ADVANCES IN CATALYSIS VOLUME 10, Volume 10

ADVANCES IN CATALYSIS AND RELATED SUBJECTS

VOLUME X

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ADVANCES IN CATALYSIS AND RELATED SUBJECTS VOLUME X

D. D. ELEY Nottingham, England

EDITED BY W. G. FRANKENBURG Lancaster, Pa.

V. I. KOMAREWSBY Chicago, I l l .

ASSOCIATE EDITOR PAULB. WEISZ Paulsboro, N . J .

ADVISORY BOARD PETERJ. DEBYE Ithaca, N . Y .

W. JOST Gottingen, Germany

P. H. EMMETT Baltimore, Md, E. K . RIDEAL London, England

W. E. GARNER Bristol, England P. W. SELWOOD Evanston, I l l .

H. S . TAYLOR Princeton, N . J.

1958

ACADEMIC PRESS INC., PUBLISHERS NEW YORK AND LONDON

COPYRIQHT 0 1958, BY ACADEMIC PRESS, INC. 111 FIFTHAVENUE,NEWYORK3, N. Y. ACADEMIC PRESS INC. (London) Ltd., Publishers 40 Pall Mall, London 5. W. 1

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MICROFILM, OR ANY OTHER MEANS WITHOUT WRITTEN PE:RMISSION

FROM THE PUBLISHERS.

Library of Congress Catalog Card Number: 49-775!5

PRINTED I N T H E UNITED STATES OF AMERICA

CONTRIBUTORS TO VOLUME X A. A. BALANDIN, N . D. Zelinskii Institute of Organic Chemistry, U.S.S.R. Academy of Sciences, and Moscow State University, Moscow, U.S.S.R. F. BERGMANN,Department of Pharmacology, The Hebrew University, Hadassah Medical School, Jerusalem, Israel

ROBERTE. CUNNINGHAM, Department of Chemistry, University of Virginia, Charlottesville, Virginia

R. P. EISCHENS, Texaco Research Center, Beacon, N e w York ALLANT. GWATHMEY, Department of Chemistry, University of Virginia, Charlottesville, Virginia

EDWINK. ,JONES, Universal Oil Products Co., Des Plaines, Illinois

W. A. PLISKIN,Texaco Research Center, Beacon, N e w York G. C. A. SCHUIT, KoninklijkelShell-Laboratorium, Amsterdam, Netherlands L. L. VAN REIJEN,KoninklijkelShell-Laboratorium, Amsterdam, Netherlands

E. R. S . WINTER,J o h n & E. Sturge Ltd., Birmingham, England

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Preface The publication of this volume takes place in the shadow of the sudden and unexpected loss, to the Advances in Catalysis, of two of its prominent founders, and, to the world, of two beloved members of the community of scientists, Dr. W. G. Frankenburg and Dr. V. I. Komarewsky. It seems appropriate to turn back to the Preface of Volume I of the Advances, and to examine the views and hopes which the editors expressed one decade ago concerning the status of the catalytic science and the role which they visualized for the then new-born publication. I n viewing the scope of knowledge concerning catalytic phenomena they noted the dominance of empirical method in catalysis, and expressed the view that “a science of catalysis has to be erected on foundations which still have to be laid.” They visualized the Advances as serving as a link and point of concentration of some of the most significant developments in knowledge of catalysis which then was ‘(scattered throughout various journals and handbooks, covering the range from theoretical physics to descriptions of industrial plants.” It would be folly t o assert that the ten years which have passed have seen the displacement of empiricism by rigor of scientific method. However, we feel that we have witnessed a vigorous movement toward a search of basic principles, toward extraction of essential knowledge from the disciplines of chemistry and physics for the laying of the foundation which our founding Editors anticipated. In this process, catalysis is coming to be recognized as one of the most challenging bridges spanning the borderline area of not one but a number of our “sciences” as classified by such conventional terms as physics, chemistry, biology, etc. The inquiry into phenomena and mechanisms of catalysis is truly unfolding as an inquiry into the fundamentals of nature, a t a level where we are a t various times called upon t o engage the knowledge, the skills, and the laws taught in chemistry as well as physics, the logic and the art of mathematics, and the analogies and experiences of biology. In the Advances in Catalysis, the editors have strived to bring together the diverse concepts, investigations, and studies which it is hoped will form an integrated picture of current knowledge in the field of catalysis: The advancement of catalysis as a science has been-and still is-dependent on the development of appropriate and deep reaching tools and methods for probing the catalytically active materials, such as the magnetic susceptibility method, field emission microscopy, or the crystal face inspection vii

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PREFACE

method and infrared surface techniques reviewed in this volume. Catalysis as a science must develop intimate familiarity with the chemistry and the physics of solids at the surface of which the catalytic chemist and the solid state physicist are beginning to have encounters; Volume VII was to a large part devoted to this arena. These meetings will undoubtedly become more frequent and more specific in the future. Kinetics studies remain important means for deriving mechanistic steps by practicing mathematical logic on observations of catalytic behavior. I n this connection we ase glad to have, in this volume, the opportunity to present a picture of extensive kinetic studies of the Russian catalytic investigators, in a review by Professor A. A. Balandin, which is reproduced in its original form. We find it gratifying that we can state our present and continued aims for the Advances by way of much direct quotation from the first Preface of our founding Editors : The Advances shall present “contributions from all parts of the world as far as the (present) world situation permits,” “. . . contributions from scientific and industrial workers . . . ,” not only “on the specific materials and products for which catalytic reactions have been developed, but . . . on new scientific theories and methods which promise to become valuable for a better understanding of catalytic phenomena.” We hope that the Advances have served and will continue to serve a most important function in the development of a science: The integration of knowledge, from various facets of catalytic and related study, from the various scientific disciplines, and from many prominent laboratories and scientists of our globe. We wish to acknowledge the aid and cooperation of Dr. Charles H. Riesz of the Armour Research Foundation and Dr. David Miller of the Argonne National Laboratory in connection with the preparation of this volume. I).D. ELEY F’. B. WEISZ April, 1968

Walter G. Frankenburg 1893-1957 Walter G. Frankenburg was born on September 7, 1893 in the old German city of Nuremberg, and died on July 4, 1957, in Lancaster, Pennsylvania. He grew up in cultivated surroundings. His father was the founder and director of the Viktoria Werke in Nuremberg and was greatly interested in the arts and sciences. In 1912 Frankenburg terminated his college education with the “Abitur” from the Maximilian Gymnasium in Munich. Then he enlisted in the Bavarian Army and was commissioned as an officer after one year of service. He then started his studies a t the University of Munich. But in the first days of World War I he was called back to active duty in his field artillery regiment. He had a very good military record, participated in 30 skirmishes and battles, was wounded and decorated with the Iron Cross first class. After the end of the war he continued his studies a t the University of Munich and in 1922 obtained the Ph.D. degree summa cuni laude with a thesis on “I. Ueber die Besetzungsdichte bei der Adsorption von SilberIonen an Bromsilber. 11. Ueber die spektrale Empfindlichkeit des Bromsilbers und ihre Beeinflussung durch adsorbierte Stoffe.” Soon afterward Frankenburg joined the staff of the Research Laboratory Oppau of the Badische Anilin- & Soda-Fabrik in Ludwigshafen/Rhein (later part of the I. G. Farbenindustrie AG.). In this position he followed his interest in photochemical reactions, a field he was familiar with from his doctoral thesis. But very soon he started fundamental investigations on ammonia catalysis, a topic which, in those times, was of highest significance in Oppau where the first technical ammonia synthesis was carried out. The way he studied this problem is characteristic of Frankenburg’s way of thinking. Whereas the effort of another group of chemists was still directed to improvements of the technical process and the catalyst itself in a more empirical way, i.e., the way which had led to great technical SUCCASS, Frankenburg attacked the basic problem of the catalytic mechanism in a purely fundamental manner. He studied, e.g., the reaction between Nz and atomic Fe and W, between Nz and Li, the adsorption of Hz on W, the interaction between Nz and W and found that “surface nitrides” are formed as intermediates a t the NHB-catalysis on W-catalysts. These “nitrides” which he identified as chemisorbed Nz molecules, react slowly with hydrogen under formation of other intermediates. These basic investigations of Frankenburg were vigorously supported by men like Bosch and Mittasch, and Frankenburg soon was in the position to work as a group leader with a staff of excellent collaborators. Thus, he was able a t the same time to ix

X

WALTER G . FRANKENBURG

proceed with his studies on photochemical reactions. At a time when the Hz was transferred to a techrlical stage synthesis of methanol from CO in Oppau, he investigated the reactioii between Hz arid CO in the presence of optically excited Hg-atoms, another striking example of his deep interest in basic research. But he also studied problenis such as the formation of erythema of the skin by sunlight and developed a very simple photochemical device to measure the physiologically active radiation. Finally, Carl Bosch asked him to develop an improved process of color photography, including the process of making colored prints. In spite of the fact that this field of applied photochemistry was not closely related to Frankenburg’s original work on photochemical problems, he worked very successfully on this problem. He interrupted this work when he decided to immigrate to America, but his ideas and results were stimulating to the work carried out in the same field in the AGFA which finally led to the process used st present. Besides letding his group in the large research laboratoiry a t Oppau where, of course, many investigations were carried out which did not result in scientific publications, he found the time for literary activity: he wrote a book on “Katalytische Umsetzungen in homogenen und enzymatischen Systemen” (Akademische Verlagsgesellschaft 1937). I n this volume all the pertinent literature of that time was discussed, arid in its introduction he excellently presented the status quo of catalytic knowledge. After his immigration he continued his connection with the I. G. Farbenindustrie A.G. as consultant for the General Aniline and Film Corporation and worked for about two years in the Department of Chemical Engineering of the Johns Hopkins University. The result of this period was a n important paper published in the Journal of the American Chemical Society, Vol. 66, entitled T h e Adsorption of Hydrogen on Tungsten.” On one of his trips he met, in Cuba, B. G. Meyer, then President of the General Cigar Co., who offered him a position as research consultant to the cigar manufacturer, and in 1942 Frankenburg became a full-time chemist. There followed a highly successful period in his career. Besides organizing the Advances in Catalysis (togetheie with V. I. Komarewsky and Sir Eric Rideal), he established a Research Laboratory in Lancaster, Pennsylvania, realizing that since literature pertaining to the cigar industry was scanty and methods were mainly empirical he would have to inquire continuously into every step of cigar making. He sought a chemical explanation of the problem of why crops responded differently to fermentation. After finding out part of the chemical conversion that occurred in leaf tobacco during fermentation, he sought a catalyst to accelerate and improve this process. The first indication for the presence of alkaloid transformation products in the fermented leaves was

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WALTER G. FRANKENBURG

xi

observed in the form of certain anomalies in the course of the analytical work. Frankenburg and his collaborators succeeded in isolating and identifying : oxynicotine, nicotinic acid, methyl-3-pyridyl ketone and 2,3’dipyridyl. The fractionation of the nicotinic breakdown products was accomplished by the use of ultraviolet spectroscopy. More recent work of his laboratory has emphasized the chemical nature of the presumed complexes between amino acids and oxidized polyphenols and the compositional changes of tobacco leaf proteins during fermentation. The majority of the fundamental results of these investigations was deposited in the Archives of Biochemistry and Biophysics and in the Journal of the American Chemical Society. After having tested over 300 catalytically active substances, he discovered one which acted as a “sweat promotor” and caused cigars to have a milder taste. Studies of shadegrown wrapper tobacco helped in finding a new tobacco leaf. His most notable contribution to the tobacco industry was the development of homogenized tobacco leaf, which was announced by General Cigar Company in 1954. Cigar binder sheet was made by him by pulverizing leaf tobacco and re-forming it into sheets. From 1943 to 1948 he was consultant to Hydrocarbon Research, Inc., and from 1948 until his death a consultant to the Socony-Vacuum laboratories in connection with the synthesis of gasoline and the cracking and re-forming of petroleum by catalytic processes. His research activities attracted the attention of scientific workers in plant chemistry throughout the world. Dr. Frankenburg stimulated discussion and exchange of ideas among specialists which resulted in the annual Tobacco Chemists Conference. Within the space of three decades major but unrelated advances in the understanding of catalysis and tobacco fermentation have come from one single laboratory. Frankenburg was a man of encyclopedic knowledge, a pioneer in the literal sense and a pathfinder; he was an excellent lecturer. Withal he was modest arid endowed with human qualities beyond comparison. R. BRILL F. F. Norzu

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VASILIILYICH KOMAREWSKY 1895-1957

It is inevitable that in an established area of chemistry such as catalysis, the time must come when the pioneers must leave us one by one. In the case of Vasili Ilyich Komarewsky, his untimely death on June 21, 1957, came as a shock; it is difficult to think of him other than as an extremely active, young man even though he had passed his 62nd year. Komarewsky was, above all, a brilliant teacher who well deserved the title of Professor. He had a unique ability to sift through a bulk of facts and theories and then to propose an exceedingly logical presentation which made a lasting impression. As a research investigator, he had a marvelous intuitive sense which overlooked the daily problems of the laboratory, but which quickly recognized an important experimental finding. As a man, he had a keen sense of humor, a love of outdoor sports, and an appreciation of the simple things of life which made him a delightful companion. Professor Komarewsky was born in Moscow in 1895. With characteristic acceptance of the vicissitudes of fate, he let neither service in a bomber squadron in the Imperial Russian Navy nor the turmoil of a bloody social revolution interrupt his early scientific education and development in Russia and Germany. He was particularly fortunate in his associations with two of the great masters of catalysis. First, as a pupil of N. D. Zelinsky, he received an invaluable indoctrination in catalysis, particularly in regard to dehydrogenation and low-pressure reactions. Second, as a fellow worker with V. N. Ipatieff, he became familiar with high-pressure techniques applied to catalytic reactions. In 1931, he came to the United States, where he made important contributions to catalytic reactions which literally revolutionized the petroleum refining industry and laid the foundation for a synthetic rubber industry. In connection with the reactions of polymerization, alkylation, dehydrogenation, reforming, and cracking, his name will be found as a frequent contributor. From 1936 until his death, he was a Professor a t the Illinois Institute of Technology. His Catalysis Laboratory, the first established a t a university in the United States to devote its effort solely to catalysis, became an important center for catalytic research. As a founder and an editor of Advances in Catalysis, he had a firm hand in establishing principles for consolidating, critically examining, and elucidating the wealth of catalytic knowledge which has built up and which continues to grow, seemingly, without limit. With the passing of Professor Komarewsky, we are losing a standard bearer, who has carried the torch of Catalysis high and in transmitting it to others, his example will serve to inspire US all. CHARLES RIESZ

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CONTENTS CONTRIBUTORS TO VOLUME X................................................. v PREFACE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .vii 1893-1957,. ........................................ ix WALTERG. FRANKENBURG VASILIILYICH KOMAREWSKY ................................................ xiii The Infrared Spectra of Adsorbed Molecules

BY R. P. EISCHENSA N D W. A. PLISKIN I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Chemisorption on Supported-Metal Samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . 111. Chemisorption on Acidic Oxides.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. Spectra of Adsorbed Water and Surface Hydroxyl Groups on Nonacidic Oxides.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Infrared Spectra of Physically Adsorbed Molecules.. . . . . . . . . . . . . . . . . . . . VI. The Pressed-Salt Method for Obtaining Spectra of Adsorbed Molecules.. VII. Spectra of Molecules Adsorbed on Unsupported Metals.. . . . . . . . . . . . . . . . VIII. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . .................................

2 2 27 29 41 45 49 53 54

The Influence of Crystal Face in Catalysis BY ALLANT. GWATHMEY A N D ROBERTE. CUNNINGHAM

I. Introduction.. . . . . . . . . . . .......................................... eveloping a Theory of the Influence of Face. 11. Factors t o be Considered 111. The Single-Crystal Method of Studying Surface Reactions.. . . . . . . IV. Results:. . . . . . ... .......................... V. Discussion.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

57 60 63 69 91 94

The Nature of Active Centers and the Kinetics of Catalytic Dehydrogenation BYA. A. BALANDIN

I. Quasi-Homogeneous Surfaces. ....................... 11. Flow Method Kinetics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111. Experimental Results on Dehydrogenation. . . . . ..................... IV. Discussion of Results and the Multiplet Theory. . . . . . . . . . . . .

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References.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

126

The Structure of the Active Surface of Cholinesterases and the Mechanism of Their Catalytic Action in Ester Hydrolysis By F. BERGMANN

I. Introduction ........................................................... 11. Enzyme Sources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111. Methods.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. Catalytic Effects of Cholinesterases . . . . . . . . . . . V. Mechanism of Hydrolysis by Cholinesterases . . . . . . . . . . . . . . . . . . . . . . . . . . . VI. Structure of the Esteratic S i t e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII. The Anionic Sites in the Active Centers of Cholinesterases. . . . . . . . . . . . . VIII. Concluding Remarks. ....... .......... References. . . . . . . . . . . . . . . . . . . . . . . . .......... xv

131 131 136 139 147 161 162

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CONTENTS

Commerical Alkylation of Paraffins and Aromatics BY EDWIN K. JONES I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Sources of Feed ................................ 111. Basic Factors in ................................. IV. Alkylation Reac ................................. V. Alkylation Procemes. . . . . . . . . . . . . . . . . . . . ........ ........... VI. Materials of Construction. . . . . . . . . . . . . . . ....................... VII. Future Outlook for Alkylation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . .: . . . . . . . . . . . . . . . ....... ...

165 I66 170 171 188 193 195 195

The Reactivity of Oxide Surfaces BY E. R. S. WINTER

I. Introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Experimental Method.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111. Exchange Kinetics. . . . . . . . . . . . . . ...................... IV. Interaction between Oxygen and de Surfaces.. . . . . . . . . . V. Exchange Reactions of CO and C O S . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 VI. Application t o Catalytic Reactions.. . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 The Structure and Activity of Metal-on-Silica Catalysts BY G. C. A. SCHUITA N D L. L. VAN REIJEN I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 ..................... 244 11. Texture and Structure of Ni-SiOl Catal sts 111. The Adsorption Bond and the Statistical hermodynamics ' of the Adsorption Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 IV. The Catalytic Activity of Ni-SiO2 Catalysts.. . . . . . . V. Some Properties of Other Metal-SiOz Catalysts.. . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

?

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 AuTnoR INDEX... . . . . . . . SUBJECTINDEX.. .............................. ...... . . . . . . . . 325

The Infrared Spectra of Adsorbed Molecules R . P. EISCHENS

AND

W . A . PLISKIN

Texaco Research Center. Beacon. New York

I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Chemisorption on Supported-Metal Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 . Experimental Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 . Hydrocarbon Molecules on Nickel a . Spectrum of Chemisorbed Ethylene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b . Spectrum of Chemisorbed Ethane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . c . Spectrum of Chemisorbed Acetylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . d . Polymerization of Chemisorbed Hydrocarbons . . . . . . . . . . . . . . . . . . . . . e . Double-Bond Isomerization in Chemisorbed Olefins . . . . . . . . . . . . . . . f . Hydrogenation of Chemisorbed Higher Olefins . . . . . . . . . . . . . . . . . . . . 3 . Carbon Monoxide Chemisorption a . Spectra of Chemisorbed CO on Palladium and Platinum . . . . . . . . . . . b . Effect of Carrier on the Spectrum of Chemisorbed CO . . . . . . . . . . . . . c . Spectrum of CO Chemisorbed on Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . d . Spectrum of CO Chemisorbed on Copper . . . . . . . . . . . . . . . . . . . . . . . . . . e . Effect of Hydrogen on Chemisorbed CO . . . . . . . . . . . . . . . . . . . f . Effect of Other Gases on Chemisorbed CO . . . . . . . . . . . . . . . . . . . . . . . . g . Spectrum of CO Chemisorbed on Rhodium., . . . . . . . . . . . . . . . . . . . . . . 4. Catalyzed Oxidation of Carbon Monoxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Failure t o Observe a Band Due t o Chemisorbed Hydrogen . . . . . . . . . . . 111. Chemisorption on Acidic Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . Ammonia Chemisorbed on Silica-Alumina Catalysts . . . . . . . . . . . . . . . . . . 2 . Ammonia Chemisorbed on 7-Alumina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV . Spectra of Adsorbed Water and Surface Hydroxyl Groups on Nonacidic Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . Stability of Surface Hydroxyl Groups ............. .... 2 . Effect of Adsorbed Molecules o n the Surface Hydroxyl Groups . . . . . . . V. Infrared Spectra of Physically Adsorbed Molecules . . . . . . . . . . . . . . . . . . . . . VI . The Pressed-Salt Method for Obtaining Spectra of Adsorbed Molecules . . 1 . Advantages and Disadvantages of the Pressed-Salt Method . . . . . . . . . . 2 . Study of the Metal-Carbon Bond of Carbon Monoxide Chemisorbed on Platinum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V I I . Spectra of Molecules Adsorbed on Unsupported Metals . . . . . . . . . . . . . . . . . 1 . Spectrum of Carbon Monoxide on Evaporated Platinum Films . . . . . . . 2 . Emission Spectra of Chemisorbed Molecules . . . . . . . . . . . . . . . . . . . . . . . . . 3 . Reflection Spectra of Chemisorbed Molecules . . . . . . . . . . . . . . . . . . . . . . . . V I I I . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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R . P. EISCHENS AND W. A. PLISKIN

I. INTRODUCTION A large variety of problems related to the nature of the adsorption processes have been studied by infrared spectroscopy. The most extensive and productive application of this method has been in studies of chemisorption on supported-metal samples. Spectra of physically adsorbed molecules have provided important information on the interaction of these molecules with the surface of the adsorbent. Experimental developments have reached a state where it is evident that the infrared techniques are adaptable to practically all types of samples which are of interest to catalytic chemists. Not only are the infrared tevhniques applicable to studies of chemisorption and physical adsorption systems but they add depth and preciseness to the definitions of these terms. Interpretation of the spectra of chemisorbed moleculec; presents some difficulties berause the surface compounds formed during chemisorption have no exact counterparts among conventioiial compounds. Although some general principles can be applied, interpretations of the spectra of unknown species are usually based on empirical comparison with ispectra of compounds of known structure. Experience has shown, however, that these difficulties are more philosophical than practical. Interpretations of spectra of chemisorbed molecules by comparison with the spectra of compounds of known structure have produced results which are self-consistent and reasonable in a wide range of applications. 11. CHEMISORPTION ON SUPPOI~TED-METAL SAMPLES 1. Experimental Considerations

The details of the sample preparation and studies of the nature of the supported-metal samples have been described in a paper dealing with the effect of surface coverage on the spectra of carbon monoxide chemisorhed on platinum, nickel, and palladium ( I ) . The samples consist of small particles of metal dispersed on a nonporous silica which is produced commercially under the names Cabosil* or Aerosi1.t This type of silica is suitable as a support because it is relatively inert and has a small particle size (150-200 A.). Thesmall particle sizeis important because it reduces the amount of radiation which is lost by scattering. A nonporoiis small particle form of gamma-alumina, known as Alon-C, is also available.* This material is not so inert as the silica and will react with gases such as CO and COZ a t elevated temperatures. The concentration of metal in the reduced sample is usually 9.2 wt. %. Higher concentrations have been tried. It appears, however, that the ad* 0btainat)le from Godfrey L. Cabot, Inc., Boston, Mavsachuse t t s .

t Obtainable from Ileutsche Gold- und Silber Scheideanstalt, Frankfurt a.M.

INFRARED SPECTRA OF ADSORBED MOLECULES

3

vantages of going to higher metal concentrations are vitiated by increased difficulty in obtaining a small metal-particle size. The particle sizes of the metal are in the 50-100-A. range in satisfactory samples. The metal-particle size is extremely important. If the metal particles are large (above 300 A.) an excessive amount of radiation is lost by absorption. The infrared studies of adsorbed molecules can be carried out most efficiently in an in situ cell, which makes it possible to reduce the sample, treat it with gases, and observe the spectra of the adsorbed molecules in a single apparatus. A schematic diagram of a cell of this type is shown in Fig. 1. The sample is supported on a salt plate which is suspended in a tungsten-wound quartz furnace. This type of cell can easily attain temperatures above 500" C. The ability to attain such temperatures is desirable to facilitate reduction and drying and because many of the carbon deposition reactions require temperatures in this range. A simpler cell may be made by tapering the outer wall inward and placing the windings on the outside. This eliminates the internal furnace but is not satisfactory for high temperatures unless the entire cell is made of quartz. In all of the spectral studies of gases chemisorbed on supported metals, a background spectrum has been recorded prior to the chemisorption of the gas. The spectrum of the chemisorbed gas is then obtained by subtracting out the background by means of an automatic per cent transmission recorder. The background contains bands due to the surface hydroxyls and Si-0 or A1-0 bands of the carrier material. It is necessary that both the

SOURCE

GAS

QUARTZ FURNACE

't---,YI --

FIG.1. Cell for infrared ytudy of chemisorbed gases.

4

R. P. EISCHENS AND W. A. PLISKIN

background and the final spectrum be obtained a t the same temperature, since the background changes with temperature. In general the carrier bands are shifted to longer wavelengths and the over-all transmission is decreased as the temperature is increased. There is a general decrease in transmission when molecules are physically or chemically adsorbed on the metal or carrier components of the samples. This change is attributed to an increase in scattering caused by changes in the optical properties a t the solid-gas interface. The inagnitude of this decrease is fairly uniform over the spectral region investigated, and the shift does not cause serious difficulties unless attempts are made to observe broad shallow absorption bands. This decrease in transmission explains why spectra such as those in Fig. 2 appear to be based on a background which is displaced from the 100% transmission level. 2. Hydrocarbon Molecules on Nickel

a. Spectrum of Chemisorbed Ethylene. Because of the large number of simple hydrocarbons of known structure and the degree of flexibility afforded by the posihility of substitution of deuterium and other elements, the study of adsorbed hydrocarbons offers a fascinating field of application of the infrared method. The question of whether ethylene chemisorption is associative, i.e., involves opening of the double bond to form single bonds between each carbon and the surface and leaviing a single bond between the carbon atoms, or dissociative, i.e., accompanied by breakage of C-H bonds, has long been of interest to catalytic chemists. The infrared spectra of ethylene chemisorbed on nickel (2) show that the adsorption can be either associative or dissociative depending on the experimental conditions used. The important variables are temperature, hydrogen pressure, and the presence or absence of a preadsorbed hydrogen layer. Associative chemisorption of ethylene is obtained a t 35" C. when a preadsorbed layer of hydrogen has been left on the nickel surface. The spectrum of this species is shown as A in Fig. 2. The conclusion that the ethylene chemisorption is associative in this case is based on the fact that the C-H stretching bands are mainly in the 3.4-3.5-p region, whiclh is characteristic of C-H bonds when the carbon is saturated, and on observation of a C-H deformation band atj 6.91 p. The latter band is due to a wissorslike vibration which changes the H-C-H angle. Its presence shows there are a t least two hydrogens on the carbon. Although this band is weak in A , it is of about the intensity expected on the basis of the intensity of the C-H stretching bands. There is also a small band near 3.3 p in A . This band is attributed to a n olefinic C-H. Even though this band is weak, it cannot be ignored, because it represents more carbons than would be expected from a simple comparison of the integrated intensities of the olefinic and

5

INFRARED SPECTRA OF ADSORBED MOLECULES

3000

2500

FREQUENCY (crn-'1 1500

1400

9oL

Q 70

70

3.5

4.0

6.5

7.0

WAVELENQTH ( P I

FIG.2. ( A ) Spectrum of ethylene chemisorbed on hydrogen-covered nickel; ( B ) after treatment with H t .

paraffinic bands. The olefinic species has fewer hydrogens per carbon and the intensity of olefinic C-H's is about one-third that of paraffinic C-H's. Thus, the intensity of the olefinic band should be multiplied by a factor of 6 when the olefinic and paraffinic bands are compared (3, 4). In the spectrum A the olefinic species is not a major factor. In some cases, however, especially after dehydrogenation of adsorbed ethyl radicals, spectra are obtained which indicate that almost half of the carbons are unsaturated. The conclusion that ethylene chemisorbed on nickel, containing a pre-

**

adsorbed hydrogen layer, is mainly in the associative structure, HzCCHz, does not agree with the conclusions of Jenkins and Rideal ( 5 ) .On the basis of quantitative chemisorption measurements, these workers concluded that ethylene formed a surface complex with a composition of about (CIH1), when ethylene was chemisorbed on either bare or hydrogencovered nickel. As will be discussed later, the infrared spectra clearly show that on Cabosil-supported nickel the same species is not obtained on the bare as on the hydrogen-covered surface. The differences are so apparent that they are independent of any specific interpretation given to the spectra. It is conceivable that the conflict between the infrared conclusions and the chemisorption conclusions is due to differences in experimental methods or type of samples. The experimental results of the chemisorption experiments do not, however, necessarily conflict with the infrared spectra. Jenkins and Rideal found that when ethylene was added to a hydrogencovered nickel film a t 20" C., there were 6 molecules of ethane formed for each 10 molecules of ethylene added. These results fit the equation 10(CnH,)g

+ lONiH

+ 6(CzH,),

+ 3(NinCzH,)+ (Ni&,HZ)

G

It. P. EISCHENS A N 0 W. A . PLISKIN

** This equation would imply three molecules adsorbed as H2CCH2 for each

** **

HCCH. The latter could still be saturated if each carbon were bonded to two nickel atoms. This equation shows 12 C-H

**

**

bonds in H2CCH2 units

for every 2 C-H bonds in non-HZCCH2 units. The above equation shows that the conclusions of Jenkins and Rideal are not the only ones which can be drawn from their data for hydrogencovered nickel and that the infrared conclusions are not necessarily in conflict with their chemisorption results.

**

When the chemisorbed ethylene, HzCCHz, is treated with 4 mm. hydrogen at 35"C., significant changes occur and lead to the spectrum B of Fig. 2. The peak C-H stretching absorption shifts firom 3.46 to 3.40 p. (Higher resolution spectra show that the band referred to as 3.40 is actually composed of two bands of about equal intensity at 3.38 and 3.42 p.) The C-H deformation bands grow more intense and shift from 6.91 to 6.86 p , and a new band appears at 7.25 p . All these changes indicate that

*

the hydrogen treatment produces adsorbed ethyl radicals, CHZCHs. The information on the nature of adsorbed ethylene obtained from A and B are in general agreement with the theory suggested by Horiuti and Polanyi (6)*

In the studies discussed above, the samples were cooled in hydrogen after reduction and the excess hydrogen was removed from the system by evacuation a t 35" C. It is assumed that under these conditions the nickel remains almost completely covered with a monolayer of adsorbed hydrogen. Samples which will be referred to as bare nickel were obtained by evacuation of the hydrogen at 350" C. for 45 hr. prior to cooling to 35" C. When ethylene is adsorbed on bare nickel a t 35" C. or on either bare or hydrogen-covered nickel at 150"C., the intensity of the C--H bands, shown as A of Fig. 3, is small compared with those of the associated chemisorbed ethylene shown in Fig. 2. When the species represented by A is treated with H2 a t 35" C., the band intensities increased as is shown in B of Fig. 3. This behavior shows that A is due to a dissociatively chernisorbed ethylene in which the number of hydrogens per carbon is low (7). The species obtained by dissociative chemisorption will be referred to :LS a surface complex. It is doubtful whether the surface complex has a specific stoichiometric composition. Rather it appears that the carbon-hydrogen ratio will depend on the severity of the dehydrogenation conditions. In some cases it appears that a surface carbide, which has no hydrogens, is obtained. Even in this case the carbons appear to be easily rehydrogenated t o adsorbed alkyl groups.

INFRARED SPECTRA O F ADSORBED MOLECULES

FREQUENCY (cm-9

3100 3000 2900

3.2

3.3

3.4

2800

3.5

WAVELENGTH

3.6

(u)

FIG.3. ( A ) Spectrum of ethylene chemisorbed on bare nickel; ( B ) after treatment with

H2

.

It is apparent that the species, referred to here as a surface complex, is closely related to the “acetylenic complex” observed by Beeck (8).There are two major conceptual differences between the infrared surface complex and Beeck’s acetylenic complex. I t is commonly assumed that the carbons in the acetylenic complex would be unsaturated, although this point was not emphasized by Beeck. The infrared results indicate the carbons are saturated even though the number of hydrogens per carbon is low. It is likely that this saturation is attained by formation of C-C bonds between adsorbed Cz units, by bonding of a carbon atom to two nickel atoms, or by both. The most important difference is in the concept of the activity of the complex. Beeck concluded that the acetylenic complex was inactive because only small amounts of hydrocarbon appeared in the gaseous phase after treatment with hydrogen. The infrared results show that the surface complex can be hydrogenated to adsorbed alkyl radicals at 35” C. The extent of this rehydrogenation depends on the hydrogen pressure. Rehydrogenation is detectable at a pressure of about 2 mm. and is practically complete at 55 atm. There is some indication, but no positive ++ proof, that the HzCCHz species can be hydrogenated a t lower pressures than can the more dehydrogenated species. If the hydrogen

8

R . P. EISCHENS AND W. A . PLISKIN

pressure is lowered to mm., dehydrogenation occurs :and the adsorbed radicals revert to the surface complex. The ease of dehydrogenation appears to depend on the amount of surface which is covered by adsorbed hydrocarbon. If the coverage is high, the dehydrogenation is slow. The adsorbed hydrocarbons can be carried through a series of hydrogenationdehydrogenation cycles. During each cycle a minor amount (about 20 %) is lost from the surface. I n experiments which are limited to studies of the gaseous phase these hydrogenation-dehydrogenation cycles could not be distinguished from a weak chemisorption of hydrogen. Recent work by Selwood (Q), based on changes in the magnetization of nickel during chemisorption of ethylene, indicates that ethylene is associatively adsorbed on bare nickel. He suggests that, the discrepancy between this result and the dissociative chemisorption indicated by the infrared experiments is due to factors such as the relative activity of the sample surfaces and temperature effects caused by the heat of chemisorption. Low-temperature infrared experiments in which et,hylene is studied a t -78" C. are expected to provide evidence on the iinportance of the above factors in determining the course of ethylene chernisorption. b. Spectrum of Chemisorbed Ethane. The study of ethylene chemisorption shows that a preadsorbed layer of hydrogen is of critical importance in producing associative rather than dissociative adsorption. Since chemisorption of ethane can only be dissociative, a preadsorbed hydrogen layer retards ethane chemisorption. No infrared evidence for chemisorbed hydrocarbons was observed when a hydrogen-covered nickel surface was exposed to ethane a t 35" C. When, however, bare nickel was exposed to ethane a t 35" C., a spectrum similar to that of ethylene on bare nickel ( A of Fig. 3) was obtained, and subsequent treatment with hydrogen produced a spectrum similar to B of Fig. 3. c. Spectrum of Chemisorbed Acetylene. When acetylene is chemisorbed on either hydrogen-covered or bare nickel a t 35"C., spectra similar to A of Fig. 4 are observed. This spectrum is identical to that attributed to adsorbed ethyl radicals as B of Fig. 2. Since this result was also obtained on bare nickel, it appeared that the ethyl groups were produced by selfhydrogenation of the acetylene. This was substantiated by the fact that the same spectrum ( A of Fig. 4) was obtained when acetylene was adsorbed on a deuterium-covered surface. This infrared evidence is consistent with the exchange experiments of Douglas and Rabinovitch (10) which indicated t,hat self-hydrogenation might be a factor in acetylene chemisorption. Production of ethyl radicals from acetylene by self-hydrogenation implies that there must be three carbon atoms which have lost hydrogens for each ethyl radical which is formed. Hydrogen treatment of chemisorbed acetylene produces the increase in band intensities expected from hydro-

INFRARED SPECTRA OF ADSORBED MOLECULES 30:O

2700

CM"

lS,OO

14,OO

9

/I

(el

+

6ohr (A)

50

WAVELENGTH

FIG.4. (A) Spectrum of acetylene chemisorbed on nickel; (B) after treatment with H e ,

genation of the surface carbide. This is shown in B of Fig. 4. If the chemisorbed acetylene is treated with deuterium rather than with hydrogen, the original bands of the adsorbed ethyl radical do not decrease and new bands appear at 4.55 and 4.75 p . These bands are due to adsorbed alkyl radicals formed by deuteration of the surface carbide. d. Polymerization of Chemisorbed Hydrocarbons. High resolving power in the C-H stretching region (3.0 to 3.5 p ) makes it possible to determine the relative intensities of the bands a t 3.38 and 3.42 p . This is important because the ratio of these intensities can be used to estimate the relative number of CHZ and CH, groups in adsorbed alkyl radicals. The term "adsorbed alkyl radical" will refer to the species, *(CH2),CH3, where the first CH, is bonded to the surface. In these radicals the 3.38-p band is due to the asymmetric C-H stretching in CH, groups, and the 3.42-p band is due to the asymmetric C-H stretching in the CH2 groups. The

**

CH2 groups in the species HzCCHz absorb a t 3.46 rather than at 3.42 p. The band at 3.49 p in B of Fig. 3 is due to an unresolved combination of CH3 and CHZsymmetric stretching bands. The spectra shown in Figs. 2 and 4 were obtained using a Perkin-Elmer Model 12 spectrometer fitted with a CaFz prism. The CaF2prism is suitable for studies with wavelengths out to 8 p. It is an important advantage to be able to scan this spectral region because of the deformation bands which are observed a t 6.86, 6.91, and 7.25 p . The CaFz prism does not provide good resolution in the C-H stretching region (3.0 to 3.5 p ) , however, because the spectrometer slits have to be opened quite wide (200 p ) in order to get sufficient radiation through the samples. Because its resolv-

10

It. P. EISCHENS AND W. A. PLISKIN

ing power is superior to that of CaF2 in the C-H stretching region, a LiF prism was used in some runs although its range is limited to about 5.0 p. The spectra shown in Fig. 3 were obtained with a LiF prism. The LiF spectrum of the species obtained after hydrogenation of associatively chemisorbed ethylene shows bands a t 3.38 and 3.42 p of equal intensity. In attempting to determine the relative number of CH3 and CH2 groups from these band intensities, one encounters the inherent difficulty of obtaining accurate comparisons with compounds of known structure. If the intensities of the bands of the chemisorbed alkyl radicals are compared with those of ethyl groups in lead tetraethyl, it appears that the CHz/CH3 ratio in the adsorbed species is 0.9. Comparison with ethyl groups in paraffinic hydrocarbons indicates that when the 3.38- and 3.42-p bands are of equal intensity, the CH2/CH3ratio is 1.6. Despite this difficulty, significant information can be obtained from the relative band intensities. In Fig. 3 it can be seen that the intensity of the 3.42-p band in the adsorbed alkyl radical is 1.32 times as intense as the absorption a t 3.38 p . On the basis of absorption intensity measurements, this would indicate a large fraction of :idsorbed butyl groups,

*

.

CH2CHzCH2CH3 The species obtained during the self-hydrogenation of acetylene has a CH?/CH, ratio expected for an adsorbed ethyl radical, but the species obtained by hydrogenation of the surface carbide appears to have three CH2 groups per CH3 These results show that polymerization occurs prior to or during the hydrogenation of the surface complex. Since the surface complex ( A of Fig. 3) appears to be saturated even though the number of hydrogens per carbon is low, it is likely that some polymerization occurs prior to the hydrogen treatment. The tendency toward polymerization decreases with chain length. According to rough estimates, it appears that nearly all of the Cz units in surface complexes are converted to C4units and about half of the Cs units are converted to Cg units. The infrared studies, which show that either ethyl radicals or poly-

.

*

merized species of the type CH?(CH*).CH3 are formed during hydrogenation of adsorbed hydrocarbons, are limited to observation of the fate of a single adsorbed monolayer. They do not necessarily predict products of catalyzed bulk hydrogenations when the amounts of reactant and product are large compared with the amount in a monolayer It is interesting, however, to note that the formation of surface carbide is inherent in acetylene chemisorption and that hydrogenation of this surface carbide produces a polymerized product. The bulk hydrogenation of acetylene also produces significant amounts of polymerized product (11).

I N F R A R E D S P E C T R A O F ADSORBED MOLECULES

11

e. Double-Bond Isornerization in Chernisorbed Olefins. On the simplest basis it would be assumed that the spectra of the chemisorbed double-bond isomers of n-hexene would differ from each other. The position of the double bond would determine the relative number of CH2 and CHX groups in the adsorbed species. For example 1-hexene would be expected to chemi-

**

* *

sorb as H2CCHCH2CH2CHsCH3 , 2-hexene as CH3CHCHCH2CH2CH3,

* *

and 3-hexene as CH&H&HCHCH&H,. The spectra of chemisorbed 3-hexene and 2-hexene would be expected to be similar, since they have the same one-to-one CH2/CH3 ratio. The spectra of chemisorbed 1-hexene, however, would be expected to be markedly different because it would have three CH2’s to one CH, . The latter would produce a relatively strong CH, band at 3.42 p. The spectra of chemisorbed 1-hexene ( A ) ,trans-2-hexene (B),and trans3-hexene (C) in Fig. 5 clearly show that no significant differences can be observed. Therefore the assumed structures discussed above are not corFREQUENCY (cm-l) 3100 3000 2900 2000

A

60

-

l

3.2

.

I

3.3

.

l

.

l

3.4 3.5 WAVELENGTH ( p )

.

l

,

3.6

FIG.5 . Spectra of ( A ) 1-hexene, ( B ) 2-hexene, (C) 3-hexene, and (D)I-butene chemisorbed on nickel.

12

R. P. EISCHENS AND W. A . PLISKIN

rect. Instead, the structure of the adsorbed species is independent of the position of the double bond in the gaseous starting material. In determining the structures of the chemisorbed hexenes, a threecomponent analysis was used in which the intensities a t 3.38, 3.42, and 3.46 p were considered. The later band is attributed mainly to structures

* *

*

*

of the type -CHor -CHCHZ . The CHZin the latter differs from those in an adsorbed alkyl radical in that it is bonded to a carbon which is attached to the surface. The essential results of the three component analysis can be seen qualitatively by inspection of either A , B , or C of Fig. 5. The strong band a t 3.46 p shows that there are a relatively large number

*

* *

or -CHCHz. This indicates that the adof groups of the type -CHsorbed species is not that expected for two-point attachment of either 2-hexene or 3-hexene and that most of the molecules are adsorbed by attachment of three or more carbons to the surface. Similar results were found in studies of the spectra of chemisorbed double-bond isomers of the normal butenes and pentenes. The spectra of chemisorbed 1-butene is identical to that of chemisorbed 2-butene and that of 1-pentene is identical to that of 2-pentene. The three-component analyses for these compounds indicate that three or four carbon atoms are bonded to the surface. Spectrum D of Fig. 5 can represent either 1-butene or 2-butene. The absence of a discrete band a t 3.42 p shows there are relatively few unstrained CHZ groups. The number of CH, groups is also small compared with the number

*

* *

of -CHand --CHCH2 groups, indicating that in some of the chemisorbed butene molecules, all four of the carbon atome are bonded to the surface. The spectra of the adsorbed butenes, pentenes, and hexenes discussed above were obtained by chemisorbing on a hydrogen-covered surface at 35°C. The results show that some dehydrogenation must occur under these conditions, since it is impossible t o get four-point adsorption without having some dissociation. When higher-molecular-weight olefins (or paraffins) are chemisorbed on a bare nickel surface, spectra similar to A of Fig. 3 are obtained, and no distinguishing characteristics are observed. 5. Hydrogenation 05 Chemisorbed Higher Olefins. When chemisorbed butenes, pentenes, and hexenes are treated with hlydrogen a t 35" C., spectra are obtained which show that in all cases the hiydrogenated species

*

is predominantly --CHz(CHz) ,CHa . The spectra of hydrogenated chemisorbed butenes (L4), pentenes ( B ) , and hexenes ( C ) are shown in Fig. 6. These adsorbed alkyl radicals can be dehydrogenated by pumping a t 35" C. The species obtained during chemisorption of acetylene and by hydrogenation of chemisorbed acetylene can not be easily dehydrogenated by pumping. The intensity of the bands obtained after hydrogenating

INFRARED SPECTRA OF ADSORBED MOLECULES

90

13

FREOUENCY (cm-'~ 2900 2800

, 3100 , , SO00

3.2

I

I

35 34 3.8 WAVELENQTH(LI

'I

sa

FIG.6. Spectra of hydrogenated (A) butenes, (B)pentenes, and ( C ) hexenes on nickel.

chemisorbed acetylene indicates the surface coverage obtained by acetylene chemisorption is at least twice as great as obtained with any of the olefins including ethylene. Thus, it appears that the difficulty in dehydrogenation is associated with the extent' of surface coverage rather than with the number of carbons in the adsorbed species. This suggests that it is necessary to have nickel sites which are not occupied by hydrocarbon in order to facilitate the dehydrogenation process. 3. Carbon Monoxide Chemisorption.

a. Spectra of Chemisorbed CO on Palladium and Platinum. Carbon monoxide chemisorbed on Cabosil-supported palladium produces bands, in the 5.2-5.5-p region, which are attributed to CO chemisorbed in the bridged structure ( I ) 0

II

C

/ \

Pd

Pd

14

11. P. EISCHENS AND W. A. PLISKIN

FIG.7. Effect of increasing surface coverage on the spectrum of CO chemisorbed on Pd.

The number of bands which are observed is a function of the amount of CO which is on the palladium surface. This is shown in Fig. 7. In this experiment the CO was added in small doses and the spectrum observed after each addition. Spectrum A shows that a band at 5.45 p is first to appear. In B a second band appears a t 5.3 p . New bands a t 4.85 and 5.2 j~ are observed in the third spectrum C. These bands become more intense in the fourth D and fifth E spectra with the 4.85-j~band showing a greater increase than the 5.2-p band. Although there is some overlapping, it is evident that the longer-wavelength bands are produced first and in some cases may be complete before the shorter-wavelength bands are detectable. Evacuation removes the bands in the reverse order of their appearance. The surface coverages represented by the spectra are ( A ) 20%, ( B ) 45%, (C) 65%, (D) 85%, and ( E ) 100%. These values are probably accurate to about 10 % of the value quoted. They are based on the integrated intensities of the bands and the assumption that E represents complete coverage. The fact that the number of bands increases with surface coverage shows that the Pd surface is heterogeneous. The heterogeneity, which is indicated, is the type in which the surface is divided into a relatively few portions which dift'er from each other but which are relatively homogeneous within themselves. On this basis, each band represents a homogeneous portion. The specific nature of these portions has not been established, but it is plausible to identify them as the major crystal faces. The band due to weakly held linear CO, Pd-CkO, at 4.85 p should be considered separately from the bridged bands above 5 p because the appearance of

INFRARED SPECTRA O F ADSORBED MOLECULES

;5

the linear CO may merely reflect the fact that adjacent double sites have been filled, and thus has no relationship to the type of crystal face exposed. I t is assumed that the small Pd particles are well-formed single crystals. This assumption is supported by electron-micrographic studies of deBoer and Coenen (12), who found that 50-100-A particles of nickel supported on silica had straight edges, indicating that the particles were either cubic or octahedral. The study of the effect of surface coverage of CO on Cabosil-supported platinum requires a different approach from the one used with palladium because the number of bands is not affected by the surface coverage of CO on platinum. In this case, there is only one strong band, and it is attributed to the linear structure Pt-C=O. Shifts in the position of this band can be observed when the surface coverage is varied. It was not possible, however, to determine whether these shifts are due to surface heterogeneity or t o interactions between molecules by observation of only one band. This difficulty was overcome by chemisorbing a mixture of CI3O and Cl2O. The spectrum of this mixture, chemisorbed on platinum, has bands a t 4.82 p due to linear Pt-C120 and a band at 4.97 p due t o linear Pt-CI3O. The changes in these bands were measured as the chemisorbed CO was pumped off a t 200°C. Figure 8 shows a plot of the absorbance (the ab-

FIG.8. Effect of removing n chemisorbed C12 0-Cl3O mixture from Pt by pumping a t 200" C.

16

R . P. EISCHENS AND

W.

A . PLISKIN

FIG.9. Effect of surfuce coverage on the ( A ) at)sortmnce and ( B ) wcivelength of CO chemisorhed on Pt.

sorbance is log l o / l ) vs. time. The absorbance of the CI20band decreases rapidly at first and then follows a straight line. The C130 absorbance first increases and then decreases almost in a straight line. The initial rise in the CI3O absorbance cannot be explained on the basis of surface heterogeneity, because changes in the strength of the metalcarbon bond would be expected to affect both types of carbon monoxide in the same way. The difference between the behavior of the C120 and C130 absorbances is due to interaction effects which increase the Cl20 absorbance and decrease the C130 absorbance a t high surface coverage. These interactions are believed t o produce coupling of the motions of adjacent carbon monoxide molecules, In the case of two CI2O molecules, coupling would produce two new vibrational modes-a high-frequency inphase mode, 0

0

T

T c

(I:

I

M-M

I

and a low-frequency out-of-phase mode, 0

0

1 1 I 11

c c M-M

The out-of-phase mode would be infrared inactive because there would be no net change of dipole moment perpendicular to the surface. When 5

INFRARED SPECTRA OF ADSORBED MOLECULES

17

Cl2O and Cl3O are coupled, two bands are observed because the difference in zero-order frequencies makes the coupling less effective. The out-ofphase vibration produces a slight change in dipole. On this basis the band referred to as the CL20band is enhanced by coupling and the out-of-phase band, previously referred to as the C130 band, is reduced by coupling. Thus, interaction can produce an opposite effect on the two bands. This hypothesis could be tested by experiments in which the ratio of Cl20 and CI3O were varied over a wide range. I n order to get the values of absorbance and wavelength position as a function of surface coverage, experiments were conducted in which the chemisorbed CO was removed by oxidation rather than by pumping. This was carried out by starting a t full coverage and adding small measured doses of O2 a t 200" C. to convert the chemisorbed CO to COZwhich could be removed from the system by freezing into a liquid nitrogen trap. By this method, accurate values of the surface coverage can be obtained. Figure 9 shows the results of oxidation experiments in a case where only normal CO was chemisorbed. The absorbances are indicated by triangles ( A ) and the wavelengths b y squares ( B ) . The horizontal portions of the curves starting a t 0 = 1 result from the fact that part of the monolayer which has been desorbed in raising the temperature from 35 to 200" C. remains in the system. This point will be discussed later in more detail. The curve is horizontal because the amount of chemisorbed CO does not change appreciably while the gaseous CO is being oxidized. In Fig. 9 it is seen that the absorbances and wavelengths are not a linear function of 0 and the absorbance per molecule of adsorbed carbon monoxide is markedly increased above 0 = 0.66. Figures 8 and 9 show that spectral changes are produced by changes in e which are due to the interactions between chemisorbed molecules. The interactions are most pronounced a t values of 0 2 0.66. At coverages of less than 0 = 0.66, interactions are also present, but the effects are smaller in magnitude. Interactions below 0.66 are shown by the fact that the wavelength varies with 0 even a t low coverages where molecules would not be expected to be adsorbed in nearest-neighbor positions. The persistence of interaction a t low surface coverage could be explained by clustering, dipole-dipole interactions between adsorbed molecules a t distances further apart than nearest-neighbor positions, or b y changes in the electronic nature of the adsorbent surface induced by the adsorbed CO. At present, the latter explanation is favored because it is consistent with the fact that other gases (discussed in 11, 3f) affect the position of the CO bands. I n a plot of the infrared data vs. surface coverage, as in Fig. 9, the determination of the point a t which the chemisorbed monolayer is complete

18

R. P. EISCHENS A N D W. A . PLISKIN

is of critical importance. The difficulty of obtaining a vaiid experimental definition of a chemisorbed monolayer is not unique to the infrared method. In practically all experimental studies of chemisorption , the “complete monolayer” is somewhat arbitrary. In Fig. 9 the initial holrizontal portions of the curves are due to the fact that the monolayer was defined as the amount rhemisorbed a t 35” C. rather than the amount rhemisorbed at the temperature of the experiment (200’ C.). The temperature of 200” C. was chosen for the oxidation because each dose of oxygen appeared to be consumed rapidly and completely a t this temperature. When CO is exposed to oxygen a t 35” C. the disappearance of the CO bands is slow, and there is a marked difference tietween samples. Part of the CO appears to resist oxidation a t 35” C. It is interesting to note that when the platinum is covered with oxygen and CO is added in small doses a t 200°C. (the reverse of the above experiment), the first doses of CO do not produce chemisorbed CO bands. There is a lag until the oxygen has been removed, and then the CO bands begin to build up. During this build-up, the band shifts are in the reverse order of those found in the removal of the CO. When CO is admitted to oxygen-covered platinum a t 35” C., bands are seen when a dose is first added, but these fade away as the CO combines with oxygen. This behavior persists until all of the oxygen has been removed. The final stages of this process are slow. If an excess of CO is admitted to the oxygen-covered platinum a t 35’ C., the CO bands immediately attain one-half to twlo-thirds of their maximum intensity. However, it takes as long as 24 hrs. to reach the maximum values. The latter could be cited as an example of “slow chemisorption.” In this case, there is little doubt that the slow chemisorption is a result of retardation by oxygen on the surface. b. E$ect of Carrier on the Spectrum of Chemisorbed CO. It is well known that the nature of the carrier will affect the course of catalyzed reactions even though the carrier has no effect of its own. Selwood (13) and his co-workers have shown that the carrier can modify the oxidation state of the metal atoms of supported metal oxides. It is more difficult to visualize how the carrier could affect the nature of supported metal particles when there is little more than a point contact between the metal and the carrier. A specific example of how the nature of the carrier can influence the structure of gases adsorbed on dispersed metals is shown in Fig. 10. In this figure, A is the spectrum of CO chemisorbed on Cabosil-supported platinum and B is the spectrum of CO chemisorbed on y-alumina(Alon-C) supported platinum. These spectra show definite differences in the position of the bands near 4.9 p , which are attributed to linear Pt-CEO, and a marked difference in the amount of bridged CO. The intensity of the 5.5-p band in B indicates

INFRARED SPECTRA O F ADSORBED MOLECULES

19

WAVELENGTH, MICRONS

FIG.10. Spectrum of CO chemisorbed on ( A ) silica-supported Pt and ( B ) aluminasupported Pt .

that about half of the chemisorbed CO is in the bridged structure, while not more than 15% of the CO on Cabosil-supported platinum is bridged. An exact interpretation of the basic reasons for the effect of the carrier on the structure of chemisorbed CO cannot yet be advanced. Qualitatively it appears that the alumina makes it easier for the platinum to provide electrons for bonding between the CO and the surface. This is based on a comparison of several possible electronic structures of the chemisorbed CO. The bond between the surface and the carbon of the linear CO could be either a double or a single bond,

.. ..

6'.

0:

c

..

or

..

**

C

..

Pt

Pt-

In the former structure, two of the bonding electrons would be provided by the platinum, while in the latter the CO would be coordinated to the platinum. The force constant for the metal-carbon bond for CO on Cabosilsupported platinum was found to be 4 x lo6 dynes/cm. On the simplest basis this force constant would indicate a single platinum-carbon bond. There is, however, considerable uncertainty in going from force constants to bond order so that a contribution, perhaps as high as 50 %, of the doublebond species could not be ruled out. The simple bridged structure 0:

..

c

.. .. Pt

Pt

would necessitate donation of electrons by the platinum. Therefore, it is reasonable to assume that the r-alumina carrier causes the CO to adsorb

20

R. P. EISCHENS AND W . A. PLISKIN

in the bridged structure by making it easier for the pl.atinum to donate electrons for bond formation. If this argument is carried further, it would appear that the y-alumina would also tend to cause the linear GO to be of the double-bonded type. This point could be checked qualitatively by determining the force constant of this bond when the CO is chemisorbed on y-alumina-supported platinum. Differences in the chemical behavior of the Cabosil-supported and y-alumina-supported platinum samples are evident. The initial reduction of the chloroplatiriic acid will start a t 35" C. when Cabosil is used, but it requires a temperature of 200' C. before reduction is started on the aluminasupported samples. The CO chemisorbed on y-alumina-supported platinum is much more difficult to oxidize, especially the bridged form, than is CO on Cabosil-supported platinum. c. Spectrum of CO Chemisorbed on Iron. Although spectra of CO chemisorbed on metals such as platinum and nickel can be obtained easily, difficulty has been experienced in obtaining the spectrum of GO chemisorbed on supported iron. This difficulty is probably due to the fact that iron is not easily reduced at low temperatures ( < 350" C.) . At higher temperatures (-500" C.), there appears to be a reaction with the imrier. This difficulty is observed, but to a lesser extent, with supported nickel. I n the latter case satisfactory samples can be ruined by attempting to re-reduce a t 500" C. The iron sample to be discussed here was reduced by gradually bringing the temperature u p to 380°C. over a 16-hr. period and then continuing the reduction a t this temperature for 2 hrs. Spectrum A of Fig. 11 was obtained when the reduced iron was cooled to room temperature and exposed to GO a t a pressure of mm. ( 1 4 ) . The band a t 5.10 p is attributed to GO in the linear structure, Fe- CEO. At higher CO pressures, 1 mm., a second band is observed a t 4.95 p . This second band may be due to iron carbonyl or to a weakly chemisorbed form of linear GO. When a layer of GO chemisorbed 011 iron is exposed to oxygen, the CO is not displaced even though sufficient oxygen is added to form several monolayers (15). In order to determine the fate of the rhernisorbed GO, oxygen was slowly added to the system represented by A of Fig. 11. This caused the band a t 5.10 p to gradually disappear and be replaced by a band a t 4.70 p . The 4.70-p band will be discussed later iin the section on the effect of added gases on chemisorbed CO. The iron-CO system is extremely sensitive to small traces of oxygen and the shift from 5.10 to 4.70 p will occur slowly when the chemisorbed GO is allowed to stand in a hydrogen or nitrogen atmosphere at 35OC. This slow change is attributed to small amounts of oxygen present as an impurity.

INFRARED SPECTRA OF ADSORBED MOLECULES

z

-

0

v)

$6%

z a a f

21

90 -80-

?v--

-

-

I

,

I

I

t

l

,

I

I

I

I

I

I

d. Spectrum of CO Chemisorbed o n Copper. The spectrum of CO chemisorbed on copper was studied early in the infrared work on chemisorbed molecules (16). It showed a single band a t a wavelength shorter than that found for CO on Pt, Pd, or Ni. This system was reexamined and the spectrum of CO on reduced copper is shown in A of Fig. 12. When oxygen was added t o chemisorbed CO or when oxygen was adsorbed prior to CO, the CO band (in B of Fig. 12) was observed at 4.72 p instead of 4.76 p . Thus, in contrast to the results over iron, the spectrum of CO on copper shows only a slight shift on addition of oxygen. This point will be discussed later. e. Eflect of Hydrogen on Chemisorbed CO. The effect of hydrogen on chemisorbed CO is of interest because there is evidence that the chemisorption of hydrogen is enhanced by a preadsorbed layer of CO. For example, Sastri and Viswanathan (17) found that a preadsorbed CO layer enhanced the hydrogen adsorption several fold on cobalt a t 53°C. A similar but smaller effect was found for carbon monoxide-hydrogen adsorption on nickel (18). Part of the enhancement of the hydrogen chemisorptioii could he attributed to complexes of the type OH

H

\ / C

II

M

22

R. P. EISCHENS AND W. A . PLISKIN

FIG.12. ( A ) Spectrum of CO chemisorbed on copper; ( B ) afher treatment with oxygen.

These complexes were postulated as intermediates in the Fischer-Tropsch synthesis (19). I n all cases so far studied, there is no evidence for a hydrogenatedcarbon monoxide complex of the type illustrated above. When the CO chemisorbed on iron (represented by A of Fig. 11) is treated with hydrogen a t 35" C., no major changes are observed in the 5.1 p band. A typical result of adding hydrogen to chemisorbed CO is n slight shift, of the band to longer wavelengths. This is illustrated in Fig. 13, which shows the spectrumof CO on Cabosil-supported platinum before ( A )and after@) treatment with hydrogen at 35' C. This shift of the cnrbonyl frequency is not consistent with formation of a H

OH

\ / C

II

I't

23

INFRARED SPECTRA OF ADSORBED MOLECULES

1 4.5

1

1

1

1

1

1

1

5,O WAVELENGTH(C 1

1

1

1

1

1

5s

FIG. 3. (A) Spectrum of CO chemisorbed on silica-supported 1%; ( B ) a ter treatment with H 1 .

complex. Formation of this complex would result in disappearance of the carbonyl band and formation of a hydroxyl band. The infrared results do not necessarily mean that the complex has no role in the Fischer-Tropsch synthesis. They do rule out, however, the possibility of any appreciable quantities of this complex having formed under the specific conditions used for the infrared experiments. It is likely that the failure to observe hydroxyl bands, when hydrogen is added to chemisorbed CO, is closely related to the failure to observe hydroxyl bands in the hydrocarbonyls. In these compounds the presence of hydrogen attached to oxygen with hydroxyl formation is definitely ruled out (20, 21). f. Effect of Other Gases on Chemisorbed CO. In most cases addition of a gas to a chemisorbed CO system will shift the frequency of the C-0 stretching band. This shift can be to either longer or shorter wavelengths. Oxygen shifts CO on iron by -0.4 p and CO on copper by -0.04 p. Hydrogen, ammonia, and HC1 shift CO on Pt by +0.09, +0.03, and -0.01 p , respectively. It is likely that these shifts are due to the effect of the added gas on the electronic nature of the metal rather than t o a change in structure of the chemisorbed CO. An effect of this type would be similar to those postulated by Boudart (22). A shift to shorter wavelengths indicates a strengthening of the carbon-oxygen bond and a weakening of the metalcarbon bond in the structure 0

111

C

I

M

On this basis, this carbon-oxygen band probably could not be shifted to a wavelength shorter than 4.67 p , since this is the position of the carbonoxygen stretching vibration for gaseous CO.

It. P. EISCHENS AND W . A . PLISRIN

24

TABLE I Hand Positions i n Spectra o j CO on Rhodiuni ~

~~

0 0 Sample preparation

R h unsintered 8 or 16% R h u n sintered 8 or 16% Rh sintered 2%

0

c c \/

Coverages All

Low High Low High

-RhTwo CO's per site 4.03 mid 4.77 -

-

4.93 and 4.77 -

-

4.90 and 4.74

c

0

I1

c \/ \/

-AhSingle linear -Rh-1thC0 Bridged CO -

-

4.90 4.87 4.89 4.85

5.25 __

-

5.19

Since adsorbed gases appear to change the electronic nature of the metal in a manner which affects the chemisorbed species of another gas, it is logical t o expect a similar effect with changes in surface coverage of a single gas, but the spectral shifts (11, 3a) observed a t coverages of more than e = 0.66 could not be due to changes in the electronic nature of the metal, since this factor could not explain the difference in behavior of Cl20 and CI3O. g. Spectrum of CO Chemisorbed on Rhodium. The spectrum of GO chemisorbed on Alon-C-supported rhodium has been studied by Yang arid Garland (23). They used experimental methods similar to those used in previous studies of GO chemisorbed on supported metals. Their results are summarized in Table I. The structure, which they attribute to the species producing each band, is indicated at the top of the columns in which the band positions (in microns) are listed. The unsintered samples are those which have not been heated over 200" C. in either the drying or reduction steps of the sarnple preparat>ion. The samples referred to as sintered were heated to 400" C1. Addition of oxygen to GO chemisorbed on rhodium produced an effect similar to that observed for GO on iron. The original C--0 bands disappeared and a new band appeared a t 4.70 p . Addition of water to CO on sintered samples produced a small shift to longer wavelengths. Thus, the difference between the sintered and urisintered samples may be due only to the differencc in :mount of water present. In the react:ion of chemisorbed CO with hydrogen, sites with two CO's bouded t o :L single l i h reacted slowly a t room temperature but rapidly :It 150" C. Some CO is displaced into the gas phase. A band appeared a t 4.88 p which Yarig and Garland attributed to a single unreacted GO remaining on the original site.

25

INFRARED SPECTR4 O F ADSORBED MOLECULES

4. Catalyzed Oxidation of Carbon Monoxide The infrared methods, when combined with use of an in situ cell, are ideally suited for study of reactions in progress. It is likely that the major contribution of the infrared technique to an understanding of catalysis will develop from this type of work. The greatest interest lies in the determination of the structures of species which can be classed as adsorbed intermediate complexes. It is assumed that a true intermediate complex would be in equilibrium with both the starting materials and the product and thus could exist only while the reaction was in progress. The best example of a study of this type of intermediate is found in the oxidation of CO over a nickel-nickel oxide catalyst (24). The latter term is used because there is doubt as to the specific nature of the catalyst surface. The spectrum in Fig. 14 was obtained during the oxidation of CO over nickel-nickel oxide a t 35" C. The band at 4.56 p is tentatively attributed to an intermediate complex having the structure Ni. .O-C=O. The bands a t 6.5 and 7.2 p are due to COZ chemisorbed on the catalyst surface. This COn is considered to be adsorbed product rather than as a reaction intermediate because these bands remain after the reactionis completed. The 4.56- p band in Fig. 14 is attributed to the asymmetrical 0-C-0 vibration rather than to the C-0 vibration of chemisorbed CO. This interpretation implies that there should be a second band vibration due t o the symmetrical vibration. The symmetrical 0-C-0 of COZ produces a Raman band a t 7.2 p . The symmetrical 0-C-0 vibration of Ni. . .O-C=O would be expected to produce a n infrared band near 6 or 7 p . Thus far this band has not been observed. This failure is not considered a serious obstacle to the structure assignment,

-

I

I

1

t

0 ln E a!

0, -0

Y$

Q

ln

0

2 4

a I-

Ti/

l o o - ~ ~ g di

90

-

ao

-

d

0.-a

Ni

I

3

1

1

I

I

I

I

I

I

.

I

I

I

I

I

I

1

1

I

FIG. 14. Spectrum obtained during catalyzed oxidation of CO over nickel-nickel oxide.

26

R . P. EISCHENS AND W. A . PLISKIN

since it is probably weak compared with the asymmetric band, and the latter is not an intense band. I n addition, the symmetric stretching band may be masked by the bands a t 6.5 and 7.2 p due to chemisorbed COz . The structure Ni * .O;r;C=O could be regarded as COS adsorbed through one of the oxygen atoms or as CO adsorbed on a preadsorbed oxygen atom. In the latter case, this intermediate would be similar to those predicted by the Rideal-Eley mechanism. This mechanism is consistent with the reaction illustrated in Fig. 14 because the oxygen and carbon monoxide were introduced simultaneously, and there is an opportunity for the oxygen to be adsorbed first. The same intermediate is observed, however, when oxygen is added to chemisorbed CO. In the latter case, it is likely that oxygen adds across the metal-carbon bond as though the latter were an olefinic double bond. In order to observe the complex, it appears essential that the oxygen be chemisorbed rather than surface oxygens of nickel oxide because adsorption of CO on bulk oxide does riot produce a band a t 4.56 p.

-

5. Failure to Observe a Band Due to Chemisorbed Hydrogen

Reduction with hydrogen is a preliminary step in practically all of the experiments dealing with chemisorption on metals. In most cases the sample is cooled to 35’ C. in an atmosphere of hydrogen. Usually an attempt is made to observe a band due to chemisorbed hydrogen even though this may not be related to the major objective of the experiment. Despite many such attempts with a wide variety of samples, no band has been observed which could be attributed to chemisorbed hydrogen. These failures to observe an absorption band due to metal-hydrogen stretching suggest that hydrogen is not covalently bonded to a single H

metal atom in a structure of the type,

1 . Such a

bond would be ex-

M

pected to produce a band in the 3-5-p region. It is reasonable to expect that the bands due to hydrogen which is bonded to two or more metal atoms would be of lower frequency and lesser intensity that the simple covalent M-H band. The “negative” infrared evidence suggests that chernisorption of hydrogen on metals involves multicenter bonds. Multicenter bonds have been discussed by I’itzer in connection with the structures of boron hydrides (25). The bridged hydrogens of decaborane produce bands in the 5.2-5.7-p region (26). Since no bands were found in this region for chemisorbed hydrogen, it is likely that the hydrogen is bridged between three or four metal atoms. Chemisorbed hydrogen heis been discussed thoroughly by Broeder et al. (2’7). They concluded that, chemisorbed hy-

INFRARED SPECTRA O F ADSORBED MOLECULES

27

drogen was predominantly covalent. The multicenter bond concept implies, however, that the hydrogen loses electrons and has a predominantly protonic character. Observation of a band attributable to chemisorbed hydrogen in the region above 7 p would support the idea of bridging between three or more metal atoms. Unfortunately, carrier-supported samples are not well suited to studies in this region, because of absorption of radiation by the carrier, and improvement in sensitivity is required before weak bands can be detected on carrier-free samples. In some cases, especially with platinum samples a band is observed near 4.9 p after reduction with hydrogen. However, this band is not shifted when deuterium is used instead of hydrogen. It is due to contamination with CO which has been desorbed from the glass walls or is present as an impurity in the hydrogen.

111. CHEMISORPTION ON ACIDICOXIDES 1. Ammonia Chemisorbed on Silica-Ahrnina Catalysts

The spectrum of ammonia chemisorbed on a silica-alumina cracking catalyst was studied to determine whether the acidity of these catalysts is due to a Lewis (nonprotonic) or a Bronsted type of acid (28,29). This work was based on the premise that ammonia chemisorbed on Lewis sites would retain a NH3 configuration while ammonia chemisorbed on a Bronsted site would form NHT. The NH3 configuration was expected to have bands near 3.0 and 6.1 p and the NH; near 3.2 and 7.0 p . A commercial cracking catalyst was used in this work, so that it was necessary to obtain small particles by grinding and water sedimentation. The sample was deposited on a salt plate by evaporation of an alcohol slurry. The average thickness was 1.5 mg./cm.2, but the intensity of the Si-0 bands near 5.0 jt indicated that, the thickness was about 4 mg./cm.2 in the area penetrated by the infra-red beam. The problem of getting uniform thin samples of catalyst on salt plates is serious when attempts are made to correlate infrared intensity measurements with quantitative chemisorption measurements obtained on bulk samples. An improvement in the technique of depositing small catalyst particles on salt plates has been developed which has considerable advantage over the simple evaporation of slurries. I n the improved method isopropyl alcohol is used as the suspending liquid in the sedimentation step and the portion of the liquid containing the desired particles is sprayed onto a heated salt plate (SO). Spectrum A of Fig. 15 is due to ammonia chemisorbed on the cracking catalyst which had been dried under vacuum for 70 hrs. at 500" C. Chemisorbed ammonia was defined as that retained by the catalyst after evacua-

28

:R. P. EISCHENS AND W. A. PLISKIN

2.5

3.0

55

4.0

4.5

50

5.5

6.0

6.5

7.0

7.5

WAVELENGTH IN MICRONS

FIG.15. ( A ) Spectrum of NH3 chemisorbed on dried silica,-alumina; ( B ) and (C) after rehydration.

tion for 2 hrs. a t 175" C. Spectrum A shows bands a t 3.(D and 6.1 p which were attributed to ammonia chemisorbed as NH3 and small bands at 3.2 and 6.9 p which were attributed to KHZ . It was concluded that most of the ammonia was chemisorbed on Lewis sites. This was confirmed by observation in B and C of increases in the intensity of the 6.9-p band when the samples were exposed to moisture. The 3.0-11 band of 1,he NH3 decreases when water is added. This decrease is difficult to see, since the NHJ band is superimposed on an OH band which increases as water is added. The decrease is evident, however, if the most reasonable level of the background due to OH is used. The increase of NH: and decrease of NH3 is attributed to reaction of NH3 with protons formed during adsorption of water on the Lewis sites. The results, which indicate that the N-H stretching and unsymmetrical bending vibrations produce bands at 8.0 and 6.1 p , indicate that the positions of these bands are not markedly affected by forrnation of a bond between the nitrogen and the surface. This result is romparable to the observation that in coordinated ammonia of complex ammines the N-H stretching and unsyrnmetrical bending modes are not sensitive to formation of bonds between the nitrogen and metal atoms (31, 3%').The N-H unsymmetrical bending bands are found near 6.1 p, arid the N-H stretching bands are found in the 3.0-3.2-c( region for a wide variety of ammines in which the NH3 is coordinated to metals such as Ag, Cu, Nil Cr, and Pt. When H2S is chemisorbed on platinum or water adsorbed on silica, the S-H and 0-H stretching bands appear a t positions close to those observed for HZS and HzO in the gaseous state. These results, together with those found for chemisorbed ammonia, indicate that the stretching vibrations of the single bonds to hydrogens are not markedly affected by formation of coordinate bonds between a surface and the central atom of the molecule.

INFRARED SPECTRA O F ADSORBED MOLECULES

29

Although the N-H stretching and unsymmetrical bending vibrations of NH3 are not markedly affected by coordinate bonding of the nitrogen to other atoms, the symmetrical N-H bending vibration, which shows up as a doublet near 10.5 p in gaseous NHs , is shifted to shorter wavelengths. In cobalt hexammine I11 chloride the NH3 symmetrical deformation band is found a t 7.5 p (32). There is no way to predict how far this band would be shifted for ammonia chemisorbed on cracking catalysts. I n a few instances a band has been observed a t 7.5 p when ammonia was chemisorbed on cracking catalysts, but generally no band is observed which could be attributed to the NH3 symmetrical deformation vibration. This is not surprising because the region above 8.0 p is obscured by a strong Si-0 or A1-0 band, and the NH3 band could not be detected unless it had shifted to below 8.0 p . 2. Ammonia Chemisorbed o n y-Alumina When ammonia is chemisorbed on y-alumina (Alon-C), a strong band is observed a t 7.8 p in addition to the bands near 3.0 and 6.0 p (high-resolution spectra show that the band referred to as 3.0 is composed of bands a t 2.96 and 3.06 p ( 3 3 ) ) .The 7.8-p band is attributed to the N-H symmetrical bending vibration. If, however, the Alon-C is heated to 500" C. prior t o chemisorption of ammonia, no band is observed at 7.8 1.1. Alon-C is often contaminated with aluminum chloride, and it is possible that the N-H bands observed a t 7.8 p are due to ammonia coordinately bonded t o the aluminum chloride rather than adsorbed on the alumina. The failure to detect the NH3 symmetrical deformation band for ammonia chemisorbed on cracking catalysts is disappointing because the position of this band is strongly influenced by bonding of the nitrogen to other atoms and its position might provide direct evidence on the relative acid strength of different catalysts. WATERAND SURFACE HYDROXYL GROUPS IV. SPECTRAOF ADSORBED ON NONACIDIC OXIDES 1. Stability of Surface Hydroxyl Groups

The earliest observations of infrared bands due to hydroxyl groups on surfaces were made by workers who were primarily interested in the spectra of the solid rather than spectra of adsorbed molecules. Despite this difference in point of view, their interpretation of the hydroxyl bands is important to later studies of adsorption. Buswell and associates (34) studied the effect of variation in drying conditions on the spectrum of montmorillonite-water systems. The use of infrared t o study the state of water in minerals had previously been suggested and briefly explored by Coblentz (35). The effect of drying on the

30

R. P. EISCHENS AND W . A. PLISKIN

Fro. 16. Spectrum of montmorillonite: ( A ) dried a t 100" C.for 36 hrs., ( B ) dried a t 100" C. for 42 hrs., (C)after exposure to air saturated with water vapor, ( D ) redried at 110"C. for 6 hrs., ( E ) redried at 110" C. for an additional 24 hrs.

spectrum of water on montmorillonite is shown in Fig. 16. These spectra show bands a t 2.75 and a t 3.0 p which were attributed to free (nonhydrogen-bonded) and hydrogen-bonded hydroxyl groups, respectively. This interpretation was based on work which showed that alcohols had strong bands a t 2.75 in dilute CC14 solution and bands in the 2.95-3.0-p region in more concentrated solutions (36). The shift from 2.75 to 3.0-p with increasing concentration of alcohol was attributed to hydrogen bonding. Figure 16 shows that the "associated" OH groups are more easily removed from the montmorillonite than are the free OH'S. The presence of two 0-H stretching bands shows that there are a t least two types of OH groups on the montmorillonite. Work which will be discussed later shows that interpretation of the spectra of the OH stretching bands is more complicated than would be expected. On the simplest basis the OH bands can be separated into those due to adsorbed water and those due to surface OH groups. These species can be distinguished in cases where it is possible to scan the spectral region past 6.2 11. Adsorbed water

INFRARED SPECTRA OF ADSORBED MOLECULES

31

has a deformation band near 6.1 p . This band is due to the change in dipole moment associated with a change in the H-0-H angle. Its observation is proof that there are two hydrogens bonded to a single oxygen. The species producing the 2.95-3,O-p band in Fig. 16 can probably best be described as physically adsorbed water. The band a t 2.75 p is due to surface OH groups of the type H 0

I t

-Si-

The 0-H

stretching band of surface O - H groups is usually found in the p when small particles of silica were exposed to water vapor a t temperatures close to the fusion point (presumably > 1600" C.) of the silica. This band disappeared when the water vapor was removed. He attributed the 2.72-p band t o surface OH groups resulting from a reaction between the water and silica, i.e., chemisorption. Chevet (38) found that commercial silica gel which had been heated to 350" C. has a broad band a t 2.90 p and a sharp band a t 2.65 11. In this case it is not clear whether there is any significance in the fact that the surface OH band was found at 2.65 rather than at 2.72 p. When her samples were subjected to progressively more severe heat treatment, the 2.90-11 band disappeared and the 2.65-p band remained. The latter was still detectable in samples which had been heated to 800" C. These experiments in themselves do not show whether the OH bands observed after heating to 800" C. are due to surface OH groups or to OH groups embedded in the interior of the silica. Results of other experiments indicate, however, that the former explanation is the more reasonable. Sidorov (39) found that part of the surface hydroxyl groups of porous glass can be replaced by OCH3 groups of methanol (Fig. 17). Spectrum A , which is due to porous glass heated for 3 to 4 hrs. at 400-500" C., has a strong surface hydroxyl band a t 2.67 p . Spectrum B was obtained after the porous glass had been treated with methanol at 20" C. and then evacuated for several hours a t 450" C. In B the surface hydroxyl band is smaller than in A , and bands are observed a t 3.38 and 3.50 p which are due to the unsymmetrical and symmetrical C-H stretching vibrations of the CH3 groups. If the OCH, groups are exposed to HzO vapor, the intensity of the CH, bands is decreased. An interesting aspect of the behavior of surface OH groups is revealed in studies of exchange with gaseous deuterium. The surface OH groups of Cabosil or the OH groups of adsorbed water do not easily exchange with 2.65-2.75-p region. Garino-Canina (37) observed a band a t 2.72

32

R . P . EISCHENS AND W . A. PLISKlN

2.5

3 .O 3.5 WAVELENGTH ( P 1

FIG.17. Effect of treating porous glass with methanol: ( A ) prior to addition of methanol, ( B ) after addition of methanol.

D2 a t room temperature. If, however, the Cabosil is impregnated with platinum, the exchange goes rapidly (14). The results of such an exchange are shown in Fig. 18. Spectrum A is due to partially dried Cabosil which is used as a support for 9.2 wt. % platinum. The bands at, 2.67 and 2.80 p are due to surface OII groups and adsorbed water. The b md s in the 3,4+ region are due t o hydrocarbon impurities on spectrometer windows and the bands near 4.27-p are due to atmospheric GOz. Spectrum B was obtained 10 min. after the sample had been exposed to g,aseous Dz . The

301

I

3.0

I

I

3.5 4.0 WAVELENGTH (p)

I

-

4.!5

FIG. 18. ( A ) Spectrum of OH groups of platinized Catwsil; ( R ) after treatment with 1 1 2 .

INFRARED SPECTRA OF ADSORBED MOLECULES

33

2.67-p band of A has been decreased, and a surface OD band has appeared a t 3.63 p . The 2.80-p band has shifted to 3.83 p . These results show that both the surface OH and the OH groups of adsorbed water are replaced by OD even though only a small fraction of these groups are in direct contact with platinum. It appears that the platinum particles promote the exchange between OH groups and gaseous D, . After the groups adjacent to the Pt have exchanged, there must he a second mechanism by which exchange is transmitted through the adjacent OH groups to those OH groups not in direct contact with the platinum. It has been suggested that the exchange is carried out by way of gaseous H,O which exchanges with deuterium on the platinum and then with the surface OH groups (40).

2. Efect of Adsorbed Molecules on the Surface Hydroxyl Groups

The surface OH groups can be considered as small probes sticking up from the surface. The position of the OH stretching band of surface OH groups is affected by the presence of adsorbed molecules. Thus, a study of the surface OH band position provides evidence on the nature of the adsorption process even though the spectrum of the adsorbed molecule is not observed. This interesting effect was first reported by Yaroslavskii and Tereiiin ( 4 1 ) and by Kurbatov and Neuimin (42). Yaroslavskii and Terenin found that the 1.365-p band, due to surface OH groups on porous glass, decreased in intensity and in some cases disappeared when benzene, toluene, aniline, phenol, or pyridine were adsorbed at 20" C. I n this work, samples of porous glass several millimeters thick were used. These samples cut off transmission above 2 p . Thus, it was necessary to study the overtones rather than the fundamental bands. It is a satisfactory approximation merely to multiply these overtone positions (in microns) by 2 when comparing them with fundamental band spectra. The effect of adsorbing aniline is shown in Fig. 19. The surface OH band at 1.365 p disappears, and the N-H band of aniline at 1.52 p shows a slight shift from the position of this band in dilute solutions of CCl, (1.50 p ) . Yaroslavskii aiid Terenin attributed the disappearance of the OH band to the formation of hydrogen bonds, O-H. . .N. The disappearance of the surface OH bands has also been observed in other experiments. I t is difficult to accept hydrogen bonding as the basic cause of this disappearance. The typical hydrogen bonding effect is a shift to a longer waveleiigth toget,her with :HI incwase in tiand width. 1Jsually the integrated intensity is incrc:isrd. In some ~ a w sthe ovcrtonc OH I)and does not behave exactly the same :LS t,he furidurnciital when hydrogen boiiding occurs (43).There is, however, no infrared experience which indicates that hydrogen boiiding could cause the OH bands to disappear completely.

34

R. P. EISCHENS AND W. A . PLISKIN

At first thought one might attribute the disappearance of the OH hand to a displacement of the OH groups from the surface or to a reaction of the type that Sidorov observed with methyl alcohol. Kurbatov and Neuimin (42) found, however, that acetone also caused the surface OH bands of silica aerogel to disappear. Moreover, this disappearance was easily reversible, and the OH band reappeared when the acetone was removed by freezing into a liquid nitrogen trap. The results obtained by Kurbatov and Neuimin (42) during adsorption of CHCls are shown in Fig. 20. Spectrum A , which has a single sharp surface OH band a t 1.37 p , was obtained after heating silica gel a t 400" C. for 3 to 4 hrs. Spectrum B was obtained after exposing; the sample to CHCI, . This shows a new hand a t 1.39 p, and a small band has appeared a t 1.69 p. The latter is due to the C-H stretch of CHCI, . Its intensity increases in C, D , and E as more CHClx is adsorbed. The 1.37-p band has disappeared in C, and the 1.39-p band decreases with increasing CHCla adsorption. Kurbatov and Neuimin attributed the 1.39-p hand to adsorbed CHCI, . They account for its decreasing intensity by postulating that the band is unique for the adsorbed state and disappears when liquid CHC13 is formed 0 -H

D

E 1.2

1.4

1.6

1.8

FIG. 19. Effect of aniline on the OH band of porous glass: ( A ) dried porous glass, ( B to E ) with increasing amounts of adsorbed aniline.

35

INFRARED SPECTRA OF ADSORBED MOLECULES

E 0 \

I

I

1.30

I

I

1.40

------___ I

I

1.50

I

I

I

1.60

I

t

c

I

C 0 A

---I

1.70

I

I

1.80 P

FIQ. 20. Effect of chloroform on the OH band of porous glass: ( A ) dried porous glass, ( B t o E ) with increasing amounts of adsorbed chloroform.

by capillary condensation. If correct, this interpretation would have important implications because it suggests that new bands appear during physical adsorption which can not be attributed to the known infrared or Raman bands of the gaseous or liquid species. The 1.39 p band cannot, however, be assigned to any specific CHCl, vibration, and it seems morelikely that it resultsfrom a shift of the 1.37-p OH band. With acetone and phenol Kurbatov and Neuimin found that the 1.37-p band almost disappeared before bands due to the adsorbed molecules were observed. In these cases the spectra do not show the OH band a t intermediate stages during its disappearance, so that it is not possible to determine whether a shift similar to that shown in Fig. 20 had occurred. Kurbatov arid Neuimin (4%')also studied the adsorption of water. Spectrum A of Fig. 21 was obtained after degassing the sample a t 400" C. for 3 hrs. After exposure to water vapor, B was obtained. This shows a trace of the original 1.37-p band plus bands a t 1.41 and 1.47 p . The 1.47-p band is probably due to physically adsorbed water, and the 1.41-p band is probably due to surface hydroxyl bands which are involved in hydrogen bonding with the adsorbed water. After heating B for 4 hrs. in vacuo a t 900" C., C was obtained. This has a single sharp band at 1 . 3 7 ~ .When C was exposed to water vapor, the resulting spectrum D shows that less water had been adsorbed than in B. The intensity of the 1.37-p band in D is about one-half that in A or C. It appears that the 900" C. evacuation has affected the sample even though the intensity of the OH band in C was about the same as in A . It is likely that sintering had occurred which closed off some of the pores without removing the OH groups from the pore surfaces.

36

R. P. E I S C H E N S A N D W. A. PLISKIN

I

I

1.30

I

I

1.40

I

I

1.50

I

l

1.60

l

1 1 1 1 :

1.70 1.8OP

FIG.21. Kffect of adsorbing water: ( A ) aerogel dried a t 400" C., ( B ) with capillary condensed water, (C) redried a t 900" C . , (11) after readsorbing water.

It should be noted that B is an example of what would be called t~ typical hydrogen bonding effect. The appearance of this effect with water suggests that extreme care must be taken in drying organic compounds used in this type of work. If this is not done, hydrogen bonding effects may be erroneously attributed to the organic compound when they are really due to water present as an impurity. One of the most interesting series of spectra, showing the effect of adsorbed molecules on surface OH bands, is shown in Fig. 22. These spectra were obtained by Yaroslavskii and Karyakin (44) when N, was adsorbed on porous glass. Spectrum A was obtained a t - 180" C. after degassing the porous glass a t 550" C. Spectrum B was obtained 1 min. after the admission of N, at 1-atm. pressure. Spectra C, D,arid h ' were obtained after 10 min., 20 min., and 2 hrs., respectively. These spectra show that the 1.365-p baiid disappears after 20 min. The 1.365-p hand is replaced h y a band a t 1.378 p. This appears a t 1 miri. and increases with time. An increase in the intensity of the 1.378-p band is observed in the period hetween 20 min. and 2 hrs. even though the 1.365-p band was not detectable after 20 min. The 1.365-p hand (spectrum F ) was restorcd by bringing the temperature up to 20" C. This suggests that the shift of the OH band is caused by physically adsorbed nitrogen. The effect of adsorbing oxygen is shown in Fig. 23. Spectrum A represents the degassed porous glass. When O2 is added at - 180" C., the 1.365-p

I N F R A R E D S P E C T R A O F ADSORBED M O L E C U L E S

37

t

z

2

I-

n

a 0 m m a

-I PIC.22. Effect of adsorbed nitrogen on t,he OH band of porous glass: ( A ) dried porous glass, ( B ) 1 min. after admission of nitrogen a t -180" C., (C) after 10 min., (D)after 20 min., ( E ) after 2 hrs., ( F ) after heating to 20" C.

barid disappears instantaneously and a band appears in B a t 1.378 p. In contrast to the results with Nz , the 1.365-p band is not restored ( C ) when the temperature is brought up to 20" C. The 1.365-p band was restored by heating t o 200" C. as is shown in D.Since physically adsorbed O2 would be desorbed a t 20" C., it is evident that the shift in the OH bands cannot be attributed only to the presence of physically adsorbed gases. The latter point is emphasized by the fact that the shift of the 1.365-r band was observed in samples of porous glass which had been degassed and sealed in a tube for two years. Figure 24 shows the spectra of the original porous glass ( A ) and the sample after two years (B).I n this case the 1.380-p band of B was not removed by heating to 550" C. This indicates that the results cannot be explained on the basis of inadvertent adsorption of gases evolved from the tube in which the sample was sealed. The original spectrum was restored by exposure t o water vapor followed hy evacuation. Yaroslavskii arid Karyabin (44) concluded that the slow aging produccs a partial perturbation of the vibrations of the surface OH groups, possibly :LS a result of sintering. It is evident that the results obtained with N B, 02, and aging cannot all be explained on any basis of simple interactions between adsorbed molecules arid surface OH groups. It appears that the adsorbed molecules aid in relieving surface strains. There may be a connection between this phenomenon and the observation by Yates (45) that porous glass expands when gases are physically adsorbed on it. By using very thin (0.14.7 mm.) disks of porous glass, Sidorov (39) was able to extend the study of the effect of adsorbed molecules on surface OH groups into the fundamental wavelength regions. The results of his

38

R. P. EISCHENS AND W. A . PLISKIN

FREQUENCY(em-3

FIQ.23. Effect of adsorbed oxygen on the OH band of poro'us glass: ( A ) dried porous glass, ( B ) after adsorption of oxygen a t -1180" C., (C) after heating t o 20" C., ( D ) after heating t o 200" C.

work with methyl alcohol have been discussed previously in connection with Fig. 17. In addition to methyl alcohol, he studied the effect of adsorbed benzene, toluene, ethyl benzene, acetone, benzaldehyde, acetaldehyde, chloroform, pyridine, and ammonia. Toluene and ethylbenzene produced shifts of 120 cm.-l, but benzene did not produce any shift. The carbonyl-containing compounds produced unusually large shifts of 290 cm.-l for benzaldehyde and acetaldehyde and 370 cm.-' for acetone. This result with acetone does not agree with that of Kurbatov and Neuimin (42) who found that acetone causes the OH overtone at 1.37 p to disappear rather than shift. Sidorov found that ammonia produced a shift of 120 ern.-' and pyridine a shift of 80 cm.-'. In a brief review of the Russian work, Terenin (46) appears to consider that hydrogen bonding is the most important factor in producing the shift of the surface OH bands. The importance of hydrogen bonding was also emphasized by McDonald (47), who made a systematic attempt to clarify the OH shift by taking into account the polarizability of the adsorbed molecules.

INFRARED SPECTRA O F ADSORBED MOLECULES

39

McDonald found that the OH shift occurs wheii rare gases are adsorbed on Cabosil which had been degassed at 300-350" C. for several days. The OH band of the degassed Cabosil is shown as A of Fig. 25. When argon was adsorbed a t a low relative pressure, the band shifted by 8 cm.? (B). The integrated intensity of the OH band in B is increased by a factor of 1.5 over that in A. When the temperature was reduced to -200°C. and argon condensed on the Cabosil, the band (shown in C) is shifted by SL total of 40 cm.-1 and its integrated intensity is 2.5 times as great as in A . Argon, krypton, and xenon have polarizabilities of 16.5, 25.4, and 41.3 X respectively. McDonald found that these gases produce shifts of 8, 16, and 19 cm.-l. Nitrogen, oxygen, and methane, which have polarproduce shifts of 24, 12, and izabilities of 17.6, 16.0, and 26.0 X 32 cm.-l, respectively. McDonald interpreted these results as showing that the polarizability is not the only factor involved and that the frequency shifts depend on an additional factor related to the chemical nature of the adsorbed molecules. He concluded that the frequency shifts cannot be completely explained in terms of macroscopic dielectric properties.

1.365~

n

1.380~

FIQ.24. Effect of aging on the OH band of porous glass: (A) original porous glass, (R) after aging for two years.

40

R . P. EISCHENS A N D W . A . PLISKIN

FIG. 25. ICffect of adsorbed argon on the OH band of C:tl)osil.

McDonald also studied the effect of benzene and found that it produced a shift of 120 cm?. This is riot in agreement with Siclorov’s observation that benzene does not produce a shift in the OH bands of porous glass. It is evident that there is no single explanation that will clarify 2111 c:Lses in which the surface OH vibrations are shifted by adsorbed molecules. In some cases, such as with adsorbed water, the effects can be attributed to hydrogen bonding. However, in many of the cases witb nonpolar adsorbents, it appears th:tt the shift of the OH could more properly be attributed to a “solvent efiect” due t o the dielectric properties of the adsorbed gas rather than to hydrogen bonding. Josien and Fusoii (48) used an equation, derived by Kirkwood and by Bauer and Magat, to predict the vibrational frequency shift for an oscillatii~gdipole when the shift results from instantaneously induced polarization of the surrounding medium by the vibrating dipole. Josierl and Fuson found that the equation

- _vu

vu

VB

- C ( D - 1) 2D+l

’

where v, and v, are the frequencies of the particular infrared adsorption band measured in the vapor arid solution states, respectively, C is a con-

I N F R A R E D SPECTRA O F A D S O R B E D MOLECULES

41

stunt which depends upon the dimensions of the solute molecule and on the dipole moment, and D is the dielectric constant of the solvent, was obeyed in nonpolar solvents. In polar solvents, discrepancies were observed which they attributed to hydrogen bonding. This equation applied to the OH shift observed with nonpolar adsorbates having a dielectric constant of 2.0 (or an index of refraction of 1.4) predicts a shift of about 40 cm.-’. It would appear that a t this stage only order of magnitude agreement could be reasonably expected because the experimental results are not completely reliable. The latter is shown by the discrepancies observed between the results of different workers. The dielectric shift cannot explain the total disappearance of the OH bands observed in some cases, but hydrogen bonding is also incapable of explaining this disappearance. It is evident that the effect of adsorbed molecules on the surface OH vibrations is an interesting and important phenomenon and warrants considerably more work. Recent publications (48a, 48b) indicate that the Russian workers are continuing their studies of this effect. l’imentel and co-workers (49) have studied the spectrum of D 2 0 adsorbed on silica gel using a technique in which they attempted to overcome scattering by embedding the sample in paraffin wax. Spectra were obtained a t surface coverages of 0.35, 0.45, 1.0, and 1.8 which show broad bands with maxima around 4.0 p . These bands were attributed to D2O adsorbed on the silica gel. At a surface coverage of 0.1, a broad diffuse band without a distinct maximum was observed. It was suggested that the diffuse character of this band might be related to the difference in energy of the adsorption sites on the silica surface. It is doubtful, however, whether any conclusion based on this band is warranted in view of the questionable nature of the experimental technique. McDonald (47) has expressed the view that addition of extraneous material t o reduce scattering is likely to complicate results involving OH (or OD) bands. This criticism is valid even when no simple chemical basis for objection, such as the danger of ion exchange, is apparent. UIIfortunately, I’imentel and his co-workers do not mention the sharp OH band which can be observed on dried silica. The position and shape of this band would have given an indication of the effect of embedding the sample in paraffin.

v.

INFHARFX) SPECTIZA OF

I’HYSICALLY

ADsoIzmm MOLECULES

Most of the work discussed in connection with the shift of the surface OH bands concerned systems in which physical adsorption occurred. In this work the spcctra of the adsorbed molecules were not emphasized. In

42

R. .'1

EISCHENM A N D W. A . PLISKIN

some cases the spectrum of the adsorbed molecule was not observed or, if observed, could not be differentiated from that of the liquid state. I n other cases, small differences could be observed in the spectrum of the physically adsorbed molecules. These differences were of the magnitude of those which are found when compounds are dissolved in various solvents. It is likely that thcse small shifts will eventually be explained in terms of induced polarization interactions of the type described by the Kirkwood-Bauer-Magat equation. At this time, however, it is not clear whether these interactions are between adjacent adsorbed molecules or between adsorbed molecules and surface OH groups. All studies of physical adsorption have involved surfaces which have OH groups. Further studies of physical adsorption on surfaces which are free of OH groups appears to be highly worth while. The most extensive study of these small changes was carried out by Sidorov (39) who used disks of porous glass as thin as 0.1 mm. The spectrum of physically adsorbed benzaldehyde is a typicall example of his findings. All of the C-H wavelengths in the adsorbed benzaldehyde were about 1 % shorter than the equivalent band of the liquid. The wavelength of the carbonyl vibration was increased by 0.7 %. Changes of similar magnitude were found for all of the molecules he studied (listed above) except for benzene, toluene, and ethylbenzene, where no changes were detectable. Sidorov observed the N-H stretching vibrations of physically adsorbed ammonia a t 2.92 and 2.94 1.1. Yates et al. (50) also studield the spectrum of ammonia physically adsorbed on porous glass. They found a new band at 3.45 p . They could not study the 2.9-p region, because their glass was relatively thick (2.5 mm.) and this region was obscured by a, strong OH band. They assume, however, that the N-H stretching bands observed by Sidorov were also present in their experiment and that Sidorov did not detect the 3.45-p band, because he had a smaller quantity of physically adsorbed ammonia in the beam path. The 3.45-p band was attributed to the OH vibration in the hydrogen bonded grouping O-He * - N . Yoshino (51) studied the abundance ratios of the gauche and trans forms of dichloroethane and the keto-enol isomers of acetyl acetone when these compounds were adsorbed (less than monolayer quantities) on silica gel. The gauche-trans ratio, which is unity in chloroform solution and 1.4 in the pure liquid, was found to be 1.9 in the adsorbed state. The 1430-~m.-~ gauche band and the 145O-cm.-' trans band were not affected by the adsorption. The absorption hands of acetyl acetone were rrieasured for n 1.5 % chloroform solution, the pure liquid, liquid saturated with water, arid the adsorbed state. The relative intensity of the lGOO-cm.-' enol hand and the 700-cm.-l keto band were used to determine the isomer ratio. The 1600-

INFRARED SPECTRA OF ADSORBED MOLECULES

43

em.+ band is increased by 25-cm.-l in the adsorbed state, but the 1700cm.-' band is not shifted. The apparent keto-enol ratios were found to be unity in chloroform solution, 2.4 in the pure liquid, 2.4 in the water-saturated liquid, and 8.6 in the adsorbed state. Extremely interesting infrared studies of physically adsorbed molecules were carried out by Sheppard and Yates (52).These workers studied the spectra of methane, ethylene, acetylene, and hydrogen on porous glass. They found that the perturbing effects of surface forces made it possible to detect bands which are found in the Raman spectra but are not observed in the normal infrared spectra. This indicates that the degree of symmetry of the adsorbed molecule is less than in the gaseous state because of the "one-sided" nature of the surface forces. This effect was discovered independently by Karagounis and Peter ( 5 . 2 ~in) studies 1 , 3 ,Btrichlorobenzene physically absorbed on silica. The spectrum of physically adsorbed hydrogen is shown in Fig. 26. The band due to physically adsorbed hydrogen at a surface coverage of 0.2 is found at 4131 cm.-'. The corresponding Raman band is at 4160 cm.-'. The infrared spectrum of physically adsorbed methane had a band a t 2899 cm.-' which is not present in the equivalent path-length of either the gas or the liquid. This corresponds to the symmetrical C-H stretching vibration which produces a Raman band at 2916 cm.-'. The spectrum of adsorbed ethylene shows an extra weak shoulder a t 3010 cm.-' which was assumed to be the normally infrared inactive v 1 vibration in which all four hydrogens are vibrating in phase and which produces a Raman band a t 3019 cm.-l. Sheppard and Yates (52) also studied the degree of rotation in physically adsorbed methane. The shape of infrared stretching bands is modified by rotation of the molecule so they calculated the band shapes expected for (1) no free rotation, (2) one degree of rotational freedom-rotation about an axis perpendicular to the surface-and (3) three degrees of rotational freedom. Comparison of the shapes of the calculated and observed bands

FREQUENCY em-'

FIG.26. Spectrum of hydrogen physically adsorbed on porous glass.

44

R . P. EISCHENS A N D W. A . PLISKIN

led to the conclusion that (3) could be ruled out hut that they could not distinguish between (1) and (2). The spectrum of COz physically adsorbed on Cabosil has also been used to study the question of rotation in physically adsorbed molecules ( 2 4 ) . Figure 27 is due to COn physically adsorbed on Cabosil a t 30" C. with surface coverage, 0, less than 0.01. The half-width of this hand is 14 cm.-l. This is less than one-half of the 30 cm.-l that would be expected for rotation about a fixed axis a t room temperature. These results indicate that, there is no free rotation of any kind in this case. This does not mean that the molecule is locked in one position. Free rotation as used here refers to a quantized rotation. The type of movement in which the molecule rotates in short irregular spurts is not ruled out. Figure 27 shows that the absorption band of physically adsorbed COs is quite intense even though the surface coverage is less than 0.01. This illustrates the high sensitivity of infrared in certain cases. The case of physically adsorbed COz is one of the most favorable, since COz has a high extinction coefficient. Using the same sample with the same optics (CaF, prism in Model 12 Perkin-Elmer Spectrometer), a band could be detected for CO, on Cabosil at EL coverage of 0.0001. If an effort were made using porous glass arid the instrumentation which is now available, coverages down to 0 = 1 X 10-7 could probably be detected.

60

4.2 43 WAVELENGTH (C)

FIG.27. Spectrum of carbon dioxide physically ndsor1)od on Csl)osil

INFRARED SPECTRA O F ilDSORBED MOLECULES

45

VI. THEPRESSED-SALT METHODFOR OBTAINING

MOLECULES SPECTRAOF ADSORBED I . Advantages and Disadvantages of the Pressed-Salt Method

The pressed-salt method has attained wide application in studies of the infrared spectra of solids. In this method the solid sample is mixed with a powdered halide salt such as K I or KBr and the mixture is pressed into a disk a t high pressures (53-55). This method reduces scattering because solid-gas interfaces are replaced by solid-salt interfaces. When this method is used; the particle size of the solid is not of critical importance and most ordinary silica or alumina catalysts can be used without the necessity of any particle-size separation. Although it is simple experimentally, the pressed-salt method will probably never attain a major importance in catalytic work, because once the sample is embedded in the salt, it cannot be subjected to further treatment. A recent paper describes the use of self-supporting disks of alumina as samples for the infrared study of adsorbed water (56).These self-supporting disks were prepared in the same type of die as is used for the pressedsalt method. The self-supporting disks will undoubtedly prove to be far superior to pressed-salt disks because the former are pervious t o gas and the complications caused by the presence of salt are eliminated. Satisfactory self-supporting disks were made by pressing 0.2 gm. of alumina in a 1-in. diameter die a t a pressure of 30-40 tons/sq. in. No preliminary particlesize separation is necessary, and the alumina may be impregnated with metal salts either before or after pressing in the die. The pressed-salt method was used by French and co-workers (57) in studies of problems of interest to the fields of flocculation, flotation, and catalysis. The flocculation studies were concerned mainly with the effect of acids and bases on the relative intensities of bands due to hydrogenbonded OH groups and free (not hydrogen-bonded) OH groups in bentonite clay. The relative amount of bonded OH groups appeared to increase when HC1 was added to the clay and decreased when NaOH was added. The flocculation of the clay is known to be increased a t low pH’s, so that it was assumed that the tendency to flocculate is related to the phenomenon producing the increase in the bonded OH’S. Although such a relationship is possible, the large body of work, which was discussed above, shows that surface OH groups are subject to an apparent hydrogen bonding effect in practically all cases where any material is adsorbed on a surface containing OH groups. Therefore, the possibility that the latter type of effect is being observed in this case must also be considered. The study of flotation by the pressed-salt method was illustrated by the spectrum of oleic acid adsorbed on CaFz . This spectrum shows a band a t

46

It. P. EISCHENS AND W. A . PLISKIN

6.4 p , which was attributed to oleic acid chemisorbed as an oleate arid associated with surface Ca atoms. The bands due to the C = C double bond were not observed in the spectrum of chemisorbed oleic acid. The failure to observe bands due to the C = C double bond was attributed to lateral polymerization of the chemisorbed species. It appears more reasonable to assume that these bands were not observed, because the extinction coefficient of the C = C band is small compared with that of the carboxylate ion. As an application of the pressed-salt method to catalysis, French and co-workers studied ammonia on cracking catalysts (57, 58). Other workers (33) have shown that this was an unfortunate choice because ion exchange occurs between the cracking catalyst and the halide salt. As a result of this ion exchange, spectra are observed which are due to ammonium halide rather than chemisorbed ammonia. This ion exchange is shown in Fig. 28, where A is the spectrum of Alon-C, B is due to Alon-C plus chemisorbed ammonia obtained by direct observation, and C was obtained after the Alon-C: with chemisorbed ammonia was pressed into a KBr disk (33). In B the ammonia is ill the NHt form because the sample had been exposed to the atmosphere. It is seen that the direct method shows the NHa band a,t 6.84 p , while the pressed-salt method shows this band at 7.14 p . Although this difference in wavelength is not large, it is important because it shows that the pressed-salt method cannot be used to study adsorption systems where ion exchange is likely to be encountered. Eyring and Wadsworth (59) used the pressed-salt method to study the adsorption of hexanethiol on ZnS, ZnO, and willemite (Zn,SiO,). The ZnS FREQUENCY (CM-')

I800

1600

1700

100

60

1500 (A1

-

z $60;-

$40-

f

IY

20

tc1

nto

0 5.4 ~~

5.8

6.2

(8) 6.6

NH:

1.0

1.4

FIG.28. Effect of pelleting technique on the spectra of Alon-C exposed to ammonia: ( A ) Alon-C!, (B) Alon-C after ammonia exposure, ( C ) ammonia,-exposed Alon-C in a KBr disk.

I N F R A R E D S P E C T R A O F ADSORBED M O L E C U L E S

47

and ZriO have surface OH groups. When the hexanethiol is adsorbed on these materials, the bands due to the surface hydroxyls at 2.9 p and the S-H bands of the hexanethiol a t 3.95 p are both decreased. This was attributed to a reaction between the surface OH arid the hexanethiol with formation of water and adsorbed hexanethiol, ZiiOH RSH -+ Zn-S-R H20. When the hexunethiol is adsorbed on willemite, the S-H band is decreased but the OH band intensities are increased. Eyring and Wadsworth attribute this to the reaction

+

+

2. Study of the Metal-Carbon Bond of Carbon Monoxide Chemisorbed o n Platinum All of the information on the structure of adsorbed molecules, which has heen discussed previously, has been obtained from consideration of bands due to bonds within the adsorbed molecules. I n that work, bands due to bonds between the surface and the adsorbed molecule were not observed, The failure to observe the latter bands was due primarily to experimental difficulties caused by the fact that silica and alumina have poor transmission in the regions where the adsorbent-adsorbate bands are expected. The pressed-salt method can be used to overcome this difficulty because it can be used to observe spectra of carbon monoxide on platinum particles which are not supported on silica or alumina (14). It is possible to use commercial platinum black, but best results are obtained if the platinum particles are formed by reduction of chloroplatinic acid while it is dispersed on the powdered salt. After reduction in hydrogen a t 300" C., the sample was treated with CO and transferred to the die. An essential feature of this technique is a final treatment with CO after the sample is in the die. The die is sealed with Apiezon Q wax to prevent exposure to the atmosphere between the time of the final treatment with CO and the application of pressure to form the salt disk. The isotope shift observed with chemisorbed C130 indicated that the force constant, Icl , of the platinum-carbon stretching vibration was 4 X lo5 dynes/cm. and the carbon-oxygen force constant, ICZ , was 16 X lo6 dynes/cm. Use of these values of the force constants in the equation (60)

( 3 . 5 5 1x ~ L;L;

klk2 -~

M2M3

R. P. EISCHENS A N D W.

18

A . PLISKIK

where L1 is the wavelength of the phtirium-carbon band, Lz is the \v:ivehig t t i of the carhi-oxygen 1)alid of 1itie:ir CO, :uid R 1 2 and h l , :LI% the masses of the carbon and oxygen :Ltoriis, leads to the prediction that the band due to the platiuum-carhii stretchiiig vihratioii would he forrid in the 2&25-p region. In deriving this equation, it is assunied that the effective mass of the surface l’t atom is iiifinite. Figure 29 shows the 4-8-p spectrum of CO chemisorhed on platinum iii a KBr disk. This spectrum was obtained with a NaC1 prism. Spectrum A of Fig. 30 shows the 14.5-16.5-p region and B shows the 20.0-22.0-p region. Figure 30 was obtained with a KBr prism. The bands near 5.0 and 5.4 p in Fig. 29 are due to the carbon-oxygen stretching vibrations of the linear and bridged structures of chemisorbed CO. The band a t 4.3 p is due to the unsymmetrical stretchiiig vibration of COz . This COz , which is formed by reaction of CO and 0 2 , is trapped in the salt disk. The band at 6.1 p is due to water. There is some uricertainty with respect to the interpretation of the hand a t 15 p in Fig. 30. A t least part and perhaps all of this barid is due to the bending vibration of the trapped CO, . The hand a t 21 I.( is of greatest importance because it is attributable to the Pt-C stretching mode of the linear CO which this experiment was designed to locate. It is not clear whether the 1%-C vibrations of the bridged CO are also represented by the 21 -I.( band. The apparent agreement bet ween the calculated and observed positions of the Pt-C stretching band is of interest because it indicates that force constants, which were measured a t low surface coverages, where interaction effects

50

WAVELENGTH IN

MICRONS

FIG. 29. Spectrum of CO chemisorlml on 1% in KBr disk (4-8-p region).

INFRARED SPECTRA OF ADSORBED MOLECULES

,~ ----__ .

z50 Y)

2 (L

40

49

14.5

-".-

IS.0

15.5 WAVELENGTH

IN

WAVELENGTH

16.0 MICRONS

IN

16.5

MICRONS

FIG.30. Spect>rumof CO chemisorbed on l't, in KBr disk: ( A ) 14.5-16.5-p region, ( B ) 20.0-22.0-p region.

are small, can be used to predict the band position even when the surface coverage is high. Thus far no band has been observed which can be attributed to the Pt-CSO bending vibration. This band is either a t a wavelength longer than can be scanned by the KBr prism or the intensity of the bending vibration is repressed by the close packing of the CO on the surface. I t is clear that the pressed-salt method has made it possible to observe bands due to the bonds between a surface and adsorbed molecules. This has important implications because it shows that the bond between the carbon and a metal atom of the surface is similar to that of conventional compounds. There is a possibility that the advantages of salt as a dispersing agent could be utilized, without the disadvantages of the pressed-salt method, by using high-area evaporated salt. This type of salt was used as an adsorbent by deBoer (61) in his studies of the color and ultraviolet spectra of adsorbed dyes.

VII. SPECTRA OF MOLECULES ADSORBED ON UNSUPPORTED METALS 1 . Spectrum of Carbon Monoxide on Evaporated Platinum Films

Evaporated metal films represent a type of sample which is of major interest in fundamental catalysis research. Moreover, these samples are not subject to the wavelength limitations imposed by the presence of silicious carriers. Therefore, development of infrared techniques suitable for observing spectra of molecules adsorbed on evaporated films is of vital

50

R. P. EISCHENS AND W. A. PLISKIN

2100

80 -

FREQUENCY (cm-1) 1900 1800

2000

I

I

I

l

4.8

l

l

5.0

1700

I

l

l

I

l

l

5.2

5.4 5.6 5.8 WAVELENGTH ( p )

l

l

l

-

l

l

~

€1.0

Fro. 31. Spectrum of carbon monoxide chemisorhed on films of evaporated platinum.

importance. Work on this type of sample has been limited but sufficient to show that such studies are possible. This conclusion is based on spectra such as that of CIO chemisorbed on evaporated platinum shown in Fig. 31. The sample consisted of four films of platinum deposited on the faces of two CaFz plates. After the films had been evaporated, the plates were mounted in a glass cylinder which was fitted with CaF2 windows and a gas inlet tube. The platinum was deposited from a tungsten heating wire in apparatus used in preparing samples for an electron microscope. The thickness of each film was estimated to be about 400 A, based on previous experience with this evaporator. A single CaF2 plate, with two films, transmitted about 50% of thLe radiation a t 5 p. The films were exposed to the atmosphere during transfer to the sample cell, so that it was necessary to displace chemisorbed oxygen with CO. At room temperature this displacement is slow. Figure 31 was obtained with a CO pressure of mm. after the films had been exposed to CO a t 1-atm. pressure for 18 hrs. (14). The spectra were obtained with a PerkinElmer Model 21 Spectrophotometer. The sensitivity of this instrument was increased by a factor of 5 by cutting out all but one of the metal fingers in the optical wedge. This is equivalent to suppressing the zero so that full scale deflection represents 80-100 % transmission rahher than &lo0 %. This important method of increasing the sensitivity of these instruments

INFRARED SPECTRA OF ADSORBED MOLECULES

51

was described by Coates and Anacreon (62).A blank sample of four Pt films was used in the reference beam. Figure 31 shows a strong band a t 4.9 p which was attributed to the C-0 stretching vibration of CO chemisorbed in the linear structure, Pt-CrO. This band is similar to that observed on Cabosil-supported platinum. On the basis of the intensity of the 4.9 p band in Fig. 31, it was calculated that the sample contained the equivalent of 20-40 monolayers of carbon monoxide. A value of 300 X lo-*”cm?/mol. was used as the extinction coefficient for the chemisorbed CO. The range of 20-40 monolayers results from the uncertainty as to what crystal faces are exposed. Since there were four films in the sample, each film chemisorbed the equivalent of 5-10 monolayers. 2. Emission Spectra of Chemisorbed Molecules

In infrared absorption spectroscopy a fraction of the sample molecules is raised to higher-energy states by absorption of radiation from the infrared beam. In emission spectroscopy, the sample molecules are raised to high-energy states and spectra can be observed which are due to spontaneous transitions from higher to lower energy states. Absorption and emission spectra of any specific compound will have bands a t about the same positions. Infrared emission spectra are not commonly used because the experimental methods are more difficult than for absorption spectra. However, the emission method has a unique advantage in the study of adsorbed molecules because bulk pieces of heated metal can be used as adsorbents. At temperatures above 150” C. heated metal rods will provide enough radiation to operate a Perkin-Elmer Model 21 Spectrophotometer if they are positioned close to the entrance slits and the slits are open to their maximum width. Differential emission spectra can be obtained if the rod in front of one slit is covered by a thin layer of the material to be studied and the rod in front of the other slit is left bare. The idea of obtaining emission spectra of molecules adsorbed on metal was suggested by French and Wadsworth (58). They indicated that they had used the method t o obtain spectra of “surface complexes” but did not give specific details of these spectra. Because of the potential importance of this method, an attempt was made to confirm its utility for the study of adsorbed molecules (14). Preliminary results indicated that it was quite easy t o obtain spectra of thin films of oleic acid which were left on aluminum rods after wiping lightly with a crocus cloth. However, the bands in the spectra of oleic acid obtained in this way were a t the same positions as in bulk oleic acid and no new bands were observed. Thus, these spectra were not due to chemisorbed molecules but rather to layers of acid about 10 monolayers thick.

52

R . P. E I S C H E N S .4ND W. A . PLISKIN

It was assumed that oleic acid could not be considered as chemisorbed unless the band near 5.9 p due to acid carbonyl had been replaced by carboxylate bands near 6.4 and 7.0 p . Subsequent attempts to obtain emission spectra of clhemisorbed oleic acid were made using highly polished aluminum rods which were 55 in. in diameter and 1 ft. long. Each rod was inserted about half of its length into an electric furnace and heated to 200" C. At this temperature thin films of oleic acid were not retained more than a few minutes on rods which were exposed to the atmosphere. The spectra in Fig. 32 were obtained by the following procedure ( I d } . The sample rod was maintained a t 200" C. until acid from preceding runs was removed. Scanning was started and 3 min. before the 5.0-p region was reached, a daub of acid was placed on the rod about 2 in. from the slit. This produced A , which shows bands a t 6.5 and 7.15 p . :No band was ohserved in the 5.8-5.9-p region. Spectrum B was obtained 15 min. later. Spectrum A is attributed to oleic acid which has migrated over the surface of the rod and been chemisorbed as an ionic soap. In order to obtain this spectrum, it was necessary to open the slits to 1800 p . 'Thus any irregularities of width smaller than the breadth of the horizontal line shown in Fig. 32 are due to noise and have no significance. The bands observed in A are so small that it is doubtful if they could have been detected if the increased sensitivity wedge had not been used. Since the background of infrared emission spectrosco.py is not so well established as for absorption spectroscopy, it is more difficult to predict the intensity of infrared emission bands. However, simplified calculations involving Planck's radiation law and Kirchhoff's law (63))and Einstein's emission and absorption coefficients (64)) show that a n emission band

6.2

6.4

6.6 6.8 7.0 WAVELE NGTH(pL)

7.2

7.4

FIG.32. Emission spectrum of oleic acid chemisorbed on an aluminum rod (oxidecoated).

INFRARED SPECTRA OF ADSORBED MOLECULES

53

with the intensity given by spectrum A could be expected from a monolayer of carboxylate ions on a gray body with absorptivity = 0.02. This corresponds to a reflectivity of 0.98, which is a reasonable value for the polished aluminum rods used in the present experiment (65). Attempts were also made to observe emission spectra of CO chemisorbed on platinum by wrapping one rod with platinum foil and enclosing it in a gas-tight cell. These efforts were not successful. The significance of these failures has not been determined. It was possible that the foil was not covered by a layer of CO or that the intensity of the emission bands of chemisorbed CO are not as great as those of the corresponding absorption bands. The failure may also have been due to the orientation of the CO molecules. If they were oriented perpendicular to the surface, the radiation would be emitted perpendicular to the direction of the change of dipole moment and would be parallel with the surface of the rod, so that little would enter the slits of the spectrometer. Because of the failure of the attempts to observe an emission spectrum of chemisorbed CO, there is some doubt as to the scope of application of this method. The method is worthy of considerable attention because it provides a means of obtaining spectra of gases adsorbed on wires. The best chance for successful application lies in systems that can be heated to high (400" C.) temperatures without desorbing the chemisorbed gas. 3 , Rejection Spectra of Chernisorbed Molecules

One of the advantages of the emission method is that it makes possible studies of molecules adsorbed on massive pieces of metal. This advantage is shared by the reflection method by which spectra of monolayers adsorbed on mirrorlike surfaces are obtained. Work on the development of reflection techniques has been carried out by Pickering and Eckstrom (66) and by Francis (67). Pickering and Eckstrom evaporate catalyst metals, such as rhodium or nickel, onto the mirrors of a White multiple (20-40) reflection cell (68). A series of scans are taken before and after adsorption of the gases. The comparison of the two sets of curves is accomplished by a method described by King et al. (69), in which the detector output is fed to an analog-to-digital converter for recording on punched cards. The final graphs are plotted with an IBM accounting machine. This technique makes it possible to study absorption bands of the order of 0.03 to 0.05% of the radiation.

VIII. CONCLUSION Not only is the infrared method applicable to studies of both chemisorption and physical adsorption systems, but it also provides a means of distinguishing between chemisorption and physical adsorption. The infrared

54

R. P. EISCHENS AND W. A . PLISKIN

spectra of physically adsorbed molecules are similar to the spectra of the gaseous, liquid, or dissolved states. While minor differences in band positions may be observed during physical adsorption, the,se differences are of the same magnitude as the differences between the various unadsorbed states. No new bands are observed in the spectra of physically adsorbed molecules which cannot be attributed to vibrations which provide infrared or Raman bands in the liquid state. The infrared spectra of chemisorbed molecules are markedly different from those of the unadsorbed states. This is true whether the heat of chemisorption is high or low. Although in some cases, such as ammonia on cracking catalysts, the spectrum of the chemisorbed species may resemble that of the gaseous state, this is an artificial similarity caused by the fact that only a small portion of the spectrum is observed. When longer wavelengths are scanned, differences will be noted. The ultimate experimentally defined criterion of chemisorption is observation of a band due to the bond between the surface and the adsorbed species. The broad scope of the problems discussed in this review emphasizes the flexibility of the infrared methods. Because of this flexibility, the direct and unambiguous nature of the spectral evidence, and the ease and rapidity with which information can be obtained, it is obvious that the study of surfaces by the infrared technique will play an important role in future developments in the field of surface chemistry. REFERENCES 1 . Eischens, R . P., Francis, S. A., and Pliskin, W. A., J . P h y s . Chem. 60, 194 (1956). 2. Pliskin, W. A., and Eischens, R. P., J . Chem. Phys. 24, 482 (11956). 3. Golike, R . C., Mills, I. M., Person, W. P., and Crawford, B., Jr., J . Chem. P h y s . 26, 1266 (1956). 4 . Francis, S. A., J . Chem. P h y s . 18, 861 (1950). 6. Jenkins, G. I., and Rideal, Sir Eric, J . Chem. Sac. p. 2490 (1955). 6 . Horiuti, J., and Polanyi, M., Trans. Faraday Sac. 30, 1164 (1934). 7. Pliskin, W. A., and Eischens, R. P., from a paper to be su1)mitted for publication in 1958. 8. Beeck, O., Discussions Faraday Sac. N o . 8 , 118 (1950). 9. Selwood, P. W., J . Am. Chem. Sac. 79, 3346 (1957). 10. Douglas, J. E., and Rabinovitch, B. S., J . A m . Chem. Sac. 74, 2486 (1952). 11. Sheridan, J . , J . Chem. Sac. p. 133 (1945). 18. deBoer, J. H., and Coenen, J. W. E., University of Delft, I M f t , Holland, private communication, April 1957. 13. Selwood, P . W . , i n “Catalysis” (P. H. Emmett, ed.), Vol. I, p. 353. Reinhold, New York, 1954. 14. Eischens, R . P., and Pliskin, W. A., unpublished results 19551-1956. 16. Brunauer, S., and Emmett, P. H., J . Am. Chem. Sac. 62, 1732 (1940). 16. Eischens, R . P., Pliskin, W. A., and Francis, S. A,, J . Chem. E’hys. 22,1786 (1954). 17. Sastri, M. V. C., and Viswanathan, T. S., J . A m . Chem. SOC.7 7 , 3967 (1955). 18. Griffin, C. W., J . A m . Chem. Sac. 69, 2431 (1937).

INFRARED SPECTRA O F ADSORBED MOLECULES

55

19. Storch, H. H., Golumbic, N., and Anderson, R. B., “The Fischer-Tropsch and Related Synthesis,” p. 592. Wiley, New York, 1951. 20. Edgell, W. F., Magee, C., and Gallup, G., J . A m . Chem. SOC.78,4185 (1956).

I . , Shufler, S. L., and Sternberg, H. W., J . Am. Chem. 77, 3951 (1950).. 22. Boudart, M., J . Am. Chem. Sac. 74, 3556 (1952). 23. Yang, A. C., and Garland, C. W., J . Phys. Chem. 61, 1504 (1957) 24. Eischens, R . P., and Pliskin, W. A., Advances in Catalysis 9, 662 (1957). 26. Pitzer, K . S., “Quantum Chemistry,” p. 191. Prentice-Hall, Englewood Cliffs, New Jersey, 1953. 26. Keller, W. E., and Johnston, H. L., J . Chem. Phys. 20, 1749 (1952). 27. Broeder, J . J., van Reijen, L. L., Sachtler, W. M. H., and Schuit, G . C. A., 2. Electrochem. 60,838 (1956). 28. Mapes, J. E., and Eischens, R. P., J . Phys. Chem. 68, 1059 (1954). 29. Eischens, R . P., 2. Electrochem. 60, 782 (1956). SO. Jones, A. C., and Benasi, H. A., Shell Development Company, Emeryville, California, private commrinication. 51. Svatos, G. F., Curran, C., and Quagliano, J. V . ,J. Am. Chem. SOC.77,6159 (1955). 52. Faust, J . P., and Quagliano, J. V . , J . Am. Chem. Sac. 76, 5346 (1954). 33. Pliskin, W. A., and Eischens, R . P., J . Phys. Chem. 69, 1156 (1955). 34. Buswell, A. M., Krehs, K., and Rodebush, W . H., J . Am. Chem. Sac. 69, 2603 (1937). 56. Coblentz, W. W., J . Franklin Znst. 172, 309 (1911). 36. Buswell, A. M., Deitz, V., and Rodebush, W. H., J . Chem. Phys. 6, 501 (1931). 37. Garino-Canina, V., Compt. rend. 239. 705 (1954). 58. Chevet, A , , J . phys. radium 14, 493 (1953). 59. Sidorov, A. N., Doklady Akad. Nauk S.S.S.R. 96, 1235 (1954). 40. Weiss, P . B., private communication, based on mathematical treatment devcribed in Science 123, 887 (1956) and 126, 31 (1957). 41. Yaroslavskii, N. G., and Terenin, A. N., Doklady Akad. Nauk S.S.S.R. 66, 885 (1949). 42. Kurbatov, L. N . , and Neuimin, G. G., Doklady Akad. Nauk S.S.S.R. 68, 341 (1949). 43. Greinacher, E., Luttke, W., and Mecke, R., 2. Electrochem. 69, 23 (1955). 44. Yaroslavskii, N . G., and Karyakin, A . V . ,Doklady Akad. Nauk S.S.S.R. 86, 1103 (1952). 46. Yates, D . J. C., Proc. Roy. Sac. A224, 526 (1954). 46. Terenin, A. N., Mikrochim. Acta 2, 467 (1955). 47. McDonald, R . S., J . A m . Chem. SOC.79, 850 (1957). 48. Josien, M. L., and Fuson, N., J . Chem. Phys. 22, 1169 (1954). 48a. Filimonov, V. N., Optika i Spektroskopiya 1,490 (1956). @b. Filimonov, V. N., and Terenin, A. N., Doklady Akad. Nauk S. S. S. R. 109, 982 (1956). 49. Pimentel, G. C., Garland, C. W., and Jura, G., J . Am. Chem. Sac. 76, 803 (1953). 60. Yates, D. J. C., Sheppard, N., and Angell, C. L., J . Chem. Phys. 23, 1980 (1955). 61. Yoshino, T . , J . Chem. Phys. 23, 1564 (1955). 62. Sheppard, N., and Yates, 11. J . C., Proc. Roy. Sac. B 3 8 , 69 (1956). 62a. Karagounis, G., and Peter, O., 2. Electrochem. 61,827 (1957). 65. Cook, E. S., Kreke, C. W., Barnes, E . B., and Motzel, W . , Nature 174,1144 (1954). 64. Stimson, M. M., and O’DonnelI, M. J., J. Am. Chem. SOC.74, 1805 (1952). 66. Schiedt, U., and Reinwein, H., Z . Naturforsch. 7b, 270 (1952). 21. Friedel, R . A., Wender,

56

R. P. EISCHENS .4ND W . A . PLISKIN

56. Lindyuist, R. H., and Rea, I). G., paper presented a t 132nd meeting of the American Chemical Society, New York, Sept. 1957. 57. French, R . O., Wadsworth, M. E., Cook, M. A , , and Cutler, I B., J . Phys. Chern. 68, 805 (1954). 58. French, R. O., and Wadsworth, M. E., I’reprint of General Papers Number 34, presented before the Division of Petroleum Chemistry, American Chemical Society, 1955. 59. Eyring, E. M . , and Wadsworth, M. E., Mining Eng. 6, 531 (1956). 60. Herzherg, F., “Infrared and Raman Spectra of Polyat,omic Molecules,” p. 173. Van Nostrand, l’rinceton, New Jersey, 1945. 61. deBoer, J. H., 2 . p h y s i k . Chenh. (Leipsig) B16, 397 (1932). 62. Coates, V. J., and Anacreon, R., Conference 011 Applied Spectroscopy, I’ittshrgh, Pennsylvania, 1957. 69. Joos, G., “Theoretical Physics,” 2nd ed., pp. 608-614. Hafner, New York, 1951. 64. Pauling, I,., and Wilson, E . B . , Jr., “Introduction to Quanhim Mechanics,” p . 301. McGraw-Hill, New York, 1935. 65. Hass, G., J . Opt. SOC.A m . 46, 945 (1955). 66. Pickering, H . L., and Eckstrom, H. C., Pan American Petroleum Corpmition, Tulsa, Oklahoma, private communication, April 1957. 67. Francis, S. A., Texaco Research Center, Beacon, New York, private communications, May 1956. 68. White, J . U., J . Opt. Soc. A m . 32, 285 (1942). 69. King, G. W . , Blttnton, E. H., arid Frowley, J., J . Opt. Soc. A.m. 44, 397 (1954).

The Influence of Crystal Face in Catalysis * ALLAN T. GWATHMEY

AND

ROBERT E. CUNNINGHAM

Department of Chemistry, University o j Virginia, Charlottesville, V i r g i n i a Page

I. Introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Factors to be Considered in Developing a Theory of the Influence of Face. . 111. The Single-Crystal Method of Studying Surface Reactions.. . . . . . . . . . . . . . . 1. Growth of the Crystals 2. Prep IV. Results 1. Oxid 2. Type I Catalytic Reactions in Which the Surface of the Catalyst Rearranges: the Reaction of Hydrogen and Oxygen on Copper.. . . . . . . . . . . . . a . The Reaction Rates on Different Crystal Faces.. . . . . . . . . . . . . . . . . . . . ti. The Formation of Oxide Films on the (111) Face of Copper and the Relation of the Films to Catalytic Properties.. . . . . . . . . . . . . . . . . . . . . . c. The Formation of Copper Powder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . d. The Effect of Foreign Atoms Added to the Surface. . . . . . . . . . . . . . . . . e. Summary of Type I Catalytic Reactions.. . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Type I1 Catalytic Reactions in which Solid Reaction Products Deposit on the Surface: the Reaction of Carbon Monoxide onNickel and Other Metals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a. Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b. Summary of Type I1 Catalytic Reactions. ............. the Structure 4. Type I11 Catalytic Reactionsin which No Visi of the Surface Takes Place: the Reaction of Hydrogen and Ethylene on Nickel.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a. Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b. Summary of Type I11 Catalytic Reactions.. . . . . . . . . . . . . . . . . . . . . . . . . V. Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

57 60 63

70 72 76 78 80 83

85 85 88

89 89 91 91 94

I. INTRODUCTION The identification of the regions of high and low activity on the surface of a catalyst is of paramount importance. The purpose of this chapter is to show that for many reactions the different crystal faces have markedly different activities, the relative order of activity among faces being dependent on the conditions of the reaction. The identification of the active regions of a catalyst has long remained unnecessarily mysterious. There are several possible reasons for this.

* Most of the experimental work described in this paper was supported by the Office of Naval Research or the Petroleum Research Fund of the American Chemical Society. Grateful acknowledgement is made t o the donors of said fund. 57

58

ALLAN T. GWATHMEY AND ROBERT E . CUNNINGHAM

Since the time of the discovery of catalysis, the fact that a relatively small amount of catalyst could control so effectively the rate of reaction of a very large amount of material, without apparently being changed itself, has naturally tended toward mystery. Second, the methods of manufacture of industrial catalysts were for many years highly guarded secrets known only to those responsible for their preparation, so that the catalyst surface itself in the past has often been a kind of “posted property.” Catalysts with the most striking characteristics were not available for general study. Third, the popular method of studying catalysts has consisted of measuring the amounts of gas adsorbed and the rates of reaction on powders, followed by attempts to deduce indirectly the nature of the active regions from plots of these measured quantities. An undue emphasis on such indirect observations has tended to obscure the possibilities of direct observation of the surface. Finally, the necessity of using the catalyst in the form of very small powders or highly porous masses in both commercial processes and laboratory studies, in order to obtain the maximum surface area, has made direct observation exceedingly difficult. For a scientific study, however, there is no reason why the catalyst must be in the form of a powder. With the aid of special techniques, it is possible to prepare some surfaces whose structure and purity are known to a fair degree of accuracy. This is equivalent to picking the “battleground” on one’s own terms, and there are decided advantages in doing so. The regions of high and low activity can then be directly observed, and changes in activity and structure followed. The identification of these regions on the catalyst surface is a simple requirement which must be fulfilled before it can be hoped t o answer the more difficult question of .why these regions have specific activities. If it can be demonstrated that certain faces of any one catalyst are areas of high activity while others are areas of low activity, and further that within any one face special structures, such as imperfections or kinks, may have special activity, some of the unnecessary mystery of catalysis will be removed. Attention can then be directed toward explaining why a particular structure has a certain activity. Attempts may even be profitably made to control the activity of the catalyst by controlling the faces exposed and the type and number of imperfections within any one face. Correlations of various kinds between the geometry of :a solid surface and catalytic activity have been proposed in the past in varying degrees by such workers as Langmuir ( I ) , Adkins (a), Burk ( 3 ) , Balandin (4, Beeck ( 5 ) , Rideal(6), Maxted (7), and Gwathmey (8).A first guess would suggest that a particularly favorable fit of the adsorbed molecules on the crystal array of the surfaces, not necessarily a one-to-one correspondence, would promote reaction. There have been few cases, however, in which a deliberate effort

INFLUENCE O F CRYSTAL FACE

59

has been made to control the structure a t the surface of the catalyst which was being studied. The work of Beeck ( 5 ) ,Wheeler, Smith, and Ritchie on evaporated films of known orientation is too familiar to require a summary here. Much valuable information was obtained which has profoundly influenced the understanding of a solid catalyst. I n relation to the studies to be described here, however, it should be pointed out that films of only one orientation with copper or nickel could be obtained, that the porous nature of a film with its many intriguing aspects has not been directly observed or explained, and that the exact faces exposed a t the surface could not be determined with electron diffraction. Sachtler et al. (9) have shown with the aid of the electron microscope that the facets exposed at the surface of an evaporated film of nickel prepared in a similar manner are not parallel to the plane of the glass backing. It will be shown later that catalytic reactions can produce great changes in the topography of the surface. Recent studies of tungsten and nickel points with the field-emission microscope by Muller (IO),Becker ( I I ) , and Gomer (1.2) have indicated that the different faces of a small metal crystal may have different chemical activities. During the period of 1934-1938, one of the authors attempted to determine the nature of the regions of high activity on a catalyst surface with the aid of the diffraction of low-voltage electrons from the surface of a single crystal of copper. Although diffraction maxima were obtained, it was soon realized that the limiting factors in these studies were the cleanliness and smoothness of the single crystal surface. Not only was the initial surface, as placed in the apparatus, not sufficiently smooth and free of contaminants, but thin films of oxide, oil vapors, and unknown contaminants collected on the surface during the experiments in spite of efforts to use pure gases and cold traps. A deliberate attempt was then made to improve the preparation of the surface of the crystal, and, in trying to find the etching reagent which gave the smoothest surface, it was noticed that for any one reagent one face would give a much smoother surface than another. The different etching characteristics of the different faces had long been known and used in the preparation of polycrystalline specimens for microscopic examinations. This variation in the reaction of the different faces with liquids suggested that the reaction of metals with gases might likewise differ with crystal face. Such effects had not been previously observed, partly because of the inability t o obtain smooth surfaces on the different faces of the same crystal. About this time electrolytic polishing was developed by Jacquet ( I S ) , and, when it was applied to a single crystal, it was possible to obtain a highly polished surface on all faces a t the same time. Through the use of the crystal in the form of a sphere, a specimen could be obtained which ex-

60

ALLAN

T.

GWATHMEY AND ROBERT E . CUNNINGHAM

posed all possible crystal faces, and by cutting the crystal along specific crystal planes, specimens could be obtained which largely exposed one crystal face. The details of the preparation of the surfaces will be described later. With the aid of such specimens, it was possible to study the catalytic activity of any one face, or to compare the activities of the different faces under the same conditions, and simultaneously to follow changes in the structure of the surface during reaction. The action of promoters, in the form of thin layers of foreign atoms, in controlling these rearrangements was investigated, and some information was obtained on the role played by imperfections within any one face. Although much of this paper will be concerned with methods of preparing the surface and with the determination of the surface topography, there is much that remains to be done in defining the structure of the catalyst surface on an atomic scale. This must be done for each reaction, the mechanism of which it is desired to understand. The primary purpose of this work is to show the importance of face and of the complex changes in the structure of the surface which take place as the reaction proceeds. It may be concluded that a careful rate measurement, if it is to be used for purposes of interpretation, must be accompanied by an equally careful definition of the surface topography of the catalyst. Although this is a difficult and tedious requirement, it a t least makes it possible in future experiments to bring under control one of the most important of variables in catalysis, surface structure.

11. FACTORS TO BE CONSIDERED IN DEVELOPING A THEORY O F THE I NF LUENC E O F FACIE At the present time there is no theory which can interpret quantitatively the influence of fact:, or surface structure, on the activity of a catalyst. Thus, there is no theoretical explanation of why the activity of any one catalyst will w r y with the crystal face exposed a t the surface or why the order of the relative rates among faces will vary with the reaction. There is no theory that mill even predict which face will be stable on the surface of a particular catalyst during a particular reaction. Since the number of atoms per unit area and their arrangement on the surface depend 011 the crystal face exposed, it seems reasonable to expect that the catalytic activity for any one reaction would depend on the face exposed a t the surface. Some experimental evidence will be presented later which suggests that crystal orientation, which involves the arrangement of atoms both a t the surface and in a few layers beneath the surface, may control catalytic activity. Sherman and Eyring (14) from theoretical considerations proposed that in case of the chemisorption of hydrogen on carbon the activation energy

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depended on the lattice spacing, being a minimum a t 3.6 A. Similar calculations were made by Okamoto and co-workers (15) for the adsorption of hydrogen on nickel. The general level of catalytic activity of some metals has in the last few years been explained in terms of the electronic interaction between the metal surface and the reacting molecules. This subject has been well discussed elsewhere (16). Studies of electronic interactions, however, have been concerned with explaining the general level of catalytic activity of polycrystalline materials and not the nature of the active regions in any one catalytic material. At this point it should be emphasized that the differences in activity between different faces on one catalyst with a particular d character can be much greater than the differences in measured activity between two catalysts with appreciably different d characters. It is of interest that the greatest differences in rate with face have been found with the transition metals, the activity of which in the polycrystalline form has been interpreted in terms of the d character. The important question arises as t o what is the relationship between the geometric factor and the electronic factor in controlling catalyst activity. In using the term “geometric factor,’’ a distinction must be made between the lattice spacing, which in general is the edge length of the fundamental cube, and the arrangement of the atoms in the face exposed a t the surface. The influence of the lattice spacing on catalysis and its relation t o the electronic character will be considered first. The rates of the reaction between ethylene and hydrogen on evaporated films of the transition metals, as determined by Beeck (5) and associates, can be plotted against either lattice spacing or per cent d character of the metal bond. When plotted against lattice spacing, rhodium with a spacing of 3.75 A. has the greatest activity. The rates decrease for metals which have lattice spacing either greater or smaller than that of rhodium. This type of plot suggests that lattice spacing determines catalyst activity. When the same rate measurements are plotted against the per cent d character of the metallic bond, the points for all of the metals, with the exception of tungsten, fall on a smooth curve, and rhodium, which has the highest per cent d character, has again the highest catalytic activity. Thus, these measurements seem to support the hypothesis that catalytic activity is determined by both the geometric factor and the d character. The reason for this appears to be the fact, emphasized by Pauling (17), that the d character controls the lattice spacing. Trapnell (18) has suggested that a different mechanism is probably involved in the operation of the geometric and electronic factors and that ethylene hydrogenation probably proceeds most readily on rhodium not because the 3.75 A. is most favorable, but because the high d character of rhodium enables it to adsorb gases weakly. Thus, in the case of the transition metals, one aspect of the geometric

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ALLAN T. GWATHMEY AND ROBERT E. CUNNIKGHAM

factor, lattice spacing, may be ail expression of the electronic character of the metal, but lattice spacing is only one aspect of the geometric factor and is by no means a true measure of the surface structure of a catalyst. It is not easy to find one term which will describe satisfactorily the geometry of a surface and all its structural features which control chemical activity. The forces a t the surface of a solid will depend on the number of atoms per unit area in the surface layer, on the arrangement of the atoms with respect to each other in the surface layer, and on the number and arrangement of atoms in a t least two layers below the surface. The term “arrangement” includes both the distance between the atoms and the directional orientation of the position of one atom with respect to that of another. Also, the number of steps and kinks in a surface is determined by it:3 crystallographic orientation. All these structural factors are included in the use of the term “crystal face,” and this term will be generally used in this sense in this paper. I n addition, other factors, such as edges between faces and imperfections of various kinds, must also be taken into account in considering the activity of a surface. A comment should be made about the use of models. Often in the past in discussing adsorption and chemical activity, models, consisting of two atoms separated by a distance equal to that between atoms in the close packed direction, or consisting of a two-dimensional array of dots representing atoms in a plane, have been used. Such models may at times be helpful in analyzing the possibilities for reaction, but they give a misleading picture of a metal surface, especially when it is actually taking part in a chemical reaction. A large three-dimensional model, showing the structure of possible faces a t the surface, is helpful, especially in considering the number of nearest neighbors, but even this model must be used with care, for still other factors, the importance of which are shown in experiments with single crystals, must be taken into consideration. As mentioned above and to be described in detail later, certain faces only may be stable on the surface of a working catalyst. The faces which are stable vary strikingly with the reacting gases and the conditions of reaction, and the surface will rearrange to expose these faces. Powders will form and disappear in some cases, and the rate of formation of these powders will vary with face. Thus, the nature and structure of a catalyst surface during reaction is different from the nature and structure during adsorption. Adsorption measurements are, of course, of value in themselves, but even when made on carefully prepared single crystal surfaces, they may be of doubtful value in interpreting catalytic reactions. This subject will be discussed later. The importance of the d character and lattice spacing in controlling the general activity of the transition metals has been em:phasized. Various

I S F L U E S C E OF CRYSTAL FACE

63

structural features of the surface which control its activity have also been discussed. The question naturally arises as to what is t.he relation, if any, between the d character of the bulk metal and t'he crystal face exposed a t t,he surface. The dangers of assuming that bulk and surface properties are the same have been emphasized (26), but it seems reasonable to assume that the electronic interaction between the surface and the reacting molecules would depend 011 t.he face exposed. I t even seems reasonable that the d character, or an equivalent electronic. property, which depends on the number of nearest. neighbors, and therefore influences bond hytlridization, would vary 1vit.h the crystal face. In connection with the import,ant. question of the stability of the different faces in different chemical environments, it is conceivable that the rearrangement of the surface and the stabi1it.y of t.he faces might be interpreted in part in terms of the d character. It is possibie to be more definit,e about the influence of face on the work function, which is a measure of the electron affinity of the metal. The work function has been shown to depend 011 crystal face (19). The question of the relationship of the work function and the specific surface energy of the surface t o its chemical activity has been discussed by Suhrmann (20). In concluding t'his section, it should be emphasized t,hat there is a t present no satisfactory theory of the influence of face on t.he catalytic activity of a metal. This section has been concerned with those factors which must be taken into considerat.ion in t.he development, of a theory of the influence of surface structure on catalytic activity.

111. THESISGLE-CKYSTAL METHODOF STUDYIXG SURFACE I~EACTIOSS The single-crystal method of study is in principle a simple one, and it has wide application in the study of many types of surface react.ions. I t consists essentially of preparing the metal specimen in the form of a single crystal of sufficient size so that the individual faces can be easily identified and studied. The surface is given :t very high polish, generally by means of electrolytic polishing, and great care is taken in the removal of foreign cont'aminants which might influence the reactivity of the surface. The rates of reaction of t.he different cryst,al faces may be measured, and the initial high polish makes it possible to observe slight changes in t,he structure of the surface during reaction. If the crystal is prepared in the form of a sphere, all possible faces will be exposed a number of times on t.he surface. In t.he case of the face-centered cubic metals, for example, the (100) face will appear 6 times, the (111) face 8, t,he (110) face 12, and so on. Therefore, all faces appear on the surface a number of times, and from t,he symmet.ry of the patterns which form during rea.ction, as described later, t.he different crystal faces may be

6‘4

ALLAN T. GWATHMEY AND ROBERT E. CUNNINGHAM

easily identified. Theoretically, a face would appear on the surface at :i tangent point only, but if the sphere is sufficiently large, there will be an appreciable area where the array of surface atoms nearly corresponds to each of the crystal faces. Several different types of “reaction patterns” may form on the sphere as the reaction proceeds. In cases where the reaction products actually build up on the surface of the metal, as in oxidation, the oxide film will form more rapidly on one face than on another. When the films are in the range of 200 to 2,500 A. and the sphere is examined by placing a tube of white paper over the crystal, the different thicknesses of oxide on the different faces appear as a regular pattern of interference colors of great beauty. The symmetry of one of these highly colored patterns is shown in Fig. 1. In electrodeposition on a crystal sphere a t a low current density the metal will deposit more rapidly on one face than another, and ibe sphere is converted into a polyhedron, or small facets are formed on the different faces which may be seen under the microscope. I n the case of catalytic reactions where the reaction products do not build up on the surface but pass on, as in the case of the reaction of hydrogen and

FIG.1. Interference color pattern on a copper single crystal oxidized in a.1 ntmosphere of oxygen for 20 min. a t 250”.

INFLUENCE OF CRYSTAL FACE

65

oxygen on the surface of copper to form water, small facets form on certain faces because of the rearrangement of the surface atoms. If such a crystal is examined in a dark room with the aid of a flashlight, patterns of brilliant reflections can be seen over the surface of the sphere, and again from the symmetry of the patterns, various crystal faces can be identified. Etching by liquids also produces facets and gives similar reflection patterns. The rate of film formation and the type of facets formed on the different faces depend on the metal, the nature of the reacting gases, and the conditions of the experiment. These patterns are highly specific and remarkably sensitive to impurities. In some catalytic reactions, such as the decomposition of carbon monoxide on nickel to form carbon, the solid products build up on the different faces at different rates and a striking pattern of these deposits may be seen. The use of spheres is particularly advantageous in making a preliminary study of a surface process in order to compare rates on the different fates under the same conditions of experiment and in order to pick out the most interesting regions for further study. When larger areas of a particular face are desired, flat surfaces may be cut parallel to a particular crystal plane, and methods have been developed as described later, whereby measurements may be carried out on a single face and the effect of edges and other faces may be eliminated. A particularly convenient specimen is a spherical crystal with plane surfaces cut on it parallel to the particular faces which it is desired to study. The actual measurements can be made on the flat faces, and the pattern on the spherical surface can be used as a sensitive test for possible contaminants and improper control of experimental condit,ions. The surface structure of the different faces should be examined with both optical and electron microscopes, and it is interesting that the rearrangements which have been observed on large single crystals are confirmed by recent observations of small metal points made with the field emission arid positive ion microscopes. The method of using large single crystals has the decided advantage of being able to measure the rates of reaction while changes in the surface structure are also observed. These crystals may be used for studying, in addition to oxidation and catalysis, a large number of surface processes such as wetting, electrochemical processes, friction and wear, diffusion, and bonding between crystals. It is helpful to build models of such surfaces, especially of a spherical surface, so that the arrangement of atoms and the number of nearest neighbors on the different faces can be seen. In describing the various reaction patterns, there are certain terms which will be used. Most of the patterns are quite complicated, and the minute details will not be described. They must, however, be taken into consideration in future studies. I n experiments with spheres the term “(111) region,”

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ALLAN 1‘. GWATHMEY AND ROBERT E . CUNNINGHAM

for example, will refer to a region in which there are constant activity and surface structure for any one set of conditions, with the (111) pole being located a t the center of the region. The size of the region will vary with the reaction. Sometimes it may be small, just a dot, and include only the face referred to, but often it may be large, for example, >iin. in diameter on a sphere $6 in. in diameter, and include many faces near the (111). The size may be determined by examining the photographs The term “(111) face” will be used to refer to a plane face which has been cut parallel to the (111) plane. Similar expressions will be used for other faces. A particular face may remain smooth during the reaction, or it may develop facets parallel to other planes. Thus, (111) facets may develop in a (100) region. In most studies of surface chemistry, it is common practice to devote a small fraction of the total effort to the preparation of the surface and then much effort to an elaborate measurement of rates and other surface properties. Obviously, the measurement is no better than the preparation of the surface, and if the surface is contaminated, strained, or has an unknown crystal orientation, the measurements can have very little meaning. Furthermore, since metals as ordinarily used consist of minute crystals randomly arranged, measurements made on such specimens are composite quantities and tell little about the reactivity of a particiilar type of structure. It is therefore highly important that the preparation of the catalyst specimen be carried out with great care, and this is generally a tedious undertaking. I n this method of study there are two important steps in the preparation of the specimen. The first is the growth of the single crystal itself, and the second is the preparation of the surface. Large single crystals of metals have been used for about fifty years largely for tlhe study of physical and mechanical properties of metals, and it is only recently with the advent of electrolytic polishing that it has been possible to use them for the study of reactions between metals and gases. 1. Growth of the Crystals

There are four principal methods of growing metallic single crystals: solidification from the melt in a vacuum, electrodeposition, condensation and decomposition from the vapor, and the strain-an’d-anneal method. Crystals of most of the common metals can be grown by solidification from the melt. Iron crystals of an appreciable size are the most difficult to grow, and because of the change in crystal structure a t the transition temperature, they may best be grown by the strain-and-anneal method. It is especially difficult to grow iron crystals of equal length in three dimensions for the preparation of spheres. The copper and nickel crystals in this study were grown from the melt in the form of rods, >$ to 1 in. in diameter. The

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details of various methods of growing crystals have been described by Holden (21) and Buckley (22). Spheres exposing all possible crystal faces or slices exposing only one face except for the edges were carefully machined from the rods. For convenience in handling, each sphere had a small shaft about >iin. in diameter extending from one side. Since single crystals of pure metals are generally quite soft, great care was required in the machining. A ball-turning outfit attached to the tool post of the lathe was used for machining the spheres. A few copper crystals were grown in the form of spheres by using a graphite mold consisting of two halves, each of which had a hemispherical opening machined in it.

2. Preparation of the Surface After machining, the surfaces were mechanically polished with metallographic polishing papers, No. 1 through 4/0, followed by polishing with a soft cloth impregnated with levigated alumina. The strained and contaminated layers of the mechanically polished surface were removed by etching in dilute nitric acid until a definite etch pattern appeared on the surface of the sphere. The crystal was repolished with 3/0 and 4/0 emery papers and then given a very careful electropolish. Different polishing solutions have been developed for different metals (23).Phosphoric acid was generally used for copper, sulfuric acid for nickel, and perchlorjc acid for iron. The final preparation of the surface after electropolishing varied with the metal and type of reaction to be studied. For copper, many methods of removing the last traces of the phosphate solution were tried, and the following procedure was adopted. After electropolishing, the crystal was immediately washed with distilled water, then immersed in a 10 % solution of phosphoric acid in water for 1 min., washed in a stream of water for 1 min., and again immersed in the acid solution for 1 min. Copper phosphate is more soluble in acid than in water. Finally, the crystal was washed in a stream of water for 4 min. and dried with a jet of pure oxygen. The wash water was prepared by distilling demineralized water. Several supplementary experiments were carried out to show that the above procedure removed any traces of phosphate which might affect the reactivity of the surface. A crystal was electropolished in a bath of nitric acid and ethyl alcohol. Since the color pattern obtained on oxidation is extremely sensitive to traces of contamination, it was used as a test for the cleanliness of the surface. Since there was no chance for phosphate to be in the solution of nitric acid and ethyl alcohol and since it was very unlikely that nitrate, if it remained on the surface, would affect the oxide pattern in the same manner that the phosphate did, it was assumed that the surface which was prepared by this method of washing was free

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ALLAN T. GWATHMEY AND ROBERT E. CUNNINGHAM

of contaminants from the polishing solution. After such treatment, the regular oxidation pattern was obtained. Immediately after washing the crystal as described above, it was annealed in hydrogen for 1 hr. a t a pressure of 1 atm. and a temperature of 500" C. I n the case of the hydrogen and oxygen reaction, the cryskal was cooled to the temperature of the reaction, and oxygen was added to obtain the desired ratio of hydrogen to oxygen. In the case of nickel and iron, the crystals after electropolishing were washed thoroughly for 4 min. in a stream of distilled water, dried, and generally cleaned by hydrogen in a glow discharge. The crystal was annealed in hydrogen a t a pressure of 1 atm. and a temperature of 500" C, and it was then cooled to the temperature of the reaction when the reacting gases were admitted. Any significant variations in these ]procedures will be described in the section on results. There are no general rules which can be given for the preparation of the final surface t o assure that it is chemically clean and sufficiently smooth for the purpose. Each system must be considered separately, and care must be taken to remove any foreign material which might affect the rearrangement or the activity of the surface, without a t the same time giving the surface such drastic treatment that its structure would be changed. Because of the rearrangements which take place in the surface, many catalytic reactions in effect prepare their own surface, and the initial smoothness of the surface is not as critical as it is in the case of oxidation. The surface of an electropolished crystal of copper appears smooth a t a magnification of 84,000, as shown by electron micrographs previously published (24. Such photographs, of course, do not prove that there is no waviness or that the surface is atomically flat, but because of the rearrangements produced by most catalytic reactions, these surfaces should be sufficiently smooth for the purposes of these studies. It may also be that rearrangement removes some contamination from the surface by simply covering it with metal. In a few reactions, such as the reaction of hydrogen and ethylene on nickel, the surface does not rearrange appreciably, and great care must be taken to remove contaminants from the surface. As indicated above, this was generally done by removing metal by hydrogen ion bonnbardment followed by annealing. The copper crystals used in these studies were made from copper of both 99.999% and 99.94% purity, the nickel crystals from metal 99.92% pure nickel plus cobalt, and the iron crystals were made from A m c o iron rods of the following approximate composition: iron, 99.8; carbon, 0.018; manganese, 0.027; phosphorus, 0.005; sulfur, 0.029; silicon, 0.005; copper, 0.11 %. The question of the degree of perfection of the cryfitals should be mentioned. Crystals grown with a reasonable degree of care in respect to purity

INFLUENCE OF CRYSTAL FACE

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and rate of cooling are known to contain many imperfections, and, as it will be shown later, imperfections will influence both oxidation and catalytic activity. Attempts are now being made to grow more nearly perfect crystals and to determine more carefully the influence of imperfections of various kinds on the catalytic activity.

IV. RESULTS Studies of the oxidation of a single crystal of copper will be described briefly for two purposes. The reaction of hydrogen and oxygen on copper, subsequently t o be described, is intimately related to the reaction of copper with oxygen alone. Also, the oxidation patterns vividly show the variation of rate with face, and the importance of imperfections in the structure of the metals. The catalytic reactions which have been studied with the aid of single crystals can be divided into three types for convenience in discussion. Type I includes those reactions in which the surface rearranges to form facets, Type I1 those in which solid reaction products build up on the surface, and Type 111 those which apparently leave the surface unaffected.

i. Oxidation of Metals The pattern obtained by heating a copper crystal in oxygen at atmospheric pressure a t 250" is shown in Fig. 1. The different crystal regions can be identified from the symmetry of the pattern. There are several methods of measuring the thickness of thin oxide films. Interference colors may be used as an approximate measure in the range of 200 to 2500 A. Oxide films from less than 10 to about 1000 A. in thickness can be measured by the change in the ellipticity of reflected polarized light, and with this method it was found that the rates of oxidation varied greatly with the crystal face exposed a t the surface (25). For example, after 50 min. a t 178", the thickness of oxide on the (100) face was 1000 A., while that on the (311) face was about 60 A. Rhodin (26) has also studied the oxidation of copper crystals with a microbalance. In addition to these large differences in rate with face, there are also significant differences in rate within any one face. Oxide films were electrolytically stripped from several faces of a copper crystal oxidized a t 150", and examined with an electron microscope. Figure 2 shows an electron micrograph of a film stripped from the (100) face (24). Several types of oxide particles can be seen which suggest the presence of imperfections in both the oxide and metal crystal. Fortunately, the edge of the film curled in handling, and some oxide particles can be seen standing in relief above the base film.

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ALLAN T. GWATHMEY AND ROBEET E. CUNNINGHAM

FIQ.2. Oxide film from the (100) face of copper. Crystal oxidized 45 min. at 150". 20,Ooox.

2. Type I Catalytic Reactions in Which the Surfact: of the Catalyst Rearranges: the Reaction of Hydrogen and Oxygen on Copper Of the reactions in which the surface rearranges, the combination of hydrogen and oxygen on the surface of a copper single crystal has been studied most extensively. It was found (2'7) that when a 3 : l mixture of hydrogen and oxygen was passed over a spherical single crystal of copper a t 260°, the surface was oxidized to a sufficient thickness to exhibit interference colors. Reduction of such an oxide film by he:tting in hydrogen a t 300" gave a roughened surface which had an irregular appearance under the microscope. If the same mixture was passed over a copper crystal at,

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360', no oxide could be detected with the unaided eye, but water began to form and the metal atoms in the surface seemed to rearrange to produce a definite pattern of smooth and roughened regions. Bright reflections were obtained when a beam of light was directed normal to the surface a t the (111) pole, and under the microscope facets parallel to the (111) planes could be seen in the (111) region. This formation of regular facets in which the amount of metal evaporated is small compared with that involved in facet formation is termed rearrangement. This process is to be contrasted with gas-etching, in which evaporation is large. Rearrangement occurred even with oxygen concentrations as low as 36 %. Experiments concerned with the detection of very small amounts of oxide on the (111) face during reaction will be discussed in a subsequent section. These showed that oxide was present on this face a t 300 to 400" when the oxygen concentration was greater than about 5%. With a lower oxygen concentration, no oxide could be detected. It seems probable that a similar situation exists in the case of the other faces, although as discussed later, it was impossible to measure the thickness of thin films except on the (111) face. Figure 3 shows the appearance of a crystal rearranged by this reaction. The facets which developed on each face and the consequent rearrangement pattern depended upon the temperature and gas composition. Thus a t 350' with 5 % oxygen, strong reflections were obtained from facets of several orientations, but a t 400" with 1236 % oxygen the (111) reflections predominated. Microscopically, the appearance of the surface depended on the symmetry of the face and the particular facets developed. Faces, in which the arrangement of surface atoms have a high degree of symmetry, exposed facets of

FIG.3. Rearrangement pattern formed on a copper single crystal by the reaction of a mixture of 1235% oxygen and 87>&y0 hydrogen, for 30 min. at 350".

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GWATHMEY A N D ROBERT E. CUNN~NGHAM

only one kind, but, faces having less symmetry exposed facets of several kinds. The details of the various rearrangement patterns are too numerous and complex to describe in this presentation of the subject, nor is it necessary for these purposes. In order to illustrate the details of facet formation, however, typical results are as follows: The reaction of 5 % oxygen and 95% hydrogen a t 400” on a crystal of copper produced the following surface structure. On the (100) face, facets were formed parallel to the (101), (1 lo), ( l O i ) , and (170) planes, all of which have the same surface structure. The (111) face did not rearrange, and the (110) face rearranged to give two similar facets, those parallel to the (210) and (120) planes. The (311) face exposed a t least four kinds of facets: those parallel to the ( l l l ) , (20l), and (210) planes, (113) and (131) planes, and (207) and (2iO) planes. Bright light reflections indicated a large total area of the ( l l l ) ,(201), and (210) facets. Weaker reflections indicated a small total area of the (113), (131), (20I), and (270) facets. Similarly, the rearranged (221) face had very large (111) facets, which are seen as the large flat areas between steps in Fig. 4. Several facets of other kinds made up the faces of the steps in this figure. It should be emphasized that for any one reaction, these details of structure will vary with the experimental conditions. Micrographs of several faces after reaction of a mixtiire of 5% oxygen and 95 % hydrogen a t 400” are shown in Fig. 4.A single face, after rearrangement, consisted of one or several kinds of surface in addition to edges and corners of various types of structures. In general only facets parallel to low index faces, especially the ( l l l ) ,(110), (210), and (311), were formed. It is interesting that all of these faces rearranged except the ( l l l ) , which remained smooth over a wide range of conditions. It was also found that at oxygen concentrations above about 5%, the surface became covered with a reddish copper “powdler.” An electron micrograph of a particle of this powder is shown in Fig. 5. The formation of powder was relatively slow. Small amounts were visible under the microscope after a few minutes, but its formation continued over a period of .many hours. This powder was probably copper with some oxide on its surface, although X-ray diffraction showed only copper. a. The Reaction Rates on Diflerent Crystal Faces. The rates of this reaction on the (100) and (111) faces of copper crystals were studied over a wide range of conditions by Leidheiser and Gwathmey (8) and under a few conditions on the (loo), (110), ( l l l ) , (311), and (221) faces by the present authors (28).No one face was found to be the most active or the least active under all conditions. In order to determine the activity of a single plane face, the apparatus shown in Fig. 6 was used. The crystal rested on a ground glass flange, and the reacting gas was passed over the lower sur-

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FIG. 4. Surfitccs produced by rearrangement of a copper single crystal a t 400" for 20 hrs. by a mixture of 5% oxygen and 95% hydrogen: (a) (100) face, (b) (111) face, (c) (110) face, (d) (311) face, (e) (221) face. 1050X.

face while the rest of the crystal was exposed to hydrogen alone. The reaction rates of the five plane faces, determined a t 400" with 5 % oxygen and 95% hydrogen a t rates of flow of 40 ml./min., are shown in Fig. 7. The rates are expressed in terms of the same starting area for all faces. The difference in rate between different faces was about eightfold in this case. The changes of reaction rate with time and the variation in rate with crystal face were not merely due to changes in the surface area, but also to the

4

ALLAN T. GWATHMEY AND ROBERT E. CUNNIXGHAM

FIG. 5. Particle of copper powder removed from the (110) region. 40,OOOX.

ti2 ti 02

FIG.6. Vessel for determination of reaction rates on plane faces.

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INFLUENCE O F CRYSTAL FACE

I

I

od

5

I

I

I

1

I

10 I5 TIME IN HOUR8

I

I

PO

I

J

FIG.7. Reaction rates on plane faces of a copper crystal a t 400" with 5% oxygen (continuous-flow system).

kind of facets which were developed by the rearrangement. For example, a (100) face of copper, catalyzing the reaction of a mixture of 10% oxygen and 90% hydrogen, showed an initial decrease in activity as (111) facets were developed, followed by an increase in activity accompanied by the formation of (110) facets. The formation of (111) facets produced an increase in area over the initial surface, while the formation of (110) facets produced a decrease in area over a surface exposing (111) facets. It was shown that changes in gas composition can lead to inversion of the relative activities of the (100) and (111) faces a t 400°, the (111) face being the more active below 2.5 % oxygen and the (100) face the more active between 2.5 and 20 % oxygen in the inlet gas. Similar inversions in the relative activities of different faces can be brought about by changing the temperature. For example, at 400" with 5 % oxygen in the gas the (221) face was among the most active tested. At 350" with the same oxygen concentration, however, the (221) face was much less active than the (loo), (110), and (311) faces. In the same series of experiments, the (100) face behaved in the opposite manner. At 400" its activity was appreciably less than that of all except the (111) face, but at 350" it was one of the most active faces. These rate measurements were reproducible within about 5 %. From these and other results, it is seen that the facets developed and the catalytic activities of the faces of a copper single crystal are a complex

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ALLAN T. GWATHMEY AND ROBERT E. CUNNIPIGHAM

function of the experimental conditions. Thus, rate measurements on such surfaces do not depend on the mechanism of the reaction alone, but are also affected by the changes in the surface which may be produced by changes in gas composition or temperature. b. The Formation of Oxide Films on the (111) Face of Copper and the Relation of the Films to Catalytic Properties. In this catalytic reaction of hydrogen and oxygen, it is important to know whether an oxide film exists on the surface and, if so, what influence it has on rearrangement, formation of powder, and catalytic activity. Some recent experiments have been carried out to answer these questions. Although the study is incomplete, interesting facts have been revealed and the results suggest several possibilities for further investigation. Such studies have been carried out only on the (111) face, since it remains smooth during the reaction. The thickness of oxide films can be measured with elliptically polarized light only on smooth surfaces. The thickness of the films was measured a t five temperatures from 325 to 425" by Cunningham and Young (29). In every case with low oxygen concentration no oxide could be detected, but with oxygen concentrations greater than 5 t o 7%, films up to about 75 A. in thickness were found. Values of film thickness, as indicated on the right-hand scale, are plotted against oxygen coilcentration in Fig. 8. The reaction rates are also given. The accuracy of these measurements of film thickness is subject to some uncertainty, as discussed in the original publication, but for purposes of determining the relationship between the oxide film and, the catalytic properties of the surface, the thicknesses given should be significant. Although powder did not form near the (111) pole of a spherical crystal, powder did form on the large flat (111) face which was used for determining reaction rates and which had a few scratches from imperfect polishing. It is more difficult to electropolish large flat faces than spherical surfaces. It was found that powder formed along the scratches on ,such a surface when the oxygen concentration was high enough to cause the formation of an oxide film. Once this effect of scratches was discovered, a few were deliberately produced in the surface for study. When the reaction was carried out with oxide on the surface, the reaction rate increased continuously for a period of 2 or 3 hrs. because of powder formation and the consequent increase in surface area. The rate more than doubled during this period. If the oxygen concentration was reduced to 4% so that no oxide film was present, the rate slowly decreased, requiring about three days to approach its initial value. At the end of this time it was found that the copper powder had disappeared and the surface was smooth except for the few scratches and pits froin the polishing. No comparable amount of metal was found on the surface of the vessel, indicating that the copper atoms in the powder

7777

INFLUEKCE O F CRYSTAL FACE INFLUEKCE O F CRYSTAL FACE

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78

ALLAN T. GWATHMEY AND ROBERT E. CUKNINGHAM

had rejoined the copper lattice. The powder could not be made to disappear in the same period of time by heating in hydrogen alone, even a t 500". Rates of reaction were determined before appreciable amounts of powder could form. Measurements were also made over the entire range of oxygen concentration with powder on the surface. The results of both sets of rate measurements, with and without powder, a t 400" are given by the circles in Fig. 8, the left-hand ordinate giving the rate scale. The lower curve gives the rate with no powder on the surface, and the upper curve gives the rate when there was powder on the surface. The vertical arrow upward a t 8 % oxygen indicates the rate during the formation of powder. The vertical arrow downward a t 4 % oxygen indicates the rate during the disappearance of powder. At the other temperatures powder also formed, but the rates were determined only for the powder-free surfaces. At 350 to 400", as shown in Fig. 8b-d, the rate was independent of the oxygen concentration in the range where no oxide was present but increased with oxygen concentration where oxide was present. There was no abrupt change in the rate between these two oxygen concentration ranges. At 425", a s shown in Fig. 8e, the rate in the absence of oxide corresponded to nearly complete conversion of the oxygen to water, but the rate was much slower when oxide was present. Apparently new factors, which must be investigated further, enter into the reaction a t this temperature. Thus, it was indicated: that an oxide film might or might not be present m the (111) face of copper during the catalytic reaction of hydrogen and oxygen, depending on the temperature and percentage of oxygen in the gas; that powder formed when an oxide film was present and disappeared when no oxide was present; and that the reaction kinetics changed according to whether or not there was oxide on the surface. The effect of the formation of such an oxide film may be even more important on the other faces which do rearrange, since the nature of the rearranged surface appears to be affected by film formation. In the rearrangement of copper by the hydrogen-oxygen reaction a t low oxygen concentrations where no oxide was present, facets were developed parallel to the (110), (210), (311), and (111) faces. At higher oxygen concentrations, where oxide films were found on the (111) faces, light reflections indicated that (111) facets predominated over the surface of the sphere. Since reaction rates vary markedly with crystal face, the reaction conditions or the manner of preparation can greatly affect the properties of a catalyst. The formation of surface films, such as oxide, which stabilize facets parallel to certain crystal faces may be an important factor in determining catalytic properties. c. The Formation of Copper Powder. In the growth of powder the important factors are the source of energy, the method of initiating growth, and the means of transporting material. It was found that the formation of

INFLUENCE OF CRYSTAL FACE

79

powder was observed on the (111) face only in the presence of oxide films, and it seems likely that the formation of the oxide films supply the necessary energy. From the heat of formation of bulk cuprous oxide, it is estimated that the energy released by the formation of an oxide film, a few Angstrom units in thickness, would exceed the difference between the surface energy of copper and the sum of the surface energy of cuprous oxide and the interfacial energy of copper-cuprous oxide. Thus, in the presence of an oxide film, the total energy would be reduced by an increase in surface area, such as that caused by the formation of powder. Wagner and Gwathmey (30) found by examining surfaces on which the reaction had continued for a short time that the formation of powder was associated with steps in the rearranging surface, suggesting that the initiation of powder growth occurred especially where the crystal lattice was rearranging or growing. An interesting type of surface formation which was observed by the present authors (28) during study of the reaction of hydrogen and oxygen in the ratio of four to one by volume a t 400' for several days is shown in Fig. 9. The dimensions of this formation, which occurred in the smooth (111) region, are about 0.1 mm. across and about

FIG.9. Object which grew near a (111) pole during three days reaction of a 4 : l hydrogen-oxygen mixture at 400". 400X.

80

ALLAN 3‘. GWATHMEY AND ROBERT E. CUNNINGHAM

0.02 mm. high. It was found that such formations could be produced on the surface of a sphere intentionally by electropolishing the crystal a t a high current density to induce pitting. The pits would till up with metal during the catalytic reaction and continue to grow above the original surface. Seventeen recognizable formations of this type were produced on one 3g-in. copper sphere. Four of these had treelike growths of cop‘per powder a t their apex which extended as high as 0.035 mm. above the original surface. These growths indicate the magnitude of diffusion of atoms on the surface. If the pits which initiated these structures had been tilled by diffusing copper during reaction without the formation of irregularities in the crystal lattice, the diffusion of copper into the pit should have stopped when the level of the original surface was reached. The continued growth above the surface was indicative of the formation of an imperfect lattice. The formation of powder a t the apex of several of these figures and a t oth,er places where the crystal was growing may indicate that the initiation of powder growth W ~ connected with imperfections in the crystal lattice. The ease with which metal can move on the surface and the means of transport of material to the growing powder is indicated by the rearrangement of the surface which occurs during this reaction. I n the absence of an oxide film, the surface energy of the metal provides a driving force for the disappearance of the powder. The disappearance, however, is much slower than the formation. The energy involved is somewhat less than that provided for powder growth by a moderately thick oxide jilm (20-30 A.), but it also seems that the ease of movement of the metal atoms is reduced in the absence of oxide. Since the amount of powder was not appreciably reduced by heating in hydrogen for 3 days, the movement of metal is certainly easier under the conditions of reaction. The fact that a surface can either increase in area or be “catalytically sintered” with only minor changes in the operating conditions indicates that the !formation of surface films can be very important in both the preparation and operation of some types of catalysts. d. T h e Effect of Foreign Atoms Added to the Surface. When a small amount of a foreign metal or metal oxide was added to the surface of a copper crystal before the reaction, the final appearance of the crystal, the facets produced by the rearrangement, the catalytic activity of’ the different faces, and the amount of copper powder formed were markedly different from the results obtained with pure copper (28, 31). Silver, zinc, and chromium trioxide were the three added substances which were investigated most, completely, since they are different chemically and had marked effects on the appearance of rearranged spheres. The metals were added to the surfaces by electrodeposition or by evaporation. Chromium trioxide was added by immersion in solution. The amounts added were equivalent to a

F

INFLUENCE OF CRYSTAL FACE

81

FIG.10. Effect of foreign atoms on the rearrangement pattern. 1235% oxygen and 8735% hydrogen for 20 hrs. at, 350": (a) nothing added, (b) silver added, (c) zinc added, (d) CrO, added.

layer 10 to 20 A t@ck and, since rearrangement involves a layer of metal several thousand Angstrom units deep, these amounts correspond to less than 1% of the surface composition. Figure 10 shows the rearrangement pattern on pure copper when no foreign atoms were added to the surface and the changes in the pattern when silver, zinc, and chromium trioxide were added. The presence of silver appeared to decrease the roughness of the rearranged surfaces and also to affect markedly the facets formed during the reaction. A thin layer of electrodeposited zinc led to the formation of copper powder over the entire surface except in the neighborhood of the (111) poles. Chromium trioxide increased the formation of copper powder in certain areas (the four dumbbell-shaped areas in Fig. 10d). The remaining surface, as in the case of rearrangement with added silver, displayed a regular pattern which differed from that of pure copper. Such foreign atoms added to the surface of a

82

ALLAN T. GWATHMEY A N D ROBERT E. CUNNINGHAM

crystal may act in several ways to control the rearrangement. First, since they change the chemical composition of the surface, they should affect adsorption and catalytic reactions on the surface. Secondly, the alloys formed rnay have different crystal structures or hal,:ts, and thirdly, the added material may interfere physically with regular growth of the lattive. Silver, since it is easily reduced and soluble in copper, would be expected to affect the rearrangement only by the first two means. With zinc or chromium trioxide, however, it is probable that the oxides of these elements would be present on the surface. These materials may thus interfere with regular rearrangement and thereby induce powder formation, since they would not fit into the crystal lattice. Rate measurements were carried out on plane faces of copper in the presence of these added solids at 350" with a gas composition of 95 % hydrogen and 5 % oxygen (28). Both the surface structure and the catalytic activity of the different faces were affected by the addition of these substances. Figure 11 shows photomicrographs of these surfaces after reaction for 20 hrs. Each face is shown with no added solid, wi1,h a 10-A. layer of silver, with a 20-A. layer of zinc, and with a small amount of chromium trioxide added before the reaction. Figure 12 shows graphs of catalytic activity against time, the starting area being the same in all cases. The microscopic appearance of the surfaces and the facets developed with the addition of the foreign solids were quite different from those of pure copper. In view of the surface anisotropy of metal crystals with respect to chemical reactions, the variation in catalytic activity with these added substances is not surprising. Although the silver was added to the surface to the extent of only four monolayers and although most of this might be expected not to remain on the surface once rearrangement began, this small amount was able to control the surface properties so as to alter the catalytic activity and the type of facets produced by the rearrangement. As with silver, most of the zinc added to the surface probably did not remain there. That which did remain brought about major changes in the catalytic surface. Aside from changing some of the facets developed, the addition of zinc led to the formation of copper powder on all but the (111) face. Chromium trioxide did not cause the formation of powder in this set of experiments, probably because of the low oxygen concentration, but did lead to changes in the facets developed and the catalytic activities of the plane faces. As indicated above, foreign atoms may change the activity of a surface because of the difference in composition and also because facets may be formed parallel to different crystal planes. I n addition, by favoring metal powder growth, they may help maintain a large surfttce area under conditions which would ordinarily lead to sintering of the catalyst with conse-

83

INFLUENCE O F CRYSTAL FACE

Nothing added

Silver added

Zinc added

Chromium trioxide added

(221)

FIG.11. Effect of foreign atoms on the rearrangemelit of copper by hydrogen and oxygen. 5% oxygen for 20 hrs. a t 350". 1,OOOX.

quent loss of area. These factors should be important in understanding the action of certain types of promoters. e. Summary of T y p e I Catalytic Reactions 1. Certain catalytic reactions, such as the reaction of hydrogen and oxygen on the surface of copper, produced marked rearrangements of the surface to expose facets parallel to certain crystal planes. The surface of such a catalyst appeared to be extremely mobile a t temperatures as low as 325".

84

4LL.4N T. G W A T H M E Y A N D ROBERT E. C U N N I N G H A M

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(b) FIG.12. 1Sffect of foreign atoms on the reaction rate of hydrogen and oxygen on copper. 350" with 5% oxygen: (a) (111) face, (b) (221) face.

INFLUENCE O F CHYSTAL FACE

85

2. The rate of the reaction varied with the face exposed. The nature of the facets formed arid the relative activities of different faces varied rapidly with changes in the experimental conditions. 3. The formation of a thin invisible film, such as a film of oxide, on the surface greatly affected the properties of the catalyst and brought about powder formation. 4. If a very small amount of foreign solid was added to the surface, the catalyst rearranged in a different manner and the tendency to form powder was altered. This suggests several ways in which promoters may act. 5 . Because of the rearrangement, withinlimits the reaction prepared its own surface. This fact must be appreciated in defining the surface structure of a catalyst and in following structure changes as the reaction proceeds. 3. T y p e 11 Catalytic Reactions in W h i c h Solid Reaction Products Deposit m the Surface: the Reaction of Carbon Monoxide o n Nickel and Others

a. Results. The second type of catalytic reaction which was studied with the aid of metal single crystals was that in which solid reaction products built up on the surface. Examples are the deposition of carbonaceous material when the crystals were heated in carbon monoxide, in carbon monoxide-hydrogen mixtures, or in ethylene. The carbon monoxide and the carbon monoxide-hydrogen mixtures are of interest in connection with the Fischer-Tropsch reaction, and ethylene in connection with catalytic processes involving hydrocarbons. Single crystals of several of the catalytically important metals, nickel, iron, and cobalt, were used in these studies. Leidheiser and Gwathmey (8) found that, when a spherical nickel crystal was exposed to an atmosphere of carbon monoxide at 550°, carbon deposited, as shown in Fig. 13, in the (111) region but not in the (100) or

FIG.13. Carbon deposited on a nickel crystal at 550" in carbon monoxide

86

ALLAN T. GWATHMEY AND ROBERT E. CUNNIKGHAM

(110) region. Even when the deposit was very thick in some regions, no carbon could be detected in others. Analysis of a sample of the deposit formed on polycrystalline nickel foil treated in a similar manner and carefully removed showed that it contained about 11 wt.% nickel. Mixtures of carbon monoxide and hydrogen were found to give sirnilar results when the carbon monoxide was in excess, but with carbon monoxide in concentrations of 10 % or less, a t atmospheric pressure, no carbon deposit was found. Instead the nickel surfaces were rearranged. Kehrer and Leidheiser (32), in a study of the Fischer-Tropsch reaction, passed vaxious mixtures of carbon monoxide and hydrogen over nickel crystals. The deposition pattern obtained varied with the gas composition. Figure 14 shows a nickel crystal sphere which was exposed to an eight-to-one mixture of hydrogen and carbon monoxide. The difference between this patkern and that obtained with carbon monoxide alone is striking. Wagner ($3) studied the effect of pretreatment of the surface on the deposition pattern. He found that, within certain limitts, the pattern was not affected by surface treatments which included electrolytic etching and catalytic rearrangement by hydrogen and oxygen. Abrading with steel wool, as would be expected, did make some difference in the pattern. These results do show, however, that within certain limits the reaction, as in the case of hydrogen and oxygen on copper, prepares its own surface. Kehrcr and Leidheiser (32)also studied the deposition of carbon from carbon monoxide and from hydrogen-carbon monoxide mixtures on iron

FIG.14. Carbon deposited on a nickel crystal at 600" in .an 8: 1 hydrogen-carbon monoxide mixture. [According t o Kehrer and Leidheiser (32)I.

INFLUENCE O F CIiYSTAL FACE

87

and cobalt crystals. The studies with cobalt were carried out at 420" and 600". At the lower temperature cobalt has a hexagonal close-packed structure, and a t the higher a face-centered cubic structure. It was found that the temperature could be passed through the transition point to change the crystal from one structure to the other without converting it into a polycrystal. The pattern of deposited carbon depended on the structure of the crystal a t the temperature of the reaction. No carbon deposited on the (0001) region of the hexagonal structure, but considerable carbon deposited on the (111) region of the face centered cubic structure, although the (0001) face of the former and (111) face of the latter are located a t the same position on the sphere and have the same arrangement of atoms in the firsttwo layers. The two faces have approximately the same spacing between atoms. The primary difference between the two structures is the arrangement of the atoms in the third layer with respect to the atoms in the first two layers. This result suggests that the difference in activity with face cannot be attributed t o the spacing, or arrangement of the surface atoms alone, but may be influenced in part by the orientation of the crystal. This suggestion assumes that there is no change in surface structure during the transition. I n the study of the reaction of hydrogen and ethylene on single crystals of nickel, to be described in the next section, it was observed that carbon deposited on a spherical crystal when it was heated in an atmosphere of ethylene alone a t 450".As shown in Fig. 15, very sharp and beautiful deposition patterns were obtained which were very different from those obtained with carbon monoxide. With ethylene no carbon was found in the (111) or (100) regions, and electron diffraction indicated that these regions were very smooth. The remaining regions on the surface were covered with carbon, the amount varying with crystal face. Visible deposits of carbon did not form until the surface had begun to rearrange, after which the carbon

E'io. 15. Carbon deposited on a nickel crystal at 450" in ethylene.

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ALLAN T. GWATHMEY AND ROBERT E. CUNNINGHAM

deposition was very rapid. This suggests that in this reaction the carbon deposition was controlled by the rearrangement pattern. The carbon deposits from these reactions were examined with an elec. tron microscope, and they appeared to grow in the form of threadlike filaments. In several other cases of filamentary growth, it was found that the deposit contained appreciable amounts of the catalyst material. For example, in the formation of cuprene by the reaction of acetylene on copper oxide, copper was found in the cuprene a t appreciable distances from the oxide surface (34).Similarly, in the deposition of soot on firebrick, iron has been found in the carbon. These facts suggest that the formation of a filamentary deposit may require the superposition of a small amount of the catalyst material on some of the reaction product. This material could then be carried away from the surface as the filament grew. The superposition of catalyst on the reaction product could be accomplished by the rearrangement process. It was seen in the reaction of hydrogen and oxygen on copper that the rearrangement, pattern depended on the temperature and composition of the gas. If carbon deposition is controlled by a rearrangeinent process, the experimental conditions might also be expected to affect t’he carbon deposition. In commercial practice it is generally desired to avoid carbon deposition. If the rearrangement could be avoided or possibly kept within limits, carbon deposition might not occur. A catalyst exposing only certain crystal faces might reduce greatly this carbon formation. Also, since rearrangement patterns are markedly affected by the addition of small amounts of foreign atoms, i t might be possible to add the proper materials in order to change the rearrangement and to avoid or reduce the formation of carbon. b. Summary of Type 11 Catalytic Reactions 1. I n certain catalytic reactions, such as the reaction of carbon monoxidc on nickel and of ethylene on nickel, carbonaceous deposits built up on the surface, and the rate of formation of these deposits varied greatly wit,h the face exposed. In some cases, even when the deposit was very thick on certain faces, no carbon could be detected on others. 2. I n the reaction of carbon monoxide on a cobalt crystal which could exist in two crystal structures depending on the tempera ture, the pattern of deposits followed the symmetry of the structure existing a t the temper:&ture of the reaction. 3. I n the reaction of ethylene on nickel, visible deposit*#did not form on the nickel until the surface had bcgun to rearrange, even though very slightly, after whicsh the deposition was very rapid. 4. When the deposits were examined with an electron nkroscoptx, they appeared to grow in threadlike filaments, and in some cases the deposit contained appreciable amounts of the catalyst material.

INFLUENCE OF CRYSTAL FACE

89

4. Type 111 Catalytic Reactions in Which N o Visible Change in .$he Structure of the Surface Takes Place: the Reaction of Hgdrogen atzd Ethylene on Nickel a. Results. I n contrast to the reactions discussed in the preceding two sectiom, there were some reactions which had no visible effect on the structure of the surface. An example is the reaction of hydrogen and ethylene on single crystals of nickel. Studies of this system were undertaken (35) both in order to complement previous studies of Types I and I1 reactions, and in hopes that the results could be more readily interpreted with surfaces which did not rearrange during the reaction. It was found, however, that, in contrast to reactions which prepare their own surface, this reaction was extremely sensitive to very small amounts of contaminating substances. Unknown substances, possibly arsenic, in the Pyrex glass reduced greatly the activity of the surface, so that it was necessary to outgas the glass thoroughly in a high vacuum prior t o the construction of the final vessel. Also, the order in which the reacting gases were added to the surface greatly affected the activity. The crystal was first annealed in hydrogen and then heated in argon and cooled in argon to the temperature of the reaction. When hydrogen was introduced first, the activity was greater, but when ethylene was added first, the activity was greatly reduced. This effect of the order in which the gases were added might be explained if the prior addition of hydrogen satisfied some of the chemical activity of the surface, so that its reactivity with ethylene might be greatly reduced. Such reactions as the disproportionation of ethylene to form acetylenic complexes might be decreased. The rates obtained in this study and shown in Fig. 16 did not follow any simple laws. It is probable that side reactions which left products on the surface were important in affecting the reaction velocity. In all cases the (321) face was the most active and the (100) the least active. The change in activity with time differed for the different faces, and the effect of temperature on the reaction rates also varied with face. The maximum difference observed at any temperature was about fivefold. These results indicate that the crystal face exposed a t the surface may affect the rate when the reaction produces no visible change in the surface. The most obvious explanation is again the differences in the structure of the surface, but it is important to determine what particular aspect of the structure is the controlling factor. As indicated previously, Beeck, Smith, and Wheeler in the study of evaporated films, assumed that the controlling factor was the lattice spacing, or the distance of nearest approach of metal atoms, but this spacing is exposed to some extent on all crystal faces and the (100) face, which is a close-packed square array abundantly exposing this spacing, is the least active. Twigg and Rideal (6) suggested that a

90

ALLAN T. G R A T H M E Y AND ROBERT E. CUNNINGHAM

oh

io

40

60

TIME IN MINUTES

80

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(b) of reaction of hydrogen and ethylene on nickel: (n) lOo", (b) 200"

INFLUENCE O F CRYSTAL FACE

91

surface which was completely covered with adsorbed molecules would be inactive. They calculated that this would be the case for the (111) and (110) faces, but in the present studies both of these faces were found to be fairly active. Thus, structural aspects other than the simple one of lattice spacing and coverage must control the activity. There have been several other investigations of catalytic reactions of this type using massive single crystals. Nakada et al. (36) studied the recombination of hydrogen atoms on copper single crystals. They used crystals on which two plane faces had been cut and disposed them so that the two faces could be alternately turned toward the source of atomic hydrogen. The change in temperature of the crystal, according to the side exposed, was taken as an indication of the activity. The faces, in order of decreasing activity, were (1lo), (loo), and (1 11). Sosnovsky (37) studied the decomposition of formic acid on single-crystal sheets of silver a t several temperatures. The specimens were used as they came from the mold, and there was no special preparation or examination of the surface. Thus, the over-all orientation of the crystal was the only controlled structural factor. The rates of reaction between faces were found to vary by less than a factor of 10, and for most temperatures by less than a factor of 2. Activation energies, varying from 16.0 to 30.4 kcal. for the different specimens, were obtained. Kummer (38) measured the rate of oxidation of ethylene on single crystals of silver prepared parallel to the (111) face, nearly parallel to the (110) face, and nearly parallel to the (211) face. The relative reaction rates between faces were found to vary by a factor of 2 a t the most, and the fraction of ethylene oxidized to ethylene oxide varied between 31 and 38 %. The surfaces were prepared by mechanical polishing followed by electropolishing and were determined by an electron microscope to be smooth within the limits of 100 A. b. Summary of T y p e I I I Catalytzc Reactions. When no visible change took place in the surface during reaction, the difference in rates on different crystal faces was very large for some reactions and small for others. No particular face was universally the most active, the relative activity of the different faces depending on the reaction and the conditions of the experiment. The results emphasize the significance of crystal face in catalytic reactions of this type.

V. DISCUSSION

It seems appropriate in this final section to attempt to evaluate the significance of the results obtained with large metal crystals. Care must be taken that the complexities of describing the surface structure do not obscure the relationship of the studies to the present state of knowledge of

92

ALLAN T. GWATHMEY AND ROBERT E. CUNNINGHAM

heterogeneous catalysis. Some of the results have already been more conveniently discussed in Section IV. The original question for which an answer was sought in studies with single crystals was: What is the nature of the regions of high activity on the surface of any one catalyst? There are obviously several factors which contribute to activity, but these studies have shown that for any one catalyst the activity depends on the crystal face exposed a t the surface. The differences between faces for any one catalyst may vary anywhere from a few per cent up to factors exceeding a thousand. These studies have also shown that within any one face there are regions which differ in activity, and these differences appear to be due to imperfections in the crystal. With the aid of highly polished crystal spheres, it has been shown that the surface of the catalyst may be mobile in the case of copper a t measured gas temperatures of 325". During the reaction the surface may rearrange to expose certain facets and other structures which depend on the nature of the reactants and the experimental conditions. Experiments recently carried out in this laboratory on bonding, or sintering, two polished single crystals of copper have shown that bonding and surface diffusion in an atmosphere of hydrogen may take place a t temperatures in this neighborhood. These results, showing the importance of mobility and structure, suggest that three steps may now be profitably taken, the first concerned with further experimeiits, the second with a theoretical understanding of the results, and the third with the practical preparation of catalysts. First, additional experiments must be carried out to determine the stable faces and the activities of these faces for catalytic reactions of each important type. This is a tedious and complicated undertaking which cannot well be avoided if catalysis is to be understood. To attempt to understand catalysis without defining the surface structure is analogous to trying to understand catalysis without knowing the chemical composition of the catalyst. The nature and the number of the imperfections within any one face which might cause pits, oxide nuclei, powder, or carbon filaments must d s o be determined experimentally. Second, an attempt must be made to explain theoretically why ccrtairi faces are stable in particular environments and why one face will have a greater activity than another. These important questions must be considered on the basis of both structure and electronic interaction. Third, attempts might now profitably be made in some cases to prepare catalysts in which the activity is determined by controlling the faces and imperfections a t the surface. Small amounts of foreign atoms, in the nature of promoters or poisons, appear to play an important role in controlling these structural factors. The mobility of the surface during reaction arid its rearrangement to

INFLUENCE O F CRYSTAL FACE

93

form structures, characteristic of the reaction, raise a question as to the use of adsorption measurements in understanding the kinetics of catalytic reactions. There are, of course, cases when adsorption can be profitably used for such purposes, but the surface during adsorption may be very different from that, during the catalytic reaction of two gases. For example, as previously stated, the surface of an electropolished copper crystal appeared smooth when examined under an electron microscope. It remained smooth while being heated in hydrogen. When such a crystal was heated in oxygen at 150°, the surface structure shown in Fig. 2 was obtained, and after reduction in hydrogen the surface was rough and irregular. On the other hand, when a mixture of hydrogen and oxygen reacted catalytically, definite facets on the different faces, such as those shown in Fig. 4, were obtained. These structures varied with the reactants and experimental conditions. Thus, not only does the surface affect the reaction between gases but the gases also affect the structure of the surface, and these effects obviously must be taken into consideration. Since copper powder generally sinters into large aggregates on being heated in hydrogen or a vacuum, the formation of powder from a massive single crystal during a catalytic reaction is an interesting phenomenon. The formation of powder raises the question as to whether there is any fundamental connection between powder formation and catalysis. Since the powder seems to form preferentially a t relatively unstable structures, such as pits, scratches, and edges between stable faces, it would appear that the process is simply one of crystal growth, facilitated by surface mobility resulting from the heat evolved by the reaction. On the other hand, the disappearance of this powder when the percentage of oxygen in the oxygenhydrogen mixture is reduced indicates that the picture is not so simple. This interesting phenomenon must be investigated further. A few words should be said about imperfections and their relation to catalysis. It was the primary purpose of this study to investigate the influence of crystal face, but the formation of pits, oxide nuclei, metal powders, and carbon filaments are evidence that imperfections play a n important role in contact catalysis. Imperfections originally present in the crystal and those formed during the reaction may both be important in determining the properties of a catalyst. Grain boundaries, dislocations, vacancies, and impurity atoms originally present in the lattice present unusual structural features in which the arrangement of atoms and the possible electronic levels differ from the rest of the surface. These regions will, therefore, have unusual catalytic properties. Dislocations, because of their importance in crystal growth, probably play an important role in the rearrangement of the surface. Imperfections formed during the reaction may also be important, es-

94

ALLAN T. GWATHMEY A N D ROBERT E. CKJKNINGHAM

pecially in the formation of overgrowths on the surface.. The dependence of powder formation and carbon deposition on rearrangement suggests that they are related to imperfections formed during the rearrangement process. Each method of study has its special advantages, arid each gives information on some particular aspect of a subject which must be related to the general body of knowledge in the field. There are many factors which can control the activity of a surface. The single-crystal meth.od gives information on one important factor, surface structure, which has been greatly neglected in the past. The fact that the regions of different activity can be easily seen and that movements on the surface can be fo:llowed makes possible a more direct attack on this intriguing question of catalyst activity. Experiments can now be devised in which activity, as determined by st,ructure, can be controlled and studied. These studies have necessarily been concerned with a limited number of reactions but they were selected for the purpose of covering a broad range of catalytic reactions. It is believed that the phenome:na discussed have general application to the reaction of gases on metal surfaces. Other materials, such as alloys and metallic oxides, should be investigated by this method, and many of the surfaces described in this study should he examined further with an electron microscope in order to detect the fine details of structure. These studies clearly show the importance of ex:tmining the catalyst surface and of following changes in the surface structure as the reaction proceeds. The single-crystal method is especially suited for these purposes.

REFERENCES 1. Langmnir, I., Tra.ns. Paraday Soc. 17, 617 (1921). 2 . Adkins, H., J . A m . Chem. Soc. 44, 2175 (1922). 9. Burk, It. E., J. I’hys. Chem. 30, 1134 (1926). 4 . Balandin, A . A., Z. physik. Chem.. ( L e i p z i g ) B2, 289 (1929), 6. Beeck, O., Advances in Catalysis 2, 151 (1950). 6. Twigg, G. H., and Rideal, E. K., Trans. Faraday SOC.36, 533 (1940); Herington, E. F. G., and Rideal, E. K . , Proc. Roy. Soc. A184. 434, 447 (1947). 7 . Maxted, E. B., Advances in Catalysis 3, 129 (1951). 8. Gwathmey, A. T., and Benton, A . F., J . Phys. Chem. 44, 35 (1940); Leidheiser, H., Jr., and Gwathmey, A . T., J . A m . Chem. Soc. 70, 1200, 1206 (1948). 9. Sachtler, W . H . M., Ilorgelo, G., and van der Knaap, W . , , J . chirn. ph,ys. 61, 401 (1954). 10. Miiller, 5 :. W . , Ergeb. exact. Naturw. 27, 290 (1953). 11. Becker, J. A . , Advances in Catalysis 7, 135 (1955). 12. Gomer, R., Advances in Catalysis 7, 93 (1955). 19. Jacquet, P. A., Nature 136, 1076 (1935). 14. Sherman, A , , and Eyring, H., J . A m . Chem. Soc. 64, 2661 (1932). 16. Okamoto, G., Horinti, J., and Hirota, K., Sci. Papers Inst. Phys. Chem. Research. (Tokyo) 29, 223 (1936).

INFLUENCE OF CIZYST,PL FSCE

95

Baker, M. M., and Jenkins, G. I . , Advances in Catalysis 7, 1 (1955). Pauling, I,., Proc. Roy. Soc. A196, 343 (1949). Trapnell, B. M. W., “Chemisorption”. Academic Press, New York, 1955. Herring, C., and Nichols, M. H., Revs. Modern Phys. 21, 185 (1949). Suhrmann, R., Advances i n Catalysis 7, 303 (1955). 21. Holden, A . N., American Society of Metals, Reprint No. 35 (1949). 22. Buckley, H. E., “Crystal Growth.” Wiley, New York, 1951. 23. Jacquet, P. A., Met. Revs. (The Institute of Metals) 1, 157 (1956). 24. Harris, W. W., Ball, F. I,., and Gwathmey, A. T., Acta Met., 6, 574 (1957). 26. Young, F. W., Jr., Cathcart, J. V., and Gwathmey, A. T., Acta Met. 4, 145 (1946). 26. Rhodin, T. N., J . A m . Chem. Soc. 72, 5102 (1950). 27. Gwathmey, A . T . , and Benton, A, F., J . Chem. P h p . 8, 569 (1940). 28. Gwathmey, A . T . , and Cunningham, R. IC., J . chim. phys. 61,497 (1954). 29. Cunningham, R . E., and Young, F. W., Jr., J . Phys. Chem. 61, 769 (1957). 30. Wagner, J. B., Jr., and Gwathmey, A . T., J . Am. Chem. Soc. 76, 390 (1954). 31. Cunningham, R . E., and Gwathmey, A. T . , J . Am. Chem. Soc. 76, 391 (1954). 3.9. Kehrer, V. J., Jr., and Leidheiser, H., J r . , J . Phys. Chent. 68, 550 (1954). 3.9. Wagner, J . B., Jr., Dissertation, University of Virginia, Charlottesville, Virginia, 1955. 34. Watson, J. H . Id., and Kaufman, K., J . Appl. Phys. 17, 996 (1946). 36. Cunningham, R . E., and Gwathmey, A. T., Advances zn Catalysis 9, 25 (1957). 36. Nakada, K . , Sato, S., and Shida, S., Proc. Japan Acad. 31, 449 (1955). 37. Sosnovsky, H. M. C., J . Chem. Phys. 23, 1486 (1955). 38. Kummer, J . T . , J . Phys. Chem. 60, 666 (1956). 16. f7. 18. 19. 20.

The Nature of Active Centers and the Kinetics of Catalytic Dehydrogenation A. A. BALANDIN N . D. Zelinskii Institute of Organic Chemistry, U.S.S.R. A c a d e m y of Sciences, and Moscow State University, Moscow, U.S.S.R.

I. QUASI-HOMOGENEOUS SURFACES In my report on the active centers of catalysts certain new conclusions will be considered, drawn from our kinetic studies on dehydrogenation. I n accordance with the multiplet theory of catalysis, proposed by the author of this report (1, 6,S), owing to the small effective radius of chemical forces, reacting atoms must be in contact with the catalyst. Of interest is the catalysis of complex molecules that are unable to superimpose on the surface. Klabunovskii and the author (4) have found that in spite of their complicated structure certain tripticene derivatives can be hydrogenated on Raney Ni already a t 45’ and 1 atmosphere. It follows from this that the active centers of the catalyst are elevations on its surface; it is on them that the reacting portion of the molecule locates itself, the rest of it falling into the depressions. (In Fig. 1 a and b represent the hydrogenation of the benzene ring and of the carbonyl bond in contrast to c, representing the hydrogenation of the double bond that can take place on a plane as well.) Even isopropanol and other secondary alcohols cannot superimpose on the surface during dehydrogenation (Fig. 2). Since the difference in the dehydrogenation of the secondary and primary alcohols is only a quantitative one, the latter should also react on the elevations in the form of Taylor’s peaks ( 5) or Volkenshtein’s “biographical” active centers (6). But the author (7) believes, moreover, these elevations to be carriers of crystal facets. Such islets are metastable and, depending 011 their differences in heights and surface areas and consequently in the deformations caused by the neighboring lattice atoms, are subject t o statistical treatment. If their distribution bears an exponential character, then this can explain the appearance of parameter h in Equation (1) (8-11). All data on the kinetics of the catalytic dehydrogenation of hydrocarbons, amincs, and alcohols obtained in our laborrttory are expressed by the equation ( I d ) : 96

ACTIVE CENTERS AND CATALYTIC DEHYDROGENATION

97

Here dx/dt is the rate of reaction, E the activation energy, T the temperature in OK, p the partial pressure, and r the substance number, being 1 for the reactant, 2 for the product, 3 for hydrogen, and 4 for a n extraneous substance; k d and zr are constants. The fractional factor with p of Equation (1) according to the accepted view corresponds to Langmuir’s adsorption isotherm for mixtures on an almost covered homogeneous surface or on like active centers and the z are the relative adsorption coefficients. This fraction was so interpreted for other cases by Hinshelwood ( I S ) , Schwab (9),the author ( I , 1 4 , Frost (15),Hougen ( I B ) , and others. Two questions now arise: 1. Is the factor in general an adsorption isotherm, i.e., do the z’s have the significance of relative adsorption coefficients? 2. How is one to reconcile the shape of the adsorption isotherm deduced for a uniform surface with the fact that, as we have seen here, the surface is heterogeneous?

FIG.1. Hydrogenation of tripticene derivatives (4)

98

A . A . UALANDIN

FIG.2. Dehydrogenation

: (a) isopropanol,

(b) cycloliexanol

Pshexhetskii et al. (17) have recently expressed the view that in the equation for the rate of dehydrogenation, containing the function p in the form of the Langmuir isotherm, z is the ratio of the rate constants of some partial reactions. Calculations by the author and Kiperman (18) have shown that both interpretations of x are limiting cases and for x to possess the usual meaning of the relative adsorption coefficient, it is necessary that the desorption rate constant k, of the unchanged reactant molecules exceed the rate constant k of the dehydrogenation. That z actually is an adsorption coefficient has been demonstrated by Balandin et al. (19).A mixture of butane and butylene was dehydrogenated, isotope. It one of the compounds being labeled with the radioactive mas found that in the consecutive reaction C,Hio

4

CaHs + C4Hs

(1)

the butylene formed from the butane is first practically completely desorbed so that only the butylene molecules adsorbed m e cv are dehydrogenated to hutadiene, This can be seen from the fact that on dehydrogenating a. I : 1 butane-active butylene mixture, the specific activities of the butylene and of the butadierie formed are found prttcticdly to coincide. In the case of a mixture of active hutane and inactive butylene, the specific activity of the butadiene was found to be somewhat lower than that of the butylene. Runs were made on a chromium catalyst a t 635" and times of contact from 0.77 to 3.22 sec.

ACTIVE CENTERS AND CATALYTIC DEHYDROGENATION

I

99

A I

I I

I I I I

I

I

I ------

B

FIG.3. Potential curve for the consecutive dehydrogenation of butane and butylene-l (19) : (1) CaHlogas, (2) C4Hlnads., (3) C4H,oactive compl., (4)C,Hs ads., ( 5 ) ClHs gas, (6) G H s ads., ( 7 ) C 4 H active ~ compl., (8) C4Hs acts., (9) C4H6 gas.

Figure 3 shows a profile of the potential surface of the consecutive stages of (I).Let k be the rate constant of the reaction. Then, in conformity with the results obtained, k45 >> ka7. It is clear, however, that the level 4 of the adsorbed butylene in the process of dehydrogenation of butane is the same as the level 6 of the butylene in the dehydrogenation of the latter. Hence, k~ = ~ C H , and consequently k65 >> k 6 7 . Thus, in dehydrogenation the desorption rate constant of the initial substance is considerably larger than that of its dehydrogenation. It is thus shown that x has the meaning of a relative adsorption coefficient. But

zr

=

a7/al

(2)

where a designates the absolute adsorption coefficients, these lat,ter being the equilibrium constants of adsorption (20):

A (gnfi) -I-

A!O(free surfnce)

Therefore, for exchange on the surface

*

AS(stcrface,

(1 1)

100

A. A. BALANDIN

Then according to thermodynamics one can obtain the change in the standard molar free energy (at constant pressure), in the enthalpy and in the entropy for adsorptional exchange (21): AF" = -4.57T log z AH" = 4.57T'd log z/dT

AS"

=

(AH" - A F o ) / T

(5) (6)

(7)

and by means of the same equations with a instead of z, for adsorption. It should be emphasized that the quantities a, z, AF", AH", and AS" determined from Equations ( 2 ) to (7) refer to the catalytically active centers and not to the surface as a whole, wherein lies the advantage of the method now being considered. Their determination is of importance in catalysis because all the molecules taking pnrt in a catalytic reaction must pass through the stage of adsorption. The contradiction in Equation (1) concerning the homogeneity and heterogeneity of the surface is eliminated by accepting the theory of yuasihomogeneous surfaces developed in some detail by the author (22). A quasi-homogeneous surface is one, the two different active centers of which are characterized by a constant ratio ar/a,o =

@

(8)

independent of the substance number r . If a,, is a , for the less active, predominant centers, then w is the degree of unsaturation. It follows from the theory that the AF" of adsorption is here the arithmetic sum of AF a t the basic level (this quantity depending upon the chemic:d nature of the catalyst) and of AF" ( w ) , depending only upon the genesis of the catalyst. Applying Langmuir's adsorption law to each kind of active centers, the theory leads to a hitherto unknown logari thmic adsorption isotherm for mixtures. It shows that the expression for adsorption

a t saturation does not depend upon the shape of the adsorption isotherm, provided only that the exchange law remains valid. At the same time the x r are constant over the entire quasi-homogeneous .surface : z, = a,/al = a7w/alow = aro/alo= const.

(9)

ACTIVE CENTERS AND CATALYTIC DEHYDROGENATION

101

Therefore, in the general expression for the rate of reaction on heterogcnous surfaces dz

-=

dt

1

hTlds (10)

where

is the distributioll function, wc havc ds

=

@(a,,a2

, . . .,

UP)

daldaz

, . , dar = @(a,)dal * *

=

a(€)de ( 1 1 )

The last part of Equation (11) is true because by our method the adsorption measured is that taking place on the catalytically active centers. S e a r saturation and when @(t)

=

hr

cye

(121

Equation (10) become::

This integral is easily taken and we obtain

Thus, we have derived the fundamental equation (I), since the term in the first brackets of Equ:it,ion ( I 1) may tw considrred as the constant 1.id and the second exponential function, owing to its rapid decay, may be neglected if h > 1IRT, whereas the first one may be neglected if h < 1/RT. Equation (14) holds also for other cases of monomolecular catalytic reactions.

11. FLOWMETHODKINETICS Our ospc>rirrieiit:il results on drhydrogc'ii:Ltiot~were ohtnincd by a flow method undcr statioiinry conditions. We shall briefly describe the method

44FIG.4. Cross section of catalytic tube

102

-4. A. BALANDIN

of calculation which we have had newly to derive (23, 24) The change in the number of moles of reactants when the gas has passed out of an elementary section dl of the tube with the catalyst (Fig. 4), owing to the reaction, is dm = krludl, where I’l is the adsorption of the reactant, k the rate constant of the reaction, both quantities referring to one ml of the catalyst filled tube, and u the cross section of the tube. At nearly complete surface coverage I’l = spl/ p , (where s stands for the surface) and, therefore,

c

But the ratio of the partial pressure p r of the rth substance in the given section to the total pressure P in the tube equals the ratio of the number of moles N r of the rth substance passing through this section per second Nr passing per second through the same to the total number of moles section :

c

pr/P

=

N r / C Nr

(16)

Let the substance T be passed into the tube a t the cons1,ant velocity A , nioles/sec. Then, owing to reaction, the given section will pass

Nr

=

A , - vrrn

(17)

moles of the substance T per second, where v, is the stoichiometric coefficient. lcor the reactant v1 = - 1; for the product of the dehydrogenation v2 = 1; for hydrogen v 3 varies for different cases and for the extraneous substance v4 = 0. From Equations (15) to (17), we obtain dm/dl = uks(Al - m ) / [ z z p ( A r

+ v,m)]

(18)

and integrating between the limits 0 and I, we obtain for the general case [Ai(zz V3Z.3) Azzz A323 A4241 111 IAi/(Ai - m)l

+

+

+

+

- (z2 + v3z3 - l ) m = KV

(19)

where thc constant K = ks arid the volume of the catalyst V = ~ l For . derivation of an exprcssion corresponding to Equation (19) when r is the Langmuir isotherm, see reference (24). If the reactant alone is passed through the tube, A P == A s = A 4 = 0 and Equation (19) assumes the form (22

+ v & 4 1In [Ar/(A1 - mo)] - (z2 + vjza - l)mo

=

Z(V

(20)

Introducing the designations mo/A1 = y and A1 = u0 , Frost applied Equation (20) to different flow rates, giving it the form uo In [1/(1 - 1/)1 = cx

+ PUOY

(21)

If uo In [l/(l - y)] is plotted against u0y, a straight line is produced, with

ACTIVE CENTERS AND CATALYTIC DEHYDROGENATION

103

+

+

the constants a = K V / ( z , Y&) arid /3 = (z2 v3zS - 1 ) . Such straight lines have actually been obtained mainly for the reactions of dehydration and cracking by Antipiria and Frost (25),Orochko (as),l’anchenkov (27) (who further developed the theory of kinetics undcr flow conditions) , Topchieva (28), Nagiev (29), and others, which also confirms Equation (20)* Since, when z 6 0.1, -In (1 -

2) M 2,

(22)

Equation (21) can not be applied for small yields, because a straight line will then always be formed with (Y = 0 and /3 = 1. On the other hand, working with small yields mo/A1 permits one, a t constant rates, to obtain the rate constant and the activation energy. Indeed, from Equations (20) and (22) it follows that in that case mo = KV

(23)

For yields > 10 % the results will be approximate. Small volumes of gas are not difficult to measure with sufficient precision. Special cases are met with when z2 = 1 and 2 3 = 0. Then Equation (20) transforms to A1 In [Al/(A1 - m,)]

KV

(24)

+ KV/2)

(25)

=

whence mo = AIKV/(A1

since (up to 30% conversion) the relation In (a/b) M 2(a - b ) / ( a

+ b)

(26)

is applicable. One can pass binary mixtures containing p % reactant and measure m and m, a t constant flow rates. Then A1 A, = N , lOOAl/N = p , and from this and from Equations (20), ( 2 2 ) , and (23), there follows a formula for evaluating zr (30) :

+

(cf. Fig. 5). For large yields the next approximation or the complete Equation (20) should be used. In order to ascertain whether zr belongs to the substance r introduced to the mixture and not to a poison which may have been formed, constant zr values should be obtained on increasing the flow rates or on diminishing 1. Now we can introduce the values of z, obtained in the manner described for smalI values of m / A l , into Equation (19). Then already for any A1

104

A . A . BALANDIN

m0

1

I m

I

Y

P

-

3

1

I

FIG.5. Schematic curve of ndsorlhve exchange at the active centers

and m we obtain K , which should remain constant (for t: given temperature). Thus, the use of the flow method allows one to obtain the usual kinetic expression for constant volume conditions :

dx -- K dt

Pl

pi

+ z2pz + zspa + 24p4

(28)

From the z and their temperature dependence, the AF", AH", and AS" of adsorption exchange can be obtained using Equations (5) to (17) and from the value of K for different temperatures, the quantities K O ,E , and h.

111.

EXPERIMENTAL lxEsULTf3 ON DEHYDROGENATION

The validity of the equations given and of the methods of computation is illustrated by the following examples. The independence upon the total pressure P within wide limits as is required by Equations (18) and (19) is evident from Fig. 6. The variations in m for binary mixtures of different composition can be seen on Fig. 7, where the experimental points fit the theoretical curves well. It can be observed in Fig. 8 that with decreasing 1, the quantity z gradusilly assumes a quite constant (true) value. Figure 9 shows the temperature dependence of z ; straight lines are obtained for log z as a function of l/T, demonstrating that AH" is practicalty independent of the temperature. Figure 10 gives the linear Arrhenius relationship for log K vs. l / T and Fig. 11 shows that this holds only for log K , since the

ACTIVE CENTERS AND CATALYTIC DEHYDROGENATION

2o

105

t 200

0 a)

200 0 0

mo m1/5'

400

e

800

600 Rmm

Y

"

200

0

b)

-

600

400 _c)

0

O

0

800

P,mm

FIG.6. The pressure P and rate of dehydrogenation mo : (a) methylcyclohexane on l't (31), (b) cyclohexanol on Cu ( 2 3 ) .

function log m vs. 1/T although linear and parallel to the first line in its lower portion is bent down somewhat in the upper, in conformity with theoretical expectations. Figure 12 shows the logarithmic relationship log K Ovs. E ; for groups of related reactions straight lines are obtained, the slopes of which equal h/2.3. Table 1 summarizes the experimental results obtained in our laboratory on the kinetics of the normal dehydrogenation of hydrocarbons (hexahydroaromatics to aromatics, the open chain compounds butylene to butadiene, and ethylbenzene to styrene), of amines to ketimines, and of alcohols to aldehydes or to ketones, respectively, in the presence of metallic or oxide catalysts. Equation (1) was found to apply in all cases. KOand h are given by log KO = log K f/4.57T (29

+

h

=

which follows from Eq. (1).

2.3 (log K O - B ) / E

(30)

I00

Aldohydr

50

100 Alcohd

m

20

1

10

0

25

50

15

100

Flu. 7 . 1~~xcti:iiige curv(:3: (a) met1iylc~clohes:~iie with t,olucnc! on Xi (52), (1)) et.liyI rilcohol with :icct,uldchyde o n (hi (SS), ( c ) ethyl ulcoliol with hydrogen 011 (!u ( S 4 ) , (11) I)utylt:iie with wat.er ( 9 6 ) . (e) isopropyl irlcohol with cicetonr! ant1 ( f ) with hydrogcr1 011 Z I I O ( 4 4 ) .

107

ACTIVE CENTERS AND CATALYTIC DEHYDROGENATION

Aldehyde

Alcohol

FIG. 8. Stabilization of the value z for ethyl alcohol and acetaldehyde on Cu, by shortening the length 1 of the catalyst, bed. Full curve, 0, low-space velocity; dashed curve, X, two high space velocities. 1.0

-

T

15

Ib

a)

16

17

--*

I

1

18

18

I

15

16

17

I

18

I/T. 10

-I b)

FIG.9. The independence of the heats of exchange on the temperature (a) alcohols on oxide catalyst (36).Reading from left to right: allyl, n-propyl, ethyl, i-propyl, i-amyl, n-butyl, phenyl ethyl alcohols. (b) Curve 1-isopropyl alcohol with acetone, curve 2-isopropyl

alcohol with hydrogen on MnO (66).

108

A. A. BALANDIN

n Pr

1.6 -

1.4 -

1.2 1.0 -

f

lg k o'8 -

0.6 -

0.4 -

0.2 01 13

'

'

I

I

I

I

( I

17 18 19 20 +I/T. lo4 FIG. 10. Arrheiiius lines for dehydrogcn:ttion of :ilcohols on an oxide catnlyst. (56) ; numerstiori corrcsponds t o Table 1.

0

16

15

I4

.

-

\ 21 2 1 22 23 I/T.104 FIG.11. <:omparison of the log k (curve 1) and log mu (curve 2) vs. l / T relationships for the dehydrogonation of drohol on ('u (33).

"17

l'8

I9

20

I09

ACTIVE CENTERS AND CATALYTIC DEHYDROGENATION

10

12

14

16

I

Cal

N&

I

I

12

I4

16

+E Cal

20

18

fb)

(a)

I

L

25

30

6E Cal

35

10

15

20

25

4 & Col

(c) (4 Vio. 12. Straight lincs confirming thc 0-rulc; numer:ition corresponds t o Table 1 : (:I) smines on Ni and Pd (41, is), (1)) alcohols on :in oxide catalyst ( S6 ) , (c) cyclic hydrocarbons on CrzOt (39, 4O), (d) iaopropyl alcohol on ZnO prepared by different methods (44).

I n view of the extensiveness of the material the values for m o, z, AFO, AS", and K are given for only one temperature, their temperature dependencies being apparent from the magnitudes of AH" and E. For comparison the values for E', i.e., E obtained using Equation (23) are given in italics. The data in Table I refer to the kinetic and not to the diffusion regions, as evidenced by random tests on catalysts of varying granule size.

CH~C~HI Ht

lcyclohexane lcyclohexane

imethylcyelone in

lcyclohexane

hexane

H%

aa Nos. 1-3

CH.CsHr

(CHddMh

I

I

1 CsHs

imethylcyclont.

lcyclohexane

hexane

245

245

236

1

I

1

0

1

0

I

0

I 0

I

0

1

0

1

I

1

1

0

-

1

0

1

-

1 "-> I

Same as Nos. 1-3

0

Ni-MrOa ; V = 60 ml (Ref. 46)

1

Ni-AlrOt ; V = 100 ml (Ref. 32)

I

Ni-AlrOa ; V = 72 ml (Ref. 45)

Pt-asbestas; V = 2 ml (Refs. 3:. 48)

1

1

1

25.6

31.2

34.9

28.8

1

1

1

I

0.84 0.85 0.96 0.92 0.35

415 472 446 442 425

10.0 -2.08

0 1140

I

20.3 -0.17

0 115

20.7

2.0

22.3 -0.08 0

55

2.2 5.6 2.7

0 240

21.2

0.76

7.0

12.6

5.6

-0.35

CrL)z-asbestos; V = 16 ml (Refs. 39, 4O).B = -2.68

I ii: 1

I

1

1

12.0

0.610

0.075

2.13

10.5

5.92

7.08

0.081

7.44

l -

5.72

5.78

5.64

8.03

0.385

0.131

0.153

0.0852

0.108

I

I

2.49 6.83 2.96

0.77

7.87

15.3

6.08

1

69,7

32,3

22,2

22,9

2S, 7

25.8

25,9

28,4

18,2

-

15,6 14,7

15,6 12,3

orption Coeficients ( z , ) , Free Energies ( A F " ) , Enthalpies (-AH"), and Entropies (AS") of Adsorptive Exchange Rate ation Energies (e), and the h Parameters i n Catalytic Dehydrogenation. AF", AH', and c cal./mole; A S " e.u.; A1 R pply, and mo Reaction Rate ml. Substance Vaporlmin., K-ml./(ml. m i n . ) .All Valuesf o r N . T . P . Original Data Are e Units.

TABLE I

I

I

1

CHa

I

(CHa)rCHCHzC=NH

CHs

I

300

300 CHa(CHz)rC=NH inoheptane

CHI

7

O i

O

1

23.8

30.7

37.7

29.8

-

37.7

29.8

-30 Pd-mbestw; V = 3 ml (Refs. 41-48)

0

0

0

0

0

0

298

I

3

22 ml (Ref. 49)

3 ml (Refs. 41-49) B = -0.96

I

=

(CzHs)zN (CHt)r-C=NH

=

I

0

270-33

thyl-4-aminotane

0

Ni-Alas ; V

I

0

300

300

650

Copper-chromium catalyst; V

2 ml (Refs. 80, 80)

302

I

=

0

0

[(CHa)zCHlzC=NH

CH:

I

(CHr)zCHCHC=NH

CHI

CHa(CHt)rC=NH

Cot

HP

I

€la

CHFCH-CH%Hn Hz

Chromium catalyst; V

20.7 20.7 -4.73 -4.57

3260 3160

425

405

0

0

0.092 0.100

Hz Ht

461 517

21.2 22.3 20.3 -5.17 -5.37 -5.65

0

3580 3720 3900

0.073 0.067 0.058

466

Hz Hz HI

20.7

-2.20

0

1520

0.33

448

CHaCioHi

as Nw. 23-26

imethyl-3nopentane hylamino-4nopentane

hyl-4-aminotane

inoheptane

enzene

ne-1 ne-1 ne-1

hyl-5,6,7.8hydronaphe bexane leyclohexane methylcyclone in thyl-S, 6,7,8hydronaphe

6.45

8.20

8.30

8

6.40

-

12.44

1

9.4

1

7.00

-

17.43

7.21

9.20

9.64

11.1

I

I

2.33

2.06

-

4.16

4.03

-

9,9

9,7

10,7 4.87 5.81

9,5

9,3

8,6

4.14

4.09

I

-

33’w

90,40

3.74

I

-I

!

2.74

3.07

3.21

0.250

17,6

17,i

I 2.9

~

I

,

I

9,140

5.06

!

11,400

12.2 1

I

76.8:

!

I

i

I

!

i

1

I

I

I

I

5.09

I

12,200

~

,

i

!

!

I342

12.3

1

23501 -22.000~-31.9

Oxide catalyst; V = 11 ml (Ref. 35) B = 2, 37.

Ni-quartz (0 58% Ni); V = 7 ml (Ref. 3 6 )

ml (Refs. 53. 54)

n /

= 44

j

1

4.4@!0.63 4.20 10.60

ol-2 ol-2

hexanol

panol

' i I

1

1

-

3.8 3.7

1 CH:CH=O

(R.ef, .% I3 i) 2, 37. -.

]

ol

11.7j 14.7

Hs

-.

I

CsHto=O

-.

28,3301 44.2 -36.030 ,-58.5

(CHr)eC=O

= I 1 ml

360 2480

Oxide catalyst: T'

I

1

H20

1'

panol (Ref.

IO.76 0.15

Hz0

385

panol (Ref.

389

CHaCHzCK=O

1

panol (Ref.

Hs

HO CHaCHzCH=O CHaCHzCH=O

CHaCOCHeCH3

! - I

3.89

O /

ol (Ref. 50) ol (Ref. 61) ldehyde (Ref.

1

15.87

1250

(Refs. 53. 54)

O /

Ni-quartz (0.58% Nil: V = 7 ml (Rei. 36)

Butanol-2 Butanol-2

1 ~

7.7

1;:

CHaCOCH C Hs

250

= 44 ml

257

I I

1

257

[

I 1

252

Co; V 12.50

1

Co; V

1

0.40

1

970 :

0.40

I I

I (CHr)eC=O

4.9

84.7 -1.87 O

I 970

0

n

970

0.40

I 2.26 / j 5.29

6.18

30.7

5.1

3.1

84.7

-1.83

-

n n n 970

n -(o

! 0.40

Ij

1 I

6.0

2.8 88.0

0

-1.83

n n

84.3 85

73.4 83.7 0

n

n

5.1

(V = 5)

4.4

10.8

-

30

I

2 ml (Refs. 5 , S S 37,38,59)

i

i

8.@ ; O . X

0.97

2.2 2.5 4.4

11.5

-

5.29

15.87 11.63

23.8

n

=

O 1

CHsCH=O H2

1

1

j

ol (Ref. 83) ol (Ref. S 4 )

51

I

257

ns Nos. 31-34

50

I

,

!

970 , 0 01

V.4b

250

I

i

01

-(o

1 I

HzO

I

0

0

i

0.40

51 ) Isopropanol ( F k f .

,

' 1

! I

i

0'

257 257

47

CyclohPxanol

2.26

25i

1x20

40

6.78 6.10

252

n-Propnnol (Ref.

Isopropnnol

30.7

CHsCHzCH-3

46

48

257

Cu; V

56)

261 257

ni

51)

i

n

n-Propanol (Ref.

HAI

CIhC1IzCH=0 CII3CIIaCII'=

1

45

CHsCH=O H z

1

I

0

01

~

Ethnnol (Kef, 50) Ethanol (Ref. 61) ! Acetaldehyde (Ref.

i

303

CH,

n

42 43 44

!

TABLE I-Continued

Ethanol (Ref. 33) Ethanol (Ref. 3 4 )

(CZH~)~N(CHI)IC=NH

1

'

41

40

I(CHa)eCIIhC=N~

Pd-asbestos; V = 3 ml (Refs. 41-43)--Continued

34

2,4-Dimethyl-3aminopentane 1 1-Dimethylamino-4i amino-pentane ,

imethyl-3nopentane ethylnmino-4no-pentane

38

i

5.09

6.0.1

~

i

1

16.3

-14,00

12.8

5.59

16,3

ii.1n

5.06

-14,00

9,1 3.89

TABLE I-Conlinuecl

3.07

I

9,9001

12.1

-11,000

-4320

336

0.95

341

3.6

-3530

342

6.1

-4940

-

-19.3

-16,400

-18.2

-11,300

-12.7

10.2 8.3 7.9 9.5

140

60.3 66.0

14.6 10.9 10.3 13.1 12.3 11.4 3.1

4.55

-12.5

-

0.51

-11.200

-3570

8.1

9.4

65.0 51.4 43.6

0.60 1 624 1 -8,100 1-14.2 The values are close to No. 75.

i

1 I

44). B

2.6 28.8

= -0, 59.

I'

-8,500 -13.4

346 341 342

345

0.60 1 624 1 -8.300 1-14.2 The values are close to No. 75.

344.5 344.5

30.4

(30.1 t30.1

I

:" 1

1.19'

0.94 0.26

0

14.0 44.0

-

The values are close to No. 68

-8,500 -13.4

1 ml (Rei.

-215

=

16:;

1,W 19,400

-215

=

1 I

=

MnO: 1'

l.lgf

ZnO: V

1650

16i00

7 4 1-3, 5-7

36 4 5 2

I

1-3, 5-7

319

342

341

4

191400

-

-27,500

1

1 I

52. i

The values are close t o No. 68

1.2

6.1

3.5

0.95

1 6

I

--

ZnO; V

-

-515

-5,100 -4940

5

1 ml (Ref. LA). B = -0. 59.

-515

2.6 28.8

15 ml (Ref. 55)

-

-43.9

-20.3

-12.7 -11,300 -3530

-16,400 140

4

30.4 (30.4 130.4 30.4

44.0 44.0

52.7

43.6

51.4

66.0

65.0 -18.2

-12.5

-19.3 -11,000 -4320

3e

7

336

7683

1.2

0.94 0.26

2 4.55

69 70 71 72 73 74 75

Impropmol lsopropanoi lsopropanol Isopropanol Impropanol Isopropanol Isopropanol Isuyrupanul laopropmol

352

68

Isopropanol Isopropanol

-11,200

67

349

0

-3570

66

1

Same us Nos. 52-58

3.6

59-65

8- Phenylet.hano1

342

58

60.3

1

9.1

14.6 10.8

3.6

6.0 6.0

0.85

1.33

4.36

1.22 13.3

3- Methyl-1-butanol

10.4

57

48.1

n-rlutmol 32.8

56

4.6

Iaupropnol

4.7

55

342 352

19.3

__

2-Propnol-1

19.2

54

0.41

1.76

n-Propanol ~

53

-18 m and K V are given for the temperatures of column 3, which are near the temperatures of determination of zr . K, AF", and AS" are given for one temperature (419". interpol). rogenation ( 4 7 ) . 3-39, z from the validity of Equation (23). 52-65. K = Kars' . gives the Nos. of the ZnO catalystes differing in the methods of preparation. stant8 have been found from the next approximation. 68-83 column 11 gives K / s , where s is the B.E.T. surface. varying from 0.3 to 15 m2. 68-83 column 12 contains log(Ko/s). The values of log& and h were obtained from and K according to Equations (29) and (30). Column 14 umber of points sufficient for the application of Equation (30).

6.35 7.03 10.20 1.33 1.51 1.0

-

4.34

0.79

-

-

11,4 12.28 12,5 15,0 17,6 19,4 26.80 4.06 0.988

23,6 8.13

6.19

5.61

19.10

16,3 5.75

JY.J

12,8 5.09

17,50 5.70

5.68

__

__

I _

19.2 4.7

19.3 4.6

__

~

1.76

I 15,600

5.68

4.87

13,700

0.41

(3.06)

17,500

5.70

17.100

32.8

48.1

4.36

5.00

12, 800

4.88

10.4

13.3

1.22

6.75

1.77

10.8

14.6

1.33

5.61

16,300 15,000 12,900

lb. 1w

9.1

9.4

0.85

6.19

18,000 17,600

6.0 6.0

8.1

0.54

8.13

23,650

0.981

~1.06

9.5 8.6 8.5 3.0

14.6 10.9 10.3 13.1 12.3 11.4 3.1

-

-

10.2 8.3

7.9

-

(5.77)

13,500

-

-

-I

0.79

4.31

1.33 1.51 1.0

6.35 7.03 10.20

I

*I'

11,400 i2.m 12.500 15,000 17,600 19,400 26.800

1.87

0.36

-

S.OB

-

I

9 .Oi 9.04 9.21

-

i

CHXH-CH=O

(CHa)zC=O

CHGHzCHtCH=O

(CHdzCHzCHzCH=O

CsHsCHtCH=O

HP

(CHa)zC=O Hz

(CHa)KO (CHa)tCO (CHa)zC0 (CHdzCO (CHa)zCO (C Ha)zC0 (CHa)zCO Hz Hz

enol-1

panol

anol

hyl-1-butanol

nylethanol

as Nos. 52-58

panol panol

panol panol panol panol panol panol panol panol panol

'

CHzCHzCH=O

panol

1 6

In Nos. 9-18 m and K V are given for the temperatures of column 3, which are near the temperatures of determination of zr . R, d F ",and AS" are given for thesake of comparison far one temperature (319". ioterpol). From h3-drogenation (47). C I n Nos. 23-39, z from thc validity of Equation (23). For Nos. 8 - 6 5 . K 2 Karn' . Column 2 givaa t.hc Nus. of the Z n 0 cxtalystes differing iu thc mothoda of prepRru.tiun. 1 The z constants have been found from the next ayyroximatiun. 0 For Noa. 6&83 column 11 gives K i 8 , where 8 is the B.E.T. surface. varying from 0.3 to 15 m2. For 3 0 s . 68-83 column 12 contaias log(Ko/s). The values of lagKO and h were obtained from and K according to Equations ( B Y ) and (30). Column 14 gives tr for cases with a number of points sufficient fur the application of Equation (30).

114

A . A. BALANUIN

IV. DISCUSSION OF RESULTS AND

THE

MULTIPI,ETTHEORY

Table I shows the following: 1. The Nature of Catalysts. The kinetics of dehydrogenation depend upon the nature of the catalyst. The catalysts used divide sharply into two groups-metals and oxides. For the latter the reaction temperature and e are higher, whereas h is lower than for the former. This is nmtural, for catalysis proceeds under the influence of chemical forces, i.e., valence electrons. According to the model described further below (Fig. 18) metal atoms participate in the dehydrogenation process; in the oxides the electrons belonging to the metal are displaced toward the oxygen atoms located in the lower layer (or, less probably, laterally adjacent). It is for this reason that the method of preparation (Nos. 68 to 83) of the catalysts affects the kinetic characteristics; upon it depends the profile of the catalyst surface, i.e., the number and the location of the atoms adjacent to the active centers and influencing them. 2. The Nature of Reactants. No less effect on z and e is exerted by the nature of the reacting atoms of the molecule. The values of z and E are distinctly different for hydrocarbons, amines, and alcohols (on the same or similar catalysts). This complies with the multiplet theory of catalysis, according to which in the three reactions the following atomic groups, in which the vertical bonds pass over to the horizontal, are in contact with the catalyst: C-C

I 1

H

H

C-N

I 1

H

H

C-0

I 1

H

(IV)

H

I n harmony with t.he multiplet theory is the observed dependency on the nature of t$e catalyst, since the atoms of the latter are involved in the multiplet complex. 3. The Influence of Substituents. The substituents a t the groups indicated have a markedly smaller effect on z and 6 . This is evidence for the orientation of the molecules with their reacting atoms toward the catalyst (Fig. 13), as is required by the theory. The influence is particularly slight in the case of metals. For hydrocarbons (Nos. 1-3), amines (23-29), and alcohols z1 = 1 and 23 = 0. From a crisscross examination of the data of Nos. 40-47, it can be seen that in the case of copper not only the balues of z2 but also those of uz are practically the same for ethyl, n-propyl, isopropyl alcohols, acet- and propionic aldehydes, and also acetone. This was explained by the author (66) in a treatment of the above-mentioned model from the standpoint of statistical mechanics. It was found that

ACTIVE CENTERS AND CATALYTIC DEHYDROGENATION

115

H,\C CH, CH3-CH

CH

\/

CH-NH

w

/

Pd 9700

E = 9920

CH-NH

Pd

I I400

CH-CH,

-

crz 0,

C

28500

cr2 0, 26600

Cr, 03 24800

FIG.13. Diagram of molecular orientation in catalytic dehydrogenation

where the numerator contains the partition function of the adsorbed molecules and the denominator of the molecules in the gas. Owing t o the canceling of both the masses m and the moments of inertia I , to the close values of AH, and to the constant values of the other quantities, a: remains constant for all the above molecules. The activation energy E for the cyclic hydrocarbons on Ni-A1203 are practically independent of the number of methyl side groups (Nos. 1-3). For amines on Ni and Pd (Nos. 23-39) the E are also quite constant. Studying the dehydrogenation kinetics of ethyl, n-propyl, n-butyl, and n-amyl alcohols on Cu prepared by a different procedure, Palmer and Constable (8) did not measure z, presumably because they were working under conditions when Equation (23) was applicable. They found for these alcohols a constant e = 22,000 cal./mole, which they interpreted as the result of the same orientation of the OH group. In our experiments the values of E for C2H,OH and C3H70Hare also quite close, although there is a noticeable difference of 600 cal./mole, which is beyond the limits of experimental error. The close values between the reaction rates for the alcohols were explained by the author (56) from the standpoint of the theory of absolute reaction rates and the multiplet theory, which may serve as illustration of their combined application; from the former one can compute velocities using models of the latter. In the case of oxides 23 # 0. For MnO and isopropyl alcohol 2 3 increases from 0.1 a t 316" to 0.36 a t 366'. The substituents exert a stronger influence on zz. Thus, on CrZ03, zz has a tendency to increase with the methyl

116

A. A. BALANDIN

groups introduced into cyclohexane (Nos. 9-11). The value of x2 falls to % of its initial value when the cyclohexane ring is condensed with benzene to tetralin. Formation of the conjugate systems of but,adiene (No. 19) raises the value of zz . An extensive study has recently been made with respect to the effect of structure on the dehydrogenation of alcohols in the presence of an oxide catalyst. It is seen from Table I that the structure influences greatly the 22 value. The formation of conjugate bonds in acrolein again raises 22 (No. 54). With increase in open chain length z2 rises ( C z H 6 0 H , z2 = 2.3; C 3 H 7 0 H , z2 = 3.4; C4H90H,x2 = 3.5; n-C6Hl10H, zz = 5.8 (Nos. 52, 53, 56, 57); for the secondary, isopropyl alcohol z2 is lower (22 = 0.2) (No. 55). The activation energy E also changes here regularly. E falls by 4900 cal. on substituting the a-hydrogen in ethyl alcohol by a CH3 group and by 2100 cal. for substitution in the P-position. On the contrary, substitution by phenyl and methylene radicals somewhat raises the E value. Such regularities in dehydrogenation remind one of the rules of Dohse (57) and Bork and Tolstopyatova (58, 59) in the dehydration of alcohols (on bauxite and on A1203). The singular position occupied by the dehydrogenation of alcohols on oxides can perhaps be explained by intermediate formation of hydrogen bonds. 4. Enthalpy and Entropy of the Adsorption Exchange. Experimental evidence shows that, in dehydrogenation the calculated A H o is practically independent of the temperature over the range of 50 or 100" (see, e.g., Fig. 9). The entropy change ASo does not vary markedly with the temperature in the cases which were investigated. The AHn and ASo of adsorption exchange approximate those for chemical reactions. The large values for A H o speak of the possibility that the adsorption required for catalysis originates from the formation of a one-electron bond. With accumulation of experimental material it would be of interest to treat the entropy of adsorptional displacement from the standpoint of statislkal mechanics as an approach t o the construction of adsorption models in catalysis. As in the case of some chemical reactions (GO), a parallelism is observed between the AH0 and ASn of adsorptional displacement (Fig. 14). 5. Activation Energy. If one works with small yields, there should be no difference between E and E' [Equation (23)]. In practice cine has sometimes to deal with larger yields. As is shown by Table I the clifference between E and E' will then not exceed several kilogram calories and, if this is taken into account, E' may also be used in suitable cases. The values of h remain sufficiently constant for groups of related reactions, which confirms the 8-rule. That the 8-rule holds true is especially clear from Fig. 12b, where all experiments were carried out on a single sample of catalyst of constant activity. The total surface in this case was,

ACTIVE C E N T E R S AND CATALYTIC DEHYDROGENATION

0-

I

I

I

I

-

t

-AHIDk

I

I

I

-

117

0

- -10

/

f

--m-AS

- -30 -40 -50

-

therefore, strictly constant. The points fit the straight line well, except for ally1 alcohol, which is not surprising if one considers the specific structure of the acrolein formed, and for isoamyl alcohol. Figure 12c shows that a straight line is obtained if log (Ko/s)be taken, where s is the specific surface area of Z n O determined by the B.E.T. adsorption method. It is characteristic that for dehydrogenation often h > 1/RT. According to Equation (14), this means that dehydrogenation is carried out predominantly not by a. few highly active centers, but, on the contrary, by centers of lowest activity, which, however, are very large in number. 6. Application of the Structure Correspondence Principle. The kinetic parameters, z, AFO, AHo, K , and E confirm the difference in the dehydrogenation mechanism of six-membered rings over Ni on A1203 and over Cr2O3 (Table I, Nos. 1-6 and 9-18). We shall enumerate now the experimental evidence in support of the concept that the elevated active centers end in crystalline sites. This includes the sextet mechanism for dehydrogenation of the cyclohexane ring on the (111) faces of Ni, Pt, etc. (Fig. 15) ( 1 ) . The sextet mechanism has been confirmed by Long et al. (61);Emmett and Skau ( 6 d ) , Beeck (63)) Trapnell (64), Rienacker and Unger (65), and others. In contrast to the sextet mechanism is the duplet niechanism for the dehydrogenation of cyclohexane on Cr2O3and other oxides, in which the ring is turned with its edge t o the surface (66) (Fig. 16). I n respect of this reference is made t o

118

.4. A. RALANDIN

FIG.15 FIG.16 FIG.15. The sextet model for the dehydrogenation of cyclohexune ( 1 ) FIG.16. The duplet mechanism for t,hc dehydrogenation of cyclohexane (66)

the recent review by Trapnell (64) concerning the structural aspect of the multiplet theory. However, certain reservations should he made. Firstly, it is preferable not to modify the sextet model according to Trapnell. In Trapnell’s variant the hydrogen atoms do not permit the cyclohexane ring to superimpose on the lattice. Secondly, according to the inultiplet theory catalysis is caused not by van der Waals’ but by chemical valence forces, which is the basis of the energetics aspect of the multiplet theory, not included in Trupnell’s review. The sextet mechanism is also confirmed by the following new facts. Rubinshtein and associates (67) found that as Y t is dispersed more and more on carbon, the rate of dehydrogenation of cyclohexane parallels the intensity of X-ray reflections from the (111) faces and not from the others. Sachtler et al. (68) established by careful electron difrraction and electron microscopic studies that in hydrogenation (and consequently in dehydrogenation) on Ni the (111) [and possibly the (lO0)l faces are active, but not the (110) ones as was found previously by Beeclr (69). The author and Isagulyants (70) investigated the dehydrogenation of cyclohexane (I) and decalin (11) A

A

over Ni on A1203 and over Crz03. In the presence of 1 5 about twice as many molecules of I dehydrogenate as 11, because if the molecules lie flat on the surface, then more molecules of I can occupy the same surface area than of I1 (Fig. 17). On the contrary, in the presence of the Cr203both I and I1 are dehydrogenated a t about the same rate, because the molecules are located on the catalyst edgewise with their sides A B , the hydrogenation of which is the slowest stage of the reaction, determining the over-all rate. The dehydrogenation activation energies of I and I1 h.sve been found to

ACTIVE CENTERS -4ND CATALYTIC DEHYDROGENATION

119

FIG. 17. Diagram of the planar orientation of cyclohexane (I) and decalin (11) on the (111) faccs of P t (70).

FIG.18. Model for the dehydrogenation of n-butyl alcohol. The model is of general significance for the hydrogenation of double bonds and the dehydrogenation of hydrocarbons, amines, and alcohols.

be the same for the same catalyst, equaling 12,500 cal./mole on Ni arid 26,000 cal./mole on Cr203. This is evidence for the same orientation of the molecules of I and I1 on each of the catalysts. The well-known studies of Rideal ( 7 l ) ,Twigg (721, and Herington (73), in which the rate of hydrogenation on Ni is treated as a function of the cross section of the molecules being hydrogenated, a t complete surface coverage by the latter, also speak in favor of catalysis proceeding on planes. Recently the author ( 3 , 7 4 )has proposed a duplet model for the catalytic dehydrogenation of open chains (Fig. 18). Here four atoms of the catalyst are active, in the valleys between which are situated four reacting atoms. The intermediate complex is similar to a surface alloy. The reacting atoms do not get into the valleys simultaneously but in stages, for example, for the dehydrogenation of alcohol : (1)

(2) (3)

+ VzWp = RHCHOV + HW + VW RHCHOV + VW = RRCVOV + HW RIICVOV + ZHW = RHCO + H P + VzWo.

RNCflOII

120

A . A. BALANDIN

The slowest stage is the disruption of the C-H bond in the alcohol. This stage possesses the highest potential barrier. Stronger adsorption in the deep pits M (Fig. 18) leads to poisoning. One of the atoms of the active center (the shaded portiori on Vig. 18) differs in its nature or in the surroundings. Therefore, distinction should be made between two types of valleys V and W . The V sites adsorb less strongly than the W and hence in dehydrogenation, when the temperature is high, they are practically free. The model on Fig. 18 covers a very extensive experimenical material considered elsewhere (75). It is mentioned here because it d s o requires the presence of a planar group of four atoms for the active center. This model forms the basis for the construction of Fig. l b . Thus, it appears that the catalytically active centers of dehydrogenation are peaks on the one hand and crystal faces on the other. This can be explained only by assuming them to be flat islets with steep banks. Such is the result obtained hy the method considered here, which may be called the method of kinetic molecular probing. With the aid of this method the author and Rubinshtein (76) have found that the activity of a mixed Ni-AlzOS catalyst is located a t its interfaces. Indeed, on specimens of this catalyst prepared by different methods, the activation energies of the dehydrogenation and dehydration of isoamyl alcohol were falund to be related by a direct proportionality. But dehydrogenation takes place on Ni, whereas dehydration occurs on A1203. Consequently, the catalysic; must proceed a t places where the Ni and AlzOa are geometrically connected, i.e., a t the boundaries between the phases. Therefore, one may believe that the active center islets on mixed catalysts are adjacent to the boundaries of the other solid phase. 7. Application of the Enerqy Correspondence Principle. The kinetics of dehydrogenation and dehydration may also be applied to obtain bond energy values between the reactants and the active centers and to determine the effect upon them of neighboring atoms. I n the catalytic active complex (Fig. 19) six layers (3, 77) are to be distinguished: (I) the bulk catalyst atoms adjacent to the active centers and differing in nature, number, and position; (11) the active center; (111) the reacting group of atoms in the molecule; (IV) the substituents at thefre atoms; (V) the adsorption layer, considered in the present report; (VI) the layer of molecular adsorption and lateral diffusion. Consider a duplet reaction of the type: A

1

B

D I-+IiI-+ C

A--I>

(V) B-C

for example, one of those schematically represented in (IV) or on Fig. 18. According to the energy part of the multiplet theory the interaction he-

ACTIVE C E N T E R S AND CATALYTIC DEHYDROGENATION

121

V

IV

m I1 I FIG.19. Stratification of the active complex in catalysis (77)

tween the atoms of layers I1 and I11 (that essentially determines the chemical reaction) in the limit is given by the equations for the heats of formation E‘ and of disruption E” of the multiplet complex M:

E’

=

E”

+

(-QAR

=

QAK

-

(&AD

QAK

+ -

QBK) QDK)

+ +

+

(-Qco (QAC

-

QCK

QBK

+ -

QDK)

QCK)

(32) (33)

where Q is the bond energy and K the catalyst. The more easily the reaction proceeds, that is, the higher the specific rate, and the lower the temperature a t which i t takes place, the smaller the potential barrier ( - E ) . E is the smaller of the two quantities E! and E”. Thus, the reaction should proceed more easily, the larger the value of E. The specific activity and selectivity of catalysts depends upon the nature of the atoms A, B, C, D, and K, since on them depends the value of Q. But the Q are variable quantities; QdB , Q c o , Q A D , and Q B C (layer IJI), having their tabular values as basic, vary with the substituents a t A, B, C . and D (layer IV) ; Q A x , Q B K , Q C K , and Q D K , analogously vary with the neighboring catalyst atoms (layer I), i.e., with the method of preparation, the promotors etc. (cf. Figs. 12c and 12d). Introducing the designations, u

=

-&A

s =

QAB

q =

QAK

+ + + + + + +

B

- QCD

+

QAD

Qnc

(34)

Q ~ D

&AD

Qnc

(35)

QBK

QCR

QDK

(36)

where u is the heat of reaction ( V ) ,s the sum of the energies of the dis-

122

A . A. BALANDIN

0

- 10 - 20 - 40 - 50 - 60

- 70 1 FIG.20. Volcano-shaped curves for the dehydrogenation of hydrocarbons (I j and alcohols (11) and dehydration of alcohols (111) on C r r 0 3 (85). For E arid q, the units are kg. cdories.

rupting bonds and of the bonds being formed in the molecule, and q the absorption potential. Equations (32) and (33) then give

B’

=

(42)

E” = ( 4 2 )

- ($2)

+q

+ (42) - q

(37) (38)

This is graphically represented by a broken volcano-shaped line (Fig. 2 0 ) . ?he cuordinates of the peaks of these curves are Eo = u/2, qo = s / 2 . From the peaks straight lines extend a t an angle of f 4 5 ” to the axis of abscissae. Each reaction has its particular volcano-like curve, arid each catalyst (as well as the group of atoms A , B , C , and D)its secant E, for example, FG, Fig. 20. It is found that the activation energy EZ

-XE

(39)

Since the optimal ordinate E, equals u/2, this permits one from the value of E to judge how far the given catalyst stands from its maximum specific activity. Since the optimal abscissa qo is s/2, then, by selecting the proper sum of s in Equation (37) and b y varying the bond energy values Q A K, QBK, Q C K ,and Q D K , we can expect to obtain the optimum catalyst for a given reaction. The mean bond energies with the catalyst may be obtained from thermochemical measurements. By means of these and Equa,tion (30) the author (2, 3) had already some time previously calculated the sequence in the ease of rupture of bonds on Ni, namely, N-0

>

C-Cl

>

C-0

> C-N > C--C

where > stands for “easier.” This permitted calculation of the sequence of hydrogenolysis for hundreds of complex conipounds, almost without excep-

ACTIVE CENTEES AND CATALYTIC DEHYDROGENATION

123

tions (2, 3 ) . Recently, the author and Ponomarev (79) by means of Equations (37) and (38) have calculated the analogous sequence for approximately a hundred conversions of furane compounds on Ni. Thus, the theory in agreement with experimental facts easily shows that on hydrogenating over Ni, 1-(alpha-furyl) pentene-1-one-3

there are formed consecutively: (1) 1-(alpha-furyl) pentanone-3; (2) 1(alpha-furyl) pentanol-3; (3) l(a1pha-tetrahydrofuryl) pentanol-3; (4) 2-ethyl-l , 6-dioxa(4,4) spirononane; (5) n-nonane and (6) methane. We have also been able to predict and carry out new reactions (1.4, 21, 41, 80). Quite recently in the investigation by Balandin et al. (81) it was shown that hydrogenation of the peroxide bond on Ni precedes that of the olefin and the latter that of the C-0 bond, in accord with the theory. For example, cyclohexene hydroperoxide a t 30" and 1 atm. first yields cyclohexenol and then only cyclohexanol. Besides the thermochemical method the author (78) has proposed a kinetic method for measuring the energies of binding with the catalyst, possessing the advantage that the results refer to the atoms of the active centers. The essence of the method is the following: Consider three types of duplet reactions, for example, dehydrogenation of a hydrocarbon (I), dehydrogenation of an alcohol (11),and, dehydration of an alcohol (111). From data on runs with tracer atoms, it was shown (82) that on certain oxides these reactions possess the same mechanism which is atomic but riot ionic. The basic process underlying them is concentrated in the groups:

c-0

C-C 1 1

H

I

H

11

I

H

C-C

I

H

111

I

I

H

O

(VII)

Here there are three atomic species and in accordance with the number of reactions three equations may be written of the type of Equation (32). l+om kinetic measurements, determining cI , eII , and eIII and making use of the expression (39) we can solve these equations with respect t o QaK , QcK , and Q o ~ since , the energies of the bonds between the atoms H, C, and 0 are known. The method is treated more fully elsewhere (82). This method was applied in our laboratory to a number of oxides (82), in particular to ZrO:, in the work of Balandin et al. (83).I n the investigations of the author and Tolstopyatovtl (84,85,86)the effect of the method of preparation of Crz03on the bond energy with the catalyst was deter-

35

70.0 68.6

134.1 192.4 135.( 192.2 147.5 46.0

53.9 54.9 53.7 54.7

11.3 10.2 11.1 10.0

73.5 72.0 74.1 72.6

130.1 130.: 129.t 129.'

50.6 50.6 51.5 51.5

8.5 8.5 8.9

17.5 20.0 17.5 20.0

53.4

14.1

66.2

135.( 187.1 143.8 46.0

14.0

19.1

52.2

12.9

69.6

130.:

186.C 147.6 50.6

14 .O

19.7

55.8 56.1

20.0 19.8

52.9

152

51.5

152

184 183

16.5 18.6: 17.6 2 0 . 3

43.6

32.2

39.0

152

158

147

29.3

60.7

11.9

44.9

145

178

129

35.8 't2.8 38.1

48.8

23.8

33.2

145

155

129.6 35.9

155

129

158

178

147

149 147 184 183

145

145

44.9

33.2

11.9

23.8

-

152

39.0

I 152 152

192.6 192.0 192.6 192.0

129.6

55.8 56.1

29

29 29 52.9 51.5 20.0 19.8

12.4 11.4

I

69.6

12.9

52.2

130.2 186.9 147.6 50

135.0 187.1 143.8 46

14.1 66.2

50 50 51 51

Fi5.0 56.1

150.0 147.3 150.0 147.3 130.4 130.2 129.6 129.4 11.3 73.5 10.2 72.0 11.1 74.1 10.0 72.6

QO K

53.9 54.9 53.7 54.7

134.8 192.4 149.8 46 135.0 192.2 147.5 46

12.4 70.0 11.4 68.6

- -.

13.1 15.0 13.1 15.0

_______ __

TABLE 11 rimental Activation Energiesa t; Bond Energies of Atoms i n the Reacting Molecules with the Catalyst Q a ;~Adsor d the Heights of the Potential Barriers E k g . cal./mole on Chromias of Diflerent Methods of Preparation; the Subsc I-Dehydrogenation of Hydrocarbons, 11-Dehydrogenation of Alcohols and Acids, 111-Dehydration of Al

QCK

TABLE I1

__ __

48.8

32.2 43.6

53.4

60.7

192.f 192.( 192.t 192.1

150.0 147.3 150.0 147.3

on asbestos, from chromium nitra te precipitated with sodium carbonate (Ref. 88)

__

r,O3

on asbestos, from chromium nitrate preeipitated with sodiumcarbonate (Ref. 88)

88

4. (:rpOa

r,03 from chromium nitrate precipitated with ammonia (Ref.

88 1

atalyst No. 7

3. Cr,Ol from chromium nitrate precipitated with ammonia (Ref.

6.4 6.7 6.4 6.7

2. Catalyst X o . 7

55.0 56.1

nitrate preripitat,ed with sodium carbonate

r,03 from chromium nitrate precipkated with sodium carbonate

I . CrzC)3 from chromium

QCK

- - - -. -

Qaic __

-

Catalysts

Catalysts

- -

h'zperiinenlal Bctivation Energies" e; Bond Energies of Aton~si n the Reacting Molecules with the Catalyst Q.aK ; Adsorption Pvlentials q and !he Heights of the Polenlid Barriers E k g . cal./mole on Chromim of Diflerent Methods of Preparation; the Subscripts Designate: I-Dehydrogenation of Hydrocarbons, II-Dehydrogenation o Alcohols and A c i d s , IIZ-Dehydration of Alcohols:

8.5

8.9

8.9

42.6

46 . O

17.5 20.0

20.4

38.1

-

35.9 36.1 122

45

13.0 20 40.6

165

i-CaH,OH

i 57.8

I

I 52.2

145

57.7

55.4

1 1

1; 55.8 i

33.9

157.7

1 55.4

l

ained by applying Equation (23)

os, oblcining bi-

'

13.0

20 .o

I

17.0

55.7

149.6 188.3) 147.5 31.6

13.0

20.0

17.6

54.0

151.4 188.01 147

29.6

13.0

20.0

141.6 188.11 147.3 39.6

13.0

20 .O

40.6

13.0

20.0

36.1

35.9

45.2

I

15.0

61.5

I

14.8

62.1

140.4 188

/ 147 1 122 I

I

33.9

1

9.gi-C3f;10Hi5.3 i-CaH70H 269.8 , g i - c a ~ ~ 15.3 oH

1

I

i-C,HTOH 9.8 1 15.3 i-CaH7OH 9.8 15.3 i-CaH7OH 9.8 15.3

i

146.5 188.01 147.4) 34.5

14.8

35.1

145

165

i

a

bestos, ium bieduced in ipiammo-

57 . 9

__

I

I

I

The are obtained by applying Eqiiation (23). * Tetralin.

16.3

\

-

14.8 35.1

14.8

61.5

i-CsH7OH 9.8 15.3 i-CJ170H 26.9

1

; 56.9

I

9.8 15.3 i-CsH70H 9.S 15.3 62.1

20 13.0

147

140.4 188

1

15.0

141.6 188.1 147.3 39.6

I

i-CSHrOH 9.8 15.3

9.8i-C3yH15.3

55.8

13.0 20 29.6 151.4 188.1 147 54.0

17.6

17.0 57.8

16.3 56.9

6. Cr& on asbestns, obtained by calcining ammonium bichromate

55.7

nia

52.2

with CH,OH in H&h , precipitated with ammo-

20 13.0 149.6 188.: 147.5 31.6

20 13.0

I

147.4 34.5

5 . Cr,Oa on asbestos, from ammonium bichromate reduced

-

-

126

A . A . BALAKDIN

mined. I n Table I1 are reproduced the results of the last study (86). On Fig. 20 the experinierital volcano-like curves and a number of secants for this case are presented, drawn to scale. Table I1 shows the following: 1. The method does enable determination of the bond energies Q H C r , QCCr, QOCr, and they are of reasonable value, since, for example, therniochemical methods give for nickel QHNi = 55, Q C N i = 19, QONi = 59 kcal. The reactions (VII) are actually retarded owing to the formation of M, for if Equation (33) instead of Equation (32) is taken for the disruption of M, then improbable values would be obtained. 2. Saturated hydrocarbon suhstituents in the molecule have little effect on the bond energies with the catalyst. On the contrary, the appearance of stabilization energy, for example, of the carboxyl groups, has a profound influence on Q A K . 3. The method of preparation of chromia greatly affects the bond energy with the catalyst. This can not be explained otherwise than by the influence on the active center of adjacent catalyst atoms. Especially sensitive is QOCr, perhaps because of the influence of the oxygen a t o m of the chromia. Thus, the active centers are to be conceived of as small, separated parts of the surface and not as the smooth continuous surface as a whole, for in case of the latter, such factors as change in the microrelief of the surface or possible unnoted microimpnrities could not exert such strong action on the bond energy. Hence, the kinetic measurements on deliydrogenatiori and dehydration also confirm the concept of the active centers as developed in the present report.

V. SUMMARY The geometry and energy aspects of the multiplet theory are useful in interpreting the kinetics of catalytic dehydrogenation and in throwing light on the nature of the active centers. The possibility was shown of the experimental determination of change in free energy, enthalpy, and entropy of adsorption on the active centers and of the bond energies between the reacting atoms of the molecule and the atoms of the active centers of the catalyst.

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ACTIVE CENTERS AND CATALYTIC DEHYDROGENATION

127

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ACTIVE CENTERS AND CATALYTIC DEHYDROGENATION

129

67. Rubinuhtein, A . M., Shuikin, N . I., and Minachev, K h . M., Doklady Akad. Nauk S . S . S . R. 67, 287 (1948). 68. Sachtler, W. M. H., Dargelo, I,., and van der Knaap, W., J. chim. phys. 61, 491 (1954). 69. Beeck, O., Smith, A. E., and Wheeler, A., Proc. Roy. SOC.A177, 62 (1940); Beeck, O., Discussions Faraday SOC.8, 118 (1950). 70. Balandin, A. A . , and Isagulyants, G. V., Doklady Akad. Nauk S. S . S . R. 64, 207 (1949). 71. Rideal, E. K., Chem. & Ind. (London) p. 335 (1935). 72. Twigg, G. H. T., and Rideal, E. K., Trans. Faraday SOC.36, 533 (1940). 73. Herington, E. F. G., Trans. Faraday SOC.37, 361 (1941). 7.4. Balandin, A . A . , Doklady Akad. Nauk S . S . S . R. 97, 449 (1954). 7 5 . Balandin, A. A , , Zhur. Obshcher Khim. 16, 608, 619, 770, 781 (1945); 16, 557 (1946). 76. Balandin, A. A . , and Rubinshtein, A . M., Zhur. Fiz. Khim. 6, 576 (1935); J . A m . Chem. SOC.67, 1143 (1935). 77. Balandin, A . A . , Doklady Akad. Nauk S . S . S . R. 97, 667 (1954). 78. Balandin, A. A . , Zhur. Obshche'l Khim. 16, 793 (1946). 79. Balandin, A . A , , and Ponomarev, A . A . , Doklady Akad. Nauk S. S . S . R. 100, 917 (1955); Zhur. Obshch. Khim. 26, 1146 (1956). 80. Balandin, A . A . , and Vaskevich, D. N., Zhur. Obshchei Khim. 6, 1878 (1936). 81. Balandin, A . A . , Freidlin, L. Kh., and Nikiforova, N. V., Izuest. Akad. Nauk S . S. S. R., Otdel. Khim. Nauk 4, 443 (1957); Doklady Akad. Nauk S . S . S . R. 112, 649 (1957). 82. Balandin, A . A,, Izuest. Akad. Nauk S . S . S. R., Otdel. Khim. Nauk 4, 624 (1955). 83. Balandin, A . A . , Tolstopyatova, A . A . , and Ferapontov, V. A., Doklady Akad. Nauk S . S . S. R. 103, 611 (1955). 84. Balandin, A . A . , and Tolstopyatova, A . A . , Doklady Akad. Nauk S . S . S . R . 94, 49 (1954). 86. Balandin, A . A., and Tolstopyatova, A. A . , Zhur. Fiz. Khim. 30, 1367 (1956). 86. Balandin, A. A . , and Tolstopyatova, A . A . , Zhur. F i z . Khim., 30, 1636 (1956). 87. Balandin, A. A., and Kukina, A . K., Doklady Akad. Nauk S. S. S . R. 64, 65 (1948). 88. Tolstopyatova, A. A . , Balandin, A. A., and Dulitskaya, K. A . , Zzvest. Akad. Nauk, Otdel. Khim. Nauk, 10, 1256 (1956).

The Structure of the Active Surface of Cholinesterases and the Mechanism of Their Catalytic Action in Ester Hydrolysis F. BERGMANN Department of Pharmacology. T h e Hebrew IJniversily. Hudas&ah Medical School. Jerusalem. Israel

I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Enzyme Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Page

131 131 131

1 . True Cholinesterase from Erythrocytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 . True Cholinesterase from the Electric Organs of Ebctrophorus clectricus or Torpedo marmorata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 132 3 . Pseudo-Cholinesterase from Plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I11. &lethods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 1 . Manometric Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 2. Titration Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 3 . Colorimetric Met.hod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 4 . Iodometric Titration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 5 . Spectrophotometric Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 6 . Indicator Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 7 . Biological Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 IV . Catalytic Effectjs of Cholinesteraves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 1. Type of Reactions Catalyzed by Cholinesterases . . . . . . . . . . . . . . . . . . . . 133 2 . Behavior of Cholinesterases towards Homologous Choline Esters . . . . . 134 3 . Reaction of Cholinesterases with Organic Phosphates . . . . . . . . . . . . . . . . 134 4 . Enzymatic Attack of Esters with Special Structural Features . . . . . . . . 135 a . Thioesters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 b . Carbamates and Carbonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 c . Halogenoacetates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 V . Mechanism of Hydrolysis by Cholinesterases . . . . . . . . . . . . . . . . . . . . . . . . . . 136 1 . ISnzymatic Ester Hydrolysis vs . Chemical Hydrolysis . . . . . . . . . . . . . . . . 136 2 . The Course of the Enzymatic Hydrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 VI . Structure of the khteratic S i t e . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 1 . pH-Activity Curves of Cholinesterases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 2 . 1nterpret.ation of the First Dissociation Constant. pK, . . . . . . . . . . . . . . . 140 3 . Interpretation of the Second Dissociation Constant of the Rsteratic 141 Site p& . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 . The Possible Role of Seririe in the Esteratic Site . . . . . . . . . . . . . . . . . . . . 144 5 . On the Mechanism of Carbamate Inhihition . . . . . . . . . . . . . . . . . . . . . . . . 146 VII . The Anionic Sites in the Active (:enters of Cholinester.tses . . . . . . . . . . . 147 1. Experimental Evidence for thc Presence of Negatively Charged Groups in the Act.ive Centers of Cholinesterases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 2 . Characterization of the Anionic Sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 130

ACTIVE SURFACE OF CHOLINESTERASES

3. The Number of Anionic Sites in True and Pseudo-Cholinesterase... . 4. Other Views on the Difference between True and Pseudo-Cholinestera se . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Interpretation of the pS-Activity Curves of True and Pseudo-Cholinesterase.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. On the Forces Acting between Quaternary Ammonium Ions and the Anionic Sites of Cholinesterases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VIII. Concluding Remarks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R.eferences. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

131 151 154 156 157

161 162

I. INT~IODUCTION Enzymatic ester hydrolysis is a common and widespread biochemical reaction. Since simple procedures are available to follow the kinetics of hydrolytic reactions, great efforts have been made during the last years to explain this form of catalysis in chemical terms, i.e., in analogy to known non-enzymatic reactions, and to define the components of the active sites. The ultimate aim of this research is the synthesis of an “artificial enzyme” with the same substrate specificity and comparable speeds of reaction as the natural catalyst. Special interest adheres to the group of cholinesterases (ChE), not only in view of their physiological role in conductive tissues, but also because their specific behavior towards substrates and inhibitors and their high efficiency towards cationic substrates permit exact kinetic measurements. In spite of an enormous amount of experimental work, the exact structure of the active surface of cholinesterases is still controversial [see the review of Whittaker ( l ) ] The . following representation will discuss the results already achieved and point out the many problems in this field still awaiting solution.

11. ENZYME SOURCES I n view of the high rate of hydrolysis by cholinesterases, it is usually possible to work with tissue homogenates, whole cells (erythrocytes), or partly purified enzymes. Only in a few cases have methods for the preparation of highly purified or crystalline ChE’s been developed. 1. T r u e Cholinesterase from Erythrocytes The enzyme is present in the stroma of red blood cells and can be isolated, after hemolysis by distilled water, according to the method of Cohen and Warringa ( 2 ) .The procedure gives a 250-400-fold purification. 2. True Cholinesterase from the Electric Organs of Electrophorus electricus or Torpedo marmorata The electric organs of various fish are the richest source of true cholinesterase and yield very pure enzyme preparations by the method of Rothen-

132

F. BERGMANN

berg and Nachmansohn ( 3 ) .From the final solutions, which are stable over prolonged periods, one may obtain by lyophilization solid material which hydrolyzes 20-25 g. of acetylcholine per hr. per mg. of protein. 3. Pseudo Cholinesterase from Plasma

This cholinesterase has been brought to the highest stage of purification by Edsall (4) and co-workers during their work on plasma fractionation. Pseudo-cholinesterase is contained in fraction IV-6-3 and has been obtained in crystalline form. Erythrocyte and plasma cholinesterases are also commercially available.

111. METHODS A considerable number of procedures has been developed to measure ChE activity. Some of them are directly applicable to tissue homogenates or blood, but others require highly purified enzyme preparations. 1 . Manometric Method

The carbon dioxide, evolved from a bicarbonate buffer upon liberation of free acid during ester hydrolysis, is measured by the Warburg manometric method ( 5 ) . 2. Titration Method

The acid liberated during hydrolysis is neutralized continuously with concentrated alkali, using either indicators or-more conveniently-electrometric titration (6, 7). 3. Colorimetric Method

This method is based on the fact that only esters, but not free carboxylic acids, react with hydroxylamine to form hydroxamic acids, which can be determined colorimetrically as complex with ferric 'chloride (8). The method-in contrast to most other procedures-measures the concentration of remaining substrate instead of products of hydrolysis. It also requires purified enzymes because of the interference of colored contaminants in the colorimetric measurements. An entirely different principle has been used by Ravin et al. (9). 0-Carbonaphthoxycholine is split by pseudo-cholinesteruse into choline and 0-naphthyl hydrogen carbonate, which spontaneously decarboxylates to &naphthol. The latter is determined by coupling with a suitable diazonium salt to produce a colored azo dye.

4. Iodometric Titration Since ChE's hydrolyze esters of thiocholine, the sulfhLydry1 group of the latter, liberated in the reaction, can be measured iodometrically (10, 11).

ACTIVE SURFACE OF CHOLINESTERASES

133

5. Spectrophotometric Method

The same type of substrates may also serve for ultraviolet spectrophotometry, since acyl thiocholines show a maximum a t 226 p . In practice, it is more convenient to follow the change in optical density a t about 250 p ( l a ) .The method, which works only with purified enzymes, measures substrate concentrations of 5 X lop6 M . 6 . Indicator Method

Free thiocholine, one of the hydrolysis products of acylthiocholines, lends itself to another analytical procedure, viz., the use of redox indicators [e.g., dichlorophenol indophenol : Meyer and Wilbrandt ( l o ) ]The . method, which has been used so far only for semi-quantitative measurements, lends itself easily to further refinements and may reach a concentration range of 10-6-10-7 M .

7. Biological Methods Since choline esters have a marked effect on many muscular organs, producing either contraction or relaxation, whereas choline (or thiocholine) possesses a several hundredfold weaker activity, biological reactions can be used to measure the concentration of choline esters, remaining after a given period of enzymatic interaction. Although biological methods are less accurate than chemical ones, they enable one to measure much smaller concentrations than any of methods 1 through 6. Thus, using the frog rectus abdominis muscle, one can determine quite accurately acetylcholine concentrations of about 10-7 M ( I S ) . IV. CATALYTIC EFFECTS OF CHOLINESTERASES 1. Type of Reactions Catalyzed by Cholinesterases Cholinesterases hydrolyze a wide variety of substrates, but differ from other esterases in that they split a t a very high rate esters derived from

+

the cationic alcohol choline, (CHJ SNCH2CH20H.This property serves to distinguish ChE’s from other, so-called “unspecific” esterases, which hardly attack such substrates. The catalytic action of ChE’s is, however, not limited to ester hydrolysis. Thus, these enzymes catalyze also the reverse reaction, i.e., esterification of acids with cholirie ( 1 4 ) . They promote transesterification (15) and the condensation of hydroxylamine with acids (14) or esters (15). Anhydrides of carboxylic acids are also substrates of ChE’s (16, 17) and can undergo all the reactions, mentioned with esters, i.e., hydrolysis, esterification, and hydroxamation.

134

F. BEIGMANN

3

2

1

PS FIQ.1. pS-activity curves for the hydrolysis of acetylcholine by cholinesterases. After Augustinsson ( I r a ) . 0 -- 0 true ChE from electric eel. __ pseudo-

+

+

ChE from human plasma. Ordinate: b30 = microliters carbon dioxide, evolved in 30 min. Abscissa: pS = - log substrate concentration.

2. Behavior of Cholinesterases towards Homologous Choline Esters The reaction of ChE’s of different origin towards a homologous series of choline esters permits a, further classification within this group. This is exemplified in Pig. 1 for two representative types, the “true” ChE from the electric organ of Electro~horuselectricus and the “pseudo” ChE from human plasma, both acting on acetylcholine as substrate. The curves show hydrolytic rates as function of pS = -log (substrate concentration) and demonstrate a fundamental difference between the two enzymes: The “true” type exhibits a bell-shaped pS-activity curve, ir tdicative of sutoinhibition a t high substrate concentrations (18). The pseudo enzyme, on the other hand, possesses a S-shaped curve; i.e., the :maximum rate is reached st and beyond an optimal substrate concentration. It should be stressed here that this difference is valid only with regard to choline esters. For other substrates, both types of ChE may show either form of pS-activity curve. With the exception of choline esters, no systematic difference has been observed so far. 3. Reaction o f Cholinesterases with Organic Phosphates

In contrast t o carboxylic esters, the esters of phosphoric acid with aliphatic alcohols are not enzymatically attacked. ChE’s do, however, react

135

ACTIVE SURFACE OF CHOLINESTERASES

with phosphoric anhydrides of the homogeneous (P-0-P) or heterogeneous (P-X) type and likewise with certain phenolic esters (19). The activation energy for the hydrolysis of esters is considerably higher than that of anhydrides. This may explain why ChE’s are unable to split neutral phosphate esters, but do attack esters with certain phenols. Organic sulfates, e.g., methanesulfonyl fluoride, behave like phosphates (20). Whereas most carboxylic derivatives, after enzymatic hydrolysis, dissociate from the active surface, so as to permit attack on new substrate molecules, this is not true for the reactive derivatives of phosphoric acid. Here, after the initial step of hydrolysis (see V, 2), a portion of the molecule adheres t o the enzyme in zt form, which cannot be easily split and thus produces irreversible inhibition.

4. Susceptibility to Enzymatic Attack of Esters with Special Structural Features Small structural changes within the ester group or its immediate environment may have pronounced effects on the availability of a compound as substrate of ChE’s. (a) Thioesters. When the ethereal oxygen of the ester group is replaced by sulfur (thiol esters), the resulting compound shows either equal or superior rates relative to its oxygen analog. On the other hand, the isomeric thiono esters, in which the carbonyl oxygen has been replaced, are not attacked at all (21). Analogous observations have been made in the phosphate series. Thus, the thionophosphate I is not an effective inhibitor of ChE’s, unless it rearranges t o a thiol derivative or becomes converted into the 0x0 derivative I1 ( = paraoxon), a process taking place in vivo (22): =-NO,

(C2HSO)zP-0-

II

S

+ (C~H~O)ZP-O- 0\ - N O z

II

0

I

I1

On the other hand, the corresponding thiolphosphonates I11 and IV are active inhibitors as such (23): ( C 2 Ha0 )? P-S-

II

a - N O ?

0

C2 H t, 0-P-0-

/ II

CzHKS 111

--NO2

0 IV

(b) Carbamates and Carbonates. When the carboxyl carbon bears another negative substituent, such as OCHs or NH2, the resulting esters are no longer attacked by ChE’s. These groups, by virtue of their mesomeric effect, decrease the formal positive charge on carbon, as seen in the resonance

136

F. BERGMANN

forms VI and IX of carbamates and carbonates, and i,hus diminish the polarization of the carbonyl bond. However, in phenyl esters of carbonic acid, the aromatic ring may serve as electron sink and thus “neutralize” the above mesomerio effect; e.g., P-carbonaphthoxycholine is split by serum cholinesterase (9). 0-

0

O-

V

VI

0-

0

0-

I

I/

VII

I +

RO-C-OR

RO-C-OR

RO-G=OII

t

x

Ix:

VIII

Likewise, in analogy to the observations in the phosphate series, anhydrides do react with ChE’s and then may produce irreversible inlhibition (20),e.g., (CzHe)zNC-F

I/

+E

-+

(C2Hs)zNC-W

/I

+ F-

0

0

Unfortunately, the corresponding fluorocarbonates, FCOOR, have not yet been tested. (c) Halogenoacetates. The reverse effect is obtained with esters of halogenoacetates. Here the electron-attracting character of tlhe substituent increases the formal positive charge on the carboriyl carbon. Thus, pseudocholinesterase of human plasma hydrolyzes chloro- and bromo-acetates much faster than acetates (24). For eel esterase, the following order h m been found by Bergmann and Shimoni (25): fluoroacetate

> bromoacetate >

v. MECHANISM OF

chloroacetate :> acetate

HYDROLYSIS BY

CHOLINES’CERASES

1. Enzymatic Ester Hydrolysis vs. Chemical Hydrolysis The chemical modes of bimolecular ester hydrolysis, :is represented by Ingold (26) and his school, are related to either acidic c r basic catalysts. They involve attack a t the carbonyl portion of the ester group, giving as intermediates or transition states derivatives of the ortho acid, which may undergo reversible ( A ) or practically irreversible ( B ) changes :

[ +[

137

ACTIVE SURFACE O F CHOLINESTERASES

RC-OR’ I1

+ [H301+

0

HOH

(A)

R‘OH

OH

OH

OH

+

’,

II

RCc0]

’

+ IHaOl+ 0

OH

I + OH- + RC-OR’

+ R’O-

I

0-

0

/ RC \\

+Ha0

OH

( B ) RC-OR’

g

R~:;R]

RpORl

+ RC

\\

+OH-)

RCOO-

0

I n contrast to these reaction schemes, enzymatic hydrolysis shows optimal rates at pH 7-8, a t least as far as animal esterases are concerned. Increasing concentrations of either Hf or OH- slow the hydrolytic reactions down. Therefore, a different catalytic mechanism, not included in Iiigold’s reaction schemes, must be operative in enzymatic systems. 2 . The Course of the Enzymatic Hydrolysis An ester can be split either a t point A (acyl oxygen fission) or a t point B (alkyl oxygen fission) :

0 A

B

I n both schemes of V,1, the bond A between the carbonyl carbon and the ethereal oxygen is broken; i.e., acyl fission is involved. That the same should apply to the reactions catalyzed by ChE’s had been derived from theoretical considerations by Burgen (2’7) and by Wilson and Bergmann (28). Direct proof was advanced by experiments which were designed in analogy to those which had served in the elucidation of the pathway of chemical hydrolysis. (a) Acyl oxygen fission was demonstrated for ACh hydrolysis by Stein and Koshland (291, using H20l8 in the medium. The isotopic oxygen was absent from choline, but was found in the acetic acid, which showed 50% of the 0l8concentration of the medium. The same conclusion can be derived from the well-known fact that acetylthiocholine (AThCh),

138

F. BERGMANN

+ (CH;OaN CH2CH2-S-C

I1

CH3

0 yields thiocholirie as one of its hydrolysis products (YO). In an analogous way, Wilson (31) showed the splitting df thioacetic acid into hydrogen sulfide and acetic acid under the catalytic influence of eel esterase: CH3COSH

enzyme

CHaCOOH

+HzO

+ H2S

( b ) If DFP is labeled by P32,the radioactive di-isopropylphosphate moiety becomes bound to the enzyme, whereas the fluoride ion is lost in the solution (32).This proves the following reaction mechanism (scheme C) : (RO)*P-F

I

+enzyme

+ F-

(RO),P--enzyme

It

ii

0

and likewise indicates that the positive phosphoryl radical combines with the enzyme in an electrophilie attack. The most reasonable explanation of this reaction consists in substitution of the phosphoryl radical for a proton. The reaction mechanism can thus be specified as follows: (RO),P-F

(D)

/I

+ EH + H+ + F- + (R0)2P--E /I

0

0

For pyrophosphates of the type

an analogous mechanism must be operative, since Aldriclge nnd Dnvihoti (33) have shown the identity of the inhibited enzynir, formcd from w r responding dialkyl fluorophosphonates and tetra-alkyl pyroph0sph:ttcs. The same is true for other types of inhibitory phosphates, such as p-nitrophenyl esters of dialkyl phosphates. The above results lead to the conclusion that hydrolyjis of c*art)oxylic arid phosphoric acid derivatives by ChE’s proceeds in the same fashion. Therefore, we can now subdivide ester fission into two steps: RC-OR’

It

+ EH

-+

RC-E

II

0

+ R’OH

0

RC-E+HgO

II

J

--$

RCOOH+EH

139

ACTIVE SURFACE OF CHOLINESTERASES

Only the second step is prevented in the hydrolysis of phosphates by ChE’s. The actual picture is more complicated, however, since phosphate inhibition itself proceeds in several stages (34) (see VI, 4). Elucidation of the nature of the irreversible enzyme-phosphate linkage evidently would be of the greatest importance for an understanding of the hydrolytic mechanism of ChE’s in general. VI. STRUCTURE OF

THE

ESTERATIC SITE

1. p H Activity Curves of Cholinesterases ChE’s, like all animal esterases known so far, do not contain any specific prosthetic group. Upon hydrolysis, either by acids or by proteolytic enzymes (35),only amino acids or oligopeptides can be isolated. The enzymic activity decreases considerably during the degradation process (36), indicating that a complex structural arrangement is responsible for the catalytic effect. Therefore, the structure of the esteratic site can be derived only indirectly, i.e., by studies on the complete enzyme. I n this respect, the most important information has been obtained from the effect of pH changes on enzymic activity. In Fig. 2 the pH-activity curves are represented for true cholinesterase (from Torpedo marmorata) and pseudo-ChE (from human serum), with ACh as substrate. The two curves are not only similar to each other, but also to the curves, characteristic for other, unspecific esterases (37). For the correct interpretation of such curves, it is important to make sure that only the protein in the

5

6

7

8

PH

9

1

0

FIG.2. The influence of pH on the hydrolytic activity of cholinesterases. After Bergmann et al. (47). 0 - - - - 0 True ChE from Torpedo marmorata; ACh, 3.3 X M . 0 -- 0 Pseudo ChE from human plasma; ACh, M.

140

F. BERGMANN

enzyme-substrate complex changes with pH. This is the case for ACh hydrolysis. Therefore, the two slopes of the bell-shaped curves in Fig. 2 can be interpreted as titratioii curves of components of the esteratic site. In conclusion, one group with tt pK about 5.8-7.0 and another with pK 8.5-9.5 must be present, the former being more common and more characteristic for all esterases than the second.

2. Interpretation of the First Dissociation Constant p K , Only very few among the common amino acids possess a pK within the range 5.8-7.0. Therefore, the imidazole ring of histidine was suspected very early to represent the group responsible for nucleophjlic attack on the substrate (38).The pK of free imidazol is 6.9 (39); that of imidazol, contained in histidine or its peptides, varies between 5.6 and 7.1 (40).Imidazol is well known t o form unstable acyl derivatives, which undergo spontaneous hydrolysis because of the presence of the resonating triad unit -N-C= N- (41). I n addition, imidazol and its derivatives catalyze the hydrolysis of certain esters, especially those derived from phenols (42). Likewise, the behavior of imidazol towards thio esters reflects exactly the specificity of ChE’s (see IV, 4). Thus, thiol esters are split (43),whereas thiono esters are resistant (21). If one accepts the imidazol hypothesis, it becomes posfjible to formulate a more detailed mechanism of enzymatic hydrolysis, as shown in scheme E : -N \CH -NH

/

+ RCOR’ II 0

-

R

I

-N-C-OR’ ‘CH

A-

., //

-NH

XI

R

R

-N

-N-

\CH

/

-NH

+ RCOOH

I

I

C

>CH -N

+ XI1

XIIa

Here it is assumed that the hydrogen, which is eliminated with the alcohol moiety R’OH, stems from the imidazol ring, so that the acylimidazol XI1 appears as intermediate. However, other sources of this hydrogen atom have to be considered (i,e., either other components of the active center or the solvent). If the hydrogen in question originates from

ACTIVE SURFACE O F CHOLINESTERASES

141

a source other than the imidazol ring, an acyl-imidazolinium ion XIIa, instead of XII, would be involved. A choice between these alternatives would be possible if one could identify or exclude with certainty the intermediate XI1 by spectrophotometric analysis (43). In the orthoester-like structure XI in scheme E a negative charge is placed on the carbonyl carbon. In view of the low electronegativity of the sulfur atom, an analogous intermediate can not be formulated with doublebonded sulfur. This explains a t once the resistance of thiono esters and thionophosphates towards imidazol-catalyzed as well as enzymic hydrolysis (see IV, 4). 3. Interpretation of the Second Dissociation Constant of the Esteratic Site

The second pK, 8.5-9.5, derived from the pH-activity curves, is much more difficult t o interpret. This p K is naturally absent in the system imester (21). It is also subject to much greater variation than pK, . idazol This has been demonstrated for a variety of substrates (Fig. 3), but is especially prominent when thiol esters are being studied (Figs. 4 and 5). In the system eel esterase-acetylthiocholine, no decrease of activity is observed on the alkaline side up to pH 11, and for plasma cholinesteraseacetylthiocholine the decrease is very much delayed, when compared with the oxy ester, acetylcholine (see Fig. 2). Similar observations have been made with other esterases and other thiol esters (44). They indicate that the second component Gz of the esteratic site, to which pKb has to be ascribed, may be less essential for certain substrates than for others.

+

5

6

7

PH

8

9

10

&

FIG.3. pH-activity curves of human plasma cholinesterase. After Bergmann, el M al. (97). 0 -- 0 acetylcholine, 10-2 M . 0 - - - 0 n-propyl chloroacetate, X - - - X n-propyl fluoroacetate, 3.6 X M.

142

F. BEIGMANN

6

7

8 PH

9

1 0 1 1

Fro. 4. pH-activity curve of the system eel esterase-acetylthiocholine. After Bergmann, et al. ( 2 1 ) . Substrate concentration: 4 X low3M .

PH

Fro. 5 . pH-activity curve of the system human plasma cholinesterusc-ucctylthiocholine. Substrate concentration: 3 X loe2 $1.

These findings shed light on the character and the funttion of Gz . Sulfur is well known t o form only weak hydrogen bonds. Therefore, it appears most probable that G2 attaches itself to the ethereal oxygen of the ester group by formation of a hydrogen bridge, but does so only to a minor degree in the case of thiol esters. The attachment of G2 has a favorable effect on the rate of hydrolysis, since, when G2 becomes inactivated hy increasing pH, the rate declines. In view of the value of pKb E 9, a suitable representative of G2 would be the phenolic hydroxyl of tyrosine, but other powibilities are not excluded. When hydrogen bonding with G2 is weak, the hydrolytic process becomes largely pH-independent in the alkaline range. This is not only true for thiol est,ers, but applies also to phenol esters (Fig. 6). Here the aromatic ring

143

ACTIVE SURFACE OF CHOLINESTERASES

6

7

8

9

10

PH FIG.6. pH-activity curves for the system eel esterase-phenyl acetates. According to Bergmann, et al. ( 2 1 ) . A --- A p-nitrophenyl acetate, 4 X lop4M . 0 -0 phenyl acetate, M . 0 - - - - 0 p-methoxyphenyl acetate, 10-3 M .

decreases the electron density of the ethereal oxygen and in this way diminishes its affinity for hydrogen bonding. It is of great significance that those esters which are split by imidazol alone, such as thiol esters and phenol esters (4.9, are a t the same time largely independent of the activating influence of Gz , when hydrolyzed b y eel esterase. Esters of aliphatic alcohols, on the other hand, which are not subject t o imidazol catalysis, are dependent on the hydrogen bonding with Gz . The hydrogen bridge decreases the electron density a t the ethereal oxygen. Rupture of the C-OR’ bond in acyl-oxygen fission leaves the oxygen with a negative charge, while the electron deficiency of the carbon can be satisfied by sharing electrons with the carbonyl oxygen. Charge separation always requires a high activation energy, which is diminished through the above hydrogen bonding. This effect is thus analogous to the catalytic influence of H+ in ester hydrolysis . I n summarizing this discussion, one may represent the function of the esteratic site as a combination of two effects: (a)Nucleophilic attack of imidazol nitrogen on the carbonyl carbon of the ester (group GI , characterized by pK,). This reaction can be classified as a case of general base catalysis.

144

F. BERGMANN

(b) Electrophilic attack on the ethereal oxygen by group Gz (characterized by pI
4. The Posszble Role of Serine in the Esteratic Site The irreversible inhibition of cholinesterases by organic phosphorus compounds and demonstration of the fact that a phosphoryl residue of the form (1tO)J'f combines with the enzyme opened a new approach to thc study of the structure of the active surface. If the phosphoryl radical is isotopically labeled, subsequent degradation of the inhibited enzyme molecule would give :L labeled fragment, representing the original component that reacted with the inhibitor. In such studies, the phosphoryl radical was invariably found linked to the p-hydroxyl of serine. If the labeled enzyme was hydrolyzed by acid, free P32-phosphoryl-serine was obtained (45). If SL crude mixture of proteolytic enzymes from pancreas was used for splitting, an oligopeptide resulted, containing a di-:tlkyl-P32-phosphoryl-serine moiety (35). Naturally, the question arises whether the labeled fragment isolated represents the primary enayme-inhibitor complex, formed inside the active surface, or a secondary product, resulting from trarisphosphorylation during hydrolysis. The high yield of phosphoryl-serine, which exceeded in some experiments 70%, led Cohen et al. (35) t o consider this group as the original form of irreversible enzyme-inhibitor combination and to claim serine as an essential component of the active surface. It should be recalled here that the alcoholic hydroxyl of serine does not possess a dissociation constant within the p H range, accessible to enzymic reactions. Therefore, this amino acid cannot influence the pH-activity curve. On the other hand, it is well known that DFP inhibition is initially reversible and becomes only slowly irreversible. This has been demonstrated for true ChE from electric eel by Nachmansohri and associates (46)and for plasma ChE by Mackworth and Webb (47). Similarly, a stepwise reaction with inhibitors, containing the diethyl phosphoryl moiet,y, has been made probable by Hobbiger (34). Therefore, it appears possible that phosphates are first attacked by the imidazol moiety of the esteratic site, in conformity with the catalytic influence of free imidazol on phosphate hydrolysis (48). This step is followed by transfer to serine. The final product is a tridkyl phosphate XV, which is not split by imidazol (scheme F ) .

145

ACTIVE SURFACE OF CHOLINESTERASES

-N

(0

‘CK /

+ (RO),TAnion 0

-Nil

\

II OR

HCHO XIV

0‘CH

/

-“H

\ +//

I

$ H + 4- (R0)Pf; 0

R O ‘ : Anion

,.CH

/ + Anion’

-I-H+

\

+serine (bound to ensyme)

{H+

-N

-N-

-NH

OR

-N-P< --N

-

nI ,OR

I

YH I

-N

CHS- CH- CII l C H f 0I

-NH

/

OR P’ 11 ‘OR

0

I

0

xv

In this representation of the course of the reaction with DFP it is assumed that the hydroxyl group of serine competes with water. Owing to the close spatial arrangement of all components of the active surface, the probability of phosphate transfer to the serine hydroxyl increases enormously. Nevertheless, reaction with water takes place simultaneously and in special cases may become preponderant. Thus, Aldridge and Davison (49) observed that during inhibition by tetraethyl pyrophosphate of ChE, bound to the surface of erythrocytes, a much larger proportion of the anhydride undergoes hydrolysis than is actually bound by the enzyme. The question naturally arises why the final form XV of the phosphorylated enzyme, in which the imidazol ring is free, should be unable to catalyze further hydrolytic reactions. (a) One may consider the possibility that the serine hydroxyl is required for the hydrolytic process itself, which involves an ortho esterlike structure XVI. One would have to assume that this intermediate is unstable and undergoes spontaneous hydrolysis. Such an assumption, however, involves that XVI is even more unstable than acyl-imidazol, since otherwise nothing would be gained by participation of the serine hydroxyl. In addition, in

146

F. BERGMANN

/

-N--C-R

-N

c

-N--

\ CH 0II //

+ HO-CHz(serine)

XI1

R

\

---f

I‘OCHz-

NCHO-

-N+ H

XVI

those esterases which are not subject to phosphate inhibition (e.g., A-esterases), the pathway of the hydrolytic reaction would have to be different. (b) It is possible that the serine hydroxyl does not :participate in the enzymatic hydrolysis, but that after its phosphorylation steric hindrance comes into play. If this is true, the “screening effect” of -phosphatesshould increase with the size of the substituents in the phosphor,yl fragment. Some indications for such an effect may be found in the reactivation studies of phosphorylated ChE (50). At present, no evidence is available to indicate the intimate mechanism of phosphate inhibition. This interesting problem still awaits its solution. 5. On the Mechanism of Carbamate Inhibition

In spite of many experiments, which show complete reversibility of ChE inhibition by carbamates, such as eserine, prostigmine and carbaminocholine, a few observations indicate that-at least in partirreversible processes may be involved. Thus, Cohen and co-workerrs (61) found that ChE’s after inhibition by eserine require dialysis for several hours before activity starts to recover. Likewise, in dilution experiments with true ChE from rat’s brain inhibited with eserine, a large part of the lost activity appears to be irrecoverable. Myers and Kemp (20) observed that the splitting of dialkylcarbamoyl fluoride by ChE’s is very similar to the hydrolysis of DFP ; i.e., inhibition progresses slowly, and recovery of the inhibited enzyme follows an exponential curve. From these results the impression is gained that carbamate inhibition passes through an initial, reversible stage into the final irreverdble combination (e.g., serine-carbamate). The findings of many authors that eserine or prostigmine behave as reversible inhibitors may be explained by selection of unfavorable reaction conditions. Since the irreversible reaction progresses so slowly and spontaneous dissociation of the enzyme-inhibitor complex is rather rapid [depending on the character of the alkyl groups: (52)],short-range experiments cannot give the required information. Like phosphate inhibition, the reaction of the carbamates with ChE’s awaits further elucidation before a final decision can be made.

ACTIVE SURFtlCE OF CHOLINESTERASES

VII. THE ANIONICSITESIN

THE

147

ACTIVECENTERSOF CHOLINESTERASES

1. Experimental Evidence for the Presence of Negatively Charged Groups in the Active Centers of Cholinesterases The outstanding affinity of ChE’s to cationic substrates such as choline esters suggested very early to Zeller and Bissegger (55) that the active center includes-in addition to the ester-binding group-a negative site. Later, experimental evidence for this assumption was adduced by the following methods: (a) Quaternary ammonium ions are inhibitory to ChE activity, but have no effect on other esterases (54). (b) For tertiary (and other) amines the inhibitory effect depends mainly on the percentage of ammonium ions present and is therefore related to the pK value of the inhibitor (28, 55), as shown in Fig. 7 for eserine, a tertiary base of pK about 8.1.

7

6

8 9 I O l i PH FIG.7. Comparison of the pH-dependence of cholinesterase inhibition by eserine with the titration curve of the base. (Note, that inhibition does not disappear when the percentage of cationic form in solution approaches zero. This may be interpreted in two ways: ( a ) either the free base as such possesses a certain inhibitory power; or ( b ) the dissociation constant, measured in aqueous solution, is not identical with - Relative th a t characteristic of the environment of the active surface). inhibitory effect of eserine against human plasma ChE. After Bergmann and Wurzel 0 Relative inhibitory effect of eserine against eel esterase. After (66). 0 Wilson and Bergman (88). - - per cent cationic form of eserine. ~

+

+

2. Characterization of the Anionic Sites

The most important information about the chemical nature of the anionic sites can be obtained by measuring the pK of the responsible

148

F. BERGMANN

groups. Great confusion has arisen, however, from the experiments designed for this purpose. It is seen from Fig. 2 that enzymatic activity can be measured only above pH 5. Therefore, an anionic site with a pK below 5 would not undergo any structural changes within the experimentally accessible pH range. Comparison of Figs. 2 and 3 shows that cationic and uncharged esters both give approximately the same decrease of hydrolytic rates between pH 7 and 5. Thus, at first glance, one is led to the conclusion that the anionic sites have no influence on the shape of the pH-activity curves; i.e., the decay of activity in the acid range is due entirely to inactivation of the nucleophilic group GI by H+, as explained in VI, 2. A fruitful approach to this problem was based on the study of the inhibitory effect of quaternary ammonium ions, such as tetre,ethylammonium, as a function of pH (66). This type of inhibitors can combine only with the anionic sites, but does not possess any specific binding forces for the esteratic site, Therefore, any change in the inhibitory activity, when the pH is varied, must be ascribed to structural changes of the anionic sites, abolishing their charges. To obtain unequivocal results in such studies, the experimental procedure used is of utmost importance. If we incubate first enzyme and inhibitor at a given pH, e.g., in the acid range, two competing equilibria are established:

EH

+ H+

EH

=

EH;,

+ I = EHI,

characterized by Characterized by Ki

(EH) (I) (Em

= -~

(2)

We may assume that the quaternary ammonium ion can combine only with the active form EH of the enzyme, but does not attach itself to the inactive form EH$ , characterized by K , , the equilibrium constant of the negative site. Although a priori there exists no proof for such an assumption (evidence for its correctness will be adduced later), a combination of inhibitor and inactive enzyme cannot be measured by presently available methods and thus does not influence the kinetic results. When a substrate like ACh is added, it will compete with both the quaternary ion and the proton for the anionic site. In addition, simultaneous changes in the esteratic site, produced by protons (see VI, l), will modify the hydrolytic rate. Therefore, a very complicated picture results, which does not allow unequivocal conclusions to be drawn. If, however, hydrolysis is measured under noncompetitive conditions, only the equilibria (1) and (2) have to be considered. This can be done by extrapolating the rates to zero time, i.e., by measuring the amount of active enzyme still available after equilibration with inhibitor or protons. The total enzyme appears in the following forms, when only an acid

149

ACTIVE SURFACE OF CHOLINESTERASES

buffer is added during incubation:

If an inhibitor I is also present, we obtain

Etotal = E H 4-EH;

+ EHI

=

(

: + It.)

E H 1 4- -

-

(4)

The velocity of hydrolysis is given by v = k*EHS

Since K ,

=

(EH)(S)/(EHS), we may write also

k v = k(EHS) = - (EH)(S) Ki

(5)

neglecting for the time being the formation of an ESz complex by true ChE. Inserting into (5) the values of EH from Equations (3) and (4), respectively, we can express vo , the rate for the noninhibited enzyme, and v i , the rate in the presence of inhibitor, both as functions of Etotal.The ratio vi:vo will then be given by

- _-

vi

VO

1

+

1 f H+/Kn H+/Kn I/Ki

+

(6)

Per cent inhibition is defined as follows:

( :)

100 1 - - = 100

I

I

+ Ki + H+Ki/Kn %

(7)

Since in a given experimental series I is kept constant and only the hydrogen ion concentration varies, it follows that with decreasing p H the per cent inhibition of a quaternary ammonium ion diminishes. On the other hand, the effect of such inhibitors should remain constant in the alkaline range, provided the anionic site does not undergo any further changes. These predictions are confirmed by experiment (Fig. 8). It appears that indeed the anionic site looses its binding capacity for quaternary ammonium ions between pH 7 and 5; i.e., it becomes neutralized by H+. Therefore, the curves of Fig. 8 are titration curves and lead to the following values of pK, : pseudo-ChE from human plasma, 6.5; true ChE from Torpedo marmorata, 6.3 (37). Only very few groups in proteins bear a negative charge at neutral pH, most conspicuously the 0-carboxyl of aspartic acid (pK = 3 . 0 4 . 5 ) and the y-carboxyl of glutamic acid (pK = 4.24.5). But the dissociation constants of these groups are far beyond the range of pKn in ChE's. On the

150

F. BERGMANN c

100 0 c

.-0 c

.-C

a

5

6

7

8

9

PH FIG.8. The influence of pH on the inhibitory activity of tetramethylammonium ion against cholinesterases. After Bergmann et at. (37). 0 __ 0 True ChE from Torpedo marmorata. 0 __ 0 Human plasma ChE.

other hand, the latter values come remarkably close to the pK, values of the nucleophilic component G1 in the esteratic site (see VI, 2). In fact, they are practically indistinguishable. This is the reason that in the acid range the pH-activity curves of cationic substrates like ACh do not differ much from the curves of neutral esters, such as, e.g., n-propyl chloroacetate (see Figs. 2 and 3). These results then lead to the conclusion that K , does not measure a t all the neutralization of the anionic site by H+, but represents the inactivation of GI by H+. From the discussion in VI, 2, it appears that GI itself cannot represent the anionic site, since inactivation of G1 is due to conversion of a free imidazol group into the imidazolinium ion. The only logical way out of this situation is represented by the assumption that the curves in Fig. 8 actually measure neutralization of the anionic site by the imidazolinium ion. This interaction is preferred to neutralization by a positive ion in the medium, because of the spatial proximity of the anionic site and the imidaaolinium group. If we accept this representation, it appears also plausible to assume that the inactive form EH: of the enzyme does not attach a t all quaternary ammonium compounds (see above). The measurements described in this paragraph do not give direct information about the pK and the chemical nature of the anionic site, but reveal the interesting property of electrostatic interaction of two components of the active center.

151

ACTIVE SURFACE OF CHOLINESTERASES

3. The Number of Anionic Sites in True and Pseudo Cholinesterases

The two types of ChE's are distinguished in two respects: (a) Only the true type is subject to auto-inhibition by excess cationic substrate (see IV, 2). ( b ) Each type shows a different order for the relative rates of hydrolysis of homologous choline esters (Table I). TABLE I Relative Rates of Hydrolysis of Homologous Choline Esters b y Cholinesterases Choline ester

Pseudo ChE (human plasma)

True ChE (human erythrocytes)

Acetyl n-propionyl n-butyryl n-valeryl

100 145 216 145

100 78 2.3 3.3

The similarity of the pH-activity curves and of the behavior of cationic inhibitors towards both types of ChE's indicates that the active surfaces are closely related. What then is the reason for the different substrate specificity of the two enzymes? An answer to this problem was found by studying a homologous series of inhibitors of the general formula + (CH3hN

- (CHthCH,

(57). The inhibitory effect of these ions increases with increasing length of the fourth substituent, i.e., I s O the , concentration required to reduce the hydrolytic rate to one-half under standard conditions, decreases. A plot of pIsO( = -log Iso) vs. n, the number of methylene groups in the side chain of the fourth substituent, gives a straight line for either ChE (Fig. 9). This observation can be interpreted as follows: From Equation (6), we obtain for constant pH

If

vi =

>$iho

(at 50% inhibition), then

IsO= C K i . Therefore, for two consecutive members of the above homologous series of inhibitors we obtain

"Is0 - "Ki

"-'I~,,

(9)

152

F. BERGMANN

I

3 n

5

7

FIG.9. The inhibitory activity of quaternary ammonium ions of the general formula (CH&N-(CH&,-CH) on cholinesterases as function of n, the number of methylene groups in the fourth substituent. After Bergmann and Segal (67). (Note that n = 0 represents tetramethylammonium ion.) 0 - - - - 0 True ChE from electric eel. Pseudo-ChE from human plasma. ~

From Fig. 9 it can be concluded that log K i , like log Is0, is a linear function of n; i.e., the difference between successive log K i values is constant. We may therefore consider the free energy of complex formation between enzyme and inhibitor as composed of two contributions: (a) The free energy of a tetramethyl ammonium ion (when TZ = 0); (b) the free energy of attaching n methylene groups to the active surl'ace, which is n times the free energy of binding of a single methylene. The magnitude of O I b 0 and of OKi can be derived from the curves in Fig. 9 by extrapolating t o n = 0. (The extrapolated values differ from the figures determined experimentally.) The OK; value is related to the free energy change, accompanying the binding of tetramethylammonium to the anionic site, by Equation (10). If we assume that in this reaction the electrostatic force plays the most important role, we can express AF also by Coulomb's law and thus find

A F , , , ~=, ~ ~ RT In OKi = Nelez __ Dr

(10)

153

ACTIVE SURFACE OF CHOLINESTERASES

where N = Avogadro’s number = 6.03 X loz3 el = number of negative charges at the anionic site of the enzyme e2 = charge of the inhibitor = 1 D = dielectric constant a t the active center r = distance of closest approach of the oppositely charged particles. (Note that el and e2 are expressed as multiples of 4.8 X 10-lo e.s.u.) The effective radius r1 of the cation is obtained by adding the bond length C-N = 1.5 A. to the van der Waals’ radius of methyl ( = about 2 A.) (58). This gives a value of r1 = 3.5 A. The radius r2 of the negative site of the enzyme is more difficult to calculate. If we assume that the responsible group is carboxyl, we may use the radius of negatively charged oxygen = 1.4 A. However, resonance distributes the charge between both oxygens of the carboxylate group. We may consider the latter approximately as a sphere over the surface of which the electrons are smeared out. In such a case, according to Coulomb’s law, the electrostatic effect is equal to that of the same charge placed in the center of the sphere. Therefore, it appears more appropriate to add the distance C-0 = 1.4 A. to the van der Waals’ radius of negatively charged oxygen = 1.4 A. The e$ective radius r2 of the anionic site thus becomes 2.8 A. and the distance of nearest approach r = 6.3 A. The value of D, i.e., the dielectric constant at the active surface, is unknown, but may be approximated by the use of Schwarzenbach’s equation (59). Values between 23 and 31 have been introduced by various authors (28, SO), but it can easily be shown that the exact value neither of r nor of D is critical for determination of the number of charges in the anionic site, e l , which has to be selected from theseries 1,2,3, . . . . InTable I1 we have calculated r for various values of D,assuming el consecutively as 1, 2 , . . . . It can be safely concluded that for pseudocholinesterase el = 1 and for the true enzyme el = 2 (see also VII, 6). In the latter case, it has been shown that a single quaternary ion combines with the enzyme, a t least in TABLE I1 Interionic Distances and Number of Anionic Sites i n Cholineslerases Enzyme Pseudo-cholinesterase

23 27 31

6.9 5.9 5.3

13.8 11.8 10.6

True cholinesterase

23 27 31

3.6 3.1 2.75

7.2 6.1 5.5

Note: For explanation of symbols see text.

154

F. BERGMANN

True ChE Pseudo ChE FIG.10. The arrangement of the cationic inhibitor tetramethylammonium at the anionic site of cholinesterases. (For true ChE, the arrangement of lowest potential energy is shown, with the centers of the three ions involved lying on a straight line.)

the concentration range used for determination of 1 5 0 (5'7'). (At higher inhibitor concentrations EI, combinations may be formed.) The results in Table I1 thus indicate that a single positive nitrogen combines with both anionic sites of true ChE, as shown in Fig. 10, which illustrates the fact that the cation is larger than the anionic groups, the latter being held in sufficient distance from each other. It is also apparent that with tetramethylammonium unspecific dispersion forces are unlikely to contribute appreciably t o the total binding force ( a ) in view of the size relationships, and (b) because the two carboxyl groups must belong to side-chains of the protein structure; i.e., they must protrude from the surface and thus are free to move within certain limits, as evidenced by the formation of a n ES2 complex between true ChE and ACh. Therefore, the cationic groups are fixed a t a certain distance from the protein surface proper. In the case of pseudo-ChE likewise, the force of biiidiing for tetramethylammonium is overwhelmingly of electrostatic nature. This emerges from experiments with di-quaternary ammonium salts of the @;enera1 structure :

+

+

(CHzhN - ( C H P ) ~ N(CH3h

I n this series the 1 6 0 values for the system pseudo-ChE-ACh are practically identical with those of the monoquaternary compounds with an equal number of carbon atoms in the side chain. Therefore, it appears that the second positive charge does not contribute materially to the over-all binding force, because of the lack of a second anionic site, serving as anchor. This proves the absence of important dispersion forces acting on tetramethylammonium. The situation is, however, different with higher homologs, since free rotation in longer alkyl chains makes contact with remote parts of the protein surface possible (see VII, 6).

4. Other views o n the Difference between True and Pseudo Cholinesterase I n the foregoing paragraph, we have shown that the essential difference between the two types of ChE's lies in the number of anionic sites in the active center. This view, however, has not been generally accepted.

ACTIVE SURFACE OF CHOLINESTERASES

155

(a) Adams and Whittaker (24) observed a great difference in the hydrolytic rates by erythrocyte ChE of ACh and its uncharged carbon homolog, y ,y-dimethylbutyl acetate. On the other hand, with plasma ChE the ratio of affinities was 1.25. This led to the conclusion that with the pseudoenzyme it makes no difference whether a quaternary nitrogen or carbon combines with the active surface. The latter, therefore, accommodates quaternary structures sterically, not electrostatically. For choline inhibition Adams and Whittaker found the ratio K i (true):Ki(pseudo) = 1:29. Interpreting this as expression of the electrostatic force, acting in the case of the true enzyme, they derived the following Equation (10a) [which is quite similar to (lo)]:

AE

=

free energy of electrostatic attraction

The experiment showed that when r = 5.3 A., el = 1. It is, however, evident that this derivation gives the diference in electrostatic units between the two enzymes and leads to the reported result only if it is presumed that the pseudo-enzyme is devoid of an anionic site. The observations of Adams and Whittaker are thus not contradictory to the conclusions of VII, 3. (b) Bernhard ( G I ) , starting again from the assumption that in pseudoChE only dispersion forces attach quaternary nitrogens to the surface, tried to interpret the binding energy of 2000 cal. of the system pseudoChE-tetramethylammonium as follows: (1) three methyl groups = 3 X 500 cal. = 1500 cal., and (2) a contribution of the exposed nitrogen. As can be judged from Fig. 10, however, because of the size relationships and the spherical structure of the cation, van der Waals’ forces cannot make an important contribution to the binding force between tetramethylammonium and carboxylate ion. It has been noted before, that-in the absence of a negative charge-binding of quaternary ions to the active surface becomes negligible (1711, 3). I n addition, the mere fact that quaternary ammonium ions lose their inhibitory power for pseudo-ChE with decreasing pH is proof of the presence of a n anionic site. (c) A somewhat different method was used by Wilson and Bergmann (28), who compared the rate of hydrolysis by eel esterase of dimethylaminoethyl acetate at pH 6, where the ester exists to over 95 % in the ammonium form, and a t pH 10, where the free base alone is present in solution. Introducing the ratio of the affinity constants of charged and uncharged form = 20 into Equation (lOa), the authors arrived at the result e l = 1 for T = 6.5 A. The kinetic measurements with the tertiary substrate were not sufficiently

156

F. BERGMANN

TABLE I11 Activation Energies and Heats of Reaction for Enzyme-Catalyzed Ester Hydrolysis System investigated Enzyme Substrate Human serum ChE Horse aerum ChE Eel esterase Pancreas lipase Horse liver esterase Wheat germ lipase

ACh ACh ACh 1-caprylylglycerol Tributyrin Ethyl butyrate Monobutyrine

Activation Heat of energy re:tction (call Ical) References 7900 5000

(62)

(63)

14,900 14,000

(64l

(66) (66)

9200 7600 72001

* 1000

7800

(67) (68) (69)

accurate, however, and the ratio of affinity constants was estimated rather than measured. Reinvestigation of this case indicates a ratio of 5 rather than for 20 for the constants in question. Use of the lower value leads to r = 3.5 A. for el = 1 and to r = 7 A. for el = 2, in conformity with the conclusions of the foregoing paragraph. Therefore, it appears probable that the experiments of Wilson and Bergmann do not give unequivocal information about the number of negative charges in the anionic site of eel esterase. (d) An entirely different approach consists in the cornparison of activation energies and heats of reaction for various esteirases; e.g., Eley and Stone (62) observed that the activation energy for hydrolysis of ACh by pseudo-ChE is of the same magnitude as for the hydrolysis of alkyl butyrate by pancreas lipase. This led them to the conclusion that in the ChE under question the main point of adsorption lies in the ester group, i.e., that electrostatic forces are unimportant. The data of Eley and Stone, together with the findings of other authors are summarized in Table 111. It is evident that activation energies can not serve to discriminate between cholinesterases and other, “unspecific” esterases. Since these measurements are not very accurate, a quantitative interpretation of the values in Table 111 is not possible, but certainly they do not support the claim that only true cholinesterase is distinguished by the possession of :in anionic site. 5 . Interpretation of the pS-Activity Curves of True and Pseudo-Cholinesterase The results of VII, 3 enable us to give a rational explanation for the different pS-activity curves of true and pseudo ChE, presented in Fig. 1. I n contrast to the conception of Zeller and Bissegger ( B ) ,it now appears probable that the combination of esteratic and anionic site in the active center is by itself not sufficient to produce auto-inhibition with ACh, al-

157

ACTIVE SURFACE OF CHOLINESTERASES

though bell-shaped pS-activity curves may appear with other substrates of pseudocholinesterase, e.g., benzoylcholine (YO), where additional forces between the *-electrons of the aromatic ring and the active surface of the enzyme come into play, or with ethyl fluoroacetate (26). If, however, two anionic sites are present in the neighborhood of the esteratic site, an inactive ESB complex is formed. Here, a cationic group is bound individually to each anionic site. The nonhydrolyzability of such an arrangement is then a consequence of crowding, i.e., of competition between the substrate molecules and the bound water. Similarly, the small rate of hydrolysis of butyrylcholine by true ChE may be the result of competition between the longer acyl chain and the bound water. 6. On the Forces Acting Between Quaternary Ammonium Ions and the Anionic Sites of Cholinesterases

In the homologous series of quaternary ammonium compounds of the

+

general formula R4N the inhibitory power increases with increasing length of R, but only up to a certain limit (55, Yf) (see Table IV). On the basis of Fig. 10 one may be inclined to assume that the larger the distance between opposite charges, the smaller the electrostatic attraction. However, the structure of the higher members of this series of inhibitors is not necessarily spherical. The longer side chains possess a certain freedom of movement and thus may come into contact with more remote parts of the protein surface, where they are attracted by dispersion forces. These parts of the enzyme molecule do not exert any measurable influence on smaller ions, such as tetramethylammonium. As a result, the Ih0and K ; values in

+

the series R4N decrease with increasing chain length. When log K ; is plotted as a function of n, the number of carbon atoms in the substituents, one would expect a linear relationship, similar to the observations represented in Fig. 9. This is nearly correct for the first three members of the series (n = 4 to n = 12) in the system eel esterase-ACh (see Fig. 11). The value of log K ; for tetra-n-butylammonium is, however, too high, indicating a TABLE IV

+

The Inhibitory Activity of Quaternary Ammonium Ions of the Structure RJV against True Cholinesterase of Electric Eel

R Methyl Ethyl n-propyl n-butyl

Substrate: ACh, 4 X 1.5 X 10-2M 3 x 10-3 M 1.5 x 10-4 M 3 x 10-4 M

Iao M Diacetin: 4.3 X 10-1 M

x x 2x 5 1

7x

10-3 10-3 10-4 10-4

M M M M

158

F. BERGMANN

-I

-2

.-

Y

0 0

-3

-J

-4

,c

4

8

n

12

16

+

FIG.11. Inhibitory constants of tetra-alkylammonium salts, RaN, as function of n, the number of carbon atoms in the four substituents together (according to Table IV and V). 0 -- 0 Eel esterase. 0 -- 0 Human plasma ChE.

decline of inhibitory activity. This is probably due to the limited space available a t the active surface of true ChE, since ft similar phenomenon is not observed with the pseudo enzyme (Table V). It is also noteworthy that these inhibitors are much stronger when the uncharged ester diacetin serves as substrate, than with ACh, in spite of the fact that diacetiri was used in these experiments in concentrations exceeding those of ACh about one hundred times. In the case of the uncharged substrate, no competition takes place with the cationic inhibitor, which very effectively blocks the approach to the esteratic site. Different relationships are observed with plasma ChE. Here the ions

+

R4N inhibit the hydrolysis of ACh always more eflectively than that of un-

159

ACTIVE SURFACE O F CHOLINESTERASES

TABLE V

+

The Inhibitory Activity of Quaternary A m m o n i u m Ions of the Structure RIN against Pseudo-Cholinesterase of H u m a n Plasma 1.60

R

Substrate: ACh.

Methyl Ethyl n-propyl n-butyl

x 5x 3x

6 X

5

n-propyl fluoroacetate : 1.5 X M

M

M 10-2 M 10-3 M 10-3 M

= 1 x 10-1

(I22

M)

3 X 10P M

charged substrates, e.g., n-propyl fluoroacetate (see Table V) . Clearly, with this enzyme no spatial restrictions prevail and no crowding is produced a t the active surface. When the anionic site is occupied by a quaternary ion, neutral substrates are only little hindered in their approach to the esteratic site. I n accordance, tetra-n-butylammonium ion inhibits pseudo-ChE more effectively than the corresponding n-propyl derivative, in contrast to the true enzyme. Extrapolation of the straight lines in Fig. 11 yields “theoretical” values for log OKi (n = 0 ) , i.e., for the ammonium ion. Because all members of

+

the series R4N are nonhydrated, the extrapolated value too refers to the water-free structure NHa and therefore differs considerably from the experimental value (for eel esterase Iso is approximately 2 M , as compared TABLE V I Free Energies and Interionic Distances for the Association of A m m o n i u m I o n with the Anionic Sites of Cholinesterases Calculations are based on Equation (10) Nele2 AF = = -2.3 RT log Ki Dr where N = Avogadro’s number = 6.03 X loz3,D = 30, T = 310” abs. e2 = number of charges a t the inhibitor = 4.8 X 10-10 e.s.u. e.s.u. el = number of charges in the active center = multiple of 4.8 X ~

True ChE

Pseudo-ChE

-2.36 3210 cal.

-1.10 1570 cal.

0.302

0.141

~

Extrapolated value of log K i AFmolar

3

r

je, = 1 ‘\el = 2 Calculated distance of closest approach for carboxylate and nonhydrated ammonium ion

Y . 1 A. 14.2 A.

3.3 A. 6.6 A . T

=

5.0 A.

160

F. BERGMANN

with the “theoretical” value of 0.1 M!). Using Equation (lo), we may calculate again the number of negative charges in the active surfaces. For an interpretation of the results, given in Table VI, we may calculate the distance of nearest approach of carboxylate and nonhydrated ammonium ion as follows: rcoo- = 2.8 A. (see VII, 3); rNH: = 2.2 A., since the bond length of N-H = 1 A. and the van der Waals’ radius of H + = 1.2 A. (68). Although agreement with the experimental data is not very good, there can be no doubt that the number of anionic sites is 1 for pseudo and 2 for true cholinesterase. Similar considerations in the series of methylated ammonium ions show that the 1 6 0 values decrease for the first three members (n = 1 to n = 3) (Table VII) (72, 73). Therefore, a plot of log K , against 91, the number of methyl groups in the amine, is linear for the first three members only (Fig. 12). It can be concluded that for trimethylammonium the arrangement of oppositely charged groups, as shown schematically in Fig. 10, is nearly optimal, and ittle can be gained by adding the fourth substituent. Three methyl groups produce as good a screening effect around the positive nitrogen as four, but two methyl groups are considerably less effective. This is a straightforward result of the geometry of these ions. The dimethylammonium ion permits interaction with the polar water molmules, mainly by hydrogen bonding, and thus undergoes considerable hydration. In the trimethylated derivative, steric hindrance is already sufficient to prevent such interaction. The higher the degree of methylation, the less the amount of bound water (water of hydration) and the smaller the effective radius of the cation. As a consequence, the latter now can approach the anionic sites more closely. This effect is completely analogous to the behavior of univalent alkali ions towards cation exchangers. Here again, the larger ion binds less water of hydration and therefore is attached more strongly to the fixed f

negative charges of the polymer (74).In contrast, in the series R4N hydration is absent already in the lowest member (where R = CHX). Therefore, larger substituents increase the affinity to the enzyme surface by increasing TABLE VII Inhibitory Activity of Methylated Ammonium Ions Against Cholinesterases

Ammonium derivative Methylammonium Dimethylammonium Trimethylammonium Tetramethvlammonium

Pseudocholinesterase, substrate: ACh, 1 0 F M 1.5 M 0.22 M 0.02 M 0.065 M

Eel esterase ACh, 4 X M 7.0 X 1.2 x 1.5 X 1.8 X

10-l M 10-1 M 10-2M 10-8 M

ACTIVE SURFACE OF CHOLINESTERASES

161

0

-I

Log Ki

-2

-3 I

2

n

3

4

FIG.12. Inhibitory constants of methylated ammonium salts as function of n, the number of methyl groups substituting the hydrogen atoms of NH: (according to Table VII). 0 __ 0 Eel esterase. __ Human plasma ChE.

van der WaaIs’ attraction forces only. On the basis of these considerations it becomes understandable that extrapolation of the straight lines in Fig. 12 gives theoretical values for the log K ivalue of ammonium, which are entirely different from the results, shown in Table VI. Since the degree of hydration of the various methylated ammonium ions is not known exactly, no calculations can be based on these extrapolations regarding the number of anionic sites in ChE’s.

VIII. CONCLUDING REMARKS The extensive studies on substrate and inhibitor specificity, on kinetics of hydrolysis, and on the influence of pH variations on the reactions catalyzed by cholinesterases have given very instructive information on the structure of the active surface and the mechanism of enzymatic hydrolysis. The conclusions reached in the various chapters of the present dis-

162

F. BERGMANN

FIG.13. Schematic illustration of the ES complex formed by true cholinesterase with acetylcholine.

cussion may be summarized in a final picture (Fig. 13), which illustrates the combination of the cationic substrate acetylcholine with the active surface of true ChE. The picture demonstrates the double anionic site, the imidazol ring, and the phenolic hydroxyl as involved in the binding of the substrate by electrostatic, covalent, and hydrogen bonds. The hydroxyl group of serine is shown in close proximity, but in a nonbonding position. In spite of the enormous effort devoted to these studies the picture remains entirely hypothetical (perhaps with the exception of the serine fragment!), since the high molecular weight of these enzymes and the 1:tck of a specific prosthetic group make isolation of the components of the active surface extremely difficult and permit only an indirect approach to this fascinating problem.

REFERENCES 1. Whittaker, V . P., Physiol. Revs. 31, 312 (1951). 2. Cohen, J. A , , and Warringa, M. G. P. J., Biochim. et Biophys. Acta 10. 195 (1953). 5. Rothenberg, M. A . , and Nachmansohn, D., J. Biol. Chem. 168, 223 (1947). 4. Edsall, J . T., Advances in Protein Chem. 3, 383 (1947). 6. Ammon, R., Arch. ges. Physiol., PJuger's 233, 486 (1933). 6. Michel, H. O., J . Lab. Clin. Med. 34, 1564 (1949). 7 . Tammelin, I,. E., Scand. J. C l i n . & Lab. Invest. 6, 267 (1953). 8. Hestrin, S., J. Riol. ('hem. 180, 249 (1949). 9. Itavin, H. A , , Tsou, K. C., and Seligman, A. M., J . Biol. C h e w . 191, 843 (1951). 10. Meyer, A . , and Wilbrandt, W., Helv. Physiol. el Pharmacol. Acta 12, 206 (1954). 11. Augustinsson, K. B., Scand. J. Clin. & Lab. Invest. 7, 284 (1955).

ACTIVE SURFACE OF CHOLINESTERASES

163

12. Tabachnik, I. I . A., Biochim. et Biophys. Acta 21, 580 (1956). 13. Wursel, M., private communication (1957).

14. Hestrin, S., Biochim. et Biophys. Acta 4, 310 (1950).

I. B., Bergmann, F., and Nachmansohn, D., J . Biol. Chcm. 186, 781 (1950). 16. Wilson, I. B., J . Biol. Chem. 197, 215 (1952). 17. Bergmann, F., Wursel, M., and Shimoni, A., Biochem. J . 66, 888 (1953). Ira. Augustinsson, K. B., Arch. Biochem. Biophys. 23, 111 (1949). 18. Haldane, J. B. S., “Enzymes.” Longmans, Green, London, (1930). 1.9. Verbeke, R., Arch. intern. pharmacodynamie 79, 1 (1949). 20. Myers, D. K., and Kemp, A., Jr., Nature 173, 33 (1954). 21. Bergmann, F., Rimon, S., and Segal, R., Biochem. J . 68,493 (1958). 22. Gage, J. C., Biochem. J . 64, 426 (1953). 23. Diggle W. M., and Gage, J. C., Biochem. J . 49, 491 (1951). 2.4. Adams, D. H., and Whittaker, V. P., Biochim. et Biophys. Acta 4, 543 (1950). 26. Bergmann, F., and Shimoni, A,, Biochem. J . 66, 50 (1953). 26. Ingold, C. K., “Structure and Mechanism in Organic Chemistry.” Cornell Univ. Press, Ithaca, New York, 1953. 27. Burgen, A. S. U., Brit. J . Pharmacol. 4, 219 (1949). 28. Wilson, I . B., and Bergmann, F., J . Biol. Chem. 186, 479 (1950). 29. Stein, S. S., and Koshland, D. E., Jr., Arch. Biochem. Biophys. 46, 467 (1953). 30. Koelle, G. B., and Friedenwald, J . S., Proc. SOC.Esptl. Biol. Med. 70, 617 (1949). 31. Wilson, I. B., Biochim. et Biophys. Acta 7, 520 (1951). 32. Michel, H. O., and Krop, S., Federation Proc. 8, 320 (1949). 33. Aldridge, W. N., and Davison, A. N., Biochem. J . 66, 763 (1953). 34. Hobbiger, F., Brit. J . Pharmacol. 10, 356 (1955). 35. Cohen, J. A . , Oosterbaan, R. A., Warringa, M. G. P. J., and Jansz, H . S., Discussions Faraday Soc. 20, 114 (1955). 36. Bergmann, F., and Segal, R., Biochim. et Biophys. Acta 16, 513 (1955). 37. Bergmann, F., Segal, R., Shimoni, A., and Wurzel, M., Biochem. J . 63, 684 (1956). 38. Wilson, F. B., and Bergmann, F., J . Biol. Chem. 186, 683 (1950). 39. Kirby, A. H. M., and Neuberger, A., Biochem. J . 32, 1146 (1938). 40. Cohn, E . J., and Edsall, J. T., “Proteins, Amino Acids and Peptides.” Reinhold, New York, 1943. 41. Stadtman, E. R., and White, F. H., Jr., J . A m . Chem. SOC.76, 2022 (1953). 42. Bruice, T. C., and Schmir, G. L., Arch. Biochem. Biophys. 63, 484 (1956). 43. Bender, M. L., and Turnquest, B. W., J . A m . Chem. Sac. 79, 1656 (1957). 44. Bergmann, F., and Segal, R., unpublished results (1956). 46. Schaffer, N. K., May, S. C., Jr., and Summerson, W. H., J . Biol. Chem. 206, 201 (1954). 46. Nachmansohn, D., Rothenberg, M. A., and Feld, E. A., J . Biol. Chem. 174, 247 (1948). 47. Mackworth, J. F., and Webb, E. C., Biochem. J . 42, 91 (1948). 48. Wagner-Jauregg, T., and Hackley, B. E., Jr., J . A m . Chem. Sac. 76, 2125 (1953). 49. Aldridge, W. N., and Davison, A. N., Biochem. J . 61, 62 (1952). 60. Wilson, I . B., Ginsberg, S., and Meislich, E. K., J . Am. Chem. Soc. 77, 4286 (1955). 51. Cohen, J. A., Kalsbeek, F., and Warringa, M. G. P. J., Biochim. et Biophys. Acta 2, 549 (1948). 62. Myers, D. K., Biochem. J . 62, 556 (1956). 63. Zeller, E. A., and Bissegger, A., Relv. Chim. Acta 26, 1619 (1943). 64. Bergmann, F., and Shimoni, A., Biochim. ct Biophys: Acta 8, 520 (1952). 16. Wilson,

164

F. BERGMANN

66. Bergmann, F., and Wurzel, M., Biochim. el Biophys. Acta 13, 251 (1954). 66. Bergmann, F., and Shimoni, A., Biochim. el Biophys. Acta 9, 47’3 (1952). 67. Bergmann, F., and Segal, R., Biochem. J . 68, 692 (1954). 68. Pauling, L., “The Nature of the Chemical Bond and the Structure of Molecules and Crystals.” Cornell Univ. Press, Ithaca, New York, 1948. 69. Schwarzenbach, G., 2. physik. Chem. (Leipzig) A176, 133 (1936).

60. Pressman, D.,Grossberg, A. I,., Pence, L. H., and Pauling, I[,., J . Am. Chem. SOC.68, 250 (1946). 61. Bernhard, S. A.,J . Am. Chem. Sac. 77, 1966 (1955). 62. Eley, D.D., and Stone, G. S., Biochem. J . 49P. XXX (1951). 63. Glick, D.,Proc. Xoc. Expll. Biol. Med. 40, 140 (1939). 64. Hase, E., J. Biochem. (Tokyo) 39, 257 (1952). 66. Wilson, I. B., and Cabib, E., J . A m . Chem. Xoc. 78, 202 (1956). 66. SchGnheyder, F., and Volqvarte, K., Biochim. et Biophys. A d a 9, 306 (1952). 67. Sizer, I. W., and Josephson, E. S., Food Research 7, 201 (1942). 68. Schwert, G. W., and Glaid, A. J., J . Biol. Chem. 199, 613 (1952). 69. Singer, T. P., and Hofstee, B. H. J., Arch. Biochem. Biophys. 18, 245 (1948). 70. Kalow, W., Genest, K., and Staron, N., Can. J . Biochem. an.d Physiol. 34, 637 (1956). 7f. Bergmann, F., and Shimoni, A . , Biochim. et Biophys. Acta 10, 49 (1953). 72. Wilson, I. B., J . Biol. Chem. 197, 215 (1952). 75. Wilson, I. B., J . Biol. Chem. 208, 123 (1954). 74. Kressman, I. R. E., and Kitchener, J. A., J . Chem. SOC.p. 1208 (1949).

Commercial Ablation of Paraflins and Aromatics EDWIN K. JONES Universal Oil Products Co., Des Plaines, Illinois

Page

I. Introduction.. ..........................................................165 11. Sources of Feed Stock.. ................................................. 166 111. Basic Factors in Alkylation.. ........................................... 170 170 1. Types of Catalyst. .................................................. 2. Feed Stock Quality. ................................................. 171 ......................... 172 3. Process Variables. .................... a. Isobutane-Olefin Ratio. ............ ......................... 172 b. Reaction Time.. . . ........................... 173 c. Space Velocity. .................... ......................... 173 d. Temperature. . . . . . . . ..................................... 173 e. Acid Strength. . . . . . . . . . . . f . Mechanical Mixing. g . Acid Settling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 IV. Alkylation Reactions. . . . . . . . . . . 1. Butylene Aviation Alkylate . 2. Propylene-Butylene-Amylene 3. Motor Alkylate. . . . . . . . . . . . . 4. Products of Alkylation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 5. Quality of Alkylate Products.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 6. Petrochemical Reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a. Ethylbenzene.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b. Detergent Alkylates. . . . . . . . . . . . . . . . . . . . c. Cumene.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . d. Diisopropylbenaene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 e. Non-Ionic Detergents and Secondary Bu V. Alkylation Processes. . . . . . .......................................... 188 1. Jet-Type Sulfuric Acid Process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 2. Kellogg Sulfuric Acid Cascade Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 3. UOP Hydrofluoric Acid-Type Alkylation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 192 4. Stratford Effluent Flash Unit.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Chamber-Type Units.. . . . . . . . . . . . . . . .......................... 192 VI. Materials of Construction.. . . . . . . . . . . . . .......................... 193 1. Corrosion in Sulfuric Acid Units.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 . . . . . 194 2. Corrosion in Hydrofluoric Acid Units. . . . . . . . . . . . . . . . VII. Future Outlook for Alkylation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 References.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195

I. INTRODUCTION The commercial alkylation of isobutane by butylenes derived from cracked gases was originally used for the production of aviation gasoline. 165

166

EDWIN K. JONES

The first units utilized sulfuric acid as a catalyst and the number of these units in operation expanded greatly during World War I][.Units based on hydrofluoric acid catalyst were later installed by many refiners for producing aviation gasoline because this type of unit did not require refrigeration and the acid could be regenerated instead of having to be shipped back to acid-reclaiming plants. Alkylation of benzene for the production of ethylbenzene, the raw material for making styrene and subsequently synthetic rubber, was also greatly expanded during the war because of the shortage of natural rubber. The catalyst in most of the original ethylbenzene units was aluminum chloride, but other catalysts are now preferred by many refiners. Alkylation for the production of ethylbenzene was the first large-scale alkylation process used for the production of petrochemicals. Since that time, others, such as cumene, dodecylbenzene, alkylated phenols, diisopropylbenzene, and secondary butylbenzene, have been added to the list, and others have been developed and should soon be in commercial production. Feed stock for the first sulfuric acid alkylation units consisted mainly of butylenes and isobutane obtained originally from thermal cracking and later from catalytic cracking processes. Isobutane was derived from refinery sources and from natural gasoline processing. 1somt:rization of normal butane to make isobutane was also quite prevalent. Llater the olefinic part of the feed stock was expanded to include propylene and amylenes in some cases. When ethylene was required in large quantities for the production of ethylbenzene, propane and butanes were cracked, and later naphtha and gas oils were cracked. This was especially practiced in European countries where the cracking of propane has not been economic. Alkylatioii for the production of motor gasoline and petrachemicals is expected to show considerable growth for years to come, but alkylation for the production of aviation gasoline should remain relatively stable as one of the more important alkylation reactions. Important aviation and motor alkylate gasoline processes are the jettype sulfuric acid process, the Kellogg sulfuric acid autoref rigeration process, the UOP hydrofluoric acid process, and the Stratford eflluent refrigeration process. Petrochemical alkylation includes various processes using as catalysts solid phosphoric acid, aluminum chloride, hydrofluoric acid, boron trifluoride, and phenol and ether complexes of boron trifluoride (1).

11. $OURCES

OF

FEEDSTOCK

The original source of feed stock for the production of aviation gasoline was the butylene-isobutane portion of products from thermal cracking. Later, the thermal cracking units were replaced by catalytic cracking units, and most of the feed stocks are now derived from this source. To a

167

ALKYLATION OF PARAFFINS AND AROMATICS

limited extent, propylene and amylenes from catalytic cracking were used as feed stock in some aviation alkylation units. As various types of catalytic cracking units give different olefin quantities and qualities, the alkylate quantity and quality can vary. Table I shows the production of olefins from typical fluid catalytic cracking, Airlift Thermofor catalytic cracking (TCC), and delayed coking units. These figures are based on the liquid recovered by a typical gas recovery unit. Both of the catalytic cracking units provide relatively low percentages of butylene-1, which indicates a good alkylation feed stock, whereas the coking unit produces a poor feed stock because of its high butylene-1 content. Also, the isobutane content of the TCC material is higher and the olefin content lower than that from the fluid-cracking unit. As can be seen from Table I, only the Airlift catalytic cracking unit has sufficient isobutane to react with all of the butylenes. The isobutane requirement for alkylation is about 1.15 vol. for every volume of butylene, as indicated in Table 11. Besides this quantity required for the reaction, there is also an additional quantity of isobutane which is lost from the alkylation fractionation section. The latter can be from 2 to 25% of the normal butane leaving the unit, depending upon the separation efficiency in the deisobutanizer. Those stocks that are deficient in isobutane require either some removal of olefins or the addition of an outside isobutane stream to give an isobutane-butylene balance. Olefin removal can be accomplished either by TABLE I Typical Alkylation Feeds Airlift Thermofor catalytic cracking, liq. vol. %

Fluid catalytic cracking, liq. vol. %

0.1 21.1 16.4 8.1 4.3 10.5 30.6 8.4

0.1 14.0 7.4 10.4 4.4 6.8 16.6 5.0 15.6 17.3 2.4

0.5

Delayed coking, liq. vol. %

11.3 18.9 8.2 11.4 48.2 1.2 0.8

__ 100.0 1.33

100.0 0.77

100.0 0.30

+

96 IS0 0.9 I 94 91 IS0 0.35 IS0 f 0.9 IS

154

i3o

0.3

-

2.9 7.0 159.3 8.7 .6

100.0 116.0

-

A Butylene

Hydrofluoric acid

TABLE I1 mmercial Yields and Product Qualities When Charging Various Feedstocks to Some of the Diflerent Types of A Units for the Production of Aviation and Motor Fuels ( 2 , 3)

+

+ .3

93

IS0

+ 0.9 93 IS0 + 0.6

96

92 IS0 89

126 157

-

-

7.5

-

IS0 .3 IS0 0.9 90 93 so 0 3 IS0 0.6

+

162.8

0.4 -

0.3

126 157

93

+ +

1.o

14.2 1.2

+

-

96

126 156

156.8 11.2

116 140

0.5 2.0

-

100.0 118.0

100.0 116.0

123 141

96 IS0 4-0.9

93

2.9 7.0 159.3 8.7 .6

0.3

15.0

150.3 16.7 1.o

0.3

130

1 11U no

154

144

+

+

122

Amplene

100.0 133.0

122 146

1.0 -

16.8 143.4 19.6 2.4

155.7 17.3 4.9

6.2

-

-

-

96

1 1

Butylene

100.0 116.0

1.4 7.0

4- .3

96 IS0 -k 0.9

92

+

IS0 .2 93 91 94 90 0.16 160 f 0.6 IS0 0.35 IS0 f 0.9 JSO

IS0 .3 90

IS0

+

rerun alk. oms

so t 0.u

00

IS0

0.4 -

1.4 7.0 156,8 11.2 1.o

-

-

1.0 -

1.6 7 .O 154.0 14.0 1.2

100.0 114.0 100.0 115.0

0.5 -

Propylene

mption 0.5 bs./gal. total alk bl. total alk. alities aikyiate 126 . a t 4.6 cc. TEL 157 . a t 4.6 CC. TEL lkylate 96 tane Clear tane a t 3 cc. TEL IS0 0.9 93 tane Clear 0.6 tane a t 3 cc. TEI IS0

+

-

16.8 143.4 19.6 2.4

126

But jrlene

-

7.0 157.0 11.o 1.2

93

100.0 114.0

1.6 7 .O 154.0 14.0 1.2

-

100.0 116.0

116 140

-

100.0 132.0

2.0

-

100.0 115.0

Effl. flash

Butylene

Amylene Butylene

Autorefrigeration

4.9

100.0 115.0

e ol. yo

-+

17.3

yo

+

6.2

-

155.7

Time-tank jet Butylene

-

132.0

Propylene

~~

Sulfuric acid

-

100.0

Effl. flash

r Q,

Hydrofluoric acid

+ .2 so + 0.02

Sulfuric acid

Time-tank Autorefrigeration jet Arnglene Butylene Propylene Butylene

Feeds, vol. 70 100.0 Olefin 115.0 Isobutane Products, Vol. yo Propane n-butane 7.0 Pentane 157.0 Depent. rerun alk. 11.o Alk. bottoms 1.2 Tar Acid consumption 0.5 H2S0,, lbs./gal. total alk HF, lb./bbl. total alk. Product qualities Aviation alkyiate 126 F-3 P.N. a t 4.6 cc. TEL 157 F-4 P.N. a t 4.6 CC. TEL Motor Alkylate F-1 Octane Clear 96 0.9 F-1 Octane at 3 oc. TEI IS0 93 F-2 Octane Clear 0.6 F-2 Octane at 3 cc. TEI IS0

123 141

0.3

162.8 14.2 1.2

-

7.5

100.0 133.0

Propylene

TABLE I1 Typical Commercial Yields and Product Qualities When Charging Various Feedstocks lo Some of the Daflerent Types of Alkylation Units for the Production of Aviation and Motor Fuels (2, 3)

+

M

w 4

0

1: B

m

169

ALKYLATION OF PARAFFINS AND AROMATICS

fractionation or by polymerization; additional isobutane can come from natural gasoline, straight-run gasoline, catalytic reforming, or isomerization of normal butane. Although for a period of time after World War I1 it was not economic to isomerize normal butane, the situation is changing because of the high cost of transporting isobutane and the low cost of normal butane feed stocks. Catalytic dehydrogenation of butane has furnished butylenes as feed stocks for alkylation units in the past, but high installation and operating costs have usually limited its application to making butylenes for producing synthetic rubber. However, the operating costs have been reduced on thermal cracking of butane and on the newer fluid dehydrogenation units. This is especially true for the dehydrogenation of isobutane, and increased use of this type of dehydrogenation for the production of isobutylene can be expected. Steam cracking of various petroleum fractions is gaining widespread use for the production of olefins. These olefins are produced essentially for use as feed stock for numerous petrochemical processes, but the by-product butylenes and propylenes are sometimes used as feed stock for aviation and motor alkylation units. Ethylene is the most important of the olefins produced from this type of cracking, and propylene is second in importance. These two olefins are normally charged to either alkylation or polymerization units for the production of petrochemicals or petrochemical intermediates. Polyethylene and propylene dimers, trimers, tetramers, and pentamers are some of the more important polymers produced, while ethybenzene, dodecylbenzene, cumene, diisopropylbenzene, and alkylated TABLE I11 Typical Olefinic Feed Compositions Obtained from Steam Cracking of Propane and Straight-Run Naphtha ~~~~~

~

Compositions, mol. %

Propane cracking

H2 CHI CzHz C2H4 CZH6 C3Hs C3Ha CiHe C4Hs C4H10 (90-94 mol. wt. ) Cg

12.7 32.4 0.1 25.7 7.9 8.7 9.7 0.9 1.9

+

Straight-run naphtha cracking 15.5 28.7 0.6 32.4 3.4 11.7 0.4 3.2 3.9 0.2

_--

-_

100.0

100.0

170

EDWIN K . JONES

phenols are some of the more important alkylates produced from these olefins. Table I11 shows typical compositions of olefins produced by the steam cracking of two petroleum fractions. Higher yields of ethylene have been obtained by improved operating techniques.

111. BASICFACTORS IN ALKYLATION There are some basic factors in alkylation which should be considered in addition to the catalyst, feed stock, and normal operating variables. Although thermal alkylation can be accomplished without the presence of a catalyst, essentially all commercial processes today employ a catalyst. The alkylation reactions considered here normally take place in the liquid state. Sufficient pressure is therefore required to insure that the hydrocarbons in the reactor remain in the liquid state. This is only partially true for the autorefrigeration process, where cooling of the reaction zone takes place as a result of vaporizing some of the light hydrocarbons in the reactor. The ethylbenzene reaction is also an exception, since it normally takes place in the partial vapor state, especially when the feed contains considerable ethane. The alkylation reaction is exothermic, and this heat of reaction must be removed from the reaction zone. This heat can be removed by cooling coils, by vaporization of hydrocarbons, or in the effluent stream. Numerous side reactions take place in the reaction zone, some of which are beneficial, but many of which are harmful (4). Some catalysts act to promote olefin isomerization to some extent as well as alkylation. In aviation and motor alkylate production, where more highly branched products are desirable, this isomerization reaction is of considerabl!e benefit. This is especially true when the isomerization of butylene-1 to butylene-2 precedes the alkylation reaction. Harmful side reactions include hydrogen transfer and polymerization. The production of propane from propylene and of normal butane from butylene occurs by hydrogen transfer. Polymerization is very harmful because it not only produces the tar which spends the catalyst, but also reduces the yield of valuable products. Alkylation catalysts may have the ability of breaking down certain olefins and alkylating isobutane with the resulting olefins. A useful example of this ability is the charging of diisobutylene to an alkylat,ion unit, wherein the diisobutylene is broken down and reacts with two molecules of isobutane t o form two molecules of trimethylpentane. 1. Types of Catalyst

The first catalyst to be used widely in commercial alkylation for the production of aviation alkylate was sulfuric acid. The catalyst in the reaction zone is normally 88 to 92 wt. % HtS04 with the makeup acid usually having a strength of 98 75 or higher.

ALKYLATION OF PARAFFINS AND AROMATICS

171

When aviation gasoline was needed in very large quantities during World War 11, many of the alkylation units built used hydrofluoric acid as a catalyst. This catalyst has the advantage of not requiring refrigeration for cooling the reactor and not requiring the hauling by rail of large quantities of both spent and fresh acid during wartime. Hydrofluoric acid has an advantage in that it can be fractionated a t the alkylation unit from the polymers which tend to spend the acid, while spent sulfuric acid must be shipped back to a sulfur-reclaiming plant and made into new acid. The regeneration of spent sulfuric acid, by chilling with propane and then separating the acid and tar, has been tried in small commercial units but has so far proved commercially unsuccessful. Because water in the feed to an HF alkylation unit is absorbed by the hydrofluoric acid, this water must also be removed in the acid regeneration column along with the polymers or tars. With the advent of the alkyl aryl sulfonate type of detergent, aluminum chloride, as well as hydrofluoric acid, was used to a great extent for the production of dodecylbenzene. Cumene is produced by an alkylation reaction, and most cumene has been produced over solid phosphoric acid catalyst, although some quantities have been produced by liquid phosphoric acid and aluminum chloride. Cumene is used as a raw material for the production of phenol and acetone by oxidation. Although boron trifluoride has been known for a long time as a catalyst for alkylation reactions, it did not become commercially important until larger quantities of aromatic alkylates were required by industry as raw materials for synthetic fibers, rubber, and plastics. As the petrochemical industry finds new uses for these aromatic alkylates, the use of BF3 catalyst is expected to expand greatly. Cumene, diispropylbenzene,and ethylbenzene are among the important alkylates which can be produced by this catalyst. Other catalysts have been produced for the alkylation reaction and their commercial use is expected to expand. The phenol and ether complex forms of boron trifluoride are examples of such catalysts (1). 2. Feed-Stock Quality

The highest quality component in the alkylation product when making aviation or motor alkylate is a mixture of trimethylpentanes. Although some of the other components have high octane, most are of inferior octane number. It is therefore desirable to make as much of the trimethylpentanes as possible. On a pure component basis, isobutylene and butylene-2 will alkylate isobutane quite readily to form trimethylpentanes. However, butylene-1 has a tendency to form dimethylhexanes. Most of these dimethylhexanes are of lower octane number, and their production is to be avoided

172

EDWIN K. JONES

if possible. Alkylation catalysts usually have some cata1,ytic effect toward olefin isomerization, and various percentages of the butylene-1 can be isomerized to butylene-2, depending upon the operating conditions used. Other methods can also be used, such as passage over phosphoric acid or silica-alumina catalysts to isomerize butylene-1 or hydrofluorination. The product of the hydrofluorination reaction of butylene-1 is secondary butylfluoride, which reacts with isobutane to form trimethylpentane. Such methods are usually considered secondary to obtaining feed stocks which have a relatively low percentage of butylene-I. The better feed stocks which meet this desirable feature are usually produced from catalytic cracking units, as shown in Table I. In the production of aviation and motor alkylates, both propylene and amylenes are inferior feed stocks when compared with the butylenes. These feeds produce lighter and heavier alkylates, respectively, than the butylenes, both alkylates having a lower octane than the trimethylpentane produced from butylenes. Also, acid consumption when sulfuric acid catalyst is used is two or more times as great with propylene or amylenes than with butylenes. Hydrofluoric acid catalyst, on the other hand, is not consumed at a higher rate on propylene and amylene feeds but does make a higher percentage of tar. Impurities such as diolefins and mercaptans in the feed to the reactor affect adversely the quality of the alkylate produced. This is considered to be more critical in sulfuric acid than in HF catalyst units. Also, certain sulfuric unit designs seem to tolerate more normal butane than others. Whereas the isobutane content of the recycle to a hydrofluoric acid unit can operate at purities of around 75% without affecting product qualities appreciably, the isobutane content of the recycle stream to most sulfuric acid units should be 90 % or better. 3. Process Variables a. Isobutane-Olefin Ratio. This is one of the most important of the variables which control both octane number and the quantity of rerun bottoms. In the normal range of external ratios of between 6:l and l O : l , the F-1 clear octane can vary two numbers and the rerun bottoms can change from 3 to 10% of the total alkylate. Recycle of the hydrocarbon within the reaction zone, which increases the internal isohutane to olefin ratio, partially compensates for a decreased external ratio, hut this procedure is not nearly as efficient as operation at increased external ratios. On sulfuric acid units, high rerun bottoms usually are also indicative of high polymer produetion and, therefore, high acid consumption. Another way of indicating isobutane-to-olefin ratio is the per cent isobutane in the reactor effluent. This method of indicating the ratio takes

ALKYLATION OF PARAFFINS AND AROMATICS

173

into consideration any diluents such as normal butane and propane, which tend to reduce octane and increase polymer formation, especially in some types of sulfuric acid units. The minimum isobutane content in the effluent from a sulfuric acid unit reactor is usually considered to be 50 %. At percentages below this figure, the polymer production is quite high and may be excessive. Although the per cent isobutane in the effluent from HF alkylation units is not critical, high percentage figures will increase octane numbers and lower heavy alkylate production. b. Reaction Time. The required time for the reaction is very difficult to determine and is stated in many different ways. Hydrofluoric acid causes the alkylation reaction to take place very rapidly at 100°F. and therefore almost any reactor will provide more than the minimum required time because of the volume required for the cooling tube bundle. Although the alkylation reaction is slower below 100" F., the minimum time is still under 10 min. Sulfuric acid alkylation is always operated at low temperatures, where the alkylation reaction is relatively slow. The operating time on these units is usually expressed indirectly as space velocity. c. Space Velocity. Reference to space velocity is made mainly on sulfuric acid units. It is defined as gallons of olefin per hour in the feed per gallon of acid in the reactor. Motor and aviation sulfuric acid alkylate units normally operate a t space velocities between 0.1 and 0.4. Highest product qualities and yields are normally obtained at the lowest space velocities, with the best results being obtained when more olefin injection points and more mixing stages are added a t the lower space velocities. d. Temperature. The operating temperature ranges of sulfuric and hydrofluoric acid units vary greatly, the sulfuric acid range usually being considered 30-50" F. and the hydrofluoric range being 50-115" F. Temperatures above 50" F. cannot economically be used with sulfuric acid, because of the increased polymer formation and acid consumption. Octane numbers increase with decreasing temperatures; the greatest relative octane increase occurs when the temperatures are reduced from the highest levels. The reason for the increased octanes at lower temperatures is the increased production of trimethylpentanes and the reduction in production of dimethylhexanes. Lower temperatures favor the isomerization of butylene-1 to butylene-2 as well as decreased side reactions. e. Acid Strength. The strength of sulfuric acid is very important from the standpoint of both octane number and tar production. Although many units discard acid when it has reached 88% HzS04, some use it down to only 90-92% because of the resulting higher octane number. The effluent refrigeration process as applied to H&04 has the advantage of being able to add and discard acid in batches rather than continuously, which allows

174

EDWIN K. JONES

operation part of the time with system acid strengths above the 92 % H804 level. The system acid in commercial hydrofluoric acid units shows very little octane or yield change within the range 86-91 % HF normally used. When the strength reaches as low as 83 % HF, however, the polymer production increases markedly and regeneration requirements are increased greatly. f. Mechanical Mixing. Although mechanical mixing arid internal circulation of isobutane within the reaction zone are not necessary with hydrofluoric acid, they are very important with sulfuric acid. The older sulfuric acid units were designed with a definite horsepower per gallon of alkylate ratio which was very high. On later units it was found that large horsepower was not required, and by increasing the efficiency of the mixers, the horsepower could be reduced considerably; a 1500-bbl. per day unit currently may use as little as 100 hp. I n order to minimize acid consumption and increase octanes, increased mixing has brought the horsepower requirements in some recent designs to as high as 200 hp. for 1500 bbl. per day of alkylate production. However, units with efficient mixing are operating with high yields and high octane numbers a t 100 hp. per 1500 bbl. per day of alkylate production. g. Acid Settling. Acid settling is not a reaction variable, but is very important in the design of sulfuric acid alkylation units. Where sufficient settling time is not available, additives have been used, but have not proved to be very satisfactory. Care is also taken to prevent overmixing, which makes a very stable emulsion that prevents the acid-hydrocarbon separation in a reasonable settling time. Normally 60 nain. settling time is adequate but some units use as much as 90 min. Hydrofluoric acid settles very rapidly, and, therefore, small settlers are used only with those reactors which have mechanical circulation. Those units without circulation in the reactor require little or no settling.

IV. ALKYLATION REACTIONS Commercial alkylation operations in petroleum refineries were origiiially limited almost entirely to the alkylation of isobutane with propylene, butylenes, and amylenes for the production of aviation gasoline. It has expanded greatly since that time to include the production of motor gasoline and numerous petrochemicals. Although aviation gasoline can be made by the alkylakion of isobutane with propylene, butylenes, and amylenes, the butylenfrs are by far the predominant feed. In a few cases a mixture of di- and triisobutylene polymers, which is a byproduct of butadiene manufacture, i:s used as part of the olefinic charge. Either sulfuric acid or hydrofluoric acid can be used as a catalyst when aviation gasoline is produced by the alkylation reaction.

ALKYLATION OF PARAFFINS AND AROMATICS

175

Diisopropyl has been used to raise octanes of both aviation and motor gasolines but has not been made in large quantities. It is produced by the alkylation of isobutane with ethylene over aluminum chloride catalyst. Alkylate for high-octane motor gasoline is produced in ever-increasing quantities to satisfy the requirements of the high-compression automobile engines. It is very similar to the aviation alkylate except that it is made with a higher vapor pressure and the full boiling range product is normally used. Because the Engler end point of the product is 400” F. or less, no rerun column is required on the motor alkylate unit. Ethylbenzene was the first petrochemical to be produced by petroleum refiners in large quantities. It is made by the alkylation of benzene by ethylene. Aluminum chloride promoted with ethyl chloride was originally the predominant catalyst used for the reaction, but solid phosphoric acid has been used more recently and is becoming more popular. Some of the newer fluoride-type catalysts are expected to become quite popular. Detergent alkylates have been produced in large quantities by petroleum refiners, and larger quantities and new types are expected to be produced in the future for a rapidly expanding market. The first alkylate of this type to be produced by the refiners was dodecylbenzene or polypropylenebenzene. This was made by alkylating benzene with a propylene polymer consisting of essentially C12’swith some C16’sand small quantities of other polymers in the range of Clo through C14. In later detergents of this type, the quantity of C16’s in the feed has been decreased. The catalysts normally used for this alkylation reaction are hydrofluoric acid and aluminum chloride. Sulfuric acid has been used by several refiners. Other detergent alkylates which are gaining acceptance are benzene alkylated by pentamer and phenol alkylated by polypropylene, of which trimer is the preferred polymer. The pentadecylbenzene alkylate is sulfonated, while the nonylphenol alkylate is further reacted with ethylene oxide, the latter product going into the production of liquid non-ionic detergents. Cumene or isopropylbenzene, diisopropylbenzene, and secondary butylbenzene, although produced in smaller quantities than some of the other petrochemical alkylates, are very important petroleum refining products. Cumene is further reacted by oxidation to form cumene hydroperoxide, which is converted to phenol and acetone; it is produced by alkylating benzene with propylene catalyzed by either solid or liquid phosphoric acid. Secondary butylbenzene is made by alkylating benzene with normal butylene using the same catalysts. Diisopropylbenzene is made by reacting cumene with propylene over solid phosphoric acid or aluminum chloride catalyst,

170

EDWIN K . JONES

1. Butylene Aviation Alkylate

Aviation alkylate is still the most important and the la,rgest of the commercial petroleum refinery alkylation processes involving the olefins from cracked gases. Where the alkylate formerly was a minor blending component for upgrading the octane of aviation base stocks, it is now used as a major component in high-octane 115-145 grade aviation gasolines. For the highest octanes, the alkylate is usually merely blended with isopentane for vapor pressure control, but sometimes a minor amount of high aromatic content blending stock is added. These blending stocks are normally a product of catalytic reforming. The preferred aromatic in such blending stocks is toluene which may also be used directly. Commercial toluene usually has an F-4 blending value of 250 with 4.6 ml. of tetraethyl lead. However, the F-3 blending value of toluene is only 100 at 4.6 ml. TEL so it reduces the over-all F-3 performance number. Also, the heating value specification of grade 115-145 aviation gasoline limits the amount of toluene which can be tolerated to 10%. In grade 100-130 aviation gasoline the heating specification allows 18% toluene. Typical sulfuric and hydrofluoric aviation alkylates are shown in Table IV. TABLE IV Typical Aviation Alkylateo from Refrigerated Sulfuric and Water-Cooled Hydro$uoric Acid Units Operating conditions Reactor temp., OF. Product "API Reid vapor pressure ASTM Engler d k t . IBP, O F . 10%

20% 30% 40% 50% 60%

70%

80% 90%

E.P. F-3 P.N. at 4.6 ml. TEL F-4 P.N. at 4.6 ml. TEL

Butylene feed H*SOI unit

__

Butylene feed HF unit

40

80

70.4 4.4

71.6 4.3

134 176 190 204 214 220 226 232 238 250 330 126.0

102 176 203 212 215 217 219 221 225 231 336 129.3 152.0

156.01

ALKYLATION OF PARAFFINS AND AROMATICS

177

2. Propylene-Butylene-Amylene Aviation Alkylate

Aviation alkylate made from mixtures of propylene, butylenes, and amylenes is sometimes made in areas where isobutane is available a t relatively low cost. Amylenes are in some cases unprofitable feed stocks, since in themselves they are valuable as high-octane motor gasoline. When used, therefore, the economics usually depend upon making essentially all of the profit on the isobutane and increased use of natural gasoline for vapor pressure. Also, when a lower-value natural isopentane is available to replace the amylenes taken from the motor gasoline, the profitability of using amylenes for feed is increased. Another deterrent to the charging of amylenes to sulfuric acid alkylation units is the relatively high acid consumption. Whereas acid consumption for butylenes ranges from 0.4 to 0.6 lb of sulfuric acid per gallon of alkylate, amylenes require approximately 1.0 lb of sulfuric acid per gallon of alkylate. As already indicated, however, this is not true for hydrofluoric acid catalyst, for which the acid consumptions are approximately the same for all three feed stocks. The octane is also lower for amylene alkylate because of the relatively high percentage of nonanes produced. Refiners normally have the choice of using propylene as a feed stock for alkylation to aviation or motor alkylate, for polymerization to motor polymer gasoline, or for the production of petrochemicals. In those refineries where a polymerization unit already exists, the motor polymer or petrochemical route is usually chosen unless isobutane is relatively inexpensive. This is because alkylation units which have been designed to charge considerable quantities of propylene cost more to build and because the acid consumption on sulfuric acid units is so much higher, being approximately 2.0 lb per gallon of alkylate. Again the acid consumption is no higher than for butylenes when using the hydrofluoric acid catalyst. The quantities of heptanes made when alkylating propylene result in a lower-octane alkylate. Yields are very difficult to determine commercially when charging propylenes and amylenes, partially because they are always charged in conjunction with butylenes and partially because the composition of the feed stock to commercial units varies almost continually. The greatest difficulty is the determination of the quantity of propane and pentanes produced from the equivalent olefin by the hydrogen transfer reaction. This is also true for the production of normal butane when charging butylenes. Another difficult determination is the isobutane consumed in the reaction because of variations in the quantities of isobutane in the outside isobutane feed stream, the butane-butylene feed stream, and the normal butaneisobutane stream leaving the unit. Also, the quantities of isobutane con-

178

EDWIN K. JONES

sumed vary with changing operating conditions. In Table I1 the generally accepted yields on commercial units from the three feed stocks are shown along with the quality of the products. The yields from either sulfuric acid or hydrofluoric acid are essentially the same when operating a t the same conditions and on the same feed. 3. Motw Alkylate

Because of the great demand for motor gasoline of 100 octane or higher, alkylation has assumed increasing importance for producing this fuel. It is clean-burning and has very good tetraethyl lead susceptibility. Also, it can be leaded to as high as 110 F-1 octane and, therefore, can be added to lower-octane materials to increase the octane of the blend. Motor gasoline, being a lower-value product than amviationgasoline, requires every economy in its production for the refiner to realize a reasonable profit. High-cost feed stocks must be eliminated, outside isobutane can be used only where the delivered cost to the refinery is very low, and acid costs must be a minimum. Maintenance costs and downtime, which are very important, vary according to the design of the unit and the quality of feed stocks. A typical motor alkylate produced in a hydrofluoric acid-type of alkylation unit when operating without refrigeration at a reactair temperature of 83" F. is shown in Table V. Figure 1 shows the F-1 clear octane numbers of motor alkylates produced on an HF unit at various reactor temperatures when charging typical butane-butylenes from a catalytic cracking unit. TABLE V Typical Motor Alkylate from H F Alkylation Unit Operating without Refrigeration and without Rerun Column

Butylene feed Reid vapor pressure, p.8.i. Gravity, "API at 60°F. ASTM Engler distillation, OF. IBP 10% M% 90%

E.P. Octanes F-1 clear F-1 3 ml. T E L F-2 clear F-2 3 ml. T E L

+ +

7.8 '71.8

87

171 219 240 358 95.8 106.6 94.0 1Cl8.1

ALKYLATION O F PARAFFINS AND AROMATICS

179

Degrees fohrcnheit

FIG.1. F-1 clear octanes produced at various reactor temperatures in a n HF alkylation unit when charging butane-butylenes.

4. Products of Alkylation The primary reactions in the alkylation of isobutane produce octanes from butylenes, heptanes from propylene, and nonanes from amylenes. Also, when dimer and trimer polymers of isobutylene are used with isobutane, the polymer is broken down during the reaction, and the resulting products are branched chain octanes similar to those produced when isobutylene is charged. Sulfuric acid consumption is somewhat higher for the diisobutylenes, however, and there are more side reactions than for isobutylene. Besides the primary reactions, many side reactions take place to form other products. Some of the side reactions form beneficial products having higher octanes than those produced from the primary reaction, but most of the side reactions form inferior products and, therefore, should be avoided as much as possible. Most side reactions can be reduced by increasing the isobutane-to-olefin ratio, by providing more efficient contact between the acid and hydrocarbon, and by reducing the reaction temperature to the lowest practical point. There are some other side reactions, such as hydrogen transfer, which convert the olefins charged to saturated products, thus forming propane,

180

EDWIN K. JONES

butanes, and isopentane, depending upon the olefin charged. The isobutane produced by this reaction subsequently reacts with an olefin to form a n alkylate. The formation of these products increases a t higher isobutaneto-olefin ratios, but the quantities are not sufficiently large to attempt to prevent their formation. Because of their small production, it is almost impossible to determine their yield on a commercial unit, and many yield figures indicate that none are produced. Isomerization plays an important part in producing trimethylpentane from butylene-1 and is described in detail in the following section. 5. Quality of Alkylate Products

I n producing alkylate, the aim is to produce the highest percentage of products having the highest possible octane numbers. Advances are continuously being made in increasing both the octane number of the alkylate and the yields of high-quality alkylate. In this direction, lche aim has been TABLE VI Octane Numbers of Some of the Major Components in Aviation and Motor Alkglates ( 5 )

lsopentane n-pentane n-hexane 2,2-dimethylbutane 2,3-dimethylbutane n-heptane 2,2-dimethylpentane 2,4-dimethylpentane 2,3-dimethylpentane 2-methylhexane 3-methylhexane n-octane 2,2-dimethylhexane 2,5-dimethylhexane 2,4-dimethylhexane 2,3-dimethylhexane 3,3-dimethylhexane 2,2,4-trimethylpentane 2,2,3-trimethylpentane 2,3,4-trimethylpentane 2,3,3-trimethylpentane 2,2,5-trimethylhexane 2,3,5-trimethylhexane 2,2-dimethylheptane

Clear

3 ml.TEL

92.3 61.6 24.8 91.8 $0.3 0 92.8 83.1 91.1 42.4 52 (-15) 72.5 55.5 65.2 71.3 75.5

$1.0 84.6 65.3 $0.6 43.5 +0.4 96.6

100

$1.2

+0.3

73.2 74.7 24.8 93.3 81.6 87.3 91.7 94.6 +3.00

+0.22

-

$0.61

-

-

-

50.3

77.2

Clear 90.3 61.9 26.0 93.4 92.3 0 95.6 83.8 88.5 45.4 515 (-19) i7.4 55.7 69.9 78.9 113.4 100 !)9.9 !>5.9 ‘39.4 s91.4 85 60.5

Note: Octane numbers above 100 are indicated by isooctane

3 ml.TEL -

83.6 65.2 f2.10 $1.79 46.9 $2.43 99.1 0.29 74.5 81.o 28.1 95.2 82.9 89.0 93.7 $0.02

$3.00 $2.0 +0.7

$1.9

+ ml.TEL.

-

-

83.5

181

ALKYLATION OF PARAFFINS AND AROMATICS

Liquid volume percent

FIQ.2. Analytical distillation of a typical Cq rtlkylate (HtSOr) (6).

to increase the product.ion of trimethylpentanes. Table VI lists the various products of the alkylation reaction and the octane numbers of each. Figure 2 shows a boiling-point curve of a typical Cq alkylate produced on a sulfuric acid unit. This sample contained the following components: -

Liquid vol. % ’ ~

~

Pentanes Hexanes Heptanes Octanes Nonanes and higher

10.0 8.5 6.9 65.8 8.8 100.0

Of t h e octanes, 37% was 2,2,4-trimethylpentane.

As can be seen in Table VI, the trimethylpentanes have the highest octane numbers, and the methylhexanes and some of the dimethylhexanes have the lowest octane numbers. Although the nonanes, as trimethylhexanes, do not have the low octane numbers of the methyl- and dimethylhexanes, they are t o be avoided when attempting to make the higheetoctane alkylate. This makes butylene the preferred charge stock for producing high-octane alkylate and leaves propylene and amylene as marginal feed stocks for somewhat lower octanes because of their higher production of the methyl-, dimethyl-, and trimethylhexanes.

182

EDWIN I(. JONES

The trimethylpentanes are easily produced by alkylatiiig isobutane with isobutylene, but unfortunately, the content of isobutylene produced by catalytic cracking is only about one-third of the total butylenes in the C4 stream, the remaining butylenes being butylene-1 and but~rlene-2.Although most of the butylene-2 tends to form trimethylpentanes, the butylene-1 must be isomerized to butylene-2, either in the alkylatiori reaction or in a separate previous reaction, before it will form trimethylpentane. If not isomerized, the butylene-1 when alkylated forms the much lower-octane material, dimethylhexane. Low temperatures in the alkylation reaction zone favor the isomerization of butylene-1 to butylene-2 during the alkylation reaction. Where low temperatures are not used for economic reasons, however, as is the case with many HF alkylation units, this isomerization is some times carried out in a separate unit and the isomerized product then charged to the alkylation unit. Catalysts for this isomerization reaction are phosphoric acid and silica-alumina. Some polymer is usually made when isome rizing with these catalysts, but when the polymer production is limited to diisobutylene, it will be alkylated to produce the trimethylpentanes. Any polymer made from normal butylenes, however, is an inferior feed for alkylation units. When this type of polymer is made, it is usually removed by fractionation before charging the isomerized butylene feed to the alkylation unit. The newer Alkylation units are designed to produce oc1;ane numbers of 98 F-1 clear and 111 F-1 with 3 cc TEL. This is the case with both hydrofluoric and sulfuric acid catalysts. When operating to make these high octanes, the acid consumptions are down considerably. On these units, acid consumptions have been obtained as low as 0.3 lbs. HzS04 per gallon of alkylate and 0.1 lbs. H F per barrel of alkylate. Also, the production of heavy alkylate is considerably reduced, the Engler end point being as low as 350°F. on the total alkylate produced. 6. Petrochemical Reactions

The first large-scale production of petrochemicals by alk.ylation was the production of ethylbenzene and cumene. The ethylbenzene was produced during World War I1 for making styrene and then synthetic rubber. The ciimene was used as a high-octane additive for aviation gasoline. Dodecylbenzene for the production of the alkyl aryl sulfonate type of detergent came later and was soon an important product of those petroleum refineries going into the petrochemical field. The non-ionic detergents followed when low-sudsing liquid detergents began to show their value. The great increase in production of synthetic fibers and plastics has given a boost to numerous other petrochemicals, a large number of which will be produced by the alkylation of an aromatic and olefins. The most

ALKYLATION OF PARAFFINS AND AROMATICS

183

predominant aromatics used are benzene, toluene, and xylene, and the predominant olefins are ethylene, propylene, normal butylene, and various polymers made from propylene. Diisopropylbenzene and tetraisopropylbenzene are both produced from such a reaction and can be further processed by oxidation to give phthalic acid, isophthalic acid, terephthalic acid, and pyromellitic acid. a. Ethylbenzene. The production of ethylbenzene was expanded greatly during World War I1 as the first step in the manufacture of synthetic rubber. The ethylbenzene is dehydrogenated to form styrene, then copolymerized to make polystyrene or polymerized with butadiene to make rubber. Many of the first ethylbenzene plants were government owned because of the uncertain future of synthetic rubber. Synthetic rubber soon became a necessary part of our daily lives, however, and the plants were sold to private manufacturing companies. Ethylbenzene is manufactured by the alkylation process from ethylene and benzene feeds. The catalyst employed has mostly been aluminum chloride with a small addition of ethyl chloride promoter. Normally, aluminum chloride is somewhat corrosive and causes relatively high maintenance on the equipment. Solid phosphoric acid eliminates these corrosion problems and is becoming quite popular. The water in the feed to the reaction zone must be controlled very closely, however; otherwise, this catalyst can also cause corrosion when the water combines with the P206 to form corrosive liquid H3POa. Newer catalysts of the fluoride type promise to be much more versatile. Essentially all ethylene from catalytic cracking was once burned as fuel but can now be utilized for the production of ethylbenzene using newer catalysts. Figure 3 shows a flow diagram of a unit for the production of ethylbenzene using aluminum chloride catalyst, and Fig. 4 shows a flow diagram of a similar unit employing solid phosphoric acid catalyst. b. Detergent Allcylates. The production of dodecylbenzene was the first big start of the petroleum refineries in the manufacture of detergents. It is produced by the alkylation of benzene and (‘tetramer,)) a relatively wide-boiling-range polymer made from propylene and consisting of dodecenes (CI2H24)through “pentamer” (C15H30).Figure 5 shows a flow diagram of an HF unit for the production of dodecylbenzene. Other processes use aluminum chloride catalyst for the production of dodecylbenzene. However, HF is now quite widely used as the preferred catalyst. Figure 6 shows a flow diagram of a dodecylbenzene unit employing an aluminum chloride catalyst.

184

EDWIN K. JONES ET)I*LIENZtNL COLUYk

VLWl QAS

REICTOR

tTYYLLWL

ETHYL-

BEWZLNL

RLWW BOTTOMS

FIQ.3. Typical ethylbenzene unit for use with aluminum chloride cataIyst. ETHTLBENZEW COLUMN

CATALYST CHAMBER

ETHANE c

1,

BENZENE RECYCLE

r ETHANE ETHYLENE

POW ETHV .BENZENE ETHYLBENZENE

FIG.4. Typical ethylbenzene unit with solid phosphoric acid catalyst.

Table VII shows yields and properties of dodecylhenzenes produced with hydrofluoric acid and with aluminum chloride catalysts. Pentadecylbenzene, made by the alkylation of benzene with pentamer, is coming into demand because of the higher detergency of the sulfonate, as compared with dodecylbenzene sulfonate. c. Cumene. Cumene was formerly used as an additive in aviation gasoline

ALKYLATION O F PARAFFINS AND AROMATICS

185

BENZENE COCUYN

FIQ.5 . Dodecylbenzene unit for use with HF catalyst.

BENZENE FEED

ALKYLATE BOTTOMS

FIG.6. Dodecylbenzene unit designed for use of aluminum chloride catalyst.

for increasing the rich-mixture performance number of the gasoline, but is used presently almost entirely as the first step in the production of many petrochemicals. The main use of cumene is for the production of phenol and acetone employing an oxidation process. It can also be alkylated further with propylene to produce diisopropylbenzene. It is usually produced in units employing phosphoric acid catalyst, although aluminum chloride has been used (7). The UOP solid phosphoric acid catalyst is preferred because of its high activity. High yields of a relatively pure product can be made as is shown in Table VIII.

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EDWIN K. JONES

TABLE V I I Yields and Properties of Dodecylbenzene Detergent Alkylates Charge, lb./hr. Propylene tetramer Benzene Products Gasoline and light alkylate Detergent alkylate Bottoms Loss

HF unit 3200 1520

3140 1580

4720

4720

490 3820 340 70

384 3570 624 142

-

__

4720

Properties True boiling point distillation, "C. IBP 5% 95% E.P. Molecular weight d'6

AlCla unit

285 290 309 315 236 0.875

4720 282 286 320 329 235 0.875

When cumene is to be used as a raw material for the production of nylon, the cumene must be very pure with a bromine number of 0.2 or lower. Although liquid phosphoric acid has been used as a catalyst to make cumene, it has the disadvantage in that it is necessary to acid treat the product to reduce the bromine number of the cumene to the 0.2 level. Some cumene is also lost in the acid treating step. Copper pyrophosphate catalyst has also been used but corrosion of the equipment has been experienced, resulting in longer than average downtime. d. Diisopropylbenzene. p-diisopropylbenzene has become an alternate to p-xylene as a raw material for the production of Dacron polyester fiber because of the relatively high cost of fractional crystallization of paraxylene. p-diisopropylbenzene can be separated from the ortho and meta material by relatively inexpensive distillation. However, 0- and m-diisopropylbenzenes cannot be separated by distillation. Aluminum chloride and UOP solid phosphoric acid are commercially available for the production of diisopropylbenzene, and newer fluoride-type catalysts are becoming available. e. Non-Ionic Detergents and Secondary Butylbenzene. New petrochemicals are continuously being added to the list, many of which will probably be made with either hydrofluoric acid catalyst or some of the newer alkylation catalysts.

187

ALKYLATION OF PARAFFINS AND AROMATICS

TABLE V I I I

Yields and Product Quality f r o m Typical Cumene Unit Charge

BID 134 139 142

Propane Propylene Benzene Products

BID 134 1 216 9

Propane Propylene Cumene Bottoms Cumene Cumene Toluene Bromine No. Cumene bottoms Unsaturates (other than methyl styrene) Cumene Alpha-methylstyrene t-butylbenzene s-butylbenzene Other monoalkylbeneenes Diisopropylbeneene 0-

mPOther dialkylbenzenes 0-

mPUnaccounted

% by wt. 99.8 0.1 0.1

% by wt. 1.8

6.6 0.04 0.8

0.5 19.4

16.2 23.5 26.5 1.8 0.3 0.8 1.8

The fastest growing of these newer petrochemicals from alkylation are the non-ionic detergents. The most popular of these are the polyoxyethylated detergents. These are normally produced by alkylating phenol with a polypropylene, usually in the “trimer” or CSrange. This alkylate is then reacted with about 8 to 12 mol. of ethylene oxide per mole of alkylate t o form a mixture of several different compounds. Another of the more desirable of these non-ionic detergents is produced using tall oil rather than the trimer. The advantage of the non-ionic detergents is their superior soil-suspending powers. Secondary butylbenzene is made by alkylating benzene with normal

188

EDWIN K . JONES

butylenes. The preferred catalyst is solid phosphoric acid, although aluminum chloride or liquid phosphoric acid can be used.

V. ALKYLATION PROCESSES The sulfuric acid process was developed by several companies and is available for licensing by these companies. The first sulfuric acid units were of the “jet” design with time tanks equipped with ba,ffles for reactors. The M. W. Kellogg Company developments in the sulfuric acid process include the cascade reactor, which uses autorefrigeration. The Universal Oil Products Company found in its research laboratories that hydrofluoric acid offers advantages over sulfuric acid. These include the advantages that, hydrofluoric acid was able to operate without the aid of refrigeration and also that hydrofluoric acid could easily he regenerated for the removal of polymers and reused, while it is necessary to ship sulfuric acid back to an acid-manufacturing plant to be made into new acid. The Stratford Engineering Company furnished many alkylation reactors and were therefore interested in efficient mixing and hea-k transfer in this equipment. They subsequently developed a very efficient reactor, which makes part of the recycle by a flashing operation and is applicable to both hydrofluoric acid and sulfuric acid alkylation. This flashing differs from autorefrigeration in that the effluent stream is flashed after the reaction zone instead of in the reaction zone. 1. Jet-Type Sulfuric Acid Process Using Time ‘Tank This type of unit was the original design for making aviation alkylate. The first units were constructed with vertical vessels containing baffles through which the hydrocarbon-acid emulsion was pumped. These vessels were called “time tanks.” Many of the units are still in operation and, after certain modifications for increased efficiency, have operated as well as the most recent designs or better. A later version of the jet type of design eliminated the time tank, but this design was not very efficient, and many of these units have been changed over to other types. This type of unit was originally designed with a “chiller,” or exchanger for cooling. It contained propane refrigerant in the shell, and the hydrocarbon-acid emulsion was pumped through the tubes. Yields and product qualities from the latest type of jet units with time tanks are shown in Table 11. 2. Kellogg Sulfuric Acid Design Using Autorefrigeration (8)

The sulfuric acid alkylation process for making aviation or motor gasoline from isobutane and olefins from cracked gases requires a relatively high isobutane-to-olefin ratio in the reaction zone to insure high octanes, good yields, and low polymer formation in the acid. The sulfuric acid catalyst can make use not only of the relatively pure external isobutane

ALKYLATION O F PARAFFINS AND AROMATICS

189

that is recycled, but can also make use of an additional quantity of a relatively impure isobutane containing alkylate. The cascade reactor takes advantage of this additional recycle by dividing the reactor into several zones, charging all of the isobutane into the first zone and allowing it to flow in series through the remaining zones. The olefinic feed is charged to each zone in parallel, thus taking advantage of the total effluent from the preceding zone for its isobutane recycle. There is a certain minimum amount of isobutane which can be recycled ; otherwise, polymerization will become excessive and acid consumption will rise rapidly. This minimum is usually considered to be 50% of the total hydrocarbon effluent from the last zone. It was originally believed that three zones were sufficient for efficient operation. As acid consumption and octanes became more critical, however, five zones were used, and still later some units were designed with seven. More zones can be used but each additional zone increases the cost of the unit. Both the contact efficiency of isobutane and olefin in the zones and the mixing efficiency of acid and hydrocarbon in these reactors are very important. The flow must be such that no isobutane recycle will bypass the reaction zone and no hydrocarbon will bypass the acid-hydrocarbon mixing zone. With this type of unit, a mixer driven by a 40-hp. motor is usually used in each of the zones. A reactor containing five such mixers can normally make 1500 bbl. per day of alkylate. The seven-zone reactors can make 3400 bbl. per day of alkylate. The autorefrigeration feature of this design is a reactor temperature control in which the hydrocarbons in the reactor are caused to vaporize by reducing the pressure on the reaction zone. This reduction in pressure is brought about by the compressors taking suction on hydrocarbon vapors from the reactor, then condensing them a t a higher pressure in a watercooled exchanger, and returning the condensed hydrocarbons back to the reactor. Because these condensed hydrocarbons have a relatively high isobutane content, they can be used as part of the isobutane recycle. In order to prevent the flashed vapors in the autorefrigeration process from containing excess normal butane, it is necessary that the remaining isobutane recycle, from the deisobutanizer overhead, have a high isobutane purity. Any normal butane charged to the reaction zone decreases the alkylate quality to some extent. The temperatures of the autorefrigeration reactor are set by controlling the quantity of vapors removed by the compressor. Figure 7 shows a flow diagram of a unit employing the autorefrigeration and cascade reactor design. Table I1 shows typical yields and quality of products from this design. The caustic scrubbing part of the unit is very important in' the pre-

190

EDWIN K. JONES

a CRESH OLEFlNlC FEED

FRESH

HZS04

SPENT A C I D

*

PROPANE

BUTANE

* *

AVIATIOY ALKYLATC

ALUYLkT! B0TTI)Y

FIG.7. Kellogg cascade type of sulfuric acid alkylatiori for aviation alkylate using autorefrigeration.

vention of corrosion in the fractionating columns. Bauxite towers have been added on some units to remove some of these corirosive elements. This is described more fully under corrosion protection in VI, 1. 3. UOP Hydrofluoric Acid-Type Alkylation

Commercial hydrofluoric acid alkylation was started in an emergency to make aviation gasoline during World War 11. It was later found that maintenance on these units could be eliminated almost entirely by using standard valves and pumps with Monel trim on the moving parts. Monellined regenerators also eliminated corrosion from the hot hydrofluoric acid. Mechanical mixers are not now required. Consequently, most units now operate with the reactor merely as a heat exchanger full of acid with the combined isobutane-olefin feed jetting into the reactor near the bottom or flowing from a distributor near the bottom. Some reactors, however, are equipped with an impeller for increasing the acid circulation rate over the exchanger surface in order to give a better heat transfer rate. Figure 8 shows a typical C4 dkylation unit with impeller, for producing motor alkylate. Lower temperatures contribute to higher alkylate qua1it:y on HF alkylation units, but gasoline with somewhat lower octane numbers of acceptable quality can be made at higher temperatures if refrigeration is not desired. Most HF units are therefore operating a t 90 t o 100" F., using water cooling in the reactor. The cooling is carried out by constructing -the reactor with a tube bundle placed directly in the reaction zone and flowing cooling water

191

ALKYLATION OF PARAFFINS AND AROMATICS OEISOBUTAMIIER

DEBUIWIZLR

SLlTLER

IRLSn

1

COLYYLR

OLEFlNlC L OUTSIDL ISOWTINE

FRESM FEED

PRO-NL

HF

YOTOR

tuLarf

RUTAYL

FIG.8. “HF” alkylation process for motor fuel production.

through the tubes. The tube bundle normally cools a large pool of acid retained in the reaction zone, and the acid in turn cools the reacting hydrocarbons. When more efficient heat transfer is desired, an impeller is normally added to increase the flow of acid past the tubes. This impeller can be driven by either an external electric motor or by a steam turbine which connects with the impeller by means of a double Dura seal. Lubricating oil is pumped into a chamber between the two seals and is held a t a pressure slightly above the reactor pressure, so that any leakage tendency would be that of the lubricating oil into the reactor rather than that of the acid or reactants out of the reactor. High isobutane recycle purity is not required on HF alkylation units as is required on many Hi304 units because relatively high normal butane concentrations in the reaction zone do not appreciably affect the quality of the alkylate. Isobutane purities below 60% are usually avoided, however, since this purity definitely gives lower-quality alkylate and the cost of recycling the normal butane is considerable in heat requirements as well as fractionation equipment requirements. Because of the relatively high normal butane tolerance in HF units, all of the recycle isobutane can be made from the effluent stream with little rectification. The purity of the recycle under these conditions is 75 to 85 % isobutane. Even though such a relatively impure external isobutaiie recycle can be used without appreciably lowering the quality of the alkylate, an increase in the internal isobutane-to-olefin ratio does not improve the quality as is the case with sulfuric acid catalyst. When the external isobutane recycle is charged to the first of a series of reaction zones and the olefinic feed is divided between the individual reaction zones, there is no apparent improvement in quality. This indicates that any increase in the isobutane-to-olefin ratio over and above the external ratio which may be

192

EDWIN K . JONE S

added by internal circulation of isobutane would make 110 apparent improvement in the quality of the alkylate on the present commercial alkylation units. Table I1 shows yields and product qualities of HF alkylates made when charging various feeds, and Fig. 1 shows the F-1 clear octane of motor alkylate at various temperatures when charging a butylene feed.

4. Stratford Efluent Flash Unit (9) The effluent flash system of refrigeration differs from autorefrigeration in that the vaporization of the reactant hydrocarbons takes place after the hydrocarbons have left the reaction zone instead of taking place in the reaction zone. The flashed material contains a relatively high percentage of isobutane, which is condensed by compression and cooling and returned to the reactor as isobutane recycle. Care must be taken to limit the amount of normal butane in the reactor, however; otherwise, the purity of the flashed vapors will contain excessive normal butane and will reduce the effectiveness of this recycle stream. Figure 9 shows a flow diagram of the Stratford effluent flash alkylation unit, and Table I1 shows typical yields and product qualities from such a unit. 6. Chamber-Type Units

The most important chamber type alkylation units are the UOP type using solid phosphoric acid catalyst for making cumene and ethylbenzene

PI

FIG.9. Stratford effluent flash type of sulfuric acid alkylation for making aviation gasoline.

ALKYLATION OF PARAFFINS AND AROMATICS

193

and those for the use of aluminum chloride when making ethylbenzene, cumene, and dodecylbenzene. The UOP type is a carbon-steel chamber with grates for separating the catalyst into three to five beds, usually with quench between the beds. The chamber is charged at the top with effluent removed at the bottom. The aluminum chloride type of chamber is normally lined with acidproof brick with catalyst addition at the top and a baffle to direct the flow of catalyst to the reaction zone in the bottom. The charge enters at the bottom, and the effluent leaves from the middle or top.

VI. MATERIALS OF CONSTRUCTION The basic material of construction in both sulfuric and hydrofluoric acid units is carbon steel. Normally, neither acid is corrosive to carbon steel at temperatures below 150"F., which covers the reactor section in both types of units. Where corrosive conditions do exist, equipment is protected in sulfuric acid units by the use of stainless steel and in hydrofluoric acid units by the use of Monel. Both acids build up a thin layer of iron salt on carbon steel even in the cold. Such a layer of salt causes any moving parts to stick in time, and, therefore, most valves and pumps are protected with alloy trim under normal conditions and are solid alloy under severe conditions. Both types of units will have very little corrosion if properly designed and operated. Many sulfuric and hydrofluoric acid alkylation units have more or less corrosion in some parts of the alkylation unit. Because the types, causes, locations, and remedies for the corrosion are entirely different, it is necessary to have a separate discussion of the two acids on this subject. 1. Corrosion in Sulfuric Acid Units The first place where corrosion has been found in these units is in the contactor. The corrosion here is not very great and is thought to be caused by the water entering the contactor in the wet isobutane recycle stream. It has been found that this can be eliminated on some units by contacting the isobutane recycle with the circulating acid previous to charging the isobutane to the contactor. The first serious corrosion in these units is usually found in the deisobutanizer reboiler. When this corrosion does take place here, it is usually accompanied by fouling of the reboiler. Such corrosion is caused by sulfur products, such as esters, leaving the reactor section and passing through the caustic wash into the deisobutanizer, then breaking down, usually to sulfur dioxide, when they contact the hot reboiler. Also, neutral salts formed in the caustic wash react similarly. Changing caustic in the effluent scrubber more often helps this corrosion on many units. The caustic is

194

EDWIN K. JONES

titrated to the phenolphthalein and methyl orange end points and is normally changed when the percentage difference between the two end points changes by one number. Another indication of need for caustic change is when the pH of the water in the effluent water wash reaches 9.5. The ASTM distillation flash char test provides a good indication of both esters and heavy polymers in the reactor effluent. The addition of water dispersible filming amine corrosion inhibitors into the fractionator overhead vapor lines and the addition of mono- and triethanol amines into the feed line of the fractionators has aided in many cases. Normally, when corrosion and fouling is found in the deisobutanizer, the remaining fractionators also have more or less corrosion and fouling. This is usually caused by the remaining corrosive elements which have not been broken down in the deisobutanizer, but which do break down upon entering the other reboilers which operate a t higher temperatures. Not only the reboiler tubes and shell corrode under these conditions, but corrosion is also found to a smaller extent on the trays and vessel walls. Also, corrosion products and hydrocarbons, polymerized to a cokelike mass, are found in the reboilers and throughout some columns. Coking in reboilers and rerun columns is caused in mabnycases by operating at excessive temperatures. In these cases, the difficulty can be reduced and sometimes be eliminated by reducing the ternperature of the reboiler. Temperatures of 400°F. and above should be avoided. The breakdown of esters and other sulfur compounds in the rerun reboiler usually results in the production of corrosive alkylate and corrosion in the rerun column. A reduction in the reboiler temperature is sometimes sufficient to clear up this situation, but in some cases, an injection of mono- or triethanolamine into the feed is required. Another method which has been used for removing corrosive elements from the reactor effluent is bauxite treatment. The effluent hydrocarbons are heated and passed through the bauxite, which is usually located on the deisobutanizer feed, and the bauxite breaks down the corrosive sulfur compounds and retains them. 2. Corrosion in HydroJEuoric Acid Units

Hydrofluoric acid causes two types of corrosion, direct corrosion, in which iron fluoride is formed, and stress corrosion, in which the metal cracks. Electrolytic corrosion can also take place when the acid becomes diluted with water and two dissimilar metals or carbon are used. Because of the possibility of stress corrosion, all vessels in acid service must be stress-relieved. Monel is also subject to stress corrosion and must be stressrelieved. Care must be taken in stress-relieving Monel that sulfur or sulfur compounds do not come in contact with the Monel either before or during the heat treatment.

L.

ALKYLATION O F PARAFFINS AND AROMATICS

195

Hydrofluoric acid has another property which should be considered, that of dissolving any slag that might be in a weld. For this reason, vessels in acid service should be X-rayed. The reaction and acid storage sections of an HF alkylation unit are constructed of carbon steel, stress-relieved, and X-rayed. When properly constructed, these sections are essentially free of corrosion. Monel trim is used on all valves, pumps, and instruments. The first place where corrosion is normally found in these units is in the acid regeneration system. The preheater on this column must be stressrelieved Monel in order to prevent corrosion during the heating of the acid. All other regeneration exchangers, lines, and column internals also must be stress-relieved Monel in order to prevent corrosion. In spite of these precautions, slight corrosion is sometimes found on the bottom trays in the column. These trays are not fabricated as an integral part of the column, however, and can easily be replaced. Also, some pitting is found in the shell and tubes of the reboilers, but this has been attributed to excessive reboiler temperatures. Corrosion has been noted in the preheater which precedes the fractionation section and the top tray and vessel wall around the top tray of the first fractionator. Proper operating technique will normally eliminate such corrosion, but Monel can be used if desired.

VII. FUTURE OUTLOOK FOR ALKYLATION A tremendous growth in alkylation is expected for many years in the production of both motor alkylate and petrochemicals. The production of aviation alkylate is not expected to increase in the future, because of the increase in jet-type airplanes; on the other hand, it is not expected to decrease to any great extent. The great increase in interest in alkylation for petrochemicals has brought about renewed interest in new alkylation catalysts, and many different new catalysts have already been developed. The trend is definitely toward solid catalysts operating at temperatures which do not require refrigeration. REFERENCES 1 . General Chemical Div., Bulls. DA-34681 and DA-34711 (1957). 2. Borthick, G. D., Durland, L. V., and Pope, B. J., Oil Gas J . 64 (57), 88-89 (1956). PI. Schutt, H. C., and Zdonik, S. B., Oil Gas J . 64 (41), 98-103 (1956). 4. Ipatieff, V. N., Schmerling, L., Advances in Catalysis 1, 27-64, (1948). 6.American Petroleum Institute Research Project 45, June 30, 1956. 6. American Petroleum Institute Research Project 6 (1946). 7. Jones, Edwin K., Advances in Catalysis 8, 230-233, (1956). 8. Stiles, R., Petrol. Resner 34, 103-106 (1955). 9. Goldsby, A. R., and Putney, D. H., Oil Gas J . 64 (20), 104-107 (1955).

The Reactivity of Oxide Surfaces, E. R. S. WINTER John & E . Sturge Ltd., Birmingham, England Page

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Experimental Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111. Exchange Kinetics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. Interaction between Oxygen and Oxide Surfaces, . . . . . . . . . . . . . . . . . . . . . . . . 1. Adsorption of Oxygen.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Semiconductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. The Oxide Surface. . . . . . . . . . . . . . . . . . . . . 4. Interpretation of Results . . . . . . . . . . . . . . V. Exchange Reactions of CO and CO,. . . . . . . .......... 1. Zinc Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Nickel and Chromium Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Cuprous Oxide.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI. Application t o Catalytic Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . Oxidation of CO and COz.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a. Reaction on CuzO. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b. Reaction on C u O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . c. Reaction on N O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . d. Reaction on Crz03. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e. Reaction on ZnO. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........ 2. The Decomposition of NzO.. . . . . . . . . . . . . . . . . Acknowledgment.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

196 198

199 202 202 206

218 218 220 221 222 222 222 226 226 231 231 232 239 239

I. INTRODUCTION Considerable attention has been given for many years to the nature of the forces active in chemisorption and catalysis and in particular to the electronic nature of these forces: these investigations have in the main treated the surface as a rigid structure, often considered as energetically uniform, with no mobility of its constituent atoms a t the temperature of catalytic reactions. This assumption of rigidity is usually valid up to around room temperature and for metals is often true at quite high temperatures. There are, of course, some well-known cases where the gas dissolves in the metal, e.g., Hz into Pd; 0 2 into Zr; Nt into a-Fe, etc., and also instances where the adsorbate induces mobility in the upper layer of the adsorbent, e.g., Hz on K [see de Boer ( I ) ] , but these are exceptional. The electronic interpretation of simple reactions upon metal surfaces, although not everywhere satisfactory, is successful, for instance, in explain196

REACTIVITY OF OXIDE SURFACES

197

ing the relationships between electronic structure of the catalyst and its activity found by Beeck, Eley, and Schwab [cf. Baker and Jenkins (S)]. When one turns to reactions upon oxides, however, there is rather more uncertainty, and although some useful generalizations may be drawn regarding oxide type and catalytic reactivity (3) detailed correlations are not yet possible. We suspect that apart from experimental difficulty in defining and reproducing a clean oxide surface, this is partly due to a neglect of the possibility that lattice oxygen lying in the surface of the catalyst can take part in catalytic reactions involving oxygen-containing gases. Some of the most detailed work upon oxides has been concerned with the use of simple catalytic reactions t o investigate solid-solid reactions (4). A large body of work also exists devoted more particularly to the semiconductor aspects of oxide catalysts and the interpretation of simple reactions in terms of the boundary-layer theory ( 5 ) or similar concepts: there are several recent reviews of this work [Stone, Parravano, and Hauffe in reference (S)]. The most detailed investigations designed t o study the interaction between the surface and chemisorbed gases are those of Garner and his co-workers a t Bristol, England. Garner (6) studied the rates and heats of adsorption and desorption and later (7) the conductivity changes occurring when the gases 0 2 ,CO, and C02 were adsorbed and allowed to react with each other upon such oxides as Cu20, ZnO, MnO, and Cr203. He concluded that the CO-oxidation proceeded on transition metal oxides usually by means of oxygen extraction reactions, the oxide surface being alternately reduced by CO and oxidized by the 02. The primary purpose of this article is to present and review some of the experimental evidence showing, mainly by the use of isotopes, how lattice oxygen itself takes part in the chemisorption step and often in catalytic reactions involving oxygen-containing gases. The work includes studies of isotopic exchange in the reactions of O2 , CO, and C 0 2 with a number of metallic oxides, together with some observations upon the adsorption of these gases. The heterogeneous catalyzed oxidation of CO by 0 2 and the decomposition of N2O are also discussed in the light of results obtained with the aid of l80as tracer. The Japanese workers Morita and Titani (8) starting in 1938, published an extensive body of semiquantitative work concerned mainly with the catalytic exchange of lSO between gaseous oxygen and water vapor on the surface of various oxides: a wide range of oxides was studied covering many of the groups of the periodic table. This work was of rather limited scope, since the '*O concentration was determined by means of water density, so that a relatively large amount of material had t o be handled. This in turn forced the employment of a flow system whereby the reactant gases were passed in considerable quantity over the solid oxide. It was thus impossible

198

E. R. S . WINTER

to study the finer details of the reactions, and in particular it was not certain whether oxygen from the oxide lattice took part in the exchange. One series of experiments, however, dealing with the interaction of oxygen with Mn02/Mn203(9) showed that, at 300400" (and in the absence of water vapor) oxygen exchange did occur between gas arid solid. Similar work was reported later from Russia by Karpacheva and Rozen (10) who, also examined the exchange of oxygen between alcohol vapor and dehydration catalysts. Later, these and other workers (11) extended the technique to investigate the oxidation of CO. The isotopic exchange :reaction between CO and COz using was studied by Hayakawa (12) and the oxidation of CO on CuO by Chitani et aE. (IS).The exchange of oxygen isotopes between CO and COz on an FesOc catalyst was examined by Kulkova et al. (14). This work was apparently all performed using a flow technique, as was the more recent work of Allen and Lauder (26) on the exchange between oxygen gas and the oxides of copper, nickel, and cobalt. The general conclusion from this work is that there is considerable mobility of surface oxygen in oxide catalysts at temperatures around 200" and upwards. On Mn02/Mn20s,CuO, and Fe304,the oxidation of CO by 0 2 occurs by alternate reduction and oxidation of the catalyst surface so that oxygen from the oxide lattice appears in the product. (The possibility of a rapid exchange of the COz with the catalyst surface is not, however, completely excluded.) The reaction of water with catalytic oxides important in the petroleum industry has also been studied, and it has been shown that water vapor very readily exchanges oxygen with alumina-silica catalysts ; relationships have been found between the ease and extent of exchang,e and the degree of strain existing in the catalyst structure (16). This exchange is connected with the formation and destruction of -OH groups on the oxide surface, and it has been shown (I7, 18) that on A1203exchange occurs more readily with water vapor thah with oxygen gas.

11. EXPERIMENTAL METHOD The technique and general procedure used in the present work is based upon the method introduced by Urey (19) when studying the pyrolysis of dimethyl ether and acetaldehyde in 1942; he later (20)used the same technique to examine the isotopic exchange of 14C between CO and COz on quartz, silver, and gold surfaces. Figure 1 shows the essentials of the apparatus used. A small amount of the catalyst, enclosed in a reaction system of fixed volume R, is subjected to a reproducible and uniform outgassing treatment at a high temperature. After isolating from the pumps and cooling to the reaction temperature, the reactant gas is admitted via T I . The reaction is followed by continuously leading off a very small fraction of the gas through a glass or quartz capillary leak I,, to the ionization re-

REACTIVITY OF OXIDE SURFACES

199

Moss speclr Ion source

FIG.1. Reaction system: T,.,,stopcocks; R , reaction vessel; L , capillary leak; M, manometer.

-

gion of a mass spectrometer. The size of the capillary is such that the quantity of gas in the system is not reduced by more than 2 or 3 % even during reaction times of several hours. Thus, the reaction system may be treated as essentially constant, with consequent simplification of the kinetics. By suitable adjustments of the quantities of gas and of catalyst, a detailed study can be made of initial reactions on the catalyst surface, and these can often be largely separated from subsequent reactions owing to adsorption of fresh reactants from the gas phase. It is essential to ensure that diffusion phenomena are not rate-determining, since the gas phase is largely static. Suitable tests can be devised, for instance, the catalyst can be spread loosely over the surface of the reaction vessel or it can be tamped into a suitable containing vessel, platinum or otherwise, thus exposing a much smaller superficial area: also the size of the capillary leaks can be varied. I n all the work subsequently described, these tests have been applied, and it has been demonstrated that diffusion is in no case a rate-determining step. In the present work the reaction system had a volume of 80-200 ml., the reaction vessel being made of borosilicate glass or fused quartz : the mass spectrometer was a 120" sector instrument very similar to that of Graham, et d.(21) but with an all-metal tube. Oxides were all C.P. grade or better; details of their methods of preparation are given elsewhere (22-25).

111. EXCHANGE KINETICS During a simple exchange reaction between a gas (e.g., CO, CO, , 0,) containing excess of I8O and an oxide whose surface contains the normal concentration, we find experimentally a continuous fall of lSO concentra-

200

E. R. S . WINTER 1.4

0.2

0

c 0

-I 20

40

60

FIG.2. Exchange of

80

100 120 Time (minuter)

140

between

and

'SO

0 2

160

I80

y-Al203.

tion in the gas phase, as illustrated in Fig. 2, which is a typical curve. The portion from within 1-154 min. of zero time* to around1 the point B follows first-order kinetics (2.2) and the portion BA shows zero slope or in some cases a slight slope a t low reaction temperatures increasing somewhat a t higher. I n cases when BA is not parallel to the time axis, the line is extrapolated to zero time a t C and the arithmetic mean of the lSO content in the gas a t points C and B is taken as the end point of the exchange reaccontent is defined as a,. tion, this value of Other symbols which will be used include ffQ,ff = '*O content of the gas a t zero time and a t any time during the reaction P o , p, p, = same for the oxide surface (Po usually = 0.2 atom %) w = weight of oxide used, gm. no = No. of atoms of oxygen introduced as gas n, = No. of exchangeable oxygen atoms (ions) in the oxide surface, per gram of oxide kQ, k , , L1, k, , etc. = rate constants We will consider the kinetics of the exchange reaction between gaseous oxygen containing excess lS0and the surface oxygen of an oxide: the case of oxygen exchange between the solid and CO or COz is formally analogous. From the plot of log ( a - a,) vs. time, we obtain the experimental rate constant k , , so that

* The delay is due to the time taken for steady conditions to be s;et up in the reactor and the mass spectrometer after admission of the gas.

REACTIVITY O E OXIDE S U R F A C E S

201

and the velocity of the exchange, v, is

where +(s) is a function of the concentration of active species in the oxide surface and f(C,) is a function of the concentration of the active species in the adsorbed layer of gas; the nature of these two functions will be discussed later. k , is the rate constant for the rate-determining step; for instance, if we may write the exchange reaction as essentially

(without specifying the precise nature of 0, and 0,) and assume there is no appreciable isotope effect, then lcl = k-1 and is the quantity appearing in Equation (3). From a material balance of lSO (an

-

=

w ~ P- P o )

(5)

and

Since we have assumed k l

=

n,

= a , and

k-l,

=

2 (5)

(7)

Also i t may readily be shown that

=

kon,

(9)

where

nsko is the most useful function to use in studying the kinetics of the exchange reactions, and the rate constants are all given in terms of this expression. I n many cases (23, 24), in order to get further information about the

202

E.

R.

S. WINTER

process occurring, the experiments on the exchange reaclions of gaseous oxygen were supplemented by studying the equilibration reaction ‘“02

+

‘ 8 0 a

e 21fl0’80

-

(11)

using a 1 :1 mixture of normal oxygen and oxygen containing 30 % atom excess of 180.This mixture has a highly abnormal 32/36 ratio, and the fall of this to the equilibrium value was followed in the same manner as the exchange reactions. When the equilibration reaction was studied under conditions where no oxygen exchange occurred with the surface, it was found that if x is the excess abundance of mass 36 above the equilibrium value,

dx -_ dt

= k:X

or

so that in a similar manner to the exchange reactions we may write

OP

where the primes refer to the equilibration reaction.

Iv.

INTERACTIONS

BETWEENOXYGEN

AND

OXIDE

SURFACES

Most of the kinetic results so far obtained upon oxygen exchange are summarized in Figs. 3 to 6, which also show the kinetics of the equilibration reaction plotted to the same scale. Before attempting an interpretation of these observations, it is desirable to consider first the nature and extent of the oxide surface and the adsorption of oxygen thereon and also to review briefly other experimental evidence regarding the interaction between gas and surface. 1. Adsorption of Oxygen

The oxygen exchange results have been obtained in general a t pressures of between 2.5 and 10 cm. I n view of the parallelism between nitrogen adsorption and oxygen exchange areas (see below), it seemed advisable to look for similar correlations with the adsorption of oxygen a t temperatures and pressures approaching those used in the exchanges. Measurements of both the rate of uptake and the saturation coverage were attempted.

REACTIVITY O F OXIDE SURFACES

203

-_

4

f 18-

.

eP

m

d

I /50

I \I,

I

300

1.77

I N O

I

NO

FIG.3. Exchange and equilibration on MgO. MgO outgassed for 16 hrs. a t 540". Equilibration. 0 Exchange. I

FIG.4. Exchange and equilibration on ZnO. ZnO outgassed for 16 hru. a t 630". X Equilibration. Exchange.

It was, in most cases, found that the rate of oxygen adsorption was too fast for measurement at the temperatures of the exchange reactions, and most work was therefore done a t temperatures around 120-250". The saturation coverages at temperatures and pressures typical of the exchange experiments are given in Table I and it is seen that in all cases the coverage is low in terms of the B.E.T. monolayer capacity. In the cases of C r 2 0 3 ,N O , and Fe203it was possible (24) to follow the kinetics of the adsorption of O2 a t constant pressures around 6 cm. It was found that the uptake q followed the Elovich equation:

204

E. R . S . WINTEIt

Fro. 5 . Exchange and equilibration on Cr203, NiO, and F e 2 0 3 . Exchange and equilibration on Fes03 outgassed a t 450"; PO, := 5.7 cm. A Exchange on Cr203 sample A , outgassed a t various tempera-tures between 385 and 520"; PO, = 6.5 cm. X @Exchange and equilibration on Cr20s sample R , outgassed at, 630"; 1'0, = 6 cm. 0 .Exchange and equilibration on NiO (from NiCOa), outgassed a t 605"; PO, = 6 cm. 0

or, on integration [cf. Porter and Tompkiris (25a)l

where qo = amount instantaneously adsorbed a t t = 0. In all our runs [e~p(bqo)]/~b was zero within experimental error, so that q0 = 0 (which was confirmed by runs a t mm. 0 2 ) . From Equation (16) we have

-

(2)

In -

=

ln (r,)

=

ln (a) - bq

where rB = rate of uptake. Using this equation, ln(r,) was evaluated a t several temperatures a t known coverages O(= q/qW where qm is the equilibrium coverage a t Po, - 6 cm.). The activation energy for adsorption, E, , was evaluated from the ln(r,)s vs. l / T plots, and it was found that a t constant pressure the activation energy increased in a roughly linear fashion with coverage

REACTIVITY OF OXIDE SURFACES

205

I o5 /T

FIG.6. Exchange and equilibration on various NiO preparations. NiO from Ni(N03)2. 0 INiO 0.01 M yoLi2O. R NiO 0.1 M % Liz(). A A NiO 1.OM % AgzO. 0 NiO 0.01 M % W 0 3 . _ . _ NiO _ from _ NiCOJ (Fig. 5 ) . All samples outgassed a t 630" foi 16 hrs. Po2 = 6 cm. 0

+ + + +

+

from 0 to about 35 kcal./mole for NiO and Crz03and from 0 to well above 12 kcal./mole for F e z 0 3 . (In the case of Fe2O3, the uptake and the rate of approach to equilibrium were not fast enough to enable accurate measurements of the limiting upper activation energy to be made.) By first adsorbing a quantity of l*O-enriched oxygen, followed by the normal gas it was demonstrated that on these three oxides desorption was negligible until saturation was attained. Engell and Hauffe (86) have investigated the kinetics of adsorption of O2on NiO from room temperature t o 700" using a sample of rather low surface area, about 1 m.'/gm., and they found the Elovich equation to be obeyed, although below 300" the plots of q against log ( t 20) show discontinuities which they attribute to the presence of two processes, chemisorption and lattice incorporation, the former being the slow process at 300" and above. They found it necessary to put to equal t o values between 60 and 220 sec. and suggest E,, , the activation energy for adsorption a t zero coverage, is about 11 or 12 kcal./niole. It is interesting that our observations are in accord with Engell and Hauffe in so far as the adherence to the Elovich equation is concerned. We do not believe that the fact that we found Eo and t o both about zero, whereas Engell and Hauffe found the values given above is of importance, since

+

206

E. R . S. WINTER

our oxide was clearly much less sintered than theirs, which would indicate a much less uniform surface. Attempts were made to measure the kinetics of adsorption on the other oxides. In the case of MgO, the uptake was very small and very approximate kinetic measurements a t pressures of mm. and below gave activation energies of the order of 10-15 kcal./mole, whereas with ZnO the uptake was complicated by oxidation of Zn atoms exposed during the outgassing process; an approximate analysis gave an activation energy of 15 f 6 kcal./mole for what appeared to be true chemisorption at low pressures (-5 mm.) near saturation. 2. Semiconductivity

MgO is virtually an insulator, although it possesses a small conductivity the nature of which is not certain; thus, Lenpicki (27') and also Mansfield (28) conclude that it is a defect semiconductor containing excess oxygen, whereas indirect evidence from Bevan et al. (29) on the behavior of magnesium and zinc spinels and from Kawamura's work (SO) with strontium and barium oxides indicates that it is probable that magnesium oxide is an excess metal semiconductor. ZnO is an n-type conductor, as is FenOs, while NiO and CrzOaare p-type. Experiments upon compressed pellets of these oxides using a cell like that of Jacobs (Sf)have shown that whereas MgO has a very high resistance so that changes are not easy to follow, the other oxides, when outgassed for 18 hr. a t 500-600" and then cooled to exchange reaction temperatures show an immediate change of conductivity on admission of O2 (32). This indicates a rapid electronic interaction between the surface and the adsorbed gas. The changes of conductivity of these oxides with temperature in a constant gaseous environment were in general agreement with those found by other workers, but since these experiments appear a t the moment to have no direct bearing upon the interpretation of the isotope exchange kinetics, they will not be detailed here [cf. (24, 3311. 3. The Oxide Surface (Equation (?'), Section I l l ]

Figures 7 and 8 show that n, in most cases increases slowly with temperature; owing to the scatter of results, accurate values for the apparent activation energies of this process are difficult to obtain, but in general they lie between 0 and 5 f 3 kcal./mole. Table I compares values of n8 a t around 450" with values of monolayer capacity, expressed as nitrogen atoms per gram calculated from the B.E.T. isotherms for the adsorption of Nz a t -196" on the same oxides: the same outgassing temperatures were used in both series of experiments. Table I similarly compares the B.E.T. monolayer capacities and n, values for MgO subjected to various outgassing

REACTIVITY O F OXIDE SURFACES

207

i

I

I30

-

140

I

I

150

\\I60

FIG. 7 . Variation of ne with reaction temperature. 0 F e 2 0 a , outgassed a t 450". El MgO, outgassed a t 540". X ZnO, outgassed a t 570". A ZnO, outgassed a t 630".

I

I30

I20

140 I

150

lo5 IT

FIG.8. Variation of n s with reaction temperature. Cr203sample B, outgassed a t 630". X NiO from NiCO, outgassed at 605". 0 NiO from Ni (NO& outgassed a t 630". A NiO 0.01 M % LisO outgassed a t 630". B NiO 0.1 M % LisO outgassedat 630". 0 NiO 1.0 M % AgnO outgassed a t 630". 4 NiO 0.01 M % C r 2 0 3outgassed a t 630".

+ + + +

treatments, showing clearly the onset of sintering. It is obvious that there is a strong correlation between the two sets of results: in the case of Fe203 the adsorption of Fe3+ions from solution has also been examined b y chemical and radio-chemical (24) means and here again the monolayer capacity is in

208

E. R. S. WINTER

TABLE I

From

Oxide

ZnO Fe 2O C1-203

NiOa NiO* NiO NiO NiO NiO NiO

+LiD + Li&Y + AgzOe + WO3" + CrtOf

Outgassing From temp. Temp. of expt. exchange 866 748 640 600 540 510 630 450 520 605 605 605 605 605 605 605

460

383500 300 486 486 300480 455 505 410 360 446

0.286 1.72 3.02 4.00 4.53 6.97 0.3-1.5 6.5 1.0 0.63 0.3-0.4 0.20 0.46 0.11 0.06 0.14

Saturation coverage using Nz at 2 4 cm.u

B.E.T. 1.88 3.67 3.31 3.44 3.86 6.15 0.6 5.0 1.4 0.7 0.44 0.32 0.76 0.32 0.33 0.40

0.006 -

0. 03h 0.007 0.13 0.01

Prepared from NiC03. Prepared from Ni(NO3)2. ~0.01 M %. d O . l M %. 1.0 M %. Multiply by lozoto convert to atoms per gram. h At least 95% of this is due to reaction with Zn exposed during the outgassing treatment. a

h

agreement with that found by oxygen exchange and by nitrogen adsorption. In several cases the oxides have been found to exchange their oxygen readily with CO or COz ; these exchanges, as shown in Table 111, take place a t lower temperatures than those with O2 and yield n, values much smaller than those of Table I . The close parallelism between n, and B.E.T. areas shown in Table I, together with the very slow exchange represented by the line BA in Fig. 1 leave little doubt that both n, and the B.E.T. areas are good approximations to the true exposed surface of the oxides. The theoretical limitations of the B.E.T. method are now well appreciated, hit it is accepted that in many cases it gives a fair indication of the true area of a solid : the present results are a further endorsement of this. From Table I i t is evident that although the saturation coverage of oxygen during exchange is low (NiO, Crz03,Fez03) or very low (MgO, ZnO), the exchange reaction involves the

REACTIVITY O F OXIDE SURFACES

209

whole surface. Coupled with the increase in activation energy for adsorption with increasing coverage and with the corresponding decrease of heat of adsorption found by other workers these facts strongly suggest that oxide surfaces are nonuniform, possessing a proportion of sites of enhanced reactivity. Indeed no other picture of the surface seems able to explain the results presented in Figs. 3 to 6. The most reactive sites may be the corners or edges of crystal faces, or of dislocations, screw and glide planes, or similar points of strain, with which are probably associated the defects giving rise to the electrical properties of these materials.

4. Interpretation of Results The adsorption, desorption, and isotopic exchange of oxygen on a n oxide surface may be written schematically in a number of ways including the following, where l8O2 or ' 8 0 represents a species enriched in lSO and e l an electron from the solid: '802~gns)

e "02(sds)

'802(ada)

213, f 2e,

+

'SO&,)

2180(ads)

'60;;)

(2)

2180(ads)

s 2"00,d,)

21807~d~)

+

(1)

(3)

(4)

2180'&d8)

+

e '80;;

'w:;da)

(5

'60:*ds)

(6 1

[o:;)

represents a surface oxygen ion.] Equations (4) and ( 5 ) may be replaced by '80Gd.)

+);:0"'

+

s '80:;)

Alternatively, Equation ( 5 ) may be trivial if, as is possible, O:ads) is indistinguishable from 0;;) . Again, Equation (2) to (4)could be replaced by

or

=

180'-

(ads)

f

'80(s,d.)

(9)

and either

+

O ( R ~ B ) O ( n d s ) -+

Oz(aes)

(10)

or O(sds)

f e,

-+

()?ads)

(11)

I n addition to the above there may be rate-determining processes associated with the migration of adsorbed species or solid defects in the sur-

210

E. R. S. WINTER

face layers, or with diffusion effects in the gas phase. Not all of these steps may occur, or be separable, on each oxide, and it is clear from Figs. 3 to 6 that in practice a variety of phenomena is met with. Equation (1) represents the physical adsorption/desorption of oxygen which has been established by direct experiment (32) to occur very rapidly a t low temperatures and is not a rate-limiting process a t exchange reaction temperatures. Process (2) is unlikely to occur, since the 0-0 bond energy in the gaseous oxygen molecule is 5.1 e.v.; on the other hand, (7) in the gas phase requires electrons of only 0.07 to 0.17 e.v. (34, 35). Massey considers the electron affinity of 0 2 going to O,, both in their ground states, to be about 1 e.v., while the electron affinity of 0 to 0-, also in the ground states, is 2.09 f 0.4 e.v.: the electron affinity of 0- t o 02-is about -6.5 e.v. Gray and Darby ($6) state that the formation of 0- from 0 2 yields 8 e.v. but do not state the source of this information. Using the figures given by Massey and assuming all reactants and products are in the ground state, we may write approximately, with the energies in electron volts,

+1 0; = 0 + 0- - 4.1 0; + e- = 20- - 2.1 O2 + 2e- = 20- - 0.9 0 2

+ e-

0 2

+ e0- + e0

=

0;

= 20

- 5.1

0-

+ 2.1

=

= 02- - 6.5

Assuming that these figures are not too much affected baythe presence of a solid surface and neglecting any activation energy, we conclude that the most probable species first formed in the chemisorption of O2 is O,, although this may be only transient leading to 0-: neutral oxygen atoms, 0, are unlikely t o be formed or, if formed, to be stable. This is in general agreement with the ideas of Garner, Stone, et al., and of Hauffe and others, who usually formulate the charged chemisorbed oxygen species as 0-; the direct formation of 02-from OS(gar)by electron transfer from the solid is most unlikely. To write the charged species as 0, or 01-is, of course, a crude simplification of the true state of affairs, the interaction between adsorbed species and surface undoubtedly involving partial sharing of electrons; nevertheless, provided this is kept in mind the symbols 0, and 0are useful shorthand. There may be sufficient charge separation to permit mobility of the adsorbed oxygen species and also neutral ion pairs may be formed, as proposed for instance by Grimley and Trapnell (37) in connec-

REACTIVITY OF OXIDE SURFACES

211

tion with the mechanisms of metallic oxidation. It is clear that the isotopic exchange reaction '80&,)

+

'60:;)

=

'w:&d,,

+

'80:;)

occurs when mobile 1807,da) moves into a geometric and electrostatic environment which allows an electron switch between the two nuclei and permits the original surface ion to move away, to be subsequently desorbed. The activation energy of this exchange process is impossible to calculate, since it depends on the geometry assumed. Any reaction such as (3), (4), (71, etc., involving partial or complete transfer of electrons between the solid lattice and the adsorbed species will change the semiconductivity of the oxide [cf. (36, 38)], and such processes may be rewritten in a manner which emphasizes the part played by the solid. Thus, using the symbolism recently proposed by Rees (39); for a p-type oxide, e.g., NiO, we would have in place of (2) and (3) : !'$'*0Z(.ds)

f 0,

('8O/O,)

+ ot

+ 0 ; s ('*0/0,)+ 0: + (P/o:)

G? (1*0-/0,)

or for an n-type conductor such as ZnO, assuming the participation of intersitial zinc,

+ 0 , + 0: + (e;/Znz+/A)

3$'*0~(~d~)

;=t

A

+ (Znz+/O:) + (l8OT./Ub)

The results presented in Figs. 3 to 6 indicate that even what might be expected to be a relatively simple exchange reaction, between 0 2 and oxide surfaces, shows considerable complexity; it is not yet possible to be certain of the interpretation in all cases. Bearing this in mind, the following tentative conclusions may be drawn regarding the processes occurring in the interaction of oxygen with these oxides (23, 24, 33). Firstly, on MgO, we consider that adsorption and desorption involving equilibration occurs on a certain number of very reactive sites with an apparent activation energy of 634 f $5 kcal./mole; this is the reaction which is shown along line F'FG on Fig. 3. The equilibration involves the dissociation of the oxygen molecule, but the sites involved are not saturated in the temperature range studied (17-275"), since the reaction rate increases directly as the first power of the oxygen pressure (23).The remarkable thing about MgO is its great activity in promoting the equilibration reaction, which can reasonably be expected to involve the formation of 0-; since MgO is a very poor semiconductor, we conclude that the reaction is restricted to a very few highly active sites. From these reactive adsorption sites, a proportion of the adspecies migrates over the surface with an activation energy of 7.7 f 2.0 kcal./mole, and in the temperature range where this migration process controls the rate of oxygen exchange with the surface (line H I I ' ) , the con-

212

E. It. S. WINTER

centration of mobile species is independent of oxygen pre'elL;sure, since the measured exchange rate is pressure-independent. The mobile species exchange with oxygen ions in the surface only rather slowly; the apparent activation energy of this, the exchange process proper, is 3#6f 1.0 kcal./ mole, and this reaction is also independent of pressure; this process is only rate-determining below 420" (line J I J ' ) . All three reactions are represented by the general rate equation

Icon, = kds)f(C,) It is not known for certain what species is represented by f(C,) or $(s). f(C,) is a function of the concentration of reactive adsorbed species, and it is not possible t o determine directly how this varies with oxygen pressure nor what fraction of the total amount of adsorbed gas is in the active state. Similarly we are, as yet, unable to define the nature of the function +(s) It is therefore not possible to say whether the activation energie