ADVANCES IN CATALYSIS VOLUME 5, Volume 5

ADVANCES IN CATALYSIS A N D RELATED SUBJECTS VOLUME V This Page Intentionally Left Blank This Page Intentionally L...

Author: Author Unknown

38 downloads 1618 Views 25MB Size Report

This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!
Report copyright / DMCA form

DOWNLOAD PDF

ADVANCES IN CATALYSIS A N D RELATED SUBJECTS

VOLUME V

This Page Intentionally Left Blank


VLADIMIR NIKOLAEVICH IPATIEFF

ADVANCES IN CATALYSIS AND RELATED SUBJECTS VOLUME V EDITED BY

V. I. KOMAREWSKY

W. G. FRANKENBURG Lancaster, Pa.

Chicago, Ill.

E. K. RIDEAL London, England

ADVISORY BOARD

PETERJ. DEBYE Ithaca, New York

W. JOST Goettingen, Germany

P. H. EMMETT Pittsburgh, P a .

P. W. SELWOOD Evanston, Ill.

M. G. EVANS Manchester, England

H. S. TAYLOR Princelon, N . J .

1953

ACADEMIC PRESS A Subsidiary of Harcourt Brace Jovanovich. Publishers

N e w Vork

London Toronto

Sydney

San Francisco

Copyright, 1953,by ACADEMIC PRESS INC. ALL RIGHTS RESERVED. N O PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER.

Library of Congress Catalog Card No. 49-7755

ACADEMIC PRESS, INC. 111 Fifth Avenue, New York, New York 10003

United Kingdom Edition published b y ACADEMIC PRESS, INC. (LONDON) LTD. 24/28 Oval Road, London N W l

PRINTED IN THE UNITED STATES OF AMERICA

808182

9 8 7 6 5 4 3 2 1

CONTRIBUTORS TO VOLUME V ROBERT B. ANDERSON, Synthetic Fuels Research Branch, Bureau of Mines, Bruceton, Pennsylvania

J . A. CHRISTIANSEN, Institute of Physico-Chemistry, University of Copenhagen, Denmark

E. FIELD,Standard Oil Company (Indiana), Chicago, Illinois EARLA. GULBRANSEN, Westinghouse Research Laboratories, East Pittsburgh , Pennsylvania G . S. JOHN,Standard Oil Company (Indiana), Chicago, Illinois

MORRISKATZ,Defence Research Chemical Laboratories, Ottawa, Canada

EUGENE LIEBER,Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois

FREDL. MORRITZ, Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois ANDERSNIELSEN,HaMor Tops#e Research Laboratory, Hellerup, Denmark MILTONORCHIN, Bureau of Mines, Bruceton, Pennsylvania

T. N . RHODIN,JR., Institute for the Study of Metals, University of Chicago, Chicago, Illinois

J . G. TOLPIN, Standard Oil Company (Indiana), Chicago, Illinois

V


PREFACE Advances in Catalysis is now entering into its fifth year. It is, in the editors’ opinion, an encouraging and significant fact that the past volumes are finding increasing use in scientific and industrial laboratories and that the Advances are frequently quoted as literature sources on various topics of catalysis. Thus, the editors present this fifth volume with confidence that it too may serve the purpose of fully informing those interested or active in catalytic studies on scientific and practical progress in this field. I n the current volume a variety of subjects is treated by competent authors. These subjects deal with new techniques of surface investigations with the microbalance, with the elucidation of reaction mechanisms by the concept of intermediates, and with specialized studies of the ammonia synthesis, hydrogenations, carbon monoxide oxidation and hydrocarbon syntheses. In addition, Volume V contains an extensive critical review of Russian literature in catalysis. The correlations established in recent years between the electronic structure of metals and of semiconductors with their activities as surface catalysts will be dealt with in future volumes of the Advances.

Vii


VLADIMIRNIKOLAEVICH IPATIEFF 1867-1952

Vladimir Nikolaevich Ipatieff, one of the truly great men of chemistry and a pioneer in the field of catalysis, passed away on November 29, 1952. Our admiration of this man should be based not only on the success of his scientific work but also on the wonderful balance between his academic and industrial achievements. Ipatieff was, first of all, a brilliant and able teacher who preferred the title of Professor to any other. His research activity of a purely scientific nature brought with it unusual industrial success, and many plants, operating all over the world, are based on catalytic reactions discovered by him. Among the most important of his contributions are the introduction of high-pressure techniques in chemistry and chemical industry, destructive hydrogenation, the production of acetone from propyl alcohol, and the production of high-octane aviation fuel by the reactions of polymerization, alkylation, and isomerization. He was the first to demonstrate the specificity of catalysts and the use of mixed catalysts and promotors. Professor Ipatieff was born in Moscow in 1867. He worked in Russia under both the Czarist and Soviet Governments before coming to the United States. His scientific and industrial achievements were independent of his political environments. He was promoted to the rank of a general by the Czar and later was awarded a government title by the Soviets. In 1930 he came to the United States where his scientific and industrial achievements received complete recognition. Among the many international honors he received were the Butlerow Prize and the Lavoisier, Berthelot, and Willard Gibbs medals. He was cited by the United States Government for his work on the production of aviation gasoline, which was credited with establishing the air superiority of the Allied Forces. All his life his activities were rewarded by more material wealth than his own simple personal tastes required. He spent it instead on the proTo honor and encourage the motion of his beloved science-chemistry. work of young chemists in catalysis, he established the Ipatieff Prize. To provide better facilities for teaching students, he built and guided the Ipatieff High Pressure and Catalysis Laboratory a t Northwestern University. ix

X

VLADIMIR NIKOLAEVICH IPATIEFF

During the years of my association with Professor Ipatieff in Russia and in the United States, I grew to know him as a lively man with a wonderful sense of humor, whose enthusiasm, patience, and understanding made him a n ideal director and teacher of research workers. The life, work, and achievements of this great man can serve as an inspiration t o us all. V. I. Komarewsky

CONTENTS CONTRIBUTORS TO VOLUMEV . . . . . . . . . . . . . . . . . . . . . . . v vii EDITOR’S PREFACE. . . . . . . . . . . . . . . . . . . . . . . . . . . OBITUARY OF VLADIMIR NIXOLAEVICH IPATIEFF . . . . . . . . . . . . . . . ix Latest Developments in Ammonia Synthesis BYANDERSNIELSEN,Haldor Topsae Research Laboratory. Hellerup. Denmark I. I1. I11. IV. V.

Introduction . . . . . . . . . . . . . . . . . . . . . Unreduced State of Catalysts . . . . . . . . . . . . . Reduced State of Catalysts . . . . . . . . . . . . . . Theory of the Reduction Process . . . . . . . . . . . . . Kinetics of the Synthesis and Decomposition of Ammonia . VI . Conversion Efficiency a t Different Reaction Conditions . . VII . Theory and Practice in Industrial Utilization . . . . . . References . . . . . . . . . . . . . . . . . . . . . .

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

. . . . . . .

1 2

9

. . . . . . . 18 . . . . . . . 20

. . . . . . . 26

. . . . . . . 32

. . . . . . .

35

Surface Studies with the Vacuum Microbalance : Instrumentation and Low-Temperature Applications

BYT . N . RHODIN. JR.,Institute for the Study of Metals. University of Chicago. Chicago. Illinois

I . Microbalance Applications . . . . . . . . . . . . . . . . . . . . . . I1. Low-Temperature Studies . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

40 85

114

Surface Studies with the Vacuum Microbalance : High-Temperature Reactions BY EARLA . GULBRANSEN. Westinghouse Research hbOTatOTieS. East Pittsburgh. Pennsylvania I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Therrnochemical and Kinetic Theory Calculations . . . . . . . . . . . I11. Apparatus and Method . . . . . . . . . . . . . . . . . . . . . . . IV . Application to the Study of the Oxidation of Metals . . . . . . . . . . V. Study of the Vapor Pressure of Metals and the Effect of Oxide and Nitride Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI . Application to the Study of the Combustion of Solid Fuels . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

120 122 130 152

161 167 174

The Heterogeneous Oxidation of Carbon Monoxide BY MORRISKATZ.Defence Research Chemical Laboratories. Ottawa. Canada I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

177 179

xii

CONTENTS

I11. Promoted Catalysts in Respirator Protection . IV . Adsorption Wave Kinetics . . . . . . . . . . V. Conclusions . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . . . . . 186 . . . . . . 196 . . . . . . 211 . . . . . . 214

Contributions of Russian Scientists to Catalysis BYJ . G . TOLPIN. G . S. JOHN.A N D E . FIELD. Standard Oil Company (Zndiana). Chicago. Illinois I . Introduction . . . . . . . . . . . . . . . . . I1. Schools of Thought on Catalysis . . . . . . . I11. Investigation of Absorption Phenomena . . . . IV . Kinetics of Heterogeneous Catalytic Reactions . V . Modification of Catalysts . . . . . . . . . . . VI . Catalytic Conversions . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . 217 . . . . . . . . . . . . 224 . . . . . . . . . . . . 238

. . . . . . . . . . . . 254 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

256 266 293

The Elucidation of Reaction Mechanisms by the Method of Intermediates in Quasi-Stationary Concentrations BY J . A . CHRISTIANSEN. Institute of Physico-Chemistry. University of Copenhagen. Denmark I . Introduction: The Correspondence between Kinetics and Mechanism. . . I1. Gibbs’ Fundamental Rule of Stoichiometry . . . . . . . . . . . . . . I11. Intermediate Products and Sequences . . . . . . . . . . . . . . . . . IV . Calculation of Stationary Velocities and Concentrations . . . . . . . . . V. Integration of the Velocity Expressions and Comparison with Experiments VI . Conclusive and Historical Remarks . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

311 314 315 325 343 350 353

Iron Nitrides as Fischer-Tropsch Catalysts BY ROBERT B . ANDERSON, Synthetic Fuels Research Branch, Bureau of Mines, Brucelon, Pennsylvania

I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 355 I1. Interstitial Compounds of Iron . . . . . . . . . . . . . . . . . . . . I11. Iron Nitrides a8 Fischer-Tropsch Catalysts . . . . . . . . . . . . . . 365 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383 Hydrogenation of Organic Compounds with Synthesis Gas BY MILTONORCHIN,Bureau of Mines. Brucelon, Pennsylvania

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Homogeneous Character of the Hydrogenation of Unsaturated Compounds with Synthesis Gas and a Cobalt Catalyst . . . . . . . . . . . . . . I11. Hydrogenation of Organic Compounds . . . . . . . . . . . . . . . . I V. Properties, Structure, and Preparation of Dicobalt Octacarbonyl and Cobalt Hydrocarbonyl . . . . . . . . . . . . . . . . . . . . . . . . .

385 387 390 402

CONTENTS

V. Mechanism of the Hydrogenation References .

xiii

. . . . . . . . . . . . . . . . . . 410

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

414

The Uses of Raney Nickel

BY EUGENE LIEBERAND FRED L . MORRITZ. Department of Chemistry. Illinois Institute of Technology. Chicago. Illinois I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .

111. Special Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

417 418 429 451

AUTHOR INDEX. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

457

SUBJECT INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . .

472

I1. Preparation and Properties . . . . . . . . . . . . . . . . . . . . .


Latest Developments in Ammonia Synthesis ANDERS NIELSEN Haldor Topsae Research Laboratory. Hellerup. Denmark Page I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2.Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 I1. Unreduced State of Catalysts . . . . . . . . . . . . . . . . . . . . 2 1. Composition . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 a.Magnetite . . . . . . . . . . . . . . . . . . . . . . . . . . 5 b . r-Fe2Os, y-FeO . . . . . . . . . . . . . . . . . . . . . . . . 5 c . RhombohedralFeO . . . . . . . . . . . . . . . . . . . . . . 6 d . Oxidic State of the Catalyst. . . . . . . . . . . . . . . . . . . 6 I11. Reduced State of Catalysts . . . . . . . . . . . . . . . . . . . . . 9 1 . Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2 . Surface Properties . . . . . . . . . . . . . . . . . . . . . . . 13 IV. Theory of the Reduction Process . . . . . . . . . . . . . . . . . . . 18 V. Kinetics of the Synthesis and Decomposition of Ammonia . . . . . . . . 20 1. Energetics of the Surface of the Catalyst . . . . . . . . . . . . . . 20 2. Isotopic Exchange on Iron Catalysts . . . . . . . . . . . . . . . . 22 3. Kinetic Equations for Ammonia Synthesis and Decomposition . . . . . 23 VI . Conversion Efficiency at Different Reaction Conditions . . . . . . . . . 26 1. Dependence upon Temperature . . . . . . . . . . . . . . . . . . 27 2. Dependence upon Pressure . . . . . . . . . . . . . . . . . . . . 28 3 . Dependence upon Space Velocity . . . . . . . . . . . . . . . . . 28 4 . Dependence upon Composition of Synthesis Gas . . . . . . . . . . . 30 a . Ratio of Hydrogen to Nitrogen . . . . . . . . . . . . . . . . . 30 b . Content of Inert Gases . . . . . . . . . . . . . . . . . . . . . 30 c. Catalyst Poisons . . . . . . . . . . . . . . . . . . . . . . . 30 VII . Theory and Practice in Industrial Utilization . . . . . . . . . . . . . 32 32 1 . Composite Catalyst Bed . . . . . . . . . . . . . . . . . . . . . 2 . Reduction Procedure. . . . . . . . . . . . . . . . . . . . . . . 33 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

I. INTRODUCTION I . Background The development of an industrial ammonia synthesis was one of the early achievements of catalytic chemistry. The ammonia industry is still a key industry. with U.S. annual production at present almost two million tons . In view of this fact it deserves to be mentioned that the 1

2

ANDERS NIELSEN

catalysts used today are of the same types as those developed forty years ago by Bosch and Mittasch. This does not mean that the latest developments in ammonia synthesis are only of theoretical value, because the increased knowledge of the catalysts and of the kinetics of the reaction has resulted in improved performance and increased lifetime of the catalysts owing to refinements in manufacture, reduction, and operation of the catalysts. It will be the object of this paper t o discuss our present knowledge of the catalysts and of the kinetics, and from this information combined with the latest published data on high-pressure conversion measurements to give a brief survey of the dependence of efficiency and lifetime of catalysts upon synthesis conditions. 2. Scope

A number of substances show considerable activity as ammonia catalysts. Fe, Os, and Re and nitrides of Mo, W, and U are the best known. Iron in the form of promoted iron catalysts is by far the most important, maybe the only type in industrial use, and except for a few comparisons, iron catalysts will be the only type dealt with in this paper. Furthermore, the discussion will be limited to the type of catalysts made by fusing iron oxides together with the promoter components and subsequently reducing the catalysts. This limitation is not too important, since this type of catalyst is the one most widely used and also the type on which most fundamental work has been done. Reference is made t o earlier surveys by Emmett (1) and by Frankenburg (2) as this paper deals mainly with later developments. The-progress during the last twenty years in our knowledge of the solid state, which resulted from the introduction of quantum mechanics, was the prerequisite t o the improved understanding of catalysts that is now in sight. The state of iron ammonia catalysts is dealt with in the following chapters, and x-ray, magnetic, and electric data will be discussed together with adsorption measurements. Information about the catalysts combined with kinetic experiments has led to a fairly good qualitative understanding of ammonia synthesis on iron catalysts, but owing to the ebtremely complicated nature of the catalyst surface during reaction, a quantitative treatment based on data of catalyst and reactants will not be attained in the near future. 11. UNREDUCED STATEOF CATALYSTS 1. Composition The iron catalysts for the ammonia synthesis discussed in this article are manufactured in form of oxides, which are subsequently reduced to

LATEST DEVELOPMENTS IN AMMONIA SYNTHESIS

3

their active state. Normally the reduction is performed in the converters. The unreduced catalysts consist mainly of iron oxide, but one or more other oxides are usually added to serve as stabilizers and activators of the reduced catalyst. The iron oxide used may be a natural oxide, such as magnetite, or it may. be a synthetic oxide made from Armco iron or similar materials. The composition of a catalyst is determined both by the type and amount of promoters and by its degree of oxidation. In the following we shall discuss nonpromoted, singly, doubly, and triply promoted catalysts; catalysts called nonpromoted may, however, contain small amounts of promoters originating from impurities in the raw materials. All available evidence suggests that a ratio of Fe" to Fe"' equal to 0.5, or in the promoted catalysts at least near t o this ratio, is optimal. Almquist and Crittenden (3) have examined the dependence of activity of pure iron catalysts on the degree of their state of oxidation prior to reduction and found maximum activity with samples corresponding to Fe304. They found the dependence to be more pronounced when the catalysts were used at 100 atm. than when used at 30 atm. Similar results were obtained by Bridger, Pole, Beinlich, and Thompson (4) when investigating K20-A1203-promoted catalysts at 100 atm., 450°, and S.V. = 10,000. A large number of papers have appeared which deal with the type and amount of promoters which preferably should be used. A survey of some of the more important promoters is given in a monograph by the author (Nielsen, 5 ) . The author believes that our knowledge concerning this point can be expressed in the following way. Nonpromoted catalysts rapidly lose their activity, particularly under severe conditions of operation. This is in no way contradicted by reports stating that reduced magnetites are good catalysts without addition of promoters. The author had made a spectrographic analysis of an octahedron crystal of Ural magnetite, and the examination disclosed a content of calcium, not less than 1%, of titanium between 0.5 and 1% and smaller contents of magnesium and aluminum, which proves that wellknown promoters were present in the magnetite matrix. It is impossible to define an optimal catalyst composition, because the composition which is most effective for one set of synthesis conditions will be far from optimal for other conditions. Thus Telegin, Sidorov, and Shpulenko (6) have shown that the amount of A1203necessary to stabilize a catalyst for work at 450" and 1 atm. is far from sufficient to give it stable activity at 500" and 300 atm. The difference may be sogreat that if 0.1% A1203 is sufficient in the former case, up to 5% A1203may be required in the latter. This is not only a problem of stability of the catalysts; it is also a question of activity. A series of catalysts do not

4

ANDERS NIELSEN

necessarily have the same relative activity under different synthesis conditions, which may be explained by the assumption that, at different conditions, not always the same surface elements are the most active for the catalytic conversion. It is a generally accepted view that for industrial application the doubly promoted catalysts are superior to the singly promoted types. The promoter pair should consist of an oxide of an alkaline metal and of a difficultly reducible oxide of amphoteric or acidic character. Commonly Kz0-A12O3 is accepted as the best activator pair. The ratio of K 2 0 to A1203 should not exceed certain limits. Oxides easily reduced, such as copper oxide, decrease catalyst activity and/or stability. It is stated that promoted iron catalysts are superior to promoted iron-molybdenum catalysts. The author holds the view that triply promoted K20Ca0-A120s-ironcatalysts are superior to doubly promoted types. This is especially true in respect to thermostability, which property largely determines the lifetime of a given catalyst in industrial application. It is generally assumed that MgO may partly substitute CaO of the triply promoted catalyst. MgO and CaO enter the magnetite matrix in solid solution, replacing iron oxide. As will be mentioned in the next section this substitution increases the amount of other promoters which can enter the spinel lattice. It ought to be emphasized that a catalyst should not be stabilized for more severe conditions than those a t which it is scheduled to operate, because a decline in activity will result if a too high promoter concentration is used. In addition a too high promoter content may cause a deterioration of the mechanical properties of the catalysts. 2. Structure

I t can be readily understood that the structure of the oxide, from which the reduced catalyst is prepared, plays an important role for the properties of the catalyst. This dependence has been proved experimentally by the influence which the rate of cooling of the oxides of a given catalyst composition shows upon the catalytic properties of the reduced catalyst. This effect can be interpreted by considering that in the reduced catalyst the promoters are distributed all over the surface and that it is, of course, highly important how they are distributed. This distribution cannot be independent of the way in which the promoters are present in the oxidic state, whether in solid solution in the magnetite, as separate crystals or as amorphous glassy layers. I t should also be mentioned in this connection that the reduction process exerts a considerable influence on the behavior of the catalysts and that the course of this process is intimately connected with the structure of the oxide.


5

In Table I, x-ray data are listed for various iron oxides relevant to this type of catalysts. A few words will be said about these pure oxides before discussing the more complicated promoted catalysts. TABLE I Structures of Iron Oxides Compound

Structure

FeJOl

Spinel

-y-Fe203 r-FeO a-FeO FeO a-Fe203

Spinel Spinel Sodium chloride Rhombohedral deformation Rhombohedral

Dimension 8.37 8.3T4* = 8.33 = 8.376 = 4.2820

ao =

a0 a0 a0 a0

=

a = a =

5.42 55'17'

References Wyckoff and Crittenden (7) Clark, Ally, and Badger (8) Lihl (9) Lihl (9) BBnard (10) Rooksby and Tombs (11) Strukturberichte

a. Magnetite. The unit cell of this spinel consists of a framework of oxygen ions which deviates slightly from cubic close packing and which forms 64 tetrahedral and 32 octahedral interstices per unit cell. In the case of stoichiometric Fe304, 24 iron ions are distributed in the interstices. According t o de Boer, van Santen, and Verwey (12) magnetite is an inverse spinel as eight FeIII occupy tetrahedral interstices and eight Fe" and eight FeIII occupy octahedral interstices. Its formula should be written FeIII (Fe", Fe"')O4. This special distribution of the iron ions explains the high electric conductivity of magnetite, because according to Verwey and de Boer (13) semiconductors of this type owe their conductivity to the presence, at equivalent crystallographic positions, of ions of the same element but of different valency. This structure also conforms with the theory by NBel (14) explaining the magnetic properties of Fe304and y-Fez03. It should further be mentioned that NQel (15,16) investigating the magnetic properties of a-Fez03observed small regions, almost two-dimensionally extended, of Fe304 deformed so as to join the a-FezO3 matrix. This is, of course, a lattice distortion which can be observed very accurately. b. -y-FezO,, r-FeO. When considering the small deviation in unit cell dimension between Fe304, y-FeZO3,and -y-FeO it may well be possible that the same type of lattice imperfection as that found by NBel in a rhombohedra1 matrix may exist in spinel structures, so that regions with a deviating number of iron ions per unit cell exist coherently with the normal lattice.

6

ANDERS NIELSEN

c. Rhombohedra1 FeO. A rhombohedra1 deformation of FeO leads t o a doubling of x-ray reflections, and this point is of some interest in connection with discussion of x-ray diagrams of unreduced catalysts. d . Oxidic State of the Catalyst. I n a microscopic examination of a great number of promoted fused iron catalysts for Fischer-Tropsch synthesis three general types of grain structure were observed (Klemm, 17). I n the first type of catalysts the grains joined without separate interfacial phases, indicating th at the promoters are completely dissolved in the magnetite matrix. I n the two other types, interfacial phases were observable either as an amorphous glassy material or as separate crystals, which means that more promoters are present than the amounts soluble in the magnetite matrix. The catalysts investigated were not ammonia type catalysts, since they contained considerable amounts of acidic promoters such as SiOz, TiOz, and ZrOz. Some information relevant to ammonia catalysts is, however, obtainable from this investigation. It appears that oxides of the alkaline earth metals enter the matrix ertsily and that the presence of these oxides leads to better solubility of other promoters in the matrix, so that unreduced catalysts of the first type result. It also appears that CaO or CaO.FezOs, when entering the matrix, makes the phase less anisotropic as observable by polarized light. Further, as should be expected, the simultaneous presence of K2O and acidic oxides, especially of SiOe, favors the formation of glassy layers. I n this connection attention is also called t o the large diameter of the potassium ion. Wilchinsky (18) using a technique based on grinding, separation according t o particle size, and chemical analysis, concluded that in a K20-Si02-A1203-promotediron catalyst, AL03 is found as solid solution of either yAlZO3or Fe0.A1203in the magnetite, whereas K 2 0and SiOzare concentrated between the layers of a mosaic-like structure. This picture was supported by microscopic examination. Wilchinsky states that particles smaller than about 30 microns are normally single celled. They are, however, most likely themselves built of much smaller mosaic blocks. Maxwell, Smart, and Brunauer (19) drew the conclusion from thermomagnetic measurements that in singly promoted (Na20, KzO, BaO) and in one doubly promoted (K20-Alz03)catalysts, a maximum of one mole per cent promoter is present in solid solution in the magnetite. However, in some other doubly promoted catalysts, including a second catalyst promoted with KzO-A1203,a greater amount of promoters was found to be in solid solution in the magnetite. In a number of highly promoted catalysts there was an indication of two coexisting magnetic phases. catalysts may The somewhat intricate behavior of K~O-Al~O~-prornoted

LATEST DEVELOPMENTS I N AMMONIA SYNTHESIS

7

mean that some unaccounted factor, such as the cooling rate of the oxides when prepared from the molten state, may play a role. When compiling the information gathered by different techniques of various investigators, the view can be expressed that the promoters, among them especially CaO and A1203, are uniformly distributed in one or more spinel or distorted spinel phases, the latter partly existing as separate phases. This view is confirmed by the available x-ray data. Wyckoff and Crittenden (7), using the Debye-Scherrer technique, have investigated Fe304, and Fe304 promoted with FeOeA1203 and with KzO.Al203. Their results indicate that the promoters do not produce separate phases, but form solid solution with magnetite. Brill (20) has taken powder photographs of unreduced and reduced ALO3-promoted catalysts and of the A1203residue formed by dissolving the catalyst in acetic acid. The residue found was always -f-A1203 independent of the special methods used in the preparation. As cr-Al203 is the stable modification and as the catalyst had been heated during preparation to a temperature a t which transition from the y- to the a-form takes place rapidly, this, as pointed out by Brill, means that A1203 forms a solid solution with the magnetite. The present author has carried out a series of x-ray examinations of unreduced iron synthetic ammonia catalysts and related substances. Details of results and techniques are found in reference ( 5 ) . In Figs. 1, 2, and 3 of Plate I the diagram of a K20-Ca0-A12O3-prornotedcommercial catalyst (KM 11) is compared with diagrams of a Ural magnetite and of a synthetic spinel FeOl.311. All three substances show the lines of a magnetite structure and their lattice constants agree within the limits of experimental accuracy. The following values were found. Ural magnetite Ferro-ferric-oxide KM I1

8 ,3770 kX units 8 . 3iT8 k X units

8.377rkX units

When considering the lattice constants it may be recalled that the Ural magnetite, although a nice octahedral crystal, was not pure FesOl. It may be seen from the figures that the catalyst pattern differs from the pattern of Ural magnetite in three ways. There are some fuzzy extra reflections somewhat removed from the magnetite lines. These reflections may be satellites or may originate from separate phases. There are satellites quite near the (311) reflection, and there is a broadening of the main reflections. In patterns of other catalysts a number of weak satellites are observed nearer to the main reflections. The extra lines are not due to promoter phases, since they are also found in the pattern of the synthetic spinel Fe01.311,neither are they due

a


to Fez03. The doubling of some of the lines noticed in Fig. 2 could be due only to FeO if this had been rhombohedrally distorted; such a structure has actually been observed by Rooksby and Tombs (11). The satellites, however, cannot be due to Be0 and can best be explained by the existence of a modulated magnetite structure. The dependence of line broadening upon the Bragg angle of reflection was determined. When line broadening is due to small crystal size, the broadening increases proportionally t o X sec 8. When it is due to certain types of strain, as found in patterns of cold worked metals, it increases proportionally to tan 8. No such simple relationship could be found, the @ cot 8 values being highest for reflections with low indices. Line broadening dependent in an individual way on the indices, although of a much more regular type, has been observed by Edwards and Lipson (21) in the pattern of an imperfect structure of Co. Similar observations have been made with Co-Fischer-Tropsch catalyst by Hofer, Peebles and Bean (21a). In summarizing, the view may be expressed that the different types of anomalies found in the patterns of unreduced catalysts are due t o a complicated variation in scattering power and lattice geometry brought about by promoters entering the magnetite matrix. The regions in which a certain modification of the matrix exists need not occupy whole crystals, as the differences in oxygen spacing may be small enough for coherence between modulated and nonmodulated phases. It should be mentioned in this connection that catalysts of a high CaO content give patterns with pronounced line broadening. This fits well with the microscopic data. Satellite reflections have been encountered previously in alloy structures and reference is given to articles by Preston (22), Daniel and Lipson (23), and Hargreaves (24). Before concluding this section attention is called to the spotted character of reflections from unreduced catalysts obtained when the samples are not finely ground. The crystal dimensions indicated hereby are in agreement with the single cell dimensions found by Wilchinsky (18) .

111. REDUCED STATEOF CATALYSTS I. Structure When the oxidic catalysts are reduced with hydrogen, no or only slight shrinkage of the external volume of the particles takes place simultaneously with the rebuilding of the material. Reduced catalysts have a density of about 2.7 g./cc. afCer correction for void space, and are thus a porous form of iron. The author has made an x-ray examination of a number of doubly gnd triply promoted reduced catalysts. The method and regultq have

ANDERS NIELSEN

9

10

ANDERS NIELSEN

been described in detail in reference (5), and the main points will be discussed below. The following picture has been formed of reduced iron catalysts corresponding t o the promoted types used for ammonia synthesis. The catalyst particles are built of crystallites, the interior of which is extremely pure iron, whereas the surface layers have a high content of promoter atoms. The total crystallite surface area as will be discussed later, is some two to three times greater than that found by low-temperature surface area measurements by the BET-method. It is concluded that interfacial layers join the crystallites, and only where the distance between two crystallites is too great or their orientation too divergent is the interfacial layer split up into two separate surface layers, thus producing a rift network through the catalyst particles. It is through this rift network that the inner surface is reached by the reactants, and this inner surface is utilized as effectively as is the outer surface. This concept is in agreement with the conclusion which Maxwell, Smart, and Brunauer (19) reached from magnetic investigations and with information about the surface previously obtained by Brunauer and Emmett (25) from chemisorption measurements. With this picture of the catalyst, the stabilizing effect of the promoters is readily understood. The author is convinced that apart from creating a great specific surface during the reduction process and stabilizing it during synthesis, the promoters function as activators in the catalyst. Reasons for this view will be given in a later passage. The proof that the interior of the catalyst crystallites is extremely pure iron is derived from x-ray measurements. A few words will be inserted here regarding the x-ray technique used by the author and especially the way in which samples have been prepared. Samples of reduced rutalysts are samples of catalysts whirh have been reduced under pressure with a three to one hydrogen nitrogen mixture, using a temperature schedule similar to t h a t recommended for industrial operation. Thc same holds for samples used for surface area and chemisorption measurements performed a t the Topsge I.nhoratory. Such samples should reprcsent the aetual working state of the catalysts fairly well. When reduced, the samples were tested for ammonia synthesis under certain conditions which were normally a temperature of 450", a pressure of 330 atm. and S.V. 15,000. The tested samples were cooled down in synthesis gas, removed from the converter in an atmosphere of purified nitrogen and, still in nitrogen, transferred into either benzene or pentane. Samples uscd for obtaining x-ray diagrams were ground in benzene and specimens were prepared by mixing the wet powder with Canada balsam in xylene solution. During the exposure to x-rays hydrogen was passed through the cameras, partly to diminish absorption of x-rays, partly to shield the samples against oxygen. 5


11

The lattice constant of a-iron determined from patterns of Armco-iron was found t o be a = 2.86011110kX units based on a wavelength of 1.78529 kX for the Co-Kal-radiation The lattice constant of the triply promoted catalyst K M I1 was determined in the same camera to be a

=

2.8599 k X units

the uncertainty of this value being somewhat greater than th a t of the value for Armco iron owing t o the fuzzy appearance of the lines. It is, therefore, taken for granted th at the lattice constants of Armco iron and of catalyst K M I1 are identical within the limits of experimental accuracy, which means that the interior of the catalyst crystallites is very pure iron, As it will be seen by comparing Figs. 7, 8, and 10 of Plate 11 with Fig. 6, the lines of the catalyst patterns are very fuzzy. As mentioned previously this might a priori be due either to a small crystal size or t o distortion of the crystals. The lines from patterns of KM 11, obt’ained by radiations of different wavelengths, have been photometered and line broadenings have been calculated by using patterns of Armco iron taken in the same camera for correction. The lines photometered are those marked with arrows in Figs. 7 and 10. The values of P cos O/h proved to be constant for the different values of 6 and A, and the line broadening consequently is due to small crystal size. From the value of ,8 cos O/h found, the mean crystal size, calculated as cube edge, was found to be

tKMI I = 360 A. = 3.6 * lov6mm. equivalent t o a total crystallite surface of 21 rn.”g. (reduced catalyst). This should be compared with a surface area of 8.15 m.2/g. determined by low-temperature nitrogen adsorption on K M 11. As will be seen from the patterns of the catalysts, of which K M I1 is a K20-Ca0-AI2O3promoted and A is a K20-A1203-promotedtype, no other lines than those of the a-iron lattice are visible. Figure 8 shows the pattern of catalyst A reduced and subjected to a test for thermal resistancy (20 hours at 550”). Figure 9 shows the pattern of the same catalyst after approximately eight months service in an industrial converter, at the end of which period its activity had decreased so that the catalyst was discarded. As will be observed from a comparison of Figs. 8 and 9, the decline in activity is connected with crystal growth.

12

ANDERS NIELSEN

(310)

(220)

d

.c .

.*cd E

5

c

e

'-

(200)

i

2

a C

w

i?

b .

I-

i

4 ( 1 10)

13

LATEST DEVELOPMENTS f N AMMONIA SYNTHESlS

2. Surface Properties

The properties of catalyst surfaces are investigated to obtain as complete as possible a description of the geometry and energetical conditions of these surfaces. The following discussion will show that, in view of the very complicated nature of the surfaces of reduced iron catalysts no complete description of their geometry and energetic properties can be presented. During synthesis conditions the state of the surface is especially intricate owing to the atoms and radicals that are chemisorbed on the surface, and penetrate, to some extent, below the promoters or even into the lattice. That nevertheless, a considerable amount of informaticn about the distribution and chemistry of the promoters in the surface is available is primarily due to the chemisorption measurements carried out by Emmett and Brunauer (25,26). It has been proved that one function of the promoters is to create a large surface area, during the reduction of the catalyst and to maintain it during synthesis. For instance, as described by Brunauer, Emmett, and Teller (27), the surface of a highly Alz03-promoted catalyst 954 is more than three times as large as the surface of the Kz0-Al2O3-promoted catalyst 931 and about ten times the surface of the nonpromoted catalyst 973, although the latter was reduced a t a lower temperature. The contents of promoters of these catalysts, their conversion activities, and surface areas, as found by Brunauer and Emmett, are given in Table I1 and based on the values of the ratios of CO chemisorption to a nitrogen TABLE I1

Catalyst

Promoters ~~~

954

931 973

Activity % NH3 at 450", 100 atm. and S.V. = 5000

Surface area, m.'/g.

Equivalent iron area, m.2/g.

~

1 0 . 2 % A1203 1.59% KzO 1 . 3 % AlnOa 0.15% A1203 as impurity

8.2 12.3 3.3

13.2 3.72 -1

5.1 1.5 N1

monolayer stated in reference ( 2 5 ) ,the iron surface areas equivalent to the available number of iron sites are estimated and shown in the last column. When comparing the available iron surfaces of the doubly and singly promoted catalysts with their activities and having in mind that the ratios of rate constants are much greater than those of the ammonia

14

ANDERS NIELSEN

percentages, it is evident that the mean catalytic activity per iron site is many times greater on the doubly promoted catalyst than on the singly promoted. This lack of parallelism between the number of sites capable of COchemisorption and the catalytic activity complies well with the results of similar experiments carried out recently by the author and his associates (Nielsen and Bohlbro, 28). When investigating two triply K20-CaOAlz03-promotedcatalysts differing only in amount and ratio of promoters it was found that the more active KM I1 had the smaller total surface area of 8.15 m.2/g., compared with 12.6 m."/g. for KM I, and further the smaller number of available iron sites as determined by CO chemisorption. This indicates heterogeneity of catalytic activity. Without further information it cannot be decided whether this heterogeneity is due to the catalyst surface having sites of differing activity or whether the activity per site is constant on a given catalyst type, but differs from one type of catalyst to the next. As will be discussed later, we can conclude from other evidence that the surface of a catalyst for ammonia synthesis is actually heterogeneous in respect to catalytic activity and that, in all probability differences in the amount and nature of the promoters cause variations of electronic properties, observable by changes of the work function and of the catalytic activity. This conclusion may be expressed by stating that the activity change caused by promoters cannot be considered solely as a local action; a point which has been especially well clarified by recent work on catalysts which are semiconductors. Beeck (29) in an investigation on metal films found that a pure iron film chemisorbs nitrogen at liquid nitrogen temperature. Iron ammonia catalysts do not chemisorb nitrogen at liquid nitrogen or liquid oxygen temperatures, as it is most clearly seen by comparison of nitrogen and carbon monoxide low-temperature isotherms for a nonpromoted catalyst (Emmett and Brunauer, 30). Total CO adsorption on catalyst 973 at - 183" was approximately twice that of a nitrogen monolayer at this temperature, and half of the CO could easily be pumped off at -78" and is assumed to have been physically adsorbed. Kummer and Emmett (31) by use of a tracer technique found that unpromoted iron does not retain a detectable amount of hydrogen when reduced and carefully evacuated, thus excluding the possibility that low-temperature nitrogen chemisorption was prevented by hydrogen which had not been removed during the evacuation process. Nitrogen adsorption isotherms at low relative pressure and low temperature have been measured a t different temperatures and with different catalyst samples (Nielsen and Bohlbro, 28). These isotherms as shown


15

in Fig. 11 comprise two different curves connected by a linear, vertical, or oblique line indicating a phase transition in the adsorbed films at the gas solid interphase. In some cases the connecting line was vertical within experimental accuracy, which, using the notation by Gregg (32), means that a simple first order phase transition is taking place. I n other cases the connecting line is oblique and the transition of the diffuse type. Similar phase transitions on gas-solid interphases of other solids have previously been described and analyzed by Harkins, Jura, and co-workers (33,34). bCT-M3WUYER.9.04

cm'

Pncssunc. mm #?.

FIG.11. Adsorption isotherms for N I and CO on iron synthetic-ammonia catalyst K M I a t 79.8"K., 86.7"K., and 90.3"K. Reproduced by courtesy of The Journal of The American Chemical Society.

It is emphasized that the phase transition on the iron catalysts takes place a t so low a relative pressure that it does not invalidate the BETnitrogen method. Nielsen and Bohlbro (28) found no appreciable chemisorption of CO at -183" on the triply promoted catalysts used. At -78" CO chemisorptions between 7 and 12% of a nitrogen monolayer were found suggesting that nine-tenths of the surface of the triply promoted catalysts KM I and KM I1 is covered by promoters. The author believes that the high coverage by promoters can be explained by the high solubility of CaO in the magnetite matrix and by the high promoter content. The differences in behavior between evaporated films and impure catalyst surfaces, and the fact that no low-temperature CO chemisorption on the triply promoted catalysts was found may be due to the promoters on the surfaces of the catalysts. The experimental procedure used by Nielsen and Bohlbro was as follows.

16

ANDERS NIELSEN

A sample of catalyst KM I was reduced under pressure in a hydrogen-nitrogen mixture. Its activity was measured at 450°, 330 atm. and'S.V. = 15,000. The ammonia percentage in the exit gas was found to be 25.3%. The sample was cooled down in the convcrter in hydrogen-nitrogen and transferred to pentane in an atmosphere of purified nitrogen. Still wet it was transferred to the adsorption bulb and immediately carefully evacuated first for a number of hours a t room temperature, then during slowly heating up t o 400"and finally for nine hours a t 400'. During the heating period, the temperature rise was stopped both at 200" and a t 300". Dead space was determined with He and the surface area determined from a nitrogen isotherm a t - 195". The catalyst had a surface area of 10.2 m.g/g. in agreement for this catalyst type with the activity measured. The nitrogen was pumped off a t room temperature and after cooling down to - 183"in a n atmosphere of He, an isotherm with CO was taken. The same procedure as that used for obtaining the low-pressure adsorption isotherms with nitrogen described in reference (28) was used. When plotting the values obtained on a graph, the points fall on a straight line indicating Langmuir type adsorption (Fig. 11). As the third point in Fig. 11 is equivalent to roughly 5% of the monolayer, this is most easily understood if the adsorption takes place rather unselectively on the whole surface. The adsorption isotherm up to 200 mm. Hg was completed and afterwards thc sample was evacuated. Evacuation was started at -183" and continued for 40 minutes at that temperature. Thereafter the temperature was increased to -78" and evacuation continued a t t h at temperature for thirty minutes. The sample was cooled down in He and evacuated. A second CO isotherm was taken a t - 183". The difference between the two isotherms was determined to be 2.7% of a monolayer. To insure t h a t this sample had not, in some way or other, been oxidized, the temperature was raised to -78" with the sample still kept in the CO atmosphere. The sample was kept a t -78' for a few minutes only and the dry-ice ba th again substituted by the liquid oxygen bath. During the temperature cycle more CO had been adsorbed so that the amount of chemisorption was now equal to 10.6% of a monolayer, 7.9% having been taken up during the temperature cycle.

Extensive information concerning distribution of the promoters, penetration below the promoters of adsorbed atoms, and chemical behavior of the promoters was obtained by Brunauer and Emmett (25,26). They used chemisorption of carbon monoxide, carbon dioxide, nitrogen, hydrogen, and oxygen, individually and successively measuring the influence of one type of chemisorption upon another type. It was concluded that CO and COz were chemisorbed as molecules, H P and NS as atoms, and O2 probably as ions. COZ is chemisorbed on the alkali molecules located a t the surface, whereas Hz, N P ,CO, and 0 2 are chemisorbed on the iron atoms. From the effect of presorbed CO upon the chemisorption of COz and vice versa it was concluded that the promoters are concentrated on the surface and are distributed so effectively that most surface iron atoms are near t o a promoter atom.* Strong indication

* I n the opinion of t h e present author it is open t o question to which extent this mutual effect is due t o electronic boundary layer phenomena induced by electronic interaction between the catalyst and the chemisorbed species as discussed by Boudart


17

was found to support the view that hydrogen and nitrogen atoms can migrate to positions below the promoters. Two types of hydrogen chemisorption had previously been found (Emmett and Harkness, 35)' type A occurring quickly a t -78"' type B occurring at 100". Later a third type of hydrogen chemisorption, appearing at lower temperature, was demonstrated at Dr. Emmett's laboratory. This heterogeneity was explained from the geometry of normal crystal faces.* On the singly A120s-promoted catalyst, N2chemisorption was found to increase type B hydrogen chemisorption. It was proved that the surface of the doubly promoted catalyst 931, containing 1.3% A1203 and 1.6% KzO, contained no free A1203 as the sum of the CO chemisorption, after correction for appropriate area, and of C 0 2 chemisorption equaled a nitrogen monolayer. The promoter pair therefore must form compounds of the KAlOz type. Catalysts high in A1203 do contain surface elements which neither chemisorb CO nor COz. Singly KzO promoted catalysts or catalysts with a high KzO to A1203 ratio lose alkali during use. Concerning vapor pressures of alkali oxides and alkali metals in equilibrium with solid alkali oxides, reference is made to a recent article by Brewer and Mastick (36). Heterogeneity as a consequence of differently oriented crystal faces can explain a number of observations made by Emmett and Brunauer. It has previously been discussed that the lack of parallelism between catalytic activity and CO chemisorption also suggests a heterogeneous distribution of the catalytic activity. Three more experimental proofs for surface heterogeneity of iron ammonia catalysts will be mentioned. Taylor (37) describes the shape of adsorption isobars for hydrogen on an iron catalyst and curves showing desorption, readsorption when the temperature is increased stepwise. These observations are best explained by the existence of an a priori heterogeneity. This heterogeneity may well be connected with the existence of abnormal lattice distances on the surface. Synthesis gas for ammonia synthesis must be carefully purified from oxygen and oxygen containing compounds, as even concentrations below 100 p.p.m. will appreciably reduce catalyst activity. Almquist and Black (38) have investigated the poisoning action of oxygen and water vapor on nonpromoted, singly and doubly promoted catalysts at 1 atm. and 444". Oxygen concentrations ranging from 200 up to 2400 p.p.m. (34a) and Weisz (34b), t o which extent it is caused by lateral interactions, or by structural changes in the interface between solid and gas. * A fourth type appearing a t higher temperatures than B has been found by Sastri and Srikant (35a).

18

ANDERS NIELSEN

were used. From these experiments and work on equilibrium in Fe, FetOl, HzO (vapor), Hzsystems (Eastman and Evans, 39), Almquist (40) concluded that oxygen retainment and catalytic activity are properties of iron atoms in the same range of unsaturation. Atoms oxidized at an oxygen concentration of 0.02% correspond to an excess of free energy of approximately 10 kcal. McGeer and Taylor (41) have confirmed this view from the observed influence of the purity of the hydrogen used for reduction on the rate of exchange of nitrogen isotopes. They calculated 8 kcal. to be approximately the lower limit of excess of free energy of the energy band of high activity for the exchange reaction. Kummer and Emmett (42) have applied a differential isotopic method to give evidence of heterogeneity of the surface of a triply A1203-ZrO2SiOz promoted iron catalyst. They added radioactive and normal CO in succession and removed CO by pumping at increasing temperatures. All their results could not be interpreted conclusively. For example it should be mentioned that when the first fraction of CO was admitted to the catalyst at -78" and the second at -195" very little exchange between the two fractions occurred at - 195", whereas if both fractions were admitted a t - 195", rapid exchange occurred equivalent to a coverage of half of the iron surface. When this point is correlated with the results with CO chemisorption on the triply K20-Ca0-Al20a-promoted catalyst, previously mentioned, it is clear that CO chemisorption on promoted iron catalysts is not yet totally elucidated. Another approach to the problem has been made by Beebe and Stevens (43), who measured calorimetric heats of adsorption of CO, among other gases, on promoted iron catalysts. They found the initial differential heat of adsorption to be approximately twice as large at -78" as it was at -183" and at both temperatures they found a rapid decrease of the differential heat of adsorption with coverage of the surface. The author would finally like to mention that some of the complexity of the heterogeneity shown by the surface may be due to a cooperation of an a priori surface heterogeneity with the influence of adsorbed molecules on the electronic state of the surface. The last point may result in a continuous change in the state of an adsorbate with surface coverage. Frankenburg (44,45) found that either of these interpretations could explain the results of his eminent work on adsorption measurements. The importance of the dependence of disorder in the surface layer on temperature has recently been stressed by Volkenstein (46).

IV. THEORY OF THE REDUCTION PROCESS During the reduction process the oxide state of the catalyst described in Section I1 is transformed into the reduced state described in the


19

preceding section. A recent article by Hall, Tarn, and Anderson (47) gives information about the variation of surface area, CO chemisorption, and of densities measured by mercury and by helium, with the degree of reduction. Two KzO-MgO-Crz03-promoted catalysts were investigated. When the reduction was carried out a t 450°, the average pore diameter, calculated as the diameter of cylindrical open-end pores, remained approximately constant from 20 to 95% reduction. An average pore diameter of 330 A. was found in this period, and a final pore size of 370 A. was found in the completely reduced catalyst. This is in satisfactory agreement with the crystallite size of 360 A. determined by x-rays for the K20-Ca0-AI2O8-promotedcatalyst. The surface area increases linearly with the extent of reduction until reduction is nearly complete; however, maximum surface was reached shortly before reduction was completed. CO chemisorption increasing from the beginning proportionally to the total surface exhibits a marked increase from 80% reduction upwards. X-ray investigations of partly reduced catalysts have been carried out by the author (Nielsen, 5 ) . As will be seen from the two patterns of partly reduced samples of the doubly promoted catalyst A which are reproduced in Figs. 4 and 5 on Plate I, the reduction proceeds directly from spinel to iron, and iron crystallites exist at an early stage of reduction. Figure 4 shows the pattern of a catalyst reduced to the point at which noticeable ammonia formation sets in, while Fig. 5 shows the pattern of a catalyst somewhat further reduced. As will be seen, the iron lines are much more fuzzy than the magnetite lines, which from our knowledge about reduced catalysts can be supposed to mean that the iron crystals formed are of small dimensions. Mosesman (47a) has made in situ x-ray diffraction studies of reduction and reoxidation of an ammonia catalyst. It seems unquestionable that when reduction starts at a certain point, a part of a magnetite particle is reduced very quickly. Whether the regions transformed quickly on the start of the reduction procedure are of the size of up to 30 microns that was found by Wilchinsky to be the upper limit of single celled particles, or whether they are much smaller mosaic blocks cannot be decided from the available data. In the first case a number of iron crystallites could result from each initiated reduction; in the second case it would most likely produce only one single iron crystallite. As seen by comparing surface areas and CO chemisorption, the iron crystallites are not completely reduced in the surface at this stage. The final reduction of the surface iron sites of high free energy has to be enforced slowly and requires a high ratio of hydrogen to water vapor in the gas, each ratio leading to reduction of sites of an excess of free energy up to a certain limit. When increasing amounts of ions which cannot participate in an

20

ANDERB NIELSEN

electric exchange between Fe" and Fe"' are substituted into the lattice of the magnetite, the electric resistance increases (Verwey, Haaijman, Romeijn, and Oosterhout, 48). It is a well-known fact that increasing amounts of promoters like CaO and MgO make the catalysts difficult to reduce. On the other hand, it seems to be one of the functions of CuO in iron Fischer-Tropsch catalysts to secure a high degree of reduction (Kolbel, 49). Oxides as CuO and NiO, as has been mentioned previously, have a deleterious effect on ammonia catalysts.

V. KINETICSOF

THE

SYNTHESIS AND DECOMPOSITION OF AMMONIA

1. Energetics of the Surface of the Catalyst

The very complex picture of the surface formed by compiling the information discussed above would appear even more intricate if by some means it were possible to take a look at the surface when ammonia synthesis is going on. In the surface we have the promoters in a high degree of dispersion. When KzO an Aln03are present in equivalent amounts, they have reacted. We do not know for certain whether more oxygen is left in the catalyst than equivalent to the promoter molecules. From the experiments by Almquist and Black, and McGeer and Taylor we know that removal of the oxygen latest removed from the surface increases the activity of the catalyst considerably both for ammonia synthesis and for nitrogen exchange. It is a near conclusion that the surface atoms of highest excess of free energy are those at which the ammonia synthesis takes place with highest frequency. However, alternative explanations of the experimental observations can be brought forward. It may be that the last small amounts of oxygen to he removed from the lattice change the valence properties of the surface atoms considerably. Had it not been for the large mean pore size, oxygen atoms a t strategic points may be considered to block extensive areas. The first two of these explanations are considered the more appropriate ones, each of them having relation t o one activation effect by the promoters. As proposed by Frankenburg* regions containing unreduced Fe304may well be the sites of promoter action in nonpromoted catalysts. As previously mentioned, the promoters render the catalysts more difficuit to reduce and their effect seems to be a double one, being related to the bulk reduction as well as to the final clean up of the surface. The promoters are assumed to create and stabilize surface sites of high exce8D of free energy. Fe, Mo, W, Re, and 0s mentioned in the introduction as ammonia

* W. G . Frankenburg.

PrivaEe communication.


21

catalysts all belong to the transition elements, and according to Pauling (50), they all have dsp-hybrid bond orbitals of a rather high d-character. Frankenburg (2) discussed the heat of formation of nitrides and their relation to the catalytic activity. Iron has a heat of formation of FezN of approximately 3 kcal., and according to the data discussed on page 18, the active surface energy band should form FezN with a lower limit of the heat of formation of approximately 11 kcal. As pointed out by Frankenburg the following steps in the synthesis reaction probably have low energy of activation as the heat of formation of ammonia is not much different from that magnitude. It is easily seen that V, although a transition metal, cannot be an active ammonia catalyst, as its nitride, VN, is formed with a heat of 78 kcal. and the following steps, therefore, would be slow. Whereas it seems justified to assume that the most unsaturated parts of the surface of iron catalysts are the most active in the synthesis, it may well be that on Mo and W, which are supposed to bind nitrogen stronger, an intermediate part of the energy spectrum functions most actively in the catalytic reaction, and it may therefore be more nearly correct t o say that in this case surface nitrides are the catalysts. If we can consider the most unsaturated sites on the iron surface as being also the most active ones for the synthesis, it is justified to speak of nitrogen chemisorption as the rate-determining process. Brunauer and Emmett have shown that in the temperature range for ammonia synthesis nitrogen is chemisorbed as atoms. The author found the lattice constant of the catalyst crystallites, after use for synthesis, to be exactly equal to that of pure a-iron, which means that the lattice showed no signs of expansion by a retainment of nitrogen. Chr6tien and Mathis (51) have shown that at a constant temperature in the temperature range for ammonia synthesis FezN will slowly give up its nitrogen, with this nitrogen release slowing down temporarily a t the stages Fe3N and Fe4N. Krichevskii and Khazanova ( 5 2 ) have calculated the nitrogen pressure in equilibrium with the eiron nitride a t 400-600” to be approximately 13,000 atm. Although nitrogen is chemisorbed as atoms, it may well be that especially on a surface of high degree of coverage, resonating bonds may establish fractional valencies between these nitrogen atoms. During synthesis we have to consider the influence of hydrogen, NH radicals, and ammonia on the surface. So it is very probable that a nitrogen molecule in the process of chemisorption will not split into nitrogen atoms but go directly into N H radicals if hydrogen is chemisorbed on neighboring sites, Christiansen and Knuth (53) have shown that the N H radical plays an important role in the decomposition of ammonia on a quartz surface, and they emphasized that the NH radical is an “even” radical and, conse-

22

ANDERS NIELSEN

quently, may have a fairly long lifetime. * In earlier work, Frankenburg and Hodler (53a) proved the formation of NH radicals during ammonia decomposition on tungsten surfaces. 2. Isotopic Exchange on Iron Catalysts

As has been mentioned an ammonia synthesis catalyst must be capable of activating, the nitrogen molecule which according t o most recent statements is supposed t o have a heat of dissociation of 225 kcal. (Kistiakowsky, Knight, and Malin, 54). It has been proved by Taylor and Jungers (55) by way of elimination that the rate-determining step in the ammonia synthesis reaction on a promoted iron catalyst must be the step in which nitrogen is taken up by the catalyst surface. This was proved by experiments showing ready exchange between ammonia and deuterium a t room temperature, progressing to the formation of all deutero-ammonias. Emmett and Brunauer (56,57) from measurements of adsorption and synthesis rates have shown that the adsorption of nitrogen is sufficiently fast to be the first step of the reaction. Kinetics of the ammonia-deuterium exchange has recenily been investigated by Weber and Laidler (58), who used a microwave technique. They concluded that the exchange takes place between ammonia and deuterium both adsorbed on the surface, and that a t low deuterium pressures only one deuterium atom enters the transition state of the exchange reaction. The conditions prevailing on the surface of a promoted iron catalyst are still better elucidated when the experiments on exchange between nitrogen isotopes are considered. Kummer and Emmett (59) found the rate of exchange on singly and doubly promoted catalysts in agreement with the observed rate of nitrogen desorption assuming that the desorbed gas is in equilibrium with regard to the exchange reaction. This assumption was shown to be justified by experiments proving that the nitrogen atoms migrate rapidly over the entire surface, or to be quite correct, on all parts degasified by pumping for 16 hours a t 500". Kummer and Emmett found acceleration of the exchange by hydrogen. The effect by hydrogen was, however, less pronounced than that observed previously by Joris and Taylor (60) when investigating the exchange between nitrogen isot,opes on a surface that had not been completely reduced. When the catalyst is not completely reduced, the exchange is much slower. It is not quite clear whether the acceleration by hydrogen in this case is due t o an easier migration of NH radicals than of nitrogen atoms, or the hydro* Sastri and Srikant (53b) have found strong evidence of complex formation during mixed adsorption of Ht and Na at elevated pressures on a Fe-Kz0-A1~O~-TiO2 catalyst.


23

gen promotes breaking of the nitrogen-nitrogen bond. In the author’s opinion the experiments on totally reduced catalysts and on nearly reduced catalysts when viewed in conjunction are thus illustrative of the conditions on the surface. 3. Kinetic Equations for Ammonia Synthesis and Decomposition

The free-energy spectrum of the adsorption sites of reduced promoted catalysts is not known in detail. Nor do we know the band of the spectrum active for synthesis and decomposition under definite conditions of temperature, pressure, ratio of hydrogen to nitrogen, etc. The sites most active under given conditions are not necessarily the most active a t all conditions, a point calling for caution when experimental activation energies are calculated. The facts that the last few per cent of the reduction greatly enhance the activity, and on the other hand poisoning with small amounts of oxygen diminish it greatly, may indicate that the sites of highest free energy are the most active. However, it should be remembered that the total number of surface iron atonis is only a small fraction of all iron atoms. A quantitative correlation of oxygen retainment and activity over a wide range of oxygen concentrations would be very indicative. The stepwise increase in activity encountered with stepwise improvement of the gas purification suggests that the most unsaturated sites, if uncovered, exert a high activity. As previously discussed the removal of oxygen from the surface may cause a change of the electronic properties in the total surface of the iron crystallite in question. We also know that chemisorption of reactants and intermediates influences the energy picture. From the exchange reactions between isotopes we know that N, NH, and H migrate easily, but as the surface is heterogeneous this migration could hardly be unrestricted, and calculation of the entropy of chemisorption of nitrogen (Kemball, 61) has shown the adsorbed nitrogen to be localized. The possibility that nitrogen and hydrogen atoms located in the catalyst surface may exert mutual bond forces may well be a further complicating factor. It is almost surprising that in spite of this a simple kinetic equation has been formulated which gives a good approximation to experimental data under widely different conditions of temperature, pressure, gas composition, and space velocity and for different types of catalysts. An equation has been suggested by Temkin and Pyzhev (62,63) and the simple form in which it was originally presented is given in Equation (I)

24

ANDERS NIELSEN

I n Equation ( l ) ,w is the conversion rate, i.e., the difference between synthesis and decomposition rates, k, and k2 are the rate constants for synthesis and decomposition respectively, and a and P are constants.

N2

+ 3Hz + 2NH3 k,/k, = K

a+p=1

Temkin and Pyzhev take as their starting point a paper by Winter (64). Winter studied the decomposition of ammonia with excess hydrogen over a n iron catalyst and found the expression (5)

which is not much different from (1) when p = 0.5. Winter concluded that the slow step in the decomposition is the desorption of nitrogen and that equilibrium exists according to (6).

+

NHa (gas) + N(*ds) 1.5Hz (gas)

(6)

Winter’s treatment leading from (6) to ( 5 ) is not correct, as pointed out by Temkin and Pyzhev; one of its consequences would be an activation energy for the decomposition amounting to approximately 100,000 cal. Following Temkin and Pyzhev, the empirical equation (1) when combined with the assumption of equilibrium according to (6) leads to a logarithmic adsorption isotherm for nitrogen

in which 8 is the fraction of surface sites covered, p is the pressure of nitrogen, and f and a0 are constants. This calls for a linear dependence of the heat of adsorption of nitrogen upon the degree of coverage of the sites of the band, active in the reaction. This treatment also involves the fact that differences in activation energies between different sites are proportional to differences in heats of adsorption on these sites and that the proportionality factor is a. Temkin and Pyzhev state th a t a is approximately 0.5. When (1) is integrated for use for flow reactors, assuming a = 0.5, simple equations result, which have been checked when using doubly and triply promoted catalysts showing excellent t o fairly good agreement even a t high pressures and widely different compositions of synthesis gas. A number of experimental data will be quoted below. Brunauer, Love, and Keenan (65) measured adsorption and desorption rates of nitrogen and derived an adsorption isotherm for a


25

doubly promoted catalyst. As reported by Emmett and Kummer (66) those measurements lead to 0 = 0.276(a = 0.724), equivalent t o the rate equation (8)

Equation (8) is in satisfactory agreement with measurements by Love and Emmett (67), who used the same catalyst. Brill (67a) has found Equation (1) valid for a doubly promoted, a singly Alz03-promoted,and a nonpromoted catalyst a t temperatures between 300" and 350" using a-values between 0.6 and 0.7, depending upon the composition of the catalyst. One of the consequences of (1) and a = 0.5 is th a t when we can disregard the backward reaction, the conversion will be maximum for the gas composition P,,/P,, = 1.5. Temkin and Pyzhev tested this conclusion using a Kz0-A12O3-promoted catalyst, 1 atm., 400°, S.V. = 30,000, and ratios of hydrogen t o nitrogen from 0.5 to 8.5. They found excellent agreement with the theoretical prediction. The author repeated this procedure, using a triply promoted catalyst; the results are shown in Table 111. TABLE I11 K20-Ca0-Aln03-Promoted Catalyst E Conversion percentage as a function of PHp/PNpat 400",1 atm., and S.V. = 30,000 PEa

Per cent ammonia in exit gas

0.5 1 .o 1.5 2.0 3.0

0.032 0.045 0.048 0.038 0.031 0.030 0.022

PNX

6.0 12.0

Another consequence of (1) and a

=

0.5 is that at low conversions,

z 2 (S.V.) ought to be constant, when z is the ammonia percentage in the

exit gas. Series of experiments a t 330 atm. have indicated that when the space velocity is increased t o values above 100,000, z 2 (S.V.) although not constant approaches a limiting value. I n some cases, the author found a decrease of z2 (S.V.) a t very high space velocities, but in such experiments the initial values ot lower space velocities could not be reproduced, which showed that the catalyst had been damaged. Temkin and Pyzhev also checked Equation (l),using a = 0.5 on the experimental data published by Larson and Tour (68), which cover a range of pressures up to 100 atm., and later Emmett and Kummer (66)

26

ANDERS NIELSEN

extended the investigation to 3: 1, 1 : 1, and 1 :3 hydrogen-nitrogen mixtures at 370, 400, and 450' and 33.3, 66.6, and 100 atm. The constancy of the rate constant at different space velocities and gas compositions was fairly good. Sidorov and Livshits (69) have shown the validity of expression (1) (a = 0.5) for a doubly promoted catalyst a t 300 atm. and 425-525", but using a 3: 1 hydrogen to nitrogen mixture only. The rate constant at 450"and 300 atm. was determined to be k T - - P = 0.94 lo4, and the apparent energy of activation of the decomposition to be 42,000 cal. Temkin (70) has recently modified Equation (1) to make it applicable for high pressures. The new Equation (9)

is derived from (1) by substituting partial pressures with fugacities and further introducing the factor e - - ( ~ n - a ~ a ) p / R Tin, which P, and 7,are the molar volumes of adsorbed nitrogen and of the transition state, and P is the total pressure. The appropriate values of and 7. are, of course, open to question. By use of activity coefficients according t o Newton (71) and by assuming Paand 7,to be equal to the molar volume of solid nitrogen, Temkin calculates that the value of = 0.94 * lo4found by Sidorov and Livshits at 450' and 300 atm. is equivalent to kT--P = 1.25 * lo4 a t 450" and 1 atm. This value should be compared with the value of 1.15 lo4found by Temkin, Kiperman, and Lukjanova (72) who recently investigated the kinetics over the same catalyst by a flow-circulation method. de Bruijn (73) in a somewhat different approach to the problem takes the same starting point as Temkin, assuming that all processes except adsorption and desorption of nitrogen may be considered to be in equilibrium. Adsorption and desorption rates are formulated for all types of sites, assuming adsorption and desorption rates on i-sites proportional to (1 - BJ2 and ei2 respectively and using Langmuir isotherms for each type of sites. By integration over all types of sites this leads to an equation which except, for an inhomogeneity factor F = [ea(1 - e)@pconforms to Equation (1). When deducing the rate expression it is assumed that adsorption and desorption rates of nitrogen are uninfluenced by adsorbed H and NH. As previously mentioned, Emmett and Brunauer found that this was not the case for the singly promoted catalyst in agreement with results by Love and Emmett (67), showing that decomposition on the AlzOrpromoted catalyst does not follow the TemkinPyzhev equation (unless assuming a negative value of a).

v,

VI. CONVERSION EFFICIENCY AT DIFFERENT REACTION CONDITIONS The dependence of the synthesis conversion upon temperature, pressure, space velocity, and ratio of hydrogen to nitrogen reported below


27

has been measured for a triply K~0-Ca0-Alz03-promoted commercial catalyst E of moderate activity and high thermal stability (Nielsen, 5 ) . Similar data for other catalysts are found in the same reference. The data can be considered relevant for the changes in performance t o be expected from changes of synthesis conditions in many industrial installations. By conversion percentage will be understood the percentage of ammonia by volume in the exit gas. By catalyst efficiency is understood, according to Larson and Tour (68)) “the ratio of the actual percent ammonia in the gas to that theoretically possible under the conditions in question.” Equilibrium percentages used for calculation of efficiencies have been obtained from a nomograph constructed by Toniolo and Giammarco (74). 1 . Dependence upon Temperature The effect of a change in temperature is a double one, as it affects both equilibrium percentage and reaction rate. While catalyst efficiency XNH3

t 24 22

I

I

n

I

20

s.y.:

18

14

I

d

3.50

*oo

450

so0 Y _c

FIG. 12. Dependence of ammonia percentage upon temperature. 330atm., H2:Nz = 3:l.

Catalyst E:

always increases with increasing temperature provided no catalyst deterioration takes place, conversion percentages go through a maximum when the temperature is increased. In Fig. 12 are shown the conversion percentages as a function of temperature in the interval from 400” to 500”, this being the most important temperature interval from an industrial point of view at pressures around 330 atm., for which pressure catalyst E

28

ANDERS NIELSEN

is especially adapted. As seen from Fig. 12 the maximum conversion was obtained a t a temperature not far from 450", somewhat dependent upon the space velocity used. 2. Dependence u p o n Pressure

The equilibrium ammonia percentage increases with pressure and so does the rate of conversion. Hence, it is impossible to know beforehand

I 100

1

I40

J80

220

260

SO0

S40

FIG. 13. Dependence of ammonia percentage upon pressure. 450"C., S.V. = 15,000, Hz:NZ = 3 : l .

Catalyst E:

how conversion efficiency will change with pressure. The author found that conversion efficiency decreased some 10% when the pressure was increased from 130 t o 360 atm. This observation is i n accordance with the results found by Larson and Tour (68) for the pressure range from 1 t o 100 atm. Figure 13 illustrates the dependence of ammonia percentage upon pressure. 3. Dependence u p o n Space Velocity

As previously discussed, the theory by Temkin and Pyzhev predicts that a t low conversions, the ammonia percentages in the reacted gas vary inversely proportional to the square root of space velocities. At higher conversions the decrease in conversion percentages with increase in space velocity is still less, as is clearly seen from Table I V showing conversion percentage, efficiency, z 2 (S.V.), and S.T.Y. as functions of S.V. a t 450' and 330 atm.

29


TABLE IV Ammonia Percentage, Esciency, z2 (S.V.),and S.T.Y. as Functions of S.V. at 450' and 330 atrn. Catalyst E 22

S.V.

Ammonia, %

Efficiency

25.7 24.5 22.5 21.5 20.4 19.6 19.2

0.67 0.64 0.59 0.56 0.53 0.51 0.50 0.49 0.43 0.34

1,400 3,100 7,900 15,500 22,300 29,800 36,800 43,500 71,000 129,000

19.0

16.5 12.9

S.T.Y., kg. NH,/l. cat. /hr.

(S.V.), % NH,?

= ___ 100

0.22 0.43 1.03 1.94 2.67 3.46 4.18 4.92 7.11 10.45

92 186 400

720 930 1120 1360 1570 1930 2130

As is evident from Fig. 14, the ammonia percentage a t high conversions is much more influenced by a change in space velocity at low temperatures than a t higher temperatures. It is also evident from Fig. 14

I

o

10

zw

30

40

50

60

70

80

90

ioo

iio

FIG.14. Dependence of ammonia percentage upon space velocity. 330 atm., H 2 : N 2= 3:l.

im

sLFio7

Catalyst E:

that using catalyst E a t 330 atm. a higher conversion is obtained a t a temperature of 450' than a t temperatures of 400" or 480'. It should be kept in mind, however, that the reaction in an industrial converter is never isothermal and that, owing t o the release of the heat of reaction, an unfavorable temperature distribution tends to develop.

30

ANDERS NIELSEN

4. Dependence upon Composition of Synthesis Gas a. Ratio of Hydrogen to Nitrogen. I n Figs. 15 and 16 are shown conversion percentage and efficiency as functions of the proportion of hydrogen to nitrogen in a pure hydrogen-nitrogen mixture used as the synthesis gas. In agreement with the concept that nitrogen adsorption is the rate-determining process, the maximum conversion percentage was not obtained at a ratio of 3 : 1, but at a higher nitrogen content. X NH. [

t 21

iy)

I9

I8

I7

I6

Fro. 15. Dependence of ammonia percentage upon ratio of hydrogen to nitrogen. Catalyst E: 450"C., 330 atm., S.V. = 15,000.

b. Content of Inert Gases. When inert gases make up part of the total pressure of the synthesis gas, this diminishes the equilibrium ammonia percentage. When compensating for this effect by increasing the pressure, a decrease of the rate constant of the reaction results, as discussed above. The equilibrium percentages in the various cases may be obtained from the nomograph by Toniolo and Giammarco (74), and Pastonesi (75) has given an estimate of the percentage of inert gases in the circulating gas of a synthesis unit operating without purge from the amount of inert gases in supply gas. In this paper, Pastonesi makes use of solubility data of argon in liquid ammonia. c. Catalyst Poisons. Catalyst poisons may diminish activity of the catalysts by blocking the active sites. This blocking can be of a


31

macro or micro type (RoginskiI, 76). It is appropriate to distinguish between permanent poisons causing a lasting irreversible reduction of the catalyst activity and temporary poisons causing a temporary reduction in activity only, while the initial activity is restored in a relatively short time after removal of the poisonous compound from the gas. I n ammonia synthesis we have to deal with permanent poisons as well as with temporary poisons. The most important group of poisons in ammonia synthesis are oxygen-containing compounds. These act as strong

I

0

I

2

3

4

s

6-H

FIG.16. Dependence of efficiency upon ratio of hydrogen to nitrogen. E: 450"C., 330 atm., S.V. = 15,000.

Catalyst

temporary poisons. Apart from this they cause a much smaller permanent decline in activity, which is really due to a rebuilding of the lattice. This rebuilding takes place most easily at higher temperatures. When the oxygen-containing compound is removed from the synthesis gas the catalyst will be fully reduced, but all the iron sites or regions oxidized do not completely revert to their initial state. Even at a stationary oxygen concentration reduction and reoxidation of iron sites is going on continuously. Equivalent concentrations of oxygen, water vapor, carbon monoxide, and carbon dioxide show equal deterioration effects at normal synthesis conditions. It has been shown by the author that such catalysts which have an outstanding thermal resistancy are also less sensitive toward the permanent effect of oxygen compounds. I n Table V

32

ANDWRS NIELSEN

are shown poisoning experiments with the two triply promoted catalysts KM I and KM 11. As will be discussed later, KM I has exceptionally high thermal stability, while KM I1 is less thermostable, but of exceptionally high activity.

S.V. =

TABLE V E$ect of Oxygen on Conversion Percentage 16,000, pressure 330 atm., and constant temperature 450' Ammonia, %

Catalyst

KM I K M I1

Initial conversion, pure gas

25.0 26.1

After 6 hr. run with gas containing 1000 p.p,m. O2

After 24 hr. run with pure gas

5.8 7.4

24.9 24.4

Hydrogen sulfide causes a permanent poisoning of iron catalysts. Methane does not poison ammonia catalysts under normal synthesis conditions. Equilibrium data (Browning, De Witt, and Emmett, 77; Browning and Emmett, 78) should be mentioned in this connection.

VII. THEORY AND PRACTICE IN INDUSTRIAL UTILIZATION 1. Composite Catalyst Bed

Even when operated on pure synthesis gas, ammonia catalysts do not retain their activity unchanged as time goes on. This decline in activity is a consequence of a rebuilding of the catalyst structure caused, among other factors, by recrystallization. This effect goes on much more quickly at high than at low temperatures; 500-550" should be considered high temperatures in this respect, while 400" is a low temperature. The rebuilding is accelerated by oxygen compounds, as mentioned above. As it is not possible to combine in one catalyst the highest degree of activity with the highest thermal stability it is preferable to take advantage of the fact that different types of catalysts can be developed suitable for the zones of lower and higher temperatures. In an earlier paper (Nielsen, 5 ) the author has used the term thermoresistancy to characterize the quality of the catalyst to keep its activity, and a certain set of experimental conditions were standardized as thermoresistancy test conditions. For catalysts designed for use at about 450°, it was chosen to measure the difference between the activities measured, at 450", 330 atm., and S.V. = 16,000, before and after a 20-hour run at 550". I n Table VI are shown the results of thermoresistancy tests on the

33


two triply promoted catalysts KM I and KM 11; the results should be compared with the results of poisoning experiments with the same catalysts, which are shown in Table V. From these data it is evident TABLE VI Thermoresistancy Tests on Catalysts K M I and K M I I

Catalyst

Ammonia Thermoresistancy percentagetest conditions catalyst prior to thermore- Temp., Pressure, sistancy test0 "C. atm. Hours

KM I KM I1 1 = 450"; p =

25.9 27.3

550 550

150 150

20 20

Ammonia percentagecatalyst after thermoresistancy test'

Decline in ammonia percentage - A % NHI

25.9 25.0

2.3

0.0

330 atrn.; S.V. = 16,000.

that the catalyst KM I, although less active than KM 11, is better suited to operation at high temperatures. 9. Reduction Procedure

The method of reduction influences the properties of ammonia catalysts. A generally appropriate reduction schedule cannot be prescribed because different types of catalysts call for different reduction procedures to reach their most active state. It has previously been mentioned that the promoters used in ammonia catalysts have a retarding effect on the reduction. According to the author's experience, oxides of the alkaline earth metals, especially CaO, make the catalysts especially difficult to reduce. As will be remembered these oxides enter the magnetite matrix readily. An important series of papers on the influence of temperature, pressure, space velocity, and gas composition during the reduction period has been published by Latchinov and co-workers. Latchinov (79) divides the reduction in an induction, an "autocatalytic," and a residual reduction period. Latchinov and Wwedenski (80) and Latchinov (81,82) examined the temperature range from 375 to 500". They state that other factors being equal, an increase in the reduction temperature reduced the activity of the catalysts and, naturally, also the time required for the reduction. Concerning pressure, Latchinov and Wwedenski (80) state that catalyst activity is almost independent of the reduction pressure, a small negative influence from increase in pressure could easily be compensated by using higher space velocity. Concerning space velocity, Latchinov and Wwedenski state that the higher it is, the more active the

34

ANDERS NIELSEN

catalyst will be, and they suggest the use of space velocities in the range 15,000-30,000. Latchinov (79) when investigating a Kz0-Al2O3-promoted catalyst made the interesting observation that during reduction at 100 stm., 500°, and S.V. = 90,000, the catalyst gave a higher conversion a t a reduction degree of 30% than when completely reduced. This observation, as pointed out by Latchinov, may be explained either by the rebuilding accompanying re-oxidation-re-reduction processes or by the disappearance of supporting or promoting Fe304. During the large number of reductions performed by the author and his co-workers this phenomenon has never been encountered; it should be mentioned, however, that increasing temperatures terminating at 450" were used. The author supports the view of Latchinov and co-workers that all conditions tending to increase the concentration of water vapor in the gas during the reduction diminish the activity of the catalysts, thus high space velocity and low temperature and low pressure yield the most active catalysts. Water vapor influences areas of the catalyst already reduced in the same way as water vapor influences a totally reduced catalyst, which means that it inflicts it with a progressive permanent deterioration. Reduction temperatures have to be kept below a certain limit to avoid a too rapid reduction, which results in too high concentrations of water vapor in the gas, and to avoid rebuilding of such parts of the catalyst already reduced, this rebuilding being accelerated by water vapor. Reduction temperatures should always be kept below the temperatures a t which the catalyst is subsequently going to operate. This means for the technical reduction schedule that such catalyst layers which during operation shall work at a temperature between, say, 350' and 400' must be reduced at temperatures not exceeding 400°, nd advantageously they are reduced at a somewhat lower temperature. As to pressure, i t may be as high as the working pressure of the synthesis, provided the space velocity is sufficiently high. The higher the space velocity, the higher the starting activity of the catalyst will be. Frequently, space velocities down to about 1000 are used and, provided sufficient time is used for the reduction, only a slightly lower activity will be obtained than at S.V. 15,000. When utilizing circulating gas for the reduction, as is the case in most industrial installations, it is very important that when reduction has started, the circulating gas is cooled almost down to zero centigrade, as otherwise a gas with a high concentration of water vapor reenters the catalyst bed. In this connection it should be emphasized that the saturation concentration of water vapor in the gas is much higher than

-


35

would be expected from the vapor pressure-temperature curve, as the activity coefficients in the compressed gas mixture are far from unity. As soon as some formation of ammonia has started, this permits the cooling down of the circulating gas even below zero, and as the liquid ammonia extracts water from the gas, the concentration of water vapor is reduced to an innocuous level.

REFERENCES 1. Emmett, P. H., Fixed Nitrogen. Edited by H. Curtis. Chem. Catalog Co.. New York, 1932. 2. Frankenburg, W. G., 2. Elektrochem. 39, 45, 97, 269 (1933). 3. Almquist, J. A., and Crittenden, E. C., Ind. Eng. Chem. 18, 1307 (1926). 4. Bridger, G. L., Pole, G. R., Beinlich, A. W., and Thompson, H. L., Chem. Ens. Progress 43,291 (1947). 5. Nielsen, A., An Investigation on Promoted Iron Catalysts for the Synthesis of ammonia. Jul. Gjellerups Forlag, Copenhagen, 1950. 6. Telegin, V. G., Sidorov, N. V., and Shpulenko, K. B., J. Applied Chem. (U.S.S.R.) 13,823 (1940); C . A . 36, 2286 (1941). 7. Wyckoff, R. W. G., and Crittenden, E. D., J . Am. Chem. Soc. 47, 2866 (1925). 8. Clark, C. L., Ally, A., and Badger, A. E., Am. J . Sci. 22, 539 (1931). 9. Lihl, F., Monatsh. 81, 632 (1950). 10. BBnard, J., Compt. rend. 206, 912 (1937); Bull. SOC. chim. (France), D,80 (1949). 11. Rooksby, H. P., and Tombs, N. C., Nature 167, 364 (1951). 12. de Boer, F., van Santen, J. H., and Verwey, E. J. W., J . Chem. Phys. 18,1032 (1950). 13. Verwey, E. J. W., and de Boer, J. H., Rec. t r w . chim. Pays-Bas 66,531 (1936). 14. NBel, L., Ann. phys. [12] 3, 137 (1948). 15. NBel, L., Ann. phys. [12] 4, 249 (1949). 16. NBel, L., Compt. rend. 228, 64 (1949). 17. Klemm, R., T. 0. M. Microfilm Reel 278, Item 2; U.S. Dept. Commerce, Off. Tech. Serv., PBL 82, 353. 18. Wilchinsky, 2. W., Anal. Chem. 21, 1188 (1949). 19. Maxwell, L. R., Smart, J. S., and Brunauer, S., J . Chem. Phys. 19, 303 (1951). 20. Brill, R., 2. Elektrochem. 38, 669 (1932). 21. Edwards, 0. S., and Lipson, H., Proc. Roy. SOC.(London) A180, 268 (1942). 21a. Hofer, L. J. E., Peebles, W. C., and Bean, E. H., J . Am. Chem. SOC.72, 2698 (1950). 22. Preston, 0. D., Proc. Roy. SOC.(London) A167, 526 (1938). 23. Daniel, V., and Lipson, H., Proc. Roy. SOC.(London) A181, 368 (1943). 24. Hargreaves, M. E., Acta Cryst. 4, 301 (1951). 25. Brunauer, S., and Emmett, P. H., J. Am. Chem. Soc., 62, 1732 (1940). 26. Emmett, P. H., and Brunauer, S., J. Am. Chem. SOC.69, 310 (1937). 27. Brunauer, S., Emmett, P. H., and Teller, E., J. Am. Chem. SOC.60, 309 (1938). 28. Nielsen, A., and Bohlbro, H., J. Am. Chem. SOC.74,963 (1952). 29. Beeck, O., Advances in CataEysis 2, 151 (1950). 30. Emmett, P. H., and Brunauer, S., J. Am. Chem. SOC.69, 1553 (1937). 31. Kummer, J. T., and Emmett, P. H., J. Phys. & Coololoid Chem. 66,337 (1951). 32. Gregg, S. J., Surface Chemistry. SocibtB De Chimie Physique and Faraday Society joint meeting. Butterworths Scientific Publications, 1945.

36

ANDERS NIELSEN

33. Jura, G., Loeser, E. H., Basford, P. R., and Harkins, W. D., J . Chem. Phys. 14, 117 (1946). 34. J w a , G., Harkins, W. D., and Loeser, E. H., J . Chem. Phys. 14, 344 (1946). 34a. Boudart, M., J . Am. Chem. SOC.74, 3556 (1952). 3ib. Weisz, P. B., J . Chenr. Phys. 20, 1483 (1952). 35. Emmett, P. H., and Harkness, It. W., J . Am. Chem. Sac. 67, 1631 (1935). 35a. Sastri, M. V. C., and Srikant, €I., Cum. Sci. 19, 313 (1950). 36. Brewer, L., and Mastick, D. F., J . A m . Chem. Sac. 73, 2045 (1951). 37. Taylor, H. S., Advances zn Catalysis 1 (1948). 38. Almquist, J. A., and Black, C. A., J . A m . Chem. Sac. 48, 2814 (1926). 39. Eastman, E. D., and Evane, R. M., J . Am. Chem. SOC. 46, 888 (1924). 40. Almquist, J. A., J . Am. Chem. Sac. 48, 2820 (1926). 41. McGeer, J. P., and Taylor, Ii. S., J . Am. Chem. SOC.73, 2743 (1951). 42. Kummer, J. T., and Emmett, P. H., J . Am. Chem. SOC.73, 2886 (1951). 43. Beebe, R. A., and Stevens, N. P., J . Am. Chem. SOC.62, 2134 (1940). 44. Frankenburg, W. G., J . Am. Chem. SOC.66, 1827 (1944). 45. Frankenburg, W. G., J . Am. Chem. Sac. 66, 1838 (1944). 46. Vol’kenshtein, F. F., J . Phys. Chem. (U.S.S.R.) 23, 917 (1949). 47. Hall, W. K., Tarn, W. H., and Anderson, R. B., J . Am. Chem. SOC.72,5436 (1950). 47a. Mosesman, M. A., J . Am. Chem. SOC.73, 5635 (1951). 48. Verwey, E. J. W., Haaijman, P. W., Romeijn, F. C., and van Oosterhout, G. W., Philips Research Repls. 6, 173 (1950). 49. Kolbel, H., Chem.-Zng.-Technik 23, 153 (1951). 50. Pauling, L., Proc. Roy. SOC.(London) A196, 343 (1949). 51. ChrBtien, A., and Mathis, M., Campt. rend. 228, 91 (1949). 52. Krichevskii, I. R., and Khazanova, N. E., Doklady Akad. Nauk S.S.S.R. 71, 677 (1950); C.A. 46, 1410 (1951). 53. Christiansen, J. A,, and Knuth, E., Kgl. Danske Videnskab. Selskab, Mat.-fys. Medd. 13, [12] (1935). 53a. Frankenburger, W., and Hodler, A., Trans. Faraday Sac. 28, 229 (1932). 53b. Sastri, M. V. C., and Srikant, H., C w r . Sci. 20, 15 (1951). 54. Kistiakowsky, G. B., Knight, H. T., and Malin, M. E., J . Am. Chem. SOC.73, 2972 (1951). 55. Taylor, H. S., and Jungers, J. C., J . A m . Chem. SOC.67, 660 (1935). 56. Emmett, P. H., and Brunauer, S., J . Am. Chem. SOC.66, 1738 (1933). 57. Emmett, P. H., and Brunauer, S., J . Am. Chem. Sac. 66, 35 (1934). 58. Weber, J., and Laidler, K. J., J . Chem. Phys. 19, 381 (1951). 59. Kummer, J. T., and Emmett, P. H., J . Chem. Phys. 19, 289 (1951). 60. Joris, G. G., and Taylor, H. S., J . Chem. Phys. 7, 893 (1939). 61. Kernball, C., Advances in Catalysis 2, 233 (1950). 62. Temkin, M. I., and Pyzhev, V., Acta Physicochim. (U.R.S.S.) 12, 327 (1940). 63. Temkin, M. I., and Pyzhev, V., J . Phys. Chem. (U.S.S.R.) 13, 851 (1939). 64. Winter, E., 2. physik. Chem. B13, 401 (1931). 65. Brunauer, S., Love, K. S., and Keenan, R. G., J . Am. Chem. Sac. 64, 751 (1942). 66. Emmett, P. H., and Kummer, J. T., Znd. Eng. Chem. 36, 677 (1943). 67. Love, K. S., and Emmett, P. H., J . Am. Chem. SOC.63,3297 (1941). 67a. Brill, R., J . Chem. Phys. 19, 1047 (1951). 68. Larson, A. T., and Tour, R. S., Chem. Met. Eng. 26, 647 (1922). 69. Sidorov, I. P., and Livshits, V. D., J . Phys. Chem. (U.S.S.R.) 21, 1177 (1947). 70. Temkin, M. I., J . Phys. Chem. (U.S.S.R.) 24, 1312 (1950).

L A T E S T DEVELOPMENTS I N AMMONIA S Y N T H E S I S

37

71. Newton, R. H., Ind. Eng. Chem. 27, 302 (1935). 72. Temkin, M. I., Kiperman, S. L., and Lukjanova, L. I., Doklady Akad. Nauk S.S.S.R. 74, 763 (1950); C.A. 46, 1854 (1951). 73. Bruijn, H. de, Heterogeneous Catalysis-The Faraday Society 1950. The Aberdeen University Press Ltd., 1950. 74. Toniolo, C., and Giammarco, G., Giorn. chim. ind. applicata 16, 219 (1933). 75. Pastonesi, G., Chimica e industria (Milan) 19, 123 (1937). 76. RoginskiI, S. Z., J . Phys. Chem. (U.S.S.R.) 22, 655 (1948). 77. Browning, L. C., Witt, T. De, and Emmett, P. H., J . A m . Chem. SOC.72,4211 (1950). 78. Browning, L. C., and Emmett, P. H., J . Am. Chem. SOC.73, 581 (1951). 79. Latchinov, S. S., J. Phys. Chem. (U.S.S.R.) 14, 1260 (1940). 80. Latchinov, S. S., and Wwedenski, A. A., J . Applied Chem. (U.S.S.R.) 10, 435 (1937). 81. Latchinov, S. S., J. Applied Chem. (U.S.S.R.) 10, 19 (1937). 82. Latchinov, S. S., J . Applied Chem. (U.S.S.R.) 10, 1847 (1937).


Surface Studies with the Vacuum Microbalance :Instrumentation and Low-Temperature Applications T. N . RHODIN. JR.* Institute f o r the Study of Metals. University of Chicago. Chicago. Illinois Page 40 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 a Historical Background . . . . . . . . . . . . . . . . . . . . . . 41 b . Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 c . Recent Technical Advances . . . . . . . . . . . . . . . . . . . 42 2. Design of Vacuum Microbalances . . . . . . . . . . . . . . . . . . 42 a Types of Instruments . . . . . . . . . . . . . . . . . . . . . . 43 ( 1 ) Cantilever Type . . . . . . . . . . . . . . . . . . . . . . 43 (2) Knife-edge Type . . . . . . . . . . . . . . . . . . . . . . 45 47 (3) Torsion Type . . . . . . . . . . . . . . . . . . . . . . . . 53 (4) Spring Type . . . . . . . . . . . . . . . . . . . . . . . . b Application to Surface Studies . . . . . . . . . . . . . . . . . . 55 ( 1 ) Fabrication of Instrument . . . . . . . . . . . . . . . . . . 55 (2) Installation of System . . . . . . . . . . . . . . . . . . . . 56 (3) Calibration of Balance . . . . . . . . . . . . . . . . . . . . 56 3. High Vacuum Operation . . . . . . . . . . . . . . . . . . . . . . 57 a . Microbalance Housing . . . . . . . . . . . . . . . . . . . . . . 57 b . Special Techniques . . . . . . . . . . . . . . . . . . . . . . . 62 (1) Magnetic Coupling . . . . . . . . . . . . . . . . . . . . . 62 (2) Radioactive Deflection . . . . . . . . . . . . . . . . . . . . 62 (3) Getters and Outgassing . . . . . . . . . . . . . . . . . . . 62 c Sample Insertion and Manipulation . . . . . . . . . . . . . . . . 64 d . Auxiliary Operations . . . . . . . . . . . . . . . . . . . . . . 65 (I) Temperature Control . . . . . . . . . . . . . . . . . . . . 65 (2) Pressure Control . . . . . . . . . . . . . . . . . . . . . . 66 (3) Weighing Observation . . . . . . . . . . . . . . . . . . . . 67 68 4. Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . a . Sample Requirements . . . . . . . . . . . . . . . . . . . . . . 68 b . Polycrystalline Foils . . . . . . . . . . . . . . . . . . . . . . 69 c . Single Crystal Plates . . . . . . . . . . . . . . . . . . . . . . 72 d . Evaporated Films . . . . . . . . . . . . . . . . . . . . . . . . 78 (1) Formation Techniques . . . . . . . . . . . . . . . . . . . . 79 (2) Structural Properties . . . . . . . . . . . . . . . . . . . . 82 (3) Additional Considerations . . . . . . . . . . . . . . . . . . 83 e . Contamination Control . . . . . . . . . . . . . . . . . . . . . 84

I . Microbalance Applications . . . . . . . . . . . . . . . . . . . . . .

. .

.

.

* Present address. E . I . du Pont de Nemours 39

& Co., Wilmington. Delaware .

40

T. N. RHODIN, JR.

Page 11. Low-Temperature Studies. . . . . . . . . . . . . . . . . . . . . . . 85 1. Surface Characterization. . . . . . . . . . . . . . . . . . . . . . 85 a. Adsorption Isotherms: General. . . . . . . . . . . . . . . . . . 85 b. Adsorption Isotherms: Specific. . . . . . . . . . . . . . . . . . 86 (1) Calibration of a Single Surface . . . . . . . . . . . . . . . . 86 (2) Oxidation-reduction Cycle . . . . . . . . . . . . . . . . . . 87 c. Progressive Oxidation and Surface Area. . . . . . . . . . . . . . 90 2. Oxidation Study of Copper Surfaces. . . . . . . . . . . . . . . . . 92 a. Physical Mechanism of Low-Temperature Oxidation . . . . . . . . 93 b. Reaction Rate Anisotropy of Oxidation . . . . . . . . . . . . . . 96 (1) Physics of Film Formation. . . . . . . . . . . . . . . . . . 97 (2) Energetics of Overgrowth Formation . . . . . . . . . . . . . 98 3. Anisotropy of Physical Adsorption. . . . . . . . . . . . . . . . . . 99 a. Anisotropic Adsorption on Copper and Cuprous Oxide. . . . . . . . 100 (1) Heats of Adsorption as a Function of Surface Coverage . . . . . 105 (2) Variation of IIcats of Adsorption on Different Major Crystal Faces of Copper . . . . . . . . . . . . . . . . . . . . . . . . . 107 b. Comparison of Adsorption on Single Crystals of Zinc, Copper, and Iron 109 c. Comparison of Adsorption of Nitrogen and Argon. . . . . . . . . . 111 4. Outlook for Future Surface Studies Using Microgravimetric Techniques . 112 Rcferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

I. MICROBALANCE APPLICATIONS 1 . Introduction

The layman, the design engineer, and the research scientist are devoted t o a considerable extent to observation of surface effects. There exists today a relatively small but rapidly growing knowledge of the properties of ideal surfaces. The basic surface properties of concern to the catalyt'ic chemist, the electron physicist, or the metallurgical engineer find long-range application in the practical fields of catalysis, corrosion, electron emission, adhesion, welding, mechanical wear, and lubrication. The importance of a careful study of the fundamental surface properties that, promote favored structures and accelerate specific chemical reactions is rapidly becoming an increasingly active field of modern research. Many metals, for example, in high vacuum or in carefully controlled atmospheres a t high or low temperatures show unique surface properties that can often be advantageously extrapolated t o more moderate conditions of temperature and pressure. In recent years, electron microscopy, electron diffraction, multiple interferometry, polarizing spectrometry, optical metallography, vacuum microgravimetry, gas adsorption, and chemical analysis of surface films have been most effective in the characterization of solid surfaces. It is recent advances in the application of vacuum microgravimetry to the study of surfaces of solids with which this chapter will be concerned.

VACUUM MICROBALANCE : LOW-TEMPERATURE APPLICATIONS

41

a. Historical Background. The pioneer work of Emich and Donau (1) continued by Benedetti-Pichler (2) has contributed immensely to the background of submicrochemical technique essential to the modern development of precision surface studies. The aim of the pioneer chemists was t o develop methods of weighing microgram and submicrogram amounts of material. Their balances, suited to their own particular needs, were of a delicate nature, often with very limited capacity and invariably unmanageable in high vacuum. Nevertheless they were the forerunners of the modern vacuum balances. Recent advances in the field of quantitative surface studies have been made by designing instruments relatively insensitive to temperature change, radiation, and vibration while suitable for routine use in high vacuum without making costly sacrifices in sensitivity. Progress in microgravimetry constitutes to a considerable degree an index t o progress in instrumentation. Applications of microgravimetry t o the measurement of physical properties is traditional. Any physical or chemical variation in a system attended hy a change in the mechanical moment associated with a given phase or component is susceptible to precise microgravimetric measurement. The measurement of liquid surface tension by the DeNouy method, of magnetic susceptibilities by the Gouy balance, of vapor pressures, and of gas densities by buoyancy balances, as well as microanalysis, are a few examples of the many classical uses of applied precision microgravimetrp. b. Definitions. Prior to a discussion of microbalance design and application, consideration of some general characteristics and definitions pertinent t o vacI;um microgravimetry can be made. The capacity of a balance, one of its primary characteristics, is the maximum load th a t can be weighed with a given accuracy in addition to the weight of the beam, pans, and other members of the balance without, injury t o those members. The range of a vacuum balance is the maximum variation i n weight change of a given load that can be measured with a given accuracy i n situ, that is without opening up the vacuum system. The sensitivily may be defined as the magnitude of reversible or elastic response 1 1 1' uii' variation in weight. The response is usually expressed in micrograms g., equivalent to 1 gamma (r)]per unit length or per unit of arc. This characteristic is often confused with sensibility which is the minimum variation in weight which may be experimentally observed in a reproducible manner with a given accuracy. Whereas the former is a n intrinsic property of the elastic modulus of the weighing system, the latter is subjective insofar as it depends on the skill of the operator and the response of the instruments used t o detect the balance deflection. T h e thermal, pressure, electrical and mechanical stabilities are all critical

42

T. N. RHODIN, JR.

characteristics of vacuum microbalance signifying the relative lack of dependence of sensibility on temperature, pressure, etc. Vacuum technique applied to microgravimetry, manipulation of sample, auxiliary physical measurements in situ and heating and cooling of the sample are more specialized aspects of a vacuum microbalance system and will be discussed with reference to specific applications. c. Recent Technical Advances. The progress in micromanipulation (Kirk, 3) in the last decade in general and the publication of techniques developed in the study of the transuranium elements (Seaborg, 4;Cunningham, 5 ) in particular have contributed much to the recent maturity of vacuum microgravimetry as a tool for surface studies. The development and refinement in high vacuum practices and equipment, and the development of new uses of well-known materials for balance construction such as quartz, tungsten, and the phosphor bronzes are specific examples. New techniques for bonding of metals with non-metals using, for example, selenium, silver chloride, and silver-platinum cements or for metallizing non-metallics have also played a part. The availability in this country of better optticsfor precision measurement of beam deflection has also promoted precision microgravimetry. Finally and probably most important of all are the advances made in the design of the microbalance itself by various investigators in the past five years. The importance of the progress in associated physical techniques used to prepare and characterize solid surfaces has also given an impetus to the application of vacuum microgravimetry to surface studies. The use of electropolishing techniques, electron microscopy, optical advances in polarization spectrometry and multiple interferometry applied to surface films and the preparation of polycrystalline surfaces by evaporation or of single crystal surfaces by slow cooling of metal melts, exaggerated grain growth, or recrystallization methods have helped considerably in the vacuum microgravimetric study. The development of high-temperature porcelains capable of being joined directly to Pyrex glass, and the availability of heating wires and ribbons of refractory metals and of glassquartz graded seals have simplified the designing of vacuum furnaces. 2. Design of Vacuum Microbalances

All vacuum microbalances suitable for surface studies can best be described in terms of four different classes with operating and performance characteristics unique to each class. The advent of bhe simple precision microbalance seems to be with Warburg and Ihmori (6) in 1886. From then on numerous designs have been described. Excellent reviews cover the steady progress in precision microgravimetry from those of Pettersson (7) (Sweden, 1915), through Emich (8) (Germany,

VACUUM MICROBALANCE : LOW-TEMPERATURE

APPLICATIONS

43

1921) and Gorbach (9) (Germany, 1936), to those of Ingram (10) (English, 1949) and Kirk et al. (3) (American, 1950). The fact that vacuum microbalances for surface studies have been comparatively rare until recently lies not in lack of development in the field of precision balance design, but in the complications introduced by the difficulties of balance manipulation and preservation of uncontaminated specimen surfaces in high vacuum. The former will be discussed in the following two sections on design types and operating characteristics. A following section will endeavor to review pertinent facts on vacuum applications and surface control and characteristics. Finally in Section I1 some specific examples of surface studies a t low temperatures using the vacuum microbalance will be considered, a. Types of Instruments. (1) Cantilever type. In the simplest type of balance, often described as the cantilever (Friedrich, 11; Bazzoni, 12) or

U

FIG.1. Salvioni balance. 47 (1924).]

[After Predvoditelev and Witt, 2.physik. Chem. lS2,

Salvioni type, the determination of the weight depends on the measurement of the amount of bending of a single fiber. I n its simplest form as described by Salvioni (13) one end of a beam is fixed and the other end is loaded. The resulting deflection measures, within the elastic limits of the beam, a given weight since the lever length and orientation of the bending moment are constant. A calibrated scale a t the end of the fiber tip permits accurate readings of the deflection with the use of a telescopic lens. A typical application of the Salvioni balance to surface reactions is illustrated in Fig. 1. The change in weight of the powdered sample in the basket (2) is followed by the deflection of the fiber (4) observed by an optical lever acting on the mirror (3) and observed through the window (1) cemented on with a high-temperature cement. The cell is evacuated through ports in the ground insert ( 5 ) not indicated in the figure. The simplicity of this arrangement has much to recommend it for adsorption studies on powdered catalysts on which adsorption effects are too small to be observed by the simpler methods of volumetric measurement. In this application it would be more desirable t o fuse a suitably ground optical flat onto the cell for optical observation.

44

T. N. RHODIN, JR .

The characteristics of some typical prototypes of cantilever balances are summarized in Table I. TABLE I Characteristirs of Typical Cantilever Balances Ihte

Designer

1901 1928 1941 1947

Salvioni Predvoditelev 1.owry Lord

1919 Korenman

Fiber

Diameter

Quartz 25 Quartz 250 Quartz 250 Phosphor 89 X 0.501 bronze X 0.098 mm. ribbon Steel wire 40 p

Load

Sensitivity Reference

5X g. 1 X lo-* g. 11, 12, 13 3 X lo-* 2 X 10-6 14 3 X 5 X 16 4X 1 X 10-6 15

6 X

2 X

17

The virtues of these balances are simplicity, freedom from vibrational troubles, and the ease with which they are adaptable to measurements a t unusual pressures and temperatures. Their extremely light construction and cantilever character make them particularly susceptible to electrostatic charges and to air currents. These balances are often more readily manipulated in a vacuum than in air. Predvoditelev (14) used his balance to study the surface reactions between Cu2S04and NH3 t o form the pentammoniate; Lowry (16) applied his instrument t o quantitative histochemical studies of tissue sections; Lord (15) perfected a n advanced modification which he applied to hydration studies of textile fibers. Lord improved capacity and robustness by replacing a fiber with a ribbon and using a microscope instead of a telescope. This is often a useful method of obtaining capacity without excessive sacrifice in sensitivity. The very small working focal length of the microscope does make certain demands on the balance housing and indicator position. I n addition Lord used a zero-point adjustment device and a fine spring adjustment at the fixed end t o bring the pointer into a convenient measuring range for a given load. This type of balance has always been used as a deflection balance and is not adaptable to zero-point balancing. It is an unfortunate necessity that in this type of balance the total load be weighed to the limiting sensitivity. In other words the beam deflection corresponds to the total load and not to the relatively small variation in the load that may occur by virtue of surface reactions occurring on, for example, a sample plate. The natural consequence is that high sensitivity is only obtained by limiting oneself to very small samples, the weight variation of which can only be followed over a small range for a given fiber or ribbon. T h e usable range of Lord's modifications, for example, varied from 10 g. t o g. as the sensitivity varied from lo-' t o lo-' g. for beams of various

VACUUM MICROBALANCE

: LOW-TEMPERATURE APPLICATIONS

45

dimensions. Theoretical equations describing the behavior of elastic systems employed in these balances have been worked out (Cunningham, 5). The equations are highly useful in solving problems of design and construction. For a cantilever balance of quartz fibers of uniform circular cross section the sensitivity can be approximated by the following equation :

D

=

0.643mgLm/3EB4

(1)

where D is deflection, m is mass of beam plus mass of weight, g is gravitational constant, L is beam length from support to hang-down, E is Young’s modulus for beam, and B is beam diameter. It is evident th a t sensitivity is dependent on the load. Actually it is also important th a t the total deflection be small, the hang-down* fiber diameter be relatively small, and the free end of the beam be near the same height as the fixed end, and the beam modulus be corrected for diameter effect which becomes important for very fine beams. For fibers of very small diameter, the selfdeflection from the weight of the beam alone becomes considerable and imposes a practical limit on the sensitivity that can be obtained. With quartz fibers, for example, g. is the maximum sensitivity for a beam deflection of 1 micron. In summary, the cantilever vacuum microbalance is simple to construct and operate in high vacuum and is capable of high sensitivity but is critically limited in capacity and range. ( 2 ) Knife-edge t j p e . Warburg and Ihmori in 1886 were probably the first t o apply the principle of center-point balancing t o a precision microbalance. The outstanding merit of this principle is th a t a relatively large mass can be balanced with very high reproducibilty but without a n absolute determination of that mass. This property is particularly pertinent t o microbalance design for surface studies where the weight of the beam and the sample is often many magnitudes greater than the weight effects caused by the surface reactions under study. The second requirement, whereby a small superimposed weight caused by a surface reaction can be balanced by an absolutely determinable force with high accuracy, is not nearly as well satisfied for this type of balance. Many methods have been used for precision absolute compensation of a small superimposed weight, but the most common one for knife-edge balances is that using the compensating weight of a micro-rider with all the errors attendant on the precise and reproducible location of the rider. The natural limitations of this type of force compensation compared t o other methods and the errors inherent in its use have been considered (Felgentrager, 18). Surface studies in which knife-edge balances with rider compensation are manipulated in high vacuum have been achieved

* The hang-down

is the fiber connecting the end of the beam to the sample

46

T. N. RHODIN, JR.

(Monk, 22), but knife-edge balances do not lend themselves to vacuum manipulation. The possible gain in accuracy obtainable by replacing the rider technique with a more precise method of force compensation is always limited by the perfection to which quartz knife edges may be fabricated and aligned. This is especially true when knife-edge balancing is also employed for the hang-down wires at the beam end where the intrinsic problem remains of fixing the location precisely of three knife edges along the beam and yet allow each the highest possible freedom of rotation about a set of three parallel horizontal axes. The natural limitations of the knife-edge principle for precision microbalances is described in considerable technical detail in an authoritative treatise by Felgentrager (18).

FIG.2. Buoyancy balance. 182, 580 (1909).]

[After Steele and Grant, Proc. Roy. SOC.(London)

The knife-edge balance most significant for the development of vacuum microbalances was that of Steele and Grant (19), who werethe first to construct a balance exclusively from fused quartz and to employ fused-on quartz fibers for suspensions of the pans. The use of quartz meant that a design of simple nature could be followed. Their model consisted of a quartz beam balanced on a quartz knife edge resting on a polished quartz plate. The balance was enclosed in an airtight case. Their balance is illustrated in Fig. 2. It is typical of the general technique of buoyancy balancing as a method for the precise compensation of small deflections. The sample ( 5 ) is hung on the same hang-down as a sealed quartz bulb (4)at one end of a beam resting on a precision quartz knife edge (3) and counterweighed by a fused quartz ball (1). Deflection is observed by light reflected from the mirror (2). The balance is restored to a null point by adjusting the pressure inside the balance case and so changing the buoyancy of the quartz bulb. By reading a manometer to 0.1 mm. mercury with a cathetometer the high sensitivity of

VACUUM MICROBALANCE

4X


47

g. can be achieved. The balance has the unusually high capacity, for such a sensitive instrument, of 1 g. On the other hand the balance of Steele and Grant was very complicated and required careful control of pressure during the run as well as other additional complicated manipulations. The use of vacuum operation, the elimination of rider compensation, and the use of all-quartz construction and fine fused-on quartz fibers for the hang-down wires represent an outstanding contribution to precision vacuum microgravimetry. In summary, although the knife-edge principle permits the measurement of unusually high loads in the microgravimetric range, mechanical difficulties in the construction and manipulation of high precision knifeedge microbalances precludes their extensive application to vacuum operation and surface studies. (3) Torsion type. The outstanding advantages of fused quartz for torsion microbalance construction were recognized soon after the turn of the twentieth century. Its advantages for this application have been reviewed in detail elsewhere (Neher, 23). Suffice to recount here the following characteristics: chemical inertness, low density compared to all metals except aluminum, high tensile strength, low coefficient of thermal expansion, high purity and homogeneity, capacity of being easily manipulated, drawn, and welded, and readily cleaned and outgassed. Its low thermal and electrical conductivity are disadvantages in some applications, on the other hand. The only other material of construction equivalent in strength and elasticity t o that of quartz fiber is fine tungsten wire. According to Gorbach, Weber (24) in 1892 was the first to substitute a quartz fused-on fiber for the knife-edge support of a microbalance beam. He also experimented with fused-on metal fibers. There is little information about the performance of these balances except that they possessed the extremely small capacity of about 1 mg. and were very fragile. Nevertheless the principle of the torsion fiber support of the beam is the basis of most contemporary precision microgravimetry. In 1906 Nernst and Riesenfeld (25) applied the Weber feature t o a very simple balance made of a quartz beam balanced on a torsion fiber. Emich and Donau (26) improved the design by substituting a micrometer eyepiece for a graduated scale and enclosed the whole unit in zt glass g.) were quite envelope, but both the capacity (0.2 g.) and the range ( limited by the character of the fine quartz torsion fiber used for the main support. The use of ground tapered glass seals to introduce samples and t o make adjustments was made a t this time as well as improvements of the working vacuum in the balance case. At the same time Pettersson (7) achieved a most amazing performance at the expense of a complicated

48

T. N. RIIODIN, JR.

and fragile instrument utilizing the quartz torsion fiber feature. It is reported that he weighed 0.2 g. with an accuracy of 1 part in lo9. He presented a detailed discussion of the theory of such instruments and during the course of a lifetime made many contributions to the design of these instruments, some of which are still appreciated. Pettersson constructed the balance entirely of quartz and presented a det,ailed discussion of t,he theory of such instruments. H e exploited the principle of compensating balancing by not only making the balance assembly symmetrical in every detail except for the difference between the sample and the counterweight, but by making the balance housing symmetrical also, hanging both sample and counterweight deep in quartz tube wells which could be heated and cooled over a considerable temperature range without interfering with the high-fidelity operation of the balance. He used the Steele and Grant method of buoyancy compensation but improved the sensitivity by using a small buoyancy differential between the sample weight and the counterweight. The vacuum technique was very advanced for th at period and magnetic coupling was employed t o activate the arrests. His balance was the prototype of the modern vacuum balance in many features except for the principle of buoyancy compensation which is obviously limited in surface studies where the gas pressure is an important variable. Aston (27) and others (Taylor, 28; Guye, 29; Ramsay and Whytlaw-Gray, 30) used a modified balance of this type to measure the atomic weights of neon and other gases and in this application it was outstanding. It seems that Stromberg (31) in 1928 was one of the few to report studies of gas adsorption on solid surfaces using the Pettersson type precision vacuum torsion microbalance. He reproduced the performance of the Pettersson balance and in addition modified the beam and the external housing toward even greater symmetry. Stromberg improved the general design stability and operational use of the instrument considerably a t the expense of some loss in capacity and sensitivity which were 0.050 g. and 5 X lo-* g./div. respectively. He measured the adsorption of water vapor on thin plates of glass, quartz, gold, and platinum and made some shrewd distinctions between multilayer physical adsorption and solubility. In many ways his experiments were the forerunners of current microgravimetric surface studies. One of the classical applications of the vacuum microbalance for adsorption studies was that of Barrett, Birnie, and Cohen (32) in 1940. They measured the physical adsorption of water vapor on silica surfaces a t 30°C. by direct weighing. They employed a quartz beam balance using a cemented-on tungsten wire (0.001 in. diameter) and enclosed in a high vacuum housing. The capacity of the balance was relatively high (0.5 g.)


49

as well as the sensitivity, which was g./O.Ol mm. deflection. No compensating force was used. The adsorption was followed by the weight change balanced by the torsion induced in the tungsten wire corresponding t o a given deflection. Considerable difficulty was encountered from shifting zero points, electrostatic charge displacement of the beam, buoyancy effects, and temperature dependence of balance sensitivity. For these reasons it was not possible to operate the balance a t any other temperature than 30°C. with the required accuracy. In 1946, Bradley, Evans, and Whytlaw-Gray (33) successfully designed and used a deflection-type vacuum torsion microbalance for the measurement of the evaporation rate of small droplets of dibutyl phthalate and butyl stearate as a function of droplet diameter. They operated effectively over a range from 0.1 mm. to 760 mm. mercury pressure. The very small load

FIG. 3. Donau torsion microbalance. 13, 155 (1933).]

[After Donau, Miikrochemie 9, 1 (1931);

of 100 X g. permitted the use of a very fine 15 p quartz fiber yielding a balance sensitivity of 2.9 X g./p deflection. Donau (34) took the Weber feature and applied it t n the simplified Nernst design by adding yokes and torsion fibers at the beam ends in addition t o the center which resulted in a beam design which has much t o recommend i t from the viewpoint of contemporary vacuum application in the simplicity of its design and operation. The Donau version of the torsion balance in Fig. 3 is the primitive forerunner of the modern quartz vacuum torsion balance. The precision ground quartz beam (2) is fused onto a fine quartz fiber (3) and enclosed in a completely sealed glass case (7). The sample is contained in the basket (6) on the hangdown (5), which in turn is fused onto the fine fiber cross wire and rigid yoke (4). Deflection is measured with the micrometer eyepiece (1). This balance may be considered as the prototype of the torsion-type deflection balance used today. For comparison a modern version of th e Donau balance is illustrated in Fig. 4 to indicate the extensive modifica-

50

T. N. RHODIN, JR.

tions introduced by Gulbransen, Podgurski, and Rhodin to make it an instrument applicable to routine surface studies. Some of the features introduced are the differential balancing, the complete symmetry of design, the high vacuum furnace, and the improved optics. The diagram in Fig. 4 is self-explanatory except for the triangle at the beam balance point which is a schematic representation of a torsion fiber. Gulbransen (35) and Podgurski (36) added many features important for high-temperature studies of metals in high vacuum such as precision ground quartz beam, simple styling for vacuum operation, and high-temperature outgassing design. Rhodin (37) applied it to adsorption studies at low VACUUM B A L A N C E

ALANCL C O N T A I M E I

IOIIIZA1IOM OAUW

VICUUM SVST

VACUUM

FURNACE

VACU UM MI CR OBAL A N C E

FIQ.4. Vacuum microbalance assembly indicating function of typical deflectiontype quartz torsion vacuum microbalanre. [After Rhodin, J. Am. Chem. SOC.72,4343 (1950).]

temperatures by increasing sensitivity, using differential sighting, symmetrical balancing, platinized members, and magnetically activated precision ground quartz arrests. Figure 5 is a photograph of a Donautype fiber balance used by the author. It is reproduced to illustrate the construction and operation of the arrests (l), the effectiveness of the platinizing treatment which gives the instrument the metallic appearance (2), the yokes (3), offset construction of the beam (4),and the indicating fibers, suspension wires, cross wires and hang-down hooks (5). The hangdown fibers are not indicated but will be added when the balance is installed in the vacuum system illustrated in Fig. 4. Some precise dimensions of a beam are indicated in Fig. 6. This is the beam used in the balance shown in Fig. 5. The dimensions are indicated because they determine the characteristics of the balance to a considerable extent and


51

their proper choice in any particular design requires much experimentation. Some of the more critical dimensions are the effective beam length and diameter, the beam offset, and the distance between the suspension

FIG.5. Photograph of a Donau-type deflection balance of contemporary design.

point and the center of gravity of the beam. Repeated mention will be made of this instrument. Using principles previously described, Kirk, Craig, Gullberg, and Boyer (38) engineered an outstanding torsion microbalance to permit the ALL DIMENSIONS IN CENTIMETERS

I6 0

* 0

1

k-3 m O E G NtTNuo*>- \

' A

Y

FRAME

FIG.6. Critical dimensions of beam design for deflection-type torsion vacuum microbalance.

quantitative chemical study of the milligram quantities of the transuranium elements in the period from 1942 to 1946. The balance combined the torsion principle of Neher, the pan suspension of Steele and Grant, the pan well of Pettersson, and the comparison microscope of Emich and Donau for determining the beam position. By using a cantilever beam

52

T. N. RHODIN, JR.

of great rigidity, the main portion of the load and the pans were balanced by a counterweight. Small differential weight changes were measured by rotation of the wheel which applied sufficient torsion to the fiber t o restore the beam to the null position it had occupied previously and which had been observed by means of a comparison microscope. The design and operational principles of this instrument are presented elsewhere in an excellent account (38). The sensitivity, capacity, and range of this instrument vary considerably with design features, especially the length and diameter of the quartz fiber. One model was adapted by the author t o vacuum operation using a 18 1.1 torsion fiber of a sensitivity of g./minute of arc and a capacity of only 0.050 g. This small 6X capacity was unfavorable for surface studies where samples with sufficiently large surfaces usually weigh about ten times as much (about 0.3 g.). In addition, vacuum operation was complicated by the necessity of activating the arrests and the torsion wheel through the vacuum wall with adequate delicacy of control. The use of stationary arrests eliminated the first difficulty but narrowed the weighing range of the instrument by limiting the free beam swing to a smaller arc. The capacity was determined essentially by the cross-sectional area of the torsion fiber, whereas the sensitivity varied inversely with the square of the crosssectional area. Hence a 50 p fiber torsion balance with a capacity of about 0.5 g. and a sensitivity of approximately 1 y could be used for surface studies. A more useful extension of the torsion balance to higher capacity is that typical of the Garner balance, in which heavier vertical fibers are used to carry the weight in addition to the much finer torsion fiber which is used to supply the torque necessary t o restore the beam to its null position. An instrument of this type with vertical suspension was preferred by the author to a comparable simple torsion balance because of its sturdier construction and flexibility of weighing characteristics in the heavier weighing range using plates 0.5 g. in weight upon which weight changes of of this quantity could be measured. The operation of a quartz torsion microbalance applied by the author (10) to the oxidation studies of evaporated metal films is illustrated in Fig. 7. The plate upon which the film is formed by the evaporated metal beam (11) from the source (13) is hung on the hang-down fiber (6). The balance itself is that designed by Kirk-Craig-Gulberg-Boyer with the hang-up fiber (4)introduced by Garner. Tension on the static fiber (2) and the torsion fiber (5) is maintained by the bow (1). The beam (3) is balanced by observing the image (7) and rotating the graduated wheel (8) to a null point until balance is restored. I n summary, some important properties of torsion balances for surface studies may be emphasized. First, the sensibility of torsion balances

VACUUM MICROBALANCE: LOW-TEMPERATURE APPLICATIONS

53

is unusually high. Second, the accuracy of the weighing is often limited by the precision with which the restoring force can be measured for either null-point or deflection observation. The former operation is capable of high accuracy but is also attended by more complicated instrumentation. This is often a most serious consideration for routine high vacuum operation, Third, balances are obviously fragile instruments but when used with trained personnel are not subject to excessive breakage. Finally, the construction of precision quartz fiber devices requires skill in the forming and joining of fine quartz members in addition t o special tools, jigs, and working conditions. Techniques in the manipulation of quartz

tQ

a

QUARTZ TORSION MlCROBAlANCE

FIG.7. operation.

Schematic diagram of torsion-type microbalance illustrating design and

have been published (53), and custom-built quartz balances are available commercially. (4) Spring t y p e . Spring balances of the general type originated by Emich (39) in 1915 have found widespread use in microanalysis and biochemistry. Helix balances of quartz and other materials have been in general use particularly for sorption studies for a number of years, especially since McBain and Bakr (40) applied the spring balance t o surface studies in 1926. It consists essentially of a coil spring hung vertically in a tube. Weight change is followed by measuring the vertical displacement of a pointer on the spring. I n design and operation it is one of the simplest of all microbalances and one that is most easily incorporated into a vacuum system. Numerous designs of helical balances have been constructed and used for adsorption studies over a wide range of temperature and pressure (McBain and Britton, 41; Morris and Maas, 42; Drane, 43; Stamm and Woodruff, 44; McBain, 45; Dunn

54

T. N. RHODIN, JR.

and Pomeroy, 46). A modification of a spring balance used for vacuum weighings by Kirk and Schaff er in precision microbiological studies is indicated in Fig. 8. The fine quartz spring (1) with the pointer (3), scale (4)and the sample contained in a pan (5) are enclosed in a glass tube (2). The principle of the helical balance, however, imposes a severe limitation on its use, namely, that the ratio of load to sensitivity is limited and relatively small compared to symmetry-type balances of the knife-edge or torsion type. In other words the total load must be weighed to the maximum precision in the absolute sense to the full accuracy. On the other hand, it is capable of rapid use and for vacuum studies is preferable to instruments, the use of which depends on the bending or distortion of mechanical members such as the cantilever beams previously discussed. It is readily constructed and inserted into a vacuum envelope, readily outgassed and employed a t high temperatures and remarkably adaptable for sorption studies at high pressures. Quartz spring balances are particularly useful for surface studies with corrosive substances since no corrosion susceptible materials are required for’ the construction of such balances (Makolkin, 47). The mechanical properties of quartz as a material for beam construction previously mentioned apply equally well to the design of spring balances. Kirk and Schaffer (48) have written an excellent review of the construction and special uses of quartz helix balances to which the *. Spring reader interested in their application to surface studies microbalance for high vacuum oper- is recommended. The use of springs of phosphor ation. [After Kirk bronze, beryllium copper, molybdenum, stainless steel, and Schaffer, Rev. and Pyrex have also been described (Jarefowitz and fJci.1n8truments19, Othmer, 49; Chipalkatti and Giles, 50; Makolkin, 51). 785 (1948).] Coiled springs (Bang, 52) where deflection occurs by circular movement instead of elongation are also useful for rapid easy weighings but are less adaptable f6r surface studies. Depending on the spring material, fiber diameter, helical diameter, spring length, and spring pitch, vertical springs have been designed which range in sensitivity from 1 y to 1 mg./mm. deflection. It is obvious that the total load for a 10-cm. spring of corresponding sensitivities would vary correspondingly from about 100 y to 1.0 g. for a total elongation susceptible of convenient manipulation.

m.


55

In summary, it is clear that the balance design most suitable for surface studies in a particular system should, first of all, be chosen from the four basic designs discussed, depending on the total load, sensibility, and range in which the weight changes associated with the surface effects are to be studied. However, the different designs vary considerably in other important factors such as ease of fabrication, installation, calibration, and operation. These latter considerations may often be of paramount importance, depending on the laboratory facilities and personnel available for surface studies. These additional factors will be discussed in brief in the next section. b. Application to Surface Studies. (1) Fabrication of instrument. The cost and effort involved in fabrication of microbalances vary considerably among the different designs. The cost of construction materials is usually negligible in comparison to the fabrication costs. Fabrication is by far the simplest for the cantilever type, which can be constructed for less than a hundred dollars. Helices of glass and quartz can also be constructed a t moderate cost once the techniques of drawing and bending are mastered. Generally the fabrication of all quartz fiber devices is greatly facilitated by the design and construction of suitable jigs (Kirk and Craig, 53). This is an indispensable requirement for the construction of the torsion balances, especially when torsion compensation is used. A vacuum-operated model of the Kirk-Craig-GullbergBoyer balance has never been constructed, to the author's knowledge, specifically for surface studies. A model originally designed and constructed for microanalysis and provided wit'h a vertical suspension was modified by the author (Rhodin, 54) for vacuum operation, but this practice is not recommended because this design does not lend itself to vacuum adaptation. The cost of a Kirk-Craig-Gullberg-Boyer balance amounts to several thousand dollars, depending on the number of and kind of additional features incorporated in its design. It is likely that simplified balances of this type can be designed for vacuum use a t lower cost. The fabrication of the beam structure and the assembly of fine quartz fibers requires the skill of an expert quartz worker. A considerably less expensive Kirk-Craig-Gullberg-Boyer balance has been constructed by B. B. Cunningham (5) but was not designed for high vacuum operation. It now seems likely that quartz vacuum balances with torsion compensation will be designed and applied to adsorption studies because of the impetus given to this type of design by (Kirk, 3). It may also be noted that commercially available knife-edge microbalances can be adapted to vacuum work (Monk, 22) but a t a high cost compared with that involved in the design and construction of a balance initially designed for vacuum application.

56

T. N. RHODIN, JR.

The most outstanding borsion balance for surface studies is that of the simple quartz beam type keynoted by Donau, improved by Gulbransen and Podgurski, and applied to adsorption studies by the author. The simplicity and reliability of this balance highly recommend it for vacuum studies. Although the precision construction of the beam requires the precise skill of a quartz worker, it is a simpler design and less costly than a balance using torsion compensation. Balances of this type can now be obtained commercially a t about one-tenth the cost of a KirkCraig-Gullberg-Boyer balance. This lower price is achieved in part by the elimination of certain refinements in the various mechanisms used for beam arrest, pan release, and optics. (2) Installation of system. The cantilever and simple torsion balances operate satisfactorily in nonthermostated rooms (Steyermark, 55), although air conditioning improves considerably the dependability of surface studies from the viewpoint of eliminating contamination during the preparation of the samples and of the vacuum housing. Thermostating the balance itself to +O.O2"C. is considered good practice (Kerner, 56). Static electrical effects may sometimes disturb the weighings, and it is common practice to maintain an ionizing source inside the vacuum housings such as some radioisotope of a suitably low vapor pressure. The author prefers metallizing all members of the balance by sputtering or evaporating platinum. Metallizing can be done readily, does not interfere with the operation of the balance, and serves well in cancelling spurious electrostatic charges when suitably grounded. I n general, elastic microbalances of the torsion or cantilever type are less subject to environmental disturbances than either spring or knifeedge balances. The latter require careful thermostating for precision work, and the former have to be calibrated a t every temperature t o be used experimentally. Special shock mountings are not required, although torsion and spring balances are more sensitive t o vibrational disturbances than knife-edge balances. Oscillations in torsion balances from external vibrations usually damp out quickly unless they are of a special frequency and amplitude peculiar to the given balance. I n such cases careful shock mounting or relocation of the balance is required. (3) Calibration of balance. Calibration in the higher than microgram scale by interchange and comparison of standard weights and riders has been well described in the literature (Wilson, 57; Felgentrager, 58). No standard weights are available in the range of the torsion, fine spring or cantilever balances, however, so that indirect methods must be used. A standard technique of microchemistry (Kirk, 3) is to deliver a known microvolume of solution to the balance pan, evaporate the liquid, and compare the deflection to that of a quantity determined by some other


57

method such as microtitration. This technique suffers from errors in pipet delivery as well as from titrimetric errors, but both can be made tolerably small with suitable precise microtechniques. Calibration using microquantities measured by their radioactivity has also been employed. It is a useful method but is limited by the accuracy to which the counters can be calibrated. A more laborious technique is to weigh long sections of very fine quartz or tungsten wire of uniform diameter and subdivide them into small well-defined lengths (Kirk, Craig, Gullberg, and Boyer, 38). This technique is capable of considerable accuracy by suitable grouping and interchanging of the fine weights during calibration assuming the variation in wire diameter to be tolerably small. Precise division of a fine wire is subject to error and this is the main objection to this technique. Systematic errors can be minimized by using large volumes of liquid for the titration method, or long sections of wire for the wire method or, even better, cross calibrating with both methods. This is obviously a long and tedious procedure and is rarely done to the limits of precision of which the method is capable. Fortunately the calibration of torsion as well as spring balances follows a linear law within experimental error over the range for which the balances are customarily designed. Furthermore these balances usually remain correctly calibrated over extended periods. Once a balance of this type is calibrated, its calibration can be readily checked by means of moderately few observations. The optics of every microbalance is also a critical factor in its calibration. It is essential that the fiber image of the Kirk-Craig-GullbergBoyer balance, for example, be observed in a reproducible and precise manner. In similar fashion, it is important that the cathetometer or micrometer eyepiece used for observing index fibers in deflection-type balances be focused properly and mounted in a stable fashion. It is equally important that reference fiduciary markers remain well fixed in space. Absolute calibration of vacuum balances for surface studies, however, is not as critical as for microanalysis, since very often for surface studies it is only important to determine accurately the changes in weight relative to the initial weight of the sample characteristic of some starting condition of the surface. Reproducible preparation of the surface is more critical and will be discussed in detail in a subsequent section. 3. High Vacuum Operation

a. Microbalance Housing. The housing for a vacuum microbalance devoted to surface studies is usually of Pyrex glass or of brass with suitable windows and ports. The housing consists essentially of a horizontal tube to enclose the balance itself which is usually thermostated just

58

T. N. RHODIN, JR.

above room temperature. From this tube hang one or two sufficiently long hang-down wells made of quartz or high-temperature resistant porcelain to permit high-temperature outgassing of the sample. A simple glass envelope for containing a beam balance is indicated in Fig. 9. The dimensions have been inserted in this figure not only to indicate precise size and shape, but also to illustrate the manner in which the instrument is inserted and located in the glass envelope. In some cases, a polished glass flat is welded on the envelope to permit precise observation of the ,

FIG.9. Microbalance envelope showing dimensions of the instrument and location in the homing. (All dimensions are in millimeters except fiber diameters which are in inches as indicated.)

beam deflection without errors caused by optical aberation. The balance is inserted at the open end of the large tube and the hang-down wires dropped down through the smaller vertical tubes and engaged onto the hooks hanging from the yokes at the ends of the beam. When this operation is successfully completed the three openings are sealed off by an experienced glass blower. The sample can be inserted by either dropping it down the well when the hang-down wire is inserted or by allowing the bottom end of the well to remain open until after the hang-down wire is inserted and then hanging the sample on the lower end and sealing off the bottom end of the well. An ingenious dual-purpose reactor tube has been applied by Gulbransen and Andrew (59) to surface roughness studies on artificial

59


graphite. The reactor used by Gulbransen and Andrew is illustrated in Fig. 10. The sample (4) hangs from the balance beam into a tube constructed of a high-temperature porcelain (3) such as mullite. The tube is heated by a bifilar ribbon of molybdenum or tantalum (5) insulated by a magnesia insert (2) during the outgassing operation. Prior t o the adsorption operation this insert is removed and a suitable refrigerant is introduced into the Dewar vessel (1)) which is an integral part of the reactor. The authors report th at this design is very effective in minimizing I ' I * eddy current effects and in facilitating the outgassing-adsorption operation. The eddy A: currents are minimized by placing a roomI temperature zone both below and above the sample space. Pyrex is readily fabricated into almost any shape, is easily outgassed, and is transparent, obviating the need for windows. I n any case facilities have t o be provided for inserting the balance, observing the beam deflection, activating the arrests, exchanging the samples, and in some cases for applying a balancing force. It is best to avoid greased seals and joints as well as gasketed ports for more precise adsorption work. This requires breaking the glass t o repair the balance or t o exchange the samples. The shape indicated in Fig. 11 has been found by practice t o be one of the most convenient for FIG. 10. Microbalance the installation and operation of a deflectionreactor for operation over an type torsion balance. Innovations in design extended temperature range : were introduced by the author t o make the -195°C. to 1000°C. [After instrument suitable for adsorption studies Gulbransen and Andrews, prirequiring the highest precision. They were vate communication (1952).] aimed a t making not only the balance but also its envelope completely symmetrical. The beam itself as well as the hang-down wires and the tubes in which they were hanging were made and assembled with that feature in mind. This is illustrated in Fig. 11, which indicates how the balance housing is fitted to the overall assembly. The elevator platform (1) lifts the Dewars (4) u p t o the sample (3). The balance (6) is shock mounted (7) and grounded ( 5 ) . The evaporator (2) can be used to deposit a metal film onto the substrate

.'

60

T. N. RHODIN, JR.

(3) or t o form an active film for scavenging the reactor volume from oxygen and nitrogen. This will be discussed in a later section. Pressures are measured by the ionization gage (9), the McLeod gage (13), and manometers which are not indicated. Purified gases (10) are introduced into the system by the gas buret (14). The balance section of the vacuum system is isolated with mercury cutoffs (12). Symmetry was found to be important not only because it minimized thermal eddy current effects but also permitted the elimination of seriously large buoyancy corrections, a s described in a subsequent section. Under carefully controlled

FIG. 11. O-*era11 assembly of microbalance and high vacuum system used for precision adsorption studies. [After Rhodin, J . Am. Chem. SOC.72, 4343 (1950).]

conditions differential changes in weight caused by adsorption were measured over a large pressure range with an apparent precision of 1 part in 10 thousand with this design. Reference has been made to a modified Kirk-Craig-Gullberg-Boyer torsion balance enclosed in an 18-in. bell jar in which the beam arrests and the torsion wheel were activated through the vacuum wall by magnetic coupling. A photograph illustrating the application of the Kirk-Craig-Gullberg-Boyer torsion balance type vacuum operation is indicated in Fig. 12. The delicacy of the actual beam assembly relative t o the housing and the manner in which magnetic coupling is used to activate the torsion wheel are clearly indicated. During operation the bell jar is lowered over the whole assembly and

VACUUM MICROBALANCE:

LOW-TEMPERATURE APPLICATIONS

61

FIG. 12. Photograph of Kirk-Craig-Gullberg-Boyer microbalance with Garner modification adapted to vacuum operation.

evacuated. One advantage obtained with metal housing in comparison with glass housing is the use of “O”-ring gaskets on ports, vacuum wall manipulators, rotary seals, packless valves, and other high vacuum accessories now commercially available. One reason the deflection-type torsion balance is preferred to the nulltype is the complication introduced in activating a torsion wheel or

62

T. N. RHODIN, JR.

other compensating device through a vacuum wall in addition to the complication introduced by the operation of gears, bearings, and general optics in a high vacuum. On the other hand, an advantage of the spring balance is that it requires insertion in a single tube only, for vacuum operation. b. Special Techniques. (1) Magnetic coupling. K. Angstrom (60) in 1895 was one of the first to make use of the principle of electromagnetic compensation for weighing as little as g. of material. Emich (61) later modified the instrument so that it would indicate weight changes of 2 X lo-* g. Ever since that time electromagnetic compensation has been an attractive method for applying a null measurement to vacuum microgravimetry. The rather elaborate instrumentation required for high precision has limited its application (Riesenfeld and Moller, 62; Humphrey, 63). This technique was also applied t o a vacuum microbalance study of transition properties of hydrated salts at high temperatures by Urbain (64) and by Manigaut and Tsai (65). An ingenious but less elaborate microbalance has been applied to rates of adsorption studies by Gregg (66). Magnetic coupling has been used to advantage in a cruder sense to adjust the position of glass-covered riders by Barrett, Birnie, and Cohen (32). Magnetic coupling for activating beam arrests has also been used by the author. Vieweg and Gast (67) in 1944 designed a recording vacuum microbalance with a resolution of 10 y operating on the principle of magnetic coupling between an energized moving wire coil mounted on the microbeam and the stationary ones mounted on each side. The current induced in the outer coils by movement of the inner one was amplified and adjusted t o restore balance and a t the time t o signal the recorder. (2) Radioactizie deflection. Feuer (68) in 1948 applied radiation from a radioactive material to the construction of a new t.ype of microbalance which lends itself t o vacuum adaptation. He placed a very small quantity of an alpha emitter at the end of the microbeam of a knife-edge microbalance and measured the beam deflection in terms of the variation of the ionization current of a three-plate ionization chamber placed in near proximity to the beam end. The dependence of counting efficiency on counter geometry is well known. Using a spotlight galvanometer t o measure the ionization current a reproducible sensitivity of g./mm. deflection was obtained. Feuer also reported that the electrical sensitivity of his counting circuit was acceptably stable. This type of microbalance has never been reported applied to vacuum operation. (3) Getters and outgassing. It is often particularly critical to preserve sample surfaces free of contamination in surface studies using the vacuum

VACUUM MICROBALANCE

:

LOW-TEMPERATURE

APPLICATIONS

63

microbalance because the surfaces are usually. very small. Outgassing the reactor tubes at temperatures up to 1000-1100°C. is readily achieved when they are of quartz or high-temperature porcelain construction. The connecting tubes of Pyrex glass can be baked out a t a suitably high temperature (5OO0C.), but outgassing the balance and its envelope presents difficultybecause of the limited temperatures that precision balances can stand. The spring balance does not suffer from this limitation but for balances of more complicated structure, especially those in which joining of the members is achieved with selenium or silver chloride cements, it is unwise to outgas a t higher temperatures than 300°C. and usually wise to stay a t 200°C. to avoid any plastic flow. Fortunately adsorbed water can usually be completely removed at this temperature from glass surfaces (Dushman, 69). A high-melting (m.p. 500°C.) solder of silver-platinum (50-50) has been used by the author to join tungsten fibers to quartz, but it is not easy to get a good joining of fine members with this solder. It is necessary to eliminate all greased ground glass seals or gasketed metal ports when outgassing the balance envelope at any temperature above 60°C. It is essential to use a glass-quartz system with no stopcocks, greased seals, or gasketed ports, and to bake out the reactor tube at lOOO"C., the connecting tubes at 500"C., and the balance envelope a t 300°C. for precision surface studies. In general, outgassing the balance itself is feasible because of its quartz construction, but any mechanical or optical instruments that must operate in the vacuum add difficulties to a point where it is wise to avoid the smallest complication in design wherever possible. The use of getters can be applied with considerable advantage (Farkas and Melville, 70). The author has used barium-aluminum (50-50) with success but has found that freshly evaporated metal surfaces are even more effective for the removal of oxygen and in some cases of nitrogen. It may be noted that the limitation of dead-space allowance is not a serious consideration in vacuum microgravimetric studies. A bulb with a large internal freshly evaporated metal surface can be coupled in close proximity to the sample surface. This is a remarkably efficient procedure, especially since the evaporator can be "flashed" as often as required during the pretreatment, cooling down, or adsorption stages. It is usually desirable to evaporate the same metal as that of the sample surface on the assumption that the vapors most likely to contaminate the sample will be those most effectively scavenged by the getter. The use of suitable plates and baffles to avoid contaminating the sample surface is recommended. An application of this technique is illustrated in Fig. 13. The sample (10) hanging from one end of the beam is enclosed in a molybdenum shield (8) heated electrically (7) for outgassing. A chemi-

G4

T. N . BHODIN, JR.

cally active metal vapor either identical with th a t of the sample (10) or of a more reactive metal, such as an alkali metal where more intensive purging is required, is evaporated from the molybdenum trough (1 1) and forms a condensed film (6) with an active surface area 100 t o 1000 times as large as that of the sample surface. The dummy plate (9) and thermocouple lead-ins (4) are useful for precision measurement of the sample surface temperature. The whole assembly is enclosed in a quartz bulb (3) and immersed in a refrigerant (1) contained in the Dewar (2). The reactor bulb is sealed onto a quartz tube which in turn connects

n F

FIG. 13. Microbalance reactor for use with getters of evaporated metal films formed in situ. (After Rhodin, J . Phys. & Colloid Chem., to be published in 1953.1

to the Pyrex glass well through which the hang-down wire swings. Items (5) and (12) are suitable lead-in wires. As previously described i t C. Sample Insertion and Manipulation. has often been found desirable to load the balance a t a distance from the working parts themselves. It is convenient to design the bottom section of the well so that i t may be removed for insertion of the sample. I n case of very precise adsorption studies using small single crystal surfaces it is essential to insert a highly polished spherical counterweight of the same weight and material in the adjoining well. The beam is usually arrested during this operation t o facilitate coupling the fine fiber hook of the sample to the hang-down hook. I n nonarrested balances this operation can be a difficult one. Since the delicacy of the hang-down suspendon often deter-

VACUUM MICRORALANCE: LOW-TEMPERATURE

APPLICATIONS

65

mines the sensitivity of the balance, it is critical not t o apply a sudden force on the hang-down hook since it is suspended on the finest possible cross wire. A mechanical micromanipulator can be used for this operation but with some care and skill manual manipulation is satisfactory. Manipulation of the sample once it is loaded is not readily achieved. It would be desirable, for example, to hold the sample stationary if supplementary observations are to be made of the surface by x-ray diffraction or optical spectrometry. It is possible that ferromagnetic samples may be lined up by the application of a suitably oriented magnetic field, but efforts to achieve this have so far not been too successful. The placing of temporary arrests around the sample has also been found to be inadvisable because of the difficulty of not injuring the hang-down connections. d. Auxiliary Operations. (1) Temperature control. The measurement and control of temperature can be achieved by the usual methods of high vacuum technique, but certain techniques found particularly useful for vacuum microgravimetry should be noted. Temperatures can be determined with a chromel-alumel or platinum-rhodium thermocouple mounted on a dummy specimen identical in size, shape, and material to the sample, and placed as close as possible to the sample hanging on the balance. This technique compares favorably with that of a tungsten wire resistance thermometer wound around the inside wall of the reactor. The latter has been found to be somewhat faster in its response to temperature variation. The resistance thermometer also facilitates the reliable measurement of temperature in the liquid nitrogen temperature range. Surface temperatures can be measured concurrently with weight changes by using a fine double hang-down wire itself as part of a thermocouple circuit but not without difficulty. Variations of this method have been tried in efforts to use the balance as an adie')atic calorimeter (Gregg, Rhodin, 71). A resistance heating device enclosed in a vacuum wall similar to that described by Gulbransen (72) was found satisfactory for high-temperature outgassing and pretreatment operations. Induction heating of the sample as an alternative method is rendered difficult by the geometry of the sample and by the poor coupling with the field caused by the requirement that the sample swing free. For low-temperature operation, liquid gas refrigerants contained in Dewars surrounding the reactor effectively maintain the sample temperature, but it is critical that the level be maintained constant for precise adsorption studies. Temperatures intermediate between the boiling or critical temperatures of nitrogen can be maintained by controlled pumping of the volume over the refrigerant gas, either as such, or

66

T. N. RHODIN, JR.

mixed with oxygen, in a manner described in detail elsewhere (Rhodin, 73). I n all cases the temperature of the refrigerant is measured periodically with either a resistance thermometer or a gas thermometer and in some cases recorded continuously. I n precision adsorption studies local pressure fluctuation caused by thermal eddy currents must be minimized. Not only are the beam, hang-downs, weights, and tubes balanced, but it is also often desirable to maintain the temperature of sample and counterweight equal during an adsorption run. The tube ends containing them are immersed in the same manner in matched Dewars and the liquid levels, volume, and composition of the refrigerants kept the same for both parts. When the liquid level is kept constant during a run, the temperature may differ by less than 0.007"C. between the two arms of the balance and by less than 0.05"C. during the determination of an isotherm. Although some of these precautions may be modified, the maintenance of carefully matched temperatures is important for precision microgravimetric adsorption studies. (2) Pressure control. Gas pressures can be measured over the range of mm. to 760 mm. with the usual combination of McLeod gage, ionization gage, and precision bore manometers containing either dibutyl phthalate or mercury, depending on the pressure range. Pressures lower than 10 cm. are usually best observed with a microscope slide or micrometer eyepiece for precision measurements. Cross calibration of the gages facilitates switching from one range t o another, continuously, during a run and is desirable when a self-consistent set of isotherms is being measured. Buoyancy corrections are often required since surface studies are seldom pursued a t constant pressure, and often several gases and vapors may be used in one experiment. When the weight effects are of the same order of magnitude as the buoyancy effects, very spp:ial techniques are required t o prevent excessive errors (Rhodin, 73). Such difficulties are inherent in single crystal studies because of the small surface effects and high precision required. The sample in the form of a single crystal plate, approximately 0.5 g. in weight, is balanced by a spherical counterweight of identical material and matched to within g. in weight. Hence the volumes displaced b y each are essentially the same, although the surface of the sample may be from ten t o a hundred times as large as that of the counterweight. The spherical surface of the counterweight is highly polished. Weight changes caused by surface reactions are magnified in this manner. A positive correction of 1 t o 10% has to be made t o the observed weight change to compensate for the increased weight of the counterweight a t each measurement. The error in the correction is from 0.1 t o 0.01% of the weight change, and it is possible to obtain reproduci-


67

ble points for the adsorption of 10-7 g. of gas with a n uncertainty of f200/,. Although this technique is a very effective one for eliminating what would otherwise be impossibly large buoyancy corrections, it is a laborious procedure because the counterweight must be prepared as carefully as the sample and matched for each run. It is indispensable only for the most precise work with the smallest surfaces. Wherever possible, it is desirable to some degree t o magnify the weight effects by using larger surfaces and thus avoiding this exacting procedure described above. It may be noted that in the more sensitive vacuum balances, the damping time is increased when the envelope is evacuated, but this effect does not introduce any serious difficulties. It may also be noted that rapid changes of pressure in the balance assembly may injure the fine structure of the balance especially if air is introduced either intentionally or inadvertently by sudden failure of the housing. The technique of pressure balancing used so effectively for measuring gas densities and determining molecular weights by Cawood and Patterson (74), Leadbeater and Whytlaw-Gray (75) and Lambert and Phillips (76) has been applied to surface studies but is limited in that the surface study must be made at the pressure required for balancing. However, it is a simple effective and remarkably precise technique for weight compensation in studies where small variations in the pressure of the gas or vapor is not of critical importance. The advantage of no dead-space limitation characteristic of vacuum microgravimetry can also be exploited by inserting traps containing various vapors to be tested in the system and by heating the trap to obtain the desired vapor pressure when required, or freezing out the liquid with suitable refrigerants when its elimination from the system is required. (3) Weighing observation. It has already been pointed out that the most effective vacuum microbalances depend on the elastic deformation of a quartz or tungsten member by bending, twisting, stretching, or otherwise distorting it in a reversible and measurable manner. Whereas most of the weight from the pan, hang-downs, and sample is best balanced independently, the weight change caused by the surface reaction can be observed in either one of two general ways. Application of a precisely controlled compensating torsion or electromagnetic force, for example, will return the beam to a null position. Alternat.ively, the weight change can be followed in terms of the beam deflection. The author has tried both techniques and recommends the latter because it allows for greater simplicity in the construction and operation of the balance. It is not as capable of great accuracy as the null method of observation, but it is adequately precise over a limited range using special techniques. Using

68

T. N. RHODIN, JR.

a Bausch and Lomb filar micrometer eyepiece mounted on a Geneva cathetometer, a precision of deflection measurement better than 0.003 mm. over a total vertical displacement of 1 cm. can be attained. Precise observations of another beam with a greater beam swing were made by vertical adjustment of the cathetometer which can be adjusted to a maximum precision of 0.01 mm. It is important t o check the horizontal swing of the cathetometer against the two levels of a mercury IT-tube, the menisci of which arc located sufficiently near to the beam ends as to be in the same focus as the indicating fibers. Sights arc taken of tungsten or quartz indicator fibers, 25 p diameter or smaller mounted a t each end of the beam. The total beam deflection is obtained in this manner from the difference of two readings, independently of any small movement th a t may have occurred in the balance position after the last preceding reading. The weight change ran be calculated from the sensitivity factor, and corrected for adsorption on the counterweight as described previously. I n this way it is possible to observe weight changes of g. k 2Q% in a reproducible and reliable manner without the necessity of making a large number of measurements. When all the instruments are mounted, focused, and calibrated, about 10 seconds is required to make a reading and i t is not necessary t o wait for the balance oscillations to subside. It must be admitted that precision of this order is not attained without exercising due care. Another balance with a lower center of gravity and a somewhat heavier torsion fiber showed greated stability g. in vaczco with _ + l o %accuracy or and still measured changes of better. I n the author’s opinion, success in making precision adsorption studies with the vacuum microbalance depends t o no small extent on the simplicity of the instrument and on the exploitation of differential techniques in the elimination of errors caused by thermal eddy currents, buoyancy effects, beam distortion, and optical aberration.

4. Sample Preparation a. Sample Requirements. The preparation of solid surfaces of a type suitable for surface chemistry studies using precision vacuum microbalance techniques is unique to such studies in the sense that the effectiveness of the technique depends to a considerable extent on the care and the ingenuity with which the sample surfaces are prepared. Weight increments caused by surface effects such as adsorption, reaction, solution, or evaporation are often small and, even for an exceedingly sensitive instrument, it is desirable for the samples to possess a high surfaceto-volume ratio, In other words the optimum geometry of the sample is that of an exceedingly thin plate or foil. Sample surfaces suitable for such studies have been successfully prepared in these shapes, and it seems


69

likely that future progress will be directed toward samples consisting of polycrystalline foils, powder compacts, single crystal plates, or evaporated films, the latter usually of metals. The preparation and characteristics of three of these classes of samples will he discussed. Microbalance studies can be extended t o finely divided catalysts if a device is provided for suspending the powders on the hang-down wire. Boats and other containers are undesirable since they contribute excessively to the total load on the system. In many cases fine powders can be compressed at reasonable pressures into compact masses to permit their suspension without a container (Gulbransen, 77). Little information is available now as to applications of this technique, and hence discussion will be focused on the three other classes, although powders can be studied when contained in lightweight boats of platinum or quartz. b. Polycrystalline Foils. Thin metal foils present a compromise between evaporated films and single plates in that they possess a relatively well-defined surface structure compared with the former, and a larger surface area per unit weight than the latter. It seems t o be a general dilemma characteristic of samples suited for microgravimetric surface studies that either the surface structure is ill defined, or the surface area and hence the gravimetric resolution is poor. Beeck (78) and his associates have shown how useful studies of evaporated metal films can be, and the possibility of precision surface measurements of small surfaces has also been demonstrated. It is not unlikely that metal foils may be best suited, of the three classes discussed here, for studies with the vacuum microbalance. Metal foils 0.025 mm. thick can be readily prepared from most metals and alloys using a Sendaimer mill. A layer of physically adsorbed nitrogen weighs about 0.04 X 10V g./sq. cm. This is about one-fifth the value of the corresponding weight of a monolayer of chemisorbed oxygen or of a metal oxide. One must use either a microbalance capable of measuring 0.04 X 10V with an acceptable accuracy, or of increasing the weight increment, e.g., the surface area, to a value measurable with an available instrument. Let us consider the second alternative, by making use of a vacuum microbalance of convenient characteristics, e.g., with a capacity of 1.0 g. and a sensibility of 1 X lop6 g. & 10%. A 1-g. sample, 0.025 mm. thick and with a roughness factor of 1 would have a surface area of 120 sq. cm. for aluminum, 40 sq. cm. for nickel, copper, and iron, and 20 sq. cm. for tungsten. The roughness factor is the ratio of actual or molecular surface area to the apparent or geometric surface area. The relative error in the determination of the weight of a monolayer would be correspondingly _+ 2, f6, and ?z 12%. Unless the samples are very carefully prepared, the roughness

70

T. N. RHODIN, JR.

factors may be about tenfold higher than unity if their surface have been oxidized and reduced, and higher by a magnitude of 100, if the surfaces have been chemically etched (Brunauer, 79). These factors would increase the weight effect of adsorbed gases, etc., and hence decrease the weighing error proportionately. It is customary to prepare sample surfaces by one or more of the following treatments: polishing, abrading, pickling, annealing, or hydrogen reduction. All these treatments have a profound effect on the surface structure, in that not only the extent but the nature of the surfaces are affected. I n some cases preferred orientations may be induced on the surfaces of metal foils by suitable metallurgical treatment, so that although the surface remains polycrystalline, a specific crystalline surface orientation predominates. This method has been applied by the author in the study of copper surfaces, and photomicrographs a t 2000X of two such surfaces used for surface studies are reproduced in Fig. 14. The cubic texture of the surface in Fig. 14b was obtained by annealing a sample of foil a t 300°C. in a good vacuum. The foil had been obtained by cold rolling some bar stock to a 98.5% reduction. The octahedral surface texture shown in Fig. 14a was obtained by annealing the surface shown in Fig. 14b for 100 hours a t 500°C. in a pure hydrogen atmosphere. This technique is a very effective one for obtaining specific surfac.e orientations without the serious sacrifice in surface area characteristic for single crystal plates. It has been used seldom in surface studies because of the complexities characteristic of this type of recrystallization phenomena. The crystalline orientation of the surface may be considered an intrinsic property, whereas surface roughness may be classed as a n extrinsic property, in that, it usually defines merely the extent of the surface, although it usually is, in some way, related t o the specific nature of the surface. It is quite evident that a variety of surface characteristics can be developed by the kind and sequence of pretreatments used for a given sample. Etch patterns, rolling textures, recrystallization, and exaggerated grain growth can be employed singly or in combination to prepare useful sample surfaces. Recrystallization and grain growth have been studied in detail in some cases (Harker and Parker, 80; Burke, 81 ; Smith, 82) from a metallurgical viewpoint and will continue to furnish a useful background for the preparation of metal surfaces. It is not the point of this presentation to discuss recent advances in surface metallurgy, for this field has been well reviewed in the literature and continues t o be extended daily. We rather prefer to indicate special metallurgical techniques for preparing unique surface structures useful for microgravimetric study. There is much to be known about the very thin surface layers on


71

FIG.14. Photographs of rccrystallized surface textures on copper foil a t 2000 X : octahedral orientation; (h) cubic orientation.

(inless steels, the Hastelloys, and other corrosion-resistant alloys. Although it is generally believed that passivity is a surface phenomenon (Speiser, Beck, Fontana, and Lassettre, 83; Uhlig, 84) efforts to assess the relative importance of physical adsorption, chemisorption, oxidation, and surface enrichment of alloy componentJscontain many ambiguities. The study of oxidation and adsorption reactions on stainless steel surfaces in

72

T. N. RHODIN, JR.

terms of surface structure and composition continues to be important from both a fundamental as well as a commercial viewpoint. c. Single Crystal Plates. In the past five years there has been considerable progress in the preparation and characterization of single crystals of metals, ceramics, and organic salts (Bridgman, 85 ;Gwathmey, 86). The surface chemistry of solids in general and of metals in particular has also been pursued with renewed interest in recent years and applied to some considerable extent to single crystal specimens. The surface of a metal crystal may, in a region where the macroscopic surface has a given orientation, approximate the ideal crystal plane of the corresponding orientation. This is not necessarily true in many cases (Langmuir, 87; Tonks, 88; Hering and Nichols, 89) and probably very rarely true in the complete sense. Increasing attention is being given today t o analyzing surface effects in terms of an interface with its own unique and variant properties rather than in terms of an essentially inert solid substrate. Gwathmey (86) and his associat :s have concerned themselves with the problem of studying metal suriaces in terms of chemical reactivity. They have been particularly concerned with the surface structure in so far as it is modified or otherwise influenced by the fluid with which it is in contact. The reader is referred to their work for details concerning the specific manner of preparing single crystal metal surfaces for the study of surface reactions. The definition of surface structure on the molecular scale is a critical prerequisite to fundamental surface studies. It is exceedingly difficult to do so when mechanical surface polishing techniques often tend t o distort the structure of the surface layers (Elam, 90; Dobinski, 91). There is good reason to believe that electropolishing, on the other hand, is remarkably effective in removing irregularities on a very fine scale but the hazard of surface contamination from the electrolyte remains (Jacquet, 92). Reflection electron diffraction studies have been useful in analyzing surface structures (Heidenreich, 93; Gregg, 94; Hickman and Gulbransen, 95), but the question is still unsettled as to whether crystal surfaces, which are essentially planar on a molecular scale, would give coherent diffraction pat,terns since many authors assume that reflection patterns arise from electrons that are coherently diffracted by suitable surface projections. Schulz, working with the author, was unable to obtain diffraction patterns on carefully electropolished, cleaned, annealed, and reduced single crystal surfaces of copper (Schulz and Rhodin, 96). Single crystal patterns are, however, obtained when the surface is slightly roughened by a mild etch or oxidation-reduction cycle. Raether (97) has made extensive structural studies of metal surfaces, and Fig. 15 shows an electron micrograph of a replica of an electropolished single crystal



73

- 0

10,000A

FIG. 15. Electron micrograph of silica replica of electropolished (100) Single crystal face of copper a t 160,OOOX. [After Raether, Optik 1, 69 (1946).]

copper surface. It illustrates the degree of planarity characteristic of electropolished single crystal plates of copper. In this particular sample more than 90% of the surface atoms may be assumed to be arranged in the cubic face. The sample had been prepared as described in the Raether’s text. In this case an evaporated silica replica was used t o

74

T. N. RHODIN, JR.

obtain the electron micrograph. The magnification in the figure is 160,000 diameters. It also may be mentioned that single crystal patterns have been obtained by electron diffraction reflection from single crystal plates of mica, which are known to be flat with steps and risers of atomic dimensions, and from very thin evaporated metal films deposited on mica as a substrate (Schulz, 98). The problems involved in the preparation and characterization of undistorted single crystal metal surfaces have been considered in the literature at great length, and yet the complexities of the problem are often

SINGLE CRYSTAL Cutting Holder

FIG.16. Single crystal jig for cutting and orienting single crystal metal plate from single crystal cylinder. [After Rhodin, J . Applied Phys. 21, 971 (1950).]

underestimated. Typical difficulties are illustrated by the preparation of thin relatively undistorted single crystal plates of copper isolated from large sound single crystals. A suitable procedure for mounting and electropolishing such samples has been described (Rhodin, 99). A single crystal jig and cutting holder which has been found t o facilitate determining the orientation, and sectioning, is illustrated in Fig. 16. The single crystal (3) in the form of a cylinder 6 in. long and 2 in. in diameter is held tenuously by the spacer (9) in the jig (4). The crystal is rotated around the horizontal axis (6) and the vertical axis (5) to line up the desired crystal planes parallel to the film (1) as indicated by the pattern resulting from diffraction of the x-ray beam emerging from the pinhole (2) and striking the film (1). Without disturbing the orientation the whole assembly is transferred to a cutting wheel which rotates in a plane parallel

VACUUM MICROBALANCE


75

to that of the film as well as to that of the desired set of crystal planes. This jig has been found to be very useful in the preparation of single crystal metal plates. It is, of course, necessary t o remove carefully the disturbed surface layers after the single crystal plate is removed from the jig without changing the surface orientation. This step is difficult and not always possible. Good preparation of a single crystal sample surface inevitably requires the collaboration of a n expert physical metallurgist. The technique of controlled secondary recrystallization developed by Dunn (100) for the preparation and surface studies of single crystal plates of silicon ferrite has also been applied to copper by Rhodin (103). It is known that the low-temperature recrystallization of very heavily rolled copper produces a fine grained structure with a high degree of preferred orientation (Bowles and Boas, 101 ; Burgers, 102). Additional heating to within a few hundred degrees of the melting point may induce an abrupt and pronounced increase in the grain size, with the resulting crystals having new and remarkably well-defined orientations. This behavior a t high temperatures is commonly called secondary recrystallization. The orientation before the grain growth corresponds to a statistical distribution of crystal faces (100)[OOl], the so-called cubic texture. There is a variety of new crystal orientations after recrystallization, and different experimental conditions yield different orientations. Similar effects are known to exist for silver, aluminum, and nickel, but the technique for preparing single crystal plates for surface studies of these metals by this method has not been well explored. Single crystal plates of copper suitable for surface studies have been prepared by the author (Rhodin, 103). The use of controlled secondary recrystallization for controlling the surface orientation of single crystal metal plates is best illustrated in Fig. 17. “ A ” represents a section of copper foil 0.10 in. thick, 1 in. wide and 2 in. long with a tail on which a small crystal with a (110) orientation has been induced to grow. The rest of the sample consists of heavily rolled material in which the cubic orientation has been developed by low-temperature annealing. In “ B ” the seed crystal has been oriented in space by careful manipulation in a jig so that its (110) pole is normal to the surface of the whole sample. In ‘ I C ” the seed crystal has been induced to grow throughout the specimen by feeding the strip slowly through a temperature gradient of maximum temperature near the melting point of copper. At times it was possible to control the surface orientation of the large crystal by altering the position of the seed crystal after it had formed, but unfortunately this was usually unsuccessful because of localized recrystallization induced often at the twist by the cold work. In spite of considerable studies the technique never

76

T. N. RHODIN, JR.

improved past the point of growing seed crystals of random orientations on a large number of strips and cultivating only those that had useful orientations. Nevertheless, the method proved to be a useful technique for the preparation of a number of single crystal plates of copper with a minimum of surface distortion. A similar method has been used by the author for inducing ferritic grain growth in iron by a strain-anneal method. It was developed with the assistance of Bergman and Cerny (104), and although the factors defining the large grain growth were far from clearly understood it proved to be remarkably effective for the preparation of strain-free single crystal A

B

C

FIG.17. Predetermined surface orientation of single crystal metal plates by secondary recrystallization.

plates of iron. This general method is excellently suited for the preparation of single crystal samples for surface studies since a minimum of handling is required after crystal growth preliminary to insertion in the microbalance. It is the author's experience th a t the final annealing temperatures used prior to grain growth were sufficiently high for the surfaces t o assume a high degree of planarity. This was achieved either by sandwiching them between highly polished quartz or often by a simple process of firepolishing. Reference should be made to another useful technique in the preparation of single crystal zinc surfaces. It can be used for single crystals susceptible t o cleavage along predefined crystallographic planes such as zinc and bismuth (Rhodin, 105). Commercial zinc of the highest degree of purity was repurified by vacuum casting it six times for 24-hour periods at mm. mercury and 430°C. Spectrographic analysis indicated 0.002% lead, 0.0002% copper, and 0.0003% iron as residual impurities. Single crystal rods (1 X 15 cm.) were grown using the Bridgman technique of slow movement (0.1 mm./minute) through agradient furnace of a maximum furnace temperature of 430°C. The rods were coated with


77

a very thin film of a urea-formaldehyde resin, cooled to liquid nitrogen temperature and cleaved along the basal plane. For this, it was sufficient to form a very small needlepoint depression on the rod surfaces, and induce cleavage by a series of cooling-heating cycles (-195 to 25OC.) without the application of additional mechanical stress. This technique was remarkably effective in minimizing surface twinning and in avoiding kink band formation. The original crystal plates (1 X 3 X 0.1 cm.) were electropolished in a nitric acid-methyl alcohol mixture (1 :4) over a period of 10 hours a t 0°C. with a current density of 1 to 5 centiamperes/ sq. em. The final crystal plates (0.03 cm.) showed negligible edge attack and no twinning lines were observable on microscopic examination. A group of these plates formed a typical specimen for adsorption studies. It weighed about 1g. and had an apparent surface area of about 10 sq. cm. Test samples were characterized by x-ray and electron diffraction and by electron microscopy to indicate the effectiveness of the preparation from the viewpoint of creating a planar undistorted surface. The roughness factor of the surface indicated by adsorption studies were in good agreement with the expectation that the surfaces actually did possess planarity on a submicroscopic scale. Because of the very small actual surface area of the adsorbent it was particularly important to keep the system and the specimen surface free of contamination to insure precise and reproducible measurements. The samples were subjected t o an annealing and firepolishing treatment in situ in addition to the more general precautions previously described. The vapor pressure of zinc and the stability of the oxide in about 2.5 atm. of hydrogen a t 375°C. are such that firepolished reduced zinc surfaces can be obtained by careful adjustment of temperature and hydrogen pressure. Zinc vapor from an independent source was exploited to “getter” oxygen from reaction volume without contaminating the specimen surfaces with spurious zinc vapor as shown in Fig. 13. I t illustrates the arrangement of the sample, dummy sample, and the shielding and heating elements around the sample. No increase of weight was observed with this sample for 15 minutes during which a relatively large surface area of evaporated active zinc was produced by condensation of vapor on all the reactor walls. Use was made of the straight flight of the evaporating zinc atoms in the evacuated vessel to insure, by suitable shielding, that no measurable amount of the evaporated zinc condensed on the sample surface, thus afflicting its known surface structure. I n the study of the surface anisotropy of adsorption (Rhodin, 73) and oxidation (Rhodin, 106) of single crystal copper surfaces no other technique was as effective as “gettering.” It may be noted that although the production and preservation of clean metal surfaces is extremely difficult,

78

T. N. RHODIN, JR.

by the same token the presence of such surfaces can be determined very rapidly from their high reactivity. Thus, the practically instantaneous and irreversible formation of a stable monolayer of oxide a t low temperatures on many metal surfaces prepared by special treatments such as long-term reduction with high-purity hydrogen, evaporation, or firepolishing is a good indication that the surface had been successfully kept free of excessive contamination by chemisorbed or reacted layers during its cooling to the adsorption temperature. The reproducibility of adsorption effects unique to a surface of given crystallographic structure is also an indication that the surface has been preserved intact. In certain cases, of course, this does not preclude the possibility of systematic disturbances. d. Evaporated Films. The formation of metal films by evaporation in a high vacuum has much to recommend itself as a technique for the preparation of experimental surfaces (Olsen, Smith, and Crittenden, 107 ; Beeck, Smith, Wheeler, 108; Rhodin, 109). It is advantageous to generate the surface in situ under conditions of minimum contamination. Films of this type have usually a considerable surface area. A great disadvantage, however, lies in the ambiguity of the surface structure of such films, compared with electropolished and hydrogen reduced surfaces. A related difficulty derives from the thermal instability of the evaporated films. A great number of metal films formed by condensation onto a solid support are unstable, in the sense that they will aggregate into crystals, provided the atoms possess sufficient surface mobility. The crystalline structure of the film is often strongly influenced by the nature of its support, and, in some cases, a single orientation prevails when the force fields around the atoms or ions in the supporting material are sufficiently strong (Barrett, 110). Relatively little data are available in the literature concerning the nature of these forces and the role they play in promoting preferred orientation of metal atoms condensing on the surface of other materials. Evaporation of metals in a high vacuum onto carefully chosen supports under conditions ideal for preferred orientation appears to be usable in some cases for the preparation of oxide-free oriented films on which t o study surface reactions (Rhodin, 109). Of the many factors that influence film structure some of the dominating ones are film thickness (Dixit, 111), growth rate (Estermann, 112), nature, condition, and temperature of the support (Briick, 113; Rudiger, 114; Finch, Quarrell, and Wilman, 115), and gas atmosphere effects (Beeck, 108). The structural properties unique t o such films (Van der Merwe, 116) and their application to surface studies have been reviewed (Beeck, Smith, and Wheeler, 108). Nevertheless, there remains more


79

work t o be done before films of known and reproducible surface structures can be prepared by evaporation in a manner sufficiently reliable for fundamental surface studies. This is not meant to indicate, however, th a t evaporation techniques do not furnish a very useful method for preparing essentially clean metal surfaces in situ under proper experimental conditions. The usefulness of this technique in surface studies has been amply illustrated. (1) Formation techniques. The film can be produced by evaporating a highly purified metal from a microcrucible or filament in a good vacuum (10-8 mm.) and condensing the vapor on an independently heated support. With a suitable slit system, a uniform and concentrated molecular beam can be obtained. The heating of the crucible can be achieved very satisfactorily by an arrangement for direct electronic heating as indicated in Fig. 18 (Shaw and Rall, 117). The microcrucible is indicated at the top of the diagram. A top view of the assembly is shown in the middle of the figure, the inside circle corresponding to the crucible, the second circle t o the heating filament, the third circle to the tantalum shield and the fourth, and outmost circle to a Pyrex glass flange t o which the assembly is sealed. Side and end views of the assembly are shown in the lower part of Fig. 18. The substrate is mounted about 25 mm. away from the vapor source in a copper frame which holds it in place, backed by a n externally heated copper block. I n this manner the lower face of the substrate is exposed to the vapor beam and maintained at the desired temperature by heating it a t its upper face. The temperature is regulated within about 1yo by a proportionating potentiometer controller. The substrate consists of a square plate of an ionic salt with sides approximately 5 mm. long and 1 mm. thick. A freshly cleaved face of the salt crystal is exposed just prior t o a run. A thermocouple in contact with the lower face of the support indicates the film temperature. The source of the beam is the small crucible focused directly on the substrate as indicated in Fig. 19. It shows how the electronic evaporator can be used in conjunction with a microbalance system. The evaporator is shown mounted on the tube in such a manner th a t it produces films directly on the substrate hanging on the fiber. After closing the shield by sliding it around with an external magnet, the evaporator can be used t o form a n active film in the side arm without contaminating the substrate. A Dewar indicated on the diagram t o be used in adsorption studies can be replaced by a suitably shaped furnace as indicated b y the dotted lines in the figure. The microcrucible holds a charge of 100 mg. The inside diameter of the crucible is 3 mm., but the beam is actually emitted through a 0.1-mm. orifice in a tantalum cap placed over the top

80

T. N. RHODIN, JR.

4';hngsten .064

Crucible

Fro. 18. Details of electron heater used for precision evaporations in microgravimetric studias. [After Rhodin, Disc. Faraday SOC.6, 215 (1949).]

VACUUM MICROBALANCE

:


81

of the microcrucible. The cap prevents splattering and also promotes thermal equilibrium of the atoms before they are emitted. The charge is outgassed by prefusing it in situ before evaporation. During this procedure, the support is protected by an externally manipulated shield. Electrons emitted from an incandescent 30-mil tungsten filament around

FIG. 19. Microbalance reactor assembly with evaporator.

the crucible are accelerated toward it by a positive potential. A tantalum shield around the assembly reduces heat losses, and a tantalum lid shields the support from direct exposure to the filament. The temperature of the microcrucible can be accurately adjusted and maintained a t any temperature up to 1500°C. within 2 t o 3 % by regulating the filament emission and the accelerating potential. The support is kept a t the same potental as the crucible to eliminate the possibility that metal ions formed in or around the crucible may be accelerated toward the substrate.

a2

T. N. RHODIN, JR.

( 2 ) Structural properties. The structure of metallic films can be conveniently determined using x-ray diffraction with a surface reflection pinhole technique in a vacuum camera as indicated in Fig. 20. This device resulted from extensive efforts to develop a camera th a t could be used to determine the surface orientation of thin metal films with a minimum of manipulation of the instrument, of exposure time and of calculation required to obtain the data on orientation from the diffraction pattern. The film (6) mounted with a Lucite spacer (7) onto the turn table (8) is activated by the shaft (10). The beam (3) enters through the

FIG.20. Vacuum camera for x-ray diffraction studies of thin evaporated metal films. [After Rhodin, Metals Transactions 186, 371 (1950).] beryllium window (4) through the pinholes ( 5 ) and is diffracted by the metal film onto the photographic film covering the internal wall of the cylindrical film case (2). The case is rapidly dismantled by loosening the lock nuts (14) and removing the end plates (1). The whole assembly is pumped out through the outlet (13) and mounted on the track of a x-ray machine with the track clamp (16). The metal film is rotated to insure that all crystal planes come into a position for diffraction and the whole case is pumped out t o facilitate picking up the weakly diffracted beam with a minimum exposure. This is particularly desirable because the metal films are usually very thin and it is especially important when the support upon which the film forms is a crystal causing strong fluorescence. A Picker-Waite diffraction unit is used with a water-cooled chromium target. The exposure time varies from 2 to 15 hours for film thicknesses from 5000 t o 500 A. with an accelerating potential of 50 kv. and a space current of 10 ma. The sample is anchored flat on the turntable and rotated around an axis normal t o the surface of the sample. T h e axis of

VACUUM MICROBALANCE

:


83

rotation is inclined away from the incident beam by a n angle corresponding t o the Bragg angle for reflection from the plane of preferred orientation. The camera is particularly designed to suit the geometry and orientation unique to thin evaporated films. Figure 21 illustrates how well the camera is designed to satisfy these requirements. Preferred orientation is characterized by segmentation of the lines into localized marks as shown by the heavy marks of 5 degrees length on the dashed reflection lines in the schematic representation of the exposed film on the right side of the figure. The vertical distance from the center line measures the orientation azimuths characteristic of a preferred orientation. I

I

11 I I I

I

__-* -I t

I

I

I

I

I

t

.

100

Fro. 21. Schematic illustration of typical X-ray diffraction pattern from oriented film. [After Rhodin, Metals Transactions 186, 371 (1950).]

The distance along the abscissa measures the angle characteristic of reflection from a given set of crystal planes. The diagram on the left side of the figure indicates schematically how the cones of reflected radiation intersect the cylindrical film in the camera before it is removed for analysis. The actual analysis of the pattern obtained in this manner can be made in the usual manner (Barrett, 110) and will not be described here. (3) Additional considerations. The degree of orientation depends, strongly for many film-support combinations, on the film thickness. Generally, the degree of orientation depends also on the temperature. A typical case is illustrated in Fig. 22 where the per cent orientation for aluminum deposited films on a (100) face of rock salt as a support is plotted for various thicknesses and temperatures of the support. The dependence of orientation on film thickness is exponential. It is interesting t o note t ha t there is indicated the existence of a critical film thickness for perfect orientation a t each temperature, by extrapolating the straight line of degrees of orientation toward small thicknesses. A striking

84

T. N. RHODIN, JR.

temperature dependence is also indicated by the much smaller slope of curve 2 compared to that of curve 3. The less the degree of the maximum orientation the smaller is the dependence on the temperature. e. Contamination Control. Interpretation of surface phenomena in terms of surface structure is greatly facilitated b y using sample surfaces 10'

I' 0

FILM TH

\

-

1

s

ii

2 10' 0

d u)

a3

f

lo'

I c

f

O Q

Id

I

\

T

z

\

\

\ \ \

(1001 Sodium C

10'

10

TI

;NESS-O IENTATI

I I

0

wid.

200.C

300.C

\ \ \

\

-

400.C

500.C

20

PERCENT ORIENTATION

FIG.22. Dependence of film Orientation on film thickness and substrate teniperaLure. [After Rhodin, Metals Transactions 186, 371 (1950).]

of a sufficiently small extent as t o he conveniently manipulated in the laboratory in the process of preparation and characterization. Specific reference has been made to the preparation of single crystal metal plates suitable for microgravimetric studies. The smallest degree of surface contamination may be sufficient t o mask surface structures which were


LOW-TEMPERATURE

APPLICATIONS

85

obtained with considerable difficulties. It must be admitted that even with the most scrupulous efforts to prevent spurious surface contamination this remains a very serious hazard. Contamination of the relatively large surfaces of metal powders, pellets made of fine particles, and evaporated films is in itself critical but much less so than for plates having relatively small surfaces. With the large-surface materials, there remains often a considerable surface fraction uncontaminated over a long enough period to permit the execution of adsorption measurements. Working a t a vacuum of mm. it is possible t o preserve small surfaces free of contamination only if a relatively large and equally active surface of a “getter material” is located as close as possible to trap oxygen or nitrogen as previously discussed. The techniques and difficulties involved in the preparation and characterization of single crystal metal surfaces have been considered here in detail because the evaluation of chemical activity in terms of surface structure, particularly of the crystallographic surface structure, is one of the most promising applications of the vacuum microbalance. The preparation of flat, clean, undistorted single crystal samples suitable for surface studies is a difficult and tedious assignment. 11. Lon -TEMPERATURE STUDIES This section will describe specific surface studies using the vacuum microbalance from the viewpoint of elucidating some characteristics of solid surfaces which lend themselves particularly well to this type of approach. 1. Surface Characterization

a. Adsorption Isotherms: General Much of the research that has been done in the past on the corrosion of metals and on the reactions between gases and metals has been handicapped by a lack of knowledge of the surface structure in general and of the true surface area in particular. It is now recognized that the physical adsorption of gases near their boiling points provides a method for determining the true surface area under certain circumstances using the Brunauer-Emmett-Teller (118) evaluation. The method of Harkins and Jura (119) for the determination of surface area makes it possible t o evaluate the effective crosssectional areas of some molecules used as adsorbates with a minimum of theoretical assumptions. The methods using gases such as nitrogen near their boiling points as adsorbates are ordinarily capable of giving a reproducible surface area if the sample has an area of the order of 1 sq. m. and a bulk volume of only a few cubic centimeters. When the particles are large and the surface areas

86

T. N. RHODIN, JR.

as well as the adsorption effects are small, the simple technique of following the adsorption by measuring the change in pressure of the gas at constant volume in the presence of the adsorbent becomes inaccurate. This difficulty can be avoided by using a vapor having a low vapor pressure at the temperature of the run. Using ethylene, for example, at 78.1"K. and very sensitive pressure gages, the sensitivity of the method can be extended to permit the measuring of surfaces as low as 100 sq. cm. (Wooten and Brown, 120). It is evident that this method is still not sufficiently sensitive for the surface study of metal specimens of small enough surface areas to be accuractely characterized. Very sensitive beam microbalances have a sufficiently high sensitivity to measure adsorptions less than cc. of gas (STP), but their use in adsorption studies has been relatively limited. The main sources of error in their application as previously discussed have been the large buoyancy corr'ections required and the presence of thermal eddy currents which obscure the rest-point determination. The following section describes the application of microbalances to the measurement of adsorption isotherms and of roughness factors and the study of the progressive oxidation of metallic surfaces. b. Adsorption Isotherms: Specific. (1) Calibration of a single surface. The adsorption isotherms of nitrogen and argon at 78.1 and 89.2"K. on the same surface of a freshly reduced polycrystalline copper specimen are plotted in Fig. 23. The smooth curves obtained from different runs using adsorption and desorption points indicate the reproducibility of the data. It is observed that the curves are of the typical sigmoidal shape, characteristic of the adsorption of nitrogen on iron catalysts and various metallic oxides where the amounts adsorbed are ten thousand times as great (Emmett, 121). Multilayer adsorption at high saturation is clearly indicated. Although the roughness factor was found as 2.5 for this sample, multilayer adsorption is also indicated in subsequently measured isotherms on surfaces with roughness factors close to unity. The roughness factor ( R ) of the surface has already been defined as the ratio of the true surface area, to the geometric surface areas apparent on a macroscopic scale. It has been established that if the amount adsorbed (2) is plotted as a function of ( p / p o ) where p is the pressure in the gas phase and p o the vapor pressure of the liquefied adsorbate, at the temperature of the isotherm measurement, a straight line indicates the applicability of the Brunauer-Emmett-Teller multilayer adsorption interpretation, according to

VACUUM MICROBALANCE


87

where c is exponential ( E l - E L ) / R T ,El is the heat of adsorption in the first layer, and EL is the heat of liquefaction. Equation (1) proved to be applicable in the pressure range p / p o from 0.05 to 0.40, for all the adsorption isotherms considered in this study. The surface areas were calculated in the usual manner from these plots. The values obtained for the surface areas from Equation (2) that are listed in Table 11, column 5 , for four different adsorption isotherms Adsorption Isotherms of Nitrogen ond Arqon on Reduced Polycrystolline Copper First Run Points Second Run Points A Ossorotion Points L o

P/Po

FIG.23. Adsorption isotherms of nitrogen and argon on reduced polycrystalline copper. [After Rhodin, J . Am. Chem. SOC.72, 4343 (1950).]

(Nos. 1, 5, 6, 7) on the same sample, agree within &6%. The crosssectional areas for the nitrogen and argon molecules were taken to be 16.1 and 14.2 sq. %..,respectively. These values are known to yield the most reliable surface areas for nonporous adsorbents of this type. (2) Oxidation-reduction cycle. A sample of freshly electropolished polycrystalline copper with an initial roughness factor of almost unity was treated by an oxidation-reduction cycle in which an oxide film effective thickness of 200 A. was formed and removed. This was done (1) to illustrate the usefulness of the adsorption technique for defining the

88

T. N. RHODIN, JR.

@, G

Run

Sample

Isot,herm

I

X 106 g./g.

sq. cm./ 0.5 g.

2,

sq. cm./ 0.5 g.

AH, cal./ mole _

Polycrystal reduced Cu 2 Polycrystal 75 A. c u z o 3 Single crystal rediiccd CLI 4 Single crystal 10 A. cuzo 5 Polycrystal reduced Cu 6 Polycrystal reduced Cu 7 Polycrystal reduced Cu 1

_

R _

_

_

N z 78.1"K.

3.5

120

103

N I 78.1"K.

1.3

45

50

3000

N 1 78.1"K.

0.7

26

19

3500 2 . 1 0

N t 78.1"K.

0.4

13

11

3650

N,89.2"K.

3.3

117

-

2500 2 . 4 0

A 78.1%.

4.6

110

-

2650 2 . 3 2

A 89.2"K.

4.5

107

-

2600 2 . 2 7

2500 2.46 1.03

1.20

roughness factor and the changes of the surface areas of samples with initially small areas and (2) t o characterize the surface itself prior t o oxidation studies. The thickness of the oxide film corresponding to a given weight change was calculated for a determined surface area, assuming the oxide film to be uniform in thickness and continuous. The density of the oxide layer was assumed to be equal to the density of the bulk oxide. The calculated thickness of the oxide film was based on the average of the surface areas measured before and after oxidation or reduction in each case. Actually, the oxide film does not strictly follow the contours of the original metallic sample, b u t this deviation is not critical in this particular case. Such effects will be discussed in more detail in the following section. The surface of a single crystal plate of copper with the cubic face exposed was treated in the same manner except that the average thickness of the oxide film added and removed was about 10 A. The values of the surface areas, roughness, factors and isosteric heats of adsorption for the two samples are listed under runs 1 , 2 , 3, and 4 in columns 5 , 7, and 8 of Table I1 for comparison. Nitrogen adsorption isotherms at two temperatures for the reduced and oxidized polycrystal sample are plotted in Fig. 24, and the corresponding four isotherms for the single crystal sample in Fig. 25. I n both cases it is significant th at the roughness factors that had increased about twofold upon reduction, decreased to unity for the thick oxide film and approached unity for the thin oxide film. In other words,

~

~


89

the effect of oxidation in this particular case was to make t,he surface smoother. It is not unreasonable that an oxide film of 200 A. average film thickness might tend t o obscure the irregularities of the substrate. It is surprising, however, that an oxide film with an average film thickness corresponding to only three monolayers was almost sufficient t o mask the surface irregularities. Detailed consideration as t o the precise physical

Adsorption of Nitrogen on Polycrystoline Copper and Copper Oxide o x Adswphon Points

0

I00

200

300

400

500

600

Prrssure, mm

FIG. 24. Adsorption of nitrogen on polycrystalline copper and cuprous oxide. [After Rhodin, J. Am. Chem. SOC.73, 4343 (1950).]

manner in which such thin oxide films form on metal surfaces is a complex problem and is not to be included here. Some doubt has been indicated as to the proper choice of a value for the cross-sectional area of the nitrogen molecule involved in the calculation of the surface area from the B.E.T. plot. The data were replotted according t o the following equation rerived from a n analysis of Harkins and Jura (119). log, p / p o = B - A/x2 (3) where x is the weight adsorbed a t pressure p , and A is a constant, t h e square root of which is directly proportional t o the absolute surface area

90

T. N. RHODIN, JR.

(2). Plotting log p / p o against 1/s2yielded a series of unbroken straight lines. The values of the surface areas were readily calculated from the slopes of the straight lines by the Harkins-Jura method and found to be in good agreement with those calculated from the Brunauer-EmmettTeller equation (columns 5 and 6 in Table 11). A comparison of the two methods has been adequately given elsewhere (Duncan, Liang, 122). It is sufficient to observe that the agreement confirms the value for the cross-sectional area of the nitrogen molecule previously used.

30

1

-

I

I

I

I

Adrorplion 01 Nitrogen on 1100) crystol l a c e of Copper ond Copper Oxide,

B

78 I*K

d

b I5 P

3 : 10 05

0

100

200

300

400

500

600

0

Prrrrura. mm

FIG.25. Adsorption of nitrogen on single crystal cubic copper and cuprous oxide. [After Rhodin, 6.Am. Chem. SOC.72, 4343 (1950).]

c. Progressive Oxidation and Surface Area. It has already been observed that the effect of oxidation on a roughened copper surface was to decrease the true area until it approached that of a completely smooth surface. This effect was systematically studied for a polycrystalline copper surface with an original roughness factor of 2.45. The originally smooth surface was roughened by an, oxidation-reduction cycle at 400°C. in which an oxide film of about 300 A. average thickness was formed and removed. Subsequent stepwise oxidation a t 100°C. of the reduced surface showed a progressive decrease in surface area as the oxidation progressed. This is indicated in Fig. 26 in which the true surface area is plotted against the average oxide film thickness. It is noteworthy that a considerable decrease of the surface area occurred in the first stages of


91

oxidation. For this sample the surfa,ce was approaching flatness a t an average oxide film thickness of 100 A. When the oxidation-reduction treatment was less severe, the surface resumed its initial flatness a t a smaller average oxide film thickness. A number of such experiments indicated this effect to be a typical one for the surfaces studied. It may be recalled that all surfaces studied were initially flat and even for the most severe treatment the average oxide film thickness produced did not exceed 300 A. It is possible that the oxidation-reduction treatment developed a distribution of protuberances on the surface; the more severe EffOCt

of O x # d o f # o n On I U f f O C I

o r t o of Reduced

copper

- 2 28

-

-204%

*

0

- t60

-

I-

- I

50-

0

as:

a I 20

I

I

I

40

60

80

I00

Oaidr F l l m T h ~ c h n r s s ~ b

FIG.26. Effect of oxidation on surface area of polycrystalline reduced copper. [After Rhodin, J. Am. Chem. SOC.72, 4343 (1950).]

the treatment, the greater the maximum size of the protuberances. Pyramidal protuberances have been actually observed in this laboratory on oxidized copper surfaces annealed in an atmosphere of hydrogen for 4 hours at 400" (see Fig. 14). Their size was about that of the wavelength of light, but smaller ones may occur with the milder treatments used in this study. In the initial stages of oxidation the metal surface is only partly covered with oxide. It is generally considered that the oxidation starts out a t a limited number of points and spreads out laterally (Evans, 123). It is possible that the oxidation selectively attacks the edges and points of the protuberances in such a manner as to smooth out the surface. A detailed analysis of the mechanism by which the surface structure

92

T. N. RHODIN, JR.

changes during oxidation is beyond the scope of this discussion. It seems .experimentally established, however, for the surfaces here considered, that the smaller irregularities are the first t o disappear during oxidation. If this is true, the rapid initial decrease of surface during oxidation may suggest that the smallest irregularities contribute the most to the area of the roughened surface. A rough clue as to their dimensions may be obtained by constructing a hypothetical surface of a suitable distribution of regular pyramidal protuberances. Their size would vary from 5 t o 100 A. in height for the surface of Fig. 26, and their distribution would have to be chosen so as to reproduce the observed surface area effect, assuming th at the effect of oxidation is to smooth out increasingly larger protuberances. This particular hypothetical surface would suffEr a decrease in surface area of 82% when all the protuberances of 35 A. in height or less disappeared. Since the structure of such a model surface is quite strictly given by the observations made on the corresponding real surface, it appears that the value of the average maximum protuberance size calculated for the artificial surface has real significance for the actual surface a t least so far as its order of magnitude is concerned. This analysis is a very simple illustration of a possible technique for systematically defining surface irregularities on a molecular scale using data on controlled oxidation together with surface area data. A corresponding analysis is applicable t o the sintering of fine particles for which a series of adsorption isotherms are evaluated. The results of studies of copper surfaces by low-temperature adsorption isotherms may be summarized as follows. True surface areas of metallic specimens as small as 10 sq. cm. can be derived with a precision of 6 % from low-temperature adsorption isotherms using vacuum microbalance techniques. This method is of special value in determining the average thickness of corrosion films formed by the reaction of gases or liquids with solids. The effect of progressive oxidation of a rough polycrystalline metal surface is to decrease the surface area t o a point where the roughness factor approaches unity. 2. Oxidation Study of Copper Surfaces

The ambiguity of the experimental evidence in the field of the oxidation of metal surfaces can be considerably diminished by a precies evaluation of the physical characteristics of some metal-oxygen systems. Some of the ambiguities have been caused by neglect of the following general factors: the extent and structure of the metal surface, the physical and compositional nonuniformity of the oxide film, and the lack of proper consideration of boundary conditions imposed by the range of film thicknesses and temperatures involved in analysis of the kinetics of


93

oxidation. A low-temperature oxidation study of single crystal copper surfaces is a promising object to study from this point of view. The thin oxide films formed a t low temperatures are essentially physically and mechanically homogeneous, and there exists considerable information on the semiconductor properties of copper and its oxides (Mott and Gurney, 124). I n addition, the structure and orientation characteristics of cuprous oxide films formed a t low temperatures have been thoroughly investigated and defined (Mehl, McCandless, and Rhines, 125; Van der Merwe, 126). It is very likely th at an insight into the fundamental mechanism of oxidation under these favorable conditions may well be extrapolated t o higher temperatures with suitable modifications. a. Physical Mechanism of Low-Temperature Oxidation. A t low temperatures, oxidation of many metals such as copper, aluminum, and zinc occurs rapidly at first but practically ceases in a relatively short time, with the formation of highly protective films of stable oxide ranging in thickness between 10 and 100 A. The rate of oxidation is relatively insensitive t o temperature and oxygen pressure, the latter being true over a wide range even for oxides which are deficit semiconductors at high temperatures. It has been demonstrated that at higher temperatures the oxidation mechanism involves the migration of electrons and cations t o the reaction zone (Wagner and Gruenewald, 127). It has also been demonstrated that, during the growth of the cuprous oxide layers, for example, it is the copper ions that diffuse and not the oxygen ions (Bardeen, Brattain, and Shockley, 128). The transfer of metal ions actually occurs by the formation of vacant cationic sites a t the oxideoxygen interface, followed by their diffusion t o the metal-oxide interface. An interpretation of low-temperature oxide film formation consistent with these phenomena was given by Mo t t (129) and elaborated by Cabrera and Mott (130). This hypothesis postulates that a very strong electric field is set u p through the oxide film between the metal and the oxygen adsorbed a t the gas interface which forces the metal ions to move through the oxide layer t o the reaction zone without requiring very much kinetic energy. The rate-determining step in the formation of very thin oxide films is assumed to be the transfer of metal ions through this strong electric field t o the reaction zone according to the expression

dx/dt

=

A exp. ( V / x k T - W / k T )

(4)

where x is the film thickness at time, W is an energy related to the energies of solution a nd diffusion of the cations into and through the oxide, respectively, V is the contact potential difference between the metaloxide interface and the oxygen adsorbed a t the oxygen-oxide interface T is the absolute temperature, and A is a proportionality constant th a t

94

T. N. RHODIN, JR.

can be calculated from fundamental constants of the oxide film. I n order t o evaluate their speculations, they have applied them i o the anodic oxidation of aluminum with some success. The task of a precise evaluation of the boundary conditions of temperature and of film thickness for which their treatment is valid and of making a systematic correlation of the theory with precise experimental data over an appreciable temperature range is one for which vacuum microbalance techniques are well suited. This behavior is characteristic of many metals. A study of the low-temperature oxidation characteristics of single crystal faces of copper are described in some detail in the following paragraph as a typical example. Two typical oxidation runs for two samples each of different crystalline orientations of the surface were made, each a t four different temperatures. They are plotted in Fig. 27. The average oxide film thickness plotted as ordinate is calculated by the method, and the assumptions of the Mott and Cabrera approach. The scatter of experimental points is typical, and all the extreme points are included. The apparent trends are not significant except for the enhanced rate of 323°C. where a n apparent change in the oxidation or chemisorption of oxygen occurs. The good agreement over the wide range of thicknesses and temperatures covered by the experimental data and the values predicted by Equation (4) suggests the possible validity of the Cabrera and Mott hypothesis under the aforementioned conditions. The fact that smooth oxidation curves are predicted even when the average oxide film thickness is as small as a single monolayer must be considered but an approximate description of the surface. Actually, parts of the surface are probably covered with patches whose thickness varies in units of a monolayer and parts which are bare of any oxide. It can be shown that the Mott and Cabrera function is a satisfactory approximation even for such a system. The prediction of correct values for the effective limiting film thickness (X,) listed in Table I11 provides additional evidence. Allan and Mitchell (131) have also made some studies of the lowtemperature oxidation of evaporated copper surfaces. They find the TABLE I11

T,"K.

X L (calcd.), A.

78 195 273 298 323 353

4.0 7.2 13 .O 17 .O 32.0 70 .O

X L (expt.), A. 4.0 7.5 12.0 16.0 28.0

200.0

d x l d t , A./hr,

0.10 0.12 0.13 0.20 0.25 0.30

t , hr.

1 6

12 20 74 1000


95

oxygen uptake to depend on the temperature and the structural properties of the surfaces in a manner which is in fair agreement with the results of this study. A more precise comparison is invalidated by the difficulties of evaluating the true surface area and crystallographic

Time (Seconds)

FIG.27. Oxidation isotherms of single crystal copper surfaces 78-323°K: run No. 1; 0 run No. 2. Samples No. 1 to No. 24 of crystal faces (loo), (110), and (111). Isotherms curves are calculated. [After Rhodin, J . Am. Chem. SOC.73, 3143 (1951).] 0

orientation of the metal atoms in the surfaces of their evaporated metal films. Some cautioning remarks, however, should be added. Although an expression of the form of Equation (4) is in good agreement with the lowtemperature oxidation data on copper, the physical significance of the two parameters, W and especially V , remains somewhat speculative. The general validity of Equation (4)should be evaluated systematically

96

T. N. RHODIN, JR.

by studying the low-temperature gas oxidation of more metals, some of which form excess semiconductors (Zn), as well as of others forming oxides with no appreciable semiconductor properties (Al). Furthermore, unless there exist compensating factors hitherto undefined, the Cabrera and Mott hypothesis is based on somewhat idealized surface considerations, seldom, if ever, encountered. As applications of their theoretical treatment accumulate, modifications of Equation (4) will probably be required t o account for structural and other characteristics specific to a given metal-oxide system. b. Reaction Rate Anisotropy of Oxidation. There are a t least two important aspects to the manner in which the reactivity of a given metal surface in the formation of protective oxide films is affected by surface structure. First, the chemical activity of the metal surface is dependent on the atomic spacing and geometry of the surface atoms. Second, the mechanical properties, such as permeability and cohesion, of the very thin oxide films formed in the initial stages of attack are influenced by the shortrange forces of the underlying metal and by the mechanism b y which the surface metal lattice is converted into an ordered oxide lattice. It is desirable to evaluate the reactive nature of metal oxide films on the basis of fundamental considerations of this type. Should these fundamental considerations regarding the formation of thin oxide films a t low temperatures prove valid, they promise t o be of importance in understanding oxidation phenomena of a more general character. It appears, for instance, possible to develop a theory of oxide formation a t higher temperatures, on the basis of the less complicated oxidation effects observed at low temperatures. For this reason, our studies on the reaction of crystal copper surfaces in an oxygen atmosphere at low temperatures previously described were extended (Rhodin, 132). The results may be summarized: (1) The rate is initially extremely rapid and then decreases sharply with time. Extension of the isotherms t o months indicates that the leveling off is a real effect. (2) The oxidation rate and the limiting oxide film thickness are greatest for the cubic face and smallest for the octahedral face with intermediate values for the rhombohedral face. (3) The limiting values for the oxide film thickness found were in fair agreement but somewhat lower than those previously reported. This difference is attributed to the marked uniformity of the surfaces in this study and to the fairly precise correction for surface roughness that was possible. (4) The dependence rate of oxidation on orientation is in agreement with th at found by others (Gwathmey and Benton, 133; Lustman and Mehl, 134) where it appears that the (100) face tends t o suffer maximum oxidation and the (111) face minimum


97

oxidation, a t low temperatures. Thik dependence was observed to change in a complicated manner a t higher temperatures (above 323°K.) as previously reported by others (Gwathmey and Benton, 135; BBnard and Talbot, 136). One should note th at precise definition of surface crystallography and structure on a microscale is often very difficult and this may be the basis for certain discrepancies in the literature on anisotropy of metal surface oxidations (Hering and Nichols, 137). The temperature coefficient of the reaction rate follows a relationship in which

ABSOLUTE TEMPERATURE CK)

FIG. 28. Dependence of limiting oxide film thickness on temperature. Rhodin, J. Am. Chem. SOC.73, 3143 (1951).]

[After

the reciprocal of the effective limiting average oxide film thickness varies inversely, and in a linear manner with the absolute temperature. The data for the (loo), the ( 1 lo), and the ( 1 11) faces are illustrated in Fig. 28. The values a t absolute zero on a clean plane copper surface correspond to approximately one monolayer for all orientations. (1) Physics of film formation. It is regrettable th a t so few quantitative data are known about the interface energies between metal surfaces and the oxide films that form upon them by chemisorption or reaction. Such considerations could eventually permit a precise evaluation of chemisorption in terms of surface structure. At present, interpretations of reaction rate anisotropy can be of a limited character only. The film

98

T. N. RHODIN, JR.

growth is often complicated by nucleation and growth processes in recrystallized regions of the oxide as well as by other structural and compositional transitions occurring during the film formation. Many efforts have been made to understand the formation of chemisorbed films. The recent treatment of Mott and Cabrera of an idealized system attempts to incorporate these effects into a generalized theory but fails t o account for many aspects of chemisorption phenomena. (2) Energetics of overgrowth formation. When a second solid phase forms on a solid support by adsorption or reaction from the vapor phase, often sufficiently strong structural relationships exist between the two layers to consider the film as an “overgrowth” of the support. The oxidation rate anisotropy should also be considered in terms of the mechanical properties of the oxide film insofar as they affect the continuity of the film and its tendency to adhere t o the underlying metal, Although no quantitative evaluation of metal oxide overgrowths are available, the theoretical considerations of Frank and Van der Merwe (138) of interfaces involving misfitting monolayers and oriented overgrowths may be helpful in a qualitative evaluation. Rather than stressing the geometrical aspects for the formation of oriented crystalline overgrowths, which has been the custom heretofore, they treated the energetics of film formation. I n terms of their model the degree of misfit may be defined in terms of surface dislocations. When the misfit exceeds a critical value the strains developed during the thickening of the oxide are no longer balanced by the attractive forces of the substrate, above this value the density of dislocations has increased t o a point where the film breaks away spontaneously from the substrate. The critical linear theoretical misfit a t low temperatures has been calculated t o be approximately 14 % whereas the actual corresponding linear misfit for the copper-cuprous oxide interface is approximately 18%. The view is favored that the condition for critical misfit may be satisfied for the copper-cuprous oxide system and pseudomorphic growth would be expected. There exists a fair amount of experimental evidence that this tendency does actually exist for cuprous oxide overgrowths. However, although pseudomorphic oxide films may form in the immediate vicinity of the single crystal copper surface, the large strain permissible in very thin oxide films cannot persist into thicker films; hence, as the film thickens a transition occurs from a pseudomorphic structure t o an oxide structure which, although, it may be highly oriented, will have the equilibrium lattice parameter of the bulk oxide. Flaws and perhaps reaction paths are generated in the oxide film in the region where this transition occurs. It may be that the closer this region is to the underlying metal, the less effective the oxide film is as a barrier to the trans-


99

fer of cations to the 'reaction zone. The critical mismatch and the density of surface dislocations will vary in a manner dependent on the structural relationships between the oxides and the metal for different crystal faces. For example, consideration of the relative mismatches for the oxide on the (111) and the (100) faces of copper based on a simplified linear Frank and Van der Merwe model suggests that approximately half as many surface dislocations per unit length occur in the former case. It is not unlikely that the tendency to form oxide pseudomorphic after the underlying metal is more favored for the octahedral than for the cubic face. If one considers that the tendency t o form oxide films pseudomorphic after the underlying surface, even though such films are very thin, favors the protective nature of the oxide film, this type of consideration may provide a starting point for interpreting the lowtemperature oxidation rate anisotropy observed in this study. This approach is compatible with the hypothesis of Cabrera and Mott since the values of their parameters W and V must be intimately influenced by the existence of regions of structural transition in the oxide near the metal interface. A detailed theoretical treatment of the speculations discussed, particularly the manner in which the two mechanisms may cooperate, is postponed for another communication. 3. Anisotropy of Physical Adsorption

It has been shown that certain metal crystal faces have higher catalytic activity than others (Leidheiser and Gwathmey, 139). Similarly, one would expect the adsorption of gases on solids to depend on the distances and geometrical configurations characteristic of different crystal faces (Schlier and Farnsworth, 144). The closest approximation to an ideal surface is probably a plane through a metal single crystal. Until recently the heat of adsorption on a definite face of a crystal has not been measured experimentally because ideal plane surfaces large enough to test adsorption theories with available techniques were not available. Suitable single crystal copper surfaces which approach flatness on an atomic scale have now been prepared. They are composed of a single species of atoms arranged in a well-ordered surface array and are good surfaces for precision adsorption studies (Gwathmey, 86; Rhodin, 99; FGrland, 140). Although nonspecificity is generally considered characteristic of van der Waals adsorption, ordinarily heats of physical adsorption vary considerably with the amount of gas adsorbed in a manner characteristic of a given adsorbent. Roberts (141) has shown that interaction between adsorbed molecules can lead to a complex variation of the differential heats of adsorption with increasing coverage of the surface even when the

100

T. N. RHODIN, JR.

adsorption takes place on a uniform surface. In addition, the variation of packing of adsorbed molecules, variation of surface electron distribution, and variation in the equilibrium distance of approach of the molecules t o the surface, characteristic of different crystal faces may also be important in determining the adsorption potential of a plane uniform surface (Halsey, 142; McMillan and Teller, 145; Frankenburg, 146). a. Anisotropic Adsorption on Copper and Cuprous Oxide. I n order to define these effects for a simple gas-surface system, the adsorption isotherms of nitrogen a t 78.1, 83.5, and 89.2%. on single crystal and polycrystal surfaces of copper have been determined (Rhodin, 73), using

-9

I .2

:*

1.0

p

0.8

0

2i0.6

H

.z

0.4

c

0.2 0

0 Pressure (ctn.)

FIG.29. Adsorption of nitrogen on electropolished and hydrogen-reduced (1 10) crystal face of copper a t 78.1, 83.5, and 89.2"K. Size of circles indicates error in microweighings. Scatter of points indicates typical distribution: 0 sample 1; '8 sample 2; 0 desorption points. [After Rhodin, J. Am. Chem. SOC.73,5691 (19501.1

precision differential microgravimetric techniques. All surface area values were obtained by a linear plot of the adsorption data according t o the Brunauer-Emmett-Teller equation as previously described. The adsorption isotherm for the whole pressure range up to 150 cm. gas pressure is indicated in Fig. 29 for nitrogen on the rhombohedra1 crystal face of copper. It is also typical in its general shape for adsorption isotherms of other gases such as argon on other crystal faces of copper as well as on those of zinc and iron. Adsorption of nitrogen and argon on single crystal surfaces of all three metals have been studied in the temperature range 78-90°K., using microbalance techniques. Da ta of Fig. 29 are plotted against actual pressure to facilitate calculation of the isosteric heats of adsorption. Figure 30 indicates that the adsorption of nitrogen on the rhombohedra1 crystal face of copper in the low-pressure range is linear with nitrogen pressure u p to 2 cm. nitrogen pressure. This iso-


101

therm is typical of the isotherms obtained for nitrogen and argon adsorption on other single crystal surfaces of both copper, iron, and zinc. The isosteric heat of adsorption ( H ) is determined in the usual manner by comparing the pressures of gas ( P ) a t equilibrium with a given weight of gas (a) adsorbed on the surface a t different temperatures (2')

[a In P/a

(l/T)la= -H/R

(5)

Actually the treatment of Joyner and Emmett (143) is used to calculate the values of ( H ) for different amounts of adsorbed gas (a). The isotherms of Figs. 29 and 30 were replotted on a large scale with the loga-

Presswe (cm.)

FIG.30.

Insert from Fig. 1 in low-pressure region (symbols as in Fig. 1).

rithm of the pressure ( p ) as abscissa against the amount of adsorption (a) as ordinate. The ratio of the horizontal distances corresponding to the two pressures, P:!and PI, for a given adsorption (a) a t two temperatures, T 2 and TI, was then substituted in the following integrated form of Equation ( 5 ) .

Characteristic data for the three major crystal faces are summarized in Table IV. Adsorptions were measured and compared a t pressures from 0.01 t o 150 cm. at each of three temperatures in the range of 78.189.2"K for each of two samples of a given crystal face. The variation of heat of adsorption with coverage of the surface was determined and

102

T. N. RHODIN, JR.

TABLE IV Characteristic Data for the Three Major Crystal Faces of Copper

Crystal face

Temp.,

O K .

78. 1 83.5 89.2 78. 1 83.5 89.2 78.1 83.5 89.2

zm X

10-6g.

0.54 0.48 0.44 0.47 0.45 0.43 0.44 0.43 0.42

2 , '

X 10-6g.

0. 54 0.49 0.45 0. 47 0.46 0.45 0.44 0.44 0.44

Apparent roughness factor

Apparent number Cu/N2 per unit area

1.42 1.29 1.19 1.35 1.32 1.29 1.20 I .20 1.20

1.8 2.0 2.2 2.7 2.8 2.8 3.3 3.3 3.3

plotted in typical heat ( H ) vs. coverage (0) curves. The thermodynamic interpretation of the isosteric heats of adsorption is now generally believed to be on a sound basis. Hill (146) has recently demonstrated a direct relationship between the isothermal differential heat of adsorption and the isosteric heat. It is essential for the valid application of the Clausius-Clapeyron type equation that the adsorbate be in strictly comparable states a t the various temperatures of comparison. This point will be considered in more detail in the discussion of the experimental results. There is good reason t o believe that a stricter application of Equation ( 5 ) should involve comparisons a t constant coverage of the surface or constant spreading pressure of the adsorbed gas. Interpretation of the data on this type of basis is, however, full of uncertainties at the present time. It seems desirable a t present still to use the classical basis for calculating the heats, but any interpretation of the results should he made with caution. The isosteric heats of adsorption for nitrogen on the (110), (loo), and (1 11) single crystal faces of copper and on polycrystalline copper surfaces calculated from the adsorption isotherms by the author a t 78.1-83.5, 78.1-89.2, and 83.5-89.2"K. are plotted as a function of surface coverage in Fig. 31. The horizontal and vertical lines indicate the maximum experimental uncertainties in the values of (11) and (e), respectively. The average of the corrected xm'values from Table I V was used for each temperature pair to calculate values of (e). The values for xm'are the values for xmcorrected for the variation of the density of the adsorbate with temperature below the critical temperature. Representative curves were drawn through a very large number of points. The latter


103


are not indicated because of the complexity of the diagram. The heatcoverage curve for nitrogen on polycrystalline iron a t 90°K. has been calculated from the calorimetric and adsorption date of Beebe and Stevens (147). The limited scope of their data presents ambiguities as t o the location of the curve with reference to the coverage axis but does not substantially affect the shape of the heat-coverage plot. Large heat values for a sparsely covered surface are apparently characteristic for the polycrystalline surfaces, but not a t all for the single

c L

p'ooo

2

0

4

6

8

I0

12

14

'

16

18

2

eFroction of Surface Covered

FIG.31. Differential heats of adsorption for nitrogen on (110), (loo), and (111) single crystal faces of copper and on polycrystalline copper surfaces calculated from the adsorption isotherms by the author a t 78.1-83.5, 78.1-89.2 and 83.5-89.2"K. (Vertical lines indicate total spread of the values calculated from the isosteric data. Horizontal lines indicate spread in values introduced by t h e error in the microweighings.) [After Rhodin, J . Am. Chem. SOC.72, 5691 (1950).]

crystal surfaces. This evidently indicates absence of active heterogeneous sites on the latter type of surfaces. The heat values are about the same for all surfaces as the coverage increases beyond one monolayer. The heat values tend to decrease continuously with coverage of the polycrystalline surfaces before one monolayer except for a slight maxima around monolayer coverage (Fig. 31). The heat values on the single crystal surfaces, however, go through distinct maxima which correspond for all three orientations to about one monolayer of coverage, within the experimental error. The height and sharpness of the maxima vary with the type of crystal face in such a manner as to be greatest for the least densely packed face (110) and the smallest for the most densely packed crystal face (111). Careful examination of the heat values for single crystal surfaces a t surface coverages corresponding to about two monolayers gave no indication of a distinct second maximum similar to that

104

T. N. RHODIN, J R .

reported on graphitized carbon (Beebe, Biscoe, Smith, and Wendell, 149). The heat of adsorption plots of all the copper surfaces descend for coverage exceeding one monolayer and approach a value corresponding to the heat of liquefaction of nitrogen, characteristic of normal multilayer adsorption. The heats of adsorption of nitrogen were also determined for the oxidized surfaces, and the corresponding data are plotted in Fig. 32. For the sake of brevity the corresponding adsorption isotherms are

0

2

4

1.0 1.2 1.4 0Fraction of Surface Covered

6

8

I6

I8

u)

FIG.32. Differential heats of adsorption for nitrogen on the oxidized (110), (loo), and (1 11) single crystal faces and the polycrystalline surface of copper calculated from the adsorption isotherms by the author at 78.1-83.5,78.1-89.2 and 83.5-89.2"K. The heat-coverage curve for nitrogen adsorption on polycrystalline chromic oxide a t 90°K. has been calculated from the calorimetric and adsorption data of Beebe and Dowden. The experimental errors are indicated as in Fig. 31. [After Rhodin, J. Am. Chem. SOC.72, 5641 (1950).]

omitted. The oxidation treatment was such a s t o form a thin coherent oxide film with an average thickness corresponding to about ten monolayers of cuprous oxide. Electron diffraction studies of thin oxide films of this type indicate that they are composed of small oxide crystallites oriented in a preferred pattern depending on the orientation of the metal surface on which the oxide layer forms. The heat-coverage curves of the oxidized single and polycrystal copper surfaces and of a chromic oxide surface resemble each other. The heat-coverage curve for nitrogen adsorption on polycrystalline chromic oxide at 90°K. was calculated by the author from calorimetric and adsorption date of Beebe and Dowden (148). The differences in the orientation of the oxide over-

VACUUM MICROBALANCE


105

growths appear to result in a small but distinct effect on the heat values, although this effect seems not to depend on surface coverage and on the crystallographic orientation of the support. The heat values for the oxides remain much more nearly constant for varying surface coverages than those for the metal and are, generally, somewhat higher. High values for a sparsely covered surface are observed only for the oxidized polycrystalline surfaces. Grain boundaries, edges, and other irregularities characteristic of polycrystalline materials in contrast to single crystal surfaces may contribute to this high initial heat of adsorption. The heats of adsorption of nitrogen on evaporated metal films of nickel and iron have been reported to decrease from 10 to 5 kcal./mole as the surface coverage increased from about 0.1 t o 1.0 monolayer. Beeck (150) states that nitrogen is unsuitable for the evaluation of surface areas of evaporated iron and nickel films by the Brunauer-Emmett-Teller method because of its high heat of adsorption a t 78°K.) which would yield a value for monolayer adsorption too high by 50%. The author feels th a t this objection does not apply to the nitrogen adsorption on reduced electropolished planar copper plates for the following reasons. 1. Outgassing of the surfaces a t 750 or 1000" is believed to free the surface of chemisorbed hydrogen and nitrogen. No preadsorption of nitrogen a t low pressures was observed by means of microweighings. 2. The isosteric heats of adsorption in this study are shown to vary from 2 t o 4 kcal./mole. I n vivo of the smallness of these values, nitrogen may be considered t o be a well-behaved gas for the area measurement of relatively planar nonporous copper surfaces. 3. The observed roughness factors (ratio of true to geometric surface) of the surfaces employed in this study varied from 1.2 t o 1.4 (Table IV). This checked with electron microscope pictures of alumina and silica replicas from electropolished single crystal copper surfwes. It is unlikely th a t the surface areas determined by the Brunauer-Emmett-Teller analysis are too high by 50% or even by 25%, because in several cases this would lead to a roughness factor of less than unity. 4. Contrary to iron and, to a lesser degree to nickel, copper does not form nitrides in nitrogen atmospheres a t elevated temperatures. I n spite of these reasons the possibility that a chemisorbed nitrogen monolayer may form a t low temperatures cannot be completely excluded. Should this occur, it is not unreasonable t o expect the nitrogen monolayer t o form a strongly oriented structure which would tend to replicate the structure of the underlying metal face in a unique manner. (1) Heats of adsorption as a function of surface coverage. If one associates the maxima in the heat coverage curves of Fig. 31 with a strong contribution from horizontal interaction, the lack of maxima

106

T. N. RHODIN, JR.

observed for the reduced and oxidized polycrystalline copper surfaces is understandable. This hypothesis may also explain, in a rough manner, the lack of distinct maxima in the heat coverage curves of Fig. 32 for oxidized single crystal copper surfaces if one assumes that sufficient nonuniformity is introduced by the formation of the oxide crystals to obscure cooperative adsorption interactions. It is not likely, however, that this explanation can be pushed so far as to explain the differences in the cooperative effect observed on the three major single crystal faces investiga,ted in this study. Although the roughness factors of the 6 separate single crystal samples studied varied from 1.2 to 1.4, there was no strict correlation between the observed maxima and the roughness factor. Secondly, the reproducibility of the heat-coverage relationship for each crystal face would indicate that the preparative technique employed always yielded a more uniform surface for the least dense face. It is not obvious why this should be so. It is therefore likely that the distinct differences in the heat-coverage curves observed between the three major single crystal surfaces are related, in each case, to the crystallographic orientation of the surfaces. This relationship will be considered in more detail in the next section. A flat uniform single crystal surface is unlikely to possess a singularly active fraction of the surface. Horizontal interactions may make a considerable contribution to the heat of adsorption for neighboring nonpolar molecules. In the case of nitrogen the horizontal interactions are probably attractive, and this contribution will tend to cause the heat of adsorption to pass through a maximum at a coverage corresponding approximately to one monolayer in a manner similar to that observed in this study. Orr (151) has supported this hypothesis both by theoretical and experimental findings for nitrogen and argon adsorption. The differential heats of adsorption of nitrogen on a partially graphitized carbon black at liquid nitrogen temperatures have been carefully measured calorimetrically by Beebe, Biscoe, Smith, and Wendell (149). Their results were confirmed by calculations of the isosteric heats from adsorption isotherms in the temperature range from 68.4 to 90.0°K., measured for the same system by Joyner and Emmett (143). I n both cases a maximum was observed in the heat of adsorption when the surface was covered with approximately 0.75 monolayer. It must be noted, however, that the apparent contribution to the maximum for the heat of adsorption on graphitized carbon appears to be about 0.5 kcal./mole at most, whereas the corresponding apparent contribution to the heats of adsorption on single crystal copper faces varies from 0.6 to 1.8 kcal./mole. It is possible that the maximum in the heat-coverage plots for the former system would be considerably more accentuated were the physical struc-


LOW-TEMPERATURE

APPLICATIONS

107

ture of the graphitized carbon much more uniform. The greater increase of the heat of adsorption a t about monolayer coverage observed for the single crystal surfaces of copper may be caused by the fact that they are relatively more uniform and planar. It is not unlikely, however, that other effects in addition to that of attractive horizontal interactions are contributing t o the observed maxima for single crystal copper surfaces (Harkins, 152). It is possible that a rearrangement of adsorbed nitrogen to a more stable structure occurs somewhere within the temperature range for which adsorption isotherms were measured. (2) Variation of heats of adsorption o n diferent major crystal faces of copper. The effect of the crystal face of the adsorbent on physical adsorption has been treated theoretically by Barrer (153) for covalent surfaces and by Orr (151) and by Lenel (154) for dielectric surfaces. The vertical interactions between a nonpolar molecule and a polycrystalline metal surface have been independently treated by Lennard-Jones (155), by Bardeen (156), and by Margenau and Pollard (157). Considering the theoretical limitations involved in the last treatment, the observed agreement between the values calculated theoretically and the experimental values is acceptable. At present no explicit theoretical treatment of the physical adsorption of a nonpolar gas molecule on a single crystal metal surface in terms of the crystal parameter and geometry of the latter is available. A geometric consideration of the arrangements in which a nitrogen molecule may sit on the various major crystal faces of copper indicates that the packing may be most effective on the least dense (110) face. The dimensions of the nitrogen molecule may be rough1 compared to and with an that of a dumbbell with an internuclear distance of 1.09 effective maximum cross-sectional area of 16.1 A. when the dumbbell is oriented with both spheres touching the surface. Tc? and side views of the manner in which such a molecule may sit on the various major crystal faces of copper are drawn to scale in Fig. 33. The positions are those that give the smallest molecule t o surface distance for each face using the very crude models described. Examination of this type of model indicates that the effective cross-sectional area for a nitrogen molecule would be approximately 16.1 8. if it lay flat, and 12.2 A. if it lay with its long axis normal to the support. If the former orientation holds, then the greatest packing occurs for the least dense (110) crystal face and the poorest packing for the most dense (111) crystal face, This is in agreement with the data summarized in Table IV where the xmml values are observed to vary with the type of crystal face in a corresponding manner. The large values of xmmlobserved especially at low temperatures for the (110) crystal face suggest that an even more efficient packing

i.,

108

T. N. RHODIN, JR.

of the nitrogen molecules may take place on this crystal face as the temperature decreases, namely, a tendency for the adsorbed nitrogen to be oriented normal to the support. A reorientation of this t,ype would not only account for the increase of zm’values with decreasing temperature but would also influence the lower temperature isotherms sufficiently to account for the unusually high maximum observed in the heat-coverage plot. This trend is in agreement with the apparent number of copper atoms per nitrogen molecule calculated for each face (Table IV). By the same token the observed roughness factors indicated in Table I V seem to be highest for the least dense (110) face. If one assumes the

I

SIDE

SIDE

SlDL

FIG.33. Some possible positions of a nitrogen molecule physically adsorbed on single crystal copper surfaces. (Projections are constructed to scale in directions normal and parallel to the substrate. Copper atoms are schematically indicated as rigid close packed sphercs and nitrogen molecules as spherical pairs.) [After Rhodin, J. Am. Chem. SOC.72, 5691 (1950).]

actual roughnesses of the various crystal faces t o be about the same, this trend again may mean that the packing is more efficient on the least dense face, and that the higher observed roughness factors are only apparent. Actually, i t does not seem desirable to speculate further until more is known about the steric factors in adsorption on single crystal surfaces. The results of studies of the crystallographic dependence of heats of adsorption of nitrogen or copper can be summarized as follows. 1. The isosteric differential heats of physical adsorption of nitrogen near its boiling point on reduced and oxidized electropolishing single crystal copper surfaces can be evaluated from adsorption isotherms determined gravimetrically. 2. The adsorption isotherms are measured with a quartz beam vacuum microbalance of high sensitivity. The surfaces studied are

VACUUM MICROBALANCE : L O W - T E M P E R A T U R E

APPLICATIONS

109

believed to be good approximations to flat, clean, undistorted single crystal faces. 3. The heats of adsorption were found to vary in a unique manner with the coverage, uniformity, and crystallographic nature of the surfaces. 4. The physical significance of these studies can be discussed qualitatively within the frame of the established theoretical concepts of adsorption phenomena on single crystal metal surfaces. The approximations and assumptions inherent in the present theory of adsorption prevent a quantitative application of the theory. b. Comparison of Adsorption on Single Crystals of Zinc, Copper, and Iron. The data in Table V summarize adsorption studies on single TABLE V Characteristic Data for the Adsorption of Nitrogen and Argon on the Octahedral Crystal Faces of Copper and Zinc Surfaces (11

Adsorbate Nitrogen Nitrogen Nitrogen Argon Argon Argon

(2)

(3)

Ratio of metale atoms to gas atoms/unit area

Adsorption temp., OK. 78.1 83.5 89.2

78.1 83.5 89.2

(4)

.

(5)

(6)

Differential& heat ads. unit coverage. cal./mol.

(7)

(8)

Roughness6 factor

(9)

(10)

Weight monolayerd X 108 g./cm.l

Copper

Zinc

Copper

Zinc

Copper

Zinc

Copper

Zinc

3.3 3.3 3.3 2.9 2.8 2.9

3.0 3.1 3.0 2.6 2.6 2.6

3100 3250 3000 3200 3110 3000

2600 2670 2620 2690 2740 2600

1.20 1.20 '1.20 1.18 1.18 1.16

1.17 1.15 1.16 1.14 1.11 1.13

3.41 3.39 3.44 5.61 5.65 5.53

3.32 3.28 3.18 5.19 5.22 5.15

a This ratio was determined from the number of metal atoms per unit area, from crystallographic data. to the number of adsorbate molecules per unit area, from the adsorption data. I t is the number of nitrogen molecules and of argon atoms per unit area. b The maximum isosteric differential heat of adsorption interpolated from the heat-coverage plots. c The roughness factor is defined as the ratio of the experimental values of the weight of adsorbate required for monolayer coverage to t h a t calculated for the eame coverage of a geometrically planar surface. It can be interpreted as the ratio of the real area t o the apparent area of the adsorhent. d The weight of adsorbate required for a monolayer coverage of the adsorbent at a given adsorption temperature calculated from a Brunauer-Emmett-Teller plot and corrected for the variation of adsorbate density with temperature.

crystal surfaces belonging t o the hexagonal crystal class instead of the face-centered cubic class dealt with in the preceding text. In addition, the adsorption of a monatomic gas (A) is compared to that of a diatomic gas (N2),and the results are compared in terms of the surface geometry and shape characteristic of the adsorbents for both copper and zinc. Assuming that there exist no explicit differences between the metal surfaces as a function of preparation, the data may be examined in terms of the differences of the atomic distances and electron densities characteristic of the two metal surfaces. The maximum molar heats of

110

T. N. RIIODIN, JR.

adsorption for nitrogen and argon are about the same for both gases, but the heats of adsorption are by about 500 cal./mole,higher for the adsorption on the copper than on the zinc surfaces (columns 5 and 6, Table V). Examination of the data on the ratio of metal atoms to gas molecules per unit area surface (columns 3 and 4) indicates th a t the accommodation of the surface for gas molecules per unit area surface is greatest for the argon-copper system, and lowest for the nitrogen-copper system is explained in terms of the differences in atomic structure of the adsorbents. Zinc crystallizes in a curious modification of the close-packed hexagonal structure which differs from the normal structure in that it is extended in the direction of the c axis so that the axial ratio is much greater than th a t for close-packed spheres (1.860 instead of 1.633). Like the octahedral crystal face of copper each zinc atom has six closest neighbors in the basal plane except th at the next lower layer is much closer in the case of copper. Although the atomic packing a n d electron configuration characteristic of the outer basal planes of the two metals may have a considerable effect on the physical adsorption, the influence of the next lower layer probably should also be contemplated. This influence on the field of force, a t the surface, can be expected t o be more pronounced for copper than for zinc. Furthermore, the periodicity of the fields of force on the two surfaces differs in th at zinc has a more “open” surface structure than copper (Hume-Rothery, 158). On passing from copper t o zinc by the addition of a second s-electron to the fourth shell, the electron clouds associated with the surface can be expected t o contract rapidly. The same holds for the analogous transitions from silver t o cadmium and from gold to mercury. This effect is indicated for all these three pairs by t,he crystallographic data for the individual metal lattices on the distances of closest approach, and from the lattice distortions produced in solid solution. Parallel with the contraction of the electron clouds associated with the transition from the one metal of each pair to the other the distances of closest approach of the atoms in the crystal lattices show a corresponding increase. I n other words although the zinc ions are farther apart than the copper ions, the electron clouds can be envisaged t o be more closely associated with each zinc ion than with each copper ion. The dispersion attractive forces characteristic of van der Waals interaction might be expected to be higher in the case of copper since not only are the adsorbate molecules somewhat nearer t o the metal ions for copper and associated with more of them per unit area but the ionic polarizabilities of copper surfaces can also be expected to be greater than those of zinc surfaces. All these effects are in qualitative agreement with the experimental observation that the heat of adsorption tends to be higher t o a relatively small but distinct degree on octahedral


111

copper surfaces than on octahedral zinc surfaces. The effect of the atomic surface configurations on the sharpness of the maxima in the heat-coverage curves is probably also indirectly influenced by the electron distribution on the surface. Preliminary adsorption studies have also been made of nitrogen and argon on various single crystal surfaces of iron (99.95%) prepared by the strain-anneal method previously described. This study is yet incomplete but definite evidence has been obtained that the heat of adsorption of nitrogen on iron is about 25% higher than on either zinc or copper single crystal surfaces. The heat of adsorption of argon, however, is about the same. Maxima of the heats of adsorption appeared around monolayer coverage, on iron, for both gases as on copper and zinc. The heats of adsorption of nitrogen were much higher for fractional monolayer coverage on all major crystal iron faces than on any of the single crystal surfaces of copper or hexagonal surfaces of zinc. c. Comparison of Adsorption of Nitrogen and Argon. Reference has already been made to the data of columns 3 and 4, Table V., which indicate how the surfaces of the two adsorbents are being covered with nitrogen and argon, in terms of numbers of metal ions associated with single gas molecules and gas atoms. It depends upon which particular values are employed for the ecective cross-sectional areas of nitrogen and 14.2 A.2) what apparent surface coordination and argon (16.1 number is obtained. These area values were selected because the extensive literature on gas adsorption on all kinds of surfaces, including metals, yields these particular cross areas for most of the systems. It is true that the effective cross-sectional areas may depend on the nature of the adsorbent and on the temperature. The values mentioned above are the basis of all the experimental determinations of surface areas from adsorption data by the B-E-T method. It would be appropriate to define them in terms of every adsorption system under consideration. The cross section value for nitrogen especially was confirmed in adsorption studies evaluated by theories other than the B-E-T theory. Use of this value leads to absolute surface area values in good agreement with areas found by independent methods. All this tends to confirm the correctness of the value of 16.1 A. for the cross-sectional area of an adsorbed nitrogen molecule. Metallurgical measurements indicate that the metal surfaces of our single crystal specimens are planar on a molecular scale, and this is in good agreement with the unusually low roughness factors calculated from the adsorption data and indicated in columns 7 and 8 of Table V. This furnishes additional evidence that the molecular area for nitrogen chosen in the deviation of the roughness values are nearly correct. The fact that the packing per unit area of the adsorbed gas is denser

112

T. N. RHODIN, JR.

on copper than on zinc can be explained by assuming that the area occupied by every particle depends primarily on the number of metal atoms in the surface rather than on the surface area itself, and by taking into account that there are just about 10% more copper ions than zinc ions present in every unit of surface. Th at packing density of argon appears to be higher on both surfaces than that of the nitrogen (columns 3 and 4, Table V) is more significant. It is probably best understood in terms of the smaller area of the argon (14.2 Any interpretation of the packing mechanism is further complicated by the dumbbell-like shape of the nitrogen molecule compared to the probably spherical symmetry of the argon atom. Finally, the theory of surface packing will have t o take into account the relative polarizabilitics of the two gases, as influenced by the periodic electron fields of the interface between metal and adsorbed gas. Quantitative considerations of this electronic interaction a t the interfaces are beyond the scope of this paper. So far, the theoretical treatments of this problem suffer from the lack of precise data for polarizabilities and other surface force parameters. The remarks on variation of surface packing are also pertinent to the data (columns 9 and 10, Table V) on the weight of gas corresponding to monolayer coverage for each gas-metal system. The higher values of this weight for copper-argon compared with zinc-argon are in direct relation to the greater surface ion density in the copper surface. This does not apply t o nitrogen adsorption where the monolayer values for nitrogen are essentially the same for both metals, probably because of the special shape of the nitrogen molecule. The higher weights of the argon monolayers on both metals are caused by the smaller size and higher atomic weight of argon. Contrary to the considerable temperature dependence of xn for the adsorption on the cubic and rhombohedra1 crystal faces of copper (22) the corresponding values for adsorption of both gases on the octahedral faces of both copper and zinc are practically insensitive to temperature in the range studied. This may indicate that the steric arrangement of both gases as adsorbed on the closely packed octahedral crystal faces of both metals corresponds closely to the arrangement of the gas atoms or gas molecules, in the liquefied state.

4. Outlook for Future Surface Studies Using Microgravimetric Techniques The performance of the vacuum microbalance described in these pages illustrat,es the advantages of precision microgravimetric techniques in surface studies. T h e instruments are sensitive and make direct and continuous measurements of gravimetric efects attending rate processes OT equilibrium conditions. They are adaptable t o high vacuum operation. Methods are available for preparing sample surfaces of well-defined


1 13

structures but of such small area that only microgravimetric techniques permit their study. The instruments are relatively inexpensive and essentially simple in their operation. I t appears advisable, however, to point here also to certain limitations of these techniques in order to clarify the fields for which the use of properly designed microbalances can be recommended. A decisive point for the reliable interpretation of microgravimetric effects is that all factors causing weight effects be resolved and understood, and that Idisturbing effects are practically eliminated. Fortunately, such disturbing effects as influences of solubility, evaporation, or dissociation of adsorbed gases are unlikely to occur a t appreciable rates in metal-gas adsorption systems during the measurement of adsorption or reaction rates a t low temperatures. Precision measurements of weight changes of microgram or smaller quantities require the proper design of a microbalance for a given problem and can only be made successfully with considerable care and understanding of the errors involved. Although vacuum balances with sufficient range, capacity and sensitivity are known, the design and construction of specific instruments require skill and experience. Finally, although the cost of vacuum microbalance for surface studies is reasonably small, the investment in vacuum equipment, optical measuring devices, construction jigs, instruments for the preparation and characterization of sample surfaces, and instruments for the control and measurement of temperature and pressure can add considerable cost to the total investment. The instrumental design is usually highly specialized and one microbalance installation specifically designed for a given surface study may be inapplicable t o another. In future advances of surface chemistry, precision vacuum microgravimetry can play a significant part by contributing t o our understanding of surface phenomena. ACKNOWLEDGMENT The writer is greatly indebted t o C. S. Smith, C. S. Barrett, and E. A. Gulbransen for many fruitful discussions pertinent t o surface studies pursued over a n extended period. Much helpful assistance in the preparation and characterization of sample surfaces from the viewpoint of surface structure was provided a t one time or another by Joseph Cerny, Walter Bergmann, Donald Clifton, Kaye Ikeuye, and James Hess. Without their assistance the tedious assignment involved in the meticulous preparation of sample surfaces could not have been achieved. The useful collaboration with L. P. Schulz on the characterization of metal surfaces by electron microscopy and electron diffraction techniques was an essential part of the surface studies. Assistance in the design and construction of techniques for t h e controlled evaporation of metals was provided by H. E. Shaw. Many of the microbalances and related instruments used b y the author were designed with the assistance of T . O’Donnell and J. Getzholz and are in part the

114

T. N. RHODIN, JR.

result of their many years of experience in the fabrication of precision instruments. The construction of the balances was achieved in no small part by the craftsmanship of W. Mankowich whose skill is also greatly appreciated. Much of the research described was carried out while the author was on the staff of the Institute for the Study of Metals at the University of Chicago. It was supported in part by U. S. Army Air Force Contract AF-33 (038)-6534.

REFERENCE s 1. Emich, F., Lehrbuch der Mikrochemie. W. Bergman & Co., Munich, 1926. 2. Benedetti-Pichler, A. A., Microtechniques of Inorganic Analysis. John Wiley & Sons, New York, 1942. 3. Kirk, P. L., Quantitative Ultramicroanalysis. John Wiley & Sons, New York, 1950. 4. Seaborg, G. T., Chem. Eng. News 24, 1193 (1946). 5. Cunningham, B. B., Nucleonics 6 (5), 62 (1949). 6. Warburg, A., and Ihmori, E., Ann. Physik. Chem. 27, 481 (1886); 31, 145 (1887). 7. Pettersson, H., Proc. Phys. SOC.(London) 32,209 (1920); Dissertation, University of Goteborg, 1914. 8. Emich, F., Abderhalden 9, 55 (1919). 9. Gorbach, G., Mikrochemie 20, 254 (1936). 10. Ingram, G., Metallurgia 39,232 (1949). 11. Friedrich, A., Mikrochemie 16, 35 (1934). 12. Bazzoni, M., J. Franklin Znst. 180, 461 (1915). 13. Salvioni, E., Dissertation, University of Messina, 1901; Lenz, W., Apoth. 2. 21, 23 (1912). 14. Predvoditelev, A., and Witt, A., 2.physik. Chem. 132,47 (1942). 15. Lord, E., J. Textile Znet. 38 (2), T84-T93 (1947); Shirley Institute F m o i r s 21, 1 (1947). 16. Lowry, O., J. Biol. Chern. 140, 183 (1941). 17. Korenman, I. M., and Fertelmeister, Ya. N., Zauodskaya Lab. 16,785 (1949). 18. Felgentrager, W., Feine Waagen, Wagungen und Gewichte, 2nd Ed. J. Springer Co., Berlin, 1933; Nature 132, 730 (1950). 19. Steele, B. D., and Grant, K., Proc. Rov. SOC.(London) A82,580 (1909). 20. Kuck, J., and Lowenstein, E., J. Chem. Education 17, 171 (1940). 21. Wilson, D., Metallurgia 34, 279 (1946). 22. Monk, G., J. Applied Phys. 19, 485 (1948). 23. Strong, J., Procedures in Experimental Physics. Prentice-Hall, New York, 1938, Chapter 5. 24. Weber, W., Werke (Berlin) 1, 497 (1892). 25. Nernst, W., and Riesenfeld, E. A., Ber. 39, 381 (1906). 26. Emieh, F., and Donau, J., Handb. Biol. Arbeitsmeth. 1 (3), 183 (1921). 27. Aston, F. A., Proc. Roy. Soc. (London) A89, 439 (1914). 28. Taylor, F. S., Phys. Rev. 10, 653 (1917). 29. Guye, Ph. A., J. chim. phys. 16, 46 (1918). 30. Ramsay, W. R., and Whytlaw-Gray, R., Proc. Roy. soc. (London) A84, 536 (1911). 31. Stromberg, R., Svenska Vetenskapsakod-Akad. 3-6 (2), (1929). 32. Barrett, H., Birnie, A., and Cohen, M., J . Am. Chem. Sac. 62, 2839 (1940). 33. Bradley, R., Evans, M. G., and Whytlaw-Gray, R., Proc. Roy. SOC.(London) A186, 368 (1946).

VACUUM MICROBALANCE: LOW-TEMPERATURE

APPLICATIONS

115

34. Donau, J., Mikrochemie Emich Fest. 39, 10 (1930); 3, 1 (1931); Mikrochemie 9, 1 (1931); 13, 155 (1933). 35. Gulbransen, E. A., Rev. Sci. Instruments 16, 201 (1944). 36. Aluminum Co. of America. Private communication (1945). 37. Rhodin, T. N., Jr., J . Am. Chem. SOC.72, 4343 (1950). 38. Kirk, P. L., Craig, R. E., Gullberg, J. E., and Boyer, R. Q., Anal. Chem. 19, 427 (1947). 39. Emich, F., Monatsh. 36, 436 (1915). 40. McBain, J., and Bakr, A., J. Am. Chem. SOC.48, 690 (1920). 41. McBain, J., and Britton, G. T., J. Am. Chem. SOC.62, 2198 (1930). 42. Morris, H. E., and Maas, O., Can. J . Research 9, 240 (1933). 43. Drane, H., Phil. Mag. 6, 559 (1928). 44. Stamm, A. J., and Woodruff, S. A,, Znd. Eng. Chem., Anal. Ed. 13,836 (1941). 45. McBain, J., J . Phys. Chem. 34, 1439 (1930); J. Am. Chem. SOC.62, 2668 (1930). 46. Dunn, R. C., and Pomeroy, H. H., J . Phys. & Colloid Chem. 61,981 (1947). 47. Makolkin, I. A., Zauodskaya Lab. 16, 1209 (1949). 48. Kirk, P. L., and Schgffer, F. L., Rev. 81%. Instruments 19 (11), 785 (1948). 49. Jarefowitz, S., and Othmer, D., Znd. Eng. Chem. 40, 739 (1948). 50. Chipalkatti, V., and Giles, C., Nature 166, 735 (1950). 51. Makolkin, I. A., Zauodskaya Lab. 16, 1209 (1949); C. A. 44, 996 (1950). 52. Bang, J., Biochem. 2. 87, 248 (1918). 53. Kirk, P. L., and Craig, R. E., Rev. Sci. Instruments 19 ( l l ) , 777 (1948). 54. Rhodin, T. N., Jr., J . Metals Trans. 186, 343 (1949). 55. Steyermark, A., Ind. Eng. Chem., Anal. Ed. 17, 523 (1945). 56. Kerner, W., Znd. Eng. Chem., Anal. Ed. 17, 523 (1945). 57. Wilson, C., Metallurgia 34, 219 (1946). 58. Felgentrager, W., 2. Anal. Chem. 83, 422 (1931). 59. Gulbransen, E. A., and Andrew, K. F., private communication (1951). 60. Angstrom, K., Oefuersigt Kongl. Vetenskaps-Akad. 643 (1895). 61. Emich, F., Abderh. Handb. Biol. Arbeitsmeth. 1-3, 261 (1921). 62. Riesenfeld, E. A., and Moller, H. F., 2. Elektrochem. 21, 131 (1915). 63. Humphrey, J., J. Textile Znst. 16, T10-Tl3 (1925). 64. Urbain, G., Compt. rend. 164, 347 (1912). 65. Manigaut, P., and Tsai, B., C'ompt. rend. 214, 658 (1942); Chem. Zentr. 11, 2254 (1943). 66. Gregg, S. J., J. Chem. SOC.1946,561, 564. 67. Vieweg, R., and Gast, T., Kunststofle 34, 117 (1944). 68. Feuer, I., Anal. Chem. 20, 1231 (1948). 69. Dushman, S., Vacuum Techniques. John Wiley &Sons, New York, 1949, Chapter IX. 70. Dushman, S., Vacuum Techniques. John Wiley & Sons, New York, 1949, Chapter X ; Farkas, A., and Melville, H. W., Experimental Methods in Gas Reactions. The Macmillan Company, New York, 1935. 71. Gregg, S. J., J . Chem. SOC.,1946,561; Rhodin, T. N., Jr., unpublished. 72. Gulbransen, E. A., International Conference on Surface Reactions. Corrosion Publishing Co., Pittsburgh, 1948, p. 223. 73. Rhodin, T. N., Jr., J. Am. Chem. SOC.72, 5691 (1950). 74. Cawood, W., and Patterson, H., Trans. Roy. SOC.(London) A242,77 (1936). 75. Leadbeater, B., and Whytlaw-Gray, R., Proc. Intern. Cong. Pure and Applied Chem. (London) 11, 189 (1947). 76. Lambert, B., and Phillips, C., Trans. Roy. SOC.(London) A242,77 (1936).

116

T. N. RHODIN, JR.

77. Gulbransen, E. A., private communication (1951). 78. Beeck, O., Advances in Catalysis, Vol. 11. Academic Press, New York, 1950. 79. Brunauer, S., The Adsorption of Gases & Vapors. Princeton University Press, Princeton, N. J., 1943. 80. Harker, D., and Parker, E., Trans. A m . SOC.Metals 34, 156 (1945). 81. Burke, J. E., Am. Znst. Mining Met. Engrs., T.P.3472 (1948). 82. Smith, C. S., Am. Inst. Mining Met. Engrs., T.P.2387 (1948). 83. Speiser, R., Beck, F. H., Fontana, M. G., and Lassettre, E. N., J. Chem. Phys. 18,772 (1950). 84. Uhlig, H. H., Corrosion Handbook, John Wiley & Sons, New York, 1948. 85. Bridgman, P. W., Proc. Am. Acad. Arts Sci. 68, 165 (1923); 60, 305 (1925). 86. Gwathmey, A. T., International Conference on Surface Reactions. Corrosion Publishing Co., Pittsburgh, 1948. 87. Langmuir, I., Phys. Rev. 22, 357 (1923). 88. Tonks, L., Phys. Rev. 38, 1030 (1931). 89. Hering, C., and Nichols, M. H., Revs. Modern Phys. 21, 258 (1949). 90. Elam, C. F., Trans. Faruday SOC.32, 1604 (1936). 91. Dobinski, S., Nature 138,685 (1936). 92. Jacquet, P. A., Rev. de Met. 48, 1, 537 (1951). 93. Heidenreich, R. D., J . Optical SOC.Am. 36, 146 (1945). 94. Gregg, S. J., Surface Chemistry of Solids. Reinhold Publishing Corp., New York (1950), Chapter VIII. 95. Hickman, J. W., and Gulbransen, E. A., Ind. Eng. Chem., Anal. Ed., 18, 640 (1946); 20, 158 (1948). 96. Schula, L. G., and Rhodin, T. N., Jr., unpublished. 97. Raether, H., Optik 1, 69 (1946). 98. Schula, L. G., J . Chem. Phys. 18,996 (1950). 99. Rhodin, T. N., Jr., J . Applied Phys. 21, 971 (1950). 100. Dunn, C. G., Metals Transactions (Jan. 1949). 101. Bowles, J. S., and Boas, W., J. Inst. MetaZs 74, 501 (1948). 102. Burgers, W. J., Rekristallisation, Verformter Zustand und Erholung, 111, Handbuch der Metallphysik. Edwards Bros., Ann Arbor, Michigan, 1944. 103. Rhodin, T. N., Jr., unpublished. 104. Bergman, W., Cerny, J., and Rhodin, T. N., Jr., unpublished. 105. Rhodin, T. N., Jr., J. Phys. & Colloid Chem. (March, 1953). 106. Rhodin, T. N., Jr., J. Am. Chem. SOC.7 2 , 5102 (1950). 107. Olsen, L., Smith, C., and Crittenden, E. D., J . Applied Phys. 16, 425 (1945). 108. Beeck, O., Smith, A., and Wheeler, A., Proc. Roy. SOC.(London) A62 (1940). 109. Rhodin, T. N., Jr., Disc.Faraday Soe. 6,215 (1949). 110. Barrett, C. S., Structure of Metals, Crystallographic Methods, Principles and Data. McGraw-Hill Book Co., New York, 1943, p. 441. 111. Dixit, R., Phil. Mag. 16, 1049 (1933). 112. Estermann, I., Z. Elektrochem. 31,411 (1925). 113. Briick, L., Ann. Physik 26, 233 (1936). 114. Riidiger, O., Ann. Physik 30, 505 (1937). 115. Finch, G., Quarrell, A. G., and Wilman, H., Trans. Faraday SOC.31, 1051 (1935). 116. Van der Merwe, J. H., Disc.Faraday SOC.4, 201 (1949). 117. Shaw, H. E., and Rall, W., Rev. Sci. Instruments 18,278 (1947). 118. Emmett, P. H., Brunauer, S. J., and Teller, E., J. A m . Chem. SOC.60,309 (1938). 119. Harkins, W. D., and Jura, G., J . A m . Chem. Soc. 66,1362, 1366 (1944).

VACUUM MICROBALANCE


117

120. Wooten, L. A., and Brown, C., J . Am. Chem. Soc. 66,113 (1943). 121. Emmett, P. H., Advances in Catalysis, Vol. I (1948). 122. Duncan, J. F., Trans. Faraday SOC.46,879 (1949). Liang, S. C., J . Phys. & Colloid Chem. 66, 1410 (1951). 123. Evans, U. R., Metallic Corrosion, Passivity and Protection. Arnold Co., London, 1948, Chapter 2. 124. Mott, N. F., and Gurney, R. W., Electronic Processes in Ionic Crystals. Oxford University Press, New York, 1940, p. 152. 125. Mehl, R. F., McCandless, E. H., and Rhines, F. N., Nature 134, 1009 (1934). 126. Van der Merwe, J. H., Disc. Paraday SOC.6, 208 (1949). 127. Wagner, C., and Griinewald, K . , 2. physilc. Chem. B40, 455 (1938). 128. Bardeen, J. W., Brattain, W. H., and Shockley, W., J . Chem. Phys. 14, 714 (1946). 129. Mott, N. F., Trans. Faraday SOC.43,429 (1947); J. chim. phys. 44, 172 (1947). 130. Cabrera, N., and Mott, N. F., Repts. Prog. Phys. 12, 163 (1949). 131. Allan, R., and Mitchell, L., Disc. Faraday Soc. 8,1815 (1950). 132. Rhodin, T. N., Jr., J . Am. Chem. SOC.73, 3143 (1951). 133. Gwathmey, A. T., and Benton, A. F., J . Phys. Chem. 46, 969 (1942). 134. Lustman, B., and Mehl, R. F., Trans. A m . Inst. Mining Met. Engrs., 143, 1 (1941). 135. Gwathmey, A. T., and Benton, A. F., J . Chem. Phys. 8, 431 (1940). 136. Bknard, J., and Talbot, J., Compt. rend. 226, 411 (1948). 137. Hering, C., and Nichols, M. H., Revs. Modern Phys. 21, 257 (1949). 138. Frank, F. C., and Van der Merwe, J. H., PTOC.Roy. SOC.(London) A198, 205 (1949). 139. Leidheiser, H., and Gwathmey, A. T., J . A m . Chem. SOC.70, 1200, 1206 (1948). 140. Fgrland, K., Tids. Kjemi Berguesen Met. 10,260 (1950). 141. Roberts, J. K., Trans. Faraday SOC.34, 1342 (1948); Roberts, J. K., and Orr, W. J. C., Trans. Faraday Soc. 34, 1436 (1938). 142. Halsey, G. D., Disc. Faraday SOC.8, 54 (1950). 143. Joyner, L. C., and Emmett, P. H., J . Am. Chem. SOC.70,2353,2359 (1948). 144. Schlier, E., and Farnsworth, H. E., Bull. A m . Phys. SOC. 26, 8 (1950). 145. McMillan, W. G., and Teller, E., J . Phys. & Colloid Chem. 66, 17 (1951). 146. Frankenburg, W. G., J . Am. Chem. SOC.66, 1827 (1944); Hill, T., J. Chem. Phys. 16, 767 (1949); 16, 181 (1943); 17, 520 (1949). 147. Beebe, R. A., and Stevens, L., J . Am. Chem. SOC.62,2134 (1940). 148. Beebe, R. A., and Dowden, L., J . Am. Chem. SOC.60,2912 (1938). 149. Beebe, R. A., Biscoe, J., Smith, R., and Wendell, C., J . Am. Chem. SOC.69, 95 (1947). 150. Beeck, O., Advances in Catalysis, Vol. 11, 155 (1950). 151. Orr, W. J. C., Trans. Faraday SOC.36,1247 (1939); Proc. Roy. SOC.(London) A173, 349 (1939). 152. Harkins, W. D., et al., J . Chem. Phys. 13,535 (1945); 14, 117, 344 (1946). 153. Barrer, R. M., Proc. Roy. SOC.(London) A161, 476 (1937). 154. Lenel, L., 2. physik. Chem. B23, 379 (1933). 155. Lennard-Jones, J. E., Trans. Faraday SOC.28, 333 (1932). 156. Bardeen, J. W., Phys. Rev. 68, 727 (1940). 157. Margenau, H., and Pollard, W., Phys. Rev. 60, 128 (1941). 158. Hume-Rothery, W., Atomic Theory for Students of Metallurgy. Institute of Metals, London, 1946, p. 249.


Surface Studies with the Vacuum Microbalance : High-Temperature Reactions EARL A. GULBRANSEN Westinghouse Research Laboratories. East Pittsburgh. Pennsylvania Page I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 TI . Thermochemical and Kinetic Theory Calculations . . . . . . . . . . . . 122 1 . TypeReactions . . . . . . . . . . . . . . . . . . . . . . . . . 122 2 . Thermochemical Calculations . . . . . . . . . . . . . . . . . . . 123 a . Gas Formation or Removal in Vacua . . . . . . . . . . . . . . . 124 b . Gas Exchange Reactions . . . . . . . . . . . . . . . . . . . . 127 3. Kinetic Theory Calculations . . . . . . . . . . . . . . . . . . . . 129 111. Apparatus and Method . . . . . . . . . . . . . . . . . . . . . . 130 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 2 . Theory of the Beam Balance . . . . . . . . . . . . . . . . . . . . 131 a . Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . 131 b.Period . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 c . Torsional Moment of Resistance of Supporting Wires . . . . . . . . 132 d . Bending of the Beam . . . . . . . . . . . . . . . . . . . . . . 132 3 . Description of the Balance . . . . . . . . . . . . . . . . . . . . . 133 4 . Vacuum System and Furnace Tube . . . . . . . . . . . . . . . . . 137, a . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 137 b . Removal of Contaminating Gases . . . . . . . . . . . . . . . . 138 c . Ceramic Materials for Furnace Tubes . . . . . . . . . . . . . . 139 d . Glass to Ceramic Seals . . . . . . . . . . . . . . . . . . . . . 139 e . Double-Walled Furnace Tube and Vacuum System . . . . . . . . . 140 f . Measurement of Performance of the Vacuum System and Furnace Tube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 g. Leak Rate Studies . . . . . . . . . . . . . . . . . . . . . . . 146 5 . Experimental Method for the Study of Oxidation, Vapor Pressure Studies and Related Investigations . . . . . . . . . . . . . . . . . 151 IV . Application to the Study of the Oxidation of Metals . . . . . . . . . . . 152 1 . Previous Work . . . . . . . . . . . . . . . . . . . . . . . . . . 153 153 2 . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . 3 . Discussion of Results . . . . . . . . . . . . . . . . . . . . . . . 154 a . High Vacuum Reaction . . . . . . . . . . . . . . . . . . . . . 154 b . Reaction with Oxygen . . . . . . . . . . . . . . . . . . . . . 155 V. Study of the Vapor Pressure of Metals and the Effect of Oxide and Nitride Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 1 . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . 161 163 2. Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 . Discussion of Results . . . . . . . . . . . . . . . . . . . . . . . 163 a . Effect of Oxide and Nitride Surface Films on the Vapor Pressure of Beryllium . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 119

120

EARL A. GULBRANSEN

Page VI. Application t o the Study of the Combustion of Solid Fuels. . . . . . . . 167 168 1. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Discussion of Results. . . . . . . . . . . . . . . . . . . . . . . 169 a. Effect of Time and Temperature. . . . . . . . . . . . . . . . . 169 b. Effect of Pressure . . . . . . . . . . . . . . . . . . . . . . . 172 c. Interpretation. . . . . . . . . . . . . . . . . . . . . . . . . 173 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

I. INTRODUCTION I n recent years increasing interest has been shown in the mechanism of reactions between gases a t solid interfaces and between gases and solids a t elevated temperatures. These reactions include many of interest to catalysis, the protection of metals and the combustion of solid fuels. From the practical viewpoint these are some of the most useful reactions to mankind in our present state of industrial development. These reactions have been studied extensively in the past but owing to lack of precise experimental techniques and the lack of adequate theoretical interpretation, progress in the understanding of the mechanism of the reactions has been slow. Owing in a large measure to the work of Wagner and co-workers, the theoretical understanding of gas-solid reactions has been put on a sound physical basis. This has greatly stimulated the development of experimental methods for the study of gas-solid reactions. In the case of gases reacting on solid surfaces no comparable theoretical development has occurred. However, the development of new experimental methods has stimulated new work in this field. One of the most important of these new experimental tools has been the development and application of the vacuum microbalance technique in which the sensitive microbalance operates directly in the vacuum or reaction system. The success of the method depends upon the coordination of a number of different experimental as well as theoretical disciplines. Thus, from an experimental point of view precise weighing techniques on properly prepared specimens must be coordinated with high vacuum techniques and the use of ceramic materials at high temperatures. From a theoretical viewpoint thermodynamic calculations must be made for all of the reactions involved and the results interpreted in terms of diffusion process for gas-solid reactions in which a film is formed or the gas diffuses into the solid, or in terms of the absolute reaction rate theory or its equivalent for gas reactions on solids including catalytic reactions and the combustion of fuels. It should be realized that a complete study of a particular surface reaction involves a correlation of the chemical factors with the physical

VACUUM MICROBALANCE : HIGH-TEMPERATURE REACTIONS

121

properties of the surface and surface films. The chemical factors can be classified as follows: 1. Kinetics a. Effect of time, temperature, pressure, gas composition, surface preparation etc. 2. Structural factors a. Crystal structure or geometrical arrangement of the atoms, ions, or molecules b. Primary structure, denoting the size and shape of the crystallites c. Secondary structure, denoting size and shape of the particles (agglomerated crystals) and the nature of the boundary regions between particles 3. Composition factors a. Atomic composition b. Lattice defects, vacancies, and impurities

The physical properties include the conductivity, both electronic and ionic, of the surface film and the nature of the diffusion process. A complete study of one particular surface reaction following this analysis would be an exhaustive undertaking and would involve many experimental difficulties. Our experience with many techniques indicates that the most fruitful approach to the understanding of many surface reactions is to study the kinetics of the reaction together with spot checks on the several structural factors. In considering the available experimental techniques and the development of new techniques for the study of surface reactions, the following factors should be considered: (1) the method should give a continuous or semicontinuous record of the reaction over a wide range of time, temperature and pressure, (2) sufficient sensitivity should be available so that a monolayer or less of reaction product can be determined on specimens of practical size and thickness] and (3) the experimental information should be directly correlated with a minimum of assumptions. Before applying the vacuum microbalance or any similar method to the study of the rate of a particular surface reaction it is essential to understand as much as possible concerning the chemistry of the main reaction and the possible side reactions which may occur in a given system. This requires detailed thermochemical calculations to be made for all conceivable reactions to determine the specifications for the vacuum system and furnace tubes, the preparation of the specimens, the experimental procedures, and the interpretation of the data obtained. Kinetic theory calculations should be applied to aid in interpretation of the rates of certain vacuum and low-pressure reactions. To set down the necessary equations the composition of the solid including both metallic] nonmetallic impurities, and surface impurities must be determined or estimated when not known. It is also essential to know the composition and impurities present in the gaseous atmosphere

122

EARL A. QULBRANSEN

since certain impurities may react much more readily with the metal than the gas under study. 11. THERMOCHEMICAL AND KINETICTHEORY CALCULATIONS 1. T y p e Reactions For our purposes the chemical reactions which occur between gases and metals, gases and solids, and solids and solids can be classified (1) in four groups: (a) reactions in which a gas is formed as one of the products, (b) reactions in which one of the reactants is a gas and is therefore removed from the atmosphere or from the vacuum, (c) reactions in which one gas is exchanged for another gas as a result of reaction, and (d) reactions between solids in which a new phase or phases are formed but no gas is involved. We are not classifying reactions in which one of the phases involved is a liquid. This classification may be rather arbitrary, but we have found it very useful. Since high vacuum is used so extensively in the vacuum microbalance method for pretreating the material and as a method for evaluating the zero point of a reaction, we will consider here detailed thermodynamic calculations for the reactions which a number of active metals containing carbon undergo when heated in a high vacuum system. Thus, if we were to study the kinetics of oxidation of chromium a t temperatures between 800" and 100O'C. it can be shown that: (1) the initial oxide cannot be effectively removed by hydrogen treatment below 1000°C.; (2) the metal will act as a getter to oxygen at these temperatures; (3) the oxide will react with carbon in the metal above 800°C.;and (4) the metal will vaporize at an appreciable rate above 850°C. It can be seen, therefore, that this type of thermodynamic information is essential t o planning, carrying out, and interpreting the results of a particular study. For purposes of simplicity, in the case to be discussed, reactions involving sulfur, phosphorus, and other volatile components are assumed to be absent. Materials containing these elements must be avoided in vacuum systems which are to be used at high temperatures. The following reactions will be considered. 1. Gas forming reactions:

(a) metal (gas)

+ metal + gas

+$

degassing of the metal

decomposition of oxide (b) M,O,(s) ~ 'zM(s) t Oz(g) MzO,-i(s) %02(g) (c) M,O,(s) decarburization by surface (d) M,O,(s) yC(meta1) & zM(s) yCO(g) oxides vaporization of the metal (e) zM(s) d zM(g) volatilization of the metal oxide (f) M,O,(s) + M,O,(g) (g) Decomposition of hydrides, nitrides, etc., to form hydrogen, nitrogen, etc.

* +

+

+


123

2. Gas removing reactions: (a) zM(s)

+ $Ol(g) F? M,O,(s)

+

oxidation

(b) M=Ov-l(s) >POz(g) F! MzO,(s) (c) zM(s) yCO(g) M,O,(s) yC (metal)

+ + (d) zM(s) + f COz(g) + M,O,(s) + yC (metal)

+

+

+

yHzO(g) F? M,-,Oll(s) M,H,&) (e) sM(s) formation (f) Reactions t o form nitrides, hydrides, etc. (9) Chemisorption of gases

oxidation oxidation oxidation and hydride

3. Gas exchange reactions:

+ +

(a) zM(s) yHzO(g) F? M,O,(s) (b) zM(s) yCOz(g) M,O,(s) 2Hz(g) Ft. metal (c) C(meta1)

+

+

+ yHl(g) + yCO(g) + CH4(g)

oxidation oxidation decarburieation by hydrogen

It is often assumed th at heating a metal in high vacua is a rather simple process of desorbing gases from the surface of the metal. This may be true for certain very pure metals under certain conditions of temperature and pressure. However, a consideration of the previous equations as a function of temperature and pressure would show th a t many metals react in high vacua by removing some gases, forming others, and exchanging still other gases. I n fact, most metals are dynamic systems even in high vacua. The degree of reactivity will depend upon the presence or absence of an oxide film and the presence or absence of carbon and other impurities. Even the presence of a n oxide film at high temperature is determined not only by the rate of the several oxidation reactions in high vacuum but by the rate of solution of the oxide in the metal, its rate of reaction with carbon and its rate of decomposition. 2. Thermochemical Calcuiations

It is important to calculate from thermochemical data the direction and equilibrium concentrations of the components involved in the reactions previously outlined. The recent work of Kubaschewski and Evans (2) is of special interest in this regard. It should be recognized that these calculations are limited in many cases by kinetic and rate considerations and the presence of reaction inhibitors such as protective films. The free energy of formation of many of the oxide systems and the several reacting gases is well known. However, the data for the nitrides, hydrides, and carbides of the metals have only in a few cases been determined. The vapor pressure data for the metals with which we are concerned are available with varying degrees of accuracy. However, the data on the vapor pressure of the oxides of the metals are incomplete.

124

EARL A . QULBRANSEN

To illustrate these types of calculations, the various reactions previously outlined will be calculated and tabulated for the metals zirconium, tungsten, chromium, iron, and magnesium. The gas-forming and gasremoval reactions are considered as one group, since under certain conditions the reaction may remove gas and a t a higher temperature the reaction may form gas. The exchange reactions will also be considered as a group. The free-energy data for zirconium dioxide are calculated from equations given by Thompson (3), while the data for chromic oxide and ferrous oxide are calculated from equations given by Chipman (4,5). The data for tungstic oxide and magnesium oxide are calculated from equations given by Seltz, Dunkerly, and DeWitt (6) and Thompson (3), respectively. The free energy data for the gases and the vapor pressure data for the metals iron and magnesium are calculated from equations given by Kelley (7). The vapor pressure of chromium is calculated from the data of Speiser, Johnston, and Blackburn (8) while the vapor pressure of tungsten is given by Jones, Langmuir, and Mackay (9). In all the calculations where carbon is involved, the activity of the carbon is assumed to be proportional to the mole fraction. a. Gas Formation or Removal in Vacua. The results of the calculations are shown in Table I. The logarithms of the equilibrium gas pressure in atmospheres for the several reactions are tabulated as a function of temperature. Such calculations are useful for determining whether the metals concerned are reacting with a formation, or with a removal of gases in a vacuum. Table I(a) on equilibrium pressure of oxygen over the oxide and metal shows that the equilibrium gas pressure of the oxides of zirconium, tungsten, chromium, iron, and magnesium a t temperatures up to 1200"C., with the exception of tungstic oxide, is less than 10-lo atm. Thus the metals will react with oxygen in a high vacuum to form their oxides. For instance, zirconium will remove oxygen gas a t 1000°C. t o a partial pressure of 10-a6.6 atm. Whether this reaction will actually occur or not will depend upon the presence or absence of oxide films that are frequently present on these metals, and on the protective properties of these films. The oxide film on zirconium, for example, is soluble in the metal a t elevated temperatures and has been described by Fast (10) as being permeable. Oxide films can only be formed if the rate of solution of oxide in the metal is smaller than the rate of formation of the oxide. The rates of solution of the oxides of tungsten, chromium, and iron in the metals is probably slow and their solubility limits are small as compared to those of zirconium. The reaction of these metals with the gases in

VACUUM MICROBALANCE

: H.IGH-TEMPERATURE REACTIONS

TABLE

125

r

Equilibria Calculations for Formation or Removal of Gases in Vacua (a) Equilibrium pressure of oxygen over the oxide and metal in atmospheres except for magnesium oxide, where thc equilibrium constant is given

Temp., "C.

ZrOz log P

WO3 log P

Crz03 log P

FeO log P

MgO log K R

600 800 1000 1200

-56.3 -43.8 -35.6 -29.9

-24.0 -17.9 -13.8 -8.8

-34.8 -26.4 -20.7 -16.5

-24.6 -18.7 -14.7 -11.8

-42.0 -24.8 -19.6

(b) Formation or removal of carbon monoxide by reaction of surface oxides with carbon in the metal; C = 0.001 %; pressure in atmospheres except for magnesium oxide ~

Temp., "C.

ZrOz log P

600 800 1000 1200

-15.9 -12.7 -10.5

~~

wos

CrzOa

log P

log P

FeO log P

MgO log K R

-8.08 -6.01 -4.54

-10.4 -7.5 -5.5 -4.1

-5.2 -3.5 -2.4 -1.6

-24.4

(c) Formation or removai of carbon dioxide by reaction of surface oxides with carbon in the metal; C = 0.001 %; pressure in atmospheres except for magnesium oxide

Temp., "C.

ZrOt log P

600 800 1000 1200 1400

-28.6 -23.4 -19.9 -17.5

WOa log P

-4.00 -2.34 -1.30

Cr203 log P

FeO log P

MgO log K R

-5.19 -3.93 -2.76 -2.05

-43.2

-12.46 -9.56 -7.56

(d) Vapor pressure of the metals; pressure in atmospheres

Temp., "C. 600 800 1000 1200 1400

W log P

Cr log P

Fe log P

Mg log P

-19.8 -15.4 -12.2

-15.96 -11.63 -8.61 -6.43 -4.77

-16.5 -12.1 -8.8 -6.7 -5.4

-6.0 -3.0 -1.5 -0.3

126

EARL A. GULBRANSEN

vacua below mm. of mercury at temperatures of 1000°C. or lower is not expected to be as rapid as for zirconium. Table I(b) on the formation or removal in vacua of carbon monoxide by reaction of surface oxides with carbon in the metal shows the results of these calculations. The reactions are feasible thermodynamically in vacua of the order of 10-l0 atm. at temperatures of 600°C. or higher for the metals tungsten, chromium, and iron. Thus, carbon monoxide will be formed by the diffusion of carbon to the surface and subsequent reaction with the surface oxides. This reaction has been discussed for the case of steels by Holm (1 1). The effect of carbon content on the reaction is not shown in the table. However, the effect can be seen from the expression for the equilibrium constant K for the reaction of ferrous oxide with carbon in the iron: log P c o = log K

+ log N c

where N C is the mole fraction of carbon, log P o is a linear function of log N C for a definite temperature. Therefore, the greater the carbon content the higher the equilibrium pressure of carbon monoxide. The reverse reaction is also of interest and may occur on metals like zirconium and magnesium. Carbon monoxide will be removed from the system by zirconium at pressures of 10-lo atm., forming metal oxide and carbon. This reaction is also affected by the carbon content of the metal. Thus, zirconium acts as a getter for carbon monoxide a t elevated temperatures, while tungsten, chromium, and iron may form carbon monoxide gas. Table I(c) on the formation or removal in vacua of carbon dioxide by reaction of the surface oxides with carbon in the metal shows the results of these calculations. The reactions are feasible for tungsten and iron but not for zirconium and magnesium. Chromium presents an intermediate case with an equilibrium pressure of 10-12.46 a t 8OO0C., at 1000°C., and a t 1200°C. The reverse reaction is feasible for zirconium and magnesium and for chromium at low temperatures. From a kinetic viewpoint the probability that this reaction will occur is small compared t o the reaction to form carbon monoxide gas. I n this case zirconium will act as a getter for carbon dioxide, while tungsten, iron, and chromium will be relatively inert to carbon dioxide molecules. If the metals form carbides which have a negative free energy of formation, then a new reaction is involved. These reactions are not calculated here because of the lack of available data. The results of Guldner and Wooten (12) indicate that carbides are probably formed. Table I(d) on vapor pressure of the metals shows the results of the calculations for the metals tungsten, chromium, iron, and magnesium.

VACUUM MICROBALANCE: HIGH-TEMPERATURE REACTIONS

127

With the exception of tungsten and zirconium (not calculated) the metals are volatile at elevated temperatures in vacua of the order of atm. At 1000°C. the vapor pressure of chromium is 10-S.61 atm., that of iron 10-s.s atm., and that of magnesium atm. Thus at 1000°C. the vaporization of chromium and iron is appreciable. Several of the reactions listed under type reactions are not possible to calculate because of the lack of available information. Decomposit.ion and formation of hydrides and nitrides can, respectively, lead to the evolution and disappearance of hydrogen or nitrogen. The removal of adsorbed gas from the surface, from grain boundaries, or from the lattice is important but not amenable to calculation. The volatilization of the oxides, with the exception of molybdic oxide, is also impossible t o calculate because of the lack of data. The oxidation reaction of water vapor with a metal to form the oxide and a hydride is also of interest, but no accurate thermodynamic data are available on hydrides of these metals. b. Gas Exchange Reactions. The results of the calculations are shown in Table 11. The logarithms of the equilibrium pressure ratios of the two gases considered are tabulated as a function of temperature. From the point of view of maintaining high vacuum these reactions must also be considered, since it is an advantage to know which particular gas is present and how it is formed. If condenbible gases are present, the use of traps can be very effective in maintaining high vacuum conditions. The results of the reaction with HzO(g) to form oxide and H2(g) are shown in Table II(a). This reaction is feasible for zirconium, chromium, and magnesium in high vacuum and in hydrogen atmospheres where the water concentration is greater than the equilibrium value. The reverse reaction is only feasible for iron and tungsten if an atmosphere of pure hydrogen is used and if the concentration of water is maintained below the equilibrium value. The results of the reaction of metal with COz(g) to form oxide and CO(g) are shown in Table II(b). In an atmosphere of carbon dioxide the reaction is feasible for zirconium, chromium, and magnesium. Mixtures of carbon monoxide and carbon dioxide are formed with tungatm. carbon monoxide is formed with sten and iron. In vacua of zirconium, chromium, and magnesium. Mixtures of carbon monoxide and carbon dioxide are formed with tungsten and iron. The kinetics of this reaction have not been investigated experimentally. It is possible that the carbon monoxide formed may react immediately with zirconium, forming additional oxide and carbon or a carbide. The results of the calculations of the reaction of Hp(g) with C (in metal) to form CH4(g) are shown in Table II(c) for a carbon content of

128

EARL A. GULBRANSEN

TABLE I1 Equilibria Calculations for Gas Exchange Reactions (a) Reaction of metal with HzO(g) to form oxide and Hz(g); pressure ratio tabulated

Temp., "C. 400 600 800 1000 1200 1400

Zr

-12.7 -10.6 -9.1 -8.1

W -0.42 -0.02 $0.25

+O .38

Cr

Fe

-4.53 -3.50 -2.76 -2.21

-0.35 -0.19 -0.11 -0.03 -0.00

Mg -25.1 -19.1

(b) Reaction of metal with CO,(g) to form oxide and CO(g); pressure ratio tabulated

pco log __ PCO,

Temp., "C. 400 600 800 1000 1200 1400

Zr

12.7 10.8 9.4 8.6

W -0.65 -0.39 -0.e9 -0.16

Cr

4.48 3.71 3.13

Fe

Mg

0.0

24.6 18.7

-0.13 -0.31 -0.42

(c) Reaction of Hz(g) with C (in metal) to form CHa(g); log- 'OH'

' is tabulated;

( P H %)

c = 0.001% Temp., "C.

Zr

W

Cr

Fe

Mg

400 600 800 1000 1200

-2.87 -4.42 -5.43 -6.14 -6.66

-2.56 -4.11 -5.12 -5.83 -6.36

-3.11 -4.66 -5.67 -6.38 -6.90

-3.08 -4.63 -5.64 -6.35 -6.87

-3.44 -4.99

0.001%. For low pressures of hydrogen and for vacuum reactions the equilibrium pressure of methane is very low and the reaction is not feasible from a thermodynamic point of view. The reverse reaction of methane, forming hydrogen with the deposition of carbon, is feasible and may occur if a proper mechanism exists for the reaction. The free-energy data for methane are given by Kelley (13).

VACUUM MICROBALANCE

: HIBH-TEMPERATURE

REACTIONS

129

3. Kinetic Theory Calculations

Several fundamental calculations should be made in order to interpret the rates of reaction in high vacua. These involve the calculation of the amount of gas striking a unit area in a unit of time. For the pressure range of 10-5 to mm. of mercury the mean free path of the molecules is large compared to the dimensions of our vessels. From the calculation of the amount of gas striking a unit area in a unit of time it is possible to calculate the time necessary to form one oxygen layer or 1 A. of the oxide lattice. The latter calculation assumes that every molecule hitting the surface will stick. This is the case for zirconium at elevated temperature, since the oxide is soluble. For other metals the formula must be corrected for those which hit the surface but do not react. From such calculations it is possible to establish the effect of pressure in limiting the reaction. Calculation of the pressure from the weight gain can be readily made. The interpretation of this pressure should be made clear. It is the approximate pressure in a system consisting of a source (the walls of the ceramic tube) and three sinks (namely, the mercury pumps, the liquid air trap, and the specimen of zirconium). No intention is made to relate this pressure to the pressure in the system without the zirconium specimen. (1) Weight of gas colliding with a surface. Consider a metal sample of 1 area intercepting a column of gas of average velocity B, mass m per molecule, and concentration n molecules per cubic centimeter. The weight intercepted is given by W = nBm/4. Let (Y be the fraction of the molecules hitting which react; then the observed weight gain is given by

This formula assumes that the mean free path is large compared to the dimensions of the vessel and that the area of the specimen is small compared to the area of the walls of the furnace tube. For our system, the specimen surface is about 5 % of the wall area of the furnace tube. . (2) Time to form one layer of oxygen atoms at 900°C. on zirconium. The reaction of zirconium with oxygen will be calculated. Similar calculations can be made for other metals. Zirconium has a B.C.C. lattice at 900°C. and a unit cell length of 3.61 8. Let us assume that the 110 plane is available for reaction. The area of this plane for each unit cell in the surface is 18.4 A2. Two atoms of zirconium are available for reaction. In 1 cm.* of surface we have 1.09 x 1015atoms. If each zirconium atom reacts with two oxygen atoms, then 2.18 X 1015oxygen atoms are needed

130

EARL A. OULBRANSEN

to form a monolayer. Let 7 be the time in seconds t o form a monolayer. Then 7 = 2.18 X 1016/nB. (3) T i m e to form 1 d. zirconium dioxide on surface. The density of zirconium dioxide is 5.71 or 1 A. = 5.71 X g. of zirconium dioxide. Since only oxygen is added, this is equivalent to 1.48 X g. of oxygen per Angstrom added. r =

1.48 X lo-* 5.92 X lo-* W n5m

Table 111 shows the results of calculation. The weights of gas striking 1 cm.2 of surface per second and the times necessary t o form one oxygen layer or 1 A. of the oxide are tabulated as a function of the pressure. TABLE I11 Calculated Weight of Gas Striking Surface T = 1173.l"K. P , m m . Hg 10-6 10-6 10-7 10-8 10-9

Weight of gas, g./cm.*/sec. 1.11 1.11 1.11 1.11 1.11

x x x x x

10-7 10-8 10-0 10- 10

10-11

Time for one oxygen layer on Zr, sec.

1.55 X 1.55 1.55 X 1.55 X 1.55 X

10-1

101 10'

10'

Time for 1 A. of ZrO,, sec.

1.33 x 1.33 1.33 X 1.33 X 1.33 X

10-1

10' 10% 10'

For reactions with a = 1 the pressure is the limiting factor in the chemical reaction. For cases where a is less than 1, other factors such as the diffusions of ions, electron, or oxygen atoms through the film are important in limiting the rate of the reaction.

111. APPARATUS AND METHOD 1. Introduction

During the past few years four methods have been developed for the study of the kinetics of oxide film formation, all being semicontinuous in nature. The polarimetric method has been adapted by Lustman and Mehl (14) for the measurement of the oxidation of copper. A spectrophotometric method has been applied by Waber, Sturdy, Wise, and Tipton (15) to the study of the oxidation of tantalum while a differential pressure method has been developed by Campbell and Thomas (16) for a study of the oxidation'of a series of metals and alloys a t elevated temperatures. We have chosen to use a sensitive weight gain method (17,18) in which a quartz microbalance is placed directly in the vacuum system.


131

Since all the reactions noted &der type reactions involve weight changes on the surface of the solid the vacuum microbalance method has perhaps the broadest range of applicability to surface reactions of a n y of the above methods. The method in brief is t o suspend a thin sheet of metal or solid from the beam of a very sensitive, quartz beam microbalance operating in a n all glass or ceramic ware vacuum system. The weight change of the specimen is followed continuously as various operations are performed upon it. When applied t o the oxidation of metals these operations include evacuation to pressures of 10-6 mm. Hg or lower, degassing, preliminary reduction of the initial oxide film with pure hydrogen, and oxidation of the metal specimen under widely varying conditions of temperature and pressure. I n the paper by Rhodin in this volume a general discussion was given of microbalances. It is the purpose of this section to discuss the basis for our particular design and how it is applied to high-temperature studies. This will require, in addition, a rather detailed consideration of vacuum systems a t high temperatures. 2. Theory of the Beam Balance

Figure 1 shows a diagram illustrating the theory of the balance. Following Glazebrook (19) we have the following equations for a beam type of balance.

I

POINT OF SUPPORT

TERMINAL SUPPORT

+ + ? j ? r f

A

TERMINAL SUPPORT

ANGLE OF BEAM M"PO'NT

OF AB

P tz \LOAD ON LEFT SIDE

'CENTER OF GRAVITY P OF BEAM LOAD ON RIGHT SIDE

FIG.1. Theory of the balance.

a. Sensitivity.

Neglecting torsional resisting moment d4 dz

where 4 P P z 1

1

Gs

+ a(2P)

turning angle in radians, load in grams on right side, = load in grams on left side, = one-half the length of beam in centimeters, G = weight of beam in grams, s = cs, a = CD = 0 for coplanar system.

+

= =

132

EARL A. GULBRANSEN

b. Period. This equation is obtained from the differential equation of motion of the system and neglecting torsional moment. =

+ + +

2d

2P(a2 12) GK2 g(Gs 2aP)

where t = period in seconds, K = radius of gyration, g = gravitational constant. Two other equations are of interest in the design of this type of microbalance. c. Torsional Moment of Resistance of Supporting Wires:

T = -EdJ4 m

(3)

where T = twisting moment, I = moment of inertia, d 4 = turning angle in radians, E, = shearing modulus, m = one-half the length of supporting wire. Since i t is desired to make the balance a gravity type, the torsional moment of resistance should be made equal t o or one-tenth of the resisting moment due to the weight of the beam or T >ioGs sin d. Bending of the Beam (approximate calculation) :

ion of such defects is confined t o the surface layers of atoms. Ions and electrons associated with these defects may give rise to surface energy levels which can facilitate electron exchange reactions or chemical bonding of atoms and molecules to the surface. Garner (26,79) has shown t ha t in the reduction of the oxide surface by carbon monoxide, the removal of oxygen ions from the external surface a s carbon dioxide, sets free electrons which, with their associated metal ions, diffuse across the oxide layer to build up a metallic phase. I n a rapid surface reaction the concentration of electrons a t the surface may become sufficiently great t o build up metallic nuclei. This process of nucleation gives rise t o a fresh metal-metal oxide interface much closer to the site of the surface reaction than the original interface. Many investigators have concluded that the existence of a n induction period in some catalytic reactions with oxides is due to the initial absence and the subsequent formation and growth of nuclei.

HETEROGENEOUS OXIDATION O F CARBON MONOXIDE

-

213

A number of recent publications deal with the properties of oxide catalysts per se. Huttig (80) has shown th at in the interaction between mixtures of oxides, the transformation to a chemical compound proceeds through individual characteristic states by diffusion of the oxide of lower melting point over the surface of the less mobile component, the former becoming more strongly bound and localized at a later stage in the process. Other stages, which are correlated with changes of catalytic activity during activation of binary oxide systems, include internal diffusion of the mobile component, formation of disordered crystalline aggregates, and finally the filling of crystal defects. Winter (81) has used the exchange reaction between 01* in the gas phase and solid oxides t o study reactivity. The surface layers of oxide undergo an exchange reaction which is kinetically first order a t catalytic temperatures. The oxygen ions in the surface of C r z 0 3are mobile and considerable mobility of oxygen from the first layer of atoms into the adsorbed state, and vice versa, was also found in the case of Alz03 and MgO. Selwood et al. (82-86) have shown by magnetic susceptibility measurements the oxidation state and the atomic environment of active oxides of the transition group elements. He has used the phenomena of valence induction, and nuclear induction in the elucidation of the type of structure which determines catalytic activity. Transition metal oxides such as those of manganese, nickel, and iron, when supported on magnesia, y-alumina, and rutile, tend t o aggregate into ions of several layers in thickness which have high magnetic susceptibilities and high catalytic activity. The transition oxides show a tendency t o assume the crystal structure and valency of their support. The results indicate th at the d-shell may be involved in the surface reactions. The foregoing remarks have stressed the importance of the structure of the oxide and the state of the surface in catalytic oxidation. The relation between chemisorption and heterogeneous catalysis has been reviewed by Taylor (88), Rideal (87), and Garner (79). T h e surface of the metal oxide in technical catalysts is rarely clean in the sense of that of a metallic wire or film, the surface of which can be readily freed of impurities. The oxide surface is more likely to be a heterogeneous one and chemisorption followed by catalytic oxidation may take place on active sites or active centers. Some oxides show a reversible type of adsorption a t low temperatures but the adsorbate may be released on raising the temperature and readsorbed again irreversibly a t a higher temperature. The heats of adsorption of CO and the behavior on desorption indicate that there are two types of bonding to the surface which may be represented as M-CO and MOC02. The irreversibly

214

MORRIS KATZ

adsorbed CO thus forms surface carbonate ions attached t o the surface by electrovalence forces. Only a fraction of the total surface covered may be active catalytically. The compounds discussed in this review fall into two broad classes on the basis of their behavior toward water vapor. The highly reactive Hopcalites are extremely sensitive to poisoning by water vapor, whereas promoted silver peroxide and activated silver permanganate are not affected by water vapor. In fact, a little moisture is necessary to initiate the reaction between CO and activated permanganate. The Hopcalites in the presence of dry gas come closer to the definition of a true catalyst than any other compound considered here. Furthermore, the Hopcalites and similar oxides which have been poisoned by adsorption of water vapor may be regenerated by heating to appropriate temperatures. N o method has, thus far, been discovered of regenerating silver permanganate which has become inactive through exposure to gas. Part of the difficulty lies in the formation of end products which are noncrystalline in character. The kinetics of the adsorption wave have provided a useful tool for evaluating the activity of activated silver permanganate and might be profitably extended to other systems where the concept of active cent,ers is applicable.

REFERENCES 1. Langmuir, I., J. Chem. SOC.1940, 511. 2. Langmuir, I., J . Am. Chem. SOC.40, 1361 (1918). 3. Langmuir, I., J. Am. Chem. SOC.38, 2221 (1916). 4. Benton, A. F., Trans. Faraday SOC.28, 202 (1932). 5. Burwell, R. L., and Taylor, H. S., J . Am. Chem. SOC.68, 1753 (1936). 6. Rrunauer, S., Emmett, P. IF., and Teller, E., J . Am. Chem. SOC.60, 309 (1938). 7. Taylor, H. S., 2nd Ogden, G., Trans. Faraday SOC.30, 1178 (1934). 8. Taylor, H. S., and Kistiakowsky, G. B., J . Am. Chem. SOC.49, 2468 (1927). 9. Taylor, H. S., and Liang, S. C., J . Am. Chem. Soc. 69, 1306 (1947). 10. Damkohler, G., and Edse, R., 2. physik. Chem. B63, 117 (1943). 11. Katz, M., Chemistry in Canada 3, 181 (1951). 12. Whitesell, W.A., and Frazcr, J. C. W., J . Am. Chem. SOC.46,2841 (1923). 13. Roginskii, S. Z., Acta Physicochim. U.R.S.S. 9, 475 (1938). 14. Roginskii, S. Z., and Zeldowitch, J., AclaPhysicochim. U.R.S.S. 1,554, 595 (1934). 15. Elovich, C. U., and Kachur, L. A., b.Gen. Chem. U.S.S.R. IX, No. 8,714 (1939). 16. Charachorin, F., Elowitz, S., and Roginsky, S., Acta Physicochim. U.R.S.S. 3, 503 (1936). 17. Charachorin, F., and Elowitz, S., Acta Physicochim. U.R.S.S. 6, 325 (1936). 18. Zeldowitch, J., Acta Physicochim. U.R.S.S. 1, 449 (1934). 19. Elovich, C.U., and Korndorf, V. A , , J.Gen. Chem. U.S.S.R. IX,No. 8,673 (1939). 20. Williamson, A. G., J . Am. Chem. SOC.64, 3159 (1932). 21. Frazer, J. C. W., and Greider, C. E., J . Phys. Chem. 24, 1099 (1925). 22. Pitzer, E. C., and Frazer, J. C. W., J . Phys. Chem. 46, 761 (1941).

HETEROGENEOUS OXIDATION O F CARBON MONOXIDE

215

Hofmann, K. A., Ber. 61, 1526 (1918). Palmer, W. G., Trans. Am. Electrochem. SOC.61, 445 (1927). Bessalow, P., and Koboeew, N. I., J . Phys. Chem. U.S.S.R. 9, 815 (1937). Garner, W. E., Gray, T. J., and Stone, F. S., Proc. Roy. SOC.(London) A197, 294 (1949); Disc. Faraday SOC.No. 8, 246 (1950). 27. Gray, T. J., Proc. Roy. SOC.(London) A197,314 (1949). 28. Engelder, C. I., and Miller, L. E., J . Phys. Chem. 36, 1345 (1932). 29. Frazer, J. C. W., and Albert, C. G., J . Phys. Chem. 40, 101 (1936). 30. Lory, E. C., J. Phys. Chem. 37, 685 (1933). 31. Finkelstein, V., Chrisman, I., and Rubanik, M., J . Phys. Chem r1.S.S.R. 4, 19 (1934). 32. Draper, H. D., J. Am. Chem. SOC.60, 2637 (1929). 33. Hoskins, W. M., and Bray, W. C., J . Am. Chem. SOC.48, 1457 (1926). 34. Jones, H. A., and Taylor, H. S., J . Phys. Chem. 27, 623 (1923). 35. Pieters, H. A. J., Chem. Weekblad 28, 250 (1931). 36. King, J. G., and Edgecombe, L. J., Dept. Sci. Ind. Research (Brit.) Fuel Research Tech. Paper 33, 28 (1931). 37. Jones, N. C., J . Phys. Chem. 33, 1415 (1929). 38. Engelder, C. J., and Blumer, M., J. Phys. Chem. 36, 1353 (1932). 39. Benton, A. F., Trans. Faraday SOC.28, 202 (1932). 40. Wright, C. R. A., and Luff, A. P., J . Chem. SOC.33,535 (1878). 41. Merck, F., and Wedekind, E., 2. anorg. Chem. 186, 49 (1930). 42. Frazer, J. C. W., U.S. Patent 1,937,689 (Dec. 5, 1933). 43. Bone, W. A., Proc. Roy. SOC.(London) All2, 474 (1926). 44. Eckell, J., 2. Ekktrochem. 38, 918 (1932); 39, 807, 855 (1933). 45. Erchak, M., and Ward, R., J. Am. Chem. SOC.68,2093 (1946). 46. Erchak, M., Fankuchen, I., and Ward, R., J. Am. Chem. SOC.68, 2085 (1946). 47. Wagner, C., and Hauffe, K., 2.Elektrochern. 44, 172 (1938). 48. Neumann, B., Kroger, C., and Iwanowski, R., 2. Elektrochem. 37, 121 (1931). 49. Frazer, J. C. W., J . Phys. Chem. 36, 405 (1931). 50. Keuntzel, W. E., J . Am. Chem. SOC.62, 437, 445 (1930). 51. Benton, A. F., and Bell, R. T., J . Am. Chem. SOC.66, 501 (1934). 52. Pieters, H. A. J., Z. anal. Chem. 86, 50, 113 (1930). 53. Paal, C., Ber. 49, 548 (1916). 54. Taylor, H. S., and McKinney, P. V., J . Am. Chem. SOC.63, 3604 (1931). 55. Taylor, H. S., Kistiakowsky, G. B., and Perry, J. H., J . Phys. Chem. 34, 748 (1930). 56. McKinney, P. V., J . Am. Chem. SOC.64, 4408 (1932). 57. McKinney, P. V., J . Am. Chem. SOC.66,3636 (1933) ; J. Am. Chem. SOC.66,2577 (1934). 58. Lamb, A. B., Bray, W. C., and Frazer, J. C. W., Ind. Eng. Chem. 12,213 (1920). 59. Hurst, W. W., and Rideal, E. K., J . Chem. Sor. 126, 685, 694 (1924). 60. Goggs, A. B., J . Chem. SOC.1928, 2667. 61. Scalione, C. C., and Frazer, J. C. W., British Patent 166,285 (Aug. 11, 1921). 62. Garner, W. E., and Maggs, J., Trans. Faraday Soc. 32, 174 (1936). 63. Garner, W. E., and Kingman, F. E. T., Nature 126, 352 (1930). 64. Shurmovskaya, N., and Bruns, B., J . Phys. Chem. U.S.S.R. 9, 301 (1937). 65. de Boer, J. H., and van Ormondt, J., U.S. Patent 2,478,166 (Aug. 2, 1949). British Patents 579,817 and 579,809 (1946). 66. Katz, M., and Halpern, S., Ind. Eng. Chem. 42, 345 (1950). 23. 24. 25. 26.

216

MORRIS KATZ

67. Katri, M., Grant, G. A., arid Riberdy, R., Can. J . Research B28, 798 (1950). 68. Katz, M., and Katzman, J., Can. J . Research F26, 318 (1948). 69. Grant, G. A., Katz, M., and Haines, R. L., Can. J. Technol. 29, 43 (1951).

Katz, M., Wilson, L. G., and Riberdy, R., Can. J . Chem. 29, 1059 (1951). Grant, G. A., Katz, M., and Riberdy, R., Can. J . Technol. 29, 511 (1951). Klotz, I. M., Chem. Revs. 39, 241 (1946). Bohnrt, G. S., and Adams, E. Q., J . Am. Chem. SOC.42, 523 (1920). Dubinin, M. M., J . Russ. Phys. Chem. SOC.62, 683 (1930). Mecklenburg, W., Z. Elektrochem. 31, 488 (1935). Wicke, E., Kolloid-2. 93, 129 (1940). Amundson, N. R., J . Phys. & Colloid Chem. 62, 1153 (1948). Danby, C. J., Davond, J. G., Everett, D. H., Hinshelwood, C. N., and Lodge, R. M., J . Chem. SOC.1946,918. 79. Garner, W. E., Disc. Faraday SOC.8, 211 (1950). 80. Hiittig, G. F., Disc. Faraday SOC.8, 215 (1950). 81. Winter, E. R. S., Disc. Faraday SOC.8, 231 (1950). 82. Selwood, P. W., Ellis, M., and Davis, C. F., J . Am. Chem. SOC.72,3549 (1950). 83. Selwood, P. W., Lyon, L., and Ellis, M., J . Am. Chem. SOC.73, 2310 (1951). 84. Selwood, P. W., and Lyon, L., Disc. Faraday SOC.8, 222 (1950). 85. Spooncr, It. B., and Selwood, P. W., J . Am. Chem. SOC.71, 2184 (1949). 86. Selwood, P. W., and Schroyer, F. K., Disc. Faraday SOC.8, 337 (1950). 87. Rideal, E. K., Nature 161, 461 (1948); Disc. Faraday SOC.8, 96 (1950). 88. Taylor, H. S., Disc. Faraday SOC.8, 9 (1950); in Advances in Catalysis 1, 1-26 (19$8). 70. 71. 72. 73. 74. 75. 76. 77. 78.

Contributions of Russian Scientists to Catalysis J . G . TOLPIN. G . S. JOHN. AND E . FIELD Standard Oil Company (Zndiana). Chicago. Illinois Page 217 1 . Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 2 . Generalsurvey . . . . . . . . . . . . . . . . . . . . . . . . . 219 I1 . Schools of Thought on Catalysis . . . . . . . . . . . . . . . . . . . 224 111. Investigation of Adsorption Phenomena . . . . . . . . . . . . . . . . 238 1. Determination of the Distribution Function of the Heats of Adsorption and Interaction Function . . . . . . . . . . . . . . . . . . . . . 238 2. Determination of the Distribution Function of the Activation Energies of Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . 242 3 . Adsorption upon Structural Defects . . . . . . . . . . . . . . . . . 249 IV . Kinetics of Heterogeneous Catalytic Reactions . . . . . . . . . . . . . 254 V . Modification of Catalysts . . . . . . . . . . . . . . . . . . . . . . 256 1 . Promoters and Poisons . . . . . . . . . . . . . . . . . . . . . . 256 264 2 . Selective Poisoning . . . . . . . . . . . . . . . . . . . . . . . . VI Catalytic Conversions . . . . . . . . . . . . . . . . . . . . . . . . 266 1 . Dehydrogenation-Hydrogenation . . . . . . . . . . . . . . . . . . 266 272 2 . Dehydrocyclization . . . . . . . . . . . . . . . . . . . . . . . . 3 . Theory of Methylene Radicals . . . . . . . . . . . . . . . . . . . 275 4. Hydrogenation of Carbon Monoxide . . . . . . . . . . . . . . . . 276 5 . Hydration-Dehydration . . . . . . . . . . . . . . . . . . . . . . 279 6. Acetylene Chemistry . . . . . . . . . . . . . . . . . . . . . . . 281 7. Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . 282 8. Isomerization . . . . . . . . . . . . . . . . . . . . . . . . . . 283 9. Alkylation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286 10. Cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288 11 . Oxidation. . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 12. Other Reactions . . . . . . . . . . . . . . . . . . . . . . . . . 292 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293

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

.

I . INTRODUCTION 1. Scope Critical reviews of Russian contributions to various fields of science are necessary today because of the limited availability and utilization of the Russian technical literature. Under conditions of normal contacts between scientists of various countries it would be undesirable to segregate the contributions from any single country, as this tends t o give a distorted perspective of scientific progress . 217

218

J. G . TOLPIN, G . S. JOHN, AND E. FIELD

This paper attempts to survey the more significant Russian contributions t o the study of catalysis th at have come to the attention of the writers, with special emphasis being placed upon recent researches. Older studies and those researches carried out by scientists who no longer reside in Russia, for instance by Professor V. N. Ipatieff, are either omitted or used only as background information. Considerable attention is being paid by Russian chemists and physicists t o catalytic phenomena. The various publications on the subject coming from many Soviet research institutes bear witness to this. These publications have appeared in a number of Russian journals as well as in book form. Several symposia have resulted from special meetings devoted t o catalysis. One such meeting in 1947 was reported t o have been attended by 800 participants. Another meeting commemorating earlier contributions by L. V. Pisarzhevskil was held in Kiev in 1948 and resulted in a Pisarzhevskii memorial volume (303). Nearly all phases of the physics and chemistry of catalysis are under active study in Russia. Some phases have considerable tradition of research behind them; others are only in the formative stage, stimulated by reports in the non-Russian scientific literature. The series of publications entitled “Problems of Kinetics and Catalysis” edited by S. Z. Roginskii is a noteworthy source of information on the fields of catalysis under discussion. The latest two volumes of this series, namely VI and VII, were published in 1949 (333). Of these, Volume V I was not available to the writers. Surveys appearing in Uspekhi Khimii (Progress of Chemistry) and in other Russian journals have also been useful in presenting the point of view of the Russian contributors to the field. If they discuss Russian researches specifically, they overemphasize them. In this category belong the reviews published on various state occasions, even when they are written by competent scientists. The survey of Russian researches on isomerization during 1917-47 by S. N. Danilov is a typical example (55). Books on catalytic chemistry, whether textbooks (64) or surveys of special fields (291,305), are not uniformly useful sources of information on original Russian contributions to catalysis. The status of Russian studies on catalysis immediately before World War I1 is shown by the Transactions of the Meeting on Catalysis of May 13-16, 1940 (387) and by the volume describing the progress of Soviet chemistry during 1917-42 (362). A bibliography of publications of the Karpov Institute during 1934-43 lists many researches on catalysis among its 825 items (12). Reports on Russian researches on catalytic conversions of possible industrial value are now withheld from free circulation outside of Russia and it remains to speculate on the value and the exact nature of some researches referred t o in the publications which are available t o us.

CONTRIBUTIONS O F RUSSIAN SCIENTISTS

219

2 . General Survey

Catalytic conversions were experimentally studied in Russia toward the end of the nineteenth century, and especially in the twentieth century, and regularities were empirically established in a number of cases. The work of A. M. Butlerov (1878) on polymerization of olefins with sulfuric acid and boron trifluoride, hydration of acetylene to acetaldehyde over mercury salts by M. G. Kucherov (1881) and a number of catalytic reactions described by V. N. Ipatieff beginning with the turn of the century (139b) are widely known examples. S. V. Lebedev studied hydrogenation of olefins and polymerization of diolefins during the period 1908-13. Soon after World War I he developed a process for the conversion of ethanol t o butadiene which is commercially used in Russia. This process has been cited as the first example of commercial application of a double catalyst. Lebedev also developed a method for the polymerization of butadiene to synthetic rubber over sodium as a catalyst. Other Russian chemists (I. A. Kondakov; I. Ostromyslenskii) were previously or simultaneously active in rubber synthesis. Lebedev’s students are now continuing research on catalytic formation of dienes. The industrial utilization of certain catalytic processes in the U.S.S.R. is indicated by the following figures. TABLE I Production in the U.S.S.R. Thousands of tons

Synthetic Fuela Synthetic Rubber6 Sulfuric Acidb Plasticsb Hydrogenation of vegetable oils and animal fatse

1938

1950

85 1500-1600

900” 250 2500’ 30’ 250

-

1lfJd

Source: Shvernik, et al. (416s). Source: Vinokurov (416b). 0 Source: Dolgov (66). d Figure for 1940. This figure IS given in the official five-year plan (1946-1950). It includes coal hydrogenation, but also shale processing; a more recent estimate (416c) indicates that Soviet synthetic 011 production, both by the Fischer-Tropsch process and by coal hydrogenation, has reached 2-3 million tons per year. f Figure for 1949. 5

b

The Institute of High Pressures founded by V. N. Ipatieff, the Institute of Organic Chemistry headed by N. D. ZelinskiI, Moscow State University, the “ Khimgas ” Institute, the Institute of Applied Chemistry, the Institute of Coal Chemistry, and the Institute of Mineral Fuels have carried out many studies on catalytic conversions in the 1930’s and

220

J. G . TOLPIN, G . S. JOHN, AN D E. FIELD

1940’s. I n recent times the Karpov Institute of Physical Chemistry and the Institute of Chemical Physics have been among the major contributors of research on catalytic phenomena and the theory of catalysis. In the field of hydrocarbon conversions, N . D. Zelinskii and his numerous co-workers have published much important information since 1911. Zelinskii’s method for the selective dehydrogenation of cyclohexanes over platinum and palladium was first applied to analytical work (155,351,438,439), but in recent years attempts have been made t o use it industrially for the manufacture of aromatics from the cyclohexanes contained in petroleum. I n addition, nickel on alumina was used for this purpose by V. I. Komarewsky in 1924 (444) and subsequently by N. I. Shuikin (454,455,456). Hydrogen disproportionation of cyclohexenes over platinum or palladium discovered by N. D. Zelinskii (331,387) is a related field of research. Studies of hydrogen disproportionation are being continued, and their application is being extended to compounds such as alkenyl cyclohexanes. The dehydrocyclization of paraffins was reported by this institute (Kazanskii and Plate) simultaneously with B. L. Moldavskii and co-workers and with Karzhev (1937). The catalysts employed by this school have also been tested for the desulfurization of petroleum and shale oil fractions by hydrogenation under atmospheric pressure. Substantial sulfur removal was achieved by the use of platinum and nickel on alumina (392). At the end of the nineteenth century, A. F. Fttvorskii and his students were studying catalytic conversions of acetylene, including those occurring in the presence of alcoholic potash. His students have continued this series with work on vinylation, hydrolysis of some of the vinyl alkyl ethers (M. F. Shostakovskii), and conversions of vinylethynylcarhinols (1. N. Nazarov). Studies of hydrogenation, including destructive hydrogenation (Nemtsov, Prokopets, Dyakova), reported in the 1 9 3 0 ’ ~may ~ have been utilized by now for industrial processes. A. V. Frost has been conducting research on the kinetics of cat,alytic reactions and on catalytic cracking. Frost and M. D. Tilicheev are co-editors of a series of publications on physical constants of hydrocarbons which may be used as a source of information on the synthesis of individual hydrocarbons. Other Russian groups have contributed (N. D. Zelinskii, A. D. Petrov) to this field. Some of this work involves catalytic reactions; however, in this review mere mention of it may sufice. Duriiig the last two decades vigorous development of several schools of thought on the nature of catalysis has been taking place in Russia. The major contributors to these schools have been A. A. Balandin, S. Z. Roginskil, and N. I. Kobozev.

CONTRIBUTIONS OF RUSSIAN SCIENTISTS

22 1

Soviet physicists and chemists have been reporting research on the peculiarities of adsorption on solid surfaces and the apparent deviations from the derived laws expressing adsorption phenomena, e.g., deviations from the Langmuir laws. S. Z. Roginskii has treated the nonuniformity of the surface and adsorption under conditions of limited extent of coverage of the surface in a number of articles. Of historical importance are the researches of A. F. Ioffe and Ya. I. Frenkel’ on the physics of solid bodies which began in the early 20’s of the present century and were concerned with the electrical and mechanical properties of crystals and the mechanism of origination of voids and distortions in a crystal lattice under the influence of thermal motion (137b). It is likely that Ioffe’s researches served as the starting point for present-day researches of some Russian investigators such as F . F. Vol’kenshtein, dealing with properties of solids of significance in catalysis. Ioffe’s observation that the ultramicroscopical cracks affected the surface conductivity of salts and the mechanical strength of a crystal was used by P. D. Dankov (58) to explain the formation of catalytic surfaces. Active platinum, nickel, and iron catalysts (27) were prepared by sublimation and were examined as to the effect of lattice parameter, atomic radii and dispersion (crystallite size) upon their catalytic activity. Dankov (58) stressed the importance of atomic radii in a review published in 1934, basing some of his conclusions upon the earlier studies of L. V. Pisarzhevskii. Dankov also pointed out that the layers between metal crystals, which probably consist of amorphous material, are visualized by some metallographers as being important for the mechanical strength of the metal, since they are the most rigid element of a polycrystalline body (38,388). This may be considered in connection with Kobozev’s theory of catalysis described elsewhere in this paper, which regards atoms of the amorphous phase of the catalyst, constituting an “ensemble,” as the carrier of catalytic activity; the crystalline phase serves as the catalyst support. Dankov recently returned to electron diffraction studies of crystals and their orientation in thin films and surface layers of solid bodies (343). The influence of the crystallite size of catalysts upon such reactions as hydrogenation or dehydrogenation over platinum or nickel has been investigated by Eubinshtein and others (376). Roginskii’s school has applied mathematical statistics to systems formed by primary monocrystals of a catalyst; the cracks and pores of varying dimensions created by these crystals predetermine the nature of the resulting porosity. The application of the statistical method t o the theory of adsorption and catalysis was recently described by V. I. Levin (200) and an equation for adsorption on nonuniform surfaces derived by Ya. Zel’dovich and S. Z.

222

J. Q. TOLPXN, 0. 5. JOHN, AND E. FIELD

Roginskii was given by the former in a treatment of his statistical theory of the Freundlich isotherm of adsorption (433). Further research resulted in a theory of processes occurring on nonuniform surfaces and a statistical treatment of such surfaces by Zel’dovich (432). M. I. Temkin independently developed a theory of adsorption on nonuniform surfaces. Temkin’s work a t the Karpov Institute of Physical Chemistry was directed toward an interpretation of the adsorption isotherms of hydrogen on platinum obtained by A. N. Frumkin and co-workers. The observed deviations from the Langmuir isotherm were explained by energetic nonuniformity of the surface (386) which gave rise to a logarithmic isotherm (125). The further development of the theory of nonuniform surfaces in the U.S.S.R. was helped by the mathematical methods of Zel’dovich and Roginskii (200,201,331). A. V. Frost analyzed some work on the subject (mostly Russian) in a recent review (10) and concluded that a n equation derived by him on the assumption that the reactants are adsorbed on a uniform surface and that no significant interactions take place between the adsorbed molecules, satisfactorily described many reactions on nonuniform surfaces including cracking of individual hydrocarbons and petroleum fractions, hydrogen disproportionation, and dehydration of alcohols. From the experimental results i t was concluded that the catalytic centers on the surface were not identical with the adsorption centers. The catalysts used consisted of different samples of silicaalumina and pure alumina. The formal connection of the views of present day chemists with those of L. V. Pisarzhevskii (301,341) is now stressed by some Russian writers on catalysis (458). The electronic mechanism of catalysis postulated by Pisarzhevskii without much experimental evidence in an early (1925-28) attempt a t correlating the physical attributes of a solid with its catalytic activity stated th at the ability of a metallic catalyst to promote hydrogenation depended on the ability of a hydrogen molecule to penetrate the crystal lattice of the metal and consequently depended upon the interionic distances in this metal. The existence of highly mobile, free (conduction) electrons in metals, as well as in oxides, was thus of great significance in catalytic phenomena, according t o Pisarzhevskii (302). Adsorption was regarded by Pisarzhevskii as a n interaction between the adsorbate molecule and the entire structure of the catalytic particle. The adsorption process could occur on the surface or in the interior of the crystal. For the process to occur in the interior of the crystal the magnitude of the lattice parameter and the atomic radii must satisfy certain restrictive conditions which have been discussed in detail by Dankov (58).


223

Many of the ideas advanced by Pisarzhevskii were also expressed in Nyrop’s work (262) published between 1931 and 1937. The views of Nyrop were unfavorably received by many catalytic chemists a t that time, as is indicated by the criticism of Emmett and Teller (101). LennardJones (199) and Schmidt (361) realized that a catalytic solid could be regarded as an electron source or sink during the course of a catalytic reaction necessitating an electron transfer in ion or radical formation. I n the presence of hydrogen-containing materials, the catalytic solid could also act as a proton reservoir. I n the hands of Roginskii and Vol’kenshtein, these views have provided an electronic theory of catalysis wherein electrons of the crystalline lattice participate in physical and chemical reactions occurring on the surface. The effect of foreign inclusions and temperature upon the nature and formation of lattice “defects” has been studied by Vol’kenshtein. The effect of the catalyst support on resistance to poisoning and sintering was investigated in the 1930’s by I. E . Adadurov and co-workers (1-9). They emphasized the favorable effects of a large difference between the ionic radii of the cations of the support and that of the catalyst. Increasing the charge of the cation raised its stability to poisoning. Adadurov’s conclusions did not greatly influence the thinking of Russian students of catalysis. No special significance was ascribed by Adadurov t o the change in the lattice constants of the catalyst (platinum, chromic oxide), resulting from the adsorption of a poison. Several other students of catalysis in the U.S.S.R. have been investigating various aspects of the influence of the support on the catalyst. These include A. A. Balandin and co-workers (30), A. S. Shekhter (363), N. Z. Kotelkov (177), and others. Of the methods employed by the workers of the Institute of Organic Chemistry who supported Ralandin’s theory, x-ray analysis of catalysts is noteworthy. A. M. Rubinshtein studied (346-349) catalysts employed in hydrogenation, dehydrogenation, and dehydration and correlated the effects of dispersion (particle size) and lattice parameters with their catalytic properties. He gave data (346) indicating that for nickel supported on alumina or for platinum on charcoal, definite sizes of the primary crystallites favor a given reaction: for hydrogenation it is approximately 40 A.; for dehydrogenation 70-80 A. These studies have been extended t o other catalysts including oxides. Rubinshtein’s x-ray data (353) supported Balandin’s theory by indicating that the octahedral faces [lll]of platinum supported on charcoal are the only active faces and no other faces are of significance in the catalytic performance of platinum. The relative activity of catalysts of low metal content (0.034.0% Pt, Pd, Co, and Ni supported chiefly on charcoal) has been compared in

224

J. G. TOLPIN, G. S. JOHN, AN D E. FIELD

dehydrogenation of cyclohexane and hydrogenation of benzene (261,353,354,373,374). For palladium it was demonstrated that upon the absorption of the proper amount of hydrogen the parameters of its lattice were deformed to a favorable extent and this was more significant in raising the activity of the catalyst than increasing the palladium content to 10 %.

11. SCHOOLS OF THOUGHT ON CATALYSIS The last two decades have been marked by a n ever increasing interest in the theory of solids as is indicated by the development of the electronic band approximation which has been so successful in the understanding and interpretation of the optical, electrical, and magnetic properties of solids. This and similar advances made by the physicists have provided new tools for the interpretation and elucidation of the catalytic efficacy of a solid. These tools had been sorely needed and long awaited, for it was RoginskiI and Schultz (337) and Russell (357) who had emphasized the importance of the electronic factor even before the introduction of the geometric factor by Balandin (13) and the ensemble principle by Kobozev. The three principal Russian schools of thought concerning the mechanisms of heterogeneous catalysis have been identified with different theories on the subject, namely Balandin’s multiplet theory, Kobozev’s ensemble theory and Roginskii’s theories of supersaturation and micropromotion. Of the three theories, the pultiplet theory of Balandin has received the most attention in the non-Russian literature. The multiplet theory postulates that bond rupture is accomplished by the adsorbed atoms being attracted to different catalyst atoms whereas bond formation arises from the adsorbed atoms being attracted to the same catalyst atom. On the surface of the catalyst the course of the reaction is influenced by the interactive forces between the reactant molecules and the catalyst atoms, these forces being determined largely by the geometry of the catalytically active structure and the spatial configuration of the reactant molecule(s). The catalytically active structure is regarded by Balandin as a small group or “multiplet” of atoms contained in the crystalline lattice of the catalyst. I n contrast, Kobozev regards the activity as being centered in a n amorphous precrystalline phase which forms a number of “migration” cells containing mobile surface atoms whose lateral displacements are limited by microfissures. These microfissures constitute the geometrical boundaries of the “migration” cells. The smallest group of catalytically active atoms in these cells forms an ensemble. The theories of Balandin and Kobozev are largely geometric in nature and consequently possess many similarities with regard to the active


225

configurations required for specific catalytic reactions. The two theories do differ, however, in that Balandin assumes that the catalytically active structure is a part of a stable crystalline phase and immobile, whereas Kobosev assumes that the active structure is composed of mobile atoms contained in an amorphous precrystalline phase. Quantitative information concerning this precrystalline phase is difficult to obtain; consequently the influence of spatial geometry upon the course of a catalytic reaction is more readily interpretable in terms of Balandin’s theory than Kobozev’s. The latter theory does have the advantage that the action of poisons and promoters can be formulated in terms of their influence upon the activity of the ensemble. The theories developed by Roginskii consider the thermodynamic and electronic properties of the catalyst rather than the purely geometric attributes of its structure. His school has put forward two concrete views which are supported by recent studies of certain Russian physicists. These are the effect of minute quantities of impurities on the catalytic activity of a surface and the principle of “supersaturation in a solid surface.” Systems with an excess of free energy are termed supersaturated systems by Roginskii. The excess free energy per gram-mole is a measure of supersaturation. The theory of supersaturation was formulated by Roginskii following the observations made by Dobychin on nickel oxide used in hydrogenation, dehydrogenation, and oxidation reactions. It was noted that the activity of the catalyst was greatly enhanced when its structure possessed free energy in excess of that possessed by the bulk material. By preparing catalysts by simultaneous condensation of vapors of a metal with that of the promoting admixture on cold surfaces in a high vacuum, Roginskii and co-workers demonstrated th a t pure films of a number of metals are totally inactive in reactions such as low-temperature hydrogenation of ethylene, and they become active when gases such as oxygen, hydrogen, nitrogen are occluded on their surfaces in optimal amounts (328,331,343). Above that amount the promoter may poison the catalyst, which explains, according to Roginskii, some contradictory data on the effect of the same substance on a given catalyst. Whether the occluded admixture creates a new active surface or stabilizes distortions of the lattice of the solid already existing there, is a problem under study. An active catalyst should be prepared, on the basis of these views, under conditions as far from equilibrium between the reactants in question as possible. It was stated by Roginskii that his theory of supersaturation was qualitaavely verified on a number of catalysts and was employed in practice with success, for instance, in the preparation of nickel catalysts for fat hydrogenation, ircn catalysts for ammonia synthesis, and molybdenum dioxide for destructive hydrogena-

226

J. G. TOLPIN, G. S. JOHN, AND E. FIELD

tion. We do not have complete details on all these researches. The study of micropromotion has been extended to other systems, and the influence of promoters has been interpreted in terms of the changes produced in the specific rate constant and activation energy of the reaction. As a result of these studies the basic concepts have been broadened and the general phenomenon has been termed “modification of catalysts” by Roginskii. It should be recognized that the geometric theories of Balandin and Kobozev and the theories of RoginskiI represent different approaches to the same phenomena and must, therefore, contain large areas of agreement and internal consistency. The work of Balandin has been recently reviewed by Trapnell (405). A significant supplement to this theory was published in 1945 and 1946 by Balandin and Eidus (20,22). It pertains to chemisorption of unsaturated molecules. For the dehydrogenation of cyclohexanes the multiplet theory requires that the six-membered ring be fitted upon a sextet of atoms and a catalytically active metal should possess face-centered cubic or hexagonal lattices with atomic radii between 1.244 and 1.385 A. These investigators have considered the principle of conservation of valence angles first applied by Twigg and Rides1 (413) to the chemisorption of ethylene on nickel and by Herington t o acetylene (135). However, unlike the British investigators, they applied it to the transitional state of ethylene adsorbed on the catalyst. It was concluded that acetylene molecules are chemisorbed with greater ease on a crystal plane abundant in Ni-Ni bonds of 3.50 A. in length whereas ethylene molecules prefer those with 2.47 A. Experimental hydrogenation of olefins on nickel reported by Twigg and Rideal(413) and Beeck and co-workers (37) were quoted in support of this principle. This principle of conservation of the valence angles extends the list of metals on which hydrogenation of olefins can occur t o include those having atomic radii approximately within the limits mentioned, but crystallizing in a body-centered cubic lattice (W, Mo, Cr, and Fe). The literature clearly indicates that Balandin has received support from Russian as well as non-Russian investigators. Among the Russian works lending confirmation to Balandin’s hypothesis the studies of Dankov and Rubinshtein are particularly notable (346,347,348,349,350). Since modern views of catalysis regard active centers as local distortions of the primary crystalline lattice it is important to characterize these elementary crystallites by the following attributes: ( a ) Radii of atoms or ions which compose the lattice. ( b ) Type of lattice and forms of crystal. (c) Parameters of the crystalline lattice.


227

(d) Dispersion, i.e. dimeriaions of the crystallites. ( e ) Extent of distortion of the lattice. These factors have been studied by many workers; however, no mutually acceptable concepts with regard to the significance of each of these factors have been developed. This has been due in part to the contradictory data appearing in the literature. The factor (a) above has been considered by Balandin (14),Schmidt (360), PisarzhevskiI(302), Adadurov (1-9), Huttig (136), Beeck (36), and others. Adadurov’s studies of the factor (a) are unique in that the catalyst support rather than the catalyst was the center of attention. The first studies were devoted to the platinum catalysts used in the oxidation of SO2. The metallic platinum was supported on the surface of the sulfates of the di-, tri-, and tetravalent elements, and investigations were made upon the activity, the sensitivity to poisoning with AsZ03, and regenerability of the platinum catalyst after poisoning. Among the divalent metals studied (Be, Mg, Ca, Sr, Ba) the sulfates of beryllium and of magnesium demonstrated the greatest activity, minimal resistance t o poisoning and maximal regenerability after poisoning. The trivalent elements (Al, Cr, Fe) and tetravalent metals (Si, Ti, Sn, Zr, Th) were less susceptible to poisoning; the most favorable results being obtained with Sn(S04)2and Zr(S0,)2. Similar studies were carried out with a nickel hydrogenation catalyst supported upon the sulfates of divalent metals and the oxides of the tetravalent elements. Adadurov correlated these results with the ionic radius of the cation in the catalyst support and concluded that the greater the difference between the radii of cations of the support and of the catalyst, the lower the susceptibility of the platinum to poisoning with As203 and the lesser the sintering tendency of the nickel catalyst. An increase in the charge of the cation of the support was found to increase the activity of both of these catalysts. The factor ( b ) above was investigated by Bredig and Allolio (43) in the hydrogenation of ethylene over nickel. Of the two modifications of nickel, cubic and hexagonal, only the first was active as was also found by LeClerc (191,192) and his co-workers and recently reaffirmed by the work of Freldlin and Ziminova (113,114) who also established that in hydrogenation reactions, hydrogen is an essential promoter of the nickel catalysts. A linear interdependence was established between the catalytic activity and the amount of hydrogen dissolved in the nickel. These data may be considered as contradictory to the results recently reported by Sherburne and Farnsworth (365) concerning the activation of nickel metal by extensive outgassing at temperatures considerably above the usual baking temperature. Similar studies were made upon both Raney and Sabatier cobalt by

228

J. G . TOLPIN, G . S. JOHN, AND E. FIELD

Dupont and Piganiol (70) who observed that the Sabatier cobalt had a cubic structure and was quite active whereas the Raney cobalt had the structure of the hexagonal modification and was considerably less active in the hydrogenation of allocymene. I n contrast, Rubinshteh arid Pribytkova (356) have recently observed that in typical hydrogenation and dehydrogenation reactions the activity of the hexagonal modification of cobalt prepared by reducing its oxide a t low temperatures ( T 5 360°C.) was much higher than the face-centered cubic modification obtained during high temperature reduction (5" 5 6OO0C.). The crystallites of cobalt were supported on either charcoal or metallic cobalt during the test reactions which consisted of the hydrogenation of benzene, cyclohexene, acetone, ethylene, or carbon monoxide, and of the dehydrogenation of cyclohexane and ethanol. It was thought by the authors that the high reduction temperature reduced the extent of lattice distortion in the cubic modification and consequently reduced its activity. The loss in activity due to possible crystal growth during hightemperature reduction was not considered. In the very early studies on catalysis little attention was paid to the influence of factors ( c ) , ( d ) , and ( e ) . It is notable, however, that Fricke and co-workers (118,119) did connect the pyrophoric nature of copper and of nickel preparations with their lattice distortions and dispersion. Taylor, Kistiakowsky, and Perry (385) studied the effect of dispersion of platinum in the oxidation of carbon monoxide and sulfur dioxide and obtained the greatest activity with primary crystallites of the order of 30 A. Recently, Rubinshtein, Minachev, and Shuikin (353) investigated the minimal concentration of supported metal catalysts effective in hydrogenation and dehydrogenation reactions. In the case of platinum, concentrations from 0.1 to 1.0% on carbon were studied. The estimated size of the platinum crystals was 40-50 A. in agreement with the findings of Dankov and Kochetkov (59). The x-ray data indicated that the activity of the catalyst was due predominantly to the [ill] octahedral faces of platinum which is in agreement with Balandin's multiplet theory. I n comparison, elementary crystallites of palladium, 40 A. in size, produced by vacuum evaporation were found to be active in the hydrogenation of ethylene. A drop in activity was associated with recrystallization and growth of the crystallites in the fatigued layers. The extreme importance of the dispersion and extent of distortion of the lattice has been established, principally through the extensive experimental studies of Rubinshtein which were recently presented in summary (350). The systems studied were (a) nickel supported on alumina, used in the hydrogenation of benzene and carbon monoxide, dehydrogenation


229

of cyclohexane and formic acid, and dehydration and dehydrogenation of alcohols; and (b) magnesia used for the dehydration and dehydrogenation of normal butyl alcohol. Activity-dispersion curves for selected dehydrogenation reactions over nickel on alumina are depicted in Fig. 1. The structure of the molecule undergoing dehydrogenation affects only the course of the curve but does not alter the occurrence of the maximum in activity a t a dispersion of 70-80 A. The studies of magnesia were carried out in great detail with special attention having been paid to (i>

70 80 90 100 110 120 DISPERSION in A. Activity-dispersion isotherms for dehydrogenation. 40

FIG.1.

50

60

1. CsH1?a t 320°C. 2. HCOOH a t 200°C. 3. i-CsH1,OH at 240°C. 4. i-CsH,OH a t 260°C.

the effect of dispersion at constant lattice parameter and (ii) the effect of deformation at constant dispersion. This study differentiated the specific influence of these factors upon the catalytic attributes of the material. The following series of preparations of magnesia were studied by Rubinshteh:

( a ) Preparations having the same degree of dispersion, 28 k 5 A. ( b ) Preparations having: (1) The lattice parameter a = 4.17 & 0.01 A. (2) The lattice parameter a = 4.20 i 0.01 A. (3) The lattice parameter a = 4.23 & 0.01 A.

230

J. Q. TOLPIN, 0. 8. JOHN, AND E. FIELD

The selectivity and activity of these preparations in the dehydrogenation and dehydration of n-butyl alcohol were determined. The reactions were conducted a t atmospheric pressure with temperatures between 400" and 46OoC., and an hourly space velocity of 1.3 which precluded thermodynamic equilibrium. The activities were expressed in milliliters (STP) of gaseous product formed by each of the reactions per milliliter of alcohol

+-* \

I

I

25

I

I

I

I

30 35 40 DISPERSION in A.

I

I

45

50

FIG.2. Activity-dispersion isotherms a t constant lattice parameter a. 1. Dehydrogenation: a = 4.20 i-0.02 A. 2. Dehydrogenation: a = 4.17 k 0.01 A. 3. Dehydration: a = 4.20 k 0.02 A. 4. Dehydration: a = 4.17 f 0.01 A.

per gram of catalyst. The selectivity coefficient for dehydrogenation was defined as the ratio k , / ( k , lcz), where kl is the reaction velocity for dehydrogenation and k 2 is the reaction velocity for dehydrogenation. The effect of dispersion upon the activity a t constant lattice parameter is shown in Fig. 2 a t 440' and 460", whereas the effect of lattice parameter upon the activity a t ronstant dispersion is depicted in Fig. 3. The lattice parameter also influenced the selectivity of the catalyst. At all temperatures the selectivity coefficient for dehydrogenation decreased with increasing lattice parameter, which indicates that the

+

23 1


dehydrogenation was favored by a compressed lattice and low operating temperature whereas dehydration was favored by an expanded lattice and high temperature. The influence of dispersion a t coiistant lattice parameter upon the selectivity is indicated by the isothermal curves of Fig. 4. The compressed lattice is given by curves ( l ) , the normal lattice by curves (2),

O U - ' 4.16 4.11 4.18

I

4.19

I

I

4.20 4.21

I

1

I

4.22 4.23 424

I

4.25

LATTICE PARAMETER in A.

FIG, 3. Activity-lattice parameter isotherms a t a constant dispersion of:% 1. 2. 3. 4.

k 5 A.

Dehydrogenation at 460°C. Dehydrogenation a t 440°C. Dehydration a t 460°C. Dehydration a t 440°C.

and the expanded lattice by curves (3). The expanded lattice favored dehydration only at the higher temperature where the coefficient of selectivity was only slightly influenced by dispersion. The compressed lattice (curves (1)) showed most markedly the influence ,of dispersion. A sharp minimum occurred in these curves a t ca. 40 A. which indicates that a dispersion of this order favored dehydration. The minimum in the selectivity-dispersion curve occurred a t lower dispersion (25-30 A.) for the undeformed lattice and was not as clearly expressed as that for the compressed lattice.


232 0 95

T = 440' C 0.90

-

0.75

-

W

0

I

I

I

I

I

>

I

1

T = 460. C

20

25

30

35

45

40

50

55

60

65

DISPERSION in A..

FIG.4. Effect of dispersion upon selectivity at constant lattice parameter a. 1. a = 4.17 = 4.21 3. a = 4.23

2. a

- 0.01 A. - 0.01 A. - 0.01 A.

400' C

w0

0.95-

E 0.90-

4.16

4.17

4.18

4.19

4.20

4.21

4.22

4.23

4.24

4.25

LATTICE PARAMETER in A.

FIG.5 . Effect of lattice parameter upon selectivity of the catalyst at constant dispersion.


233

Figure 5 contains isothermal selectivity-lattice parameter curves. A simple linear dependence was established between the selectivity and the parameters of the crystalline lattice a t all temperatures investigated; however, the slope of these isothermal lines did increase with increasing temperature. The curves of Fig. 4 demonstrate excellently the interrelationship th a t was found t o exist between the dispersion and extent of distortion of the lattice and further emphasize the importance of these factors in the formation and selection of catalysts. A similar study was carried out by Kukina (183) upon chromic oxide used in the simultaneous dehydrogenation and dehydration of isopropyl alcohol. Thermographic as well as x-ray analyses were carried out upon individual preparations of the catalyst. The method of preparation had a pronounced effect upon the specificity and selectivity of the chromic oxide as well as on the thermograms. However, a parallelism was not observed between specificity and the average change of the lattice parameters. This was previously shown by Balandin and by RubinshteIn t o be in accord with the multiplet theory; the latter regard the enhanced dehydration only as a result of increasing lattice parameter (183). The importance of dispersion and distortion in catalysis was recognized by Dankov as the result of his very early investigations. I n this connection his studies prompted Kobozev to interpret the phenomenon of optimal dispersion in terms of his theory of fluctuation and active ensembles. The conditions for attaining the optimal dispersion were given a theoretical interpretation by Roginskii in terms of his theory of supersaturation. Balandin examined (30) the influence of two types of asbestos on the formation of a platinum (8%) catalyst and concluded that the nature of the asbestos determined both the number and the nature of active centers on the catalyst and that chrysotile asbestos was preferable t o amphibole asbestos for dehydrogenation. Electron-microscopical studies of asbestos and other materials used for catalyst supports are being made by Shekhter (363a). Atoms of platinum, palladium, silver, and gold were found to creep on several supports with rise of the temperature. The final distribution of the catalyst, the extent of dispersion and the shape of the particles depended upon the nature of the support (36313). Different faces of a palladium and perhaps platinum crystal may exhibit different stabilities to this lateral displacement of atoms. Furthermore, the extent of changes in the different (‘zones” on a palladium surface is influenced by the reaction itself (363c). For zinc oxide, electron-microscopical, x-ray, and adsorption data were correlated with the effectiveness of the samples in decomposition of methanol a t 328”. The results were interpreted as showing no definite correspondence between the catalytic

234

J. G . TOLPIN, G . 6 . JOHN, AND E. FIELD

activity and the x-ray structure of the catalysts, but pronounced differences in the electron-microscopical data corresponded to differences in the performance (363c). Recently Roginskil reviewed this field including Shekhter’s contributions (334) and concluded that electron microscopy is of no decisive advantage over other methods in the study of dimensions of individual particles, but is superior to other methods for a statistical study of distribution of larger aggregates of particles and pores. The value of oxidized nichrome as a support for platinum catalysts has also been explored by a group of workers (26,126,177). Criticisms of Balandin’s purely geometric multiplet theory have arisen in the Russian and in the non-Russian literature; one of the most recent arose as the result of work carried out at the Karpov Institute of Physical Chemistry with platinum, nickel, copper, and chromia. Work reported by Kagan (145a) was interpreted to show that the results on dehydrogenation of cyclohexane over platinum contradicts the multiplet theory. According to Kagan cyclohexene was first formed and underwent subsequent hydrogen disproportionation. This process is not compatible with the mechanism envisioned on the basis of Balandin’s sextet model. Similarly copper, which is unsuitable for either the hydrogenation of benzene or dehydrogenation of cyclohexane, hydrogenates cyclohexene or promotes hydrogen disproportionation in it. This behavior of copper is thought by Kagan to be contrary to Balandin’s concepts. Dehydrocyclization of paraffins was also used by Kagan as a means by which his disagreement with the multiplet theory was demonstrated (145b). On the basis of limited experimental data on the conversions of heptaneheptene mixtures over chromia-alumina at 484”, Kagan and co-workers rejected the applicability of the multiplet theory to the explanation of the mechanism of dehydrocyclization. Instead of this theory and of the views of Herington and of Pitkethly and co-workers, Kagan postulates random orientation of adsorbed paraffinic radicals, leading t o cyclization or dehydrogenation alone, depending upon the position of the second carbon atom adsorbed. Hydrogenation of some of the olefins charged may also occur. Kobozev’s inductive theory of active ensembles (l68,169,171,172a,b) postulates that the carrier of catalytic activity is a phase present in high dilution on the support. This phase, which is in the amorphous precrystalline state, consists of a number of cells separated by geometrical barriers (microfissures) which are impenetrable to molecules for movement from one group of cells to another. Thus there is no exchange of catalyst atoms, reactant molecules or catalyst poisons between these cells. The smallest group of catalytically active atoms in the cells form an ensemble which constitutes the carrier of the catalytic activity and to


235

a first approximation is independent of the nature of the support. The number of atoms in each ensemble and the area within which the catalyst atoms can migrate can be determined from experimental kinetic data and physical properties of the active component of the catalyst by means of formulae developed by Kobozev. Other formulae developed by him make it possible t o calculate the activity of an ensemble as a function of temperature, the number of ensembles and of “migration cells” present on the surface of the catalyst and the poisoning coefficient of a poison adsorbed onto the catalytic surface. The cells act as “potential valleys” within which active atoms congregate into ensembles, a t the point of the highest adsorption potential. The distribution of these atoms is governed by the laws of probability. Resonance between closely adjacent atoms of an ensemble activates the latter. The “migration cell” concept was connected by Kobozev (172b) with some modern views of Russian and non-Russian students of the crystalline state, indicating that crystals are built u p of microcrystalline units of the order of magnitude of cm., which is close t o the size of the “migration cell” (137a). The catalytically active structures were grouped by Kobozev into the following basic classes: (a) Structures conforming to the ensemble principle, namely, the combination of 2, 3, or more atoms attached t o a carrier. These-structures resemble the multiplets postulated by Balandin. Palladium, platinum, and iron which may be employed in ammonia synthesis, and molybdenum used in the hydrogenation of olefinic linkages constitute members of this category. The catalysts of this class are essentially unaffected by the nature of the support. (b) Structures complying with the aggravation principle, i.e., active elements attached to a catalytically inactive group or t o a supporting substrate. The aggravation principle appears to be a new term devised t o represent the well-known effects of supports on the activity of a catalyst. Since the formulation of his concepts in 1939 Kobozev has carried out many investigations in the attempt t o verify their general validity. I n the decomposition of HzOzand the oxidation of NaZS03, the specific effect (173) of small amounts of iron (0.0005 t o 40%) added to copper on carbon and the converse in which small amounts of copper were added t o iron on carbon were studied. The activity of these catalysts was very effectively promoted by these additives, the extent of promotion being proportional t o the concentration of the additives. The catalytic synthesis of ammonia by iron supported on carbon or asbestos was also studied. The results of this study and similar studies of catalytic

236

J. G . TOLPIN, G. S. JOHN, AN D E. FIELD

oxidation-reduction reactions and hydrogenation processes were interpreted by Kobozev as supporting his ensemble principle (1 70). The nature of the active centers of supported platinum catalysts for hydrogenation of olefins and aromatics was investigated by Koboxev arid Reshetovskaya (172a). They concluded that the double bonds of benzene hydrogenate consecutively and that the active ensemble is identical with that involved in the hydrogenation of olefins. These conclusions are in disagreement with those of t,he Balandin school. Experiments were also performed comparing catalysts made from H2PtC1, with those from K2[C13Pt-CH2-CH=CH-CH2-PtC13].H20. Presumably the latter can yield only even numbers of Pt atoms in an ensemble and the observed differences in behavior were ascribed to this fact. Applying Kobozev’s method of calculation of the number of platinum atoms in an active ensemble they found that activation of one type of molecules (e.g., hydrogen molecules in the reduction of nitro compounds) is involved in reactions occurring on ensembles of one or five platinum atoms; when two types of molecules are activated, as in hydrogenation of olefinic (maleic acid) or aromatic (phenol) compounds, the ensemble consists of two or six atoms of platinum. According t o their data, secondary catalytic structures are occasionally formed by deposition of a single platinum atom or a doublet upon a catalyst face consisting of four platinum atoms, rather than on the catalyst support; this results in a n imperfect new structure of high catalytic activity. Limited support of Kobozev’s theory has appeared in the Russian literature with little or no mention in the non-Russian literature. Among his Russian advocates is Frost, a thermodynamicist who has been studying the kinetics of the reactions involved in catalytic cracking. Frost and his co-workers found evidence of the existence of a n ensemble of two aluminum atoms in hydrogen disproportionation over catalysts prepared by impregnating pure silica gel with acidified aqueous aluminum sulfate (132a) and in dehydrogenation of cyclohexane over palladium and nickel supported on charcoal and on magnesia (132b). The data for palladium agreed with those of Kobozev (172b). The theory of Kobozev is open to criticism of the character recently stated by Kummer and Emmett (184), who observed that there was a very rapid exchange between the isotopes of nitrogen in the presence of singly and doubly promoted iron synthetic ammonia catalyst. Kobozev (167) had concluded earlier that iron synthetic ammonia catalysts consisted of an ensemble of iron atoms to which were attached the promoter molecules. Each ensemble was capable of adsorbing only one nitrogen molecule; further, in accordance with his theory, these ensembles, separated from one another by geometrical barriers, would make the


237

exchange of nitrogen between them impossible. The data of Kummer and Emmett cannot be reconciled with the ensemble principle as it stands. Several theories with regard to mode of preparation, promotion, poisoning, and modification of catalysts have been advanced by Roginskii (324-329,336,339). Very early Roginskii postulated the significance of the role of supersaturation in reaction kinetics; recently this concept has been extended to include catalyst preparation. During the preparation of a catalyst a number of chemical, mechanical, and physical effects are operative. The relative significance of each of these factors has been discussed by Roginskii in terms of the phenomenon of supersaturation (excess free energy). Supersaturation may be achieved in a number of different ways, several of which are: (1) catalytic preparations by crystallization : (a)From solution-have the solutions containing the reactants forming the catalyst as concentrated as possible. ( b ) From vapor-have the surface upon which condensation occurs as cold as possible and the condensation rate high. (2) Catalysts prepared by a gaseous reaction through: ( a ) Decomposition-maintain low concentration of products a n d high concentration of reactants. For example, the preparation of nickel catalyst by decomposition of N i(C 0 )4 : Ni(CO)a--+ Ni

+ 4CO.

An attempt should be made to maintain a low concentration of co. ( b ) Reduction-maintain minimum concentration of reaction products and maximum Concentration of reactants. For example, in the reduction of metal oxides with hydrogen or carbon monoxide, the product gas is water or carbon dioxide; the concentration of these substances in the exit stream should be held to a minimum. Likewise it is equally import a nt that oxygen and product gases be absent from the entrant gas. The importance of removing traces of impurities from the reducing gas has recently been demonstrated by Taylor (384) and McGeer (229). In the investigation of the isotopic exchange reaction of NZ3Oand NZz8 over an iron-alumina ammonia synthesis catalyst,l McGeer observed th a t the rate of isotopic exchange was strikingly dependent upon the purity of the hydrogen employed in reducing the catalyst prior to its use. The

238

J. G . TOLPIN, G. S. JOHN, AND E . FIELD

presence of minute traces of oxygen and water vapor in the hydrogen made it impossible t o free the catalytic surface completely of the effective oxygen poison. Roginskil suggests that during the formation of a catalyst by reactions in the solid state the addition of a substance which forms a solid solution with the reaction products resulting from the formation of the catalyst, but not with the reactants, is beneficial to the production of supersaturation, i.e., excess free energy. This excess free energy possessed by a solid catalyst is essential to the formation of active structures which are principally responsible for the catalytic activity. The effect of free energy of formation on the catalytic activity has been investigated for a number of systems by Roginskii and his co-workers. 111. INVESTIGATION OF ADSORPTION PHENOMENA

A fundamental advance in the theory of adsorption phenomena was made in 1916 when Langmuir suggested that adsorption was to be regarded as a chemical process occurring on an energetically uniform surface and that the adsorbed phase was a unimolecular layer of noninteractive molecules. However, it soon became impossible to describe some important phenomena of catalysis without denying the concept of a uniform surface. Modern developments of the theory of adsorption have been directed toward studies of the interactions between adsorbed molecules and of the energetic heterogeneity of the surface. Following the pioneering work of Constable (1926), Roginskil and Zel’dovich introduced (328,331,362) statistical concepts for the quantitative description of the nonuniformity of catalysts. I n these studies the energetic nonuniformity of the surface has generally been characterized by distribution functions for the energies of activation and heats of adsorption. Methods for the determination of these distribution functions from equilibrium and kinetic data have been one of the principal goals. 1. Determination of the Distribution Function of the Heats of Adsorption

and Interaction Function The theory of equilibrium adsorption and kinetics of adsorption upon the surface of adsorbents have been thoroughly treated in the Russian literature since experimental results were obtained which did not agree with Langmuir’s initial assumptions: (1) that the surface of the adsorbent is energetically uniform; (2) that the adsorbed particles do not interact with one another; (3) that the surface is covered by only a monolayer of the adsorbate, If the third assumption is retained then three alternative possibilities are offered: (a) assumption (1) is rejected and assumption (2)


239

is retained; ( b ) assumption (1) is retained and assumption (2) is rejected; (c) assumptions (1) and (2) are both rejected. From a catalytic point of view the situation (a) which pertains to rejection of the uniformity condition is of greatest interest. This particular situation was investigated by Zel’dovich (432), Temkin (386), and recently by Levin (202). In 1935 Zel’dovich (432) carried out a special theoretical treatment concerning the origin of the parabolic isotherm typified by the type I1 isotherm. It was demonstrated that this type of isotherm could result from the nonuniformity of a surface characterized by an exponential distribution function p ( Q ) with respect to the heat of adsorption defined by p(Q) = Ccn* where Q is the heat of adsorption and C and a are constants. A similar study was carried out by Cremer and Flugge (51) and later by Halsey and Taylor (134). Temkin (386) assumed an equilibrium distribution on the surface with respect to the heat of adsorption given by p(Q) =

const. = C

and theoretically founded the logarithmic isotherm of adsorption @(PI = b In (ap)

which was empirically obtained by Frumkin and Shlygin (125). If the distribution function p ( & ) for the heat of adsorption Q is known, it is not difficult in principle to calculate the adsorptive properties of the surface through the adsorption isotherm @ ( p ) . Far more difficult is the inverse problem of deducing the distribution function p ( Q ) from the adsorption isotherm @ ( p ) . The latter problem, important both to adsorption and to catalysis, has been investigated principally by RoginskiI, Zel’dovich, Elovich, and Temkin. The study has extended over a number of years and has resulted in the development by Roginskii of a general theory of processes occurring on nonuniform surfaces. The progress of this work was held up for some time due largely to mathematical difficulties involved in the solution of the integral equations which arise in the determination of p ( & ) from a general experimental isotherm. Roginskii suggested a simplified method of analysis for processes occurring on a nonuniform surface which made it possible to surmount these mathematical difficulties without excessive distortion of the physical model. The method has general applicability to statistical processes; however, its application to adsorption equilibrium only will be discussed here.

240

J . G . TOLPIN, G . S . JOHN, AND E. FIELD

The adsorption isotherm @ ( p ) on a nonuniform surface was expressed b y Roginskii as a function of the pressure p in the form

where p ( Q ) is the distribution function of the hcat of adsorption &, Q1 and Q2 are the lower and upper limits of the heats of adsorption on the surface,

FIG.6. Graph of the function B ( Q ) .

and bo is a quantity which is independent of Q. The distribution function p(Qj is defined such that

Let us define a function

e(Q) as follows:

It is seen from Fig. 6 that e(Q) has an inflection point, the abscissa of which is QrI = RT In (bolp) and the ordinate is BI1 = 45. A typical distribution function p ( Q ) is plotted in Fig. 7 together with 6(Q) . p ( Q ) . The shaded area limited by this latter curve with abscissa Q I and Qz amounts to the integral given in Equation (1). Substituting this shaded area by the equal area ABQ2QDRis equivalent t o substituting the integral in Equation (1) by the equal integral


24 1

If p ( Q ) does not change too rapidly and if the interval between Q1 and is sufficiently large, one can assume approximately th a t the point Q,, coincides with the abscissa of the turning point of O(Q), that is, Q2

QGR 02 Q Graphical interpretation of Roginskil's method.

QI FIG.7.

When Equation (2) is evaluated for the distribution functions p(Q) =

or

p(Q) =

const. Ce-aQ

one obtains $(p)

=

const. (Qz - RT In bo

+ RT In p ) = C1 + Cz In P

or

respectively which is in agreement with the results obtained by other more rigorous methods.

242

J. G . TOLPIN, G . 8. JOHN, AND E. FIELD

If Equation (2) is differentiated with respect t o the lower limit we obtain

which makes the determination of p ( Q ) possible. A number of investigations have been made along the line of improving the Langmuir theory by assuming a uniform surface and introducing the possibility of interaction between the particles. Russian and nonRussian investigators participated in this effort, including Frumkin, Langmuir, Wang, Roberts, Kobozev, and Temkin. Recently Vo1’kenshteh (417) reinvestigated the problem with the object of finding a convenient method for the determination of the form of function +(r) from the experimental isotherms ‘P(p), where +(r) is the function representing the interaction energy of two adsorbed molecules in terms of their distance of separation r. The experimental isotherm does not provide sufficient information t o indicate which one of the two basic assumptions of the Langmuir theory should be rejected. Consequently Vol’kenshtein (417) has undertaken the task of deducing the interrelationship between the functions 4(r) and p ( Q ) which give rise t o the same isotherm ‘P(p). The heat of adsorption Q per molecule has been related t o the energy of a molecule upon the surface, this energy being expressed in terms of the interaction energy of the adsorbed molecule with the lattice of the adsorbent and of the interaction energy with other adsorbed molecules. This relationship in turn was used in the adsorption isotherm to deduce the adsorbate interaction energy function. Vol’kenshteln has evaluated the distribution function p ( Q ) of heats of adsorption and the adsorbate interaction energy function +(r) = Av(r) for a number of typical isotherms and his results are summarized in Table 11. I n this table H , n, bo, and c are constants. Qo is the heat of adsorption of the free surface, Qm., is the maximum heat of adsorption, and T is the effective diameter of the adsorbate molecule on the surface. 2. Determination of the Distribution Function of the Activation Energies

of Adsorption In addition to the theoretical studies of equilibrium adsorption, noginski1 has conducted extensive investigations of the kinetics of adsorption on nonuniform surfaces and has shown that the character of the kinetics of adsorption is dependent upon the presence or absence of surface migration. Migration of the adsorbate molecules brings about a redistribution which conforms with the values of the adsorption coeffi-

TABLE I1 Distribution function

p(&)

11. Uniform distribution, a particular case of the above, where n = 0 p = H

Law of interaction Au(r)

A L ~= 011

=

QO

+ r7 where

Isotherm 8 ( p )

e

Q2

a1

-7

- Qmax

~1

=

HkT =

= c1 In p

- c2, where

kT

-YO' a2

c2 = K k T In boeQmax/kT =

a1

+ ro2kT In boe-Qo/kT a2

IV. Exponential distribution p

=

He-4

Au =

7

- c2, where

where

c1 =

244

J. G . TOLPIN, G . S. JOHN, .4ND E. FIELD

cients on the individual portions of the surface. The nature of the functional dependence between the energy of activation E and the heat of adsorption Q on each section of the surface determines the kinetics of adsorption. Thus Roginskii has considered the kinetics of (1) adsorption without redistribution of the adsorbate molecules, and ( 2 ) adsorption with redistribution of the adsorbate molecules, in the absence of a functional connection between the activation energy E and heat of adsorption Q. In the absence of redistribution of molecules on the surface the velocity of the adsorption process is determined solely by the distribution function of activation energies. The velocity of adsorption on sections of the surface characterized by the activation energy E is expressed in terms of the change in the fractional surface coverage e(E,t) with the time t by do( h',t ) = lcop(l - O)e-E/RT. --dt

(3)

Integration of Equation (3) under conditions of constant pressure gives 1 - @(E,t)= exp [ - k ~ p t e - " / ~ ~ ]

(4)

which represents the fraction of the free surface of a section characterized by the activation energy E a t time t. Let us now multiply this expression (4) by p(E)dE, where p(E) is the distribution function of activation energies, arid integrate overall values'of E between h'1 and Ez. The result of this operation gives the fraction of the total free surface 9 a t the time t , that is

where +(t) is the fractional coverage of the entire surface. In adsorption with redist,ribution in which there is no functional relationship between E and Q , it is assumed that the velocity of redistribution is larger than the velocity of adsorption. Consequently the various sections of the surface will be occupied in the order of decreasing heats of adsorption and a t all times all sections will be involved in the adsorption irrespective of the magnitude of their activation energy. However, due to the exponential dependence of the velocity of adsorption upon the activation energy the character of the adsorption will be determined principally by the processes occurring upon sections with minimal values of E. These sections form on the graph p ( E ) , as shown in Fig. 8, a relatively narrow vertical band the position of which determines the

245


velocity of the process. RoginskiI has termed this band the controlling band since this band controls the nature of the adsorption. With increasing coverage of the surface the controlling band will not alter in position but will decrease in height, i.e., the number of sections with small values of the activation energy will decrease continually, and a s a

FIG.8. The kinetics of adsorption when E is independent of &. (The area under curve t l is the fraction of the free surface a t the time t l ; that under curve t 2 the fraction a t the time tS. The letters C.B. denote the controlling band.)

consequence the velocity of the total process will decrease. of adsorption will be expressed by d+ - = kop(l dt

+)p(E,,,)e-Emin’RT

= k,,’(1 -

The velocity

@)e--EmidRr.

which is equivalent to the kinetics observed on a uniform surface. Roginskii also treats the cases in which the functional relationship between E and Q is prescribed. If E’ and Q vary in the same respect from section t o section the kinetics of adsorption are defined by

@ = dt

kOPP(Emin)e-Emin’~~

=

ko’e-Ernin/RT.

On the other hand, if E and Q vary conversely to one another then the controlling band shifts with coverage toward higher values of E and the kinetics will coincide practically with the kinetics of adsorption on a nonuniform surface without redistribution discussed previously.

246

J . G . TOLPIN, G . S. JOHN, AND E. FIELD

Roginskif (331) has treated the problem of determination of the distribution function p ( E ) of activation energies by this approximate statistical method. The details are very similar t o those discussed above for the determination of p ( & ) from the experimental adsorption isotherm. The energetic nonuniformity of catalytically active nickel oxide was investigated by Keier (161). The kinetics of activated adsorption was studied under constant pressure a t several temperatures and the data were described by the expression

At'/" (6) where A and l / n are constants, and t is the time required to adsorb the q =

quantity, q, of adsorbate. Employing the approximate theory of non-unif orm surfaces developed by Roginskii the distribution function p ( E ) of activation energies E was determined to be p ( E ) = lieuE where H = A ~ ( T @ and~ a~ = 1/nRT. quantity T O is defined by the relation

In the expression for H , the

~ , A,, nT,are the characteristic constants The quantities ATI, n ~ and of Equation (6) evaluated at temperatures T Iand T zrespectively. Of equal importance in the characterization of a catalytic surface is the determination of the dependence of the change of activation energy of adsorption upon the extent of surface coverage. This change in energy of activation with coverage has been related by Roginskii to the change of log t of adsorption in the following manner t

E = 2.3RT log -*

70

Substituting into this equation values of log t corresponding t o given extents of adsorption, the energy of activation as a function of coverage can be found. Levin (202) presented a more exact solution of the problem with regard to the determination of the distribution function p(E) of activation energies which does not possess the limitations imposed by Roginskii's met,hod. The method employed the Laplace transform to solve the integral Equation (5). The following transformation was carried out: y=lnl

x=-

E'

RT


which yielded J/(eU)= assuming E l

=

J0

0 and E , =

O0

00.

247

exp [ - ? ~ ~ p e v - ~ ] p ( s J d z Both sides were then multiplied by e-”d y

and integrated from - 00 t o

+

00,

thus

Interchanging the order of integration on the left and integrating the resulting expression with respect t o y the following was obtained

where r (-8) is a gamma function. When the inversion of the Laplace transform was used, the expression

was obtained. Levin then gave means of evaluating the integral over y when experimental data concerning are available. Thus an extremely useful method for the determination of p ( E ) from experimental data was rigorously developed. I n the case of the adsorption of acetylene upon nickel oxide it is observed that the activation energy increases pronouncedly with surface coverage. From this study Keler states that the adsorption characteristics on nickel oxide are explicable on the basis of a nonuniform surface without the imposition of interactive forces between adsorbed molecules. However, final judgment has been reserved by Keler until a study employing the differential isotopic method developed by Roginskii (338) has been carried out. This technique, formulated by RoginskiI in 1946, consists essentially in utilizing tracer molecules t o distinguish between energetically uniform and nonuniform surfaces. On a uniform surface, in the presence of interactive forces, all adsorbed molecules are under identical conditions. However, on an energetically nonuniform surface this is not the case. Thus the method cQnsists of covering a portion of the surface with one isotope and a subsequent portion with another. The composition of successive samples of gas desorbed from these labeled sites reveals the desired energetic information.

+

248

J . G. T O L P I N , G. S. J O H N , A N D E. F I E L D

Keier and Roginskii have experimentally investigated the heterogeneity of the surface of metallic nickel and ZnO (163) catalysts on sugar charcoal (162). In the activated adsorption of hydrogen on sugar charcoal it was shown that the connection between the activation energies of adsorption and of desorption is quite complicated. In the region of small surface coverage an increase in the activation energy of adsorption is accompanied by an increase in the activation energy of desorption whereas a t large surface coverages the converse is true. I n the application of this method it should be pointed out th a t mobility of the adsorbed phase on a nonuniform surface will yield results th a t are equivalent to those of a uniform surface. However, making use of information obtained from the kinetics of adsorption and from studies employing the differential isotopic method, these difficulties can frequently be resolved. I n their study of metallic and ZnO catalysts, Roginskii and Keier obtained results conclusively demonstrating the surface heterogeneity TABLE I11 Conditions of adsorption

DZ%

H2%

4 5

5 0 0 40 95

95 100 100 65 5

6 7

100 100

0 0

5

100 100 100 55 0

0 0 0 45 100

Room temp. Room tcmp. 63-78 84-100 197-253

6

5

95

380

Portion KO.

T'C.

Step I11 Desorption Step I Adsorption of Df p = 1.48 mm. of ITg Q D ~= 0.095 CC. (STP) t = 9 min. 1 = room temp. Step I1 Adsorption of Hz p = 3.08 mm. of Hg yuz = 0.04 CC. (STP) t 9 min. I" = room temp. , I

1 2

3

Room temp. 20-65 170-220 300-320 420-470 520 530

=i

Step I Adsorption of Hz p = 1.51 mm. of Hg YE, = 0.06 CC. (STP) t = 8 min. I" = room temp. Strp I1 Adsorption of D, p = 2.02 mm. of Hg ( I H ~= 0.01 CC. (STP) t = 8 min. T = room temp.

Step I11 Desorption

1 2 3

4


249

of these catalysts. The ZnO was studied in the same manner as the sugar charcoal discussed above. In the investigation of the metallic nickel the adsorption was carried out at room temperature upon a sample which had been previously outgassed a t 580°C. The desorption of the gas was produced by gradually increasing the temperature from room temperature t o 500°C. The desorbed gas was pumped from the reaction chamber and analyzed. Two portions of gaseous hydrogen differing in isotopic composition (light hydrogen and 99% deuterium being used in these studies) were initially adsorbed. Table I11 gives the results of some experiments in which the adsorption was carried out in various orders to insure the absence of an isotope effect. As is seen, the composition of the gas first desorbed agrees with that of the gas last adsorbed. This initial desorption is followed by a region of gaseous mixtures, of the two isotopes and the last portion desorbed is of the same or nearly the same composition as the first gas adsorbed. 3. Adsorption upon Structural Defects

Vol’kenshtein (422) has also treated adsorption phenomena on solids in terms of defects in the periodic lattice of the adsorbent. These disturbances embrace all macroscopic and microscopic distortions of the strictly periodic structure of the lattice. This classification was categorically made on the basis of the range of their perturbing influences. The macroscopic distortions extend over regions many times larger than the unit cell and are characterized by cracks, dislocations and even faces and edges of a crystal. On the other hand, the microscopic distortions are confined to domains that are of the same order of magnitude as the unit cell of the crystalline solid and arise from Schottky and Frenkel defects, abnormal valency in an ion a t its normal position in a heteropolar lattice and foreign atoms in substitutional or interstitial sites in the lattice. The microscgpic distortions or micro-defects possess a mobility which is characterized%y an energy of activation. This energy of activation is dependent upon the nature of the defect, lattice structure, and direction of migration of the defect. Micro-defects also possess the ability of interacting with one another, giving rise to new defects which possess different properties. These interactions may be regarded as reactions between the defects having characteristic heats of reaction and activation energies. The totality of defects in a unit volume of the crystal is termed the ‘(disorder” of the crystal. This disorder is assumed to be small and composed of defects having either a biographical or a thermal origin. The biographical disorder, denoted by X , is irreversible and preserved

250


down t o 0°K. The thermal disorder is reversible and superimposed upon the biographical. Thus the total disorder a t any temperature T is the sum of the biographical disorder existing at 0°K. and thermal disorder characteristic of the temperature T ; consequently a t T = T,,, the total disorder Y is a maximum of which X is biographical and ( Y - X ) is thermal. This latter number is a characteristic constant for a given surface. Clearly two extreme cases can exist: (1) that in which +,he disorder is purely biographical and (2) that in which it is purely thermal. For the intermediate cases the relative importance of the two types of defects is dependent upon temperature and previous history of the material. Vol’kenshtein regards these micro-defects as sites for adsorption and is thus free from the basic assumptions of conventional adsorption theories, (1) constancy of number of adsorption centers with temperature and coverage, and (2) immobility of adsorption sites. The presence of defects does not imply that the surface is energetically heterogeneous. Nonuniformity of the surface arises from adsorption sites having different heats of adsorption. Vol’kenshtein assumed that only one definite type of defect characterized by the heat of adsorption q is involved in adsorption and that only one adsorbate molecule can be attached t o each site. The adsorption process is regarded by Vol’kenshtein as a reaction between the adsorption site A with the gaseous adsorbate molecule G . This reaction which produces a new defect B may be written symbolically

A+G+B,

AH=q.

(8)

If the concentration of the defects A , G, and B at any given temperature T is denoted respectively by N L , No, and Ns,then their equilibrium concentrations are related in the following manner

Assuming that the total number of adsorption sites a t any temperature is given by N = N, N B (10) then

+

which is equivalent to the conventional Langmuir expression if N remains constant for all temperatures, which implies that the defects have a biographical origin.


25 1

I n addition t o the adsorption reaction it is admitted that the defects

A can undergo reactions involving other defects on the surface. These reactions among the defects may be either monomolecular, bimolecular, or more complex. Examples of the first two cases have been discussed in detail by Vol’kenshtein. I n the monomolecular case it is assumed th a t in addition t o the defects A and B a third type of defect C exists which undergoes the following reaction AH = -u.

CGA,

(12)

At equilibrium Equation (11) is supplemented by

where N c denotes the concentration of the defects C. Evidently the maximum thermal disorder Y is given by

Y

=

NA

+ NB + Nc

(14)

and the biographical disorders X a t T = 0°K. is given by X = 0. Making use of the latter expression and Equation (10) it is possible t o write Equat,ion (13) in the form

N - Ns --

-

Y - N

0.

Using Equations (11) and (15) it is possible t o solve for the unknowns NB, the number of adsorbed molecules and N , the total number of adsorption centers. I n the bimolecular case it is assumed th at in addition to the defects A and B there are defects of the type C and D characterized by the reaction C&A+D, AHZ-U. (16) As in the previous case, at equilibrium Equation (11) is supplemented by

N AND -p NC

=

POg-u/kT

(17)

where ND is the concentration of the defects D. I n the most general case the biographical disorder X * a t T = 0 will be equal t o

X = Nn

+ Ng -

whereas the maximum disorder Y a t

Y = NA

T

=

N D

T,,, is given by

+ N s + Nc.

* The coefficient of N D is the negative of

(18)

that given by Vol’kenshtein.

(19)

252

J. G . TOLPIN, G . 5. JOHN, AND E. FIELD

The quantities X and Y are invariant with temperature. Therefore, making use of Equations (lo), (18), and (19) it is possible to rewrite Equation (17) in the form

which may be solved for N giving

N

= +{

Ne - P

+ X - d ( N B - /3 - X)’ + 4 B Z }

(21) where 2 = ( Y - X) is the thermal disorder. This expression* gives the dependence of the total number N of adsorption sites upon the number N e of occupied sites. If the expression (21) is substituted into Equation (11) and the resulting expression solved for N R there is obtained

where

If it is assumed that the disorder is predominantly biographical in origin, i.e., 2 = 0, the conventional Langmuir isotherm is obtained. If, on the other hand, the disorder is assumed t o have a thermal origin, i.e., X = 0, then the characteristic Freundlich isotherm 9 = kp”” is obtained. The differential heat of adsorption is readily calculated from the change in the total energy E of the crystal produced by increasing the extent of adsorption. On the surface of the crystal there are N defects of which N n are free and NB are occupied. Of the total number of defects X are biographical defects and N - X are thermal defects. The production of each defect consumed the energy u. Adsorption on each of these No sites liberates the energyq; therefore the total energy E is given by E = ( N - x)U - NBQ. The differential heat of adsorption Q will be expressed by Q = - -

dE dN = -u-+q. dNB dNe

* This expression in Vol’kenshtcin’s article has a positive sign before the radical; this root, however, gives a physically unreal result when 2 = 0. The root with the negative sign before the radical, on the other hand, gives the correct number of adsorption sites under the above condition.


253

Differentiating Equation (21) with respect to N, and substituting the resulting expression* into Equation (23) gives

When Z = 0 (purely biographical disorder), Equation (24) becomes Q = q = const.

Thermal disorder introduces the dependence of Q upon N B ; the greater the value of Z the stronger the effect. The kinetics of adsorption upon defects have also been studied by Vol'kenshteIn. Defects which are manifested through a bimolecular reaction will give the conventional Langmuir kinetics, if the disorder is purely biographical. On the other hand if the disorder is purely thermal, deviations from the Langmuir kinetics are obtained in the region of sufficiently low temperature where p > N B the kinetic expression coincides with the Langmuir expression. Defects which arise as the result of a monomolecular reaction will yield the characteristic Langmuir kinetics if /3 >> 1; however if /3 > p-CH3 > m-CH3, p-tert-butyl > H > p-C1 > m-OCH3>> m-CFs. This sequence is in the decreasing order of the ability of the substituent t o release electrons to the ring and the relative rates of reaction are those to be expected if the formation of a carbonium ion were the rate-determining step. The formation and subsequent decomposition of a carbonium ion from pinacol: CHI-

r 8"' [ __

AH

-CH3+

bH

JH3E]

CH3 - -CHI

+

Products

under conditions similar t o those used with the benzyl alcohols has been postulated t o accounl; for the four products isolated from the reaction mixture (Wender, Metlin, and Orchin, 45). The formation and subsequent reaction of a carbonium ion intermediate has also been postulated

412

MILTON ORCHIN

to account for the reaction products from 2-butene-1,4-diol under 0x0 conditions (Craig, Elofson, and Resa, 46). A kinetic study of the hydrogenation of benzhydrol to diphenylmethane (Wender, Greenfield, Metlin, Markby and Orchin, 21) in different solvents showed that the rate decreased in the following order: ethanol > benzene > cyclohexane. This is th e solvent effect t o be expected in a reaction with an ionic intermediate. Furthermore, th e reduction is completely inhibited with pyridine a s a solvent, a fact consistent with acid catalysis. The cited evidence supports the hypothesis that a carbonium ion intermediate can be formed under 0x0 conditions. The subsequent course of reactions leading to the reduced product is more speculative. With benzhydrol, for example, the diphenylmethyl carbonium ion can be formed from dicobalt octacarbonyl and benzhydrol via the following reactions :

+ IIz +.i 2HCo(CO)4

[Co(CO)a]z PhZCHOH PhzCHOHa'

+ HCo(CO)r ti PhzCHOHz+ + Co(C0)r% PhzCH+ + HzO

At this stage, the existence of the cation Ph2CH+and the anion CO(CO)~is postulated and if present the ions probably associate: €'hzCH+

+ Co(CO)(- % PhzCHCo(C0)c

The subsequent formation of diphenylmethane can be most simply and satisfactorily written by assuming this compound t o be split homolytically by hydrogen or preferably by additional hydrocarbonyl : PhzCHCo(CO)4

+ HCo(CO)4

-+

PhzCHz

+ [CO(CO)~]~

In an alternate view of this reaction (Wender, 47), it can be assumed th a t the cation is reduced by the electron donor, CO(CO)~-,t o give a diphenylmethyl radical: PhlCHf

+ 1 e - i Ph2C.

The ion-radical sequence leads to the same overall reaction as the homolgtic split: im-radical mechanism :

+

PhzCH+ PhiCH.

+ CO(CO)a--+ + HCo(C0)r-t

PhzCH+

+ CO(C0)r- + HCo(C0)4

PhzCH' .cO(c0)4 PhzCHz .cO(c0)4 2.Co(CO)r + [cO(co)4]2

+

+ [CO(CO)~]~ ~

-+

PhzCHz

~

~~

A somewhat analogous ion-radical reduction of carbonium ions to free radicals has been shown to occur (Conant and Chow, 48) in the reduction of various triphenylmethyl carbonium ions with chromous or titanous ion.

HYDROGENATION

OF ORGANIC COMPOUNDS WITH SYNTHESIS GAS

413

It will be noted that the complex Ph&HCo(C0)4 is analogous t o the complex postulated for the reactions of nuclear substituted benzyl alcohols. The benzyl alcohol complex was postulated t o react with carbon monoxide t o give homologated alcohol. With benzhydrol, no homologation occurs, conceivably owing to steric factors. The reduction of conjugated olefins under 0x0 conditions is reminiscent of the reduction of the same systems by sodium and liquid ammonia and by metal-acid combinations. These reductions are thought to consist of electron transfers, involving one or two electron shifts, the electrons being supplied by the metal going into solution (Hammett, 20). With stilbene and sodium in liquid ammonia, the reaction may be represented (Schlenk and Bergmann, 49) : 0

PhCH=CHPh

+ N i -+

[PhCH-CHPhI..

Na -----)

2H+

[PhCH-CHPhI-

-t

PhCHZCHzPh

However, in reductions which proceed b y this mechanism, and in which radical formation (one electron transfer) is rate-controlling, dimeric substances are frequently formed and thus far their presence in the reductions under 0x0 conditions has not been reported. A two electron reduction where formation of the carbanion is rate-controlling does not give dimeric substances. If a carbanion were a n intermediate in the hydrogenation, it might be expected th at in the reaction of a substituted benzyl alcohol such as PhCHOHCH, where both homologation and hydrogenation may occur, the hydrogenation would be retarded owing t o the electron releasing properties of the methyl group. However, PhCHOHCH3 reacts relatively fast and gives nearly all ethylbenzene. An additional interesting fact uncovered by the kinetic study of the hydrogenation of benzhydrol (Wender, Greenfield, Metlin, Markby and Orchin, 21) is the accelerating effect of small quantities of metallic-copper. The addition of copper t o the reaction mixture increased the rate of reduction about tenfold. The exact significance of this fact which probably involves the ability of copper t o act as a n electron donor, Cuo-+ Cuf l ( e ) , is yet t o be explained. Recently (Wender, Sternberg, and Orchin, 38) i t has been demonstrated that triphenylcarbinol and benzhydrol may be reduced t o triphenylmethane and diphenylmethane, respectively, by means of cobalt hydrocarbonyl. With triphenylcarbinol the reaction may be written :

+

+

+

Ph3COH HCo(C0)r % Ph3COH2+ Co(C0)dPhSCOHz+ % Ph3Cf HzO PhaC+ CO(CO)~-=PhaC. .CO(CO)~ Ph3C. HCo(C0)r + PhaCH *CO(CO)a 2.Co(CO)r + [Co(CO),],

+ +

+

+

+

414

MILTON ORCHIN

Again, a n alternate way to write the reaction consists of the homolytic split of Ph&Co(CC)d with IICo(CO)4. The reaction was carried out in acetone solution a t 0' to room temperature and a 98% yield of triphenylmethane was secured. The absence of dimeric products argues against radical formation. The reduction of aliphatic carbonyl compounds is most simply expressed by assuming the addition of HCo(C0)4 to the carbonyl group in the manner of other HA addenda, followed by reaction of the adduct with additional hydrocarbonyl: /H R-CH&=O

+ HCo(C0)d ti R-CHZClIOHCo(CO)r

R-CI-I~CHOH-CO(CO)~

+ HCo(C0)r

---*

+

R-CHzCHOH+ Co(C0)rR-CHzCHzOH [C0(C0)4]2

+

Alternately, the ion-radical mechanism, as with benzhydrol, may be written. It will be recalled that in the aliphatic series, the hydrogenation stops at the alcohol stage; hydrocarbons have been isolated in only one example (methanol). On the other hand the aromatic carbinols give hydrocarbons in good yield. The difference in products may perhaps be ascribed as the greater stability of the intermediate ions such as Ph&H+ as compared The many resonance possibilities responsible for the to R-CHZCH2+. stabilization are absent if the carbon with the electron sextet is not conjugated t o an aromatic ring or other unsaturated group. Resonance possibilities would also stabilize corresponding radical forms so that, as is frequently the case, one cannot distinguish between ionic a n d radical mechanisms because of the possibility of resonance-stabilization lofzan inter mediate. On the basis of present published information it seems fruitless t o carry the discussion of mechanism into greater detail. The increasing interest in this unusual type of reduction will undoubtedly stimulate the activity that eventually may lead to the solution of the problem.

REFERENCES 1. Debus, H., Ann. 128, 200 (1863). 2. Sabatier, P., and Senderens, J. B., Compt. rend. 128, 1173 (1899). 3. Normann, W., British Patent 1,515 (1903). 4. Adkins, H., Reactions of Hydrogen. University of Wisconsin Press, Madison, Wis., 1937. 5. Maxted, E. B., J . Chem. SOC.1949, 1987. 6. Pease, R. N., and Stewart, L., J. Am. Chem. SOC. 47, 1235 (1025). 7. Adkins, H., and Kresk, G., J . Am. Chem. SOC.71, 3051 (1949). 8. Taylor, A,, and Turner, G., British Patcnt 633,184 (1949).

HYDROGENATION

OF ORGANIC COMPOUNDS WITH SYNTHESIS GAS

415

Wender, I., Levine, R., and Orchin, M., J . A m . Chem. SOC.,72, 4375 (1950). Wender, I., Orchin, M., and Storch, H. H., J . Am. Chem. SOC.72, 4842 (1950). Natta, G., Chimica e industrid (Milan) 24, 389 (1942). Wheland, G. W., The Theory of Resonance. John Wiley & Sons, New York, 1944. Brooks, R. E., U. S. Patent 2,517,383 (1950). Wender, I., Levine, R., and Orchin, M., J . Am. Chem. SOC.71, 4160 (1949). Adkins, H., and Kresk, G., J. Am. Chem. SOC.70, 384 (1948). Wetzel, L. A., McKeever, C. R., and Levesque, C. L., J . A m . Chem. SOC.72,4349 (1950). 17. Wender, I., and Orchin, M., unpublished work. 18. Wender, I., Greenfield, H., and Orchin, M., J . Am. Chem. SOC.73, 2656 (1951). 19. Wender, I., Greenfield, H., Metlin, S., and Orchin, M., J. Am. Chem. SOC.74, 4079 (1952). 20. Hammett, L. P., Physical Organic Chemistry. McGraw-Hill Book Co., New York, 1940. 21. Wender, I., Greenfield, H., Metlin, S., Markby, R., and Orchin, M., unpublished work. 22. Wender, I., Greenfield, H., and Orchin, M., unpublished work. 23. Pino, P., private communication. 24. Mond, L., Langer, C., and Quincke, F., J . Chem. SOC.67, 749 (1890). 25. Mond, L., Hirtz, H., and Cowap, M. D., Chem. News, 98, 165 (1908). 26. Mond, L., Hirta, H., and Cowap, M. D., J . Chem. SOC.97, 798 (1910). 27a. Hieber, W., Angezu. Chem. 49, 463 (1936). 27b. Hieber, W., Z e d . Electrochem. 40, 158 (1934). 28. Coleman, G. W., and Blanchard, A. A., J . A m . Chem. SOC.68,2160 (1936). 29. Sternberg, H. W., Wender, I., Friedel, R. A., and Orchin, M., J . Am. Chem. SOC., in press. 30. Hieber, W., and Schulten, H., 2.anorg. u. allgem. Chem. 232, 32 (1937). 31. Armit, H. W., J. Hyg. 7 , 525 (1907); 8, 565 (1908). 32. Langmuir, I., Science 64, 65 (1921). 33. Sidgwick, N. V., Electronic Theory of Valency. Oxford University Press, London, 1927. 34. Ewens, R. V. G., Nature 161, 530 (1948). 35. Ewens, R. V. G., and Lister, M. W., Trans. Faraday SOC. 36, 681 (1939). 36. Wender, I., Sternberg, H., and Orchin, M., J. Am. Chem. SOC.74, 1216 (1952). 37. Hieber, W., Muhlbauer, F., and Ehmann, E.A., Ber. 66, 1090 (1932). 38. Wender, I., Sternberg, H. W., and Orchin, M., unpublished work. 39. Calvin, M., Trans. Faraday SOC.34, 1181 (1938); J. Am. Chem. SOC.61, 2230 (1939). 40. Sternberg, H. W., Wender, I., and Orchin, M., Anal. Chem. 24, 174 (1952). 41. Orchin, M., and Wender, I., Anal. Chem. 21, 875 (1949). 42. Hieber, W., Schulten, H., and Marin, R., 2. anorg. u. allgem. Chem.240,261 (1939). 43. Gilmont, R., and Blanchard, A. A., Inorganic Synthesis, Vol. 11. McGraw-Hill Book Co., New York, 1946. 44. Natta, G., Pino, P., and Ercoli, R., J . Am. Chem. SOC.74, 4496 (1952). 45. Wender, I., Metlin, S., and Orchin, M., J . Am. Chem. SOC.73, 5704 (1951). 46. Craig, L. E., Elofson, R. M., and Resa, I. J., J . Am. Chem. Soc. 72, 3277 (1952). 47. Wender, I., private suggestion. 48. Conant, J. B., and Chow, B. F., J . Am. Chern. SOC.66, 3752 (1933). 49. Schlenk, W., and Bergmann, E. D., Ann. 463, 1 (1928). 9. 10. 11. 12. 13. 14. 15. 16.


The Uses of Raney Nickel E U GENE LIEBER

AND

F R E D L . MORRITZ

Department of Chemistry. Illinois Institute of Technology. Chicago. Illinois Page I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417 I1. Preparation and Properties . . . . . . . . . . . . . . . . . . . . . . 418 1 . Preparation of Raney Nickel Catalysts . . . . . . . . . . . . . . 418 2 . Properties of Raney Nickel Catalysts . . . . . . . . . . . . . . . . 419 420 3 . Other Raney Catalysts . . . . . . . . . . . . . . . . . . . . 4. The Effect of Additives . . . . . . . . . . . . . . . . . . . . . . 420 a . Platinic Chloride . . . . . . . . . . . . . . . . . . . . . . . . 421 b . Alkali . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422 5 . Selectivity of Raney Nickel Catalyst . . . . . . . . . . . . . . . . 424 6. Reductions with Nickel-Aluminum Alloy and Aqueous Alkali . . . . . 426 I11. Special Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . 429 429 1. Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . a . Raney Nickel as a n Oxidation-Reduction Catalyst . . . . . . . . . 429 b . The Cannizzaro Reaction . . . . . . . . . . . . . . . . . . . . 432 c . Dehydrogenation . . . . . . . . . . . . . . . . . . . . . . . 433 2. Amines by Reductive Alkylation . . . . . . . . . . . . . . . . . 434 3 . Rearrangements . . . . . . . . . . . . . . . . . . . . . . . . . 438 4 . Dehalogenation . . . . . . . . . . . . . . . . . . . . . . . . 440 5 . Desulfuration . . . . . . . . . . . . . . . . . . . . . . . . . . . 444 6. Hydrogenations without the Use of Added Hydrogen . . . . . . . . . 447 7. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . 449 a . Ozonide Decomposition . . . . . . . . . . . . . . . . . . . . . 449 b . Raney Nickel in an Acid Medium . . . . . . . . . . . . . . . . 449 c . Catalysis of Hydrazine Decomposition . . . . . . . . . . . . . . 450 d . Reduction of a Hydroperoxide . . . . . . . . . . . . . . . . . . 450 e . Decarboxylation . . . . . . . . . . . . . . . . . . . . . . . . 450 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451

I . INTRODUCTION I n 1925 Murray Raney (la) was granted a patent covering a new method of preparation of a nickel catalyst . A pulverized nickel-silicon alloy was reacted with aqueous sodium hydroxide to produce a pyrophoric. brownish nickel residue with superior catalytic properties. Upon investigation of other alloys of nickel and alkali-soluble metals. it was found that the aluminum alloy could be made with ease ( l b ) and was easily pulverized . The catalyst which is prepared by the action of aqueous sodium hydroxide on this nickel-aluminum alloy is known as 417

418

EUGENE LIEBER AND FRED L. MORRITZ

Raney nickel. Similar methods have been patented by workers in Germany (2), England (3), and the U.S.S.R. (4). Raney nickel is very probably the most commonly used nickel catalyst. It is also the most versatile of catalysts. In one or other of its several modifications it has been used for hydrogenations over a wide range of pressures varying from high t o subatmospheric, for desulfuration, for dehalogenation, and for very many other reactions. This chapter will not concern itself with the topic of hydrogenation of specific functions. Such topics as the hydrogenation of the double bond, the carbonyl group, and the cyanide group have been well covered in recent reviews (5,6,7). It is the authors' intention to demonstrate the versatility of Raney nickel by citing the many uses t o which i t can be put over and above the simple addition of hydrogen t o an unsaturated function. No attempt is made, however, to ignore this best-known reaction of Raney nickel. Examples and references are cited where they most aptly fit into the context. 11. PREPARATION AND PROPERTIES 1. Preparation of Raney Nickel Catalysts

The process used for the preparation of Raney nickel was originally described in a patent issued to Murray Raney in 1927. The catalyst is prepared from Raney alloy which is commercially available and which consists of approximately equal weights of nickel and aluminum. Schroter ( 5 ) lists several ways in which the alloy may be treated after pulverization. That method in which the alloy is decomposed by the action of caustic alkali gives rise t o Raney catalyst. Several methods of processing the alloy to give the catalyst have been described in the literature. I n the original process recommended by Raney (la) and developed by Covert and Adkins (8), there was involved a prolonged period of digestion of the alloy at 115'. Mozingo (9) later improved the process, shortening and lowering the temperature of the digestion [period. At the suggestion of Adkins, the catalyst of Covert is known as W-1 and that of Mozingo is known as W-2, a similar designation being applied t o the improved preparations described by Adkins and his co-workers (10,ll). The W-6 Raney nickel catalyst of Adkins and Billica (11) has been claimed to be the most active form of Raney nickel known. The leaching and the digestion processes are accomplished a t about 50°, and the washing is done by a continuous flow method under pressure of hydrogen. W-7 catalyst is alkaline in nature and has given good results in the hydrogenations of ketones, phenols, and nitriles for which alkali in the reaction mixture is beneficial (12,13).

THE USES OF RANEY NICKEL

419

Smith et al. (14) have claimed that different temperatures of solution of the inactive portion of the alloy, different temperatures and lengths of digestion, and different methods of washing have little effect on the catalytic activity as measured by the rate of hydrogenation of d-limonene. I n their procedure the alcohol was evaporated under vacuum from the catalyst after which the terpene was added. With the probable large loss of hydrogen under these conditions, it is doubtful that these authors were investigating actual Raney nickel of the W-6 type. Adkins and Krsek (15)) in comparing the various types of Raney nickel catalyst, found their activities to vary considerably in the hydrogenation of @-naphthol. Pattison and Degering (16) have prepared a catalyst of the Raney type from a nickel-magnesium alloy. These authors used acetic acid to dissolve the inactive portion of the alloy. The catalyst was found to be as active as the W-4 Raney nickel of Pavlic and Adkins (10) and twice as active as the W-2 catalyst of Mozingo (9). Paul (17) has described nickel catalysts of the Raney type which were prepared from alloys of nickel, aluminum and cobalt, or chromium, or molybdenum. The alloy contained 52% aluminum to 48% of the other twp metals. Bernstein and Dorfman (18) have prepared a modified catalyst by allowing the reaction mixture to stand at room temperature overnight after the addition of the alloy. Bougault, Cattelain, and Chabrier (19) and Mozingo (20) have prepared similarly modified catalysts. Delepine and Horeau (21) and Reichstein and Gatzi (22) received good results using small amounts of Raney nickel catalyst prepared in amounts just sufficient for use. The procedures consumed very little time because of the small quantities concerned. The advantage of always working with freshly prepared catalyst is obvious. 2. Properties of Raney Nickel Catalysts

The composition of the catalyst will, of course, vary with the method of preparation. Aubrey (23) indicates the following composition for his catalyst: A1 1-3 %, Fe 1%, Cu 0.1 %, Co 0.05%, Mn 0.04 %. The aluminum content is a function of the duration of the treatment with the alkali. Paul ( 6 ) has used catalysts which titrated to 17% aluminum. Heublen (24) has used catalysts even richer in aluminum. Adkins and Billica (11) reported their W-6 catalyst to contain 11% aluminum, the remainder being nickel. Ipatieff and Pines (25), however, found 21% alumina, 1.36% aluminum, 0.5% sodium aluminate, and about 77% nickel in W-6 catalyst.

420


There has been some conflict in the literature concerning the deterioration of the activity of Raney nickel as the preparation increases in age. Aubry (23) reports that, when the surface of Raney nickel is exposed t o water, it is oxidized t o the hydroxide. Mozingo (9) cautions against the preparation of more than a six-month supply because the catalyst deteriorates on standing, whereas Paul (6) assumes that the catalyst is easily preserved. I t has been claimed (5b) that the catalyst can be stored in a sealed container without much loss in activity. Adkins and Billica (11) maintain that the high activity of their W-6 modification is due t o the large amount of adsorbed hydrogen and that on standing the catalyst reverts to ordinary Raney nickel. Many writers have found relatively old catalyst to be quite active. Recently, Fanta (26)’ used year-old Raney nickel for the conversion of 5-nitro-2-phenyl pyrimidine to 2,2’-diphenyl-5,5’-azoxypyrimidine. Pattison and Degering (27) have shown that oxygen is responsible for the major part of the loss in activity. Smith et al. (14) studied the influence of aging and were able to obtain a correspondence of the surface area, which lessened with age, and the catalytic activity. Dupont and Piganiol (28) have applied x-ray analysis to the study of Raney nickel. The crystal dimensions lie between 40 and 80 A . , a magnitude ten times smaller than that of the reduced nickel of Sabatier and Senderens, the dimensions of which lie between 400 and 1090 A. The spectral lines of Raney nickel are also more diffuse than those of reduced nickel. Raney nickel catalyst contains hydrogen, most of which is probably bound by van der Waals forces. A good part of the hydrogen can be removed by heating. After prolonged heating of the catalyst at a temperature of 95”, some hydrogen is still held by the catalyst. Unlike ordinary nickel, Raney nickel can form an amalgam. This property can be attributed to the hydrogen which acts as a third component. 3. Other Raney Catalysts

Other Raney catalysts have been prepared. Raney cobalt has been described by several authors (28,29). The active cobalt has been claimed t o be especially suitable for the reduction of nitriles. The preparation of an active copper has been described by Faucounau (30). Paul and Hilly (31) have described the preparation of Raney iron. It is claimed that Raney iron reduces acetylenic bonds to ethylenic bonds with no further hydrogenation occurring.

4. The E$ed of Additives There are two general methods for the activation of Raney nickel catalysts. Many examples can be found in the literature of the addition

THE USES OP RANEY NICKEL

421

of a base to activate the catalyst, or of the addition of a noble metal. More recently, combinations of the two have been used. a. Platinic Chloride. I n 1936, Lieber and Smith ( 3 2 ) found th a t the addition of small amounts of platinic chloride to Raney nickel catalyst just prior t o the start of reduction produced a marked enhancing effect on the activity of the catalyst for a variety of functional groups. DelBpine and Horeau (21) independently proposed a similar use of platinic chloride whereby, prior to the reduction, the catalyst was shaken in a solution of platinic chloride for a relatively long period. The platinized catalyst was used in an alkaline medium. More recently, Jayme and Sartre (33) have described a catalyst activated in a manner similar t o th a t of Deldphe. This catalyst was used with good results for the hydrogenation of oxidized xylan. Both types of preparation have been described as having a high degree of activity. The two methods of activation by platinic chloride have been compared (34), and it is claimed that the method of Delepine is superior. Workers in G.B.L. Smith’s laboratory, however, have failed to confirm this fact. It is to be noted that the difference in opinion probably lies in the fact th at the two groups studied different types of compounds and that the French school, unlike the American, used alkali in conjunction with their platinized Raney nickel. Further studies dealt with the reduction of nitro and carbonyl compounds (35,36). Voris and Spoerri (37) used platinized Raney nickel to effect the reduction of 2,4,6-trinitro-m-xylene. Platinic chloride, high temperature, and high pressure were used because of the difficulty of reducing the third nitro group. Using the platinized Raney nickel catalyst of DelBpine, Decombe (38) succeeded in hydrogenating triphenyiacetonitriIe, the end product being 2,2,2-triphenyIethyIamine. More recently, Samuelson, Garik, and Smith (39) have used their platinized catalyst in conjunction with alkali to effect the hydrogenation of nitro compounds. Since the earlier investigations there have been marked improvements in the methods of preparation of the catalyst (11). Accordingly, the activating effect of platinic chloride on the improved catalysts, in particular that designated as W-6, was investigated (40,41). Adkins and Billica had noted that the addition of a small amount of triethylamine had a beneficial effect in the hydrogenation of carbonyl compounds. The time required for the hydrogenation of aldehydes and ketones was approximately cut in half. Lieber and his co-workers investigated the hydrogenation of a variety of compounds using platinic chloride and triethylamine separately and together. In the ketones the use of platinic chloride alone resulted in complete poisoning, whereas the use of

422


triethylamine reduced the time for complete hydrogenation. A mixture of triethylamine and platinic chloride also lessened the reaction time. A variety of aldehydes, aliphatic and aromatic, were reduced in the presence of the amine alone and in the presence of the mixture. I n general, the time for reduction in the presence of the mixture was less than in the presence of triethylamine alone. An exception was cinnamaldehyde, in which molecule the conjugated double bonds may be a complicating factor. The mixture proved also to have a beneficial effect in the hydrogenation of nitroethane, p-nitrotoluene, and phenylacetonitrile. The use of preformed triethylamine chloroplatinate was also studied, and, though the data are sparse, it seems that the preformed basic chloroplatinate is the more effective promoter for nitro compounds, whgreas the best results are obtained for benzaldehyde by using an excess of the amine over the platinic chloride. It has been reported (42) that ammonium chloroplatinate used in conjunction with alkali showed a favorable effect in the reduction of the fatty acids in butter. Campbell and O’Connor (43) have used platinic chloride alone and in conjunction with alkali for the reduction of substituted acetylenes. When the platinic chloride and the alkali were used together, the reduction occurred more rapidly in the case of amyl acetylene, but there was no significant change in the reduction velocity of dibutylacetylene. Although the action of most additives on a catalytic surface is somewhat obscure, it seems safe to assume that the action of platinic chloride is a promoter action through the formation of metallic platinum under these reductive conditions. The platinum is plated out on the nickel surface. Promoter action has been observed when platinic chloride in amount sufficient to provide only 0.4 mg. of platinum is mixed with as much as 3 g. of Raney nickel catalyst (41). b. Alkali. The additive used probably most frequently for the activation of Raney nickel catalyst is sodium hydroxide. Delepine (21), noting that pinonic acid could be hydrogenated only if alkali were added in excess of the amount needed for neutralization, extended the investigation t o the reduction of many carbonyl compounds. When small amounts of alkali are added to the reduction mixture, there is a severalfold increase in the rate of hydrogenation. This activation is even more apparent when the catalyst has been platinized. Smith and Lieber (36) reported that, although the use of alkali activated Raney nickel for the hydrogenation of carbonyl groups, it had a deterring effect on the reduction of aromatic nitro compounds. I n particular, alkali retarded the reduction of nitrobenzene and the three isomeric sodium nitrobenzoates, whereas it increased the velocity of reduction of the methyl and ethyl esters of nitrobenzoic acid.

423

T H E USES O F R A N E Y NICKEL

To check the .general applicability of activation by alkali, Paty (44) extended the investigation t o several functional groups. The addition of small amounts of sodium hydroxide considerably reduced the time for reduction of 2-methyl-2-butene, phenylacetonitrile, and anethole. Larger amounts of alkali, in the case of phenylacetonitrile and in the case of anethole, increased the time for complete hydrogenation. P a ty considers that the activating effect is due t o the action of sodium hydroxide on the residual aluminum or nickel-aluminum alloy in the catalyst. Ungnade and McLaren (45) studied the effect of alkali on the catalytic reduction of phenols, using Raney nickel catalyst. The ease of reduction was not affected by substitution in the ring unless two ethyl or n-propyl groups occupied the ortho positions. I n the latter case, with no alkali, no cyclohexanols were formed. 2,6-Di-n-propylphenol was recovered unchanged, but 2,6-diethyl-4-me t hylphenol underwent hydrogenolysis and reduction of the ring to give l-methyl-3,5-diethylcyclohexane. When the hydrogenations were carried out in the presence of a small amount of sodium hydroxide, the corresponding cyclohexanols were formed. With other phenols, the addition of alkalies led t o a slight promotion effect, inasmuch as a lower initial temperature was required for reduction. It was also found (46) that the hydrogenation of alkyl phenols is promoted by small amounts of their sodium salts. Chabrier and Sekera (47) found th at in the presence of alkali the sodium salt of the semicarbazone of a-keto-y-phenyl butyric acid was hydrogenated four times as rapidly as in the presence of Raney nickel alone. Similarly, the semicarbazone of P-benzoylpropionic acid was hydrogenated slowly in neutral solution and four times as rapidly in 0.1 N alkali. CsHaCH(=NNHCONH,)CH2CH2COOH---* CeHsCHCH2CHtCONHCONHNH I

I

Newman, Underwood, and Reno11 (48) studied the reduction of 1,2-epoxydecane. Over Raney nickel the product was 1-decanol. I n the presence of alkali, however, the main product was 2-decanol. Chemical reduction also leads to 2-decanol. It is interesting t o note that, when styrene oxide is reduced over Raney nickel, the primary alcohol is received as the main product whether or not alkali is present. Heilmann (49) reported the use of sodium hydroxide in conjunction with platinum and Raney nickel for the reduction of the double bond in a,@-unsaturatedketones. Burnette (50) used sodium hydroxide in the hydrogenation of silvan over Raney nickel. Delepine (51) has shown that many hydrogenations over Raney nickel, for which high temperatures and pressures have been used, can be run under ordinary conditions, particularly if sodium carbonate is added.

424

E U G E N E LIEBER A N D FRED L. MORRITZ

Smith (52,53) has investigated the affect of various additives on the rate constants for the hydrogenation of terpenes. The most effective additive tried was palmitic acid. Soaps, amines, sulfated alcohols, quaternary ammonium salts, and amides also increased the rate. 6. Selectivity of Raney Nickel Catalyst

One of the most important and intriguing tasks before the catalytic chemist today is the selective hydrogenation of one functional group in the presence of a second, also reducible, functional group. The role of Raney nickel in this respect is a bit obscured because of the many methods of preparation. Although the activity of the older forms of the catalyst toward the carbonyl function was not great, it is evident that the newer types of Raney nickel, the W-6 and W-7 catalysts of Adkins and Billica (1l),can smoothly reduce aldehydes and ketones to the respective alcohols. I n the following examples it is not claimed that no other catalyst will better perform the task but, rather, it is sought merely t o point out some of the uses t o which this catalyst has been put. The effect of temperature upon selectivity has been shown in a striking manner by Zafiriadis (54,55) in the hydrogenation of cinnamylidenemethylhexyl ketone with Raney nickel. At 40" the ethylenic bonds were reduced; a t 130" the carbonyl function was reduced to the alcohol; and at 260" the phenyl ring was reduced. AcOEt, 5 % catalyst

C~H~CH=CH-CH=CH-COC~€II~ ------+ AcOEt, 5 % catalyst

CsH s(CHz)&OCeH 13

40°, 100 atrn., 3 hr.

CeHs(CHz)rCOCeHla

CaHs(CH2)4CHOHCsH13

130". 100 atm.. 3 hr. AcOEt, 10% catalyst + CsHs(CHz)rCHOHCeH13 260°, 100 atrn.. 12 hr.

CeH1 I (CH2)dCHOHCeHtr

Cornubert and Phelisse (56), using Raney alloy as catalyst, studied the selective hydrogenation of a,ðylenic ketones. The reduction of benzalacetone gave w-phenyl-Bpentanol when the reaction was run in absolute alcohol. When, however, the reaction was run in a solution of ethanol containing chloroform in a concentration of 2 g./l., or HC1 in a concentration of 1.1 g./l., benzyl acetone was obtained. Similar results were noted with dibenzalacetone. Difurfurylideneacetone adds seven molecules of hydrogen in absolute ethanol or in ethanol containing 0.6 g./l. of chloroform. In ethanol containing 10 to 500 g./l. of chloroform, only two molecules of hydrogen are taken up. With higher concentrations of chloroform, the catalyst is inactive. Blout and Silverman (57) have reported a good example of selective reduction with Raney nickel. It was found th a t Raney nickel effectively catalyzes the hydrogenation of aromatic nitro compounds in

425

THE USES O F RANEY NICKEL

preference t o aliphatic double,bonds conjugated to the benzenoid ring. The hydrogenations of the isomeric nitrocinnamic acids and esters were studied. Since these compounds are not readily soluble in organic solvents, i t was found necessary to use an alcoholic suspension. When the suspension was shaken with Raney nickel under a pressure of 2-3 atm. and a t 20-30°, rapid reduction took place, the rate remaining steady until three equivalents of hydrogen were taken up, after which the rate fell to 0.3-0.01 of its former value. The esters were hydrogenated more rapidly than the acids. The data in Table I illustrate the case. TABLE I The Selective Hydrogenation of Nitrocinnamic Acids and Eslers" Acceptor p-Nitrocinnamic acid Methyl-p-nitrocinnamate m-Nitrocinnamic acid Methyl-m-nitrocinnamate a-Methyl-p-nitrocinnamic acid o-Nitrocinnamic acid Methyl-0-nitrocinnamate

Product p-Aminocinnamic acid Methyl-p-aminocinnamate nz-Aminocinnamic acid Methyl-m-aminocinnamate a-Methyl-p-aminocinnamic acid o-Aminocinnamic acid Methyl-o-aminocinnamate

Hours

Yield, %

6 3 12 4.5 6

73 76 76

5 1.5

37 74

81

a4

OAbstracted from Blout and Silverman, J . Amer. Chem. SOC.,66, 1442 (1944), with the perniission of the authors and copyright owner, the American Chemical Society.

The o-nitro compounds show a much smaller decrease in reduction rate when the nitro group has been hydrogenolyzed, and it was necessary to stop the reduction immediately after absorption of the calculated amount of hydrogen. On further reduction hydrocarbostyril was formed in 80% yield (if R = H, the yield is 90%).

Hilditch and Pathak (58) studying the catalytic reduction of methyl eleostearate, found that the reaction a t 110" and a t 170" was extremely selective in the presence of Raney nickel. No methyl stearate was formed until 90% of the linoleate had been transformed into octadecenoates. Ehrhart (59) used Raney nickel to hydrogenate compounds of the type RCH(CN)NHCOR'. The nitrile was reduced t o the amine, but the amide was untouched.

426


6. Reductions with Nickel-Aluminum Alloy and Aqueous Alkali

A rather unique variation in the application of Raney nickel to the reduction of organic compounds is the use of nickel-aluminum alloy in the presence of aqueous alkali. The procedure has the advantage of ease of application. The reduction is effected as part of the procedure for the preparation of the Raney nickel hydrogenation catalyst itself. The technique has been developed largely through the efforts of Papa, Schwenk, and co-workers (60). The reduction is probably due to the activation, by the freshly formed nickel catalyst, of the hydrogen liberated by the action of the alkali on the aluminum component of the alloy. Similar reductions have been observed when aluminum was used along with previously prepared Raney nickel catalyst. In these cases, if the nickel catalyst is omitted and only aluminum is used, either no hydrogenation occurs or amorphous products are obtained from which no pure substances can be isolated. GENERAL PROCEDURE

(60a)

Ten grams of the compound is dissolved in 300 ml. of 10% sodium hydroxide and heated to 90" after which 30 g. of Raney's nickel-aluminum alloy is added in small portions with stirring. The reaction mixture is stirred for an additional hour, the temperature being maintained a t 90". The original volume is maintained by the addition of water. A few drops of octyl alcohol are added occasionally to prevent any excessive foaming. Although this treatment is usually sufficient to complete the reduction, further heating of the reaction mixture with the addition of 5 g. of alloy and 50 ml. of 10% sodium hydroxide generally results in increased yields, especially with alkali-insoluble compounds. The hot solution is filtered and the residue washed thoroughly with water in such a manner that it is always covered with liquid. If the nickel residue is allowed to become dry, it will ignite. The filtrate is cooled and made acid to Congo red paper with concentrated HC1. It is desirable to effect the acidification by adding the alkaline solution to the acid with stirring. The reduction product is isolated by filtration or by extraction of the acidified solution. For the alkali-insoluble compounds, the reductions are carried out in a 1-1. flask equipped with an adaptor and a reflux condenser. During the addition of the alloy the reaction mixture is shaken frequently. In some cases toluene may be added to retain the compound in a uniform surface layer. Alcohol may also be used in sufficient amount to keep the compound in solution. The reduction product may be isolated either by steam distillation or extraction of the alkaline solution. One of the first reports (60b) on this method described the reduction of

427


estrone t o a mixture of a- and P-estradiol. Further investigation of many carbonyl compounds showed that the method gives results comparable t o those obtainable by the Clemmensen reduction (61). The use of the alloy with aqueous alkali is not specific. The carbonyl compounds are converted either to the corresponding carbinol or to the hydrocarbon, the extent of reaction depending only upon the structure of the compound. Carbonyl compounds of the general formula I yield the hydrocarbon, whereas carbonyl compounds of the general formula I1 yield the carbinol. CsH6COR

R = H, alkyl or aryl

I

CE.H~(CH,),COR’

R‘

=

H or alkyl

I1

I n order t o obtain reduction of the carbonyl group to the hydrocarbon, it is necessary that the carbonyl group of I be directly attached t o a n aromatic nucleus, thus forming a conjugated system. When the carbonyl group, even a s part of a conjugated system, is not directly attached to an aromatic carbon atom, the reduction of the carbonyl group proceeds only as far as the alcohol. I n Table I1 are listed the carbonyl compounds which Papa and his co-workers have reduced by this method. It will be noted that, whereas TABLE I1 Reduction of Carbonyl Compounds by Nickel-Aluminum Alloya Compound Benzaldehyde Salicylaldehyde p-Hydroxybenzaldehyde Cinnamic aldehyde Acetophenone m-Nitroacetophenone p-Hydroxyacetophenone p-Hydroxypropiophenone p-Hydroxybenzophenone 2-Methylcyclohexanone Dibenzyl ketone Salicylacetone pHydroxybenzalacetophenone Desoxybenzoin Benzoin Anisil (toluene as solvent) Anisil (EtOH and toluene as solvent)

Reduction product Toluene o-Cresol p-Cresol Hydrocinnamyl alcohol Ethylbenzene m-Aminoethylbenzene p-Ethylphenol p-Hydroxy propylbenzene p-Hydrox ydiphenylmethane 2-Methylcyclohexanol Dibenzyl carbinol 4-(o-Hydroxyphenyl)-butanol-Z p-Hydroxydiphenylpropane Dibenzyl Dibenzyl Anisoin Hydroanisoin

Abstracted from Papa, Schwenk, and Whitman, J . Ore. Chem., 7, 587 (1042). 10 g. of carbonyl compound.

* Based on the reduction of

Yield, 60 75 80 50 70 76 72 78 90 80 70 85

50 70 50 80 80

%b

428


benzaldehyde gives toluene and acetophenone gives ethylbenzene, cinnamic aldehyde and salicylacetone are reduced to hydrocinnamyl alcohol and 4-(o-hydroxyphenyl)butano1-2, respectively. I n addition to the carbonyl group, other functional groups, such as the nitro group which is reduced to the amine, are reduced as usual with no interference with the reduction of the carbonyl function. Table I1 also illustrates an interesting effect resulting from change of solvent in the reduction of an alkali-insoluble compound. Anisil, which is not reduced without solvent, gave anision as the only reduction product in the presence of toluene, while hydroanision was obtained by using toluene and ethanol as solvent. The use of nickel-aluminum alloy in aqueous alkali for the reduction of organic compounds has been observed (6Oc) to briny about the displacement by hydrogen of methoxy, halogen, and sulfonic acid groups from several types of benzenoid compounds. Table I11 summarizes the TABLE I11 Displacement of Halogen and Sulfonic Groups by Nickel-Aluminum Alloy i n Ayueous Alkali" Compound

Reduction product

Halogen Compounds Bromobenzene Benzene m-Chlorobeneoic acid Benzoic acid p-Chloronitrobeneene Aniline Toluene p-Chlorobenzaldehyde 5-Chloro-2-hydroxybenealdehy de o-Cresol Ethylbenzene p-Bromoacetophenone @-(p-Chlorobenzoy1)propionicacid 7-Phenylbutyric acid Sulfonic Acids Benzenesulfonic acid Benzene o-Sulfobenzoic acid Benzoic acid rn-Sulfobenzoic acid Benzoic acid Naphthalene-@-sulfonic acid Naphthalene 2-Naphthol-6-sulfonic acid @-Naphthol 2-Naphthol-3,6-disulfonicacid @-Naphthol 0

Yield, %

100 100 65 60 75 67 70 10 40 50 40 30 30

Sohwenk, Papa, Whitman, and Ginsberg, J . Ore. Chem., 9. 1 (1944).

results of observations on the displacement of halogen and sulfonic acid groups. I n general the displacement of sulfonic acid groups has been limited t o the a-naphthalenesulfonic acids, only a few instances of a similar displacement being observed for the beta compounds (62). By means of nickel-aluminum alloy in aqueous alkali, sulfonic groups are displaced from a- and 0-naphthalenesulfonic acids as well a s from the benzenesulfonic acids. The low yields in some cases are probably due t o


429

a poisoning of the nickel catalyst by the sulfite or sulfide formed during the reaction. The displacement of an alkoxy group by hydrogen using nickelaluminum alloy in aqueous alkali depends upon the nature and position of the other substituents in the benzenoid ring (60c). When subjected t o this reduction procedure, p-anisidine and o-, m-, and p-cresyl methyl ether were recovered unchanged. When the ortho-para-directing methyl or amino groups in these compounds are replaced by the meta-directing carboxyl group, quantitative displacement of the methoxy group takes place in o- and p-methoxybenzoic acid. The meta acid, however, is recovered unchanged, and in no case has a meta-substituted methoxy group been observed to be displaced. Similar displacement of the methoxy group takes place in compounds having other meta-directing groups such as -NOz, -CHO, and -COCHs. In addition t o the displacement reaction, the meta-orienting groups in these compounds are subject t o reduction to the ortho-para-orienting amino or alkyl groups. I n a compound containing such a reducible substituent, elimination of the methoxy group occurs only before the reduction has converted the meta-directing into an ortho-para-directing group. For example, in the reduction of p-nitroanisole, a 20% yield of aniline and a 70% yield of p-anisidine were obtained. The aniline must result from a n initial displacement of the methoxy group followed by reduction of the nitrobenzene thus formed, whereas the p-anisidine arises from the initial reduction of the nitro group.

111. SPECIAL REACTIONS 1. Oxidation

a. Raney Nickel as a n Oxidation-Reduction Catalyst. If Raney nickel is added t o a slightly alkaline solution of sodium hypophosphite, immediate frothing occurs with liberation of hydrogen. Bougault, Cattelain, and Chabrier (19) demonstrated th at the sodium hypophosphite was oxidized t o sodium phosphite. This is a remarkable reaction in th a t a reduction catalyst is used to effect an oxidation. It was observed th a t NaH2POz

+ H ~ 0 - tNaH2P03 + H2

0.5 g. of Raney nickel was capable of transforming 35 g. of sodium hypophosphite t o sodium phosphite. It is probable th a t the oxidation is

430

EUGENE LIEDER AND FRED L. MORRITZ

effected by a dehydrogenation of the hydration product, the last step being the one catalyzed. I n a second example, Bougault et al. showed

/

H

HO

O=P-H

\

+HzO+ ONa

H

\ /

/H -+O=P-OH

P-H

HO

/ \

\

ONa

+Hz

ONa

that sodium stannite is oxidized t o sodium stannate under the same conditions. Sn/OH

+ 2H20

+

Sn(OH),

+ Ha

\OH

By conducting the oxidation in the presence of a hydrogen acceptor, the first oxidation-reduction reaction to be catalyzed by Raney nickel was effected. Sodium phenylisocrotonate was readily converted to sodium phenylbutyrate. Ashida (63) observed that glucose in a saturated solution in cyclohexanol was reduced to d-sorbitol by heating a t 130-135" NaHzPOz

+ CsH&H=CHCH2COZNa + HzO

---f

NaHZP03

+ C6H6(CH2)&02Na

in the presence of Raney nickel. I n the absence of the catalyst no reduction was observed. Ashida, however, failed to determine whether the hydrogen for the reduction came from the nucleus or from hydrcjxyl groups of the cyclohexanol. A report (64) of the work of H. Ruschig has described the catalytic conversion of pregnenolone (I) to progesterone (11) in the presence of a special Raney nickel catalyst, with cyclohexanone as a hydrogen acceptor. Kleiderer and Kornfeld ( 6 5 ) , in an interesting investigation, determined the generality of this oxidation and studied the

I

I1

possibility of using it in the reverse sense, i.e., as a reductive method in the presence of a hydrogen donor. Preliminary attempts to oxidize cholesterol to cholestenone with cyclohexanone as hydrogen acceptor showed that the special aerated Raney nickel as prepared by Ruschig was considerably less effective than the usual Raney nickel kept under toluene. Because of its favorable oxidation potential (66), cyclohexanone is an

43 1


excellent hydrogen acceptor to use in these types of Raney nickel catalyzed oxidations. The method merely involves the refluxing of a mixture of the compound t o be oxidized, the hydrogen acceptor and the catalyst in toluene. Table IV shows the variety of secondary alcohols which may be TABLE I V Catalytic Oxidation of Secondary Alcohols with Raney Nickel in Presence of Cyclohexanone as Hydrogen Acceptoro Compound oxidized

g./g. compound

Time of reflux, hr .

Product

Yield, % '

Cholesterol Benzoin Benzhydrol Dihydrocholesterol Fluorenol Epieoprostanol

2.0 2.0 2.0 2.5 2.5 1.5

24 24 22 24 24 24

Cholestenone Bend Benzophenone Cholestenone Fluorenone Coprostanone

80 35 30 80 76 50

a

Catalyst,

Kleiderer and Kornfeld, J . Org. Chem.. lS, 455 (1948).

converted t o ketones by this procedure. The oxidation of cholesterol, i t may be noted, involves a simultaneous shift of the A 5double bond t o the A4 position in conjugation with the carbonyl group. This shift occurs likewise in the Oppenauer oxidation of cholesterol (67). When, next, the redox reaction was studied as a preparative reduction method, i t was found that a wide variety of compounds could be reduced in the presence of any of several hydrogen donors. Table V summarizes some typical results which can be accomplished by this type of reduction. I n general, the types of conversion obtained are similar t o those brought TABLE V Catalytic Reductions with Raney Xickel in the Presence of Various Types of Hydrogen Donors" Compound reduced

Hydrogen donor

Product

Yield, % ~

Diphenylacetylene Laurone 3-Acet ylquinoline

Ethanol Isopropanol Isopropanol

3-Ethyl-5,6,7,8-tetrahydro-

Cholestanone Benzoin Benzophenone Desoxyanisoin Stilbene Ethyl o-benzylbenzoate

Cyclohexanol Cyclohexanol Diethy lcarbinol Cyclohexanol Diethylcarbinol Isopropanol

quinoline Dihydrocholesterol Dibenzyl Diphenylmethane p,p'-Dimethoxydibenzyl Dibenzyl o-Benzylbeneoic acid

a

Dibenzyl Diundecylcarbinol

Abstracted from Kleiderer and Kornfeld, J . Org. Chem., 19,455 (1948).

77 80 62 50

53 75 80 60 86


432

about by high-pressure reduction (7) or by the action of alkali on nickelaluminum alloy (68). Carboriyl groups, activated ethylenic bonds, and acetylenic bonds in varied environments are smoothly reduced. Hydrogenolysis of the carbon-oxygen bond occurs when it is alpha to a n aromatic ring. Related reductions have been carried out by Bougault (19) and by Mozingo (69) with Raney nickel, the latter using ethanol as solvent. These workers believed that the hydrogen for the reduction was supplied by the catalyst and have not mentioned the possibility that the hydroxylic solvent employed might act as hydrogen donor. Support for this latter view was obtained (66) in the isolation of acetone from hydrogenation experiments in which isopropanol was used as a hydrogen donor. Under similar conditions Wolfrom (70) has isolated acetaldehyde when using ethanol as solvent. It is probable that both the hydrogen donor and the hydrogen held by the catalyst play a part in these reductions, especially when only small amounts of catalyst are used. Kleiderer and Kornfeld (65), however, found that, in the presence of excess Raney nickel, stilbene may be reduced in 80% yield to dibenzyl when dioxane is the solvent. It is evident, therefore, that reduction may be effected entirely by means of the hydrogen adsorbed on the catalyst and that the role of the hydrogen donor solvents is an accessory one. b. The Cannizzaro Reaction. Raney nickel has been known to catalyze the Cannizzaro reaction of aldehydes. As is well known, this reaction, in the ordinary sense, is catalyzed by strong bases and proceeds when the carbon alpha to the carbonyl group has no hydrogen bound to it. In the Raney nickel induced reaction, however, the presEnce of a n a-hydrogen seems to have no effect. Butyraldehyde, undergoes an aldol condensation in the presence of alkali alone, but in the presence of both Raney nickel and alkali disproportionation takes place a t ordinary temperatures (71). Formaldehyde is entirely disproportionated in 70 minutes in the presence of the Raney nickel catalyst, whereas in its absence it is only 50% transformed in 30 hours. The data below are due to Del6pine (71). Formaldehyde, g. 3

3 3 6 12

Solveni, 2% NaOI1, ml. 100 100 100 200 400

Catalyst, Raney nickel None 4.5 4.5 4.5 4.5

Time, min. 1800

70 30 180 65

Transformed, % 50 100 97 94.5 52.5

Benzaldehyde, glucose, galactose, and arabinose also undergo the reaction. The reaction has been formulated as follows:

THE USES O F RANEY NICKEL (1) (2)

(3)

+

433

RCHO NaOH + RCH(0Na)OH followed by dehydrogenation RCH(0Na)OH -+ RCOONa HZ RCH(0Na)OH H 2 + RCH9OH NaOH

+

+ +

Reaction (3) is assumed to be fast, since there is no observed release of hydrogen. The Cannizzaro reaction can be avoided if an easily reducible substance is added to the reaction mixture. Thus Delepine reacted Raney nickel with a mixture of galactose and the sodium salt of either cinnamic or crotonic acid in the presence of alkali. This reaction has also been noted by Tomkuijak (72), who, when hydrogenating D-xylose in water RCH(0Na)OH

+ R'CH=CHC02Na

-+ RC02Na

+ R'CH&H2C02Na

with 30% Raney nickel, found that d-xylonic acid was produced. The results of Delepine were confirmed by Ashida and Bebiko (73). I n the presence of Raney nickel, the Cannizzaro reaction was observed with formaldehyde, butyraldehyde, D-glucose, benzaldehyde, and furfural in 2% NaOH solution. In the absence of Raney nickel formaldehyde, D-glucose and furfural did not react in 2% NaOH solution. Evidence has also been presented (74) that the Cannizzaro reaction can occur in the absence of alkali if sufficient Raney nickel is present. c. Dehydrogenation. There does not exist a large literature concerning the activity of Raney nickel as a dehydrogenation agent. Palfray and Sabetay (75) and Paul (76,77) have made a study of the dehydrogenation of alcohols using Raney nickel as a catalyst. It was found that dehydrogenation could be accomplished with Raney nickel at lower temperatures than with reduced nickel. By distillation from a suspension of Raney nickel in alcohol, Paul was able to dehydrogenate sec-butyl alcohol at go", receiving a 90% yield of butanone. Hexanol-3, dehydrogenated a t 130°, gave an 80% yield of hexanone-3. Octanol-2 and octanol-3 a t 176" gave 95% yields of the respective octanones. Isopropyl alcohol at 80" gave a 30% yield of acetone. Secondary alcohols are easier to dehydrogenate than primary alcohols. I n the primary alcohols, hydrocarbons are formed along with carbon monoxide, condensation products, and products of crotonization. When dodecanol was heated a t 200" with hydrogen under 220 lb. pressure in the presence of Raney nickel, an almost theoretical yield of undecane was obtained (78). The fact that the same result is obtained when the hydrogen is replaced by nitrogen is evidence for this not being purely a hydrogenolysis. Since a small amount of lauryl aldehyde could be obtained at ordinary pressures, it is inferred that it is the first formed product and later dissociates into undecane and carbon monoxide. The carbon monoxide reacts with hydrogen to form methane and water.

434


Tishchenko (79)) using a modified form of Raney nickel, obtained a 95.7%yield of camphor from the dehydrogenation of borneol. Rutovskii, (80) received a 93.5% yield of camphor with Raney alloy. Reeves and Adkins (81)) studying the dehydrogenation of primary alcohols, removed the hydrogen with ethylene. It was found that, though Raney nickel could be used for a catalyst for the reaction, the yields were low and, in general, the Raney nickel was inferior t o a catalyst composed of copper, zinc, nickel, and barium chromite. Moretti (82,83) has studied the catalytic dehydrogenation of fatty acids over a Raney nickel catalyst. The fatty acid, in a nitrogen atmosphere, was spread in a thin layer over the catalyst and was heated without agitation. Part of the fatty acid was dehydrogenated t o form an unsaturated acid; part reacted with the nickel t o form a salt. It is postulated that the hydrogen liberated by these two reactions reduces the acid to the aldehyde and to the alcohol. Although the aldehyde was not found to be present in determinable quantity, a yield of 5 t o 6% of the alcohol as the stearic ester was found. The aldehyde and the alcohol are degraded to carbon monoxide and a hydrocarbon. Thus, though the fatty acids can be dehydrogenated by means of Raney nickel, the yields are poor because of the complex ensuing reactions. Harlay (84) has used Raney nickel to dehydrogenate dihydropapaverine to papaverine in 50% yield. He found Raney nickel t o be more satisfactory for this purpose than the nickel of Sabatier and Senderens, but not as effective as a palladium catalyst. Mosettig and Duvall (85) used Raney nickel to transform the tetrahydrophenanthrone-1and -4 into the respective phenanthrols a t the boiling point of benzene, but also found this catalyst less advantageous than palladium.

2: Amines by Reductive Allcytation By the action of ammonia, primary or secondary amines, carbonyl groups can be converted, in the presence of hydrogen and a suitable catalyst, to primary, secondary, or tertiary amines. This reaction has proved to be a useful tool in the field of synthetic organic chemistry. The conversion can be formulated as follows:


(1) (2) (3) (4)

+

435

RR’CO RNHz + RR’CZNR’’ RR’C=NR” H, + RR’CHNHR” or R”R’”NH RR’CO R”R’”NH + RR’C(0H)NR”R”’ RR’C(0H)NR’’R”’ H P-+ RR’CHNR’””‘

+

+ +

The conditions can be adjusted so as to give primary amines in the case of ammonia or secondary amines in the case of a primary amine. Whether the aldimine, as in (l),is formed or the hydroxy compound, as in (3), is of little consequence, since either species can be hydrogenated to the desired product and since, in this procedure, there is no necessity of isolating and purifying intermediates. Experimentally (86), the carbonyl compound and the amine together with a condensing agent are condensed and hydrogenated in one step. Robinson and Snyder (86) used acetophenone and ammonia over W-2 Raney nickel a t 150” and 3500-5000 lb. pressiire t o produce a-phenethylamine in 44 to 52% yield. By this general

+ Hz + C~HF,CH(NH~)CH,+ HzO

C6H5COCHa f NHa

method, first observed by Mignonac (87) in 1921, it is not necessary to prepare and t o isolate imines, hydroamides, or Schiff bases; they are formed during the reaction as intermediates. The catalysts usually encountered in this reaction are platinum oxide and Raney nickel. Cope and Hancock synthesized several alkylaminoethanols, using both catalysts (88). Although they prepared the compounds with Adams’ catalyst, they found Raney nickel to be also suitable a t elevated temperatures and pressures. An 86% yield of 2-sec-butylaminoethanol was obtained from methyl ethyl ketone and CH3COCH2CH3

+ HzNCHzCHzOH + HP Raney nickel

150°,

t

CsH5CH(CHa)NHCHzCH20H

+ HzO

1000-2000 p.s.i.

ethanolamine. Emerson and Walters (89) found that, for the reductive alkylation of aniline, Raney nickel and sodium acetate as condensing agent gave the best results. A 58% yield of N-ethylaniline was received when 58 g. of W-1 Raney nickel and 1 g. of sodium acetate were used. With 0.2 g. of platinum oxide and 1 g. of sodium acetate, a 41 % yield was obtained. With Raney nickel the following results were obtained: Aniline Ethyl n-Propyl n-€3 u ty 1 n-Amy1 n-Heptyl Benzyl

Yield 58 52 47 62 65 50

436


Emerson (90,91,92) has extended the reaction to include aromatic nitro compounds and azo compounds as starting materials. When a n alcoholic solution of an aromatic nitro compound and an aldehyde are reduced with hydrogen over W-1 Raney nickel in the presence of sodium acetate, the corresponding secondary amine is formed in good yield. Azo compounds treated with hydrogen and Raney nickel in the presence of an aldehyde and sodium acetate give secondary amines. When activating groups such as hydroxyl or diethylamino are ortho or para to the azo group, tertiary amines are produced. Table V I summarizes the significant results of these investigations. It seems probable that with azo compounds the reaction is a reduction to the hydrazo compound, condensation of the latter with the aldehyde, followed by reduction and further alkylation to the secondary or tertiary amine. Schwoegler and Adkins (93) reacted alcohols with primary and secondary amines over Raney nickel to form secondary and tertiary amines, respectively. Since tertiary alcohols do not undergo the reaction, it is assumed that the catalyst dehydrogenates the alcohol to a carbonyl compound which reacts with an amine to give a product th a t can be readily hydrogenated to a more complex amine. Piperidine was reacted with ethyl, n-butyl, and n-dodecyl alcohols to give 82,70, and 69% yields, respectively, of the corresponding alkylpiperidines. RzCHOH

R”H2

---f

RZCO --+

R&(OH)NHR’

Ha +

RZCHNHR’

R = H, alkyl

Couturier (94) obtained a rather good yield of dl-ephedrine by reacting a diketone, phenylpropanedione, with methylamine in the presence of Raney nickel. Schwoegler and Adkins (93), on reducing acetonylacetone in ammonia, received a 28% yield of 2,5-dimethylpyrrolidine and a 59 % yield of 2,5-dimethylpyrrole. From acetylacetone a quantitative yield of acetamide was obtained. Winans (95) has studied the hydrogenation of aldehydes in the presence of Raney nickel in alcoholic ammonia solution. Aldehydes with no hydrogen on the a-carbons were used, and the amounts of ammonia were varied. When a 1 : 1 ratio of equivalents of ammonia to aldehyde was used, the main product was the primary amine. With a 1:2 ratio, a high yield of the secondary amine was obtained. I n the former case o-chlorobenzaldehyde gave an 85% yield of the primary amine and a 7.G% yield of the secondary amine. In the latter case there was obtained a 3.6% yield of the primary and an 84.6% yield of the secondary amine. Henze and Humphreys (96) have used the reaction for the preparation of several mixed secondary amines in yields varying from 26 to 56%.

TABLE VI Reductive Alkylations Using Nitro Compounds and Azo Compounds"

N Compound

Yield, % 50 57-63 94-96 84

Nitrobenzeneb

Aldehyde Formaldehyde Acetaldehyde n-Butyraldehyde n-Valeraldehy de n-Heptaldeh y de Benzaldehyde n-Butyraldehyde n-But yraldehyde n-Valeraldehyde n-Butyraldehyde n-Heptaldehyde n-But yraldeh y de

Product N-methvlaniline N-eth ylaniline N-n-butylaniline N-n-amylaniline N-n-heptylaniline N-benz ylaniline N-n-buty 1-p-anisidine N-n-butyl-a-naphthylamine N-n-am y 1-a-naphthylamine N-n-butyl-p-toluidine N-n-heptyl-p-toluidine N, N ,di-n-butylaniline

~ - N = N - - P ~ ' ( C HJ~

n-Butyraldehyde

a N H C 4 H p

73

(n-CdH9)r N O N ( C H 2 )P

76

N-n-bu tylaniline

71

Nitrobenzene

p-Nitroanisole 1-Nitronaphthalene p-Nitrotoluene

= - N = N - ~

n-Butyraldehyde

n-Butyraldehy de

'

40 34 31

60 43 85 35 63

41

N, H-Di-n-butyl-p-aminophenol .Emeraon, J . Am. Chem. Soc., 62, 69 (1940); 63, 749, 751 (1941). Trimethylamine hydrochloride was used as condensing agent.

~

46

438

E U G E N E LIEBER AND FRED L. MORRITZ

Metayer (97) has alkylated, over Raney nickel, such nitrogen compounds as indole and N-formyl-1-phenethylamine. For a Raney nickel catalyzed acylation of an amine, see Blout and Silverman (57), who obtained hydrocarbostyril when the hydrogenation of o-nitrocinnamic esters was continued after three equivalents of hydrogen had been taken up. 3. Rearrangements

It was noted by Delepine and Horeau (98) that in the hydrogenation acid over Raney nickel a migration of of 2,6-epoxy-3-heptene-3-carboxylic the double bond occurs. The acid 2,6-dimethyl-5,6-dihydro-3-carboxypyran was received in 21 % yield a t ordinary temperatures. I n 1 hour a t 100' a 60 % isomerization was observed.

The migration of the double bonds of asymmetric octahydrophenanthrene when in contact at 130" with Raney nickel was reported by Durland and Adkins (99). A 15% yield of the symmetric octahydrophenanthrene was obttLined after 5 hours, and a 28% yield was obtained after contact for 19 hours.

f - l

O-D-C% When dipropenylglycol is distilled under reduced pressure from a suspension of Raney nickel, the diketone, dibutyryl, is obtained in 30% yield and the acyloin, propenylbutyrylcarbinol, is obtained in 15% yield (100). CHsCH=CHCII (OH) CH (OH)CH=CHCHa 4 CsIIrCOCOC3H7

+ CHaCH=CH-CH(OH)COCjHT

Divinylglycol reacts with greater difficulty to give a 20% yield of dipropionyl. The reaction can be formulated as a 1,3 shift of a hydride ion followed by ketonixation of the resulting enol. H

H

n y -CH=CH-C-

u

0

-D

-CHz-CH=C-

I

+

-CH&H2CO-

In the catalytic hydrogenation of aldoximes with Raney nickel Paul (101) observed a small amount of rearrangement of the aldoxime to the acid amide. Raney nickel induces the rearrangement which takes place

439


even under cold conditions. The reaction is complete with a yield of 75 to 100% after a few hours in refluxing alcohol. Some of the results obtained are summarized below. The role played by the catalyst has not been clearly established. When Raney nickel is introduced to the oxime Oxime Acetaldoxime Heptaldoxime Benxaldoxime Furfuraldoxime

Arnide Acetamide Oenanthamide Benzamide Fiiramide

Yield, % 60-86 90 75 80-96

or to the oxime solution, a deep red color due to solution of the metal is noted. After separation of the undissolved Raney nickel, it is found that the dissolved metal (i.e., the complex) is capable of carrying the reaction to completion. Bryson and Dwyer (102) have provided evidence for Paul's suggestion that the aldoxime-nickel complex is an intermediate in the reaction. By means of this reaction Caldwell and Jones (103) obtained citronellamide in 50% yield when 20 g. of citronellaldoxime and 3 g. of Raney nickel were heated a t 100-105" for 2 hours, and, after dilution with ether, the catalyst was removed by filtration through activated alumina. The oxime of tetrahydrocitral, when heated for 2 hours a t 110-120' with Raney nickel, gave a 70% yield of the amide of 2,6-dimethyloctanoic acid. Among the products resulting from the catalytic hydrogenation of dinitroneopentane with Raney nickel, Rockett and Whitmore (104) found a 67% yield of the expected diaminoneopentane and a 5% yield of the diamide of dimethylmalonic acid. It is thought that a small amount of oxime, which is an intermediate in the hydrogenation, rearranges t o the diamide. There is a possibility that the hydrogenation of a nitro compound or an oxime t o the corresponding amine proceeds by way of the CHs

CHzNOz

CH1

\c/

-+

C'

/ \

CH2 Ha

CHzNOz

CHz

/ \

CH=NOH

CH=NOH CH3

-+

CONHg

+

\C/ CH3/

CH3

'CONHg

CHzNHz

\C/

CH,

/ \

CHzNH2

intermediate amide. The difficulty, however, of reducing amides t o amines would indicate that this is not the most important reaction. Although Raney nickel will induce the rearrangement of an aldoxime to the amide, it will not effect the reaction with a ketoxime. This type of rearrangement has been effected with reduced copper with which

440

E UGE NE LIEBER AND F RE D L. MORRITZ

both aldoximes and ketoximes can be used. The yields, however, are small. When heated with Raney nickel at 200', substituted formamides rearrange with the elimination of ammonia to form a carbonyl compound (105). This is t,he reverse of the Leuckart-Wallach reaction. The reaction is postulated to take the following course: Raney nickel, 200'

RR'CHNHCHO

;-RR'CO ---+

HCONIIz, 180'

HCONHCHRR'

---f

CHzNHCRR' -+ OCRR' 0 ''

I I

I1 = I1 or C-

Substituted formamides react below 185' by scission of the formyl groups t o give amines. Above 1 8 5 O , carbonyls are formed. When the carbon bearing the nitrogen is monosubstituted, aldehydes are received. Raney nickel. Hz 185O, 750 p.8.i.

--

CsHIsCH(CH3)NHCHO ------+

C

H

,

~

~ ( cCH ~NHCHO )H

3GO p.8.i.

CS+H~~CH(CH~)NII~

O C O C H , among other products

C H 3 0 0 C H & H ( C H 3 ) N H C H 0 185'

C H 3 O ~ C I l ~ C O C H ~

600 p.8.i.

-

When the carbon is disubstituted, ketones are received. trisubstituted, only the amine is formed.

If the carbon is

4. Dehalogenation Reduction is a well-known method for the dehalogenation of organic compounds. The application of reductive methods for the quantitative determination of halogens is based on the conversion of the halogens into salts of the halogen acids and their subsequent determination by standard methods. Molecular hydrogen, derived for example, from zinc and acid, is not generally applicable for this conversion, since only a limited number of organic compounds quantitatively undergo the halogen displacement when treated in this manner. The use of catalytically activated hydrogen as an analytical tool for the dehalogenation of organic compounds was discovered by Husch and Stowe (106)) who used a palladium-ralcium carhonate catalyst. Later, Kelber (107) reported a similar procedure, using a reduced nickel catalyst. Whitmore and Revukas (108) observed that, in the reductive splitting of substituted azo compounds with hydrogen in the presence of Raney


44 1

nickel at 1 to 3 atm. of pressure, halogen was quantitatively displaced if alkali was present in sufficient amount to combine with the halogen acid formed. I n the absence of alkali the hydrogen acted only upon the azo bond to produce the normal fission. Paty (109) reported the use of Raney nickel in the debromination of several aromatic compounds of the type

where R is an aliphatic group. The reaction seemed to be facilitated if potassium hydroxide was added in amount slightly in excess to that required t o neutralize the hydrogen bromide formed. I n each case the halogen was replaced by hydrogen without the aliphatic functions being reduced. When R = -CHzOH, the percentage yield was 55%; R = -CHzOCzHs, 88%; R = -COOH, 88%; R = -CHzCOOH, 87%; R = -CHzCONHz, 95%. Halogens in the ally1 position seem to be readily replaceable by hydrogen. Anglade (110) treated bis-2,4-(chloromethyl)anisole with Raney nickel and hydrogen in alcohol a t ordinary temperatures. A 25% yield of 2,4-dimethylanisole was obtained, with ethyl ether formed as a by-product by interaction with the solvent. Aromatic halogens, however, seem t o possess a much greater degree of stability. Winans (111) was able t o reduce aromatic halogen compounds over Raney nickel catalyst without displacement of the halogen by being careful t o maintain the temperature below 150". An exception is such a compound as 2,4-dinitrochlorobenzene,which gives a 91 % yield of m-phenylenediamine a t 40". Shriner (112), in describing a procedure for the dehalogenation of 2-chlorolepidine to produce lepidine, noted that the reduction did not run smoothly a t room temperatures. At higher temperatures, with palladium on charcoal as catalyst, the theoretical amount of hydrogen was absorbed in 1.5 to 2 hours with a yield of 81 to 87 %. Raney nickel in alcohol can be used a t room temperatures, but 15 hours is required for the hydrogenolysis. Grigorovskii (I 13a), investigating the dehalogenation of chloroacridines over Raney nickel, found that halogen atoms in position other than 9 were unaffected. Chloroacridine (10 g.) and Raney nickel paste (10 g.) in 250 ml. of methanol were boiled with stirring for 4 hours. Upon treatment of the reaction mixture, a 66% yield of 9,9'-biacridine was isolated. When 3,9-dichloroacridine was treated in a similar manner, 3,3'-dichloro-

442


9,9'-biacridine was obtained in 70% yield. This Wurtz type of condensation was also found to occur in the dehalogenation of benzyl chloride (113b) over large amounts of Raney nickel. When bensyl chloride (10 g.) was heated with Raney nickel (20 g.) in boiling methanol for 4 hours, dibenzyl (0.6 g.) and toluene (2.6 g.) were obtained. With less catalyst more dibenzyl was formed, and with more catalyst more toluene was formed. Alkali, without an external supply of hydrogen, inhibited the dehalogenation. In the cold, in the presence of alkali and with hydrogen supplied, the tendency toward condensation disappeared almost completely. Little is known of the effect of nuclear substituents on the dehalogenation of aromatic halogen compounds. Schwab (114) debrominated several 4-bromoanthraquinones by means of hydrogen over Raney nickel. A p-hydroxyl group causes a much more rapid removal of bromine than a p-methoxy group. Methoxy groups in the 6 or 8 positions had very little inhibitory effect. The application of the displacement of halogen by nickel-aluminum alloy in aqueous alkali to the quantitative determination of halogens in organic compounds (115) is of considerable interest. Many of the above methods, when adapted for analysis, require the use of hydrogen gas, a prepared catalyst, and a hydrogenation apparatus of the conventional type. These requirements are obviated by the use of nickelaluminum alloy in aqueous alkali, and the method is advantageously used for the quantitative determination of halogen in many aliphatic, aromatic, alicyclic, and heterocyclic compounds. GENERAL PROCEDURE

Approximately 0.3 g. of sample, accurately weighed, is added to 100 ml. of 5% aqueous sodium hydroxide contained in a beaker. Three grams of the nickel-aluminum alloy is added in three or four portions over a period of 10 minutes. When the reaction has subsided, the mixture is heated on a steam bath for 1 hour and then filtered, the residue being carefully washed. An aliquot portion of the filtrate may now be analyzed for halogens by any standard procedure. Ethanol may be used to facilitate the reduction of alkali insoluble compounds,

443

THE U S E S OF RANEY NICKEL

Table VII summarizes some of the results that have been obtained by this method. The fact that only the usual laboratory apparatus is required to carry out the analyses gives the procedure a definite advantage over other reduction methods for the determination. The results obtainable compare favorably in both accuracy and precision with those of other methods. Its ease of manipulation over such methods as the Carius nitric acid sealed-tube procedure and the peroxide bomb procedure is obvious. TABLE V I I Quantitative Determination of Halogen by Means of Nickel-Aluminum Alloy in Aqueous Alkalia ~

Halogen Compound

Theory, 9% Found, %

3,5-Diiodo-4-hydroxyphenylaceticacid ~-(3,5-Diiodo-4-hydroxyphenyl)-~-phenylpropionic acid 8-Bromopropionic acid

62.84 51.40 52.21

Bromobenzene

50.85

2-Bromopyridine m-Chlorobenzoic acid

50.61 22.67

p-Nitrochlorobenzene

22.52

2-Chloropyridine

31.25

62.15 51.61 52.33 52.06 51.01 50.87 50.76 23.04 23.06 22.57 22.68 31.02

,. Reprinted from Ind. Eng. Chem. Anal. Ed.. 16, 576 (Sept. 1943), hy permission of the copyright owner. the American Chemical Society.

Raney alloy has also been used (116) for the dehalogenation of 4-chloro-5-methylquinaldine and 4-chloro-7-methylquinaldine t o form the 2,5- and 2,7-dimethylquinaldines. Zinc, in alkaline solution, in the presence of Raney nickel has been used to dehalogenate organic compounds (117). The halogen is then determined by a Volhard determination. GENERAL PROCEDURE

For compounds containing chlorine or bromine, a weighed sample is dissolved in 10 ml. of methanol; 10 ml. of 20% sodium hydroxide, 2 g. of zinc, and 0.5 g. of Raney nickel are added, and the mixture is heated with reflux for 1 hour over a water bath. It is then cooled and decanted. After acidification with nitric acid, 20 ml. of standard silver nitrate solution and 5 ml. of a ferric alum solution are added, and the solution is titrated with standard potassium thiocyanate. The procedure is modi-

444


fied for iodine-containing compounds by filtration of the silver iodide prior t o the titration. Although this method was unsatisfactory for DDT, excellent results were obtained with a wide variety of halogenated derivatives including acyclic, phenolic, aromatic, and steroid types. 5. Desulfuration

Sulfur can be effectively removed from a compound, organic or inorganic, by contact with Raney nickel. The action has been described by Aubry (118) as being noncatalytic in nature. One atom of sulfur is removed from sodium thiosulfate in the cold, yielding sodium sulfite from which the sulfur can be completely removed a t 100". Although sulfur can be completely removed from stannous sulfide, it can be only partially removed from antimony sulfide. Following are some of the inorganic compounds from which sulfur removal by use of Raney nickel has been observed (119). NazS

+ 2Hz0 + Ni(Hn)

+ 2NaOH + ( n / 2 + l ) H z + 2NaOH + Ha0 + NazS03+ (n/2)Hz + NazSOs + (3n/2 - 3)Hz + 3ILO

NiS NiS Ni(Hn) 3 NiS Ni(H,) -+ 3NiS Ni(H,) 3 NiS 3Ni(H,) 3 NiS 3Ni(H,) 3 NiS Ni(H,) 3 NiS

+ Ni(H,)

Na2S03 Na2S203 NazS40e NatSaO, Ass3

+ + + + SbzSa+ SnS +

3

+ Na2SOa+ ( n / 2 - 1 ) I L + NiAsS + (n/2)Hz + 2NiSbS + (n/2)Hz + Sn(OH)2 + (n/2 + 1)H2

Raney nickel acts to remove sulfur from organic compounds even in the cold (119,120). The use of Raney nickel for the removal of thiophene from benzene and methylthiophene from toluene has also been reported. Mozingo (20) has shown with a wide variety of compounds that Raney nickel catalyst in the presence of a solvent and with only its adsorbed hydrogen can cleave either reduced or oxidized sulfur from the remainder of the molecule a t a moderate temperature. Two courses for the reaction can be postulated: RSR'

+ Ni(H)-

+ RR + R'R' RH + R'H

,-+(A) RR'

+(B)

Similar postulates can be written for the reactions of disulfides, sulfoxides, and sulfones. Only reaction B was observed by Mozingo, who used sufficient catalyst to contain a large excess of adsorbed hydrogen. It is of interest to note the ease with which a carbon-sulfur bond in an aromatic sulfide is cleaved. Simple refluxing of the compound in a n ethanol suspension of Raney nickel is sufficient t o effect hydrogenolysis,

445


the conditions being so mild that the ring does not react with the hydrogen. Diphenyl sulfide yields benzene in 68% yield, and di-p-tolydisulfide yields toluene in 87 % yield. A similar case of cleavage is observed in oxidized sulfur compounds. The cleavage of oxidized sulfur compounds has also been reported by Shah et al. (121), who converted J-acid to 6-amino-1-naphthol, and by Kenner and Murray (122), who converted alkyl esters of p-toluenesulf onic acid t o the corresponding alcohols. A new method for the transformation of a carboxyl group t o a methyl group has been developed (123,124). I n an intermediate step of this method, a thiol ester is cleaved with Raney nickel t o the alcohol. The method may be outlined as follows: Raney

-CO2H

----t

-COSCHa y+

nickel

Raney

-CH&H

+ -CH,I

---+ nickel

-CHa

Note that the method also involves the use of Raney nickel for dehalogenation. The method is also applicable t o the reduction of a single carboxyl group of a polycarboxylic acid. Wolfrom and Karabinos (125) CHaSH

HOzC(CHZ)iaCOzH 4 CH~OCO(CHZ),~COZH -+ CHaOCO(CH2)14COCl--

Py ridine

Raney

CH30CO(CHz) irCOSCH3 7- CH,OCO(CHJ 14CH20H nickel

developed a similar procedure a t about the same time t o receive the aldehyde. Spero, McIntosh, and Levin (126) using W-1 Raney nickel catalyst, in like manner converted a thiol ester to the alcohol, receiving also traces of aldehyde. With W-4 catalyst (127,128) the thiol ester (I) was rapidly OCHO CH-CH2COSEt

c6c'":I,?

4

Cholane alcohol

+ Cholttnic aldehyde

HCO I

I1

111

and quantitatively reduced and desulfurized to the alcohol (11). When a catalyst deactivated in boiling acetone (126-128) was used, the aldehyde (111)was obtained in good yield. Miescher and Heer (129) have received aldehydes from the reaction in water and alcohols from the reaction in absolute alcohol.

446


A novel method of converting a carbonyl t o a methylene group has been reported by Wolfrom and Karabinos (70). The carbonyl group is converted to the mercaptal or mercaptol which is hydrogenolyzed over Raney nickel to give the methylene group. This method has found applications in the fielda, among others, of steroids (130-132) and streptomyces antibiotics (133).

Mozingo postulated two courses by which Raney nickel might react with an organic sulfur compound to split out sulfur, Reaction A (p. 444) was not observed by Mozingo, but shortly afterwards Campaigne (134) showed such a Wurtz type of reaction to occur. The thiocarbonyl group in thioacetophenone was found to react with W-2 Raney nickel to form a carbon-carbon double bond. Bergmann (135), seeking to form 9-methylphenanthrene, treated phenanthrene-9-thioaldehyde with Raney

CH3

nickel. The sole product was that of coupling. Hauptmann et al. (136,137), using a Raney nickel catalyst which had been freed of hydrogen

HS

\

\

\

CH=CH

/ /

by heating under a vacuum at 200°, found that all mercaptals of benzaldehyde gave stilbene in varying yields. Benzaldehyde diethylmercaptal gave a 70% yield of stilbene, and the dibenzylmercaptal gave dibenzyl. When the diphenyl mercaptal was used, some diphenyl sulfide was formed in addition to stilbene. Diphenyl sulfide was formed from formaldehyde diphenyl mercaptal in 71 % yield. Di-p-tolysulfide and di-9-naphthylsulfide are similarly made. The formation of thioethers under these conditions was observed only when the sulfur was directly bound to an aromatic ring. When benzaldehyde dibenzylmercaptal was used, only stilbene and sym-diphenylethane could be isolated. In one of the early papers (119) describing work in the field there was reported the hydrogenolytic cleavage of simple sulfides, disulfides, and mercaptans. Snyder and Cannon et al. (138,139), studying the desul-

447


furation of disulfides, found evidence of carbon-to-carbon cleavage in the fact that, when RSCHzCHzSR is reacted with Raney nickel, not only ethane but also methane is formed. Karrer (140) synthesized methyltocols using cleavage of a sulfide in the final step. King and Campbell

CHs

CHI

(141) used W-1 catalyst for the thiohydrogenolysis of a thiourea derivative of a steroid, and Rosenkranz et al. (142) used W-2 catalyst for the hydrogenolysis of 3-thioenol ethers of A4-3-ketosteroids. Mozingo, Folkers, and co-workers (143) used the desulfuration reaction in the determination of the structures of biotin and its derivatives. 0

0

/I

/I

HN%H

H h ’ h

I 1 1 1 ’ /I-‘CH214C02H ,,,pH

c-c’ ‘s

lH

-+

,,/

c-c

A H 3

,,,pH

bHz[CHz]4COzH

Blicke and Sheets (144), using Raney nickel in sodium carbonate solution, were able to eliminate the sulfur from a thianaphthene.

Modest and Szmuszkovicz (145) applied the same reaction to make naphthalene and phenanthrene derivatives, and Papa, Schwenk, and Ginsberg (60e) used the reaction to cleavage thiophene derivatives utilizing, however, Raney alloy and aqueous alkali in the reaction mixture. Thiohydrogenolysis with Raney nickel has been used as a tool in the elucidation of structures of organic compounds. The technique has been used in establishing the configuration of streptose (146), the pyranose structure of ethyl-1-thio-a-d-mannoside (147) and the structure of tetraacetylpolygalitol (148). 6. Hydrogenations without the Use of Added Hydrogen

In one of the earlier communications (8) dealing with a preparation of Raney nickel catalyst, it was shown that this catalyst when mixed with

448

E U G E N E LIEDER AND FR E D L. MORRITZ

nitrobenzene or p-nitrophenol in an open beaker deoxygenates the compounds with 38 to 50% yields of azo- and azoxybenzene. Although the mechanism of the reaction is obscure, it is probably safe to assume that the hydrogen held by the catalyst surface plays some part in the reaction. Bougault et al. (19) ch im that 1 g. of Raney nickel catalyst holds the equivalent of 140 ml. of hydrogen. With Raney nickel catalyst and its large amount of loosely held hydrogen, a large number of organic compounds can be hydrogenated without adding hydrogen from a n external source. Orchin (149) has described a novel method for the preparation of small quantities of tetrahydroanthracene from 9,lO-dihydroanthracene or from anthracene. PROCEDURE

To about 0.54 g. of 9,lO-dihydroanthracene in 25 ml. of ethanol, 8 g. of Raney nickel catalyst was added. After refluxing for 2% hours in an atmosphere of nitrogen, the mixture was filtered and the filtrate was concentrated. On cooling, about 0.33 g. of 1,2,3,4-tetrahydroanthracene was obtained. The mother liquor, when treated with 0.3 g. of s-trinitrobenzene, yielded about 0.25 g. of the 1,2,3,4-tetrahydroanthracene-trinitrobenzene complex. The total yield was 81% of the theoretical. When about 0.34 g. of anthracene in 50 ml. of ethanol was treated with 10 g. of Raney nickel, a 71 % yield of the same product was obtained. Mozingo et al. (143) has hydrogenated several organic compounds, using only the hydrogen contained on the catalyst. The following is a typical procedure. PROCEDURE

Five grams of cyclopentanone, about 25 g. of Raney nickel catalyst, 100 ml. of ethanol, and 35 ml. of water were refluxed for 2 hours. The nickel was removed by filtration, and the product extracted with benzene. The benzene and the alcohol were carefully distilled through a fractionating column, the residue being taken as the reduction product. From half the residue and 7.0 g. of 3,5-dinitrobenzoyl chloride in pyridine, 5.1 g. of cyclopentyl-3,5-dinitrobenzoatewas obtained. The yield was 61 % of the theoretical. The process is not applicable to simple aromatic compounds such as benzene and toluene. Table VIII summarizes the results. I n the authors’ laboratory, approximately 3 g. of W-6 Raney nickel catalyst was mixed with 5 ml. of cyclohexene in a test tube. A vigorous reaction took place a t once. Mass spectrographic analysis showed a 43 % conversion to cyclohexane (156).

449


TABLE VIII Hydrogenations without Added Hydrogen' Hydrogen acceptor Toluene Cyclopentanone Hydrazobenzene Azoxybenaene Ethyl acetoacetate Benzalacetone Benzaldehyde Acetone Eugenol Cholesterol

Product No reaction Cyclopentanolb N-ethylnnilinec N-eth ylaniline" Ethyl-p-hydroxybutyrate 4-Phenyl-2-butanol Toluene Isopropanol Dihy droeugenol" Incomplete reduction

Yield, %

61 43 36 96d 73 78 78 75

Abstracted from Moringo, Spencer, and Folkers, J. Am. Chem. SOC.,66, 1859 (1944). Isolated as the 3,5-dinitrobenzoate and as the p-nitrobenroate. C Isolated as the hydrochloride. d Ten grams of acceptor was used with 50 g. of catalyst. I n the case of eugenol a n d of rholesterol 2 g. of acceptor was used with 25 g. of catalyst. I n the other cases 5 g. of acceptor was used along with 25 g. of catalyst. * Isolated as the p-nitrohensoate. 0

b

7. Miscellaneous

a. Ozonide Decomposition. Cook and Whitmore (150) have found that Raney nickel reacts with ozonides t o give aldehydes or ketones and nickel oxide. The reaction is vigorous at 35". The yields obtained are comparable t o those obtained by the less convenient method of Fischer. I n the actual procedure, 75 g. of the nonenes prepared by the dehydration of methylethylneopentylcarbinol was ozonized. The ozonide, dissolved in 200 ml. of pentane, was slowly added t o 48 g. of Raney nickel in 100 ml. of pentane. Upon being stirred overnight with the catalyst, the mixture showed no test for the ozonide. The yields of aldehydes and ketones were about 75 %. Boord and his co-workers (151) used this reaction as a proof of the identity of a reaction product believed to be vinylcyclopropane. The

aCH=CH~ carbonyl compound was obtained in 8%' yield. It is assumed th a t the remainder of the carbonyl compound was hydrogenated or decarbonylated by contact with the hot catalyst. b. R a n e y Nickel in a n Acid M e d i u m . Adkins (7) has claimed that "acids may not be used with base metal catalysts," Noble metals have

450


almost exclusively been used when an acid medium is desired. Wenner (152), however, has reported the use of Raney nickel for catalytic hydrogenations in solutions with pH as low as 3. For the hydrogenation of Schiff bases of type I to the corresponding alcohols of type 11, Raney RzNCHzC[CH31zCHO

R2NCHzC[CHa]zCHzOH

I

I1

nickel was not only found to be an efficient catalyst but it proved superior to the noble metal catalysts. When noble metal catalysts were used for the hydrogenation of the hydrochlorides of amines of type I, only poor yields of the alcohols resulted. In all cases some hydrogenolysis of the amine portion took place. With Raney nickel, however, the hydrogenations proceeded smoothly and high yields of the alcohols were obtained. The pH of the hydrochlorides in aqueous solution varies from about 3.5 to 4.5. The results are important in the sense that they show that Raney nickel catalyst, contrary to generally accepted opinion, can be used efficiently for hydrogenations in distinctly acid solutions. Strongly acid solution containing a large amount of free mineral acid cannot, of course, be used. c. Catalysis of Hydrazine Decomposition. It has been postulated that hydrazine in the presence of platinum black decomposes according to the following equation: 2NzH4-t 2NHa

+ Nz + HZ

I t was found, however, by Irrera (153) that the volume of gas obtained from the decomposition of hydrazine in the presence of Raney nickel is always greater than that indicated by the equation above. An increase in the amount of catalyst increases the quantity of gas obtained, which approaches the volume required by the following equation : 3NzHd 4 2NHs

+ 2N2 + 3Hz

d. Reduction of a Hydroperoxide. A recent patent (154) describes the application of Raney nickel for the catalytic reduction of hydroperoxidcs to give alcohols of the type R2ArCOH. The hydroperoxide of cumene is converted to dimethylphenylcarbinol. Platinum and cobalt are also described as catalyzing the reaction. e. Decarboxylation. Raney nickel has been shown to catalyze the decarboxylation of 2-furanacrylic acid in quinoline (155). When 0.1 of the catalyst is used, only trace amounts of vinylfuran are obtained, but when 3% of the catalyst is used, the yield is 41%. Several types of catalyst were examined in this study, but Raney nickel appeared to give the best yield.

T H E USES O F RANEY NICKEL

451

REFERENCES 1. Raney, Murray, (a) U. S. Patent 1,563,787 (Dec. 1, 1925); (b) U. S. Patent

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38.

1,628,191 (May, 1927); (c) U. S. Patent 1,915,473 (June, 1933); (d) Canadian Patent 315,299 (Oct., 1934); (e) French Patent 729,357 (Dec., 1932). German Patent 408,811 (Apr., 1923). British Patent 282,112 (Dec., 1927). Russian Patent 38,127 (1933). Schroter, R., (a) Newer Methods of Preparative Organic Chemistry, p. 61, Interscience Publishers, New York, 1948; (b) Angew. Chem. 64, 229, 252 (1941). Paul R., Bull. SOC. chim. France 7, 296 (1940). Adkins, H., Reactions of Hydrogen, Wisconsin Press, Madison, Wis., 1937. Covert, L. W., and Adkins, H., J . Am. Chem. SOC.64, 4116 (1932). Mozingo, R., Org. Syntheses 21, 15 (1941). Pavlic, A. A., and Adkins, H., J . Am. Chem. SOC.68, 1471 (1946). Adkins, H., and Billica, H. R., J . Am. Chem. SOC.70, 695 (1948). DelBpine, M., and Horeau, A., Compt. rend. 201, 1301 (1935). Fluchaire, M., and Chambret, F., Bull. soc. chim. France 11, 22 (1944). Smith, H. A., Bedoit, W. C., Jr., and Fuzek, J. F., J . A m . Chem. SOC.71, 3769 (1949). Adkins, H., and Krsek, G., J . Am. Chem. SOC.70, 412 (1948). Pattison, J. N., and Degering, E. F., J . Am. Chem. SOC.72, 5756 (1950). Paul, R., Bull. S O C . chim. France 1946, 208. Bernstein, S., and Dorfman, L., J . Am. Chem. SOC.68, 1152 (1946). Bougault, J., Cattelain, E., and Chabrier, P., Bull. SOC. chim. France 6, 1699 (1938). Mozingo, R., Wolf, D. E., Harris, S. A., and Folkers, K., J . Am. Chem. SOC.66, 1013 (1946). DeMpine, M., and Horeau, A,, Bull. SOC. chim. France 4, 31 (1937). Reichstein, T, and Gatzi, K., Helzf. Chim. Acta 21, 1497 (1938). Aubry, J., Bull. S O C . chim. France 6 , 1333 (1938); 4, 1974 (1937). Heublen, R., Deut. Parfum. Ztg. 21, 30, 63 (1935) [see ref. 61. Ipatieff, V. N., and Pines, H., J . Am. Chem. SOC.72, 5320 (1950). Fanta, P., J. Am. Chem. SOC.72, 5343 (1950). Pattison, J. N., and Degering, E. F., J . Am. Chem. Soc. 73, 486 (1951). Dupont, G., and Piganiol, P., Bull. soc. chim. France 6 , 322 (1939). (a) Faucounau, I,., Bull. SOC. chim. France 4, 63 (1937); (b) Fischer, F., Ber. 67, 253 (1934); (c) U. S. Patent 2,166,183. Faucounau, I,., Bull. soc. chim. France 4, 58 (1937). Paul, R., and Hilly, G., (a) Bull. soc. chim. France 6 , 218 (1939); (b) Compt. rend. 206, 608 (1938). Lieber, E., and Smith, G. B. L., J . Am. Chem. SOC.68, 1417 (1936). Jayme, G., and Sartre, M., Ber. 77, 248 (1944). Heilman, R., Dubois, J. E., and Beregi, L., Compt. rend. 223, 737 (1946). Reasenberg, J. R., Lieber, E., and Smith, G. B. L., J . A m . Chem. SOC.61, 384 (1939). Scholnik, S. S., Reasenberg, J. R., Lieber, E., and Smith, G. B. L., J. Am. Chem. Soc. 63, 1192 (1941). Voris, S., and Spoerri, P., J . Am. Chem. SOC.60, 935 (1938). Decombe, J., Compt. rend. 222, 90 (1946).

452

EUGENE LIEBER A N D FRED L. MORRITZ

39. Samuelson, G. S., Garik, V. L., and Smith, G. B, L., J . Am. Chem. SOC.72, 3872 (1950). 40. Levering, D. R., and Lieber, E., J . Am. Chem. SOC.71, 1515 (1949). 41. Levering, D. It., Morritz, F.L., andLieber, E., J . Am. Chem. SOC.72, 1190 (1950) 42. Kentre, A , , and Nauta, W. W., Rec. trav. chim. 64, 159 (1945). 43. Campbell, K. N., and O’Connor, M. J., J. A m . Chem. SOC.61, 2897 (1938). 44. Paty, M., Compt. rend. 220, 827 (1945). 45. Ungnade, H. E., and McLaren. A. D., J . Am. Chem. SOC.66, 118 (1944). 46. Ungnade, H. E., and Nightingale, D. V., J . Am. Chem. SOC.66, 1218 (1944). 47. Chabrier, P., and Sekera, A,, Compt. rend. 226, 819 (1948). 48. Newman, M. S., Underwood, G., and Renoll, M., J . Am. Chem. SOC.71, 3362 (1949). 49. Heilmann, R., and Beregi, L., Compt. rend. 223, 904 (1846). 50. Burnette, L. W., J . Sci. Instruments 19, 9 (1944); C. A . 39, 831 (1945). 51. DelBpine, M., Compt. rend. 224, 1396 (1947). 52. Smith, H. A,, and Fuzek, J. F., J . A m . Chem. SOC.72, 3454 (1950). 53. Smith, €1. A., Fuzek, J. F., and Meriwether, H. T., J. Am. Chem. SOC.71, 3765 (1949). 54. Zafiriadis, Z., Compt. rend. 230, 452 (1950). 55. Mastagli, P., and Zafiriadis, Z., Compt. rend. 226, 184 (1948). 56. Cornubert, R., and Phelisse, J., Compt. rend. 227, 1131 (1948). 57. Blout, E., and Silverman, D., J . A m . Chem. SOC.66, 1442 (1944). 58. Hilditch, T. P., and Pathak, S. P., Proc. Roy. SOC.(London) A198, 323 (1949). 59. Ehrhart, G., Chem. Ber. 82, 387 (1949). 60. (a) Papa, D., Srhwenk, E., and Whitman, B., J . Org. Chem. 7, 587 (1942). (h) Whitman, B., Wintersteiner, O., and Schwenk, R., J . B i d . Chem. 118,792 (1937). (c) Schwenk, E., Papa, D., Whitman, B., and Ginsberg, H., J . Org. Chem. 9, 1 (1944). (d) Papa, D., Schwenk, E., and Rreiger, I€., J . Org. Chem. 14, 366 (1949). (e) Papa, D., Schwenk, E., and Ginsberg, H., J . Org. Chem. 14, 723 (1949). 61. Martin. E., in Organic Reactions, edited by Roger Adams, Vol. I, p. 155, John Wiley & Sons, New York, 1942. 62. (a) Friedlander, P., and Lurht, P., Ber. 26, 3028 (1893). (b) Claus, A,, Ber. 10, 1303 (1877). (c) Friedlander, P., and Kielbasinski, W., Ber. 28, 1535 (1895); 29, 1978 (1896). 63. Ashida, N., J . Agr. Chem. SOC.Japan 20, 621 (1944); C. A . 44, 7780 (1950). 64. Kleiderer, E. C., Pharmaceutical Activities of the I. G. Farbenindustrie Plant,

65. 66. 67. 68.

Hochstam Main, U. S. Department of Commerce, Ofice of the Publication Board, Report P B 981 (1945). Iileiderer, E. C., and Kornfeld, E. C., J . Org. Chem. 13, 455 (1948). Baker, R., and Adkins, H., J. Anz. Chem. SOC.62, 3305 (1940). Oppenauer, R., Ree. truv. chim. 66, 137 (1937). Schwenk, E., Papa, D., Whitman, B., and Ginsburg, H., J . Org. Chem. 7, 587

69. 70. 71. 72. 73.

Mozingo, R., Spencer, C., and Folkers, K., J . Am. Chem. SOC.66, 1859 (1944). Wolfrom, M., and Karabinos, J., J . Am. Chem. SOC.66, 909 (1944). Delkpine, M., and Horeau, A,, Bull. SOC. chim. Prance 4, 1524 (1937). Tomkuljak, D., Chem. Zvesti 3, 209 (1949); C.A. 44, 7245 (1950). Ashida, K., and Bebiko, T., J . Agr. Chem. SOC.Japan 23, 174 (1949); C . A . 44,

(1942); 9, 175 (1944).

7781 (1950).


453

103. 104. 105.

DelQpine, M., and Hanegraaff, C. J., Bull. S O C . chim. France 4, 2087 (1937). Palfray, L., and Sabetay, S., Compt. rend, 208, 109 (1939). Paul, R., Compt. rend. 208, 1319 (1939). Paul, R., Bull. SOC. chim. France 6 , 1592 (1938). Gault, H., Palfray, L., and Hsu, P., Compt. rend. 209, 999 (1939). Tishchenko, V. E., Grekhnev, M. A,, and Eliseeva, A. A., J . Applied Chem. (U.S.S.R.) 14, 393 (1941); C.A. 36, 2537 (1941). Rutovskii, B., and Mulyar, P., J . Applied Chem. (U.S.S.R.) 14, 173 (1941); C.A. 36, 442 (1942). Reeves, W. and Adkins, H., J . Am. Chem. SOC.62, 2874 (1940). Raymond, E., and Moretti, J., Compt. rend. 222, 893 (1946). Moretti, J., Compt. rend. 226, 188 (1948); Bull. SOC. chim. France 1948, 1154. Harlay, V., Compt. rend. 224, 568 (1947). Mosettig, E., and Duvall, H., J . Am. Chem. SOC.69, 367 (1938). Robinson, J., and Snyder, H., Orq. Syntheses 23, 68 (1943). Mignonac, G., Compt. rend. 172, 223 (1921). Cope, A., and Hancock, E., J . Am. Chem. Soc. 64, 1503 (1942). Emerson, W., and Walters, P., J . Am. Chem. Soc. 60, 2023 (1938). Emerson, W., and Mohrman, H., J . Am. Chem. SOC.62, 69 (1940). Emerson, W., and Uraneck, C., J . A m . Chem. SOC.63, 749 (1941). Emerson, W., Reed, S., and Merner, It., J . Am. Chem. SOC.63, 751 (1941). Schwoegler, E., and Adkins, H., J . Am. Chem. Soc. 61, 3499 (1939). Couturier, P., Compt. rend. 207, 345 (1938). Winans, C. F., J . Am. Chem. SOC.61, 3566 (1939). Henze, H., and Humphreys, D., J . A m . Chem. SOC.64,2878 (1942). Metayer, M., Bull. SOC. chim. France 1948, 1093. DelEpine, M., and Horeau, A,, Bull. SOC. chim. France 6, 339 (1938). Durland, J. R., and Adkins, H., J . Am. Chem. SOC.60, 1501 (1938). Wieman, J., and Laude, G., Compl. rend. 226, 345 (1948). Paul, R., Bull. SOC.chim. France 4, 1115 (1937). Bryson, A., and Dwyer, F. P., J . Proc. Roy. SOC.N . S. Wales 74, 471, 9 C.A. 36, 3250 (1941). Caldwell, A,, and Jones, E., J . Chem. SOC. 1946, 599. Rockett, J., and Whitmore, F., J . Am. Chem. SOC.71, 3249 (1949). Metayer, M., Compt. rend. 226, 457 (1947); 226, 500 (19t.8); Ann. chim. (12) 4,

106. 107. 108. 109. 110. 111. 112.

Busch, M., and Stowe, H., Ber. 49, 1063 (1916). Kelber, C., Ber. 60, 305 (1917). Whitmore, W., and Revukas, A., J . Am. Chem. SOC.62, 1687 (1940). Paty, M., Bull. SOC. chim. France 6 , 1600 (1938). Anglade, M., Bull. SOC.chim. France 6, 473 (1939). Winans, C. F., J . A m . Chem. SOC.61, 3564 (1939). Neumann, F. Sommer, N., Kaslow, C., and Shriner, R., Org. Syntheses 26, 45

74. 75. 76. 77. 78. 79.

80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99. 100. 101. 102.

196 (1949).

(1946). 113. (a) Grigorovskii, A., J . Gen. Chem. (U.S.S.R.) 17, 1124 (1947); C.A. 42, 1593 (1948). (b) Grigorovskii, A., and Federov, V., J . Applied Chem. (U.S.S.R.) 21, 529 (1948); C.A. 43, 646 (1949). 114. Schwab, W., Ezperientiu 3, 28 (1947); C.A. 41, 4366 (1947). 115. Papa, D., Ginsberg, H., and Schwenk, E., Ind. Eng. Chem., Anal. Ed. 16, 576 (1943).

454

E U G E N E LIEBER AND FRED L. MORRITZ

Spivey, A., and Curd, F., J . Chem. SOC.,1949, 2656. Pesez, M., and Pokier, P., Bull. sac. chim. France, 1948,379. Aubry, J., Bull. SOC. chim. Prance 6, 1333 (1938). . . Bougault, J., Cattelain, E., and Chabrier, P., Bull. sac. chim. France 7, 781 (1940). 120. Bougault, J., Cattelain, E., and Chabrier, P., Compt. rend. 208, 657 (1939); Bull. sac. chim. France 6, 34 (1939); 7, 780 (1940). 121. Shah, K., Tilak, B., and Venkataraman, K., Proc. Indian Acad. Sci. A28, 142 (1948); C.A. 44, 3958 (1950). 122. Kenner, G., and Murray, M., J . Chem. Sac. 1949, Suppl. Issue, No. 1, S178. 123. Prelog, V., Norymberski, J., and Jeger, O., Helv. Chim. Acta 29, 360 (1946); Jeger, O., Norymberski, J., Szpilfogle, S., and Prelog, V., ibid., 29, 684 (1946). 124. Ruzicka, L., Szpilfogel, S., and Jeger, O., Helv. Chim. Acta 29, 1520 (1946). 125. Wolfrom, M., and Karabinos, J., J . Am. Chem. Sac. 68, 724, 1455 (1946). 126. Spero, G., McIntosh, A., Jr., andLevin, R., J . Am. Chem. SOC.70, 1907 (1948). 127. McIntosh, A., Jr., Meinzer, E., and Levin, R., J . Am. Chem. SOC.70, 2955 (1948). 128. McIntosh, A., Jr., Searcy, A., Meinzer, E., and Levin, R., J . Am. Chem. SOC. 71, 3317 (1949). 129. Miescher, K., and Heer, J., U. S. Patent 2,494,253 (Jan., 1950). 130. Bernstein S., and Dorfman, L., J . Am. Chem. Sac. 68, 1152 (1946). 131. Hauptmann, H., J. Am. Chem. Sac. 69, 562 (1947). 132. Huffman, M., and Lott, M., J . Am. Chem. Sac. 71, 719 (1949). 133. Kuehl, F., Flynn, E., Brink, N., and Folkers, K., (a) J . Am. Chem. Sac. 68, 2096 (1946); (b) ibid. 68,2405 (1946); (c) ibid. 70, 2085 (1948). 134. Cline, J., Campaigne, E., and Spies, J., J. Am. Chem. Sac. 66, 1136 (1944). 135. Bergmann, F., and Israelashwili, S., J . Am. Chem. SOC.67, 1951 (1945). 136. Hauptmann, H., Wladislaw, B., and Camargo, P., Ezperientiu 4, 385 (1948); C.A. 43, 2596 (1949). 137. Hauptmann, H., and Wladislaw, B., J . Am. Chem. SOC.72, 707 (1950). 138. Snyder, H., Howe, E., Cannon, G., and Nyman, M. A., J . Am. Chem. Sac. 66, 2211 (1943). 139. Snyder, H., and Cannon, G., J . Am. Chem. SOC.66,155 (1944). 140. Karrer, P., and Dutta, P., Helv. Chim. Acta 31, 2080 (1948). 141. King, L. C., and Campbell, J. A,, J . Am. Chem. SOC.71,3556 (1949). 142. Rosenkranz, G., Kaufman, D., and Romo, J., J . Am. Chena. SOC.71,3689 (1949). 143. (a) Vigneaud, V. du, Melville, D., Folkers, K., Wolf, D., Mozingo, R., Keresztesy, J., and Harris, S., J . Biol. Chem. 146, 475 (1942) ; (b) Harris, S., Mozingo, R., Wolf, D., Wilson, A,, Arth, G. E., and Folkers, K., J . Am. Chem. Sac. 66, 1800 (1944); (c) Harris, S., Mozingo, R., Wolf, D., Wilson, A., and Folkers, K., J . Am. Chem. Sac. 67, 2102 (1945). 144. Blicke, F., and Sheets, D., J . Am. Chem. Sac. 70, 3768 (1948). 145. Modest, E., and Szmuszkovicz, J., J . Am. Chem. Sac. 72,577 (1950). 146. Wolfrom, M., and DeWalt, C., J . Am. Chem. SOC.70, 3148 (1948). 147. Fried, J., and Walz, D., J . Am. Chem. Sac. 71, 140 (1049). 148. Richtmeyer, N., Carr, C., and Hudson, C., J . Am. Chem. SOC.66, 1477 (1943). 149. Orchin, M., J. Am. Chem. Sac. 66, 535 (1944). 150. Cook, N., and Whitmore, F., J . Am. Chem. SOC. 63,3540 (1941). 151. Volkenburgh, R., Greenlee, K., Derfer, J., and Boord, C., J . Am. Chem. SOC. 71, 3595 (1949). 152. Wenner, W., J. Org. Chem. 16, 301 (1950). 116. 117. 118. 119.


455

153. Irrera, L., SOC. ital. progress0 sci. Atti 27th Riunione,Bologna 6,357 (1939); C.A. 34, 314 (1940). 154. Lorand, E., and Reese, J., U. S. Patent 2,491,926 (Dec., 1949). 155. Breault, E., and Dermer, O., Proc. Oklahoma Acad. Sci. 28, 82 (1948); C . A . 43, 2615 (1949). 156. Lieber, E., Bernstein, R. B., and Morritz, F. L., unpublished results.


Author Index Numbers in parentheses are reference numbers. They are included to assist in locating references in which the authors’ names are not mentioned in the text. Numbers in italics refer to the page on which the reference is listed at the end of each article. For ezample: Adarns, E. Q . . 196(73), 216‘ indicates that this author’s article is reference 73 on page 196 and is listed in the bibliography on page 216.

A

Atroshenko, V. L., 227(2), 293 Aubry, J., 419, 420, 444, 451, 454

Abel, E., 343, 353 B Adadurov, I. E., 223, 227, 293 Adamova, V. A., 285(410), 308 Badger, A. E., 5, 35 Adams, E. Q., 196(73), 216 Biickstrom, H., 324, 352, 353 Adcock, W. A., 359, 383 Bag, A. A., 271, 293 Adkins, H., 386, 388, 392, 394, 395, Bagdasar’yan, Kh. S., 218(12), 293 414,’415, 418(7), 419, 421(11), 424, Baker, R., 430(66), 432(66), 452 430(66), 432(7, 66), 434, 436, 438, Bakhshiev, A. A., 292(215), 301 447(8), 449, 451, 452, 453 Bakr, A., 53, 115 Albert, C. G., 182(29), 215 Balandin, A. A., 220, 221(27), 223, 224, Allan, R., 94, 117 225, 226, 227, 233, 234(26), 235, 267, Allolio, R., 227, 294 268(23, 25, 29), 269, 270, 273(15), Ally, A., 5, 35 276, 278, 290, 292(110), 293, 294, Almquist, J. A., 3, 17, 18, 20, 35, 36 297, 300 Alpert, D., 139(22), 174 Amundson, N. R., 197, 198, 216 Ballod, A. P., 289, 294 Bang, J., 54, 115 Andersen, V. S., 346, 348, 349(33a), 353 Bardeen, J. W., 93, 107, 117 Anderson, R. B., 19, 36, 355, 356(4), 357, Barrer, R. M., 107, 117, 151, 161(29), 175 360(15), 362, 363, 364, 365(21, 22, 4, Barrett, C. S., 78, 83, 116 5, 23, 24, 25), 367(4, 25), 368(4, 26), Barrett, H., 48, 62, 114 371(27), 374(28, 29, 21, 31), 380(28, Basford, P. R., 15(33), 36 29), 383 Andrew, K. F., 58, 59, 115, 122(1), Bashkirov, A. N., 278, 294 143(27), 152(30), 155(27), 159(27), Batuev, M. I., 262(304), 272(304, 92), 296, 30.4 161(43), 162(49), 168(51, 52), 174, Batuev, N. I., 274(151), 298 175 Bazzoni, M., 43, 44(12), 114 Anglade, M., 441, 453 Bean, E. H., 8, 35 Angstrom, K., 62, 115 Anokhin, 262 Bebiko, T., 433, 452 Antipina, T. V., 222(10), 255, 295 Beck, F. H., 71, 116 Antsus, L. I., 283(292), 304 Bedoit, W. C., Jr., 419(14), 451 Bedworth, R. E., 158(36), 175 Aristoff, E., 380, 382, 384 Beebe, R. A., 18, 36, 103, 104, 106, 117 Armit, H. W., 404, 415 Arrhenius, S., 330, 331, 353 Beeck, O., 14, 35, 69, 78, 105, 116, 117, Ashida, K., 430, 433, 452 226, 227, 294 Aston, F. A., 48, 114 Beilby, G. T., 221(38), 294 457

458

AUTHOR INDEX

Beinlich, A. W., 3, 36 Belen'kaya, A. P., 269(198), 300 Bell, R. T., 184, 916 B h a r d , J., 5, 35, 97, 117 Benedetti-Pichler, A. A,, 41, l l 4 Benson, H. E., 381(34), 384 Benton, A. F., 96, 97, 117, 178, 183, 184, 214, 216, 291, 294 Beregi, L., 421(34), 423(49), 461, 462 Bergman, W., 76, 116 Bergmann, E. D., 413, 416 Bergmann, F., 446, 464 Berlin, T. S., 275(225, 227), 301 Bernstein, S., 419, 446(130), 448(156), 451, 464, 466 Bessalow, P., 181, 216 Bezprozvannaya, F., 272(237), 3 O I Billica, H. R., 418(11), 419, 421(11), 424,

461 Birnie, A., 48, 62, l l 4 Biscoe, J., 104, 106, 117 Bitepazh, Yu. A., 280(40), 289(40), 294 Black, C. A., 17, 20, 36 Blackburn, P., 124, 174 Blanchard, A. A., 403, 409, 416 Blears, J., 139(23), 174 Blicke, F., 447, 464 Blout, E., 424, 425, 438, 462 Blumer, M., 183, 216 Boas, W., 75, 116 Bodenstein, M., 312, 316(5), 321(1 6), 334(16), 351, 352, 353 Bogdanova, 0. K., 269(32, 33), 294 Bohart, G. S., 196(73), 216 Bohlbro, H., 14, 15, 16(28), 36 Bone, W. A., 183, 216 Boord, C., 449, 464 Borisov, P., 264, 310 Borisov, P. P., 220(392), 284, 294, 308 Borisova, G . P., 290(216), 301 Boudart, M., 16, 36 Bougault, J., 419, 432, 444(119), 446 (119), 448(19), 461, 464 Bowles, J. S., 75, 116 Boyer, R. Q., 51, 52(38), 55, 56, 57, 60, 61, 116 Bradley, R., 49, 114 Brattain, W. H., 93, 117 Braude, G., 277, 294, 360, 383

Bray, W. C., 182, 185(58), 186, 191(58), 216, 343, 365 Breault, E., 450(155), 466 Bredig, G., 227, 294 Breiger, H., 426(60), 427(60), 462 Brewer, L., 17, 36 Bridger, G. L., 3, 36 Bridgman, P. W., 72, 116 Brill, R., 7, 25, 36,36 Brink, N., 446(133), 464 Britton, G. T., 53, 116 Brooks, R. E., 392, 416 Brown, C., 86, 117 Browning, L. C., 32, 37 Briick, L., 78, 116 Bruijn, H. de, 26, 37 Brunauer, S., 6, 10, 13, 16, 21,22, 24, 26, 36,36, 70, 85, 86, 116, 179, 214, 356, 383 Bruns, B., 189(64), 216,277, 294,360,383 Brusov, I. I., 267(18, 19), 293 Bryson, A., 439, 463 Bulanova, T. F., 272(158), 299 Burgers, W. J., 75, 116 Burke, J. E., 70, 116 Burmistvova, M. S., 281(108), 297 Burnette, L. W., 423, 462 Butlerov, A. M., 219, 279(44), 280, 286, 294 Burwell, R. L., 178(5), 214 Busch, M., 440, 463 C

Cabrera, N., 93, 94, 98, 99, 11 7 Caldwell, A., 439, 463 Calvin, M., 408, 416 Camargo, P., 446(136), 464 Campaigne, E., 446(134), 464 Campbell, J. A., 447, 464 Campbell, K. N., 422, 462 Campbell, W. E., 130, 174 Cannon, G., 446, 464 Carr, C., 447(148), 464 Cattelain, E., 419, 444(119), 446(119), 448(19), 451, 464 Cawood, W., 67, 116 Cerny, J., 76, 116 Chabrier, P., 419, 423, 444(119), 446 (119), 448(119), 461, 452, 464

AUTHOR INDEX

Chambret, F., 418(13), 451 Chapman, D . L., 316, 353 Charachorin, F., 180(16, 17), 214 Charlot, G., 262, 264, 294 Charskaya, K. N., 280(189), 300 Chel’tsova, M. A., 284, 294 Cherkasov, A. S., 286(69), 295 Cherniak, M. I., 267(205), 300 Chernyshewski‘i, N. G., 284, 303 Chinaeva, A. D., 269(414), 309 Chipalkatti, V., 54, 115 Chipman, J., 124, 174 Chow, B. F., 412, 415 Chrisman, I., 182(31), 216 Christiansen, J. A., 21, 36, 311, 315(3, 4, 7), 319(11, 12), 320, 322(9), 326(19), 331, 334(7), 337(7), 345(30), 346(33a), 349(33a), 351, 352, 353 Churbakov, A. N., 280, 294 Ciapetta, F. G., 380, 382, 384 Clark, C. L., 5, 35 Cline, J., 446(134), 454 Cohen, M., 48, 62, 114 Cohn, E. M., 358, 374(31), 383 Coleman, G. W., 403, 415 Conant, J. B., 412, 415 Cook, N., 449, 454 Cope; A., 435, 453 Cornubert, R., 424, 452 Corson, B. B., 281, 294 Couturier, P., 436, 455 Covert, L. W., 418, 447(8), 461 Cowap, M. D., 402, 403, 409, 416 Craig, L. E., 412, 415 Craig, R. E., 51, 52(38), 53(53), 55, 56, 57, 60, 61, 115 Craxford, S. R., 277, 278 Cremer, E., 239, 260, 264, 294 CrBtien, A., 21, 36 Crittenden, E. C., 3, 5, 7, 35 Crittenden, E. D., 78, 116 Crone, H. G., 279(258), 309 Crowell, J. H., 381(34, 35), 384 Cunningham, B. B., 42, 45, 55(5), 114 Curd, F., 443(116), 454

D Damkohler, G., 179, 214 Danby, C. J., 197, 198, 203, 207(78), 208, 210, 216

459

Daniel, V., 8, 35 Danilov, N. I., 291(52), 294 Danilov, S. N., 218, 280, 282(53), 283, 294, 295

Dankov, P. D., 221, 222, 226, 228, 233, 295, 304,306

Darling, S., 337, 553 Dashkovskaya, F. A., 269(198), 500 Davis, C. F., 213(82), 216 Davond, J. G., 197(78), 198, 203, 207, 208, 210, 216 De Boer, F., 5, 36 De Boer, J. H., 5, 36, 190, 215 Debus, H., 385, 414 Decombe, J., 421, 461 Degering, E. F., 419, 420, 451 DelBpine, M., 418(12), 419, 421, 422, 423, 432, 433, 438, 4.51, 452, 453 Del’tsova, 266(442), 310 Demenev, N. V., 221(376), 307 Dement’eva, V. V., 292(215), 301 Derfer, J., 449(151), 454 Dermer, O., 450(155), 4.56 DeWalt, C., 447(146), 454 DeWitt, B. J., 124, 174 DeWitt, T. W., 360, 383 Dieter, W. E., 362, 363, 364, 383 Dintses, A. I., 292(60), 295 Dixit, R., 78, 116 Dobinski, S., 72, 116 Dobranski‘i, A. F., 282(389), 288, 295, 508

Dobychin, I).P., 225 Doladugin, A. I., 287(63), 296 Dolgov, B. N., 218(64), 219, 269, 270, 271(64), 280, 286, 291, 296 Donau, J., 41, 47, 49, 51, 56, 114, 116, 133(20), 174 Dorfman, L., 419, 446(130), 461, 454 Dornte, R. W., 161(48), 175 Dowden, D. A., 264, 295, 380, 384 Dowden, L., 104, 117 Drane, H., 53, 115 Draper, H. D., 182, 216 Drost-Hansen, W., 345, 353 Dubinin, M. M., 196(74), 216 Dubois, J. E., 421(34), 451 Dufraisse, 324, 352, 353 Duncan, J. F., 90, 11 7 Dunkerly, F. I., 124, 174

460

AUTHOR INDEX

Dunn, C. G., 75, 116 Dunn, J. S., 158(39), 176 , Dunn, R.C., 53, 115 Dupont, G., 228, 295,420, 461 Durland, J. R., 438, 465 Dushman, S., 63, 115 Dutta, P., 447(140), 464 Duvall, H., 434, 463 Dwyer, F. P., 439, 463 D’yakova, M. K., 220 Dzis’ko, V. A., 227(5), 293

E Eastman, E. D., 18, 36 Echeistova, A. I., 233(363b), SO6 Eckell, J., 183, 212, 215 Eckstrom, H. C., 359, 583 Edgecombe, L. J., 182, 216 Edse, R., 179, 21.4 Edwards, 0. S.,8, 55 Egorova, G. M., 283(394), 287(394), SO8 Egupov, T. P., 271(11), 293 Ehmann, E. A,, 406, 416 Ehrhart, G., 425, 462 EIdus, Ya. T., 226, 276, 277, 278, 279, 295,295,296 Elagina, N. V., 277(85, 86), 296 Elam, C. F., 72, 116 von Elbe, G., 168(50), 176 Eliseeva, A. A., 434(79), 463 Element, N. I., 259, 296 Elliott, M. A., 365(25), 367(25), S83 Ellis, M., 213(82, 83), 216 Elofson, R. M.,412, 416 Elovich, S. Ye., 239, 262, 264(98), 296 Elovich, S. Yu., 239, 262, 291, 296,SO6 Elowitz, C. U., 180(15), 181(15, 19), 214 Elowitz, S., 180(16, 17), 214 El’tekov, A. P., 286(99, loo), 296 Emerson, W., 435, 436, 437, 45.9 Emich, F., 41, 42, 47, 51, 53, 62, 114,116 Emmett, 1’. H., 2, 10, 13, 14, 16, 17, 18, 22, 25, 26, 32, 55,36,S7, 85, 86, 116, 117, 179, 214, 223, 236, 237, 296,300, 356, 360, S83 Engelder, C. I., 182, 183, 216 Engelhardt, F., 278, 299 Epifanskir, P. F., 220(392), 277(86), 296, 308

Erchak, M., 183, 215 Ercoli, R., 411, 416 Erivanskaya, L. A., 234(145b), 267(145), 298 Ershov, N. I., 279(91, 92), 296 Estermann, I., 78, 116 Evans, E., 123, 174 Evans, M. G., 49, 114 Evans, R. M., 18, 36 Evans, U. R., 91, 117 Eventova, M. S.,284(41), 294 Everett, D. H., 197(78), 198, 203, 207, 208, 210, 216 Ewens, R. V. G., 405, 415 Eyring, H., 174(53), 175,331, 332, 553

F Fankuchen, I., 183(46), 215 Fanta, P., 420, 451 Farkas, A., 63, 115 Farnsworth, H. E., 99, 117,227, SO7 Fast, J. D., 124, 139(24), 174 Faucounau, L., 420, 461 Favorskaya, T. A., 281 Favorskii, A. E., 220, 280, 281, 296,297 Feder, E. A., 267(372), SO7 Federov, V., 442(113b), 463 Fedotova, 0. Ya., 282(208), SO1 Feldman, J., 371(27), 383 Felgentriiger, W., 45, 46, 56, 114, 115 Fertelmeister, Ya. N., 44(17), lf4 Feuer, I., 62, 115 Field, E., 217 Field, J. H., 381(34), 384 Filaretov, A. N., 271(311), 273(311), 304 Filatova, N. N., 292(366), SO7 Finch, G., 78, 116 Finkelstein, V., 182(31), 216 Fischer, F., 276, 297,355, 364, 365(20), 383, 420(29), 461 Flid, P. M., 234(145a), 267(145), 298 Flinn, E., 446(133), 454 Fluchaire, M., 418(13), 461 Flugge, S., 239, 294 Fokin, S. A., 262, 291(117), 297 Folkers, K., 419(20), 432(69), 446(133), 447, 448(143), 449, 451,452,464 Fontana, M. G., 71, 116 Fplrland, K., 99, 117

461

AUTHOR INDEX

Foss, o., 345, 353 Frank, F. C., 98, 117 Frankenburg, W. G., 2, 18, 20, 21, 35, 36, 100, 117 Frankenburger, W., 22, 36 Frazer, J. C. W., 179, 181(21, 22), 182, 183, 184(22, 49), 185(58, 61), 186, 187, 188, 191(58), 212, 214, 215 Freidlin, L. Kh., 227, 265(114, 115), 266(113), 292, 297 Frenkel’, Ya. I., 221, Fricke, R., 228, 297 Fridman, G. A., 292(110, 111, 112), 2997 Fridman, S. D., 234(145a), 267(145), 298 Fried, J., 447(147), 454 Friedel, R. A., 404, 405, 410, 415 Friedlander, P., 428(62), 452 Friedrich, A., 43, 44(11), 11.4 Frost, A. V., 220, 222, 235, 236, 255, 271, 279, 288, 289(123, 124), 290, 293, 297, 301, 303, 305 Frumkin, A. N., 222, 239, 242, 297 Fry, H. S., 315(2), 352 Fuzek, J. F., 419(14), 424(52, 53), 451, 452

G Gal’pern, G. D., 270, 302 Garik, V. L., 421, 452 Garner, W E., 182, 183(26), 185(62, 63), 189, 212, 213, 215, 216 Garrett, A. B., 161, 165, 175 Gast, T., 62, 115 Gatzi, K., 419, 451 Gault, H., 433(78), 453 Geil, G. G., 153, 161, 175 Gershenovich, M. S., 234(126), 297 Giammarco, G., 27, 30, 37 Gibbs, J. W., 314, 352 Giles, C., 54, 116 Gilmont, R., 409, 416 Ginsberg, H., 426(60), 427(60), 428, 432(68), 442(115), 447, 462, 453 Ginzberg, A. S., 268(127), 297 Gladshtein, B. M., 300 Glasstone, S., 174(53), 176 Glazebrook, R., 131, 17.4 Godnev, I. N., 292(366), 307

Goggs, A. B., 185(60), 215 Gol’dfel’d, Ya. M., 234(171), 299 Golovanova, M. V., 284, 298 Golovina, 0. A., 287, 304 Golumbic, N., 365(22), 383 Goodeve, C., 361, 383 Gorbach, G., 43, 47(24), 114 Gorin, I. K., 280(130), 297 Gorin, Yu. A., 280(190), 297, 300 Gracheva, E. P., 281(368), 307 Gragerov, I. P., 292(428), 309 Grant, G. A., 191(67), 192(71, 67), 198(67), 199(69), 216 Grant, K., 46, 47, 51, l I 4 Gray, T. J., 182, 183(26, 27), 189. 212(26), 216 Greenfield, H., 395, 396, 399, 400, 401, 409, 412, 413, 415 Greenlee, K., 449(151), 464 Gregg, S. J., 15, 35, 62, 65, 72, 115, 116 Greider, C . E., 181(21), 214 Grekhnev, M. A,, 434(79), 453 Grigorovich, N. M., 227(6), 293 Grigorovskii, A., 441, 442(113b), 453 Grosse, A. v., 272(131), 297 Gruenewald, I

ADVANCES IN CATALYSIS VOLUME 5, Volume 5

Advances in Geophysics, Volume 5

Advances in Immunology Volume 5

Advances in Computers, Volume 5

Advances in Cycloaddition. Volume 5

Advances in Agronomy, Volume 5

Advances in Dendritic Macromolecules, Volume 5, Volume 5

ADVANCES IN CATALYSIS VOLUME 11, Volume 11

ADVANCES IN CATALYSIS VOLUME 39, Volume 39

ADVANCES IN CATALYSIS VOLUME 16, Volume 16

Advances in Catalysis, Volume 38

Advances in Catalysis, Volume 27

Advances in Catalysis, Volume 52

Advances in Catalysis, Volume 48

ADVANCES IN CATALYSIS VOLUME 2, Volume 2

ADVANCES IN CATALYSIS VOLUME 7, Volume 7

ADVANCES IN CATALYSIS VOLUME 10, Volume 10

ADVANCES IN CATALYSIS VOLUME 1, Volume 1

ADVANCES IN CATALYSIS VOLUME 14, Volume 14

Advances in Catalysis, Volume 34

Advances in Catalysis, Volume 46

Advances in Catalysis, Volume 29

Advances in Catalysis, Volume 32

Advances in Catalysis, Volume 37

Advances in Catalysis, Volume 42

Advances in Catalysis, Volume 28

Advances in Catalysis, Volume 33

Advances in Catalysis, Volume 24

Advances in Catalysis, Volume 23

Advances in Catalysis, Volume 23

Advances in Catalysis, Volume 31

ADVANCES IN CATALYSIS VOLUME 5, Volume 5

Advances in Geophysics, Volume 5

Advances in Immunology Volume 5

Advances in Computers, Volume 5

Advances in Cycloaddition. Volume 5

Advances in Agronomy, Volume 5

Advances in Dendritic Macromolecules, Volume 5, Volume 5

ADVANCES IN CATALYSIS VOLUME 11, Volume 11

ADVANCES IN CATALYSIS VOLUME 39, Volume 39

ADVANCES IN CATALYSIS VOLUME 16, Volume 16

Advances in Catalysis, Volume 38

Advances in Catalysis, Volume 27

Advances in Catalysis, Volume 52

Advances in Catalysis, Volume 48

ADVANCES IN CATALYSIS VOLUME 2, Volume 2

ADVANCES IN CATALYSIS VOLUME 7, Volume 7

ADVANCES IN CATALYSIS VOLUME 10, Volume 10

ADVANCES IN CATALYSIS VOLUME 1, Volume 1

ADVANCES IN CATALYSIS VOLUME 14, Volume 14

Advances in Catalysis, Volume 34

Advances in Catalysis, Volume 46

Advances in Catalysis, Volume 29

Advances in Catalysis, Volume 32

Advances in Catalysis, Volume 37

Advances in Catalysis, Volume 42

Advances in Catalysis, Volume 28

Advances in Catalysis, Volume 33

Advances in Catalysis, Volume 24

Advances in Catalysis, Volume 23

Advances in Catalysis, Volume 23

Advances in Catalysis, Volume 31

Recommend Documents