013

u

0 2.16

G

O

G

M

n

G

2.23

M

0

G

0.667 0 G 0.675 M O G

-

M

-

0.18

G M

0.876 -

M G bl

0 I .09 0 1.20 3.55

G M G M G M

0

G

4.04 0

M ci M

-

-

G M

0

G

3.41

M

0

(;

1.18

M G M

0

omI2 0.0086 0.0270

0

0.650 -

0.090 G

-

M

G

-

0.0022

Ivl G

-

0

M

~-

0.117 G -0.62 M 0.117 Ci 0.93 Ivl

0.670 0.492 0.349 0.492

- 1.59

75.89

C;

O O 0.50 -

0 G 0.0420 M -

G

-

M

0

6537 years 22 hr '"Pu, 3.8 x lo5 years 'J"Pu.

'%p.

'""Ta, 8 hr

G

'"Ta, I I5 days M 0.0070 ci 153 Ta. 5.1 days 0.0390 M ['BJRe.71 days) 0.0390 ci '"'Ta. 5.1days 0. I565 tbl !'"Re. 71 days) G "'Re, 38 days 0 0.257 h.1 ('s41*IKe.IbY days) 0 (; '"Re, 90 hr 0.221 M 0.271 G '"'1. 1.7 x 107ycars 0.237 M 0.119 G I 3 ' L 8 days

0.248

M

0

0

6.15

M

0.4 I 5

w

0 6.20 0

G

0

G

M ci M G M

0.410 0

w

00362

0

0.0276

2.19 0

-

-

M

G ' ' I Lu. 8.3days hi ("'Trn, 1.92years) G "'Lu, 6.7days

0 5.40

0 0244

G

M G M G M G M G

0.0738

0.0292

0 -

2640. -.

-

0 0.370 0.148 0.I05 0.289 0.594

G \I G

1

'"Tm, 130 days

"'Tm, I .92years

M

171

L.u, 3.6years

G

CE

2810.

M G

"Ga. 78 hr

-

$1

Appendix II : MBssbauer Isotope Feasibility for Catalyst Studies

A source or a temperature enclosed in parentheses means that these have not been widely used. Final eslimates of R ,

R,

Temperature of study

Source half-life, I

g2

R3

-.

6" Isolupc

"'Ag 24'Am "'Au "'Ba I 3JCS

140Dy '"Dy 16'Dy

16'Dy

"'Dy '64Dy "'Er 166Er 16?Er

(kcV) ~ < l d a ! I d a y < r < 5 d a y s 93.1 83.9 77.34 12.29 8I.M) 86.79 25.65 43.84 74.57 80.7 73.39 91.5 80.56 79.32

"'Er

79 8 793

'"Eu

21.64

'"Eu "'Eu '"Eu "Fe i'Fe

83.37 97.43 103.2 14.41 136.3 123.1 60.00

'68Er

"'Gd "'Gd

5days
rr-!Odays

6 5 hr 3.98 hr 18hr

Liquid Liquid Room He N 1 kmp.


>I.cl(l

>I0

*:I

>1.<10

>I0


-1

>5

Number of publications in 1971xnd1972

X X X

39 hr

7.2 ycars

X

72 days 6.9

X

CE 125 hrl

X

6.9

X

Nucl. R x n 37min 37min

X

x X

27 hr

X

CE (3.1 hr)

X

85 days

CE

X X

IW 41 hr

47 hr

days

(93 years) (242 days) 242 days

X

1?42 days)

X

270 days 270 days Ib?ean 1.syears

X

1

0~33

( X )

0.3

0,3 10 -

x

X

x

-

0.2

X X

-

I

0.2

-

-

-

10 -

-

-

0.2

2

3.

6

5 x

0.4

5. -

3.

- 0.3 0.02

920 1

86.54 105.3 88.97 54.5 64.0 79.51 75.3 13.3 67.03

i .8 ycars ;.8 vears

15

-

X

-

X

100

CE

X

5hr

X

CE

X

138.9

-

X

-.

-

-

-

-

0.2 0.3 0.4 O W

1.h

-

I1 hr IW days

0.06 11.8

X

X

15

X

31 hr 31 hr

(101 hr)

170 days!

X

I8 years

X

X

2.4 1

1.

6

2.

I -

0.R 0.1

0.m

12

X

0.3

0.I

-

X

-

X

.-

.

..

-

8

X

0.M

x

-

x

2 1 -

X

-

-

X

-

0.01

2 8 14 2 2

60

X X

I2

1.

150

X

33 days I83 days I83 days 30 years

1 I

I.

13 13 13

X

28

0.2

38

-

7

4

0.2 0.I 0.1 0.4

I1

I

0.08

0.7 0.5 0.2

X

26 hr

3

50

0.6

x

458 year5

5

0.03

0.03 0.3 0.1

18 yean

90 hr

.~

38

(83 days! -

~

om2

0.116

0.2

X

-

1h.

5

x

.~

0.2 0.02

0.07

X

2 2

I,

6 3

33 days (15)

~

f 200

0.6

1.b X

3

0.5

390

_. -

I 1 -

80

0.6

0.2

X

6.7 21

-

900 I .9 2.2

29.4

KucI. Rnn. 9.40 2.4 hr 94.70 130min 113.8 CE 67.25 72.50 67.40 99 min 59.54 137.2 155.0 I? hr 36.30 69.59 92.3 18h.7 84.2 9 1-03 145.2 98.84 129.8 57.3 134.2

0.2

X

4 -

0.I 0.5 0.02

I 2.0

X

15 hr Ihr CE

15 hrl 88.36 3.7 hr I 13.0 93.17 93.33 5.5 hr .32.l!J 57.60 27.75 82.40 129.40 I 3.03

I.

h X

-

-

2 1 I -

-

-.

3. 3.

-

I

3

0.04 0.2 0.w 0.01

4

3 1 ~~~

(conlinued)

Appendix Il-continued

Source half-life. T

lnalope

E:, (keVI

r
Iday
89.36 127.2 37 15 122.1 22.5 121.8 81.99 CE 23.83 6.23 136.3 58.0 18 hr 140.5

5da)s
39.58 80.16 84.26 66.14 75.89 18.67 76.5 82.1 CE 93.31

Liquid N,

Room remp.

x

250 days 140 days 140 days 1144 days)

67 hr 60 days 110' years) (32 days) 6500 years 4 x los years

90 hr 2 x 10' years 8

130 days 1.9 years I 1 9 years) 3 6 years

78 hr


>1,<10

>I0

x

x

x

x X

x

X

x X

X

01

0.3 0.03

x

X

IX )

008 -

0.02

x

0.004

0.01

X

-

x

004 0.02 0.01 0.01

x

0.01

x

X

-

x

-

0.5

27 I

0.7 -

-

-

12

0.04 -

14

02

I .2

X

-

2.3

0.005

X

-

0.4

-

0.001 0.02

4 .-

004 0.09

-

-

0.4 0.8

104

X

X

I2 2 36 I 3

-

-

Number of publications in 1971and1972

248

0.7 -

-

X

>5

0.2 2 -

-

X

-1

x

x

X


17

x

x

I15 days (71 days) (71 days) 38 days (169 days)

83 67

>I0

_.

X

5 5

>I.clO

w,

6,

-~ 10%days I? years

94


X

L n ' .

111.1

122.5

Liquid He X

3 years 76 years

100.1

46.48 99.08

r>30days

16

35 46 49.75 CE 8.42 45 3 22 hr 44.7 103. 8 hr

k

Temperature o f stud?

-

1 4

0.04

2 8 3

0.3 0.3

2 5

0.2 0.3

6 1 1 12 2 4 2

0.1

19

-

-

2 2 -

_ 900

04 0. I 0.6 0.4 0.4

-

A

-

-

-

2800

3

M~SSBAUERSPECTROSCOPY APPLICATIONS

239

ACKNOWLEDGMENTS We would like t o gratefully acknowledge Steen M@rup,Haldor TopsQe, and JQrgen Villadsen, who thoroughly read the various drafts of this paper and who were responsible for significant improvements. We are also indebted to Ruth Dumesic and the Haldor Tops$e Research Laboratories for helping in the preparation of this review chapter. We wish to acknowledge Michel Boudart, at whose laboratory we were introduced to and first studied catalysis and Mossbauer spectroscopy. Finally, one of us (J.A.D.) acknowledges financial support from the National Science Foundation (Grant No. GK17451X) while at Stanford University.

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MOSSBAUER SPECTROSCOPY APPLICATIONS

24 1

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Compensation Effect in Heterogeneous Catalysis A . K . GALWEY Department ?/Chemistry Queen’s University Belfast. Northern Ireland

I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Compensation Behavior . . . . . . . . . . . . . . . . . . . . . . . A . Theoretical Explanations ofCompensation Behavior . . . . . . . . . B. Surface Reactions . . . . . . . . . . . . . . . . . . . . . . . . C . Surface Concentrations of Reactants . . . . . . . . . . . . . . . . D . Arrhenius Parameters for Heterogeneous Reactions . . . . . . . . . E . The Common Surface Equilibrium Model . . . . . . . . . . . . . F . Quantitative Recognition ofCompensation Behavior . . . . . . . . . I11. Compensation Behavior in Reported Kinetic Data . . . . . . . . . . . A . Scope of Literature Survey and Sources of Data . . . . . . . . . . . B . Reactions on Metals . . . . . . . . . . . . . . . . . . . . . . . C . Reactions on Alloys . . . . . . . . . . . . . . . . . . . . . . . D . Reactions on Oxides . . . . . . . . . . . . . . . . . . . . . . . E . Reactions Involving Clays . . . . . . . . . . . . . . . . . . . . IV . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix 1. Compensation Behavior Resulting from TemperatureDependent Variations in Concentrations of Surface Reactants . . . . . . . . . . . . . . . . . . . Appendix I1. Statistical Formulas Used in Linear Regression (Least Squares) Analyses . . . . . . . . . . . . . . . . List of Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

247 250 252 256 258 261 264 267 271 271 274 294 298 304 307

311

314 315 315

.

1 Introduction

Studies of chemical kinetics are often undertaken to elucidate the mechanisms of reactions. including identification of the factors that control the reaction rate. characterization of the intermediates involved. and determination ofthe rates at which these are formed from the reactants and transformed into products . From such investigations a theoretical reaction mechanism 241

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may be developed, which conveniently expresses the more important features of the experimental observations and also provides a basis for the prediction of behavior in hitherto untested systems. It is this predictive capacity of the theoretical analysis that is important in the development of our understanding of chemical reactivity and underlies the search for laws of general application. The purpose of the present article is to discuss one particular correlation, the compensation effect in heterogeneous catalysis, which expresses the existence of interrelated kinetic behavior within a group of rate processes. The occurrence of this effect has been widely reported in many and diverse surface reactions, but it has not been responsible for providing general insight into the mechanisms of such processes, and few significant predictions have been based on the use of the compensation relation. It is appropriate, therefore, to examine the significance of this topic within the wider context of the chemistry of surface reactions and current ideas about the mechanisms of such rate processes. Since many of the kinetic concepts used in discussions of heterogeneous catalysis (including compensation behavior) have been applied through modifications of the theories of the kinetics of homogeneous reactions, it is appropriate also to consider here some aspects and consequences of this use of parallel treatments based on comparable reaction models. Very many rate processes that proceed at a gas-solid interface obey the Arrhenius equation, which expresses the variation of the specific reaction rate constant k with temperature k

=

A exp( - E / R T )

(1)

where the preexponential term A is often referred to as the reaction frequency factor and E is the activation energy: both descriptive titles are derived from the theory of homogeneous reaction rates. Kinetic studies of heterogeneous processes frequently report values of the Arrhenius parameters A and E . This article is primarily concerned with the significance of a particular interrelationship sometimes observed between A and E values whereby, within a group of comparable rate processes, the effective influence of a change of one Arrhenius parameter is offset by a corresponding (compensatory) change of the other. Such behavior (the compensation eflect) is usually expressed by log A

=

B

+ eE

(2)

where Band e are constants, characteristic ofthe particular group ofreactions to which the relationship applies. Obedience to Eq. (2) has been reported in the literature for many groups of heterogeneous rate processes. The earliest reported instance of a linear relationship between log A and E

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

249

(i.e., compensation behavior) in heterogeneous catalysis was that given by Constable ( I ) , from studies of the dehydrogenation of ethanol on copper. Subsequently, a large number of further examples of comparable patterns of kinetic behavior have been described for many and diverse surface reactions. The developing subject was reviewed in 1955 by Cremer (2), who listed the many reported examples of obedience to Eq. (2) and discussed theoretical explanations of the phenomena. In a later (1962)comprehensive review of the catalytic properties of metals, Bond (3)mentioned many examples of compensation behavior in these reactions and also discussed characteristic features and possible explanations of the effect. Since these appraisals of the literature, reports mentioning the occurrence of compensation relations have continued to appear. In many such articles, however, the theoretical and/or mechanistic implications of this particular observation are passed over with little or no discussion. Authors do not always attempt to identify which of the various suggested explanations of compensation behavior might be applicable to their results. In consequence, the recognition of the existence of this particular pattern of kinetic behavior has not significantly increased our understanding of the mechanisms of reactions proceeding at solid surfaces. Indeed, during personal discussions with workers in the field, the reviewer has gained the impression that several of his colleagues believe that the compensation effect may not be a valid kinetic observation in all cases, but could arise for other reasons, for example, as an experimental artifact. Support for this viewpoint has been given by the suggestion (4),albeit in a slightly different context, that apparent obedience of data to a compensation relation can arise as a result of the method of treatment of observations and be without physical significance. At the present time, therefore, doubts remain concerning the general theoretical implications of the compensation relation despite the very many reported instances of obedience to Eq. (2). Accordingly, this review emphasizes the interrelation between kinetic characteristics and the chemistry of the participating surface rate processes. The survey is presented in two parts. In the first, we discuss the theoretical models that have been proposed to explain obedience to Eq. (2), with particular reference to the properties of surfaces and adsorbed species, and also the factors controlling the magnitudes of the observed values of A and E . This section includes some discussion of the statistical methods used in the recognition of compensation phenomena. The second part summarizes and discusses reported examples of compensation behavior, and describes some new instances of the effect that became apparent while making the present search of published kinetic data. A semiquantitative statistical analysis of these trends has been undertaken to provide a common basis for comparisons and to attempt to define criteria that enable meaningful relationships to be recognized. Some implications of these patterns of results

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K . GALWEY

are discussed with reference to the significance of the compensation effect in theoretical and mechanistic studies of heterogeneous catalytic reactions. While the present article is almost exclusively concerned with the compensation effect in surface reactions, it must be pointed out that such behavior has also been described for a wide variety of other different groups of related rate processes. The diversity of systems for which obedience of kinetic data to Eq. (2) has been reported is illustrated by the following list: homogeneous reactions in solution (5,6a, 6b),reactions on electrodes (7), decomposition of hydrogen peroxide on copper foils (a), solid phase reactions (9) including the decomposition of calcium carbonate (lo),decomposition of cobalt complexes ( I ] ) , the oxidation of metal films (12), the catalyzed oxidation of carbon (13), desorption reactions ( I # ) , viscosity of aqueous solutions (15), and the conductivity of both inorganic (16a, 166)and organic ( I 7) semiconductors. These examples are cited only to indicate the generality of compensation behavior and will not be discussed in detail here. It is clearly apparent, however, that the effect is not to be regarded as a feature characteristic of catalytic reactions alone.

II. Compensation Behavior Although not always explicitly stated, the constituent reactions of any group of catalytic rate processes, which together exhibit compensation behavior, generally possess in common (18) one or more of the following features : reactant(s), product(s), overall chemical change, and/or the catalyst phase (including constituents, as, for example, in alloys). Where one or more of these factors provides the unifying parameter in the several rate processes, we shall refer to the assemblage as a group of related reactions. For some examples of compensation behavior the relationships between the reactions, from which the A and E values were obtained, are particularly close. This is the case for the decomposition of formic acid on silver (/9,20), where the reactant, products, and catalyst phase are common. In the exchange of methane with deuterium on alloys (21), kinetic measurements refer to reactions on active metallic phases that contain the same two metals, although in different proportions. In other systems, compensatory behavior is apparent within groups of related reactions where there are changes, for example, in the catalyst phase [as in the decomposition of formic acid on several metals (3, p. 422)] or in the volatile reactant [as in the cracking of various hydrocarbons on nickel carbide (241. Although the term "compensation effect" is now in general current usage, such behavior has also been referred to as the isokinetic eject (23)and as the

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

25 1

6-rule (24, 25). This alternative nomenclature derives from the property of any group of reactions which obeys Eq. (2) that there exists a temperature at which the rates of all these are equal (the isokinetic temperature p ) (6a): E = 2.303RP log A - B/e

(3)

The magnitude o f a may not be of fundamental significance (6a)and it may or may not be experimentally accessible. A consequence of the occurrence of the isokinetic temperature within the interval studied, which may be of considerable significance when mechanistic conclusions are based on kinetic comparisons, is that at this point the relative rates of reactions within the group undergo an inversion. The term anticompensation efSect (3, 26,27) has been applied to groups of related reactions that obey Eq. (2) but for which the value of e is negative, so that an increase in E is accompanied by a decrease in log A . This behavior is uncommon. Other special cases, which may be formally regarded as compensation behavior, arise in series of rate processes for which either E or A remains constant while the other varies: here e(log A - B) = const o r e = 0, respectively (28,29).A further possibility, the potential of which has not been fully explored, but which is considered again below, is that there may be a nonlinear relationship between log A and E (30).While the existence of a functional relation between the Arrhenius parameters is usually deemed to be worthy of mention, the absence of any such correlation, or where the data contain appreciable scatter, is not usually particularly remarked upon. Therefore, the recognition and delimitation of the scope of meaningful application of the compensation relation to kinetic data on the basis of existing literature reports is neither easy nor straightforward. None of the mechanistic explanations of compensation behavior have enabled the values of Arrhenius parameters for untested systems to be predicted. Thus, every compensation plot consists of a number of individual points (log A l , E l ; log A 2 , E,; log A 3 , E,; . . . ; log Ai,E , ; . . .) each point is defined by a single reaction, and the line through these yields the characteristic values of B and r for that series of related reactions. In the absence of control over the magnitudes of A and of E, Eq. (2) is not a realizable continuous function. In principle, this might be achieved by appropriate variations in conditions if a meaningful mechanistic explanation of the surface behavior were available. It appears to the reviewer that the compensation effect has been incompletely exploited as a method of making quantitative comparisons of catalytic activity in different systems. The parameters B and e provide a more generalized measure of the reactivity characteristic of a series of related reactions (22)

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than, for example, the temperature at which the rate constant for a particular reaction reaches an arbitrarily selected value. The use of Compensation behavior in the provision of a unifying common basis for the interpretation of a series of related reactions has been discussed (13).

A. THEORETICAL EXPLANATIONS OF COMPENSATION BEHAVIOR In this section we give descriptions of the several theoretical explanations of compensation behavior that have been provided in the literature. Each such account is limited to a brief summarizing statement, since more detailed explanations may be readily found in the references cited. The various models mentioned below are not necessarily mutually exclusive, so that there is the additional possibility that behavior in any particular group of related reactions may not be fully specified by a single explanation. Different factors may be important in controlling a specific chemical change proceeding under alternative conditions, or in similar reactions under identical conditions. Again, it may be envisaged that there could be mixed or transitional behavior arising through the occurrence of concurrent rate processes or a change in reaction mechanism within the temperature range of measurements. The establishment of whether or not a particular explanation of compensation behavior is a meaningful representation of surface phenomena in a group of reactions exhibiting the effect is beset with problems, since the identification of the parameters that control surface kinetic properties is experimentally very difficult. This is a recurrent theme in the text below. Accordingly, it is not surprising that none of the explanations mentioned has been accorded general acceptance, and few reports in the literature provide convincing arguments in favor of one explanation in preference to all others. 1. Characteristic Temperature of Onset of Reaction

It has long been appreciated that the occurrence of compensation effects in kinetic data could result from the specific selection of reaction systems for study on the criterion that conveniently measurable rates are obtained within the same selected temperature interval ( 4 , 5 ) . If either A or E varies significantly within such data, appropriate magnitudes of k are only possible if there is a measure of compensation. An alternative, and comparable, qualitative explanation of the same pattern of behavior is that there is a characteristic temperature at which the chemisorption, dissociation, or mobility of a necessary common reactant or intermediate becomes sufficiently great to allow significant participation in the catalytic process. This is the characteristic temperature of onset of reac-

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

253

tion within the group of related rate processes, and again rates are comparable within a common temperature interval with the consequent appearance of compensatory behavior (31,32). 2. Energetically Heterogeneous Catalyst Surfhce If the surface is represented as an array of energetically different sites, with the property that the catalytic reaction on any particular group of sites n, requires the activation energy Ei,then the overall rate constant k is expressed by the summation k = k c n i exp( - Ei/RT) (4)

If it is further assumed that there is an exponential dependence of ni on E i , and the distribution of site energies is such that the activation energy values extend between the limits El to E,, it may be shown that the preexponential term includes a factor {exp(+ E,/y)} and, in consequence, there is compensatory behavior. More complete treatments of this model, including further references, are given by Laidler [(33),pp. 119 and 1951 and by Bond [(3), p. 1431. 3. More than One Active Surface

If the overall chemical change is the summation of several independent contributory processes (occurring, perhaps, on different active areas of a solid surface), we may write k

= i

xiAi exp( - Ei/RT)

where Ai and Ei are applicable to a reaction proceeding on a fraction xi of the surface. The simplest case (only two active areas) can be expressed as

k = xiA, exp(-EJRT)

+ (1 - x i ) A , exp(-E,/RT)

(5)

It can be shown, through substitution of appropriate values into Eq. (5), that variations in the values of xi in a series of related reactions results in compensatory behavior, subject to certain further conditions. Since the properties of Eq. (5) have been described particularly extensively in previous articles (13, 34-38), the analysis will not again be repeated here. It is worth mentioning, however, that the composite reaction does not strictly obey the Arrhenius equation, although the error present may be below the limits that can be

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detected experimentally. If the number of contributory terms in the summation is increased, this representation may become similar to that described in the previous paragraph (Section 2) for an energetically heterogeneous catalyst surface.

4. Ei?thulpl.-~i~tlop!, Relntionskip Kemball(39),from consideration of the approximate form of the Langmuir equation 1 - 8, = PX' (exp(- AS/R) exp(AH/RT)) has shown that the activation energy for a reaction involving a bare site is increased by the diflerential heat of adsorption ( - AR) and the frequency factor is increased by the entropy term exp(AS/R). Everett (40) has pointed out that entropy and cnthalpy of adsorption are often related in a linear manner: AAi = A S i / R h (6)

+

If reactant adsorption can be described by the above equations it is to be expected (39) that groups of related reactions will exhibit compensation behavior. A physical representation of the relationship expressed in Eq. (6) is that the greater the binding energy of the molecule to the surface then the more restricted is the vibrational and rotational freedom. The restriction of movement of surface species has also been discussed by Kwan (41) in a consideration of the kinetics of gas adsorption. 5. Vuriufiorz iri Auuilahility of Surface Reactant If (i) the sequence of steps that precedes product formation results in a systematic variation in the frequency of occurrence of the precursor state for the rate-limiting step in the overall catalytic reaction (i.e., the preexponential factor) across the temperature interval used in the determination of the Arrhenius parameters, and (ii) the magnitude of this variation is different in each reaction of a group of related rate processes, then compensation behavior can be expected (31). When these conditions obtain, uppurent values of A and E are found by application of the Arrhenius equation to data for the overall change, and these parameters are not necessarily identified with the surface rate-limiting bond redistribution step. (This conclusion contrasts with the generally accepted mechanistic significance of the Arrhenius parameters in many homogeneous reactions.) Unlike some of the other models mentioned here, it would appear that the theoretical implications of this pattern of kinetic behavior have not been discussed in detail, and accordingly

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

255

it is appropriate to describe some relevant features of this model in the present article. The reaction rate constants can be expressed as k

=

aA, exp( - E,/RT)

(71

where the values of A , and Es refer to the rate-limiting step on the active surface. The term a is included to make due allowance for variations, with reaction conditions, in the frequency of occurrence of the surface reaction situation (i.e., the probability of a collision between two reactant entities or a change in the effective concentration of the transition complex for product formation). The magnitude of M will depend inter a h upon the concentrations and mobilities of the surface intermediates. A quantitative discussion of the kinetic consequences of the variation of a with temperature is given in Appendix I. Further consideration of the effective concentrations of, and equilibrium between, adsorbed species in heterogeneous reactions and of the significance of measured values of A and E is given in Section 11, B. 6. Rate Law f i x Surjuce Reactions Moro-oka and Ozaki (42)have attributed observed compensation behavior to a surface reaction proceeding through a bimolecular rate-controlling step on the active solid in which the concentrations of participating adsorbed reactants are determined by a power law involving variation of the reaction orders. Theoretical values of A were estimated through use of the transition state theory and reactant concentrations derived from the differentiated form of the Langmuir equation [(42), see also (441. For the series of related reactions described (42),the values of A thus found exhibited a linear correlation with those measured for reactions believed to proceed through the same slow step. Determinations of the heats of adsorption during reaction, required to enable estimations of E, await further experimental investigation. 7. Other Explanations

The following list mentions several factors that may control or influence the magnitude of one or both Arrhenius parameters and, in consequence, possibly result in the appearance of compensation behavior. Some of these parameters closely resemble, or represent alternative variations of, the reaction models described in Sections 1-6. (i) The concentration of surface-active centers may be temperature dependent (44). (ii) Changes in the activity coefficients of the adsorbed species may arise through lateral interactions (45).

256

A. K . GALWEY

(iii) Surface dipoles may influence the energetics of reaction (46). (iv) The transition complex may change from an immobile state to a mobile state as the reaction temperature is increased (46). (v) The transmission coefficient for the breakdown of the activated complex may vary with temperature (14). (vi) The tunnel effect may operate (2,44). (vii) Catalyst doping may influence the concentrations of electrons in the solids (47). (viii) The Arrhenius equation may not be correctly applied to heterogeneous rate processes (48). (ix) There may be a manifestation of the law of conservation (49). (x) Other discussions of the compensation effect have also been given ( I7,50-53). 8. Discussion

The theoretical and mechanistic explanations of compensation behavior mentioned above contain common features. The factors to which references are made most frequently in this context are surface heterogeneity, in one form or another, and the occurrence of two or more concurrent reactions. The theoretical implications of these interpretations and the application of such models to particular reaction systems has been discussed fairly fully in the literature. The kinetic consequence of the alternative general model, that there are variations in the temperature dependence of reactant availability (reactant surface concentrations, mobilities, and active areas; Section 5 ) has, however, been much less thoroughly explored. No single theoretical explanation of compensation behavior has been recognized as having general application. It is appropriate, therefore, to consider in this context the conditions obtaining on a catalyst surface during reaction, with particular reference to the factors that control the rate of product evolution and to the interpretation of kinetic measurements. This discussion of surface behavior precedes a critical assessment of the significance of measured values of A and E. B. SURFACE REACTIONS A heterogeneouscatalytic reaction, by definition,necessitates the participation of at least one chemisorbed intermediate (54) and involves a sequence of interlinked and interdependent (55,56) steps, which include the adsorption of reactant(s), one or more surface rearrangements, and the desorption of product(s). More than one area of the solid may be active in promoting reaction; the activity of such regions may vary from one crystallographic

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

257

plane to another and also with compositions of different crystallites, if these are not all identical. Moreover, as shown by Thomson and Wishlade (57), all molecules adsorbed by a catalyst are not equally reactive. There may also be groups of surface sites of different reactivities (58).A part of the surface may therefore be occupied by material that does not yield products but which can, nonetheless, influence the chemical properties of other surface-held material. Kemball(59)has pointed out that a small but sufficiently reactive proportion of the adsorbed phase may provide the pathway whereby a large fraction of product formation proceeds. These features of catalytic reactions must (inter alia) be incorporated into any general theoretical discussion of the mechanistic interpretation of kinetic observations. In specific instances it may be possible to represent the overall change as proceeding through a single, identified, surface-bonded intermediate. More generally, however, a heterogeneous reaction must be more realistically regarded as a complicated process involving the interdependent interactions of several intermediates and perhaps proceeding at different rates on different surfaces. The constituent steps and controlling factors in these concurrent changesare not readily separated and individually identified by kinetic investigations alone. Accordingly,a variety of techniques have been applied to the recognition and/or estimation of the concentrations of chemisorbed species (adsorption, exchange studies, infrared measurements, LEED, and others). While the identities and concentrations of the most abundant molecular groupings at the surfaces of particular solids have been determined for a number of systems of interest, further information regarding the reactivity of each entity is required before the data can be inserted meaningfully into kinetic expressions. It is often not possible to specify the effective surface concentrations of those groups which participate in the rate-limiting step of a catalytic reaction. This feature of heterogeneous processes contrasts with a property of homogeneous reactions in which all equivalent molecules are regarded as being equally reactive. Despite the possible contributions from several interrelated concurrent processes, the kinetic expressions obeyed by many catalytic reactions are simple. This is attributable to a high degree of interdependence between the interactions involved in the sequence of successive steps between reactant adsorption and product desorption and equilibria existing between the various surface intermediates. To establish a complete kinetic description of a heterogeneous reaction, which is to be regarded as analogous with those used in rate studies of homogeneous processes, it would be necessary to measure the eflective concentrations of those entities which participate in the ratelimiting step for product formation. This is not in general possible, and so, in Section 11, C, we consider the characteristic properties, behavior, and interactions of both the surfaces and the surface-bonded species.

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A. K . GALWEY

C. SURFACE CONCENTRATIONS OF REACTANTS Wc here review the factors that control the kinetics of product formation through reaction at an active surface. This includes consideration of the availability of those adsorbed intermediates which participate in the ratelimiting step (this term is analogous to concentration in a homogeneous reaction) and the mobility of the same species, which may determine, or at least influence, the frequency of occurrence of the reaction situation. The discussion is given under three broadly interpreted general headings, between which there is considerable overlap. 1. Mobility of Sur-uce Species

The frequency with which two surface-bonded species may meet (a “surface collision”) in a configuration suitable for product formation is influenced by the concentrations, mobilities, and distributions of the reactants on the surface of the solid. At sufficiently low temperatures, all chemisorbed entities are immobilized at specific surface sites. During a progressive temperature increase, each surface species will become mobile within a characteristic temperature range, which is probably also influenced by (inter. a h ) the crystallographic plane, surface coverage, and other species present. Activated surface migration will further increase with heating, so causing changes in disposition of surface species and an increase in the frequency of collisions. A rise of temperature may also result in the progressive dissociation of multiatomic groupings and the desorption of adsorbed material (both reactants and/or products) before and during the onset of sintering, which precedes melting. There is evidence that during many catalytic reactions at elevated temperatures the lattice constituents of the solid may enter into and participate in the reactions of the adsorbed phase. Adsorbed material may also penetrate the lattice of the solid. The catalytic activity of some solids may be regarded as a manifestation of the increase in reactivity of surfaces that precedes lattice disintegration before decomposition, dissociation, sintering, or melting. Ifthe appearance of catalytic properties is a consequence of the onset of surface reactivity or mobility at a characteristic temperature, the occurrence of compensation behavior in these systems may be ascribed to explanations of Sections 11, A, 1 and/or 11, A, 5. Aspects of the distribution of species on surfaces have been reviewed (38) and our understanding of the disposition, composition, and properties of the adsorbed phase is increasing through applications of recently developed high-vacuum techniques, for example, LEED (60, 61). Some information about the mobility of adsorbed material is available (62u-e) and the significance of surface diffusivity in reaction kinetics has been discussed (63).The behavior of supported metal catalysts may be influenced by the transfer of material between the two phases (metal and support) by diffusion (64- 66).

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

259

2. Properties and Beliuvior of Adsorbed Species As previously emphasized, the positive identification of the specific entities that participate in the rate-limiting step of a catalytic reaction is a matter of considerable difficulty. Aspects of this problem have been discussed by Knozinger et al. (67) and by Webb (66).Thus, the interpretation of kinetic observations for heterogeneous processes must include consideration of the following possibilities. (It is not intended to imply, however, that every heterogeneous reaction necessarily includes each effect mentioned.) (i) The effective concentration of any constituent species in the adsorbed phase may be dependent, to a greater or lesser extent, on the temperature, the pressures of all gaseous compounds present, the quantities of all other adsorbed substances, including poisons, the catalyst support (if any), and any other additive. There may be a significant diminution in the quantity of volatile material retained on the surface as the temperature is increased, e.g., hydrogen on nickel (43). (ii) Chemisorption may be accompanied by extensive dissociation of multiatomic groups, as, for example, in the reactions of hydrocarbons on nickel (68-72). During the catalytic reaction, there may be equilibration of the surface phase with reactants or products (73,74),and an initial period may be required before stable activity is achieved (75). The compositions of the adsorbed species are dependent on the gases present (76.- 78). A particular reactant gas may be accepted into more than a single bonding state on an active surface (79 82). There is strong evidence [(57, 66) and references therein] that not all surface-bonded material is equally reactive. The number of active surface sites may be temperature dependent (83).Catalytic activity is not always directly related to the quantity of gas adsorbed (84). There is the possibility, in appropriate systems, that different chemical transformations proceed concurrently on the active surface(s), as in the hydrogenation of ethylene on nickel (85)and of cyclopropane on the same metal (86),the exchange and cracking of ethane, again on nickel (61,87),and the stepwise and multiple exchange ofmethane on alloys (21).The relative rates ofsuch alternative reactions may be temperature dependent and such concurrent chemical changes may or may not involve common surface intermediates. (iii) Measured values of Arrhenius parameters for catalytic or for desorption reactions may be influenced by impurities (88,89),surface coverage, as in the desorption of hydrogen from iron ( 4 9 , and the quantity’ of material adsorbed, as in the influence of oxygen on the silver-catalyzed decomposition of nitrous oxide (90). (iv) Many heterogeneous reactions may be composite, including contributions from rate processes occurring on different crystallographic planes and/ or involving different mechanisms ( Y I , 92). Reactions proceeding on different exposed lattice arrays of the same solid phase may exhibit different kinetic

260

A. K. GALWEY

-

characteristics: such behavior has been discussed with reference to the reactions of formic acid on silver (19,20)and for the isomerization of isobutane on platinum (93a). (v) Reactions may occur preferentially at surface imperfections (93b). The use of chemisorption measurements to estimate the surface concentrations of particular species may be satisfactory at low temperatures where coverage is high and there is little breakdown of the chemisorbed entities. Useful applications of this approach have been made in studies of hydrogendeuterium exchange and ethylene hydrogenation. At higher temperatures, however, the situation is less simple and due account must be taken of dissociative adsorption of the reactants and the desorption of the products of such breakdown (e.g., gaseous hydrogen is often detected from the dissociative chemisorption of hydrocarbons on nickel). While many adsorption studies have been concerned with temperatures below those characteristic of the onset of catalytic activity, attempts have been made to extend this approach to include such conditions, for example, in studies of cracking reactions (92)and the decomposition of formic acid on metals (94).It may ultimately prove practicable to estimate concentrations of participating surface intermediates through measurements of radioactivity following introduction of labeled reactants (95). 3. Participation of the Catalyst in Surface Processes

It is undoubtably unrealistic to regard heterogeneous catalysts as invariably behaving as an immobile foundation beneath an active surface that enhances the reactivity of adsorbed material. Constituent elements from the catalyst may participate in surface rate processes and elements of the adsorbed phase may penetrate the crystal lattice of the solid (96,97).Such processes involve activation and are thus temperature dependent. There is ample evidence that many heterogeneous reactions are accompanied by reorganization of the active solid phase@) involved (60, 98- 101). During ammonia oxidation a platinum catalyst is etched (102), a silver surface is pitted during ethylene oxidation (103), and copper particles are retextured during decomposition of formic acid (104).Impurities within crystals may diffuse to surfaces and there influence reactions (105-107). The number and activity of surface sites on silica-alumina catalysts are influenced by pretreatment (108).Catalytic reactions may be accompanied by the appearance of a new phase (109),iron is nitrided during reactions of ammonia ( ] l o ) there , is the stable coexistence of two oxides during oxidation of hydrocarbons over cobalt oxides (111).Nickel metal is formed during the cracking of hydrocarbons on nickel carbide, despite the net deposition of carbon (22). Appar-

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

26 1

ently the converse reaction, or approach to a comparable equilibrium, also occurs since nickel carbide is formed during the hydrogenation of methylamine on this metal (112), a process in which a constituent of the adsorbed material penetrates the catalyst lattice. The kinetics of hydrogenation of carbon adsorbed on nickel depends on the quantity of such carbon present (96). Chemisorption has been shown to modify the surface composition of certain alloys (113); the ratio of elements at the phase boundary may be different from that in the bulk (29, fZ4).(It may be added here that the composition of the alloy surface may be established either during preparation, where such equilibrium is frozen in, or during catalysis, where there is a dynamic equilibrium, or be the resultant of contributions from both effects so that there is a progressive change in behavior.) The mobilities of species on clean metal surfaces have also been studied, and boundary rearrangements have been detected on rhenium and ruthenium above 450 K (115). There is, therefore, much evidence that the constituents of many solids attain mobility during participation in heterogeneous reactions and this mobile material may enter directly [e.g., (104)], or possibly indirectly, into the steps required for the conversion of reactants to products. The absorption ofgaseous reactants is expected to modify the electronic structure of the solid, thus influencing surface properties, including both quantities and reactivities of adsorbed species. As the temperature of a solid is raised above the range of significance in studies of catalytic reactions, even greater mobility of constituents becomes apparent (116): there may be segregation of constituents (decomposition or dissociation) and sintering, through surface diffusion (117), precedes melting. This reorganization removes the most reactive atoms or ions, located at nonlatlice positions or in disordered regions of greatest local strain (118):it is often suggested that these unstable surface configurations are the active sites at which heterogeneous reactions proceed. D. ARRI-IENIUS PARAMETERS FOR HETEROGENEOUS REACTIONS

From the above considerations of surface properties of catalysts, we may conclude that the quantity of adsorbed material is not necessarily a measure of the number (concentrations) of reaction intermediates present on that surface. The reactivity of particular species may vary with both surface position (crystallographic plane, or edge, corner, jog, etc.) and degree of occupancy of that surface. In addition, the effective concentration of those entities capable of reacting to yield product may be temperature dependent. In these several important respects, the kinetic behavior of adsorbed material differs from that usually regarded as characteristic of the homogeneous reactant. Since many of the terms used in discussions of rates of heterogeneous

262

A . K . GALWEY

processes were developed from previous work concerncd with homogeneous reactions, it is appropriate to rccxamine the implications of the use of certain concepts in discussions of reactions at surfaces. The terms “reaction frequency factor” and “activation encrgy” are of particular interest here; these are identified with the frequency of occurrence of the reaction situation and with the encrgy requirement of the slow bond redistribution step, respectively. Both concepts represent important contributions in the successful development of the theory of kinetics of homogeneous, particularly gas, reactions. For the bimolccular gas reaction, the “reaction situation” is usually identified as a collision between two molecules, quantitatively expressed as

where the collision number Z , , provides a measure of A (sometimes these two terms are apparently considered almost synonymous). The values of N , and N , in Eq. (8)(where N is the number of molecules per unit volume) are effectively constant, when due allowance has been made for product formation. Meaningful estimations of the effective values of collision frequency factors in surface reactions ( A s ) arc, however, not readily obtainable. Morcovcr, it has not been demonstrated conclusively for many systems that the availability and reactivity of the participants in thc rate-limiting step in the surface reaction are necessarily constant over the range of conditions investigated. Indeed, from the evidence mentioncd above, there are strong indications that in many instances the effective values of the concentrations of rcactants (let these be c, and cz for a bimolecular surface rate-limiting step) vary across the temperature interval used in the determination of E . By analogy with the homogeneous reaction, the rate of the catalytic reaction can, therefore, in general be represented [see Eq. (7)] as

k

= c,c,A, exp( - E , / R T )

(9)

The values of A, and E, are not necessarily idcntical with, or to be identified as those calculated from, measurements for the overall reaction, A and E , since the latter are composite terms that may include contributions from the temperature dependences of c,, c,, and A s , as described in Appendix I. The surface reaction is not completely represented by the consideration of this single step (the surface collision) and rate expressions should be more realistically regarded as the resultant of several contributory factors in the sequence of interdependent (55, 119,120) processes required to convert the reactants into products. In general, the overall surface reaction is composite kinetic behavior and thus more complicated than many of the homogeneous processes that have attracted greatest interest. In the heterogeneous reactions,

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

263

influences from the steps that precede the slowest and, indeed, all surface equilibria may exert some control over the apparent values of A and E. It is reasonable to suppose that between the members of a group of related reactions there will be modifications, but not drastic changes, in the positions of surface equilibria and in the temperature dependences of cl, c2, and A,. Such variations, when subject to appropriate constraints, are capable of providing an explanation of compensation behavior (Appendix I and Section 11, A, 5). From this it follows that the compensation effect appears as a general or at least a widely occurring characteristic feature of surface processes, rather than an exceptional phenomenon that requires an exceptional explanation. The above mechanistic interpretation of compensation behavior, suggesting the existence ofa cummun surface equilibrium in groups of related reactions that obey Eq. (2), does not necessarily exclude the operation of the other explanations mentioned in Section 11, A. The concurrent operation of two such effects in parallel reactions or other comparable types of intermediate behavior are, in principle, theoretically possible. The positions of surface equilibria may vary from one exposed surface lattice plane to another, allowing surface heterogeneity and variations in reactivity. The concept of phase boundary equilibria involving several constituents accords well with the observed reorganization of surfaces during reactions where the constituents of the solid may enter catalytic rate processes. This analysis does, however, strongly call into question the validity of widely accepted interpretations of the Arrhenius parameters for many heterogeneous reactions. The quantitative conclusions reached through application of the transition state theory to those reactions for which the identities of the activated complex have not been positively established should be considered with due regard for the limitations imposed by the nature of the underlying assumptions. The theory of absolute reaction rates has been applied to heterogeneous processes through equations of the form (3, 121) k"T k = r-c,c,-exp[-(E, h f;.f*

-

e)/RT]

where r is the transmission coefficient and E the contribution from the heats of adsorption of reactants and poisons. Since individually and independently measured values of c,, c2, E,, and E are not usually available, this approach is subject to many of the shortcomings described above. Indeed, the difficulties that attend the meaningful (or rigorous) application of this analysis [i.e., Eq. (lo)] are seen in perspective when it is remembered that, even for some of the most fully investigated heterogeneous reactions, the identity of the rate-limiting step remains in doubt [see, for example (67,77,87,122-126)].

264

A. K . GALWEY

Moreover, before such interpretation is attempted, it is important to be certain that Arrhenius parameters under consideration specifically refer to the reaction in question. This js not always as straightforward as might appear at first sight. Values of A and E measured for a freshly prepared catalyst sometimes differ from those found for aged material, perhaps due to surface poisoning (127) or other effects during the “break-in” period (101). Activity is expected to change during sintering (128). While mechanistic kinetic studies are usually designed to avoid a significant influence of diffusion effects on rate data (129), less consideration would appear to have been directed toward the consequences (on A and E ) of local self-heating during exothermic reactions on finely divided and inhomogeneous solids (130). Perhaps the most unexpected factor that has been reported as influencing surface kinetic behavior is that resulting from the presence of an inert gas (131,132).

We would, therefore, agree with Bond’s conclusion ( 3 )that application of the transition state theory to heterogeneous reactions has not so far provided insight into the mechanisms of surface reactions and that the failures of the theory are generally more significant than the successes. We do not accept that the use of the theory of absolute reaction rates in the interpretation of kinetic data provides a general and reliable method for the estimation of the concentration of surface active sites but conclude that results should always be considered with reference to appropriate quantitative supporting evidence (133). Several other discussions of the significance of the activation energy in heterogeneous reactions have been given (34,37,53, 119,134,135). Garn (32) has concluded that values of E measured for solid phase decomposition reactions have very questionable meanings and aspects of his theoretical analyses are equally applicable to other reactions at the surfaces of solids.

E. THECOMMON SURFACE EQUILIBRIUM MODEL In Section 11, D we propounded a case to suggest that values of A,, cl, and c2 may vary with temperature; we now discuss the probable form of this variation of the value of c1 in Eq. (7) with reference to compensation behavior (Appendix I). Application of the collision theory of reaction rates to surface processes is not straightforward. The meaningful definition of a surface collision is difficult and the necessary assumptions, inherent in any quantitative treatment based on this approach, make the results of dubious validity and restricted usefulness. The movement of surface entities within the temperature range of interest could necessitate activation, but (in different systems) may alternatively be a rapid and facile process, and the expression defining the

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

265

temperature dependence of the occurrence of the reaction situation (A,) could be quite different from that applicable to gaseous collisions [Eq. (S)]. Furthermore, in many rate processes of interest, we have little direct knowledge concerning (i) the area of the catalyst that participates in the reaction, (ii) the reaction entropy, including the steric constraints on surface interactions between chemisorbed species, and (iii) the transmission coefficient Y P I . (1011 (14). It is usually assumed (again by analogy with homogeneous rate processes) that there is a single, relatively slow rate-controlling step in a heterogeneous reaction, involving bond redistribution, and all other steps (adsorption, desorption, and dissociation processes) are, by comparison, relatively rapid. During catalysis, equilibrium may be established between gaseous molecules and certain, perhaps not all, surface molecular groupings. An increase in temperature may then be expected to displace the relative proportions of the participants in surface interconversions in the direction of more extensive dissociation and this may be accompanied by a reduction in total surface coverage through desorption and/or lattice penetration. Thus, depending on the chemical properties of the system considered, it is theoretically possible for the magnitude of the term c,cz either to increase or to decrease with a rise in reaction temperature. No equation of general applicability expressing these changes is readily formulated, but the following approximate relations may be acceptable within a limited range of conditions o r provide a semiquantitative indication of the type of behavior that might be expected.

(i) The Langmuir adsorption isotherm may be expressed (3)as

where g = 1 exp( - A H , / R T ) . When the surface coverage is small, 6' << 1, and the gas pressure is low, P << g, we may write

0

=

IP exp( - A H , / R T )

Allowing for the possibility of dissociation (71) and assuming that, within appropriate limits, c1 cc 0, it follows that, approximately,

Reaction pressure dependences are not always unity ( k changes with P then the value of E also changes (22,77). (ii) Equilibrium in the surface dissociation reaction AB2eAB+ B

K

Px")and if n

266

A. K . GALWEY

may, within appropriate limits, be approximately expressed as In(cABcB/cAB,) = - AGo/RT so that again, subject to relevant restrictions, Eq. (1 1) follows.

A more rigorous treatment of adsorption equilibria would include due allowance for the total number of surface sites available, which sets an upper limit on the surface concentrations to bc used in the above equations, the variation in the heat of adsorption with coverage, and surface heterogeneity. The significant feature of Eq. (1 I), relevant to the present discussion of compensation behavior, is that this predicted temperature dependence of variations of c1 and c2 results in no deviation from obedience to the Arrhcnius equation. If a given set of kinetic results obey Eq. (1) the condition for a fit to Eq. (9) is c,c2,4, = A exp[-(E - E s ) / R 7 ]

which is fulfilled by Eq. ( 1 1) for c2 as well as c I. When the values of c I and c2 are large, so that surface coverage is extensive, or complete, the influence of temperature on the magnitude of c,c2 may be small, and therefore E, E . Such conditions may occur in some heterogeneous reactions proceeding at low temperatures. When c 1 and c2 are small but not temperature independent, data may still, however, obey the Arrhenius equation, through the relationship given in Eq. ( 1 I), and minor deviations from strict obedience to these kinetic expressions are not readily detected. Equation (1) is relatively insensitive to systematic inaccuracies so that deviations from linearity are not easily demonstrated. Thus the consequence of variations in the temperature dependence of the product clc2 within a series of related reactions is the appearance of compensation behavior. Since the experimentally measurcd values of E are almost invariably positive, it follows that the effect of such variations in (dct/dT) are almost invariably less thar. those resulting from E,. Several experimental approaches have been used to obtain information concerning the identity, concentrations, and reactivities of intermediates in catalytic reactions. Tamaru (136),during measurements of catalytic activity, concurrently determined the total quantity of gas adsorbed. One possible limitation to this method is that a proportion of the material bonded to the surface may not be involved in the surface reactions (57).The use of labeled reactants and monitoring the radioactivity in the region of the active solid may (95)provide a potentially useful technique for the estimation of cI and/or c2. An alternative, and perhaps complementary, approach is through the individual investigation of the kinetics of product formation from reactions of known amounts of adsorbed material. This method has been used to elucidate some of the elementary steps in the breakdown of methanol on platinum (80). The independent preparation of a postulated intermediate,

-

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

267

followed by the investigation of the reactivity under appropriate conditions is a general technique for the elucidation of reaction mechanisms. Care must be taken, however, to ensure that such measurements of reactivity are applicable to the conditions that obtain during catalysis. At present we have evidence for the complexity of higher temperature adsorption/desorption phenomena while, in general, the kinetic characteristics observed for many catalytic reactions are perhaps deceptively simple. The estimations of the concentrations of the participating surface intermediates are, in contrast, experimentally very difficult. Mechanistic investigations of many heterogeneous catalytic processes yield insufficient information to allow clear distinctions to be drawn between alternative reaction models(l25). Compensation Constants us Comparison Parunzrters for Catalytic Activity

One aspect of compensation behavior that would appear to have received less attention than perhaps it deserves is the use of the constants B and e, or the isokinetic temperature fl and the isokinetic reaction rate constant k,, as quantitative measurements of reactivities between series of related reactions. In the literature, comparisons of relative reaction rates are often based on the values of k at a particular temperature, arbitrarily selected, though often within the range of measurements, or the temperature a t which a specified value of k is attained (137).It can be argued, however, that where compensation exists, a more complete description of kinetic behavior is given by B and e. The magnitudes of these parameters define the temperature range within which reaction rates become significant and that at which these become comparable: there is also the possibility that such behavior may be associated with the operation of a common reaction mechanism or intermediate.

F. QUANTITATIVE RECOGNITION OF COMPENSATION BEHAVIOR

All kinetic measurements are subject to experimental inaccuracies. Consequently, before a given set of data can be meaningfully described as exhibiting compensation behavior, it is necessary (or at least desirable) to specify the criterion by which such obedience is to be recognized. Perhaps the most widely used method of identification of a compensation effect in the literature is from the observation that a linear relation exists between the available data on a plot of log A against E, together with a small scatter of the measured points about this line. It would appear that judgment as to whether the compensation law [Eq. ( 2 ) ] is obeyed is often subjective, depending principally on the limits of variation of data within which the individual research worker concerned regards the relationship as retaining validity. Such an analysis should also properly consider the minimum number of points required to enable the trend to be meaningfully recognized.

268

A. K. GALWEY

A detailed consideration of the application of statistical methods to the recognition of a linear relationship between values of log A and E has been given by Exner and his co-workers, in a most valuable series of articles ( 6 4 23, 238, 139). Although these discussions are primarily concerned with homogeneous reactions, the treatment of data used and concepts developed are also relevant in the consideration of heterogeneous rate processes. This analysis provides a useful starting point for the meaningful recognition of compensation behavior. In this approach, rate constants and reaction temperatures are used to determine the isokinetic temperature [p in Eq. (3)], and the reliability of /3 is estimated by a statistical (least squares) method. The variables k and T are preferred for use in this treatment since errors in T are relatively small (in the examples considered),so permitting the incorporation of values of this variable into the regression formulas as if (statistically) free of error. In contrast, log A and E values both contain uncertainty and, of greater significance, these are mutually dependent, a feature of the data that reduces the reliability of the regression analysis. The articles cited (6u, 23, 138, 139) discuss the calculation and significance of b, the limits of error within which this quantity may be determined from a given set of measurements, and the assumptions made in the analysis. This account need not be reiterated here. Exner concludes that many instances of reported isokinetic behavior in the literature are not correctly so described. Good et ul. (15,30,140)have also considered these problems and extended Exner's approach in a reassessment of the validity of previously reported relationships between the energies and entropies of activation for the fluidity of certain aqueous solutions. These workers show, for the particular systems considered, that the relationship is nonlinear. Good et ul. (30,140)emphasize the necessity, in the general statistical analysis, of distinguishing between two alternative objectives, which are (i) to determine whether a given set of data are best described by a linear function, and (ii) to calculate the best straight line through a given set of data. In the further development of this treatment (140) it was concluded that it is essential to establish the physical basis upon which the variation in the specific reaction rate itself depends and this information is then carried through to the enthalpy-entropy plane. The linear enthalpy - entropy relation is a particular subset of possible valid functional relationships. Equation (2) is not the only expression capable of demonstrating an interdependence between A and E. [Attention has also been drawn to the additional possibility that there may be a nonlinear relation between log k and T - ' (14/).]

It is apparent from an examination of the literature that there have been few attempts to make use ofstatistical methods in the quantitative recognition

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

269

of compensation trends in groups of related heterogeneous reactions. Instances of obedience to Eq. (2) are often reported as a plot that includes a “best line” through the points considered. Such examples of the compensation effect range from the accurate fit of many and wide-ranging values of (log A , E) to the identification of a just discernible sympathetic variation of these quantities. [The paucity of rigorous analysis here is perhaps a manifestation of the generally infrequent use of statistical methods for curve fitting in kinetic studies of heterogeneous reactions, though confidence limits for reported values are given in some instances, e.g., (103).] Section I11 summarizes the results of the application of a semiquantitative statistical analysis of the compensation trends found in kinetic data collected from literature sources. We have calculated best-fit straight lines on plots of log A against E for groups of related reactions using the linear regression (least squares) method and tabulate values of B and e, the standard deviations of these quantities ( G and ~ oe, respectively), and the squared deviations of the line ((T~). Further details of the statistical method are given in Appendix 11. This approach was adopted after consideration of the discussions given by Exner et al. ( 6 4 23, 138,139), Good et al. (15,30,140), and others (142), and represents a compromise between these more rigorous analyses and the inherent problems apparent in making a quantitative assessment of the significance of trends present in existing published material. While other methods of comparison are also possible, the technique described is applicable to many and diverse assemblages of related reactions, provides a comparative measure of the accuracy of fit of data to Eq. (2), and indicates one possible starting point for more rigorous analyses of experimental observations. The results of any statistical analysis using the formulas given in Appendix I1 contain the shortcomings inherent in the method: reliable and accurate quantitative conclusions may not be obtained where the available kinetic data consists solely of a list of Arrhenius parameters. Neither variable can be regarded as being free of error and, moreover, the inaccuracies of log A and Eare interconnected by a compensation-type relation (2). Unfortunately, the approach described by Exner ( 6 4 23,138,139)cannot be applied directly to many of the reported data for heterogeneous reactions since values of (log k, T-‘) are not usually recorded. Also, and more significantly, one cannot regard the values of T for many of the higher temperature reactions as a term statistically free of error. The quantitative recognition of compensation behavior in catalytic reactions requires, therefore, the development of an appropriate statistical analysis, specifically directed toward this objective. Such an analysis would probably start from yield-time measurements for isothermal reactions, where the latter parameter is the more accurately known and might be regarded, in this context, as if statistically free of error. The extended

270

A. K . GALWEY

treatment could also include consideration of inaccuracies in the functional dependence of product yield upon time (the kinetic equation) due to scatter of measurements and, more significantly, systematic trends arising from catalyst aging, poisoning, etc. It would also be possible (if thought appropriate) to include a test for the linearity of obedience of data to the Arrhenius equation. Additional statistical problems arise when the relationships being investigated include observations by different workers using different experimental techniques, with consequent variations in the accuracy of kinetic measurements. Thus, due to the shortcomings of currently available statistical procedures and the restricted data included in many reports of kinetic studies, it is at present impracticable to calculate a parameter that provides a realistic measure of the accuracy of obedience of (log A , E ) values to the compensation equation. While this objective may become realizable in the future, we are at present restricted to the use of the linear regression formula as a semiquantitative approximation. Results obtained using this approach, in a comparative analysis of the kinetic data available in the literature for a wide variety of surface reactions, are tabulated in Section I I1 and some judgments concerning the relative accuracy offit of data for diffcrent systems to Eq. (2) can be made. Interpretation of the significance of the observed trends must include consideration of the possibilities that the observed relationships (i) are true compensation relations and are not artifactual (4, 143), (ii) do not arise through the use of incorrect statistical procedures, (iii) are not more correctly described by some nonlinear relationship hetween log A and E (140),and (iv) provide information useful in the elucidation of the mechanisms of surface reactions or in the comparisons of levels of activity in groups of related reactions.

When E = 0, log A is equal to B, and this corresponds to the specific rate constant for reaction at the isokinetic temperature k,. I t is also easily shown that e = @/I-’. The variation of p with r over the temperature range of interest, in the reactions considered in the present review, is given in Fig. 1, which includes dashed lines for values C J ~= +0.05r,O.lOe, and 0.20e, corresponding to the influence of various levels of inaccuracy of data upon the calculated precision with which fl may be determined. For example, when oC/r= 0.05 and [I’ = 700 K the deviation is 2 3 0 K and this reduces to - 15 K when [I’ = 300 K : other values are very readily found.

+

-

27 1

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS 90(

801

701

60(

P 50(

40(

30(

2 0c 0.04

0.08

0.12

0.16

0.20

0.24

e FIG. I . The slope of the compensation line c is rclated to the isokinetic temperature 11 by the full line E = ( R [ j ) - ' . The influence of scatter of data o n the accuracy with which /j is deterrnincd is shown by the dashed lines, which corrcspond t o uncertainty in a,/e = k0.05, 0.10. and 0.20.

111. Compensation Behavior in Reported Kinetic Data

A.

SCOPE OF

LITERATURE SURVEY

AND SOURCES OF

DATA

A search was made of the literature concerned with heterogeneous catalytic reactions to collect all kinetic data appropriate for the identification and quantitative comparison of compensation behavior. This survey necessarily sought every report in which the authors concerned described an observed

272

A. K . GALWEY

relationship, between Arrhenius parameters, as exhibiting a compensation effect. In addition, accurate and meaningful values of (log A, E ) were recorded and the accuracy of fit of data for appropriate groups of related reactions to Eq. (2) was investigated using the regression formulas given in Appendix 11. Identical methods of analysis were used in the quantitative comparisons of both the previously reported and the newly found trends. The results of this analysis are summarized below in tabular form: for each group of related rate processes we include the calculated best line (B, e), the relative accuracy of data to this line (cg,ue,and uL),and the range of applicability (AE). Some theoretical and mechanistic implications and interpretations of these trends are discussed. Although the number of reported instances of compensation behavior is large, this feature of kinetic observations is not often mentioned in the title of an article. Accordingly, the necessary literature search could not be primarily based on an examination of the indexes of Chemical Abstracts, in which there are relatively few entries referring to compensation. Therefore, the present survey was made through perusal of those journals which report kinetic and mechanistic studies of heterogeneous reactions. It is difficult to ensure comprehensive coverage of the literature by this method and, in consequence, there is the possibility of some (unintentional) bias of emphasis toward particular reaction systems and the reports of certain groups of workers. It is hoped, however, that the coverage is sufficiently widely based to be representative of catalytic reactions in general. Results are presented below in a form that allows both comparison with information which was omitted from the original survey and later extension to include new observations. It was considered that the simple compilation of a list of every mention of compensation behavior would serve little useful purpose. Instead, attention has been directed toward the significance of obedience to Eq. (2), the possible mechanistic implications of this behavior, and the comparison of activity in groups of related reactions as expressed by (B, e) values. Throughout the survey we have emphasized the most fully characterized systems as being ofgreatest inherent interest; in these studies supplementary information concerning the reactions and/or the surface conditions was available to provide a basis for the mechanistic interpretation of the kinetic observations. Since not all discussion in each article mentioned can be included here, emphasis is largely directed toward those specific features which were relevant to the present comparative analysis. Further information is usually available in the original reports. Many of the references mentioned were selected to facilitate access to related earlier work. This review is primarily concerned with those kinetic measurements for which reaction rates are referred to equal areas of solid, so that a consistent

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

273

and meaningful basis for the comparison of both Arrhenius parameters is provided. This important criterion excludes from consideration many otherwise useful kinetic studies. The rate processes for which the greatest amount of relevant information is available concern the reactions of hydrocarbons, formic acid, hydrogen (deuterium),and oxygen on metals, alloys, and oxides. By alternative definitions of the common feature of related reactions, two classifications of the relevant data are possible, based on either the catalyst phase or the chemical transformation occurring. Sometimes all, or most, of the kinetic observations available within a particular group of related reactions are provided by a small group of workers. In this review, comparatively little attention is devoted to reactions that involve liquid reactants or catalytic processes that occur on mixed oxides, zeolites, halides, and other solids.There is, however, no reason to suppose that the kinetic properties of reactions on these, and other less extensively studied catalyst phases, are necessarily different from those found for reactions on metals, alloys, and oxides. Similar mechanistic conclusions may, in principle, be applicable. Comparisons made below refer to kinetic data obtained for processes proceeding under similar conditions. All available values of (log A, E ) within each group of related reactions were included in the linear regression analysis (Appendix 11) and the compensation line was calculated using these formulas. Unless otherwise stated, the units of A are always molecules m-' sec-' at 1 Torr pressure of reactants and those of E are kJ mole-'. The compilation of Arrhenius parameters referred to identical reaction conditions is not always easy (or, indeed, possible in some instances) and it may be necessary to recalculate data from literature sources using an extrapolation. Not all details of the necessary corrections are recorded below, but such estimations were always minimized to preserve the objectivity of the conclusions reached. The general form of the Arrhenius equation as used in studies of heterogeneous reactions normally includes pressure dependence terms of the form k = P,"P,"

. . . A exp( - E/RT)

where P, and P, are the pressures of reactants X and Y, respectively. The following problems were encountered when making quantitative comparisons between data derived from different sources. (i) The exponents rn, n (etc.) may be pressure dependent and interrelated so that extrapolation of reported values to a standard condition (say P, = P, = 1.0 Torr) is not necessarily justified or realistic. Some such extrapolation may, however, be required to allow comparison of values of (log A , E ) from different studies. Additionally,it is not possible to ascertain from certain published articles the value of P, or P y to which a reported magnitude of A refers.

274

A . K . GALWEY

(ii) The value of A may be detectably influenced by whether the reaction rate is referred to the number of molecules of reactant or of product. (iii) Surface areas of catalyst may change during reaction and in some systems it may be necessary to determine this quantity both before and after rate measurements. If sintering is a factor in controlling kinetic behavior, there may be a dependence of effective area on temperature, and values of (log A , E ) are then influenced by whether or not the catalyst was aged before use. Similarly, the surface composition of binary alloys may change during reaction. Problems also arise in the meaningful comparisons of activities per unit area of different catalyst preparations, e.g., metals in the form of wire, foil, or film (144). (iv) It is not usually possible to predict values of (log A, E ) for catalytic rate processes, and no general method is available whereby the magnitudes of these parameters may be systematically varied over the whole range covercd by the compensation behavior. In consequence, the test of obedience to Eq. (2) may be restricted to a narrower range of limits than is potentially reali7able.

Despite these difficulties and limitations (which are not necessarily applicable or significant in all systems) very many instances of apparent obedience to Eq. (2) are found in the literature, which apply to data compiled from observations reported by difierent workers. This suggests that the level of catalytic activity, rather than the absolute values of A and E, is the dominant characteristic feature of catalytic behavior.

B. REACTIONS ON METALS The largest number of reported kinetic observations, available for comparative analyses in the present context, refer to reactions on nickel metal. These data are considered in some detail here to illustrate the several characteristic properties of typical rate measurements, including the complexities and inherent limitations that become apparent when attempting to discern general unifying trends. Some theoretical implications of these results are discussed to assess the possible mechanistic significance of the observations. Reactions on palladium, platinum, and tungsten, which have similarly attracted particular interest, are also considered, though at shorter length. Observations for the same rate processes on these and other metals are then reanalyzed through the use of the alternative relating feature, the chemical change occurring (cracking, exchange, oxidation, formic acid decomposition). The selection of data meaningfully cncornpassed within each group of related reactions thus identified may contain a degree of subjectivity in some instances, although, in general, a broad intcrpretation of each grouping has been used.

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

215

1. Nickel (Kinetic Results) From the some 150 values of (log A , E ) found in the literature for reactions on this metal, compensation effects were discerned in related processes grouped under the following general headings. a. Cracking Reactions. The compensation plot found for cracking reactions on nickel is shown in Fig. 2 and calculated (Appendix TI) parameters for the line given in Table I, A (i.e., Table I, row A). The term “cracking” is taken for the present purposes to include all rate processes in which there is believed to be dissociative adsorption of surface-bonded methane. The points in Fig. 2 refer to the exchange (39, 145-147) and dissociative adsorption (68, 148) reactions of methane, ethane, and propane; hydrogenolysis of

25C

20c

15C

E

100

50

0 15

20

25

30

35

40

log A

FIG.2. Compensation plot for cracking and related (see text) reactions on nickel (Table I, A). The error in the individual points is indicated by the size of each cross; the line was calculated by the least squares method (Appendix Il),and standard deviations of slope (oe)and intercept ( o B are ) indicated.

TABLE 1 Compmsafion Beharior .for Reactions on Nickel arid on Nickrl Carbide" Number

Keaction'

Solid

P

A B

C C C

Ni Ni,C Ni,C

I D 11 E 111 F IV G V H VI I

E

NI

E E E H E + H D + H D F

Ni Ni Ni N1 Ni Ni Ni Ni

0.1015 0.0859 0.0856 0.0901 0.09 1 I 0.1052 0.1718 0.0849 0.0878 0.1064 0.08 10 0.1019

Refcrencc"

c

J

K L

0,.

n

0.0038 0.00I6 0.0033 0.0133 0.01 12 0.01 25 0.0232 0.0240 0.0104 0.009 15 0.00606 0.0125

16.950 15.909 16.206 21.045 21 -488 20.421 16.086 21.072 2 1.259 20.350 18.608 18.471

--

flB

0.501 I0.232 0.4R1

0.552 0.167 0.808 0.909 0.949 0.423 0.536 0.533 1.198

0I.

P(K)

of points

1.283 0.338 0.717

527 620 622

13

1.590

59 1

15 20

1.297 0.917 0.525 1.459 1.382 1.29I 0.617 1.498

584

16

506 310 627 607 500 657 522

6 4 18 34 24 14 I?

12

Values and standard deviations of parameters that define mmpensation effects, calculated using methods described in Appendix IJ Roman nurntrals refer to lines on Fig. 4. ' Reactions: C,cracking; U, dehydrogenaiinn; E,exchange; F. formic acld decomposition; H, hydrogenation. a

Range ofAE

35-244 20-235 20-235 0-118 0- 118 30-1 18 27-56 13-66 0-118

13-123 32-132 40-140

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

277

log A

FIG.3. Compensation plot for cracking reactions on nickel carbide (see text, Table I, B), line calculated by least squares method (see Appendix 11). Points for reactant mixtures containing hydrogen, 0 ; line for cracking reactions on nickel metal (from Fig. 2) shown dashed.

hydrocarbons [(74,92,122, 147,149-152) and, using estimated areas of the amines (112, 156,157), ether (158), hydrocyanic nickel catalysts (153- m)], acid (159), steam reformation of hydrocarbons (160), and nickel carbide formation (161). Cracking reactions on nickel carbide in the absence ofadded hydrogen (22), hydrogenation of nickel carbide (162), and the reactions of water and of sulfur dioxide with this solid (163)exhibited a different compensation line (Fig. 3, full line, and Table I, B) from that for cracking reactions on the metal (Fig. 3, dashed line). When data for the reactions of propane on nickel carbide in the presence of some added hydrogen (0on Fig. 3) (22) are included in the calculation, the position of the line is almost unchanged, but the values 2, Table I, C). of (T are significantly increased (by the factor x

-

b. Exchange and Hydrogenation Reactions. The available data are summarized on the composite compensation plot, Fig. 4, and literature citations (164-183) are specified in the legend. A variety of criteria may be used for

278

A . K . GALWEY 150 EXCHANGE

HYDROGENATION

I

II

P

m m

100

/:m

50 I)

C

5

lE

20

22

24

log A

26

28

30

32

20

22

24

26

I

t

28

30

1

log A

FIG. 4. Kinctic data for cxchangc and hydrogenation reactions on nickcl and calculated [Appendix 11) compcnsation lines for groups o l related reactions. Exchange reactions: H,:D,, o-p-H,:0(3, 164- /66); hydrocarbons, acetone, ether: 0 ( 1 6 7 - / ? / ) ; amines and mcthanol: x (172 174). Hydrogenation reactions: olcfins. cyclic compounds: * (33, 171, 175 183). Compensation lines (see also Tahlc I ) were cdlculatcd for the following groupings: 1.0.0, x ; I1,O.o; III..:IV. x:V.*:Vl.o,., x.*

the identification of groups of related reactions and the trends, which might be regarded as compensation effects, are represented by the calculated lines I-VI on Fig. 4. Calculated parameters (Table I, D-K) show that most values of 0, are - 0 . 1 2 ~and, as seen rrom Fig. 4, this corresponds to a considerable scatter of data. Indeed, it is doubtful whether these compensation trends can be regarded as being meaningfiil at this level of uncertainty. I f the data for line V on Fig. 4 (and Table I, H) for hydrogenation are considered together with comparable observations for cyclohexane dehydrogenation, the compensation trend is improved (Table I, J). The line calculated from the combined data for dehydrogenation reactions of cyclic hydrocarbons (184) and t-butanol (185) (Table 1, K ) gave an improved value of crV ( =0.075e), compared with those found for the individual compensation plots (re= 0.13r and 0.20e, respectively). While this relationship (Table I, K) might appear to represent an acceptable demonstration of compensation behavior, it is also apparent that the individual points fall within the scatter of values on the plot for cracking reactions (Fig. 2, Table I, A). There would appear to be no satisfactory criterion that would enable a decision to be made from the evidence available as to whether these two sets of data (Table I, A and K ) are different compensation lines that cross or are to be regarded as a single compensation effect.

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

279

There are other reports that mention compensation behavior on nickel catalysts (186- 1881, but since A values are not given in appropriate units, these data cannot be compared quantitatively with the above results. c. Dccompositio/z of Formic Acid. There is some evidence of a measure of compensation in the decomposition reactions of formic acid on nickel metal (3, 189-194) and on the doped metal (1911, but again the scatter of data (Table I, L) was appreciable. 2. Nickel (Throretic~ilDiscussion) a. Compensution Behavior. While at first sight the value of C T would ~ appear to provide the most satisfactory criterion for a test of the obedience of data to Eq. (2), in practice the magnitude of this parameter must always be considered with reference to the range of values (AE and A log A ) for which measurements are available and the number and spread of points recorded. From the critical consideration of all data in Table I and Figs. 2-4, it may be seen that both crL and oB are less than satisfactory measurements of the scatter of data about the compensation line. The value of the latter parameter is sensitive to the distance of points from the intercept of the line on the log A axis. We conclude, however, that the value of o ~ ,considered , with reference to the range of values AE for which observations are available, provides the most realistic quantitative assessment of the accuracy of fit of data to the compensation relation. Using this criterion we have reached the opinion 0.1~ is indicative of the existence of a weak correlation that a value of cr', between log A and E (as for most ofthe hydrogenation and exchange reactions given in Fig. 4). When oQ < 0.05~there is a much more positive demonstration of the existence of a compensation eflect (as in the cracking reactions, Figs. 2 and 3). These interpretations of the significance of observed trends are used as a working hypothesis throughout this review, so that the recognition of compensation behavior is based largely on the accuracy with which e in Eq. (2) is defined by a given set of data. This parallels the approach developed by Exner (6a),who used for a similar purpose the reliability of the measurement of the isokinetic temperature /?,since e = (R/?)-'. If it can be accepted that the compensation trends given in Table I for reactions on nickel metal are a meaningful representation of the presently available kinetic data, the following conclusions can be drawn. Values of e are 0.08-0.1 I (except for the reactions of amines, Table I, G). Exchange and hydrogenation reactions are relatively more rapid than cracking reactions because of the greater values of B, but fi values for the former are often the larger.

-

280

A . K. GALWEY

b. Hydrogenation Reactions. The reason for undertaking the detailed investigation of the applicability of Eq. (2) to the data in Fig. 4 was that compensation behavior has already been described for certain hydrogenation reactions. For example, in a study of the hydrogenation of ethylene on nickel, Tuul and Farnsworth (187)report a compensation line (e = 0.17 and B = 18.78) that passes between the points on Fig. 4:the value of e is larger than those in Table I, while the scatter of data is appreciably less. Clearly, further observations are required to characterize behavior in these reactions and to identify the reasons for the various differences found. The trends given in Fig. 4 are, at best, very approximate and no interpretation of the pattern of kinetic behavior can be based on the presently available data. This comparison demonstrates that a meaningful compensation relationship is not readily identified for these reactions, since alternative selection of data results in marked variations in the calculated values of B and e. All except one of the values of log A in Fig. 4 are significantly less than the theoretical maximum [ - 3 2 (3)]. While Laidler (121) has ascribed such differences to a steric factor, we wonder whether this effect is sufficiently large 4 (187)l. to explain observed compensation behavior [A log A N

c. Crucking Reactions. The many and diverse rate processes, encompassed within the compensation effect shown in Fig. 2, are related through possession of the common mechanistic feature that adsorbed hydrocarbon species undergo partial or complete breakdown with the formation of chemisorbed methane and hydrogen. It is well known that hydrocarbons are readily and extensively dissociated on nickel surfaces (68, 72), and hydrogenolysis (125) and exchange (39) reactions may involve surface monocarbon units and/or carbide formation. Interconversion processes that may participate in the surface reactions of surface-bonded methane include the following (Scheme I), CH&s) Reactant

11

--*

CH3

I

S

HJgas)

11

+ yH+CHZ + (y + l ) H +CH + (y + 2 ) H e C I / I 1 I t s s s s

t (y

+ 3)H I

S

sNi,C i(bulk carbide)

(1)

where S represents the metal surface. There is evidence in the literature, from which the data in Fig. 2 were obtained, that some, or all, of the species mentioned in Scheme I are present during the reactions considered. Even when these are not probable intermediates or participants in the dominant transformation studied, there are sometimes indications that surface monocarbon units are present. For

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

28 1

example, methane is evolved during ethane exchange (145) and during ethylamine hydrogenolysis (156); also nickel carbide formation accompanies methylamine hydrogenolysis ( I 12). It seems reasonable to conclude, therefore, that the common feature ofthese rate processes, which behave as a group of related reactions (Fig. 2), is the dissociative chemisorption of hydrocarbon and the consequent coverage of an appreciable (perhaps large) fraction of the surface by the species of Scheme I equilibria. Those data in Fig. 2 which refer to hydrogenolysis reactions exhibit relatively large value Arrhenius parameters ; the surface H/C ratio is probably high, while carbon deposition (and possibly total surface coverage) is small. When the availability of hydrogen is reduced, as, for example, in the dissociative adsorption reactions of hydrocarbons, the values of log A and E are generally smaller. The point found for the hydrogenation of nickel carbide [Fig. 3 and (162)l is close to the line for the cracking on nickel (though omitted from Fig. 2), there is undoubtedly a high surface occupancy by carbon here, and the Arrhenius parameters are the lowest of all those measured. These changes are paralleled by the tendency for there to be a diminution of the hydrogen pressure dependence exponent (nin Pk2)with increase in E (162). A mechanistic explanation of this pattern of kinetic behavior is provided by a reaction model involving common surface equilibria (Section 11, E), in which the species given in Scheme I dominate or at least occupy a significant fraction of the active catalyst area. A possible interpretation of the compensation behavior of Fig. 2 is that there are, in the various reactions, different temperature dependences of the surface concentrations of those surface entities (Scheme 1) which participate in the product evolution step. The variations in the relative availabilities of these species in the presence of different gaseous reactant mixtures and conditions provide the link in the pattern of comparable kinetic behavior. This model is also capable of explaining (Appendix I) the abnormally large values of A that have been reported for some cracking reactions. We also note that the isokinetic temperature for these reactions on nickel ( p = 527 K, Table I, A) is somewhat below the temperature of nickel carbide decomposition [ -630 K (162)l but presumably within the range of onset of mobility and reactivity of carbon in the nickel lattice. The catalytic activity of nickel in these cracking-type processes is, therefore, associated with properties of the probable intermediate, surface carbide, which can undergo stepwise hydrogenation. It is appropriate to discuss this pattern of kinetic behavior, and the interpretation offered here, with reference to problems that arise in the elucidation of the mechanisms of cracking reactions of hydrocarbons on nickel. The hydrogenolysis of ethane has been the subject of many studies and it is believed by (inter &a) Sinfelt ( 7 4 , T6tCnyi (87, 123), Shopov (I24),and their co-workers that the rate-limiting step is carbon-carbon bond rupture. In

282

A. K . GALWEY

contrast, Anderson and Baker (92), Free1 and Calwey(22),and Shephard (122) have suggested that the rate-controlling process may be methane desorption. In a more recent contribution to this discussion, Frennet et ul. (125) conclude that a mechanism cannot be established from the kinetic evidence currently available and more detailed information is required concerning the identities of the surface species involved and their interactions. The detailed arguments for these conflicting viewpoints are given in the articles cited and need not be reiterated here. It is relevant to point out, however, that kinetic data for these cracking reactions fall on the more widely defined cracking line (Fig. 2) and if this is attributed to the existence of several interlinked surface equilibria and interactions, then it follows that observed kinetic parameters A, E are not necessarily to be meaningfully identified with a single rate-controlling step. We conclude, with Frennet et ul. (125),therefore, that sufficient evidence is not at present available to allow the unambiguous identification of a unique rate-limiting step in the cracking reactions of hydrocarbons on nickel. We also believc that the occurrence of the isokinetic behavior in this group of related reactions is indicative of the participation of one or more comparable surface intermediates. Thus, compensation behavior can be regarded as providing a unifying concept in the representation of these kinetic data and we ascribe this to similar patterns of surface behavior. However, until more direct information is obtained concerning the concentration and reactivities of the important participants in product evolution, the identity of the slow process can only be inferred from indirect evidence. Care is required in making such mechanistic interpretations on the basis of kinetic comparisons alone, since here the isokinetic temperature is within the range accessible to measurements and the relative values of rate constants may depend on the valuc ot'tcmpcraturc uscd for the comparisons. I t is also relevant to mention that surface equilibria and reactions may include surface species in addition to those mentioned in Scheme I. The carbon-carbon bond rupture step is not necessarily irreversible: Anderson (38)has pointed out that there is some propensity for carbon-carbon bond Jbrrnufionon nickel since ethane is produced during nickel carbide hydrogenation (162),presumably resulting from reactions of monocarbon fragments. We also note that reactants containing nitrogen and oxygen gave Arrhenius parameters that defined points close to the compensation line (Fig. 2), showing that the presence of these elements, presumably as hydrogenated species on the surface, did not markedly disturb the postulated adsorption equilibria or reactivity of the participating intermediates. d. Crrrcking Rtwcrions on Nickel Carbide. Observations for these reactions (22) may also be described by the same mechanistic representation as that given above for cracking on nickel metal. The point located by the

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

283

Arrhenius parameters for nickel carbide hydrogenation (162)is close to both lines on Fig. 3. Compensation behavior for reactions on the carbide phase must include an additional feature in the postulated equilibria, to explain the removal of excess deposited carbon, if the active surface is not to be poisoned completely. The relative reduction in the effective active area of the catalyst accounts for the lower rates of reaction on nickel carbide, and the difference in the compensation line from that ofthe metal (Fig. 3) is identified as a consequence of the poisoning-regeneration process. After any change in reaction conditions, a pcriod of reestablishment of surface equilibria was required before a new constant reaction rate was attained (22). I n these reactions, me!allic nickel is formed under conditions of net carbon deposition, indicative of mobility of one (or both) elements during the catalytic process. When small amounts of hydrogen were included in the gaseous reactant mixtures, the values of (log A , E ) were significantly closer to the compensation line characteristic of reactions on the metal, as expected for displacement of catalyst equilibria in the direction of an increase in the area of the active metal phase present, presumably due to surface reduction. We conclude, therefore, that the mechanisms of catalytic cracking reactions on nickel metal and nickel carbide are closely comparable, but that the latter process is subject to an additional constraint, since a mechanism is required for the removal of deposited carbon from the active surfaces of the catalyst. Two phases are present during reactions on the carbide, the relative proportions of which may be influenced by the composition of the gaseous reactant present, but it is not known whether the contribution from reactions on the carbide phase is appreciable. Since reactions involving nickel carbide yielded products other than methane, surface processes involved intermediates other than those mentioned in Scheme I, although there is also the possibility that if cracking reactions were confined to the metal present, entirely different chemical changes may proceed on the surface of nickel carbide.

3. Palludiznm Analysis of the available kinetic data for catalytic reactions on palladium revealed several points of similarity with the behavior described for nickel (see above) and, accordingly, observations need not be discussed at length. Compensation behavior was apparent for cracking reactions (Fig. 5 , Table 11, B), again using the broad definition of this term to include the exchange of methane (21, 39, 195) and ethane (145), in addition to the hydrogenolysis of hydrocarbons (74, 93a. 151, 196) and nitrogen-containing compounds (112, 156, 157, 159). A different line was discerned for exchange and oxidation reactions: combination ofthese two sets of data resulted in a slight reduction

284

A.

16

18

20

22

24

K . GALWEY

26

28

30

32

34

36

38

40

log A

FIG. 5. Compensation behavior in reactions on palladium metal. The lines (Table 11) were calculated (Appendix 11) for cracking ( 0 ) and for exchange ( x ) and oxidation ( 0 ) processes (data references given in text).

in the value of or, which is an indication of the possible occurrence of a common compensation line for the alternative types of chemical process. The line Table 11, C refers to the exchange of various hydrocarbons (3, 168, 171,197-202), acetone (169),amines (etc.) (159,172,I74),ether (170), alcohols ( I 73), and water (204, and the oxidation of hydrocarbons (204-209~).No significant trends could be discerned in the Arrhenius parameters reported for hydrogenation and dehydrogenation reactions. The calculated compensation line (e = 0.1292 +_ 0.0083; B = 16.452 +_ 0.668; oL = 0.466) for the epimerization reaction of cis- 1,4-dimethylcyclohexane on palladium (209b) is almost parallel with the line for exchange and oxidation on this metal (Table 11, C), and data exhibit a systematic trend across the line characteristic of the cracking process (Table 11, B).

4. Platinum The general pattern of compensation behavior found for platinumcatalyzed reactions was appreciably different from that described above for nickel and palladium. Moreover, since some of the trends were less well defined, these may not represent meaningful obedience to Eq. (2).

TABLE 11 Compensation Behattior for Reactiorrs on Various Metuls"

Reference

Reaction*

A R

c c

c

E t O

D E

C

F G H I J K L M N 0

E

P

c

]

fJe

0.1015 0.0779 0.1 188

0.0038

0.0838 0.062 1 0.108h 0.1 593 0.1171

0.0059 0.0055 0.0045

2:

r 8 3

MoS,

0.1127

0.0060

rt

C + O C + O

e

0.0033 0.0153 0.0203 0.0169 0.0203 0.0117 0.0059 0.0067 0.0593 0.0151 0.0081

Pt

D + H

E

Ni Pd Pd Pt PI

E 0

C E H C

Solid

Pt Pt Ru Rh W W W

0.0551 0.1767 0.1109 0.1335 0.2600 0.1066

Number of points

Range of A E

43

2.009 1.390 1.028 0.669 2.019 1.099 0.597

527 6x3 448 624 842 4x5 3 29 447 948 472 672 392 210 490 532

12 8 14

35-244 74- 239 22-136 41-243 96-222 43 105 14-106 48-1 19 26- 136 94-197 45-218 28-136 24-54 20-98 42-119

0.455

464

12

4-86

BW) 16.950 20.192 18.356 17.495

18.024 19.071 22.017 9.587 17.401 16.131 14.127 15.959 14.881

0.501 0.774 0.405 0.663 0.436 1.255 1.220 1.375 1.243 1.721 0.721 0.512 2.215 1.047 0.596

15.431

0.276

17.232 17.486

1.28.3 1.270

0.886 1.341 0.356 0.929 1.680

1.019

19 32 22 6

IS 9

10

16 16 13

8

Valucs and standard deviations of parameters that dcfinc compensation effects, calculated using methods described in Appendix 11. I, Reiictinns: C, cracking: D, dehydrogenation; E, exchange; H. hydrogenation; 0, oxidaiicm.

286

A. K . GALWEY

Using here the more restrictivc definition ofa cracking reaction as a process in which there IS rupture of at least one carbon-carbon bond in a hydrocarbon (7’4,93o,127.148,150,210-213), ethylamine (156),or ether (214, the compensation trend Table 11, D was found. Data reported by Tktinyi et al. (215)were represented by the different line Table 11, E. Exchange reactions were more satisfactorily considered in two groups: (i) reactions of C,, C2, and C3 hydrocarbons (21, 39,145,215,216) and hydrogenolysis of nitrogen-containing compounds ( 1 12, 156,157’,159),which gives the line Table 11, F, and (ii) reactions of other compounds (C, and above) and nitrogen-containing compounds (159, 168, 169, 17’1, 174,198,200,215), which give the line Table 11, G. This pattern of behavior resembles those described for nickel and for palladium in recognizing two groups of exchange processes but differs in that the data for the lighter molecules do not coincide with the line for the (more broadly defined) cracking reactions. A compensation trend has also been described for exchange reactions of butanes (217): from these measurements we calculate v = 0.1 126 and oe = 0.01 16. A compensation trend was apparent in data for oxidation reactions on platinum (205, 20%) (Table 11, H). Obedience to Eq. (2) was, however, less than satisfactory for hydrogenation and dehydrogenation processes (Table 11, I). Tktknyi et [I/. (184) report compensation behavior for the dehydrogenation of cyclic hydrocarbons (for these data we calculate r = 0.0843 and oc = 0.0035). Various combinations ofthe data for exchange (C, and above), hydrogenation, and oxidation reactions yielded no improvement in the calculated values of cr?. More systematic investigations, including further experimental measurements, are necessary to establish whether the pattern oftrends of log A and E values described are to be regarded as meaningful compensation behavior.

5. Riithvriitrrn Parameters for the compensation line found for cracking reactions (74,151, 212, 213,218),methane exchange (21),and also including data for the oxida) this metal are given in Table 11, J. tion of ethylene and propylene ( 2 0 9 ~on

6. Rhodium Data for the common compensation line on this metal for cracking of hydrocarbons (74,151,212,213)and ethylamine (156),also including methane ) reported in Table exchange (39,219) and oxidation reactions ( 2 0 7 , 2 0 9 ~are 11. K .

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

287

7 . Tungsten Cracking reactions on tungsten (68,92),including methane exchange (39), exhibited significant compensation behavior, Table 11, L. Obedience of exchange reactions of hydrocarbons (145, 168, 171, 197,220),nitrogen-containing compounds (159, 174), and acetone (169) to Eq. ( 2 ) was, however, less satisfactory, Table 11, M, since changes in the magnitude of E with log A were relatively smaller than those characteristic of comparable reactions on other metals (Tables I and 11). Hydrogenation reactions of amines (112,156, 157),cyclopentanone (1761, and ethers (221)exhibited an approximate compensation trend, Table 11, N ; the line found was close to that for cracking reactions. Values of e = 0.1 262 and (T, = 0.01 13 were calculated for the combined data for cracking and hydrogenation processes.

Tzrngsten DiszrlJlile and Molyhdenirm Disuljide. Arrhenius parameters reported for reactions on both these catalysts (222- 225) were close to two compensation lines, characteristic of cracking and exchange processes (Table 11,Oand P).Since the reactants studied in this work were different from those used in kinetic measurements on tungsten metal, no direct comparison of activities is appropriate. Data for reactions of thiophene did not correlate with either line.

8. Other Metals Although values of (log A , E ) have been reported for reactions on several further metals (in addition to those specifically mentioned above, e.g., Cu, Au, Ir, Fe, Ag), the data now available are insufficient to permit the meaningful recognition of compensation trends. There is no indication, however, that the kinetic characteristics of reactions on these metals are significantly different from the patterns of behavior described in the previous paragraphs. Data for an extended range of metals have been included in the section below, where related reactions are grouped on the basis of a common chemical transformation.

9. Cracking Reuctions a. Hydrocurbons. A compensation trend is present in the large number of values of (log A , E ) reported for the hydrogenolysis reactions of normal and branched C,-C, hydrocarbons catalyzed by various transition metals. The line Table 111, A was calculated from the following data [(38),p. 180, and (74, 127, 148,149, 151, 152,207,210-212,218,226)l. The relatively small value of C T is ~ attributable to the extensive range covered by the Arrhenius parameters; the considerable scatter of the data is reflected in the large oL term. Similar calculations were performed using the data listed by Anderson and Baker

TABLE 111 Compensation Bekarior f o r Rearlions u f e r Croups of DiFerent .bfetal.f

Reactionb

B

6,

B

UB

cL

b(KI

A

C

B

C

0.1030 0.0868 0.0773 0.1223 0.0407 0.1204 U.0999 0.1219 0.0959

0.0072 0.0048 0.0067 0.0171 0.01 11 0.0175 0.0055 0.0103

17.228 18.532 20.713 14.869 27.534 16.228 17.897 18.595

1.064 0.703 0.982 2.431 2.029 1.351

0.0068 0.0098

18.348 20.601 16.243 13.344 7.422 17.628 17.244 19.820 18.683 18.463 20.354 18.329 17.013

0.599

2.855 1.380 2.574 1.325 1.100 1.316 0.962 0.892 0.551 1.321 0.417 0.249 0.794 0.690 0.307 0.792 1.296 1.452 0.628 0.380 0.628

580 603 677 428 1280 434 523 329 54 5 574 406

Metals

Reference

Ni: Co, Fe, Ptl Pdl W, Rh, Ru, and others

C D E

C C

F G H I J K

H E

I.

F

M

F F

See ref. (681 Various metals Pt: Pd A&, .4u, Ru. Rli Pt, Pd Agilll! 4g( I 10) .4g(lMIj 4g

N 0 P

Ni, Cu, Au. Co. Fr Ni Cu,Au Pd, Pt:Rh, Ir Ni. Ru, Cu, Fe, Col Au, Ag

Q R S T U

c 0 0 0

1.

F F F F F D D

0.0911 U.1290 0.1418 0.1804

0.0124

0.0064

0.1072

0.M73 0.0061

0.11Y2 0.0920

0.0027 0.0123

0.1023 0.1020 0.0816 0.0862 0.0793

0.0081 0.0121 0.0095 0.0086 0.0032

0.581

0.860 0.893 0.970

0.832 6.439 0.687

0.282 1.340 0.754 1.149 0.619 0.445 0.279

Number ofpoints

Range ofAE

56 23 50 8 6 17 24 19

29-239 29 -239 29-239 88-183 131 -239 40-101 36-190

11

51-118 51 -150 49-84 94-142 123-145 49--145 73-144 65-127 42-148 42-139 30-95 24-78 33-190

30 6 ti 3

369 290 488

16

439

46 7 32 18 7 10 20

564 51 I 513 641 606 660

Values and standard deviations of parameters that define compensation effects. calculated using methods described in Appendix 11.

’ Reactions: C, cracking; D, dehydrogenation; E, exchange; F, formic acid decomposition; H, hydrogenation; 0, oxidarion.

55-119

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

289

[(38),p. 180 and a later list given by Anderson ( 2 2 7 ~p. ) ~711; parameters for the compensation lines so defined are given in Table 111, B and C, respectively. Although there is extensive overlap of the observations used in these analyses, the best-fit lines were not coincident, indicating a dependence of the magnitudes of B and e on the selection of data and representing, therefore, an unsatisfactory correlation. Calculated parameters for the compensation effects mentioned by Sinfelt (74) are given in Table 111, D [for the dashed line in Fig. 5 of(74), eight metals] and Table 111, E (the full line on the same graph, six metals). No acceptable obedience to Eq. (2) could be found in other Arrhenius parameters for catalytic cracking reactions on a group of different metals (151, 212, 213). We conclude that, while there is a tendency among values of (log A , E ) for cracking reactions on various active metals to show some compensation, the correlation is at best only qualitative and may not be significant. From the critical comparisons of the above observations, we conclude that obedience to Eq. (2) is more satisfactory when the rate processes considered relate to cracking reactions on a single metal. The scatter of data on the most general compensation line (including reaction on several metals, Table 111, A) is therefore at least partially attributable to variations of the values of B and/or e on the individual metals concerned. Arrhenius parameters for the methanation reaction on alumina-supported Group VIII metals (227h) were close to the line for cracking reactions on several metals (Table 111, A). Activity was based on the numbers of surface metal atoms and a compensation relation was described: from these data we calculate e = 0.1185 f 0.0117, B = 15.216 k 1.068, and cL = 0.491.

b. Amines. Data for amine hydrogenolysis (112,156, 157) exhibited considerable scatter and, in consequence, there was only very approximate obedience to the compensation relation (Table 111, F). Points were close to the line (Table 111, A) characteristic of hydrocarbon cracking. The extrapolated line (Table 111, F) did not pass through the results for ammonia decomposition (228) and no compensation trend was discerned in these latter observations.

10. Deuterium Exchunge Reactions The exchange reactions of methane with deuterium over several metals (21,39,146,147,195, 215,216,229) exhibited compensation behavior (Table 111, G), as has been pointed out previously (3).No such relationship was found in the data available for exchange reactions of C2-C6 hydrocarbons. This is consistent with discussion given above (see Section 111, B, 4) in which it was concluded that the differences in the kinetic behavior of the exchange reactions between methane and the other hydrocarbons varied with the individual metals concerned.

290

A. K . GALWEY

The trend between increases in log A with E for reactions of amities (174) represented only approximate obedience to Eq. (2). The most notable feature of the exchange reactions of ammonia on eight metals (172)was the relatively small range of A ( A log A = 0.8) compared with the variation in E (21 < E < 58). 1 1. Hydrocarbon Oxitlntion Rructions

From examination of data for the oxidation of olefins over the noble metals, it was concluded that the observations were most satisfactorily represented by two compensation relations that refer to reactions with oxygen ovcr platinum and palladium (204-209a),Table 111, H, and the same rate processes over silver, gold, ruthenium, and rhodium (103, 207, 209a), Table 111, 1. Arrhenius parameters for the oxidation of hydrocarbons on platinum and palladium, reported by Moro-oka r r ul. (42,230),defined points that were between the compensation lines Table 111, H and I. When these latter observations were included with other results for reactions on the same two metals, the new line, Table 111, J, was found. Values of B and e were appreciably changed so that lines of different slope (i.e., Table 111, H and J) intersected near the region of maximum density of points on the compensation plot. Again, the calculated values of B and e were sensitive to the selection of results for quantitative consideration. 12. Decomposition Ructions of Formic Acid

Several compensation effects have been reported for the decomposition of formic acid on metals (231);particular interest has been shown in the silvercatalyzed reaction (19, 35, 232, 233). The available data for this rate process do not, however, define a single compensation relation, and, even for groups of closely related reactions, it is difficult to decide which of the various possible calculated lines provides the most meaningful representation of the kinetic measurements. Values of B and e, obtained from various sets of reported results, extend over a significant range and, moreover, there is an apparent tendency for an increase in the value of one of these parameters to be offset by a diminution in the other. Such behavior can, in the widest sense of the word, be described as compensatory. To illustrate the difficulties inherent in this analysis of the published data, some calculated values of B and r derived from possible alternative groups of observations are discussed here. Values of (log A , E ) reported by Sosnovsky (35),for the decomposition ol formic acid on silver, may be considered either as the three different lines, Table 111, K, L, and M for reactions on the (1 1 I), (110),and (100) planes, respectively, or as the single line,Table 111, N.These combined data (Table 111, N and x on Fig. 6 )intersect with the line obtained from a different set of results

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

16

20

24

28

32

29 1

36

log A

FIG.6 . Compensation behavior including calculated (Appendix 11) lines (see also Table 111) for the dccomposition of formic acid on silver metal. Data obtained from: (35), x and dashed line: ( I Y , 233), and solid line.

(19,233)(Table 111, 0,and 0 on Fig. 6) reported for the same reaction and characterized by a comparable range of A and E values. A further line, obtained from another group of reported measurements (3,234, 235), is given in Table 111, P. Values of 8 and e are sensitive to the selection of data for the analysis. Compensation trends found for decomposition of formic acid on metal (and other) catalysts are represented diagrammatically in Fig. 7. Line I (Table 111, Q) refers to reactions over nickel and copper (3, 190, 194, 236), gold (3,189,237), cobalt (137,194),and iron (194):the observations included in this group were obtained by selection, since other metals, which showed large deviations, were omitted [see also (3),p. 4221. Line I is close to that calculated for the reaction catalyzed by nickel metal (Table 111, R) (3, 137, 189- 194, 238). Lines I1 (19,233) and 111 (3, 234, 235) (Table 111, 0 and P) refer to decomposition on silver. The other lines were found for the same rate process on: IV, copper--nickel alloys (190); V, oxides (47, 137); VI, tungsten bronzes (239);and VII, Cu,Au (Table 111, S ) (240~1). The kinetics and mechanisms of catalytic decomposition of formic acid on metals (and oxides) have been extensively described ( 3 , 137, 232, 240h) and these references provide access to the considerable literature. Here we discuss only those aspects of reaction rate which impinge upon compensation phenomena.

292

A.

K. GALWEY

150

100

E

50

0 15

20

30

25

35

log A

FIG.7. Compensation lines (see also Table 111) calculated (Appendix It) for the decomposition of formic acid o n various solids: I, metals (Ni, Cu, Au. Co, Fe); 11, silver (19, 233): Ill, silver (3,234, 235); IV, Cu-Ni alloys ([YO): V, oxides (47, 137); VT, tungsten bron7es (239); and VII, Cu,Au ( 2 4 0 ~ ) .

Recent studies of the copper-catalyzed decomposition of formic acid (104) and of the pyrolysis of copper (11)formate (241 )have provided strong evidence that both reactions involve the formation of an unstable and volatile intermediate, probably copper (I) formate. Concurrent copper sublimation during both decomposition processes indicates sufficient mobility of a participating species for this to migrate beyond the confines of an active reaction interface. Compensation behavior (3) in the decomposition of formic acid on copper can, therefore, be attributed to a common equilibrium model and, in this instance, the entities involved arc not restricted to species chemisorbed on metal. The concentration of the volatile participant, which may control the reaction rate and which certainly influences the effective area of the catalyst metal, depends on the prevailing conditions including temperature, catalyst geometry (size, disposition, etc., of particles), and gases present. There is appreciable catalytic activity when there is a significant rate of formation of this intermediate and the characteristic kinetic behavior must be influenced directly or indirectly by the effective mean lifetime of this species. We have here, therefore, some experimental justification for the occurrence of the reaction model of the type described in Appendix I, since the effective concentration (and lifetime) of the intermediate and the catalyst surface area may be expected to vary with temperature to an extent that depends on reaction conditions. Compensation behavior in the decomposition of formic acid on silver has been ascribed to the participation of (at least) two active areas of surface

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

293

[the (1 1 I) plane and other crystallographic surfaces for which the value of E is different (19,35,233)].It has also been concluded that the apparent value of E for formic acid breakdown varies with surface coverage (242).It would appear that no quantitative comparisons have been made between the rate of this reaction and that of silver formate decomposition. Since there is evidence that the latter process involves the formation of a volatile intermediate (243), it would be appropriate to make a detailed kinetic and mechanistic comparison of these two rate processes to determine whether or not the pattern of behavior and, in consequence, the reaction mechanisms are generally comparable with those described above for copper. The possible role of nickel formate as an intermediate in the breakdown of formic acid on nickel has been extensively discussed (3,232,240b,244): this is another catalytic reaction in which there is compensation behavior (Table 111, R). While the observed obedience to Eq. (2) does not identify the reaction mechanism, it is probably significant that catalytic activity becomes apparent in the temperature range of onset ofsalt instability. Again it may be envisaged that the temperature dependence of effective concentration of nickel formate intermediate may vary with reaction conditions. The compensation relationships mentioned here for the decomposition of formic acid on metals (Table 111,K-R and Figs. 6 and 7) cannot be regarded as established, meaningful kinetic descriptions of the reactions concerned, since the magnitudes of the calculated values of B and e depend on the selection of data to be included in the calculation. While there is evidence of several sympathetic interrelationships between log A and E, the data currently available do not accurately locate a specific line and do not define values of B and e characteristic of each system, or for all such systems taken as a group. The pattern of observations is, however, qualitatively attributable to the existence of a common temperature range within which the adsorbed formate ion becomes unstable. The formation of this active intermediate, metal salt, or surface formate, provides a mechanistic explanation of the observed kinetic behavior, since the temperature dependence of concentration of such a participant may vary with the prevailing reaction conditions. 13. Other Reactions Other groups of Arrhenius parameters obtained for reactions on two or more metals and which have been shown to exhibit compensation effects include para-hydrogen conversion (on Cu, Ag and Au) (144),hydrogenation of methylacetylene (188) and of allene (245),and the inverse compensation behavior in decomposition of acetylene on metal f i l m (26). Compensation parameters calculated from kinetic data reported by T6ttnyi et al. (184, 246, 247) for the dehydrogenation of cyclohexane and of

294

A . K. GALWEY

cyclohexene are reported in Table 111, T and U. The two lines refer to reactions over the two groups of metals distinguished previously (246), although the r a n g A E used to find Table 111, T is relatively smaller than is appropriate for this analysis. It would require a more soundly based statistical method than that used here to decide whether this is meaningfully described by the two lines, or if a single line would be more appropriate (for which r = 0.0733 and oc = 0.0045).

C. REACTIONS ON ALLOYS The relationships betwcen the individual reactions in each group of rate processes considered in this section are particularly close, since the measured Arrhcnius parameters refer to the same chemical transformation occurring over the samc metallic phase(s), although the proportions and/or composition of the latter may change. In general, the magnitudes of log A and E do not exhibit a systematic dependence on catalyst composition and do not vary progrcssively from values characteristic of the reaction on one metal to those for the other [see, e.g., (219)l. The point defined by the reaction on one of the pure constituents may be appreciably removed from the compensation line characteristic of the alloy [see, e.g., Fig. 3 of (207)J Furthermore, the spread of measurcd (log A , E ) values for reactions on an alloy of a particular composition can be greater than that which may result from variations in the proportions of the constituent elements (27). These effects have not always been specifically investigated, since kinetic studies often omit measurements of the changes in Arrhenius parameters that result from variations in such factors as reaction conditions, the method of catalyst preparation, or the pretreatment. Other considerations, which are relevant in the characterization of the properties of alloy catalysts, include the possibilities that the compositions of the active surfaces may be different from those of the bulk (114)and be influenced by chemisorption ( I 1 3 ) ;there may also be a miscibility gap in the compositions of alloys (248).

McKee (21, 195) and McKee and Norton (219, 249, 250) have reported compensation effects in the exchange reactions of methane on several pairs of binary noble metal alloy catalysts. For each combination of elements kinetic measurements were made at a number of different compositions. Although the compensation behavior was generally very similar, there were pcrceptible differences in the values of B and r calculated for the various alloy combinations. The parameters found, by use of the formulas given in Appendix 11, itre summarized in Table IV, A-E, and are subject to the following comments. In consideration of data for the Pd-Rh alloys, the point for

Kefcrcncc

F!cactionh

Alloy

e

fJe

Pd-Rh Pd-AU Pd R u Pd-Pt Pt- Rh Ni-Cu Ni-Cu Ni-Cu

0.1201 n. I 304 I). 1246 0.0974 t). 1332 0.07 18 0.1036 0.1592

0.0039 0.01 13 0.0027 0.0070 0.0023 0.0052 0.0108 0.0236

R

fJH

0,.

16.253 IS.h'3 15.898

0.466 0.875 0.275

19.131 1 h.bb2

0.9 14

0.357 0.365 0.529 0.473 0.210 0.292 0.665 0.61 7

19.953 15.000 14.026

0.204 0.347 0.898 2.313

B(K)

-436 401 420 534 393 728 504 328

Nurnhcr of pr>IIlt\

Kangc of Ah

7

45- 151 51 Y4 38- 190 84-172 37 117 33-89 51-114 82- 110

7 24 8 10

8 10 9

a Values and standard deviations of parameters that define compensation effects, calculated using methods described in Appendix 11. Reactions: C, cracking; E, exchange.

'

296

A. K . GALWEY

reaction on pure palladium, not on the compensation line, was omitted from calculation. The compensation effect in Pd-Au alloys was restricted to Pd > 50'7,: in gold-rich alloys the value of E increased without a corresponding rise in log A. Variations in values of (log A , E ) for reactions on Pt-Ru alloys were too small (249) to allow compensation behavior to be characterized. The authors of these articles discuss the influence of the band structures of the metallic phases on the measured values of E . The energetics of the stepwise and multiple exchange reactions are also considered; the relative importance of the latter process increased with temperature. The interpretation of the observations does not, however, provide a mechanistic explanation of the compensation behavior. A compensation effect (Table IV, F) has also been found in reported data (251) for cyclopropane-deuterium exchange on Cu-Ni alloys.

2. Hydrogenation Reactions Compensation trends are found in the kinetic data reported for the catalytic hydrogenation of several hydrocarbons on bimetallic systems, including the reactions of ethylene on Ni-Cu and on Ni-Au alloys (252), buta-1,3-diene on Ni- Cu alloys (183, benzene on Ni-Cu (but the behavior of benzene on Cu-Pd was less obviously compensatory) (253), and the liquid phase hydrogenation of nitrobenzene on Pd-Ag alloys (56). The kinetic characteristics of but-Zyne on Pd-Au alloys (28) was different in that E underwent little variation, while the value of log A systematically diminished as the proportion of palladium in the alloys was reduced. It was concluded that palladium was the active constituent and gold was an inactive diluent.

3. Oxidation Reactions Compensation effects have been reported for the oxidation of ethylene on Pd-Ru and on Pd-Ag alloys (207, 254, 255); discussion of the activity patterns for these catalysts includes consideration of the influence of hydrogen dissolved in the metal on the occupancy of energy bands. Arrhenius parameters reported (208) for ethylene oxidation on Pd-Au alloys were an appreciable distance from the line calculated for oxidation reactions on palladium and platinum metals (Table 111, H). Oxidation of carbon monoxide on Pd-Au alloys also exhibits a compensation effect (256).

4. Other Reuctions Variations in the activation energies for ethane hydrogenolysis on Cu-Ni alloys (147) were relatively small and, while the compensation plot showed appreciable scatter, the line found (Table IV, G) was close to that for cracking

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

297

reactions on nickel metal (Fig. 2, Table I, A). Arrhenius parameters reported (147) for methaneedeuterium exchange on these alloys were also close to the cracking line for nickel. The variations of log A and E for the decomposition of methanol on Pt-Ru alloys (257) could only be described as very approximate obedience to Eq. (2). Formic acid decomposition on Cu-Ni alloys (190) exhibited compensation behavior (Table IV, H) but no such trend was apparent for the same reaction on Pd-Au alloys (237). Compensation behavior occurs in the decomposition of hydrogen peroxide on Ag-Au alloys (25)and, unlike most other alloy systems, there is a systematic change in the Arrhenius parameters with proportions of metals present. This behavior is ascribed to the progressive transformation, with alloy composition, of the reaction mechanism from that characteristic of one metal to that which occurs on the other. In contrast, decomposition of hydrogen peroxide on Pd-Au alloys (27) does not correlate with ratios of metals present in the catalyst, and kinetic parameters are sensitive to surface pretreatment. Other instances of compensatory behavior in reactions on alloys have been described and discussed in the literature [see, for example, Bond (3): para-hydrogen conversion (pp. 170- 172), olefin hydrogenation (pp. 248 250), benzene hydrogenation (pp. 321), and decomposition of formic acid (pp. 426-429)l. 5. Discussion

A stated objective of many of the reported studies of the catalytic properties of alloys has been to elucidate the significance of the band structure of the metallic phase (i.e., the energy levels of the d electrons) in determining the energetics of reaction (i.e., the value of E ) . While significant correlations of the values of E with band structures have been found in several instances [e.g., (25,258)],the interpretation of results is not always straightforward (237)and it may be necessary to incorporate due allowance for other factors that may exert some control over the mechanisms of reactions. Such factors include the possible presence of more than one alloy phase (207),dissolution of hydrogen in the alloy (207), and the composition and disposition of elements in the active outer surface of the alloy under reaction conditions (28,113,208). While kinetic measurements are available for reactions on a number of alloy systems, detailed mechanisms of the surface steps involved have not always been established and in some system have only been partially characterized. The identification of Arrhenius parameters with specific processes is not always practicable since several factors may be involved; these include the possible influences of electronic, elemental, and crystallographic structures of the active catalyst surfaces. Compensation behavior could arise

298

A. K . GALWEY

through variations in the proportions of different active crystal faces (as in reaction model, Section 11, A, 3) or through a common surface equilibrium in which the alloy contributes two elements (and, in addition, there is the possibility that there may be dissolution of elements of chemisorbed reactants), and, in consequence, there is at least one additional variable in the interpretation. Detailed discussions of surface interactions have been put forward for only a few reactions on binary metal alloys, and no generally acccpted explanation of compensation behavior on these reactant systems has so far been provided.

D. REACTIONS ON OXIDES I . Eschrrnyr of’ 0s)grw w i d Dccnmpositiori of’ N 2 0 arid NO The mean compensation line for exchange reactions of oxygen on oxides, calculated from the extensive data reported in comparable studies by Winter (259, 260) and by Boreskov et 01. ( 2 6 / ,262) is given in Table V, A. As part of the critical comparisons of these data, similar calculations were performed on nine groups of rate processes selected as follows: oxygen exchange on (i) rare earth sesquioxides, (ii) all “other” oxides, and (iii) both sets combined from the results reported by Winter and by Boreskov rt (11. taken separately and together. With a single exception, all calculated values of the compensation parameters were within the limits 0.485 < e < 0.525 and 17.9 < B < 18.8. Since this variation is small, it was concluded that within the limits of meaningful identification all reported results (259-262) could be represented by the single line Table V, A. [Observations for reactions on Ago, CuO, Mn,03, and IrO, were, however, omitted from consideration here for reasons discussed in the report (259), and, likewise, the value for A120, was discounted from (26/).] Compensation parameters calculatcd from data reported by Winter for the decomposition of nitrous oxide (263) and nitric oxide (264) on various oxides are given in Table V, B and C, respectively. The variation of data in the latter reaction is relatively small (0,.= 0.795, Table V, C) and the values of B and E can be reliably determined. htcrpretation of the results for nitrous oxide decomposition is, however, less straightforward since the compensation trend for reactions on the rare earth sesquioxides ( B = 19.831 and e = 0.057 1 ) was significantly different from that for “all other” oxides studied ( B = 23.226 and e = 0.0321) and the combined data give the intermediate values of Table V, B. Thus, we are unable to discriminate between the possibilities that either there are two distinct lines or the number ofpoints available is insufficient to characterize fully the compensation effect at the observed level of standard deviation.

TABLE V Compenscrtion Behiirior for Reactions on Oxides"

Reference A B c' D E F' G Ii I

0, ( U Y I O (RI NO (R)

Propene (01 4cetylene ( 0 ) LIydrocarbon (0) Kydruc;irhoii ( E + 0) I lydrocarbon (0) Hydrucdrbon (E + 0) Hydrocarbon (E + 0)

J K L

M N 0 P

Q R S

Reactionb

Formic acid (F) i-Propanol (R) r-Propanol (R) C,-Alcohol (R) C,-Alcohol (R) It-Butene (R) f -Hcxane (1)) Hydrocarbon (K) Dehydration ( R )

Solids Oxidcs Oxides Oxides Oxides Oxides Oxidcs NiO C:r, () J

MnO, Cohaltites, chromites Oxides Oxides TiO,

e 0.0494

0.0430 0.0522 0.1017 0.1042 0.0XhO

0.0899 0.1193 0.0871 0.0558 0.07 15

0.1012

Oxides Oxides Clays

0.1091 0.0727 0 0773 0.1519 0.0697 0.1012

ClilYS

0.056 1

A1203

Al*O,

B

oe

0,0031 i).i~)75 0.0045 0.0079 0.0075 0.0083 0.0069 0.0125

0.0087 0.0044 0.0059 0.0061 0.0042 0.0062 0.0035 0.0072 0.0050 0.0028 0.0020

18..<22 21

.x:

16.778 15,469 19.01 7

18.396 15.301 j .1.338 17.604 i 8.872 19.298

15.604 13.401 14.446 17.941

14.337 12.73' 15 4% 20.224

gn

UL

B(K)

Number of points

Range of AE

0.384 1.042 0.457 0.929 0.885 0.994 0.706 1.290 0.995 0.395

1.266 1.207 0.795 0.960 0.774

105X

ae

41 2x5

1217 1001 519

34

82-190

311 18

so2

(I

1.890

609

1.671 1.154 1.647 0.875

63 2H

43 x 600 937

16 16

37 169 61-170 51 171 51-171 4-185 48 -127 16-227

42

33-153

0.723 0.943 0.447 0.66 1 0.361 0.613 0.752 0.404 0.704

0.782 1.984 0.730 0.235 0.262 0.940 0.843 0.309 1.906

732 517 480 720 677 388 751 517 934

I: 29 21 13 16

70-192 31-235 29- 147 87-126 71-120 25-138

9

70- I82

(r

75-195 I 6- 1060

5x2

8

15

' Values and standard deviations of parameters that define compensation effects, calculated using methods described in Appendix I1 Reactions: D. dehydrogenation: E, exchange; F, formic acid decomposition: 0. oxidation: R. other reactions.

300

A. K . GALWEY

From the critical comparison of all observations included in the determination of the compensation trends in Table V, A-C, and also remembering the scatter of data and the interrelationship between errors in B and e, we conclude that the kinetic results for all three rate processes discussed can probably be regarded as a single compensation trend. Allowing for standard error, there is overlap of the values of e at -0.049. This common pattern of kinetic behavior is consistent with the conclusion reached by Winter (263, 264) that the three rate processes involve the same rate-limiting step, oxygen desorption. No mechanistic interpretations of these examples of compensation behavior have been provided and it is not known which of the theoretical models described in Section 11, A gives the most satisfactory explanation of the systematic variation of log A with E. 2. Oxidution Reactions

Moro-oka el a/. (42,230)have reported kinetic data for the oxidation reactions of acetylene, ethylene, propene, propane, and isobutene on up to twelve different oxides and also palladium and platinum metals. Calculated parameters for the two compensation effects mentioned by these authors [i.e., those oxidation reactions of propene for which the oxygen pressure dependency exponent was <0.6 (42)and the oxidation of acetylene (230)l and for all the data given in both references are given in Table V, D, E, and F, respectively. Although similar calculations were completed for several other selected groups of related reactions, no additional significant instances of obedience of data to Eq. (2) were detected. Compensation trends were also found in some groups of rate processes catalyzed by a particular metallic oxide. Nickel oxide (as also the metal) has attracted particular interest and compensation parameters calculated for the oxidation reactions of several hydrocarbons, hydrogen, carbon monoxide (42, 230, 263--267),and also including oxygen exchange on this phase are given in Table V, G [see also (268)l.A similar trend is present in the oxidation reactions of hydrocarbons on chromium (111) oxide (42, 230,269), although the range of results available here is less extensive and the line (Table V, H) did not include data for oxygen exchange. Manganese (IV) oxide apparently occupied an intermediate position in that data for oxidation (42, 230,270) and for exchange (259, 261,264) reactions were near a single line (Table V, I) although there were also indications that the two groups of Arrhenius parameters were more correctly represented by two close, and perhaps parallel, lines. Derefi et u1. (271)have reported compensation behavior in the oxidation of carbon monoxide on nickel oxide containing various amounts of chromium oxide; the effect is attributed to the modification of the Fermi level at

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

30 1

the surface by the additive. A compensation trend has also been found in the combined data (2720)for the exchange and oxidation reactions of hydrogen and of methane on various chromites and cobaltites. The calculated line, Table V, J, crossed but was not coincident with that for oxygen exchange on oxides.

3. Decomposition of Formic Acid Trillo et al. (47,137 ) have reported Compensation behavior in oxidecatalyzed decomposition of formic acid and the Arrhenius parameters for the same reactions on cobalt and nickel metals are close to the same line, Table V, K. Since the values of E for the dehydration of this reactant on titania and on chromia were not influenced by doping or sintering, it was concluded ( 4 7 )that the rate-limiting step here was not controlled by the semiconducting properties of the oxide. In contrast, the compensation effect found for the dehydrogenation reaction was ascribed to a dependence of the Arrhenius parameters on the ease of transfer of the electrons to the solid. The possibility that the compensation behavior arises through changes in the mobility of surface intermediates is also mentioned (137). Krupay and Ross (2726),in a study of the decomposition of formic acid on manganese (11) oxide, demonstrate that manganese (11) formate is produced during reaction and discuss the probable role of this participant in the catalytic process. The reported Arrhenius parameters (log A, E ) for the dehydration and dehydrogenation reactions were (28.7, 132) and (24.9, 87), respectively: both points were close to the compensation line (Table V, K) characteristic of the breakdown of formic acid on oxides. The decomposition of formic acid on alumina and on magnesia also exhibits compensation behavior; estimated values of B and e [from Fig. 2 (273)l are 15.7 and 0.103, and 15.4 and 0.086, respectively. It is tentatively suggested that the observed variations in log A and E may arise from different proportions of exposed crystallographic planes in the various catalyst preparations. Moody and Taylor (239) have studied the breakdown of formic acid on tungsten bronzes, of a range of different compositions, and it is shown that the Arrhenius parameters exhibit compensation behavior (B = 21.6 and e = 0.093). It is concluded that the catalytic activity is controlled by geometric, rather than electronic, properties, including the occupancy of surface lattice sites by sodium and retention of some of the product hydrogen by the solid. Further kinetic data relating to the decomposition of formic acid on oxides are summarized by Mars et al. (232),who also discuss the mechanisms ofthese heterogeneous reactions. The pattern of calculated compensation lines (Tables 111-V) for the decomposition of formic acid on metals and oxides is summarized on Fig. 7.

302

A. K. GALWEY

As pointed out previously, several of the lines shown intersect in the region of maximum availability of reported data, indicating some correlation between the values of B and e. Clearly, the variations in behavior between the different systems are not large and for these to be identified precisely it is necessary to obtain many accurate values of log A and E extending over the maximum realizable range. More experimental observations are required to clarify the interrelationships of Arrhenius parameters in different reaction systems and to identify the mechanisms of the contributory surface processes, since several distinct explanations of the reported interdependences of log A and E have been advanced in articles relating to these reactions (47,137,239, 273).

4. Dehydrogrnutioii urzd Dehydrutiori of Alcohols Arrhenius parameters reported for the dehydrogenation and dehydration reactions of isopropanol on the alkaline earth oxides ( 2 7 4 ~and ) other oxides (265) exhibit compensation behavior. The parameters calculated from the combined data given in both these studies is the line Table V, L. Further measurements are given by Szab6 et u / . (274h).Values of (log A , E ) for reactions of the same alcohol on samples of pure and doped titania (275) were close to the same line, but calculations based on this system alone defined the somewhat different compensation relation, Table V, M. Other Arrhenius parameters for several further chemical processes (276,277) showed scatter in which no significant obedience to Eq. (2) could be recognized. A systematic increase of log A with E was apparent in the four observations reported (129) for the dehydration and dehydrogenation of /3-phenyl ethanol on zinc oxide. Compensation behavior, of an accuracy comparable with many trends so described in the literature, was found in data reported by Knozinger et a/. (278) for the dehydration of alcohols over alumina. Values of A were converted to units used throughout the present review from the catalyst surface area, kindly supplied by Knoingcr (279).Analyses of the results in Table 2 of (278)yielded two compensation effects; one of these was for alcohols that contained a four-unit carbon chain and the other where there was a five unit carbon chain; the compensation parameters are given in Table V, N and 0, respectively. The accuracies of these two obediences are both significantly better than that found by the same calculation using the combined data. Compensation effects have also been reported for the dehydration of alcohols on alumina (280) and on alumina modified with 10% potassium chloride (281). Values of A referred to unit area of active surface were not included in these reports, but for data in the latter article (281)we calculate e = 0.0836 and ocL= 0.0075.

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

303

5. Reactions on Zeolites Zeolite catalysts may also be regarded as mixed oxides, but the crystallographic structures differ from the solids discussed above in that active sites for catalysis occur within the open lattice framework. In consequence, rate data are not directly comparable with similar observations for other heterogeneous reactions since the preexponential factors are calculated and reported on a different basis. For completeness, however, it is appropriate to mention here that instances of compensation behavior on zeolite catalysts are known. Taylor and Walker (282) described such an effect for the decomposition reactions of formic acid and of methyl formate on cation-exchanged 13X molecular sieves, and comparable trends may be found in data reported for reactions of propene on similar catalysts (283). 6. Other Reactions Compensation behavior found for the decomposition of hydrogen peroxide on preparations of chromium (111)oxide, which had previously been annealed to various temperatures, was attributed to variations in the energy states of the active centers (here e 0.165). Compensation behavior has also been observed (284) in the decomposition of hydrogen peroxide on cobalt-iron spinels: the kinetic characteristics of reactions on these catalysts were ascribed to the electronic structures of the solids concerned. The compensation trend present in data reported by Shannon ef a!. (285) for the isomerization of n-butenes over a number of different oxide catalysts is given in Table V, P (omitting from the calculation the point for MnO, which shows a marked deviationj. Dehydrogenation of cyclohexane over oxides (286) exhibited similar behavior; the calculated line is given in Table V, Q. Hydrocarbon exchange over alumina (287) also gave a slight compensation trend, for which e = 0.132 and oe = 0.024.

-

7. Discussion The features of behavior described above for the reactions on oxides bear a close resemblance to the kinetic characteristics of reactions on metals; these include, for example, ranges of obedience to Eq. (2j, the magnitudes of the calculated values of B, e, and 0,and other features. Again compensation trends were found within groups of rate processes that involved either a common chemical transformation catalyzed by several oxides or related reactions on a single oxide. Representative instances of such observed obedience to Eq. (2) are included in Table V. The quantity of kinetic information available for reactions on particular oxides was, however, often insufficient to enable values of B and e to be estimated meaningfully. Also in the

304

A. K . GALWEY

analyses of such data, it was necessary to consider the possible existence of more than a single oxide phase. One reason for the particular interest which has been shown in the rates of exchange of oxygen on oxides (259-262) has been the suggestion that the catalytic properties ofsuch oxides may be controlled by the strength of surface oxygen bonding (288,28Y). A consequence of the existence of a compensation effect in reactions on different oxides (Table V, A) could be the appearance of similar behavior in all other chemical changes controlled by the same surface step. While such comparisons may indicate the occurrence of closely related processes in these reactions, more direct evidence concerning the identities, the concentrations, and the energetics of interactions of the adsorbed intermediates is required to provide a quantitative mechanistic explanation of the compensation pattern. As can be seen from the articles cited above, several such proposals have been advanced: these include variations in the reactivities of different surface sites, of crystallographic planes, and of electron energy levels in the solids. Thus, as in the discussion of the reactions on metals, we again conclude that the complete elucidation of the kinetics of surface processes necessitates information regarding the identity, availability, and reactivity of those particular species which participate in the rate-limiting step on the surface. This information is not always known or, indeed, readily obtained. The following example illustrates one particular quantitative application of compensation behavior for the comparison of levels of activity between different systems. The Arrhenius parameters for the steam reformation reaction over nickel-alumina catalysts (290) are log A = 17.25 and E = 29.0. The position of this point on compensation diagrams would appear to be more realistically represented by the compensation relation found for oxidation and exchange processes on nickel oxide (Table V, G) than that for cracking on the metal (Table I, A). One possible mechanistic explanation for this distinction is that the active catalyst is an oxide phase [possibly including NiAI,O, (290)J.

E. REACTIONS INVOLVINGCLAYS The reactions described and discussed here have been selected t o extend the present analysis beyond surface processes and to include some consideration of certain chemical changes that also involve interactions within lattices of solids. The examples selected include reference both to surface catalytic properties and to dehydration reactions of clay minerals. Adsorbed n-dodecanol and stearic acid react on the surfaces of illite, kaolinite, and montmorillonite to yield hydrocarbons and the pattern of kinetic behavior exhibits a compensation effect (39,291), Table V, R. The

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

50' 15

I

I

I

I

20

25

30

35

305

log A

FIG.8. Compensation plot for reactions ofn-dodecanol and ofstearic acid on illite, kaolinite, and montmorillonite (0).Points have also been included for the reaction of the alcohol in the absence of volatile material ( x ) and the reactions of cyclohexanol in presence (0)and absence (@) of volatile products (31, 291, 292) on montmorillonite. Reproduced with permission from J . Chim.Phys. (291).

Arrhenius parameters for these reactions are shown on Fig. 8, which also includes results for the reactions of n-dodecanol in the absence of volatile products ( x ) (292)and of cyclohexanol in the presence (0) and the absence ( 0 )of volatile material (291).This pattern of kinetic observations emphasizes the sensitivity of the values of log A and E determined for these reactions to the availability of volatile reactants, including water vapor. The values of A extend beyond the range attributable to a simple rate-limiting desorption process; such magnitudes cannot be directly identified with the frequency of occurrence of the reaction situation and do not provide a measure of the concentration of surface-active sites. Interpretation of the mechanisms of the hydrocarbon desorption reactions mentioned above was considered (31,291) with due regard for the possible role of clay dehydration. While this water evolution process is not regarded as a heterogeneous catalytic reaction, it is at least possible that water loss occurs at an interface (293)so that estimations of preexponential factors per unit area can be made. On this assumption, Arrhenius parameters (in the units used throughout the present review) were calculated from the available observations in the literature and it was found (Fig. 9, Table V, S) that compensation trends were present in the kinetic data for the dehydration reactions of illite (+) (294),kaolinite ( 0 )(293,295-298), montmorillonite ( x ) (294)and muscovite (0) (299).If these surface reactions are at least partially reversible,

306

A. K . GALWEY

4001 300

100

1

+ x

0

20

Y

I

I

I

I

I

I

25

30

35

40

45

50

loa

A

FIG.9. Compcnsation behavior for thc dehydration reactions of kaolinite ( O ) , illite (+)% rnonlinorillonite ( x i . and muscovite (0). estimated from published kinetic data (2Yj-2YY).

the observed kinetic characteristics may be sensitive to the prevailing pressure of water vapor and, through the mechanism given in Appendix I, this can result in compensation behavior and large values of A . The interrelationship between log A and E is therefore attributed to the onset of a significant rate of reaction within a common, but narrow, temperature interval for the group of related reactions. In support of this model we cite other reported instances of sensitivity of the apparent value of E to conditions that obtain during reactions of solids (10,300,301). From the discussion presented in the previous paragraphs, we identify the kinetic characteristics of the hydrocarbon evolution reactions (31,291,292) and the clay dehydration processes with the common mechanistic features: reversibility and similar characteristic temperatures of onset of the water evolution step. The compensation effects observed for the two groups of related rcactions (Table V, R and S) were not identical, however, since the species participating in the equilibria on the surfaces (believed to be represented by the kinetic characteristics described in Appendix I) are different. Undoubtedly, the interaction of hydroxyl groups to yield water was common to both types of reaction (surface desorption and lattice dehydration) and the properties and reactivities of these species probably determine the temperature at which significant surface activity and product evolution becomes apparent. This surface reaction is 2 ( -OH),,,

+ ( > O ) + H,O(gas)

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

307

and K = [>O][H,0]/[-OH]2

=

exp(-AG/RT)

Among other influences, it is to be expected that AG will vary with surface coverage and [H,O] may be controlled by prevailing conditions, including rates of diffusion from the site of evolution and subsequent evacuation. Thus, the surface concentration hydroxyl groups, which participate in catalytic and dehydration reactions, will be influenced by both kinetic and equilibrium factors and perhaps vary with position on crystallite surfaces and within the reactant mass. The overall rate of product evolution is not controlled by a single identified process, involving a particular bond redistribution step, and the complete mechanistic description of the kinetic characteristics requires supplementary observations. Although all contributory influences have not been elucidated, a theoretical explanation can be provided (Appendix I) whereby the pronounced compensatory behavior (Figs. 8 and 9) is attributed to the combined influences of kinetic and equilibrium interactions. IV. Conclusions

The present survey has listed and compared representative examples of the numerous instances of reported obedience of kinetic data to the compensation relation in diverse heterogeneous rate processes. The work discussed includes behavior described as an acceptable fit to Eq. (2) by the authors concerned and, in addition, comparable relationships were found during the present analysis of reported kinetic data. No doubt additional trends could be recognized if this survey was extended further or experimental measurements obtained for additional systems, perhaps related to those discussed above. However, despite these many and various examples of compensation behavior, there remain important difficulties in establishing the range of meaningful application and the usefulness of Eq. (2) in understanding the significance of kinetic observations. These may be discussed under the following headings.

a. Acciirucy ojobedience of’Data to Eq. (2). It has been emphasized above that the statistical basis for the recognition of compensation behavior in literature reports is usually less than rigorous and has hitherto often been largely subjective. Little consideration has been given to the possibilities that the interrelationships between log A and E (and indeed log k and T - ’ ) may not be linear and to the consequences of the levels of inaccuracies inherent in the measurements. Most articles probably do not contain sufficiently detailed information to allow a strict statistical treatment of compensation behavior, and accordingly the semiquantitative approach undertaken here is a compromise between what is desirable and what is practicable.

308

A . K . GALWEY

From the data listed in Tables I-V, we conclude that most authors would probably accept that there is evidence for the existence of a compensation relation when oe < 0.le in measurements extending over AE 100 and when oL < 1.5. The magnitude of the ce/eratio, which is related to the accuracy of determination of the isokinetic temperature /?,would appear to be the most uscful criterion for assessing the excellence of fit of Arrhenius values to Eq. (2). The value of oL,a measure of the scatter of data about the line, must always be considered with reference to the distribution of data about that line and the range AE. As the scatter of results is reduced and the range AE is extended, the values of cr dimin 1, and for the most satisfactory examples of compensation behavior that we have found oe < 0.03.~.There remains, however, the basic requirement for the advancement of the subject that a morc rigorous method of statistical analysis must be developed for treatment of kinetic data. In addition, uniform and accepted criteria are required to judge quantitatively the accuracy of obedience of results to Eq. ( 2 ) or, indeed, any othcr relationship.

-

b. Range of' App/ic'niion qf' Eq. (2). The compensation relation would appear to bc equally applicable to many different types of heterogeneous reactions (and also a variety of other chemical processes). Criteria for the recognition of groups of reactions, to which this interrelationship is applicable, can alternatively be either that there is a common chemical change or that reactions occur on the same active solid : appropriate examples have been given above. There are not, however, readily recognizable features that define the groups of processes to which Eq. (2) is applicable, so that the excellence of fit of Arrhenius parameters to this function often varies with the data selected for inclusion in the quantitative calculations. A degree of subjectivity enters the selection of such data. Accordingly, it is not clear whether this is a law of general, of restricted, or of specific application or, alternatively, a consequence of the method of treatment of kinetic observations. c. Selection o f D m i . It is pointed out in Section 111 that in those groups of related reactions for which the largest numbers of (log A , E ) values have been reported (e.g., the decomposition of formic acid on silver, the cracking of hydrocarbons on metals) alternative selections from the data available give different compensation parameters. The method of comparative analysis used did not, therefore, identify a single correlation applicable to all apparently comparable results. Although there was this irreproducibility of behavior on the compensation diagram, the calculated lines tended to intersect in the region of maximum density of measurements, This indicates that levels of reactivity were usually approximately constant although there were also systematic differences within the groups considered. It is alternatively possible that thc observed variations could arise from shortcomings in the method

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

309

of analysis or inherent differences in the kinetic data obtained in different studies. It must be concluded, however, that the further accumulation of measurements does not necessarily result in an increase in the accuracy with which values of B and e are obtained; indeed, in the instances referred t o above, it would seem that the opposite is true. d. The Mechanistic Busis of’ Compensurion Behavior. Although several mechanistic explanations of compensation behavior in heterogeneous reactions have been proposed in the literature, the quantitative demonstration of the relevance of these to particular chemical processes has not always been provided. Care must be taken in the interpretation of observations for surface reactions, using theoretical concepts developed from homogeneous kinetics (reaction frequency factor, activation energy). The sequence of interlinked and interdependent changes occurring during processes at the gassolid interface may be subject to constraints that are not applicable to homogeneous rate processes (dissociative equilibria on a restricted area of surface, alternative interactions on different crystallographic planes, etc.). It is necessary, therefore, in any complete mechanistic description of a heterogeneous reaction, to establish the identities and concentrations of the precursors to the rate-limiting step. Once the factors controlling the values of A and of E in the individual reactions have been identified, the significance of the compensation behavior should be more readily elucidated. In the absence of a quantitative mechanistic basis for the compensation effect this well-defined feature of kinetic behavior has not been responsible for extensive advancement of our understanding of the chemistry of surface processes. The foregoing critical appraisal of the present status of the compensation effect is not intended to imply that such behavior is necessarily without interest or application. The very existence of this pattern of data requires an explanation, even if this is only the positive demonstration that it arises as an experimental or interpretative artifact. There are, however, indications that the effect is more significant than this and may be capable of providing quantitative comparative measurements of levels of activities and/or yielding information of mechanistic significance, through indicating groups of related rate processes that involve comparable surface intermediates. Common features in the various theoretical explanations of compensation behavior referred to in Section 11, A, 1-7 are the occurrence of parallel reactions that are characterized by different values of the kinetic parameters ( A , E ) and/or a systematic change in the effective concentrations of reactants across the temperature interval used in the measurements of the Arrhenius parameters. Both influences are based on reaction models for which the kinetic behavior cannot be represented as a single desorption step and, indeed, the overall surface interactions could be much more complicated.

3 10

A. K . GALWEY

If the catalytic properties of a solid derive from the reactivity of a specific surface species, the route for chemical transformation becomes detectable at the temperature for which the rate of formation/desorption of this intermediate becomes appreciable. Several examples of the appearance of catalytic properties at temperatures somewhat below the occurrence of a related chemical transformation of the active solid have been included in the present account. (This is consistent with the generally greater reactivity of surfaces than the bulk solid.) Moreover, such reactions of the solid are sometimes associated with compensation behavior, as illustrated by the following examples. Nickel catalyzes cracking reactions somewhat below the temperature of nickel carbide decomposition, the catalytic decomposition of formic acid on copper exhibits several parallels with the decomposition of copper formate, and the catalytic properties of clays become appreciable somewhat below the temperature of onset of dehydroxylation. One interpretation of the compensation behavior observed in these systems is that there are different rates of change of concentration of the participating surface intermediates, across the temperature intervals studied, in the different reactions of each group of related rate processes (Appendix I). This model associates obedience to Eq. (2) with a characteristic temperature of onset of reaction, although this is not the only possibility, and the other theoretical explanations given may dominate, contribute toward, or influence the overall kinetic properties of any rate process. We conclude, therefore, that the identification of A and E with the concentration of the surface precursor to product formation and the energy barrier to a bond redistribution process in the dominant step of a surface reaction, respectively, is not always or necessarily justified and may not be a realistic representation of the kinetics of a surface change. More direct information concerning the concentrations and reactivities of surface intermediates is required to substantiate meaningfully the kinetic properties of reactions proceeding on surfaces. Such considerations also call into question the application of the transition state theory to systems for which the transition complex has not been characterized unambiguously. In conclusion, we may summarize the lines of future development that could be expected to advance our understanding and useful application of compensation behavior in heterogeneous reactions as follows: (i) the provision of an acceptable and rigorous statistical basis for the recognition of the effect, (ii) reassessment of the significance of measured values of A and E together with other relevant features of the kinetic characteristics of surface processes, (iii) determination of the concentrations and reactivities of surface intermediates,

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

31 1

(iv) establishment of the mechanistic basis for the occurrence of such behavior in specific groups of related reactions, and (v) investigation of the potential usefulness of B and e as comparative measurements of catalytic activities between different systems. Such developments should be considered with respect to the kinetic and mechanistic implications of compensation behavior in homogeneous reactions. Theoretical models initially advanced to account for the reactions occurring in specific systems may be capable of more general application. The compensation effect remains largely an empirical method of classifying kinetic observations, and this has not yet been developed into a theoretical representation capable of predicting activities and patterns of reactivity in hitherto untested catalytic processes. Development of the usefulness of the correlation necessarily requires reexamination of many of the basic tenets of heterogeneous kinetic behavior, although it may be confidently anticipated that changes arising from any such reappraisal would be more than compensated for by the appearance of new representations and models that would provide more information concerning the mechanisms of heterogeneous reactions.

Appendix I : Compensation Behavior Resulting from Temperature-Dependent Variations in Concentrations of Surface Reactants

Some kinetic consequences of the systematic variation of clc2 [Eq. (9)] across the temperature range used for the determination of the Arrhenius parameters are demonstrated in the following analysis. (cl and c2 are the effective concentrations of those intermediates which undergo a bimolecular surface reaction to yield product.) Replacing c1c2 by CI, we obtain Eq. (7), and, taking for the purposes of discussion ct, = 0.01 at TI and a2 at T 2 , we have k1 = O.OIAs exp(-E,/RT,),

k z = a2A, exp(-E,/RT,)

If A, = lo3'.' (molecules m-' sec-') and E, = 82.4 (kJ moles-') then the apparent values of the Arrhenius parameters calculated from the observed magnitudes of k , and k , vary as shown in Fig. 10 with changes 1.0 < ciZ < 1.0 x and the compensation relation is accurately obeyed: logA = 19.0

+ 0.121E

312

A. K. GALWEY 164.8

123.6

r

/

0.0001

0.0002 0.0005/

Values of

E 82.4

41.2

I

0 20

25

I

30 log A

I

I

35

40

FIG.10. Theoretical compensation plot for Arrheniiis parameters calculated for rate constants: k1

--

loz9.' exp( - 82,400/R 365),

k,

=

aA, exp( - 82,400/R 439)

where the valuc of c( varies from 10' to l W 4 ; for more detailed explanation see text.

For this variation in u2 (10°-104). the apparent values of A and E show very marked changes (0 < E < 2E, and 1019 < A < lo3') and the consequence of a tenfold change in L Y ~is a lo5 change in A . This treatment is applicable to other values of A, and Es, and some illustrative, representative examples of B and e, similarly obtained using ctl = 0.01 and 1.0 < a2 < are shown in Table VI. The compensation relation was obeyed by all examples. Systematic changes in the values of a, therefore, are capable of providing a reasonable explanation of the often relatively large differences in measured magnitudes of A and E reported for comparable reactions. This model also satisfactorily accounts for the very large values of log A ( > 3 3 ) sometimes found; the magnitudes of these are greater than can be explained by the simple desorption process (3).Abnormally high frequency factors have been reported and discussed by Anderson and Baker (92) and by Metcalfe and Rowden (56). The above representation of compensation behavior is consistent with reactions involving the surface equilibrium model. In this model the con-

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

313

TABLE VI Parumeters in Compensation Equation Calculated by Making Appropriate AIlowances for the Occurrence of Temperuture-Dependent Changes in Concentration of' Surface Reactants'

31.0 31.0 31.0 31.0 33.0 a

41.2 82.4 123.6 164.8 123.6

183 365 548 731 470

219 439 658 877 548

19.0 19.0 19.0 19.0 17.0

0.243 0.121 0.081 0.061 0.097

For more detailed explanation see text

centrations of surface reactants may be influenced by interactions with all other surface entities, and values of c1 and c2 may be expected to vary both absolutely and/or relatively among the individual members of a group of related reactions. This approach can be extended beyond the above consideration of two points T , and T2 by noting that there are indications that we may represent [at least approximately; see Eq. (1 l)] the temperature dependence of the surface concentrations of dissociatively adsorbed precursors to the rate-limiting step by an expression of the form cL = a, exp(- E , / R T ) The rate constant for the catalytic reaction is k = u,a2Asexp[-(Es - E l - E 2 ) / R T ]

so that there may not be significant deviation of measured values of k from obedience to the Arrhenius equation. Since reactions at comparable concentrations of surface species and involving an almost identical bond redistribution energy requirement Es must be expected to exhibit at least approximate isokinetic behavior, it follows that the observed values of A must compensate for changes in E ( = E s - E l - E2). While exploring the kinetic consequences of variations in surface occupancy upon reaction rate, a further mechanistic explanation of compensation behavior, of particular relevance in the consideration of adsorption kinetics, became apparent. If the total quantity of gas adsorbed by a surface Of varies with temperature and the rate of adsorption d8/dt is proportional to the

314

A.

K . GALWEY

quantity of gas yet to be adsorbed 0, - 0, then the values of log A and E determined for adsorption at equal volumes of gas ( O , , 02, 8,, . . .) taken up at different temperatures exhibit compensation behavior. This is conveniently illustrated by reference to the second example in Table VI: it is assumed that at T , ( = 3 6 5 K) 0, = 1.00 and k , = lo”, while at T , (=439 K) 8, = 2.08 and k2 = Values of A and E calculated for rates of reaction corresponding to equal volumes of gas adsorbed (Oi= 0.58, 0.68, . . . , < 1.00) at T , and T , obeyed the compensation line B = 19.3 and e = 0.117. If the values of 0, were interchanged for the same calculations the different compensation line, B = 16.5, e = 0.140, was defined. This effect arises through a progressive but unequal change in adsorption rate with increasing coverage at the two temperatures: the 8-time relationship for gas uptake is different at T , and T,. Appendix II : Statistical Formulas Used in Linear Regression (Least Squares) Analyses

The standard statistical formulas summarized below were incorporated in a program used in a desk calculator to find the best line log A = B

+ eE

(2)

through the I points (El, log A , ; E,? log A , ; . . . ; E,, log A , ) available in each series of related reactions. - _ _ SSD refers to the sum of square deviations; E, log A represent mean values : SSD(E2)= CE,’ - E C E l

C log A ,

SSD ((log A)’)

=

C(log Ai)’ -

SSD(E log A )

=

C E , log A , - T0g;l C E

e = SSD(E log A)/’SSD(E2),

Standard deviation of slope: Standard deviation of intercept:

B =

- CE

crv = a,[SSD(E2)]-’

{: +

crB = -

-__ls&2J1’’

Standard deviation of line: ffL

A

{SSD(E log A))’)]’’’ SSD(E2)

= {!__ (SSD{(log A)’) - - _ _ _ _ _ _ -

COMPENSATION EFFECT IN HETEROGENEOUS CATALYSIS

LISTOF

E

SYMBOLS

B

Arrhmius purumc~trrs A

calculated reaction frequency factor (obtained from experimental data) (molecules m - * sec- ‘) calculated reaction activation energy (obtained from experimental data) (kJ moles ’) values for bimolecular surface rate-controlling step in reaction specific reaction rate constant reaction temperature (K) gas constant ( k = Aexp(-E/RT)j

isokinetic temperature ( e = (Rp)-’i oB,u,, u L defined in Appendix 11 Kinetic parameters

~

A,, E ,

k T R

e

(‘X

px rn, 11 0,

Compensution parumelers B

315

constant in compensation equation (value of log A when E = 0) constant of proportionality in compensation equation: (log A = B + e E )

r c(

effective surface concentration of species X participating in a heterogeneous catalytic reaction pressure of constituent X in reaction mixture reaction rate pressure dependence exponents fractional surface coverage of catalyst by species X transmission coefficient for surface reaction term to give due allowance for changes in frequency of occurrence of surface reaction situation

ACKNOWL~DGMENTS

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Transition MetaI-Cata lyzed Reactions of Organic Halides with CO, Olefins, and Acetylenes R. F. HECK Department o j Chemistry Universit?;of Deluware Newark, Delaware

I . Introduction. . . . . . . . . . . . . . . . . . 11. Carbonylation Reactions . . . . . . . . . . . . 111. Olefinic Substitution Reactions. . . . . . . . . IV. Substitution Reactions of Terminal Acetylenes . . V.Conclusions. . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . .

. . . . . . . . . .

323

. . . . . . . . . . 324 . . . . . . . . . . . 336 . . . . . . . . . . . 345 . . . . . . . . . . 347 . . . . . . . . . . 348

1. Introduction

Metal-catalyzed reactions of CO with organic molecules have been under investigation since the late 1930s and early 1940s, when Roelen (I)discovered the hydroformylation reaction and Reppe (2) the acrylic acid synthesis and other related carbonylation reactions. These early studies of the carbonylations of unsaturated hydrocarbons led to extremely useful syntheses of a variety of oxygenated products. Some of the reactions, however, suffered from the serious problem that they produced isomeric mixtures of products. For example, the cobalt-catalyzed hydroformylation of propylene gave mixtures of n-butyraldehyde and isobutyraldehyde. CH3 CHjCHzCH2

+ C O + Hz

C”

CHjCH2CHZCHO

I

+ CH3CHCHO

Numerous attempts have been made to cause this reaction to go more selectively, and in recent years these attempts have met with considerable success either by modifying the catalyst with an organophosphine(3, 4) or by using a rhodium catalyst (5). While these discoveries have important commercial applications, they only allow the specific synthesis of straightchain aldehydes. The synthetic chemist would like to have methods available for the specific formation of other isomeric aldehydes or other oxygenated products with widely different structures as well. There are, of course, numerous completely “organic” routes to these types of compounds, but many 323

324

R. F. HECK

of these routes involve multiple steps, suffer from low yields, and may not be generally applicable. In recent years, several new, metal-catalyzed carbonylations have been discovered that provide practical alternatives to the organic routes. These reactions generally employ organic halides as starting materials, and the halide group is specifically replaced by the carbon monoxide in the reactions. Several other closely related reactions of organic halides with olefins and acetylenes have also been discovered in the past few years. These new reactions offer simple, convenient methods for the preparation of a wide variety of esters, amides, aldehydes, olefins, and acetylenes. This review will discuss these new reactions, emphasizing those that are catalytic and developed to the extent that their utility is clear. II. Carbonylation Reactions

The carbonylation of ally1 chloride with a nickel carbonyl catalyst appears to be the first useful example of an organic halide reaction to be reported (6). In alcohol solution at 50 atm pressure and loo", mixtures of esters of 2- and 3-butenoic acid were obtained in about 50(x, yield with the 3-isomer predominating. Such high pressures and temperatures are probably not necessary for this reaction, however (7): CH2-CHCH2CI

+ CO + ROH

Ni(COln -------+

CH2-CHCHzCOOR

+ CH,CH=CHCOOR + HCI

A transient orange complex is formed in this reaction, which is very probably chloro-h3-allylnickel(11) dimer, a complex that can be obtained from nickel carbonyl and ally\ chloride in an inert solvent (8). The probable mechanism of the carbonylation then becomes clear with the additional facts that bromoh3-allylnickel(II) dimer reacts with carbon monoxide to give unstable bromodicarbonyl(3-butenoyl)nickel(II), which then reacts further with CO to form nickel carbonyl and 3-butenoyl bromide (6). Six separate steps are believed to be involved: Ni(CO), Ni(CO),

+ Ni(CO), + CO

+ CHL=CHCH,CI -

co

CH,~CHCH~NI(CO)~CI

+LO

+

CH2=CHCH2Ni(C0)3CI

(1) (2)

co + co

CH~-CHCH~NI(CO)~CI h'-C3H,Ni(CO)CI

CH2-CHCH2Ni(CO),CI

- CO

+ co

Lh-C,H,NiCI],

+ CH2=CHCH2CONi(CO),C1 co

CH2=CHCH2CONi(CO),CI CH,=CHCH,COCI + ROH

+

CH2=CHCHzCOCI CH2;CHCHlCOOR

+ Ni(CO), + HCI

(3)

(4) (5)

(6)

CATALYZED REACTIONS OF ORGANIC HALIDES

325

Nickel tetracarbonyl is known to dissociate into the more reactive tricarbony1 readily [step (l)] and this species is known to react readily with a variety of halides by oxidative addition presumably as shown in steps (2) and (3). Subsequent loss of CO would give an equilibrium mixture of the four complexes shown in (3). Step (4)is the well-known carbon monoxide insertion reaction. The acylnickel complex formed in this step then may reductively eliminate acid halide [step (5)], which then alcoholizes [step ( 6 ) ] or it may react directly with alcohol to form ester and a hydridonickel complex (7), which then reacts with CO and decomposes to nickel tricarbonyl and HCl(8): CH,=CHCH,CONi(CO),CI

+ ROH + CHz=CHCH,COOR + HNi(CO),CI

+ co HNi(CO),CI r--+ HNi(CO),CI + HCI - co

+ Ni(CO),

(7)

(8)

The nickel hydride species would be present in either case because of the equilibria in (8).The hydride is likely responsible for the isomerization of the initially formed 3-butenoate ester into the 2-butenoate. If a base such as a tertiary amine is added to the carbonylation reaction mixture, only the 3butenoate is formed, since the HCl is then neutralized preventing formation of significant concentrations of the hydride. The nickel carbonyl-catalyzed carbonylation of allylic halides is probably general, although apparently only allyl chloride has been investigated in detail. Saturated aliphatic halides apparently do not give esters under the same conditions. More recently it has been reported that aryl halides are readily carbonylated stoichiometrically with nickel carbonyl (9) and most recently catalytically with that reagent and calcium hydroxide as a base in aqueous solution to form calcium salts of the carboxylated products (10).The catalytic reaction, occurring at 100-1 lo" and 1 atm of CO, is presumably taking place by a mechanism analogous to that of the allyl chloride carbonylation discussed above: 2ArX

+ 2CO + 2Ca(OH),

Ni(CO),

(ArCOO),Ca

----+

+ 2 H , 0 + CaX,

H,O

The nickel-catalyzed reaction often proceeds in high yield under mild conditions and appears to be a useful synthetic procedure. Examples of the reactions are given in Table I. Palladium catalyzes the carbonylation of allylic, vinylic, benzylic, and aromatic halides in alcohols to form esters under conditions similar to those required by the nickel carbonyl catalyst ( I I). The palladium-catalyzed reaction offers the advantage of not requiring the use of highly toxic and volatile nickel carbonyl, and perhaps higher catalyst activity, although accurate comparisons have not been made. Like the nickel reaction, the palladium reaction

TABLE I Nickel Carhunyl-Catalyzed Carboalkox?;lationsof Ally1 and Aromatic Halides ~~~~~

Organic halide

Hydroxylic reactant

CH,=CHCH2C1

CH,OH

1-CIC 1OH7

H2O

2-BrC,H40CH,

H,O

4-BrC6H,CN

H2O

3-BrC,H4C1

H2O

Reaction conditions IOO", 50 atm CO, 5 hr 1lo', I atm CO, Ca(OH),. DMF. 6 hr loo", 1 atm CO, Ca(OH),, DMF. 8 hr IOO', 1 atm CO, Ca(OH),, DMF, 3 hr 100". 1 atm CO, Ca(OH),. DMF, 5 hr

Product

(2 yield)

~~~~

Reference

CH,=CHCHzCOOCH3 ( - ) CH3CH=CHCOOCH3 (-) Ca( 1-C, H (95)

I Oa 10

Ca(2-CH30C,H4COO)z (80)

10

Ca(4-CNC6H4C00)2(80)

10

Ca(3-C1C6H,C00), (88)

I0

8

n z

m

rl

x

CATALYZED REACTIONS OF ORGANIC HALIDES

327

is highly tolerant of a variety of functional groups such as ester, nitro, cyano, and ether groups. The hydrogen halide formed in the palladium-catalyzed reactions must be neutralized with a base such as a tertiary amine or sodium acetate. The catalyst may be added as a simple palladium(I1) salt such as the acetate or as a bis(triorganophosphine)palladium(II)complex. Which ofthese catalysts is used depends on the halide being reacted. Substituents may affect the halide reactivity substantially, but in general vinylic iodides, aryl iodides, and allylic and benzlic chlorides, bromides, and iodides will carbonylate with the palladium acetate catalyst, while a bisphosphine-palladium complex is required to cause aromatic and vinylic bromides and chlorides to react. Aromatic chlorides often do not react well even with the phosphine catalysts. Apparently, however, chlorobenzene can be carbonylated to benzoyl chloride with a palladium(I1) catalyst at high temperatures (160") under 80 atm pressure (I2,13). Some examples of the homogeneous, palladium-catalyzed carbonylation of organic halides are given in Table 11. In the absence of hydroxylic solvents, allylic chlorides produced 3-butenoyl chloride derivatives as products. Other halides apparently do not react under the same conditions unless a hydroxylic solvent and a base are present to form esters. The reaction rates varied considerably with the structure of the organic halide. Generally, organic chlorides were less reactive than bromides or iodides, and allylic, vinylic, and benzylic halides were more reactive than aromatic halides. Electron withdrawing groups on the double bond of vinylic halides or on the aromatic ring in aryl halides tended to increase the rates of carbonylation. Reaction rates were decreased if large substituents were present near the reacting halide group. Attempts to carbonylate saturated aliphatic halides with palladium (and presumably nickel) catalysts fail because the intermediate alkylmetal complexes undergo P-hydride eliminations forming olefins, much more easily than they insert carbon monoxide. The effect of varying the triorganophosphine ligands in the palladiumcatalyzed carbonylations has been looked at briefly. In general, triarylphosphines are much better than triorgano phosphites and trialkylphosphines. Changes in the substituents in the triarylphosphine have only minor effects upon reaction rates ( I 1). The palladium-catalyzed carbonylation of isomeric vinylic halides shows the reaction to be reasonably stereospecific and proceed with retention of the original halide structure. The degree of specificity, however, depends somewhat on the reaction conditions. Low reaction temperatures and/or excess triarylphosphine favor the stereospecific reaction (1I). The mechanism of the palladium-catalyzed reaction is similar to that of the ally1 chloride-nickel carbonyl reaction described above, but more complex, at least when phosphine ligands are present (14).The first step is believed

TABLE I1 Pollodium-Curalyzd Curhwlkylarions of Allylic, Aromatic, and Vinylic Halides

Organic halide

Catalyst

Hydroxylic reactant

[h3-C3H5PdC1]2

None

CH,CH=CHCH,CI

[h3-C3H,PdCI],

None

CHz=CHCH2CI

[h3-C3H5PdCI]z

CH,OH

Pd(OAc), PdBr,(PPh,), PdBr,(PPh,), PdCI,( PPh,),

n-BuOH n-BuOH n-BuOH n-BuOH n-BuOH n-BuOH

Pd1Z(PPh3)Z Pd12(PPh3)2

Reaction conditions

90",77 atm, benzene solvent, 5 hr 90", 77 atrn, benzene solvent, 6 hr 90", 77 atm, 5 hr

Product (% yield)

Reference

CHz=CHCH*COCI (80)

131.7

CHjCH=CHCHICOC1(72J

130 131.7

lOO", 1 atm, 80 hr, n-Bu,N loo", 1 atm, 40 hr, n-Bu,N

CHZ=CHCH2COOCH3 (60) CH,=CHCH,COOH( 12) C6H5COOBu-n(70) 4-CNC,H,COOBr-n (89) l-CloH,COOBu-n (57) C6HSCHzCOOBu-n (69)

loo", 1 atrn, 2 hr, n-Bu,N

(E)-CH,(CHZ),CH=CHCOOBu-n (83)

11 11 11 11 11

a',1 atm, 40 hr, n-Bu,N

(E)-CZH,CH=C(C~H,)COOBU-~~ (74) (Z)-C,H,CH=C(C,H,)COOBu-n (6)

11

IOO", I atm, 20 hr, n-Bu,N

a", 2 atm, 14 hr, n-3u3N

CATALYZED REACTIONS OF ORGANIC HALIDES

329

to be a reduction of the palladium(I1) complex to a palladium(0) complex, probably the bisphosphinemonocarbonyl. The last complex may then undergo oxidative addition with the organic halide. The five coordinated intermediate so formed then may either lose carbon monoxide, lose a phosphine, or rearrange to the acyl-metal complex. The loss of carbon monoxide is reversible and does not lead to product. The loss of phosphine produces a halo-phosphine-carbonyl-organometal complex, which probably rearranges to a halomonophosphineacylmetal derivative. The last complex then would react with phosphine in solution to form the halobisphosphineacylpalladium complex, the same product as is obtained directly from the five coordinated intermediate by migration of the organic group. The acylpalladium complex then either alcoholizes directly to ester and metal hydride or possibly produces acid halide first by reductive elimination, which then alcoholizes. The acylpalladium complexes do readily alcoholize directly but it is not known if the reductive elimination is competitive with it or not:

COR

R'OH

(PPh,),Pd(COR)X

(PPh,),Pd(H)X

a (PPh,),PdCO

+ R'OCOR

+ RCOX RCOX + R'OH -+ RCOOR' + HX (PPh&Pd(H)X + CO (PPh,),PdCO + HX HX + base + H-base' + X -

(PPh,),Pd(COR)X

-

330

R. F. HECK

Amides are produced from the palladium-catalyzed carbonylation of organic halides if primary or secondary amines are present rather than only an alcohol ( 1 5 ) : RX RX

+ 2R1NHZ+ CO

(PPh3IIPdX,

+ 2Rz'NH + CO -

(PPh,12PdX,

RCONHR' RCONR,'

+ R'NH,+X+ R2'NH2+X-

A strongly basic amine is required to neutralize the hydrogen halide formed. Amides may be formed from weakly basic amines (e.g., aniline) if a strongly basic tertiary amine is also present. The reaction proceeds readily with benzylic, vinylic, and aromatic halides at 100" or lower and atmospheric pressure with a palladium-phosphine catalyst. The reactions are highly stereospecific with vinylic halides. Rates of reaction at 100" are generally faster than in the corresponding ester-forming reaction with n-butanol. The mechanism of the amidation reaction may be similar to the carboalkoxylation reaction except that the acylmetal complex reacts directly with the primary or secondary amine to form amide and metal hydride. Some examples of the amidation reaction are given in Table 111. Nickel carbonyl has been reported stoichiometrically to convert vinylic and aromatic halides and amines into amides (9). Presumably, this reaction could be made to be catalytic, also. Another variation of the palladium-catalyzed carbonylation reaction occurs when hydrogen is added rather than an alcohol or a primary or secondary amine. This variation leads to aldehyde formation; the hydrogen reduces the acylpalladium intermediate to aldehyde and metal hydride (16). A basic tertiary amine is also added as in the ester-forming reaction to neutralize the hydrogen halide formed in the dissociation of the hydride: RCOPd(X)(PPh,)z HPd(X)(PPh,),

+ H2

+ CO + R J I N

-+ --t

+ HPd(X)(PPh3)2 (PPh,),PdCO + R,'NH+X RCHO

The formylation reaction of vinylic, aryl, and heterocyclic halides proceeds in good yields, generally at 115 to 150" under several hundred pounds pressure. The hydrogenation does not take place at atmospheric pressure. Examples of the formylation reaction appear in Table IV. Neither the palladium nor nickel catalyst described will promote the carbonylation of saturated aliphatic halides as noted above. However, this reaction can be catalyzed with cobalt (17) or iron ( 1 7 ) and probably with manganese (18) carbonyl anion salts. These carbonyl anions are strongly nucleophilic species and readily displace halide or other good leaving groups from primary or secondary positions giving alkylmetal-carbonyl complexes.

Organic halide

Catalyst

CJ15Br 4-CH,C)COC,,H,Rr 2-Bromothiophene (2)-C,H,CH=CHBr (Z)-CH3OCOC(CH,1=.CH3r CH,=C(CI)CH3 E-C,H5CH=CHBr

PdBr,(PPh,), PdBr,(FPh,), PdBr,(PPh3), PdBr,(PPh,), PdBr,(PPh,), PdCI,(PPh3), PdBr,(PPh3)2

Aniinc C,H,CH,NH, C,II,NH, and C6H,NH2 and C,HjNHZ and C6H,NH2 and C,H,NH, and C,H,Nb

Reacl.ion conditions

n-Uu,N n-Bu,N n-Bu,N n-Bu,N n-Bu,N

100". I atrn, 3 hr 100'. 1 alm. 3.5 h r 100'. 1 atm. 2 hr 60". 1 atm. 4 hr loo-, 1 atm, 1.3 hr 135', 40 atm, 10 hr 60'. 1 atm. 2.5 hr

Product IS;, yicld) Ci,H j C O N ~ l C € 1 2 C :(79) ~l~5

4-CH,0COC,H,COh'HC,Hs (Xh) 2-C,H,SCONHC6H, (63) (Z)-C,H,CH=CHCONHC,Hs (80) (Z)-CH,OCOC(CH,)=CHCONHC,H, CH,=C(CONHC6H,lCH3 (74) E-C6H,CH=CHCONC,HB (91)

' Data from Schoenberg and Heck (15).

' Pyrrolidine.

Organic halide C,Il,Br 4-CH30C6H4Br l,CC6H,Br, 3-Bromopyridine CH,(CH,I3C(I)=CH,

a

Catalyst PdBr,( PPh 3 )2 PdBr,(PPh,), PdBr,(PPh,), PdBr,(PPh,), PdIz(PPh,),

Data from Schoenberg and Heck (16).

Initial pressure of 1 : I H,:CO (psi) 1350

1450 1375 1350 1380

Reaction conditions

Product (% yield)

125-, 24 hr, n-Br,N C,H,CHO (94) 150". 10 hr, ~ I - B U ~ N 4-CH,OC,H,CHO (84j 140'. 25 hr. n-Bu,N 1,4-C,H4(CH0), (83) 145", 26 hr. Et3N 3-CSNHdCHO (80) 100'. 3 hr, Et,N CH3(CH2),C(CHO)=CH, (53) CH3(CH2),CH=C(CHO)CR3 (20)

(80)

332

R. F. HECK

These complexes readily insert carbon monoxide between the alkyl and metal groups giving acylmetal complexes. As in the preceding examples, these complexes undergo alcoholysis readily to form esters and a hydride. The reaction is then made catalytic in the metal by adding a base to convert the hydride back into the carbonyl anion :

+ RX RM(C0)" + X RM(C0)" + CO + RCOM(CO), RCOOR' + HM(CO), RCOM(CO), + R'OH HM(C0). + base H-base' M(CO),M(C0)"-

-+

--t

4

The reaction does not occur with tertiary halides since elimination takes place more readily. Examples of the carboalkoxylation of saturated halides are given in Table V. The carboalkoxylation of saturated aliphatic halides may give mixtures of isomeric products if carried out above about 75", at least with tetracarbonylcobalt anion as catalyst. Isomerization occurs because the intermediate alkylcobalt complex isomerizes competitivelywith the carbonylation at the higher temperatures. The isomerization probably involves stepwise loss of carbon monoxides to the tricarbonylalkylcobalt(1) stage. This complex then may reversibly rearrange by a hydride elimination to a hydride-olefin-n complex. The hydride may also add back in the reverse direction and produce an isomeric alkyl. Subsequent readdition of carbon monoxides and alcoholysis would produce isomerized ester:

co -co

c'o CH,

I

CH3

CH3

Several useful variations of the tetracarbonyliron dianion-alkyl halide reactions are known. These employ the iron complex stoichiometrically, however. Since the dianion is much more nucleophilic than the alkyliron monoanion, the reaction can easily be stopped at the monoalkylated stage. Carbonylation or addition of another good ligand such as triphenylphosphine to the monoalkylated complexes produces monoacyliron anions. Pro-

CATALYZED REACTIONS OF ORGANIC HALIDES

333

TABLE V Catalytic Carbomethoxylation of Saturated Aliphatic Halides"

Organic halide I-CaH, 71

Catalyst

NaCo(CO),

1,4-(CICH2)2C6H,

NaCo(CO),

I-CICH,CioH,

NaCo(CO),

I-CICSHI 7

NaCo(CO),

2-CaH171

Na,Fe(CO),

a

50", 1 atm, 20 hr,

NaCo(CO),

2-c8~',1

Product (% yield)

Reaction conditions

l-CVH17COOCH3 (56)

DCHEA,b CH,OH 25", 1 atm, 16 hr, NaOCH,, CH,OH 50", 1 atm, 2 hr, DCHEA,b CH,OH 5 0 , l atm, 20 hr, DCHEA,b CH,OH loo", - 2 atm, 25 hr, DCHEA,b CH,OH 25", 1 atm, 20 hr, NaOCH,, CH,OH

2-CSH17COOCH3 (41) 1,4-(CH,OCO),CsH, (32) l-CH30COCH2C1oH7 (71) 1-C8HI7COOCH, (56) 2-C,Hl7C0OCH3 (12) 2-CsH17COOCH3 (43)

Data from Heck and Breslow (17).

* DCHEA, dicyclohexylethylamine.

tonation of these anions with acetic acid presumably gives transient hydrides that decompose into aldehydes and a polynuclear iron carbonyl complex. Yields of aldehydes are quite good by this procedure (19).

+ R X + RFe(CO),-

Fe(C0);'

[H(RCO)Fe(CO),L]

--+

-+ L

RCOFe(CO),L-

RCHO

HOAc

[Fe(CO),L],

In other variations ketones are produced. The acyliron monoanion may be alkylated again with another alkyl halide to form a transient acyl-alkyliron intermediate, which rapidly decomposes into ketone and the polynuclear iron carbonyl complex. This reaction is limited, however, because only very reactive alkylating agents such as methyl, allyl, and benzyl halides will react with the weakly nucleophilic acyliron monoanions: R' RCOFe(CO),L-

+ R'X

I

--*

[RCOFe(CO),L-]

+

RCOR'

+ (Fe(CO),L),

A more general ketone synthesis involves the reaction of the alkyliron monoanions with (highly reactive) acid chlorides or anhydrides. COR' RFe(CO),L-

+ R'COCI

I

-+

[RFe(CO),L]

+

RCOR'

+ (Fe(CO)3L),

Good yields of a variety of ketones have been obtained by means of this reaction (20).

334

R. F. HECK

Numerous metal-catalyzed reactions of organic halides with carbon monoxide and olefins, acetylenes, aldehydes, etc., have been carried out (21). Only two of these, however, appear to have been developed into generally useful reactions. One is the reaction of allylic halides with carbon monoxide and acetylene in alcoholic solution with a nickel catalyst (22,23).This reaction produces cis-2S-hexadienoate esters at atmospheric pressure in good yields : CH2-CHCH2CI

+ HC-CH + C O + ROH

*

N'KoL

CHI=CHCH>

COOR \

c=c

/'

H

/

+ HCI

\

H

The mechanism of this reaction appears similar to the ally1 chloride carbonylation discussed above, with an additional insertion of acetylene in the allylnickel intermediate before the CO insertion. A possible formulation is the following: HC=CH h3-C3H,Ni(CO)C1 CH2=CHCH2 \

H

1

+ HCSCH

c=c

/

CH,=CHCH~NI(CO)CI

Ni(CO)CI

\

H

co CHl-CHCH2 co

CONI(CO)~CI \

-

/

c-c

/

ROH

- H N I [ C O ~ ~ C'I

\

H

H

The reaction is only partially catalytic because nickel chloride is formed in side reactions. If a reducing agent such as iron powder is added to reduce nickel(I1) dichloride to nickel carbonyl with CO, higher catalytic activity is observed (22). Some examples of the 2S-hexadienoate ester synthesis are given in Table VI. The second possibly useful reaction in this group is the tetracarbonylcobalt anion-catalyzed conversion of alkyl halides with a base, CO, and conjugated dienes into acylated dienes (24).In this reaction the alkylcobalt intermediate RX

+ CH2=CHCH-CH,

+ CO

cU(c0I.base

RCOCH=CHCH=CH2

+ HX

reacts with the conjugated diene to form a 1-acylmethyl-h3-allylcobalt complex which then undergoes elimination of a hydridocobalt group with the base (usually a hindered tertiary amine) to form the acylated diene.

335

CATALYZED REACTIONS OF ORGANIC HALIDES

TABLE VI Nickel Curbonyl-Catalyzed Carbomethoxyuinyfation of Alfylic HalidePb

Product (% yield)

(Z)-CHz=CHCH,CH=CHCOOCH3

CHZ=CHCHzCl (E)-CH3CH=CHCHzCI CH3

(70)

(E,Z)-CH,CH=CHCHzCH=CHCOOCH3(81) CH 3

1

I

CH,=CCHzCI (E)-CH3OCOCH=CHCH,CI (E)-NCCHzCH=CHCHZCI

(Z)-CHZ=CCHzCH=CHCOOCH3

(80)

(E,Z)-CH30COCH=CHCHzCH=CHCOOCH 3 (40) (E,Z)-NCCHZCH=CHCHzCH=CHCOOCH,

H

r;

(79)

H \

c=c

/

Chiusoli and Cassar (23). Reactions carried out at 20" in methanol solution at atmosphere pressure with a 1 : 1 CO:C,H, mixture with Ni(CO), as catalyst.

The hydride, with the base and CO, reforms the carbonyl anion catalyst: RX

+ Co(CO),-

+

RCo(CO),

+X CH,COR

RCo(CO),

-+

RCOCo(CO),

butad'ene

I

'

CH

4 CH \,

.*._

,cO(Co)3

,,'

CHZ

H-base+ Co(CO),-

CO(CO), -

+ co

---*

+ RCOCH=CH-CH=CH2

~

ba,r

co

1

CO(CO),

Some examples of the acyldiene synthesis appear in Table VII. Finally, it should be mentioned that there is one important commercial application of the organic halide carbonylation. This is in the rhodium and methyl iodide-catalyzed conversion of methanol and carbon monoxide into acetic acid (25).The mechanism of the reaction appears to involve the oxidative addition of methyl iodide to the rhodium(1) catalyst followed by CO insertion and hydrolysis : CH3OH

+ CO

Rh cat

CH3COOH CH,I

336

R.

F. HECK

TABLE VII Tetracurbonylcobtrlt(-I)Anion-Catalyzed Acylution of Dienes"

Organic halide

Dime

Product (% yield)

Reaction conditionsb

~~

~~

CH,l

Butadiene

CH3CI

Butadiene

50 psi, 50 , 6 hr, THF, DCHEA 130 PSI, 85", 24 hr, EtOH, DCHEA

CH ,COCH=;CHCH=CHz

(56)

CH,COCH-CHCH=CH,

(35)

CH 3 CH31

50 psi, 7 5 , 3 5 hr,

Isoprene

I

CH,COCH-CHC=CH,

(> 10)

THF, DCHEA CH3 CH,I

(Z)-Piperylene

50 psi, 7 0 ,20 hr, THF, DCHEA

1

CH,COC=CHCH=CHz

(50)

" Ddta from Heck ( 2 4 ) DCHkA, dicyclohexylethylamine

The hydrolysis forms a metal hydride, which decomposes into the catalyst and hydrogen iodide. The hydrogen iodide reacts with the methanol to form the water needed for hydrolysis, and methyl iodide again (2.5~): [I,Rh(CO)J

+ CH31

--t

[CH,Rh(CO),I,]-=

[HRh(CO),I,][HRh(CO),13]HI

+ CHaOH

--t

4

[CH3CORh(CO),I3]-

+ CH,COOH + [I,Rh(CO),] CH31 + HZO HI

111. Olefinic Substitution Reactions

Olefinic compounds will often insert into carbon-transition metal bonds as CO does, and this reaction is an important step in many catalytic syntheses. When this step is combined with an oxidative addition of an organic halide to a palladium(0) complex in the presence of a base, a very useful, catalytic olefinic substitution reaction results (26-29). The oxidative addition produces an organopalladium(I1) halide, which then adds 1,2 to the olefinic reactant (insertion reaction). The adduct is unstable if there are hydrogens beta to the palladium group and elimination of a hydridopalladium salt occurs, forming a substituted olefinic product. The hydridopalladium salt then reforms the

337

CATALYZED REACTIONS OF ORGANIC HALIDES

palladium(0) catalyst with the base present as it did in the carbonylations discussed above. Simple palladium salts such as the acetate may be used with the more reactive halides such as aryl and vinylic iodides but bisorganophosphinepalladium complexes are required for less reactive halides, at least for reactions under mild conditions ( < 125"). The palladium salts are reduced under the reaction conditions by the olefinic compound. The reaction is believed to take place according to the following equations, exemplifiedwith the reaction of bromobenzene and methyl acrylate to form methyl cinnamate:

+ CHZ=CHCOOCH3

(PPh,)2Pd(OAc),

--t

(PPh,),Pd(H)OAc Catalyst formation:

(PPh,),PdH(OAc)

+ R,N + CH2=CHCOOCH,

+ AcOCH=CHCOOCH, -+

(PPh,),Pd-

CH2

(1

CH

+ R,NH'OAc-

COOCH,

I

Br

COOCH,

1

Br

COOCH,

PPh,

CH,OOC

H \

/

C=C

'1

H Ph3P

'C6H, H

Pd /

Br

H / ChHS

HPd(Br)(PPh,),

PPh,

COOCHj \

+

\

C=C

/

+ HPd(Br)(PPh,),

\

H

+ R,N + CH,=CHCOOCH,

--t

(PPh,),Pd -

CHI

11

CH I COOCH,

+ R,NH+Br-

The reaction is highly catalytic in most instances and yields of substituted olefinic products are generally good to excellent at 100" or less. As in the related carbonylation reactions only aromatic, benzylic, and vinylic halides are useful in the reaction. Typical examples are shown in Table VIII.

TABLE VlIl Olrfinic Subst it urion Rcluctions

Organic h a1ide

--

C6HJ 4-CH,C)COCbH,Br

Olefinic compound

CHz=CHCOOCH,

CbH,I (CH,),C=CHBr

Pd(OAc)z

Reaciion cunditionf

I hr, loo'. n-Bu,N 8 hr, IW. n-Bu,N 12 hr, 100'. n-Bu,N

4-CIC,H4Rr

C6H51

ratalv%t

(Z)-CII,CH=CHC,II, (E)-CH,CH=CHC, H 5 CH,=CHCOOCH,

34 hr, 100'. Et,N 34 hr, 100'. Et,N 70 hr. 100'. Et3N 19 hr. lOO', Et3N,

(E)-CH,(CH,),CH=CHI

CH,=CHCOOCH,

PPh, 38 hr. loo', Et,N

(E)-CH,OCOCICH, j=CHBr

CHz=CHC6 H 5

21 hr, 100". Et,N

3-BrC:, NH,

20 hr, IOO', Et,N

Product (Y,, yicldl

Referenw

(E)-CH,OCOCH=CHC,H, 181) (E)-4-CH30CC)CbHd-CHCIIC,H, (77 5 ) (E)-44-ICbHh CH= CHCOOCH, 154) (Z)-CeH,C(CH,)=CHC,H, ('0) (E)-C6H5C(CH3)=CHC6H5 (71) (E)-(CH,),C=CHCH= CHCOOCH,(75) (E,Z)-CH,(CHz),CH=CHCH=CHCOOCH3 ( 8 2 ) (E,E)-CH,(CH,),CH=CHCH=CHCOOCH3 (45) (E,E)-CH30COC(CH3]=CHCII=CIIC,H, (78) (E)-3-CsNH,CH-CHC,H 5 (79)

26 27 27 27 27 28

28

28 28 280

3 39

CATALYZED REACTIONS OF ORGANIC HALIDES

The reaction generally gives mainly or exclusively the product with the new substituent on the least substituted carbon of the double bond. The direction of addition is believed to be predominantly sterically controlled with the organic group acting as the larger part of the organopalladium complex which is adding. Electron-supplying substituents on the double bond tend to produce some reverse addition, but this is usually minor. The rates of the reaction depend on the organic halide used in approximately the same manner as observed in the palladium-catalyzed carboalkoxylation reaction. Additions to monosubstituted olefins generally give trans products. Additions to 1,2-disubstituted olefins proceed steriospecifically. The steriospecificity is believed to arise as a result of a cis addition of the organopalladium species followed by a cis elimination of the hydridopalladium moiety. Thus, (Z)-1-phenyl-1-propene and iodobenzene give mainly (Z)-1,2-diphenylI -propene, while (E)-1-phenyl-I-propene yields mainly the (E)-product. Some isomerization (-25%) is encountered in each case because some of the hydridopalladium-product complex reverts to a sigma palladium complex with palladium going on a different carbon than it was on initially. A final elimination then gives an isomerized product:

Pd

‘\X

H/’ L

L

340

R. F. HECK

Substitution reactions with (E)- or (Z)-vinylic halides usually show predominant retention of structure in the olefin substitution as they do in the carboalkoxylation, but the specificity is quite dependent on reaction conditions. Low reaction temperature, excess organophosphine, and dilution with excess trialkylamine and/or olefin all appear to improve the specificity. Under favorable conditions, for example, (Z)-l-bromo-1 -hexene and methyl acrylate give an 82% yield of the (E, 2)and only 10% of the (E, E) isomer of methyl 2,4-nonadienoate (29). H

H \

/

c-c

+ CH2=CHCOOCH, + Et,N

\

/'

CHdCH2h

p

d

'

+

~

Br

H

H /

'\

H

C=C /

\

CHdCH,),

c=c

/

/

+ Et3NH'Br-

\

H

COOCH3

The homogeneous catalytic olefinic substitution, like the carboalkoxylation, does not generally proceed in high yield with aromatic chlorides under the usual conditions. A heterogeneous catalyst, palladium on charcoal, has been reported to cause chlorobenzene and other aromatic chlorides to react with styrene and styrene derivatives, with sodium carbonate as a base at 100" (30, 31). In our laboratory, we have found the reactions occur as described, but the catalyst is apparently rapidly deactivated. Two reactions closely related to the olefinic substitution with organic halides have been reported that are useful in certain instances for the preparation of substituted olefins. In one reaction, organomercurials are used rather than organic halides. The palladium is required in the divalent state in this case and it is reduced in each cycle. The reaction is made catalytic by adding cupric chloride, which reoxidizes the palladium(0) to palladium(I1) again (32).The organopalladium reagent is produced by an exchange reaction of the organomercury compound with the palladium(l1) salt. The organopalladium product then reacts with the olefinic compound as in the organic halide case. The hydrogen chloride formed in the metal hydride decomposition does not need to be neutralized in these reactions, probably because they are done at 0 to 2.5" rather than 100". RHgCl RPd(CI)Li

+ PdCI,

L

+ CHz=CHCOOCH,

HPd(Cl)L,

Pd

where L is the solvent.

+ ZCuCI,

[RPd(Cl)LZ]

+

+ HgClz

RCH=CHCOOCHS

+ Pd + 2L -, PdCI, + 2CuCI

-+

HCl

+ HPd(CI)LZ

~

~

~

341

CATALYZED REACTIONS OF ORGANIC HALIDES

This reaction is limited to organomercurials not having sp3-bonded flhydrogens as in the previous reactions. The reaction succeeds with aryl as well as with methyl-(33), neopentyl-(33), 2-methyl-2-phenylpropyl-(33),and carboalkoxy- (32,34)groups in the mercurial. The last four groups have not been added by the preceding methods. A variation of this reaction, not possible by the other olefinic substitution methods, is the use of allylic chlorides as the olefinic reactant. This variation produces allylic derivatives catalytically. For example, p-anisylmercuric chloride and ally1 chloride with cupric chloride and a catalytic amount of palladium chloride at 25" produced 4allylanisole in 47% yield (35).In these reactions apparently palladium chloride is eliminated much more easily than palladium hydride since substituted allylic chlorides are not formed in significant amounts : RPd(CI)L,

+ CH,=.CHCH,CI

+

RCH,CHCHZCI

I

+

RCH,CH=CH,

+ PdCI2L2

Pd(CI)L,

The use of mercurials as the source of the organic group is a major deterrent from using these reactions. Some other organometallics may also be used, such as lead (32),tin (32),silicon (36),and boron (29) compounds, but they are generally more difficult to obtain than the organic halides. Thus, these reactions are generally less useful than the organic halide method. The olefinic substitution reaction may also be effected in some instances by using a palladium(I1) salt and an aromatic compound instead of an organic halide. Palladium(I1) salts are apparently able to metallate some aromatic hydrocarbons directly. The reaction succeeds best with aromatics activated with electron-supplying substituents producing, with certain olefinic compounds, vinylically substituted products. For example, benzene and styrene with palladium acetate in boiling acetic acid produce stilbene in 90% yield (37).

C,H,Pd(OAc)L,

+ Pd(OAc),

L

C,H,Pd(OAc)L, + HOAc + C6H5CH=CH2 + C6H5CH=CHC6H5 + HPd(OAc)L, HPd(OAc)L, + HOAc + 2L + Pd

C,H6

The reaction can be carried out at least partially catalytically under oxygen pressure (38).In addition to the problem of making the reaction catalytic in palladium, it appears that only certain olefins react well and only activated aromatic hydrocarbons are useful. Even with activated aromatics the palladation is often not very selective and isomeric mixtures result. Thus, this variation is not as generally useful as the organic halide reaction either. The use of olefinic compounds with methylene groups adjacent to the double bond in the substitution reaction may lead to the formation of allylically substituted olefinic products as well as the vinylic product, since the hydride elimination may now occur (in one of the possible adducts) in

342

R. F. HECK

either of two directions. For example, the reaction of iodobenzene with 2-phenyl-1-propene with a palladium acetate catalyst produces 24% of 2,3diphenyl-1-propene as well as 68% (E)-and 8% (2)-1,2-diphenyl-I-propenes : CH 3 C,HSC=CH,

t CeHSPdLZX 1

CH3

CH 3

I

C H C - CHZCbHS

5l

/

\

'C ~ H S

PdL2X

/

C-C

C6H 5

C6H5

+

',

\ /

H

c-c

/

C6H3

\

CHJ

H

C~HS

I

+ CHZ-C-CHZC6H5 The elimination of the hydridopalladium group to the allylic position causes the formation of 3-substituted carbonyl compounds when allylic alcohols are reacted with organic halides (39). For example, when iodobenzene is reacted with methallyl alcohol, 2-methyl-3-phenylpropanalwas formed in 9 1% yield along with 4% of 2-methyl-2-phenylpropanal : CH,

C6HsPdLzI

+ CH,=C-I

CH,

CH2OH

+

I I

C6HsCH2CCH20H

>

PdL,I

We do not know if the vinylic alcohol is actually an intermediate or whether a hydride-n complex of it rearranges directly to the aldehyde as probably happens in the palladium-catalyzed oxidation of ethylene to acetaldehyde. The formation of 4% 2-methyl-2-phenylpropanal is unexpected. This product must arise from a reversed addition of the phenylpalladium group followed by a hydrogen transfer from the hydroxyl-bearing carbon to the palladium, followed by reductive elimination of a hydridopalladium group. An alkyoxypalladium intermediate has been proposed (39). CH,

/

\ '

CH,

C

1

/

IL,PdCH2CCH20H

CH,

I

I CH3

I

+ CH,CCHO I

C6H5 [L,Pd]

\

CH,

L,Pd -0

C6H5

[L,Pd]

C6H5

+ C,H,I

-+

C,H,PdL,I

CH3

-

\

/

C6HS

C I'

CH2

I

L,Pd-H

\

C-H

II

0

343

CATALYZED REACTIONS OF ORGANIC HALIDES

The allylic alcohols generally give significant amounts of the addition of the organic group from the halide to the more substituted carbon in the double bond (reversed addition) because of the electron-donating character of the hydroxymethyl substituent. Still, the major products are always the isomers with the organic group on the least-substituted position. Organic iodides with palladium acetate as catalyst generally give the highest yields of carbonyl compounds with allylic alcohols, while organic bromides with phosphinepalladium catalysts with excess phosphine may give large amounts of substituted allylic alcohols as products. For example, iodobenzene reacts with 3-buten-2-01 and a palladium acetate catalyst to form 90% 4-phenyl-2butanone and 10% 3-phenyl-2-butanone,while the corresponding reaction of bromobenzene, adding 18 equivalents of triphenylphosphine per palladium acetate, produced 58% 4-phenyl-3-buten-2-01and 14% 3-phenyl-3-buten-2-01 along with 26% 4-phenyl-2-butanone and 2% 3-phenyl-2-butanone at the same reaction temperature. The bromide reaction, however, is much slower. 0 0 I1 I1 C6H5CH2CH2CCH3 CH3CHCCH3

+

OH C,H,X

I

C6H5

+ CH~=CHCHCH,’ \

OH

OH

I

C,H.jCH=CHCHCHj

+ CH,=C

1

- CHCH,

I

0

5

0

0

II

/I + C6H5CH2CH,CCH3+ CH3CHCCH3 I

C6H5

The formation of the 3-phenyl-2-butanone indicates a rearrangement of the intermediate palladium adduct is occurring, since only 3-phenyl-3-buten-2-01 could have been formed by a simple P-hydride elimination from the 3-phenyl4-pallado intermediate. It has been shown that the allylic alcohols are stable under the reaction conditions. Therefore, the 3-phenyl-2-butanone probably is being formed by an internal metal hydride elimination-readdition sequence as follows: C6H,

‘

OH

I

C6H5

I

XL2PdCH2kH--CHCH3 s CH2=C-

1

H-PdL2X - HPdL,X

C,H,

I

OH

I

CHZ=C--CHCH,

//

OH

1

CHCH,

C6H5 $

I

OH

I

CHIC-CHCH, PdL2X

I

Pd

C6H1

-HPdL,X

7

CHBCHCCH3

C6H5 OH

I

1

CH,C=CCH,

I

H---PdLZX

I

Pd

TABLE, 1X Olefinir Subst it ut ion Rrarrians of AlIj-lic rl1cohol.f Organic halide

AIlylic alcohol

Catalyst

Reaction conditions

----

C,W

CHZ-CHCHIOH

Pd(0Ac)z

loo", Et,N, 0.5 h r

C,H,I

CH,CH=CHCH,OH

Pd(OAc)I

IOO", E t 3 N CH,CN, 12 h r

Products (o,; yield)

C,H,CH,CH,CHO (6oj CH3CH(C,H,)CHO (1 I ) CH,CH(C,H,)CH,CHO (62) CH,CH2CH(C6H,)CHO (22)

OH 4-CH30C6H41

CH,=CHCHCH3

7 Pd[OAc)?

IOO', Et3N. CH,CN, 12 hr

4-CH30C6H,CH2CH,COCH3 (84) CH3CH(4-CH3OC,HJCOCH3 (1 2) OH 4-CF3C,H,CH=CHCHCH3 (23) 4-CF3C6H4CH,CH2COCH3(64) CHZ=C(4-CF,CsHd)CH(OH)CH3 (5) CHJC:H(CCF,C6H,)COCH, (1) C6H5CH=CHC(CH,);OH (97) ( Z ) - C , H , C H = C H C ( C H S ) = ~ ~ (~1 )

OH

I

4-CF3C6H4Br

CH,=CHCHCH,

Pd(PPh&(OAc),

loo", Et,N,44 h r

C6IJ51

CH,=CHC(CH3),0H

PdlPPh, j,(OAc),

100L,Et,N, 4 hr

' Data from Melpolder and Heck (39).

P

I

Rx

345

CATALYZED REACTIONS OF ORGANIC HALIDES

In spite of these complications, the allylic alcohol substitution reaction provides a simple method for preparing a variety of carbonyl compounds and alcohols often not readily accessible by other methods. Some examples of the reaction are shown in Table IX. The allylic alcohol substitution reaction may also be carried out in DMF solution with sodium bicarbonate as the base at 100 to 125" with palladiumphosphine catalysts, in which case only carbonyl products are formed. With this catalyst combination nonallylic, unsaturated alcohols also react to form carbonyl compounds in good yields. For example, in an extreme case, 9decen-1-01 and bromobenzene gave some 10-phenyldecanal (40):

+ CH,=CH(CHz),CHZ0H + NaHCO, C,H5(CH2)9CH0 + COz + H 2 0 + NaBr

C,H,Br

Pd(PPh,),Br,

+

A few experiments have been tried with conjugated dienes in the substitution reaction. Preliminary results indicate that they too may react normally. Using palladium acetate in a stoichiometric reaction, benzene and butadiene were found to form 1-phenylbutadiene in about 25% yield (41). Iodobenzene and isoprene react with triethylamine and Pd(PPh,),(OAc), as catalyst at 100" to form (E)-3-methyl-l-phenyl-l, 3-butadiene in 52% yield (42):

IV. Substitution Reactions of Terminal Acetylenes

Since the substitution reaction succeeded so well with olefins, the obvious extension to acetylenes was tried. Of course, only terminal acetylenes could be used if an acetylenic product was to be formed. This reaction has been found to occur but probably not by a mechanism analogous to the reaction of olefins (4444).It was found that the more acidic acetylene phenylacetylene reacted with bromobenzene in the presence of triethylamine and a bisphosphine-palladium complex to form diphenylacetylene, while the less acidic acetylene, 1-hexynedid not react appreciably under the same conditions. The reaction did occur when the more basic amine piperidine was used instead of triethylamine, however (43). Both reactions occur with sodium methoxide as the base (44).It therefore appears that the acetylide anion is reacting with the catalyst and that a reductive elimination of the disubstituted acetylene is

TABLE X Termitid Ace1 ylene Substitulion Rewirons"

Organic halide C6H,Br C6H,Br 4-OCHC,H,Br 2-CaH4SBf 4-N02C,H,Br C6H51

CH,=CBrCH,

Acetyleiic

C6H,GCH C6H5C=CH C6H5C-CH C6HsCECH t-BuCECH HOCH2C=CH C6H5C~CH

' Catalyst was cither Pd(PPh,),(OAc),

' C , N H , , , pipmdine. ' 2-Bromothiophene.

Uasc

C5NH11 NaOCH, Et3N C5NH11b Et,N NaOCH, Et ,N or Pd(PPh3),

Rcaction conditions loo-, 2.5 hr SO", DMF, 4 hr loo", 1 hr loo",0.5 hr lW", 0.5 h r 50". DMF, 3 hr lW", 1 hr

Product uoyield)

Reference

C,H5C=CC6H5 (64) C6H,C=CC6H, (88) ~ - O C H C ~ H ~ C E C C(661 ~H~

43 44 43

w

2-C4H4SC~CC,H5(53) ~-NO,C,H,CEXBU-~ (88) C,H,C==CCH,OH (55) CH2=C(CH3)C=CC6H5 (851

43 43 44 43

0

?I

7:

347

CATALYZED REACTIONS OF ORGANIC HALIDES

involved rather than an addition-elimination sequence: C,H5PdL,Br

+ HC=CC,H,

R,N

C,H,PdL,(C=CC,H,) [PdL,]

+ C,H,Br

C6H5PdL,(C=CC6H,)

-+

C,H5CrCC6H,

+

C,H,PdL,Br

+ HBr

+ [PdL,]

The acetylene substitution reaction proceeds much more rapidly than the related olefin reaction. The acetylene products and starting materials also undergo side reactions such as polymerization concurrently with the substitution. The best yields are obtained when the reactants are diluted with a large excess of amine, or carried out at lower temperatures in methanol with sodium methoxide as the base. Vinylacetylene derivatives can also be prepared by this reaction starting with vinylic halides. For example, (E)-methyl 3-bromo-2-methylpropenoate and t-butylacetylene react in 2 hours at 100" to form the expected vinylacetylene derivative in 59% yield : Br

CH.3 \ /

CH,OC

I1

c=c

/

+ HC_C-C(CH3)3 + Et,N

\

Pd(PPh

,;,

) (OAc),

H

0

cn,oC

/

+ Et,NH+Br-

c=c

I/

\

H

0

Some examples of the terminal acetylene substitution reaction are given in Table X.

V. Conclusions Numerous useful transition metal-catalyzed reactions of organic halides are now known. Practical syntheses of esters, acids, amides, aldehydes, olefins, ketones, and acetylenes have been described. In many instances the metalcatalyzed reactions are superior to more conventional, purely organic routes to the same compounds.

348

R. F. HECK REFERENCES

1. Roelen, O., Angew. Chem. 60,213 (1948).

2. 3. 4. 5.

Reppe, W., Justus Liebigs Ann. Chem. 582, 1 (1953). Slaugh, L. H., and Mullineaux, R. D., J . Organometal. Chem. 13, 469 (1968). Tucci, E. R., Ind. Eng. Chem., Prod. Res. Develop. 9, 516 (1970). Yagupsky, G., Brown, C. K., and Wilkinson, G., J . Chem. Sor. A , 1392 (1972). 6. Chiusoli, G. P., Chim. Ind. (Milan) 41, 503 (1959); Caiz. Chirn. Ital. 89. 1332 (1959). 7. Heck, R. F., J . Am. Chem. Soc. 85,2013 (1963). 8. Fischer, E. 0..and Burger, G., Z. Naturforsch. B 17,484 (1962). 9. Corey, E. J., and Hegedus, L. S., J . Am. Chem. SOC.91, 1233 (1969). 10. Cassar, L., and FOB, M., J . Organometal. Chem. 51,381 (1973). IOU. Chiusoli, G . P., and Merzoni, S., Z. Naturforsch. B 17, 850 (1962). 11. Schoenberg, A., Bartoletti, I., and Heck, R., J . Org. Chem. 39. 3318 (1974). 12. Scheben, J. A., Conf: Catal. Org. Synth. Sth, 1975. 13. National Distillers and Chem. Corp., British Patcnt 1, 145). 359 (1969). I3a. Medema, D., van Helden, R., and Kohll, C.F., Inorg. Chrm. Acta 3, 255 (1969). 14. Garrou, P. S . , and Heck, R. F., J . Am. Chem. SOC.In press. 15. Schoenberg, A., and Heck, R. F., J . Org. Chem. 39, 3327 (1974). 16. Schoenberg, A., and Heck, R. F., J . Am. Chem. Soc. 96, 7761 (1974). 17. Heck, R. F., and Breslow, D. S., J. Am. Chem. Sac. 85,2779 (1963). 18. Filbey, A. H., Wollensak, J. C., and Keblys, K. A,, ADstr. 138th Natl. Meeting Am. Chem. Soc. 1960. p. 54P. 19. Cooke, M. P., Jr., J . Am. Chem. SOC.92, 6080 (1970). 20. Collman, J. P., Winter, S . R., and Clark, D. R., J . Am. Chem. SOC.94, 1788 (1972). 21. Heck, R. F., “Organotransition Metal Chemistry,” pp. 201 -264. Academic Press, New York, 1974. 22. Chiusoli, G. P., Dubini, M., Ferraris, M., Guerrieri, F., Merzoni, S . , and Mondelli, G., J . Chem. Soc. C p . 2889 (1968). 23. Chiusoli, G. P., and Cassar, L., Angew. Chem., Int. Ed. Engl. 6, 124 (1967). 24. Heck, R. F., J . Am. Chem. SOC.85,3383 (1963). 25. Schultz, R. G., and Montgomery. P. D., J . Catal. 13, 105 (1969). 25a. D. Forster, J. Am. Chern. Sbc. 98, 846 (1976). 26. Mizoroki, T., Mori, K., and Ozaki, A,, Bull. Chem. Soc. Jap. 44, 581 (1971). 27. Heck, R. F., and Nolley, J. P., Jr., J . Org. Chem. 37, 2320 (1972). 28. Dieck, H. A,. and Heck, R. F., J. Am. Chem. Soc. 96, I I33 ( I 974). 28a. Frank, W., and Kim, Y . , unpublished work. 29. Dieck, H. A., and Heck, R. F., J . Org. Chem. 40,1083 (1975). 30. Julia, M., and Duteil, M., Bull. SOC.Chim. Fr. p. 2790 (1973). 3/. Julia, M., Duteil, M., Grand, C., and Kuntz, E., Bull. Soc. Chim. Fr. p. 2791 (1973). 32. Heck, R. F., J . Am. Chem. Soc. 90, 5518 (1968). 33. Heck, R . F., J. Organometal. Chem. 37, 389 (1972). 34. Heck, R. F., J . Am. Chem. SOC.91,6707 (1969). 35. Heck, R. F., J . Am. Chem. Soc. 90, 5531, (1968). 36. Weber, W. P., Felix, R. A,, Willard, A. K., and Koenig, K. E., Tetrahedron Lett. p. 4701 (1971). 37. Fujiwara, Y., Moritani, I., Danno, S . , Asano, R., and Teranishi, S., J . Am. Chem. Soc. 91, 7166 (1969). 38. Shue, R. S., Chem. Commun. p. 1510 (1971). 39. Melpolder, J. B., and Heck, R. F., J . Org. Chem. 41,265 (1976).

CATALYZED REACTIONS OF ORGANIC HALIDES

349

40. Personal communication from Dr. A. Chalk; Chalk, A. J., and Magennis, S. A,, J . Org. Chem. 42,213 (1976). 41. Danno, S., Moritani, I., and Fujiwara, Y . , Tetrahedrun Lett. p. 4809 (1969). 42. Dieck, H. A,, unpublished work. 43. Dieck, H. A,, and Heck, R . F., J. Organometal. Chem. 93, 259 (1975). 44. Cassar, L., J. Organometal. Chem. 93,253 (1975).

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Manual of Symbols and Terminology for Physicochemical Quantities and Units-Appendix II Definitions, Terminology and Symbols in Colloid and Surface Chemistry

Part II : Heterogeneous Catalysis* Adopted by the IUPAC Council at Madrid, Spain, on 9 September 1975 Prepared for publication by

ROBERT L. BURWELL, JR.

PREFACE

This Part I1 of Appendix IIt to the Manual of Symbols and Terminology for Physicochemical Quantities and Units$ (hereinafter referred to as the Manual) has been prepared by the Commission on Colloid and Surface Chemistry of the Division of Physical Chemistry of the International Union of Pure and Applied Chemistry. It is the outcome of extensive discussions within the Commission5 and its Task Force headed by Professor Burwell, with other * Republished from Pure Appl. Chem. 45, 71-90 (1976) by permission of the International Union of Pure and Applied Chemistry. + Part I of Appendix 11, Definitions, Trrminologp and Symbols in Colloid and Surface Chemistry, prepared for publication by D. H . Everett, Pure Appl. Chrm. 31, 579-638 (1972). Manuul of’ Symbols and TerminoloqjJir Phjsicoc,hemic.ul Quantiries and Units (1973 Edn.), prepared for publication by M. L. McGlashan and M. A. Paul, Butterworths, London (1975). The membership of the Commission during this period was as follows: Chairman: -1973 D. H. Everett (UK);1973- K. J. Mysels (USA) Secretary: H. van Olphen (USA) Titular Members: S. Brunauer (USA); R. L. Burwell, Jr. (USA); R. Haul (Germany); V. B. Kazansky (USSR); 1971- C. Kemball (UK); -1973 K. J. Mysels (USA); -1971 M. Pretre (France); G . Schay (Hungary). Associate M rmhers ; R. M. Barrer (UK); -1973 G. K. Boreskov (USSR); A. V. Kiselev (USSR); -1973 H. Lange (Germany); 1973- J. Lyklema (Netherlands); A. Scheludko (Bulgaria); G. A. Schuit (Netherlands); 1971 K. Tamaru (Japan). Ohseruer : -1971 Sir Eric Rideal (UK). National Represmrafii;rs: 1972- K. Morikawa (Japan): 1971 -74 Sir Eric Rideal (UK) (deceased); 1975 w. Schirmer (DDR).

*

35 1

352

COMMISSION

1.6 OF IUPAC

IUPAC Commissions, and with persons outside IUPAC during the period 1970-1975. Among the latter, special mention must be made to Professors M. Boudart (USA),J. B. Butt (USA), and F. S. Stone (UK). A tentative version of these proposals was issued as Appendix 39 (August 1974) on Tentative Nomenclature, Symbols, Units and Standards to IUPAC Information Bulletin. The text has been revised in the light of the criticisms, comments, and suggestions which were received, and the present version was prepared by the Commission and formally adopted by the IUPAC Council at its meeting in Madrid, Spain, in September 1975. It was felt that the use of unambiguous terminology would promote communication and avoid misunderstandings among workers in heterogeneous catalysis and that a list of preferred symbols would be useful in many respects. Heterogeneous catalysis is primarily a branch of physical chemistry but it has substantial overlap with organic and inorganic chemistry and with chemical engineering. The Commission agreed that no term or symbol should be used in heterogeneous catalysis in a sense different from that in physical chemistry in general or, as far as possible, in a sense different from that in other branches of chemistry. The present proposals are based on the same principles as those used in the Manuul’ and in Part I of this Appendix and are consistent with them. The most pertinent definitions of Part I are summarized and quoted in sections 1.2.1 and 1.2.2. Historical and common usage of terms has been retained as far as is compatible with the above principles. Since the present proposals should be considered as one of the sub-sets of the set of terms and symbols of physical chemistry, the general principles are not repeated here. Attention must be called, however, to one point, namely the restriction of the term “specific” to the meaning, divided by mass. This necessitates either the repetitive use of “per unit area” or the introduction of a new term having this meaning. After careful consideration the Commission recommends that the term areal, meaning divided by area, be used. This is, however, at this time, a provisional recommendation subject to a decision on this and related terms by ICSU, the International Council of Scientific Unions. La Jolla, California 29 December 1975

KAROL J. MYSELS Chairman Commission on Colloid and Surface Chemistry

t Manual of Symbols and Terminology jor Physicochemical Quuntities and Units (1973 Edn.), prepared for publication by M. L. McGlashan and M. A. Paul, Butterworths, London (1975).

TERMINOLOGY IN HETEROGENEOUS CATALYSIS

353

CONTENTS

Section 1 . Definitions and Termino1og.v . . . . . . . . . . . . . . . . . . 353

1.1 Catalysis and Catalysts . . . . . . . . . . . . . . . . . . . . . . . 1.2 Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 General terms . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Chemisorption and physisorption . . . . . . . . . . . . . . . . 1.2.3 Types of chemisorption . . . . . . . . . . . . . . . . . . . . 1.2.4 Sites for chemisorption . . . . . . . . . . . . . . . . . . . . . 1.2.5 Uniformity of sites . . . . . . . . . . . . . . . . . . . . . . 1.2.6 Active site, active centre . . . . . . . . . . . . . . . . . . . . 1.2.7 Adsorption isotherms . . . . . . . . . . . . . . . . . . . . . 1.2.8 Bifunctional catalysis . . . . . . . . . . . . . . . . . . . . . 1.2.9 Rates of adsorption and desorption . . . . . . . . . . . . . . . 1.3 Composition, Structure and Texture of Catalysts . . . . . . . . . . . . 1.3.1 General terms . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Porosity and texture . . . . . . . . . . . . . . . . . . . . . . 1.4 Catalytic Reactors . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Kinetics of Heterogeneous Catalytic Reactions . . . . . . . . . . . . . 1.5.1 General terms . . . . . . . . . . . . . . . . . . . . . . . . 1.5.2 Selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.3 Rate equations . . . . . . . . . . . . . . . . . . . . . . . . 1.5.4 Kinetic aspects of mechanism . . . . . . . . . . . . . . . . . . 1.5.5 Non-uniformity of catalytic sites . . . . . . . . . . . . . . . . . 1.6 Transport Phenomena in Heterogeneous Catalysis . . . . . . . . . . . 1.7 Loss of Catalytic Activity . . . . . . . . . . . . . . . . . . . . . . 1.7.1 Poisoning . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7.2 Deactivation : general . . . . . . . . . . . . . . . . . . . . . 1.7.3 Types of deactivation . . . . . . . . . . . . . . . . . . . . . 1.8 Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . I .8.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8.2 Elementary processes in heterogeneous catalysis . . . . . . . . . . 1.8.3 Nomenclature of surface intermediates . . . . . . . . . . . . . . I .9 Nomenclature of Catalytic Reactions . . . . . . . . . . . . . . . . . Section 2 . List oj’Symbo1.Y and Abbreviations . . . . . . . . . . . . . . . . Section 3. Alphabetical Index . . . . . . . . . . . . . . . . . . . . . .

SECTION

1.

353 355 355 356 358 360 361 362 362 365 365 366 366 367 369 371 371 372 373 375 376 376 377 377 378 378 379 379 380 381 383 384 386

DEFINITIONS AND TERMINOLOGY

1.1 Catalysis and catalysts Catalysis is the phenomenon in which a relatively small amount of a foreign material. called a catalyst. augments the rate of a chemical reaction without itself being consumed. Cases occur with certain reactants in which the addition of a substance reduces the rate of a particular reaction. for example. the addition of an inhibitor in a chain reaction or a poison in a catalytic reaction. The term “negative catalysis” has been used for these

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COMMISSION 1.6 OF IUPAC

phenomena but this usage is not recommended; terms such as inhibition or poisoning are preferred. A catalyst provides for sets of elementary processes (often called elementary steps) which link reactants and products and which do not occur in the absence of the catalyst. For example, suppose the reaction A = C

to proceed at some rate which might be measurable but might be essentially zero. The addition of X might now provide a new pathway involving the intermediate B, A t X + B

B+C+X

If reaction by this pathway proceeds at a rate significant with respect to the uncatalysed rate such that the total rate is increased, X is a catalyst. In this sense, a cutalytic reaction is a closed sequence of elementary steps similar to the propagation steps of a gas-phase chain reaction. The catalyst enters into reaction but is regenerated at the end of each reaction cycle. Thus, one unit of catalyst results in the conversion of many units of reactants (but see $1.7). A catalyst, of course, may catalyse only one or some of several thermodynamically possible reactions. It is difficult to separate Nature into water-tight compartments and probably no operational definition of catalysis can be entirely satisfactory. Thus, water might facilitate the reaction between two solids by dissolving them. This phenomenon might appear to constitute an example of catalysis but such solvent effects are not, in general, considered to fall within the scope of catalysis. The kinetic salt effect in solution is also usually excluded. Further, a catalyst must be material and, although an input of heat into a system usually augments the rate of a reaction, heat is not called a catalyst, nor is light a catalyst in leading to reaction between chlorine and hydrogen. A catalyst should be distinguished from an initiator. An initiator starts a chain reaction, for example, di-t-butylperoxide in the polymerisation of styrene, but the initiator is consumed in the reaction. It is not a catalyst. In homogeneous catulysis, all reactants and the catalyst are molecularly dispersed in one phase. In heterogeneous cutalysis, the catalyst constitutes a separate phase. In the usual case, the catalyst is a crystalline or amorphous solid, the reactants and products being in one or more fluid phases. The catalytic reaction occurs at the surface of the solid and, ideally, its rate is proportional to the area of the catalyst. However, in practical cases, transport processes may restrict the rate (see 91.6).

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355

Most examples of catalysis can be readily characterised as homogeneous or heterogeneous but there are examples of catalysis which overlap the two types. Consider a system in which intermediates are formed at the surface and then are desorbed into the gas phase and react there. Such intermediates might generate a chain reaction in the gas phase, i.e., chain initiation and chain termination occur at the surface but chain propagation occurs in the gas phase. Enzyme catalysis may share some of the characteristics of homogeneous and heterogeneous catalysis, as when the catalyst is a macromolecule small enough to be molecularly dispersed in one phase with all reactants but large enough so that one may speak of active sites on its surface. This manual deals with heterogeneous catalysis. Other types of catalysis will receive no further attention.

I .2 Adsorption 1.2.1 General terms Although adsorption exists as a subject of scientific investigation independent of its role in heterogeneous catalysis, it requires particular attention here because of its central role in heterogeneous catalysis. Most or all catalytic reactions involve the adsorption of at least one of the reactants. Many terms related to adsorption have already been defined in Appendix 11, Part I, $1.1. These include surface, interface, area of surface or interface, and specijic suYface area. Appendix 11, Part I, recommends A or S and a or s as symbols for area and specific area, respectively. A , and a, may be used to avoid confusion with Helmholtz energy A or entropy S where necessary. Other terms are sorption, sorptive, sorbate [a distinction being made between a species in its sorbed state (sorbate) and a substance in the fluid phase which is capable of being sorbed (sorptive)], absorption, absorptive, ubsorbate, absorbent; and adsorption, adsorptive, adsorbate, adsorbent.? The term adsorption complex is used to denote the entity constituted by the adsorbate and the part of the adsorbent to which it is bound. Appendix 11, Part 1, gl.l.5, treats the adsorbent/fluid* interface as follows. “It is often useful to consider the adsorbent/fluid interface as comprising two regions. The region of the fluid phase (i.e., liquid or gas) forming part of the adsorbent/fluid interface may be called the adsorption space, while the portion of the adsorbent included in the interface is called the surface layer qf the adsorbent.” The use of substrute for adsorbent or support is to be discouraged because of its general use in enzyme chemistry to designate a reactant. Appendix 11, Part I, recommends: The use of a solidus to separate the names of bulk phases is preferred to the use of a hyphen which can lead to ambiguities.

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COMMISSION 1.6 OF IUPAC

When used to denote the process in which moleculest or dissociated molecules accumulate in the adsorption space or in the surface layer of the absorbent, adsorption has as its counterpart the term desorption which denotes the converse process (see Appendix 11, Part I, 41.1.4). Adsorption is also used to denote the result of the process of adsorption, i.e., the presence of adsorbate on an adsorbent. The adsorbed state may or may not be in equilibrium with the adsorptive [see §1.2.2(c)]. Adsorption and desorption may also be used to indicate the direction from which equilibrium has been approached, e.g., adsorption curve (point), desorption curve (point). 1.2.2 Chemisorption and physisorption For convenience, the relevant portions of $41.1.6 and 1.1.7 of Appendix 11, Part I, are reproduced here. “Chemisorption und physisorption Chemisorption (or Chemical Adsorption) is adsorption in which the forces involved are valence forces of the same kind as those operating in the formation of chemical compounds. The problem of distinguishing between chemisorption and physisorption (see below) is basically the same as that of distinguishing between chemical and physical interaction in general. N o absolutely sharp distinction can be made and intermediate cases exist, for example, adsorption involving strong hydrogen bonds or weak chargetransfer. Some features which are useful in recognising chemisorption include: (a) the phenomenon is characterised by chemical specificity; (b) changes in the electronic state may be detectable by suitable physical means (e.g., u.v., infrared or microwave spectroscopy, electrical conductivity, magnetic susceptibility); (c) the chemical nature of the adsorptive(s) may be altered by surface dissociation or reaction in such a way that on desorption the original species cannot be recovered; in this sense chemisorption may not be reversible; (d) the energy of chemisorption is of the same order of magnitude as the energy change in a chemical reaction between a solid and a fluid: thus chemisorption, like chemical reactions in general, may be exothermic or endothermic and the magnitudes of the energy changes may range from very small to very large; (e) the elementary step in chemisorption often involves an activation energy; The term molecules is used in the general sense to denote any molecular species: atom, ion, neutral molecule or radical. +

TERMINOLOGY IN HETEROGENEOUS CATALYSIS

357

(f) where the activation energy for adsorption is large (activated adsorption), true equilibrium may be achieved slowly or in practice not at all. For example, in the adsorption of gases by solids the observed extent of adsorption, at a constant gas pressure after a fixed time, may in certain ranges of temperature increase with rise in temperature. In addition, where the activation energy for desorption is large, removal of the chemisorbed species from the surface may be possible only under extreme conditions of temperature or high vacuum, or by some suitable chemical treatment of the surface; (8) since the adsorbed molecules are linked to the surface by valence bonds, they will usually occupy certain adsorption sites on the surface and only one layer of chemisorbed molecules is formed (monolayer adsorption). Physisorption (or Physical Adsorption) is adsorption in which the forces involved are intermolecular forces (van der Waals forces) of the same kind as those responsible for the imperfection of real gases and the condensation of vapours, and which do not involve a significant change in the electronic orbital patterns of the species involved. The term van der Waals adsorption is synonymous with physical adsorption, but its use is not recommended, Some features which are useful in recognising physisorption include : (a’) the phenomenon is a general one and occurs in any solid/fluid system, although certain specific molecular interactions may occur, arising from particular geometrical or electronic properties of the adsorbent and/or adsorptive ; ( b ) evidence for the perturbation of the electronic states of adsorbent and adsorbate is minimal; (c’) the adsorbed species are chemically identical with those in the fluid phase, so that the chemical nature ofthe fluid is not altered by adsorption and subsequent desorption; (d’) the energy of interaction between the molecules of adsorbate and the adsorbent is of the same order of magnitude as, but is usually greater than, the energy of condensation of the adsorptive; (e’) the elementary step in physical adsorption does not involve an activation energy. Slow, temperature dependent, equilibration may however result from rate-determining transport processes; (f‘) in physical adsorption, equilibrium is established between the adsorbate and the fluid phase. In solid/gas systems at not too high pressures the extent of physical adsorption increases with increase in gas pressure and usually decreases with increasing temperature. In the case of systems showing hysteresis the equilibrium may be metastable. (8‘) under appropriate conditions of pressure and temperature, molecules from the gas phase can be adsorbed in excess of those in direct contact with the surface (multilayer adsorption or filling of micropores).

358

COMM~SSION1.6 OF IUPAC

Monoluyer und multilayer adsorption, micropore,filling and capillary condensation In monolayer adsorption all the adsorbed molecules are in contact with the surface layer of the adsorbent. In multilayer adsorption the adsorption space accommodates more than one layer of molecules and not all adsorbed molecules are in contact with the surface layer of the adsorbent. The monolayer capuciry is defined, for chemisorption, as the amount of adsorbate which is needed to occupy all adsorption sites as determined by the structure of the adsorbent and by the chemical nature of the adsorptive; and, for physisorption, as the amount needed to cover the surface with a complete monolayer of molecules in close-packed array, the kind of closepacking having to be stated explicitly when necessary. Quantities relating to monolayer capacity may be denoted by subscript m. The swfure cooeraye (0) for both monolayer and multilayer adsorption is defined as the ratio of the amount of adsorbed substance to the monolayer capacity. The area occupied by a molecule in a complete monolayer is denoted by a, ; for example, for nitrogen molecules a,(N,). Micropore filling is the process in which molecules are adsorbed in the adsorption space within micropores. The micropore oolume is conventionally measured by the volume of the adsorbed material which completely fills the micropores, expressed in terms ofbulk liquid at atmospheric pressure and at the temperature of measurement. In certain cases (e.g., porous crystals) the micropore volume can be determined from structural data. Capillary condensution is said to occur when, in porous solids, multilayer adsorption from a vapour proceeds to the point at which pore spaces are filled with liquid separated from the gas phase by menisci. The concept of capillary condensation loses its sense when the dimensions of the pores are so small that the term meniscus ceases to have a physical significance. Capillary condensation is often accompanied by hysteresis.”

1.2.3 Types of clzenzisorption Non-dtssocuriw, dissociatiue. If a molecule is adsorbed without fragmentation, the adsorption process is non-disociatiue. Adsorption of carbon monoxide is frequently of this type. Ifa molecule is adsorbed with dissociation into two or more fragments both or all of which are bound to the surface of the adsorbent, the process is dissociatioe. Chemisorption of hydrogen is commonly of this type. H&) -, ZH(ads)

or

The asterisk represents a surface site.

H2(g) + 2*

+

ZH*

TERMINOLOGY IN HETEROGENEOUS CATALYSIS

359

Homolytic and heterolytic relate in the usual sense to the formal nature of the cleavage of a single bond. If the electron pair in the bond of the adsorptive A : B is divided in the course of its dissociative adsorption, the adsorption is homolytic dissociative adsorption. If A or B retains the electron pair, the adsorption is heterolytic dissociative adsorption. Examples follow. (a) Homolytic dissociative adsorption of hydrogen on the surface of a metal : H,

+ 2* -+

2H*.

(b) Heterolytic dissociative adsorption of hydrogen at the surface of an oxide where the surface sites M"' and 0'- are surface sites in which the ions are of lower coordination than the ions in the bulk phase: H2

+ M"' + 0 ' -

H-M"'

---t

+ HO-

Where clarity requires it, the equation may be written HZ(gj+ M:+

+ 0:-

+

H-M:'

+ HO,

where the subscript s indicates that the species indicated are part of the surface. The notation H-M"' is used, as in conventional inorganic terminology, to indicate that the oxidation number of M has not changed. (c) Heterolytic dissociative adsorption of water at the same pair of sites as in (b): H,O

+ M"' + 0'-

HO-M"'

+

+ HO-.

Reductive and oxidative dissociative adsorption involve usage analogous to that in coordination chemistry in which one speaks of the following reaction as an oxidative addition L,M(I)

+ H,

+

L,M(III)H,

Here, M represents a transition metal atom and L a ligand. H as a ligand is given an oxidation number of - 1. If reductive, the electron pair which constitutes the bond in the sorptive, A:B, is transferred to surface species; if oxidative, a pair of electrons is removed from surface species. One would say that dissociative adsorption of CI, on a metal is oxidative if chlorine forms C1- ions on the surface of the adsorbent. A dissociative adsorption would be reductive if, for example, it occurred thus (note that H2 -+ 2Hf + 2e here), H2(g)

+ 2[M(III)02~],

+

2[M(IIj(OH)-],.

Charge transjer adsorption represents oxidative or reductive chemisorption where reductive and oxidative refer to electron gain or loss on species in the solid. In simple cases it is non-dissociative, i.e., there is a mere transfer of

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COMMISSION 1.6 OF IUPAC

charge between adsorptive and adsorbent in forming the adsorbate. Two examples follow. Reductive X + * +. X + * - , where X represents an aromatic molecule of low ionization potential such as anthracene or triphenylamine and * a site on silica-alumina. Oxidative 0,

+ * + O,-*'

The term, charge transfer adsorption, has also been applied to adsorption which resembles the charge transfer complexes of Mulliken. Immobile, mobile. These terms are used to describe the freedom of the molecules of adsorbate to move about the surface. Adsorption is immobile when kT is small compared to AE, the energy barrier separating adjacent sites. The adsorbate has little chance of migrating to neighbouring sites and such adsorption is necessarily localised. Mobility of the adsorbate will increase with temperature and mobile adsorption may be either localised or non-localised. In localised mobile adsorption, the adsorbate spends most of the time on the adsorption sites but can migrate or be desorbed and readsorbed elsewhere. In non-localised adsorption the mobility is so great that a small fraction of the adsorbed species are on the adsorption sites and a large fraction at other positions on the surface. In some cases of localised adsorption the adsorbate is ordered into a twodimensional lattice or net in a particular range of surface coverage and temperature. If the net of the ordered adsorbed phase is in registry with the lattice of the adsorbent the structure is called coherent, if not it is called incoherent (see also 9 1.2.4). Each of the various processes of adsorption may have desorptions of the reverse forms, for example, dissociative adsorption may have as its reverse, associative desorption. However, the process of chemisorption may not be reversible [§1.2.2(c)]. Desorption may lead to species other than that adsorbed, for example, ethane dissociatively adsorbed on clean nickel gives little or no ethane upon desorption, 1-butene dissociatively adsorbed to methylallyl and H on zinc oxide gives mainly 2-butenes upon desorption, and some W 0 3 may evaporate from tungsten covered with adsorbed oxygen. Photoadsorption, photodesorption. Irradiation by light (usually visible or ultraviolet) may affect adsorption. In a system containing adsorptive and adsorbent exposure to light may lead to increased adsorption (photoadsorption) or it may lead to desorption of an adsorbate (photodesorption). 1.2.4 Sites for chemisorption Sites may be classified according to their chemical nature in usual chemical terminology.The following terms are simple extensions of ordinary chemical usage: basic sites, acidic sites, Lewis acid sites, proton or Brbnsted acid sites,

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361

electron accepting sites and electron donating sites (possible examples of the last two appear under charge transfer adsorption). It is often useful to consider that sites for chemisorption result from surface coordinatiue unsaturation, i.e., that atoms at the surface have a lower coordination number than those in bulk. Thus, for example a chromium ion at the surface of chromium oxide has a coordination number less than that of a chromium ion in the bulk. The chromium ion will tend to bind a suitable adsorptive so as to restore its coordination number. An atom in the (100) surface of a face-centered cubic metal has a coordination number of 8 vs 12 for an atom in bulk; this, too, represents surface coordinative unsaturation. However, of course, there are sites to which the concept of surface coordinative unsaturation does not apply, for example, Br$nsted acid sites. One is rarely sure as to the exact identity and structure ofsites in adsorption and heterogeneous catalysis. However, some symbolism is needed for theoretical discussion of possible sites. On the one hand one may wish to use a description which is general and non-specific. For this * and (ads) are recommended as, for example, H* and H(ads). Or one may wish to use a symbolism which is as specific as possible. General chemical symbols may be useful in this case. A symbolism useful for metals involves the specification of Cj and B, where Cj denotes a surface atom with j nearest neighbours and B, denotes an ensemble of n surface atoms which together constitute an adsorption site, for example, the adsorption site lying above the centre of three surface atoms constituting the corners of an equilateral triangle is a B, site [for details see van Hardeveld and Hartog, Surface Sci. 15,189 (1969)l. Cases of chemisorption are known in which at high coverages the net (twodimensional lattice) of the adsorbate is not in registry with the lattice of the adsorbent. In such situations, the concept of sites of precise location and fixed number may not be applicable. Similar difficulties about the definition of sites will occur if surface reconstruction takes place upon interaction of adsorbate and adsorbent. Because of various difficulties which often appear in knowing the identity of surface sites, it is frequently convenient, particularly for metals, to define the surface coverage 0 as the ratio of the number of adsorbed atoms or groups to the number of surface atoms (cf. $1.2.2). 1.2.5 Uniformity of sites Variations in the nature of the sites for adsorption or catalysis can occur even with pure metals where there is no question of differences in chemical composition between one part of the surface and another. These variations arise not only because of defects in the metal surfaces but also because the nature of a site depends on the structure of the surface. Uniform sites are more likely to be encountered when adsorption or catalysis is studied on an individual face of a single crystal; but even individual faces may present more

362

COMMISSION 1.6 OF IUPAC

than one kind of site. Nun-uniform sites will normally occur with specimens of metal exposing more than one type of crystal face. There are two main kinds of non-uniformities. Intrinsic non-unijormity is a variation due solely to the nature of the adsorbent. Induced non-unqormity arises when the presence of an adsorbate molecule on one site leads to a variation in the strength of adsorption at a neighbouring site. Thus, a set of uniform sites on an individual crystal face may become non-uniform if the surface is partially covered with a chemisorbed species. When the catalytic properties of metals are examined, the importance of the non-uniformity of sites depends on the reaction under study. For some reactions, the activity of the metal catalyst depends only on the total number of sites available and these are termed structure-insensitive reuctiuns. For other reactions, classified as structure-sensitive reactions, activity may be much greater on sites associated with a particular crystal face or even with some type of defect structure. The alternative names of,fucile or demanding have been used to describe structure-insensitive or structure-sensitive reactions, respectively. The terms of $1.2.5 have been discussed with reference to metallic surfaces but they can be applied to other adsorbents and catalysts and, in particular, to the pair-sites involved in heterolytic dissociative adsorption,

1.2.6 Active site, active centre The term active sites is often applied to those sites for adsorption which are the effective sites for a particular heterogeneous catalytic reaction. The terms active site and uctive centre are often used as synonyms, but active centre may also be used to describe an ensemble of sites at which a catalytic reaction takes place. 1.2.7 Adsorption isotherms An udsorption isotherm for a single gaseous adsorptive on a solid is the function which relates at constant temperature the amount of substance adsorbed at equilibrium to the pressure (or concentration) of the adsorptive in the gas phase. The surface excess amount rather than the amount adsorbed is the quantity accessible to experimental measurement, but, at lower pressures, the difference between the two quantities becomes negligible (see Appendix 11, Part I, $1.1.11). Similarly, when two or more adsorptives adsorb competitively on a surface, the adsorption isotherm for adsorptive i at a given temperature is a function of the equilibrium partial pressures of all of the adsorptives. In the case of adsorption from a liquid solution, an adsorption isotherm for any preferentially adsorbed solute may be similarly defined in terms of the equilibrium concentration of the respective solution component, but the isotherm usually

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363

depends on the nature of the solvent and on the concentrations (mole fractions) of other solute components if present. Individual solute isotherms cannot be derived from surface excesses except on the basis of an appropriate model of the adsorption layer; when chemisorption occurs it is generally adequate to assume monolayer adsorption. Amounts adsorbed are often expressed in terms of coverages 4. In chemisorption, Bi is the fraction of sites for adsorption covered by species i. Types of adsorption isotherms of interest to heterogeneous catalysis follow. The linear adsorption isotherm. The simplest adsorption isotherm is the analogue of Henry’s law. For a single adsorptive, it takes the form

0

=

Kp

6 = Kc,

or

where p and c are the pressure and concentration of the adsorptive, 8 is the coverage by adsorbate and K the linear adsorption isotherm equilibrium constant, or Henry’s law constant. Most adsorption isotherms reduce to Henry’s law when p or c becomes small enough provided that simple adsorption occurs, i.e., adsorption is neither dissociative nor associative. That is, at low enough coverages Henry’s law usually applies to the first of the following equations but not the second and third.

+ *$A*; + 2* + 2A*:

A A,

2A t * & A , * .

The Langmuir adsorption isotherm,

0

= ___ Kp

1

+ Kp

or

0 p ( 1 - 8) = K’

or the equivalents in terms of concentrations, is commonly taken to result from simple (non-dissociative)adsorption from an ideal gas on a surface with a fixed number of uniform sites which can hold one and only one adsorbate species. K is called the Langmuir adsorption equilibrium constant. Further, the enthalpy of the adsorbed form must be independent of whether or not adjacent sites are occupied and consequently the enthalpy of adsorption is independent of 8. The second form of Langmuir’s isotherm given above, emphasizes that the constant K is the equilibrium constant for A + * + A*. Since the constancy of enthalpy with coverage is analogous to the constancy of enthalpy with pressure in an ideal gas, the adsorbed state in a system following Langmuir’s isotherm is sometimes called an ideal adsorbed state.

364

COMMISSION 1.6 OF IUPAC

If chemisorption is dissociative,

Langmuir’s equation takes the form

For simple adsorption of two adsorptives A and B competing for the same sites, Langmuir’s isotherm takes the form

where K A and K , are the equilibrium constants for the separate adsorption of A and B, respectively. This equation can be generalised to cover adsorption of several adsorptives and to allow for dissociativeadsorption of one or more adsorptives. In the Freundlich udsorption isotherm, the amount adsorbed is proportional to a fractional power of the pressure of the adsorptive. For a particular system,the fractional power and the constant of proportionality are functions of temperature. In terms of coverage the isotherm assumes the form

8 = up’/”, where n is a number greater than unity and a a constant. In the region of validity of the isotherm the (differential)enthalpy of adsorption is a linear function of In 8. In the Temkin adsorption isotherm, the amount adsorbed is related to the logarithm of the pressure of the adsorptive

8 = A lnp

+ B,

where A and B are constants. In the region of validity of the isotherm the (differential)enthalpy of adsorption is a linear function of 8. The Brunauer-Emmett- Teller (or B E T ) adsorption isotherm applies only to the physisorption of vapours but it is important to heterogeneous catalysis because of its use for the determination of the surface areas of solids. The isotherm is given by the following equation,

TERMINOLOGY IN HETEROGENEOUS CATALYSIS

365

where c is a constant which depends upon the temperature, the adsorptive and the adsorbent, I? is the amount adsorbed, n, is the monolayer capacity and p o is the saturated vapour pressure of the pure, liquid adsorptive at the temperature i n question. According to this equation, which is based on a model of multilayer adsorption, 6, exceeds unity when p / p o is sufficiently large. 1.2.8 Bfunctioriul catalysis Some heterogeneous catalytic reactions proceed by a sequence of elementary processes certain of which occur at one set of sites while others occur at sites which are of a completely different nature. For example, some of the processes in the reforming reactions of hydrocarbons on platinum/ alumina occur at the surface of platinum, others at acidic sites on the alumina. Such catalytic reactions are said to represent bifunctional catalysis. The two types of sites are ordinarily intermixed on the same primary particles (51.3.2) but similar reactions may result even when the catalyst is a mixture of particles each containing but one type of site. These ideas could, of course, be extended to create the concept of poljlfunctional catalysis. 1.2.9 Rates of adsorption untl desorption Sticking coefficient is the ratio of the rate of adsorption to the rate at which the adsorptive strikes the total surface, i.e., covered and uncovered. It is usually a function of surface coverage, of temperature and of the details of the surface structure of the adsorbent. Sticking probubility is often used with the same meaning but in principle it is a microscopic quantity concerned with the individual collision process. Thus the sticking coefficient can be considered as a mean sticking probability averaged over all angles and energies of the impinging molecules and over the whole surface. The mean residence time of adsorbed molecules is the mean time during which the molecules remain on the surface of the adsorbent, i.e., the mean time interval between impact and desorption. While residing on the surface the molecules may migrate between adsorption sites before desorption. If the residence time of an adsorbed species refers to specified adsorption sites, it would be called the mean life time of the particular adsorption complex. When the rate of desorption is first order in coverage, the residence time is independent of surface coverage and equal to the reciprocal of the rate constant of the desorption process. In this case it can be characterised unambiguously also by a half-life or by some other specified fractional-life of the desorption process. If the desorption process is not first order, e.g., due to mutual interactions of the adsorbed molecules and/or energetic heterogeneity of the surface, the residence time depends upon surface coverage and the operational definition of “residence time” needs to be specified precisely.

366

COMMISSION1.6 OF IUPAC

Unuctirated and activated adsorption. If the temperature coefficient of the rate of adsorption is very small, the adsorption process is said to be unactiuated (i.e.,to have a negligible activation energy). In this case the sticking coefficient at low coverages may be near unity particularly for smaller molecules. If the temperature coefficient of the rate of adsorption is substantial, the adsorption process is said to be activuted (i.e., to have a significant activation energy). In this case, the sticking coefficient is small. In general, the activation energy of activated adsorption is a function of coverage and it usually increases with increasing coverage. A number of relations between rate of activated adsorption and coverage have been proposed. Of these, one has been particularly frequently used, the Royinskii-Zeldovich equation sometimes called the Elovich equation, dO -- ue ~- bO _

dt

7

where 0 is the coverage, and a and b are constants characteristic of the system. 1.3 Composition, btructure and texture of catalysts

I .3.1 General terms Catalysts may be one-phase or multiphase. In the first case, they may be composed of one substance (for example, alumina or platinum black) or they may be a one-phase solution of two or more substances. In this case, the components of the solution should be given and joined by a hyphen (for example, sil ica-a1umina). Support. In multiphase catalysts, the active catalytic material is often present as the minor component dispersed upon a support sometimes called a currier. The support may be catalytically inert but it may contribute to the overall catalytic activity. Certain bifunctional catalysts ($1.2.8) constitute an extreme example of this. In naming such a catalyst, the active component should be listed first, the support second and the two words or phrases should be separated by a solidus, for example, platinum/silica or platinum/silicaalumina. The solidus is sometimes replaced by the word “on,” for example, platinum on alumina. Promoter. In some cases, a relatively small quantity of one or more substances, the promoter or promoters, when added to a catalyst improves the activity, the selectivity, or the useful lifetime of the catalyst. In general, a promoter may either augment a desired reaction or suppress an undesired one. There is no formal system of nomenclature for designating promoted catalysts. One may, however, for example, employ the phase “iron promoted with alumina and potassium oxide.”

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A promoter which works by reducing the tendency for sintering and loss of area may be called a textural promoter (see 41.7.3). Doping. In the case of semiconducting catalysts, a small amount of foreign material dissolved in the original catalyst may modify the rate of a particular reaction. This phenomenon is sometimes called doping by analogy with the effect of similar materials upon semiconductivity.

1.3.2 Porosity and texture Many but not all catalysts are porous materials in which most of the surface area is internal. It is sometimes convenient to speak of the structure and texture of such materials. The structure is defined by the distribution in space ofthe atoms or ions in the material part ofthe catalyst and, in particular, by the distribution at the surface. The textzire is defined by the detailed geometry of the void space in the particles of catalyst. Porosity is a concept related to texture and refers to the pore space in a material. With zeolites, however, much of the porosity is determined by the crystal structure. An exact description of the texture of a porous catalyst would require the specification of a very large number of parameters. The following averaged properties are often used. With respect to porous solids, the surface associated with pores may be called the internal surface. Because the accessibility of pores may depend on the size of the fluid molecules, the extent of the accessible internal surface may depend on the size of the molecules comprising the fluid, and may be different for the various components of a fluid mixture (molecular sieve eflect). When a porous solid consists of discrete particles, it is convenient to describe the outer boundary of the particles as external surface. It is expedient to classify pores according to their sizest (i) pores with widths exceeding about 0.05 ym or 50 nm (500 A) are called macropores ; (ii) pores with widths not exceeding about 2.0nm (20A) are called micropores ; (iii) pores of intermediate size are called mesopores. The terms intermediate or trai~sitionalpores, which have been used in the past, are not recommended. In the case of micropores, the whole of their accessible volume may be regarded as adsorption space. The above limits are to some extent arbitrary. In some circumstances it may prove convenient to choose somewhat different values. Pore-size distribution is the distribution of pore volume with respect to pore size; alternatively, it may be defined by the related distribution of pore See Appendix 11, Part I, 51.15.

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area with respect to pore size. It is an important factor for the kinetic behaviour of a porous catalyst and thus an essential property for its characterisation (see $1.6). The computation of such a distribution involves arbitrary assumptions and a pore-size distribution should always be accompanied by an indication as to the method used in its determination. The methods usually involve either or both of the following: (i) adsorption-desorption isotherms of nitrogen or other adsorptives in conjunction with a particular model for conversion of the isotherm into a pore-size distribution, (ii) data obtained by the mercury porosimeter. The isotherm gives a pore-size distribution for mesopores. The mercury porosimeter gives a distribution covering macropores and larger mesopores. In both cases what is measured is, strictly speaking, not the exact volume of pores having a given pore size, but the volume of pores accessible through pores of a given size. The relationship between these two functions depends on the geometrical nature of the pore system. The specijc pore volume is the total internal void volume per unit mass of adsorbent. Some of the pore volume may be completely enclosed, and thus inaccessible to molecules participating in a catalytic reaction. The total accessible pore volume may be measured by the amount of adsorbate at the saturation pressure of the adsorptive, calculated as liquid volume, provided the adsorption on the external surface can be neglected or can be evaluated. The accessible pore volume may be different for molecules of different sizes. A method which is not subject to the effect of the external surface is the determination of the dead space by means of a non-sorbable gas (normally helium) in conjunction with the determination of the bulk volume of the adsorbent by means of a non-wetting liquid or by geometrical measurements. Primury particles. Certain materials widely used as catalysts or supports consist of spheroids of about 10 nm (100 A) in diameter loosely cemented into granules or pellets. The texture of these resembles that of a cemented, loose gravel bed. The 10 nm (100 A) particles may be called primary particles. Percentage exposed in metallic catalysts. The accessibility of the atoms of metal in metallic catalysts, supported or unsupported, depends upon the percentage of the total atoms of metal which are surface atoms. It is recommended that the term percentuge exposed be employed for this quantity rather than the term dispersion which has been frequently employed. Pretreatment und activation. Following the preparation of a catalyst or following its insertion into a catalytic reactor, a catalyst is often subjected to various treatments before the start of a catalytic run. The term pretreatment may, in general, be applied to this set of treatments. In some cases the word uctiuution is used. It implies that the material is converted into a catalyst or

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into a very much more effective one by the pretreatment. Outgassing is a form of pretreatment in which a catalyst is heated in uacuo to remove adsorbed or dissolved gas. Culcination is a term which means heating in air or oxygen and is most likely to be applied to a step in the preparation of a catalyst. 1.4 Catalytic reactors The vessel in which a catalytic reaction is carried out is called a reactor. Many different arrangements can be adopted for introducing the reactants and removing the products. In a batch reactor the reactants and the catalyst are placed in the reactor which is then closed to transport of matter and the reaction is allowed to proceed for a given time whereupon the mixture of unreacted material together with the products is withdrawn. Provision for mixing may be required. In a,flow reactor, the reactants pass through the reactor while the catalysis is in progress. Many variations are possible. The catalyst may be held in a packed bed and the reactants passed over the catalyst. A packed bed flow reactor is commonly called aJixed bed reactor and the term plug-Jow is also used to indicate that no attempt is made to back-mix the reaction mixture as it passes through the catalyst bed. The main modes of operation of a flow reactor are differential involving a small amount of reaction so that the composition of the mixture is approximately constant throughout the catalyst bed, or integral involving a more substantial amount of reaction such that the composition of material in contact with the final section of the catalyst bed is different from that entering the bed. In a pulse reactor, a carrier gas, which may be inert or possibly one of the reactants, flows over the catalyst and small amounts of the other reactant or reactants are injected into the carrier gas at intervals. A pulse reactor is useful for exploratory work but kinetic results apply to a transient rather than to the steady state conditions of the catalyst. Several alternative modes of operation may be used to avoid the complications of the changing concentrations along the catalyst bed associated with integral flow reactors and each of these has a special name. In a stirred flow reactor, effective mixing is achieved within the reactor often by placing the catalyst in a rapidly-rotating basket. If the mixing achieved in this way is efficient, the composition of the mixture in the reactor will be close to that of the exit gases. The same result can be reached by recirculation of the gas around a loop containing a fixed bed of catalyst, provided that the rate of recirculation is considerably larger then the rate of flow in and out of the loop. Under these circumstances, a substantial conversion to products can be obtained even though conditions in the bed correspond more closely to those associated with a differential rather than with an integral reactor. Another mode of operation involves ajuidised bed in which the flow of gases

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is sufficient to cause the bed of finely divided particles of catalyst to behave like a fluid. In a fluidised bed, the temperature is uniform throughout, although mixing of gas and solid is usually incomplete. It has special applications in cases where the catalyst has to be regenerated, e.g., by oxidation, after a short period of use. Continuous transfer of catalyst between two vessels (one used as reactor and the other for catalyst regeneration) is possible with a fluidised system. The stirred flow and the recirculation reactors are characterised ideally by very small concentration and temperature gradients within the catalyst region. The term, gradientless reuctor, may be used to include both types. All reactors, batch or flow, may be operated in three main ways in regard to temperature. These are isothermal, adiubatic and temperature-programmed. For the last, in a batch reactor the variation of temperature with time may be programmed, or in a fixed bed reactor the variation of temperature along the length of the bed may be controlled. When reactors are operated isothermally the batch reactor is characterised by adsorbate concentrations and other aspects of the state of the surface which are constant in space (i.e., uniform within the catalyst mass) but which change with time. In the integral flow reactor with the catalyst at steady state activity, the surface conditions are constant with time but change along the bed. In the gradientless reactor at steady state, the surface conditions are constant in space and, if the catalyst is at a steady state, with time. In the pulse reactor, the catalyst is often not in a condition of steady state, concentrations change as the pulse moves through the bed, and there may be chromatographic separation of reactants and products. In general, if heterogeneous catalytic reactions are to be conducted isothermally, the reactor design must provide for heat flow to or from the particles of catalyst so as to keep the thermal gradients small. Otherwise, temperatures within the catalyst bed will be non-uniform. The differential reactor and the various forms of the gradientless reactors are advantageous in this regard. The types of reactors described above can, in principle, be extended to reactions in the liquid phase although the pulse reactor has been little used in such cases. Reactions in which one reactant is gaseous, the other is in a liquid phase, and the catalyst is dispersed in the liquid phase, constitute a special but not unusual case, for example, the hydrogenation of a liquid alkene catalysed by platinum. A batch reactor is most commonly employed for laboratory scale studies of such reactions. Mass transport from the gaseous to the liquid phase may reduce the rate of such a catalytic reaction unless the contact between the gas and the liquid is excellent (see $1.6).

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1.5 Kinetics of heterogeneous catalytic reactions

1.5.1 General terms Consider a chemical reaction

where vB is the stoichiometric coefficient (plus for products, minus for reactants) of any product or reactant B. The extent ofreaction 5 is defined (see $1 1.1 of the Manual) d4 = v g l dn,, where ng is the amount of the substance B. If rate ofreaction is to have an unambiguous meaning, it should be defined as the rate of increase of the extent of reaction

4

=

dc/dt

=

v R 1 dnB/dt,

whereas the quantity dn,ldt may be called the rate of formation (or consumption) o j B To facilitate the comparison of the results of different investig?tors, the rates of heterogeneous catalytic reactions should be suitably expressed and the conditions under which they have been measured should be specified in sufficient detail. If the rate of the uncatalysed reaction is negligible, the rate of the catalysed reaction may be given as 1 r = -dQdt.

Q

If Q, the quantity ofcatalyst, is in mass, 1 r = rm = -dt/dt m and rm is the specijic rate of reaction which may be called the specific activity ofthe catalyst under the specified conditions. If Q is in volume, 1 r = rv = -d5/dt. V

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The volume should be that of the catalyst granules excluding the intergranular space. If Q is in area, r

=

1

r, = -d[/dt, A

where r, is the urealf rate ofreaction. If the total surface area of the catalyst is used, it should be preferably a BET nitrogen area. However, other types of specified areas may be employed, lor ex iple, the exposed metal area of a supported metallic catalyst. The exposed metal area is often estimated by selective chemisorption of a suitable sorptive, e.g., hydrogen or carbon monoxide. The turnoverjrequency, N , (commonly called the turnover number) defined, as in enzyme catalysis, as molecules reacting per active site in unit time, can be a useful concept if employed with care. In view of the problems in measuring the number of active sites discussed in $1.2.4, it is important to specify exactly the means used to express Q in terms ofactive sites. A realistic measure of such sites may be the number of surface metal atoms on a supported catalyst but in other cases estimation on the basis of a BET surface area may be the only readily available method. Of course, turnover numbers (like rates) must be reported at specified conditions of temperature, initial concentration or initial partial pressures, and extent of reaction. In comparing various catalysts for a given reaction or in comparing various reactions on a given catalyst, it may be inconvenient or impracticable to compare rates at a specified temperature since rates must be measured at temperatures at which they have convenient values. Therefore, it may be expedient to compare the temperatures at which the rates have a specified value. In reactors in which the concentrations of reactants and products are uniform in space, the rate is the same on all parts of the catalyst surface at any specified time. In integral flow reactors, however, the rate on each element of the catalyst bed varies along the bed. 1.5.2 Selectivity The term selectivity S is used to describe the relative rates of two or more competing reactions on a catalyst. Such competition includes cases of different reactants undergoing simultaneous reactions or of a single reactant taking part in two or more reactions. For the latter case, S may be defined in two ways. The first of these defines a ,fractional selectivity S , for each The term weal meaning per unit area is tentative (see preface)

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373

product by the equation

The second defines relutive selectivities, SR, for each pair of products by:

In shape selectivity, which may be observed in catalysts with very small pores, the selectivity is largely determined by the bulk or size of one or more reactants. On zeolites, for example, the rate of reaction of alkanes with linear carbon chains may be much greater than that of those with branched chains. 1S.3 Rate equufions Gaseous systems in which all concentrations are uniform in space and in which the reaction is irreversible will be considered first. The rate 4, besides being proportional to the quantity of catalyst, Q, is also in general a function of temperature T and the concentrations ci or partial pressures pi of reactants, products and other substances if present:

The statement of this equation is commonly called the rate equatioii or the rate law. Frequently, in heterogeneous catalysis, the function J is of the form

where k is the rate constant which is a function of temperature but not of concentrations and ui (integral or fractional ;positive, negative or zero) is the order ofthe reuction with respect to component i. This form of the rate law is called a power rate luw. Often, however, a rate expression of different form is used. For example, for a reaction A + B + products, the rate equation might be

This equation can be interpreted in terms of Langmuir adsorption isotherms. It is assumed (see $1.5.4) that both reactants must be adsorbed in order to react and that K , and KB are the respective Langmuir adsorption equilibrium constants. The denominator allows for competition for sites between

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reactants and other substances (diluents, poisons and products) present in the system at concentrations c, with related adsorption equilibrium constants K,. A rate law ofthis type is appropriately called a Lungmuir rate law although it was made popular by Hinshelwood, Schwab, Hougen, Watson and others. Such rate laws are frequently used for systems in which the adsorptions may not obey the Langmuir adsorption isotherm. Under these circumstances, the rate laws can still provide a useful means of correlating experimental results but the values of the derived constants must be interpreted with caution. For a single elementary process, k

=

A exp( - E/RT),

where A is the frequency facfor and E the activation energy. Even though heterogeneous catalytic reactions rarely if ever proceed by a single elementary process, the same relation often applies to the overall rate constant. In such a case, however, A is not a frequency factor but should be called the prerxponentialfuctor and E should be called the apparent activation energy. Sometimes A and E exhibit compensation, i.e., they change in the same direction with change in catalyst for a given reaction or with change in reaction for a given catalyst. A special case of compensation called the 6-rule occurs when, at least approximately, In A

=

const

+ --,E

RT,

where T , is the isokinetic temperature, the temperature at which all k's would be identical. These considerations can be extended to reversible processes. They also apply to single phase, liquid systems. For the case, rather common in heterogeneous catalysts, in which one reactant is in a gas phase and the others and the products are in a liquid phase, application of the principles given above is straightforward provided that there is mass transfer equilibrium between gas phase and liquid phase, i.e., the fugacity of the reactant in the gas phase is identical with its fugacity in the liquid phase. In such case, a power rate law for an irreversible reaction of the form

may apply where the quantities have the same significance as before except that the gaseous reactant g is omitted from the ci's and entered as a pressure term with order a,.

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The determination of rate of reaction in a flow system requires knowledge both of the feed rate, u, of a given reactant and of thefraction converted, x. The definition of feed rate as the amount of reactant fed per unit time to the inlet of the reactor is consistent with $1.5.1. The rate of reaction is then given by d5 - x - 2’-, dt “B ~

where vB is the stoichiometric coefficient of the reactant of which the fraction x is converted. Alternatively, one may proceed from r,, ri, and r, rather than d(/dt by defining the space velocities, v,, u,, and v, where the ui(s represent the rate of feed of the given reactant fed per unit mass, volume or surface area of the catalyst. The relation,

r,

=

X

v, -, VB

gives the specific rate of’ reaction or, under specified conditions, the specific activity of the catalyst. Substitution of v, or u, gives the areal rate of reaction or the rate divided by volume of the catalyst, respectively. Alternatively, space times, T,, z, and T ” , the reciprocals of the space velocities, may be used. “Contact time” and “residence time” are terms which may be misleading for flow systems in heterogeneous catalysis and should be avoided. 1.5.4 Kinetic aspects of mechanism Of general convenience in the treatment of mechanisms are the notions of rate determining process or step and most abundant surface intermediate. The rate determining process is defined, as is usual in kinetics in general, as that single elementary process in the catalytic sequence which is not in equilibrium when the overall reaction is significantly displaced from equilibrium. If the surface of a catalyst has one set of catalytic sites, a particular intermediate is said to be the most abundant surface intermediate if the fractional coverage by that intermediate is much larger than coverages by the other intermediates. Of course, there is no guarantee that either a rate determining process or a most abundant surface intermediate will exist for any particular reaction under a particular set of conditions. The term reaction centre may be used to include both vacant and occupied catalytic sites. The sum of the surface concentrations of reaction centres on the surface of a catalyst is a constant L. Thus, if species m at a surface concentration L, is the most abundant surface intermediate, L, L, N L, where L, is the surface concentration of vacant reaction centres.

+

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COMMISSION 1.6 OF IUPAC

Lungmuir-Hinshrlwoocll mechunism. This represents a somewhat anomalous use of the term mechanism to specify relative magnitudes of rate constants. In a Langmuir- Hinshelwood mechanism, all adsorption-desorption steps are essentially at equilibrium and a surface step is rate determining. Such a surface step may involve the unimolecular reaction of a single adsorbate molecule or the reaction of two or more molecules on adjacent sites with each other. Where the adsorption processes follow Langmuir adsorption isotherms, the overall reaction will follow some kind of a Langmuir rate law (tj1.5.3). However, the term Langmuir --Hinshelwood mechanism may covcr situations in which Langmuir adsorption isotherms do not apply. 1.5.5 Non-unifbrmity uf cutalytic sites A characteristic of a catalytic surface is that its sites may differ in their thermodynamic and kinetic properties. In the kinetic description of catalytic reactions on non-uniform surfaces, a parameter ci is frequently used to connect changes in the activation energy of activated adsorption with the enthalpy of the adsorption

where E:d5 is the energy of activation and - q" is the enthalpy of adsorption on the uncovered surface. Eadsand q apply to the surface with the same value of 0. In practice the equation may apply only over a restricted range of 8. Sometimes 01 is defined as in the equation above but in terms of Gibbs energies o l activation and adsorption, respectively. The name trunsfer cotficient has been used by electrochemists to represent ci in another related situation. 1.6 Transport phenomena in heterogeneous catalysis This section will not attempt to cover the more technical aspects of chemical reactor engineering. A unique feature of heterogeneous catalytic reactions is the ease with which chemical kinetic laws are disguised by various transport phenomena connected with the existcnce of concentration and/or temperature gradients in the hydrodynamic boundary layer surrounding the catalyst particles ( e x t c m d yruclirnts) or in the porous texture of the catalyst particles themselves (internal yrudients). Additional difficulties arise in batch reactors and in stirred Row reactors if agitation is inadequate to maintain uniform concentrations in the fluid phase. Agitation is particularly critical where one of the reactants is a gas and the catalyst and other reactants and products are in condensed phases, for example, in the hydrogenation of a liquid alkene. Here the agitation must be adequate to maintain the fugacity of the dissolved gaseous reactant equal to that i n the gaseous phase.

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When external gradients correspond to substantial differences in concentration or temperature between the bulk of the fluid and the external surface of the catalyst particle, the rate of reaction at the surface is significantly different from that which would prevail if the concentration or temperature at the surface were equal to that in the bulk of the fluid. The catalytic reaction is then said to be influenced by external mass or heat transfer, respectively, and, when this influence is the dominant one, the rate corresponds to a regime of external mass or heat transfer. Similarly, when internal gradients correspond to differences in concentration or temperature between the external surface of the catalyst particle and its centre, the rate in the particle is substantially different from that which would prevail if the concentration or temperature were the same throughout the particle. The catalytic reaction is then said to be influenced by internal mass or heat transfer, and, when this influence is the dominant one, the rate corresponds to a regime of internal mass or heat transfer. Terms such as diffusion limited or diffusion controlled are undesirable because a rate may be larger in regimes of heat or mass transfer than in the kinetic regime of operation, i.e., when gradients are negligible.

1.7 Loss ofcutalytic activity 1.7.1 Poisoning and inhibition Traces of impurities in the fluid to which the catalyst is exposed can adsorb at the active sites and reduce or eliminate catalytic activity. This is called poisoning and the effective impurity is called a poison. If adsorption of poison is strong and not readily reversed, the poisoning is called permanent. If the adsorption of the poison is weaker and reversible, removal of the poison from the fluid phase results in restoration of the original catalytic activity. Such poisoning is called temporary. If adsorption of the poison is still weaker and not greatly preferred to adsorption of reactant, the reduction in rate occasioned by the poison may be called competitive inhibition or inhibition. Here, of course, the poison may be present in much larger than trace amounts. There are, of course, no sharp boundaries in the sequence permanent poisoning, temporary poisoning, competitive inhibition. In selective poisoning or selective inhibition, a poison retards the rate of one catalysed reaction more than that of another or it may retard only one of the reactions. For example, there are poisons which retard the hydrogenation of olefins much more than the hydrogenation of acetylenes or dienes. Also, traces of sulphur compounds appear selectively to inhibit hydrogenolysis of hydrocarbons during catalytic reforming. A product of a reaction may cause poisoning or inhibition. The phenomenon is called selflpoisoniny or autopoisoning.

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1.7.2 Deactivation: general The conversion in a catalytic reaction performed under constant conditions of reaction often decreases with time ufrun or time on stream. This phenomenon is called catulyst deactizution or carulyst decay. If it is possible to determine the kinetic form of the reaction and, thus, to measure the rate constant for the catalytic reaction k, it is sometimes possible to express the rate of deactivation by an empirical equation such as

-dk/dt

=

Bk",

where t is the time on stream, n is some positive constant, and B remains constant during a run but depends upon the temperature and other conditions of the reaction. Alternatively, the decline in k may be assumed to result from elimination of active sites and L may be substituted for k in the preceding equation where L is considered to be the effective concentration of surface centres. It is then common practice to define a time of deactivution (or decay time) as the time on stream during which k falls to a specified fraction of its original value, often 0.5. Times of deactivation may vary from minutes as in catalytic cracking to years as in hydrodesulphurisation. Catalytic deactivation can sometimes be reversed and the original catalytic activity restored by some special operation called regeneration. For example, coked cracking catalyst is regenerated by burning off the coke (see $41.7.3, 1.9). If the catalytic reaction is a network of various processes, deactivation can lead to a change in the distribution of products. In such cases, the deactivation not only reduces the overall rate but it changes the selectivity. 1.7.3 Types of deactivation Catalyst deactivation can result from deactivation of catalytic sites by poisoning either by impurities or by products of the catalytic reaction (41.7.1). Many reactions involving hydrocarbons and particularly those run at higher temperatures lead to the deposition on the catalyst of high molecular weight compounds of carbon and hydrogen which deactivate the catalyst. This phenomenon is called coking or fouling. Catalysts so deactivated can often be regenerated. Catalyst deactivation may also result from changes in the structure or in the texture of the catalyst. Changes of this kind are usually irreversible and the catalyst cannot be regenerated. This type of deactivation is often called catalyst ageing. Sinrering and recrystallisution. Catalysts often suffer during use from a gradual increase in the average size of the crystallites or growth of the primary particles. This is usually called sinteriny. The occurrence of sintering leads to

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a decrease in surface area and, therefore, to a decrease in the number of catalytic sites. In some cases, sintering leads to a change in the catalytic properties of the sites, for example, for catalysts consisting of highly dispersed metals on supports, catalytic properties may change on sintering due to a change in the relative exposure of different crystal planes of the metallic component of the catalyst or for other reasons. Thus sintering leads to a decrease in rate and perhaps also to a change in selectivity. Similar phenomena can occur in oxide catalysts as used in catalytic oxidation. The crystal size increases, or the initial structure of the crystals changes. For example, a binary solid compound may decompose into its components or an amorphous mass may crystallise. These processes may be called recrystallisation. In some cases the terms sintering and recrystallisation may refer to the same process. The removal of surface defects may accompany these processes. In some cases, as for example in catalytic cracking on silica-alumina, processes similar to those involved in sintering and recrystallisation can lead to a change in the texture ofthe catalyst. Surface areas are diminished and the pore-size distribution is changed. 1.8 Mechanism of catalytic reactions 1.8.1 General

A chemical reaction proceeds by a set of elementary processes (the Manual, $11.3) which are in series and perhaps also in parallel. These processes start and terminate at species of minimum free energy (reactants, intermediates and products) and each elementary process passes through a state of maximum free energy (the transition state). To specify the mechanism, one must specify the elementary processes. This specifies the intermediates. One must also give the nature (energetics, structure, charge distribution) of the transition state. So much is true for chemistry in general. The special features of mechanism in heterogeneous catalysis are those which involve reactions between sorptives and active sites, reactions among adsorbates, and processes which regenerate active sites to give a type of chain reaction. In general, only partial approaches to the specification of mechanism as given above have been possible. Mechanism is sometimes used in different senses. For example, consider the two situations. A+B+C

C-D

vs

A f B - C C S D

It may be said that the two situations have different mechanisms or that they are two variants of the same mechanism.

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1 3.2 Elementary processes in heterogeneous catulysis

There are many more types of elementary processes in heterogeneous catalysis than in gas phase reactions. In heterogeneous catalysis the elementary processes are broadly classified as either adsorption-desorption or surface reaction, i.e., elementary processes which involve reaction ofadsorbed species. Free surface sites and molecules from the fluid phase may or may not participate in surface reaction steps. There is no generally accepted classification of elementary processes in heterogeneous catalysis. However, names for a few types of elementary processes are generally accepted and terminology for a partial classification [see M. Boudart, Kinetics of Chemical Processes, Chap. 2 (1968)] has received some currency. The particular reactions used below to exemplify this terminology are ones which have been proposed in the literature but some have not been securely established as occurring in nature at any important rate. Au’sorprion-desorptioil. This includes the process of physical adsorption as well as non-dissociative chemisorption.

+ NH,(g) $ H 3 N * * + H(g) $ H*

*

Dissociutive cidsorption and its reverse, associutive desorpt ion. 2* + CHJg) 5 CH3* + H*

The methane might be supposed to react either from the gas phase or from a physisorbed state. Dissociatiw st4rfuc.e react ion and its reverse, associative surfuce reaction. 2*

+ C2H5* S H* + *CHzCH,*

This involves “dissociative adsorption” in an adsorbate. Sorptiue insertion. This is analogous to the process of ligand insertion in coordination chemistry. H*

+ C,H,(g)

+

*CzH5

This reaction might also be imagined to proceed by adsorption of CzH4 followed by ligand migration (an associative surface reaction). Reactiue cidsorption and its reverse, reactive desorption. This resembles dissociative adsorption but one fragment adds to an adsorbate rather than to a surfwe site. HZC--CH,

** ~

+ D-D(&

/

ZSH2C I

*

CH2D

I> I

*

TERMINOLOGY IN HETEROGENEOUS CATALYSIS

38 1

In abstraction and extraction processes, an adsorptive or adsorbate species extracts an adsorbed atom or a lattice atom, respectively. Abstraction process

*H

Extraction process

+ H(g)

+

+ CO(g)

0,’

+ H,(g)

*

--t

2e

+ CO,(g)

The following elementary process occurring either on one site or, as shown, on two sites is called a Rideal or a Rideal-Eley mechanism:

* *

+ *

*

D2 may also be considered to be in some kind of a weakly adsorbed state. It will be noted that one D atom is never bonded to the surface in any minimum Gibbs energy intermediate. It is recommended that the term Rideal or Rideal-Eley mechanism be reserved for this particular elementary process. However, the term has been used for analogous processes in which there is a reactant molecule and a product molecule of nearly the same energy in the fluid phase or in some weakly adsorbed state and in which one or more atoms are never bonded to the surface. An example is the following elementary process H,C-CH=CH,(g)

*

4

H,C=CH-CH,D(g)

D

H

*

*

*

which has been called a switch process. The term might well be used generically for similar processes. The term Rideal or Rideal-Eley mechanism has been further extended to include all elementary processes in which a molecule reacts from the fluid phase or from some weakly adsorbed state. Even the sorptive insertion process and the abstraction process illustrated above fall within this extended definition.

1.8.3 Nomenclature of surface intermediates Surface intermediates should be named in ways compatible insofar as possible with chemical nomenclature in general. Adsorbed species may be treated as surface compounds analogous to molecular compounds. For example, *H may be called surface hydride, *=C==O may be called a linear surface carbonyl and 0

/I *

A

*

may be called a bridged surface carbonyl. H2N* may be called a surface amide and H3C*,a surface methyl or a surface a-alkyl. The species *H may also be called an adsorbed hydrogen atom and *CO, adsorbed carbon monoxide.

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1.6 OF

IUPAC

Organic adsorbates pose a particular problem because quite particular structures of some complexity are regularly discussed. A nomenclature is recommended in which the surface is treated as a substituent which replaces one or more hydrogen atoms. The degree of substitution is indicated by monoadsorbed, diadsorbed, etc. This terminology does not specify the nature of the chemical bonding to the surface nor does it restrict, a priori, the valency of the surface site *. Thus, both of the following species

are named 1,3-di-adsorbed propane. Other examples are : *CH, *CH2CH,CH,

monoadsorhed methane 1-monoadsorbed propane

CH,

I

CH,- C CH3

2-monoadsorbed 2-methylpropane

1 *OCH,CH, *CH,CH,OH CH,-$H-CH,

-$H-CH,-CH3

H G

H C

C

eclipsed 1,2-diadsorbcd ethane

*'

*/ *

0-monoadsorbed ethanol 2-monoadsorbed ethanol 2,4-diadsorbcd hexane

= CH

CH3 or (*)2CH-CH3

*

= NH or (*),NH *COCH,

1,l-diadsorbed ethane

diadsorbedammonla 1-monoadsorbed acetaldehydc

Species adsorbed as 7c-complexes are described as 7c-adsorbed: H,C=CH,

i

n-adsorbed ethylene

H H HZC,CrCH2

*1 1*

4 ~\ or H,C

' ' * d

CH,

n-adsorbed ally1

TERMINOLOGY IN HETEROGENEOUS CATALYSIS

383

The substitution system of nomenclature should be viewed as showing only how atoms are connected and not as indicating the precise electronic structure. Thus .n-adsorbed ethylene is one representation of 1,2-diadsorbed ethane. Nomenclature based upon the process of formation of a particular adsorbate is to be discouraged. Thus, H* may be “dissociatively adsorbed hydrogen” but the same species is formed in dissociative adsorption of CH,, NH3, HzO. 1.9 Nomenclature of catalytic reactions In general, a catalytic reaction may be named by adding the adjective “catalytic” to the standard chemical term for the reaction, for example, catalytic hydrogenation (or, if clarity demands, heterogeneous catalytic hydrogenation), catalytic hydrodesulphurisation, catalytic oxidative dehydrogenation, catalytic stereospecific polymerisation. In general, special terminology for reactions is to be discouraged. However, certain catalytic processes of technological interest have special names in common use. Where such processes involve the simultaneous occurrence of two or more different chemical reactions, special names for the processes are probably inevitable. Some important examples of such processes of technological interest are: Catalytic cracking. In this process, a higher boiling cut of petroleum, for example, gas oil, is converted substantially into a lower boiling material of high octane number. Among the processes which appear to be involved are skeletal isomerisation of alkanes followed by their cleavage into alkane and olefin, and hydrogen transfer reactions which reduce the amount of olefin formed and which lead to coke and aromatic hydrocarbons. Catalytic hydrocracking. This is similar to catalytic cracking in its industrial purpose but it is effected under hydrogen pressure and on a catalyst containing an ingredient with a hydrogenating function. Catalytic reforming. Catalytic reforming is a process for increasing the octane number of naphthas. It involves isomerisation of alkanes, dehydrogenation of cyclohexanes to aromatic hydrocarbons, isomerisation and dehydrogenation of alkylcyclopentanes, and dehydrocyclisation of alkanes. The following reactions may be mentioned because they are rare except as heterogeneous catalytic reactions and have somewhat specialised meanings in catalysis. Catalytic methanation. This is a process for removing carbon monoxide from gas streams or for producing methane by the reaction CO

+ 3H2 + CH4 + H 2 0

384

COMMISSION 1.6 OF IUPAC

Catalytic dehydrocyclisution. This is a reaction in which an alkane is converted into an aromatic hydrocarbon and hydrogen, for example, heptane

+

toluene

+ 4H2

Catalytic hydrogenolysis. This is ordinarily used for reactions in which + Hz gives ESCH HC=, for example,

+

=C-C=

+ H, -+ + H2 butane + H2

propane

toluene

ethane

+ methane + methane

+

benzene

-+

2 ethane

However, it may also be used for cleavage of bonds other than r C - G , for example, benzyl acetate bemylamine

+ H 2 toluene + acetic acid + H, + toluene + NH3 -+

Catalytic hydrodesulphurisation. This is a process in which, in the presence of hydrogen, sulphur is removed as hydrogen sulphide. SECTION

2.

LIST OF SYMBOLS AND ABBREVIATIONS

Constant in Langmuir's adsorption K isotherms Constant in Langmuir's 2.2 Adsorption adsorption Area of surface A , A,, s isotherms for Specific surface area a, a,, s t3 Surface coverage substance i Ki Constants in Area per molecule in Freundlich complete monolayer of isotherms a, n Constants in Temkin substance i a,(i) Surface site * isotherms A, B Constant in BET Ion M"' (or atom M) C isotherms of adsorbent or Monolayer capacity n, catalyst at the Constants of surface M," Roginskii(or MJ Zeldovich Constant in Henry's law K equation a, b 2.1 Catalysis and catalysts

+

385

TERMINOLOGY IN HETEROGENEOUS CATALYSIS

2.3 Composition structure and texture of catalysts 2.4 Catalytic reactors

2.5 Kinetics of heterogeneous catalytic reactions Stoichiometric coefficient of substance B Extent of reaction Rate of catalysed react ion Quantity of catalyst Specific rate of reaction Specific activity of the catalyst Rate of reaction per unit volume of catalyst Areal rate of reaction Turnover frequency (turnover number) Selectivity Rate constant Order of the reaction Frequency factor Activation energy Isokinetic temperature (Kelvin scale) Fraction converted Feed rate Space velocities Space times Sum of surface concentrations of reaction centres

Surface concentration of surface intermediate m L, Surface concentration of vacant reaction L" centres Energy of activation for activated Eads adsorption Energy of activation for activated adsorption on uncovered surface EaOds (Differential) enthalpy of -4 adsorption (Differential) enthalpy of adsorption on uncovered surface - qo Transfer coefficient c1

2.6 Transport phenomena in heterogeneous catalysis 2.7 Loss of catalytic activity Constants in equation for rate of deactivation Time of run (on stream) 2.8 Mechanism 2.9 Nomenclature of catalytic reactions

B, n t

386

COMMISSION

SECTION

Symbol

Term absorbate absorbent absorption absorptive abstraction process accessible pore volume acid site activated adsorption

3.

1.6 OF IUPAC

ALPHABETICAL INDEX

area occupied by molecule in complete monolayer A , A,, S area of interface 1.8.2 A , A,, S area of surface areal areal rate r, 1.3.2 of reaction 1.2.4 associative desorption 1.2.2,1.2.9, associative surface 1S.5 activation reaction 1.3.2 E, Eads activation autopoisoning energy basic site 1.5.3,1.5.5 active centre 1.2.6 batch reactor *, (ads) active site 1.2.2,1.2.6 BET adiabatic adsorption 1.4 reactor isotherm adsorbate 1.2.1 bifunctional adsorbed state 1.2.I catalysis adsorbent Brqinsted acid 1.2.1 adsorption 1.2.1 site adsorption Brunauercomplex 1.2.1 EmmettadsorptionTeller desorption adsorption process 1.8.2 isotherm adsorption calcination isotherm 1.2.7 capillary con*, (ads) adsorption site 1.2.2 densation adsorption carrier space 1.2.1 catalysis adsorptive 1.2.1 catalyst ageing catalyst ageing 1.7.3 E apparent catalyst activation deactivation 1.5.3 energy (decay1 Section 1.2.1 1.2.1 1.2.1 1.2.1

am

1.2.2 1.2.1 1.2.1 preface 1.5.1 1.2.3,1.8.2

1.8.2 1.7.1 1.2.4 1.4

1.2.7 1.2.8 1.2.4

1.2.7 1.3.2 1.2.2 1.3.1 1.1 1.1 1.7.3 1.7.2

387

TERMINOLOGY IN HETEROGENEOUS CATALYSIS

catalytic cracking catalytic dehydrocyclisation catalytic hydrocracking catalytic hydrodesulphurisation catalytic hydrogenation catalytic hydrogenolysis catalytic methanation catalytic oxidative dehydrogenation catalytic reaction catalytic reactors catalytic reforming catalytic stereospecific POlYmerisation charge transfer adsorption chemical adsorption chemisorption coherent structure coking

1.9 1.9 0, Oi

compensation competitive inhibition composition of catalyst contact time coverage

1.9

1.9

1.9

1.9

1.9

1.9 1.1 1.4 1.9

1.9 1.2.3,1.2.4 1.2.2 1.2.2, 1.2.3, 1.2.4 1.2.3 1.7.3

deactivation decay time demanding reaction desorption diadsorbed differential flow reactor diffusion 1im ited (controlled) dispersion dissociative adsorption (chemisorption) dissociative surface reaction doping eclipsed diadsorbed electron accepting site electron donating site elementary process elementary step Elovich equation

1.5.3 1.7.1

1.3 1.5.3 1.2.2,1.2.4, 1.2.7 1.7.2 1.7.2 1.2.5 1.2.1,1.2.3 1.8.3 1.4

1.6 1.3.2

1.2.3,1.8.2

1.8.2 1.3.1 1.8.3 1.2.4 1.2.4 1.1,1.8.2 1.1

1.2.9

COMMISSION 1.6 OF IUPAC

388

5

U

X

A

K

enzyme catalysis extent of reaction external gradient external surface extraction process facile reaction feed rate fixed bed reactor flow reactor fluidised bed reactor fouling fraction of reactant converted frequency factor Freundlich adsorption isotherm gradientless reactor Henry’s law constant heterogeneous catalysis heterogeneous catalytic hydrogenation heterolytic dissociative adsorption homogeneous catalysis

1.1 1.5.1 1.6 1.3.2 1.8.2 1.2.5 1.5.3 1.4 1.4 1.4 1.7.3

1.5.3 1.5.3

Te 1.2.7 1.4 1.2.7 1.1

1.9

K, Ki 1.2.3 1.1

homo1ytic dissociative adsorption ideal adsorbed state immobile adsorption incoherent structure induced nonuniformity inhibition initiator integral flow reactor interface intermediate pores internal gradient internal surface intrinsic nonuniformity isokinetic temperature isothermal reactor kinetic aspects of mechanism kinetics of heterogeneous catalytic reactions kinetic regime Langmuir adsorption equilibrium constant

1.2.3 1.2.7 1.2.3 1.2.3 1.2.5 1.7.1 1.1

1.4 1.2.1 1.3.2

1.6 1.3.2 1.2.5 1.5.3 1.4

1S.4

1.5 1.6

1.2.7,1.5.3

389

TERMINOLOGY IN HETEROGENEOUS CATALYSIS

K

Langmuir adsorption isotherm 1.2.7 Langmuir rate law 1.5.3 LangmuirHinshelwood mechanism 1.5.4 Lewis acid site 1.2.4 linear adsorption isotherm 1.2.7 linear adsorption isotherm equilibrium constant 1.2.7 localised adsorption 1.2.3 loss of catalytic activity 1.7 macropores 1.3.2 mean life time of adsorption complex 1.2.9 mean residence time 1.2.9 mechanism of catalytic reactions 1.8 mesopores 1.3.2 micropore filling 1.2.2 micropore volume 1.2.2 micropores 1.3.2 mobile adsorption 1.2.3

molecular sieve effect monoadsorbed monolayer adsorption nmS,n,, monolayer V, capacity most abundant surface intermediate multilayer adsorption negative catalysis net nomenclature of catalytic reactions nomenclature of surface intermediates nondissociative chemisorption non-localised adsorption non-uniform site Qi order of reaction outgassing oxidative dissociative adsorption packed bed reactor percentage exposed

1.3.2 1.8.3 1.2.2 1.2.2,1.2.7 1.5.4 1.2.2 1.1 1.2.3 1.9

1.8.3

1.2.3 1.2.3 1.2.5,1.5.5 1.5.3 1.3.2 1.2.3

1.4 1.3.2

390

VP

Q k

COMMISSION

permanent poisoning photoadsorption photodesorpt ion physical adsorption ph ysisorption x-adsorbed plug-flow reactor poison, poisoning pol yfunctional catalysis pore volume pores pore-size distribution porosity power rate law preexponential factor pretreatment primary particles promoter proton acid site pulse reactor quantity of catalyst rate constant ratedetermining process (step) rate equation

1.6 OF IUPAC

1.7.1 1.2.3 1.2.3 1.2.2 1.2.2 1.8.3

dn,/dt

t

I .4

r" 1.7.1 1.2.8 1.3.2 1.3.2 1.3.2 1.3.2 1.5.3

1.5.3 1.3.2 1.3.2 1.3.1 1.2.4 1.4 1.5.1 1.5.3

1.5.4 1 S.3

rate law rate of adsorption and desorption rate of formation (consumption) of B rate of reaction rate of reaction per unit volume of catalyst reaction centre reactive adsorption reactive desorption reactor recirculation reactor recrystallisation reductive dissociative adsorption regeneration regime of external mass (or heat) transfer regime of internal mass (or heat) transfer

1.5.3

1.2.9

1.5.1 1.5.1

1.5.1 1.5.4 1.8.2 1.8.2 I .4 1.4 1.7.3 1.2.3 1.7.2

1.6

1.6

391

TERMINOLOGY IN HETEROGENEOUS CATALYSIS

residence time Rideal (or RidealEleY) mechanism RoginskiiZeldovich equation selective inhibition selective poisoning S,, S selectivity (as fraction) S, S selectivity (as ratio) self-poisoning shape selectivity sintering *,(ads) site for chemisorption sorbate sorption sorptive sorptive insertion ,z, z,, z, space time per unit mass, area, volume of catalyst v,, v,, v, space velocities per unit mass, area, volume of catalyst rln specific activity of catalyst UP specific pore volume

I .2.9,1.5.3

r,

a, s, a, 1.8.2

1.2.9 1.7.1 1.7.1 1.5.2 1.5.2 1.7.1 1.5.2 1.7.3

1.2.4 1.2.1 1.2.1 1.2.1

L

1.8.2

6, Bi 1.5.3

1.5.3 1.5.1 1.3.2

specific rate of catalysed reaction specific surface area sticking coefficient sticking probability stirred flow reactor structure of adsorbent (catalyst) structureinsensitive reaction structuresensitive reaction substrate support surface surface concentration of reaction centres surface coordinative unsaturation surface coverage surface layer of the adsorbent surface step switch process Temkin adsorption isotherm

1.5.1 1.2.1 1.2.9 1.2.9 1.4

1.3,1.3.2 1.2.5

1.2.5 1.2.1 1.2.1,1.3.1 1.2.1

1S.4 1.2.4 1.2.2,1.2.4, 1.2.7 1.2.1 1.5.4 1.8.2 1.2.7

392

a

COMMISSION 1.6 OF IUPAC

temperature programmed reactor temporary poisoning texture of adsorbent (catalyst) 8-rule time of deactivation time of run (on stream) transfer coefficient transitional pores

1.4

1.7.1

N 1.3,1.3.2 1.5.3 1.7.2 1.7.2

1.5.5 1.3.2

transport processes in heterogeneous catalysis turnover frequency (turnover number) unactivated adsorption uniform sites van der Waals adsorption

1.6

1.5.1

1.2.9 1.2.5

1.2.2

Author Index Numbers in parentheses are reference numbers and indicate that an author's work is referred to although his name is not cited in the text. Numbers in italics show the page on which the complete reference is listed.

A

Appleby, A. J., 250(7), 315 Arnold, D., 124(39), 187(168), 211, 213, 240, 244,246 Arroyo, A. C., 287(220), 320 A n , H. E., 250(10), 306(10), 315 Asano, R., 341(37), 348 Ashmore, P. G., 259(68), 275(68), 280(68), 287(68), 31 7 Avery, N. R., 100(118), 118, 260(93a), 227 (180), 278(180), 283(93a), 286(93a), 317, 319

Aaron, H. B., 72(10d), 115 Abragam, A,, 124(27), 240 Abras, A., 150(80), 241 Addink, C., 56(43), 57(43), 68 Afanaser, A. M . , 192(185), 244 Aharoni, A., 145(64), 147(74), 241 Aizumi, Y., 92(92b), 117 Akitt, J. W., 124(8, 9, lo), 239 Akselrod, S., 180(137), 243 Aldag, A. W., 52(36e), 67, 95(103a), 118, 196 ( I96), 245 B Aleksandrov, A. U., 222(246), 246 Aleshin, V. G . , 104(121d), 118 Babernics,L., 259(84),278(184,185),286(184), Ali, A. H . , 99(114), I18 293(184), 317,319 Allison, E. G., 87, 100(61), 116 Babeshkin, A. M., 187(170), 244 Bacaud, R., 229(255), 246 Al-Noori, M. K., 251(26), 293(26), 316 Alsdorf, E., 92(90), 117 Baddour, R. F., 258(62a), 260(101), 264(101), Amariglio, A,, 259(89), 317 279(191), 291(191), 317,318,320 Amariglio, H., 259(89), 317 Baverstam, U., 163(90,91), 242 Amenomiya, Y., 260(108), 318 Bagg, J., 253(37), 264(37), 268(141), 290(233), 291(233, 234), 292(233, 234), 293(233), Amulyaviehus, A. P., 181(150), 243 Anderson, J. H . , 75, 116 316,319,320 Anderson, J. R., 87(65b),98(113),99,100,117, Baird, T., 260(107), 318 118, 253(38), 258(38), 259(92), 260(92, Baker, B. G., 253(38), 258(38), 259(92), 260(92), 277(92), 282(38), 287(38, 92), 93a). 275(145, 146). 277(92, 157,159, 168, 173, 175, 180), 278(168, 173, 175, 180), 289(38), 312,316,317 281(145), 282, 283(93a, 145, 157, 159), Baker, R. T. K., 261(116), 318 284(159, 168, 173, 198), 286(93a, 145, Balooch, M., 42,67 157, 159, 168, 198), 287(38, 92, 145, 157, Baltrunas, D. A,, 222(246), 246 159, 168, 227a), 289(38, 146, 157), 312, Bancroft, G. M., 124(32), 126(32), 128(32), 316,317,319 134(32), 140(32, 54), 141(54), 157(32), 159(32), 160(32), 161(32), 167(32), 168 Anderson, P. W., 104(128), 110, 118 (32), 190(184), 240, 241, 244 Anderson, R. B., 286(218), 287(218), 320 Ando, K. J., 123(1), 154(1), 181(143),239,243 Banks, B. E., 249(4), 252(4), 270(4), 315 Anthony, G. D., 305(298), 306(298), 322 Baron, K., 8(5), 35(5), 53(36c), 57(36c), 66, Apel'baum, L. O., 291(238), 321 67,258(61), 259(61), 317 3s13

394

AUTHOR INDEX

Barrett, P. H., 197(197), 245 Bartek, J. P., 259(85), 317 Bartholomew, C. H., 73(17), 75,115, 165(99), 195, 197(195), 226,242, 245 Bartoletti, I., 325(1 I), 327(1 I ) , 328(1 I), 348 Basset, J. M., 259(83), 317 Bauer, D. J., 261(1 IS), 318 Beachell, H. C., 75(37), 116 Bean, C. P., 143(58), 147(72), 241 Beekman, F. W., 259(90), 317 Beelen, J. M., 95(102), 117 Belleman, A., 74(20), 83(20), 115 Benczer-Koller, y.,157(86), 164(86), 166(86), 24 I Benedict, W. S., 277(153), 319 Benson, J. E., 52(36e), 67,95 (103a), 118 Berinek, V., 268(138), 269(138), 318 Berglund, S., 75(28), 83, 116 Berkowitz, A. E., 208, 209, 245 Bernard, G., 82(48), 87(48), 116 Bernasek, S. L., 27(23), 39, 41(31), 42, 53(31), 67 Bertrand, P. A., 72(12), 83(12), 11.5 Best, R. J., 91, 92(87), 117 Bethe, H. A , , 79(46), 116 Biegen, J. R., 259(81), 317 Bischof, H., 251(25), 297(25), 316 Blakely, D. W . , 8(5), 25(22a), 35(5), 44(35), 49(35), 51(22a. 36b), 53(35, 36a), 66, 67, 258(61), 259(61), 260(93b), 317,318 Blume, M., I82(159, 160), 244 Bommel, H., 183(162), 184(162), 185(162), 186(162), 220(162), 244 Boersma, M. A. M., 75(36), 83, 87(36), 101, I16 Bohm, C., 163(90,91), 242 Bommel, E. A,, 91(86), 94, 117 Bonchev, Z. W., 194,245 Bond, G. C., 87(64d), 91(64d), 100(61), 116, 249, 251(3), 253, 258(65), 263(3), 264, 265(3), 279(186), 284(3), 291(3), 292(3), 293(3), 297, 312(3). 315,317,319 Bonhoeffer, K. T., 90, 117 Bowel, H. P., lI(lOc), 67, 72(10d), 115, 261 ( I 17), 318 Boolchand, P., 166(101a), 242 Boon, M. R., 270(143), 319 Boreskov, G. K., 88(71), 117, 200(204), 245, 298, 300(261), 301(272a), 304(261, 262, 289), 32/, 322 Boronin, V. S., 56(42), 68

Both, E., 167(1I3), 242 Boudart, M., 52, 63, 67, 68, 73(17) 75(29), 86, 87(65a, 66), 95(103a), 105(133b), 115, 116, 117, 118, 119, 124(36, 43), 146, 162, 164(94, 95, 97), 174(95), 175(95), 176, 177, 178, 185(97), 186(165), 187(97, 169), 188(94,176,177,178), 189(178), 190(178), 191(178), 195, 196(196), 197(97, 195), 198(94, 176, 177, 178), 199(178), 200(97, 176, 206), 201(165. 207), 203(165, 206, 215), 204(215, 216), 205(165, 206, 216, 217), 206(165, 216), 207(165), 208(165, 216, 217, 222), 216(237), 226(95), 240, 242, 244, 245, 246. 258(64), 286(212), 287(212), 289(212), 317, 320 Bouwman, R., 71(6), 72(11, 15), 73(15), 75 (6, 15, 32), 77(42, 43), 78(11), 83(52). 84 (52), 85, 86,115, 116, 261(113), 294(113, 248), 197(113), 318, 321 Bowles, B. J., 197, 245 Bradley, T. L., 41(33), 67 Brandenberger, S. J., 75(33), 116 Breakspere, R. J., 89, 117 Breslow, D. S., 330(17), 332, 348 Bridger, G. W., 259(91), 317 Brill, R., 208(220), 245 Brindley, C.W., 305(295,297), 306(295, 297), 322 Brown, C. K., 323(5), 348 Brown, M. E., 292(241), 293(244), 321 Brown, W. F., Jr., 145(63), 24i Brownlie, I. C., 260(100), 318 Brumbach, S . B., 26(22b), 27(22b), 67 Brunauer, S. J., 174(123, 124, 125),243 Brundege, J. A., 256(48), 316 Brydon, J. E., 305(299), 306(299), 322 Buchanan, D. N. E., I76( 133), 243 Buhl, H., 302(278), 322 Bukshpan, S., 180( I37), 243 Bunbury, D. St. P., 151(82), 241 Burger, G., 324(8), 348 Burton, J. J., 251(29), 261(29), 316 Burton, J. W., 137(50), 201(50), 240 Bussiere, P., 229(255), 246

C Cadenhead, D. A., 71(5), 97, 115, 118, 296 (253), 321 Campbell, A. D., 186(166), 244 Campbell, J. S., 75(30), 92,116, 117,296(252), 321

395

AUTHOR INDEX

Candia, R., 226(251 b), 246 Cant, N. W., 284(208, 209a), 296(209a) 290(208,209a), 297(208), 320 Carr, P. F., 96(104), 118, 277(183), 278(183), 296(183, 251), 319, 321 Carra, S., 264(129), 318 Carter, J. L., 87(63), 93(63, 95a), 94(63), 95 (63), 100(63), 105(63), 116, 117,277(152), 286(213), 287(152), 289(213), 319, 320 Cassar, L., 325(10), 326(10), 334(23), 335, 345(44), 346(44), 348, 349 Ctrny, S., 94(99a), 117 Chalk, A., 345(40), 349 Chdnning, D. A,, 214, 215, 246 Chappert, J., 166(101b), 242 Charpurey, M. K., 92(93b), 117 Chen, S. C., 259(81), 317 Chesters, M. A,, 29(27), 37(29), 58(29), 67 Chiusoli, G. P., 324(6), 325(10a), 334(22,23), 335,348 Chornet, E., 255(45), 259(45), 316 Christmann, K., 42, 67, 86, 107, 116 Ciapetta, F. G., 87(64e), 116, 286(217), 320 Cimino, A,, 63,68 Cinneide, A. O . ,97, 118, 284(202), 320 Clark, D. R., 333(20), 348 Clark, M. G., l90( 184). 244 Clark, N. J., 277(157, 159), 283(157, 159), 284(159), 286(157, 159), 287(157, 159) 289(159), 319 Clarke, J. K. A.,87(64g),96(104),97,116,118, 275(147), 277(147, 170, 183), 278(170, 183), 284(170, 202, 209b), 289(147), 291(237), 296(147, 183, 251), 297(147, 237), 319, 320, 321 Clausen, B. S., 226(25l b), 246 Clausen, C. A., 166(102b), 242 Cochran, D. R. F., 167(106), 242 Coenen, J. W. E., 262(119), 264(119), 318 Cohen, R. L., 157(88), 159(88), 160(88), 161(88), 164(88), 165, 166(88), 242 Cohen, S. G., 124(21), 239 Collins, D. W., 182(158), 244 Collman, J. P., 333(20), 348 Conn, P. J., 75(33), 116 Conrad, H., 58(47), 68 Constabaris, G., 181(143), 183(162), 184(162), 185( 162), 186(162), 220( 162), 226(253), 243,244,246 Constable, F. H., 249,315 Coogan, H. M., 123(1), 154(1), 239

Cooke, M. P., Jr., 333(19), 348 Corciovei, A,, 203(213), 205(218), 245 Corcoran, W. H., 260(103), 269(103), 290 (103), 318 Corey, E. J., 325(9), 330(9), 348 Cormack, D., 259(79), 31 7 Costacke, G., 205(218), 245 Coughlin, R. W., 255(45), 259(45), 316 Couper, A,, 90, 100(78), 103(78), 117 Cranshaw, T. E., 174(130), 197,243,245 Cranstoun, G. K. L., 261(115), 318 Cremer, E., 249, 255(44), 256(2, 44), 315, 316 Criado, J. M., 256(47), 267(137), 291(47, 137), 292(47, 137), 301(47, 137), 302(47, 137), 316,318 Crosbie, K . D., 124(10), 239 Crucq, A., 263(125), 267(125), 280(125), 282 (125), 318 Cunningham, R. E., 260(98), 318 Cusamano, J. A., 56(43), 68, 93(95a), 95 (103b), 117, 118, 286(213), 289(213), 320 Cvetanovic, R. J . , 260(108), 318 Cyrot-Lackmann, F., 109(141), 119 Czanderna, A. W., 259(81), 317

D Dabiri, A. E., 41(33), 67 Daglish, A. G . , 296(256), 321 Dalla Betta, R. A., 95(103b), 118 Damjanovic, V., 249(4), 252(4), 270(4), 315 Danno, S., 341(37), 345(41), 348,349 Danon, J., 138(52), 139,240 Deans, H. A,, 55(37), 68 Defay, R., 74, 83(20), 84, 115, 116 Degols, L., 263(125), 267(125), 280(125), 282(125), 318 Dehn, J. T., 182(158), 244 Deibert, M. C., 279(191), 291(191), 320 De Jong, W. A,, 260(109), 318 Delbouille, A,, 63(51c), 68, 164(97), 185(97), 187(97), 197(97), 200(97), 242 Delgass, W. N., 124(36,43), 164(94), 187(169), 188(94, 176, 177), 198(94, 176, 177), 200(176), 240, 242, 244 Dembinski, G . W., 56(43), 57(43), 68 de Neufville, J. P., 166(101a), 242 Denisor, D. A., 250(6b), 315 Dere6, J., 300, 321 Derouane, E. G., 63 ( ~ I c )68 , Desjonquires, M. C., 109(141), 119

396

AUTHOR INDEX

Dessing, R. P., 75(31), 81(31), 87(31), 88(31), 89(31), 116 DeVoe, J. R., 124(15, 16, 17, 18), 239 Dezsi, U., 124(19). 239 Dickerson, S. M., 215(234), 246 Dickson, B. L., 188 (179, 180, 181), 191(179. 180, 181), 244 Dieck, H. A,, 336(28, 29), 338(28), 340(29), 341(29), 345(42,43), 346(43), 348, 349 Dobbie, R. C., 124(10), 239 Dobson, C. M.. 215,246 DolejzEk, Z., 94(99a), 117 Dolgopolov, V. A,, 169(117). 242 Dollimore, D., 287(225), 320 Dominguez, J., 256(47), 291(47), 292(47), 301(47), 302(47), 316 Donaldson, G. R., 57(45), 68 Dorgelo, G. J. H., 71(4a, 4b, 4d), 73(4a, 4b, 4d), 74(4a, 4b, a), 75(4a, 4b), 76, I15 Dorn, R., 63(52a), 68 Dougharty, N. A,, 52(36e), 67, 68, 95(103a), I18 Dowden, D. A., 99(116b), 103(116a, 116b), 108, 118, 119, 259(91), 317 Dowie, R. S., 264(127), 286(127), 287(127), 318 Doyen, G., 109,119 Dubini, M., 334(22), 348 Dumesic, J. A., 146(215), 162(215), 164(95, 97), 166(102a), 174(95), 175(95), 185(97), 186(165), 187(97), 197(97), 200(97, 206), 201(165), 203(165, 215), 204(215, 216), 205(165, 206, 216), 206(215), 207, 208 (217, 222), 216(37), 223, 226(95), 242, 244, 245, 246 Duncan, J. F., 172, 173,243 Durigon, D. D., 260(99), 318 DuS, R., 257(58), 290(58), 317 Dutartre, R., 229(255), 246 Duteil, M., 340(30, 31). 348 Dzhuntini, B., 269(142), 319

E Eastman, D. E., 23(20), 67 Ehrenreich, H., 70(2), 104(2, 126, 127, 130a, 130b), 105, 115, 118 Ehrlich. G., 11 l(145). 119 Ehrman, J. R., 167(104), 242 Ekdahl, T., 163(90, 91), 242

Eley, D. D., 88, 89(72), 90(70, 80), loO(78). 103(78, 80), 104(80), 117, 250(8, 17), 251 (27), 256( 17), 274( 144). 277( 164,165), 278 (164, 165), 293(144), 294(27), 296(256), 297(27. 258), 315, 316, 319,321 Elford, L., 75(25), 99(25), 115 Elliott, J. A., 151(82), 241 Ellis, W. P., 11(10b), 67 Emmett, P. H., 75(30), 92(93a,93b), 116, 117, 174(123, 124, 125), 243, 296(252), 321 Erickson, D. J., 182(157), 244 Erickson, N. E., 211(226), 245 Erkelens, J., 259(76), 317 Errington, W., 124(8,9, lo), 239 Ertl, G., 42(34c), 58(47), 67, 68, 72(10a), 86, 107, 109,115, 116,119 Escoubes, M., 277(161), 319 Everett, D. H., 74(20), 83(20), 115, 254, 316 Exner, O., 250(6a, 23), 251(6a), 268(6a, 23, 138, 139), 269(23, 138, 139), 279, 315, 316,318,319 Eyraud, C., 277 (161), 319 Eyzerikhin, E. I., 97(108), 118

F Fabrichnyi, P. B., 187(170), 244 Fahrenfort, J., 279(189), 291(189), 320 Fain, S. C., 73(16), 86, 115 Fairclough, R. A., 250(5), 252(5), 315 Falconer, J. L., 291(240b), 293(240b), 321 Falicov, L. M., 60, 61, 62,68 Farach, H. A., 144(59), 241 Farkas, A., 90, 117 Farnsworth, H. E., 92(92a), 117, 279(187), 280,320 Farnsworth, M. E., 72, 115 Farrell, H. H., 16(17), 67 Faulkner, Y.S . , 104(130c), 118 Feates, F. S., 250(13), 252(13), 253(13), 316 Fedak, D. G., 11(9a), 67 Felix, R. A,, 341(36), 348 Ferrante, J., 72(1Oc), 115 Ferraris, M., 334(22), 348 Fichte, P. M., 306(300), 322 Figueras, F., 229(255), 246 Filbey, A. H., 330(18), 348 Firsova, A. A., 218(239), 219(239), 221, 222(246), 226,246 Fischer, E. O., 324(8), 348

AUTHOR INDEX

Flanagan, T. B., 260(97), 306(300), 318, 322 Flank, W. H., 75(37), 116 Flinn, P. A., 202(2 lo), 245 Flynn, P. C . , 264(128), 318 FOB,M., 325(10), 326(10), 348 Forester, D. W., 208(223), 209(223), 245 Forster, D., 336(25a), 348 Franco, S . , 167(109, IIO), 242 Frank, W., 336(28a), 338(28a), 348 Franken, P.E. C . , 77, 116 Frauenfelder, H., 124(25), 167(106), 240, 242 Freel, J . , 2S0(22), 251(22), 260(22), 26S(22), 277(22), 282, 283(22), 316 Frennet, A., 259(75), 263(125j, 267(125), 280(125), 282, 289(229), 317, 318, 320 Friedel, J., 104(129), 118 Friedt, J. H., 167(112), 242 Fripiat, J. J . , 305(296), 306(296), 322 Fryer, J. R., l60( 100). 318 Fujiwara, Y., 341(37), 345(41), 348. 349

G Gager, H. M., 124(47), 181(145), 185(164), 200, 210, 21 I , 240, 243, 244, 245 Gaidai, N . A , , 260(96), 261(96), 318 Gallard-Nechtschein, J., 188(178), 189(178), l90( l78), l91( 178), l98( 178), 199(178), 244 Galwey, A. K., 2S0(22), 251(22), 253(31), 254(31), 259(69, 70, 76), 260(22, 104). 261(104), 265(22), 277(22, 163, 164), 281(162), 282(22,162), 283(162), 287(222, 225), 292(24, 104), 293(244), 304(291), 305(291,292), 306(31,291,292), 316,317, 318,319,320,321,322 Gambhir, B. S . , 258(63), 317 Gardner, N. C . , 55, 68 Garn, P. D., 253(32), 264, 305(298), 306(298), 316,322 Garrou, P. S . , 327(14), 328 Garten, R. L., 124(43), 164(94), 188(94, 176, 177, 178), 189, 190, 191(178), 198(94, 176, 177, 178), 199(178), 200(176), 226, 227, 228, 240, 242, 244, 246 Garzanor, 1. Ya., 217(238), 218(238), 246 Gaspard, J. P., 109, 119 Gastuche, M. C., 305(296), 306(296), 322 Gates, B. C., 226, 252 Gault, F. G., 283(196), 284(199), 320

397

Gelatt, C . D., 104(130b), 105, 118 Gen, M. Ya., 180(136), 243 Gentry, S . J . , 302(275), 321 Gerasimor, Ya. I., 74(23b), 115 Gerberich, H. R., 284(206), 290(206, 208), 296(208), 297(208), 320 Gibb,T. C., 124(8,9,10,30), 126(30), 128(30), 134(30), 136(30), 140(30), 148(30), 149 (30), 152(30), 155(30), IS6(30), 157(30), 159(30), 160(30), 161(30), 162(30), 166 (30), 167(30), 168(30), 202(212), 239,240, 245 Gibbens, H. R., 284(207), 286(207), 287(207), 290(207), 294(207), 296(207), 297(207), 320 Gilliland, E. R., 258 (62a), 317 Giner, J., 279(192), 291(192), 320 Girvin Harkins, C., 95(103a), 118 Gjostein, N. A,, 11(9a), 67 Gland, J. L., 28(24), 34(24), S3(36c), S7(36c), 67 Godivin, R. P., 137(50), 201(50), 240 Gol’danskii, V. I., 124(29, 38,40,48), I26(29), 128(29), 130(49), 133(49), 134(29), 136 (29), 138(49), 140(29), 148(49), 149(49, 76), 1S0(49), 151(78), 169(11S, 116, 120), 170(1IS), 171(1 IS), 172(120), 180(134, 136), 188(173), 192(I S ) , 220(242, 243), 223(248), 240, 241, 242, 243, 244, 246 Goldsmith, R. L., 260(101), 264(101), 318 Goldstein, J. R., 303(284), 322 Golodets, G. I., 250(18), 315 Comer, R., 89(76), 117 Goncharuk, V. V., 250(18), 316 Gonser, U., 124(33), 166(103), 194(194), 202, 240,242,245 Gonzalez, F., 256(47), 291(47), 292(47), 301(47), 302(47), 3-16 Good, M. L., 166(302b), 242 Good, W., 250(1S), 251(30), 268, 269, 270 (140), 316, 3 / 9 Goodman, F. O . , 264(132), 318 Gorbatchev, V. M., 250(9), 315 Gorodinskii, G. M., 149(76), 241 Goszner, K . , 251(25), 297(25), 316 Graham, M. J., 214,215(233,234), 246 Grand, C . , 340(31), 348 Gray, T. J., 87(64b), 116 Graydon, W. F., 259(83), 317 Greatrex, R., 124(11, 12, 13), 239

398

AUTHOR INDEX

Greco, A,, 264( I29), 318 Greenwood, N, N., 124(8, 9, 10, 11, 30, 35, 41), 126(30), 128(30), 134(30), 136(30), 140(30), 148(30), 149(30), 152(30), 155 (30), 156(30), 157(30), 159(30), 160(30), 161(30), 162(30), 166(30), 167(30), 168 (30), 172, 202(212), 239, 240, 243, 245 Grimley, T. B., 109(143a), 119 Grintzos, Ch., 200(203), 245 Gruverman. 1. J., 124(14), 239 Gryaznov, V. M., 99(115), 118 Guczi, L., 259(87), 263(87, I23), 281(87, 123), 286(215), 289(215), 317, 318, 320 Gudkov, B. S., 259(87), 263(87), 281(87), 317 Guerrieri, F., 334(22), 348 Guggenheim, E. A,, 83(53), 116 Guindy, N. M . . 305(294), 306(294), 322 Gwathmey, A. T., 260(98), 318

H Haensel, V., 57, 68 Hall, H. E., 151(82), 241 Hall, W. K., 92(93a, 94). 117 Halsey, M. J., 220, 246 Hanna, S. S . , 166(101a), 242 Hansen, M., 74(22), 88(68), 115, 117 Hansen, R. S., 55, 64, 68, 208(221), 245 Hardy, W. A., 86, 105, 106, 116 Hargrove, R. S., 176(132), 243 Harkins, C. G . , 52(36e), 67 Harper, R. J., 277(171), 278(178), 284(171, ZOO), 286(171, 200), 287(171), 319, 320 Harris, P . S . , 250( 13), 252( 13), 253( 13), 261(116), 270(143), 316,317,319 Harson, M. S . , 263(124), 281(124), 318 Hartog, F.. 208(219), 245 Harvey, B. G., 147(75), 148(75), 241 Hazony, Y.. 157(87). 159(87), 160(87), 161 (87). 164(87). 165, 166(87), 167(87), 168 (87), 241 Heberle, J., 167(108. 109, IlO), 242 Heck, R. F., 324(7), 325(11), 327(11, 14), 328(1 I), 330(15, 16, 17), 331,332,334(21, 24), 336(27, 28, 29), 338(27, 28), 340(29, 32), 341(29, 33, 34, 35, 39), 344, 345(43), 346(43), 348,349 Hedman, J., l04(123), 118 Hegedus, L. L., 52,67 Hegedus, L. S., 325(9), 330(9), 348

Henzler, M., 12(12), 67 Herber, R. H.,124(29, 34). 126(29), 128(29), 134(29), 136(29), 140(29), 157(86, 87), 159(87), 160(87), 161(87), 164(86, 87), 165, 166(86, 87). 167(87), 168(87), 240, 24 I Herman, Z., 94(99a), I 1 7 Herz. R., 51(36b), 67 Hichs, J. M . , 167(105), 242 Hinshelwood, C. N., 250(5), 252(5), 315 Hirschwald, W., 258(62c), 317 Hoare, F. E., 105(134), 119 Hobert, H., 124(39), 187(168), 211, 213,240, 244,246 Hobson, M. C., Jr., 124(44, 45, 46, 47), 181(144, 145). 185(164), 186(166), 200, 210, 21 1(227), 240, 243, 244, 245 Hodges, C. H., 105(135b), 119 Hoekstra, P., 56(43), 57(43), 68 Hod, E. M., 180( 138). 243 Hoffman, D. W., 75, 116, 198(199, 200), 245 Holbrook, C. M., 55,68 Holscher, A. A,. 72(11), 78(1 I), 115 Honex, C., 97, I18 Horn, K., 63(52a), 68 Hosemann, R., 174,243 Houston, J. E., 13, 67 Howe, A. T., 172,243 Howe, R. F., 291(240a), 292(240a), 321 Hrynkiewicz, A. Z., 124(24), 138(51), I81 (146), 240, 243 Hucl, M., 124(22), 240 Hudgins, R. R., 264(131), 318 Hiifner, S., 104(121c, 122), 118 Hurkin, A. A,, 97(108), 118 Hurwitz, H., 57(44), 68 Hyman, E., 251(29), 261(29), 316

I Ibach, H., 63(52a), 68 Ignatiev, A,, 1 l(9a). 29(28), 67 Ikoma, H., 187,244 Imai, H., 287(221), 320 Imelik, B., 259(72), 280(72), 317 Inanu, S. H., 97(112), 118 Indovina, V., 63(51c), 68 Ingalls, R., 190(183), 244 Ingham, D. B.. 251(30), 268(30, 140). 269(30, 140), 270(140), 316,319

AUTHOR INDEX

Inglis, H. S., 279( 194), 320 Inoue, Y., 55(39), 68 Isozumi, Y., 163(93), 242

399

Kehl, W. L., 202(210), 245 Keith-Hall, W., 284(206, 208), 286(209a), 290(206, 208, 209a), 296(208), 297(208), 320 Keller, A,, 293(243), 321 J Kellerman, E., 215(235), 246 Jacobs, 1. S., 147(72), 208(223), 209(223), Kemball, C., 254, 257, 259(68, 69, 71, 76), 260(110), 261(112), 264(127), 265(71), 241,245 275(39, 68, 1 4 9 , 277(112, 155, 156, 158, Jaeger, H., 250(19), 260(19), 290(19, 233), 167, 168, 169, 170, 171, 172, 173, 174, 291(19, 233, 234), 292(19, 233, 234), 176, 178), 278(167, 168, 169, 170, 171, 293(19, 233), 316,320 172, 173, 174, 176, 178), 280(39, 68), Jamieson, D. M.,260(104),261(104),292(104, 281(112,145,156), 283(39,112, 145,156), 241), 300(268), 318, 321 284(168, 169, 170, 171, 172, 173, 174, Jerman, Z., 264(134), 318 197, 198, 199,200,204,205), 286(39, 112, Jiru, P., 88(69), 117 127, 145, 156, 168, 169, 171, 174, 198, Johansson, A , , 193,244 200, 205, 214), 287(39, 68, 112, 127, 145, John, G . S., 256(49), 316 156, 168, 169, 171, 174, 176, 197, 220, Johnson, C . E., 174(130), 243 221, 222, 223, 224), 289(112, 156, 228), Johnson, H. B., 305(293), 306(293), 322 290(172, 174, 204, 205), 302(276, 277), Joke, B. J., 96(106), 118 303(283,285, 287), 304(39), 316,31 7,318 Jones, A. V., 11(9a), 29(28), 67 319,320,321,322 Jones, H., 104(124), 118 Kemeny, G.,270(143), 319 Jongepier, R., 71(4c, 4d), 73(4c, 4d), 74(4c, Kempling, J. C., 286(218), 287(218), 320 4d), 76(4c), 77(4c), 87(107), 94(107), Kerttsz, L., 259(84), 293(247), 317, 321 115,118 Kesmodel, L. L., 60,3(1), 8(6,7), 16(1), 18(1), Jordanov, A,, 194(191, 192), 245 21(1), 60,66, 67, 68 Jbvtr, B., 302(274b), 321 Kessler, F., 305(293), 306(293), 322 Joyner, R. W., 5(3), 12(13), 13(13), 15(15), Keure, W., 181(151), 194(194), 243,245 26(3), 55(41), 66,67, 68 Khaffar, M., 99(115), 118 Julia, M., 340(30, 31), 348 Khait, Yu. L., 256(52), 316 Khammouma, S., 164(97), 185(97), 187(97), 197(97),200(97,206), 203(206), 205(206), K 242,245 Kadar, I., 163(93), 242 Khazhzhar, E., 99(115), 118 Kahn, D. R., 5(4), 26(4), 51(4), 66 Khomenko, A. A., 291(238), 321 Kalvius, M., 154(85), 164(98), 166(98), 186 Khovanskaga, N. N., 221(245), 226(245), 246 Khrapov, V. V., 149(76), 241 (98), 241, 242 Khulbe, C. P., 91(84a, 84b), 117 Kaminska, T. J., 8(7), 67 Kieran, P., 287(224), 320 Kankeleit, E., 198(201), 245 Karas, W., 145(65), 241 Kikuchi, E., 277(151), 283(151), 286(151), Karasev, A . N., 212, 246 287(151), 289(151), 319 Karyagin, S. V., 149(76, 77), 151(78), 202 Kilty, P. A., 101(119), 118 (209), 203(209), 241, 245 Kim, Y., 336(28a), 338(28a), 348 Kasatkina, L. A , , 298(262), 304(262), 321 Kinza, H., 101(120), 118 Kassel, L. S., 264(135), 318 Kiperman, S. L., 260(96), 261(96), 318 Katzer, J. R., 58, 68 Kirkpatrick, S., 70(2), 104(2, 127), 115, 118 KdtZU, 87(65~),117 Kittel, C., 145(60), 241 Kautz, M., 124(20), 239 Klaasson, M., 104(123), 118 Kawai, T., 258(62d), 317 Klein, M. P., 143(56), 144(56), 182(56), 241 Keblys, K. A,, 330(18), 348 Klier, K., 88(69), 117, 304(288), 322

400

AUTHOR INDEX

Klissurski, D. G., 300(265, 268, 270), 321 Kljushnikor, 0. I., 104(123), 118 Knauer, R.C., 150(81), 241 Kneller, E., 147(73), 241 Knozinger, H.,259, 263(67), 302,317,322 Knor, Z., 94(99a), 117,256(55), 262(55), 317 Kochloefl, K., 259(67), 263(67), 302(278), 31 7,

322 Kodama, H., 305(299), 306(299), 322 Koenig, K . E.,341(36),345 Korosy, F., 293(243), 321 Koezuka, J., 187(167), 244 Kohll, C. F., 327(13a), 328(13a), 348 Kokes, R.J., 259(85), 317 Kolbanovskii, Yu. A,, 212(230). 246 Kolchin, I. K., 199(202),245 Kondo, S., 74(21), 115 Kondow, T.. 258(62d), 31 7 Konvalinka, J. A,, 259(90), 317 Kordynk, S. L., 169(119), 243 Korecki, J., 145(65), 241 Korneev, V. P., 192(186, 187), 244 Korytko, L. A., 149(76), 217(238), 218(238),

220(242), 241,246

L Lahut, J. A,, 208(223), 209(223), 245 Laidler, K. J., 253,263(121),277(177), 278(33,

177), 280,316,318,319 Lake, 1. J. S., 302(276,277). 321,322 Lane, B.C., 302(281),322 Lane, R.M., 302(281), 322 Lang, B., 5(3), 12(13), 13(13), 15(15), 26(3),

55(41), 66,67,68 Lang, G., 167(1 I I), 242 Lang, N.D., 104(126, 127). 118 Latshaw, G. L., 152(84), 241 Latta, E. E., 58(47), 68 Lauer, J., 181,243 Laurouskii, K. P.. 219(240). 224(249), 225

(249),246 Lawson, A., 250(20), 260(20), 316 Leach, H.F.,303(285), 322 Lee, D. I., 163(93), 242 Lefelhocz, J. F., 181(145), 243 Leidheiser, H., Jr., 215(235), 246 Lemm, K., 174( 127), 243 Levin, K., 104(130a), 118 Levinson, L. M., 208(223), 209(223), 245 Levy, R.M., 260(111), 261(118), 318 Lewis, G. N.,73(18), 115 Lewis, R.. 89(76), 117 Lienard, G., 263(125), 267(125), 280(125),

Kozak, E., 25(22a), 51(22a), 67 Krafczyk, B., 259(78), 317 Kraft, M., 56(43), 57(43), 68 Krawowski, R.A,, 194(190), 244 Kreber, E., 202,245 Krizhanskii, L. M.,149(76), 241 282(125), 289(229), 318,320 Krop, K.,145(65), 241 Liengme, B.V., 188( 175), 244 Krupay, B. W.. 301,321 Likhtenshtein, G. I., 253(36), 316 Krupiansky, Yu.F., 192(186), 220(244), 244, Liljequist, D., 163(90, 91), 242 246 Lindgren, R. G., 260(103), 269( 103), 290(103), Krylov, 0. V., 223(248), 246 318 Ku, R.,11(1Oc), 67 Lindquist, R. H., 181(143), 183(162), Kubaschewski, O., 75(25), 99(25), 115 184(162), 185(162), 186( 162). 220(162), Kubokawa, Y., 293(242), 321 226,243,244,246 Kiindig, W., 145(66), 176(132), 181(143), Linford, R. G., 73,115 183, 184. 185(162), 186(162), 220(162), Linnett, J. W., 86,105, 106,116 226(253), 241,243,244,246 Lippits, G. J. M., 72(15), 73(15), 75(15), Kiippers, J., 72(IOa), 86,ll.5 85(15), 86(15), 115,261(113), 294(113), Kullman. D., 166(100), 242 297(113), 318 Kummer, J. T., 390(266),321 Lisichenko, V. I., 169(119), 243 Kunimori, K., 258(62d), 317 Livingston, J. D., 143(58), 241 Logan, S. R., 260(110), 289(228), 318, 320 Kuntz, E., 340(31), 348 Kuyers, F. J., 75(31), 86(31), 87(31), 88. 89, Logvinenko, V. A., 250(9), 315 Lu, K. E., 42,67 I16 Kuznetsov, V . S., 256(53), 264(53), 317 Lungu, M., 250(1 l), 315 Kwan, T.,254,316 Lupis, C. H.P., 82(48), 87(48), 116

AUTHOR INDEX

Luss, D., 64(54), 68 Luth, H., 63(52a), 68 Lyubarski, G. D., 97, 118

M Maak, F., 74(23a), 115 Maatman, R. W., 56(43), 57,68,264( 133), 318 McAllister, J . , 208(221), 245 McAllister, 1. W., 260(106), 318 McCabe, R. W., 260(102), 318 Mccdffrey, E. F., 300(265, 270), 302(274a), 321 McCosh, R., 303(283), 322 McDavid, J . M., 73(16), 86, 115 McDonald, R. J., 98(113), 99(113), 118, 275(146), 289(146), 319 McGinn, M. J., 293(244), 321 McKee, D. W., 87(64c), 91(82), 116, 117 250(21), 259(21), 275(148), 283(21, 195), 286(21, 148, 210, 211,219), 287(148, 210, 211), 289(21, 195), 294(219), 296(249), 297(257), 316, 319. 320, 321 MacKenzie, K. J . D., 172(122), 173(122), 243 McLean, D., 83, 116 MacMahon, D. M., 250(8), 251(27), 294(27), 297(27), 315,316 McMahon, E., 296(251), 321 McNab, T. K., 183(161), 197(197), 244, 245 Madden, H. H., 18(19b), 67 Madden, W. F., 277(178), 278(178), 319 Madix, R. J., 291(240b), 293(240b), 321 Magennis, S. A., 345(40), 349 Mahaffy, P., 56(43), 57(43), 68 Maire, G., 283(196), 320 Maksimov, Yu. V., 124(48), 166(102a), 219, 223(102a), 224,225,240, 242,246 Mal’donado, K., 99(115), 118 Maletta, H., 167(112), 242 Mann, R. S., 87(64d), 91(64d, 84a, 84b). 116, 117, 279(186, 188), 293(188, 245), 319,320,321 Manogue, W. H . , 58,68, 87(65c), 117 Maradudin, A., 62(52b), 68 Margolis, L. Ya., 199(202), 218(239), 219 (239), 221(245), 222(246), 223(248), 226 (245), 245, 246 Margulies, S., 167(104), 242 Markarov, E. F., 130(49), 133(49), 138(49), 148(49), 149(49, 76), 150(49), 169(115,

401

117, 118), 170(115, 118), 171(115, 118), 180(136), 192(185), 201(208), 202(208), 203(208), 217(238), 218(238), 220(243), 240, 241, 242, 243, 244, 245, 246 Mars, P., 290(232), 291(232), 293(232), 301 (273), 302(273), 320, 321 Marshall, S. W., 180(135), 182(157), 243,244 Marshreva, V. I., 304(289), 322 Martin, G. A., 229(255), 246,259(72), 280(72), 31 7 Martin, M. R., 11(9b), 67 Marzke, R. F., 182(157), 244 Masse, N. G., 87(64b), 97, 116, 118,296(253), 321 Matloob, M. H., 250(12), 315 Mavrakis, N., 200(203), 245 May, J. W., 258(60), 260(60), 317 May, L., 124(31), 240 Medema, D., 327(13a), 328(13a), 348 Meisel, W., 180, 194(193), 243, 245 Melera, A,, 104(121c), 118 Melpolder, J. B., 341(39), 344,348 Menon, P. G., 301(273), 302(273), 321 Menzel, D., 259(73), 317 Merrill, R. P., 55(37), 68 Merta, R., 94(99d), 11 7 Merzoni, S., 325(10a), 326(10a), 334(22), 348 Metcalfe, A,, 90, 117, 256(56), 259(88), 291(240a), 292(240a), 296(56), 312, 317, 321 Meye, W., 259(67), 263(67), 317 Meyer, A. W., 250( 16a), 316 Meyer, E. F., 284(201), 320 Meyering, J. L., 82(49), 84(49), 116 Micka, T. A,, 302(274a), 321 Miedema, A. R., 105(135a), 119 Mikhail, R. S., 305(294), 306(294), 322 Miller, R. B., 194(190), 244 Minkova, A,, 194(191, 192), 245 Miyata, H., 293(242), 321 Miyatanu, D., 92(92b, 92c), 117 Miyoshi, I., 72(7c), 75(7c), 89(7c), 90(7c), 115 Mizoroki, T., 336(26), 338(26), 348 Modell, M., 260(101), 264(101), 318 Mossbauer, R. L., 124(42), 240 Momma, N., 291(236), 321 Mondelli, G., 334(22), 348 Montgomery, P. D., 335(25), 348 Moody, S. S., 291(239), 292(239), 301, 302 (239), 321

402

AUTHOR INDEX

Morgan, A. E., 9(8), 67 Mori, K., 336(26), 338(26), 348 Morice, J. A., 188(174), 244 Morikawa, K., 277(153, 154). 319 Morikawa, Y . ,290(230), 300(230), 320 Morita, Y . , 277(151), 283(151), 286(151), 287(151), 289(151), 319 Moritani, I., 341(37), 345(41), 348, 349 Moro-oka, Y . , 255, 290, 300,316,320 Morrish, A. H., 146(67. 68), 241 Mlrup, S., 151(83), 167(113), 183, 185, 226(25l b), 241,242,244,246 Moss, R. L., 75(39), 86, 87, 116, 259(79), 260 (110), 261(112), 277(112, 156), 281(112, 156), 283(112, 156). 317, 318, 284(207), 286(112, 156, 207), 287(112, 156, 207), 289( 1 12, 156), 290(207), 294(207), 296 (207, 254, 255), 297(207), 317, 318, 319, 320, 321 Mott, N. F.. 104(124), 118 Moyes, R. B . , 97(111), 118 Miiller, F., 75(25), 99(25), 115 Muir, A . H., Jr., 123(1), 154(1), 239 Mulay, L. N., 182(158), 244 Mullen, J. G., I50(80), 241 Mulliken, J., 107, 119 Mullineaux, R. D., 323(3), 348 Mundt, W. A , , 166(100), 242 Munuera, G., 256(47), 267(137), 291(47, 137), 292(47, 137), 301(47, I37), 302(47. 137), 316,318 Myers, C. G., 286(217), 320 Myers, H. P., 104(121b), 105, 118

N Nagle, D. E., 167(106), 242 Naik, S. C., 279(188), 293(188), 320 Nakahira, M., 305(295), 306(295), 322 Nakashima, Y.,92(92b), 11 7 Nakayama, K . , 75(27b, 27c), 116 Narahari, B. N., 305(297), 306(297), 322 Nason, D., 83(50), 84(50), 85(50), 116 Niel. L., 145(61,62), 147,206(69,70), 241 Neimark, I. E., 169(120), 172(120), 243 Neldel, H., 250(16a), 316 Nemmonov, S . A,, 104(123), 118 Nemoshkalenko, V . V.. 104(121d), I18 Neumann, C., 258(62c), 317 Nichitaile, A. I., 223(248), 246

Nicholas, J. F., 253(34), 264(34), 316 Nieuwenhuys, B. E., 37(30), 38(30), 58(30), 67, 107(138), 119 Nikolaev, A. V., 250(9), 315 Nilsson, R., 104(123), 118 Nininger, R. C., Jr., 181(153), 243 Nishiyama, Y . ,259(80), 266(80), 317 Nolley, J. P., Jr., 336(27), 338(27), 348 Nordling, C., 104(123), 118 Norris, C., 104(121b), 105, 118 Norton, F. J., 286(219), 294(219), 296(249), 320,321 Norton P. R., 87(64c), 88, 89(72), 90(70), 116, 117, 277(165), 278(165), 319 NovikovB, J., 88(69), 117 Nowotny, J., 300(271), 321

0 Oates, W. A,, 260(97), 318 O’Connor, D. A., 167(107), 242 Ohmacht, R., 302(274b), 321 Okamoto, K., 92(92c), 117 O’Keefe, D. R., 41(32), 67 O’Keefe, M. A,, 291(234), 292(234). 320 Oleynikov, N. N., 263(126), 318 Oliver, R. G., 96, 118 Ollis, D. F., 226, 227, 228, 246 Oluoch-Okeio, D. K., 99(115), 118 Onishi, T., 258(62d), 317 Ono, M., 75(27b, 27c), 116 Ono, S., 74, 115 Oswin, H. G., 87(64b), 116 Otto, K., 300(267), 321 Overbury, S. H., 72(12), 83, 115 Ozaki, A,, 255, 290(42, 230), 300(42, 230). 316,320, 336(26), 338(26), 348

P Paal, Z., 94, 117, 260(107), 318 Palazov, A. N., 263(124), 281(124), 318 Palmberg, P. W., 23(21b), 29(26), 67, 107 (139), 119 Palmer, R. L., 41, 67 Pareja, P., 259(89), 317 Park, R. L., 13, 18(19b), 67 Parravano, G., I87(169), 198(169), 244, 256(48), 262(120), 316,318 Pasternak, M., 124(21), 180(137), 239, 243 Patat, F., 256(50), 316

AUTHOR INDEX

403

Patterson, J. H., 305(297), 306(297), 322 Rapp, R. A,, 74(23a), 115 Patterson, W. R., 284(204, 205), 286(205), Rees, L. V. C., 188(174, 179, 180, 181), 191(179, 180, 181), 244 290(204, 205), 320 Pauling, L., 105(132a, 132b), 118 Reman, W. G., 99,118 Reppe, W., 323,348 Perdereau, J., 12(1l), 67 Petermann, L. A,, 250(14), 256(14), 265(14), Reuben, B. G . , 250(13), 252(13), 253(13), 316 316 Reynolds, P. W., 99, 103(116b), 118 Petersen, E. E., 5(4), 26(4), 51(4), 52(36d), Rhead, G. E., l2(11),67,258(62b), 317 66,67 Rhodin, T. N., 11(9a), 29(28), 67 Pfannes, H. D., 166(103), 202,242 Riassian, M., 258(62e), 317 Phillips, C . S. G., 302(281), 322 Richardson, P. C., 303(286), 322 Pignet, T., 42(34c), 67,260(102), 318 Richter, E. L., 208(220), 245 Plachinda, A. S., 169(115, 116, 117, 118), Rickett, G., 287(225), 320 170(115, 118). 171(115, 118), 172, 188 Ridout, M. S., 174(130), 243 (173, 182), 191(182), 192(185, 187), 242, Riedl, F. J., 57(45), 68 243, 244 Riekert, L., 259(73), 317 Platt, R. H., 140(54), 141(54), 241 Rienacker, G., 91(86), 94, 116, 117 Plouidy, G., 283(196), 320 Ringstrorn, B., 163(90, 91), 242 Plunkett, T. J., 275(147), 277(147), 284(209b), Rissmann, E., 279(192), 291(192), 320 289(147), 296(147), 297(147), 319, 320 Robbins, M., 176(133), 243 Polak, L. S., 212(229, 230). 246 Roberti, A., 95(100), 96(100), 99(100), 117 Poltorak, 0. M., 56(42), 68 Roberts, G. G., 250(16b), 316 Ponec, V., 77, 86(60), 87(62, 64a, 67), 91(64a, Robertson, A . J . B., 260(105), 318 83), 92(64a, 67), 94(99a, 99d), 95(60, 62, Robertson, P. J., 303(287), 322 102), 96(60, 62, IOO), 97(62), 99(60, loo), Robertson, W. D., 72(10b), 75(10b), 115 100(62), 113(146), 116, 117, 119 Roelen, 323,348 Poole, C. P., Jr., 144(59), 241 Rog, G., 300(271), 321 Popovskii, V. V., 301(272a), 321 Roggwiller, P., 145(66), 241 Portis, A. M., 226(252), 246 Roginskii, C. Z., 256(52), 316 Pouteau, R. M. L., 284(203), 286(216), Roiter, V. A,, 250(18), 316 289(216), 320 Rol, N. C . , 101(119), 118 Preisinger, A,, 174(126), 243 Rooney, J. J., 96(106), 118 Prigogine, I., 74(20), 83(20), 84, 115, 116 Rosenberg, B., 270(143), 319 Pritchard, A. M., 215,220,246 Ross, J. R. H., 304(290), 322 Pritchard, J., 29(27), 67 Ross, P. N., Jr,, 187, 188, 244 Prudhomme, J. C . , 283(196), 320 Ross, R. A., 300(265, 268, 270), 301, 302 Ptak, L. D., 286(212), 287(212), 289(212), 320 (274a), 321 Prustowka, A. J., 181(146), 243 Rossington, D. R., 75(38), 116, 274(144), Pyke, D. R., 261(115), 318 293(144), 303(286), 319, 322 Rostrup-Nielsen, J. R., 277(160), 319 Roth, E., 200(203), 245 Q Roth, S., 180(138), 243 Quinn, D. F., 279(190), 291(190), 292(190), Roukens, J . J., 256(46), 316 Rovida, G . F., 37(30), 38(30), 58(30), 67 297( 190), 320 Rowden, M. W., 256(56), 296(56), 312, 317 Quinto, D. T., 72(10b), 75(10b), 115 Rozen, A. M., 250(6b), 315 Rubashov, A. M., 187(170), 244 R Ruby, S. L., 167(105, 1I2), 168(114), 202, Radescu, E., 203(213), 245 242,245 Rafter, E. A,, 291(237), 297(237), 321 Ruch, E., 208(220), 245 Randall, M., 73(18), 115 Rudharn, R., 302(275), 321

404

AUTHOR INDEX

Ruetschi, P., 256(51), 316 Ruppin, R., 180(141), 243 Rushford, H. G., 91(85), 100(85), 117, 251 (28), 296(28), 297(28), 316 Russell, W. W., 89(74), 91,92(87), 117 Russer, B., 300(271), 321 Ruthven, D. M., 259(86), 317 Rye, R. R., 42,67

S Sachtler, W. M. H., 71(3, 4a, 4b, 4c, 4, 6), 72(13, 14, 15), 73(4a, 4b, 4c, 4d, 15), 74(4a, 4b, 4c, 4d, 14), 75(4a, 4b, 6, 15, 31, 32), 76(4c, 40), 77(4c, 41, 42, 43). 78(14, 45), 80(45), 81(14, 32, 40, 45), 82(14), 84(45), 85(15, 4 9 , 86(15, 50), 87(32, 62, 107), 88(32), 89(32, 75a, 75b), 94(107), 95(60,62, 100, 101, 102), 96(40, 60, 62, 100, I O I ) , 97(60, 62), 98, 99(60, 100), 100(62), 101(119), 102(14, 41), 103(3, 41), 104(3, 40, 101). 107(138), 112(75a, 75b), 113(146), 115, 116, 117, 118, 119, 261(113), 277(182), 278(182), 279(189), 291(189), 294(113, 248). 297 (113), 318, 319, 320 Sagert, N. H., 284(203), 286(216), 289(216), 320 Sakaguchi, M., 71(7b), 72(7c), 75(7c), 89(7c), 90(7c), 115 Saleh, J. M., 250(12), 251(26), 293(26), 315, 316 Saltsburg, H., 41(32), 67 Sams, J. R., 188(175), 244 Sanches, A , , 99(115), 118 Sanders, J. V., 290(233), 291(233), 292(233), 293(233), 320 Sandler, Y. L., 260(99), 318 Sarkiny, A,, 263(123), 281(123), 286(215), 289(215), 318,320 Savchenko, V. I., 88(71), 117 Sawicka, B. D., 181(146), 243 Sawicki, J. A,, 124(24), 181(146), 240, 243 Sazonov, V. A,, 301(272a), 321 Schachter, K., 278(185), 293(246, 247), 294(246), 319, 321 Scheben, J. A., 327(12), 348 Schmidt, L. D., 64(54),68,260(102),264(130), 318 Schnorr, H., 124(20), 239

Schoenberg, A., 325(11), 327{11), 328(11), 330(15, 16), 331,348 Scholten, J. J. F., 259(90), 290(232), 291(232), 301(273), 302(273), 317,320,321 Schonfeld, A,, 174(127), 243 Schrieffer, J. R., l09(143b), 119 Schroeer, D., 181(153), 182(154, 156, 157), 243,244 Schuit, G. C. A,, 99(114), 105(133a), 118, IIY, 226,246, 255(43), 259(43), 316 Si-hultz, R. G., 335(25), 348 Sdiutz, J. M., 175(131), 243 Schwab, G. M., 100(117), 118, 200, 245, 251(24), 290(231), 316,320 Schwoebel, R. L., 11(10b), 67 Scott, J. C., 188(175), 244 Scurrell, M. L., 303(287), 322 Seib, D. H., 104(121a), 105, 118 Selwood, P. W., 147(71), 241 Senkevich, A. I., 104(121d), 118 Sermon, P. A,, 258(65), 317 Sexton, B. A,, 25(22a), 51(22a), 67 Shannon. I. R., 302(277), 303,322 Sharp, J. H., 305(297), 306(297, 301), 322 Shelef, M., 300(267), 321 Shenoy, G. K., 154(85), 167(112), 241, 242 Shephard, F. E., 263(122), 277(122), 282,318 Sherwood, R. C., 176(133), 243 Shibata, F , 71(7a), 115 Shield, L. S., 89(74), 117 Shimizu, H., 75(27c), 85, 91, 116, 117, 261(114), 294(114), 318 Shimoyama, Y., 87(65b), 98(113), 99(113), 117, 118 Shimulis, V. I., 269(142), 319 Shinjo, T., 181(151), 243 Shinohara, H., 55(39). 68 Shirley, D. A,, 124(37), 143(56), 144(56), 182(56), 240, 241 Shkarin, A. V., 220(242, 243), 222(247), 246 Shlikhter, E. B., 212(229,230), 246 Shooter, D., 277(164), 278(164), 319 Shopov, D. M., 263(124), 281,318 Shpinel, V. S., 212(229), 246 Shtyrkov, L. G., 169(116), 188(173), 242, 244 Shue, R. S., 341(38), 348 Shulrnan, R. A., 57(44), 68 Shumyantzer, A. V., 263(126), 318 Sibbett, D. J., 286(217), 320 Siegbahn, K., 23(21a), 67, 194(189), 244

405

AUTHOR INDEX Siegel, S., 277(171), 278(171). 284(171), 286 (171). 287(171), 319 Silverston, P. L., 264(131), 318 Simmons, G . W., 215,246 Simons, J. W., 306(300), 322 Sinfelt, J. H., 56(43), 57(43), 68, 75, 87(63), 93(63, 95a, 95b, 96), 94(99b, 99c), 95(63, 103b), 100(63), 105(63), 116, 117, 118, 259(74), 277(149,150,152,179),278(179), 281, 283(74), 286(74, 150, 213), 287(74, 149, 152), 289(213), 317, 319, 320 Singwi, K. S., 150(79), 151(79), 241 Sjolander, A,, 150(79), 151(79), 241 Skalkina, L. V., 199(202),245 Skapski, A. S., 84, 116 Sladek, K. J., 258(62a), 317 Slaugh, L. H., 323(3), 348 Sloczyhski, J., 300(271), 321 Smith, J. N., 41(32), 67 Sokolovskii, V. D., 304(289), 322 Solbakken, A,, 174(125), 243 Soibakken, V., 174,243 Solymosi, F., 279(193), 291(193), 320 Soma-Nota, Y., 89(75a, 75b), 112(75a, 75b), 113, 117 Somorjai, G . A,, 3(1, 2), 5(3, 4), 8(5, 6 , 7), 9(8), 11(9b), 12(13), 13(13), 15(15, 16), 16(1, 17), 18(1, 18), 21(1), 23(2), 25(22a), 26(3, 4, 22b), 27(22b, 23), 28(24, 25), 34(24), 35(5), 37(29, 30), 38(30), 39, 41(31), 42,44(35), 49(35), 51(4,22a, 36b), 53(31, 35, 36a, 36c), 55(41), 57(36c), 58(29, 30), 66, 67, 68, 72( 12). 75(28), 83(12), 115, 116, 258(61), 259(61), 260(93b), 317,318 Sorduna, M. F., 104(123), 118 S q h m , H., 74(24), 115 Sosnovsky, H. M. C., 253(35), 290(35), 291(35), 293(35), 316 Soven, P., 70(1), 104(1), I15 Spicer, W. E., 104(121a), 105, 118 Spijkerman, J. J., 124(15, 16, 17), 163(89,92), 164(92), 168(92), 239, 242 Spindler, H., 56(43), 57(43), 68 Squire, R. C., 97(11 I), 118 Sridhar, T. S., 259(86), 317 Staffin, H. K., 277(181), 278(181), 319 Staib, M., 259(73), 317 Stair, P. C., 8(7), 67 Steel, M. C. F., 304(290), 322

Stefansson, V., 163(90), 242 Steinbruchel, Ch.. 264(130), 318 Stephan, J. J., 113(146), 119 Stern, E. A., 104(125), 118 Stevens, J. G., 123(2, 3, 4, 5 , 6, 7a, 7b), 124(18), 152(7a, 7b), 154(2, 3,4, 5 , 6, 7a, 7b), 156(5, 6), 239 Stevens, V. E., 123(2,3,4,5,6,7a, 7b), 152(7a, 7b), 154(2,3,4,5,6,7a, 7b), 156(5,6), 239 Stewart, D. J., 172(122), 173(122),243 Stickney, R. E., 41, 42,67 Stocks, G. M., 104(130c), 118 Stoddart, C. T. H., 277(169, 176), 278(169, 176), 284(169), 286(169), 287(169, 176), 319 Stone, A. J., 190(184), 244 Stone, F. S., 63, 68 Stone, J., 250(15), 251(30), 268(15, 30), 269(15, 30), 316 Stott, M. J., 105(135b), 119 Strakhov, B. V., 187(170), 244 Straughan, B. P., 124(8, 9, lo), 239 Sundaram, V. S., 72(10b), 75(10b), 115 Suzdaler, I. P., 124(38, 40, 48), 149(76), 166(102a), 169(115, 116, 117, 118, 119, 120), 170(118), 171, 172(120), 180(134, 136), 181(150), 188(173, 182), 191(182), 192(185, 186, 187), 199(202), 201(208), 202(208), 203(208), 217, 218, 219(240), 220(243, 244), 221(245), 222(246), 223 (102a, 248), 224(249), 225(249), 226(245), 240,241,242,243,244,245,246 Svensson, S. O., 151(83), 241 Szabb, Z. G . , 302,321 Szalkowski, F. J., 3(2), 23(2), 28(25), 66, 67

T Takabatake, T., 72(7c), 75(7c), 89(7c), 90(7c), 115 Takada, Y . , 92(92c), 117 Takasu, T., 85, I16 Takasu, Y., 72(9), 75(27b), 91(81), 115, 116, 117, 261(114), 277(166), 278(166), 294(114), 318, 319 Takayasa, D., 91(88), 92(88), 117 Takeuchi, T., 71(7a, 7b), 72,75(7c), 89(7c, 73), 90, 91, 92(92b, 92c), 115, 117 Tamaru, K., 87(64f), 116, 258(62d), 260(94), 266, 291(235), 292(235), 317, 318, 320

406

AUTHOR INDEX

Tarmg, M. C., 75(27a), 116 Taylor, D., 279(190, 194), 291(190. 194, 239), 292(190, 239), 297(190), 301, 302(239). 303,320, 321,322 Taylor, H. S . , 277(153, 154, 155). 319 Taylor, R . D., 167(106). 242 Taylor, W . F., 94(99b, 99c), 117, 259(77), 263(77), 265(77), 277(71, 149, 150, 179, 181), 278(179, 181), 286(150), 287(149), 317,319 Teraniski, S . , 341(37), 348 Tesuka, Y., 91(88), 92(88), 92(88, 92b), 117 Tktenyi, P., 94, 117. 259(84, 87), 263(87, 123). 278(184, 185), 281, 286(215), 287(226). 289(215), 293, 294(246), 317, 318, 319, 320,321 Thomas, D. H., 86, 116. 284(207), 286(207), 287(207), 290(207), 294(207), 296(207, 254). 297(207), 320.321 Thomas, R. B., 261(116), 318 Thomson. S. J . , 257, 259(57), 260(95, 107), 266(57, 95). 317, 318 Tjon, J. A,, 182(160),244 To, D. E., 293(245). 321 Tompkins, F. C., 256(54), 257(58), 290(58), 317 Toneman, L. H., 72(11), 78(11), 83(52), 84(52), / I 5 TopsBe, H., 146(215), 162(215), 164(95, 96, 97), 174, 175, 176, 177, 178, 183, 185(97), 186(165), 187(97), 197(97), 200(97, 206), 201(165), 203(165, 206, 215), 204(215, 216), 205(165, 206, 216, 217), 206(165, 216), 207(165), 208(165, 216, 217), 216, 226(251b), 242, 244, 245, 246 Toussaint, F., 305(296), 306(296), 322 Townshend, R. E., 277(177), 278(177), 319 Tracy, J. C., 29(26), 67, 107(139), 119 Trambouze, Y., 259(82), 317 Trapezsnikov, V. A,, 104(123), 118 Travis, J. C., 124(18), 140(53), 239, 240 Trenner, N., 277(154), 319 Tretyakov, Yu. D., 263(126), 318 Trillo, J . M.. 256(47), 267(137), 291(47, 137), 292(47, 137). 301, 302(47, 137). 316, 318 Trimm, D. L.,258(62e), 317 Triplett, €3. B., 166(101a). 242 TrBnsdal, G. D., 74(24), 115 Trooster, J. M., 180(139), 243 Trumpy, G . , 151(83),241

Tsang. Y. W., 61,62,68 Tseung, A. C. C., 303(284), 322 Tsurumi, M., 277(151), 283(151), 286(151), 287(151), 289(151), 3IY Tsyganov, A. D., 221(245). 226(245), 246 Tucci, E. R., 323(4), 348 Tucker, C . W., II(lOa), 67 Turlier, P., 259(82), 317 Tuul, J., 72, 115, 279(187), 280,320

v Vail, J., 203, 245 Vamanu, D., 205(218), 245 Van Aardenne, 0 . G . , 107(138), 119 Van Barneveld, W. A. A., 87(67), 92(67), 117 Van der Kraan, A. M., 181, 243 Van der Plank, P., 71(3), 76(40), 81(40), 95(101), 96(40, 101), 103(3), 104(3, 40, I O I ) , 115, 116, 117, 277(182), 278(182). 319 Van Deventer, M. M., 180(139), 243 Van Eijkeren, J. C. H., 180(139), 243 Van Hardeveld, R.,208(219), 245, 301(273), 302(273), 321 Van Helden, R., 327(13a), 328(13a), 348 Van Herwijnen, T., 260(109), 318 Vannice, M . A., 196(196), 245,289(227b),320 Van Reijen, L. L., 255(43), 259(43), 316 Van Santen, R. A,, 75(36), 78(45), 80(45), 81(45), 83(52), 84(45, 52). 85(45), 87(36), 101. 106(136), 109(136), 112(136), 116, I18 van Wieringer, J. S., 180(140), 243 Varrna, M. N . , 198(199,200), 245 Vasserberg, V. E., 302(280), 322 Vaughan, R. W., 182, 244 Vecher, A. A., 74(23b), 115 Velicky, B., 70(2), 104(2, 127), 115, 118 Verbeek, H., 72(14), 74(14), 78(14), 81(14), 82(14), 98, 102(14), 103, 115 Vernon, C. A., 249(4), 252(4), 270(4), 315 Vertes, A., 21 3,246 Vickers, D. E., 259(88), 31 7 Viegers, M. P. A,, 180(139), 243 Visser, C . , 91(83), 117 Vissher, W. M., 167(106), 242 Volter, J., 92(90), 117 Vogel, W., 174(126), 243 Volta. J. C., 259(82), 317

407

AUTHOR INDEX

w Wagner, N . J., 71(5), I15 Wagstaff, K. P., 302(275), 321 Waite, R. J., 261(116), 318 Walker, D. R., 303,322 Wallace, N. D., 87(64e), 116 Wallis, R., 62(52b), 68 Walter, G., 259(78), 317 Walters, A. B., 63(51c), 68 Wanke, S. E., 264(128), 318 Watson, A. M., 290(231), 320 Weast, R. C., 111(144), 119 Webb, G., 258(66), 259, 260(100), 317, 318 Weber, W. P., 341(36), 348 Wedd, R. W. J., 188(175), 244 Wegener, H., 124(28), 130(28), 133(28), 143, 240,241

Wehmer, G. K., 75(27a), 116 Weinberg, W. H., 55,68 Weiss, A. H., 258(63), 317 Wells, P. B., 96(106), 118 Wernick, J. H., 104(121c, 122), 118 Wertheim, G. K., 104(121c, 122), 118, 124 (26), 143(55), 145(55), 150(55), 157(88), 159(88), 160(88). 161(88), 164(88), 165, 166(88), 176(133), 240,241, 242,243 Whalley, L., 75(39), 87, 116, 296(255), 321 Whan, D. A,, 91(85), 100(85), 117, 251(28), 264(127), 277(158), 286(127, 214), 287(127, 221), 296(28), 297(28), 316,318,319,320 White, J. M., 260(106), 318 Wickman, H. H., 143(55), 144, 145(55), 150(55), 182(56), 241 Wiedemann, W., 166(100), 242 Wilenzik, R. M., 180(135), 182(157), 243, 244 Wilke, W., 174(127), 243 Wilkinson, G., 323(5), 348 Willard, A. K., 341(36), 348 Willhoft, E. M. A., 260(105), 318 Williams, F. L., 75(29), 83(50), 84(50), 85(50), 86, I16 Williams, P. M., 258(62e), 317 Williams, R. W., 104(130c), 118 Wilson, G. R., 96(106), 118 Wilson, M. C., 253(31), 254(31), 304(291), 305(31,291), 306(31,291), 316,322

Wilson, R. L., 287(222,223), 320 Winter, E. R. S., 298(259, 260, 263, 264), 300(259), 304(259,260), 321 Winter, S. R., 333(20), 348 Wise, H., 55, 68, 97(112), 118, 259(80), 266(80), 317 Wishlade, J. L., 257, 259(57), 266(57), 317 Wold, S., 268(139), 269(139), 319 Wolf, F. J., 277(174), 278(174), 284(174), 286(174), 287(174), 290(174), 319 Wollensak, J. C., 330(18), 348 Wood, E. A,, 18(19a), 67 Woodward, J. W., 260(103), 269(103), 290 (103), 318 Wright, P. G., 259(68), 275(68), 280(68), 287(68), 31 7 Wurzbacher, G., 259(78), 317

Y Yagupsky, G., 323(5), 348 Yamashira, T., 72(9), 75(27b), 91(81), 92(92a), 115, 116, 117, 277(166), 278(166), 319 Yampol’skii, Yu. P., 219(240), 224(249), 225(249), 246 Yao, Y. Y., 300(266, 269), 321 Yasumori, I.. 55,68, 291(236), 321 Yates, D. J. C., 87(63), 93(63, 95b), 94(63), 95(63), 100(63), 105(63), 116, 117, 277 (149, 150, 152, 179), 278(179), 286(150), 287(149, 152), 319 Yeramian, A. A,, 264(131), 318 Yoshioka, T., 187(167), 244

Zanderighi, L., 264(129), 318 Zemcik, T., 124(22), 240 Zhabrova, G. M., 220(242), 222(247), 246 Ziman, J. M., 105(131), 118 Zsak6, J., 250(10, l l ) , 306(10), 315 Zuidwijk, J. G. P., 91(83), 117 Zukrowski, J., 145(65), 241 Zwietering, P., 256(46), 290(232), 291(232), 293(232), 301(232), 316, 320 Zyryanov, V. G., 104(123), I18

Subject Index A

Aluminum-nickel alloys, see Nickel--ahminum alloys Amidation reactions of organic halides, palladium-catalyzed, 330, 331 Amines, cracking reactions, 289 Arrhenius parameters, 315 for heterogeneous reactions, 261 -264 Auger electron spectroscopy (AES), 3,21-24, 72, 86, 91

Abstraction process, 381 Acety lene(s) reaction with organic halides, 323-347 substitution reactions, 345-347 Acylation reaction of dienes, 335, 336 Adsorption, 355 366, 380 activated, 366 charge transfer, 359, 360 immobile, 360 mean residence time, 365 B mobile, 360 rates of, 365, 366 Boron compounds in olefinic substitution, 341 reactive, 380 Brunauer- Emmett-Teller (BET) adsorption sites for, 360-362 isotherms, 364, 365 active, 362 uniform, 361, 362 sticking coefficient, 365 surface intermediates, 381 - 383 C unactivated, 366 Calcination, 369 work function changes, 30-33 Adsorption isotherms, 362 -~365, see also Carboalkoxylation reactions of organic halides specific types nickel-catalyzed, 326 linear, 363 palladium-catalyzed, 328 Alcohol, dehydrogenation and dehydration Carbomethoxylation reactions of organic haof, 302 lides, 332 Alloy catalysts, 69- I 15, see also specific alloys Carbomethoxyvinylation reactions of organic biphasic, 75-77 halides, 334, 335 compensation behavior, 294-298 Carbon monoxide, reaction with organic ensemble effect, 100-103 adsorption energies, 106 halides, 323-347 Cdrbonylation reactions, of organic halides, heat of adsorption on, 106-1 14 324-336 ligand effect, 103-106 cobalt-catalyzed, 332-336 adsorption energies, 109 iron-catalyzed, 333 monophasic ordered, 78-82 nickel-catalyzed, 324-326, 334 selectivity of, in hydrocarbon reactions, palladium-catalyzed 325-332 87-100 rhodium-catalyzed, 335, 336 solid solutions, 82-87 Catalysis, 353-355 surface composition, 71 -87 bifunctional, 365 surface enrichment, 73, 74 correlations between homogeneous and Allylic alcohols, olefinic substitution of, 343heterogeneous, 65.66 345 408

SUBJECT INDEX

definitions, 353-383 heterogeneous, 354, 355 active sites in, 1-66 compensation effect, see Compensation behavior elementary processes in, 380, 381 kinetics of, 371 -377 catalytic sites, 376 rate equations, 373-375 selectivity, 372, 373 transport phenomena, 376, 377 homogeneous, 354 negative, 353, 354 polyfunctional, 365 symbols and abbreviations, 384-392 terminology, 353-383 Catalyst(s), 353-355, see also specific types activation, 368, 369 aging of, 378 composition, structure, and texture, 366369 elementary processes, 354 loss of activity, 377-379 Mosshauer spectroscopy, see Mossbauer spectroscopy porosity, 367, 368 pretreatment, 368, 369 primary particles, 368 promoter, 366 support, 366 Catalytic reaction, 354, see also specific types mechanism of, 379-383 nomenclature of, 383, 384 Catalytic reactors, 369, 370 batch, 369, 370 flow, 369,570 Charge transfer adsorption, 359, 360 Chemisorption, 356-358 on alloys, 72, 73 dissociative, 358, 380 heterolytic, 359 homolytic, 359 oxidative, 359 reductive, 359 of hydrocarbons, 28-39 in Mossbauer spectroscopy, 209 -229 nondissociative, 358, 359 sites for, 360, 361 uniformity of, 361, 362 types of, 358-360

409

Clays, compensation behavior, 304-307 Cobalt carbonyl as catalyst, 332-336 Coking, 378 Compensation behavior, 247-31 5 active surface, 253, 254 Arrhenius parameters, see Arrhenius parameters availability of surface reactant, 254, 255 compensation parameters, 267, 315 energetically heterogeneous catalyst surface, 253 enthalpy-entropy relationship, 254 in kinetics, 271 -307 quantitative recognition of, 267-271 rate law for surface reactions, 255 surface concentration of reactants, 258-261 surface equilibrium model, 264-267 surface reactions, see Surface reactions temperature at onset of reaction, 252, 253 temperature-dependent, 31 1-314 theoretical explanations of, 252-256 Compensation parameters, 267, 315 Copper, high Miller index, 12 Copper-gold alloys, see Gold-copper alloys Copper-iron alloys, see Iron-copper alloys Copper-nickel alloys, see Nickel-copper alloys Copper-osmium alloys, see Osmium-copper alloys Copper-palladium alloys, see Palladiumcopper alloys Copper-platinum alloys, see Platinum-copper alloys Copper-ruthenium alloys, see Rutheniumcopper alloys Copper-zinc alloy, 74 Cracking catalytic, 383 reactions, 275-277,280-283, 287-289 Crystal surfaces, 4, 5 bond breaking on, 53, 54 cleaning and preparation of, 27, 28 fcc metal, 6 low and high Miller index, 5-15 Cyclohexane dehydrogenation and hydrogenolysis of, 43-49, 51, 52 mechanism of dehydrogenation, 56 -58 Cyclohexene dehydrogenation and hydl ogenolysis Of, 49,50

410

SUBJECT INDEX

mechanism of dehydrogenation, 56-58 Cyclopropane hydrogenolysis of. 5 I 52 rate of, 52

Deactivation of catalyst. 378 types of, 378. 379 Dehydrocyclization. catalytic. 384 Desorption, 356, 380 associative, 360, 380 rates of, 365. 366 reactive, 380 Deuterium exchange reactions, compensation behavior, 289,290 Dienes, acylation of. 336 Doping, 367 Doppler effect, 125. 152. I57 Doppler velocity. 125. 158, I59

E Electron spin reasonance, 144 Elovich equation, 366 Enzyme catalysis. 355 on metal surfaces. 64, 65 Exchange reactions. 277-279 on alloys, 294-296 of oxygen, 298-300 Extraction process, 381

F Formic acid, decomposition of, 279,290-293, 30 I , 302 Formylation reactions of organic halides, palladium-catalyzed, 330, 33 l Fouling, 378 Freundlich adsorption isotherm, 364

Gold, chemisorption on. 37-39 Gold copper alloys, 82, 83 Gold-nickel alloys, see Nickel gold alloys Gold palladium alloys, see Palladium-gold alloys Gold-platinum alloys, see Platinum-gold alloys Gold-silver alloys. see Silver-gold alloys Group IB metals as catalysts, 87, 104 Group VIII metals as catalysts, 87. 104

H n-Heptane, dehydrocyclization of, 51 -53 Hydrocarbon(s) chemisorption of, 28-39 cracking reactions, 287, 289 oxidation, 290 selectivity of alloys in reactions, 87-100 Hydrocracking, catalytic, 383 Hydrodesulfurization, catalytic, 384 Hydrogenation, 277-280 o n alloys, 296 Hydrogenolysis, catalytic, 384

1 Mite, 304-307 Indium-lead alloys, see Lead- indium alloys Inhibition. 354. 377 Iridium, chemisorption on, 37-39 Iron, as supported catalyst, 186- 193 Iron carbonyl. as catalyst. 333 Iron-copper alloys. 82 Iron- platinum alloys, .we Platinum-iron alloys lsokinctic effect, see Compensation behavior Isomer shift. 126. 132. 138-140

K Kaolinite, 304 307 Kinetic parameters. 315

G L Langmuir adsorption isotherm, 265, 363, 364 Langmuir-Hinshelwood mechanism. 376

41 1

SUBJECT INDEX

Lead compounds in olefinic substitution, 341 Lead-indium alloys, 83 Linear regression analysis, 314 Low-energy electron diffraction (LEED), 3, 16-21

M Magnetic hyperfine interaction, 126, 135, 142- 147 Magnetocrystalline anisotropy, 146, 147 Mercury compounds in olefinic substitution, 340,341 Metals, see also specific elements groups of, compensation behavior of, 288 reactions of, kinetics, 274-294 Methanation, catalytic, 383 Miller index, see Crystal structure Mossbauer isotopes, 124, 125, 151-168 chemical information about, 153-157 as chemical probe, 226-229 classification of, 153 feasibility for catalytic studies, 236-238 nuclear data for, 230-235 source of radiation, I51 - 153 Mossbauer resonance, 126 Mossbauer spectrometer, 157-163 Mossbauer spectroscopy, 124-126 advantages of technique, 122, 123 of alloys, 73, 74 applications to heterogeneous catalysis, 121-238 catalyst preparation, 169-193 chemical perturbations of nuclear levels, 130-136 chemisorDtion and reaction, 209-229 data processing, 168 detectors and nuclear counting system, 160, 161 geometry, 162, 163 interaction of surface sites with gases, 209213 kinetics of slow processes, 213-220 line intensities and shapes, 147-151 linewidth, natural, 128 literature concerning, 123, 124 particle size and size distribution, 179- 186 recoil-free fraction, 136-138 resonant absorption and, 126-129 sample cells and mounting, 163-166 sample preparation, 163-168

sample thickness, 167 source-detector distance, 167, 168 stationary-state effects, 221 -225 supported-metal catalysts, 186- 193 surface and bulk mobility, 169-173 surface chemical state, 193-201 surface properties of catalysts, 193-209 surface structure, 201 -209 textural and chemical promoters, 173-179 Molecular-beam surface scattering, 26, 27 Molybdenum disulfide, compensation behavior of, 285 Montmorillonite, 304-307 Muscovite. 305-307

Nickel, compensation behavior of, 275-283, 285 Nickel-aluminum alloys, 77 Nickel carbide compensation behavior, 276, 277 cracking on, 282,283 Nickel carbonyl as catalyst, 324-326, 334 Nickel-copper alloys, 71-77, 81, 85, 86, 90100, 104-106, 112,296 Nickel-gold alloys, 75, 86,92 Nitric oxide, decomposition of, 298-300 Nitrous oxide, decomposition of, 298-300

0 Olefin(s) reaction with organic halides, 323-347 substitution reactions, 336-345 Organic halides, metal-catalyzed reactions of, 323-347 Osmium-copper alloys, 94 Outgassing, 369 Oxidation, 290, 300, 301 on alloys, 296 Oxides, compensation behavior, 298 -304

P Palladium, 107 as catalyst, 325-332, 336-347 compensation behavior, 283-285

412

SUBJECT INDEX

Palladium-copper alloys, 97 Palladium -gold alloys, 96.97, 100. 296 Palladium--silver alloys, 85, 86, 90, 96, 100, 104-106, 112, 113,296 Palladium-rhodium alloys, 294 Palladium-ruthenium alloys, 294 Photoadsorption, 360 Photodesorption, 360 Physisorption, 356-358 Platinum, 100, 101, 114 bond breaking on, 53, 54 carbonaceous overlayer on, 54,55 chemisorption on, 28--38 I I 1 and 100 faces, 28-35 stepped face, 35--38 compensation behavior of 284-286 crystal surfaces active sites on, 53. 54 hydrocarbon catalysis on. 58-60 dehydrogenation and hydrogenolysis of cyclohexane on, 43 - 49 of cyclohexene, 49,50 (l00)faceof,aIomicsurbcestructure, 9-11 ( 1 10) face of, atomic surface structure, I I ( I 11) face of, 7 atomic surface structure, 8, 9 H,-D, exchange on, 39-43 high Miller index, 12-15, 35, 36 hydrocarbon reactions of cyclopropane, cyclohexane, and n-heptane, 51 -53 reactions o n crystal surfaces, 39-53 stepped surfaces, 13- 15 structure sensitivity, 56-58 Platinum-copper alloy, 75 Platinum-gold alloys, 75, 77. 81. 87-91, 113 Platinum-iron alloys, 75 Platinum-ruthenium alloys, 77 Platinum silver alloys, 75 Platinum tin alloy, 72, 74, 78-82, 102, 103 Poisoning, 49, 55, 92 --94,97, 99, 354, 377

Quadrupole splitting, 126, 134, 140-142

R Recrystallization, 379 Reforming, catalytic, 383

Rhodium as catalyst, 335, 336 compensation behavior of, 286 Rhodium-palladium alloys, see Palladiumrhodium alloys Rhodium-silver alloys, 75 Ridedl- Eley mechanism, 381 Roginskii-Feldovich equation, 366 Ruthenium, compensation behavior of, 285, 286 Ruthenium-copper alloys, 94 Ruthenium- palladium alloys, see Palladiumruthenium alloys Ruthenium platinum alloys, see Platinumruthenium alloys

S Silicon compounds in olefinic substitution, 341 Silver-.gold alloys, 75, 82, 86, 87, 101 Silver-palladium alloys, see Palladium-silver alloys Silver-platinum alloys, see Platinum-silver alloys Silver-rhodium alloys, see Rhodium-silver alloys Sintering, 378 Sorptive insertion, 380 Substitution reactions olefinic, 336-345 of terminal acetylenes, 345-347 Superparamagnetism, 142, 145 Surface composition, techniques to study, 1628 Surface reactants properties and behavior of, 259, 260 mobility of, 258 Surface reactions, 256-258 CdtalyStS in, 260, 261 rates at low and high pressures, 25. 26 Surface structure active sites on, 1-66 low-coordinat ion number, 60 -63 on nonmetals, 63, 64 atomic, 5-15 crystal, see Crystal structure solid, 5 techniques to study, 16-28 static, 16-24 transport, 25-27

41 3

SUBJECT INDEX

T Temkin adsorption isotherm, 364 Tin compounds in olefinic substitution, 341 Tin-platinum alloy, see Platinum-tin alloy Transition metal(s), see also specific metals as catalysts for reactions of organic halides, 323-347 Tungsten, compensation behavior of, 285,287

Tungsten disulfide, compensation behavior of, 285,287

Z Zeolites$ lg8-I91, 367 reactions on, 303 Zinc-copper alloy, see Copper-zinc alloy

Contents of Previous Volumes Volume 1 The Heterogeneity of Catalyst Surfaces for Chemisorption HUGHS. TAYLOR Alkylation of Isoparaffins v. N. LPATIEFF A N D LOUIS SCHMERLING Surface Area Measurements. A New Tool for Studying Contact Catalysts P. H. EMMETT The Geometrical Factor in Catalysis R. H. GRIFFITH The Fischer-Tropsch and Related Processes for Synthesis of Hydrocarbons by Hydrogcnation of Carbon Monoxide H. H. STORCH The Catalytic Activation of Hydrogen D. D. ELEY Isomcrization of Alkanes HERMAN PINES The Application of X-Ray Diffraction to the Study of Solid Catalysts A N D I. FANKUCHEN M. H. JELLINEK Volume 2 The Fundamental Principles of Catalytic Activity FREDERICK SEII z The Mechanism of the Polymerization of Alkenes LOUISSCHMERLING AND v. N. IPATIEFF Early Studies of Multicomponent Catalysts ALWINMITTASCH Catalytic Phenomena Related to Photographic Development T. H. JAMES Catalysis and the Adsorption of Hydrogen on Metal Catalysts OTTOBEECK Hydrogen Fluoride Catalysis J. H. SIMONS

Entropy of Adsorption CHARLES KEMBALL About the Mechanism of Contact Catalysis GEORGE-MARIA SCHWAB Volume 3 Balandin’s Contribution to Heterogeneous Catalysis B. M. W. TRAPNELL Magnetism and the Structure of Catalytically Active Solids P. W. SELWOOD Catalytic Oxidation of Acetylene in Air for Oxygen Manufacture J. HENRYRUSHTON A N D K. A. KRIECER The Poisoning of Metallic Catalysts E. B. MAXTED Catalytic Cracking of Pure Hydrocarbons VLADIMIR HAENSEL Chemical Characteristics and Structure of Cracking Catalysts A. G. OBLAD,T. H. MILLIKEN, JR., AND G. A. MILLS Reaction Rates and Selectivity in Catalyst Pores AHLBORN WHEELER Nickel Sulfide Catalysts WILLIAM J. KIRKPATRICK Volume 4 Chemical Concepts of Catalytic Cracking R. C. HANSPORD Decomposition of Hydrogen Peroxide by Catalysts in Homogeneous Aqueous Solution J. H. BAXENDALI~ Structure and Sintering Properties of Cracking Catalysts and Related Materials HERMAN E. RIES,Jn. 414

CONTENTS OF PREVIOUS VOLUMES

Acid-Base Catalysis and Molecular Structure R. P. BELL Theory of Physical Adsorption TERRELL L. HILL The Role of Surface Heterogeneity in Adsorption GEORGE D. HALSEY Twenty-Five Years of Synthesis of Gasoline by Catalytic Conversion of Carbon Monoxide and Hydrogen HELMUT PICHLER The Free Radical Mechanism in the Reactions of Hydrogen Peroxide JOSEPHWEISS The Specific Reactions of Iron in Some Hemoproteins PHILIPGEORGE

Volume 5 Latest Developments in Ammonia Synthesis ANDERSNIELSEN Surface Studies with the Vacuum Microbalance: Instrumentation and Low-Temperature Applications T. N. RHODIN,JR. Surface Studies with the Vacuum Microbalance: High-Temperature Reactions EARLA. GULBRANSEN The Heterogeneous Oxidation of Carbon Monoxide MORRISKATZ Contributions of Russian Scientists to Catalysis J. G. TOLPIN,G. S. JOHN,AND E. FIELD The Elucidation of Reaction Mechanisms by the Method of Intermediates in QuasiStationary Concentrations J. A. CHRISTIANSEN Iron Nitrides as Fischer-Tropsch Catalysts ROBERT9 . ANDERSON Hydrogenation of Organic Compounds with Synthesis Gas MILTONORCHIN The Uses of Raney Nickel EUGENELIEBERAND FREDL. MORRITZ

Volume 6 Catalysis and Reaction Kinetics at Liquid Interfaces J. T. DAVIES

415

Some General Aspects of Chemisorption and Catalysis TAKAO KWAN Noble Metal-Synthetic Polymer Catalysts and Studies on the Mechanism of Their Action AND F. F. NORD WILLIAM P. DUNWORTH Interpretation of Measurements in Experimental Catalysis P. 9 . WEISZAND C. D. PRATER Commercial Isomerization 9 . L. EVERING Acidic and Basic Catalysis MARTINKILPATRICK Industrial Catalytic Cracking RODNEY V. SHANKLAND

Volume 7 The Electronic Factor in Heterogeneous Catalysis M. McD. BAKERA N D G. I. JENKINS Chemisorption and Catalysis on Oxide Semiconductors G.PARRAVANOAND M.BOUDART The Compensation Effect in Heterogeneous Catalysis E. CREMER Field Emission Microscopy and Some Applications to Catalysis and Chemisorption ROBERTGOMER Adsorption on Metal Surfaces and Its Bearing on Catalysis JOSEPHA. BECKER The Application of the Theory of Semiconductors to Problems of Heterogeneous Catalysis K. HAUFFE Surface Barrier Effects in Adsorption, Illustrated by Zinc Oxide S. ROYMORRISON Electronic Interaction between Metallic Catalysts and Chemisorbed Molecules R. SUHRMANN

Volume 8 Current Problems of Heterogeneous Catalysis I. ARVIDHEDVALL Adsorption Phenomena J. H. DE BOER

416

CONTENTS OF PREVIOUS VOLUMES

Activation of Molecular Hydrogen by Homogeneous Catalysts S . W. WELLER AND G. A. MILLS Catalytic Syntheses of Ketones V. I. KOMAREWSKY AND J. R. COLEY Polymerization of Olefins from Cracked Gases EDWINK. JONES Coal-Hydrogenation Vapor-Phase Catalysts E. E. DONATH The Kinetics of the Cracking of Cumene by Silica-Alumina Catalysts CHARLESD. PRATERAND RUDOLPHM. LAGO

Gas Reactions of Carbon P. L. WALKER, JR., FRANKRUSINKO,JR., AND L. G. AUSTIN The Catalytic Exchange of Hydrocarbons with Deuterium C. KEMBALL Immersional Heats and the Nature of Solid Surfaces J. J. CHFSICK AND A. C. ZETTLEMOYER The Catalytic Activation of Hydrogen in Homogeneous, Heterogeneous, and Biological Systems J. HALPERN

Volume 9

The Wave Mechanics of the Surface Bond in Chemisorption T. B. GRIMLEY Magnetic Resonance Techniques in Catalytic Research D. E. O’REILLY Bare-Catalyzed Reactions of Hydrocarbons PINESAND LUKEA. SCHAAP HERMAN The Use of X-Ray and K-Absorption Edges in the Study of Catalytically Active Solids ROBERTA. VANNORDSTRAND The Electron Theory of Catalysis on Semiconductors TH. WOLKENSTEIN Molecular Specificity in Physical Adsorption D. J. C. YATES

Proceedings of the International Congress on Catalysis, Philadelphia, Pennsylvania, 1956 Volume 10 The Infrared Spectra of Adsorbed Molecules R. P. EISCHENS AND W. A. PLISKIN The Influence of Crystal Face in Catalysis ALLANT. GWATHMEY AND ROBERTE. CUNNINGHAM The Nature of Active Centres and the Kinetics of Catalytic Dehydrogenation A. A. BALANDIN The Structure of the Active Surface of Cholinesterases and the Mechanism of Their Catalytic Action in Ester Hydrolysis F. BERGMANN Commercial Alkylation of Paraffins and Aromatics EDWINK . JONES The Reactivity of Oxide Surfaces E. R. S. WINTER The Structure and Activity of Metal-onSilica Catalysts G. C. A. SCHUITA N D L. L. VAN REIJEN Volume 11 The Kinetics of the Stereospecific Polymerization of cl-Olefins G. NATTAAND I. PASQUON Surface Potentials and Adsorption Process on Metals R. V. CULVER AND F. C. TOMPKINS

Volume 12

Volume 13 Chemisorption and Catalysis on Metallic Oxides F. S. STONE Radiation Catalysis R. COEKELBERGS, A. CRUCQ, AND A. FRENNET Polyfunctional Heterogeneous Catalysis PAULB. WEISZ A New Electron Diffraction Technique, Potentially Applicable to Research in Catalysis L. H. GERMER The Structure and Analysis of Complex Reaction Systems JAMESWEI AND CHARLFB D. PRATER Catalytic Effect in Isocyanate Reactions A. FARKAS AND G. A. MILLS

CONTENTS OF PREVIOUS VOLUMES Volume 14 Quantum Conversion in Chloroplasts MELVIN CALVIN The Catalytic Decomposition of Formic Acid P. MARS, J. J. F. SCHOLLEN,AND P. ZWIETERING Application of Spectrophotometry to the Study of Catalytic Systems H. P. LEFTINAND M. c. HOsSON, JR. Hydrogenation of Pyridines and Quinolines MORRISFREIFELDER Modem Methods in Surface Kinetics: Flash, Desorption, Field Emission Microscopy, and Ultrahigh Vacuum Techniques GERTEHRLICH Catalytic Oxidation of Hydrocarbons L. YA. MARCOLIS Volume 15 The Atomization of Diatomic Molecules by Metals D. BRENNAN The Clean Single-Crystal-Surface Approach to Surface Reactions N. E. FARNSWORTH Adsorption Measurements during Surface Catalysis KENZITAMARU The Mechanism of the Hydrogenation of Unsaturated Hydrocarbons on Transition Metal Catalysts G. C. BONDAND P. B. WELLS Electronic Spectroscopy of Adsorbed Gas Molecules A. TERENIN The Catalysis of Isotopic Exchange in Molecular Oxygen G. K. BORESKOV Volume 16 The Homogeneous Catalytic Isomerization of Olefins by Transition Metal Complexes MILTONORCHIN The Mechanism of Dehydration of Alcohols over Alumina Catalysts PIN= AND JWST MANASSEN HERMAN n Complex Adsorption in Hydrogen Ex-

417

change on Group VIII Transition Metal Catalysts J. L. GARNETTAND W. A. SOLLICHBAUMGARTNER Stereochemistry and the Mechanism of Hydrogenation of Unsaturated Hydrocarbons SAMUEL SIECEL Chemical Identification of Surface Groups H. P. BOEHM Volume 17 On the Theory of Heterogeneous Catalysis NAKAMURA JUROHORIUTIAND TAKASHI Linear Correlations of Substrate Reactivity in Heterogeneous Catalytic Reactions M. KRAUS Application of a Temperature-Programmed Desorption Technique to Catalyst Studies R. J. CVETANOVIC AND Y. AMENOMIYA Catalytic Oxidation of Olefins R. ADAMS HERVEY H. VOGEAND CHARLES The Physical-Chemical Properties of Chromia-Alumina Catalysts CHARLES P. POOLE,JR. AND D. S. MACIVER Catalytic Activity and Acidic Property of Solid Metal Sulfates Kozo TANABE AND TSUNEICHI TAKESHITA Electrocatalysis S. SRINIVASEN, H. WROBLOWA, AND J. O’M. BOCKRIS Volume 18 Stereochemistry and Mechanism of Hydrogenation of Napthalenes in Transition Metal Catalysts and Conformational Analysis of the Products A. W. WEITKAMP The Effects of Ionizing Radiation on Solid Catalysts ELLISON H. TAYLOR Organic Catalysis over Crystalline Aluminosilicates P. B. VENUTOAND P. S. LANDS On the Transition Metal-Catalyzed Reactions of Norbornadiene and the Concept of n Complex Multicenter Processes G. N. SCHRAUZER

41 8

CONTENTS OF PREVIOUS VOLUMES

Volume 19

Volume 21

Modern State of the Multiplet Theory of Heterogeneous Catalysis A. A. BALANDIN The Polymerization of Olefins by Ziegler Catalysts M. N. BERGER,G. BOOCOCK, AND R. N. HAWARR Dynamic Methods for Characterization of Adsorptive Properties of Solid Catalysts L. POLINSKI AND L. NAPHTALI Enhanced Reactivity at Dislocations in Solids J. M. THOMAS

Kinetics of Adsorption and Desorption and the Elovich Equation AND F. C. TOMPKINS C. AHARONI Carbon Monoxide Adsorption on the Transition Metals R. R. FORD Discovery of Surface Phases by Low Energy Electron Diffraction (LEED) JOHNW. MAY Sorption, Diffusion, and Catalytic Reaction in Zeolites L. RIEKERT Adsorbed Atomic Species as Intermediates in Heterogeneous Catalysis CARLWAGNER

Volume 20 Chemisorptive and Catalytic Behavior of Chromia ROBERT L. BURWELL, JR., GARYL. HALLER, AND JOHNF. READ KATHLEEN C. TAYLOR, Correlation among Methods of Preparation of Solid Catalysts, Their Structures, and Catalytic Activity KIYOSHIMORIKAWA, TAKAYASUSHIKASAKI,A N D MASAHIDE OKADA Catalytic Research on Zeolites J. TURKEVICH A N D Y. ONO Catalysis by Supported Metals M. BOUDART Carbon Monoxide Oxidation and Related Reactions on a Highly Divided Nickel Oxide P. C. GRAVELLE AND S. J. TEICHNER Acid-Catalyzed Isomerization of Bicyclic Olefins JEAN EUGENEGERMAINA N D MICHEL BLANCHARD Molecular Orbital Symmetry Conservation in Transition Metal Catalysis FRANK D. MANGO Catalysis by Electron Donor-Acceptor Complexes KENZITAMARU Catalysis and Inhibition in Solutions of Synthetic Polymers and in Mieellar Solutions H. MORAWETZ Catalytic Activities of Thermal Polyanhydroa-Amino Acids DUANE L. ROHLFING A N D SIDNEY W. FOX

Volume 22 Hydrogenation and Isomerization over Zinc Oxide R. J. KOKESAND A. L. DENT Chemisorption Complexes and Their Role in Catalytic Reactions on Transition Metals 2. KNOR Influence of Metal Particle Size in Nickel-onAerosil Catalysts on Surface Site Distribution, Catalytic Activity, and Selectivity R. VANHARDEVELD AND F. HARTOG Adsorption and Catalysis on Evaporated Alloy Films R. L. MOSSAND L. WHALLEY Heat-Flow Microcalorimetry and Its Application to Heterogeneous Catalysis P. C. GRAVELLE Electron Spin Resonance in Catalysis JACK H. LUNSFORD Volume 23 Metal Catalyzed Skeletal Reactions of Hydrocarbons J. R. ANVERSON Specificity in Catalytic Hydrogenolysis by Metals J. H. SINFELT The Chemisorption of Benzene R. B. MOYESAND P. B. WELLS The Electronic Theory of Photocatalytic Reactions on Semiconductors TH. WOLKENSTEIN

CONTENTS OF PREVIOUS VOLUMES Cycloamyloses as Catalysts DAVID W. GRIFFITHSAND MYRONL. BENDER Pi and Sigma Transition Metal Carbon Compounds as Catalysts for the Polymerization of Vinyl Monomers and Olefins D. G. H. BALLARD

Volume 24 Kinetics of Coupled Heterogeneous Catalytic Reactions L. B E R ~ N E K Catalysis for Motor Vehicle Emissions JAMES WE1 The Metathesis of Unsaturated Hydrocarbons Catalyzed by Transition Metal Compounds J. C. MOLAND J. A. MOULIJN One-Component Catalysts for Polymerization of Olefins AND V. ZAKHAROV Yu. YERMAKOV The Economics uf Catalytic Processes J. DEWINGAND D. S. DAVIES Catalytic Reactivity of Hydrogen on Palladium and Nickel Hydride Phases W. PALCZEWSKA Laser Raman Spectroscopy and Its Application to the Study of Adsorbed Species R. P. COONEY, G . CURTHOYS, AND NGUYEN THETAM

A

8 7 C D

8 9

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6 2 H 3 1 4 1 5

419

Analysis of Thermal Desorption Data for Adsorption Studies MILOS SMUTEK, SLAVOJ CERN?, AND FRANT~SEK BUZEK

Volume 25 Application of Molecular Orbital Theory to Catalysis ROGERC. BAETZOLD The Stereochemistry of Hydrogenation of UJ-Unsaturated Ketones ROBERTL. AUGUSTINE Asymmetric Homogeneous Hydrogenation J. D. MORRISON,W. F. MASLER,AND M. K. NEUBERG Stereochemical Approaches to Mechanisms of Hydrocarbon Reactions on Metal Catalysts J. K. A. CLARKE AND J. J. ROONEY Specific Poisoning and Characterization of Catalytically Active Oxide Surfaces HELMUT KNOZINGER Metal-Catalyzed Oxidations of Organic Compounds in the Liquid Phase: A Mechanistic Approach AND JAY K. KOCHI ROGERA. SHELDON

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