Preparation of Catalysts: 2nd: International Symposium Proceedings (Studies in Surface Science and Catalysis)

Studies in Surface Science and Catalysis 3 PREPARATION OF CATALYSTS II Scientific Bases for the Preparation of Heterog...

Author: B. Delmon | P. Grange | P.A. Jacobs | G. Poncelet

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Studies in Surface Science and Catalysis 3

PREPARATION OF CATALYSTS II Scientific Bases for the Preparation of Heterogeneous Catalysts

Volume 1

Preparation of Catalysts 1. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium held a t the Solvay Research Centre, Brussels, October 14-17, 1975. Second Impression edited by B. Delmon, P. Jacobs and G. Poncelet

Volume 2

The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon

Volume 3

Preparationof Catalysts II. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-la-Neuve, September 4-7, 1978 edited by B. Delmon, P. Grange, P. Jacobs and G. Poncelet

Explanation of the cover design The figure gives a pictorial representation of surface analysis techniques. Eight basic input probes are considered, which give rise t o one or more o f four types o f particles that leave the surface carrying information about it to a suitable detector. The input probes can be particle beams of electrons, ions, photons, o r neutrals or nonparticle probes such as thermal, electric fields, magnetic fields or sonic surface waves. All o f the input probes (with the exception of magnetic fields) give rise t o emitted particle beams, i.e. electrons, ions, photons, or neutrals. The various surface analysis techniques can therefore be classified according to the type of input probe and the type of emitted particle (e.g. electrons in, ions out; thermal in, neutrals out, etc.). In analyzing the emitted particles, one can consider four possible types of information; identification of the particle, spatial distribution, energy distribution and number. Any or all of these forms o f information are then used t o develop a better understanding o f the surface under study.

Studies in Surface Science and Catalysis 3

PREPARATION OF CATALYSTS II Scientific Bases for the Preparation of Heterogeneous Catalysts Proceedings of the Second International Symposium, Louvain-la-Neuve, September 4-7,1978

Editors

B. Delmon, P. Grange, P. Jacobs and G. Poncelet

ELSEVl ER SCI ENTl FIC PUBLISHING COMPANY 1979 Amsterdam - Oxford - New York

ELSEVIER SCIENTIFIC PUBLISHING COMPANY 335 Jan van Galenstraat P.O. Box 211, 1000 AE Amsterdam, The Netherlands

Distributors for the United States and Canada: ELSEVIER/NORTH-HOLLAND INC,

52, Vanderbilt Avenue New York, N.Y. 10017

Library of Congress Cataloging In PublIcatIon D a t a

Main entry under title: Preparation o f catalysts 11. (Studies in surface science and catalysis j 3) Includes bibliographical references and index. 1. Catalysts--Congresses. I. Delmon, Bernard. 11. Series. QD505.P722 91l.395 79-1397

ISBN 0-444-41733-8

ISBN 0444417338 (Vol. 3) ISBN 0-44441801.6 (Series) 0 Elsevier Scientific Publishing Company, 1979 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical. photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Scientific Publishing Company, P.O. Box 330,1000 AH Amsterdam, The Netherlands. Printed in The Netherlands

V

CONTENTS Organizing Committee Foreword Acknowledgements Financial Support

The design d catalysts (D.L. Trim)

VIII IX XI XI11

1

Effect of impregnation and activation conditions of A1203/Cu0 supported monolith catalysts in the reduction of NO (M. Engler, K. Unger, 0. Inacker, C. Plog and M. Seidl)

29

Effect of pretreatment on activity, selectivity and adsorption properties of a Fischer-Tropsch-catalyst (G. Lohrengel, M.R. Dass and M . Baerns)

41

The influence of impregnation, drying and activation on the activity and distribution of CuO on a-alumina (f4. Kotter and L. Riekert)

51

A study of the chemisorption of chromium(V1) , molybdenum(V1) and tungsten(V1) onto y-alumina (A. Iannibello, S. Marengo, F. Trifiro' and P . L . Villa)

65

Dispersion and compound formation in some metathesis catalysts and fluorine containing alumina's, studied by XPS and Laser-Raman spectroscopy (F.P.J.M. Kerkhof, J.A. Moulijn, R. Thomas and J.C. Oudejans)

77

Reduction of silica supported nickel catalysts (J.W.E. Coenen)

89

Interaction of nickel ions with silica supports during depositionprecipitation (L.A.M. Hermans and J.W. Geus)

11.

Crystallite size distributions and stabilities of homogeneously deposited Ni/SiO2 catalysts (J.T. Richardson, R.J. Dubus, J.G. Crump, P. Desai, U. Osterwalder and T.S. Cale)

131

The preparation and pretreatment of coprecipitated nickel-alumina catalysts for methanation at high temperatures (E.C. Kruissink, L.E. Alzamora, S . Orr, E.B.M. Doesburg, L.L. van Reijen, J.R.H. Ross and G. van Veen)

143

Controlled catalyst distribution on supports by co-impregnation (E.R. Becker and T.A. Nuttall)

159

Nulticomponent chromatographic processes during the impregnation of alumina pellets with noble metals (L.L. Hegedus, T.S. Chou, J.C. Summers and N.M. Potter)

171

Factors controlling the retention of chlorine in platinum reforming catalysts (S. Sivasanker, A.V. Ramaswamy and P. Ratnasamy)

185

Preparation of alumina or silica supported platinum-ruthenium bimetallic catalysts (G. Blanchard, H. Charcosset, M.T. Chenebaux and M. Primet)

197

VI

Preparation of catalysts by adsorption of metal complexes on mineral oxides (J.P. Brunelle)

211

Analysis of steps of impregnation and drying in preparation of supported catalysts (V.B. Fenelonov, A.V. Neimark, L.I. Kheifets, and A.A. Samakhov)

233

Some mechanistic correlations between impregnation and activation operations for the preparation of high-selectivity supported metal catalysts (H. Shingu and T. Inui)

245

The impregnation and drying step in catalyst manufacturing (G.H. van den Berg and H. Th. Rijnten)

265

Study of the interaction of Fe203-Mo03 with several supports (L. Cairati, M. Carbucicchio, 0. Ruggeri and F . Trifiro)

279

Relationship between average pore diameter and selectivity in iron-chromium-potassium dehydrogenation catalysts (Ph. Courty and J.F. Le Page)

29 3

Preparation of highly-dispersed ruthenium on magnesium oxide supports : comments on the advantages of non-aqueous catalyst preparations (L.L. Murrell and D.J.C. Yates)

307

Catalyst activation by reduction (N. Pernicone and F. Traina)

321

The activation of iron catalyst for ammonia synthesis ( A . Barahki, M. Xagan, A, Pattek, A . Reizer, L.J. Christiansen and H. Topsgfe)

353

Preparation and characterization of small iron particles supported on MgO (H. Tops#e, J.A. Dumesic, E.G. Derouane, B.S. Clausen, S . Mgfrup, J. Villadsen and N. Topsgfe)

365

Ruthenium catalysts for ammonia synthesis prepared by different methods (A. Ozaki, K. Urabe, K. Shimazaki and S. Sumiya)

381

Limiting factors on the structural characteristics of Ru/Si02 and RuFe/SiOz catalysts (L. Guczi, K. Matusek, I. Manninger, J. Kirdly and M. Eszterle)

39 1

Preparation aspects of Ru-supported catalysts and their influence on the final products (A. Bossi, F. Garbassi, A. Orlandi, G. Petrini and L. Zanderighi)

405

E1.G.

High surface area oxide solid solution catalysts (A.P. Hagan, Lofthouse, F . S . Stone and M.A. Trevethan)

417

Tentative classification of the factors influencing the reduction step in the activation of supported catalysts (B. Delmon and M. Houalla)

4 39

Preparative chemistry of cobalt-molybdenum/alumina catalysts (A. Iannibello and P.C.H. Mitchell)

469

The influence of the support on Co-Mo hydrodesulfurization catalysts (H. Topsgfe, B.S. Clausen, N. Burriesci, R. Candia and S. Mgfrup)

479

Study of some variables involved in the preparation of impregnated catalysts for the hydrotreatment of heavy oils ( 0 . Ochoa, R. Galiasso and P. Andreu)

49 3

VII

Preparation and properties of thiomolybdate graphite catalysts (G.C. Stevens and T. Edmonds)

507

Catalyst stabilization and deactivation compared with catalyst preparation (R E flontarnal)

519

..

Preparation and properties of monodispersed colloidal metal hydrous oxides (E. Matilevid) 555 Process for the production of spherical catalyst supports (R.M. Cahen, J.M. Andre and H.R. Debus)

585

Methods of saturation with alkali ions. Influence of the properties of oxides (R. Hombek, J. Kijenski and S. Malinowski) 595 Mechanism of formation of a catalytically active phase in the reaction of Cr02C12 with silica gel (D. Mehandjiev, S. Angelov and D. Damyanov)

605

The use of transport reactions for_dispersing supported species (R. Haase, H.-G. Jerschkewitz, G. Ohlmann, J. Richter-Mendau and J. Scheve)

615

Synthesis of new catalytic materials : metal carbides of the group VI B elements (L. Leclercq, K. Imura, S. Yoshida, T. Barbee and M. Boudart)

627

A novel method for the preparation and production of skeleton catalysts (J. Petrd)

641

MINISYMPOSIUM ON CATALYST NORMALIZATION

Normalization of catalyst test methods (L. Moscou)

659

Mesopore determination from nitrogen sorption isotherms: fundamentals, scope, limitations (J.C.F. Broekhoff)

663

Metal surface area and metal dispersion in catalysts (J.J.F. Scholten)

685

Progress report on the work of the SCI/IUPAC/NPL working party on catalyst reference materials (C.C. Bond, R.L. MOSS, R.C. Pitkethley, K.S.W. Sing and R. Wilson)

715

Organization and functions of ASTM Committee D-32 on catalysts (A.H. Neal)

719

Measurement of the activity of solid state catalysts (G.K. Boreskov)

723

The Council of Europe Research Group on Catalysis (E.G. Derouane)

727

Concluding remarks List of participants Author index

729 735 761

VIII ORGANIZING COMMITTEE Members

Dr. s.P.S.

ANDREW, Imperial Chemical Industries

Ltd., Great Britain. Dr. H. BOHLBRO, Haldor Topsbe, Denmark. Dr. R. CAHEN, Labofina S.A., Belgium. Prof. J.W.E.

COENEN, Unilever Research,

The Netherlands. Prof. B. DELMON, Universite Catholique de Louvain, Be 1gium

.

Prof. B. DEROUANE , Universite de Namur, Belgium. Prof. J.W. HIGHTOWER, Rice University, Houston, Texas, U . S . A . Dr. P. LAMBERT, Union Chimique Belge, Belgium. Dr. A. LECLOUX, Solvay et Cie., Belgium. Dr. J.L. LE PAGE, Institut Franpais du Petrole, France. Dr. R. MONTARNAL, Institut Francais du Petrole, France. Dr. L. MOSCOU, AKZO-Chemie Nederland, The Netherlands. Prof. E. PLUMAT, Universite Libre de Bruxelles, Belgium. Secretaries

Dr. P. GRANGE, Universite Catholique de Louvain, Belgium. Dr. P.A. JACOBS, Katholieke Universiteit Leuven, Belgium. Dr. G. PONCELET, Universite Catholique de Louvain, Belgium.

IX FOREWORD As a result of the First International Symposium on "The Scientific Bases for the Preparation of Heterogeneous Catalysts" it clearly appeared to the local organizers that a large international audience was interested by the topics treated.

However, the scope of the

symposium was probably too large, and possibly did not allow sufficient discussion and interaction between participants. It was clear from the beginning that for the Second Symposium a smaller number of scientific domains involved in the preparation of industrial heterogeneous catalysts should be discussed. The scientific program was set up by an Organizing Committee formed by representatives of industries, universities and research institutes.

This committee was composed by experts in the field of catalyst preparation from different countries. At the first meeting of the Organizing Committee, it was felt that the symposium should focus on two unit processes, IMPREGNATION and ACTIVATION of supported catalysts, and more particularly on

:

the chromatographic effect, transport in pores, calcination, activation by reduction and sulfidation, carrier effects and compound formation.

It was also decided that new trends in the preparation of real

catalysts should be included in the scientific program as they were in the First Symposium. Over 90 papers treating these topics were submitted.

The Organizing

Committee at a second meeting selected 36 communications which fitted most closely the imposed topics.

I t was decided not to discuss the

selected unit processes separately but rather to structurate the program sessions around distinct groups of catalysts treating impregnation and activation.

The symposium topics were introduced by four

plenary lectures and three extended communications, each of them delivered by scientists who, with a remarkable cooperation spirit, accepted to adapt their contribution to the audience and to the subject and style of the Symposium. The Organizing Committee also felt that the standardisation of methods of catalysts characterization remained a topic of much interest, and that progress in international agreement and cooperation was important.

Encouraged by the success of an almost informal dis-

cussion included in the program of the First Symposium, it was found necessary to devote half-a-day to catalyst normalisation.

Three

scientific papers and reports from national and international

committees were scheduled. A round-table discussion w a s planned to evaluate the importance of a scientific approach for the impregnation and activation of catalysts.

The discussion also focused on the role of this symposium in

fostering scientific research in the field and for enabling a better interaction between industrial and fundamental investigation.

A

summary of these discussions is given by the Symposium Chairman in his concluding remarks. Approximately 350 participants representing 30 nations attended the Symposium.

Almost 60% of the participants came from industry.

IUPAC accepted to sponsor the symposium.

In his introductory

remarks IUPAC's President, Prof. G. Smets, mentioned that this sponsor ship is only awarded provided three prerequisites are fulfilled:

.

fair geographical representation of the contributors

. quality . quality

of the geographical site of the organisation.

There is no doubt that the first condition was met.

Whether the

two other ones were fulfilled, the appreciation is left to those who attended the symposium. If the participants agree that the organisation of the symposium went on smoothly, this is the result of a high number of personal efforts, which the local organizers would like to acknowledge.

P. GRANGE P.A. JACOBS G. PONCELET

XI ACKNOWLEDGEMENTS In the name of the Organizing Committee, we want to thank the Rector of the Universite Catholique de Louvain, Mgr. E. Massaux, who agreed that the Second International Symposium could be held in Louvain-la-Neuve.

For all the facilities generously provided by the

university, we also sincerely thank the Rector. The Organizers acknowledge the sponsorship of IUPAC and the benevolent appreciation of its President for the project of this 2nd International Symposium.

The introductory remarks of IUPAC's Presi-

dent, Prof. G. Smets, were also gratefully appreciated by the local Organizers, the Scientific Committee and all those who participated to the set-up of the Symposium. We also thank the Societe Chimique de Belgique, Division de Catalyse, for sponsoring the Symposium, and in particular its President, Dr. A. Lecloux, for

his welcome address.

The local Organizers are pleased to mention the exceptional and enthousiastic participation of the members of the Organizing Committee. Their work and contribution go well beyond what usually is expected from members of scientific committees of symposia:

they determined

the topics of the Second Symposium and selected the papers, they structurated the scientific program, acted as session chairmen, and animated the discussions.

This symposium was really the collective

achievement of a11 the members of the Organizing Committee.

To all

o f the members of the Organizing Committee, we convey our deepest gratitude. While preparing these Proceedings, we learned the death of one member of the Organizing Committee.

All those who knew Dr. R.

Montarnal appreciated his human qualities, enthousiasm and scientific competence. Special thanks are due to Dr. R. Cahen for his efficient help in contacting several scientists of industry and collecting financial contributions. The efforts of Dr. L. Moscou and Prof. E. Derouane, who successfully took in charge the complete organization of the Minisymposium on Catalyst Normalisation, are also greatly appreciated. We also would like to thank Prof. D.L. Trimm, Dr. J.P. Brunelle, Dr. N. Pernicone and Prof. E. Matijevic for their highly stimulating lectures.

XI1 The Organizers also acknowledge all those who submitted a paper, including those whose contribution could be not accepted.

Special

thanks are due to the authors of the papers included in the Proceedings. TWO

They were the artisans of the success of the symposium. departments of the university deserve to be congratulated:

the 'Relations Exterieures de l'Universit6 de Louvain' (REUL), and in particular Mrs. F. Bex and her team, and the 'Service de Logement' headed by Mr. E. Vander Perre. Finally, we want to acknowledge the help of all those of the 'Groupe de Physico-Chimie Minerale et de Catalyse' who, in charge of different organisational aspects before and during the symposium, contributed to its success: Dr. M.A. Apecetche, Dr. N. De Keyzer, Dr. F. Delannay, Dr. P. Gajardo, M. Genet, Dr. P. Gerard, Dr. J. Lemaitre, A. Mathieu, L. Petit, C. Pierard, D. Pirotte, M . P. Scokart and Dr. M.L.

Somme-Dubru.

Rodriguez,

XI11 FINANCIAL SUPPORT

The O r g a n i z i n g C o m m i t t e e g r a t e f u l l y a c k n o w l e d g e s t h e " M i n i s t e r e d e 1 ' E d u c a t i o n N a t i o n a l e e t d e l a C u l t u r e F r a n q a i s e " and t h e "Fonds National de l a Recherche S c i e n t i f i q u e " f o r t h e i r f i n a n c i a l guarantee. The p e c u n i a r y s e c u r i t y p r o v i d e d b y t h e S o c i 6 t 6 C h i m i q u e d e B e l g i q u e i s a l s o acknowledged.

F i n a n c i a l s u p p o r t w a s a l s o c o n t r i b u t e d by t h e f o l l o w i n g c a t a l y s t s m a n u f a c t u r e r s and companies: AKZO Nederland,

b.v.,

The N e t h e r l a n d s

BASF, W. Germany C a t a l y s t s and Chemicals Europe, Condea,

Belgium

Germany

W.

Cyanamid o f G r e a t B r i t a i n , E n g l a n d Essochem Europe,

Belgium

H a l d o r T o p s d e , Denmark H a r s c h a w C h e m i e , The N e t h e r l a n d s I m p e r i a l Chemical I n d u s t r i e s L t d . , J o h n s o n M a t t h e y a n d Co. Mallinckrodt Inc.,

Ltd.,

Calsicat Division,

Catalysts International,

S.p.A.,

U.S.A.,

and

France

M e t a l l u r g i e Hoboken-Overpelt, Montedison,

England

England

Belgium

Italy

P r o c a t a l y s e , France Solvay e t C i e . , Siidchemie A G , Unilever,

Belgium

Sparte Girdler Katalysatoren,

U n i v e r s a l Matthey P r o d u c t s L t d . , U.O.P.

L a b o f i n a S.A.

W. Germany

The N e t h e r l a n d s

Processes,

England

U.S.A.

h a s c o n t r i b u t e d t o t h e c o s t s of t h e p r i n t i n g of t h e

announcing booklets. The o r g a n i z e r s a r e i n d e b i t e d t o t h e s e c o m p a n i e s .

This page intentionally left blank

1

THE DESIGN OF CATALYSTS" D.L. TRIHt? L a b o r a t o r y o f I n d u s t r i a l Chemistry, The U n i v e r s i t y o f Trondheim, Norway

R e a l i s a t i o n o f t h e importance o f c a t a l y s t p r e p a r a t i o n t o t h e a c t i v i t y , s e l e c t i v i t y and l i f e o f a c a t a l y s t has l e d t o i n c r e a s i n g i n t e r e s t i n t h e s c i e n t i f i c basis o f d i f f e r e n t preparations.

T h i s , i n t u r n , has l e d t o improved

performance, and t o t h e n e c e s s i t y o f d e f i n i n g c a r e f u l l y what c a t a l y s t s h o u l d be prepared

-

r a t h e r than o p t i n i s i n g a c a t a l y s t t h a t

can be prepared.

T h i s paper

d e s c r i b e s a g e n e r a l approach t o t h e problem o f r e c o o n i t i o n o f which c a t a l y s t should be prepared f o r a given reaction. The d e s i g n o f a c a t a l y s t i s d i s c u s s e d i n terms o f a f l o w diagram i n which means o f i d e n t i f i c a t i o n o f m a j o r and m i n o r c o n s t i t u e n t s o f a c a t a l y s t a r e i n t e r related.

F a c t o r s i n f l u e n c i n g t h e c h o i c e o f t h e chemical c o n s t i t u e n t s a r e

c o n s i d e r e d t o g e t h e r w i t h t h e c h o i c e o f a s u p p o r t , which o f t e n depends more on the desired physical characteristics o f the catalyst.

Both t h e o r e t i c a l concepts

and e m p i r i c a l o b s e r v a t i o n s can be combined i n t h e o v e r a l l d e s i g n t o o i v e a l i m i t e d number o f p o s s i b i l i t i e s f o r e x p e r i m e n t a l t e s t i n g .

Finally, correlation

o f suggestions from t h e design w i t h c a t a l y s t preparation i s considered b r i e f l y , w i t h p a r t i c u l a r r e f e r e n c e t o i m p r e g n a t i o n and a c t i v a t i o n .

INTRODUCTION It i s w e l l known t h a t t h e a c t i v i t y and s e l e c t i v i t y o f a heterogeneous

c a t a l y s t depends on t h e i n h e r e n t a c t i v i t y o f t h e components, on t h e p h y s i c a l s t r u c t u r e o f t h e c a t a l y s t and on t h e o p e r a t i n g c o n d i t i o n s f o r t h e r e a c t i o n . T h i s has l e d t o a t t e n t i o n b e i n g focused on c a t a l y s t p r e p a r a t i o n , s i n c e t h i s i s a m a j o r p o i n t f o r c o n t r o l o f chemical c o m p o s i t i o n and p h y s i c a l s t r u c t u r e o f t h e catalyst.

F o r a l o n g t i m e c a t a l y s t p r e p a r a t i o n was r e g a r d e d as one o f t h e l a s t

s t r o n g h o l d s o f alchemy, b u t t h e advent o f modern methods o f s u r f a c e a n a l y s i s ( r e f . l),c o u p l e d t o i n c r e a s i n g s c i e n t i f i c knowledge, have e l e v a t e d t h e s u b j e c t f r o m an a r t t o a s c i e n c e ( r e f s . 2, 3 ) . As a r e s u l t , i t b e g i n s t o be p o s s i b l e t o manufacture a c a t a l y s t t o a wide v a r i e t y o f g i v e n s p e c i f i c a t i o n s . A l t h o u g h o u r c a p a b i l i t y i s f a r f r o m p e r f e c t , t h i s p o s s i b i l i t y r a i s e s new

2

I n p a r t i c u l a r , given t h a t we may know how t o prepare a c a t a l y s t , i t

questions.

i s necessary t o ask which c a t a l y s t should be prepared.

Since t h i s question

covers f a c t o r s such as chemical composition, physical s t r u c t u r e and mechanical strength, the answer may w e l l i n v o l v e o p t i m i s a t i o n o f several non-related While t h e b u l k o f the proceedings o f t h i s meeting w i l l be concerned

parameters.

w i t h how t o prepare c a t a l y s t s , t h i s paper i s focused p r i m a r i l y on how t o decide which c a t a l y s t t o prepare.

As can be seen from f i g u r e 1, the i n t e r a c t i n g requirements o f a c a t a l y s t complicate the decision.

As a r e s u l t , i t i s n o t s u r p r i s i n g t h a t most successful1

c a t a l y s t s have been developed on an empirical basis.

I n recent years, however,

procedures have been developed t o place the s e l e c t i o n o f c a t a l y s t s on a more s c i e n t i f i c footing.

Generally, these i n v o l v e a t h e o r e t i c a l study i n which

general knowledge o f c a t a l y s i s , arguments by analogy and various t h e o r e t i c a l concepts are applied t o a p a r t i c u l a r problem ( r e f s . 4 , 5, 6, 7).

c8talyst

i

nigh activity and s*lmctivity

a

.ffmct

1

The chemical m t u n o f

1

of c-ntr

rctrC

Is b l f u n c t i m l i t y

-r lchlntul

an k i U l

1

OPtlU shape

WUlrad 7

b Ikh.nica1 r t m

b Effect o f rdditirar

b Porosity c Lrs a d Imt t n n r f w

C

Stability

cood contacting b o d control

tormt fla

/

c surfau L r u

Choice o f catalyst

Figure 1.

Factors i n f l u e n c i n g c a t a l y s t s e l e c t i o n .

I t i s r e g r e t t a b l e t h a t our understanding o f c a t a l y s i s i s s t i l l n o t good enough t o guarantee success, b u t the approach has been found t o be very useful i n

e x p l a i n i n g why a given c a t a l y s t i s a c t i v e and, i n some cases, i n successfully p r e d i c t i n g new c a t a l y s t s ( r e f . 5).

I n s u f f i c i e n t knowledge i s a v a i l a b l e t o be

independent o f experimental t e s t i n g , b u t the approach acts as a valuable guide t o experimentation.

As a r e s u l t , the time needed f o r c a t a l y s t i d e n t i f i c a t i o n

and t e s t i n g can be s i g n i f i c a n t l y decreased, f o r t h e investment o f o n l y a s h o r t time on c a t a l y s t design. The procedure has been discussed i n a few papers i n recent years (refs. 4, 5, 6, 7), u s u a l l y w i t h emphasis on a given r e a c t i o n .

A more general approach i s

attempted i n the present paper, w i t h p a r t i c u l a r emphasis on those aspects of design t h a t are of i n t e r e s t i n the context o f c a t a l y s t preparation. The broader

3

framework o f t h e complete d e s i g n i s p r e s e n t e d i n r e f e r e n c e 5. I n t i t i a l stages o f t h e design. The d e s i g n process d i s c u s s e d below i s summarised i n scheme 1, and v a r i o u s e n t r y points t o the design are indicated.

The e x t e n t and depth of a s t u d y depends on t h e

e f f o r t w a r r a n t e d b y a p a r t i c u l a r a p p l i c a t i o n and, i n t h i s a r t i c l e , t h e f u l l d e s i g n

w i l l be discussed.

The s t a r t i n g p o i n t f o r t h e d e s i g n r e q u i r e s t h e d e f i n i t i o n o f t h e o b j e c t i v e and a statement, i n t h e f o r m of a chemical r e a c t i o n network, o f d e s i r a b l e o r u n d e s i r a b l e r e a c t i o n s which may o c c u r ( r e f . 1 6 ) .

Thermodynamic and economic c a l c u l a t i o n s can

t h e n be used t o e s t a b l i s h t h e most a t t r a c t i v e r o u t e , and g e n e r a l t y p e s o f c a t a l y s t t h a t s h o u l d f a v o u r i n d i v i d u a l r e a c t i o n s can be i d e n t i f i e d .

T h i s r e a c t i o n network

p r o v i d e s t h e b a s i c o u t l i n e o f t h e design, t o which i t i s p o s s i b l e t o a p p l y t h e c a t a l y s t s e l e c t i o n c r i t e r i a d e s c r i b e d below. S e l e c t i o n o f t h e chemical b a s i s o f t h e c a t a l y s t :

m a j o r components

A t t h i s p o i n t we a r e concerned w i t h t h e i d e n t i f i c a t i o n o f t h e m a j o r chemical components o f t h e c a t a l y s t , and f a c t o r s such as mechanical s t r e n g t h , s u r f a c e area and p o r o s i t y w i l l be c o n s i d e r e d a t a l a t e r staoe. Experience has shown t h a t one d i f f i c u l t y can a r i s e a t t h i s p o i n t .

Because o f o u r

s c i e n t i f i c t r a i n i n g , we t e n d t o t h i n k l i n e a r l y , r a t h e r t h a n t o c o n s i d e r a l l o f t h e possibilities that exist.

I t i s b e t t e r t o c o n s i d e r (and p o s s i b l y t o r e j e c t ) a wide

v a r i e t y o f r e a c t i o n s r a t h e r t h a n t o i g n o r e one t o t a l l y new approach. The t o o l s t h a t a r e used t o i d e n t i f y p o s s i b l e c a t a l y s t s range f r o m e m p i r i c a l o b s e r v a t i o n s t o t h e o r e t i c a l l y based c a l c u l a t i o n s . The f i r s t s t e p i n t h e process i s

4

t o t r a n s l a t e the r e a c t i o n mechanism i n terms of adsorbed intermediates and surface t h i s a l l o w close d e f i n i t i o n o f what i s required from the c a t a l y s t .

reaction:

To i l l u s t r a t e t h i s process i t i s useful t o consider a s p e c i f i c example, such as the production of limonene ( r e f . 8). This i s a member o f the terpene f a m i l y t h a t have considerable importance as solvents.

A combination of thermodynamic

and economic c a l c u l a t i o n s showed t h a t the r e a c t i o n sequence was most a t t r a c t i v e .

The scheme involves hydrogenation and dehydrogenation (catalysed by metals ( r e f . 9) o r metal oxides ( r e f . 1 0 ) ) as w e l l as alkylation-. This i s normally c a r r i e d o u t over a c i d i c catalysis ( l l ) , b u t an o x i d a t i v e coupling i s a l s o possible ( r e f . 12). For the a c i d catalysed r e a c t i o n t h e surface r e a c t i o n may be summarised as ( r e f s . 11, 13).

A

A-

A

The surface o x i d a t i o n reaction, on t h e o t h e r hand, may be w r i t t e n as ( r e f s . 5, 12): CHn-CH-CHn

t

O f course, there w i l l be many o t h e r p o s s i b i l i t i e s , b u t these s i m p l i f i e d schemes

serve t o i l l u s t r a t e the t r a n s c r i p t i o n o f the reactions t o the surface. It i s now possible t o discuss d i f f e r e n t means o f i d e n t i f y i n g a c t i v e c a t a l y s t s

5

a ) Activitv n a t t e r L By f a r the most useful guide, where they a r e a v a i l a b l e , a r e p a t t e r n s of a c t i v i t y f o r reactions of t h e same type. These may vary from the simple pattern well known f o r t h e decomposition of nitrous oxide ( f i g u r e 2 ) ( r e f . l o ) , t o t h e more complicated p a t t e r n s f o r hydrogenation ( f i g u r e 3) ( r e f . 13) and oxidation ( f i y r e 4 ) ( r e f s . 15, 1 6 ) . Generally i t i s possible t o find an a c t i v i t y pattern f o r a c l a s s of s o l i d s which c a t a l y s e a type of reaction. A c t i v i t y p a t t e r n s f o r metals catalysing reactions involving hydrogen a r e shown in f i g u r e 3 and f o r metal oxides involved i n oxidation a r e shown in f i g u r e 4.

cu20

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200

300 P-tYpe

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400

500 insulators

600

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Figure 2. A c t i v i t y p a t t e r n f o r n i t r o u s oxide decomposition (lemperature a t which d i f f e r e n t oxides c a t a l y s e t h e decomposition of N20).

Figure 3.

A c t i v i t y pattern f o r reactions involving hydrogen a t 8OoC +propane dehydrogenation ( 5 5 O O C ) A e t h y l ene hydrogenation (-1 20 t o 40OoC) Ocyclohexene disproportionation (200 t o 45OoC)

OH2/D2exchange

6

6 00

soc

A

2 !!

e

-f

Figure 4.

-light

A c t i v i t y patterns f o r oxidation o f f temperature f o r Pt/Rh gauzes doped w i t h metal oxide and used t o

catalyse. the o x i d a t i o n o f ammonia ---log r a t e o f o x i d a t i o n o f propylene a t 30OoC.

A l e s s q u a n t i t a t i v e p a t t e r n has been developed f o r metal oxides i n v o l v e d i n a c i d catalysed reactions ( f i g u r e 5 ) ( r e f . 1 7 ) , w h i l e t h e a c t i v i t y o f a b i f u n c t i o n a l c a t a l y s t can be established w i t h the a i d o f diagrams such as f i g u r e 6 ( r e f . 18). The p o s i t i o n o f the desired compound on the diagram determines the s t r e n g t h o f the metal and a c i d functions desired. I t should be remembered t h a t a c t i v i t y patterns g i v e o n l y an idea o f the

r e l a t i v e a c t i v i t y o f c a t a l y s t s , unless they r e f e r t o the s p e c i f i c r e a c t i o n i n question.

7

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Figure 5.

Orders of acidity of different materials.

cracking products

increasing strength o f acid function

Figure 6.

Bifunctional catalysis

-

sio2

8

b) Correlations o f a c t i v i t y w i t h bulk p r o p e r t i e s o f the c a t a l y s t . I n the p a s t t h e r e have been several attempts t o r e l a t e c a t a l y t i c a c t i v i t y w i t h the bulk properties o f solids.

Although these have f a l l e n i n t o disrepute w i t h the

r e a l i s a t i o n t h a t the surface may have very d i f f e r e n t properties t o the bulk, i t i s possible t o o b t a i n valuable pointers f o r c a t a l y s t design from t h e approach. Thus, f o r example, the concept o f percentage d bonding ( r e f . 19) i s open t o severe c r i t i c i s m , and y e t these are some systems i n which i t i s possible t o r e l a t e c a t a l y t i c a c t i v i t y w i t h t h i s f a c t o r ( r e f s . 20, 21).

S i m i l a r l y , attempts

t o r e l a t e adsorption and c a t a l y s i s w i t h b u l k semiconductor p r o p e r t i e s o f oxides have been reasonably successful i n e x p l a i n i n g some patterns o f c a t a l y t i c a c t i v i t y ( r e f . l o ) , and should c e r t a i n l y n o t be ignored a t t h i s stage o f the design. However, more accurate p r e d i c t i o n s should be obtained by considering t h e gass o l i d interface. c) Predictions from heats o f adsorption. I n a few cases i t i s possible t o p r e d i c t the most a c t i v e c a t a l y s t on the basis o f heats o f adsorption. This approach i s more t h e o r e t i c a l l y j u s t i f i e d i n t h a t , i f adsorption i s t o o strong, then a gas w i l l n o t be displaced from the surface o r w i l l n o t react.

If

adsorption i s t o o weak, t h e residence time o f t h e adsorbed gas on t h e surface

w i l l be t o o s h o r t t o favour reaction.

Using arguments o f t h i s type, i t has been

possible t o i d e n t i f y t h e most a c t i v e c a t a l y s t f o r the hydrogenation o f n i t r o g e n ( r e f . 22) and f o r ethylene ( r e f . 18), and s i m i l a r arguments can be used f o r analogous reactions.

P r e d i c t i o n o f optimal c a t a l y s t s does depend, however, on

t h e a v a i l a b i l i t y o f heats o f adsorption data.

A somewhat s i m i l a r approach has been used f o r o x i d a t i o n c a t a l y s i s , where various p r o p e r t i e s t h a t i n d i c a t e t h e strength o f adsorption o f oxygen have been c o r r e l a t e d with catalytic activity.

Boreskov ( r e f . 23) has measured the heat o f adsorption

of oxygen on some oxides and have a l s o measured i s o t o p i c oxygen exchange between gas and oxides ( r e f . 24):

t h e i r r e s u l t s are summarised i n f i g u r e 7.

Moro-oka

( r e f . 25), on the other hand, r e l a t e s c a t a l y t i c a c t i v i t y t o t h e heat o f formation of t h e b u l k oxide ( f i g u r e 8). It w i l l be seen t h a t a l l o f these measurements may have a s a t i s f a c t o r y t h e o r e t i c a l explanation, even when t h e authors were n o t aware of t h i s . C e r t a i n l y t h e p l o t s o f f e r a guide t o t h e s e l e c t i o n o f t h e most a c t i v e c a t a l y s t , although i t must be remembered t h a t a c t i v i t y and s e l e c t i v i t y i n o x i d a t i o n are o f t e n i n v e r s e l y related.


10

d ) P r e d i c t i o n s on t h e b a s i s o f geometric c o n s i d e r a t i o n s . One o f t h e more u s e f u l methods o f p r e d i c t i n g c a t a l y t i c a c t i v i t y a r i s e s f r o m c o n s i d e r a t i o n o f oeometric factors.

The concept t h a t t h e geometry o f t h e c a t a l y s t can a f f e c t

a c t i v i t y has been r e c o g n i s e d f o r many y e a r s , and formed t h e b a s i s of t h e m u l t i p l e t t h e o r y o f c a t a l y s i s ( r e f . 26).

It i s p a r t i c u l a r l y u s e f u l i n p r e d i c t i o n , p a r t i a l l y

because d a t a is r e a d i l y a v a i l a b l e and can be e a s i l y a p p l i e d , and p a r t l y because t h e p r e d i c t i o n s t u r n o u t t o be r e a s o n a b l y a c c u r a t e ( r e f s . 5, 6 ) . P r e d i c t i o n s a r e made on t h e b a s i s o f matching bond l e n n t h s o f adsorbed s p e c i e s w i t h c r y s t a l parameters o f c a t a l y s t .

Thus, f o r example, a l k y l a t i o n r e a c t i o n s t o

produce 1 imonene c o u l d f a v o u r t h e p r o d u c t s

- 2.09

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A I 4.33.w

I

where x i s an a d s o r p t i o n s i t e . Assuming t h a t t h e p r o d u c t s come f r o m an adsorbed c y c l w - o l e f i n and adsorbed p r o p y l e n e , i t i s p o s s i b l e t o g e n e r a t e a t a b l e o f d i s t a n c e s between a d s o r p t i o n c e n t r e s which a r e d e s i r e d o r undesired, depending on t h e modes o f a d s o r p t i o n and t h e r e a c t i o n path: Mode o f a d s o r p t i o n

D i s t a n c e between a d s o r p t i o n c e n t r e s ( A )

P r o p y l ene

Cyclodiene

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* I n terms o f a r e a c t i o n l e a d i n g t o limonene. Comparison w i t h t h e l a t t i c e parameters o f m e t a l s and o f m e t a l o x i d e s ( f i g u r e s 9 and 10) g i v e s a s t r o n g i n d i c a t i o n o f which m e t a l s o r metal o x i d e s can be c o n s i d e r e d and which s h o u l d be avoided. These t y p e o f arguments can be e a s i l y extended t o c o n s i d e r t e r r a c e s o r edges on t h e c a t a l y s t surface.

One a p p a r e n t d i f f i c u l t y l i e s i n t h e f a c t t h a t t h e s u r f a c e

s t r u c t u r e U o e i n o t always m i n o r t h e b u l k ( r e f s . 1, 27), b u t t h e f a c t remains t h a t ? r e d i c t i o n s made on t h i s b a s i s a r e o f t e n a c c u r a t e .

11

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Figure 9. L a t t i c e parameters of t r a n s i t i o n metals c o r r e l a t e d t o desired (-1 and undesired (---) values f o r production of limonene. 1.8

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Figure 10. L a t t i c e parameters of metal oxides correlated t o dcsired (-) undesired ( - - - ) values f o r 1 imonene production.

and

12 e)

Nature o f chemisorbed complexes.

Even from the s i m p l i f i e d reactions on the

surface l i s t e d above, i t i s obvious t h a t the d i r e c t i o n o f r e a c t i o n must be very dependent upon t h e nature o f the adsorbed complex.

Since there i s u s u a l l y more

than one form o f adsorption t h a t i s possible, c a t a l y s t s should be selected which can be expected t o favour the desired form. Thus, f o r example, t h e formation o f a p i bond requires the overlap o f a f i l l e d ,

2

bonding p i o r b i t a l from an o l e f i n w i t h an empty sigma (dz ) o r b i t a l o f a metal i f i t i s t o be strong. Back donation from occupied d o r b i t a l s o f the metal t o xy’ dyz t h e empty p i antibonding o r b i t a l s o f the o l e f i n i s a l s o desirable. Considering,

f o r example, square pyramidal coordination on t h e surface, such adsorbed species 2 3 d , d metals i f o n l y the D o r b i t a l s a;re involved i n

can o n l y be formed on d’,

bonding ( V (2), (3), ( 4 ) , T i ( l ) , ( 2 ) , (3), C r (3), (4), (5), e t c ) and on d8, d9, d10 metals when both D and P o r b i t a l s are involved (Fe(O), Co(O), ( l ) , N i ( 0 ) , ( l ) ,

(21, Cu (11, ( 2 ) and Zn ( 2 ) ) . S i m i l a r arguments can be a p p l i e d t o other adsorbed species t o produce a t a b l e o f s o l i d s t h a t can adsorb d i f f e r e n t reactants o r products i n the desired form. Thus, f o r example, i n considering the o x i d a t i o n o f o l e f i n s , p o s s i b i l i t i e s o f adsorption o f reactants can be summarised as i n t a b l e 1.

+

Oxygen ( r a d i c a l ) (Pi bonded)

+

.

t

+

+

+

+

+

+

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This type of approach i s very useful i n l i m i t i n g the number o f c a t a l y s t s t h a t should be considered i n the design. I t does n o t take too much time since, once a v a i l e b l e , such a t a b l e i s widely applicable. S i m i l a r arguments can be a p p l i e d t o one other, more complex, method o f assessing the importance o f d i f f e r e n t chemisorbed complexes.

This involves the

a p p l i c a t i o n o f molecular o r b i t a l c a l c u l a t i o n s t o chemisorption and c a t a l y s i s . c a l c u l a t i o n s are complex, and are n o t t o be undertaken l i g h t l y .

The

However, several

13 papers have appeared i n r e c e n t y e a r s which assess t h e p r o b a b i l i t y o f f i n d i n g a g i v e n a d s o r p t i o n f o r m on a c a t a l y s t ( r e f s . 28, 2 9 ) .

One p a r t i c u l a r u s e f u l r e v i e w

o f m o l e c u l a r o r b i t a l c a l c u l a t i o n s o f chemisorbed molecules and i n t e r m e d i a t e s i n heterogeneous c a t a l y s i s has been p u b l i s h e d by Beran and Zagradnik ( r e f . 2 8 ) , c o v e r i n g m a i n l y Russian work up t o t h e second h a l f o f 1975.

F o r t h e normal

c a t a l y s t design, i n d i v i d u a l c a l c u l a t i o n s o f t h i s t y p e a r e p r o b a b l y unrewarding: where such i n f o r m a t i o n i s a v a i l a b l e , i t can be used t o good e f f e c t . The dependence o f t h e f o r m a t i o n o f chemisorbed s p e c i e s on t h e d i r e c t i o n a l p r o p e r t i e s o f bonds emerging f r o m a s u r f a c e i n an approach w h i c h combines t h e p r e s e n t concept w i t h g e o m e t r i c e f f e c t s .

I n an e l e g a n t paper c o v e r i n g t h e

p o s s i b i l i t y o f a d s o r p t i o n on d i f f e r e n t c r y s t a l faces, Bond ( r e f . 30) has c o n s i d e r e d and ipn o r b i t a l s a r e s p a t i a l l y d i r e c t e d . As a r e s u l t , l o c a t i o n El o f p a r t i c u l a r chemisorbed s p e c i e s i n p o s i t i o n s f a v o u r a b l e t o r e a c t i o n c a n be the fact that e

envisaged, and p r e d i c t i o n s based on t h i s were found t o be a c c u r a t e .

Regrettably,

f o r t h e c a t a l y s t design, t h i s approach i s o f l i m i t e d value, i n t h a t we know l i t t l e about how we s h o u l d p r e p a r e a c a t a l y s t w i t h a d e s i r e d s t r u c t u r e , and even l e s s about how we s h o u l d s t a b i l i s e t h a t s t r u c t u r e d u r i n g r e a c t i o n . There i s no doubt t h a t c o n s i d e r a t i o n o f d e s i r e d and u n d e s i r e d chemisorbed complexes can be o f g r e a t importance t o a c a t a l y s t design.

Success depends on how

a c c u r a t e l y t h e r e a c t i o n has been t r a n s c r i b e d t o t h e s u r f a c e ( b o t h d e s i r e d and u n d e s i r e d r e a c t i o n s ) and how f e a s i b l e t h e proposed s u r f a c e r e a c t i o n s a r e .

T h i s can

o n l y be determined by advanced s u r f a c e a n a l y s i s ( r e f . 1) o r , b y analogy, f r o m e x p e r i m e n t a l t e s t i n g o f proposed c a t a l y s t s . f)

Crystal f i e l d theory.

R e l a t i v e l y modern t h e o r i e s advanced t o e x p l a i n t h e

b e h a v i o u r o f i n o r g a n i c complexes have been found t o o f f e r a good d e s c r i p t i o n o f c h e n i s o r p t i o n and c a t a l y s i s : theories ( r e f . 31).

t h e s e i n c l u d e t h e c r y s t a l f i e l d and t h e l i g a n d f i e l d

The b a s i s o f t h e t h e o r i e s l i e s i n t h e f a c t t h a t d o r b i t a l s a r e

known t o have d i r e c t i o n a l p r o p e r t i e s and, i f a t r a n s i t i o n metal i o n i s a s s o c i a t e d w i t h l i g a n d s , t h e e n e r g i e s a s s o c i a t e d w i t h t h e s e o r b i t a l s can v a r y .

The n a t u r e

o f t h e l i g a n d and t h e n a t u r e o f t h e complex ( h i g h s p i n o r l o w s p i n ) can o b v i o u s l y a f f e c t t h e e n e r g i e s , b u t t h e geometry o f t h e complex, as d i c t a t e d by t h e c o o r d i n a t i o n , i s very important.

Now t h e c h e m i s o r p t i o n o f a r e a c t a n t on a metal i o n c e n t r e can

a l s o be d e s c r i b e d as t h e f o r m a t i o n o f a complex, whether t h e i o n i s i s o l a t e d ( s a y i n s o l u t i o n ) o r i s l o c a t e d on t h e s u r f a c e o f a c r y s t a l m a t r i x .

Obviously the

geometry w i l l be more c o n s t r a i n e d by t h e g e n e r a l f o r m o f t h e m a t r i x , b u t t h e p r i n c i p l e i s t h e same. The energy changes t h a t o c c u r on f o r m a t i o n o f such “complexes” depend on many factors,

o f which t h e c r y s t a l f i e l d s t a b i l i s a t i o n energy i s one.

The f i v e degenerate

d o r b i t a l s o f t h e f r e e i o n a r e s p l i t by c r y s t a l f i e l d s o f d i f f e r e n t symmetry, and t h e amount o f t h e s p l i t t i r q is measured i n terms o f an energy parameter,

D

9’

which

14

is usually obtained from optical data. Chemisorption, the addition of a ligand to a complex, results in a change in geometry of the complex, for example from square pyramid to octahedron or from tetrahedron to square pyramid to octahedron. This, in turn, alters the crystal field stabilisation energy and calculations show that a characteristic twin peak pattern is obtained (figure 11). What is interesting is that this pattern is similar to the patterns of chemisorption and catalytic activity for many metals and oxides (figures 3 and 4 ) . Since catalysis involves reaction of the chemisorbed complex (either to a new chemisorbed molecule or to a desorbed product), it is not surprising that the same effect can be seen in both cases. Indeed, if catalysis involves transfer of an electron, rearrangement of the coordination to a different complex (which has the electrons distributed to give overall lower energy) could well be an important driving force for the reaction.

number of d electrons

Figure 11. Calculation of crystal field stabilisation energy for strong field compl exes. Arqunients on this basis provide a useful diagnostic tool for prediction and provide a sound theoretical basis for many of the observed activity patterns. A full description o f the approach is given in references 15, 31 and 32 but, for the purpose of design, the information summarised in figure 1 1 is very useful indeed. Obviously all the above approaches are very interactive, and the importance o f a given approach depends on data that is available. Some idea of the inter-

15 r e l a t i o n s h i p s between approaches can be o b t a i n e d , however, b y c o n s i d e r a t ; i o n of t h e l i n e diagram shown i n scheme 2. _ -

-

Propose adsorbed

of reaction and catalyst I1

charge transfer

Conslder geometric

Look for heat of

I

-

I

-

1 Is there any

1

1correlation with

1

1

1

-1

c

Look for a c t i v r t y patterns. etther experiwntal or theoretical

Hhat form o f adsorbed reactant o r product i s desired and which Catalyst w u l d favour this?

-r

I

1 activrtyt

,

___._ - ---

adsorption data

Select major cnponents

on Chemical grounds

Scheme 2 .

-.

.

.-I

,1

A p p l i c a t i o n o f t h e d i f f e r e n t approaches summarised above can be expected t o suggest s e v e r a l p o s s i b i l i t i e s f o r t h e b a s i s o f t h e c a t a l y s t , and t h e s e can o n l y be d i s t i n g u i s h e d by e x p e r i m e n t a l t e s t i n g .

Assuming t h a t t h e p r i m a r y p r e d i c t i o n

has suggested about t w e n t y c a t a l y s t s , o f which two o r t h r e e a r e found t o be i n t e r e s t i n g , i t i s t h e n necessary t o c o n s i d e r how t h e c a t a l y t i c a c t i v i t y l s e l e c t i v i t y can be improved by t h e a d d i t i o n o f m i n o r components. S e l e c t i o n o f t h e chemical b a s i s o f t h e c a t a l y s t b ) m i n o r components. The o b j e c t i v e o f t h i s p a r t o f t h e d e s i g n i s e a s i l y s t a t e d , i n t h a t t h e c a t a l y s t

is p e r f o r m i n g l e s s w e l l i n some r e s p e c t ( s ) t h a n i s d e s i r e d .

The q u e s t i o n t h e n i s

how t o improve t h e performance. Two approaches a r e p o s s i b l e . produces r e s u l t s .

The s i m p l i s t i c approach i s easy t o a p p l y and o f t e n

Thus, f o r example, i f t h e r e a c t i o n produces chemicals v i a a

r e a c t i o n p a t h i n v o l v i n g an excess o f one r e a g e n t ( s a y oxygen), a d d i t i o n o f a m i n o r component designed t o decrease t h e amount o f oxygen adsorbed i s e a s i l y c a r r i e d o u t ( r e f . 12).

A l t h o u g h such an approach i s p r a g m a t i c , i t u s u a l l y works.

The second approach i s more i n t e l l e c t u a l l y s t i m u l a t i n g and a l m o s t c e r t a i n l y

w i l l work:

t h e d i s a d v a n t a g e i s t h a t i s u s u a l l y t a k e s c o n s i d e r a b l e t i m e and e f f o r t .

The b a s i s o f t h e method i s t o d e l v e d e e p l y i n t o t h e mechanism o f t h e r e a c t i o n , on t h e grounds t h a t u n d e r s t a n d i n g t h e mechanism a l l o w s o p t i m a l f i n e t u n i n g o f t h e catalyst.

T h i s i s g e n e r a l l y c o r r e c t , b u t t h e r e i s a c l o s e d c i r c l e i n t h e sense

t h a t a c a t a l y s t must be o f c o n s i d e r a b l e i n t e r e s t t o w a r r a n t t h e necessary a t t e n t i o n

16

and y e t the c a t a l y s t may o n l y be o f s u f f i c i e n t i n t e r e s t once i t has been improved. As a r e s u l t , d e t a i l e d studies o f t h i s k i n d are u s u a l l y c a r r i e d o u t o n l y f o r c a t a l y s t s which a r e i n c u r r e n t use, b u t which could be improved. There are, i n f a c t , two ways o f studying t h e mechanism i n order t o f i n e tune the catalyst.

The most widely used way i s t o study reactions on the surface, using

r e c e n t l y developed a n a l y t i c a l techniques.

O f these, e l e c t r o n s p i n resonance and e l e c t r o n paramagnetic resonance spectroscopy ( r e f . 33) , together w i t h i n f r a red spectroscopy ( r e f . 34) have proved p a r t i c u l a r l y useful , w h i l e e l e c t r o n spectroscopy ( r e f . 1 ) holds o u t much hope f o r the f u t u r e .

I t should a l s o be noted t h a t i s o t o p i c

l a b e l l i n g experiments, w h i l e n o t fashionable, can be very r e v e a l i n g ( r e f . 35). The second method i s l e s s d i r e c t , b u t appears t o be very i n t e r e s t i n g .

It

involves studies o f analogues o f the c a t a l y s t , i n which i t i s p o s s i b l e t o c o n t r o l , f o r example, the l o c a t i o n o r valency o f one o f the components o f the o r i g nal catalyst.

Several such systems have now been i d e n t i f i e d , varying from so i d

s o l u t i o n s t o compounds such as scheelites ( r e f . 36), perovskites ( r e f . 37 p a l m i e r i t e s ( r e f . 39) and tungstates ( r e f . 40).

Y

The approach and i t ' s b e n e f i t s

can be i l l u s t r a t e d from a b r i e f consideration o f two o f these. The s c h e e l i t e s t r u c t u r e i s represented i n f i g u r e 12 ( a ) .

The general formula

i s AM04, w i t h the M c a t i o n being t e t r a h e d r a l l y coordinated t o oxygen.

About 100

compounds o f the general s t r u c t u r e have been i d e n t i f i e d , and some phase diagrams have been presented ( r e f . 41). As a c a t a l y s t analogue, they are very i n t e r e s t i n g i n t h a t defects, c o n s i s t i n g o f

A c a t i o n vacancies, can be introduced i n r e l a t i v e l y high concentrations (up t o onet h i r d o f t h e t o t a l o f A).

A l t e r n a t i v e l y , i t i s p o s s i b l e t o replace A cations by a

second cation, 6, t o g i v e the general s t r u c t u r e

A B 0 M04 X Y Z

where 0 = defect and ( x t y t z ) = 1.

As a r e s u l t , i t i s possible t o generate an

analogue which contains two cations a t known p o s i t i o n and i n known coordination, w i t h o r without c o n t r o l l e d amounts o f defects. This analogue was used t o advantage i n an elegant study o f t h e o x i d a t i o n o f o l e f i n s by S l e i g h t and L i n n ( r e f . 36).

I n v e s t i g a t i n g the mode o f a c t i o n o f bismuth

molybdate, the analogue showed t h a t defects promoted the formation o f a l l y 1 r a d i c a l s , w h i l e t h e r o l e o f bismuth appears t o be mainly t o r e p l e n i s h the a c t i v e s i t e w i t h oxygen. Coupled w i t h o t h e r d i r e c t observations o f the c a t a l y s t ( r e f s . 42, 43), the r o l e of a d d i t i v e s i n t h e system was established i n terms o f the various processes t h a t occured during the o v e r a l l reaction. S i m i l a r arguments were used t o develop perovskite c a t a l y s t s , b u t here there i s a d d i t i o n a l i n t e r e s t i n t h a t t h e m a t e r i a l s are i n t e r e s t i n g c a t a l y s t s i n t h e i r own r i g h t ( r e f s . 37, 38).

The c r y s t a l s t r u c t u r e i s very close t o cubic ( f i g u r e 12 ( b ) ) ,

containing l a r g e anions (A = La, Nd, P r , Cay S r o r Ba) and B ions held i n an

17 octahedral c o n f i g u r a t i o n .

I n a d d i t i o n t o being useful analogues, t h i s series

o f compounds i s o f g r e a t i n t e r e s t i n the automobile exhaust gas clean up f i e l d , where Ru and Rh are s t a b i l i s e d i n the perovskite s t r u c t u r e . There i s no doubt t h a t d e t a i l e d studies are needed t o understand the mode o f a c t i o n of a d d i t i v e s , and t h a t such studies can a l s o be used t o p r e d i c t which a d d i t i v e s could be u s e f u l .

However, the time needed t o be invested i s high, and

one must be sure t h a t the c a t a l y s t w i l l be useful before undertaking the e f f o r t . Where t h i s i s n o t c e r t a i n , i t i s probably best t o use a blend o f empiricism (based, where possible, on s i m i l a r c a t a l y s t s ) and as deep a l e v e l o f thought as i s possible, based on the proposed mechanism. This l a t t e r process has been i l l u s t r a t e d f o r d i f f e r e n t c a t a l y s t s designs ( r e f s . 4, 5, 6, 12).

.A

0 0

.--

Figure 12.

Crystal s t r u c t u r e s o f s c h e e l i t e s ( a ) and perovskites (b)

18

S e l e c t i o n o f t h e p r e f e r r e d form o f t h e c a t a l y s t . The morphology o f a c a t a l y s t i s o f i n t e r e s t t o c a t a l y s t p r e p a r a t i o n f o r two reasons, which may be c a l l e d micro and macro e f f e c t s .

Micro e f f e c t s i s used as

a general t e r m c o v e r i n g t h e d e s i r e d c r y s t a l 1 i n i t y , s u r f a c e area, p o r o s i t y e t c o f t h e c a t a l y s t , w h i l e macro e f f e c t s covers such f a c t o r s as p e l l e t s i z e and s t r e n g t h . With a few n o t a b l e exceptions, macro e f f e c t s have n o t r e c e i v e d t h e a t t e n t i o n i n t h e l i t e r a t u r e t h a t they deserve, s i n c e mechanical breakdown i s a very common cause o f c a t a l y s t replacement. However, micro and macro e f f e c t s , although showing i n d i v i d u a l c h a r a c t e r i s t i c s , a r e c l o s e l y t i e d together. This i s c l e a r l y shown, f o r example, i n t h e case o f alumina, where v a r i a t i o n i n c r y s t a l l i t e s i z e , l o c a l i s e d v a r i a t i o n i n phase change o r heating, and homogenity ( i n c r y s t a l l i t e dimensions) were found t o have a major e f f e c t on a t t r i t i o n and crush s t r e n g t h ( r e f . 44). Andrew has presented a good, b u t t o o short, review on macro e f f e c t s i n c a t a l y s t s ( r e f . 45), i n which t h e d e s i r e d p r o p e r t i e s a r e r e l a t e d b y t h e diagram shown i n f i g u r e 1. While c o r r e c t l y i d e n t i f y i n g p h y s i c a l s t r e n g t h c h a r a c t e r i s t i c s as b e i n g v e r y important, t h e r e l a t i o n w i t h o t h e r f a c t o r s i m p o r t a n t i n c a t a l y s i s does n o t emerge from t h e review.

Thus, f o r example, t h e s t r e n g t h o f c a t a l y s t s i s r e l a t e d

t o s u r f a c e area and p o r o s i t y , b u t these f a c t o r s a l s o have a l a r g e i n f l u e n c e on a c t i v i t y and s e l e c t i v i t y , i n t h a t they i n f l u e n c e msss and heat t r a n s f e r i n t h e system. Considering t h e problem i n terms o f c a t a l y s t p r e p a r a t i o n , we would expect t h a t a h i g h s u r f a c e area should g i v e h i g h e s t a c t i v i t y .

However h i g h s u r f a c e areas a r e

d i f f i c u l t t o prepare, a r e d i f f i c u l t t o m a i n t a i n (because o f t h e p o s s i b i l i t y o f s i n t e r i n g ) and a r e a s s o c i a t e d w i t h h i g h p o r o s i t y .

T h i s may i n t r o d u c e mass t r a n s f e r

l i m i t a t i o n s and w i l l c e r t a i n l y g i v e r i s e t o a weaker c a t a l y s t . The problem can be n e a t l y i l l u s t r a t e d by t h e o x i d a t i o n o f methane t o formaldehyde and t o carbon d i o x i d e .

Formaldehyde i s u n s t a b l e and, a t t h e temperatures needed

t o o x i d i s e methane, w i l l f u r t h e r o x i d i s e t o carbon d i o x i d e very e a s i l y .

Since b o t h r e a c t i o n s a r e exothermic, t h e c a t a l y s t temperature w i l l t e n d t o r i s e and t h i s w i l l favour o v e r - o x i d a t i o n and c a t a l y s t s i n t e r i n g .

As a r e s u l t , i t i s necessary t o

remove formaldehyde from t h e a c t i v e c a t a l y s t r a p i d l y , and t o equal i s e temperature i n t h e c a t a l y s t bed a t as low a value as p o s s i b l e .

T h i s r e q u i r e s a low p o r o s i t y

c a t a l y s t w i t h good thermal c o n d u c t i v i t y . F o r s i m i l a r reasons, platinum-rhodium gauzes a r e w i d e l y used f o r t h e o x i d a t i o n o f ammonia and novel geometries have been developed f o r methanation c a t a l y s t s , i n c l u d i n g t h e i d e a o f p l a t i n g them on t h e

A general guide t o s e l e c t i o n o f t h e p r o p e r t i e s o f t h e c a t a l y s t i s g i v e n i n t a b l e 2. I t should be emphasised t h a t t h i s

s u r f a c e o f a heat exchanger ( r e f . 46).

19

Thus, f o r

t a b l e i s general, s i n c e p a r t i c u l a r systems may show i n d i v i d u a l e f f e c t s .

example, mass t r a n s f e r l i m i t a t i o n s may be d e s i r a b l e , i n t h a t t h e y improve s e l e c t i v i t y ( r e f . 47) o r even, i n some cases, i n c r e a s e r a t e ( r e f . 48).

I

.D.-

The v a r i a t i o n i n p o r o s i t y and s u r f a c e area t h a t can be achieved u s i n g a p u r e c a t a l y s t i s l i m i t e d b y t h e p r e p a r a t i o n methods a v a i l a b l e and by t h e f a c t t h a t such m a t e r i a l s tend t o s i n t e r r a p i d l y .

As a r e s u l t , i t i s usual t o i n t r o d u c e t h e

d e s i r e d c h a r a c t e r i s t i c s p r i m a r i l y though t h e use o f a s u i t a b l e s u p p o r t . t h e r e a r e s e v e r a l o t h e r f a c t o r s which can i n f l u e n c e t h e c h o i c e .

Here, t o o ,

These may be

d i s c u s s e d w i t h t h e a i d o f t a b l e 3.

Table 3.

Choice o f support

Chemical Factors.

Is the support required t o s h w c a t a l y t i c a c t i v i t y 1 A r e c h a i c a l Interactions w i t h tkt catalyst possible ? I f so, are these desired o r undesired 7 Can the support i n t e r a c t w i t h reactants o r products ? Is t h i s desired o r undesired 7 How resistant i s the support t o poisoning 7 Can the catalyst be deposited on the Support i n the desired form ? Does the support induce a p a r t i c u l a r coordination g e m t r y on the catalyst (52, 53, 54). Is the support stable under operating conditions ? Physical Factors. Uhat i s the desired surface area and porosity 1 Uhat i s the desired t h e m 1 conductivity ? Is the support mechanically strong (45) 7 Uould t h i s be affected by deposition o f poisons. such as carbon 1

Is the support stable under the operating conditions ? Uhat i s the desired form of the p e l l e t 7

Recent s t u d i e s have r e v e a l e d t h a t chemical

n t e r a c t i o n s between t h e c a t a l y s t

and t h e s u p p o r t may be more i m p o r t a n t t h a n was p r e v i o u s l y t h o u g h t .

O f course, i t

has been l o n g e s t a b l i s h e d t h a t b i f u n c t i o n a l c a a l y s i s i s o f m a j o r i n d u s t r i a l

20

importance (ref. 49), and that chemical interaction between the support and the catalyst may be desirable (ref. 50) or undesirable (ref. 51). What has emerged comparatively recently is that the support may be able to induce a given geometry on a catalyst without formal chemical interaction, and that this will influence adsorption and catalysis. Such effects have been reported for supported silver (ref. 52) and for supported metal oxides (refs. 53, 54), and may be widely spread. No general guide lines for catalyst design can, as yet, be listed. However, it is probably the physical properties of the support which are primarily responsible for it's selection, provided that these are consistent with desired chemical properties. Both mechanical strength and porosity/surface area are important, as well as stability with respect to temperature etc. In addition, we have to consider theenvironmentin which the catalyst will be used, both with respect to stability and with respect to the reactor. A general guide to the factors that influence the choice of the latter is given in table 4.

Considerably more is known of micro effects in catalysis, although these may be of more academic interest than applied. This results from the fact that although we begin to know how to prepare a catalyst with a given structure, we know little about how to retain this structure under reaction conditions. This may be illustrated by considering supported metal catalyst. It is generally desirable to optimise the use of the metal by preparing it in as high a dispersion as possible and, in practice, this usually means depositing the metal in small crystallites on a support. NOW, theoretically, this can have other advantages, in that small crystallites are most likely to show geometrical effects (ref. 57) in their catalytic action. The concept of structure sensitive and structure insensitive reactions is well established (refs. 58, 59), and some assessment can be made of the nature of a given reaction, using the criteria outlined in table 5. Although it should be emphasised that this table has no theoretical justification, experimental observations indicate that the greater the number of criteria satisfied by a given reaction, the greater the probability

21 that a reaction

s demanding o r i s f a c i l e .

S i m i l a r l y , on a more t h e o r e t i c a l l y

sound base ( r e f . 57), i t can b e shown t h a t t h e number o f edge and c o r n e r s i t e s on a c a t a l y s t w i 1 be much h i g h e r w i t h sma 1e r c r y s t a l 1 it e s

.

Table 5.

Structure insensitive ( f a c i l e ) reactions may be addition reactions o r elimination reactions b) reactions involving a large decrease in free energy c ) reactions involving reactants with lone pair electrons or pi bonds or strain energy d) reactions that do not require a multifunctional catalyst e ) reactions occuring on an active catalyst whose l a t t i c e parameters do not change with dispersion a)

Structure sensitive (demanding) reactions a) b)

c) d)

e) f)

may occur on certain sites. e.g. Wz chmisorbs only on Y(111) and NHZ i s produced only on W(111) involve single C-C bond breakage may involve reactants with no lone pair electrons. pi bonds or strain energy need a multifunctional catalyst need a less active catalyst whose l a t t i c e parameters change w i t h dispersion may involve reactants w i t h unpaired electrons (e.g. NO).

I n p r a c t i c e , t h e r e a r e two d i f f i c u l t i e s w i t h t h i s approach.

With t h e

e x c e p t i o n o f c a t a l y t i c r e f o r m i n g , i n which t h e o v e r a l l r e a c t i o n i n v o l v e s b o t h f a c i l e and demanding r e a c t i o n s ( r e f . 49), most r e a c t i o n s o f i n d u s t r i a l i n t e r e s t are, i n f a c t , f a c i l e .

Secondly, g i v e n t h a t a r e a c t i o n i s demanding, i t can be v e r y d i f f i c u l t t o a c h i e v e o r t o m a i n t a i n a g i v e n geometry i n a c a t a l y s t prepared

on a n y t h i n g b u t t h e s m a l l e s t s c a l e , s i n c e r e o r g a n i s a t i o n o f t h e c a t a l y s t w i t h i n t h e p a r t i c l e ( r e f . 60), o r between p a r t i c l e s ( r e f s . 61, 62) r a p i d under r e a c t i o n c o n d i t i o n s

-

-

-

either

i s often

o r even d u r i n g p r e p a r a t i o n and a c t i v a t i o n .

Gross rearrangements ( s i n t e r i n g ) can be p r e v e n t e d t o some e x t e n t by t h e use o f spacers.

C o n v e n t i o n a l l y these i n v o l v e a h i g h m e l t i n g p o i n t n o n - d e l e t e r i o u s

metal s a l t w h i c h i s c o - p r e c i p i t a t e d on a s u p p o r t t o g e t h e r w i t h t h e c a t a l y s t , t h e r e b y a c t i n g as a p h y s i c a l b a r r i e r t o a g g l o m e r a t i o n .

However, t h e r e a r e o t h e r

forms o f spacer, a l t h o u g h t h e y a r e n o t c o n s i d e r e d i n t h e s e terms.

Thus, f o r

example, a s o l i d s o l u t i o n n o t o n l y d i c t a t e s t h e geometry o f t h e s o l u t e i o n ( r e f . 63) b u t a l s o a c t s t o s e p a r a t e s o l u t e i o n s .

S i m i l a r l y , an a l l o y d i s t r i b u t e s

one metal i n another, and t h e r e s u l t i n g e f f e c t can be due e i t h e r t o d i l u t i o n o r t o chemical o r e l e c t r o n i c i n t e r a c t i o n ( r e f s . 64, 6 5 ) . F o r t h e c o r r e c t p r o p e r t i e s , one component c o u l d a l s o be regarded as a s p a c e r f o r t h e o t h e r .

22

The overall desiqn. A general programne f o r the design of a catalyst has been discussed above, b u t t h i s i s only part of the overall design process which has been described i n detail elsewhere ( r e f s . 5, 6 ) . Although the discussion has been presented only briefly, i t does attempt t o review the areas important in the context of catalyst preparation. Before the combination of these ideas with catalyst preparation i s considered, i t is worthwhile emphasising some points. Regrettably, we have insufficient knowledge t o ensure that the catalyst design i s absolutely correct, and experimental testing must be carried o u t . However, i t should be stated t h a t , on the thirty-odd occasions that the author has carried out a catalyst design, the procedure has shown up a c a t a l y s t that has been found e i t h e r previously or subsequently - t o be active f o r the reaction under consideration. I t should be hastily added, however, that several inactive catalysts have also been suggested by the design. As a reasonable assessment, the design procedure offers a guide to.experimentation which can often be successful and which requires the investment of only a l i t t l e time. As the knowledge and experience of the designer improves, so the accuracy of the design can also be expected t o improve. Secondly i t i s necessary t o emphasise the feed back cycle i n the design. Experimental testing i s necessary a t various points, and the results of these experiments can be used t o modify the conceptional basis of the design. Thus, f o r example, i f experiments show t h a t reaction p a t h A leads t o a more desirable product spectrum t h a n reaction path B y the design can be adjusted t o p u t more weight on reactions of type A. Thirdly i t must be remembered t h a t there are always factors that have not been considered. Catalysis is a complex subject, involving inter-related phenomena from a wide variety of f i e l d s . As a r e s u l t , a s e t of experiments, carried out f o r a given reason, may give the "wrong" results because of a second factor which has not been considered. Perhaps the most obvious case of t h i s was work carried out on the design of a catalyst t o convert propylene t o benzene ( r e f . 5): a t the end of the design i t was discovered that changing economics would make the reverse reaction more a t t r a c t i v e ! This, again, emphasises the importance of feedback a t a l l stages. blith these comments in mind, i t i s now possible to consider the interaction of catalyst design and catalyst preparation.

23 C a t a l y s t d e s i g n and c a t a l y s t p r e p a r a t i o n . F i n a l l y , i t i s necessary t o c o n s i d e r c o r r e l a t i o n o f t h e suggestions f r o m t h e d e s i g n procedure w i t h what can be p r a c t i c a l l y achieved by c a t a l y s t p r e p a r a t i o n . I n d e f e r e n c e t o t h e main theme o f t h e meeting, t h i s w i l l be l i m i t e d t o i m p r e g n a t i o n and a c t i v a t i o n . I m p r e g n a t i o n i s w i d e l y used as t h e f i r s t s t e p i n t h e p r e p a r a t i o n o f a c a t a l y s t , and i n v o l v e s t h e depositsion o f a metal s a l t on a s u p p o r t :

t h i s metal s a l t can be

c o n v e r t e d i n t o t h e a c t i v e c a t a l y s t d u r i n g subsequent p r o c e s s i n g , a c t i v a t i o n b e i n g one o f t h e m a j o r s t e p s .

As w i t h many o f t e n a p p a r e n t l y s i m p l e processes, i m p r e g n a t i o n may be complex. Three g e n e r a l t y p e s may be i d e n t i f i e d .

I n t h e f i r s t , t h e s u p p o r t i s soaked i n

t h e m e t a l s a l t s o l u t i o n , and t h e excess l i q u o r evaporated:

t h i s g e n e r a l l y leads

t o an even d i s t r i b u t i o n o f metal s a l t t h r o u g h o u t t h e p o r e system which can be p e n e t r a t e d by t h e o r i g i n a l s o l u t i o n . support.

T h i s need n o t i n c l u d e m i c r o p o r e s i n t h e

I n t h e second, i t i s p o s s i b l e t o p r e - t r e a t t h e metal s a l t i n s o l u t i o n

before imprepnation.

Thus, f o r example, a c o l l o i d a l suspension o f metal c o u l d

be prepared, subsequent i m p r e g n a t i o n r e s u l t i n g i n t h e c o n c e n t r a t i o n o f t h e c o l l o i d a t t h e e x t e r i o r surface o f t h e support.

T h i r d l y , and perhaps o f most i n t e r e s t i n

t h e c o n t e x t o f c o n t r o l , i m p r e g n a t i o n c o u l d r e s u l t f r o m a chemical r e a c t i o n between t h e s a l t and s u r f a c e groups on t h e s u p p o r t . T h i s can be i l l u s t r a t e d f o r t h e p r e p a r a t i o n o f a p l a t i n u m on s i l i c a c a t a l y s t . S i l i c a i s known t o have a complex s u r f a c e s t r u c t u r e , i n which t h e r e a r e a number o f d i f f e r e n t hydroxyl s i t e s o f varying r e a c t i v i t y ( r e f . 66).

I m p r e g n a t i o n by

c h l o r p l a t i n i c a c i d i n v o l v e s h y d r o l y s i s a t some o f t h e s e s i t e s t o produce p l a t i n u m a n i o n - s i l i c a groups a t t h e s u r f a c e ( r e f . 6 7 ) .

I n t h e s e terms, t h e p o s s i b i l i t i e s o f

control are large.

Thus, f o r example, c h l o r p l a t i n i c a c i d can be e a s i l y c o n v e r t e d

t o (PtCl,(OH)6_,)Z-

( r e f . 6 8 ) , and t h e c o m p o s i t i o n o f t h e s a l t c o u l d be a d j u s t e d

t o favour r e a c t i v i t y w i t h o n l y one t y p e o f s u r f a c e h y d r o x y l group.

Secondly, t h e

h y d r o x y l groups on s i l i c a c o u l d be poisoned, e i t h e r w i t h r e s p e c t t o t h e i r r e a c t i v i t y ( b y p r e - t r e a t i n g w i t h compounds o f d i f f e r e n t b a s i c i t y ) o r w i t h r e s p e c t t o t h e i r l o c a t i o n ( b y u s i n g bases i n which s t e r i c e f f e c t s may l i m i t t h e p e n e t r a t i o n o f t h e p o r e system).

As a r e s u l t , i t is p o s s i b l e t o d e p o s i t t h e metal s a l t i n v a r y i n g

d e n s i t y and a t d i f f e r e n t l o c a t i o n s on t h e s u r f a c e . From t h e above, i t i s seen t h a t t h e amount and l o c a t i o n o f t h e c a t a l y s t can be c o n t r o l l e d , a t l e a s t i n p r i n c i p l e , d u r i n g i m p r e g n a t i o n , a l t h o u g h subsequent p r o c e s s i n g may r e d i s t r i b u t e t h e c a t a l y s t ( s e e b e l o w ) .

Interaction w i t h the catalyst

d e s i g n i s p o s s i b l e i n t h e s e l e c t i o n o f t h e a c t i v e c a t a l y s t ( m a j o r and m i n o r

24

c o n s t i t u e n t s ) and i n t h e use o f mass and heat t r a n s f e r c o n s i d e r a t i o n s t o decide where t h e c a t a l y s t should be l o c a t e d .

I n addition, p o s s i b i l i t i e s o f sintering

c o u l d d i c t a t e t h e d e s i r e d s t r e n g t h o f c a t a l y s t - s u p p o r t bonding, although t h i s may be harder t o achieve i n p r a c t i c e . I n t h e present context, a c t i v a t i o n may be considered as t h e conversion o f t h e as-deposited s a l t t o t h e d e s i r e d c a t a l y s t by thermal treatment i n t h e presence o f a gas: t h i s may, o r may n o t , i n v o l v e r e a c t i o n s such as o x i d a t i o n , r e d u c t i o n o r sulphidation.

The c o n d i t i o n s and e f f i c i e n c y o f t h e process a r e easy t o assess,

i n terms of t h e n a t u r e o f t h e d e s i r e d conversion, although complete a c t i v a t i o n may be more demanding than these c a l c u l a t i o n s would i n d i c a t e ( r e f . 69). Two major problems may a r i s e d u r i n g a c t i v a t i o n .

The f i r s t o f these i n v o l v e s

t h e rearrangement of t h e c a t a l y s t o r t k s u p p o r t : e i t h e r may be d e s i r e d o r undesired. Rearrangement of t h e support g e n e r a l l y a r i s e s as a r e s u l t o f phase t r a n s f o r m a t i o n s i n t h e m a t e r i a l , causing c o l l a p s e o f pore s t r u c t u r e and decrease i n s u r f a c e area.

This i s o f t e n a c c e l e r a t e d by i m p u r i t i e s ( i n c l u d i n g t h e c a t a l y s t )

( r e f s . 70, 71), and by t h e ambient gas ( r e f . 72): d u r i n g a c t i v a t i o n , t h i s i s u s u a l l y undesirable. Estimates have been p u b l i s h e d o f t h e s t a b i l i t y o f common supports ( r e f s . 44, 45, 70) b u t , r e g r e t t a b l y , these a r e w i d e l y dispersed i n t h e l i t e r a t u r e and a comparative review i s b a d l y needed. However, i t i s p o s s i b l e t o i d e n t i f y c o n d i t i o n s under which rearrangement may occur. Rearrangement o f t h e c a t a l y s t d u r i n g a c t i v a t i o n may be d e s i r e d i f t h e c a t a l y s t migrates t o support s u r f a c e s i t e s where i t can be l o c a t e d more f i r m l y .

The

r e s u l t i n g c a t a l y s t can be expected t o be more s t a b l e d u r i n g i t ' s working l i f e . I f , on t h e o t h e r hand, rearrangement leads t o c a t a l y s t agglomeration and t o

decreased s u r f a c e area o r increased p a r t i c l e size, then t h e process should be avoided.

This means e i t h e r a c t i v a t i n g under c o n d i t i o n s where rearrangements i s

minimal o r , if t h i s is impossible, i n t r o d u c i n g a second component (such as a spacer) t h a t minimises rearrangement. The choice o f such a component depends, t o some e x t e n t , on t h e mechanism of rearrangement. As a general r u l e , rearrangement can i n v o l v e s u r f a c e d i f f u s i o n ( r e f . 73) , volume d i f f u s i o n ( r e f . 74) o r evaporation-condensation ( r e f . 75) , t h e i m p o r t a n t process depending on t h e temperature. Use o f a spacer rninimises volume d i f f u s i o n , b u t s u r f a c e d i f f u s i o n o r evaporation-condensation can o n l y be prevented by t h e a d d i t i o n o f a second component t o t h e c a t a l y s t (as, f o r example, i s t h e It i s i m p o r t a n t

case w i t h P t / l O % Rh gauzes used f o r t h e o x i d a t i o n o f ammonia).

t o remember t h a t , although a second component may be added t o s t a b i l i s e t h e c a t a l y s t , i t nay a f f e c t t h e o v e r a l l c a t a l y t i c behaviour o f t h e s o l i d . As a r e s u l t o f r e c e n t work on s i n t e r i n g ( r e f s . 76, 7 8 ) , i t i s p o s s i b l e t o p r e d i c t w i t h some accuracy t h e c o n d i t i o n s t o be avoided. From t h e v i e w p o i n t o f c a t a l y s t design, however, i t is u s u a l l y s u f f i c i e n t t o use t h e o l d r u l e t h a t s u r f a c e rearrangements a r e l i a b l e t o be important a t ca. 0.3 x m e l t i n g p o i n t and

25

volume rearrangement a t ca. 0.5 x m e l t i n g p o i n t .

I f conditions are c r i t i c a l , it

may be necessary t o c a l c u l a t e t h e t e m p e r a t u r e r i s e t h a t c o u l d o c c u r i n a c a t a l y s t bed, and h e r e a s i m p l e one dimensional model w i l l u s u a l l y s u f f i c e ( r e f . 79):

the

temperature i n a p e l l e t i s u n l i k e l y t o r i s e m a r k e d l y ( l e s s t h a n 20°C) above t h e ambient i n t h e p a r t i c u l a r p o r t i o n o f t h e bed ( r e f . 80). The second f a c t o r t h a t may be i m p o r t a n t d u r i n g a c t i v a t i o n i s t h e p o s s i b i l i t y o f catalyst-support interactions.

These, too, may be d e s i r a b l e (e.g.

chromia-

alumina ( r e f . 5 0 ) ) o r u n d e s i r a b l e (e.g. n i c k e l alumina ( r e f . 5 1 ) ) , and t h i s can be e s t a b l i s h e d f r o m t h e design.

F o r t u n a t e l y , t h e c o n d i t i o n s under which such

s o l i d - s o l i d i n t e r a c t i o n s a r e p o s s i b l e a r e u s u a l l y a v a i l a b l e ( r e f s . 74, 81 , 82), a l t h o u g h t h e r e has been some c o n s t e r n a t i o n i n r e c e n t y e a r s t o d i s c o v e r t h a t i n t e r a c t i o n between p l a t i n u m and alumina i s p o s s i b l e ( r e f . 83). A l t h o u g h t h e purpose o f t h e c a t a l y s t d e s i g n i s p r i m a r i l y t o d e c i d e w h i c h c a t a l y s t t o prepare, i t i s seen t h a t i n t e r a c t i o n between t h e d e s i g n procedure and c a t a l y s t p r e p a r a t i o n i s p o s s i b l e and can b e p r o d u c t i v e .

It is c e r t a i n l y

p o s s i b l e t o r e c o g n i s e which f a c t o r s a r e open t o c o n t r o l and t o p r e d i c t t h e p r o b a b l e e f f e c t s o f such c o n t r o l d u r i n g p r e p a r a t i o n .

C a t a l y s t design i s s t i l l

f a r f r o m p e r f e c t , b u t i t can o f f e r a l o g i c a l g u i d e t o e x p e r i m e n t a t i o n t h a t can c o n s i d e r a b l y s h o r t e n t h e t i m e needed t o develop a new c a t a l y s t . AC KNOWL EDGEHE NTS The a u t h o r acknowledges, w i t h g r a t i t u d e , v a l u a b l e d i s c u s s i o n s w i t h P r o f . J.W. Coenen.

26

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f

27

52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71

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

K.K. Kakati and H. Wilman, J . Phys. D., Appl. Phys. 6 1973)1307. V . A . Schvets and V.B. Kazansky, Kin. and Cat. 25(1972 123. 11. Goldwasser and D.L. Trimm, i n preparation. R . Ugo, Cat. Revs. Sci. Eng., 11(1975)225. Yu. I . Yermakov, Cat. & Rev. Sci. Eng. 13(1976)77. R . Van Hardeveld and F. Hartog, Surface Sci. 15(1969) 89. C . Bernard0 and D . L . Trimm. Carbon 14(1976)225. M. Boudart, Proc. VI I n t e r n a t . Cony-. 'on Catal. (London)(1976)1. B.J. Cooper, B. Harrison and E . S h u t t , S.A.E. paper 770367 (1977). A . E . B . Presland, G . L . Price and D . L . Trimm, J . Catal. 26(1972)313. R.H.J. Fiederow and S.E. Wanke, J . C a t a l . 43(1976)34. F.S. Stone, Adv. i n C a t a l y s i s 13(1962)1. V . Ponec, Cat. Rev. S c i . En:. 11(1975)41. R . L . Moss and L . Whalley, Adv. i n C a t a l y s i s 22(1972)115. J.B. P e r i , J . C a t a l . 41(1976)227. J.P. Brunelle, A. Sugier and J.F. Le Page, J . Catal. 43(1976)273. E.H. Archibald and W.A. Gale, J . Chem. SOC. 121(1922)2849. S.P. Noskova, M.S. Borisova, V . A . Dzisko, S.G. Khisanieva and Yu.A. Alabuzhev, K i n . and Cat. 15(1974)527. D.A. Dowden, I . Chem. Eng. Syrnp. Ser. 27(1968)18. N.11. Zaidman, V . A . Dzisko, A.P. Karnaukhov, L.M. Kefeli, N.P. Krasilenko, N.G. Koroleva and I.D. Ratner, Kin. and Cat. 10(1969)313. A.V. Kiselev and Yu. S. N i k i t i n , Kin and Cat. 4(1963)562. G . E . Rhead, Surface S c i . , 15(1969)353. P . Wynblatt and N . A . Gjostein, Prog. Solid S t a t e Chem. 9(1975)21. G.C. Fryburg, Trans. A . I . H . E . , 233(1973)1986. P.C. Flynn and S.E. Wanke, J . Catal. 34(1974)390. C.G. Granqvist and R . A . Buhrman, J . C a t a l . 42(1976)477. P.C. Flynn and S.E. Wanke, Cat. Rev. S c i . En!. 12(1975)93. 0 . Levenspiel , "Chemical Reaction Engineering", John Wiley (1962). D.L. Trimm, J . Corrie and R.D. Holton, Chem. Eng. Sci. 29(1974)2009. R.W.G. Wyckoff, "Crystal S t r u c t u r e s " , Interscience (1966). 3. Plellor, "A comprehensive T r e a t i s e on Inorganic and Theoretical Chemistry", Longmans (1922-37). F. Dautzenberg, Lecture: Rideal Conference: April 1977.

DISCUSSION

D.

CHADWICK

: You

r a i s e d t h e q u e s t i o n of t h e importance of

in relation t o catalytic activity.

defects

Recent experimental r e s u l t s i n

s u r f a c e s c i e n c e s u g g e s t t h a t d e f e c t s do indeed p l a y an i m p o r t a n t r o l e i n s u r f a c e r e a c t i o n s a n d t h a t s e l e c t i v i t y , f o r e x a m p l e , may b e related t o step/kink

d e n s i t y , i n which c a s e i t seems s u r p r i s i n g t h a t

one can o b t a i n a c t i v i t y p a t t e r n s w i t h s i m p l e p a r a m e t e r s such a s l a t t i c e spacing.

TRIMM : I t i s t r u e t h a t d e f e c t s p l a y a n i m p o r t a n t r o l e i n d e -

D.L.

termining c a t a l y t i c a c t i v i t y . of

I t s h o u l d be emphasized t h a t e a c h one

t h e f a c t o r s d i s c u s s e d i n t h e p a p e r may,

As a

o r may n o t , b e i m p o r t a n t .

r e s u l t one should use l a t t i c e spacing arguments ( o r any o t h e r

approach) a s an i n d i c a t o r o n l y . way t o c h e c k t h a t a n i n d i c a t o r i s

Experimental

t e s t i n g i s the only

(or i s not) accurate.

One i s ,

28 however, on much stronger ground when it is advantageous to study the mechanism of the reaction in greater detail, in order to investigate possible secondary components.

At this stage the role

of defects and the importance of different sites can be clarified. I t should be emphasized, however, that rearrangements under operating conditions may make it difficult to optimise the surface geometry, even when the desired surface structureshave been established

M.

MARTAN

:

Did you really study the direct synthesis of limonene

from toluene and propylene 7

D.L.

TRIMM

: A

design study for the catalytic production of limonene

was carried out.

N o experimental testing has been completed.

I

suspect that the partial hydrogenation of toluene will not be possible using a heterogeneous catalyst.

29

EFFECT OF IMPREGNATION AND A C T I V A T I O N C O N D I T I O N S OF A1203/Cu0 SUPPORTED MONOLITH CATALYSTS I N THE REDUCTION OF NO

M . ENGLER and K . UNGER

I n s t i t u t f u r Anorganische Chemie und A n a l y t i s c h e Chemie Johannes-Gutenberg-UniversitEit 0. I N A C K E R , C . PLOG and M .

Mainz, 6 5 0 0 Mainz

SEIDL

D o r n i e r System GmbH, 7 9 9 0 F r i e d r i c h s h a f e n

SUMMARY

Reduction of N O i n t h e p r e s e n c e of CO and A r was examined on two s e r i e s o f m o n o l i t h i c s h e e t s c a r r y i n g an a c t i v e CuO/A1203 l a y e r . In t h e impregnation p r o c e d u r e t h e r a t i o of A 1 0

2 3

t o CuO w a s v a r i e d i n a wide r a n g e . The t e x t u r e o f t h e l a y e r and

t h e d i s p e r s i o n of CuO t h e r e i n was c o n t r o l l e d by mercury p o r o s i m e t r y , s c a n n i n g e l e c t r o n microscopy and secondary i o n mass s p e c t r o m e t r y (SIMS). I t could b e e s t a b l i s h e d t h a t t h e s u r f a c e c o n c e n t r a t i o n o f CuO determined by SIMS measurements

i s t h e most d e c i s i v e q u a n t i t y of t h e c a t a l y s t c o r r e l a t i n g l i n e a r l y w i t h t h e c o n v e r s i o n of N O .

INTRODUCTION

The need of c a t a l y t i c c o n v e r t e r s t o c o n t r o l t h e emission o f automobile e x h a u s t s gave r i s e t o a wide-spread a c t i v i t y i n t h e p r e p a r a t i o n of optimum d e s i g n e d catalysts ( r e f . 1,Z).

Due t o t h e s p e c i f i c r e q u i r e m e n t s such a s m e c h a n i c a l , t h e r m a l

and t h e r m a l shock s t a b i l i t y s u p p o r t e d ceramic m o n o l i t h s made o f c o r d i e r i t e were found t o b e s u p e r i o r a s c a r r i e r s compared t o p e l l e t s a r r a n g e d i n a f i x e d bed. As t h e automobile e x h a u s t c o n s t i t u t e s of a l a r g e v a r i e t y of p o l l u t a n t s t o be removed d i f f e r e n t c a t a l y t i c systems were proposed u t i l i z i n g o x i d a t i o n , r e d u c t i o n and soc a l l e d three-way c a t a l y s t s ( r e f . 2 ) . I n t h i s c o n t e x t c o n s i d e r a b l e i n t e r e s t h a s focussed on t h e r e d u c t i o n o f NO as one h a z a r d o u s component o f e x h a u s t . T h i s s t u d y d e a l s w i t h t h e r e d u c t i o n of NO i n t h e p r e s e n c e o f CO and by means of an o x i d a t i o n c a t a l y s t composed of CuO/A1203 which i s s u p p o r t e d on a c o r d i e r i t e m o n o l i t h . The purpose of t h e work was t o s t u d y t h e e f f e c t of t h e g e o m e t r i c p r o p e r t i e s of t h e l a y e r and t h e d i s p e r s i o n o f CuO t h e r e i n on t h e c o n v e r s i o n o f NO a t c o n s t a n t r e a c t i o n c o n d i t i o n s .

30 2 . Experimental 2 . 1 S t a r t i n g Materials f o r C a t a l y s t P r e p a r a t i o n

The c a t a l y s t c a r r i e r w a s a m o n o l i t h of t y p e AlSiMag 795 (American Lava C o r p . , Chattanooga, Tenn., U S A ) w i t h about 233 h o l e s / i n c h 2 . The s p e c i f i c s u r f a c e a r e a

2

a c c o r d i n g t o BET was determined t o 0 . 1 m / g . The s p e c i f i c p o r e volume measured by t h e i n t r u d e d volume of mercury up t o 4.000 b a r amounts t o 0.064 ml/g. The p o r e volume d i s t r i b u t i o n i s of bimodal t y p e (D = 10 - 40 nm; Dmax( 2 ) = 3 000 nm) max( 1) b u t 94 % of t h e t o t a l s p e c i f i c p o r e volume i s d i s t r i b u t e d i n p o r e s l a r g e r t h a n 2 500 nm. S i n c e t h e c o a t i n g of t h e whole monolith o r p i e c e s of it l e a d s t o a

l a r g e l y inhomogeneous d i s t r i b u t i o n of t h e c a t a l y s t a l o n g t h e c h a n n e l s t h e monolith was s l i c e d i n t o s h e e t s of 40 x 10 mm i n dimensions. These s h e e t s c o a t e d a t b o t h s i d e s were used i n a l l f u r t h e r i n v e s t i g a t i o n s s u c h as c o n v e r s i o n measurements, mercury p o r o s i m e t r y , s c a n n i n g e l e c t r o n microscopy ( S E M I , secondary i o n mass spectrometry (SIMS) e t c . The s t a r t i n g m a t e r i a l s u p p o r t i n g t h e a c t i v e c o a t i n g was a f i n e l y d i v i d e d nonporous gamma-alumina o f t y p e Alu C (Degussa, Hanau, West Germany) o f mean p a r t i c l e 2 20 nm and o f s p e c i f i c s u r f a c e area SBET = 100 m / g . The c a t a l y t i c s i z e dp component CuO w a s made from c o p p e r ( I 1 ) n i t r a t e ( c r y s t . p u r e , E . Merck, Darmstadt,

West Germany). 2.2 Catalyst Preparation

The m o n o l i t h i c s h e e t s were c o a t e d i n one s t e p by c o n t r o l l e d d i p p i n g i n t o a homogeneous and s t a b l e s u s p e n s i o n made of Alu C , c o p p e r ( I 1 ) n i t r a t e and e t h a n o l as d i s p e r s i n g a g e n t . The impregnated s h e e t s were d r i e d a t 410 K f o r 8 h o u r s and f i n a l l y c a l c i n e d a t 873 K f o r one h o u r .

2 . 3 Examinations The s p e c i f i c s u r f a c e a r e a and t h e p o r e s t r u c t u r e d a t a o f t h e u n t r e a t e d and c o a t e d s h e e t s were determined by means of n i t r o g e n s o r p t i o n t e c h n i q u e s and mercury p o r o s i m e t r y ( r e f . 3 ) . The mass i n c r e a s e of t h e s h e e t s w a s monitored by weighing b e f o r e and a f t e r c o a t i n g . The b u l k c o n c e n t r a t i o n of copper i n t h e s h e e t w a s measured t i t r i m e t r i c a l l y a f t e r d i s s o l u t i o n o f CuO i n c o n c e n t r a t e d s u l f u r i c a c i d . Conversion measurements were made i n a r e a c t o r c o n s t r u c t e d by D o r n i e r System GmbH, F r i e d r i c h s h a f e n , West Germany ( r e f . 4 ) . C o n d i t i o n s were: g a s composition 10 sccm N O , 1430 sccm A r , p r e s s u r e 400 Torr, t e m p e r a t u r e 623 K. A s p e c i a l probe t r a n s f e r system developed by Dornier System GmbH ( r e f . 5 ) connected t o t h e r e a c t o r p e r m i t t e d t o t r a n s f e r t h e c a t a l y s t from t h e r e a c t o r i n t o u l t r a h i g h vacuum chamber of t h e SIMS a p p a r a t u s ( t y p e 102, B a l z e r s , L i e c h t e n s t e i n ) w i t h o u t exposing it t o t h e atmosphere. SEM measurements were made w i t h a model MSM 5 a p p a r a t u s of I n t e r n a t i o n a l S c i e n t i f i c

Instruments I n c . , Japan.

31

3. R e s u l t s and D i s c u s s i o n 3.1 V a r i a b l e s i n C a t a l y s t P r e p a r a t i o n Two s e r i e s of c a t a l y s t s were p r e p a r e d . I n s e r i e s I t h e mass o f d e p o s i t e d alumina was v a r i e d w h i l e h o l d i n g t h e c o n t e n t of CuO c o n s t a n t . This was performed by d i p p i n g

t h e s h e e t s i n s u s p e n s i o n s o f i n c r e a s i n g c o n t e n t of Alu C ( 0 . 5 t o 5 . 0 % ( w / w ) ) which

were 0 . 8 molar on c o p p e r . I n s e r i e s I1 t h e mass o f d e p o s i t e d alumina was k e p t c o n s t a n t w h i l e t h e c o n t e n t of CuO was v a r i e d i n t h e r a n g e between 0,2 t o 6 , 6 % ( w / w ) by d i p p i n g t h e s h e e t s i n s u s p e n s i o n s of i n c r e a s i n g m o l a r i t y o f copper (11) n i t r a t e from 0,04 t o 1,6 m o l / l w h i l e t h e c o n t e n t o f Alu C was m a i n t a i n e d a t 1 % (w/w). I n t h i s way i t w a s p o s s i b l e t o vary (i)

t h e d e g r e e of d i s p e r s i o n of a c o n s t a n t amount o f c u p r i c o x i d e w i t h i n t h e a c t i v e alumina l a y e r ( s . s e r i e s I )

( i t ) t h e amount o f c u p r i c o x i d e d i s p e r s e d i n a c o n s t a n t mass o f d e p o s i t e d alumina ( s . s e r i e s 11)

3.2 Pore s t r u c t u r e and Thickness of t h e C o a t i n g The d e p o s i t e d l a y e r made of Alu C a l o n e i s r e g a r d e d as a close-packed assembly of t h r e e - d i m e n s i o n a l l y l i n k e d non-porous alumina p a r t i c l e s forming a r e g u l a r p o r e s t r u c t u r e . Independent of t h e mass o f t h e mounted alumina t h e most f r e q u e n t p o r e d i a m e t e r d e r i v e d from mercury p o r o s i m e t r y measurements amounts t o 20 - 35 nm which

i s one o r d e r s m a l l e r t h a n t h a t of t h e uncoated m o n o l i t h . I n t h e p r e s e n c e o f c o p p e r ( I 1 ) n i t r a t e as a second component of t h e s u s p e n s i o n CuO p a r t i c l e s are formed through t h e c a l c i n a t i o n p r o c e d u r e . The q u e s t i o n arises i n which way t h e CuO p a r t i c l e s are a r r a n g e d w i t h i n t h e porous alumina l a y e r . I t can b e assumed t h a t a t

a h i g h r a t i o of alumina t o CuO t h e CuO p a r t i c l e s are uniformly d i s p e r s e d w i t h i n t h e alumina whereas a t a low r a t i o CuO forms c l u s t e r s r e s u l t i n g i n a lower d e g r e e

of d i s p e r s i o n . By means of mercury p o r o s i m e t r y measurements i t w a s evidenced i n b o t h s e r i e s t h a t t h e most f r e q u e n t p o r e d i a m e t e r remained c o n s t a n t a t 20 - 35 nm and hence was i n d e p e n d e n t o f t h e amount of CuO embedded i n t h e porous l a y e r . A l i n e a r r e l a t i o n s h i p was e s t a b l i s h e d between t h e c o n t e n t o f Alu C i n t h e

s t a r t i n g s u s p e n s i o n and t h e mass d e p o s i t e d a t c o n s t a n t c o n t e n t o f CuO i n s e r i e s I and between t h e molar c o n c e n t r a t i o n of C U ( N O ~ ) i~n t h e s t a r t i n g s u s p e n s i o n and t h e mass d e p o s i t e d a t c o n s t a n t c o n t e n t of alumina i n s e r i e s I1 (s. Table I ) .

32 Table I : P r o p e r t i e s of CuO/A1 0

2 3

s u p p o r r e d l a y e r s i n s e r i e s I and 11,

respectively

series I c o n t e n t o f Alu C

t o t a l mass

molar r a t i o

i n t h e suspension

deposited

A1203/Cu0

% (w/w)

% (w/w)

0

3.0

0.5

3.8

0.01

1.0

4.1

0.02

1.5

4.2

0.07

2.0

4.5

0.09

2.5

5.1

0.23

3 .O

5.4

0.21

4.0

5.6

0.35

5.0

6.0

0.66

s e r i e s I1 c o n c e n t r a t i o n of CuO

t o t a l mass

molar r a t i o

i n t h e suspension

deposited

A1203/Cu0

% (w/w)

mol/l

-

0

0.46

0.04

0.51

1.00

0.08

0.53

0.89

0.20

1.26

0.55

0.40

2.37

0.19

0.80

3.93

0.07

1.20

5.90

0.10

1.60

6.93

0.08

The t h i c k n e s s of t h e l a y e r , d s , can b e f o r m a l l y e s t i m a t e d by t h e e q u a t i o n

n

m A1203

CuO

t-

d

= ['p(Hg)

- "p(Hg, macro) 'BET

where V p ( H g ) i s t h e t o t a l s p e c i f i c p o r e volume of t h e l a y e r ,

the 'p(Hg, macro) amount of V p ( H g ) which p e n e t r a t e s macropores l a r g e r t h a n 2 500 nm, m and

*12'3 andgCuOthe

mCuO t h e mass of t h e d e p o s i t e d alumina and CuO, r e s p e c t i v e l y , g A1203

r e s p e c t i v e t r u e d e n s i t i t e s and SBET t h e s p e c i f i c s u r f a c e a r e a of t h e p a r e n t m o n o l i t h i c s h e e t determined by means of t h e BET method. Comparison of t h e ds v a l u e s

33 of series I f o r i n s t a n c e r e v e a l s t h a t d

v a r i e s from 140 t o 340 nm and hence ds

amounts t o about one t e n t h o f t h e most f r e q u e n t p o r e d i a m e t e r of t h e n a t i v e m o n o l i t h .

3.3 D i s p e r s i o n o f CuO Within t h e Porous Layer The d e g r e e o f d i s p e r s i o n o f CuO on t h e s h e e t s o f s e r i e s I was s t u d i e d a t v a r i o u s t

amounts of alumina and c o n s t a n t c o n t e n t of CuO ( 3 , 9 - 0 , 2 % (w/w)). I n F i g . 1 t h e v a r i a t i o n of t h e c o n c e n t r a t i o n o f Cu i n atom p e r c e n t determined w i t h SIMS f o l l o w i n g

a method developed by P l o g ( r e f . 6 ) is shown as a f u n c t i o n o f t h e d e p t h o f t h e l a y e r e x p r e s s e d i n numbers o f monolayer u n i t s o f Cu.

surface concentration of Cu (atom

(1 ) 0 0 ( 2 ) 0.5

% (w/w) Alu C

50

40

8

( 3 ) 1.0

% ( w / w ) Alu C

( 4 ) 2.5

% (w/w) Alu C

%

30

O ( 5 )

% (w/w) Alu C

5.0 % (w/w) Alu C

20 10

0 100

10'

102

103

lo4

-D depth inmonolayer units F i g . 1 Depth p r o f i l e o f Cu a t d i f f e r e n t amounts o f alumina

The impregnation o f t h e s h e e t w i t h CuO a l o n e w i t h o u t a s u p p o r t i n g alumina l a y e r (Curve (1) i n F i g . 1) g i v e s t h e l o w e s t s u r f a c e c o n c e n t r a t i o n . The SEM photograph t a k e n o f t h i s p a r t i c u l a r s h e e t shows r e l a t i v e l y l a r g e c l u s t e r s o f CuO of 20 nm i n diameter d i s t r i b u t e d a t t h e surface ( s . Fig. 2a). The a d d i t i o n of alumina d r a s t i c a l l y i n c r e a s e s t h e d e g r e e of d i s p e r s i o n of CuO,

a f a c t which is evidenced by t h e h i g h e r s u r f a c e c o n c e n t r a t i o n o f Cu shown by c u r v e s 2

-

4. The d i s p e r s i o n e f f e c t due t o alumina s t a r t s t o become n o t i c e a b l y a t 1 %(w/w)

of Alu C i n t h e s u s p e n s i o n whereas a t h i g h e r c o n t e n t s o f Alu C t h e s u r f a c e

c o n c e n t r a t i o n o f c o p p e r d e c r e a s e s a g a i n . By g o i n g from t h e o u t s i d e t o t h e i n t e r i o r Of

t h e l a y e r , t h e s h a p o f t h e d e p t h p r o f i l e o f Cu is s e e n t o b e s p e c i f i c a l l y

dependent on t h e amount o f alumina i n t h e r a n g e c o n s i d e r e d .

34

Fig. 2 : SEM photographs of s h e e t s c o a t e d i n d i f f e r e n t ways (magnification

i000 x l e f t hacd s i d e , m a g n i f i c a t i o n 70GO x r i g h t hand

side) a ) native ironolithic sheet

b) s h e e t c o a t e d w i t h 3.0 % ( w / w ) CuO w i t h o k t alumina c) sheet coated with 0 . 5 % (w/w)

alumina w i t h o u t CuO

d ) s h e e t c o a t e d w i t h 3 , O % ( w / w ) CuO and 3 , 0 % ( w / w )

alumina a c c o r d i n g

t o a s u s p e n s i o n c o n t a i n i n g 5 % (w/w) Alu C and 0 . 8 m o l / l C U ( N O ~ ) ~

35 3 . 4 E f f e c t o f t h e Impregnation Conditions on t h e Conversion of NO 3 . 4 . 1 Amount of Mounted Alumina a t Constant Content o f CuO v s . Conversion o f NO

I n F i g . 3 t h e conversion o f NO i s p l o t t e d as a f u n c t i o n o f t h e amount o f alumina i n t h e s t a r t i n g s u s p e n s i o n . The s h e e t w i t h o u t alumina y i e l d s t h e lowest c o n v e r s i o n . A t about 1 % ( w / w )

of Alu C t h e conversion goes through a maximum which i s

c o n s i s t e n t w i t h t h e r e s u l t s o f F i g . 1 which shows t h e h i g h e s t s u r f a c e c o n c e n t r a t i o n o f CuO i n t h i s r e g i o n . A t h i g h e r amounts of alumina t h e c o n v e r s i o n d e c r e a s e s and f i n a l l y approximates a c o n s t a n t l e v e l . A s shown i n F i g . 3 t h e s h a p e of t h e curve conversion v s . amount o f alumina i s i n c o n c e r t w i t h t h a t o f s u r f a c e c o n c e n t r a t i o n o f Cu v s . amount o f alumina. With r e g a r d t o conversion t h e optimum amount o f Alu C i n the suspension i s 1 % ( w / w ) .

3.4.2 C o n c e n t r a t i o n o f CuO i n t h e Layer a t a Constant Amount of Alumina v s . Conversion o f NO I n t h i s c a s e t h e amount of alumina was maintained a t 0.46 % (w/w) whereas t h e bulk c o n c e n t a t i o n o f CuO v a r i e d between 0 . 2 - 6.6 % ( w / w ) .

A t low CuO c o n c e n t r a t i o n s

i n t h e r a n g e between 0.05 and 1 . 9 1 % (w/w) t h e conversion i n c r e a s e s remarkably w i t h t h e CuO c o n t e n t . A t about 2.0 % (w/w) CuO t h e curve shows a downward i n f l e c t i o n and conversion i n c r e a s e s t o a lower e x t e n t w i t h i n c r e a s i n g c o n t e n t of CuO. The c o u r s e of t h e f u n c t i o n can a g a i n be e x p l a i n e d i n c o n t e x t w i t h t h e v a r i a t i o n of t h e s u r f a c e c o n c e n t r a t i o n which was measured by means o f SINS on t h e r e s p e c t i v e s h e e t s . Again t h e two curves conversion v s . copper c o n t e n t and s u r f a c e c o n c e n t r a t i o n v s . copper c o n t e n t a r e i n c o n c e r t with one a n o t h e r (s. F i g . 4 ) . F i n a l l y F i g . 5 shows a n approximately l i n e a r r e l a t i o n between t h e NO conversion and t h e s u r f a c e c o n c e n t r a t i o n o f copper.

36

surface concentration of C u ( atom

P

conversion of NO ( mol '/01

70 60

50

40

0

s u r f ace c o n c e n t r a t i o n

0

conversion

-. --

30 20 10 0

4 .j

0

1

2

3

4

5

con.ent o Alu C in the suspension ( O/O w / w l

F i g . 3 : Dependence of t h e conversion of NO and s u r f a c e c o n c e n t r a t i o n of Cu, r e s p e c t i v e l y , on t h e amount of alumina i n t h e s u s p e n s i o n

37

conversion of

NO (mol ‘/el

./ /

10

..

/

d’

0

surface concentration

o

./

/

b e

a’

F i g . 4 : Dependence of conversion of NO and s u r f a c e c o n c e n t r a t i o n of copper on t h e c o n c e n t r a t i o n of Cu(N03)* i n t h e s t a r t i n g s u s p e n s i o n

conversion of NO ( m o l O/O)

30 20 10

0 0

10 20 30 LO 50 60 70 surface concentration of Cu ( a t o m %)

F i g . 5 : Conversion v s . s u r f a c e c o n c e n t r a t i o n o f Cu s e r i e s 11)

( d a t a o b t a i n e d a t s h e e t s of

38 4 . Concluding Xemarks

The results r e v e a l t h a t

( i ) t h e r e a c t i o n of NO p r e f e r a b l y t a k e s p l a c e a t t h e o u t e r s u r f a c e of t h e c a t a l y t i c l a y e r and

( T i ) t h e h i g h e s r s u r f a c e c o n c e n t r a t i o n of CuO which a l s o g i v e s t h e h i g h e s t c o n v e r s i o n i s o b t a i n e d a t a molar r a t i o CuOjAl 0 of 100 t o 200 i n t h e 2 3 layer.

REFERENCES

1 E . K o b e r s t e i n and W . M . W e i g e r t , Angew. Chem. 88 ( 1 9 7 6 ) 657 2 J.W. tiightower, i n B. Delmon, P.A. J a c o b s and G . P o n c e l e t ( E d i t o r s ) , P r e p a r a t i o n of C a t a l y s t s , E l s e v i e r , Amsterdam, i 9 7 6 , p . 615. 3 S . J . Gregg and K.S.W. S i n g , A d s o r p t i o n , S u r f a c e Area and P o r o s i t y , Academic P r e s s , London, 1967. 4 M . S e i d l , 0 . I n a c k e r , C . P l o g and E . S t e i n h e i l . i n P r o c e e d i n g s of 3 . Symposium iiber Gas-Oberflzchenwechselwirkung, Meersburg, 1975, P a r t 11, BMVG-SBWT 76-20, p. 29. 5 M. S e i d l and 0 . I n a c k e r t o be published 6 C. Plog t o be published

DISCUSSION

S.P.S.

ANDREW

:

The a u t h o r ' s m e a s u r e m e n t s u g g e s t s t h a t t h e a c t i v i t y

i s d e t e r m i n e d by t h e c o m p o s i t i o n o f t h e o u t e r m o s t l a y e r o f t h e o u t T h i s would imply t h a t t h e r e a c t i o n i s v e r y f a s t .

side coating.

Could it be t h a t t h e s u r f a c e composition of part i n the reaction ? If

inner layers also take

so, d o e s i t n o t i m p l y t h a t s u r f a c e compo-

s i t i o n of t h e s e l a y e r s i s a l s o i m p o r t a n t ? Is t h e r e a c t i o n r e a l l y so f a s t ?

J.W.

HIGHTOWER

: I s i t p o s s i b l e t h a t t h e maximum o b s e r v e d

i n Fig.

3

may b e d u e t o a d i l u t i o n o f t h e a c t i v e i n g r e d i e n t a s t h e C u i s d i s p e r s e d w i t h i n t h e l a r g e r amount o f a l u m i n a a n d i s t h e r e f o r e n o t available t o the reactants in t h i s strongly diffusion limited reaction?

0.

INACKER

:

Both q u e s t i o n s a r e c l o s e l y r e l a t e d .

i n t e r p r e t a t i o n of o u r r e s u l t s .

They c o n c e r n t h e

The p a p e r shows o n l y t h a t a c o r r e -

l a t i o n between t h e c o n v e r s i o n r a t e and t h e s u r f a c e c o n c e n t r a t i o n of t h e a c t i v e component c a n b e e s t a b l i s h e d .

F o r t h i s t y p e of

catalyst

we d o n ' t f i n d 1 0 0 % N O c o n v e r s i o n u n d e r t h e a b o v e r e a c t i o n c o n d i t i o n s and t h e a p p l i e d c a t a l y s t arrangement,

b u t f o r o t h e r c a t a l y s t s we

39

can achieve 100% conversion under the same conditions. we exclude that the reaction is very fast.

From that

We consider two possi-

bilities for the interpretation of our results.

The reaction rate

may be limited by pore diffusion, and the reaction rate is determined mainly by the highest concentration of the active component close to the gas-solid interface, which can be concluded from the depth profiles. have a

The copper on the gas solid interface may also

much higher catalytic activity than the copper within the

cupric oxide alumina layer.

But applying the approach of Dornier

System ( 5 1 , this question can principally be answered. L. GUCZI

:

Did you investigate the stability of the layer on the

cordierite, i.e. how fast does the catalytic layer leave the monolith surface 0.

INACKER

:

?

We have not investigated the longtime mechanical sta-

bility of the active layer but fast deterioration processes occurring within several hours.

Copper is removed slowly from the ac-

tive layer via volatile compounds.

Additionally a fast deteriora-

tion of the catalyst takes place due to a redox-process or to structural changes within active layer.

V. FATTORE tions.

:

All your date were obtained using copper nitrate solu-

Did you try other copper salts ? Do you thtnk that changing

the impregnating copper salt, you can find a different dependence of the conversion of NO on the surface concentration of Cu and on the amount of alumina in the suspension

?

(Fig. 3 ) .

0. INACKER : We have used other impregnation procedures too : e.g.

water as solvent and I C U ( N H ~ ) ~ ~ ’as + copper ions.

We found diffe-

rent activities and surface concentrations of copper compared to the same copper and alumina concentration in ethanol.

Within the

limits of this research program, there was no time to establish a correlation for a series of catalysts with variable copper and alumina concentration. P.G. MENQN

:

During the use of the catalyst in the reaction, does

the surface concentration of copper change

?

Do you find any sur-

face enrichment or surface depletion of copper on the used catalyst relative to the surface concentration of copper on the fresh catalyst?

40 0. INACKER : We investigated the aging of the catalyst and found

that a fresh catalyst has a higher copper concentration on the surface than a used catalyst. the reaction.

The copper content decreases during

Copper leaves the catalyst via the gas phase.

M.V. TWIGG : The impregnation procedure described involves the use of ethanolic solutions of cupric nitrate. dangerously explosive and

Such solution can be

should be diluted with a large excess

of water before disposal.

0. INACKER : We used only small quantities of that solution and we

were careful, so we had no explosion. C.J. WRIGHT : May I point out that the two disadvantages of SIMS,

namely the requirement for vacuum conditions and the frequent possibility of non-uniform sputtering are both overcome with Rutterford Back Scattering.

This technique would have given a quantitative,

and non-destructive profile of the copper present in the catalyst you have described. M. SEIDL

:

Certainly Rutherford Back Scattering in some cases would

give valuable informations additional to SIMS, but we are not sure how far our rough and structured surfaces limit the possibilities of this method.

We think also that Rutherford Back Scattering hardly

can work in a catalytic reactor at technical gas atmospheres and pressures.

So a sample transfer in an idealized gas atmosphere

should always be necessary.

Because the change of the gas composi-

tion leads to a change of the surface composition by gas-surface interactions, we think a transfer in vacuum is better.

If you have

enough experience of non-uniform sputtering processes by SIMS, it is not such a big problem.

41

EFFECT OF PRETREATMENT ON ACTIVITYl SELECTIVITY AND ADSORPTION PROPERTIES OF A FISCHER-TROPSCH-CATALYST G. LOHRENGEL, M.R. DASS, M. BAERNS Lehrstuhl fur Technische Chemie Ruhr-Universitat Bochum, Bochum, G.F.R.

ABSTRACT The oxide precursor of a Fischer-Tropsch catalyst was thermally treated at 3OO0C and subsequently reduced with hydrogen at 3OO0C (type A) and 500°C (type C) before being used for the synthesis. The surface area and pore size distribution was only slightly effected by this pretreatment but activity and selectivity as well as adsorption properties of the catalyst were changed. Decrease of activity is related to diminishing activity of a single active site at higher reduction temperature. Higher selectivity of catalyst type C towards olefins and shortchain hydrocarbons is suggested to be due to difference in adsorption properties. To elucidate the change of activity, selectivity and adsorption properties, surface composition of the two types of catalyst was studied by ESCA.

INTRODUCTION Catalysts for Fischer-Tropsch-synthesis are obtained by either coprecipitating suitable catalytic compounds, or impregnating a support with them, or mixing the appropriate oxides and treating this catalyst precursor thermally and subsquently reducing it with hydrogen or its mixture with carbon monoxide. Different conditions of these treatments can greatly change activity and selectivity. Certain aspects of the effect of pretreatment on such catalysts have been studied by several investigators. R.B. Anderson [ l ] noted a decrease in surface area and an increase in average pore diameter as a result of increasing temperature of thermal treatment performed in the range from 1 0 0 to 550°C. H. Kolbel and G. Leuteritz [2] further observed that the presence of potassium carbonate diminishes the crystallization of Fe203 up to a temperature of 4OO0C when the catalyst precursor is thermally treated. It

42

w a s f u r t h e r s e e n by K o l b e l and S c h n e i d t [ 3 ] t h a t t h e t h e r m a l p r e t r e a t ment i n f l u e n c e s t h e p r i m a r y s t r u c t u r e o f a c a t a l y s t p r e c u r s o r c o n t a i n i n g p o t a s s i u m c a r b o n a t e : t h e a d d i t i o n of 0,2 % p o t a s s i u m c a r b o n a t e res u l t e d i n t o an i n c r e a s e of 6 0 %' i n s u r f a c e a r e a and of 5 0 % i n t h e h e a t

of a d s o r p t i o n f o r t h e r e v e r s i b l e CO a d s o r p t i o n . T h i s a d d i t i o n a l s o s t e p ped u p t h e o l e f i n p r o d u c t i o n by a f a c t o r o f t h r e e . An i n f l u e n c e of t h e r e d u c t i o n t e m p e r a t u r e on a c t i v i t y and s e l e c t i v i t y of a F i s c h e r - T r o p s c h c a t a l y s t h a s been r e c e n t l y o b s e r v e d by one of t h e p r e s e n t a u t h o r s [ 4 ] : The same c a t a l y s t p r e c u r s o r a l s o u s e d i n t h e p r e s e n t s t u d y was composed of u n s u p p o r t e d o x i d e s o f m a i n l y i r o n , mangan e s e b e s i d e s t h o s e of z i n c , c o p p e r and traces of vanadium, aluminium and s i l i c i u m ; it was f u r t h e r m o d i f i e d by p o t a s s i u m c a r b o n a t e . The p e l l e t i z e d m i x t u r e of t h e s e compounds ( p e l l e t d i m e n s i o n s : d = 3 , 7 mm,

1=

6 , 2 mm) were tempered f o r 20 h o u r s i n an a r g o n a t m o s p h e r e a t 3 O O O C and

r e d u c e d w i t h hydrogen f o r 50 h o u r s a t 300'C

( t y p e A ) and 5OO0C ( t y p e C ) .

The two c a t a i y s t s A and C showed o n l y s l i g h t l y d i f f e r e n t s t r u c t u r a l da-

t a , b u t t h e i r a c t i v i t y and s e l e c t i v i t y w e r e s i g n i f i c a n t l y d i f f e r e n t . The d a t a on s u r f a c e a r e a , p o r e s i z e d i s t r i b u t i o n and p o r e volume which w e r e o b t a i n e d by n i t r o g e n a d s o r p t i o n a t 77 K and m e r c u r y p o r o s i m e t r y a r e g i v e n i n T a b l e 1 . The r e s u l t s i n d i c a t e t h a t o n l y t h e s u r f a c e due t o micropores i s s l i g h t l y reduced.

TABLE 1 S t r u c t u r a l d a t a of t h e c a t a l y s t Catalyst A BET-surface

m2/g

Surface d i s t r i b u t i o n between p o r e r a d i i nm Mo03>W03. Can you provide any explanation for this observed pattern ? A. IANNIBELLO : The thermodynamics of these systems, except some preliminary equilibrium adsorption studies, have not been deeply investigated, At present we can only speculate that the observed adsorption equilibria mainly reflect the large difference in the activity of Cr(V1)

,

Mo(V1) and W(V1) in aqueous environment.

J.L. LEMAITRE

:

At low pH values did you observe dissolution of alumi-

na and/or formation of tungstoaluminate ions

?

A. IANNIBELLO : There is no doubt that at low pH alumina dissolves. Aluminium cations in the solution have been observed when a large excess of dilute aqueous solution of ammonium dichromate or ammonium

p a r a m o l y b d a t e are c o n t a c t e d w i t h y-alumina. C o n c e r n i n g W ( V I ) ,

we

r e c a l l t h a t t h e p H of a q u e o u s s o l u t i o n s o f ammonium d i c h r o m a t e or a m m o n i u m p a r a m o l y b d a t e are c o n t a c t e d w i t h y-alumina. C o n c e r n i n g W(VI),

w e r e c a l l t h a t t h e p H of a q u e o u s s o l u t i o n s of ammonium p a r a -

t u n g s t a t e i s about 6-6.4; in this r a n g e o f p H w e d o not e x p e c t s i g n i f i c a n t d i s s o l u t i o n o f alumina. In our opinion, in the impregnation of y-alumina with paratungstate a q u e o u s s o l u t i o n s , t h e f o r m a t i o n of aluminum t u n g s t a t e s p e c i e s ( m a i n l y as s u r f a c e p h a s e ) , r a t h e r t h a n of t u n g s t e n a l u m i n a t e s p e cies, m u s t b e expected.

77

DISPERSION AND COMPOUND FORMATION I N SOME METATHESIS CATALYSTS AND FLUORINE CONTAINING ALUMINA'S, STUDIED BY XPS AND LASER--

F.P.J.M.

Kerkhof, J . A .

Moulijn, R. Thomas and J . C .

SPECTROSCOPY

Oudejans

I n s t i t u t e f o r Chemical Technology, University of Amsterdam, Amsterdam.

ABSTRACT We have s t u d i e d t h e n a t u r e of t h e i n t e r a c t i o n between c a r r i e r and promoter f o r a number of c a t a l y s t s . I t i s concluded t h a t s u r f a c e compounds a r e formed a s well on s i l i c a a s on y-alumina.

The i n t e r a c t i o n of alumina with t h e promoter i s s t r o n g e r ,

r e s u l t i n g i n b e t t e r d i s p e r s i o n , while on s i l i c a t h e formation o f c r y s t a l l i t e s i s observed a t low promoter content.

INTRODUCTION Many c a t a l y t i c systems c o n s i s t

of a high-surface

a r e a support and a well dispersed

promoter. However, t h e n a t u r e of t h e i n t e r a c t i o n between promoter and c a r r i e r i s not always uderstood and can d i f f e r i n various systems. A number of i n t e r a c t i o n s between c a r r i e r and promoter can t a k e p l a c e i n t h e

formation of t h e p r e c u r s o r of t h e a c t i v e s i t e ( r e f . 1, 2 ) .

-

Diffusion of t h e promoter i n t o t h e c a r r i e r ( s o l i d s o l u t i o n ) . Formation o f a new bulk compound between support and promoter.

- Creation of a molecular d i s p e r s i o n of t h e promoter on t h e c a r r i e r (monolayer).

-

Growth of promoter c r y s t a l l i t e s on t h e support.

W e have s t u d i e d t h e formation of t h e p r e c u r s o r f o r a number of c a t a l y s t s . The t r a n s -

formation of p r e c u r s o r i n t o t h e f i n a l a c t i v e s i t e i s n o t d e a l t with here. Metathesis c a t a l v s u A t y p i c a l example of t h e metathesis r e a c t i o n i s t h e conversion of propene i n t o an

equimolar mixture of ethene and 2-butene.

This r e a c t i o n i s c a t a l y s e d by e.g.

tungsten-

oxide on s i l i c a o r alumina and rheniumoxide on alumina. We have s t u d i e d t h e s e c a t a l y s t s by

s e v e r a l techniques and obtained valuable information from laser-Raman

spectroscopy ( r e f . 3 , 4 ) . I n t h e case o f tungstenoxide on s i l i c a two tungsten s p e c i e s could be i d e n t i f i e d v i z . c r y s t a l l i n e t u n g s t e n t r i o x i d e with t h e main Raman l i n e s a t 809 and 715 cm-' a compound with a broad band a t 970 cm-l.

and

This band was t e n t a t i v e l y a s c r i b e d t o a

s u r f a c e compound c o n s i s t i n g ofdistortedtungstenoxide octahedrons. We t r i e d t o c o r r e l a t e t h e s e s p e c i e s with t h e r e s u l t s of reduction experiments.

78 F u r t h e r t h e presence of t h e band a t 970 cm-las a function of c a t a l y s t s y n t h e s i s and c a l c i n a t i o n temperature was s t u d i e d . I n t h e case of rheniumoxide on alumina we concluded t h a t i n t h e s e c a t a l y s t s rhenium

i s p r e s e n t a s a monolayer of rheniumoxide t e t r a h e d r o n s . Even a t high rheniumoxide content (18 w t % ) no o t h e r s p e c i e s l i k e o c t a h e d r a l l y coordinated rhenium o r Al/Re/O compounds could be found. In t h e case of tungstenoxide on alumina we found t h a t up t o 15 w t % tungstenoxide only a band a t 970 cm-'was

p r e s e n t and t h e r e f o r e t h e formation

of tungstenoxide c r y s t a l l i t e s could be excluded. I t w i l l be shown t h a t XPS confirms t h e formation of tungstenoxide c r y s t a l l i t e s i n t h e case o f t u n g s t e n / s i l i c a c a t a l y s t s and t h e presence ofamonolayer on t h e rheniumoxide/alumina c a t a l y s t s . F l u o r i n a t - ? d alumina's I t i s well know t h a t i n c o r p o r a t i o n of f l u o r i n e enhances t h e a c t i v i t y o f y-alumina

i n polymerization, cracking, isomerization and t h e d i s p r o p o r t i o n a t i o n of t o l u e n e ( r e f . 5 , 6 ) . The f l u o r i n e i n t h e s e c a t a l y s t s can be p r e s e n t as:

- Surface groups.

-

A l F 3 c r y s t a l l i t e s ( a and

6 modification).

Aluminum hydroxyfluorides.

By using XPS, we could show, t h a t f o r c a t a l y s t s prepared by impregnation of boehmite

with a s o l u t i o n of ammoniumfluoride i n aqueous ammonia, up t o 2 6 w t 8 f l u o r i n e no A l F 3 c r y s t a l l i t e s were p r e s e n t . ( r e f . 7 ) .

The binding e n e r g i e s of F ( l s ) e l e c t r o n s i n A1F3 and i n s u r f a c e f l u o r i n e were 687.5 and 685.7 eV r e s p e c t i v e l y . The binding energy of t h e Al(2p) e l e c t r o n s i n A l F 3 was 77.3 e V , whereas a value of 75.0 eV w a s found i n y-alumina.

By measuring

t h e XPS s p e c t r a we t h e r e f o r e could examine t h e presence of c r y s t a l l i t e s and s u r f a c e f l u o r i n e from a s h i f t i n t h e binding energy o f t h e Al(2p) and F ( 1 s ) e l e c t r o n s . Besides XPS, X-ray d i f f r a c t i o n (XRD) was used t o study t h e compound formation i n f l u o r i n a t e d alumina's prepared by co-impregnation

o f A l ( N O 3 ) 3 and NH4F on y-alumina

and s i l i c a . The use o f XPS-intensities The binding e n e r g i e s of t h e W(4f) and

and S i ( 2 p ) e l e c t r o n s of some tungsten-

oxide on s i l i c a c a t a l y s t s are r e p o r t e d elsewhere ( r e f . 8 ) . W e showed t h a t d i f f e r e n t i a l charging caused a l i n e broadening of t h e XPS peaks. With a new sample p r e p a r a t i o n technique t h e l i n e broadening could be avoided and e.g.

t h e broad peak

of t h e W(4f) e l e c t r o n s r e s o l v e d i n a s h a r p doublet. I n s p i t e of t h e l i n e broadening t h e i n t e n s i t i e s of t h e s i g n a l s can be used t o measure t h e d i s p e r s i o n of t h e promoter. Angevine e t a l .

( r e f . 9 ) have proposed a model which p r e d i c t s t h e r e l a t i v e XPS

i n t e n s i t y of t h e promoter i n t h e c a s e of monolayer and c r y s t a l l i t e formation. One of t h e assumptions of t h i s model i s t h e presence of t h e promoter on a s e m i - i n f i n i t e support. We t h i n k t a h t t h i s assumption i s not always j u s t i f i e d and t h e r e f o r e t h e pred i c t i o n of t h e support i n t e n s i t y by t h e model of Angevine can be t o o high.

79 The t h i c k n e s s o f t h e support can be estimated from a simple model. I f t h e support i s envisaged a s a t h i n square p l a t e , t h e t h i c k n e s s d i s given by d = 2 ~ - l S - ' ( p = t r u e

d e n s i t y of t h e support and S i s t h e s u r f a c e a r e a ) . For a t y p i c a l s i l i c a support with p = 2200 k g ~ n - and ~ S = 350 rn2g-l t h i s r e s u l t s i n d = 2.6 nm. Because t h i s i s i n t h e

o r d e r of magnitude of t h e mean f r e e path o f t h e e l e c t r o n s i n t h e s o l i d most of t h e c a r r i e r i s "seen" by XPS. In t h e case of a monolayer c a t a l y s t a l s o no l o s s of promoter s i g n a l occurs and t h e r e f o r e t h e XPS i n t e n s i t y can be p r e d i c t e d from t h e bulk atomic r a t i o and t h e photo-electron c r o s s s e c t i o n s . When a t h i n s u p p o r t i s assumed it can e a s i l y be proven t h a t t h e model of Angevine reduces t o t h i s p r e d i c t i o n ( r e f . 1 0 ) . EXPERIMENTAL Preparation of t h e c a t a l y s t s Tunustenoxide on s i l i c a and alumina by auueous m-i prepared by adding t h e c a r r i e r e.g.

'

These c a t a l y s t s were

Grace s i l i c a (190 pm) t o an aqueous s o l u t i o n of

ammoniummetatungstate ( :NH4) 6 H 2 W 1 2 0 4 0 ) .

A f t e r removal of t h e water i n a r o t a t i n g f i l m

evaporator t h e sample was d r i e d overnight a t 390 K. Than t h e samples were f l u i d i z e d i n dry a i r a t 820 K during two hours. In t h e case of t h e tungstenoxide promoted s i l i c a ' s t h e white powders turned yellow during t h i s procedure. A t higher promotercontent t h i s e f f e c t became s t r o n g e r . In case of t h e tungstenoxide on alumina c a t a l y s t s t h i s e f f e c t was n o t observed and t h e samples were white before and a f t e r c a l c i n i n g . A t u n g s t e n o x i d e / s i l i c a c a t a l y s t was prepared by aqueous impregnation with s i l i c o -

tungsten a c i d .

(Si02-12W03.26H20).

Tunqstenoxide on s i l i c a - a e r o q e l . The p r e p a r a t i o n o f s i l i c a g e l with e x c e l l e n t o p t i c a l p r o p e r t i e s i s given by P e r i ( r e f . 1 1 ) . We have synthesized t h e s i l i c a - a e r o g e l

and

t r i e d t o impregnate t h e s e supports by passing tungstenhexachloride vapour i n d r y nitrogen over t h e s i l i c a a t 470 K.

I t proved t h a t most of t h e tungsten was p r e s e n t on

t h e o u t s i d e of t h e s i l i c a p l a t e and no uniform d i s p e r s i o n could be obtained. Tunqstenoxide on s i l i c a with s o l u t i o n s o f tunustenhexachloride i n met-

Before

impregnation t h e support i s contacted with dry methanol during 40-50 days. During t h i s p e r i o d t h e methanol i s r e f r e s h e d 4 t o 5 t i m e s . After decanting t h e methanol, tungstenhexachloride i n methanol i s added. When tungstenhexachloride i s added t o t h e methanol, hydrogenchloride i s evolved, probably r e s u l t i n g from t h e formation of hexamethoxytungsten. The methanol i s removed i n an autoclave under s u p e r c r i t i c a l conditions ( P = 9-11 MPa, T = 533 K ) . A f t e r t h i s , t h e blue samples were c a l c i n e d under t h e same c o n d i t i o n s a s described before. Tunustenoxide and rheniumoxide on alumina. These impregnations are s i m i l a r t o t h e f i r s t described procedures and a r e r e p o r t e d i n d e t a i l by Kapteijn e t a l . ( r e f . 12) and Thomas e t a l .

(ref. 3 ) .

Fluorine c o n t a i n i n s alumina's. These samples were prepared by adding y-alumina

80

(Ketjen alumina grade B, calcined in dry air during 16 hours by the method given before) to an aqueous solution of aluminumnitrate and ammoniumfluoride in the molar ratio 1

:

3 . After drying in the same way as mentioned above catalysts were calcined

during 16 hours. Laser-Raman spectroscopy The spectra were recorded on a Jeol-JRS-1 or Ramanor HGZS spectrometer. The details of the experimental procedures are given by Thomas et al. (ref. 3 ) . Duplicate runs of some samples were run on both spectrometers. X-ray photoelectron spectroscopy The spectra were recorded on a AES-200 spectrometer using AlKa and MgKa radiation. The X-ray source power was 180 watt. The samples were powdered and mounted on adhesive tape. Further details are given elsewhere (ref. 8 ) . RESULTS AND DISCUSSION Laser-Raman spectroscopy and reducibility of some metathesis catalysts The Laser-Raman spectra of the tungsten on alumina and tungsten on silica catalysts prepared by the aqueous impregnation with ammoniummetatungstate are given elsewhere (ref. 3 ) . The spectra of the silica catalysts are characterized by the lines of tungstenoxide at 809 and 715 cm-' and a broad band at 970 cm-'. of a 15 wt

%

In case

tungstenoxide on alumina catalyst only a broad band at 970 cm-l is

_i_:v

observed. The fact that at least two tungsten species are present on the silica catalysts is confirmed by reduction experiments (ref. 1).

At low tungsten con-

fents a rather large fraction of the tungsten is present as a hardly reducible

compound whereas at higher contents the easier reducible tungstenoxide is formed. The amount of easier reducible material is plotted as a function of the Raman

intensity of the tungstenoxide line at 809 cm-l in figure 1. The amount of hardly reducible material versus the intensity of the band at 970 cm-'is

given in figure 2

, -

(a.u.1

0

1

''

4

Fracfzn Crystalline W03

Figure 1. Intensity of the Raman line of tungstenoxide as a function of the amount OE easily zeducible material.

-

0.10 Fraction Hardly Reducible WO,

0.05

Figure 2 . Iniensity of the Raman band at 970 cm-I as a function of the amount of hardly reducible material.

81 I n f i g u r e 1 i s shown t h a t t h e r e i s a good c o r r e l a t i o n between t h e amount of cryst a l l i n e m a t e r i a l measured by laser-Raman spectroscopy and by reduction experiments. The c o r r e l a t i o n i n f i g u r e 2 suggests t h a t t h e band a t 970 cm-l can be a s c r i b e d t o a hardly reducible compound. This i s i n agreement with t h e f a c t t h a t f o r tungstenoxide on alumina, which i s harder t o reduce t h a n tungstenoxide on s i l i c a , m l y a band a t 970 cm-l i s p r e s e n t ( r e f . 1 3 ) . By comparing t h e s l o p e s o f t h e l i n e s i n f i g u r e 1 and 2 it can be concluded t h a t t h e Raman a c t i v i t y of tungstenoxide i s about s i x times higher t h a t t h e Raman a c t i v i t y of t h e hardly r e d u c i b l e material. The presence of tungstenoxide c r y s t a l l i t e s w a s a l s o d e t e c t e d with e l e c t r o n microscopy. The electronmicrographs o f a 12 and 41 w t

%

c a t a l y s t showed t h a t t h e s i z e

of t h e c r y s t a l l i t e s i s i n t h e same o r d e r o f magnitude i n both c a s e s (k20 nm). This i s i n agreement with e s t i m a t e s from t h e l i n e broadening of t h e XRD l i n e s . I t was a l s o revealed t h a t o n t h e 4 1

%

c a t a l y s t more c r y s t a l l i t e s were p r e s e n t than would be ex-

pected from an i n t e r p o l a t i o n of t h e number of c r y s t a l l i t e s p r e s e n t on t h e 12

%

catal-

y s t . This corresponds with t h e f a c t t h a t on a 12 w t % c a t a l y s t about h a l f of t h e tungsten i s p r e s e n t as c r y s t a l l i n e m a t e r i a l whereas on t h e 4 1 % c a t a l y s t 90 % of t h e tungsten i s c r y s t a l l i n e . I n t h e s p e c t r a of t h e c a t a l y s t s prepared by impregnation with tungstenhexachloride i n methanol no i n t e n s e l i n e s could be observed. Except f o r bands a t about 800 cm-’, 700 cm-l and i n some c a s e s a weak one a t 970 cm-’

no o t h e r bands were p r e s e n t .

Because t h e s e c a t a l y s t s a r e q u i t e a c t i v e and have a good dispersion,we conclude t h a t on t h e s e c a t a l y s t s p a r t of t h e tungsten i s p r e s e n t as a Raman i n a c t i v e species. Brown e t a l .

( r e f . 14) showed t h a t f o r molybdenum on alumina c a t a l y s t s a treatment

with water vapour r e s u l t e d i n t h e appearance o f a Raman band which w a s absent i n a regenerated c a t a l y s t - W h e t h e r t h e formation of a Raman a c t i v e s p e c i e s upon water or hydroxyl coordination can be observed i n our c a t a l y s t s , w i l l be i n v e s t i g a t e d . The laser-Raman

,

spectrum of a c a t a l -

y s t prepared by aqueous impregnation o f s i l i c a with s i l i c o t u n g s t e n a c i d i s shown i n f i g u r e 3 . This f i g u r e shows t h a t t h e l i n e s o f t l i e s p e c t r u m of t h e uncalcined c a t a l y s t a r e t h e same a s f o r t h e unsupported s i l i c o t u n g s t e n acid. A f t e r c a l -

’

9’00

7’00

’

500

3b0

’

100

c cm-’-

c i n i n g t h e same spectrum i s found a s i n t h e case of t h e c a t a l y s t s prepared with

Figure 3. Laser-Raman s p e c t r a of s i l i c o t u n g s t e n acid; pure ( a ) , on s i l i c a before c a l c i n i n g (b), on s i l i c a a f t e r c a l c i n i n g (c+d) A , b and c recorded a t 100 cm’l/min;d a t 20 cm-l/min.

.

spectrum does n o t r e v e a l a band a t 970 cm-l.

anmoniummetatungstate viz. tungstenoxide l i n e s a t 809 and 715 cm-land a broad band a t 970 cm-l. Before calcining, t h e laser-Raman T h i s suggests t h a t t h e i n t e r a c t i o n

82 compound i s formed during t h e c a l c i n a t i o n . Alsowhen a c a t a l y s t w a s p r e p a r e d by impregnation of a d i f f e r e n t s i l i c a (Mallinckrodt) with ammoniummetatungstate t h e s e l i n e s were found. We t h e r e f o r e conclude t h a t when a s i l i c a c a t a l y s t i s prepared by aqueous impregnation with a tungsten compound,aswell tungstenoxide a s an i n t e r a c t i o n compound i s p r e s e n t a f t e r c a l c i n i n g . Sinterinq effects We t r i e d t o vary t h e s i z e o f t h e tungstenoxide c r y s t a l l i t e s by h e a t i n g t h e c a t a l y s t s i n a i r a t s e v e r a l temperatures.Figure 4 g i v e s t h e laser-Raman

spectra

of a 12 w t % tungstenoxide c a t a l y s t a f t e r s e v e r a l hours a t 920 K. The f i g u r e shows t h a t h e a t i n g of t h e c a t a l y s t r e s u l t s i n t h e

12%

wo3/s I o2

disappearance of t h e peak a s c r i b e d t o t h e s u r f a c e compound. A f t e r prolonged h e a t i n g

r,

t h e l i n e s of tungstenoxide become more 1

I

11

i n t e n s e , The d i f f r a c t i o n p a t t e r n s o f t h e s e c a t a l y s t s a r e shown i n f i g u r e 5.

did x

Sutfacrlrea

I

V Figure 4. Laser-Raman s p e c t r a of a 12 w t % tungstenoxide on s i l i c a c a t a l y s t before and a f t e r h e a t i n g i n a i r !at 920 K.

50

40

30

20

10

Figure 5. X-ray d i f f r a c t i o n p a t t e r n s of a 1 2 w t % tungstenoxide on s i l i c a c a t a l y s t , before and a f t e r h e a t i n g i n a i r a t 920 K.

The b e t t e r c r y s t a l l i n i t y upon h e a t i n g i s shownhere by t h e decreasing peak width. This e f f e c t i s more pronounced a f t e r h e a t i n g a t 1070 K. I n t h i s case c r y s t a l l i z a t i o n of a s well tungstenoxide a s t h e c a r r i e r i s observed. This i s shown by t h e XRD-patterns of t h e c a t a l y s t s ( f i g u r e 6 ) . A f t e r two hours t h e l i n e s o f tungstenoxide a r e observed and a f t e r 2 4 hours a l s o t h e XRD-pattern

of c r y s t o b a l i t e (a-quartz) i s found. I t i s

s t r i k i n g t h a t t h e c r y s t a l l i z a t i o n of t h e c a r r i e r i s f a r less when no tungstenoxide i s p r e s e n t . T h i s i s shown i n f i g u r e 7 which gives t h e XRD-patterns of t h e support a f t e r h e a t i n g a t 1070 K. The main e f f e c t of h e a t i n g i n t h i s case i s t h e decrease of t h e s u r f a c e a r e a from 300 t o 160 m2g-l. The i n c r e a s e of t h e p a r t i c l e s i z e o f t h e s i l i c a can be estimated by a decreasing widthof t h e s i l i c a band a t 28=22' (from 1 . 3 t o 1.5 nm)

.

83

Surface Area

,2

p-'

tlme hr

50

50

30

__

-20

Figure 6. X-ray d i f f r a c t i o n p a t t e r n of a 12 w t % tungstenoxide on s i l i c a c a t a l y s t before and a f t e r h e a t i n g i n a i r a t 1070 K .

20

30

40

+28

20

10

-

Figure 7. X-ray d i f f r a c t i o n p a t t e r n s of s i l i c a b e f o r e and a f t e r h e a t i n g i n a i r a t 1070 K.

%:::b[

Because of t h e decrease i n small angle s c a t t e r i n g ( r e f .

12'1. W 0 3 / S l O ~

15) t h i s i n c r e a s e i n p a r t i c l e s i z e i s apparently due

t o t h e disappearance of t h e s m a l l e s t s i l i c a p a r t i c l e s . The loss of surface-area i s represented i n f i g u r e 8.

From t h i s f i g u r e it can be concluded t h a t h e a t i n g a t t h e normal c a l c i n a t i o n temperature has no influence

100

1070K

on t h e s u r f a c e a r e a , while a sharp decrease i s found 30

20

u p o n h e a t i n g a t 1070 K. A f t e r 2 4 h o u r s a t 1 0 7 0 K t h e s u r f a c e a r e a of t h e c a t a l y s t i s p r a c t i c a l l y zero,

Figure 8. Surface a r e a of a 12 w t % c a t a l y s t a s a function of c a l c i n a t i o n t i m e and temperature.

whereas t h e support without tungstenoxide s t i l l has h a l f of i t s o r g i n a l

s u r f a c e a r e a . When an alumina

c a t a l y s t i s heated a t 1020 K,nor a decrease i n s u r f a c e area nor t h e disappearance of t h e s u r f a c e compound with a Raman band a t 970 cm-l

i s found.

F l u o r i n a t e d alumina' s The XRD-patterns of a number of c a l c i n e d c a t a l y s t s , prepared by co-impregnation of N H + F and A l ( N O 3 ) 3 on y-alumina a r e shown i n f i g u r e 9. By measuring t h e XPS s p e c t r a of t h e s e c a t a l y s t s t h e presence of s u r f a c e bound f l u o r i n e atoms can be measured ( r e f . 7 ) . By combining XPS and XRD we could compose t a b l e 1 which i n d i c a t e s t h e presence of s e v e r a l compounds i n uncalcined and c a l c i n e d samples. I t can be concluded t h a t up t o 10 w t 8 F, f l u o r i n e i s p r e s e n t only i n s u r f a c e

groups. From 10 t o 40 w t % F aluminumhydroxyfluorides are formed whereas a t higher f l u o r i n e content O-AlF3 i s observed. The co-impregnation was based on t h e p o s s i b l e formation of B-A1F3 on t h e c a t a l y s t . Without support i n d e a d t h e formation of @-A1F3 was observed a f t e r c a l c i n i n g . Also on a 15 w t % f l u o r i n e on s i l i c a c a t a l y s t t h e formation of

O-AlF3

was favoured. The d i f f e r e n c e i n formation of aluminumhydroxy-

f l u o r i d e s on s i l i c a and alumina, w i l l b e t h e s u b j e c t of f u r t h e r i n v e s t i g a t i o n .

84

w 48 24

u

1 2

0

2s -

Figure 9. XRD-patterns of some f l u o r i n e containing alumina‘s. TABLE 1

Compounds i n f l u o r i n e c o n t a i n i n g alumina’s % F

Compounds p r e s e n t (XRD) before calcination

-

1

a f t e r calcination

-

-

2

.

I,

-

.

A I F 1 960H1 0 4

n

*

+ + + + + +

-

A1F1.650H1,35

6 7 11 12 14

Surface f l u o r i n e p r e s e n t i u 685.7 ev) (XPS s a f t e r calcination

A1F1.960H1.04

2d 48

*Before -

A1P1-960H1.04

I,

+

F ( l s ) of t h e hydroxy-

f l u o r i d e s (687.5 ev) i s dominant.

1

8-AlI?3

c a l c i n a t i o n n o t measured.

XPS-intensities T h e , r e l a t i v e i n t e n s i t i e s of promoter and support of t h e metathesis c a t a l y s t s i s p l o t t e d as a function o f t h e bulk atomic r a t i o i n t h e f i g u r e s 10, 11 and 12.

0

0.m

004 (W*l)h+

Figure 10. XPS i n t e n s i t y r a t i o o f t h e rhenium-

oxide on alumina c a t a l y s t s .

(V*l)k&

Figure 11. XPS i n t e n s i t y r a t i o of t h e f l u o r i n e containing alumina c a t a l y s t s .

-l5W,

85

I(W4f) I(S12p)'O

05

. f rcatalyst used e s h catalyst-a -b A0

0.1

0.2

(w/sl)b"lk

Figure 12. XPS i n t e n s i t y r a t i o o f t h e tungstenoxide on s i l i c a c a t a l y s t s . a = c a t a l y s t s prepared by aqueous impregnation b = c a t a l y s t s prepared by imprgnation with a s o l u t i o n of tungstenhexachloride i n methanol. I n t h e s e f i g u r e s a l s o t h e p r e d i c t i o n based on t h e atomic bulk r a t i o and t h e photoe l e c t r o n c r o s s s e c t i o n of S c o f i e l d ( r e f . 16) a r e given. I n t h e case of rheniumoxide on alumina and f l u o r i n a t e d alumina's t h e s e predict i o n s agree well with t h e experimental values. This i s r a t h e r s u r p r i s i n g , considering t h e f a c t t h a t XPS i s a s u r f a c e technique. I n t h e i n t r o d u c t i o n we showed t h a t t h e t h i c k n e s s of t h e support i s i n t h e same o r d e r of magnitude a s t h e e l e c t r o n escape depth and t h e r e f o r e only a small l o s s of s i g n a l o f t h e carrier i s t o be expected. I n t h e case o f a monolayer c a t a l y s t no l o s s of i n t e n s i t y o f t h e promoter e l e c t r o n s w i l l occur. These two f a c t s r e s u l t i n a simple model p r e d i c t i n g t h e i n t e n s i t i e s of monolayer c a t a l y s t s . Figure 10 confirms t h e i d e a of Olsthoorn and Boelhouwer ( r e f . 17) and o f Kapteijn

e t a l . ( r e f . 12) t h a t rheniumoxi.de on alumina i s a monolayer c a t a l y s t up t o a rhenium content of 18 w t % .The same conclusion can be drawn f o r t h e f l u o r i n a t e d alumina's up t o 10 w t % f l u o r i n e . A t higher f l u o r i n e content c r y s t a l l i t e s a r e formed and a decrease i n XPS i n t e n s i t y i s observed. The f a c t t h a t t h e experimental values are lower than i s p r e d i c t e d from t h e bulk r a t i o f o r tungstenoxide on s i l i c a i s explained by t h e formatjon of c r y s t a l l i t e s from which only a p a r t i s "seen" by XPS. Figure 12 shows t h a t t h e c a t a l y s t s prepared from a s o l u t i o n of tungstenhexachloride i n methanol have a higher XPS i n t e n s i t y . We theref o r e conclude t h a t on t h e s e c a t a l y s t s t h e d i s p e r s i o n of t h e tungstenoxide i s b e t t e r . This i s confirmed by t h e f a c t t h a t t h e XRD-patterns of t h e tungstenhexachloride/ methanol c a t a l y s t s show t h e presence of c r y s t a l l i n e m a t e r i a l a t a h i g h e r tungsten content than t h e c a t a l y s t s prepared by aqueous impregnation. I t might be p o s s i b l e t h a t t h e reason f o r t h e b e t t e r d i s p e r s i o n of t h e tungsten-

hexachloride/methanol

c a t a l y s t s i s caused by an i n t e r a c t i o n o f t h e hexamethoxy-

tungsten with t h e methoxy groups p r e s e n t on t h e s i l i c a a f t e r e q u i l i b r a t i o n with methanol. Obviously, t h e i n t e r a c t i o n of t h e hydroyl groups of t h e s i l i c a with t h e tungsten compound i n water i s smaller and t h e r e f o r e t h e formation o f c r y s t a l l i t e s

86 i s favoured. CONCLUDING REMARKS

I n t h i s paper we sho.ded. t h a t alumina supported c a t a l y s t s have a s t r o n g e r i n t e r a c t i o n with t h e promoter than s i l i c a . The s t r o n g i n t e r a c t i o n a l s o seems t o r e s u l t i n a b e t t e r s t a b i l i t y of t h e compound formed between c a r r i e r and promoter e.g.

t h e t u n g s t e n / s i l i c a systems s e g r e g a t e s a t

high temperature i n a-quartz and tungstenoxide, whereas t h e i n t e r a c t i o n compound between alumina and tungstenoxide i s not decomposed. We showed how XPS can be used a s a t o o l t o observe monolayer and c r y s t a l l i t e formation. Whereas rheniumoxide on alumina i s a monolayer c a t a l y s t c o n s i s t i n g of Re04-

t e t r a h e d r o n s on t h e s u r f a c e , c r y s t a l l i t e s a r e formed i n t h e case of tungsten-

oxide on s i l i c a .

ACKNOWLEDGEMENTS We are indebted t o D r G.Sawatzky and A. Heeres (Laboratory f o r Physical Chemi s t r y , University o f Groningen) f o r u s e f u l l d i s c u s s i o n on t h e XPS s p e c t r a . Thanks a r e a l s o due t o D r B. Koch and W. Molleman (Department of X-ray Spectroscopy and D i f f r a c t i o n , University of Amsterdam). We a r e a l s o indebted t o D r J. Medema, D r D.J.

Stufkes and D r V.H.J.

de B e e r f o r h e l p i n recording and i n t e r p r e t a t i o n of

Ramanspectra. This study was supported by t h e Netherlands Foundation f o r Chemical Research (S.O.N.)

with f i n a n c i a l a i d from t h e Netherlands Organization f o r t h e

Advancement o f Pure Research ( Z . W . O . ) . REFERENCES

1. F.P.J.M. Kerkhof, R. Thomas and J . A . Moulijn, Recl. Trav. Chim.,Pays-Bas 9 6 ( 1 1 ) , 1977, M121. 2. T. Jansen, P.C. van Berge and P. Mars i n “Preparation of C a t a l y s t s “ e d i t e d by B. Delmon, P.A. Jacobs and G. Poncelet, E l s e v i e r , Amsterdam, 1976. 3. R. Thomas, J . A . Moulijn and F.P.J.M. Kerkhof, Recl. Trav. Chim., Pays-Bas, 9 6 ( 1 1 ) , 1977, M134. 4. F.P.J.M. Kerkhof, J . A . Moulijn and R. Thomas, submitted f o r p u b l i c a t i o n . 5. V.C.F. Holm and A . Clark, Ind. Eng. Chem., Prod. Res. Develop. 2(1963)38. 6. R. Covini, V. F a t t o r e and N . Giordano, J. C a t a l . 7(1967)126. 7. F.P.J.M. Kerkhof, H . J . Reitsma and J.A. Moulijn, React. Kinet. Catal. L e t t . , 7-1 (1977) 15. 8. F.P.J.M. Kerkhof, J . A . Moulijn and A. Heeres, submitted f o r p u b l i c a t i o n . 9. P . J . Angevine, J . C . V a r t u l i and W.N. Delgass i n t h e Proceedings of t h e S i x I n t e r n a t i o n a l Congress on C a t a l y s i s , volume 2 , p 611 e d i t e d by G.C. Bond, P.B. Wells and F.C. Tompkins, The Chemical S o c i e t y , Burlington House, London 1976. 10. F.P.J.M. Kerkhof and J . A . Moulijn, t o be published. 11. J . B . P e r i , J.Phys.Chem., 70(1966)2937. 12. F. Kapteijn, L.H.G. Bredt and J . C . Mol, Recl.Trav.Chim Pays-Bas 96(11),1977 M139. 13. P . Biloen and G . T . P o t t , J.Cata1. 30(1974)169. 14. F.R. Brown, L.E. Makovsky and K.H. Rhee, J. C a t a l . 50(1977)162. 15. M.H. J e l l i n e k and I. Frankuchen, Adv. Catal. 1(1948)257. S c o f i e l d , J . Electron Spectrosc. Relat. Phenom. 8(1976)129. 16. J.H. 17. A.A. Olsthoorn and C . Boelhouwer, J . C a t a l . 44(1976)197.

87 DISCUSSION

D.

CHADWICK

h a v e you c o n s i d e r e d u s i n g a f l o o d gun t o

: Firstly,

e l i m i n a t e d i f f e r e n t i a l charging ? Secondly,

i n t h e c a s e o f t h e XPS

r e s u l t s on f l u o r i n e c o n t a i n i n g c a t a l y s t s i t would b e i n t e r e s t i n g t o p l o t t h e F ( 2 s ) / F ( l s ) r a t i o a g a i n s t bulk composition.

Since

t h e s e p h o t o l i n e s have d i f f e r e n t k i n e t i c e n e r g i e s and t h e r e f o r e d i f f e r e n t sampling d e p t h s ,

s u c h a p l o t s h o u l d p r o v i d e an i n d e p e n d e n t

check on your method.

KERKHOF : 1 ) T h e a p p a r a t u s w e u s e d was n o t e q u i p p e d w i t h

F.P.J.M.

a f l o o d gun.

Moreover,

u s i n g a f l o o d g u n t h e s a m p l e may g e t a n e g a -

t i v e c h a r g e and t h i s c h a r g i n g c a n b e n o t - u n i f o r m

2 ) Using A I K a

X-rays,

too.

t h e k i n e t i c e n e r g i e s of t h e F ( 2 s ) and F ( 1 s )

e l e c t r o n s a r e 1.4 a n d 0 . 8 k e v r e s p e c t i v e l y .

Therefore the diffe-

r e n c e i n e s c a p e d e p t h w i l l b e a b o u t 30% ( 2 . 0 and 1 . 5 nm). Because of

t h i s f a c t it can b e expected t h a t a t high f l u o r i n e c o n t e n t s

( c r y s t a l l i t e f o r m a t i o n ) t h e v a l u e F (2s)/ F ( I s 1 w i l l s l i g h t l y i n c r e a s e .

M. H O U A L L A : You h a v e e l e g a n t l y shown t h a t t h e f r a c t i o n h a r d l y red u c i b l e W03/Si02 i s r e l a t e d t o t h e i n t e n s i t y o f t h e Raman b a n d a t -1 970 c m On t h e o t h e r hand, d u r i n g your s t u d y of t h e s i n t e r i n g

.

e f f e c t , you r e p o r t t h e d i s a p p e a r a n c e of t h i s b a n d b y h e a t i n g a t 920 K .

Would t h i s d i s a p p e a r a n c e c o r r e l a t e w i t h a n a b s e n c e o f

the

fraction unreducible ?

KERKHOF : W e h a v e o n e i n d i c a t i o n o f t h e f a c t t h a t i n d e e d -1 a r e easier

F.P.J.M.

c a t a l y s t s w i t h o u t a b a n d i n t h e Raman s p e c t r a a t 970 c m t o reduce, viz.

c a t a l y s t s w h i c h w e r e r e d u c e d a t 920 K a n d r e o x i d i z e d

a t 6 5 0 K r e d u c e d much f a s t e r t h a n t h e f r e s h c a t a l y s t s a n d show no b a n d a t 970 c m - I .

W e hope t o o b t a i n more e v i d e n c e on t h e r e l a t i o n

b e t w e e n r e d u c i b i l i t y a n d s u r f a c e c o m p o s i t i o n by a p p l i c a t i o n o f T . P . R .

P.G.

ROUXHET

Selim, J.P. i n press)

: We h a v e r e c e n t l y i n v e s t i g a t e d

Damon a n d P . G .

S c o k a r t , S.A.

Rouxhet, J . C o l l o i d I n t e r f a c e S c i .

(1978),

f l u o r i n a t e d a l u m i n a s p r e p a r e d b y i m p r e g n a t i o n o f Y-A1203

b y NH4F, u s i n g XPS a n d o t h e r t e c h n i q u e s . f a c e of

(P.0.

alumina i s modified.

A t

low F c o n t e n t , t h e s u r -

When t h e F c o n t e n t i n c r e a s e s a b o v e

a b o u t 5 % , f l u o r i n e c o n t r i b u t e s t o make A 1 F 3 p a r t i c l e s w h i c h a r e s e p a r a t e d f r o m a l u m i n a a n d d e v e l o p a low s u r f a c e a r e a .

We h a v e a l s o

i n v e s t i g a t e d t h e a c i d i t y p r o p e r t i e s of t h e modified alumina s u r f a c e s .

88 F o r p r o d u c t s o u t g a s s e d at 6 O O 0 C , h y d r o x y l s r e m a i n i n g o n t h e s u r f a c e h a v e a stronger acidity than h y d r o x y l s of alumina; s t r o n g e r n o n p r o t o n i c s i t e s are a l s o observed. F o r p r o d u c t s calcined at h i g h temp e r a t u r e , stored in t h e laboratory a t m o s p h e r e and r e a c t i v a t e d at 3 O O 0 C , strong p r o t o n i c s i t e s are o b s e r v e d ; they are r e s p o n s i b l e for t h e i s o m e r i z i n g activity of t h e c a t a l y s t s and are t e n t a t i v e l y assigned t o w a t e r m o l e c u l e s held by the strong non-protonic s i t e s obs e r v e d after o u t g a s s i n g at higher temperature.

F.P.J.M.

KERKHOF

: Your result

for t h e f l u o r i n e d i s t r i b u t i o n as

s u r f a c e g r o u p s and a s c r y s t a l l i n e m a t e r i a l in c a t a l y s t s t r e a t e d w i t h

NH4F i s s i m i l a r t o our r e s u l t s for t h e s e c a t a l y s t s (Kerkhof,F.P.J.M., Reakt. Kinet. Catal. Lett., 7-1

R e t t s m a , N.J.

and Moulijn, J . A . ,

(1977), 15).

Recently w e h a v e s t u d i e d f l u o r i n a t e d a l u m i n a s by in-

f r a r e d s p e c t r o s c o p y , p o i s o n i n g e x p e r i m e n t s and activity m e a s u r e m e n t s , and a l s o f o u n d a c o r r e l a t i o n b e t w e e n t h e n u m b e r o f p r o t o n i c s i t e s and c a t a l y t i c activity. T h e s e results w i l l b e reported in t h e n e a r future. F. T R I F I R O : You h a v e a t t r i b u t e d t h e o c t a h e d r a l c o o r d i n a t i o n t o t h e s p e c i e s o f t u n g s t e n p r e s e n t at t h e s u r f a c e o f y - A 1 2 0 3 b y R a m a n spectroscopy.

I n m y p a p e r at t h i s s y m p o s i u m I h a v e a t t r i b u t e d t h e

t e t r a h e d r a l c o o r d i n a t i o n b y the a n a l y s i s o f e l e c t r o n i c spectra. A r e t h e R a m a n s p e c t r a d i a g n o s t i c for t h e a t t r i b u t i o n of c o o r d i n a t i o n of t u n g s t e n in o x i d e s y s t e m s 7

F.P.J.M.

KERKHOF : Generally, the Raman technique gives more direct

i n f o r m a t i o n on t h e c o o r d i n a t i o n o f a n atom. M o r e o v e r t h e b a n d s i n t h e r e f l e c t a n c e s p e c t r a a r e r a t h e r broad.

Therefore, w e prefer the

u s e of R a m a n s p e c t r o s c o p y i n identifying c o m p o u n d s p r e s e n t in a catalyst.

S t r u c t u r a l i n f o r m a t i o n is o b t a i n e d by c o m p a r i n g s p e c t r a

o f t h e c a t a l y s t w i t h s p e c t r a o f r e f e r e n c e s a m p l e s , e.g. o f t h e W / S i 0 2 and W / A l z 0 3

the spectra

c a t a l y s t s are b o t h c h a r a c t e r i z e d by t h e -1 T h i s b a n d is a l s o t h e s t r o n g e s t o n e

p r e s e n c e of a b a n d a t 970 cm

.

in t h e spectrum of ammonium m e t a - t u n g s t a t e w h i c h i s composed o f d i s t o r t e d W06 octahedra. T h e r e f o r e w e c o n c l u d e d t h e o c t a h e d r a l coord i n a t i o n o f the W-atom, at l e a s t at h i g h e r m e t a l loading. A t e t r a h e d r a l c o o r d i n a t i o n at low m e t a l c o n t e n t ( < 3 w t % ) c a n n o t b e e x c l u d e d

89

REDUCTION OF SILICA SUPPORTED NICKEL CATALYSTS J.W.E. COENEN Unilever Research Vlaardingen, P.O. Box 1 1 4 , 3130 AC Vlaardingen The Netherlands SUMMARY The difficult reducibility of silica supported nickel catalysts has oftenbeen ascribed to chemical interaction with formation of basic silicates. Fromareview of literature data and from our own work many arguments for the importance of silicate formation will be derived. A structural model for silica supported nickel catalysts made by precipitationhasbeen evolved. Nickel silicate is seen as a "glue" layer between the silica and nickelhydroxide, which forms an epitaxial

o-

verlay on the silicate. In the first stage of the reduction or in a precalcining stage the hydroxide breaks up in an array of small oxide crystallites, stillepitaxially linked to the silicate. In subsequent reduction each oxide particle is reduced to one nickel crystallite, still epitaxially linked to the silicate glue layer. A s often reported in the literature we also find a strong inhibition of the

reduction by water vapour. The kinetics of reduction can be described by anoverall activation energy of about 90 kJ.mol-l, slightly increasing at higher degrees of reduction. The rate is close to third order in unconverted oxide. The surface area of the nickel is an almost linear function of the degree of reduction. Arguments are put forward for the assumption that in the reduction the nucleation is rate controlling. The most important reason for the low reduction rateis the low yield of reduced material per nucleus, due to the fine oxide dispersion. To explain the fast rate decline

-

third order

-

surface heterogeneity possibly

partly due to silicate has to be invoked as well as a particle size distribution for the supported oxide. It proved possible to explain the rate behaviour in a reasonably quantitative way. I . INTRODUCTION

Dispersions of various nickel compounds on silica supports, prepared by a great variety of impregnation and precipitation methods, after their acthation by hieh temperature reduction with a reducing gas, find widespread applicationas catalysts. Although the very extensive literature is hardly coherent and often contradictory, there is general agreement at least on two important issues:

-

A much finer nickel dispersion is obtained by reduction of silica supported

90

nickel compounds than with the same material without support. Average nickel crystallite sizes as small as 0.5 nm have been reported ( I ) for successful preparations, individual crystallites containing 10 atoms or less. Such fine dispersions in which virtually all nickel atoms are exposed and available for catalytic action, are completely unattainable without the aid of a support. Even if such fine dispersions could be prepared without support they would be highly unstable and their recovery from a reaction mixture would be next to impossible.

-

Reduction to the metallic form, which is the active form for most applications,

i s much more difficult for the supported material. For comparable rates thesili-

ca supported material requires reduction temperatures at least 2OO0C higher than the unsupported material. Complete reduction even at much higher temperature and long reaction time is often difficult to achieve.

A structural model and a compatible mechanism are clearly required to explain these observations. 2 . THE IMPORTANCE OF NICKEL SILICATE FORMATION

2.1. Published data Already in 1946 de Lange and Visser ( 2 ) postulated the occurrence of chemical interaction between silica supports and nickel compounds, with formation of basic silicates, as the reason for difficult reducibility of silica supported nickel catalysts. Many later investigations ( 3

-

8) substantiated silicate formation. By

quantitative x-ray analysis it was shown that more nickel is bound as silicate with lower nickel to silica ratios ( 9 , l O ) . Longuet ( 3 ) showed in 1947 that nickel hydroxide is highly corrosive to glass, so

that nickel antigorite is formed from nickel hydroxide and pyrex glass under

hydrothermal conditions. Under the conditions of catalyst preparation a similarreaction between silica support and nickel compounds should then be possible. The highly characteristic structure of kieselguhr, a commonly used siliceous support material, is often not found back in electron micrographs of the catalyst. Fig.1 gives striking evidence for this interactionforacatalystwherecomple-

tedestructionof theguhr did not take place. On the left a structure from the native guhr is shown, on the right the same

Fig.1. Electron micrographs of kieselguhr (left) and structure from a catalyst after same recovered from reduced catalyst (right) reduction and extraction

of nickel metal with carbon monoxide. In the latter picture it can be seen that the

silica surface has been extended considerably by the outgrowth of thin silicaplates remaining from the antigorite formed intermediately. The BET surface area was increased by a factor of more than 20. As shown before (6,9,10)

there is a close structural relation between nickelhy-

droxide and antigorite. The latter can b e formed from the former by replacing on one side of the hexagonal nickel layer the hexagonal array of hydroxyl ions by a network of interlinked SiO4 -tetrahedra. Thus a layer of antigorite froms an ideal "glue" between the silica support and an overlay of nickelhydroxide, which fits perfectly on the hydroxide side of the antigorite layer. In the dehydration of thehydroxide prior to reduction a shrink occurs, so that the hydroxide layer breaks up into small oxide blocks. It is logical to assume that these retain epitaxial relation with the supporting antigorite ( [I 1 4 perpendicular). In the ensuing reduction the epitaxial bonding of the nickel metal crystallites is retained. Dalmai et a1 ( P I ) proved that in Ni/SiO

2 obtained by reduction of nickel antigorite the supposed

[I 1 I] -orientation of the nickel indeed preponderates. Due to the smaller Ni-Ni dis-

tance in fcc nickel metal this induces strain. Coenen ( 9 , l O ) found about 0.4% enbarged lattice parameter for 3.5 nm nickel crystallites in Ni/SiO -catalysts. Re2 cently Sharma (12) interpreted satellite peaks in ferromagnetic resonance spectra of such catalysts ( 1 3 , 1 4 ) as evidence of the same amount of strain in the nickel metal. From a quantitative x-ray analysis of passivated reduced Ni/Si02-catalysts it was found that the amount of nickel oxide was proportional to the nickel surface area before passivation, indicating that the oxide found was present on the surface of the nickel crystallites and was formed in passivation ( 9 , l O ) .

Thus in the non-

passivated reduced catalyst no nickel oxide was present and incomplete reductionwas located in the silicate foundation structure. We are thus led to believe that an oxide particle, derived from hydroxide by dehydration, once its reduction is initiated,is reduced completely. At thehighreduction temperatures,generally applied,the resulting system nickel crystallites strongly bonded by residual antigorite to the support

-

-

small

will

strive towards minimum free energy, in this case a combination of maximum bonding surface and minimum exposed metal surface. Thus the idea of hemi-spherical nickel crystallites was born. Obviously the demands of the fcc-lattice w i l l induce faceting of the spherical surface. For this model confirming evidence was found. For a series of 22silicasupported nickel catalysts Coenen and Linsen (10) found thatthecrystallite

sizes de-

rived from x-ray line broadening were virtually equal to those calculated fromthe nickel surface areas with the hemisphere model. Horeover the total surfaceareaexpressed per gram Si02 of a reduced catalyst decreased, on extraction of nickel metal with CO (cf.fig.l), by an amount equal to half the nickel surface area, as expected for the model.

92

2.2. Reactivity of nickel in the unreduced catalyst In view of the manifest importance of silicate formation and its probable relevance also for reduction behaviour we devised a simple test for nickel reactivity in unreduced catalysts. This test comprises extraction of the catalyst with a 0.2 molar solution of EDTA at 5 5 O C . The reaction proved virtually zeroorder in complexing agent in the concentration range 0.1-0.4 m. For one catalyst of which the reduction kinetics will be discussed later the progress of nickel extraction and of reduction at 41OoC are shown in fig. 2. The two curves are surprisingly similar. The close correlation between reactivity towards EDTAandreducibility is further demonstrated in fig. 3 where standard reducibility-

% Redudkn 70 /'. Extraction

degree of reduction attained under standardizedconditions-

60-

is plotted against a similar

50-

40 -

30

-

20

-

- --

f

Y

extractibility foraseriesof

I

Catalyst 220

catalysts comprising diffe-

Reduction at 410.C. of precalcind d a l y s l Extraction with EOTA at 55% of umebced catalyst

rent methods of precipitation and supporting silica types. From the data presented in

" M w p h w MXHn n p data of 400 .nd US%

2.1.

action betweennickel compounds

Reaction time min

1w

50

F 0

150

and silica with intermediate

2M)

2. SIMILARITY OF EXTRACTION AND REDUCTION RATE BEHAVIOUR

silicate formationismostcertainly involved. The extent of

1

Degme of extraction %

it is clear that inter-

silicate formation depends on

80-

60-

'

40-

0

\Rd

20-

0stand.ndudbilily / 4f

4

,

m

A standexhactability

0

,

, 40

,

, 60

Oegree of reduction u%

,

,

m

,

,

100

3. CORRELATION BETWEEN REDUCIBILITYAND EXTRACTABILITY FW A SERIES OF CATALVSTS

0

5

x)

15

20

25

4. EFFECT OF SUPPORT REACTIVITY ON NiCKEL REKTlVlTlES IN UNREWCED CATALYST

precipitation conditions and on the reactivity of the silica used as support.The latter can be roughly assessed by a simple standardised solubility test of thesilica in alkaline solution. The data in fig.4 for five catalysts precipitated by the same method on different silica supportsfurther demonstrate the importance of

93 chemical i n t e r a c t i o n w i t h t h e s u p p o r t . The r e l e v a n c e of t h e s e d a t a w i l l b e d i s c u s s e d a f t e r c o n s i d e r a t i o n o f t h e r e d u c t i o n b e h a v i o u r . So f a r w e c a n o n l y n o t e t h a t c e r t a i n f e a t u r e s i n t h e u n r e d u c e d c a t a l y s t , w h i c h m a y b e chemical typeof nickelcompound-or p h y s i c a l -pore s t r u c t u r e w i t h a t t e n d a n t m a s s t r a n s p o r t l i m i t a t i o n - have c l o s e l y s h i l a r e f f e c t s inreductionand extraction.

3 . STRUCTURE CHANGES DURING DEHYDRATION OF THE UNREDUCED CATALYST Although t h i s i s n o t immediately e v i d e n t t h e c r y s t a l l a t t i c e s o f n i c k e l h y d r o x i d e , n i c k e l oxide a n d n i c k e l m e t a l h a v e a r a t h e r p r o n o u n c e d i n t e r r e l a t i o n . N i c k e l hyd r o x i d e h a s a l a y e r s t r u c t u r e of hexagonal symmetry c o n s i s t i n g o f s a n d w i c h e s o f two Close packed hydroxyl l a y e r s

w i t h a s i m i l a r h e x a g o n a l n i c k e l l a y e r i n b e t w e e n , perpendi-

c u l a r t o t h e hexagonal c - a x i s . Viewed a l o n g t h e [I 1 I]

-

a x i s of t h e f c c NaC1-type s t r u c -

t u r e of N i O w e f i n d r e g u l a r a l t e r n a t i o n o f h e x a g o n a l l a y e r s of o x y g e n a n d n i c k e l i o n s . Viewedalong t h e same a x i s o f t h e f c c n i c k e l l a t t i c e w e f i n d a s u c c e s s i o n o f hexagonal n i c k e l l a y e r s . The d e h y d r a t i o n s t e p d o e s n o t r e q u i r e

a second r e a c t a n t , hydrogen,

which t h e r e d u c t i o n d o e s . Hydroxyl i o n s and p r o t o n s a r e l o c a t e d i n c l o s e proximit y i n t h e hydroxide l a t t i c e . It i s t h u s n o t s u r p r i s i n g t h a t t h e d e h y d r a t i o n s t e p

i s a l r e a d y q u i t e f a s t a t 20OoC. From k i n e t i c s t u d i e s of s e p a r a t e d e h y d r a t i o n and of r e d u c t i o n of t h e c a l c i n e d c a t a l y s t we found t h e d e h y d r a t i o n s t e p t o b e about f i v e times f a s t e r t h a n t h e r e d u c t i o n . As w i l l b e d i s c u s s e d i n c h a p t e r 4 w a t e r a c t s

as a n i n h i b i t o r f o r t h e r e d u c t i o n . We may t h u s conclude t h a t a l s o when c a t a l y s t r e d u c t i o n i s performed i n a s i n g l e p r o c e s s t h e d e h y d r a t i o n s t e p w i l l p r e c e d e t h e r e d u c t i o n . I t w i l l t h e n b e u s e f u l i f we c a n v i s u a l i s e t h e s t r u c t u r e of a c a t a l y s t a f t e r decomposition b e f o r e r e d u c t i o n . O f u n r e d u c e d c a t a l y s t 5 8 , predriedtoconstantweightat 120°C,we h e a t e d samples f o r 6h a t 300, 375, 450 and 5 5 O o C and we measured w e i g h t l o s s e s and n i t r o g e n a d s o r p t i o n isotherms. Fromobservedweight l o s s e s w e c a l c u l a t e d t h e shrink involumeduetodecomp o s i t i o n o f n i c k e l h y d r o x i d e t o oxide and l o s s of a d s o r b e d w a t e r . Theseexpectedvolume changes ( t a b l e I , l i n e 1 ) m a y b e c o m p a r e d w i t h c h a n g e s i n p o r e v o l u m e ( l i n e 2 ) d e r i v e d f r o m t h e s a t u r a t i o n v a l u e o f t h e i s o t h e r m s (p/p

= 0 . 9 6 ) ( l i n e 3 ) . Apart f r o m a d i s c r e -

p a n ~ y f o r t h e 4 5 0 ~ C s a m p l e t h e a g r e e m e n t i quitegood,indicatingthatinthisparticus

l a r c a t a l y s t no s i g n i f i c a n t s t r u c t u r a l changes o c c u r i n t h e s t u d i e d t e m p e r a t u r e r a n g e , a p a r t f r o m t h e d i r e c t e f f e c t of d e h y d r a t i o n . Thenitrogen adsorptionisothermswereconvertedtoV,/t-plots

a c c o r d i n g t o DeBoer

a n d L i p p e n s (15) and t h e s e a r e shownin f i g . 5 . The i s o t h e r m o f t h e d r i e d s t a r t i n g m a t e -

r i a l i n t h i s p r e s e n t a t i o n i s a c l o s e a p p r o x i m a t i o n t o a s t r a i g h t l i n e up t o p / p o ~ O . 7 , showing t h u s o n l y s l i g h t d e v i a t i o n f r o m t h e u n i v e r s a l i s o t h e r m f o r o x i d i c m a t e r i a l s ( l 6 ) 2 239 m /g N i .

up t o porewidth of about 2.5nm.The s l o p e i n d i c a t e s a s u r f a c e a r e a of

The i s o t h e r m s of t h e c a l c i n e d samples a l l show an i n i t i a l s t r a i g h t l i n e i n t h e Va/t-plot,

which n e a r r e l a t i v e p r e s s u r e 0.7 changes t o a new s l o p e c l o s e t o t h a t

of t h e p l o t f o r t h e o r i g i n a l sample. To i n t e r p r e t t h e s e o b s e r v a t i o n s w e w i l l assume t h a t t h e e n t i r e s u p p o r t

94 s u r f a c e i n t h e unreduced c a t a l y s t i s c o a t e d w i t h a r e a s o n a b l y uniform l a y e r of n i c k e l hydroxide presumably bonded t o t h e s i l i c a by a t h i n s i l i c a t e l a y e r . By h e a t i n g t h i s hydroxide l a y e r i s decomposed i n t o N i O , which induces a s h r i n k w i t h development of c r a c k s . The s h a r p b r e a k i n t h e V a / t - p l o t s

of f i g . 5 f o r a l l i g n i -

T a b l e 1 . C a t a l y s t 58, c a l c i n e d , r e s p . reduced a t f o u r t e m p e r a t u r e s

120

300

375

450

550

P o r e volume i n c r e a s e from w e i g h t l o s s

0.19

0.24

0.31

0.33

1

P o r e volume i n c r e a s e from i s o t h e r m

0.20

0.24

0.23

0.29

2

0.53

0.73

0.77

0.76

0.82

3

239

473

436

405

297

4

220

230

260

238

222

5

2.9

3.6

4.2

10

2.7

3.5

4.0

I1

Treatment temp. OC

................................................................................ P o r e volume from i s o t h e r m st

s; (1.9 nm)

Areas i n m

2

(a)

, volumes

i n m l , w e i g h t s i n g, a l l p e r gram t o t a l n i c k e l .

a. I s o t h e r m f o r 3OO0C a l s o shows 0.8 m c r a c k s which were a l s o t a k e n i n t o a c c o u n t i n calculation b . From %ioassuming one o x i d e c r y s t a l y i e l d s one m e t a l c r y s t a l c. Detd. on same c a t a l y s t d i r e c t l y reduced i n H, a t temp. shown. t e d samples a t about t = 0.95 nm i n d i c a t e s c r a c k s o f uniform 1.9 nm w i d t h : a s soon as t h e t h i c k n e s s of t h e absorbed n i t r o g e n l a y e r r e a c h e s 0.95 nm w i t h com-

.-

1m.c

p l e t e f i l l i n g of t h e c r a c k s t h e r e m a i n i n g

Do

s u r f a c e area d r o p s t o a v a l u e of a b o u t

2 240 m /g N i , a b o u t e q u a l t o t h e area of t h e o r i g i n a l d r i e d c a t a l y s t . Table 1 a l s o m

shows t h e most i m p o r t a n t d a t a from t h e n i t r o g e n a d s o r p t i o n i s o t h e r m s : p o r e volumes s u r f a c e areas St from t h e i n i t i a l P' s l o p e s of t h e V a / t - p l o t s and S; from t h e

V m

s l o p e s above t = 0.95 nm a r e shown. Assuming t h a t t h e a r r a y of o x i d e b l o c k s r e - 5

5.Va/t-plots

ID

f o r Cat. 58

heated t o four temperatures

s u l t i n g from t h e d e h y d r a t i o n i s monodisp e r s e we can c a l c u l a t e from t h e s e d a t a and t h e known t o t a l volume of t h e o x i d e t h e d i -

95

mensions L and H of the oxide blocks. These are shown in lines 6 and 7 of table 1 . The oxide crytallite size D

NiO

=

K2His given in line 8.

With the assumption of one nickel crystallite to result from reduction of one oxide crystallite we then calculated DNi = DNi0.\3/11aNi0Ai) (line 9). These-are then compared to the nickel crystallite sizes for the same catalyst directly reduced at the same series of temperatures, as determined from hydrogen adsorption ( D ) and from x-ray line broadening (D ) . (Lines 10 and 1 1 ) . H X I n view of the rather drastic simplifying assumptions involved in the computations one should not attach too absolute value to the data obtained. Still the overall picture looks quite reasonable. First we note the close agreement between the nickel crystallite sizes derived from hydrogen adsorption DH and x-ray line broadening DX, which further strengthens the model discussed earlier, Comparing now these values with the value D (line 9) derived from the oxide crystallite size which followed from the isotherm interpretation, we are struck by their close parallelism and even their reasonable absolute agreement. But even the differences can find an acceptable explanation. We feel strongly inclined to conclude that the assumption that one oxide crystallite produces one nickel crystallite, holds good for all nickel oxide derived from nickelhydroxide which is epitaxially attached to the support. However, such hydroxide which because of local excess of precipitating agent remained unsupported or simply occluded in the precipitate, may not sinter too badly in the dehydration step but will sinter drastically upon reduction to the metal (melting point oxide 2163'K,

metal 1726'K),

especially at the higher reduction temperatures well

above the Tamman temperature of the metal). This would explain that the nickel crystallite sizes calculated from the oxide dispersion deviate the more from the actual nickel crystallite size as the reduction temperature i s higher, because of an as yet undefined fraction of badly supported material. 4 . EXPERIMENTAL OBSERVATIONS OF REDUCTION BEHAVIOUR 4.1.

The importance of water vapour

We will discuss the very extensive literature on reduction of both unsupported and supported nickel oxide later.

7

100 a%

One conclusion from all published data is that water vapour has a strong retar-

93-

ding effect and our data confirm these 70-

findings.

0

We reduced a catalyst at temperatu-

50 -

res 350 to 5OO0C with dry hydrogen and with 20% water vapour, other conditions 3;o

LM)

L50

6. EFFECT OF WATER VAPOUR IN REDUCING GAS

500

were standardized. The result is shown in Fig. 6. Especially below 45OoC the

96

inhibiting effect of water vapour is dramatic. We now recall what we have said before about the importance of silicate formation and the effect of hydrothermal treatment. The effect shown in fig. 6 may at least partly be due to increased silicate formation. To separate the effects a

10 ---A

r n b we pretreated a catalyst at three tem-

mmd;e.% m1n.g i

2

d

2.5

41

-A-A-

a8- \,

0.60.4-

water vapour during I hour. The effects

*Ao'

.-. -.

''\ /o

, /

are shown in fig. 7 , as obtained after

15

- 05

-\

0

a2

peratures in nitrogen containing 3%

-

Catalyst 58

subsequent reduction in dry hydrogen.

Line I gives the degree of reduction after a "standard'lreduction, line 2 after a prolonged reduction. In both pretreatment. Also is shown (line 3 )

7. EFFECT OF PRETREATMENT IN MOIST NITROGEN

the activity for benzene hydrogenation, expressed per unit weight nickel metal. A s will appear later, the latter quantity is almost proportional to the specific

nickel metal surface area, We must thus conclude that with the pretreatment the reducibility has decreased but the average crystallite size of the reduced metal is also smaller. Both may be ascribed to increased silicate formation. We now turn our attention to normal reductions. Fig. 8 illustrates a curious observation, which again focusses on the importance of water vapour. At three temperatures we did reductions such that the product of hydrogen flow rate and reduction time was constant

-

except

for very short reduction times

-

the

3

I 10t 20 #30 6

min 0

I

I

I

I

I

I

8

,

rnJ/min.kgNi 0.5

5 2 1 0.8 06 8. INDEPENDENCE OF DEGREE OF REWCTIW a OF FLOW RATE AND TIME FOR GIVEN TOTAL VOLUME OF GAS

so

I

40 50 60 70 80 90 100 120130

-

50 m /kg Ni. We then find that

degree of reduction is independent of either time or flow rate. In the decomposition and subsequent reduction a considerable amount of water is set free

that the average water vapour pressure in the reducing gas is always signi-

ficant. The effect is further illustrated in fig. 9 as a function of total flow and of temperature. We find again the two conflicting effects: costly measures like the use of much hydrogen and high temperatures admittedly produce a higher degree of reduction but at the same time larger nickel crystallites. 4 . 2 . The kinetics of reduction

Two experimental set-ups were used for kinetic studies. In a simple apparatus reduction was studied with hydrogen flowing over a bed of catalyst of thickness 0.5 or I cm. Parallel experiments were broken off after different ti-

97 mes and the catalyst was dissolved in 4 n sulphuric acid after degassing in vacuum at reduction temperature to eliminate adsorbed hydrogen. The degree of reduction was calculated from the volume of hydrogen evolved and the amount of nickel in solution. The procedure was very tedious if a reduction was to be fol-

11%

90 / A - x z &

80- A/A

01

-03

Benzene &it per unit weight of nickel metar

-02

1000

k g r e e d reduction

Catalet 8

O1 Reduction temperaturey 425

475

525

Loo

periments catalyst which was pre-ignited at reduc-

10 Loo

water we used in these ex-

425

475

tion temperature. For a limited number of broken

525

9.EFFECT OF TOTAL HYDROGEN N)w Ft (m3/kgNi) ON DEGREE OF REDUCTION AND SPECIFIC BENZENE ACTIVITY

Off

the degree

Of

reduction was measured

as described to calibrate the apparatus. In either apparatus a large number of runs were done and representative data are shown in fig. 10. a% Degree of reduction

’.,

-

./--

. I

5o(rc

1

A s expected and reported in the litera-

ture there is clearly some transport limitation. A thinner bed gives a better result but the difference is not spectacular. The combined effect of pre-ignition, which eliminates about two thirds of the water, and of hydrogen flowing through the bed - impracticable on a technical scale for a powder of a few pm particle size - is more noticeable. We note for later reference that the effect of lower effective water vapour pressure becomes more pronounced at higher degrees of reduction which has also been reported in the literature. Since conditions are clearly bet-

10. KINETICS OF CATALYST REDUCTION.INFLUENCE OF MASS TRANNSPORT AND TEMPERATURE

ter defined for the pre-ignited cata-

lyst and flow of hydrogen through the bed we will use the upper curves and similar ones for intermediate temperatures for a detailed kinetic analysis.

98 We have s e e n t h a t i n m o s t s i l i c a s u p p o r t e d c a t a l y s t s p r i o r t o r e d u c t i o n p a r t of tl n i c k e l i s b o n d e d a s h y d r o x i d e / o x i d e , p a r t as s i l i c a t e . I t i s t h e n o f i n t e r e s t t o s e e whether t h e a c t i v a t i o n e n e r g y s h i f t s t o h i g h e r v a l u e s as t h e r e d u c t i o n p r o g r e s s e s . F r 1 t h e k i n e t i c p l o t s t h e t i m e s r e q u i r e d f o r t h e d e g r e e of r e d u c t i o n t o p r o g r e s s from O t c 30, f r o m 3 0 t o 4 O % e t ~ . w e r e r e a d a n d t h e i r l o g a r i t h m sa r e p l o t t e d a g a i n s t r e c i p r o c a l temperature as inverted A rrh en iu s p l o t s i n f i g . 1I.Theslopes represent

E a / R . The

r e s u l t i n g a c t i v a t i o n e n e r g i e s f o r t h e average r a t e s of t h e s u c c e s s i v e e s p a n s do shok s m a l l b u t s i g n i f i c a n t tendency t o i n c r e a s e a b o v e 4 0 % r e d u c t i o n , a s shown i n f i g . 1 2 . Obv i o u s l y t h e a p p a r e n t a c t i v a t i o n e n e r g y must

averycomplexquantityandwewillpostpone d i s c u s s i o n u n t i l l a t e r . With l i t t l e s u c c e s s \ t r i e d f i t t i n g t h e r a t e d a t a tonumerous equationsproposed i n t h e l i t e r a t u r e f o r r e d u c t i a

of n i c k e l o x i d e . S u r p r i s i n g l y a g o o d f i t i s o b t a i n e d w i t h a simple t h i r d o r d e r r a t e equa-

lo

-Ea, kJ mot-’

/

P /”

100 -

Catalyst 220

go --o-o

11. ARRHENIUS PLOTS FOR REDUCTON OF CATALYST 220. RATE AS A “€TION OF DEGREE OF REDUCTION (I

1

I

I

20

40

60

a% #

80

12. CHANGE OF ACTIVATION ENERGY WITH

DEGREE OF REDUCTION a ~

t i o n : - ( d P / d t ) = k d i n w h i c h $ i s t h e f r a c t i o n of unconverted o x i d e , b = l d . T h e e x p e r i -

mental d a t a a r e p l o t t e d i n t h e i n t e g r a t e d form @ -2=

-

75

I + 2kt i n f i g . 13. IJe w i l l come backtothis empiricalcorrelatic i n a laterdiscussion. Thereacti< i s o b v i o u s l y t o o complex t o a t t a

p1-u

b a s i c s i g n i f i c a n c e t o t h i s appa

-m 10 -

r e n t l y simple r a t e behaviour. From t h e s l o p e s of t h e t h i r d o r d e r p l o t s an o v e r a l l a c t i v a t i o n

60

energy of 101 kJ/mol can b e d e r ved i n agreement w i t h t h e d a t a

-50

t mm

50

100

150

13.THIRD ORDER RATE PUlT FOR REDUCTION OF PRE-IGNITED CATALYST 220

-30

m

f i g . 12.

99 4.3. The r e s u l t of r e d u c t i o n . The reduced c a t a l y s t The s o l e purpose of c a t a l y s t r e d u c t i o n i s t h e g e n e r a t i o n of a l a r g e n i c k e l s u r f a c e a r e a and t h e r e b y p r o d u c t i o n of a h i g h c a t a l y t i c a c t i v i t y . The d a t a on n i c k e l s u r f a c e a r e a p r e s e n t e d h e r e were o b t a i n e d by measuring t h e hydrogen a d s o r p t i o n a t 2OoC and 1 Bar hydrogen

a f t e r e v a c u a t i o n of t h e f r e s h l y reduced c a t a l y s t a t re-

d u c t i o n t e m p e r a t u r e d u r i n g 2 h o u r s . For r e a s o n s e x p l a i n e d e l s e w h e r e ( l 0 ) a n equil i b r a t i o n t i m e of 16 h r s w a s a l l o Nickel surface area 140 - mz/g Nitot

120

wed f o r t h e a d s o r p t i o n , which was t h e n assumed t o r e p r e s e n t a mono-

-

l a y e r of a t o m i c a l l y adsorbed hydro100 -

gen w i t h 1 H p e r exposed n i c k e l ao6

80-

-

tom. For t h e l a t t e r a n a v e r a g e a r e a 2 of 0.0633nm was assumed. Thus 2 SNi=3.41Vm m / g N i , Vm i n m l H2

a t STP.per gram t o t a l n i c k e l . For c a t a l y t i c a c t i v i t y assess-

-02

0

02

04

06

08

i2 i4

ment we used h y d r o g e n a t i o n of ben-

a

&

(0

14 DEVELOPMENT OF NICKEL SURFACE AREA AND B 9 " E A C T l V l T v

zene i n a c o n t i n u o u s f l o w system. Details were d e s c r i b e d e l s e w h e r e ( l ) . U n l e s s o t h e r w i s e s t a t e d hydroge-

n a t i o n was done a t 7OoC, benzene p a r t i a l p r e s s u r e was 0.1 Bar, t o t a l p r e s s u r e 1 Bar. Res u l t s a r e e x p r e s s e d a s $,mmolbenzene

c o n v e r t e d p e r m i n u t e and p e r g r a m t o t a l n i c k e l o r

asmAB,e x p r e s s e d p e r g r a m metal. C l e a r l y mAB = $k.We [ S N ~mz/gNiW

mmd C,&/min,g total nickel 1.8

show a few r e p r e s e n t a t i v e r e s u l t s . rnmd C&/min gNttotAg

90-

8070

-

60 -

- 0.7 50-

- 0.6

-

- 0.5

30 -

- 0.4

40

- 0.3 M-

16.NlCKEL SURFACE AREA S N ~AND BENZENE ACTIVITY Ag IS.BENZENE ACTIVITY GROWTH WITH DEGREE OF REWCTON AS A FUNCTION OF DEGREE OF REDUCTION a I n f i g . 14 combined r e s u l t s on c a t a l y s t 58 are p r e s e n t e d . E s p e c i a l l y t h e re-

100 s u l t s on benzene hydrogenation are s t r i k i n g : w e f i n d a l i n e a r i n c r e a s e of

% with

degree of reduction. This implies t h a t successive f r a c t i o n s o f n i c k e l f o r m e d h a v e t h e samebenzene a c t i v i t y A p e r u n i t w e i g h t of metal. Although t h e s u r f a c e a r e a s s c a t t e r m B more t h e c o n c l u s i o n i s s i m i l a r : apseudo-linear i n c r e a s e o f m e t a l s u r f a c e w i t h M . T o g e t h e r t h e d a t a s u g g e s t t h a t t h e n i c k e l formed i n r e d u c t i o n i s c r y s t a l l i t e s i z e i s l a r g e r a t higher reduction temperature. The l a t t e r conclusionwas a l -

readyindicatedbytheresultsofthedecompositionstudy,discussed i n c h a p t e r ? . Fig. 15 givesbenzeneactivitydataforanothercatalyst,whichconfirm t h e e a r l i e r observations. This c a t a l y s t appears t o h a v e a f i n e r n i c k e l d i s p e r s i o n s i n c e i t h a s a val u e o f m % w h i c h i s 1.54times l a r g e r t h a n t h a t f o r c a t a l y s t 5 8 a t a l l t e m p e r a t u r e s . Fig.16

g i v e s c o m b i n e d d a t a f o r y e t a n o t h e r c a t a l y s t . Benzeneactivitiesweredeterrninedboth a t 45 andat70'C.

Thedataconfirmtheearlierpicturewiththeexceptionthatnow the best

s t r a i g h t l i n e s b o t h f o r t h e s u r f a c e a r e a s a n d f o r t h e b e n z e n e a c t i v i t i e s h a v e an i n t e r c e p t

*t \

withtheaxis. Wewillpostponedetaileddiscuss i o n o f t h e r e s u l t s t o t h e n e x t chapter. Next

thequestionmaybeaskedhowtheme-

t a l l i c n i c k e l p r o d u c e d i n p a r t i a l reduction

is

d i s t r i b u t e d i n t h e c a t a l y s t p a r t i c l e s . An average p a r t i c l e of about 3 m contains about 108 n i c k e l c r y s t a l l i t e s . Arewe t o a s s u m e t h a t a s p h e r i c a l reaction frontprogresses 17. SELECTIVfPI OF CATALYST Z O

Increase in stearic acid ccntent AS h standard hvaopeMticn

d soyl k M Oil.

intothepar-

t i c l e o r i s reducedmaterialrandomly d i s t r i b u ted? H y d r o g e n a t i o n o f f a t t y o i l s i s o f t e n mass

t r a n s p o r t l i m i t e d , whichmay impair s e l e c t i v i t y . This i s causedbyanaccumulationof s i r e d intermediateproduct i n t h e p o r e systemof t h e c a t a l y s t p a r t i c l e core

de-

, which i s then

hydrogenatedtoundesired f u l l y s a t u r a t e d p r o d u c t , s i n c e m o r e h i g h l y u n s a t u r a t e d r a w mater i a l i s d e p l e t e d i n thepore system.Thismanifests i t s e l f inthehydrogenationofe.g.soya

beanoiltyformationoffullysaturated s t e a r i c a c i d t o o e a r l y i n thehydrogenation. Test hydrogenationsunder standardised conditions t o a f i x e d end i o d i n e v a l u e (degree o f u n s a t u r a tion) canthusbeusedtoassess selectivity. I f n o w i n t h e p r o g r e s s o f c a t a l y s t r e d u c t i o n a

pseudo-spherical reductionfrontprogresses ineachparticle-ashasbeen found f o r much l a r g e r n i c k e l o x y d e c o m p a c t s (17-20)-catalystsw~thlowdegreeofreduct~onshouldhavea b e t t e r s e l e c t i v i t y than a t higher degree of reduction. Fig. 17 shows t h e r e s u l t f o r two temperatures of reduction. High A S i n d i c a t e s low s e l e c t i v i t y . We f i n d t h a t select i v i t y improves with degree of reduction. By t a k i n g i n t o account t h a t a t t h e higher r e d u c t i o n temperature t h e c r y s t a l l i t e s i z e i s l a r g e r and thus t h e n i c k e l s u r f a c e a r e a per u n i t mass of n i c k e l metal i s smaller w e can l e t t h e two l i n e s merge i n t o a s i n g l e one, i n d i c a t i n g t h a t s e l e c t i v i t y improves a t higher n i c k e l s u r f a c e a r e a . B u t t h i s i s a f a m i l i a r f i n d i n g i n fathydrogenation(21):selectivityalwaysinproveswith use of more cat a l y s t o r more a c t i v e c a t a l y s t . We may then conclude t h a t t h e d a t a do not i n d i c a t e a progressive p e n e t r a t i o n of t h e reduction f r o n t i n t o the c a t a l y s t p a r t i c l e s so t h a t

101 the distribution of reduced material in partially reduced catalyst is most probably random. 5. CONCLUSIONS. A TENTATIVE EXPLANATION OF OUR OBSERVATIONS

5 . 1 . Summing up of experimental data

Intermediate silicate formation, with sometimes drastic modification of supporting silica, is well established and appears very important. The silicate is a bonding layer between the support and hydroxide/oxide/metal through an epitaxial relation and by inhibiting sintering it aids formation of small crystals of the oxide and of the metal. On the other hand it makes reduction more difficult. EDTA extraction shows remarkably parallel behaviour with the progress of reduction, which may indicate that the chemical character of the nickel to be reduced governs both extraction and reduction. Decomposition to the oxide to a large extent precedes reduction. In a suitable experimental set up we could observe that in reduction with hydrogen the light green unreduced catalyst first turned black (oxide with excess oxygen), thenlight green (stoichiometric NiO) and finally black (nickel metal).For one catalyst we showed that the nickel containing layer (hydroxide + basic carbonate) breaks up into small oxide crystallites. The latter are larger at higher decomposition temperature.Basic carbonatedecomposestooxide almost as easilyashydroxide ( 5 6 ) . In the reduction water vapour plays an important role. Clearly reduction without water is virtually impossible since it is a product of the reaction. Its partial pressure will go down with increasing#. Some hydrothermal treatment occurs at higher water pressures, resulting in slower reduction, presumably increased silicate formation and smaller ultimate nickel crystallites. Fenomenologically the reduction is about third order in unconverted oxide, the overall activation energy

is about 9ClkJ. mol - 1 increasing somewhat at higher degrees of reduc-

tion. Both the nickel area and AB increase pseudo-linearly withaGenerally the SNd*line

has an intercept with thed-axis, sometimes also the $/d-line.

The increase of S withdis steeper at lower reduction temperature, indicaNi ting smaller crystallites. There is no indication of a spherical reaction front penetrating into the catalyst particles (A0 pm). 5.2. Relevant literature data The literature on reduction of nickel oxide, as such and supported, is extensive. We will quote some of the more important findings: Nickel oxide, Ni0,is an interesting substance. Unless prepared at very high temperature it contains excess oxygen, which only disappears at about 1000°C. 3+

The excess oxygen charge is compensated by presence of Ni ions. The crystal Structure is close to cubic, NaC1-type but only above 275OC it is really cubic.

102

Between 250 and 275OC there is a h-transformation, involving an antiferrornagnetid paramagnetic transition and a discontinuity in specific heat, thermal expansion, lattice type, electrical conductivityand s i g n i f i c a n t l y t h e r e d u c t i o n r a t e shows a maximum ( 2 3 - 2 9 , 5 0 ) . ye1low-green

NiO with excess oxygen is black, the stoichiometric form

is

.

A s for many solid state reactions, reduction of nickel oxyde requires nuclea-

tion, followed by propagation of the metal/oxyde reaction front. A s a resultoxyde reduction curves are often sigmoid. The induction time t. dependsonpretreatment (high T pre-ignition gives longer ti ( 3 0 ) ) and on reduction conditions(shorter ti at higher reduction temperature ( 1 8 , 2 9 , 3 0 )

and higher hydrogenpressure (31)).

Many investigators note the inhibiting effect of water vapour on reduction and most of them indicate that it is especially the nucleation which is inhibited. ( 2 2 , 37-41).

Lattice faults are deemed important for nucleation ( 3 4 - 3 6 ) and the re-

lative inactivity of nickel oxide ignited at high temperature is ascribed mainly to slow nucleation. By various treatments nucleation could be artifically aided: incorporation of metallic nickel, platinum and palladium accelerate reduction by creating nuclei. Gold and silver are completely inactive. Curiously, incorporation of copper is also

very effective although metallic copper hardly adsorbs hydrogen ( 3 0 , 3 1 , 1 9 , 3 5 ,

38, 41-46).

Thus the rates of nucleation and propagation could be separated ( 1 9 ,

35, 44-46).

We will come back later to the quantitative data thus obtained.

The kinetics of reduction were extensively investigated. The systems studied however, were generally very remote from our interest (large oxide particles, macroscopic oxide compacts) and it is thus not surprising that the equations derived and

fitted to published data are practically useless for our observations.

Hass transport was often found to be partly or entirely rate determining ( 1 7 - 1 9 , 47-49).

Observed activation energies vary widely: 4 2

-

134 k3.mol-l. Lower values

can generally be ascribed to mass transport limitation. (18,19,22,26,30,35,39,53, -1 5 5 ) . For reaction front propagation agreement is good: 117-120 kJ.mol

(30,3539).

For nucleation the spread is much wider, reflecting the importance ofthedetailed -1

structural perfection of the lattice and its surface: 117-188 kJmol

(26,30,19,

35, 44-46).

Although the literature on reduction of supported catalysts is also quite voluminous only a limited number. of papers gives fundamental information, relevant to our present considerations. The importance of silicates and their difficultreducibility was already referred to ( 3 - 1 0 , 5 4 ) Of particular interest are the observations of Delmon et a1 ( 5 8 , 5 7 ) who like Nowak ( 5 9 ) observe acceleration of reduction by incorporation of othermetal-s Delmon indicates the crucial importance of the high degree of dispersion of supported nickel oxide, which may make effective nucleation more difficult and may inhibit propagation of reaction fronts.

103 Telipko et a1 ( 6 0 ) found evidence from their observations in reduction of nickel on kieselguhr for two states of supported nickel varying widely in reducibility. Zapletal et a1 ( 6 1 ) found strong retardation of reduction by water vapour. They also found a pseudo-linear increase of benzene activity with d a n d higher

%

for a g i v e n 6 3 moisture bearing hydrogen is used for reduction. (see fig. 7 and 9) 5 . 4 . A tentative contribution to understanding

Based on the separate determinations of nucleation and propagation rates by Delmon et a1 ( 4 4 ) , Frety et a1 ( 3 5 , 4 5 , 4 6 ) and Yamaguchi et a1 (19) we concluded that propagation of the reaction front in the small separated oxide crystallites at the usual reduction temperatures is very fast on the time scale of catalystreduction, s o that an oxide crystallite, oncenucleated “sparks” into the metallic state. This then should mean that the reduction rate we observe is really the nucleation rate. We verified this idea by plotting the nucleation rates, observed for pure nickel oxide by Delmon et a1 ( 4 4 ) in an Arrhenius plot together withthe nucleation rate calculated for catalyst 220 from the reduction rate at 4OO0C and

the estimated nickel oxide surface area, in an Arrhenius plot. The result is shown in fig. 18. We admit that the span of extrapolation is irresponsible, nevertheless the result is encouraging. We recall that nucleation rates per unit surface area may vary by at least an order of magnitu15

16

17

18

19

2 0 1MX) i l

18. SPECIFIC NUCLEATION RATE. Delron’s and wr d a c

de, depending on the perfection of the oxide lattice and the water vapour pres-

sure. A fit within a factor of three may then be lucky chance but it is still inspiring. We recall again the picture we developed of silica supported nickel catalysts after dehydration but before reduction: an array of very small oxide crystallites “glued” to the silica support. If -for the moment- we assume the oxide to be mono-disperse, and one oxide crystallite to produce one nickel crystallite then with proceeding reduction degree the nickel surface area will grow linearly, which is close to what we observe. Thus encouragedwenow consider reduction rate behaviour. For a monodisperse system of nickel oxide particles of linear size D, or for a narrow particle size cut from a size distrfbution, the rate equation is worked out-retaining the assumption that nucleation is fully rate determining

-

in table 2. At first sight it

would appear that the large exposed oxide area would result in fast nucleation, SO

that our basic assumption that nucleation is rate determining would be invali-

dated. In fact the nucleation rate is fast but the follow up is slow, because the

104 crystals are small per nucleation event the yield is very small because the reaction front cannot jump to another oxide crystal. The result remains that what we essentially observe is the nucleation

Table 2 Reduction Kinetics of highly dispersed oxide

rate. From the equations in table 2 we

-

Initial number of oxide particles OND of size D Surface area S N fD2 f and g Mass = N:PgD3 shape factors

can see that for the simple case of a

Nucleation rate nD nucleilsec. m2 of particle size D

rate which is constant per unit surface

{

-

Partial nucleation rate

NDfD2nD nucleilsec

Assume nucleation rate determining, yield per nucleus P gD3

Partial reduction rate

NDfD2nD X pgD3 kglsec

single particle size D and a nucleaction area (n ) the rate is first order in unD converted oxide. The first order rate constant equals f.nD.D

Degree of reduction (a) rate for sire D

2

which goes far

towards explaining why supported cata-

Define BD

-

lysts reduce slowly. The situation is I-% =

remarkably similar to crystallisation

NDlOND

of fats in emulsion, recently discussed by van den Tempe1 (62) who found that Thus for monodisperse system and nD constant decline of 8 is first order in B Experimentally we find order in B to be 3 (fig. 13)

very much more undercooling was required to obtain observable crystallization

rates. Returning to rate behaviour we note first order behaviour for our simplified system, whereas experimentally we observe third order in unconverted oxide. We then must scrutinise our assumptions. It is highly unlikely that the oxide in our catalyst is really mono-disperse. Entropy certainly requires a size distribution. Secondly not all oxide crystallites will be of equal perfection, so that also nD is unlikely to be constant. For

Table 3

Effect of Oxide Crystallire Sire Distribution on Reduction Kinetics FD

-

number fraction of size D

-

cal considerations as the fact that the

Instantaneous degree of reduction aD af sire D at time t mD I exp (-fn,rJ2t) Total

o.

the moment we will ignore such practi-

-

water vapour pressure declines in the course of reduction

at rime t equals

cleation poison

-

-

and water is a nu-

and the possible ef-

fect of silicate, which being more dif-

1-FDD'dD Result plotced for a l a g normal distribution,nD in fig. 20 .B as thied order rate plot.

ficult to reduce will be "saved" to the

-

last. In table 3 we have set out what kD3"

happens if we assume a particle size

distribution and at the same time assume nD, the specific nucleation rate, to be some obscure function of D. For our thought experiment, which empatically is not meant to represent a reality for the catalyst involved, we chose a log-normal crystallite size distribution as shown in fig. 19, which was chosen in such a way that the volume-average crystallite size of the oxide was 2 . 5 nm, this value admittedly inspired by the nickel surface area development of the catalyst 220, reduced at 4 0 O o C . Still leaving nD constant the effect of the crystallite size distribution, which is cer-

105 tainly not very wide and may well be realistic, already works in the desired di2

rection. Due to the D -dependence of the "rate constant" the largest crystallites will be reduced preferentially. This in turn entails a faster decline of the rate with degree of reduction than for the first order reaction the mono-disperse system. Computer simulation showed that 12

-

xw)-2

distribution the

gE@ dD

close

expected for

for

the chosen

apparent reaction order

is

to two in unconverted oxide. An unrealis-

tically wide distribution would have to be chosen to simulate an apparent order of three, as required by the experimental data. Wereeallnow that we quoted many observations which indicate that the nucleation is promoted by lattice faults, impurities, etc. It is logical then to assume that the oxide surface, composed of the surfaces of the oxide crystallites, is heDnm

0

2

4

TlON (numbers)

6

6

terogeneous with respect to nucleation rate. Ob-

~'

viously we must expect those oxide crystallites

to disappear faster than those with a greater

perfection and purity. The size preference and the nucleation preference may be expected to be interlinked for the following reasons: A larger crystallite has obviously a greater chance of containing an impurity atom to serve as a nucleating center than a small crystallite. Similarly a dislocation or vacancy has a better chance of annealing out in a small crystal. Thus the largest crystallites, which we expected already to be the first to be reduced, have an added reason for doing so because of a higher nucleation chance. We can incorporate this idea in the equations of tables 2 and 3 by assuming nD to contain the particle size D to a power yet to be defined. It appears useful at this stage to emphasize again that the computation we will do is less an analysis of rate data but rather an illustrative test of a theory, which aims to explain at the same time the low absolute rate of reduction and the fast decline of the rate with degree of conversion. The computation comprises the following steps: we define a lognormal par-

ticle size distribution F(D) thus that the distribution on complete reduction produces the correct nickel surface area. The volume-average size D The distribution is cut in size spans of equal width which the progress of reduction -decline of#-

-

= 2.5 nm.

quasi-monodisperse

-

for

is calculated with first

order kinetics. Successive complete computations are done for powers of D f(t) is in the rate constant ranging from 2 (nD constant) to 4 (nDAID').&= then calculated by integrating over the distribution according to the last equation in table 3 . For each computation (power of D in rate constant)@

-2

is

106 plotted against t. A

linear plot is obtained for nD=kD3/2. Conversion of computer

time to real time makes the computed plot coincide with the experimental plot of fig. 13.Fig.20 shows the result. The crystallite volume distribution for the ignited catalyst and 3 stages in reduction is shown in A. There is clear shift to smaller sizes with growing a. The surface heterogeneity implied in the model,d(nDS)/dS is shown in C. The shape is a normal heterogeneity type and indicates that our approach is related to the Elovich-type reasoning of Levinson et a1 (63). 1

2

-

/*

-4

Our experiments

/-'

-3

were started by

I

Dnm

-t

/

1 Model prediction 2 Third uder Not 3 Experimental

#.*

D-2

rnin

1

B

We must now intro-

Reduction kinetics third order rate plot a/*

20 40 60 80 100 120 140 160

3 4 5 6 7 8 9 10 11

"

6

duce two additional complications.

-2

immersion of the

-1

reactor in a preheated salt bath Heating up is

c

D

Surface bet-ty implicit in assumed

fast but the first

Nick4 surtace area development

minutes are still a blind period:

.-

Fraction of surface reacted -0

0

04

02

06

08

20.MODELLlNG OF REDUCTION

T and zero time

'!

10 0

are ill-defined. 02

04

06

08

10

OF CATALYST 220 AT N O 0 C

The possibility of badly suppor

ted, fast reducing and badly sintering material was already mentioned. These combined problems we solved by arbitrary selection of a point on the reduction curve

-

18%

- where both

are certainly past. The computation was done for the remai-2 = 4.5 experimental data, theirempi-

ning 82 %. The result is shown in B. Up to B

rical 3rd order plot and the computed model coincide. The slight irregularity near the B-'-axis

+ the straight line does not pass through 8-'=1

for t=O

-

indi-

cates that the reduction up to 18% went too fast though the temperature was too low, indicating some badly supported material. The computed nickel area development finally is shown in D. The smooth curve refers to the well-supported material. For the 18% we put in an arbitrary small area contribution. In this manner we simulated

-

not necessarily for catalyst 220

-

one possible cause for a "toe"

in the SNi/a-curve. The steepening at high a is partly a computation artefact: the assumed continuous size distribution goes to zero size and contains particles of sub-atomic size. A cut-off at a minimum crystallite size might have been used. An alternative explanation for the "toe" in the SNi/ a-curve lies in the area measurement. At low

ci

the catalyst may well be loaded with water. In the pumping

prior to H2-adsorption this may partially poison the nickel surface, yielding too low adsorption values. This does not explain the more regular AB-behaviour, which

107 remains surprising. We did the same computation for 5OO0C,

allowing for the parser oxide disper-

sion: D =3.0 nm. The fit was equally good, again D3/’in n The remaining T-effect V DI1 in nD after allowing for the size effect gave E = 2 ! kJ.mol .Though the nucleation reaction involved must be complex this may be a true activation energy. As an assumption: nucleation may be equilibrium dissociative adsorption of H2 on adjacent Ni and 0, followed by rate determining H 0-desorption. In a Langmuir ap2 = E2 - (l-B)qa in which E2 is Ea

proximation the apparent activation energy E for desorption of H20 and q

is adsorption heat for H2. For 8 small Ea = E2

-

qa’

which difference may well be small. We gave a birds eye view over many part-investigations, often separated by several years, with different workers, apparatus and also catalysts. Transfer of conclusions form one chapter to the next involves risks since different Ni/Si02catalysts have much in common but show many differences. We did a last minute verification on catalyst 220. Electron micrographs showed no trace of guhr structure, only wrinkled sheets indicating heavy interaction. Nevertheless x-ray diffractionoftheunreduced catalyst showed mainly nickelhydroxide with some silicate. Decomposed at 4OO0C, quantitative x-ray analysis gave 78% nickel as oxide, 22% as 2

silicate. Mean oxide size 2 . 2 nm ( 2 . 5 assumed in computation). BET-area 1000 m /g SiO indicates 0.8 nm thick silica sheets, confirming EN-evidence. At*= 7 3 % SNi 2 2 was 135 m /g Ni, average crystallite size 2 . 3 nm. The applied model is thus amply confirmed. Cu in Ni 20 ppm, Co + Fe about 1%: in a 1 nm particle 1 impurity atom, in a 5 nm particle 125, which may be relavant for nucleation. Although with much speculation we hope to have made a case for extreme dispersion to be the main reason for slow reduction of silica supported Ni-catalysts. Regrettably the correlation with EDTA extraction remains unexplained. 6 . ACKNOWLEDGEMENT

Thanks are due to Unilever management for permission to publish this work. Many people did the hard work and contributed enormously by their ingenuity and perseverence in meticulous measurement and by provision of constructive ideas. I mention especially W.P. van Beek, J.C.P. Broekhoff, A. de Jonge, Th. J. Osinga F. Pastoor, D. Verzijl and P. van der Vlist. There were many more. REFERENCES 1 J.W.E. Coenen et al. Proc, 5th Int. Congr. Catalysis, Palm Beach, USA 1972 2 J . J . de Lange and G.H. Visser, Ingenieur 58, 24 ( 1 9 4 6 ) 3 J. Longuet: C.r. 2 2 5 , 869 ( 1 9 4 7 ) 4 Y. Trambouze and T P e r r i n , C.r. 228, 837 ( 1 9 4 9 ) 5 G.C.A. Schuit and L.L. van Reijen: Adv. Catalysis lo, 242 6 J.J.B. van Eyck van Voorthuizen and P. Franzen, Rec. Trav. Chim. 6 9 , 666 ( 1 9 5 0 ) 7 J. Francois-Rosetti and B. Imelik: Bull. SOC. chim. Fr. 1957, l l l r 8 W.J. Singley and J.T. Carriel: J . Am. Chem. SOC. 2, 778 (1953) 9 J.W.E. Coenen, Thesis Delft 1958 10 J.W.E. Coenen and B.G. Linsen:Phys.and chem.asp. of ads.and cats, Acad.Press ( 1 9 7 0 ) I 1 G. Dalmai, C. Leclercq and A, Maubert-Muguet:J.Solidstatechem. 2,129 ( 1 9 7 6 ) 1 2 V.K. Sharma, Private communication 1 3 A.A. Andreev and P.W. Selwood: J . Catal. 8, 88 ( 1 9 6 7 ) 14 A.A. Andreev and P.W. Selwood: J . Catal. 8, 375 ( 1 9 6 7 ) 15 B.C. Lippens and J.H. de Boer: J . Catal. 5, 319 ( 1 9 6 5 ) 16 B.G. Linsen in E.A. Flood, ed. the solid-gas interface,M.Lkkker,NewYork 1044 ( 1 9 6 7 )

108 17. 3. Szekely and J.W. Evans: Metall. Transactions 2, 1699-1710 ( 1 9 7 1 ) 18. J. Szekely, C.I. Lin, H.Y. Sohn: Chem. Eng. Sci ;?8, 1975-89 ( 1 9 7 3 ) 19. A. Yamaguchi and J. Moriyama: Memoirs Faculty Engineering Kyoto University 2 8 , no 4 , 389-403 ( 1 9 6 6 ) 20. Kolomasnik, J. Soukoup, V. Zapletal, V. Ruzicka and J . Vacha: Coll, Czech. Chem. Communic. 35, 819-29 ( 1 9 7 0 ) 2 1 . J.W.E. Coenen: Chem. and Ind. in the press 22. H.P. Rooksby: Nature 152, 304 ( 1 9 4 3 ) 617-20 ( 1 9 7 1 ) 23. F. Chiesa and M. Rigaud: Canad. J. of Chem. Eng. 2 4 . A. Roman and B. Delmon: C.r. 269 B, 801-4 ( 1 9 6 9 ) 25. A. Roman and B. Delmon: C.r. 77-79 ( 1 9 7 0 ) 26. B. Delmon and A. Roman: J. Chem. Soc.,Farad. Trans ( 6 9 , pt 5 ) , 941-8 ( 1 9 7 3 ) 27. R. Frlty, L. Tournayan, H. Charcosset: Ann. Chim. 9, 341-55 ( 1 9 7 4 ) 28. M. Foex: Bull SOC. Chim. Fr. 373-9 2 9 . G. Nury: C.r. 2 3 4 , 946-8 ( 1 9 5 2 ) 30. B. Delmon: BulrSoc. Chim. Fr. 590-7 31. W. Verhoeven and B. Delmon: Bull SOC. Chim. Fr. 3065-73 3 2 . T. Kurosawa, R. Hasegawa and T. Yagibashi: Transact. Japan. Inst. Metals 265-71 ( 1 9 7 2 ) 33. A.N. Kuznetsov: Russ. J. Phys. Chem. 3, 15-18 ( 1 9 6 0 ) 3 4 . H. Charcosset, G. Dalmai, R. Frlty, C. Leclercq: C.r. 264C, 151-4 ( 1 9 6 7 ) 3 5 . R. Frlty: Ann. Chim. E t 4 , 453-74 3 6 . Y. Iida and K. Shimada: Bull. Chem. SOC. Japan 33, 1194-6 ( 1 9 6 0 ) 3 7 . J . Bandrowski, C.R. Bickling, K.H. Yang and O.A. Hougen, Chem. Eng. Sci. 379-90 ( 1 9 6 2 ) 3 8 . R. Frety, H. Charcosset, Y. Trambouze: C.r. CongrZs National des SOC. Sav. Section Sciences ( 1 9 6 6 ) , 2, 91-101 ( 1 9 6 7 ) 39. H. Charcosset, R. Frgty, Y. Trambouze, M. Prettre: Proc. 6th Int. Symp. React. Solids 171-9 ( 1 9 6 8 ) 4 0 . H. Charcosset, R. FrLty, P. Grange, Y. Trambouze; C.r. 267 C, 1746-8 ( 1 9 6 8 ) 4 1 . H. Charcosset, R. FrBty, A. Soldat, Y. Trambouze: J. Catalysis 204-12 ( 1 9 7 1 ) 4 2 . A. Roman and B. Delmon: C.r. 273 C, 94-7 ( 1 9 7 1 ) 43. R. Frlty, H. Charcosset, P. Turlier, Y. Trambouze: C.r. 1451-4 ( 1 9 6 7 ) 4 4 . B. Delmon and M.F. Pouchot: Bull. SOC. Chim. Fr. 2677-82 4 5 . H. Charcosset, R. FrBty, P. Grange, G. Labbl, A. Soldat and Y. Trambouze: J. Chim. Phys. 68, 49-55 ( 1 9 7 1 ) 4 6 . H. Charcosset, P. Grange, Y. Trambouze: C.K. E ;1298-1309 ( 1 9 7 1 ) 4 7 . J. Szekely & J.W. Evans: Metall. Transactions 2, 1691-8 ( 1 9 7 1 ) 4 8 . Y. Yamashine and T. Nagamatsuya, J. Phys. Chem. 70, 3572-5 ( 1 9 6 6 ) 4 9 . T.D. Roy and K.P. Abraham: Phys. Chem. Process. Metall. Richardson Conf. Pap. (I 973), 85-93 50. J.R. Tomlinson, L. Domash,R.G. Hay and C.W. Montgomery, J.A.C.S. 7 7 , 9 0 9 - 1 0 (1955)

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c,

109 DISCUSSION A. BARANSKI

:

Your description of kinetics of reduction is made

from a "solid state" point of view.

Do you plan to extend this

description taking into account the effect of partial pressures of hydrogen and water.

There is literature evidence on the re-

tarding effect of water on the reduction of oxides. J.W.E. COENEN : It would be very satisfying to do what you suggest. The problem is that the situation is very complex.

It appears

that low water concentrations already have an effect on nucleation. It will be very difficult to define with sufficient accuracy the prevailing water concentration, which may be different in different parts of the catalyst grain and will change with time. methods you have further

The

developped in your paper may help to bring

the problem closer to a solution. H. CHARCOSSET

:

What is your hypothesis concerning the nucleation

sites on the surface of the NiO particles, with respect to their reduction by hydrogen

?

When studying the reduction of unsupported

NiO, we only could establish unambiguously that the corners and edges do not constitute preferential areas for the nucleation of the Ni particles.

But we could not determine the exact nature of

the nucleation centres i.e., cracks, impurity centres or defects 3+ (i.e. Ni ) . J.W.E. COENEN : I only can speculate on this question. Nucleation is clearly the formation of enough metallic nickel to dissociate hydrogen.

Thereafter propagation runs smoothly as your own work

has demostrated and -for our small oxide crystallites

-

very soon

complete reduction is reached. On the surface of each oxide crystal we are looking for sites which are energetically more favourable than the other parts of the surface to provide this spark.

Inspired by many literature indica-

tions that the nucleation rate is sensitive to the oxide used (its purity and pretreatment) and that black NiO reduces faster than green NiO, I feel that lattice distortion and impurities may well enhance the chance to find this favourable site in the surface of a crystal.

The chance of finding such faults should be less in

small crystals, which is a justification for our assumption that the surface nucleation rate is size dependent.

110 R. MONTARNAL :

You have presented a strong inhibition of the

re-

duction by water vapor of oxidized nickel supported on silica. On the other hand the drastic increase of sintering rate by water vapor, of nickel metal deposited on carrier, is well known. Do you think that some analogy can exist between the two phenomena, or, on the contrary, that they rather proceed by different mechanisms. J.W.E. COENEN : The influence of water on the sintering rate of supported metal is outside my experience.

We did observe that the

presence of water vapour during reduction definitely repels the reducticn but at the same time that exposure to water vapour during or prior to reduction at high temperature tends to result in more finely divided nickel.

I do not see a direct relation between

these two observations. This effect on the reduction rate may partly be due to poisoning of nucleation and to a shift of the surface equilibria in an unfavourable direction.

The hydrothermal treatment may result in the

formation of difficultly reducible silicates.

This last phenomenon

we invoke tentatively to explain that the dispersion of nickel after the "wet" reduction is improved.

Alternatively it may be

that some of the hydroxide which originally was badly

attached to

the support, adheres better after the treatment and sinters less on reduction. J.T. RICHARDSON

:

Do you have any evidence other than benzene con-

version data confirming that higher H 2 flow rates result in larger crystallites ? be a factor J.W.E.

Could the reduction precompound (oxide or silicate)

?

COENEN

:

The paper shows only benzene hydrogenation activity

to demonstrate the finer nickel dispersion after wet reduction. Also nickel surface area from hydrogen adsorption shows the same trend.

In the paper we propose the idea that the wet conditions

at high temperature give a kind of hydrothermal treatment which may well induce some additional silicate formation. take this too strictly, however.

One should not

The observed effects certainly

demonstrate some structural reorganization in the catalyst.

This

may be additional silicate formation or better attachment to the support of some badly supported hydroxide.

111 M.V.

TWIGG

e f f e c t of

: With r e g a r d t o t h e

flow r a t e of hydrogen and t h e

t h e p a r t i a l p r e s s u r e o f w a t e r on t h e r e d u c t i o n r a t e ,

are

t h e d a t a o b t a i n e d u s i n g an i n e r t c a r r i e r g a s i n q u a n t i t a t i v e agreement w i t h t h e r e s u l t s o f r e d u c t i o n s u s i n g v a r i o u s f l o w r a t e s of hydrogen a l o n e ?

J.W.E.

COENEN

: I

h a v e n o d e f i n i t e i n f o r m a t i o n on t h i s p o i n t .

Since

i n t h e p r a c t i c a l p r o c e s s d i l u t i o n o f h y d r o g e n i s u n a t t r a c t i v e we n e glected t o consider it i n laboratory studies.

My i m p r e s s i o n i s

t h a t f o r t h e r e d u c t i o n m e c h a n i s m t h e r a t i o pH 2 0 / p H z g o v e r n s t h e s i tuation.

I n e r t g a s would n o t o n l y d i l u t e t h e w a t e r b u t a l s o t h e

h y d r o g e n a n d t h e e f f e c t w o u l d n o t b e t h e same a s a d d i t i o n a l h y d r o gen.


113

INTERACTION OF NICKEL I O N S WITH SILICA SUPPORTS DURING DEPOSITION-PRECIPITATION

L.A.M.

HERMANS and J.W. GEUS

Department of Inorganic Chemistry*,

University of Utrecht, The Netherlands

ABSTRACT

The conditions t o be f u l f i l l e d t o p r e c i p i t a t e c a t a l y t i c a l l y a c t i v e components e x c l u s i v e l y onto t h e s u r f a c e of a suspended c a r r i e r a r e d e a l t with. It i s argued t h a t l o c a l s u p e r s a t u r a t i o n s must be kept s t r o n g l y l i m i t e d during t h e p r e c i p i t a t i o n . P r e c i p i t a t i o n from a s o l u t i o n kept a s homogeneous a s p o s s i b l e l e a d s t o t h e above d e p o s i t i o n onto t h e support, provided t h e support i n t e r a c t s s u f f i c i e n t l y s t r o n g l y with t h e p r e c i p i t a t i n g compound. The e f f e c t of suspended s i l i c a on t h e p r e c i p i t a t i o n of n i c k e l i o n s was i n v e s t i gated by recording t h e pH-value and t h e light-transmission.

It was e s t a b l i s h e d

t h a t t h e s p e c i f i c s u r f a c e a r e a of t h e support, t h e temperature and t h e mode of a d d i t i o n of hydroxyl i o n s a f f e c t t h e r e a c t i o n of t h e n i c k e l ions. A t 25OC adsorption of n i c k e l ions changes over smoothly i n t o growth of n i c k e l hydroxide t h a t c o n t a i n s an amount of s i l i c a t e depending on t h e r e a c t i v i t y of t h e suspended s i l i c a . A t 90°C n u c l e a t i o n of n i c k e l h y d r o s i l i c a t e proceeds a b r u p t l y and t h e s i l i c a r e a c t s appreciably t o n i c k e l h y d r o s i l i c a t e . The elementary r e a c t i o n s revealed a r e confirmed by i n v e s t i g a t i o n of t h e loaded c a r r i e r s i n t h e e l e c t r o n microscope and determination of t h e i r t e x t u r e by n i t r o g e n s o r p t i o n a t 77 K. I.

INTRODUCTION

1.1.

Production of supported catalysts

A high a c t i v i t y of a s o l i d c a t a l y s t g e n e r a l l y c a l l s f o r a l a r g e s p e c i f i c sur-

f a c e a r e a and, hence, f o r a f i n e l y divided s o l i d . Since most c a t a l y t i c a l l y a c t i v e m a t e r i a l s s i n t e r , however, r a p i d l y a t t h e c o n d i t i o n s of t h e c a t a l y t i c r e a c t i o n , t h e a c t i v e component has g e n e r a l l y t o be applied on a highly porous, thermostable support, such a s s i l i c a o r alumina. The support, t h a t i s o f t e n not a c t i v e i t s e l f , d i l u t e s t h e c a t a l y s t . The support i s used most e f f i c i e n t l y , when t h e a c t i v e m a t e r i a l i s d i s t r i b u t e d densely and uniformly over t h e s u r f a c e of t h e support. A rapid t r a n s p o r t of r e a c t a n t s and r e a c t i o n s products through t h e porous c a t a l y s t

together with a low p r e s s u r e drop i s a l s o a p r e r e q u i s i t e f o r an a c t i v e s o l i d

*

Croesestraat ??A, Utrecht, The #etherlands.

114 c a t a l y s t ( r e f s . 1-2).

As supports of t h e required porous s t r u c t u r e can be produced

p r e s e n t l y , d e p o s i t i o n of t h e a c t i v e material i n a dense and uniform s t a t e must be done without a f f e c t i n g markedly t h e porous s t r u c t u r e of t h e support. C a r r i e r m a t e r i a l s produced i n a s e p a r a t e process a r e g e n e r a l l y loaded with t h e a c t i v e component by impregnation and drying o r by p r e c i p i t a t i o n of t h e a c t i v e m a t e r i a l . A dense coverage of t h e s u r f a c e of t h e support by t h e a c t i v e component asks f o r a high degree of loading of t h e c a r r i e r . A t a high degree of loading impregnation and drying do n o t lead t o t h e required uniform d i s t r i b u t i o n of t h e a c t i v e m a t e r i a l over t h e support ( r e f s . 1-2).

A high degree of loading t o g e t h e r

with a uniform d i s t r i b u t i o n of t h e a c t i v e component can be achieved by s t a r t i n g from a compound i n which t h e c o n s t i t u e n t s of t h e a c t i v e component and t h e support a r e mixed on an atomic s c a l e . An i n s t a n c e i s t h e ammonia s y n t h e s i s c a t a l y s t , where t h e c a t a l y t i c a l l y a c t i v e i r o n i s atomically mixed w i t h aluminium ions. Reduction b r i n g s about t h e formation of t h e a c t i v e m e t a l and t h e alumina s u p p o r t , The atomicall y d i s p e r s i o n , t h a t was obtained by melting with t h e ammonia c a t a l y s t , can a l s o be a t t a i n e d by c o p r e c i p i t a t i o n ( r e f s . 3 - 4 ) . Instances a r e copper-zinc oxide c a t a l y s t s and magnesium-nickel o x a l a t e s . It i s , however, not p o s s i b l e t o c o n t r o l e f f e c t i v e l y t h e porous s t r u c t u r e of c o p r e c i p i t a t e d c a t a l y s t s . C o p r e c i p i t a t i o n of t h e a c t i v e m a t e r i a l and t h e support i s , furthermore, not g e n e r a l l y p o s s i b l e . The metal ions i n c o p r e c i p i t a t e d c a t a l y s t s , f i n a l l y , can r e q u i r e r a t h e r elevated temperatures t o be completely reduced. When t h e r e a c t o r cannot be used a t t h e s e high temperatures, i n s i t u reduction of t h e c a t a l y s t i s not p o s s i b l e , Elevated reduction temperatures l e a d , furthermore, t o s i n t e r i n g of t h e metal p a r t i c l e s . Since p r e c i p i t a t i o n of t h e a c t i v e m a t e r i a l o r i t s precursor onto t h e s u r f a c e of t h e support o f f e r s most promise, we w i l l consider t h e p r e c i p i t a t i o n of s o l i d s from a s o l u t i o n more c l o s e l y .

I .2. TheoreticaZ Background I n f i g u r e 1 t h e phase diagram of a s o l i d i n equilibrium with i t s s o l u t i o n i s represented. When a t a constant temperature t h e concentration of t h e homogeneous s o l u t i o n i s r a i s e d a s i n d i c a t e d by ( I ) ,

t h e s o l u b i l i t y curve i s reached f i r s t .

Crossing t h e s o l u b i l i t y curve does not lead g e n e r a l l y t o formation of a p r e c i p i t a t e , but a metastable s t a t e r e s u l t s . Only i f t h e c o n c e n t r a t i o n of t h e s o l u b i l i t y curve

i s exceeded by a c r i t i c a l amount, n u c l e i of t h e p r e c i p i t a t e a r e spontaneously generated. Growth of t h e n u c l e i b r i n g s about t h e t r a n s i t i o n t o t h e equilibrium s t a t e . The concentrations where n u c l e i s t a r t t o develop spontaneously i n homogeneous s o l u t i o n , a r e indicated by t h e s u p e r s o l u b i l i t y curve. The f a c t t h a t no p r e c i p i t a t e grows when t h e s o l u b i l i t y curve i s crossed i s due t o t h e considerable s u r f a c e energy of very small p a r t i c l e s of t h e p r e c i p i t a t e . A t t h e concentration of t h e s u p e r s o l u b i l i t y curve t h e l a r g e r decrease i n f r e e energy compensates f o r t h e s u r f a c e energy of t h e n u c l e i , which removes t h e n u c l e a t i o n

115 b a r r i e r . When t h e c o n c e n t r a t i o n i s r a i s e d homogeneously according t o t h e p a t h ( I ) of f i g u r e 1 , spontaneous n u c l e a t i o n s t a r t s a s t h e c o n c e n t r a t i o n of t h e supersolu-

b i l i t y curve i s touched. A l i m i t e d number of s t a b l e n u c l e i develop t h a t subseq u e n t l y grow r a p i d l y . The growth l e a d s t o t h e d e c r e a s e of t h e c o n c e n t r a t i o n of t h e s o l u t i o n i n d i c a t e d i n f i g u r e 1 ( r e f . 5 ) . When more p r e c i p i t a n t i s generated homogeneously i n t h e s o l u t i o n t h e p a r t i c l e s of t h e p r e c i p i t a t e grow and t h e concent r a t i o n of t h e s o l u b i l i t y curve i s approached. Homogeneously r a i s i n g t h e c o n c e n t r a t i o n t o t h e v a l u e of t h e s u p e r s o l u b i l i t y curve l e a d s t o a l i m i t e d number of n u c l e i . I f t h e s o l u t i o n i s maintained homogeneously, t h e c o n c e n t r a t i o n remains between t h e s o l u b i l i t y and s u p e r s o l u b i l i t y c u r v e , where no new n u c l e i can develop. A s a r e s u l t , p r e c i p i t a t i o n from a homogeneous s o l u t i o n l e a d s t o a r e l a t i v e l y small number of l a r g e p a r t i c l e s of t h e p r e c i p i t a t e . Pouring a p r e c i p i t a n t i n t o t h e s o l u t i o n , on t h e o t h e r hand, g i v e s r i s e t o an inhomogeneous s o l u t i o n . The c o n c e n t r a t i o n w i l l be l o c a l l y i n c r e a s e d f a r beyond t h a t of t h e supers o l u b i l i t y curve, as i n d i c a t e d by p a t h (2) i n f i g u r e 1 . The l o c a l l y h i g h d e g r e e of s u p e r s a t u r a t i o n g i v e s r i s e t o f o r m a t i o n of a l a r g e number of n u c l e i .

I COMPOSITION

-

F i g . 1 . Phase diagram.

Before t h e c o n c e n t r a t i o n h a s decreased by homogenizing t h e s o l u t i o n , t h e n u c l e i have grown s u f f i c i e n t l y t o b e s t a b l e a t t h e lower c o n c e n t r a t i o n . Working w i t h an inhomogeneous s o l u t i o n hence r e s u l t s i n a r e l a t i v e l y l a r g e number o f small p a r t i c l e s ( r e f s . 6-7-8).

The above shows t h a t pouring a p r e c i p i t a n t i n t o a suspension of a porous c a r r i e r l e a d s t o p r e c i p i t a t i o n of t h e a c t i v e m a t e r i a l where t h e p r e c i p i t a n t e n t e r s t h e suspension. The p r e c i p i t a t e hence w i l l n o t develop uniformly over t h e s u r f a c e of t h e s u p p o r t . P r e c i p i t a t i o n from a homogeneous s o l u t i o n proceeds e q u a l l y i n t h e p o r e s of t h e support and i n t h e b u l k of t h e s o l u t i o n , it hence can e l i m i n a t e t h e d i f f i c u l t i e s i n h e r e n t i n t h e inhomogeneous a d d i t i o n of t h e p r e c i p i t a n t . Working w i t h a homogeneous s o l u t i o n , however, g e n e r a l l y r e s u l t s i n l a r g e p a r t i c l e s of t h e p r e c i p i t a t e , whereas an a c t i v e supported c a t a l y s t system a s k s f o r v e r y small p a r t i c l e s 1 . 3 . Interaction with the Support.

Since u s i n g of a homogeneous s o l u t i o n i s n o t a s u f f i c i e n t c o n d i t i o n t o p r e c i p i t a t e v e r y small p a r t i c l e s o n t o t h e s u r f a c e of a suspended c a r r i e r , a second c o n d i t i o n

116 must be met a s w e l l , v i z . a s u f f i c i e n t l y s t r o n g i n t e r a c t i o n of the p r e c i p i t a t i n g compound with t h e support. The i n t e r a c t i o n must decrease t h e n u c l e a t i o n b a r r i e r so f a r t h a t n u c l e a t i o n a t t h e s u r f a c e of t h e support can proceed a t a concentration

between t h e s o l u b i l i t y and t h e s u p e r s o l u b i l i t y curve. Then t h e p r e c i p i t a t e can nucleate

a t t h e s u r f a c e of t h e support, whereas n u c l e a t i o n i n t h e bulk of t h e

s o l u t i o n i s prevented. Nucleation a t t h e s u r f a c e must, furthermore, proceed r a t h e r r a p i d l y , t o avoid growth of a small number of n u c l e i t o l a r g e p a r t i c l e s of t h e p r e c i p i t a t e adhering t o t h e support. The above has demonstrated t h a t t h e i n t e r a c t i o n with t h e support cannot be too weak i f a dense uniform d i s t r i b u t i o n of t h e p r e c i p i t a t e over t h e support i s t o be obtained. A s t r o n g i n t e r a c t i o n , on t h e o t h e r hand, can lead t o r e a c t i o n of a s u b s t a n t i a l f r a c t i o n of t h e f i n e l y divided support t o a compound with t h e p r e c i p i t a t i n g metal ions. Reaction of t h e support b r i n g s , consequently, about t h e adverse e f f e c t s of c o p r e c i p i t a t e d c a t a l y s t s . I n t e r a c t i o n confined t o t h e s u r f a c e of t h e support and r e a c t i o n of t h e support can be studied very well with n i c k e l ions p r e c i p i t a t i n g i n t h e presence of suspended s i l i c a . Geus has shown t h a t t h e hydrolysis of u r e a can be used t o p r e c i p i t a t e n i c k e l ions from a homogeneous s o l u t i o n onto s i l i c a ( r e f . 9 ) . The r e s u l t i n g catal y s t s displayed t h e d e s i r e d dense uniform d i s t r i b u t i o n of n i c k e l p a r t i c l e s and t h u s a high t h e r m o s t a b i l i t y . This author a l s o p r e c i p i t a t e d n i c k e l i n t h e presence

of suspended s i l i c a by i n j e c t i o n of a hydroxyl i o n s containing l i q u i d below t h e l e v e l of t h e vigorously a g i t a t e d suspension ( r e f . 9 ) . The i n j e c t i o n method can be used a t room temperature

and a t more elevated temperatures, whereas t h e h y d r o l y s i s

of u r e a proceeds with a marked r a t e only above about 70OC. To study t h e e f f e c t of t h e temperature we i n v e s t i g a t e d i n j e c t i o n s i n t o s i l i c a suspensions kept a t 25OC and a t 90°C. Nickel hydroxide d i s s o l v e s i n t o an excess of ammonia owing t o complex formation. Ammonia was t h e r e f o r e used a s t h e p r e c i p i t a n t t o be i n j e c t e d . As n u c l e a t i o n can proceed i n a l i m i t e d c o n c e n t r a t i o n range only, n u c l e a t i o n a t t h e t i p of t h e i n j e c t i o n tube w i l l be minimized. The p r e c i p i t a t i o n v e s s e l was, moreover, s p e c i a l l y designed so a s t o avoid l o c a l concentration d i f f e r e n c e s a s much a s p o s s i b l e . To e v a l u a t e t h e degree of homogeneity obtained by t h e i n j e c t i o n method, we compared i n j e c t i o n i n t o a suspension kept a t 9OOC with h y d r o l y s i s of u r e a a t 90°C. Continuous measurement of t h e pH-value and t h e l i g h t - t r a n s m i s s i o n of t h e suspension was used t o i n v e s t i g a t e t h e p r e c i p i t a t i o n process. The i n t e r a c t i o n with t h e suspended s i l i c a was e s t a b l i s h e d by comparing curves recorded with a s o l u t i o n of n i c k e l n i t r a t e alone, with suspended s i l i c a alone, and with a suspension of s i l i c a i n n i c k e l n i t r a t e . The light-transmission

i n d i c a t e d formation of a p r e c i p i -

t a t e ( n i c k e l n i t r a t e alone) o r coagulation of t h e suspended s i l i c a . Comparison of curves recorded on dosewise a d d i t i o n of hydroxyl i o n s with curves measured during continuous a d d i t i o n of hydroxyl i o n s yielded information on t h e r a t e of consumption of hydroxyl ions and, hence, on t h e r a t e of n u c l e a t i o n and growth of p r e c i p i t a t e s .

As d e a l t with above t h e n u c l e i grow a s more hydroxyl ions a r e made a v a i l a b l e

117 and, consequently, become more s t a b l e . Ageing of t h e f r e s h p r e c i p i t a t e c a n , f u r t h e r more, r a i s e t h e s t a b i l i t y of t h e p r e c i p i t a t e by rearrangement of t h e i o n s i n t h e p r e c i p i t a t e and by i o n exchange w i t h t h e s o l u t i o n and w i t h t h e s u p p o r t . We t h e r e f o r e measured t h e pH-calues a t which p r e c i p i t a t e s were r a p i d l y e s t a b l i s h e d a s w e l l a s t h e pH-values a f t e r ageing p r e c i p i t a t e s f o r 24 o r 48 hours i n t h e mother

1i q u o r

.

A t t a c k o f t h e s i l i c a support by p r e c i p i t a t i n g n i c k e l i o n s can be expected t o depend on t h e temperature and on t h e p a r t i c l e s i z e of t h e s i l i c a . We t h e r e f o r e i n v e s t i g a t e d s i l i c a o f a s u r f a c e area of 200 m2 p e r g and of 380 m2 p e r g. Subs t a n t i a l r e a c t i o n of t h e s i l i c a was i n d i c a t e d by t h e pH-curves.

To confirm t h e

i n t e r p r e t a t i o n of t h e pH-curves we i n v e s t i g a t e d t h e loaded c a r r i e r s i n t h e e l e c t r o n microscope and by measurement of t h e n i t r o g e n s o r p t i o n a t 77 K. 2. EXPERIMENTAL 2.1. Apparatus Both i n j e c t i o n of ammonia and h y d r o l y s i s o f u r e a a r e used t o i n c r e a s e homogeneo u s l y t h e hydroxyl i o n c o n c e n t r a t i o n of s o l u t i o n s and suspensions. For an e x t e n s i v e d e s c r i p t i o n of t h e a p p a r a t u s , we r e f e r t o ( r e f . 10) and t o ( r e f . 1 1 ) . 2.2. Materials

A s a s i l i c a support A e r o s i l (Degussa, West Germany) g r a d e s 200 V and 380 V were used. The s u r f a c e a r e a s were determined a s 200 and 380 m2/g r e s p e c t i v e l y . 2.3. Procedures 2.3.1.

Dosewise a d d i t i o n of d i l u t e sodium hydroxide

The t i t r a t i o n experiments w i t h d i l u t e sodium hydroxide were done i n t h e small v e s s e l f i l l e d w i t h a suspension c o n t a i n i n g 7.6 g A e r o s i l 380 V i n a s o l u t i o n of 37 g Ni(N03)2,6H20 p.a.

p e r 1 water. A f t e r adjustment of t h e pH-value a t a v a l u e

between 2 and 3 by a d d i t i o n of a few d r o p s of n i t r i c a c i d and e s t a b l i s h m e n t of t h e r e q u i r e d temperature, small d o s e s of sodium hydroxide were i n j e c t e d by means of t h e Autoburet. A f t e r a d d i t i o n of a dose t h e n e x t dose was added when t h e pH-value had remained c o n s t a n t f o r a t l e a s t 15 min. Nitrogen was bubbled through t h e susp e n s i o n t o prevent i n t e r f e r e n c e by carbon d i o x i d e from t h e a i r . 2.3.2.

Measurement of pH-values of aged p r e c i p i t a t e s .

These experiments

were a l s o done

i n t h e small v e s s e l . The p r e c i p i t a t e was

o b t a i n e d from a s o l u t i o n of 37 g of Ni(N03)2.6H20 p e r 1 water. The suspended amount of s i l i c a v a r i e d from 0.76 t o 7.6 g p e r 1 of t h e s o l u t i o n depending on t h e silica-to-nickel

r a t i o t o be e s t a b l i s h e d i n t h e p r e c i p i t a t e . A f t e r adjustment of

t h e temperature and a d d i t i o n of a few drops of n i t r i c a c i d , t h e p r e c i p i t a t i o n was done by i n j e c t i o n of 0.5 N ammonia a t a flow-rate of 2.8 ml/min. w i t h i n 30 min.

118 To i n c r e a s e t h e r a t e of ageing a small amount of n i t r i c acid was i n j e c t e d subsequently. The ageing time used was 24 h and 48 h a t 90°C and a t 25OC, r e s p e c t i v e l y . 2.3.3. Measurement of pH-values during continuous p r e c i p i t a t i o n . The small v e s s e l f i l l e d with t h e suspension containing 7.6 g a e r o s i l 380 V i n a s o l u t i o n of 37 g of NifN03)2,6H20 per 1 water was used. A f t e r adjustment of t h e temperature and a d d i t i o n of a few drops of n i t r i c a c i d , t h e Autoburet w a s used t o i n j e c t 0.5 N ammonia a t a flow-rate of 2.8 mllmin. I n t h e experiments with u r e a , 23 g u r e a per 1 of t h e suspension was added. The hydrolysis of u r e a w a s c a r r i e d o u t a t 90°C. I n a l l t h e above experiments t h e V i t a t r o n UC 200 S colorimeter was used t o record continuously t h e transmission of l i g h t of a wavelength of about 780 nm. 2.3.4.

Preparation of samples f o r e l e c t r o n microscopy and n i t r o g e n s o r p t i o n .

These samples were prepared i n t h e l a r g e v e s s e l . A f t e r a d d i t i o n of 8.5 g a e r o s i l per l i t e r and/or 41 g Ni(N03)2.6H20 p.a. per l i t e r a few drops of n i t r i c a c i d were added and t h e temperature was adjusted. Next 26 g u r e a p e r l i t e r was added o r I N ammonia was i n j e c t e d a t a r a t e of 1.4 m l per minute. When a f t e r about 20 hours t h e pH had increased t o about 8 a t 25OC and t o about 6.5 a t 90°C t h e suspension was f i l t e r e d . In t h e presence of s i l i c a t h e p r e c i p i t a t i o n of n i c k e l i o n s was q u a n t i t a t i v e , a s was evident from t h e c l e a r f i l t r a t e . The r e s i d u was subsequently washed two times with d i s t i l l e d water t o remove n i t r a t e , whereafter drying was performed a t t h e p r e p a r a t i o n temperature ( i n vacuo a t 25OC and i n a i r a t 9OOC). 2.3.5.

E l e c t r o n microscopy and n i t r o g e n s o r p t i o n .

Transmission e l e c t r o n microscopy was performed on u l t r a t h i n s e c t i o n s of t h e d r i e d p r e c i p i t a t e s embedded i n methylmetacrylate ( r e f . 12). Nitrogen s o r p t i o n a t

77 K was measured on samples outgassed a t 12OoC i n vacuum with a Carlo Erba Sorptomatic type 1810 equipped with an a c c u r a t e (+ 0.5 t o r r ) p r e s s u r e transducer. The t - p l o t was used t o analyze and i n t e r p r e t complete adsorption-desorption

isotherms

and t h e pore-size d i s t r i b u t i o n was c a l c u l a t e d according t o t h e method of Broekhoff ( r e f . 13).

3 . RESULTS AND DISCUSSION 3.1. Dosewise addition of d i l u t e sodimhydroxide. T i t r a t i o n curves, i n which t h e pH-value i s p l o t t e d a s a f u n c t i o n of t h e amount

of a l k a l i added, can provide conclusive information about t h e i n t e r a c t i o n of p r e c i p i t a t i n g ions with t h e s u r f a c e of suspended s i l i c a ( r e f s . 14-15).

In a f i r s t series

of experiments a sodium hydroxide s o l u t i o n was i n j e c t e d dosewise i n t o s o l u t i o n s

o r suspensions kept a t 2 5 O C o r a t 90°C. Measurements were done with pure water, a n i c k e l n i t r a t e s o l u t i o n and with suspensions of s i l i c a i n pure water and i n a n i c k e l n i t r a t e s o l u t i o n . With water and with s i l i c a suspended i n water t h e pH-

119 v a l u e assumed r a p i d l y an e q u i l i b r i u m v a l u e . When n i c k e l i o n s were p r e s e n t , t h e time needed t o e s t a b l i s h

a c o n s t a n t pH-value depended on t h e t o t a l amount of

sodium hydroxide added. A s l o n g a s p r e c i p i t a t i o n o f n i c k e l i o n s o r c o a g u l a t i o n of s i l i c a ( r e f . 16) was n o t a p p a r e n t , a c o n s t a n t pH-value was observed r a p i d l y , but t h e time t o reach a c o n s t a n t v a l u e r o s e s t e e p l y a f t e r t h e o n s e t of p r e c i p i t a t i o n o r c o a g u l a t i o n . A s a consequence t h e observed c o n s t a n t pH-values,

recorded a f t e r

t h e o n s e t of p r e c i p i t a t i o n o r c o a g u l a t i o n , do n o t correspond t o r e a l e q u i l i b r i u m (compare 2.3.1).

E s s e n t i a l f o r measurement of t h e s e phenomena i s t h e method of

a d d i t i o n of sodium hydroxide d e s c r i b e d above, which p r e v e n t s a s much a s p o s s i b l e , a temporary, l o c a l l y h i g h s u p e r s a t u r a t i o n . Inhomogeneous a d d i t i o n of sodium hydroxide l e a d s t o premature f o r m a t i o n of p r e c i p i t a t e . R e d i s s o l u t i o n of t h e p r e c i p i t a t e , according t o t h e e q u i l i b r i u m a t t h e f i n a l pH-value of t h e homogenized s o l u t i o n , proceeds v e r y slowly a t pH-values n e a r t o t h a t corresponding t o t h e composition of t h e s o l u b i l i t y curve. F i g u r e 2 shows t h e ( s t a b l e according t o

*I

LO :WATER

50

2.3.1)

60

pH-values recor-

ded a f t e r dosewise addit i o n o f sodium hydroxide a t 25O ( l e f t ) and a t

90°C ( r i g h t ) . A t 25OC t h e s i l i c a suspension i n water d i s p l a y e d a l e s s r a p i d r i s e i n pHv a l u e t h a n pure w a t e r , which i s due t o r e a c t i o n of hydroxyl i o n s w i t h

m

30

40

50

hydrogen i o n s set f r e e by d i s s o c i a t i o n of surf a c e hydroxyl groups:

Fig. 2. T i t r a t i o n c u r v e s .

*

Si

-

OH + H 2 0

+ f

E SiO- + H30+

The p r e c i p i t a t i o n of n i c k e l hydroxide was a t 25OC observed t o s t a r t a t a pH-value of 6.7 i n t h e absence of s i l i c a . The r e a c t i o n of hydroxyl i o n s t o n i c k e l hydroxide

caused t h e pH-value t o i n c r e a s e only s l i g h t l y on f u r t h e r a d d i t i o n of sodium hydroxide. The o n s e t of p r e c i p i t a t i o n being d i s p l a y e d a t a pH-value of 6 . 7 a g r e e s w e l l w i t h t h e s o l u b i l i t y product of n i c k e l hydroxide, which was found by F e i t k n e c h t ( r e f . 17) t o l i e between I O - l 4 a 7

and 10-'7.2 a t room temperature. I t can be seen from f i g u r e

2 t h a t b e f o r e t h e o n s e t of p r e c i p i t a t i o n t h e pH-value of t h e n i c k e l n i t r a t e s o l u t i o n i n c r e a s e d markedly l e s s r a p i d l y t h a n t h a t of p u r e water. According t o Burkov ( r e f . 18) t h i s i s due t o t h e formation of ( p o l y n u c l e a r ) hydroxyl complexes. I f a d d i t i o n

120 of hydroxyl ions does not lead t o i n t e r a c t i o n of n i c k e l ions with t h e suspended s i l i c a , a pH-curve would be measured t h a t corresponds t o t h e added amounts of hydroxyl ions consumed, with respect t o pure water, by n i c k e l ions and s i l i c a s e p a r a t e l y . This c a l c u l a t e d pH-curve has been included i n t h e curves measured a t 25OC. Figure 2 shows t h a t the curve recorded a t 25OC with a s i l i c a suspension i n n i c k e l n i t r a t e s o l u t i o n l i e s appreciably below t h i s t h e o r e t i c a l curve. The s i l i c a consequently s t r o n g l y r e a c t e d with t h e n i c k e l ions. The pH-curves

indicate that

n i c k e l ions were adsorbed markedly by t h e s i l i c a from a pH-value of about 4.2 onward. The amount of (hydrolized) n i c k e l ions, exchanged f o r protons from t h e s u r f a c e hydroxyl groups, increased smoothly with t h e pH-value.

A t a pH-level of

about 6 . 3 adsorption of n i c k e l ions onto t h e s i l i c a had decreased t h e e l e c t r o s t a t i c charge so f a r , t h a t a s l i g h t coagulation of t h e s i l i c a was evident from t h e decrease i n t h e l i g h t transmission. A t 25OC adsorption of n i c k e l ions on t h e s i l i c a s u r f a c e smoothly changes over i n t o p r e c i p i t a t i o n on t h e surface. Nucleation and growth of hydroxide a t t h e s i l i c a s u r f a c e b r i n g s about a d r a s t i c coagulation of t h e s i l i c a , a s was evident from a s t r o n g decrease i n transmission. (compare r e f . 16, f i g u r e 6 ) . The hydroxide p r e c i p i t a t e d onto t h e s i l i c a s u r f a c e i s less s o l u b l e than pure n i c k e l hydroxide a s i s evident from t h e lower pH-value where p r e c i p i t a t i o n onto s i l i c a s t a r t s . The i n t e r a c t i o n of (hydrolyzed) n i c k e l ions

and s i l i c a i s completely

d i f f e r e n t a t 90°C. Without s i l i c a t h e onset of p r e c i p i t a t i o n i s found a t a pH-value of 5.5. The s h i f t from 6 . 7 a t 25O t o 5.5 a t 90°C is mainly due t o t h e change i n t h e d i s s o c i a t i o n constant of water from

a t 25OC t o about 10-12-4a t 90OC.

The pronounced d i f f e r e n c e w i t h t h e curve recorded w i t h pure water a l s o b e f o r e t h e s t a r t of t h e p r e c i p i t a t i o n p o i n t s t o a tendency t o form (polynuclear) hydroxyl complexes which i s l a r g e r a t 900 than a t 25OC ( r e f . 19). The pH-curve measured a t 90°C with a suspension of s i l i c a i n a n i c k e l n i t r a t e s o l u t i o n coincides with t h e curve recorded f o r n i c k e l n i t r a t e alone, t o branch a b r u p t l y a t a pH-value of 4.8, where t h e l i g h t transmission i n d i c a t e d t h e onset of coagulation. The 90°C curves of f i g u r e 2 t h e r e f o r e show t h a t n i c k e l ions p r e c i p i t a t e d onto t h e s i l i c a s u r f a c e without a previous adsorption of n i c k e l ions onto t h e s i l i c a , i n c o n t r a s t t o t h e behaviour a t 25OC. That t h e n i c k e l ions i n t e r a c t i n g a t 90°C with s i l i c a a r e much l e s s s o l u b l e than pure n i c k e l hydroxide can be concluded from t h e pH-value a t which p r e c i p i t a t i o n with t h e s i l i c a proceeded, v i z . 4.8. This value l i e s appreciably below t h a t of 5.8 a t which pure n i c k e l hydroxide s t a r t e d t o p r e c i p i t a t e a t 90°C.

3.2. Aged precipitates. I f (hydrolized) n i c k e l ions are adsorbed p r e f e r e n t i a l l y on t h e s i l i c a s u r f a c e , a monolayer of n i c k e l ions should be taken up b e f o r e p r e c i p i t a t i o n of n i c k e l hydroxide onto t h e l a y e r of adsorbed hydrolized n i c k e l ions can proceed. Above we e s t a b l i s h e d t h a t adsorbed hydrolized n i c k e l ions a r e l e s s s o l u b l e than those i n bulk n i c k e l hydroxide. Together with t h e known s u r f a c e a r e a of t h e s i l i c a used i n t h i s work,

121 t h i s enabled u s t o demonstrate t h e p r e f e r e n t i a l adsorption and p r e c i p i t a t i o n of a monolayer

of n i c k e l ions experimentally. To t h i s end we used t h e pH e q u i l i b r a t i o n

measurements t h a t a l s o showed t h e reason of t h e d i f f e r e n t behaviour a t 90°C. A s d e a l t with above, t h e equilibrium pH-value was measured of suspensions of r e l a t i v e l y r a p i d l y p r e c i p i t a t e d n i c k e l hydroxide. The hydroxide was p r e c i p i t a t e d both i n t h e presence and i n t h e absence of s i l i c a . Two s i l i c a grades of a d i f f e r e n t s p e c i f i c s u r f a c e a r e a were used. During ageing of t h e p r e c i p i t a t e s t h e pH-value can change owing t o : ( i ) exchange of occluded and/or adsorbed n i t r a t e i o n s f o r hydroxyl ions ( r e f s . 20-21),

( i i ) Ostwald ripening which l e a d s t o l a r g e r p a r t i c l e s of lower

s o l u b i l i t y ( r e f . 5) and ( i i i ) ? i n c r e a s e i n t h e s i l i c a t e content of s o l i d s p e c i e s p r e c i p i t a t e d i n t h e presence of s i l i c a . Reaction with t h e s i l i c a proceeded e s p e c i a l l y

a t 90°C ( r e f . 22); it r e s u l t e d i n a l e s s s o l u b l e s o l i d and t h u s i n a lower e q u i l i brium pH-value. The r e s u l t s are represented i n f i g u r e 3, where t h e equilibrium pH-value a s a f u n c t i o n of t h e s i l i c a - t o - n i c k e l

is plotted

ratio. W e have c a l c u l a t e d t h e s i l i c a - t o - n i c k e l r a t i o s corresponding t o a monolayer of n i c k e l ions using t h e n i c k e l spacings of n i c k e l hydroxide and n i c k e l hydros i l i c a t e ( r e f s . 2 3 - 2 4 ) . For t h e two grades of s i l i c a used t h e monolayer r a t i o s a r e i n d i c a t e d i n f i g u r e 3. The n i c k e l hydroxides p r e c i p i t a t e d without s i l i c a (SiOz/Ni = 0) showed equilibrium pH-values of 7 a t 25OC and of 4 . 8 a t 90°C. The pH-value a t 25OC being s l i g h t l y higher than i n f i g . 2 i s due t o t h e f a c t t h a t i n t h e experiments of f i g u r e 3 ammonia was used a s a p r e c i p i t a n t . A t 25OC an a p p r e c i a b l e c o n c e n t r a t i o n of ammonium ions increased t h e s o l u b i l i t y of n i c k e l ions owing t o complex format i o n . Ageing of t h e p r e c i p i t a t e a t 90°C, where t h e ammonium concentration

i s low a t a pH-level of about 5, led Fig. 3. pH-values

f o r aged p r e c i p i t a t e s .

t o t h e equilibrium pH-value being markedly lower than t h a t of t h e f r e s h l y

n u c l e a t i n g n i c k e l hydroxide of f i g u r e 2. The curves i n f i g u r e 3 measured a t 25OC n i c e l y show t h a t a f t e r adsorption of a monolayer of n i c k e l i o n s t h e equilibrium pH-value gradually s h i f t e d t o t h e pHv a l u e of pure n i c k e l hydroxide. With t h e s i l i c a Aerosil 200, t h a t has a smaller

122 s u r f a c e a r e a , t h e s h i f t s t a r t e d at a higher s i l i c a - t o - n i c k e l

r a t i o than with the

a e r o s i l 3 8 0 , t h e s u r f a c e a r e a of which i s l a r g e r . The gradual i n c r e a s e i n t h e solub i l i t y of n i c k e l ions with a decreasing s i l i c a - t o - n i c k e l

r a t i o demonstrates t h a t

t h e i n t e r a c t i o n with t h e s i l i c a s u r f a c e continuously decreases. A s was i n d i c a t e d already i n f i g u r e 2 hydrolized n i c k e l ions appear from f i g u r e 3 t o r e a c t completely d i f f e r e n t l y with s i l i c a a t 90°C. When t h e amount of n i c k e l corresponding t o monol a y e r s was exceeded no d i f f e r e n t equilibrium pH-values were e x h i b i t e d . The pH-values measured with t h e d i f f e r e n t s i l i c a grades, furthermore, coincided. The equilibrium pH-value s t a r t e d t o r i s e only when t h e s i l i c a - t o - n i c k e l

r a t i o of n i c k e l a n t i g o r i t e

( r e f . 24) was approached. T h i s r a t i o i s a l s o i n d i c a t e d i n f i g u r e 3 . The r e s u l t s of f i g u r e 3 thus demonstrate t h a t a t 90°C t h e r e a c t i o n between hydrolized n i c k e l ions and s i l i c a i s not confined t o t h e s i l i c a s u r f a c e . The small s i l i c a p a r t i c l e s r e a c t t o n i c k e l h y d r o s i l i c a t e t o a considerably e x t e n t . The d i f f e r e n t grades of s i l i c a i n v e s t i g a t e d did not c o n t a i n s i l i c a p a r t i c l e s of a s i z e s u f f i c i e n t l y d i f f e r e n t t o lead t o a markedly d i f f e r e n t e x t e n t of r e a c t i o n . 3 . 3 . Measurements of pH during continuous precipitation.

Nickel-on-silica

c a t a l y s t s a r e produced by d e p o s i t i o n - p r e c i p i t a t i o n by making

a v a i l a b l e continuously hydroxyl ions. The next t h r e e f i g u r e s t h e r e f o r e show t h e change i n pH-value during continuous i n j e c t i o n o r generation of hydroxyl ions. In f i g u r e 4 curves measured a t 25OC a r e represented. The onset of p r e c i p i t a t i o n

was e s t a b l i s h e d from t h e l i g h t transmission. As s a i d above a smooth t r a n s i t i o n from adsorption of n i c k e l ions t o growth of n i c k e l hydroxide proceeds a t 25OC. This

i s apparent from t h e pH-curve of f i g u r e 4 recorded with suspended s i l i c a . The curve does n o t d i s p l a y a d i s c o n t i n u i t y i n d i c a t i n g t h e onset of p r e c i p i t a t i o n . When n i c k e l hydroxide i s p r e c i p i t a t e d without s i l i c a being p r e s e n t , an overshoot of t h e pH-value can be expected. A f t e r t h e pH-value of t h e s u p e r s o l u b i l i t y curve has been a t t a i n e d , a sudden n u c l e a t i o n and growth can proceed. The r a t e of accomnodation of hydroxyl ions i n t o t h e growing hydroxide g e n e r a l l y exceeds t h e r a t e a t which t h e hydroxyl ions are being made a v a i l a b l e . Nickel hydroxide, however, e x h i b i t s a low r a t e of growth ( r e f s . 20, 25) presumably owing t o i t s l a y e r - l i k e l a t t i c e s t r u c t u r e . A s a r e s u l t t h e growth r a t e remained l i m i t e d i n t h e experiment o f f i g u r e 4 and t h e pH-values continued t o rise s t e a d i l y . That an appreciable s u p e r s a t u r a t i o n i s required t o get a marked n u c l e a t i o n and growth r a t e i s evident from t h e pH-value

of about 7.4 where p r e c i p i t a t i o n was e s t a b l i s h e d , whereas t h e equilibrium pH-value of n i c k e l hydroxide p r e c i p i t a t e d w i t h anrmonia i s only 7.0. Figure 5 shows t h e pH-curves measured on i n j e c t i o n of armnonia a t 90°C. The curve recorded with n i c k e l n i t r a t e alone again does not e x h i b i t a maximum. The pH-level a t which t h e p r e c i p i t a t i o n proceeded, was again considerably higher than t h a t observed with e q u i l i b r a t e d pure hydroxide a t 90°C, v i z . 6.8 and 4 . 8 , r e s p e c t i v e l y . The i n t e r a c t i o n of n i c k e l i o n s with s i l i c a being a t 90°C s t r o n g l y d i f f e r e n t from

123

I

PH

r

6-

- No SILICA

---- WITH SILICA

Fig. 4. pH-curves d u r i n g p r e c i p i t a t i o n

TlME(min) 30

60

F i g . 5. pH-curves d u r i n g p r e c i p i t a t i o n

t h a t a t 250C i s e v i d e n t from f i g u r e 4 and 5 t o o . The pH-curve recorded a t 90°C w i t h suspended s i l i c a d i s p l a y s a maximum, which i n d i c a t e s a n u c l e a t i o n b a r r i e r . The d a t a of f i g u r e 3 i n d i c a t e d t h a t f r e s h l y p r e c i p i t a t e d n i c k e l h y d r o s i l i c a t e e x h i b i t s an e q u i l i b r i u m pH-value of 4 . 8 . When continuous a d d i t i o n of hydroxyl i o n s had r a i s e d t h e pH-value up t o about 5.4, r a p i d n u c l e a t i o n s t a r t e d a l l o v e r t h e l a r g e s u r f a c e a r e a of t h e s i l i c a . The h i g h r a t e of t a k e up of hydroxyl i o n s brought about by t h e sudden n u c l e a t i o n caused t h e d r o p i n t h e pH-value,

a f t e r which t h e

growth of t h e n i c k e l h y d r o s i l i c a t e proceeded a t a lower pH-level. It i s i n t e r e s t i n g t o compare t h e i n j e c t i o n w i t h t h e completely homogeneous

g e n e r a t i o n of t h e p r e c i p i t a n t . I n f i g u r e 6 pH-curves a r e r e p r e s e n t e d recorded d u r i n g t h e h y d r o l y s i s of u r e a . The pH-levels a t which n i c k e l i o n s p r e c i p i t a t e d i n t h e absence of s i l i c a ( 6 . 8 ) a r e equal w i t h i n t h e experimental e r r o r . The s l i g h t l y

less homogeneous d i s t r i b u t i o n d u r i n g i n j e c t i o n brought about, however, t h a t hydroxide c r y s t a l l i t e s were d i s c e r n e d i n t h e l i g h t t r a n s m i s s i o n a t a lower pH-value t h a n w i t h t h e h y d r o l y s i s of u r e a . With suspended s i l i c a t h e pH-curve o b t a i n e d w i t h t h e r e a c t i o n of u r e a does n o t d i f f e r markedly from t h a t of f i g u r e 5. The decomposition o f u r e a l e a d s a l s o t o f o r m a t i o n of c a r b o n a t e i o n s . Owing t o t h i s t h e s o l i d p r e c i p i t a t e d i n t h e absence of s i l i c a contained about 13 w t . % of carbon d i o x i d e . I n c o n t r a s t t o t h e p r e c i p i t a t e o b t a i n e d without suspended s i l i c a , t h e n i c k e l compound p r e c i p i t a t e d w i t h s i l i c a o n l y contained 1 w t . % of carbon d i o x i d e . T h i s a g a i n r e f l e c t s t h e l a r g e i n t e r a c t i o n between s i l i c a and t h e p r e c i p i t a t i n g n i c k e l i o n s ; r e a c t i o n w i t h s i l i c a t e i o n s p r e v e n t s t h e t a k e up of c a r b o n a t e i o n s i n t h e p r e c i p i tating solid.

124 3.4. Electron microscopy and nitrogen sorption. The above pH-measurements indicated t h a t a t 2 5 O C n i c k e l i o n s a r e f i r s t adsorbed onto t h e s i l i c a s u r f a c e t o p r e c i p i t a t e on f u r t h e r a d d i t i o n of hydroxyl ions a s a n i c k e l hydroxide l a y e r , bound t o s u r f a c e s i l i c a t e ions. A t 90°C,

on t h e o t h e r hand,

n u c l e a t i o n of n i c k e l h y d r o s i l i c a t e s e t s i n a b r u p t l y when t h e pH-value i s continuously r a i s e d . The r e a c t i o n appeared t o be not confined t o t h e s u r f a c e of t h e s i l i c a , but a considerable f r a c t i o n of t h e s i l i c a p a r t i c l e s reacted t o h y d r o s i l i c a t e . The a e r o s i l s i l i c a , used i n t h i s work, c o n s i s t s of conglomerates of very small p a r t i c l e s (about 1 5 A) t h a t a r e very t i g h t l y c l u s t e r e d t o g e t h e r t o non-porous l a r g e r p a r t i c l e s . The conglomerates and t h e non-porous p a r t i c l e s , which a r e g e n e r a l l y i n t i m a t e l y connected, have a g l o b u l a r appearance ( r e f . 26). Reaction of a s u b s t a n t i a l f r a c t i o n of t h e s i l i c a must be c l e a r l y apparent i n e l e c t r o n micrographs; t h e g l o b u l a r c l u s t e r s must be changed i n t o n i c k e l h y d r o s i l i c a t e p l a t e l e t s ( r e f . 23). We theref o r e i n v e s t i g a t e d f o u r d i f f e r e n t p r e p a r a t i o n s , a l l having a s i l i c a - t o - n i c k e l r a t i o of 1.0, i n t h e e l e c t r o n microscope. The above p i c t u r e i s s u b s t a n t i a t e d by f i g u r e 7a which repres e n t s a micrograph of a e r o s i l 380 loaded with n i c k e l ions by i n j e c t i o n of a m o n i a a t 90°C.

It can be seen t h a t t h e

o r i g i n a l globules have completel y disappeared. The s i l i c a has

1 I-NOVUCA

I

reacted t o r e l a t i v e l y w e l l c r y s t a l l i z e d f l a t and bended ( r e f . 23) p l a t e l e t s . P r e c i p i t a t i o n by h y d r o l y s i s of u r e a

77MEbnid30 60

l e a d s t o s l i g h t l y less w e l l c r y s t a l l i z e d p l a t e l e t s a s can be seen i n f i g u r e 7b. This f i g u r e shows a micrograph of a e r o s i l 380 (SiO*/Ni = 1.0)

Fig. 6 . pH-curves during p r e c i p i t a t i o n

loaded by hydrolysis of u r e a

a t 90°C. The b e t t e r c r y s t a l l i z a t i o n r e s u l t i n g from i n j e c t i o n of ammonia i s presumably due t o t h e f a c t t h a t near t h e i n j e c t i o n p o i n t t h e pH-value was s l i g h t l y higher during t h e a d d i t i o n of ammonia, which l e d t o a more easy r e a c t i o n of t h e s i l i c a ( r e f . 27).

A t 25OC t h e extent of r e a c t i o n of t h e s i l i c a should be appreciably smaller. Figure 7c shows a micrograph of a e r o s i l 380 loaded by i n j e c t i o n of ammonia a t 2 5 O C . Again p l a t e l e t s , but now being much t h i n n e r , can be seen. The p l a t e l e t s a r e s i t u a t e d a t a g r e y i s h background t h a t r e f l e c t s t h e o r i g i n a l s i l i c a s t r u c t u r e . With a e r o s i l 200

125 the platelets a r e l e s s well developed a s can be seen i n f i g u r e 7d t h a t shows a micrograph of a e r o s i l 200 onto which n i c k e l ions had been prec i p i t a t e d by i n j e c t i o n of ammonia a t 25OC. I t i s difficult to establish whether t h e platelets present i n t h e preparat i o n s of f i g u r e 7c and 7d c o n s i s t of pure n i c k e l

Fig. 7 : e l e c t r o n micrographs.

a: i n j e c t i o n 90°C ; b: u r e a 90°C ; c: i n j e c t i o n 250C, 380 V ; d: i n j e c t i o n 25OC, 200 V.

hydroxide o r c o n t a i n an

a p p r e c i a b l e f r a c t i o n of h y d r o s i l i c a t e . Nickel hydroxide c r y s t a l l i z e s with t h e b r u c i t e s t r u c t u r e t h a t contains f l a t l a y e r s of n i c k e l ions separated by two l a y e r s of hydroxyl ions ( r e f . 2 4 ) . Though t h e f l a t l a y e r s can be s l i g h t l y curved, they a r e not l i k e l y t o follow t h e l o c a l l y strong c u r v a t u r e s of t h e s i l i c a globules. The p l a t e l e t s might t h e r e f o r e c o n s i s t of pure n i c k e l hydroxide. Since t h e globules of a e r o s i l 380 have s u r f a c e s more s t r o n g l y curved than those of a e r o s i l 200, n i c k e l hydroxide p l a t e l e t s can adhere b e t t e r t o t h e globules o f a e r o s i l 200. The d a t a of f i g u r e 3 show, however, t h a t t h e equilibrium pH-value of pure n i c k e l hydroxide i s not e x h i b i t e d a t a s i l i c a - t o - n i c k e l

r a t i o of u n i t y . P l a t e l e t s of pure

n i c k e l hydroxide protruding from t h e s i l i c a p a r t i c l e s a r e hence u n l i k e l y . The p l a t e l e t s must t h e r e f o r e c o n t a i n some s i l i c a , though much l e s s than t h e n i c k e l s p e c i e s p r e c i p i t a t e d a t 90°C. Owing t o t h e l a r g e r curvature of t h e p a r t i c l e s of a e r o s i l 380 t h e s i l i c a w i l l be more r e a c t i v e . The p l a t e l e t s w i l l t h e r e f o r e c o n t a i n more s i l i c a than t h e n i c k e l p r e c i p i t a t e obtained with a e r o s i l 200 and w i l l protrude f u r t h e r

126 from t h e s i l i c a p a r t i c l e s . Preparations i n which t h e s i l i c a has been loaded by d e p o s i t i o n p r e c i p i t a t i o n a t 25OC n e v e r t h e l e s s s t i l l r e t a i n a l a r g e f r a c t i o n of t h e s i l i c a unreacted. This can be i n f e r r e d from i n f r a r e d experiments c a r r i e d out by J o z i a s s e ( r e f . 28), who observed t h a t s i l i c a samples loaded a t 25OC s t i l l d i s played t h e c h a r a c t e r i s t i c s i l i c a l a t t i c e v i b r a t i o n s . S i l i c a loaded a t 90°C d i d not e x h i b i t s i l i c a l a t t i c e v i b r a t i o n s . When t h e sample was reduced t h e n i c k e l ions migrated out of t h e h y d r o s i l i c a t e t o form small n i c k e l p a r t i c l e s adhering t o t h e remaining s i l i c a p l a t e l e t s ( r e f . l o ) . The removal of t h e n i c k e l ions led t o t h e reappearance of t h e s i l i c a l a t t i c e

v i b r a t i o n s t h a t remained a f t e r r e o x i d a t i o n of

t h e m e t a l l i c n i c k e l p a r t i c l e s t o n i c k e l oxide. Though e l e c t r o n micrographs can provide very d e t a i l e d information, e l e c t r o n microscopic r e s u l t s must be corroborated by r e s u l t s obtained on macroscopic samples. To t h i s end we used t h e adsorption and c a p i l l a r y condensation of n i t r o g e n a t 77 K. The experimental d a t a were processed according t o Broekhoff ( r e f . 13). The r e s u l t i n g d a t a a r e very well s u i t e d t o confirm t h e conclusions drawn from t h e above pHmeasurements and i n v e s t i g a t i o n s i n t h e electronmicroscope.

SBET St 1sample m'/gcat m2/gsio m'/gcat 380 'aerosil380V I42 (25) 522 1305 43 1 I 4 2 (90) 495 121 198 U4 2 256 663 20 I 200 aerosi1200V 323 808 I22 (25) 242 I22(90) 264 660 190 u22 , 245 , 613 , 186

I

-

-

Vm ml/gcat

mllgcat

Scum m2/gcat

0.043 0.043 0,029

1.515 0.845 1.294

103 56 185

0.043 0.037 , 0.027

0.180

-

V

-

-

-

-

-

1.071

, 1.155

,

94 111

-

S, m2/gcat

dm A

328 65

18 21

-

-

-

242 96 75

I

15 26 33.

I i n j e c t i o n of a m o n i a ; U h y d r o l y s i s of u r e a a t 90°C. 42 a e r o s i l 380; 22 a e r o s i l 200 (25) 25OC ; (90) 9OOC i n j e c t i o n temperature. torr. Samples previously evacuated f o r 16 h a t 120°C t o

In t a b l e 1 t h e r e s u l t s have been c o l l e c t e d . The meaning of t h e symbols i s SBET-

- s u r f a c e a r e a a s obtained - pore volume; Scum - s u r f a c e

from t-plot

area obtained by a d d i t i o n of s u r f a c e

area of pores with width l a r g e r than about 30 A; S,,, (S,,, = St

-

( r e f . 13); Vm

- volume

BET s u r f a c e a r e a ; St

of micropores; Vp

s u r f a c e a r e a of micropores

- Scum); im- mean width of micropores a s c a l c u l a t e d from S, and Vm.

Aerosil i s a l o o s e packing of s i l i c a globules t h a t has t h u s no f i x e d pore s t r u c t u r e ( r e f . 26). A s has t o be expected from t h e p l a t e l e t s apparent i n t h e e l e c t r o n micrographs t h e h y s t e r e s i s loops and pore d i s t r i b u t i o n c a l c u l a t i o n s i n d i cated the presence of slit-shaped pores i n a l l t h e samples.

127 In t h e micrographs i t can be seen t h a t t h e p l a t e l e t s i n t h e samples a r e g e n e r a l l y stacked so a s t o enclose very narrow pores. These pores a r e f i l l e d with n i t r o g e n a t very low r e l a t i v e p r e s s u r e s . This causes t h e BET-surface a r e a s t o be appreciab l y l a r g e r than t h e s u r f a c e a r e a s c a l c u l a t e d from the t - p l o t s .

The o v e r a l l p i c t u r e

from t h e micrographs i s n i c e l y confirmed. P r e c i p i t a t i o n onto a e r o s i l by i n j e c t i o n

of ammonia was observed t o lead t o r e l a t i v e l y t h i c k p l a t e l e t s ; a s a r e s u l t the s u r f a c e a r e a of I42(90) i s r e l a t i v e l y small. The smaller r e a c t i v i t y of s i l i c a by t h e absence of l o c a l l y high pH-values

( r e f . 2 7 ) i n the hydrolysis of u r e a , which

l e a d s t o t h i n n e r p l a t e l e t s , i s apparent from t h e l a r g e r s u r f a c e a r e a of U42. The v a r i a t i o n i n t h e t h i c k n e s s of t h e p l a t e l e t s a s evident from t h e s u r f a c e a r e a s i s , however, l i m i t e d . The s p e c i f i c s u r f a c e a r e a of extended p l a t e l e t s i s proportional t o t h e r e c i p r o c a l t h i c k n e s s of t h e p l a t e l e t s . A s t h e s u r f a c e a r e a s d i f f e r by a f a c t o r smaller than two, t h e t h i c k n e s s of t h e p l a t e l e t s i s not l i k e l y t o vary appreciably more than a f a c t o r of two. This i n d i c a t e s t h a t t h e p l a t e l e t s are b e t t e r stacked i n c l u s t e r s with a mutually p a r a l l e l o r i e n t a t i o n , which i s l i k e l y t o be due t o t h e more rapid d i s s o l u t i o n of t h e s i l i c a . The formation of b e t t e r stacked p l a t e l e t s with i n j e c t i o n a t 90°C

i s a l s o r e f l e c t e d from t h e pore volume of I42(90)

being markedly smaller than t h a t of U42. Figure 3 showed t h a t a t 90°C

the s i l i c a

almost completely r e a c t s and looses i t s o r i g i n a l s t r u c t u r e . This can a l s o be concluded from t h e f a c t t h a t both a e r o s i l 200 and a e r o s i l 380 being loaded a t 90°C d i s p l a y s u r f a c e a r e a s of t h e same o r d e r of magnitude. The same t r e n d is shown by t h e pore volumes. With t h e more r e a c t i v e a e r o s i l 380, i n j e c t i o n l e a d s t o b e t t e r c r y s t a l l i z e d p l a t e l e t s , a s s a i d above. A s can be expected with c l u s t e r s of p l a t e -

l e t s an appreciable f r a c t i o n of t h e s u r f a c e a r e a i s present i n pores of a width smaller than about 30 A. The conclusions drawn from t h e micrographs of c a t a l y s t s loaded a t 2 5 O C can a l s o be n i c e l y confirmed by t h e d a t a of Table 1 . The s t r u c t u r e of a e r o s i l 200 was l a r g e l y r e t a i n e d , while a number of t h i n p l a t e l e t s had grown out of t h e s i l i c a p a r t i c l e s . The loose s t r u c t u r e of t h e o r i g i n a l a e r o s i l was l o s t by t h e coagulation mentioned above, The s u r f a c e a r e a c a l c u l a t e d per g of s i l i c a i s much l a r g e r than t h a t of t h e o r i g i n a l s i l i c a owing t o t h e p l a t e l e t s protruding from t h e s i l i c a p a r t i c l e s . The p l a t e l e t s a r e enclosing mainly micropores a s evident from t h e r e l a t i v e l y small value of S t . The micrograph of a e r o s i l 380 onto which n i c k e l i o n s had been p r e c i p i t a t e d a t 2 5 O C showed very t h i n p l a t e l e t s t h a t protrude considerably f u r t h e r from t h e s i l i c a p a r t i c l e s . The s i l i c a globules of t h e a e r o s i l are kept a t l a r g e r d i s t a n c e s by t h e l a r g e r protruding p l a t e l e t s . This can be seen from t h e much l a r g e r meso-pore volume, while t h e t h i n p l a t e l e t s g i v e r i s e t o t h e l a r g e s u r f a c e area.

Since t h e p l a t e l e t s a r e r a t h e r well separated, they enclose an appreciable

s u r f a c e a r e a i n s l i t s more than 30 A i n width. The above f i n d i n g s with r e s p e c t t o t h e formation of n i c k e l h y d r o s i l i c a during d e p o s i t i o n p r e c i p i t a t i o n and/or t h e ageing of t h e p r e c i p i t a t e s , agree very well

128 with the results published by Signalov (refs.29-30) on the interaction of nickel ions with silica gels. 4. CONCLUSION This work has demonstrated, we believe, that combining careful characterization of the reaction products with procedures in which deposition-precipitation is carried out under accurately controlled conditions can explain the structure of the resulting catalysts very satisfactorily. Since precipitation from homogeneous solution can be scaled up very easily, catalysts having desired properties can be produced. This work showed, however, also that the elementary processes proceeding during the precipitation of the active components onto the support must be studied in detail. This holds also for the thermal pretreatment of solid catalysts. REFERENCES 1 G.K.Boreskov in Preparation of Catalysts (B.Delmon, ed.), Elsevier, Amsterdam, 1976, pp.223-50. 2 J. Cervello, E.Hermana, J.Jimindz, and F.Melo in Preparation of Catalysts (B.Delmon, e d . ) , Elsevier, Amsterdam, 1976, pp.251-263. 3 K. Morikawa, T.Shirasaki, and M. Okada, Adv. Catal., 20(1969) 98. 4 P. Courty, and C. Marcilly in Preparation of Catalysts (B-Delmon,ed.), Elsevier, Amsterdam, 1976, pp.119-45. 5 A.G. Walton in Dispersion of Powders in Liquids (G.D. Parfitt, ed.),Elsevier, Amsterdam, 1969, pp.122-64. 6 D. Mealor, and A. Towhnshend, Talanta, 13(1969)1069-74. 7 P.F.S. Cartwright, E.J. Newman, and D.W. Wilson, The Analyst, 92 (1967)663-79. 8 W.J.Blaede1, and V.W. Meloche, Elementary Quantitative Analyses, 2nd edn., Harper and Row, New York, 1963, pp.177-84, pp.719-29. 9 J.W. Geus, Dutch Patent Application, 1967, 6705, 259, and Dutch Patent Application, 1968, 6813, 236. 10 A.J. van Dillen, J.W. Geus, L.A.M. Hermans, and J. van der Meijden, Preprints Proceedings Sixth International Congres on Catalysis, London, 1976,B7. 11 A.C. Vermeulen, J.W. Geus, R.J. Stol, and P.L. de Bruyn, J. Coll. Interf.Sci., 51 (1975)449. 12 L. Moscou, Leitz.-Mitt. Wiss. u Techn., 11 (1962) 103-5. 13 J.C.P. Broekhoff, and B.G. Linsen in Physical and Chemical Aspects of Adsorbents and Catalysts (B.G. Linsen, ed.), Academic Press, London, 1970, pp.1-62. 14 L.H. Allen, and E. Matijevic, J. Coll. Interf. Sci., 33(1970) 420-429. 15 L.H. Allen, and E. Matijevic, J. Coll. Interf. Sci., 35(1971) 66-75. 16 T.W. Healy, R.O. James, and'R. Cooper, Adv. Chem. 79(1968) 62-73. 17 W. Feitknecht, Pure and Appl. Chem., 6(1963) 130-99. 18 K.A. Burkov, L.S. Lilic, and L.G. Sillen, Acta Chem. Scan. 19(1965) 14-30. 19 K.A. Burkov, N.I. Znevick, and L.S. Lilich, Russ. J. Inorg. Chem. 16 (1971) 926, 927. 20 W. Feitknecht, Koll. Zeitschrift, 136 (1954) 52-66. 21 0. Bagno, and J. Longuet-Escard, J. Chim. Phys. 51 (1954) 434-39. 22 J.J.B. van Eijk van Voorthuijsen, and P. Franzen, Rec. Trav. Chim. 70(1951) 793-812. 23 G. Dalmai-Imelik, C. Leclercq, and A. Maubert-Muguet, J. Solid State Chem., 16(1976) 129-39. 24 J.W.E. Coenen, and B.G. Linsen in Physical and Chemical Aspects of Adsorbents and Catalysts (B.G. Linsen, ed.), Academic Press, London, 1970,pp.473-480. 25 V.M. Chertov, and R.S. Tijutijunnik, Kolloidn. Zhur. 37 (1975) 283-7. 26 D. Barby in Characterization of Powder Surfaces (G.D. Parfitt, ed.) Academic Press, London, 1976, pp.385-93.

129 27 C. Okkerse in Physical and Chemical Aspects of Adsorbents and Catalysts (B.G. Linsen, ed.), Academic Press, London, 1970, pp.215-42. 28 J. Joziasse, thesis, Utrecht, The Netherlands, 1978. 29 I . N . Signalov, and A.P. Dushina, J. Appl. Chem. USSR, 46(1973) 1751-54. 30 I . N . Signalov, G.N. Kuznetsova, Yu. V. Konovalova, T.V. Shakina, A.P. Dushina, and V.B. Aleskovskii, J. Appl. Chem. USSR, 49 (1976) 2436-42.

DISCUSSION E.MATIJEVIC:I~ both papers only nickel nitrate was used. what the effect of other anions

would be.

I wonder

Indeed, no metal

hydroxide precipitation takes place by direct combination of metal and hydroxide ions.

Instead, a series

precede precipitation.

The composition

depends strongly on anions present.

Of

metal hydrolysis products

Of

pure complex solutes

Thus, it should be expected

that sulfate or acetate salts of nickel would give precipitate characteristics different from those obtained with the corresponding nitrate salt. J.W.

GEUS

Without suspended silica the precipitation of nickel

:

hydroxide is affected by the anions present in the solution.

It

has been observed that the structure of the precipitate depends strongly on the presence of carbonate ions in the solution.

Carbonate

ions drastically suppress the growth of nickel hydroxide crystallites (to be published). The hydrolysis of urea leads to formation of carbonate ions.

The

precipitation obtained using the hydrolysis of urea hence contains about 13 wt.% of carbon dioxide as mentioned on p.11. suspended silica the situation is completely different.

With The low

solubitility of nickel hydrosilicate brings about that the loaded carrier did not contain carbon dioxide.

The precipitation of nickel

with suspended silica is not markedly influenced by the anions in the solution. D.J.C. YATES

:

I realize that the particle sizes of the urea-pre-

cipitated catalysts are rather small, as shown by your electronmicroscope photographs.

Can you be sure that the catalyst after

precipitation is nickel silicate as distinct from, say, nickel hydroxide

?

J.W. GEUS

:

When the deposition-precipitation was carried out at

9 O o C , we observed virtually complete reaction of the silica support we used, as dealt with in our paper.

This conclusion was based on

:

[i) the pH-value of equilibrated suspensions as a function of the silica-to-nickel ratio represented in Fig. 3 ; (ii) electron micrographs which do not show the silica particles of the support with the carrier loaded at 90°C; (iiiY the infra-red evidence obtained by Joziasse; with the loaded carrier he

did not observe the characteristic lattice vibrations

of silica.

Reduction led to decomposition of the hydrosilicate and

hence to reappearance of the lattice vibrations of silica.

The

lattice vibrations remained on reoxidation. when the precipitation was done at 3 O o C, reaction of the support to nickel hydrosilicate is much less extensive.

This can be

concluded from the pH-values of Fig. 3, the electron micrographs of Fig. 7 and the infra-red results mentioned on p. 14.

The results

of Fig. 3 show that the silica content of the nickel precipitate decreases gradually with the silica-to-nickel ratio. It is difficult to get evidence from X-ray or electron diffraction. The diffraction patterns of platelets of nickel hydroxide and nickel hydrosilicate are difficult to distinguish. J.W.E.

CQENEN

:

I refer to the question of Dr. Yates on the X-ray

evidence for hydrosilicate formation and answer of Dr. Geus who found it difficult to get this information. I would like to say that first igniting the material facilitates this detection

:

nickel

hydroxide decomposes quantitatively to oxide at 4OO0C, but hydrosilicate does not. J.W. GEUS

:

Whereas pure nickel hydroxide decomposes already at

temperatures below 2 O O o C , nickel hydrosilicates dehydrate at more elevated temperatures.

Van Eijk, van Voorthuijsen and Franzen

(1951) found two different nickel hydrosilicates, one being dehydrated at a considerably lower temperature than the other. Nickel hydrosilicate decomposing at the higher temperature was assigned as nickel montmorillonite and the one decomposing at lower temperature as nickel antigorite. We wonder whether keeping the catalyst at 400°C leads to dehydration of nickel hydroxide and not of nickel hydrosilicate.

The gradual drop in the silica content of the

precipitate [Fig. 3) should lead to a continuously changing dehydration temperature.

Therefore we are investigating presently

the differently prepared nickel-on-silica catalysts by thermogravimetry and differential thermal analysis.

131

CRYSTALLITE SIZE DISTRIBUTIONS AND STABILITIES OF HOMOGENEOUSLY DEPOSITED Ni/Si02 CATALYSTS J . T. RICHARDSON, R. J. DUBUS, J. G. CRUMP, P. DESAI, U. OSTERWALDER and T. S. CALE Department of Chemical Engineering, University of Houston, Houston, Texas USA 77004 ABSTRACT Nickel on silica catalysts were precipitated with urea. A f t e r reduction the crystallite size distribution (CDS) of the nickel was determined from magnetic measurements. This technique gives very narrow CSD's and is much more reproducible than conventional impregnation methods. Nickel loading is controlled by solution concentration and precipitation time but higher values lead to broader CSD's, lower BET surface areas, and constricted pores. The amount of reduced nickel but not the CSD is a function of hydrogen flow rate and time. Increasing reduction temperatures produce broader CSD's. Calcination or passivation has no effect on the CSD's. The CSD is very stable a t 673 K but shows broadening for sintering times up to 100 hours. Above 723 K , small crystallites disappear and the CSD reaches a limiting log-normal shape, independent of initial distribution or nickel concentration. The decline of metal surface area has an order of 10 and an activation energy of 200 kJ/mole. A t temperatures approaching 873 K , the sintered distributions are bi-modal . INTRODUCTION Nickel catalysts are most effectively prepared through the optimal combination of high dispersion and metal loading (ref. 1). Small crystallite sizes ensure high specific metal areas but increasing nickel concentrations result in agglomeration. Common impregnation and precipitation procedures lead to inhomogeneous crystallite sizes except for the dilute samples (6%). Increasingly broad and even bimodal crystallite size distributions (CSD) are found as the nickel increases. This not only changes the activity and selectivity patterns for demanding-type reactions but may also affect thermal stability, since resistance to sintering depends to some extent on the initial CSD (ref. 2). Van Dillen and co-workers developed a method for the homogeneous precipitation - deposition of nickel hydroxide on silica in aqueous suspension (ref. 3).

132

Slow decomposition of urea in water is a controlled source of hydroxyl ions. In the presence of nickel ions, these hydroxyls precipitate nickel slowly and homogeneously throughout the suspension. Furthermore, nickel reacts with silica upon precipitation to form a hydrosilicate which is resistent to coalescence. Catalysts prepared in this manner exhibit a uniform dispersion of very small nickel particles. We have used this method together with CSD measurements to investigate (a) preparational parameters (precipitation time, solution concentrations and initial pH, (b) reduction parameters (time, temperature and flow rate), (c) calcination (d) oxygen passivation, and (e) sintering. EXPERIMENTAL Catalyst Preparation Urea was added to a suspension of Cab-0-Sil HS5 ( S O 2 ) in nickel nitrate solution after the solution reached 363 K. In one series of experiments, the concentrations of nickel nitrate, urea and the support were 0.14M, 0.42M and 7.6 kg/m3 respectively; in another, 0.28M, 0.84M and 10kg/m3. The pH-time curves were the same as those reported by Van Dillen g g. (ref. 3). Increasing amounts of total nickel were obtained by precipitating for longer times. The precipitate was filtered, washed and dried a t 393 K. Total nickel content on each sample was measured with a colorimetric method described by Coenen and Linsen (ref. 4). Selected samples were calcined in air at various temperatures and times. Reduction procedures The catalysts were reduced in the same cell used for magnetic measurements (Fig. 1). ROTATlNG SHAFT

COIL SETS A AND B A-SAMPLE COIL 6-FIELD COIL LEADS QUARTZ PACKING SAMPLE QUARTZ SAMPLE CELL

Figure 1. Experimental sample cell and rotating magnetometer

133

The outer 12mm quartz tube contained one centimeter of catalyst. Reducing and purge gases passed through the inner capillary tube. Each sample (2 to 81~10-~kg) was charged and the cell connected to a gas system. High purity grade hydrogen was passed through the bed at 25 to 200 cm3/min and the temperature raised to reduction temperatures a t eight degrees per minute. After reduction, the sample was outgassed in argon or helium for one hour at 25 degrees above the reduction temperature and cooled to ambient with the inert gas still flowing. The cell was transferred to the magnetometer for magnetization measurements and then to a Micromeretics Pore Size Analyzer for nitrogen BET surface area, pore size distribution and hydrogen nickel surface area measurements. Magnetic Measurements and Calculations Details of the measurement of magnetization using the rotating coil magnetometer shown in Fig. 1 are given elsewhere (ref. 5). Magnetization curves were measured at 298 K for each catalyst after reduction and sintering for various times and temperatures. This included the change of magnetization upon removal of adsorbed hydrogen (ref. 6 ) . The last step was determination of the saturation magnetization and degree of reduction after excessive sintering in helium a t 1073 K. Crystallite size distributions were calculated from magnetization data using Model calculations confirmed that this developed procedures (ref. 7). technique is sufficiently sensitive to describe changes in distributions resulting from these treatments. RESULTS AND DISCUSSION Homogeneous precipitation deposition Fig. 2 shows a comparison between samples prepared by urea precipitation and impregnation with nickel nitrate. Both samples contained about the same amount of total nickel and were reduced at identical conditions (15 hours at 673 K ) yet the precipitation CSD is very narrow and small compared to the result of the impregnation. The precipitation procedure is also extremely reproducible. For example, two separate preparations (with different operators) resulted in almost identical CSD's and reductions. Impregnation, however, was very difficult to repeat. Even specimens from the same preparation were different.

-

Effect of precipitation time The effect of precipitation time is to increase the total amount of nickel deposited, as shown in Fig. 3. Changes in sample pore structure as a result of increasing precipitation time are represented in the nitrogen isotherms of Fig. 4.

134

o , 5 r i HOMCGENEOUS PRECIPITATION

0.4-

2 0.3m

g

0.2-

;

0.1-

E

t;; 8

O-

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0.5 - 16% TOTAL NICKEL 9.8% REDUCED NICKEL

-I

0.4-

8

0.2

k

0.I

1

2

3

4

5

6

7

RADIUS, nm

Figure 2.

Effect of method of deposition

RADIUS, nm

Figure 3. Effect of precipitation time The hysteresis trends indicate pore neck constrictions that become more severe as precipitation continues. Loss of total surface area from 2.91 x los m2/kg to 1.66 x lo5 m2/kg also occurs, with the percentage of surface in pores below 10 nm decreasing from 90 to 76. The extent of reduction (29-35%) is approximately the same. The increased constrictions in the pore may be responsible for the bi-modal trend in the CSD. Since growth of crystaliite in the narrow necks are constrained against further growth, whereas those in the larger body of the pores are not. From these and subsequent results, we

135

conclude that the nickel concentration is much more important in determining the CSD than is the amount of reduced metal. This implies that crystallite growth during reduction is a consequence of nucleation and not sintering, at least up to 673 K.

4 HOURS

0.3

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r 0

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0.14M Ni(NO&

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0.42M UREA 7.6 k g S i O p / m 3 25% TOTAL Ni

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0.84M UREA 10.0kg Si02/m3 4 0 % TOTAL Ni

3 6 % TOTAL Ni

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Figure 4.

Nitrogen isotherms for increasing precipitation times

Effect of solution concentrations Increasing concentrations of urea and nickel nitrate increase nickel deposition rates with decreased dispersion. The effect of precipitation time on the CSD is much less for the higher concentrations, possibly because the amount of nickel deposited reaches a steady value for the longer times. One difference is significant. The extent of reduction is higher and increases with time although the total nickel content does not change significantly. Effect of initial pH This variable was explored in only one experiment. Two samples were prepared with 4 hours precipitation from 0.4M urea solution but sufficient nitric acid was added to one of them, lowering the initial pH from 4 to 2.5. The result is a slightly lower CSD with less total nickel for the sample with lowered pH. However, the extent of reduction increases, showing some effect on the deposition mechanism.

136

Effect of reduction time Figure 5 shows the effect of time on reduction at 673 K. The CSD is remarkably uniform but the extent of reduction increases. Sintering does not appear to involve the reduction in any way. Initially the reduction is fast so that about 35%of the nickel is reduced a t five hours. Thereafter the process is slower. This may indicate rapid reduction of the outer layer of nickel hydrosilicate followed by slower reduction deeper in the particle. Effect of reduction temperature Reduction a t temperatures higher than 673 K result in broader CSD. Many of the samples have bi-modal distributions. These effects may come from accelerated sintering during reduction or from reduction of separate precursers which are harder to reduce at lower temperatures yet result in larger crystallites . 04

5

d 03

E 0.2

2

0)

& 01 O1

J 2

3

4

0

1

-2

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RADIUS, nrn

Figure 5. Effect of reduction time Effect of hydrogen flowrate Fig. 6 demonstrates the dependence of the extent of reduction on hydrogen gas velocity. The CSD are again unchanged. Particle Reynolds numbers at these conditions are extremely small (.01 .05) so that the flow is almost in the Stokes region, yet the mass transfer coefficient for hydrogen is six orders of magnitude higher than the fastest reduction rate. The reduction process may be controlled by water removal from the pores. Varying the hydrogen flow rate is a very sensitive and reproducible method of controlling the amount of reduced nickel.

-

137

RADIUS, nrn

Figure 6. Effect of hydrogen flow rate Effect of calcination Calcination in air prior to reduction has little effect on the resulting CSD. Temperatures from 623 K to 723 K were tried with similar results. This implies that the reduction precursor, presumably nickel hydrosilicate, is not drastically altered by the air treatment. This is in contrast to other systems, such as alumina, where calcination promotes support compound formation. Effect of passivation Reduced nickel catalysts are often passivated by treatment in dilute oxygen to protect the nickel during subsequent handling operations. A sample was reduced, cleaned and measured in the usual manner. A 2% mixture of air in helium was passed over the catalyst at 298 K for one hour. Air concentration was increased slowly to one atmosphere and the cell exposed to the laboratory atmosphere overnight. Subsequent reduction and measurement resulted in a CSD almost identical to that before passivation. The following observations were made about the pyrophoric nature of dispersed nickel. After a reduced catalyst has been cleaned in argon at 25O above the reduction temperature, the magnetization curve increases about thirty percent. Adsorption of hydrogen a t room temperature returns the curve to its initial value, indicating that the surface is saturated with hydrogen after reduction. If the catalyst is now exposed to air, rapid oxidation and heating of the catalyst follows. Dispersion of the nickel is destroyed by the exothermic heat of oxidation. However, exposure of the catalyst to air after cleaning produces no temperature increase and the magnetization curve shows a decrease consistent with the adsorption of a

138

monolayer of oxygen (ref. 6). Further reduction, cleaning and measurement gives a CSD almost identical with the non-oxidized sample. These results confirm the observation of Popowicz g g. (ref. 8) that the adsorbed hydrogen and not the nickel initiates the dangerous pyrophoric oxidation of the nickel. Sintering Effects Sintering results at 673 and 773 K and are given in Fig. 7. This preparation is very stable a t 673 K . The mean radius is almost constant a t 1.5 nm, but the distribution first broadens and then moves toward higher radii. Finally, after 150 hours, small crystallites disappear. A t 773 K, the shift of SlNTERlNG AT 773 K

SlNTERlNG AT 673 K

0.4

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08 FRESH

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8

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150 HOURS 1

2 3 RADIUS, nm

4

0.4

5

0

1

2 3 RADIUS, nm

4

5

Figure 7. Crystallite size distributions for sintering a t 673 and 773 K. the mean radius toward higher values is so pronounced that all crystallites below 1.5 nm are removed. Sintering is further illustrated in the results of Table 1 and Fig. 8. TABLE 1 Total and nickel surface areas for sintered catalysts Catalyst Surface Area m2/g (sample or Ni) x 4 hrs precipitation reduced 15 hrs, 673 K sintered 30 hrs, 823 K sintered 16 hrs, 1073 K

S~~~

'CSD

sH

2.67 2.20 1.75

1.86 1.24 c.30

1.82 0.84 0.15

1.69 0.94

-

139

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FRESH AND REDUCED, 673 K

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I

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I

I

0

I

2

3

4

5

6

7

8

9

10

PORE RADIUS, nm

Figure 8. Pore size distributions for sintered catalysts There is a moderate loss of total surface area upon sintering a t 823 K , due mostly to the collapse of pores smaller than about 1.5 nm. The values of SCSD were calculated from the CSDs, SH came from hydrogen adsorption measurements and Sm were estimated from the change of magnetization after cleaning using procedures to be published elsewhere (ref. 9). For the freshly reduced catalyst, the results are in satisfactory agreement, indicating internal consistency of procedures and emphasizing the availability of metal surface. After sintering a t 823 K, SH and Sm are in agreement but are less than the calculated surface. This can only mean that some surface is inaccessible to hydrogen due to either trapping of small crystallites in collapsed pores or to the surface bonding with the support. The CSD’s show a similar trend as those at 773 K in Fig. 7 with the disappearance of crystallites smaller than 1.5

m. Other conclusions are as follows: (1) A t all temperatures the final CSD is independent of the initial CSD, (2) calculated surface areas, SCSD, fit the ds = ksn with n = 10 and an activation energy of 200 kinetic expression kJ/mole, (3) sintering rates are independent of reduced nickel concentrations up to 17 wt% Ni, and (4) for temperatures above 873 K , bi-modal CSD’s result. These results are most consistent with the model of crystallite migration (ref. 10) but the effect of changes in support pore texture during sintering must be reconciled before definite conclusions can be reached.

-

140

ACKNOWLEDGEMENTS The authors thank the Robert A. Welch Foundation and the National Science Foundation for support of this work. REFERENCES

1 J. W. Geus, Sintering and Catalysis, Plenum Press, New York, 1975, p. 29. 2 P. C. Flynn and S. E. Wanke, J. Catal., 34(1974)290. 3 J. A. Van Dillen, J. W. Geus, L. A. M. Hermans and J. Van der Mejden, Proceedings of the Sixth International Congress on Catalysis, London, July 1976, The Chemical Society, London, 1977, p. 677. 4 J. W. G. Coenen and B. F. Linsen, Physical and Chemical Aspects of Adsorbents and Catalysts, Academic Press, New York, 1970, p. 472. 5 J. T. Richardson and P. Desai, to be published. 6 J. A. Dalmon, G. A. Martin, and B. Imelik, Surf. Sci. 41(1974)587. 7 J. T. Richardson and P. Desai, J. Catal. 42(1976)294. 8 M. Popowitz, W. Celler, and E. Treszczanowicz, Int. Chem. Eng. 6(1966)63. 9 J. T. Richardson and T. Cale, to be published. DISCUSS ION S.P.S. ANDREW : One of the conclusions drawn is that the results on sintering are most consistent with the model of crystallite migration. However, one of the observations is that sintering rates are independent of reduced nickel concentrations.

This does not seem

consistent with the migration model which is a second order process. 3.T.

RICHARDSON : Two samples with identical crystallite size

distributions but differing by a factor of two

in reduced nickel

concentration, were found to sinter with the same kinetics.

You

are indeed correct that this is inconsistent with the particle migration model.

Perhaps this reflects the influence of support

texture and existing theories must be modified.

However, a factor

of two differences in nickel i s not expected to give a large difference according to current models.

This may very well be within

our experimental error. If.

CHARCOSSET

:

What do you think of the stoichiometry of oxygen

adsorption on Ni at room temperature after cleaning procedures ? Are one or two oxygen atoms chemisorbed per exposed Ni atom ? Have the authors to admit that about two 0 atoms are chemisorbed per surface nickel atom ? 3.T.

RICHARDSON

:

We have assumed one oxygen atom per nickel at the

surface but have no evidence for this.

The stoichiometry is not

important to our conclusion that one layer of nickel is affected

141 by oxygen during our passivation procedure. J.W.E.

COENEbl : DO you have quantitative information on the depth

of oxidation in passivation ? In our experience in passivation, the depth of oxidation is about 2 nickel atom layers. J.T. RICHARDSON : The l o s s of maanetization after passivation corresponds to the "demetallization" of one layer of nickel atoms. We conclude that only one layer of nickel is oxidized and that the phenomenon is equivalent to chemisorption. B. DELMON : Concerning Fig. 3 (at 14 hrs), the authors suggest that

the two categories of crystallites might correspond to two different precursors.

Another explanation is that longer reaction times

bring about a recrystallization of a single precursor, with the larger crystallites growing at the expense of the smaller ones. Pig. 3 indeed shows that, even after only 4 hrs., there is already a family of larger crytallites

:

these might be the ones

growing,

later on, at the expense of the smaller ones. Concerning Fig. 2 , the two families of crystallite sizes observed with the catalysts impregnated with nickel nitrate are very different. A s nickel nitrate impregnation is a widespread method for preparing

nickel catalysts, it would be interesting to know the origin of these families.

Have you evidences that the larger crystallites

might come from nitrate deposited at, or having migrated to, the mouth of the pores during the drying process

?

The smaller

crystallites could correspond to deposits inside the pores. J.T. RICHARDSON

:

The example was taken from a number of samples

showing similar heterogeneity.

We have no explanation for the

variation in distributions but suspect that uneven and difficult to control drying procedures are responsible. L.L. MURELL

:

What is your opinion of the structure of the unreduced

nickel portion of a nickel on silica catalyst ?

Is for Ni(N03)2

impregnated onto silica the silicate layer only a monolayer thick ? If so, why is there such difficulty in getting complete reduction of this phase as shown in your work

?

If the nickel silicate layer

is greater than monolayer thickness then difficulties in complete reduction might be expected.

142 J.T.

R I C H A R D S O N : E x a m i n a t i o n of m a n y p r e p a r a t i o n s s u g g e s t s t h a t

NilOEI),

i s easily r e d u c e d but n i c k e l h y d r o s i l i c a t e i s not.

The

nickel s i l i c a t e layer i s m o r e t h a n o n e l a y e r t h i c k and e v e n c o n s u m e s m o s t of t h e particle. reduction.

The deeper this layer, the more difficult the

143

THE PREPARATION AND PRETREATMENT OF COPRECIPITATED NICKEL-ALUMINA

CATALYSTS FOR

METHANATION AT HIGE TEMPERATURES

E . C . ICRUISSINK~,L.E.

J.R.H.

A L Z A M O R A ~ , s. O R R ~ , E.B.M. DOESBURG',

L.L. VAN REIJEN~,

R O S S ~and G. VAN V E E N ~

"Laboratory o f I n o r g a n i c and P h y s i c a l Chemistry, U n i v e r s i t y o f Technology, D e l f t ( N e t h e r l a n d s ) and

b

School o f Chemistry, U n i v e r s i t y of B r a d f o r d , B r a d f o r d BD7 1DP

(U.K.)

ABSTRACT The p r e p a r a t i o n and p r o p e r t i e s o f a s e r i e s o f c o p r e c i p i t a t e d nickel-alumina c a t a l y s t s have been examined. For 2

5

Ni/A1(

3, t h e p r e c i p i t a t e h a s a l a y e r

s t r u c t u r e o f t h e t y p e p r e v i o u s l y d e s c r i b e d by F e i t k n e c h t and by Longuet-Escard. Our r e s u l t s i n d i c a t e t h a t t h e n i c k e l and aluminium i o n s occupy i n t e r c h a n g e a b l e p o s i t i o n s i n t h e "brucite-type"

-

-

(CO;, NO3 or C l - ,

l a y e r s o f t h e s t r u c t u r e , while various anions

depending on t h e method o f p r e p a r a t i o n ) a r e found i n t h e "in-

t e r l a y e r " i n a d d i t i o n t o w a t e r o f c r y s t a l l i s a t i o n . On c a l c i n a t i o n , p o o r l y c r y s t a l l i s e d n i c k e l o x i d e , which p r o b a b l y c o n t a i n s d i s s o l v e d A13+ i o n s , i s formed; t h e c r y s t a l l i n i t y o f t h i s phase depends s t r o n g l y on t h e i o n s p r e s e n t i n t h e p r e c i p i t a t e . Phase s e p a r a t i o n o f N i A l 0

2 4 o c c u r s at t e m p e r a t u r e s around 1000 K . The

m e t a l l i c s u r f a c e a r e a s and t h e methanation a c t i v i t i e s o f t h e reduced samples depend d i r e c t l y on t h e i r degree of r e d u c t i o n ; for i d e n t i c a l c a l c i n a t i o n and red u c t i o n c o n d i t i o n s , it i s found t h a t samples o r i g i n a t i n g from t h e carbonatec o n t a i n i n g p r e c i p i t a t e s have h i g h e r a c t i v i t i e s under methanation c o n d i t i o n s t h a n t h e n i t r a t e s o r c h l o r i d e s . The p r e s e n c e o f sodium h a s a d e t r i m e n t a l e f f e c t on t h e p r o p e r t i e s of t h e c a t a l y s t s .

1 . INTRODUCTION Because of r i s i n g demand f o r n a t u r a l g a s , t h e r e has r e c e n t l y been an i n c r e a s i n g i n t e r e s t i n t h e methanation o f carbon monoxide r i c h g a s e s . The methanation r e a c t i o n CO + 3H2

-+

CHb + H20

i s h i g h l y exothermic and l a r g e t e m p e r a t u r e r i s e s w i l l occur under t w i c a l r e a c t i o n

144 conditions ( r e f s . l , 2 ) ; hence, a good methanation c a t a l y s t must have a c t i v i t y a t t h e low temperatures of t h e e n t r y t o t h e c a t a l y s t bed, but it must a l s o have high s t a b i l i t y under t h e hydrothermal conditions of t h e r e a c t o r e x i t . The methanation r e a c t i o n i s a l s o u t i l i s e d i n t h e "Adam and Eve" process under development i n West-Germany f o r t h e t r a n s p o r t a t i o n of heat from a high temperature nuclear r e a c t o r ( r e f . 2 ) ; i n t h i s , t h e helium coolant of t h e r e a c t o r s u p p l i e s t h e energy f o r t h e steam reforming of t h e methane, and t h e energy i s regained with high e f f i c i e n c y by subsequently c a r r y i n g out t h e methanation r e a c t i o n . Thermal e f f i c i e n c y r e q u i r e s a high temperature a t t h e r e a c t o r e x i t and hence again e x c e l l e n t c a t a l y s t s t a b i l i t y . Currently, t h e production of s y n t h e t i c n a t u r a l gas (SNG) i s most commonly achieved by t h e steam reforming of naphthas a t low temperatures using a coprecip i t a t e d Ni-A1203 c a t a l y s t w i t h high N i content ( r e f s . 3,

4).

Preliminary r e s u l t s

of an examination of t h e p r e p a r a t i o n and a c t i v i t i e s of such c a t a l y s t s showed t h a t t h e p r o p e r t i e s of t h e f i n a l c a t a l y s t depend markedly on t h e parameters which were changed during t h e p r e p a r a t i o n and pretreatment of t h e samples ( r e f . 5 ) . The a h of our p r e s e n t work i s t o gain a g r e a t e r understanding o f t h e i n t e r - r e l a t i o n s h i p s between t h e method of p r e p a r a t i o n of such c a t a l y s t s , t h e i r s t r u c t u r e s and t h e i r a c t i v i t i e s and s t a b i l i t i e s f o r t h e methanation r e a c t i o n with t h e hope t h a t we may be a b l e t o design a s t a b l e methanation c a t a l y s t f o r p o s s i b l e use i n t h e Adam and Eve p r o j e c t . This paper p r e s e n t s some of our main conclusions t o d a t e ; some preliminary r e s u l t s have been published elsewhere ( r e f s .

6 , 7 ) . We have found t h a t w i t h i n a

d e f i n i t e range of N i / A 1 r a t i o s , nickel-aluminium hydroxy compounds a r e formed on p r e c i p i t a t i o n and, depending on t h e conditions o f p r e p a r a t i o n , t h a t t h e s e may contain carbonate, n i t r a t e o r c h l o r i d e ions. On c a l c i n a t i o n , decomposition o f t h e p r e c i p i t a t e occurs i n two s t a g e s : ( a ) loss of water of c r y s t a l l i s a t i o n , and ( b ) loss of anions and decomposition of t h e hydroxide s t r u c t u r e ; t h e o x i d i c form of t h e c a t a l y s t probably c o n s i s t s of n i c k e l oxide w i t h dissolved aluminium i o n s , t o g e t h e r with amorphous alumina. The f i n a l s t a g e of t h e p r e p a r a t i o n i s reduction i n hydrogen; m e t a l l i c n i c k e l i s formed i n a highly d i s p e r s e d s t a t e whose high s t a b i l i t y at e l e v a t e d temperatures must be due t o i t s i n t e r a c t i o n with t h e alumina. We have examined t h e e f f e c t of t h e following v a r i a b l e s on t h e p r o p e r t i e s of the f i n a l catalyst: (a) the Ni/A1 ratio; ( b ) t h e pH of p r e c i p i t a t i o n ; ( c ) t h e type of metal s a l t used; ( d ) temperature of r e d u c t i o n . The p r e c i p i t a t e s have been c h a r a c t e r i s e d by means of X-ray d i f f r a c t i o n , by thermal techniques (D.S.C., D . T . A . ,

T.G.A.)

and by chemical a n a l y s i s ; t h e c a l c i n e d and

reduced m a t e r i a l s have been examined by X-ray l i n e broadening, by magnetic

145 s u s c e p t i b i l i t y measurements and by chemisorption of hydrogen; t h e a c t i v i t i e s (and s e l e c t i v i t i e s ) of t h e c a t a l y s t s for t h e CO + He r e a c t i o n were measured i n a low pressure r e c i r c u l a t i o n r e a c t o r and t h e i r a c t i v i t i e s and s t a b i l i t i e s were measured i n an atmospheric-pressure flow r e a c t o r .

2 . EXPERIMENTAL 2.1 Catalyst Preparation Coprecipitation w a s c a r r i e d out using two s o l u t i o n s : one contained n i c k e l and aluminium n i t r a t e s w i t h a t o t a l concentration of 0 . 9 mol d ~ n -while ~ t h e other 3 of each s o l u t i o n was added simultaneously

contained Na2C03 and/or NaOH. 175 cm

with constant s t i r r i n g t o 150 em3 of d i s t i l l e d water; t h e temperature was maintained at 353 K throughout and t h e pH w a s a l s o kept constant a t a d e s i r e d value by a d j u s t i n g t h e r a t e of a d d i t i o n of a l k a l i . I n another s e t of p r e p a r a t i o n s ,

a s o l u t i o n of Na CO w a s added t o a s o l u t i o n of t h e n i c k e l and aluminium n i t r a t e s 2 3 i n such a way t h a t t h e pH of t h e s o l u t i o n increased gradually from e i t h e r one t o f i v e or one t o seven. A f t e r p r e c i p i t a t i o n , p a r t of t h e product w a s f i l t e r e d on a g l a s s f i l t e r and d r i e d a t 353 K ; t h e remainder was hydrothermally t r e a t e d i n t h e mother l i q u i d i n a t e f l o n tube i n an autoclave a t 423 K and 5 atm f o r two days b e f o r e f i l t r a t i o n and drying. The d r i e d samples were c a l c i n e d (decomposed) i n a i r , e i t h e r i n a q u a r t z tube closed a t one end or i n a system i n which t h e sample w a s maintained i n p o r c e l a i n b o a t s ; temperatures ranged from 45OoC t o 1000°C. Several were a l s o decomposed i n

a high temperature X-ray camera ( s e e below) or using thermal a n a l y s i s techniques (D.S.C., D.T.A.

or T . G . A . ) .

The c a l c i n e d m a t e r i a l s were reduced i n H2, e i t h e r i n

t h e c a t a l y t i c apparatus described below, or i n a flow o f hydrogen i n t h e system used f o r t h e c a l c i n a t i o n s t e p ; s e v e r a l samples were also reduced i n t h e high temperature X-ray camera using T.G.A. 2.2 Catalyst Charact e r i s at i on X-ray measurements were c a r r i e d out by means of a Guinier-de Wolff camera and

a Guinier-Lennb high temperature camera, both manufactured by Enraf-Nonius ( D e l f t ) . Some samples were analysed chemically f o r n i c k e l , aluminium, sodium, carbonate and/or n i t r a t e . Nickel, aluminium and sodium were determined by atomic absorption spectroscopy, carbonate by d i s s o l v i n g t h e p r e c i p i t a t e i n phosphoric a c i d and t i t r a t i n g t h e C02 evolved, and n i t r a t e by reduction t o NH

3

which i s then t i t r a t e d

(Devarda method). Chemisorption and p h y s i c a l adsorption measurements were made volumetrically using a vacuum apparatus capable of background pressures o f b e t t e r t h a n

N m-2.

The sample v e s s e l c o n s i s t s o f a q u a r t z tube s e p a r a t e d from t h e remainder o f t h e system by a t r a p maintained a t 78 K.

146

A c t i v i t y measurements were made i n two systems, both of which u t i l i z e G.L.C. a n a l y s i s of r e a c t a n t s and products. The f i r s t of t h e s e ( r e f .

6 ) i s a low-pressure

-2

r e c i r c u l a t i o n system, o p e r a t i n g a t about 3 kN m , in which a background p r e s s u r e -I+ of -10 N mV2 can be a t t a i n e d ; water produced during t h e r e a c t i o n is trapped out at 193 K . The second ( r e f . 7 ) i s an atmospheric p r e s s u r e flow r e a c t o r of conventiona l design i n which t h e water i s removed by a condenser p r i o r t o admitting samples o f t h e r e a c t a n t s and products t o t h e G . L . C .

I n both c a s e s , t h e samples a r e reduced

i n s i t u p r i o r t o commencing t h e r e a c t i o n .

2.3 M a t e r i a l s A l l chemicals were o f Pro-analysis or Analar q u a l i t y . Gases were supplied by t h e B r i t i s h Oxygen Company; for t h e low p r e s s u r e r e a c t o r , t h e y were obtained i n s e a l e d ampoules (Grade X) and f o r t h e flow r e a c t o r , a s t a n d a r d mixture of high p u r i t y CO and H2 i n t h e r a t i o 1:3 w a s used.

3. RESULTS AND DISCUSSION

3.1 S t r u c t u r e and Comuosition o f t h e P r e c i p i t a t e C o p r e c i p i t a t i o n o f n i c k e l and aluminium gives r i s e t o badly c r y s t a l l i s e d hydroxy compounds of which t h e c r y s t a l l i n i t y i s much improved by t h e a p p l i c a t i o n of hydrothermal t r e a t m e n t . This enables b e t t e r c h a r a c t e r i s a t l o n of t h e samples t o be made, while it has l i t t l e or no e f f e c t on t h e c r y s t a l l h i t i e s o f t h e c a l c i n e d and reduced samples, o r on t h e a c t i v i t i e s of t h e l a t t e r . The hydroqy compounds appear t o belong t o a l a r g e c l a s s of hydroxide l a y e r s t r u c t u r e s made up of l a y e r s

of t h e b r u c i t e type (as found i n M E ( O H ) ~ ) s e p a r a t e d by disordered i n t e r l a y e r s containing anions and water molecules. Important unresolved problems concerning t h e s e compounds f o r t h e N i / A l system a r e ( a ) t h e r e l a t i v e p o s i t i o n s o f t h e n i c k e l and aluminium, and ( b ) t h e i r exact composition, e s p e c i a l l y with r e s p e c t t o t h e presence o f anions o t h e r t h a n hydroxyl i o n s . According t o Feitknecht ( r e f . 8 ) , n i c k e l occupies o c t a h e d r a l p o s i t i o n s within t h e b r u c i t e l a y e r while t h e aluminium

i s s i t u a t e d somewhere i n t h e i n t e r l a y e r . I n c o n t r a s t t o t h e s e conclusions, Longuet-Escard

(ref.

9 ) assumes t h a t both elements a r e s t a t i s t i c a l l y d i s t r i b u t e d

about o c t a h e d r a l p o s i t i o n s w i t h i n t h e b r u c i t e l a y e r . Both authors mention only t h e i n c l u s i o n of hydroxyl i o n s . We w i l l summarise here only t h e main conclusions of our i n v e s t i g a t i o n of t h e p r o p e r t i e s of t h e N i - U

system. The majority of t h e s e r e s u l t s have been obtained

from hydrothermally aged samples, as t h e X-ray d a t a f o r t h e s e e x h i b i t more sharply defined spacings, but s i m i l a r , l e s s q u a n t i t a t i v e r e s u l t s have been obtained f o r t h e unaged samples used f o r c a t a l y s t p r e p a r a t i o n .

As long a s t h e N i / A 1 r a t i o l i e s between two and t h r e e , a single-phase l a y e r compound i s obtained. Outside t h i s range, t h e excess Ni o r A1 forms a s e p a r a t e

147 phase, Ni(011)2 o r Al(0H)

3

r e s p e c t i v e l y . Measurements of t h e dependence of t h e

c e l l constants on t h e A1 content t o g e t h e r with magnetic measurements i n d i c a t e t h a t t h e Al ions s u b s t i t u t e f o r N i w i t h i n t h e b r u c i t e l a y e r ( r e f . 1 0 ) . I t w a s a l s o found t h a t t h e i n t e r l a y e r spacing depended markedly on t h e pH of p r e c i p i t a t i o n . Table 1 shows s e l e c t e d r e s u l t s f o r both aged and unaged samples.

Table 1 . E f f e c t of P r e p a r a t i o n Conditions on t h e Layer Spacings and Chemical Compositions o f t h e P r e c i p i t a t e s .

Ni ($)

Precipitating

pH of

Layer

NOg/

Nj.+Al

Agent(s)

precipitation

Spacing/E

wt

66” 6Ga

Na011/Na2C03

10

Na011/Na2C03

5

50b

Na2C03

7

-7.5

50b

NaOH

7

-9.0

a , hydrothermally aged;

wt%

7.58

c0.2

7.9.

8.92

18.3

0.24

0.88 13.8

---

?I

8.6

b , unaged

The r e s u l t s show t h a t t h e l a y e r spacing i s high a t low pH’s (e.g.pH decreases a t h i g h e r p H ( e . p . pH

co3/

%

‘L

5 ) but

10) when t h e p r e c i p i t a t i n g c o n d i t i o n s a r e

otherwise i d e n t i c a l ; when t h e pH w a s kept a t

7 and d i f f e r e n t p r e c i p i t a n t s were

used, it w a s found t h a t t h e lower spacing w a s obtained with N a CO as p r e c i p i t a n t . 2 3 Chemical a n a l y s i s of t h e p r e c i p i t a t e s , a l s o r e p o r t e d i n Table 1 , shows t h a t t h e higher l a y e r spacing corresponds t o s i t u a t i o n s when NO;

-

ions a r e incorporated i n

Che p r e c i p i t a t e (low pH o r i n t h e absence o f C0-) while t h e lower l a y e r spacing 3 corresponds t o i n c o r p o r a t i o n of CO; i o n s . The e q u i l i b r i a

have values of pK1 o f 6.h and of pK

-

2

o f 10.3. Hence, t h e concentration of HCO-

3

and C O i w i l l be low a t pH values much l e s s t h a n

6.

Apparently, a l a r g e amount of

n i t r a t e i o n s is only included i f carbonate c o n c e n t r a t i o n i s low; otherwise carbonate ion is p r e f e r e n t i a l l y incorporated.

If t h e p r e c i p i t a t i o n i s c a r r i e d

o u t from a s o l u t i o n o f t h e c h l o r i d e s a t low pH, c h l o r i d e i o n s a r e incorporated

in t h e s t r u c t u r e r a t h e r than n i t r a t e i o n s . S i m i l a r r e s u l t s were obtained f o r o t h e r samples; s e e t h e d a t a of Table 3 discussed below. It i s i n t e r e s t i n g t o compare our conclusion t h a t anions o t h e r than hydroxyl

i o n s a r e i n c o r p o r a t d i n t h e s t r u c t u r e with t h e work o f Merlin e t a l ( r e f . 1 1 ) ,

148 who claimed t o have prepared a w e l l defined compound Ni0.A1203.8.

H20. I n t h e i r

p r e p a r a t i o n method, n i c k e l and aluminium n i t r a t e s a r e dissolved i n excess concent r a t e d ammonia and t h e ammonia i s removed bp h e a t i n g on a water b a t h u n t i l t h e pH reaches

6-7; a p r e c i p i t a t e having an X-ray p a t t e r n s i m i l a r t o t h a t o f our

p r e p a r a t i o n s was obtained. Samples prepared by us following t h e same procedure a l s o showed considerable carbonate and/or n i t r a t e c o n t e n t s , depending on t h e exact p r e p a r a t i o n c o n d i t i o n s . Chemical a n a l y s i s of t h e p r e c i p i t a t e s prepared i n t h i s work has shown t h a t it

i s o f t e n d i f f i c u l t t o remove sodium ions from t h e p r e c i p i t a t e s , p a r t i c u l a r l y f o r samples prepared at pH = 10; t h e presence of sodium ions has a marked e f f e c t on t h e a c t i v i t i e s of t h e f i n a l c a t a l y s t s (see section 3.5).

3.2 Calcination Thermal decomposition of t h e p r e c i p i t a t e s t a k e s p l a c e i n two s t a g e s , t h e f i r s t corresponding t o loss of water of c r y s t a l l i s a t i o n and t h e second t o l o s s of CO 2 ( o r NO/N02 o r H C 1 ) and t h e decomposition of t h e hydroxide l a y e r s . For example,

T.G.,

D.T.A.

and D.S.C. r e s u l t s for t h e hydroxy carbonates a l l show t h a t t h e

f i r s t decomposition occurs a t about 520 K and t h e second a t about 625 K ; t y p i c a l D.S.C.

r e s u l t s were shown i n r e f .

5. Results obtained u s i n g t h e high temperature

X-ray camera showed t h a t t h e l a y e r s t r u c t u r e disappeared during t h e second decomp o s i t i o n , when only badly c r y s t a l l i s e d N i O w a s obtained. A t higher temperatures of around 1170 K , formation of N i A l 0 w a s observed t o g e t h e r with a r e c r y s t a l l i s a 2 4 t i o n of t h e N i O ; s t r o n g sharp r e f l e c t i o n s o f both phases were found a f t e r calcinat i o n a t 1273 K. I n t h e p r e p a r a t i o n o f samples f o r a c t i v i t y measurements, c a l c i n a t i o n was g e n e r a l l y c a r r i e d out a t 723 K i n a i r . I n agreement w i t h t h e r e s u l t s of t h e Xray camera, only N i O i s observable. The c e l l constant of t h e highly d i s p e r s e d

N i O i s somewhat t o o small, which suggests t h a t some A 1 i o n s have dissolved i n i t , r e s u l t i n g i n t h e formation o f a phase which can b e represented as NiA12601+36. There may i n a d d i t i o n be p r e s e n t some X-ray amorphous alumina phase. The c r y s t a l l i n i t y of t h e N i O formed d-pends s t r o n g l y on t h e anion p r e s e n t i n t h e p r e c i p i t a t e . The c r y s t a l l i n i t y i n c r e a s e s i n t h e o r d e r carbonate < n i t r a t e < c h l o r i d e , as i s i l l u s t r a t e d by t h e r e s u l t s of Table 2 , f o r t h r e e samples with mole f r a c t i o n N i / N i + A l = 0.50 ( s e e page 7 ) . 3.3 Reduction The e x t e n t of t h e reduction process a t any temperature w a s found t o depend on t h e conditions under which t h e reduction was c a r r i e d o u t . For example, i n t h e low p r e s s u r e r e c i r c u l a t i o n r e a c t o r , approximately 50% reduction w a s achieved a t

873 K f o r most samples, c a l c i n e d a t 723 K ( s e e Table 3, discussed below), whereas i n t h e TGA system, i n which a flow of hydrogen was used, almost 100% reduction

149 sizes

Table 2 . RelatIon between anion incorporated i n t h e p r e c i p i t a t e , p a r t i c l e of r e s u l t i n g N i O and N i , and N i s u r f a c e a r e a . Sample

‘6 B2 c1

Anion

Particle size

N i surface

of N i O i n cal-

of N i i n re-

area

cined m a t e r i a l

duced m a t e r i a l

(m2g-’ )

co;

4.0 nm

5.5 nm

NO;

c14f

-w

P a r t i c l e size”

>50

nm

Specific a c t i v i t y C atoms s lm-2 17

-

6.2 nm

49

0.137

8.5 nm

28

0.115

nm

21

0.063

>50

c a l c u l a t e d from X-ray l i n e broadening d a t a

w a s measured. The c r y s t a l l i n i t y of t h e n i c k e l i n t h e reduced m a t e r i a l appeared t o be d i r e c t l y r e l a t e d t o t h a t o f t h e c a l c i n e d samples (see Table 2 ) ; i n consequence, t h e s m a l l e s t n i c k e l c r y s t a l s a r e obtained from t h e carbonate samples. A sample c a l c i n e d a t 1273 K w a s more d i f f i c u l t t o reduce t h a n t h o s e c a l c i n e d a t

lower temperatures and w a s found t o contain NiA1204 a f t e r reduction at 873 K.

1 . 4 Surface Areas Hydrogen adsorption isotherms were obtained f o r various samples a f t e r reduct i o n over a range o f temperatures. The isotherms behaved according t o t h e Langmuir isotherm f o r d i s s o c i a t i v e adsorption and n i c k e l s u r f a c e a r e a s were c a l c u l a t e d from p l o t s of t h e l i n e a r i s e d form o f t h e equation. Fig. l a shows t h e dependence

on reduction temperature (T,) of t h e m e t a l l i c a r e a s of two samples of d i f f e r e n t n i c k e l c o n t e n t s , one derived from a n i t r a t e p r e c i p i t a t e and t h e o t h e r from a carbonate p r e c i p i t a t e ; a l s o shown are t h e corresponding degrees of reduction. Fig. lb shows t h e s u r f a c e a r e a s as a function of degree of reduction. The areas r i s e almost l i n e a r l y w i t h degree of reduction up t o about 80% reduction, when t h e a r e a decreases once more; i n both c a s e s , t h i s corresponds t o a temperature o f about 1000 K and it i s reasonable t o assume t h a t s i n t e r i n g of t h e n i c k e l c r y s t a l l i t e s has occurred at t h i s temperature. Other r e s u l t s showed t h a t t h e r e was a c l o s e correspondence between t h e n i c k e l p a r t i c l e s i z e c a l c u l a t e d from X-ray r e s u l t s and t h e n i c k e l s u r f a c e a r e a ( r e f . 1 0 ) . These r e s u l t s w i l l be discussed f u r t h e r below ( s e c t i o n 3 . 5 ) i n connectton w i t h t h e a c t i v i t y data. Table 2 shows t h e r e s u l t s o f m e t a l l i c a r e a (H2 a d s o r p t i o n ) measurements f o r s m p l e s with t h e same N i / A 1 r a t i o s ( 1 ) but prepared from p r e c i p i t a t e s containing t h e d i f f e r e n t anions. The sample prepared from t h e carbonate-containing p r e c i p i t a t e has t h e h i g h e s t m e t a l l i c a r e a . The r e l a t i o n s h i p between p a r t i c l e s i z e and m e t a l l i c a r e a i s demonstrated.

Table 3. The dependence o f n e t h a n a t i o n a c t i v i t y on t h e met;hod o f c a t a l y s t p r e p a r a t i o n and composition. Sample A1

A2

Ni/Ni+Al

pH of Precip.

0.85 0.75

10 10

Layer Spacing

7.9 7.7

8.4

NO;,

9.0

NO;,

5* 7'

8.9 7.7

NO;,

7.5

A

0.25

10

0.75 0.50

0.37

7

B1

B3

0.50

c1 Dl

D2 n3

x

0.75 0.66

10

7.7

10

0.40

10

7.7 7.5

Prepared at i n c r e a s i n g pH, t o t h e l i m i t given

I

'

P r e c i p i t a t e d with NaOH

CO?, 8.2%

7+

7

7 7"

c 1co; co; co;

N a content ( w t %)

13.8% 13.8% 16.4%

% Reduction at 873 K

A c t i v i t y Atoms of C g-1s-1.1017

48 71

5.6 9.4

64

0.07

53 55

6.1 5.3 5.8

0.05

45

6.7

?

71

5.2

?

55 46 46 45 49

3.3 3.3

?

6'

A6

0.50

*5

co;

7.5

10

7.7 7.7 7.5

A3

Anion Incorporatea

co; co; co;, 8.4% co;, 8.6% co;

0.75 0.72 0.50

A4

(8)

0.09 0.05 ?

0.21 ?

0.05 0.37 0.70 1.13

2.0

1.3

50

2.5 2.5

66

2.2

151 3.5 A c t i v i t y , S e l e c t i v i t y and S t a b i l i t y The hydrogenation o f CO was examined i n t h e low p r e s s u r e r e a c t o r ; preliminary r e s u l t s have been described elsewhere ( r e f .

6 ) . It w a s found, e s p e c i a l l y at l o w

degree of reduction, t h a t hydrocarbons o t h e r than methane were formed; F i g . 2a shows t h e e f f e c t of temperature of reduction ( T ) on t h e a c t i v i t y and s e l e c t i v i t y (methane/total carbon atoms r e a c t e d ) f o r t h e two samples f o r which t h e s u r f a c e a r e a r e s u l t s were given i n Fig.

1. Fig. 2b shows t h e r e l a t i o n s h i p between a c t i v i t y

and s u r f a c e a r e a , from which it can be seen t h a t t h e a c t i v i t y i n c r e a s e s with s u r f a c e a r e a , although t h e r e a r e probably d i f f e r e n c e s i n t h e s p e c i f i c a c t i v i t y f o r t h e two c a t a l y s t s examined. Table 3 shows a c t i v i t y d a t a f o r a number o f samples prepared i n d i f f e r e n t ways and with d i f f e r e n t N i / A 1 r a t i o s . S e r i e s A shows t h e influence o f d i f f e r e n t N i / U r a t i o s f o r p r e c i p i t a t e s containing carbonate i o n s . The a c t i v i t y i s not g r e a t l y a f f e c t e d by t h i s r a t i o . For samples prepared w i t h i n c r e a s i n g pH, only t h e f i n a l pH appears t o be important; compare sample A 4 w i t h A3. S e r i e s B w a s prepared from n i t r a t e containing p r e c i p i t a t e s ; t h e a c t i v i t i e s a r e considerably lower t h a n those of S e r i e s A. This e f f e c t appears t o be due t o s i n t e r i n g during t h e c a l c i n a t i o n process i n t h e presence o f t h e oxides of n i t r o g e n which a r e evolved. It seems t h a t t h e e f f e c t may be minimised i f t h e evolved gases a r e a b l e t o escape e a s i l y during c a l c i n a t i o n . This depends on t h e r a t e o f h e a t i n g t o t h e c a l c i n a t i o n temp e r a t u r e , and on t h e t e x t u r e of t h e powder. The c h l o r i d e containing sample C

1

had a l o w a c t i v i t y , which i s i n agreement

with t h e low metal a r e a measured. Also, i t s s p e c i f i c a c t i v i t y ( s e e Table 2 ) i s almost a f a c t o r two lower than t h a t of t h e c a t a l y s t s of S e r i e s A or B. This might be due t o t h e presence o f c h l o r i d e on t h e metal s u r f a c e . The D s e r i e s shows t h a t a l l samples with high sodium contents have low a c t i v i t i e s . The same poisoning e f f e c t of sodium has been observed by Rostrup-Nielsen f o r steam reforming c a t a l y s t s ( r e f . 1 2 ) . The s t a b i l i t i e s o f s e v e r a l of t h e samples shown i n Table 3 were examined i n t h e atmospheric p r e s s u r e flow r e a c t o r . Preliminary r e s u l t s w i l l be published elsewhere ( r e f . 7 ) . They confirm t h e o r d e r of a c t i v i t y CO

~

> NO3 > C1 and a l s o show t h a t t h e most of t h e samples l o s e less than

20% of t h e i r a c t i v i t y over two weeks o p e r a t i o n a t 773 K. Tests of longer duration and under more severe conditions a r e planned.

152

reduction

01~

Fig. 1 . ( a ) The dependence of N i s u r f a c e a r e a (open symbols) and % reduction ( c l o s e d symbols) on t h e temperature o f r e d u c t i o n f o r c a t a l y s t s Ah ( A , & ) and B 3 ( O , # ) . ( b ) The dependence o f N i a r e a on t h e % r e d u c t i o n (Ah, A; B3, 0 ) .

Fig. 2. ( a ) The dependence of a c t i v i t y f o r hydrogenation of CO under standard conditions (open symbols) and s e l e c t i v i t y f o r methane formation ( c l o s e d symbols) as a f u n c t i o n of temperature of reduction f o r c a t a l y s t s A4 ( A , A ) and B 3 (0,O). ( b ) The a c t i v i t y as a f u n c t i o n o f s u r f a c e a r e a (A4, A; B 3 , 0 ) .

153

ACKNOWLEDGEMENTS The authors acknowledge w i t h thanks an award from NATO under t h e Science Programme (Grant No. 1085). E.C.K.

L.E.A.

thanks t h e Z.W.O.

f o r a Research Fellowship,

thanks t h e B r i t i s h Council f o r a Technical Assistance Award, S.O. thanks

t h e S.R.C.

f o r a Research Studentship, and G.v.V.

thanks t h e Royal Society f o r a

V i s i t i n g Fellowship. They a l s o wish t o thank m r . Y . Timerman a t D e l f t f o r experimental a s s i s t e n c e . REFERENCES

1 G.A. M i l l s and F.W. S t e f f g e n , C a t a l y s i s Revs., 8 (1973) 159. 2 B. HEhlein, J C l . Report No. 1433, KFA J i i l i c h GmbH, 1977. 3 R.G. Cockerham, G. P e r c i v a l and T.A. Yarwood, I n s t . Gas. Eng. J . (1965), log. 4 J . R . H . ROSS, i n M.W. Roberts and J . M . Thomas ( E d s . ) , "Surface and Defect P r o p e r t i e s of S o l i d s " , Vol. I V , S p e c i a l i s t P e r i o d i c a l Reports , Chemical S o c i e t y , London, 1975, p. 34. 5 T. Beecroft, A.W. M i l l e r and J . R . H . Ross, J. C a t a l . , 40 (1975) 281. 6 E.B.M. Doesburg, S . Orr, J . R . H . Ross and L.L. van Reijen, J . Chem. SOC., Chem. Comm., (1977) 734. 7 G. van Veen, E . C . Kruissink, J . B . H . Ross bnd L.L. van Reijen, submitted f o r p u b l i c a t i o n i n Rn. Kinet. Catal: L e t t . 8 W. F e i t k n e c h t , Helv. Chim. Acta, 25 (1942) 555. 9 J . Longuet-Escard, J . Chim. Phys., 47 (1950) 238. 10 L.E. Alzamora, E . C . Kruissink, J . R . H . Ross and L.L. van Reijen, unpublished results 1 1 A. Merlin, B. Imelik and S . J . Teichner, C.R. Acad. S c i . , 238 (1954) 353. 12 J . R . Rostrup-Nielsen, Steam Reforming C a t a l y s t s , Teknisk Forlag A / S , Copenhagen (1975) 102, 122.

.

DISCUSSION D.I. BRADSHAW

:

X-ray diffraction and infrared spectroscopy carried

out by us on similar co-precipitated N i - A 1 2 0 3 catalysts indicate that vacancies exist in the "brucite" layer with aluminium bonded to carbonate ions in the interlayer. at 1 3 2 5 and 1 5 5 0 cm-l.

Carbonate bands were observed

Would the authors care to comment on this

alternative structure for the precipitate. E.C.

KRUISSINK

:

Serna et al. have shown similar infrared spectra

for samples of Mg-A1 coprecipitates in which a sodium aluminium hydrate carbonate structure [dawsonite) has separated. carbonate and A 1 3 + ions are in close proximity.

In these, the It is possible that

the samples you mentioned contain a similar phase.

Infrared spectra

obained with our samples do not contain the band at 1550 cm-l with the exception of one sample in which also dawsonite was found by X-ray examination.

In this particular case a substantial excess of

Na2C03 during precipitation led to further reaction (formation of

dawsonite) upon subsequent hydrothermal treatment. In view of these 3+ 2and C03 ions are in close proximity

data we do not believe that A1

in the remainder of our samples. Serna, C.J., White, J.L. and Hem, S . L . J. Pharm. Sci., 67, 324 ( 1 9 7 8 ) . J.W. GEUS

64,

468(1975);

Though generally anions strongly affect the precipitation

:

process, the effect of anions with silica is much smaller than with alumina because of the acid character of silica.

We accordingly

observed that the decomposition of urea leads without suspended silica to a carbonate-containing precipitate, whereas with suspended silica the nickel precipitate did not contain markedly carbonate. studying the precipitation of Al(II1) ions without other metal cations being present, we found consumption of OH- ions at pH values much lower than at which nickel ions react with hydroxyl ions.

Did you

investigate whether precipitation of Al(II1) ions induces precipitation

of nickel ions at a lower pH level than at which nickel ions alone precipitate from a homogeneous solution E.C. KRUISSINK

:

?

We carried out one experiment in which Ni(I1) and

Al(II1) i o n s were precipitated from a nitrate solution by urea

hydrolysis at 90°C.

Formation of a nickel containing phase was

observed at a p H value of 4 . 5 ,

which is lower than the value 5 . 5 ,

reported by Hermans et al. for the precipitation of nickel ions only from a homogeneous solution at 90°C, in the same range of nickel concentration.

Our concentrations were

:

Ni(I1) 0 . 4 8 M, Al(II1)

0.17 M , and initial urea concentration 3.6 M. Hermans, L.A.M. K. KOCHLOEFL

:

and Geus, J.W.

,

this symposium.

We found that not only the precipitation agent but also

the Ni/Al ratio plays an important role in the thermal stability

of coprecipitated Ni-A1203 catalysts. P.G. MENON

:

Why do you need such high concentrations of Ni, as

high as 5 0 - 7 0 wt% ?

Most of the N i ultimately ends up in large 0

crystallites of average size 140 A.

Will not a catalyst with less

Ni, but better dispersed and hence with smaller crystallite size, be equally suited, if not better for the methanation reaction ? E.C. KRUISSINK

:

Our answer to this point is speculative as we

have not yet examined in any detail the effect of nickel content

on stability.

We have reason to believe that the high stability

of the reduced catalyst results from the well defined structure of the precipitate

:

the aluminium and nickel are intimately mixed in

the brucite-like layer and remain associated with one another during calcination and reduction.

For Ni/A1>2, separate alumina

phases result and these may be detrimental to the life of the catalyst, blanketing the active components.

Catalysts with

Ni/A1=3 are preferred for industrial steam reforming applications; however, this may be because the separate phases of alumina present at low nickel contents encourage undesirable coking of the catalysts and this objection is not of such importance under methanation conditions. MATIJEVIC

:

At pH 10 aluminum hydroxide should not coprecipitate

as it is soluble as aluminate and as such it would react with nickel hydroxide.

However, in the presence of carbonate basic

sparingly soluble complexes could account for the coprecipitation. Indeed, in table 3 no data in the presence of nitrate at pH 10 are given.

Obviously, the question arises can nickel hydroxide

coprecipitate with nickel nitrate solution at this pH. E.C. KRUISSINK

:

Indeed, at pH=lO no precipitate was observed in

the absence of carbonate ions, from an aluminium nitrate solution. However, solubility of aluminium hydroxide as aluminate Al(OH)i is

2.0 l o m 3 mole/l at pH=tO, according to data of SillBn (1964). This i s small compared with the A1 concentration of our starting solution

which is about 0.2 mole/l.

Therefore we think A1(0Hl3 may be

dissolved in a collordal state. The situation is totally different when both Ni(I1) and Al(II1) ions are present.

We carried out a precipitation at pH=lO, from a

nickel and aluminium nitrate solution, rigorously excluding CO 2 ' Under these conditions

The precipitating agent was sodium hydroxide.

A1 precipitates in a nickel-aluminium hydroxy-nitrate, crystallizing in a hydrotalcite-like structure.

The same is known for the Mg-A1

system (Miyata, S. 1975). Sillen, L.G. "Stability constants of metal-ion complexes". Chem. S O C . London Spec. Pub. 17, 1964. Miyata, S . Clays and Clay Minerals 3 , 369-375(1975). P.E.H.

NIELSEN

: Could

you elaborate, i.e. give data on the observed

156 NiA12601+36phase, such as crystal size and lattice parameters. E.C. KRUISSINK

Crystal size o f the NiAl 2601+36phase depends on

:

0

the anion present in the starting material, and may vary from 40 A to > 500 A (see our paper). The well-crystallized samples show d-values equal to those of pure NiO.

For the samples consisting of small NiO particles the follow-

ing d-values were measured

:

( ? 5 ) ; 2.076

2.402

(+

5)

;

1.470

(+

5)

for respectively the 1 0 1 , 012 and 110-104 NiO reflections (indexed o n a hexagonal basis).

The corresponding d-values of pure NiO are

2.412, 2.088 and 1.477-1.476. A.

OZAKI

:

Ni(NO3I2 usually gives a better catalyst than does NiSO

when it is precipitated on Si02.

4

This effect seems to come from

ion incorporated in the precipitate. In view of this I would NO3 like to ask your idea on the role of carbonate ion although you are working on Ni/A1 0 2 3' L.L. VAN REIJEN

:

The role of carbonate ion in our preparations is

to prevent the incorporation of nitrate or chloride ions.

As we

have shown in our paper, incorporation of these ions may cause sintering of nickel oxide during the calcination stage. Probably also incorporation of sulphate ions is detrimental. P.B. Wells and coworkers have recently shown that Ni/Si02 samples prepared by impregnation with sulphate are contaminated by sulphur. D.L. TRIMM

:

What would you relate selectivity to

?

Do you feel

that it reflects crystallite size, residual anion, pore size or some other factor E.C.

KRUISSINK

:

?

We believe that the selectivity is associated with

the degree of reduction of the catalyst, low degrees of reduction favouring the formation of higher hydrocarbons.

There has recently

been renewed interest in the m e c h a n i s mof the methanation and Fischer-Tropsch (FT) reactions; a number of authors favour the participation of surface carbon, formed by dissociative adsorption of C O , both in the methanation

and FT reactions ',

the latter, van Barneveld and Ponec

whereas for

have suggested a mechanism

involving the insertion of molecular CO into

metal-carbon bonds.

Our most recent results tend to favour the van Barneveld and

157 Ponec mechanism

:

at low degrees of reduction, the presence of a

proportion of unreduced nickel ions on the surface (or, alternatively, sites where nickel is associated with the alumina) discourage dissociative adsorption of CO and hence favour the FT reaction, whereas high degrees of reduction cause the formation of larger nickel crystallites on which dissociative adsorption, followed by methanation, occurs. However, it is not necessary to invoke dissociative adsorption to explain methane formation: more extensive hydrogen adsorption may occur on the larger metal crystallites, encouraging methane formation rather than the FT reaction.

1. P.R. Wentreck, B.J. Wood and H. Wise, J.,Catal., 4 3 , 363(1976). 2. R.W. Joyner, J. Catal., 176(1977). 3 . W.A.A. van Barneveld and V. Ponec, J. Catal. 2 , 426(1978).

so,

J.L.

WHITE

:

The crystalline phase of the material described by

Kruissink et al. is the synthetic equivalent of minerals of the

pyroaurite-sdjrenite-hydrotalcite group.

The structure of the

synthetic material was worked out by Gastuche, Brown and Mortland I n 1967.

They showed Mg/A1 ratios of 3:1 and 5 : l gave stable

structures.

he Na

+

observed by the authors is probably due to

adsorption on amorphous aluminum hydroxide precipitated at a moderate to high pH. M.C.

Pertinent references include

Gastuche (1967), Clay Miner.

2,

and M.M. Mortland (19671, Clay Miner. White and S . L .

D.J.C. YATES

:

G. Brown and

193; M.C. Gastuche, G. Brown

1, 177;

Hem (1978), J. Pharm.

B. Kobo, S. Miyata,

29, 55; C.J. Sci. 67,324.

T. Kumura and T. Shimada (.1969), Yakuzai Gaku J.L.

:

Serna,

What factors led to your choice of sodium carbonate

and sodium hydroxide as precipitants in view of the fact that sodium is a well-known poison for nickel catalysts

?

J.R.H. ROSS : Initially we have adopted commercial practice, in which it is considered that the sodium can be washed out of sufficiently low levels.

Our experience agrees with this conclusion

for most conditions of preparation.


159

CONTROLLED CATALYST DISTRIBUTION ON SUPPORTS BY CO- IMPREGNAT ION

E. R. BECKER* and T. A. NUTTALL** Chemical Engineering Research Group

-

CSIR, Pretoria, South A f r i c a

ABSTRACT The performance o f c a t a l y t i c reactions, c a t a l y s t d u r a b i l i t y and r e a c t i o n s e l e c t i v i t y can be c o n t r o l l e d by appropriate c a t a l y s t d i s t r i b u t i o n on supports. Burried c a t a l y s t l a y e r s on supports enhance r a t e s o f reactions which obey negative order k i n e t i c s and extend c a t a l y s t l i f e when reactions are accompanied by pore mouth i m p u r i t y poisoning. The preparation o f c a t a l y s t s w i t h a c t i v e subsurface l a y e r s was studied v i a t h e impregnation o f alumina support spheres w i t h hexachloroplatinic a c i d i n the The i n f l u e n c e o f t h e impregnation process v a r i a b l e s on

presence o f c i t r i c acid.

platinum d i s t r i b u t i o n were determined.

Subsurface l a y e r s o f platinum on a support

I-

p a r t i c l e w i t h c o n t r o l l e d 'depth and l a y e r w i d t h were produced.

Impregnation time,

s e v e r i t y o f reduction and the c o r r e c t choice o f an immobilization and d r y i n g sequence were found t o be the c r i t i c a l v a r i a b l e s i n the process. times measured 5 minutes.

The impregnation

M i l d reduction o f the wet p a r t i c l e s i n hydrazine vapor

succeeded i n preserving concentrated, narrow platinum bands i n s i d e the support. The depth o f t h e platinum l a y e r s was reproducibly c o n t r o l l e d by t h e c i t r i c a c i d concentration i n the impregnating s o l u t i o n .

INTRODUCTION Catalysts w i t h bands o f a c t i v e ingredients concentrated near the support p a r t i c l e e x t e r i o r a r e w e l l known and have been i n commercial use f o r some time, e.g.

, reforming

catalyst.

The eggshell c a t a l y s t d i s t r i b u t i o n minimizes the d i f -

f u s i o n path o f the molecules which serves t o maximize the e f f e c t i v e u t i l i z a t i o n o f the c a t a l y s t m a t e r i a l on t h e support. I n the absence o f poisoning o r s e l e c t i v i t y considerations, t h e eggshell c a t a l y s t d i s t r i b u t i o n i s optimal f o r c a t a l y t i c react i o n s w i t h p o s i t i v e order r a t e dependence on reactant concentrations.

The eggshell

c a t a l y s t i s n o t t h e most s u i t a b l e f o r reactions which have negative order r a t e dependence on reactant concentrations o r f o r reactions which are accompanied by pore mouth poisoning. * t o whom correspondence should be addressed; a t : A i r Products and Chemicals, Inc. P.O. Box 538, Allentown, PA 18105 U.S.A. **currently w i t h Dept. o f Chem. Eng., Loughborough University, Loughborough, U.K.

160

B u r r i e d l a y e r c a t a l y s t s w i t h a c t i v e i n g r e d i e n t i n an egg white o r egg y o l k p o s i t i o n were c a l c u l a t e d t o be superior t o eggshell c a t a l y s t s o r uniform c a t a l y s t s by Becker and Wei ( l ) , ( 2 ) . The systems they studied were CO o x i d a t i o n over platinum and f i r s t order r e a c t i o n s w i t h i m p u r i t y poison deposition.

The c a l c u l a t i o n s showed

t h a t o x i d a t i o n r a t e s o f CO could be enhanced by several f a c t o r s i n c a t a l y s t p a r t i c l e s w i t h an egg white d i s t r i b u t i o n o f a c t i v e ingredient.

Experimental work i n

the General Motors Research Laboratories (3) have demonstrated the s u b s t a n t i a l advantages o f automotive c a t a l y s t performance and c a t a l y s t d u r a b i l i t y by c o n t r o l l e d subsurface platinum and paladium l a y e r s on an alumina support.

Other notable con-

t r i b u t i o n s t o the concept o f c o n t r o l l e d d i s t r i b u t i o n s on supports and t h e i r e f f e c t on r e a c t i o n rate, s e l e c t i v i t y and d u r a b i l i t y are those o f Corbett, e t a l . ( 4 ) , Shadman Yadzi, e t a l . ( 5 ) , Cervello, e t a l . (6), Michalko (7) and Roth, e t a l . ( 8 ) . Maatman and P r a t e r (9), (10) have explored the e f f e c t s o f competitive species i n a platinum s o l u t i o n t o achieve uniformly d i s t r i b u t e d platinum on alumina catal y s t supports.

Michalko ( 7 ) suggested the use o f co-impregnation t o achieve

i n t e r i o r l a y e r s o f platinum on alumina f o r automotive o x i d a t i o n c a t a l y s t s . T h i s research i s an e f f o r t t o determine the f e a s i b i l i t y o f manufacturing c a t a l y s t s w i t h accurately c o n t r o l l e d d i s t r i b u t i o n o f a c t i v e ingredients i n support particles.

An understanding o f the underlying impregnation processes i s sought t o

determine the i n f l u e n c e o f t h e processing variables on the r e s u l t a n t c a t a l y s t distributions.

CATALYST IMPREGNATION The preparation of a c a t a l y s t by impregnation o f a support c o n s i s t s o f several d i s t i n c t processing steps.

The wetting o f the support w i t h impregnating s o l u t i o n

c o n t a i n i n g the a c t i v e specie i s followed by d r y i n g and immobilization o f the a c t i v e ingredient, n o t necessarily i n t h a t order.

I n the process o f co-impregnation,

one

o r more chemical species are added t o the s o l u t i o n t o modify the deposition charact e r i s t i c s o f the a c t i v e i n g r e d i e n t i n order t o achieve a desired d i s t r i b u t i o n e f f e c t w i t h i n the c a t a l y s t support p a r t i c l e . The experiments c a r r i e d o u t i n t h i s research consisted o f w e t t i n g a d r y alumina support w i t h platinum containing solution. by c a p i l l a r y suction.

S o l u t i o n i s transported i n t o the pores

The c h a r a c t e r i s t i c time governing t h e c a p i l l a r y suction

i n a c y l i n d r i c a l pore may be characterized by: (10)

where t i s the penetration time i n seconds, a t h e surface tension o f t h e s o l u t i o n ,

r the mean pore radius, c a p i l l a r y length.

the dynamic v i s c o s i t y o f t h e l i q u i d , and x the e f f e c t i v e T = 10-3Ns/m 2 , o= 72 x 10- 3N/m and x = 10-3 in,

For r = 10-6m,

161

a penetration time o f 0.03 seconds i s predicted.

L i q u i d f l o w i n t o the pores i s

accompanied by adsorption o f the a c t i v e ingredients on the pore walls.

The r a t e o f adsorption and desorption o f the a c t i v e species and t h e i r r e l a t i v e amounts determines the shape o f the d i s t r i b u t i o n curve.

Ifl e f t t o e q u i l i b r a t e , the adsorbed species

w i l l r e d i s t r i b u t e i n s i d e the pores.

Only r a p i d immobilization, by reduction i n the case o f platinum c h l o r i d e , can preserve the d i s t r i b u t i o n s achieved during t h e impregnating step.

EXPERIMENTAL Spherical alumina support p e l l e t s from Universal O i l Products (UOP) measuring 3.2 mm and 1.6 mm i n diameter were chosen f o r impregnation.

Three-gram batches o f e i t h e r p e l l e t s i z e were d r i e d and heated a t 25OOC i n a i r f o r 8 hours preceding impregnation.

Seven impregnation s o l u t i o n s were prepared. An a l i q u o t o f 10 m l , w e l l i n excess o f the c a t a l y s t pore volume, contained a f i x e d amount o f platinum. The s o l u t i o n s contained c i t r i c a c i d varying from zero t o 37 g / l .

Compositions o f the solutions,

numbered one t o seven, are given i n Table 1. TABLE 1 Compositions o f impregnating solutions:

Platinum concentration = 5.8 g/1.

S o l u t i o n Number

1

2

C i t r i c Acid Conc. s/l it r e

0

o

C i t r i c Acid Conc. Mass o f C i t r i c Acid % Mass o f Alumina

3

4

5

6

7

0.925

1.85

3.70

9.25

18.5

37.0

0.31

0.62

1.23

3.io

6.20

12.3

~~~~~

Seven batches o f each s i z e o f dry p e l l e t s were immersed and s t i r r e d i n s o l u t i o n s 1 t o 7.

A f t e r a measured contact time t h e s o l u t i o n was f i l t e r e d o f f .

The p e l l e t s

were washed b r i e f l y w i t h d i s t i l l e d water t o remove i n t e r p a r t i c l e s o l u t i o n . p e l l e t s were transferred t o a f l u i d i z e d bed tube f o r reduction. wet p e l l e t s were d r i e d i n an unreduced state.

The wet

A l t e r n a t i v e l y , the

Impregnation times v a r i e d from 2 t o

30 minutes using s o l u t i o n 4 t o determine t h e e f f e c t o f contact time on i n t e r n a l platinum d i s t r i b u t i o n .

I n a l l other experiments an impregnation time o f 5 minutes

was chosen. The absorbed platinum c h l o r i d e was reduced i n a hydrazine containing n i t r o g e n gas stream a t ambient temperatures.

The c o l o r o f t h e p e l l e t s changed from a l i g h t

y e l l o w t o black i n d i c a t i n g t h e completion o f the reduction.

Drying o f the reduced

p e l l e t s a t 90°C completed the c a t a l y s t preparation. The platinum d i s t r i b u t i o n w i t h i n the support bead was measured i n a scanning e l e c t r o n microprobe (EMP).

The p l a s t i c mounted p e l l e t s were sectioned e q u a t o r i a l l y .

162

T h i s c r o s s s e c t i o n was analyzed by t h e scanning EMP.

Three scans were t a k e n across

t h e s e c t i o n as i n d i c a t e d i n F i g u r e 1 by a t y p i c a l scanning t r a c e .

The s i g n a l i n t e n -

s i t y was assumed t o be p r o p o r t i o n a l t o t h e p l a t i n u m c o n c e n t r a t i o n .

I

SCAN WIDTH 50um

1 4 L A T I N U M SCAN

EDGE

CENTER

EDGE 1

1

PLATINUM BACKGROUND BASE LINE

,

d

BASELINE

1

( BASE LINE 2

1 800 700 600 500 400 300 200 100 0

100 200 300 400 500 600 700 800

DISTANCE FROM CENTER OF PELLET ( p m )

FIGURE 1:

E l e c t r o n m i c r o p r o b e p r o f i l e o f an impregnated alumina sphere

Catalyst Support

Support Diameter 10-3 m

Modal p o r e radius fim

Porosity

3449-11B

196

4,7

2,35

1,73

lo5

295

3449- 1 1A

3Y2

1,7 x

1,36 x

1,49 x lo5

385

m3/kg

S u r f a c e Area m2/kg

Average bulk density kg/m3

PLATINUM DISTRIBUTION The photograph o f t h e s e c t i o n e d 1.6 mm alumina spheres, t a k e n under a microscope and shown i n F i g u r e 2, c l e a r l y shows t h e p l a t i n u m bands a g a i n s t t h e alumina background.

The r e l a t i v e sharpness o f t h e s e r i n g s i s b e l i e v e d t o depend p r i m a r i l y on

t h e e l a p s e d t i m e between i m p r e g n a t i o n and r e d u c t i o n .

163

FIGURE 2: Platinum distribution in 1.6 mm diameter alumina spheres: a: solution 1 6: Soln 2 c: Soln 3 d: Soln 4 e: Soln 5 f: Soln 6. EFFECT OF CITRIC ACID CONCENTRATION: A comparison o f the normalized EMP concentration profiles shows that the platinum band is deposited deeper towards the center of the pellet with increasing citric acid concentration. Within the errors o f measurement, the depth of platinum penetration is a linear function of citric acid concentration (11). Figure 3 shows the relative profiles for the 3.2 mm alumina "spheres". Since these pellets are more elipsoid in shape than the 1.6 mm pellets, the radii (or edge to elipse-focus paths) were scanned. A more complete description may be found el sewhere (1 1 ) . For citric acid concentrations exceeding 3.1%, no distinct layer of platinum is observed in the smaller pellets. Figure 2 shows a diffuse double ring of a grey shade in contrast to the black platinum ring in Figures 2 (a)-(c). The

164

,

,

,

,

,

I

,

,

,

,

,

,

,

,

,

l

I

I

.

I

1st RADIUS

SOLUTION I 1.1 w t '10 pt

2nd RADIUS

I

I

03

-

I

I

I

I I I

I I

z

F 2c z

W

0

z

8 4 z

0.0

t

w t % Pt

-t

n.

2 nd RADIUS

W

I

%-t W

a

1.0

,

,

1

,

,

(

1

1

1

I

,

.

I

'

SOLUTION 7

- d Irt RADIUS 0.5

I I

-

I

1 I

I I

I

' ' ' ' ' '

Q 1.0 SURFACE

0.5

a

1 2 0 CENTER

I

,

0.5

,

.

,

,

-

140

SURFACE

DIMENSIONLESS RADIUS

FIGURE 3:

Normalized radial EMP platinum profiles i n 3.2 mm diameter alumina beads.

165

width o f the platinum band varied from 50 p m i n 3.2 mm diameter p a r t i c l e s t o 430 ,urn i n the 1.6 mm alumina supports. The r e p r o d u c i b i l i t y o f the d i s t r i b u t i o n i s shown i n Figure 4 and corresponds t o a maximum v a r i a t i o n o f

10% o f the radius o f the support.

For example, the depth

o f the c a t a l y s t l a y e r i n Figure 4 measures 0.53 mm w i t h a standard d e v i a t i o n o f 0.07 mm.

SURFACE

CENTER DIMENSIONLESS RADIUS

FIGURE 4:

Experimental v a r i a t i o n o f platinum d i s t r i b u t i o n i n a 3.2 mm diameter a1 umina bead.

EFFECT OF IMPREGNATION TIME:

Experiments using d i f f e r e n t impregnation times

showed t h a t platinum subsurface l a y e r s were already evident a f t e r the f i r s t two minutes.

P e l l e t s impregnated longer than 10 minutes revealed a general grey back-

ground ( i n s t e a d o f white), suggesting a broader d i s t r i b u t i o n o f platinum throughout the support. The EMP analysis confirmed the broader d i s t r i b u t i o n and i s shown i n Figure 5. EFFECT OF REDUCTION AND DRYING:

When d r y i n g o f the impregnated p e l l e t s preceded

reduction, a "washout" peak o f platinum was observed.

This washout peak was

166

z 0 !a

I .o

,

,

I

,

I

1

I

MINS. --- 30 MINS.

-

LL

I-

z W u

SOLUTION 6

-

Z

0

-

0

I

I

-

-5

-

-

a

-

W

-

-

3

z

0.5

5

J

f

5

,-I"'-\

/'

I-

FIGURE 5 :

\ .t

E f f e c t o f impregnation time on platinum d i s t r i b u t i o n i n 3.2 mm diameter a1 umina beads.

displaced from the o r i g i n a l peak toward t h e e x t e r i o r o f t h e support bead g i v i n g r i s e t o a bimodal platinum d i s t r i b u t i o n o f wide and varying shape.

A wet reduction procedure was chosen t o avoid the r e d i s t r i b u t i o n o f platinum during the d r y i n g process.

This technique, described above, was successful i n pre-

serving what was thought t o be the o r i g i n a l c a t a l y s t d i s t r i b u t i o n .

The s e v e r i t y o f

the hydrazine reduction had t o be c o n t r o l l e d t o avoid the d i s l o c a t i o n o f reduced platinum p a r t i c l e s from t h e surface.

Reduction o f the wet supports i n a 5% hydrazine

s o l u t i o n r e s u l t e d i n loss o f platinum from t h e support and r e d i s t r i b u t i o n o f platinum i n s i d e t h e support beads.

Further d e t a i l s o f the reduction e f f e c t s on d i s t r i b u t i o n

may be found e l sewhere (1 1). GENERAL DISTRIBUTION CHARACTERISTICS:

The noticeable features e x h i b i t e d by t h e

platinum p r o f i l e s obtained w i t h the co-impregnation method are: 0

One sharp peak o f platinum per radius i.e.

concentration o f platinum i n a

0

There was i n v a r i a b l y some platinum present on the p a r t i c l e e x t e r i o r .

small volume o f the support (an exception i s shown i n Figure 3d). 0

The regions o f low platinum concentration were e s s e n t i a l l y platinum-free i n most cases.

167

DISCUSSION A catalyst impregnation technique has been demonstrated to achieve controlled distribution of platinum within an alumina support. Subsurface layers as thin as 50 p m and as deep as 0.8 of the bead radius can be obtained. The depth of a platinum subsurface layer can be predetermined to an accuracy of 10% of the pellet radius by adjustment of the citric acid concentration. Short impregnation times of 5 minutes followed by rapid, but mild reduction in hydrazine vapor were found to give the best results. Platinum bands in the 3.2 m diameter supports were thinner but less uniform than those in the 1.6 mm diameter supports. The role of the support characteristic could not be clearly determined from these experiments. The results of this exploratory research have shown that it is possible to control active ingredient distribution within a support to a degree not heretofore published. The shape of the platinum distributions and the time required to form subsurface layers suggests a mechanism of selective adsorption coupled with capillary suction. A chromatographic separation of platinum hexachloride and citrate ions is envisaged. This was in part confirmed by similar distributions measured even when the pellets were prewetted. To gain a better understanding of the impregnation mechanism, the surface chemistry and the ion-transport mode needs to be understood. The ability to accurately control internal catalyst distribution on supports adds a valuable tool to the resources o f the catalytic reaction engineer and should result in catalysts with better performance characteristics as has been demonstrated by Summers and Hegedus (3) for automotive oxidation catalysts. ACKNOWLEDGEMENTS The authors are indebted to Mr. W. G . B. Mandersloot for supporting the research. The analytical contribution of J. Thirlwall and Dr. Colborn is gratefully acknowledged. Alumina supports were kindly supplied by Universal Oil Products and the Houdry Division o f Air Products and Chemicals, Inc. U.S.A. REFERENCES 1. E. R. Becker and J. Wei, J. Cat., 46 (1977) 365-371. 2. E. R. Becker and J. Wei, J. Cat., 46 (1977) 372-381. 3. J. C. Summers and L. L. Hegedus, J. Cat., 51 (1978) 185-192. 4. W. E. Corbett and D. Luss, Chem. Eng. Sci., 29 (1974) 1473-1783. 5. F. Shadman-Yadzi and E. E. Petersen, Chem. Eng. Sci., 27 (1972) 227-237. 6. J. Cervello, J. F. Garcia de la Banda, E. Hermana and J. F. Jimenez, Chem. Ing. Tech., 48 (1976) 520-525. 7. E. Michalko, U. S. Patent 32 59 589 (1966). 8. J. F. Roth and T. E. Reichard, Jnl. of the Res. Inst. Cat., Hokkaido Univ. 20 (1972) 85-94. 9. 10. 11.

R. W. Maatman and C. D. Prater, Ind. Eng. Chem., 49 (1957) 253-257. R. W. Maatman, Ind. Eng. Chem., 51 (1959) 913-914. T. A. Nuttall, CSIR Report CENG 182, Catalyst with subsurface active layers prepared by co-impregnation, (1977) , CSIR, Box 395, Pretoria, South Africa.

DISCUSSION K. KOCHLOEFL

Do you think that e.g. formaldehyde can be used instead

:

of hydrazine ? E.R. BECKER : Y e s , other reducing compounds will give similar effects, however the conditions of treatment may be quite different from those for hydrazine. G.H. VAN DEN BERG

:

What is the explanation for the phenomenon

described,viz. the subsurface deposition of platinum occurring at zero citric acid concentration. E.R.

BECKER

I have no good explanation for this phenomenon.

:

However, it is unique to the UOP alumina support spheres.

The

homogeneity and the preparation method of these unusually light alumina supports was not determined.

Other aluminas, e.g. Rhdne

Poulenc alumina spheres, did not show this behaviour and the penetration distance of platinum “rings“ is zero in the absence of citric acid. E.A.

IRVINE

:

Are there any conditions (i.e. time length, pH) at

which platinum comes out of solution ?

If so, what effect does

this have on the platinum dispersion, and on factors such as reproducibility ? E.R.

BECKER : During impregnation there is no precipitation of

platinum from solution.

I n these experiments the pH of the

solution was between 3 and 5 .

The precipitation is observed during

reduction of the wet pellets.

Under strong reducing conditions

black platinum sludge is formed and redistributes within the support. The dispersion of platinum was not measured in the catalysts, as they have very high local concentrations of platinum, up to 20 wt%. Rapid precipitation from solution also results in poor reproducibility of platinum distribution within the support particles. D.E. WEBSTER

:

Some years ago we observed a similar “tree-ring’‘

effect with Pd(N03)2/HN03

solutions on alumina pellets.

More

recently, this has not been observed on the same supplier support, suggesting a strong dependence on the alumina base. acid reacts with citric acid at circa 90°C.

Chloroplatinic

Does this reaction

169 occur under your conditions ? What do you see as the mechanism of the time platinum distribution E.R.

BECKER

-

dependence of the

?

1. our experiments would confirm your observation that

:

the alumina base does have an important role in this process of coimpregnation. 2.

We can rule out any reaction of chloroplatinic acid with citric

acid at the temperatures of the impregnation experiments, viz. 25OC.

3. We envisage a chromatographic separation of the citrate ions from the platinum chloride ions.

The interaction of platinum ions with

the alumina surface is thought to b e chemisorption and physical adsorption of precipitate.

Redistribution occurs due to the mobility

of the platinum ions coupled with the high local concentration gradients.

The characteristic times of the impregnation processes

cover reaction times [adsorption) to diffusion times [redistribution). I feel the key to successful distribution lies in accurate time control of all steps. D.C. TRIMM

Presumably the mechanism must involve diffusion

:

of citric acid or the platinum salt.

-

either

Did you attempt to model the

phenomenon for dried or prewettedsupports in terms of diffusion effects

If so, which do you consider more important in determining

?

the width of the platinum ring ? E.R.

BECKER

:

Certainly the prewetted pellet impregnations involve

diffusion coupled with chemical reaction. role in the dry pellet experiments.

Diffusion plays a secondary

Dr. Hegedus, in the following

presentation shows a model for prewetted pellets.

The modeling of dry

pellet impregnation has not been attempted to the best of my knowledge.

M. MARTAN : Did you find any difference between the UOP and Houdry alumina ?

Did you try to correlate the amount of P t adsorbed with

the basic sites/acid sites ? E.R. BECKER

:

Yes, there was a difference in impregnation charac-

teristics from one support to another.

Houdry supports show less

penetration of platinum for an equivalent amount of citric acid. We did not correlate adsorbed platinum with acid or basic sites.

We found that the amount of platinum on the support was very close to the amount contained in the volume of solution adsorbed. H. CHARCOSSET : What about the particle size or the percent disper-

sion of platinum after reduction ?

E.R.

BECKER

:

We did not measure platinum particle size or dispersion

as the platinum levels in our experimental catalysts were very high, viz. up to 20 w/w.

These concentrations are not comparable to

industrial catalysts such as CO combustion catalysts which would have maximum platinum levels of 1 w/w platinum. R. POISSON : Does your method work with other complexing ions of alumina like F- or EDTA E.R. BECKER

:

?

If yes, do you know the mechanism

?

Dr. Hegedus shows in the following paper that F- is

indeed suitable as a competing ion in palladium impregnation.

The

mechanism is related to the transport of ionic species into the catalyst support and their relative reactivities with the surface. R.P. SIEG

:

Did you find an effect on Pt profile of impregnation time

prior to reduction ? E.R. BECKER : Yes. Times greater than 5 minutes caused broadening of profile.

171

MULTICOMPONENT CHROMATOGRAPHIC PROCESSES DURING THE IMPREGNATION OF ALUMINA PELLETS WITH NOBLE METALS L. L. HEGEDUS, T. S. CHOU, J. C. SUMMERS, and N. M. POTTER General Motors Research Laboratories , Warren , Michigan , 48090 USA

ABSTRACT A mathematical model of a competitive, multicomponent diffusion-adsorption process is presented, and applied to the impregnation of porous y-alumina pellets by a Rh complex, in the presence of HF as a site blocking agent which drives the Rh below the surface of the pellets. The model correctly predicts the measured Rh peak penetration depths and Rh uptakes from solution.

INTRODUCTION Due to its technical importance and scientific interest, the literature of catalyst impregnation has been rapidly growing. Especially interesting are problems related to nonuniform catalyst impregnation, that is, when an activity profile exists along the radius of the porous catalyst pellets. Both the experimental [e.g., Briggs, et al. (ref. l), Hoekstra (ref. 2 ) , Whitman and Leyman (ref. 3), Retallick (ref. 4), Roth and Reichard (ref. 5), Summers and Hegedus (ref. 6)] and theoretical [e.g., Kasaoka and Sakata (ref. 7), Minhas and Carberry (ref. G ) , Shadman and Petersen (ref. 9 ) , Corbett and Luss (ref. l o ) , Wei and Becker (ref. l l ) , Smith (ref. 12), Villadsen (ref. 13), Becker and Wei (ref. 14,15), Cervello et al. (ref. 16), Hegedus and Summers (ref. 17)] 1 iterature recognize the importance of nonuniform impregnation profiles in affecting the activity, selectivity, or durability of catalysts. Intrapellet impregnation profiles usually become most important when the main reactions or the poisoning reactions (or both) are significantly influenced by intrapellet diffusion resistances. The preparation of nonuniformly impregnated catalysts has a1 so received a great deal of attention in the literature. Maatman and Prater (ref. 18) provided an early analysis of the roles of adsorption and solvent exclusion in determining the activity profiles; Maatman (ref. 19) showed how Pt can be uniformly distributed

172 by the a d d i t i o n o f s a l t s o r acids t o the impregnating solution; Hichalko ( r e f . 20) showed the use o f s i t e blocking agents (various organic acids) t o achieve an impregnated section a t the p e l l e t ' s edge, below the p e l l e t ' s edge, o r i n a core a t i t s center; and f i n a l l y , Chen and Anderson ( r e f . 21,22) discussed various impregn a t i o n techniques leading t o nonuniform C r p r o f i l e s i n y-alumina spheres. The nonuniform impregnation p r o f i l e s u s u a l l y a r i s e due t o a complex i n t e r a c t i o n o f i n t r a p e l l e t flow, d i f f u s i o n , adsorption, and r e a c t i o n phenomena, depending on the p a r t i c u l a r system a t hand and on the method o f impregnation process employed.

A comprehensive treatment of heterogeneous s o r p t i on-di f f u s i o n

problems, as they apply t o c a t a l y s t impregnation by a s i n g l e component, was provided by Weisz e t a l . ( r e f . 23,24,25), processes.

i n c l u d i n g both i r r e v e r s i b l e and revers b l e

H a r r i o t t ( r e f . 26) analyzed an i n t r a p o r e p r e c i p i t a t i o n technique and

found good agreement between a d i f f u s i o n model's p r e d i c t i o n s and experiments w i t h Ag-alumina c a t a l y s t s .

Cervello e t a l . ( r e f . 27) compared a d i f f u s i o n - r e a c t Dn

model w i t h measured N i d i s t r i b u t i o n s i n a idiO-alumina c a t a l y s t .

I n another work, Cervello e t a l . ( r e f . 28) provided an experimental and t h e o r e t i c a l analysis o f the v a r i a b l e s which i n f l u e n c e the d i s t r i b u t i o n o f the a c t i v e components i n porous p e l l e t s . The e f f e c t s o f i n t r a p e l l e t f l o w during c a t a l y s t impregnation were f i r s t analyzed by Maatman and Prater ( r e f . 18), and then i n more d e t a i l by Vincent and

M e r r i l l ( r e f . 29). The purpose o f t h i s work i s t o provide a t h e o r e t i c a l and experimental analysis o f mu1ticomponent c a t a l y s t impregnation.

Such processes are important

when the c a t a l y s t i s e i t h e r simultaneously impregnated by two o r more a c t i v e components which may compete w i t h each other f o r the s i t e s o f the support, o r when s i t e blocking agents are used t o manipu a t e the i n t r a p e l l e t p r o f i l e s o f one o r more a c t i v e components. F i r s t we w i l l o u t l i n e a somewhat genera t h e o r e t i c a l d e s c r i p t i o n o f t h e mu1t icomponent d i ff us ion-adsorpti on process, and then we w i l l i l l u s t r a t e the a p p l i c a b i l i t y o f the model t o Rh-alumina c a t l y s t s prepared by the use o f HF as a s i t e blocking agent, so t h a t t h e Rh i s deposited below the c a t a l y s t p e l l e t ' s surface. THEORETICAL ANALYSIS We w i l l consider the simultaneous d i f f u s i o n of n components ( c a t a l y t i c metals and s i t e blocking agents), and t h e i r r e a c t i o n ( o r adsorption) on the s i t e s of the c a t a l y s t support.

The support i s prewetted, so t h a t i t s pores a r e

i n i t i a l l y f i l l e d w i t h the solvent. The impregnation process can be described by a s e t o f t r a n s i e n t d i f f e r e n t i a l equations.

For the l i q u i d phase,

173 Di

2

Q

asi

li

-

at

App

=

a1

,i=

c -

at

1

....

n.

I n f o r m u l a t i n g t h e s o l i d phase c o n s e r v a t i o n e q u a t i o n s , we have t o s p e c i f y t h e mechanism by which t h e i m p r e g n a t i n g s p e c i e s i n t e r a c t w i t h t h e s u p p o r t ' s As we w i l l see i n t h e e x p e r i m e n t a l p a r t o f t h i s paper, d i f f e r e n t

surface.

s p e c i e s have d i f f e r e n t s a t u r a t i o n c o n c e n t r a t i o n s o v e r t h e same s u p p o r t s u r f a c e . Consequently, we use a Langmuir a d s o r p t i o n scheme where t h e v a r i o u s s p e c i e s a r e

A physical i n t e r p r e t a t i o n

a l l o w e d t o have d i f f e r e n t s a t u r a t i o n c o n c e n t r a t i o n s .

o f t h i s mechanism i m p l i e s s p e c i e s o f d i f f e r i n g m o l e c u l a r c r o s s s e c t i o n s , c o v e r i n g d i f f e r i n g number o f s u p p o r t s i t e s upon s a t u r a t i o n , b u t anchored o n l y a t one s i t e t o the surface. W i t h t h e above mechanism, t h e s o l i d phase c o n s e r v a t i o n e q u a t i o n s f o r a l l s p e c i e s i become asi App

NA -

si

s.)

- -NA N t o t j=1 .- i. # i

"s,i

i= 1 Ntot

li ( 1 -

= App [k+,i

at

... n.

J

k

.

NA Ki Ns,i +,I

Sil,

(2)

is c o n v e n i e n t l y d e f i n e d as t h e h i g h e s t s a t u r a t i o n c o n c e n t r a t i o n o f a l l t h e

s p e c i e s which p a r t i c i p a t e i n t h e process; i n o u r experiments, t h e s a t u r a t i o n c o n c e n t r a t i o n o f t h e s i t e b l o c k i n g agent HF was found t o s a t i s f y t h i s choice. The boundary c o n d i t i o n s a r e i n p a r t a f u n c t i o n o f t h e way t h e experiments a r e conducted, so t h e y need some d i s c u s s i o n .

The s o l v e n t - s a t u r a t e d p e l l e t s

( t h e i r number i s N ) a r e p l a c e d i n t o a s o l u t i o n o f volume V and s t i r r e d . Due t o P t h e f i n i t e volume o f t h e i m p r e g n a t i n g s o l u t i o n , t h e c o n c e n t r a t i o n o f s p e c i e s i i n t h e s u r r o u n d i n g l i q u i d w i l l change w i t h t i m e . The f o l l o w i n g e q u a t i o n s r e p r e s e n t t h e i n i t i a l and boundary c o n d i t i o n s : li(r,O)

= 0, i = 1

... n.

(3)

s 1. ( r , o )

= 0, i = 1

... n.

(4)

a1

Di a r (R,t)

= kmi,

(li(m,t)

a1

ar ( 0 , t )

"

a1 i(m,t)

at

i

= 0,

= 1

N 4nR Di ar P

li(R,t)),

i= 1

... n. ali(R,t)

=

-

, i

= 1

... n.

... n.

(5)

174 where Equation ( 7 ) describes t h e conservation o f species i i n t h e b u l k impregnating s o l u t i o n which surrounds t h e c a t a l y s t p e l l e t s . Equation ( 2 ) can be used t o determine t h e a d s o r p t i o n e q u i l i b r i u m isotherm o f species i. A t e q u i l i b r i u m , and f o r a single-component system: asi

at

and so, from Equation ( 2 ) ,

= 0,

1 qF-7 =

NA

NA

1

N s , 1 . +NS,ii liTm,m)

I n E q u a t i i n ( l o ) , si(-)

and li(m,m)

centrations a t equilibrium.

are, o f course, t h e s o l i d and l i q u i d con-

Figures 2 and 5 i l l u s t r a t e how NSti

and Ki can be

determined u s i n g equation (10). Equations ( 1 ) t o ( 8 ) were solved n u m e r i c a l l y on an IBM 370 computer, u s i n g t h e PDEPACK r o u t i n e o f Madsen and Sincovec (30).

The r e s u l t s , t o g e t h e r w i t h t h e

numerical values o f t h e parameters i n v o l v e d , w i l l be discussed l a t e r on. EXPERIMENTAL PART

A commercial y-alumina support was used, e i t h e r i n t h e form o f s p h e r i c a l p e l l e t s o r as a powder.

The s u p p o r t ' s p r o p e r t i e s a r e shown i n Table 1 below.

TABLE 1 P r o p e r t i e s o f t h e y-alumina support employed Pore volume Pore s i z e d i s t r i b u t i o n Surface area (BET), A P a r t i c l e diameter, 2R Pellet density, p P e l l e t void fractyon,

E

3 0.629 cm / g Bimodql 97x1 O4 cm2/g (powder) 89x10 cm / g ( p e l l e t s ) 0.365+0.018 cm ( s p h e r i c a l p e l l e t s ) 149-177 ~9 (powder) 1.09 g/cm 0.69

Before each experiment, t h e pores o f t h e support were f i l l e d w i t h a HCL s o l u t i o n a t pH = 2.7.

( T h i s pH was chosen because t h e Rh s o l u t i o n which we used

i n t h e impregnation was a l s o a d j u s t e d t o pH = 2.7,

u s i n g HCl).

A t t h e beginning o f each experiment, t h e wet support was suddenly brought i n t o c o n t a c t w i t h a known volume o f impregnating s o l u t i o n i n t h e r e a c t i o n vessel.

The r e a c t i o n s were c a r r i e d o u t i n sealable, s t i r r e d c o n t a i n e r s

which were t h e r m o s t a t i c a l l y c o n t r o l l e d a t 3150.5OC.

T y p i c a l experiments used 4

175 3 t o 20 g alumina ( d r y b a s i s ) , and 100 t o 250 cm o f i m p r e g n a t i n g s o l u t i o n ( i n excess o f t h e p o r e volume). L i q u i d samples o f 0.5 cm

3

volume were drawn a t predetermined i n t e r v a l s .

HF was analyzed by a s o l i d - s t a t e f l u o r i d e e l e c t r o d e , w h i l e Rh was determined by a t o m i c a b s o r p t i o n . I m p r e g n a t i o n p r o f i l e s i n t h e c a t a l y s t p e l l e t s were d e t e r m i n e d by i o n m i c r o probe mass a n a l y s i s . RESULTS AND DISCUSSION a.

Rh-Alumina The i n t e r a c t i o n s o f n o b l e metal complexes w i t h alumina s u r f a c e s a r e o f t e n Recent papers which deal w i t h t h e s u b j e c t

complex, and l i t t l e i n v e s t i g a t e d .

were w r i t t e n by S a n t a c e s a r i a , e t a l . ( r e f . 31, 3 2 ) , Summers and Ausen ( r e f . 33) and by Spek and S c h o l t e n ( r e f . 3 4 ) .

I n s t e a d o f e l u c i d a t i n g t h e d e t a i l e d mechanism

o f t h e chemical i n t e r a c t i o n s , o u r purpose is t o develop a s e m i q u a n t i t a t i v e i n t e r p r e t a t i o n o f t h e process, s u i t a b l e t o g e n e r a t e i n p u t parameters f o r o u r mathematical model, The k i n e t i c s o f t h e a d s o r p t i o n o f o u r Rh complex on t h e alumina s u r f a c e was determined i n Rh u p t a k e measurements as a f u n c t i o n o f t i m e , on a powdered s u p p o r t so t h a t d i f f u s i o n e f f e c t s a r e minimized.

F i g u r e 1 shows t h e d a t a : t h e

i n i t i a l slopes were used t o determine t h e f o r w a r d r a t e c o n s t a n t k+,Rh, l o n g - t i m e asymptotes ( l R h

(my-))

while the

served t o e v a l u a t e t h e e q u i l i b r i u m c o n s t a n t KRh

and t h e s a t u r a t i o n c o n c e n t r a t i o n NSIRh. The l i n e a r i z e d p l o t o f t h e e q u i l i b r i u m i s o t h e r m is shown i n F i g u r e 2, from w h i c h KRh and Ns,Rh

were determined. I t i s i n t e r e s t i n g t o n o t e t h a t t h e s a t u r a 14 t i o n c o n c e n t r a t i o n o f Rh does n o t r e a c h a monolayer o v e r t h e alumina ( 1 . 5 ~ 1 0 2 molecules/cm BET v s . a p p r o x i m a t e l y 1015 f o r a monolayer), p a r t l y due t o t h e f a c t t h a t n o t a l l t h e s i t e s may a t t r a c t Rh, and p a r t l y t h a t one Rh complex m o l e c u l e may cover s e v e r a l s i t e s , even i f we assume t h a t i t i s anchored t o o n l y one o f them. The e f f e c t i v e d i f f u s i v i t y o f t h e Rh complex i n t h e porous alumina p e l l e t s was determined by comparing t h e r e s u l t s o f Rh u p t a k e measurements on p e l l e t s w i t h numerical d i f f u s i o n - a d s o r p t i o n c a l c u l a t i o n s [Equations ( 1 ) t o ( 8 ) ] i n which a l l t h e parameters, w i t h t h e e x c e p t i o n o f DRh, were e v a l u a t e d f r o m t h e powder 2 experiments d i s c u s s e d above. As F i g u r e 3 shows, OR,, = 3 ~ 1 0 -cm~ / s appears t o be a reasonable approximation. b.

HF-A1 umina The k i n e t i c and e q u i l i b r i u m parameters o f t h i s system were determined by

t e c h n i q u e s s i m i l a r t o t h o s e employed f o r Rh.

F i g u r e 4 shows t h e HF u p t a k e

176

V = 1 0 0 cm3 liquid 4.0 g alumina

a

k + , ~ h = 2X10-8 cm3/(cm28ET

m -

f-

g

S)

c 10.0 g alumina

10

d 10.0 g alumina

0

10

Figure 1.

20

\\

30

40

50 60 t (min)

70---------

7200

Rh uptake experiments on powdered alumina.

BET

Figure 2. Linearized plot o f the adsorption isotherm for Rh.

r

a

0.7

-

01 0

I

I

I

5

10

15

I 20

I

25 t(min)

I

I

I

I

30

35

40

45

I

50

Figure 3. Spherical catalyst pellets: comparison of theory with experiment to determine the effective diffusivity of the Rh complex.

1.5

1

0 0

V = 250 0 3 liquid

3.0 g alumina

10

20

30 40 t (min)

50

60

70

Figure 4. HF uptake experiments on powdered alumina.

178

N,,HF = 3.4 x1014 rnolecules/crn2 BET KHF = 1.3 x106 cm3/rnole

0

1

2 1 IHF (m.4 x 1o

3

4

- (crn3/rnole) ~

Figure 5. Linearized plot of the adsorption isotherm for HF.

0

5

10

15

20

t (rnin)

Figure 6. Spherical catalyst pellets: comparison of theory with experiment to determine the effective diffusivity of HF.

179

0.010

-:Calculated curves Arrows : Measured peak locations (by Ion Microprobe Mass Analysis)

0.008 0.007

-

0.006-

0 0

50

100

150

250

200

300

0 350

Penetration ( p m )

Figure 7. Rh distribution in the catalyst pellets in an experiment with HF + Rh: comparison o f theory with measurements.

rn

Measured

-Calculated 0 '0

I

I

I

10

20

30

J 40

t (rnin)

Figure 8. Rh uptake by porous catalyst pellets in an experiment with HF + Rh: comparison o f theory with measurements.

0.010

-

0.009

c 0

0)

.-C

0.008

t

O.Oi0r

A

I

b

t = 10 min

a b c

\

d

0 0.5 5.0 15.0

e

50.0

t=

IT

5 min

K

L

Z

'

\

a Reference b Effect of DRh (x1.5) c Effect of DHF (x1.5)

-LIE

d Effect Of k+,Rh(X2)

0.001-

0

e Effect of k+,HF(xlO) I

I

I

I

I

I

J

-0

50

100

150

200

250

300

350

Penetration ( r m )

Figure 9. Effects of various parameters on the subsurface Rh peak (to illustrate parametric sensitivity).

Figure 10. Effect of the initial HF concentration on the distribution of Rh in the pellets.

181 experiments on powdered alumina.

The i n i t i a l s l o p e s a r e v e r y steep, r e s u l t i n g

i n a c o n s i d e r a b l e u n c e r t a i n t y i n d e t e r m i n i n g k+,HF.

However, as we w i l l see

l a t e r on, k+,HF i s so l a r g e t h a t t h e u p t a k e of HF i n a c a t a l y s t p e l l e t i s e s s e n t i a l l y d i f f u s i o n c o n t r o l led.

Consequently, b o t h t h e i n d i r e c t l y determined

e f f e c t i v e d i f f u s i v i t y o f HF and i t s i m p r e g n a t i o n p r o f i l e a r e a weak f u n c t i o n o f k+,~~. F i g u r e 5 d i s p l a y s t h e l i n e a r i z e d a d s o r p t i o n i s o t h e r m f o r HF: n o t a b l e i s t h e a p p r o x i m a t e l y 2.3 t i m e s l a r g e r s a t u r a t i o n c o n c e n t r a t i o n o f HF t h a n t h a t o f Rh. S i m i l a r l y t o t h e case o f Rh, t h e e f f e c t i v e d i f f u s i v i t y o f HF i n t h e c a t a l y s t p e l l e t s was determined f r o m a comparison o f n u m e r i c a l c a l c u l a t i o n s w i t h HF u p t a k e r e s u l t s on s p h e r i c a l p e l l e t s .

The r e s u l t i n g e f f e c t i v e d i f f u s i v i t y o f HF

( F i g u r e 6) i s a p p r o x i m a t e l y 3.3 t i m e s l a r g e r t h a n t h a t o f t h e more b u l k y Rh complex: c.

a reasonable f i n d i n g .

Rh-HF-Alumina We determined t h e parameters f r o m independent experiments; l e t us now

compare t h e p r e d i c t i o n s o f o u r mathematical model w i t h an e x p e r i m e n t i n which b o t h Rh and HF a r e employed.

The parameters employed i n t h e c o r r e s p o n d i n g

computer r u n s a r e l i s t e d i n T a b l e 2 .

TABLE 2 Parameter v a l u e s f o r t h e Rh-HF i m p r e g n a t i o n o f s p h e r i c a l a l u m i n a p e l l e t s

0.41;g10-6 2x10 9. 8 x l g 5 3x1 0 0.003 ,4 1.5~10

D

15 . O X ~ O - ~ 2ooox~o-8 13x10-6 10x10 0.003 14 3.4~10

3 = 380, 2R = 0.365 cm, p = 1.0 g/cm P P F i g u r e 7 shows t h e computed Rh p r o f i l e s i n t h e p e l l e t s , a t 5 and 10 m i n u t e s

V = 100 cm3, N

a f t e r t h e experiment began.

The computed peak l o c a t i o n s a r e i n r e a s o n a b l e

agreement w i t h i o n m i c r o p r o b e mass a n a l y s i s measurements. Beyond t h e l o c a t i o n o f t h e Rh peak, we a r e a l s o i n t e r e s t e d i n s e e i n g i f o u r model c o r r e c t l y p r e d i c t s t h e amount o f Rh which was t a k e n up by t h e p e l l e t s . F i g u r e 8 compares t h e measured and computed b u l k l i q u i d Rh c o n c e n t r a t i o n s as a function o f time.

Again, t h e m o d e l ’ s p r e d i c t i o n i s q u i t e r e a s o n a b l e .

To t e s t t h e e f f e c t s o f i n a c c u r a c i e s i n t h e n u m e r i c a l v a l u e s of D and k, computer c a l c u l a t i o n s were c a r r i e d o u t t o see how p e r t u r b a t i o n s i n them may a f f e c t t h e l o c a t i o n and magnitude o f t h e Rh peak i n t h e p e l l e t s .

As F i g u r e 9

182

shows, r e l a t i v e l y l a r g e p e r t u r b a t i o n s i n t h e above parameters r e s u l t e d i n modest changes i n t h e peak l o c a t i o n s . Our model can be used t o s i m u l a t e a v a r i e t y o f i m p r e g n a t i o n experiments, w i t h t h e purpose o f m o d i f y i n g t h e l o c a t i o n o f t h e Rh peak i n t h e p e l l e t s . t h e many p o s s i b l e v a r i a t i o n s , we i l l u s t r a t e o n l y one here:

Of

the e f f e c t o f i n i t i a l

HF c o n c e n t r a t i o n on t h e Rh d i s t r i b u t i o n a t t = 5 m i n u t e s ( F i g u r e 10).

As t h e

r e s u l t s i n d i c a t e , t h e Rh peak can be f l e x i b l y p o s i t i o n e d t o i t s d e s i r e d l o c a t i o n b y an a p p r o p r i a t e c h o i c e o f t h e i n i t i a l HF c o n c e n t r a t i o n . ACKNOWLEDGMENTS The i m p r e g n a t i o n experiments and p a r t o f t h e analyses werz c a r r i e d o u t by

S. A. Ausen. NOMENCLATURE A Di

2

BET s u r f a c e a r e a o f t h e alumina E f f e c t i v e d i f f u s i v i t y o f species i i n t h e catalyst pell e t s Forward r a t e c o n s t a n t f o r s p e c i e s i

(cm2/s) (cm / s )

Nass t r a n s f e r c o e f f i c i e n t f o r species i E q u i l i b r i u m constant f o r species i L i q u i d phase c o n c e n t r a t i o n o f species i

NA (mo 1ecu 1es/mol e )

Avogadro’s number Number o f p e l l e t s i n beaker

14P NS,i

(molecules/cm

2

BET)

S a t u r a t i o n c o n c e n t r a t i o n o f species i o v e r t h e alumina s u r f a c e T o t a l number o f s i t e s p e r u n i t s u r f a c e ( s e e t e x t ) P e l l e t r a d i a l coordinate Average p e l l e t r a d i u s S o l i d phase c o n c e n t r a t i o n o f species i Time Pellet void fraction Pellet density

REFERENCES

1 2 3

4 5 6

W. S. B r i g g s , W. A. S t o v e r , and D. S. Henderson, U.S. P a t e n t No. 3 288 558, llovember 29, 1966. J . Hoekstra, U.S. P a t e n t No. 3 360 330, December 26, 1967. R. H. Whitman and E. Leyman, U.S. P a t e n t s No. 3 819 533, June 25, 1974, and 3 956 459, Flay 11, 1976. W. B. R e t a l l i c k , U.S. P a t e n t No. 3 901 821, August 26, 1975. J. F. Roth and T. E. Reichard, J . Res. I n s t . C a t a l y s i s , Hokkaido U n i v . , 20(2)(1972)85. J . C. Summers and L. L. Hegedus, J . C a t a l y s i s , 51 (1978) 185.

183

7 8 9 10 17 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

S. Kasaoka and Y. Sakata, J. Chem. Eng. o f Japan, 1(2)(1968)138. S. Minhas and J. J. Carberry, J . C a t a l y s i s , 14(1969)270. F. Shadman-Yazdi and E. E. Petersen, Chem. Eng. Sci., 27(1972)227. W. E. C o r b e t t and D. Luss, Chem. Eng. Sci., 29(1974)1473. J . Wei and E. R. Becker, Adv. i n Chemistry Series, 143(1975)116. T. G. Smith, I n d . Eng. Chem. Process Des. Dev., 15(3)(1976)388. J. V i l l a d s e n , Chem. Eng. Sci., 31(1976)1212. E. R. Becker and J. Wei, J. Catalysis, 46(1977)365. E. R. Becker and J. Wei, J. C a t a l y s i s , 46(1977)372. J . Cervello, J. F. J. Flelendo, and E. Hermana, Chem. Eng. Sci., 32( 1977)155. L. L. Hegedus and J . C. Summers, J. C a t a l y s i s , 48(1977)345. R. W. Maatman and C. 0. P r a t e r , Ind. Eng. Chem., 49(2)(1957)253. R. W. Maatman, Ind. Eng. Chem., 51(8)(1959)913. E. Michalko, U.S. Patents No. 3 259 454 and 3 259 589, b o t h J u l y 5, 1966. H. C. Chen and R. B. Anderson, Ind. Eng. Cliem. Prod. Res. Dev., 12( 2) (1973)122. H. C. Chen and R. B. Anderson, J. C a t a l y s i s , 43(1976)200. P. B. Weisz, Trans. Faraday SOC., 63(1967)1801. P. B. Weisz and J. S. Hicks, Trans. Faraday SOC., 63(1967)1807. P. B. Weisz and H. Z o l l i n g e r , Trans. Faraday SOC., 63(1967)1815. P. H a r r i o t t , J. C a t a l y s i s , 14(1969)43. J. C e r v e l l o , E. Hermana, J . F. Jimenez, F. Melo, in B. Delmon, P. A. Jacobs, G. Poncelet ( e d i t o r s ) : " P r e p a r a t i o n o f Catalysts," E l s e v i e r , Amsterdam, (1976)251. J . Cervello, J. F. Garcia de l a Banda, E. Ilermana, and J. F. Jimenez, Chem. -1ng.-Tech. 48(6)( 1976)520. R. C. Vincent and R. P. M e r r i l l , J . C a t a l y s i s , 35(1974)206. N . K. Madsen and R. F. Sincovec: "PDEPACK: P a r t i a l D i f f e r e n t i a l Equations Package," S c i e n t i f i c Computing C o n s u l t i n g Services, 531 Z i r c o n Way, Livermore, C a l i f o r n i a . E. Santacesaria and S. Carra, I. Adami, Ind. Eng. Chem. Prod. Res. Dev 16(1)(1977)41. E. Santacesaria, 0. Gelosa, and S. Carra, I n d . Eng. Chem. Prod. Res. Dev. 16( 1 ) (1977)45. J. C. Summers and S. A. Ausen, J. C a t a l y s i s , i n press, 1978. T. G. Spek and J. J. F. Scholten, J. Molec C a t a l y s i s , 3(1977/78)81.

DISCUSSION V. FENELONOV

:

What is the difference between experimental and

calculated data in the distribution along the pellet radius ? L.L. HEGEDUS

:

Figure 7 shows computed Rh distribution profiles in

the catalyst pellets at two selected times ( 5 min. and 10 min.). Only the penetrations at the peaks (indicated by arrows) could be compared with ion microprobe mass analysis data, because the ion microprobe instrument was not calibrated for absolute Rh concentrations.

Such a calibration would be difficult due to the

low Rh Levels and due to the possible L.L. MURRELL

:

non-linearities involved.

Do your isotherms of HF and H C 1 adsorption take into

account the fact that

A1203

in aqueous solution is quite reactive

toward acidic solutions such as oxalic and hydrochloric acid ?

184 L.L. HEGEDUS

:

Our catalyst supports were pretreated in an HC1 solu-

tion (pH = 2.7) before the impregnation experiments.

The isotherms

for HF and Rh[III) were measured in the presence of HC1 in the solutions (initial pH = 2 . 7 ) HC1 concentrations.

and should not be extrapolated to other

While the chemistry of the problem is probably

more complex than what our model implies, it seems that the simplifications we employed are permissible to predict the penetration of Rh into the pellets and the uptake of Rh from solution. M.V. TWIGG

:

Rhodium(II1) + chloride ions in aqueous solution form

a number of complex ions.

Their interconversion is not rapid, and

their ratio at equilibrium is concentration dependent.

In connection

with the theory described which assumes but one type of rhodium ion, what are the rhodium species present in your impregnation solutions

7

Were any interesting time or concentration effects observed ?

L.L. HEGEDUS

:

We are indeed aware of the fact that aqueous RhC13

solutions contain a number of partially hydrated species in equilibrium.

Our model stipulates that their reactivity and

diffusive properties are similar, s o that they can be lumped into one hypothetical species.

The good agreement of our computations

with the experimental data indicates the validity of this assumption for modeling purposes.

However, we decided to explore the adsorptive

properties of various a q u o - c h l o r o - r h o d i u m ( I I 1 ) species in further experiments. J.W. HIGHTOWER

:

It appears from your Fig. 10 that all the deposition/

penetration curves at various HF levels blend into the zero HF curve.

The total Rh content must then be decreased by the presence

of HF.

Rather than the Rh being "pushed inside" by the HF, the Rh

adsorption sites near the external surface are simply poisoned by the HF.

Would you agree with this interpretation ?

L.L. HEGEDUS : Figure 10 displays the computed Rh profiles at a fixed time elapsed ( 5 min.1.

Indeed, the presence of HF delays the

accumulation of Rh in the pellets, since they compete for the same sites on the alumina surface.

185

FACTORS CONTROLLING THE RETENTION OF CHLORINE IN PLATINUM REFORMING CATALYSTS S. SIVASANKER, A.V. RAMASWAMY and P. RATNASAMY

Indian Institute of Petroleum, Dehradun 2 4 8 0 0 5 , India ABSTRACT The influence of various preparative and operational parameters on the uptake and retention of chlorine in Pt-alumina and Pt-Snalumina is reported.

Catalysts based on eta-alumina adsorb and retain

more C1- than those based on the gamma form. ion-exchanges for OH- o n the alumina surface.

It is shown that C1During the simultaneous

adsorption o f HC1 and H2PtC16, while C1- ions do not affect the uptake of P t significantly on eta-alumina, there is a decrease i n the case o f gamma-alumina as the concentration o f C 1

-

increases.

Apart

from the type of alumina used (eta- or gamma-) the temperature of calcination/reduction and the water content of the air/H2 used are found to be the two major factors which control the retention of chlorine during the activation of these catalysts.

The nature of

chlorine held o n alumina is discussed.

INTRODUCTION Chlorine has a major influence on the performance of naphtha reforming catalysts, such a s Pt-alumina and Pt-Sn-alumina.

I t i s incor-

porated into the catalyst formulation during the preparative stage by impregnation from a solution containing chloride ions (ref. 1).

After

impregnation with chlorine and platinum,the catalyst is dried at around 3 8 3 K and then calcined at around 7 7 3 K.

Before use, the catalyst is

reduced in a stream o f hydrogen to convert the ionic platinum to the metallic state.

During the above activation and reduction processes,

the amount of chlorine retained on the catalyst is likely to change. No systematic study of the uptake of chlorine during the impregnation process and its retention during the subsequent calcination and reduction has so far been reported.

For instance, are there differences

between eta and gamma aluminas in their ability to adsorb and strongly retain chloride ions

?

How is the incorporation of P t affected by

C1-ions present in the same solution

?

During the activation and

reduction of the catalyst, how does the purity of the gases (especial-

186 ly its moisture content) affect the loss, if any, of chlorine from the catalyst ?

Does tin have any influence on the uptake and reten-

tion chloride ?

The present study is an attempt to answer some of

the questions.

EXPERIMENTAL Two modifications of alumina, the gamma and the eta forms were used The gamma alumina (SBET = 1 9 0 m 2/g) was obtained from

in this study.

Labofina and had been prepared from boehmite. The eta alumina 2 (SBET = 200 m /g) was prepared by hydrolysis of aluminium isopropoxide following the procedure of Yoldas (ref. 2). Water loss experiments indicated that this alumina contained more than 80% of the eta form. The Sn- and Pt-alumina was prepared by impregnating the support with solutions of SnC12 and H2PtC16, respectively, in the presence of small amounts of HC1 to yield catalysts containing 0.4 wt.% of the metal.

The impregnations were carried out at 293 K.

The impregnated

materials were dried at 383 K and then calcined at 673 K in air for

1.8 x 1 0

4

In the cass of Sn-Pt-A1203, the support was first

s.

impregnated with Sn and calcined at 673 K prior to Pt impregnation. Adsorption isotherms were obtained by equilibrating 5 g amounts of catalysts with 1 0 0 ml volumes of HC1 (without or with added H PtCl to 2 6 4 give 0.4% Pt on the catalyst) for 8.28 x 10 s . Chloride ion uptake was estimated by titration of both the supernatant liquid and the catalyst.

The supernatant liquid was also analysed for A1 and P t ions

by emission spectroscopy and colorimetric methods, respectively.

The

dispersion of P t was determined by the method of Benson and Boudart (ref. 3 ) . Extractions of the catalysts for chloride and P t estimations were made with six 15 ml aliquots of NH OH (1N) for each sample. The 4 extracts were mixed and analysed either for P t by a colorimetric procedure (ref. 4) or for chloride ions by titration against AgN03 after acidification with HN03 (6N) (ref. 5 , 6 ) .

This extraction

procedure was su'fficient to remove all the chlorine from the catalysts in whatever form.

For instance, in the case of 0.88% C1-gamma-alumina

sample, the first, second and third extractions removed 91.4, 7.7 and

0.9% respectively of the total chlorine present in the sample.

RESULTS AND DISCUSSION

The

uptake of chloride ions - equilibria and rates

Adsorption

isotherms of HC1 on eta and gamma aluminas.

The adsorp-

187

/

/O'

d /

I

10 Eq. CONC. HCL ( M I X10'3

20

Fig. 1. Adsorption isotherms of HC1 at 293 K o n a) eta alumina - A - , b) gamma alumina -0-,c) C1-gamma-alumina -n-, and d) Sn-gammaalumina -v-. Total C1 contents for C1-gamma-alumina -0- and Sngamma-alumina -0- are also shown.

tion isotherm of C1- ion on eta and gamma aluminas are shown in Fig. 1, curves a and b,respectively.

The number of chloride ions

adsorbed per cm2 of alumina surface i s higher on eta than on gamma alumina.

The values range between 1013 and 1014/cm2.

It may be re-

called that the concentration of surface hydrolysis on both eta and gamma aluminas is about 1015/cm2 (ref. 7 ) .

The adsorption o f mineral

acids and aqueous electrolytes on alumina surfaces has been studied by Jacimovic et al. (ref. 8) and Ahmed the occurrence of charged

( 2 ) or

(ref. 9).

In such solutions,

neutral surfaces at the oxide-solution

interface i s due to the formation of metal-aquo complexes as shown beiow

:

In species A, the anions (Cl-, in our case) do not replace the surface hydroxyl groups but stay as counter-ions outside the primary

188 hydration s h e l l of OH-

the surface.

In s p e c i e s B ,

the anions replace the

i o n s and a r e t h u s a t t a c h e d d i r e c t l y t o t h e metal c a t i o n .

l a t t e r c a s e i s a n a l o g o u s w i t h t h e i o n e x c h a n g e o f t h e s e OH-

This ions.

S i n c e o u r a d s o r p t i o n i s o t h e r m s were c a r r i e d o u t a t p H v a l u e s lower than t h a t corresponding t o t h e zero p o i n t charge of alumina, t h e s u r f a c e of alumina i s expected t o bear a n e t p o s i t i v e charge A

(species

o r B) due t o p r o t o n a d d i t i o n t o t h e n e u t r a l a q u e o u s c o m p l e x . I n o r d e r t o d i s t i n g u i s h between t h e s p e c i e s A and B ,

the kinetics

o f a d s o r p t i o n o f C1- i o n s o n b o t h e t a a n d gamma a l u m i n a s was f o l l o w e d a n d t h e r e s u l t s a r e shown i n F i g .

2 A.

The r a t e s o f a d s o r p t i o n o n

e t a a l u m i n a a r e h i g h e r t h a n o n gamma a l u m i n a .

C1- i o n s i s e s s e n t i a l l y a n i o n e x c h a n g e p r o c e s s f a c e OH-

If

t h e a d s o r p t i o n of

(exchanging f o r s u r -

i o n s ) , t h e n t h e r a t e e q u a t i o n f o r e x c h a n g e c o n t r o l l e d by

f i l m d i f f u s i o n i n t h e shallow-bed observed k i n e t i c data.

-

1

Y/Y,

method

(ref.

10)

should f i t t h e

In t h e r a t e q u e s t i o n , =

exp

(-

(2)

3Dt/RKAr)

0

!2

-0 X

N

E

0

a a L W

0

b 0

z

t 0.2

0.6 t, ks

0-6 t,

0.9

ks

F i g . 2 . K i n e t i c s o f a d s o r p t i o n o f HC1 o n e t a a n d gamma a l u m i n a s : P l o t o f c h l o r i d e i o n u p t a k e vs. t i m e . B. P l o t of d a t a according t o equation ( 2 ) .

A.

Y and Ym a r e t h e

a m o u n t s o f C1- a d s o r b e d a t t i m e t a n d a f t e r a n

i n f i n i t e time i n t e r v a l , dary film,

D is the diffusion

c o e f f i c i e n t i n t h e boun-

K i s t h e e q u i l i b r i u m c o n c e n t r a t i o n o f C1- i n a l u m i n a

189 d i v i d e d by t h e c o n c e n t r a t i o n o f of

i n the solution, R is the radius

C1-

t h e a l u m i n a p a r t i c l e s a n d Ar i s t h e t h i c k n e s s o f

f i l m .

A linear plot of

t h a t C1-

vs.

ln(l-Y/Ym)

t shown i n F i g .

i o n s i o n - e x c h a n g e w i t h s u r f a c e OH-

linked t o t h e s u r f a c e A13+ Adsorption

isotherms of

Curves c and d o f

Fig.

H C 1 o n C1-gamma-and

f u r t h e r uptake

of of

C1-

C1-

alumina.

from aqueous HC1.

comparable concentrations

T h i s i s shown i n F i g .

(i.e.,

i n equilibrium with

1 where t h e d a t a f o r t h e t o t a l C1-

a n d Sn-gamma-alumina

b,

f o r t h e p u r e gamma a l u m i n a s a m p l e .

c a p a c i t y of

t h e alumina surface

t h e amount p r e -

i s s i m i l a r t o t h a t o n gamma

c o n t e n t o n C1-gammathe isotherm data

Even

i o n s from aqueous HC1,

r e t a i n e d i n t h e alumina

C1-

impregnated p l u s t h a t s u b s e q u e n t l y t a k e n up) a q u e o u s H C 1 of

Sn-gamma-aluminas.

1 i l l u s t r a t e t h e influence of preimpregnated

the

t h e t o t a l amount o f

2 B indicates

i o n s and a r e d i r e c t l y

cations.

H C 1 a n d SnC12 bn t h e s u b s e q u e n t u p t a k e

though both reduce

t h e boundary

s a m p l e s also f a l l o n c u r v e

f o r t h e r e t e n t i o n of

C1-

Thus t h e ions in

aqueous media depends mainly on i t s i n h e r e n t s t r u c t u r a l f e a t u r e s . Multiple impregnation

techniques or pre-impregnation

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

s p e c i f i c a d s o r p t i o n of C1-

further

coordination

s h e l l of

with t i n chloride

surface.

This picture

supports t h e conclusions of

p r e c e d i n g s e c t i o n t h a t C1- i o n - e x c h a n g e s

the primary

the

w i t h s p e c i f i c OH-

t h e s u r f a c e A13+

the

groups i n

ions.

Eq. CONC. HCI (MI X Fig.

3.

Solubility of

Solubility effects.

e t a a n d gamma a l u m i n a i n a q u e o u s H C 1 . A t

the present investigation,

t h e low c o n c e n t r a t i o n s

o f HC1 employed i n

the solubility of A1203 w a s very low.

190 Jacimovic et al. (ref. 8 ) had reported negligible solubility for solutions whose pH was above 2.5.

The solubility of eta and gamma

alumina in aqueous H C 1 of different concentrations is illustrated in Fig. 3 .

At all values of the concentration of HC1, the eta

form is more soluble than the gamma form. Simultaneous adsorption of HC1 and H PtC16 on eta and gamma 2-

aluminas.

The isotherms for the simultaneous adsorption of chloride

and platinum ions on eta and gamma aluminas are shown in Fig. 4 A and B .

In these experiments enough H2PtC16 was taken to give 0 . 4 %

wt. of P t in the final Pt-alumina catalyst.

Eta-alumina adsorbs

and retains more C1- than the gamma sample (Fig. 4 concentration of C1- in the aqueous solution.

A)

at a given

An interesting

difference in the ability of adsorption and retention of platinum in the presence of C1- is evident from Fig. 4 B.

While C1- ions

do not affect significantly the uptake of P t o n eta alumina,

there is a decrease in the case of the gamma alumina as the concentration of ~ 1 increases. -

01,

\

Q

B 3

ir 20 Eq. CONC. HCI (MI XlC3 10

20

10

Eq. cow.

HCI (MI

x

Fig. 4. Isotherms for simultaneous adsorption of chloride ions (A) and P t ions ( B ) on eta and gamma aluminas.

191 TABLE

1

I n f l u e n c e of v a r i o u s c a l c i n a t i o n a n d r e d u c t i o n p r o c e d u r e s on t h e chlorine content of

the catalysts

383 K

%C1 a f t e r calcination in dry a i r a t 823 K

%C1 a f t e r c a l c i nation i n s t a t i c atm. contg.15000 ppm o f w a t e r a t 673 K 823 K

0.26

0.25

0.55

0.51

-

-

-

-

0.71

-

0.55

0.15

0.65

0.61

-

%C1

after drying a t

Catalyst

y-a lumina

n-alumina

%C1 a f t e r reduction a t 773 K i n dry H contg. H2 l ? O O ppm water

-

1.12

0.87 0.90

-

-

-

1.53 1.20

0.98

0.85

0.52

-

0.60

0.20

Sn-y-alumina

(0.79)

Pt-y-alumina

(0.70)

Pt-y-alumina

(0.54)

Pt-Sn-y-alumina

(0.71)

Pt-?l-alumina

(0.57)

1.26 0.88

0.52

0.22

-

-

0.52

0.22

0.51 0 . 4 8

0.70

0.22

0.70 0.72

0.56

-

0.56

-

0.54

N o t e : a ) The v a l u e s a r e b a s e d on a n a l y s i s , e x c e p t i n g t h o s e g i v e n i n p a r e n t h e s e s , w h i c h a r e b a s e d on t h e o r e t i c a l v a l u e s . b) Metal loadings a r e 0 . 4 % by w t . f o r b o t h P t and Sn. c) A l l d r y i n g , c a l c i n a t i o n and r e d u c t i o n w e r e d o n e f o r 1.8 x 104 s . d ) F l o w r a t e o f h y d r o g e n was 1.7 x m 3 s-l d u r i n g r e d u c t i o n .

I n f l u e n c e o f c a l c i n a t i o n and r e d u c t i o n c o n d i t i o n s on t h e r e t e n t i o n of chlorine Table

1 i l l u s t r a t e s some o f o u r r e s u l t s .

In addition,

t i o n of c h l o r i n e during t h e r e a c t i o n of n-heptane m i x t u r e o f H2 a n d n - h e p t a n e

(mole r a t i o , 2 . 7 )

c a t a l y s t b e d a t 773 K f o r 1 . 4 4 x l o 4 s.

lo9 mole s

-1

was m e a s u r e d .

The f e e d r a t e o f n - h e p t a n e

The c a t a l y s t s w e r e h e l d i n H 2

x

f o r 3.6

x lo3 s p r i o r t o i n t r o d u c t i o n o f n - h e p t a n e .

t o dry t h e n-heptane H2

s t r e a m was c o n t r o l l e d by p a s s i n g t h r o u g h a N a Q H s o l u t i o n .

c o n c e n t r a t i o n of C 1 of t h e run.

was a b s e n t .

the The

i o n i n t h e c a t a l y s t w a s e s t i m a t e d a t t h e end

I n t h e c a s e of

t h e r e was n o l o s s o f

a t 773 K

Care w a s t a k e n

The m o i s t u r e c o n t e n t o f

feed thoroughly.

A

w a s passed through t h e

was 5.6

kg-'.

the reten-

the 0.4

w t . % Pt-gamma-alumina

c h l o r i n e i n a H2

+

n-heptane

catalyst,

atmosphere i f water

The C1- c o n t e n t d e c r e a s e d f r o m 0.54 t o 0.49% by w t .

f e e d s t r e a m c o n t a i n e d 1500 a n d 25,000 ppm o f w a t e r . w h i c h t o l u e n e was r e p l a c e d f o r n - h e p t a n e

i f the

Experiments i n

d i d n o t r e v e a l any i n f l u e n c e

192 of the type of hydrocarbon on the loss of C1- from the catalyst. Temperature of calcination/reduction and the water content of the air/H2 used during the process are the two major factors which control the retention of chlorine during the activation of the Pt-alumina or Pt-Sn-alumina catalysts.

The following points may be noted

:

1) A decrease in the chlorine content with increasing temperature of

calcination is observed in the case of eta and gamma alumina supports. In the case of 0.4 wt.% Pt-gamma-alumina also, an increase from 673 to 823 K of the calcination temperature at constant H 0 content de2

creased the chlorine content from 0.52 to 0.22

%

by wt.

2) The amount of chlorine retained in eta alumina at a calcination

temperature of 823 K decreases from 0.98 to 0.52

%

by wt. when the

H 0 level is increased from essentially dry conditions to 15,000 ppm. 2 3) During reduction in hydrogen, the presence of H 0 leads to a l o s s 2 of chlorine only if the chlorine level is above 0.7

%

by w t .

Below

this value, there is no loss of chlorine in the presence of H 0 (at 2 least up to a level of 1500 ppm of H 2 0 ) . Chlorine is lost from alumina and Pt-alumina in the form of HC1 as well as A1C13. Our studies indicate that from pure alumina, chlorine is lost predominantly as AlCl in a stream of dry H2.

In moist H2 (1500 ppm of H20) evolution of

HC1 occurs. TABLE 2 Concentrations of extractable Pt complex after calcination in air Temperature of

Percentage of

calcination, Kb

P t extracted

Pt-y-alumina

383

8.0

Pt-y-alumina

673

2.0

Catalysta

Pt-0-alumina

383

2.2

Pt-0-alumina

673

3.2

Sn-Pt-y-alumina

383

24.5

Sn-Pt-Y-alumina

673

2.7

aThe catalysts contained 0.4

%

P t by wt. and 0.4

%

Sn by wt.

bAll calcinations were carried out in a static atmosphere of air containing 15,000 ppm of H20. Duration was 1.8 x lo4 s. 4)

Retention of C1- ion during the activation process is generally

more on Pt-eta-alumina than on Pt-gamma-alumina. 5 ) While P t dispersion values were insensitive to the presence of

H 2 0 (upto 1500 ppm) during reduction in H2 at 773 K in the case of

3

193 Pt-gamma-alumina and Pt-Sn-gamma-alumina, there i s a drastic reduction from 93 to 59

%

in the case of Pt-eta-alumina.

In Pt-alumina,

in addition to the chlorine held by the support, part of the chlorine will also be associated with Pt.

The variation in the concentrations

of the platinum extracted by hot NH OH (1N) after calcination in air 4

is shown in Table 2 .

Incorporation of tin increases the concentration

of extractable P t in the catalyst dried at 383 K.

Calcination at

673 K in moist air leads to a drastic diminution due to its conversion

into different species. The nature of chlorine held by alumina A s noted earlier, the impregnation of C1- on A 1 2 0 3 at low concen-

trations is essentially an ion-exchange process with surface hydroxyl groups being replaced by C1-.

What are the surface hydroxyls that

are most liable to exchange for C1-

?

A recent model for the surface

hydroxyls of alumina (ref. 7 ) classifies them into 5 groups depending on their net charge.

Type I a and I b hydroxyl groups bear negative

charges of -0.25 and -0.5, respectively and will, hence, undergo ion-exchange with C1- more easily than others.

Of the two, type ~a

forms part of the 'x-site', the site postulated to be active in reactions of hydrocarbons, like double-bond and cis-trans isomerisation (ref. 7).

In order to accomodate the larger C1- (diameter, 3.62

in the place of OH

-

i)

0

(diameter, 2.80 A) Some type IIa hydroxyl groups

which are adjacent to both the original type Ia hydroxyl group as well as the 'x-site' have to be removed (on dehydration at higher temperatures) thus increasing the coordinative unsaturation and hence the strength of the acid sites on the alumina surface (see Fig. 12 of ref. 7).

This enhanced acid strength is manifested in the

ability of chlorided aluminas to catalyse such reactions as the methyl shift and other skeletal isomerisation reactions of hydrocarbons which the relatively weaker acid sites of pure alumina are unable to catalyse (ref. 11).

The larger concentration of such OH- groups on eta

alumina and consequently its higher uptake of C1- ions are understandable in view of the lower packing density and higher concentration of stacking faults in its oxygen lattice. chlorine is also higher on the eta form.

The retention of

Under certain conditions the

presence of tin enhances the retention of chlorine on the catalyst.

CONCLUSIONS At low concentrations of chloride ions (below 1 % by wt.), the uptake of C1-ions on gamma and eta aluminas is controlled by the

194 concentration of surface OH- groups carrying a net negative charge. The larger concentration of such OH- group on eta alumina and consequently its higher uptake of C1- ions are understandable in view of the lower packing density and higher concentration of stacking faults in its oxygen lattice. higher on the eta form.

The retention of chlorine is also

Under certain conditions the presence of

tin enhances the retention o f chlorine on the catalyst.

ACKNOWLEDGEMENT We thank Labofina sample.

(Belgium) for the supply of the gamma alumina

We are grateful to our colleagues in the Analytical Physics

section for A1 estimations, and to Miss. S. S. Vishnoi for P t We thank Drs. I.B. Gulati and K.K. Bhattacharyya for

estimations.

encouragement and support.

REFERENCES

1. R.W. Maatman, Ind. Eng. Chem., 51 (1959) 913. 2. B.E. Yoldas, J. Appl. Chem. Biotechnol. 23 (1973) 803. 3. J.E. Benson and M. Boudart, J. Catal., 4 (1965) 704. 4. K. Kodama, Methods o f Quantitative Inorganic Analysis, Interscience, New York, 1963, p . 241. 5. A.I. Vogel, A Text-book of Quantitative Inorganic Analysis, 3rd Ed., ELBS and Longman, London, 1969, p. 267. 6. S. Sivasanker and L.M. Yeddanapalli, Curr. Sci., 41 (1972) 878. 7. Knozinger and P. Ratnasamy, Catal. Rev., 17 (1978) 31. 8. Lj. Jacimovic, J. Stevovic and S. Veljkovic, J. Phys. Chem., 76 (1972) 3625. 9. S.M. Ahmed, J. Phys. Chem., 73 (1969) 3546. 10. W. Rieman and H.F. Walton, Ion-exchange in Analytical Chemistry, Pergamon, New York, 1970, p. 55. 1 1 . B.H. Davis, J. Cata1.,23 (1971) 355. DISCUSSION :

R. POISSON : Did you take the precaution to analyse the properties

of bayerite in your precursor ? I mean by ATG to take 100% of A1203 into account.

P. RATNASAMY

:

Yes, the precursor hydrates were analysed by thermo-

gravimetry and found to contain more than 90% of the trihydrate.

J.W.

GEUS : Some people have mentioned that they have troubles in

getting reliable estimates of free P t surface area by using the oxygen-hydrogen

(or reversibly) titration.

They are preferring

therefore the adsorption of hydrogen onto the previously cleaned

195 P t surface.

Do you know whether the au,thors have had difficulties

with this technique 7 P. RATNASAMY

:

In spite of the extensive work reported in this area,

there is still no satisfactory method to measure accurately the dispersion of P t in typical reforming catalysts.

Both the H2 chemi-

sorption and H2-02 (or 02-H2) titrations have their own drawbacks. Factors like cleanliness of the P t surface, spill-over of H2 and/or 02,

variation in the stoichiometric values with metal crystallite

size, etc., preclude an unambiguous preference of one method over the other.

In our laboratory, we have obtained almost identical va-

lues of metal dispersions by both the HZ-02 or the reverse 02-H2 titrations using a conventional volumetric set-up.

We find that

this method gives satisfactory and reproducible values of metal dispersions for samples not differing significantly in metal loadings, dispersion levels, nature of support, etc.. H. BREMER ration

:

1) Chlorine in practice is not only added during prepa-

but also during e.g. reforming reaction in the form of CC14

and CHC1) etc...

Is its action similar to that of HC1 7

2) To what

extent dispersion is influenced by added chlorine 7 P. RATNASAMY

:

1 ) We believe that the chloro compounds added during

the reaction get first converted into HC1 and hence their action will probably be similar to that of HC1 added during the preparation stage.

In fact, chlorine compounds are added during the reforming

reaction mainly to compensate for the loss of chlorine from the catalyst under the reaction conditions. 2)

Chlorine added during the reaction is not expected to alter the

dispersion of the metal.

However, chlorine added during the rejuve-

nation of the used catalyst reportedly helps in dispersing the Pt better. P.G. MENON : With reference to H. Bremer's questions we have found

recently ( 1 ) that chlorination with CC14 affects both A1203 and Pt on the Pt-A1203 catalyst,

In addition to enhancing the acidity of

A1203, chloriding drastically suppresses the hydrogenolysis activity of Pt.

The chloride retained by the catalyst under reaction condi-

tions still depends on the temperature and moisture content in the

196 f e e d s t o c k and t h e r e c y c l e g a s , a s emphasized in t h i s paper. S i n c e c h l o r i n e adds o n t o P t a s well, H Z - c h e m i s o r p t i o n m e a s u r e m e n t s o n t h e chlorided c a t a l y s t w i l l s h o w a lower v a l u e

(1).

But it d o e s

not m e a n any change in P t d i s p e r s i o n , it only s h o w s t h a t p a r t o f t h e P t is c o v e r e d by C 1 and h e n c e i n a c c e s s i b l e t o H2.

( 1 ) . P.G.

M e n o n , R.P.

D e P a u w and G.F.

Froment. Ind. Eng. C h e m .

Prod. Res. Dev.. ( i n press)

8. D E L M O N

: C o n c e r n i n g Fig.

f e a t u r e s o f t h e C1- uptake.

2, could y o u c o m m e n t o n t h e k i n e t i c D o y o u a t t r i b u t e them to d i f f u s i o n a l

kinetics, activated exchange or other causes 7

P. R A T N A S A M Y : T h e k i n e t i c d a t a w e r e fitted t o r a t e e q u a t i o n s d e r i ved from v a r i o u s m o d e l s of a d s o r p t i o n and e x c h a n g e processes.

As

m e n t i o n e d i n t h e t e x t , t h e r a t e e q u a t i o n f o r ion-exchange c o n t r o l led b y d i f f u s i o n o f t h e c h l o r i d e ions t h r o u g h t h e b o u n d a r y f i l m w a s found t o f i t , m a t h e m a t i c a l l y , m o s t a p p r o p r i a t e l y our k i n e t i c data.

197

PREPARATION OF ALUMINA OR SILICA SUPPORTED PLATINUM-RUTHENIUM BIMETALLIC CATALYSTS G. BLANCHARD"),

H. CHARCOSSET''),

M.T. CHENEBAUX(2) and M. PRIMET( 1 )

Institut de Recherches sur la Catalyse du C.N.R.S.,

79, bd du I 1 novembre 1918

69626 Villeurbanne CQdex France Institut FranGais du PQtrole, 1 et 4 , avenue de Bois PrQau, 92506 Rueil-Malmaison, France

ABSTRACT The purpose of the study was to prepare well dispersed (Pt, Ru) bimetallic cluster particles supported by y-A1203. A (Pt, Ru)/Si02 alloy catalyst, poorly dispersed, and hence suitable for X-Ray Diffraction analysis, was also investigated for comparison. W visible spectroscopy, electron microprobe analysis, Temperature Programed Reduction and Titration, electron microscopy and Infrared Spectroscopy of chemisorbed CO were used as complementary methods. We arrive to the conclusion that the difficulty in preparing the above (Pt, Ru)/Al 0 clusters arises mainly from a microscopic 2 3 heterogeneity of the catalyst at the end of the impregnation step. Segregation of Pt and Ru may further result from an oxidizing heat treatment which gives rise to considerable sintering of Ru.

INTRODUCTION The present study deals with the alumina (1) or silica supported platinum-ruthenium bimetallic catalysts. Their reduction by hydrogen was studied, by Temperature Programmed Reduction (TPR) (Z), to determine if a l l of the Pt and Ru were reduced to the metallic state and to check the possible synergetic effects in the reduction of the Pt and Ru species. The degree of interaction between Pto and Ruo and their degree of dispersion were studied altogether, by Temperature Programmed Titration (TPT) of previously chemisorbed oxygen, by hydrogen. That method was found very useful to show the presence if any, of pure Ru particles in a bimetallic catalyst. Electron microprobe analysis, U.V. visible absorption spectroscopy, X-Ray diffraction analysis, electron microscopy and Infrared Spectroscopy of chemisorbed CO were also used. The main conclusion is that the preparation of homogeneous (Pt, Ru) bimetallic particles, highly dispersed on y-A1203, needs for further work.

198 EXPERIMENTAL I . Supports and r e a g e n t s They were : -A1 0 length ; S

%

33

Rh8ne P r o g i l GFS400 ( p e l l e t s of

%

1.5 mm i n d i a m e t e r ,

200 m / g ; broad pore s i z e d i s t r i b u t i o n w i t h a maximum a t

t o t a l pore volume

%

F

P

3 0 . 7 cm / g ) .

-Si02 A e r o s i l Degussa (powder, S

%

15 mm i n

= 60-70 A ,

2 200 m / g , non p o r o u s ) .

- c h l o r o p l a t i n i c a c i d s o l u t i o n (Merck) and c h l o r o r u t h e n i c a c i d s o l u t i o n (Comptoir Lyon Allemand). 2. P r e p a r a t i o n of t h e c a t a l y s t s 2 .1,.-1FLrz gn_aLiz? It was proceeded t o t o t a l (co) impregnation of S i 0 2 w i t h H2PtC16, H RuC16 t o have

2

a number of

(Pt+Ru) atoms e q u i v a l e n t t o 10 w t

reduced p r e s s u r e

-2

Pt.

Drying was c a r r i e d o u t under

Torr) from 25 t o 50°C. The nominal composition of t h e bime-

10

(%

X

t a l l i c c a t a l y s t was 50 a t % Ru. The A1203 p e l l e t s were c a l c i n e d i n a dry a i r flow 2 h r s a t 5OO0C, b e f o r e impregnation by water a t f i r s t and subsequently by t h e a c t i v e

species solution. Pt/A1203

(%

2 wt

X Pt)

:

-4

2.4 x 10

mole HC1/gAl2O3 were found n e c e s s a r y t o add i n

t h e H2PtC16 s o l u t i o n , t o g e t macroscopic homogeneous d i s t r i b u t i o n of P t i n s i d e t h e

A1203 p e l l e t s . 1 h r a g i t a t i o n was succeeded by water washing and by d r y i n g i n a d r y a i r flow from 25 t o l l O ° C . Ru/A1203

(%

1 w t % Ru) : The amount of HC1

(%

1.5 x

mole/gA1203) t o be added

i n t h e H2RuC1 s o l u t i o n was about t e n times as much a s used f o r t h e Pt/A1203 c a t a l y s t .

6

10 h r s a g i t a t i o n were followed by water washing and d r y i n g , a s f o r Pt/A1203

(Pt+Ru)/A1203

(Q

X P t , 0.5 w t

1 wt

% Ru) : It was n o t found p o s s i b l e t o o b t a i n a

homogeneous d i s t r i b u t i o n of P t , Ru i n s i d e t h e A1203 p e l l e t s , when u s i n g a coimpregnat i o n procedure. The b i m e t a l l i c c a t a l y s t was t h e r e f o r e prepared a s follows : a ) imp r e g n a t i o n by t h e H2RuC1

6’-

t o e l i m i n a t e t h e excess C 1

HC1 s o l u t i o n (1.5

mole HCl/gA1203), b) water washing

i o n s , c ) impregnation by t h e H2PtC16 s o l u t i o n , w i t h o u t

f u r t h e r a d d i t i o n of H C 1 , d) washing and d r y i n g a s above.

2.2-. -Rzd_uc_tjoz While t h e s i l i c a supported c a t a l y s t s were reduced d i r e c t l y by hydrogen, two modes of r e d u c t i o n were used f o r t h e alumina supported c a t a l y s t s : a ) d i r e c t r e d u c t i o n by a hydrogen flow of t h e d r i e d c a t a l y s t s , 2 h r s a t 5OO0C, b) r e d u c t i o n by hydrogen a s i n a ) b u t f o l l o w i n g 2 h r s p r e - c a l c i n a t i o n i n a d r y a i r flow a t 5OO0C. Fiom l i t e r a t u r e d a t a (3) t h e f i r s t mode of r e d u c t i o n should r e s u l t i n a b e t t e r d i s p e r s i o n of Ru

in

Ru/A1203, w h i l e t h e second mode i s c l o s e r t o t h e u s u a l l y a p p l i e d i n d u s t r i a l condit i o n s of r e d u c t i o n .

199

3. 0 supported catalysts diffraction data for the unreduced A 1 2-2 3.1 * ~ ~ ~ ~ ~ ~ The results are reported in Fig. 1 . The spectra related to Ru are for : - the chlororuthenic acid impregnation solution (atomic overall Cl/Ru ratio Q 30)(Spect 1 )

-

the solid ammonium hexachlororuthenate(1V) (Fluka,puriss)(Spect 2) -the non hydro-

lyzed orange Ru/A1203 catalyst (Spect 3) and the hydrolyzed and dried green-dark Ru/ A1203 catalyst (Spect

4). The chemistry of the Ru chloride solutions is highly com-

plex (4) (5) and dimeric species are not excluded (6). Even the spectrum ( 2 ) for the

--

, according solid NH4 chlororuthenate may not be ascribed unambiguously to RuC16 to the data in (4) (5) and some oxygenated ligands are possibly present.

pbsorhnce

1*2Pbance bsorbance

1.2-:

0.8-

0.6.-

0.4

0.2

O

L3

r

0

--t

I

\

.‘ L-.. \

m in

Fig. 1 There is a strong analogy between the non-hydrolyzed Ru/A1203 (Spect 3) and the NH4 chlororuthenate (Spect 2 ) which means that Ru is in the same state in the two solids. Hydroxychlorospecies of Ru4+ like Ru(OH)2C14 2- or the dimeric species C15RuORuC154seem to be the most probable. Comparison of Spect 4 to Spect 3 suggests that the hydrolysis of Ru/A1203 gives rise to Ru4+ species with an increased ratio of oxygen containing ligands to C1 ligands. Spect 5 for the dried Pt/A1203 catalyst was identical to that for the H2PtC16, HC1 solution, of atomic ratio Cl/Pt

‘L

7.5. Comparison of Spect 6 for the bimetallic

~

200 d r i e d c a t a l y s t t o Spect 4 and 5 f o r t h e Ru and P t monometallic c a t a l y s t s r e s p e c t i v e l y shows s t r o n g evidence of a d d i t i v i t y . The b i m e t a l l i c c a t a l y s t behaves a t l e a s t i n a f i r s t approximation a s a mixture of t h e two monometallic c a t a l y s t s . 3.2

E_1le_cct_rgn_mLcrgpprob_e _an_aaly_sis-

F i g . 2 r e p o r t s t h e Ru, P t and C 1 p r o f i l e s i n t h e b i m e t a l l i c c a t a l y s t e i t h e r d r i e d a t lIO'C

( l e f t ) o r f u r t h e r c a l c i n e d i n a i r a t 5OO0C ( r i g h t ) (Subsequent r e d u c t i o n by

H2 d i d n o t change t h e p r o f i l e s ) . It i s shown t h a t t h e procedure followed i n 2 g i v e s

a r e a s o n a b l e macroscopic homogeneity of t h e Ru, P t , C 1 d i s t r i b u t i o n s i n s i d e t h e A1203 p e l l e t s .

Fig. 2 3 . 3 ,- ;-Ja;y_

~ i f f r , a . c t ~ o ~ a ~ ~ l y ~ i ~

It was n o t p o s s i b l e t o observe any d i f f r a c t i o n l i n e f o r P t , Ru o r ( P t , Ru) s p e c i e s ,

a l s o a f t e r c a l c i n a t i o n i n a i r of t h e A1203 supported c a t a l y s t s .

4 . Temperature Programmed Reduction of t h e c a t a l y s t s TPR c o n s i s t e d t o f o l l o w t h e v a r i a t i o n i n t h e thermal c o n d u c t i v i t y of a 3 Z H2 i n N2 mixture flowing through t h e c a t a l y s t bed a t a h e a t i n g r a t e

%

10°C/min. A l i q u i d

n i t r o g e n t r a p was included between t h e c a t a l y s t and t h e catharometer t o which t h e H2-N2

mixture was d i r e c t e d through an automatized v a l v e ( c a r r i e r gas : N 2 ) .

F i g . 3 r e p o r t s t h e r e d u c t i o n curves f o r t h e s i l i c a supported c a t a l y s t s . The red u c t i o n curve f o r P t / S i 0 2 shows s t r o n g analogy w i t h r e c e n t d a t a of J e n k i n s e t a 1 . ( 7 ) . The r e d u c t i o n of P t i n t h e b i m e t a l l i c c a t a l y s t i s s t r o n g l y i n h i b i t e d presumably

201

f!

H2'g*min

4""

:LPt

5

4-300

Fig. 3 because of the cocrystallization of the Pt and Ru chloride compounds during the drying step. The overall H2 consumption was about 95 % the value calculated for the reduc-

4+, Ru4+ to Pto, Ruo, in the three catalysts.

tion of Pt

Fig. 4 reports the reduction curves for the A1203 supported catalysts, dried at llO°C. The overall H2 consumption at 5OO0C (balance between the H2 consumption due to

reduction and the H2 evolution due to thermal desorption) corresponds to 95 % reduc4+ to Pto in Pt/A1203, but only to % 55 % reduction of Ru4+ to Ruo in Ru/ tion of Pt A1203. The uncomplete reduction of Ru in Ru/A1203 at 5OODC is corroborated by a further H2 consumption near 700'C

which corresponds to a 15 % increase (absolute value)

in the degree of reduction. The bimetallic catalyst does not show any H2 consumption in this temperature range, which suggests a

4+

0

100 % reduction of Ru to Ru

%

accordingly, the overall H2 consumption at 500'C

at 500'C

;

corresponds to a nearly complete re-

duction o f Pt and Ru. When a hydrogen flow (p = 1.2 atm) was used to reduce Ru/A1203 2 hrs at 5OO0C, the subsequent TPR experiment with the 3 % H2 in N2 mixture did not show any H2 consumption near 700°C, hence the % reduction of Ru at 500'C at least

%

was certainly

70 %.

Fig. 5 reports the TPR curves for the A1203 supported catalysts, calcined 2 hrs in dry air flow at 500'C.

The reduction curve of Pt/A1203 is not modified signifi-

cantly following precalcination in air, while the reduction of Ru/A1203 occurs then at a very low temperature. The % reduction of Ru/A1203 at 5OO0C in the conditions of the TPR is about 80 and the reduction wave at crease of

%

Q

7OO0C corresponds to a further in-

15 % (absolute value) in the degree of reduction. In the bimetallic cata-

lyst, the main reduction wave of Ru is shifted to a

%

60°C greater temperature

202

Fig. 5

203 compared t o t h e monometallic c a t a l y s t . The reason f o r t h i s i s not c l e a r , we suppose t h a t t h e f i r s t amounts of H20 i s s u e of t h e r e d u c t i o n of P t oxide s p e c i e s could i n h i b i t t h e r e d u c t i o n of Ru02. There i s no f u r t h e r H2 consumption a t 2, 7OO0C and t h e % 4+ r e d u c t i o n of ( P t , Ru4+) t o (Pt', Ruo) i s i n f a c t 2, 100 a t 5OO0C from t h e H2 consumption a t t h a t temperature.

5. X-Ray D i f f r a c t i o n and e l e c t r o n microscopy d a t a over t h e reduced c a t a l y s t s reduced 2 h r s i n H2 flow a t 700'C showed X-Ray d i f f r a c -

The (Pt+Ru)/Si02 c a t a l y s t

t i o n p a t t e r n s c h a r a c t e r i s t i c of t h e c f c s t r u c t u r e of P t ( P t / S i 0 2 ) o r hcp s t r u c t u r e of Ru (Ru/Si02). The b i m e t a l l i c c a t a l y s t showed a c f c s t r u c t u r e w i t h a l a t t i c e parameter a = 3.88 A. From t h e X-Ray d i f f r a c t i o n d a t a f o r bulk Pt-Ru a l l o y s (8), l u e of a corresponds t o a

%

40 a t

X Ru i n P t s o l i d s o l u t i o n ( t o compare t o

%

t h a t va49 a t

x

Ru, t h e composition d e r i v e d from chemical a n a l y s i s ) . The r e l a t i v e l y e a s y formation of Si02 supported Pt-Ru a l l o y s h a s a l r e a d y been r e p o r t e d (9) and a l s o t h e preparat i o n of unsupported Pt-Ru a l l o y powders (10). The m e t a l l i c p a r t i c l e s i n t h e above ca0

0

t a l y s t s were r a t h e r c o a r s e (& 300 A i n P t / S i 0 2 , 150 t o 300 A i n Ru/Si02, and v e r y broad p a r t i c l e s i z e d i s t r i b u t i o n i n (Pt+Ru)/Si02). The e l e c t r o n microscopy d a t a f o r t h e A1203 supported c a t a l y s t s a r e s u m a r i z e d i n Table I. I n agreement w i t h (3)(11) p r e c a l c i n a t i o n i n a i r of Ru/A1203 b e f o r e reduct i o n leads t o a considerable decrease i n t h e

X d i s p e r s i o n of Ru.

TABLE I

Catalyst (approximate composition)

Calcined i n a i r a t 500'C b e f o r e reduction

Type of p a r t i c l e size distribution (very s t r o n g l y bimodal o r n o t )

Mean

d

(t

Not Yes

Not Not

15 15

IXRu/A1203 IXRu/A1203

Not Yes

Not Not

< 10 1000

IXPt, 0.5XRu/A1203 IXPt, 0.5%Ru/A1203

Not Yes

Not Yes

20 (broad d i s t r i b u t i o n )

The p r e c a l c i n e d b i m e t a l l i c c a t a l y s t looked s t r o n g l y l i k e a m i x t u r e of t h e p r e c a l c i n e d P t and Ru monometallic c a t a l y s t s . No i n f o r m a t i o n could be drawn about t h e e x i s tence of P t and Ru a s a mixture of P t and o f Ru c l u s t e r s o r a s ( P t , Ru) b i m e t a l l i c c l u s t e r s , i n t h e b i m e t a l l i c c a t a l y s t reduced w i t h o u t p r e c a l c i n a t i o n .

6 . Temperature Programmed T i t r a t i o n by hydrogen of t h e oxygen p r e v i o u s l y chemisorbed a t 25°C on t h e reduced c a t a l y s t s The method d e r i v e s from p r e v i o u s work about t h e (Pt+Re)/A1203 (12) and ( P t + I r ) / A1203 (13) c a t a l y s t s . S t r o n g evidence was found t h e r e t h a t c l u s t e r i n g Re, Ir respect i v e l y , w i t h P t , r e s u l t s i n a s t r o n g i n c r e a s e i n t h e r e d u c i b i l i t y of t h e oxygen

204

chemisorbed by t h e Re ( o r I r ) exposed atoms ( t h e technique was microthermogravimet r y and t h e 02-H2 The hereunder

t i t r a t i o n s were c a r r i e d out a t

T,

25'0.

r e p o r t e d experiments were c a r r i e d o u t a s f o l l o w s . The samples were

reduced 2 h r s i n H2 flow a t 7OO0C (Si02 s u p p o r t ) o r 5OO0C (A1203 s u p p o r t ) , t h e n outgassed 2 h r s i n

He flow a t 5OO0C, b e f o r e c o o l i n g i n He down t o 25'C.

f r o n t a l a n a l y s i s w i t h a 1 Z O2 i n He mixture allowed then sample was t h e n purged I h r by A r a t 25'C,

Differential

t o chemisorb oxygen. The

b e f o r e t h e H2 t i t r a t i o n w i t h a 1 Z H2 i n

A r mixture. The H2 t i t r a t i o n was c a r r i e d o u t 20 min a t t h e room temperature a t f i r s t and t h e n a t i n c r e a s i n g temperature (lO°C/min) up t o 200°C. The oven was f i n a l l y removed, a l l o w i n g t h e c a t a l y s t t o c o o l t o room temDerafure under t h e H 2 / A r flow. F i g . 6 r e p o r t s t h e H2-TPT c u r v e s f o r t h e P t , Ru, and (Pt+Ru) s i l i c a supported cat a l y s t s . The t i t r a t i o n is complete a t 25OC over P t and only very l i t t l e i f any H2 consumption o r e v o l u t i o n occurs d u r i n g t h e subsequent h e a t i n g and c o o l i n g . On t h e o t h e r hand, t h e t i t r a t i o n i s only slow a t 25OC over Ru and i t t s k e s p l a c e ( l a r g e l y ) consumption peak d u r i n g c o o l i n g i s approximately 2 equal t o t h e H2 d e s o r p t i o n wave d u r i n g t h e p l a t e a u a t 200'C. The b i m e t a l l i c c a t a l y s t

d u r i n g t h e h e a t i n g , t h e small H

shows c o n s i d e r a b l e d e v i a t i o n from a d d i t i v e behaviour between P t and Ru s t u d i e d sep a r a t e l y ; most of t h e oxygen chemisorbed on Ru i s t i t r a t e d a t t h e room temperature and o n l y a small t i t r a t i o n wave i s observed when i n c r e a s i n g t h e temperature.

i

.a.

.. .. .. a

.

. .

1

0

Fig. 6

205

I t c o u l d be due t o a small % of u n a l l o y e d ruthenium, t h e p r e s e n c e of which was suggest e d by comparison of t h e e x p e r i m e n t a l l a t t i c e p a r a m e t e r t o t h e v a l u e c a l c u l a t e d i f

a l l of t h e Ru would be a l l o y e d w i t h P t . The l a r g e H2 consumption wave d u r i n g c o o l i n g , compared t o t h e monometallic c a t a l y s t s , would s u g g e s t a d e c r e a s e i n t h e mean hydrogen

metal bond s t r e n g t h due t o a l l o y i n g , b u t o t h e r phenomena l i k e oxido-redox p r o c e s s e s could a l s o be i n v o l v e d . F i g . 7 r e p o r t s t h e H2-TPT c u r v e s f o r t h e A1203 s u p p o r t e d c a t a l y s t s , reduced d i r e c t l y by H2. It i s shown t h a t : a ) t h e PtsO r e d u c t i o n t a k e s p l a c e a t t h e room temperat u r e , l i k e over P t / S i 0 2 , b ) none of t h e r e d u c t i o n of t h e oxygen chemisorbed on Ru/A1203 t a k e s p l a c e a t 25'C,

a t t h e d i f f e r e n c e of Ru/Si02. P a r t i c l e s i z e e f f e c t s

(Ru/A1203 i s much b e t t e r d i s p e r s e d t h a n Ru/Si02), p r o b a b l y a l s o s u p p o r t e f f e c t s , a r e i n v o l v e d , c ) t h e TPT c u r v e f o r t h e b i m e t a l l i c c a t a l y s t shows c o n s i d e r a b l e e v i d e n c e o f a d d i t i v i t y between P t and Ru s t u d i e d s e p a r a t e l y ; n o t e t h a t t h e b a s e l i n e i s p r a c t i c a l l y recovered a t t

'L

1000 s e c , t h a t i s between t h e two r e d u c t i o n waves.

Fig. 7 The room t e m p e r a t u r e wave i s moEe t h a n a h a l f t h e v a l u e f o r Pt/A1203 which s u g g e s t s t h a t P t i s somewhat b e t t e r d i s p e r s e d i n t h e b i m e t a l l i c c a t a l y s t . The h i g h t e m p e r a t u r e wave i s a l s o more t h a n a h a l f t h e v a l u e f o r Ru/A1203 which i s a c c o u n t e d f o r by t h e increase i n the

Z r e d u c t i o n of Ru.

F i g , 8 r e p o r t s t h e r e s u l t s f o r t h e P t , Ru and (Pt+Ru) alumina s u p p o r t e d c a t a l y s t s c a l c i n e d i n a i r b e f o r e r e d u c t i o n . The c u r v e f o r Ru/A1203 h a s o n l y a v e r y small area,

206 a s expected from t h e e l e c t r o n microscopy d a t a . On TPT curve f o r (Pt+Ru)/A1203 n o t e t h a t : a ) t h e H2 consumption when t Ru/A1203, b) t h e H2 consumption when t

2000 s e c i s approximately a h a l f t h e v a l u e f o r 2,

1000 s e c i s c l e a r l y above t h e v a l u e f o r t h e

two monometallic c a t a l y s t s , which s u g g e s t s t h a t a small p a r t of t h e Ru could be i n interaction with P t .

200

110

20

Fig. 8

7. I n f r a r e d spectroscopy of CO chemisorbed on t h e reduced A1203 supported c a t a l y s t This technique was found u s e f u l i n determining t h e s t a t e of P t and Re i n (Pt+Re)/ A1203 c a t a l y s t s ( 1 2 ) . The Ru/A1203 ( d i r e c t l y reduced form) and (Pt+Ru)/A1203 (reduced e i t h e r d i r e c t l y o r a f t e r c a l c i n a t i o n ) c a t a l y s t s were cooled down i n H2 from 500 t o 25’C. H was then evacuated a t RT and t h e chemisorbed H d i s p l a c e d by CO (p = 50 T o r r ) . 2 The s p e c t r a over t h e two b i m e t a l l i c c a t a l y s t s , f o l l o w i n g e v a c u a t i o n of gaseous CO ,

are reported i n Fig. 9 A ( a : d i r e c t reduction ; b : reduction a f t e r calcination). The r a t i o of t h e o p t i c a l d e n s i t i e s of a t o b i s 1 . 7 , which s u p p o r t s t h e above conc l u s i o n of a l a r g e r number of (Pt+Ru) atoms exposed, a f t e r d i r e c t r e d u c t i o n than a f t e r r e d u c t i o n following c a l c i n a t i o n . The s p e c t r a A do n o t show any evidence of a band a t 2 1 4 0 cm-I which was observed over t h e d i r e c t l y reduced form of Ru/A1203,

207 corresponding t o GO adsorbed on oxidized s p e c i e s of Ru. This i s i n agreement-with t h e above c o n c l u s i o n of a more complete r e d u c t i o n of Ru i n t h e b i m e t a l l i c c a t a l y s t s than i n t h e d i r e c t l y reduced form of t h e monometallic Ru c a t a l y s t . The s p e c t r a A

@

I

2115

crn - 1

2 200

2 000

2200

2000

Fig. 9 f i n a l l y s u g g e s t s o v e r l a p p i n g of t h e CO s p e c i e s adsorbed on P t and Ru r e s p e c t i v e l y , a phenomenon

which w i l l b e d i s c u s s e d s e p a r a t e l y i n as much a s t h e i n t e r a c t i o n of CO

w i t h Ru monometallic c a t a l y s t s i s r a t h e r complex (14). The low v(C0) (2050 cm-I)

frequencies

compared t o t h e v a l u e f o r GO adsorbed on b a r e platinum (2075 cm-'1 men-

t i o n e d i n (12) i s a s c r i b e d t o t h e e f f e c t of adsorbed w a t e r i n t h e p r e s e n t experiments; t h i s e f f e c t of H20 on t h e v ( C 0 ) f r e q u e n c i e s f o r CO adsorbed on P t has been r e p o r t e d previously ( I 5 )

.

A f t e r r e c o r d i n g t h e s p e c t r a A, t h e c a t a l y s t s were submitted t o o x i d a t i o n by O 2 a t 150'C i n o r d e r t o burn out t h e chemisorbed GO. They were t h e n cooled i n O2 a t RT, O2

was evacuated and CO was chemisorbed again. CO was h e r e i n t e r a c t i n g w i t h a oxygen covered metal s u r f a c e and g i v i n g r i s e t o t h e s p e c t r a B. The main p o i n t i s t h a t t h e

e0

p r e c a l c i n e d c a t a l y s t (Spect b) shows a pure P t c a t a l y s t behaviour ( t h e band a t 21 I5 cm-I corresponds t o P t

s p e c i e s (12)) w h i l e t h e d i r e c t l y reduced form

(Spect a) shows a l i k e a d d i t i v e behaviour between P t and w e l l d i s p e r s e d Ru monometall i c c a t a l y s t s ( t h e bands a t 2075 and 2015 cm-' were observed on Ru/A1203 i n t h e same c o n d i t i o n s ) . This a g a i n does not c o n f l i c t w i t h t h e above c o n c l u s i o n t h a t t h e d i r e c t l y

208

reduced (Pt+Ru)/A1203 c o n s i s t s i n a mixture of small P t and Ru p a r t i c l e s while t h e p r e c a l c i n e d c a t a l y s t would mainly c o n s i s t i n a mixture of w e l l d i s p e r s e d P t and c o a r s e Ru p a r t i c l e s . DISCUSSION AND CONCLUSION There i s a c o n s i d e r a b l e evidence from s e v e r a l t e c h n i q u e s , i n p a r t i c u l a r t h e hydrogen p r o g r a m e d t i t r a t i o n of chemisorbed oxygen t h a t we d i d n o t succeed i n p r e p a r i n g w e l l d i s p e r s e d and homogeneous i n composition (Pt,Ru) b i m e t a l l i c c l u s t e r p a r t i c l e s on t h e y A1203 s u p p o r t . R e f e r r i n g t o our previous s t u d i e s on t h e (Pt+Re o r I r ) / A 1 0 c a t a l y s t s (12)(13), i t follows t h a t t h e (Pt,Ru) p a i r of elements behaves more 2 3 l i k e t h e ( P t , I r ) p a i r r a t h e r than t h e (Pt,Re) p a i r . Nevertheless Ru i s more sens i t i v e than Ir t o s i n t e r i n g due t o h e a t t r e a t m e n t i n an o x i d i z i n g atmosphere. The h e t e r o g e n e i t y of t h e p r e c a l c i n e d (Pt+Ru)/A1203 c a t a l y s t could be connected t o t h e degregation of most of t h e Ru a s l a r g e Ru02 p a r t i c l e s during t h e c a l c i n a t i o n ; t h e P t oxide o r oxychloride s p e c i e s do n o t co-agglomerate w i t h Ru a s (Pt+Ru) mixed oxide p a r t i c l e s . The h e t e r o g e n e i t y of t h e d i r e c t l y reduced (Pt+Ru)/A1203 c a t a l y s t may n o t be a s c r i bed t o s e g r e g a t i o n d u r i n g t h e r e d u c t i o n s t e p s i n c e t h e r e a r e only minute d i f f e r e n c e s i n t h e temperature of r e d u c t i o n of t h e two a c t i v e elements s p e c i e s ( s e e F i g . 4). Ther e f o r e , we conclude t h a t a microscopic h e t e r o g e n e i t y i n t h e r e p a r t i t i o n of P t and Ru on t h e A1203 s u r f a c e p r e e x i s t s

b e f o r e t h e r e d u c t i o n . Improvement i n t h e homogeneity

of t h e b i m e t a l l i c phase needs, t h e r e f o r e ,

f u r t h e r s t u d i e s i n order t o obtain a

more homogeneous microscopic d i s t r i b u t i o n of P t and Ru a t t h e end of t h e impregnation procedure. The q u a n t i t a t i v e a n a l y s i s of t h e Temperature P r o g r a m e d T i t r a t i o n d a t a i n terms of c o n c e n t r a t i o n s i n exposed P t and Ru atoms may be done on t h e b a s i s of 1 0 chemisorbed per. P t

S

(12) and of 2 Ochemisorbed p e r Rus ( 1 6 ) . The r e s u l t s were compatible w i t h t h e

e l e c t r o n microscopy d a t a i n Table I and a r e n o t r e p o r t e d h e r e f o r a purpose of conc i s i o n . F i n a l l y , l e t us n o t e t h a t t h e TPT technique i s n o t confined t o platinum based b i m e t a l l i c c a t a l y s t s . It may be u s e f u l every time t h e d i f f e r e n c e i n r e d u c i b i l i t y

of

t h e oxygen chemisorbed on t h e two component elements ( a t l e a s t one of which b u t even t h e two may a c t i v a t e hydrogen) i s s u f f i c i e n t l y s i g n i f i c a n t . It i s e s p e c i a l l y u s e f u l t o d i s c r i m i n a t e between s e p a r a t e p a r t i c l e s of t h e two elements and b i m e t a l l i c c l u s t e r p a r t i c l e s , a t a s k which i s w e l l known t o be n o t e v i d e n t e s p e c i a l l y over w e l l dispersed catalysts (17).

Of c o u r s e , t h e complementary use of o t h e r techniques,when i t i s pos-

s i b l e , i s very p r o f i t a b l e ; i t i s t h e reason why we make a p l u r i - t e c h n i q u e approach of our problem. Note : A f t e r the paper being f i n i s h e d , we l e a r n t from t h e s u p p l i e r t h a t t h e "ammonium hexachlororuthenat" used f o r Fig. 1 , Spect 2 was i n f a c t ammonium -oxo-

decachlorodi-

r u t h e n a t (IV), (NH 4 4 (Ru20C1,0), i n agreement with t h e presence of oxygen c o n t a i n i n g

209

dimeric species considered above. ACKNOWLEDGMENTS

The authors kindly acknowledge Mrs. M.T.

Gimenez, C. Leclercq and H. Praliaud f o r

help i n the X-Ray D i f f r a c t i o n , e l e c t r o n microscopy, U.V.

spectroscopy work r e s p e c t i -

vely. REFERENCES 1

2

G. Martino, J . Miquel and P. Duhaut, F r . P a t e n t 2.234.924, 24 Janv, 1975, (CA80, 20080 D). S.D. Robertson, B.D. McNicol, J.H. De Baas and S.C. Kloet, J. Catal. 37 (1975)

424-31. 3 4 5 6 7 8 9 10 11

12 13 14 15 16 17

R.A. Dalla B e t t a , J. Catal. 34 (1974) 57-60. Chr.K. Jdrgensen, Acta Chem. Scand. 10 (1956) 518-34. Chr.K. Jdrgensen, Molecular Phys. 2 (1959) 309-332. I . V . Prokoficva and N.V. Fedorenko, J. Inorg. K h i m 13 (1968) 1348-53. J . W . Jenkins, B.D. McNicol and S.D. Robertson, Chem. Tech. (1977) 316-20. N.W. Ageev and V.G. Kuznetsov, Izv. Akad. Nauk SSR l(1937) 753. M.F. Brown and R.D. Gonzalez, J. C a t a l . 48 (1977) 292-301. S. Engels, Nguyen-Phuong-KhuS and M. Wilde, 2. Chem. 16 (1976) 455-57. C.A. Clausen I11 and M.L. Good, J. Catal. 38 (1975) 92-100. C. Bolivar, H. Charcosset, R. F r l t y , M. Primet, L. Tournayan, C. Betizeau, G. Leclercq and R. Maurel, J. Catal. 45 (1976) 163-78. L. Tournayan, J. Barbier, H. Charcosset, R. F r l t y , C. Leclercq, G. Leclercq and P. T u r l i e r , Thermochimica Acta, i n p r e s s . R.A. Dalla B e t t a , J. Phys. Chem., 80 (1975) 2519-25. M.F. Brown and R.D. Gonzalez, J. Phys. Chem. 80 (1976) 1731-35. A.A. Davydov and A.T. B e l l , J . C a t a l . , 49 (1977) 332-44. M. Primet, J . M . Basset, M.V. Mathieu and M. P r e t t r e , J. Catal. 29 (1973) 213-23. H. Kubicka, Reaction Kinetics and C a t a l . L e t t e r s , 5 (1976) 223-28. J . H . S i n f e l t , Accounts of Chemical Research 10 (1977) 15-20.

DISCUSSION N.

PERNICONE : C o n c e r n i n g y o u r TPR d a t a , you h a v e p e r f o r m e d a

q u a n t i t a t i v e m e a s u r e m e n t o f t h e a m o u n t o f H 2 consumed i n t h e v a r i o u s reduction steps.

Y o u know t h a t t h e r e s p o n s e o f t h e r m a l c o n d u c t i v i t y

c e l l s is v e r y s e n s i t i v e t o ambient c o n d i t i o n s ( t e m p e r a t u i e , p r e s s u r e , gas flow r a t e , e t c . ) .

Have y o u e s t i m a t e d t h e a c c u r a c y o f y o u r

m e a s u r e m e n t s a n d w h i c h e x p e d i e n t s h a v e you a d o p t e d t o i m p r o v e i t ?

G.

-+

BLANCHARD

:

The a c c u r a c y o f o u r m e a s u r e m e n t s i s e s t i m a t e d t o b e

5 % f o r t h e volume o f h y d r o g e n consumed i n t h e r a n g e o f m a g n i t u d e o f

2 cm3 N T P .

The m o s t u s e f u l e x p e d i e n t t o i m p r o v e t h e a c c u r a c y h a s

been t h e u s e of an e l e c t r o n i c g a s flow r e g u l a t o r .

Of c o u r s e f u r t h e r

v e r i f i c a t i o n has been c a r r i e d o u t , t h a t i s : ( i ) absence of any base l i n e d r i f t during blank experiments,

( i i ) p r o p o r t i o n a l i t y of

the response signal of the detector t o the gas concentrations i n the concentration range considered.

N e v e r t h e l e s s a drawback o f t h e TPR

e x p e r i m e n t s s h o u l d b e n o t o m i t t e d : t h e r e d u c t i o n is c a r r i e d o u t under

210 a much lower H2 pressure than the atmospheric one.

This dis-

advantage may be overcome to some extent by carrying out the reduction under 1 atm. pressure at first, before drawing the TPR curve which is then relevant of the reduction of the unreduced part of the active species in the normal conditions of 1 atm. pressure of hydrogen. C.J.

WRIGHT

:

Am I correct in thinking that none of the evidence

you put forward could unequivocably differentiate between small particles of Pt/Ru alloy

-

and platinum and ruthenium particles in

close proximity. G. BLANCHARD

:

We think to be able to differentiate unequivocably

between two situations where the first one should correspond to small alloy particles and the second one to a mixture of small particles of pure P t and of pure Ru.

We do not think that a situation where

pure P t and pure Ru particles would be in close contact with each other is likely to occur.

With respect to the sintering of mono-

metallic catalysts it is accepted that as soon as two small metallic particles come in close contact, they co-agglomerate into a single particle at rather low temperatures.

It should be considered that

our catalysts were heat treated in hydrogen at 50OOC; interdiffusion of very small RU and P t particles would be very rapid if these particles would come in close contact.

Therefore in our catalysts

the P t and the RU particles are most likely to be separated by some positive, but unknown distance. R. MILES

:

I would like to make some comments with regard to the

technique of temperature-programmed reduction (TPR).

Firstly,

McNicol et al. have observed that, as in the case of a Si02 support, alloy formation is readily achieved in bimetallic Pt-Ru supported on graphitic carbon, as Bhown by TPR.

mixtures

Secondly, they have

found that the ease of reducibility as determined by the temperature of the maximum in the hydrogen depletion rate, is largely governed by such factors as the support interaction, oxidation state and crystallite size of the metal catalyst.

Furthermore, he has shown

that more information concerning the detailed nature of the platinum component in such systems can be gained if the thermal scan is commenced at low temperature, e.g. about 7 7 K.

Have you investigated

the TPR behaviour of your catalyst at low temperature ? H. CHARCOSSET : We are well aware of the TPR studies of McNicol et al. performed on various metallic catalysts [Ref. 2 cited as example in the present paper).

We have not yet investigated our catalysts in

the subambient temperature region.

211

P R E P A R A T I O N O F CATALYSTS B Y ADSORPTION OF METAL COMPLEXES O N MINERAL

J. P.

OXIDES::

BRUNELLE

P r o c a t a l y s e , C e n t r e d e Recherches Rh6ne-Poulenc,

Aubervilliers, France

ABSTRACT The p r e p a r a t i o n o f d i s p e r s e d m e t a l s u p p o r t e d c a t a l y s t s b y a d s o r p t i o n o f m e t a l c o m p l e x e s o n o x i d e s h a s l e d u s t o a n a l y z e t h e phenomena o c c u r ring a t t h e interface oxide-solution.

T h i s a n a l y s i s i s b a s e d on s i m p l e

p r i n c i p l e s s u c h a s s u r f a c e p o l a r i z a t i o n o f o x i d e s v e r s u s pH a n d a d s o r p t i o n of c o u n t e r i o n s by e l e c t r o s t a t i c a t t r a c t i o n . T h e t h r e e m o s t i m p o r t a n t p a r a m e t e r s w h i c h seem t o r u l e t h e a d s o r p t i o n phenomena a r e :

i s o e l e c t r i c p o i n t o f t h e o x i d e , pH o f t h e a q u e o u s

s o l u t i o n , a n d n a t u r e of

t h e m e t a l complex.

T h i s s i m p l i f i e d a p p r o a c h t o w a r d a d s o r p t i o n phenomena i s i n a g r e e m e n t w i t h t h e r e s u l t s i n l i t e r a t u r e c o n c e r n i n g t h e f i x a t i o n on a l u m i n a o r s i l i c a c a r r i e r s of

c h l o r o m e t a l l i c and m i n e

complexes of m e t a l s belonging

t o ? a , 0 and l b g r o u p s . An e x t e n s i o n o f

1.

t h i s a n a l y s i s t o o t h e r m i n e r a l o x i d e s is p r o p o s e d .

I M P O R T A N C E O F ME T AL L IC CATALYSTS

The i m p o r t a n c e o f m e t a l - b a s e d

c a t a l y s t s i n o u r economic system need

c e r t a i n l y no l o n g e r b e d e m o n s t r a t e d . rent fields as o i l refining, cal industries.

A

They a r e i n v o l v e d i n s u c h d i f f e -

automobile,

p e t r o c h e m i c a l and f i n e chemi-

l i s t of t h e p r i n c i p a l p r o c e s s e s o p e r a t i n g w i t h metal-

b a s e d c a t a l y s t s w i l l b e f o u n d i n t a b l e 1. The p r o p e r t i e s o f t h e s e c a t a l y s t s i n g e n e r a l and t h e i r a c t i v i t y i n p a r t i c u l a r a r e c l o s e l y r e l a t e d t o t h e s t a t e of d i s p e r s i o n of e l e m e n t s . T h i s e x p l a i n s why t h r e e - f o u r t h s

the active

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

b l e 1 u s e c a t a l y t i c systems i n which t h e a c t i v e phase

consists i n very

s m a l l c r y s t a l l i t e s o f a b o u t t e n a n g s t r c i m s d i s p e r s e d on t h e s u r f a c e o f a support.

I t i s w h a t we u s u a l l y c a l l d i s p e r s e d m e t a l

catalysts.

The m e t a l s u s e d i n t h e s e c a t a l y s t s b e l o n g g e n e r a l l y t o g r o u p s 7 a , 8 and l b o f t h e p e r i o d i c t a b l e , v e r y o f t e n s u c h a s p l a t i n u m and p a l l a d i u m . F0u.r m a i n s u p p o r t s a r e u s e d i n t h e i r p r e p a r a t i o n : mina,

alumina,

a c t i v e c a r b o n and m o l e c u l a r s i e v e s .

:: P u b l i s h e d i n P u r e A p p l .

Chem.,

50,

9-10

(1978),

p.

1211.

silica-alu-

212 TABLE 1

Metal catalysts used in o i l refining, automobile, petrochemical and fine chemical industries. TYPE OF CATALYST PROCESS

METAL

CARRIER

Oxydation catalysts

Pt+Pd

Alumina or Alumina coated

Three-way catalysts

PttRh

Cordierite

, Auto-exhaust gases Post-combustion

. Selective hydrogenation of : Olefins streams in ethylene plants Pyrolysis gasoline in ethylene plants B

. .

Catalytic Reforming Hydrocracking Isomerization of : Paraffins Xylenes

. . ~~

Dismutation o f Toluene Fine organical chemistry Off-gas Treatments Fuel Cells

Pd Pd Pt +Re,Ir,Au.. Pd

Pt Pt Cu, Ni Pt,Rh,Pd,Ru Ni Raney Pt ,Pd Pt ,Pd

-~

Ammonia oxidation Selective oxidation o f ethylene in ethylene oxide Selective oxidation of methanol in formol

t

Alumina

Alumina

+

C1

Y zeolite based Alumina + C1 or mordenite Alumina + C1 or Alumina-Silica Zeolite Charcoal Alumina Charcoal

a Alumina Carborundum

It is to be noted that the Raney nicke1,which is still used nowadays in several liquid phase hydrogenation processes,constitutes a particular case of a dispersed metallic catalyst which is non supported. If we consider the tonnage of catalyst involved, the most important processes are catalytic reforming, hydrogenation of different petrochemical streams,and above all, automobile post-combustion, which represents the greatest turnover for the industry.

213 I n fact, o n l y t h r e e important p r o c e s s e s use m e t a l c a t a l y s t s that d o not r e q u i r e a h i g h d i s p e r s i o n o f t h e a c t i v e element: 1 ) a m m o n i a o x i d a t i o n t o m a k e nitric acid o n a platinum-rhodium w i r e c a t a l y s t , 2 ) m e t h a n o l sel e c t i v e o x i d a t i o n into f o r m o l and 3) e t h y l e n e s e l e c t i v e o x i d a t i o n i n t o e t h y l e n e oxide. Silver-based c a t a l y s t s are used in the latter t w o processes.

2. METHOD OF P R E P A R A T I O N OF D I S P E R S E D M E T A L C A T A L Y S T S S e v e r a l s t u d i e s (1-6) have shown the interest of using t h e a d s o r p t i o n o r e x c h a n g e of c o m p l e x e s with some s u r f a c e s i t e s of m i n e r a l s u p p o r t s t o o b t a i n highly l o a d e d m e t a l surfaces. I t should b e a p p a r e n t t h a t : - T h e r e d u c t i o n o f m e t a l ions adsorbed o n t h e s u r f a c e o f t h e support m u s t lead i n i t i a l l y t o an a t o m i c d e p o s i t of t h e metal.The w h o l e p r o blem c o n s i s t s in p r e s e r v i n g a d i s p e r s i o n of t h e m e t a l l i c d e p o s i t as c l o s e as p o s s i b l e t o i t s initial s t a t e , t h u s avoiding t o o g r e a t a c r y s t a l l i z a t i o n o f t h e m e t a l d u r i n g t h e catalyst

activation.

-On t h e o t h e r h a n d , if t h e i m p r e g n a t i o n i s m a d e in c o n d i t i o n w h e r e m e t a l l i c p r e c u r s o r s d o not fix o n t h e s u r f a c e of t h e s u p p o r t , t h e s e p r e c u r s o r s a r e g o i n g t o d e p o s i t according t o a p r o c e s s of c r y s t a l l i z a tion, p r e c i p i t a t i o n o r d e c o m p o s i t i o n d u r i n g t h e drying step.

The

s i z e o f t h e c r y s t a l l i t e s d e p e n d s on a g r e a t n u m b e r of p a r a m e t e r s such as s u p p o r t t e x t u r e , p r e c u r s o r s o l u b i l i t y and d r y i n g velocity.

In t h i s

0

c a s e , c r y s t a l l i t e s s m a l l e r t h a n 50 A a r e r a r e l y obtained. F o r i n s t a n c e , f i g u r e 1 e n a b l e s t h e c o m p a r i s o n o f t h e m e a n crystallite s i z e o f t w o s e r i e s o f platinum-silica

c a t a l y s t s w i t h v a r i a b l e percentage

o f platinum and p r e p a r e d : - e i t h e r b y a c o n v e n t i o n a l m e t h o d of i m p r e g n a t i o n w i t h o u t e x c h a n g e from a c h l o r o p l a t i n i c acid solution. -or by tetramine-platinum

I 1 c a t i o n - e x c h a n g e w i t h ammonia-polarized

s u r f a c e s i t e s o f silica. T h e m e a n c r y s t a l l i t e s i z e of c a t a l y s t s p r e p a r e d by t h e c o n v e n t i o n a l m e t h o d v a r i e s b e t w e e n 60 and 170

I,w h i l e

the cation exchange method 0

leads t o a m e a n p l a t i n u m c r y s t a l l i t e s i z e of about 10 t o 20 A , f o r a platinum content v a r y i n g b e t w e e n 0.4 and 5.5 3.

wt%.

A D S O R P T I O N OF I O N C O M P L E X E S ON T H E S U R F A C E OF M I N E R A L O X I D E S A l t h o u g h s e v e r a l s t u d i e s have s h o w n t h e p r a c t i c a l i n t e r e s t o f using

a d s o r p t i o n and e x c h a n g e o f m e t a l c o m p l e x e s on m i n e r a l o x i d e s , f e w stud i e s in f a c t h a v e b e e n carried o u t o n t h i s subject. S o m e of them d e a l w i t h t h e a d s o r p t i o n o f platinum

(1,2,4,6,13), palladium

(3,5,11,13),

c o b a l t , n i c k e l , o r c o p p e r (13,14) c a t i o n i c c o m p l e x e s o n t h e s u r f a c e of

s i l i c a or a l u m i n a supports. S o m e others t r e a t t h e a d s o r p t i o n o f anionic c o m p l e x e s o f platinum

(7-10, 1 2 ,

131, palladium

( 5 , 1 1 ) or o t h e r pre-

c i o u s m e t a l s such a s g o l d , r h o d i u m , r u t h e n i u m , iridium

( 1 2 , 1 3 1 o n alu-

mina.

Fig.

1.

-

Variation of the mean crystallite size vs metal content of Pt2 of e i l i c a s u p p o r t 260 m / g .

S i 0 2 catalysts. S p e c i f i c s u r f a c e area

A c t i v a t i o n of c a t a l y s t s : d r y i n g i n air at 1 2 0 ' C

for 16 h r and r e d u c t i o n

in h y d r o g e n at 4 1 0 - C f o r 6 hr.

O u r p u r p o s e i s not t o d e s c r i b e the r e s u l t s obtained from t h e s e d i f f e r e n t s t u d i e s b u t t o treat g e n e r a l l y t h e a d s o r p t i o n o f m e t a l c o m p l e x e s and t h e e x c h a n g e p h e n o m e n a o n t h e s u r f a c e of m i n e r a l oxides. I n using s i m p l e p r i n c i p l e s s u c h a s s u r f a c e p o l a r i z a t i o n of an o x i d e as f u n c t i o n o f p H and a d s o r p t i o n of c o m p l e x ions by e l e c t r o s t a t i c a t t r a c t i o n , w e s h a l l t r y t o e x t r i c a t e t h e m o s t important t h e r m o d y n a m i c p a r a m e t e r s w h i c h seem t o c o n t r o l t h e a d s o r p t i o n p h e n o m e n a , and t o suggest t h e b a s i c p r i n c i p l e s for t h e choice of t h e s e parameters.

215 On the other hand, the problem of fixation of metallic hydrolyzable cations ( 1 5 - 4 1 )

will not be treated. This problem, though it has often

been studied, does not yet seem to be clearly explained as the conclusions are sometimes contradictory. The most common mechanism is hydrolytic adsorption of hydrolyzable cations. This mechanism, which certainly deserves to be carefully studied, involves a hydrolysis and not an adsorption of the cation on the oxide. We shall also restrict our analysis to the phenomena which occur during impregnation at the interface oxide-solution without taking into account the other phenomena which may take place afterwards during activation (reaction of the complex with the support, partial or total decomposition of the complex

...) .

Finally, we will ignore the kinetic aspect of these phenomena, even if it is significant.

3.1. ISOELECTRIC POINT OF AN OXIDE A particle of a mineral oxide in suspension in an aqueous solution tends to polarize and to be electrically charged. Most oxides are amphoteric. Thus,the nature and importance of this charge are a function of pH of the solution surrounding the particle. For example, in an acid medium,the particle is positively charged. The principle of electroneutrality implies the presence of a layer of ions with opposite charge near this particle,the two electric charges compensating each other. If we consider a schema in agreement with the G O U Y theory,counteranions will thus be located around the particle in a

thin diffuse layer, as shown in figure 2. Schematically, the equation of the surface polarization may be written

:

S-OH

+ H'A-

2

+ -

S-OH2A

where S-OH represents a surface adsorption site and H

+A -

a mineral acid.

In a basic medium, the reverse is true. The particle is negatively charged and is surrounded by compensating cations. The equation of the surface polarization may be written as follows S-OH

where B'OH-

+

B'OH-

'

2

S-O-B+

+

:

H20

represents a base.

One conceives easily that between these two cases, a given value of pH exists at which the overall charge of the particle is zero. This value, which is a characteristic of the oxide, corresponds to its zero point of charge (Z.P.C.) or its isoelectric point (I.E.P.S.)

.

216

irrelectric erirt aeir pH

Fig. 2 .

or

zari !lint rf clarle

basic pH

Schematic representation of the surface polarization of an

oxide particule as a function of the solution pH. Let us now examine two methods allowing us to estimate the nature and importance of the surface polarization of an oxide as a function of pH of the solution in which it is dipped.

3.1.1.

Electrophoresis

Electrophoresis techniques enable us to measure the velocity of a charged particle in suspension placed in an electrical field. The electrophoretic velocity is proportional to the potential difference existing between the opposites areas of the double layer (Zeta potential). For this reason it is possible to determine experimentally the sign and the importance of the polarization of a given oxide particle. In order to illustrate this method, we have shown in figure 3 four curves of Zeta potential versus pH corresponding to four different products: a silica gel, a neodyme hydroxide gel, a titania gel and a gibbsite gel ( 4 2 ) .

217

Fig. 3 . Various types of Zeta potential curves vs pH obtained by electrophoresis. The following informations can be drawn out of these curves -The I.E.P.S.

:

of silica, which corresponds to the cross-over point of

Zeta potential curve and pH-axis, is very low, around pH 1. This result indicates the acid and non-amphoteric type of this oxide.

A

negative

polarization of its surface occurs at pH higher than 1 (negative zeta potential) but becomes significant only above pH 7 . In other words, silica may only adsorb cations, and this phenomenon i s important only above pH 7 . -Neodyme hydroxide has a quite opposite behaviour compared to that of silica.Its I.E.P.S. is around pH 12,which is in agreement with the basic type of this hydroxide. Its surface polarizes positively below pH 1 2 , which makes possible the adsorption of anions on its surface. -Titania and gibbsite gels correspond typically to amphoteric compounds which results in an inversion of the polarization sign when passing from acid to basic medium. Their I.E.P.S. are respectively equal to 5.5 and 7 . 5 .

It i s then possible to adsorb cations or anions if the pH of

the solution is higher or lower than the I.E.P.S. of these two amphoteric gels.

218 3.1.2.

N e u t r a l i z a t i o n at c o n s t a n t p H

T h e second m e t h o d w h i c h p e r m i t s t o f o l l o w t h e p o l a r i z a t i o n o f an o x i d e s u r f a c e c o n s i s t s in m e a s u r i n g the capacity of a d s o r p t i o n o f t h e o x i d e s at constant p H

(2).

T h e p r o c e d u r e i s as follows: an o x i d e i s dipped i n t o an a q u e o u s sol u t i o n , w h i c h pH i s m a i n t a i n e d c o n s t a n t by using a m m o n i a o r m o n o c h l o r a c e t i c acid. T h e b a s i c o r acid q u a n t i t i e s delivered t o m a i n t a i n t h e pH at a g i v e n value are r e c o r d e d a s a f u n c t i o n of time. T h e s e q u a n t i t i e s c o r r e c t e d w i t h t h e b l a n k experiment, c o r r e s p o n d t o t h e q u a n t i t i e s neut r a l i z e d by t h e support. If t h e support d o e s not d i s s o l v e , t h e y a l s o c o r r e s p o n d t o t h e q u a n t i t i e s of ions adsorbed o n t h e s u p p o r t surface.

Fig. 4 s h o w s t h e a m o u n t s o f ammonium o r m o n o c h l o r a c e t a t e a n i o n s adsorbed o n g a m m a a l u m i n a and s i l i c a as a f u n c t i o n of pH. T h e s e r e s u l t s , obt a i n e d according t o the m e t h o d d e s c r i b e d above, lead t o t h e s a m e conc l u s i o n s as t h o s e of e l e c t r o p h o r e s i s : - A l u m i n a , d u e t o its a m p h o t e r i c p r o p e r t i e s , a d s o r b s ammonium c a t i o n s and m o n o c h l o r o a c e t a t e a n i o n s at p H r e s p e c t i v e l y h i g h e r and l o w e r t h a n 8. T h e p a r t i c u l a r p o i n t at p H 8 c o r r e l a t e s w e l l w i t h t h e I . E . P . S . s i t e in f i g u r e 3 and w i t h t h e s e v e r a l I.E.P.S.

of gibb-

m e a s u r e m e n t s of a l u m i n a

f o u n d in l i t e r a t u r e (43). - S i l i c a a d s o r b s ammonium at p H h i g h e r t h a n 6 b u t d o e s not a d s o r b monoc h l o r o a c e t a t e anions in t h e p H r a n g e investigated. Fig. 4. A m m o n i u m and m o n o c h l o r a c e t a te adsorption on y-alumina or silica supports as a function of pH 2

( 0 , V ) : y a l u m i n a o f 190 m / g 2 (o,v): S i l i c a o f 160 m /g g o f s u p p o r t + 100 m l of a 0.02 M s o l u t i o n of C H 2 C l C 0 0 N a ; a d d i t i o n of a 0.1 M s o l u t i o n of C H 2 C l C O O H

( 0 ~ 0 )5 :

0.4

(V,V) :

03 ALUMINA

0.P SILICA 0.1

0 4

5 g of support + 100 ml of a 0.02 M s o l u t i o n of NH4N03 ; a d d i t i o n o f a 0.1 M s o l u t i o n of ammonia

219 I n summary, a p a r t i c l e of o x i d e d i p p i n g in a s o l u t i o n at a p H l o w e r t h a n i t s I.E.P.S.

t e n d s t o p o l a r i z e p o s i t i v e l y and t o adsorb compensa-

t i n g anions. O n t h e c o n t r a r y , t h e s a m e p a r t i c l e d i p p i n g in a s o l u t i o n a t a p H h i g h e r t h a n the I.E.P.S.

g e t s a n e g a t i v e surface c h a r g e w h i c h

i s c o m p e n s a t e d b y a d s o r b e d cations. T h u s , t h r e e p a r a m e t e r s seem t o b e important: t h e i s o e l e c t r i c point of t h e oxide, t h e pH o f t h e impregnat i n g solution and t h e c h a r g e o f t h e ion t o b e adsorbed.

3.2.

ANIONIC AND CATIONIC METAL COMPLEXES

Our p r a c t i c a l aim i s t o p r e p a r e s u p p o r t e d m e t a l c a t a l y s t s b y adsorption. W e are now g o i n g t o s p e c i f y t h e form in w h i c h t h e s e m e t a l

com-

p l e x e s may occur. W e c a n u s e t w o well-known f a m i l i e s of c o m p l e x e s c o n c e r n i n g a n u m b e r o f m e t a l s from g r o u p 7 a , 8 and lb. - T h e f a m i l y o f c h l o r o m e t a l l i c c o m p l e x e s (MC1n)X- s h o w n i n t a b l e 2.

TABLE 2 A n i o n i c c o m p l e x e s o f m e t a l s of 7 a , 8 and lb groups.

1

Tc

I

Ru

IK '

1

RhClZ- PdCI42-

-

Ir

Pt

Au

lrCI6*'

PtClt-

AuCI;

Re 0 s ReOQ

OsClz-

I n t h i s c a s e , t h e m e t a l i s in t h e form o f a n a n i o n i c c o m p l e x in w h i c h the c o o r d i n a t i o n s p h e r e i s c o n s t i t u e d b y f o u r or s i x c h l o r i n e atoms, W e h a v e t f o r instance, t e t r a c h l o r o a u r a t e (AuC14)-,hexachloroplatinate (PtC16)2 - , h e x a c h l o r o i r i d a t e (IrC16)2- ,t e t r a c h l o r o p a l l a d a t e (PdC14)2 - , h e x a c h l o r h o d a t e (RhC16)

3-

and h e x a c h l o r o o s m a t e

(OsC16)

a n i o n s , w e can add p e r o x i d e a n i o n s s u c h a s p e r r h e n a t e

2-

a n i o n s .To these

(Re04)- and perman-

220

ganate (Mn04)- anions in which the coordination sphere is constituted by four oxygen atoms instead of chlorine atoms. X+ -The family of amine complexes IM(NH3),l shown in table 3 .

-

TABLE 3

Cationic complexes of metals of 8 and lb groups.

Ni

cu

2* Ni (NH3lx

In this family,the metal is i n the form of a cation coordinated to several amine or ammonia groups. Among the most often used complexes,we can indicate the chloropentamine complexes of ruthenium, rhodium and iridium with valence II1,the tetramine complexes of palladium and platinum with valence I1 and amine complexes of silver, copper, nickel and cobalt. 3.3.

EXAMPLES

To illustrate what has been said,we can now examine on the basis of the results given in literature,what are the possibilities of adsorption between several platinum complexes and silica or alumina. For silica,two conditions are required: a solution pH higher than one and preferentially about six and the use of a metal cationic precursor. These two conditions are not met with a chloroplatinic acid solution; this explains why chloroplatinic acid does not adsorb on silica (2,4,6). With a tetramine platinum I1 chloride solution,we have a cationic complex but the pH of the solution is not high enough. Thus,the adsorption is very small (2,4).With a tetramine platinum hydroxide, both conditions

are fulfilled

and in this case platinum adsorption (2,4) effectively

221 occurs. Likewise, the use of a solution containing tetramine platinum chloride and ammonia leads to platinum adsorption (1,2,4,6). For alumina,two conditions are also required: either an anionic precursor solution with a pH lower than about 8 , or a cationic precursor solution with a pH higher than about 8 . In the cases of chloroplatinic acid solution,tetramine platinum hydroxide solution or tetramine platinum chloride ammonia solution,both conditions are fulfilled

and consequently platinum adsorption occurs.

(1,2,4,7-10,13). On the other hand,if we impregnate alumina with a sodium chloroplatinate solution,we have only very little adsorption because the pH of the solution is close to the isoelectric point of alumina. As for the tetramine platinum chloride solution,it does not lead to cation adsorption because the pH condition is not met. Thus,the results in literature concerning the adsorption of platinum complexes on oxides such as alumina and silica are consistent with the simplified schema presented above. We think that this schema remains valid for other oxides and other metal complexes (3,5,11-13). 3.4. EXCHANGE OF ADSORBED SPECIES Until now, we have considered the case of an oxide particle in contact with a solution containing one salt. We shall now consider the case where the impregnating solution contains, for instance,two mineral acids HA and HB. If the oxide is basic or amphoteric,its surface

ly and is surrounded by two types of counterions:

polarizes positive-

A-

and B-.

Schematically,the three following reactions take place at the interface oxide-solution :

+ -)

M-OH

+

H ' A -

4 +

(M-OH2A

M-OH

+ H+B-

-+c

(M-oH~B-)

:

( M - o H ~ B - )+

( M - o H ~ A - ) + B-

(1)

(2) A-

(3)

The first and the second reactions correspond to the polarization of a surface site S-OH by HA or HB. The equilibria are displaced towards the right side. On the contrary, reaction (3) corresponds to a true exchange of Aand B- between the oxide surface and the solution. To illustrate these various c o n ~ i d e r a t i o n s ~ wcan e take the practical

example of the impregnation of alumina by hydrochloric acid and chloroplatinic acid solutions. According to ( 3 ) the equilibrium of exchange can be written as follows : 2 (")

2ads + PtC16

+

4

222 If t h e s o l u t i o n is d i l u t e d and assuming h o m o g e n e i t y of t h e a d s o r p t i o n s i t e s o n alumina, w e can e x p r e s s t h e equilibrium c o n s t a n t as f 01 lows :

J

w h e r e Kd i s the d i s t r i b u t i o n c o e f f i c i e n t of platinum b e t w e e n t h e solut i o n and alumina. Now,

f w e t a k e t h e l o g a r i t h m i c e x p r e s s i o n of t h e e q u i l i b r i u m c o n -

s t a n t ,w e should o b t a i n a l i n e a r r e l a t i o n s h i p b e t w e e n log K and [log (c1-

-

log C l a d d

,

t h e v a l u e o f the s l o p e g i v i n g t h e v a l e n c e of

t h e m e t a l anion. T h i s is what w e o b s e r v e e x p e r i m e n t a l l y , w h e n g a m m a a l u m i n a is imp r e g n a t e d w i t h a h y d r o c h l o r i c s o l u t i o n of c h l o r o p l a t i n i c a c i d , c h l o r o i r i d i c acid o r c h l o r a u r i c a c i d

( f i g u r e 5).

l i n e s a r e obtained w i t h s l o p e s of

2,

I n t h e r a n g e studied,straight

2 for p l a t i n u m and iridium and

Q ,

1

for gold.

O.5 Fig. 5. A n i o n i c exchange equilibrium

-

F o r d e t a i l s , s e e Ref.

(13).

S i m i l a r r e s u l t s are o b t a i n e d w h e n s i l i c a is i m p r e g n a t e d w i t h a n amm o n i u m b u f f e r s o l u t i o n of p l a t i n u m t e t r a m i n e c h l o r i d e , p a l l a d i u m t e t r a m i n e c h l o r i d e or a m i n e c o p p e r s a l t s (figure 6).

223

1

0

-1

+/

/

I

[Log

bHfl -Log

[SiO-NH,+l] t

0

1.5

1

0.5

Fig. 6. C a t i o n i c e x c h a n g e equilibrium

-

F o r d e t a i l s , s e e Ref.

(13).

F r o m a p r a c t i c a l p o i n t of view, t h e r e v e r s i b i l i t y of a d s o r p t i o n i s v e r y i m p o r t a n t b e c a u s e it p e r m i t s t o o b t a i n a r e d i s t r i b u t i o n of t h e ads o r b e d c o m p l e x i o n s o n t h e p o l a r i z e d s i t e s w i t h a c o m p e t i t i v e ion and t h u s , t o o b t a i n an h o m o g e n e o u s d e p o s i t of t h e m e t a l i o n s on t h e support. 3.5.

ISOELECTRIC POINT OF OXIDES

W e h a v e seen t h a t , g i v e n t h e i s o e l e c t r i c p o i n t of an o x i d e , w e m a y f o r e s e e t h e a d s o r p t i o n capability

(anionic o r c a t i o n i c ) of t h i s o x i d e

and r o u g h l y what t h e pW r a n g e ( a c i d i c o r b a s i c ) of t h e impregnating s o l u t i o n w i l l be. T h e i s o e l e c t r i c p o i n t of a l a r g e n u m b e r of o x i d e s and h y d r o x i d e s is k n o w n and a v a i l a b l e in literature. F o r t h i s p u r p o s e , w e c a n r e f e r t o t h e s y n t h e s i s a r t i c l e p u b l i s h e d in 1965 b y P A R K S (43). A number of v a l u e s are g i v e n in t a b l e

4. T h e y allow u s t o r a n k ap-

p r o x i m a t e l y t h e o x i d e s in t h r e e c a t e g o r i e s : -The f i r s t o n e c o r r e s p o n d s t o t h e o x i d e s t h a t are acidic,such a s Sb203, W 0 3 o r Si02.Their

I.E.P.S.

are v e r y l o w and w e m a y s u p p o s e t h a t t h e y are

only able t o s e r v e as a d s o r b e n t s or e x c h a n g e r s of m e t a l c o m p l e x cations.

224

- T h e second o n e c o r r e s p o n d s t o t h e o x i d e s t h a t a r e b a s i c , s u c h as L a 2 0 3

or MgO. T h e i r I.E.P.S.

a r e very h i g h , g e n e r a l l y g r e a t e r t h a n 10. W e

t h i n k t h a t t h e s e o x i d e s a r e able t o adsorb essentially m e t a l complexes. -Finally,the third and m o s t n u m e r o u s c l a s s , i n c l u d e s t h e o x i d e s t h a t a r e a m p h o t e r i c such as T i 0 2 , C r 2 0 3 o r A1203.

TABLE 4 I s o e l e c t r i c p o i n t s of v a r i o u s oxides.

I .E.P.S. td. Concentration profiles during diffusional impregnation have been calculated by several investigators (ref.2,3,6) If fo=O then the impregnation process nay be divided into two periods: capillary impregnation tcap , and diffusional impregnation td. In the case of capillary inpegnation the component may be transferred by both convection and diffusion. The duration of capillary impregnation is determined by the viscosity of the solution the value of surface tension 6 , the wetting angle 8 and structural characteristics of the support. F o r the model of a monodispersed grain with capillary length R and radius r, the following equation is valid:

r,

tcap.' %%is@ During the limited time of irnpegnation, the component distribution

236

is determined by parameter d

:

At

(* 41 .O the component, together with a solution, moves inside the grain, At d n l . 0 distribution depends on dissusion and for calculation the ratios analogous to those cited in ref. 2, 3 may be used. Peculiarities of the impregnation of bi-dispersed structures may be illustrated by the model of the support grain of radius R o y consisting of aggregates of radius R 2 , which, in turn, consist of primary species of radius R1. When the specific pore volume between aggregates (the secondary structure) equals V2 and pore volume in the aggregates (the primary structure) equals VIy the ratio between the time required for grain impregnation into pores of the secondary structure, to, and aggregate impregnation time, t l , is found by the

Here Q is the true support density. Equation ( 8 ) is valid for both capillary and diffusion impregnation. In the case of diffusional

In this equation S is the support surface area and S 2 is the sur( 1 + V , p ). Here and hereafter face area of aggregates. S -2- R 2 P

we shall assume f, = 0. According to eq. 9, at 0(>>1.0 o < , n l . O and with limited time of impregnation, we may anticipate relatively uniform distribution of the component on the external surface of the aggregates. A non-uniform distribution will be found along the radii of the aggregates. At oiL&1.0 a non-uniform distribution is possible along the radii of both grains and aggregates.

3. Drying. An analysis of the mechanisms of solution transfer and redistribution of components in the grain during drying is reported by several authors (ref. 7-9). When the rate of evaporation from the grain surface, j:, is significantly less than the characteristic rate of solution

237

t r a n s f e r i n t h e g r a i n volume under the i n f l u e n c e of c a p i l l a r y d r y i n g . Under t h e s e f o r c e s , j c a p . , we may d e s i g n a t e a s "slow c i r c u m s t a n c e s the f o l l o w i n g e q u a t i o n i s a p p l i c a b l e ( r e f . 1 1 , 1 2 ) :

rV

Here fL and r e p r e s e n t t h e deusit:.es (of s o l v e n t l i q u i d and vapor, r e s p e c t i v e l y . Nud i s the N u s s e l t c r i t e r i o n , & i s p o r o s i t y , F i s mean pore s i z e , D1 i s the m o l e c u l a r d i f f u s i o n c o e f f i c i e n t of vapor and is the t o r t u o s i t y f a c t o r . A l s o A i s the parameter chara c t e r i z i n g p o r e d i s t r i b u t i o n a l o n g the r a d i i ( f o r ur,iform d i s t r i b rmax - rmin I f , i n a d d i t i o n t o e q u a t i o n 1 0 . the f o l u t i o n A = 7). lowing c o n d i t i o n i s f u l f i l l e d

t h e n d i f f u s i o n a l g r a d i e n t s i n the g r a i n must be a b s e n t . A schemat i c r e p r e s e n t a t i o n of the movement of t h e l i q u i d e v a p o r a t i o n bound a r y a t &,>>I i n the s e c t i o n of an a r b i t r a r y porous body i s shown i n Fig. 2. T h i s movement i s due t o the removal o f l i q u i d from

Fig. 2. Scheme of l i q u i d movement d u r i n g e v a p o r a t i o n of a porous body: a - s t e p I , , 6 - 6 - s t e p I1 - f o r m a t i o n of domains, 2 s t e p I11 - l i q u i d phase i s p r e s e n t only i n micropores and adsorption films.

238

l a r g e r p o r e s a t t h e e v a p o r a t i o n boundary. I n t h i s c a s e l i q u i d i s p a r t l y e v a p o r a t e d d i r e c t l y and p a r t l y moves t o p o r e s of s m a l l e r d i a m e t e r a t t h e e v a p o r a t i o n boundary. Such o c c u r s b e c a u s e of c a p i l l a r y p r e s s u r e g r a d i e n t s . T h i s t y p e of t r a n s f e r i s p o s s i b l e o n l y between t h e p o r t i o n s of s o l i d bound by l i q u i d . The e x t e n t of b i n d i n g by t h e l i q u i d d e c r e a s e s w i t h e v a p o r a t i o n w i t h e v a p o r a t i o n . A t t h e b e g i n n i n g t h e g r a i n i s c o m p l e t e l y bound. How-ever, as e v a p o r a t i o n p r o c e e d s , i s o l a t e d domains a r e g r a d u a l l y formed ( s t e p 11). T h e i r numbers i n c r e a s e and t h e i r s i z e d e c r e a s e s . A t t h e l a s t s t e p ( 1 1 1 ) l i q u i d i s f o u n d o n l y i n l a r g e l y i s o l a t e d domains of minimum s i z e connected only w i t h the adsorption f i l m . When P 4 1 .O, ( i m p r e g n a t e d c a t a l y s t s ) , t h e s o l v e n t volume Vs a t t h e c o n c e n t r a t i o n of s a t u r a t i o n G S i s

v s= c0 vIT If v a l u e Vs

(12)

'cs

i s r e a c h e d a t s t e p s I o r I1 t h e n an a p p r e c i a b l e p o r -

t i o n of a n a c t i v e component s h o u l d b e p r e c i p i t a t e d i n a c c o r d a n c e w i t h t h e r e d i s t r i b u t i o n mechanism. A s a r e s u l t , p r e c i p i t a t i o n occ u r s a t t h e mouth o f t h e l i m i t e d number o f p o r e s cf s m a l l d i a m e t e r coming t o t h e e x t e r n a l s u r f a c e s of e i t h e r g r a i n s or macro-pores. This r e d i s t r i b u t i o n i s c a u s e d by b o t h t h e volume t r a n s f e r of t h e s o l v e n t e f f e c t e d by c a p i l l a r y f o r c e s and d i f f u s i o n a l t r a n s f e r o f t h e component toward f a i r l y l a r g e p a r t i c l e s formed a t t h e mouth o f small p o r e s . A t dq ( 1 . 0 , g r o w t h of t h e s e p a r t i c l e s p r e v e n t s t h e s u p e r s a t u r a t i o n of a s o l v e n t and t h e f o r m a t i o n of new p a r t i c l e s a t t h e boundary o f s h i f t i n g m e n i s c u s . A s a r e s u l t one may e x p e c t t h e format i o n o f a r a t h e r c o a r s e l y - d i s p e r s e d d e p o s i t of t h e component ( r e f . 9 ) . A f i n e l y - d i s p e r s e d d e p o s i t i s formed o n l y i n t h e domains of m i n h a l s i z e a t s t e p 111 of d r y i n g a t V s ( V h A c c o r d i n g t o r e f . 7,8,Vh corresponds to the c r i t i c a l l i q u i d concentration. Its value i s s i m i l a r t o t h e a d s o r p t i o n v a l u e a t t h e l o w e s t p o i n t oE t h e h y s t e r e s i s l o o p on t h e a d s o r p t i o n i s o t h e r m of s o l v e n t v a p o r s on t h e s u p p o r t , Condition f o r the formation of only a f i n e l y - d i s p e r s e d deposit i s the following (re€. 7 , 3 ) :

.

> 1 t h e p o r t i o n of c o a r s e l y - d i s p e r s e d p h a s e i s B = I-H-'. I n t h e r e g i m e of f a s t d r y i n g (or "moving e v a p o r a s i o n f r o n t t t ( r e f . 11-12)) a t d341.0, the r e d i s t r i b u t i o n of s o l v e n t under th e i n f l u e n c e of c a p i l l a r y f o r c e s mag a l s o be n e g l e c t e d . The r a t e o f

A t 13

239

d r y i n g f a l l s w i t h d e e p e n i n g of t h e e v a p o r a t i o n f r o n t and t h e c o r responding i n c r e a s e i n the r e s i s t a n c e o f the vapor d i f f u s i o n t o t h e g r a i n e x t e r n a l s u r f a c e . The f a l l i n t h e v a l u e o f p a r a m e t e r w i t h time a t t > 0 i s d e s c r i b e d by t h e e q u a t i o n :

d,(t)

=

[Ro- R( t)]6 C o s @ R ( t ) 4 D,

-

rA

f L

( - )

P.

(14)

where R ( t ) i s t h e p o s i t i o n of t h e boundary o f e v a p o r a t i o n zone. A s f o l l o w s from e q o ( 1 4 1 , t h e r e i s a p o s i t i o n of t h e e v a p o r a t i o n zone boundary R* s u c h that d =I .O and corrdi t i o n f o r t h e above r e g i m e i s n o t f u l f i l l e d . If (R*/Ro)3C( 1.0 t h e n the amount of r e s i d u a l l i q u i d may b e n e g l e c t e d . Moreover we may c o n s i d e r t h a t d r y i n g of t h e whole g r a i n p r o c e e d s i n t h e regime of moving evapo r a t i o n f r o n t . With t h i s regime we may a n t i c i p a t e a u n i f o r m c i i s t r i b u t i o n of t h e componFnt i n t h e \olurne of t h e s u p p o r t g r a i n b e g i n n i n g w i t h d e p t h RE , c o r r e s p o n d i n g t o t h e c o n c e n t r a t i o n C s of a s a t u r a t e d s o l u t i o n . The d e p t h o f zone RH may b e e s t i m a t e d from t h e b a l a n c e e q u a t i o n , for example, f o r a s p h e r i c a l mono-dispersed g r a i n w i t h r a d i u s Ro :

4. I n f l u e n c e of t h e component a d s o r p t i o n on d i s t r i b u t i o n d u r i c g

drying C o n s i d e r some p e c u l i a r i t i e s of d r y i n g o f a d s o r p t i o n c a t a l y s t s ( P > I ) when t h e component c o n c e n t r a t i n g r e s u l t s i n a d d i t i o n a l s o r p t i o n . A f t e r t h e c o n c e n t r a t i o n of a s a t u r a t e d s o l u t i o n C s i s r e a c h e d , p r e c i p i t a t i o n of t h e component s t a r t s a t t h e boundary v a p o r - l i q u i d . I n t r o d u c e t h e v a l u e of t h e r e l a t i v e m o i s t u r e a t a g i v e n moment o f d r y i n g and t h e known dependc o n t e n t u= vi/Vs e n c e of t h e s p e c i f i c a r e a a t the boundary s o l u t i o n ( s u p p o r t ) - v a p o r (S=So ( u ) ). Then we may w r i t e t h e f o l l o w i n g e q u a t i o n f o r t h e dependence o f t h e s o l u t i o n c o n c e n t r a t i o n C on t h e s p e c i f i c m o i s t u r e

7

I n t h e Henry r e g i o n t h e f o l l o w i n g a n a l y t i c a l s o l u t i o n may be i obtained:

c = co

e

(cu + & I'UJI - h U "

240

as i s the value of s o r p t i o n i n a s a t u r a t e d solu-

.

t i o n a t Cs Functions u, ? ( u ) i n eqs. 1 6 , 1 7 may be c a l c u l a t e d u s i n g models of the support s t r u c t u r e . I n the s p e c i f i c case of " s l o w " drying regime, d i s t r i b u t i o n of the s o l u t i o n i n the support volume i s c l o s e t o equilibrium. Down t o l o w s o l u t i o n c o n c e n t r a t i o n s , corresponding t o the monolayer coverage, the e n t i r e support s u r f a c e i s covered with a s o l u t i o n ,

- a,

.

-P ' = Ps c o A more r i g i d ( a s compared t o P & 1 ) condis c, csvr t i o n f o r a d s o r p t i o n t o be n e g l e c t e d f o l l o w s from eq. ( 1 8 ) :

i.e., the r a t i o between component and the t o t a l I n a " f a s t t 1 regime of d i s p e r s e d systems t h a t

1

,=

b

-

the maximum p o s s i b l e amount of adsorbed amount of i n s e r t e d component must be l o w , d r y i n g a t dA600-800 r.p.m.)

f o r example, by

i n suspensoid-catalytic batch

r e a c t o r s o r by s e l e c t i n g f l o w c o n d i t i o n s i n f i x e d - b e d with c a t a l y s t p e l l e t s of smaller s i z e What i s r e m a r k a b l e i s t h a t ';he

r e a c t o r s packed

( O ) or acceleration ( a < O ) factors specific to the catalytic surface processes, and accordingly may be referred to as either retardation or acceleration coefficient.

To confirm the applicability of homogenkinetic approximation in

255 macrokinetic regime, an alternative expression for L R S T R is preferred for comparison with homogeneous reactions, which is also suitable for treating the integral rates of suspensoid-catalytic reactions in a complete-mixing batch reactor

:

this is the linear

relationship between reciprocals of fractional conversion, x=p'/po, or product partial pressure, p', and reaction time, t, which for a homogeneous first-order reaction with the initial rate, r = kopO, can be expressed as l/x=l/kot

+

1/2

or

l/p'=l/rot + 1/2po

(2)

-

Eq. (2) proved a good approximation in the range of conversions up to 60%. Similarly, for heterogeneous catalysis for which homogenkinetic approximations in both regimes are applicable, s o that HW treatment of double-sided mechanism may be assumed with the specific rate formula

r=kp/ (l+Kp+K'p')' and ro=kpo/ ( l+Kpo) ',

:

the linear reciprocal

relation can be expressed with about the same degree of approximation as Eq.2 as l/p ' =l/rot+ ( 1/2po) X (2( l+K'po) / ( l+Kpo) -1) Comparing Eqs 1

(3).

3 and noting that tc=t, p ' s are expressed in atm,

and the adsorption terms may be approximated as (l+K'p )/(i+Kp ) = K'/K, we may obtain 160.6xCl= ( 1/2p0) (2K'/K- 1) =1/2p0+

(

.

1 /po) (K'/K- 1 ) (4)

The last term of Eq.4 signifies, or at least explains qualitatively that the particular retardation or acceleration of the integral rate, i.e., specific activity, with respect to individual catalyst particles in the stationary-reacting state, as observed in the axial profiles in Fig. 1, is caused substantially by the microkinetic concurrence of adsorption and desorption of certain molecular species relevant to reaction components, especially the reaction product.

In this respect, it i s stressed that the retardation or

inhibition due to adsorbates is the most important factor for microkinetic control of catalyst performance, expecially stability and selectivity, and that without the control over inhibitory factors it is impossible to establish the stationary-reacting state or the real working state of the catalyst. In this way, L R S T R not only provides basic correlation data for microkinetic analysis and control of catalyst performance, revealing the heterogenkinetic characteristics of surface processes involved in the stationary-reacting state of the catalyst system which may therefore, be referred to as "reacting-apparatus", but it also

256 presents in its final form, with correction due to macrokinetic control, a practical a posteriori design equation. 3.

PERFORMANCE ANALYSIS FEED BACK TO CONTROLS OF CATALYST P REP A RA TION

The co-ordination of catalyst preparation and reaction engineering in developing both the catalyst structure and performance with feedbacks to both controls of catalyst preparation and reaction parameters may be illutstrated in the scheme

:

Controls of Catalyst preparation engineering design

catalyst evaluation

process design

R.E. analysis of catalyst performance

Stability

It can be seen that the three fundamentals of catalyst performance,

as well as structural

requirements represented by the uniformity

principles for each of structural elements, are functionally a s well as structurally interrelated.

Originally, certain controls over

uniformities in structure relative to activity and stability are pre-

-

requisite for the establishment of LRSTR.

In the course of further

development of the catalyst performance, namely, in a sequential cyclic process

:

Activity

Stability

-+

Selectivity, further refinements I

in structural uniformity principles are followed up, and at a certain stage of the development where a knowledge of the structural elements postulating the realization of 100% purity of catalytic function would necessitate the use of preformed uniform-porous supports to effect activity and stability improvements, the impregnation and activation operations become mandatory as a method for High Selectivity (HS) catalyst preparation, and the controls over mechanistic correlation

between

both operations also become important in relation to the

formation, distribution, and stabilization of the active sites on the surface matrices of the selected supports.

Scientific bases for

such controls of catalyst preparation parameters will be illustrated in the following examples 3.1

:

HS SILVER CATALYSTS FOR EPOXIDATION OF ETHYLENE AND PROPYLENE

3 . 2 HS BLACK SILVER O R COPPER CATALYSTS FOR COMPLETE OXIDATION.

257 3.1 HS SILVER CATALYSTS FOR EXPOXIDATION OF ETHYLENE AND PROPYLENE 3.1.1 ESSENTIAL STRUCTURAL REQUIREMENTS ACTIVE SITES

:

Pure metal particle of 10 to 20 nm diameter for

ethylene oxide formation and of 5 to 6 nm diameter for propylene oxide formation. SITUATION OF SUPPORTING

:

Physically, sufficient seclusion of the individual metal particles from each other and heat stability (to above 600 up to 900°C) of the surrounding support materials are required for the complete prevention of thermal migration. Chemically, complete inertness not only to metallic silver but also to silver oxide at higher temperature (above 400°C) is necessary to prevent chemical migration and deterioration. SELECTION OF T H E P O R O U S SUPPORT

:

The above mentioned STRUCTURAL REQUIREMENTS could have been fulfilled at least as a laboratory curiosity with non-porous supports.

But, in order to improve the catalyst activity to

an industrially feasible level of several Space Time Yield in 1-1 - 1 h , it was necessary to make use of a porous

mole (epoxide)

0.08

r-

0.06

0.04

0.02

0

lo2

I o3

Fig. 2. SEM photomicrogram of a cross-section Fig. 3. Differential pore-volume disof the ideally uniform support preformed in tribution curve for the same spheres of 3 . 5 mm diameter, showing the isosupport as shown in Fig. 2, showing tropic three-dimensionally uniform and conthe pronounced uniformity of pore tinuous pore system of high mechanic strength. size of the unimodal support.

258 support having an i d e a l l y uniform pore system of diameter.

s u i t a b l e pore

The p o r o u s s u p p o r t s y n t h e s i z e d f o r t h i s p u r p o s e 3 . 5 mm d i a m e t e r ,

was a n u n i m o d a l o n e p r e f o r m e d i n s p h e r e s o f

t h e r m a l l y s t a b l e up t o 1 5 0 0 ° C , a n d h a v i n g a n i s o t r o p i c t h r e e d i m e n s i o n a l l y u n i f o r m a n d c o n t i n u o u s p o r e s t r u c t u r e , a s shown i n Figs. 3.1.2

2 & 3.

C O N T R O L S OF CATALYST PREPARATION PARAMETERS : IMPREGNATION A N D

ACTIVATION The u s e o f t h e a b o v e - m e n t i o n e d

porous support precludes t h e usual

c o n t r o l s of t h e adsorption o r ion-exchange chromatographic e f f e c t s ,

effects,

STRUCTURAL REQUIREMENTS c l a i m s t o t h e n e c e s s i t y o f s t r u c t u r a l promoter

However,

the

the use of c e r t a i n

( s p e c i f i c a l l y , t h i s proved t o be e f f e c t i v e i n t h e

f i n a l form a s a l k a l i n e e a r t h c a r b o n a t e s )

i n an e x a c t l y o p t i m i z e d

a t o m i c p r o p o r t i o n of t h e p r o m o t e r e l e m e n t s elements) t o s i l v e r ,

a s well a s the

i n the impregnation operation.

(i.e.,

alkaline earth

s o t h a t t h e i m p r e g n a t i o n must be conform t o t h e

a c t i v a t i o n process i n accordance with t h e a c t i v a t i o n o r promoting mechanism.

F i r s t , t h e a c t i v a t i o n p r o c e s s e s were s t u d i e d w i t h n u m e r o u s

impregnant compositions, a s w e l l a s impregnation c o n d i t i o n s . revealed,

a c c o r d i n g t o UNIFORMITY PRINCIPLES,

crystalline precipitations is prohibitive optimized impregnant compositions.

It has

t h a t any kind of

t o a proper a c t i v a t i o n with

And f i n a l l y ,

a proper procedure

f o r i m p r e g n a t i o n a n d a c t i v a t i o n was e s t a b l i s h e d , by u s i n g s u c h a n i m p r e g n a t i o n c o n d i t i o n t h a t t h e i m p r e g n a n t compounds,

after filling

Fig. 4 . Electron microgram of t h e in-pore s u r f a c e of a HS SILVER CATALYST s u i t a b l e f o r propylene oxide formation, showing t h e uniformly dispersed s i l v e r p a r t i c l e s of c a . 5 nm diameter as blackened shadow d o t s i s o l a t e d by t h e surrounding materials.

259 t h e p o r e v o l u m e w i t h t h e i m p r e g n a n t l i q u i d , may h o m o g e n e o u s l y b e p r e c i p i t a t e d i n an amorphous s t a t e of c o l l o i d a l homogeneity, t h e a c t i v a t i o n o p e r a t i o n may c o n s e q u e n t l y f o l l o w u p . c o n t a i n i n g 1-3 w t % Ag,

CATALYSTS t h u s o b t a i n e d ,

t o f i t t h e STRUCTURAL REQUIREMENTS, showing e x c e l l e n t performance Fgs.

5

&

so t h a t

The HS SILVER

have proved a c t u a l l y

a s c a n be s e e n from F i g .

4,

f i g u r e s which a r e i l l u s t r a t e d i n

6 along w i t h t h e r e s u l t s of midget p i l o t t e s t s .

100

80

-

a

c 2 6

\

B

El

-4

20

6 82

0

0

Fig. 5. Temperature jump i n s e l e c t i v i t y i n t h e s t a t i o n a r y - r e a c t i n g state opera t i o n s on 100% EO-selectivity b a s i s with an isothermal one-dimensional rea c t o r : Ptota116.2, Pc 1-70, P 0.3 -1 2 4 02 24 ( a t m ) ; SV=5500 h .

0

60 100 160 220 Catalyst Bed Length (an)

Fig. 6. Axial p r o f i l e s f o r r e a c t i o n s i n a s i n g l e tube r e a c t o r e q u i v a l e n t t o t h e i n d u s t r i a l u n i t r e a c t o r operated f o r 100% EO s e l e c t i v i t y , showing EO conc e n t r a t i o n jump i n s e l e c t i v i t y a t 110 cm c a t a l y s t bed length.

Some r e s u l t s o f m i d g e t p i l o t t e s t s f o r p u r e s y n t h e s e s o f e t h y l e n e a n d p r o p y l e n e o x i d e s a r e shown i n t h e f o l l o w i n g t a b l e R e s u l t s of Midget P i l o t T e s t s

C2H4 C

~

20.5 20.5 6.6 H 20.5 ~

2.06 0.90 0.19

0.41 1.0 0.56

0.70

0.35

Reactor I . D .

0 0

0

0.25

200 3.5 205 5.0 240 3.5

28600 18000 9000

6.2 19.0 63

8.0 5.8 5.5

100 80 70

zoo

23600

11.8

2.1

71

3.7

21.6 mm, C a t a l y s t Dia. 4 . 0 mm

:

260 3.2 HS BLACK SILVER OR COPPER CATALYSTS FOR COMPLETE OXIDATION This category of High-Selectivity (HS) supported metal catalysts is characterized firstly by its performance standards,i.e., 100% purity of catalytic function for complete oxidation of hydrocarbons and their oxygenated derivatives to the extent of the quantity of oxygen existing in the reaction mixture, with high stability and activity.

It may also be characterized by its black colour in

general, which is indicative of certain structural resemblance to the old black metal catalysts such as P t or Pd black, but it differs from these greatly in its structural uniformity due to supporting, hence in its selectivity and stability, especially at higher temperature. 3.2.1 ESSENTIAL STRUCTURAL REQUIREMENTS 0

ACTIVE SITES

:

In-pore surface metal layer of less than 10

A

thickness in the microporous region (pore diam. 2 to 5 nm) of the porous support. SITUATION OF SUPPORTING

:

Physically, thermal stability (at above 600 to 800OC) of the microsurface geometry of the microporous support materials is required to exclude in-pore surface migration and sintering. Chemically, certain chalcogenic affinity to the metal or metal oxide of the microsurface of the support materials is a requisite for a good extension and stability of the very thin metal layer, but it must not be too large. SELECTION -

OF THE POROUS SUPPORT

:

An unimodal, uniformly microporous support which conforms well to the above REQUIREMENTS can be obtained by calcining (at up to 600

-

size.

Also, bimodal preformed supports having similar

800°C) pure and uniform silica gel granules of suitable

characteristics on silica or silica-alumina basis can be synthesized, which are suitable for large scale applications with high space velocity operations (SV > 300,000 h-l). 3.2.2. CONTROLS OF CATALYST PREPARATION PARAMETERS

:

IMPREGNATION

AND ACTIVATION The selection of impregnant compounds is important in order to obtain an efficient result with these SELECTED SUPPORTS, which also determines the procedures of both impregnation and activation. The usual proceeding with the operations referring to nitrate (AgN03 or Cu(N0 ) ) solutions has yielded poor results in respect 3 2 to supporting efficiency or %dispersion---lo to 15 wt% metal loading

261

may be regarded as usual.

Complexing the nitrate solutions with

seems to cause no particular improvements in both impregnation

NH3 and activation processes.

In general, it can be said that certain

melting of the crystalline impregnant deposits, prior to activation, e.g., thermal decomposition of impregnant compounds, may profitably be applied to fit the REQUIREMENTS, provided macroporous uniformity control over the distribution of the impregnant deposites be ensured in the impregnation stage.

As a specific example, with

cyanide pyridine complexes as impregnant compounds and with SELECTED SUPPORTS, HS black silver or copper catalysts may be obtained which show with metal loading of less than 5 wt% very stable metal 2 -1 (Ag or Cu). surface area of higher than 100-200 m g REFERENCES 1. A. Skrabal, Homogenkinetic, Th. Steinkopff, Dresden, 1941. 2. O.A. Hougen, K.M. Watson, Chemical Process Principles, Part 111, J. Wiley, New York, 1947. 3. R.C. Baetzold, C.A. Somorjai, J . Catalysis, 45, 94, 1976. 4. J.A. Christiansen, Adv. Catal., 2 , 311, 1953. 5. G.N. Lewis, 2. Physik. Chem., 52, 310, 1905. 6. T. Inui, T. Ueda, M. Suehiro, H. Shingu, J . Chem. SOC. Japan, 1976, (ll), 1665. 7 . T. Inui, H. Shingu, SHOKUBAI, 14, ( 4 ) , 195P, 1972. 8. G.C. Bond, Catalysis by MetalsTAcademic Press, 1962, p. 73. 9. G. Paravano, Catal. Rev. 2 , 207, 1969. 10. D.W. van Krevelen, Chemical Reaction Engineering (Ed. K. Rietema), Pergamon Press., 1957, p.8. 1 1 . H. Shingu, T. Okazaki, T. Inui, SHOKUBAI, 5 , 32, 1964. 12. R.R. White, S.W. Churchill, A.I. Ch. E. J . , , 2 , 354, 1959. 13. T. Inui, Docter thesis, Kyoto University, Faculty of Engineering, 1971. 14. H. Shingu, T. Inui, 26th Intern. Congress IUPAC Tokyo 1977, ABSTRACTS Session 1 1 p.60. 15. H. Shingu, T. Inui, T. Okazaki, 2nd Petrochemical Symposium, Japan Petrol. Inst., Tokyo, 1971, Nov. 18, Preprints, p. 120.

DISCUSSION

A.V.

KRYLOVA : Do you think that selectivity changes may be due to

the existence of discrete adsorbed species at different temperatures? H. SHINGU : In homogeneous reactions, selectivity changes

may be

considered to be due to different reaction intermediates.

In hetero-

geneous catalysis, reaction mechanisms are much more complicated : the adsorbed species or the adsorbates are not always the only reaction intermediates which determine the selectivity of surface reactions.

This remains true even if, at the stationary state, the

homogeneous kinetic approximation i s apparently applicable.

The

selectivity is essentially a kinetic term, and is generally controlled by simultaneous and/or consecutive events. (ref. 14).

In

the latter case, the change in selectivity may not be ascribed to the change of adsorbate structure.

The adsorbate may undergo dif-

ferent surface reactions in competition with product desorption (see in Fig. 5 the second temperature jump in selectivity, at temperatures higher than 1 8 O O C .

In the former case, most commonly

observed are the simultaneous or concurrent surface reactions, due to the non-uniformity of the catalyst surface or active sites. This is clearly seen in Fig. 5 where at the first temperature jump the selectivity for ethylene oxide steadily increases with temperature. The selectivity for C02 increases independently at 160-170°C after the development of a transient chain period.

This is due to the

release of the inhibition predominant at lower temperatures.

The

possibility that selectivity changes are due to the existence of different adsorbed species, as you mentioned, cannot be excluded for a complex heterogeneous reaction.

In the present reaction of

olefin oxidation with silver catalysts, this is not the case. S.S.Y.

KRICSFALUSSY

a very high value.

:

A selectivity of 71% for propylene oxide is

Could you tell us something about the life time

of the catalyst ?

H. SHINGU

:

This selectivity may also be reached in laboratory

tests under sufficiently inhibited reaction conditions but at substantially lower activities. The catalyst life time is unlimitedly long. At least it is longer than 6 0 0 hours of operation. With a semi-continuous laboratory test apparatus completely reproducible results are obtained, provided proper reaction conditions are maintained throughout the tests. It is to be noted that with HS catalysts within several minutes the stationary-reacting state is reached. These catalysts are very stable and their performance is reproducible. S.P.S.

ANDREW

:

Your principle of uniformity reminds me of a more

ancient version of the principle are selective.

-

namely that inactive catalysts

Are your catalysts pellets operating in a state

of diffusional limitation for the ethylene oxidation ?

263 H. S H I N G U : Y o u r r e m a r k i s very partly c o r r e c t in t h i s way t h a t i n a c t i v e c a t a l y s t s w h i c h are s u f f i c i e n t l y inhibited or p o i s o n e d may r e t a i n uniform a c t i v e sites.

H o w e v e r , our HS c a t a l y s t s a r e highly

a c t i v e and s t a b l e , so that t h e r e can be n o d i f f u s i o n a l l i m i t a t i o n s w i t h r e s p e c t t o t h e operating c o n d i t i o n s ( w i t h high f l o w r a t e ) , as w e l l as t o t h e u n i f o r m p o r e s t r u c t u r e of t h e support.

J.W.

GEUS

:

Y o u m e n t i o n e d s e l e c t i v i t i e s in t h e o x i d a t i o n o f

e t h y l e n e t o e t h y l e n e o x i d e of 100%. S a c h t l e r h a s d e v e l o p e d a t h e o r y t h a t it i s i m p o s s i b l e t o g e t h i g h e r s e l e c t i v i t i e s than 86% based o n a n e x t e n s i v e body of e x p e r i m e n t a l evidence.

Did I u n d e r s t a n d

y o u w e l l t h a t y o u o b t a i n e d s e l e c t i v i t i e s o f 100% ?

H. S H I N G U : W e o b t a i n e d s e l e c t i v i t i e s o f 100% in laboratory experim e n t s , as w e l l as in p i l o t tests.

T h e y are expressed in s t o i c h i o -

m e t r i c m o l a r t e r m s and equal t o (100-E)%.

E is a definite quantity

o f inhibitory c o n s u m p t i o n ( o x i d a t i o n ) o f r e a c t a n t s p e r pass. quantity

This

is i n h e r e n t t o t h e c a t a l y t i c r e a c t o r and r e a c t i o n condi-

t i o n s and i s e s t i m a t e d u s u a l l y t o b e l e s s t h a n 0.1-0.01%.

I am

a w a r e o f t h o s e t h e o r i e s w h i c h limit t h e m a x i m a l s e l e c t i v i t i e s t o

85-86%.

T h e y h a v e b e e n f o r w a r d e d by s e v e r a l a u t h o r s i n c l u d i n g

S a c h t l e r , and a r e b a s e d o n h y p o t h e t i c a l mechanisms.

I t is easy to

r e f u t e s u c h h y p o t h e t i c a l t h e o r i e s by claiming t h e l a c k of c o n v i n c i n g surface kinetic arguments, but I consider that the experimental e v i d e n c e for 100% selectivity i s t h e m o s t c o n v i n c i n g one.

V.D.

Y A G O D O V S K I : Could y o u explain why a very n a r r o w m e t a l p a r t i c l e

s i z e d i s t r i b u t i o n i s necessary for t h e h i g h activity and s e l e c t i v i t y

of e t h y l e n e o x i d a t i o n o n s i l v e r 7

How can these data been explained

theoretically 7

H. S H I N G U : T h e n a r r o w r a n g e o f m e t a l p a r t i c l e s i z e s i s d e r i v e d from an empirical basis a s a necessary consequence of the uniformity p r i n c i p l e c o r r e s p o n d i n g t o t h e s t r u c t u r a l r e q u i r e m e n t s for 100% p u r i t y of c a t a l y t i c function.

And t h i s r e q u i r e m e n t a l s o c o r r e s p o n d s

t o t h e h i g h s t a b i l i t y , or t h e high d e g r e e o f r e p r o d u c i b i l i t y of the catalyst

s u r f a c e , as w e l l as t o t h e h i g h e s t s p e c i f i c a c t i v i t y ,

o n metal-loading

b a s i s , w h i c h are a t t a i n a b l e at t h e r e a l w o r k i n g

s t a t e o r t h e s t a t i o n a r y r e a c t i n g s t a t e o f t h e c a t a l y s t system. T h e d a t a for t h e o p t i m a l m e t a l p a r t i c l e s i z e m a y b e e x p l a i n e d a s a r e s u l t of t h e d e f i n i t e ( o p t i m a l ) s u r f a c e p o t e n t i a l , or s t a t i s t i c a l

264

d i s t r i b u t i o n o f s u r f a c e d i s o r d e r s , d e t e r m i n e d by t h e c u r v a t u r e o f t h e macroscopic surface of t h e spheroidal, uniformly heat-treated, m e t a l particles.

These surface characteristics are particle size

d e p e n d e n t a s i n d i c a t e d by o x y g e n a d s o r p t i o n m e a s u r e m e n t s on s u p p o r t e d s i l v e r c a t a l y s t s o f v a r i o u s p a r t i c l e sizes. (T. I n u i , H. S h i n g u , S h o k u b a i , f i ( 1 ) , 49 (1972).

265

THE IMPREGNATION AND D R Y I N G STEP I N CATALYST MANUFACTURING

G.H.

van den Berg and H.Th.

Akzo Chemie b.v.,

Rijnten

K e t j e n C a t a l y s t s , Research Centre Amsterdam, P.O.

Box 15,

Amsterdam, The Ne t he rl an ds.

SUMMARY Three m e t a l compound-on-carrier

systems, d i f f e r i n g i n t h e i r r e s p e c t i v e i n t e r -

a c t i o n s t r e n g t h and me t a l l o a d i n g a r e d i scusse d. D i f f e r e n t impregnation and d r y i n g processes have t o be a p p l i e d ,

I.

t o o b t a i n the required p r o p e r t i e s o f the c a t a l y s t s .

INTRODUCTION Many c a t a l y s t s i n common use today c o n s i s t o f small metal c r y s t a l l i t e s , d i s -

perse d on a h i g h s u r f a c e area c a r r i e r . These metal-on-support

c a t a l y s t s can be

produced by im p r e gn at i ng t h e c a r r i e r w i t h a s o l u t i o n o f t h e metal compounds. D u r i n g i m pr e g n a t io n and subsequent d r y i n g ,

t he metal species a r e d e p o s i t e d o n t o t h e sup-

p o r t . The c o n d i t i o n s a p p l i e d d u r i n g these st e ps can i n f l u e n c e t h e f i n a l metal d i s t r i b u t i o n th r o u g h t h e c a r r i e r p a r t i c l e . Two in te r d e p e n d e n t f a c t o r s a r e d e c i s i v e f o r t h e c h o i c e o f impregnation technique and d r y i n g c o n d i t i o n s :

1.

t h e a d s o r p t i o n s t r e n g t h o f t h e metal compound o n t o t h e c a r r i e r .

2 . t h e metal c o n t e n t o f t h e f i n a l c a t a l y s t . Two

techniques can be a p p l i e d f o r i mp reg na t i o n v i a t h e l i q u i d phase:

A. A t low a d s o r p t i o n s t r e n g t h t h e i mpre gn at i on technique chosen w i l l be t h e " dry"

im p r e g n a t io n as i n d i c a t e d e.g.

by B e r r e b i and Bernusset

113.

I n t h i s case,

t h e r e q u i r e d amount of met al compounds i s d i s s o l v e d i n a s u f f i c i e n t volume o f w a t e r t o f i l l ab ou t t h e t o t a l p ore volume o f t h e c a r r i e r b a t c h (pore volume saturation). B. A t s t r o n g a d s o r p t i o n t h e c a r r i e r can be impregnated u s i n g an excess amount

of s o l v e n t and c i r c u l a t i n g t he s o l u t i o n t hrough a bed o f c a r r i e r p a r t i c l e s . The metal compound d i f f u s e s from t he s o l u t i o n i n t o t h e c a r r i e r p a r t i c l e and i s adsorbed t h en (so aki n g p roce du re).

266 Because t h e number o f s t r o n g a d s o r p t i o n s i t e s per u n i t w e i g h t o f c a r r i e r i s l i m i t e d t h i s "soaked"

im pre gn at i on t ech ni q ue can be a p p l i e d f o r r a t h e r low metal c o n t e n t s .

For h i g h e r m e t a l c o n t e n t s (approx. 5%) g e n e r a l l y t h e d r y impregnation technique i s applied.

I f v e r y h i g h me t a l c o n t e n t s a r e r e q u i r e d ,

m et a l compounds

the l i m i t e d s o l u b i l i t y o f

may r e q u i r e two or even t h r e e c o n s e c u t i v e impregnation s t e p s .

D r y i n g o f t h e impregnated system i s g e n e r a l l y c a r r i e d o u t a t temperatures between 80 and 30OoC. T h i s s t e p can a f f e c t t h e i mpregnation r e s u l t q u i t e d r a s t i c a l l y i n t h e case o f weak a d s o r p t i o n o f t h e me t a l compound on t h e c a r r i e r . A t t h e area i n t h e c a r r i e r p a r t i c l e where e v a p o r a t i o n occurs, t h e metal compound c o n c e n t r a t i o n i nc r e a s e s . T r a n s p o r t o f t h e s o l u t i o n from t h e c a t a l y s t p a r t i c l e i n t e r i o r towards t h e e v a p o r a t i o n a rea and c o n t i n u i n g e v a p o r a t i o n lead t o an inhomogeneous d e p o s i t i o n o f t h e m e ta l compound t h rou gh t h e c a r r i e r p a r t i c l e . Even d i s t r i b u t i o n o f t h e metal compound i s o b t a i n e d when d i f f u s i o n o f t h e me t al compound o v e r t h e c a r r i e r pore s urfa c e can o c c u r , even d u r i n g t h e f i n a l s t a g e o f t h e d r y i n g process (empty p o r e system w i t h l a y e r s o f 1 i q u i d s t i 1 1 p r e s e n t ) .

When t h e metal compound i s adsorbed

s t r o n g l y , a change o f t h e metal d i s t r i b u t i o n on d r y i n g i s un i k e l y .

I n t h i s case,

t h e d i s t r b u t i o n i s determined by t h e i mp reg na tion procedure. I n t h i s paper we l i k e t o d i s c u s s t h r e e i mp r egnation systems, d i f f e r i n g i n ads o r p t i o n s t r e n g t h and me t a l l o a d i n g ( t a b l e 1 ) . These examples may i l l u s t r a t e t h e phenomena i n d i c a t e d above.

TABLE 1.

C h a r a c t e r i s t i c s o f t h e systems under c o n s i d e r a t i o n .

3. 1 Carrier A c t i v a t e d carbon Me t a 1 compound Zn (CH~COO),.ZH,O 11 M etal c o n t e n t ( w t %) Adsorption weak + s t r o n g C r i t i c a l step I mpre gn at i on + drying i m pr e g n a t io n soaked proce d u r e

3. 2

8- '1203 CUCl2

5 weak drying dr Y ( PV-sa t )

3. 3

8-H 2~P t1C 21 60 3 0.3 strong impregnation soaked L

2. EXPERIMENTAL The equipment used i s ske t ch ed i n f i g u r e 1. The d r y impregnations have been c a r r i e d o u t i n a r o t a t i n g pan, p r o v i d e d w i t h b a f f l e s (1A). p r e g n a t i o n s a g l a s s column has been used ( l a ) .

For t h e soaked i m -

I n t h i s case, t h e s o l u t i o n was

c i r c u l a t e d t h r o u g h t h e bed o f c a r r i e r p a r t i c l e s by means o f an a i r - l i f t . Some p h y s i c a l p r o p e r t i e s o f t he c a r r i e r s we used a r e g i v e n i n t a b l e 2.

267

A

flg 1 TECHNIOUE

TABLE 2.

B

OF DRY IAl A N D SOAKED iB1

a,, IMPREGNATION

Some p h y s i c a l p r o p e r t i e s o f t h e c a r r i e r s . ~

3. 3 A c t i v a t e d carbon Source TY Pe Size S u r f a c e a r e a (m /g) Pore volume (ml/g) (mercury pyknometer).

Norit

RKD-3

3 mm e x t r u d a t e s 1343 1.12

2 8-3.A1203

3. 3

Akzo Chemie K e t j e n Grade E F l u i d powder 122 0.39

Akzo Chemie K e t j e n CK 300 1.5 M e x t r u d a t e s 180 0.59

T y p i c a l im p r e g na t i o n and d r y i n g c o n d i t i o n s o f t h i s work a r e g i v e n i n t a b l e 3. TABLE

3.

I m p r eg na t i o n and d r y i n g c o n d i t i o n s

C a r r i e r (g) Water (g) M e ta l compound

3. 2

3. 3

5000 1800

50 100 H2PtCI

757 Zn(CH3COO) 2.2H20

308

Amount (9) Temperature (OC) I m p r e g n a tio n t i m e ( h ) D r y i n g temp. (OC) 3.1.

3. 1 400

25

3 130

CUCl2

565 25 1 120-300

0.65

25 3 120

ZINC ACETATE ON ACTIVATED CARBON

A z i n c a c e t a t e on a c t i v a t e d carbon c a t a l y s t i s a p p l i e d i n t h e m a n u f a c t u r i n g o f vinylacetate.

T h i s c a t a l y s t must c o n t a i n a l a r g e amount o f z i n c a c e t a t e i n o r d e r

t o ensure a lo ng c a t a l y s t l i f e under p r a c t i c a l c o n d i t i o n s (0.55 gram z i n c a c e t a t e .2H20 p e r gram o f c a r r i e r ) . volume o f 1.12 ml/g.

The a c t i v a t e d carbon i n e x t r u d e d form has a p o r e

A s a t u r a t e d aqueous z i n c a c e t a t e s o l u t i o n o f 20°C

(density

1.165 g/ml) c o n t a i n s 0.31gram metal compound p e r m l . From these data, i t can be calculated,

t h a t a maximum l o a d i n g o f 0.35 gram o f z i n c a c e t a t e 2H20 p e r gram o f

c a r r i e r can be o b t a i n e d i n a one s t e p d r y i mpregnation. To reach t h e d e s i r e d metal compound l o a d i n g by t h i s t ech ni q ue , a second d r y impregnation must be c a r r i e d o u t . The im p r e g n at i on m i g h t be c a r r i e d o u t i n one-step, technique,

however, u s i n g t h e soaking

by e x p l o i t i n g b o t h t h e s o r p t i v e p r o p e r t i e s o f t h e a c t i v e carbon and

t h e p o r e volume c a p a c i t y of t h e c a r r i e r .

The s o r p t i o n o f z i n c a c e t a t e on a c t i v e

268 c arbon has been demonstrated by measuring t h e d e n s i t y o f t h e mother l i q u o r d u r i n g a s oakin g experiment.

The i n i t i a l d e n s i t y o f

1.153

reached an e q u i l i b r i u m v a l u e o f

1.091 w i t h i n 3 h. The t o t a l u pt ake o f z i n c a c e t a t e has been measured as a f u n c t i o n o f t h e z i n c a c e t a t e c o n c e n t r a t i o n i n t h e e q u i l i b r i u m s o l u t i o n ( f i g . 2 ) . From t h e c o n c e n t r a t i o n i n t h e s o l u t i o n t h e u pt ake by t h e pore volume has been estimated. The d i f f e r e n c e between b o t h cu rves rep rese nt s t he adsorbed amount. T h i s f i g u r e i n d i c a t e s t h a t an a p p r e c i a b l e p a r t o f t he t o t a l z i n c a c e t a t e l o a d i n g i s o r i g i n a t e d from a d s o r p t i o n . 4 zinc ocetate

>HP

/ g active carbon

0 6-

COW

0

010

0 20

0 30

BY NORIT RKD 3

f j g 2 UPTAKE OF ZINC ACETATE 2H,D

IEOUILIBRIA

soln (g/mli

AT 25'CI

To g e t some i n s i g h t i n t h e l o c a t i o n o f t h e metal compound a f t e r c a r e f u l d r y i n g o f t h e impregnated c a r r i e r , t h e c a r r i e r and t h e d r i e d p r o d u c t have been i n v e s t i gated ( t a b l e

4.

TABLE

4). All

data a r e r e l a t e d t o 1 g o f c a r r i e r .

Chemical and p h y s i c a l d a t a o f c a r r i e r and d r i e d p r o d u c t . N o r i t RKD-3

Dried product

-

Z i n c a c e t a t e .2H 0 c o n t e n t (wt%) 22 Surfac e a r e a (m /g) T o t a l p o r e volume (ml/g) Pore volume m ic r o p o r es (4.75 nm) m l /g

59.2 410 0.79

1340 1.12 0.57

0.20

The mercury p e n e t r a t i o n curve s o f b o t h c a r r i e r and d r i e d p r o d u c t a r e g i v e n i n fig. '

3.

The c u r v e g i v e n f o r t h e c a r r i e r i n d i c a t e s two types o f pores t o be p r e s e n t :

macropores, h a v i n g a r a d i u s between

( K>Na ( r e f . 4 ) .

I n agreement with t h i s i d e a , Raney ruthenium

containing t h e e l e c t r o p o s i t i v e aluminum metal has been found t o be an e f f i c i e n t ammonia c a t a l y s t t h a t i s a c t i v e even a t 100°C a f t e r a d d i t i o n of potassium ( r e f . 5 ) . I t seems obvious, however, t h a t such c a t a l y s t s promoted by m e t a l l i c a l k a l i a r e

s u b j e c t t o i r r e v e r s i b l e poisoning by water, suggesting a p o s s i b l e drawback of t h e c a t a l y s t from a p r a c t i c a l p o i n t of view. r e s i s t a n t and e f f i c i e n t c a t a l y s t .

I t would be d e s i r a b l e t o g e t a water-

Thus a l t e r n a t i v e methods have been i n v e s t i g a t e d

f o r p r e p a r a t i o n of t h e ruthenium c a t a l y s t .

The p r e s e n t paper d e a l s with two methods

of potassium a d d i t i o n t o Ru/A1203, i n which potassium compounds were used a s t h e s t a r t i n g material.

382 CATALYST PREPARATION 1. Oxide-promoted

catalysts.

P o t a s h ( o r cesium oxide)-promoted ruthenium c a t a l y s t s were prepared by impregnating alumina with a mixture of RuCl3-KNO3(or CsN03)(l/lO,mol /mol ) . The alumina sample obtained from Tokai Konetsu Co. was i n a form of c y l i n d r i c a l pellet(3.5mm both i n diameter and i n length) with a c o a x i a l open hole and had a s p e c i f i c s u r f a c e a r e a 2 of 180 m /g. The alumina, i n t h e form of p e l l e t s or crushed g r a i n s of 10-20 mesh,

w a s immersed i n t h e mixed aqueous s o l u t i o n of 0.02 mol and evaporated t o dryness on a water bath.

The Ru/A1203 r a t i o w a s a d j u s t e d t o g i v e 2.0%(w/w)Ru.

The d r i e d

m a t e r i a l w a s t r e a t e d with c i r c u l a t i n g hydrogen a t i n c r e a s i n g temperatures, f i r s t a t 100°C u n t i l no hydrogen consumption w a s observed and f i n a l l y a t 45OOC f o r 40hr, during which t h e gaseous products of reduction were removed by a l i q u i d n i t r o g e n trap. Four c a t a l y s t s were used as follows: C a t a l y s t Number

Impregnation with

Support

N- 1

RUC13-KNO3

grains

N-2

RUC13-CSN03

grains

N- 3

RuC13-NH4N03

grains

X

RuCl3

pellet

2 . Cyanide-promoted c a t a l y s t s .

Two d i f f e r e n t samples,A and B , o f K4Ru(OU6 were prepared according t o t h e known methods.

The sample A was prepared by adding KCN t o K2Ru04 which w a s prepared

by a l k a l i f u s i o n of Ru metal and KNO3 ( r e f . 6 and 7 ) .

The sample B was prepared

by adding KCN t o RuC13 ( r e f . 8). B o t h samples were p u r i f i e d by r e c r y s t a l i z a t i o n from water t o g i v e a c o l o r l e s s t e t r a g o n a l p l a t y c r y s t a l .

a better yield

( @ 50% i

The former method gave

n Ru).

Both samples were examined by elemental a n a l y s i s f o r t h e i r i d e n t i t y as shown i n Table 1.

The sample B was dehydrated i n advance.

TABLE 1 Elemental a n a l y s i s of samples A and B Sample

H

C

N

K

A

found calc. as trihydrate

1.21 1.30

15.42 15.41

18.01 17.96

34.00 33.45

B

f0Wd c a l c . a s anhydride

0.05 0.00

17.41 17.43

19.73 20.31

37.82

-

There is a good agreement with t h e c a l c u l a t e d value f o r K4Ru(CN)6.

383 The sample A was f u r t h e r i d e n t i f i e d by X-ray d i f f r a c t i o n and I R absorption. Since K Ru(CN) -3H 0 and K4Fe(CNI6.3H 0 a r e isonorphous ( r e f . 9 ) , t h e i d e n t i t y 4 6 2 2 of t h e sample A was confirmed by a comparison of X-ray d i f f r a c t i o n p a t t e r n s with a known sample of K Fe(CNI6-3H20. The I R absorption spectrum of t h e sample A 4 was i n accord with t h e l i t e r a t u r e f o r K R U ( C N ) ~ - ~ H (~rOe f . 10). 4 The samples A and B were supported on t h e 10-20 mesh g r a i n s of alumina described above t o give a 2.0%(w/w) Ru/A1203, i.e.,

t h e alumina was impregnated

w i t h A o r B from i t s aqueous s o l u t i o n of 0.04 m o l / l by evaporation t o dryness. The supported samples were s u b j e c t e d t o decomposition i n c i r c u l a t i n g H2 o r 3 H /N 2

2

mixture a t i n c r e a s i n g temperatures up t o 450°C. Four c a t a l y s t samples were used as follows; Number

Reducing gas

K4Ru (CN)

c1

B

c2

B

c3

A

c4

A

3H - N 2 2 H2

H2 3H2-N2

PROCEDURES The hydrogen reduction and t h e a c t i v i t y measurements were c a r r i e d o u t i n a closed c i r c u l a t i n g system comprising a l i q u i d n i t r o g e n t r a p .

The ammonia s y n t h e s i s

gas. The numbers of s u r f a c e runs were c a r r i e d o u t a t 600torr using t h e 3H / N 2 2 ruthenium atoms were e s t i m a t e d by chemisorption of hydrogen which was determined by e x t r a p o l a t i o n o f l i n e a r p a r t of t h e isotherm a t OOC.

RESULTS AND DISCUSSIONS 1. Stoichiometry o f reduction by hydroqen

1-1 Hydrogen treatment of n i t r a t e - c o n t a i n i n q c a t a l y s t s .

A r a p i d consumption

o f hydrogen s t a r t e d a t around 100°C and was n e a r l y completed a t 200'C. consumption up t o 450'C

The t o t a l

f o r Cat.N-2 was 3.9 mol H /mol Ru which corresponded t o 94% 2

of t h e c a l c u l a t e d value f o r a r e a c t i o n ; RuC13

+

10CsN03

+

41.5H2+Ru

+

3CsC1

+

7CsOH

+

10NH3

+

23H20

I t i s obvious t h a t most of t h e n i t r a t e i s reduced t o NH3 and H 2 0 .

(1)

On t h e o t h e r

hand, t h e reduction of C a t . X (RuC1 / A 1 0 ) was much slower and t h e t o t a l consumption 3 2 3 up t o 45OOC w a s 1.38 m o l H /mol RU o r 92% of t h e c a l c u l a t e d value f o r t h e r e a c t i o n : 2 RuC13

+

1.5H2 + Ru

+

3HC1

The d e v i a t i o n from 100%would be due p a r t l y t o t h e l o s s during p r e p a r a t i o n .

(2) A t any

r a t e t h e presence of n i t r a t e c l e a r l y r e s u l t s i n t h e f a s t e r and l a r g e r consumption of hydrogen.

384 1 - 2 Hydrogen treatment of cyanide-containing c a t a l y s t s .

The r e a c t i o n of

unsupported K4Ru(CNl6 w i t h hydrogen was found t o be very slow even a t 500°C ( 2 mmol H

/mol Ru h r ) , while t h e hydrogen consumption was r e a d i l y observed a t 38OOC with t h e alumina-supported c a t a l y s t s a s i l l u s t r a t e d i n Fig. 1 f o r Cat. C-2.

500

-

L; 400

-

!-I

- 300 0

c,

a, !-I

1 rn

!-I g200

-

01 N

m

100

10

5 Time (hr)

0

Fig. 1. Time course of hydrogen consumption a t 380'C f o r c a t a l y s t C-2.

The i n i t i a l slow r e a c t i o n (66 m o l H / m o l Ru h r ) is a c c e l e r a t e d a f t e r about 6 h r 2

by a f a c t o r o f about 25.

I t i s n o t i c e a b l e t h a t t h e r a t e of r e a c t i o n is independent

of hydrogen p r e s s u r e , suggesting t h a t a s o l i d phase process is r a t e - l i m i t i n g . The temperature was r a i s e d s t e p w i s e (400, 420 and 450°C) a f t e r t h e r a t e a t a temperature was slowed down. A f t e r t h e decomposition o f C a t . C-1 at 450°C t h e products c o l l e c t e d i n t h e t r a p were i d e n t i f i e d by massspectroscopy t o be methane and a much l a r g e r amount of ammonia, i n d i c a t i n g t h a t t h e ammonia s y n t h e s i s r e a c t i o n took p l a c e because t h e reducing gas was 3H2/N2.

Thus t h e decomposition w a s made i n H2 f o r C a t . C-2

and

t h e t o t a l hydrogen consumption up t o 450'C was determined t o be 10.1 mol/mol Ru, a value corresponding t o 3CN/Ru converted t o CH4 and NH3. decomposed i n H 2 ,

t h e amounts of CH4 and NH3 formed were determined i n a d d i t i o n

t o t h e hydrogen consumption as follows; CH4/Ru

:

4.8,

I n t h e case of Cat. C-3

NH3/Ru

:

4.6,

H2/Ru

:

16.5

2-

385 The hydrogen consumption corresponds t o 4.7 CN/Ru, i n a b e a u t i f u l agreement with t h e I n t h e case of C a t . C-4 decomposed i n 3H2/N2, both t h e values of CH4/Ru and NH /Ru. 3 CH4/Ru and CN/Ru values a r e found t o be 4.6. I t i s c l e a r from t h e above t h a t more than two CN l i g a n d s a r e hydrogenated i n t h e

decomposition, which implies t h a t some c a t i o n o t h e r than Ru(I1) i s reduced by hydrogen, a s represented f o r K ( 1 ) by

+

10.5H2*Ru

+

K

K4Ru(CN)6 +

14 H -Ru 2

+

2K

K4R~(CN)6

+

3KCN

+

+

3CH4

3NH3

(3)

or i2KCN

+

4CH4

+

4NH3.

(4)

I n f a c t t h e reduction o f KCN by H2 i s not impossible thermodynamically when t h e products a r e removed continuously.

The alumina support might be p a r t l y transformed

t o cyanide during t h e decomposition, r e s u l t i n g i n a reduction t o form m e t a l l i c aluminum, on which no information i s a v a i l a b l e .

N o information i s a l s o a v a i l a b l e

about t h e reason o f t h e d i f f e r e n c e i n e x t e n t of reduction observed f o r Cat. C-2 and C-3. 2 . Ammonia s y n t h e s i s a c t i v i t i e s

The r a t e s of ammonia s y n t h e s i s over t h e c a t a l y s t s are summarized i n Fig. 2 as Those c a t a l y s t s a r e commonly 2 % f w / w ) R u on t h e same alumina support,

Arrhenius p l o t s .

while d i f f e r e n c e s may be found i n t h e e x t e n t of d i s p e r s i o n a s w e l l as i n t h e n a t u r e of promoter.

The amounts of hydrogen chemisorption a t O°C a r e given i n Table 2

t o g e t h e r with t h e r a t e s of ammnia s y n t h e s i s p e r g c a t . a t 3OOOC and t h e apparent a c t i v a t i o n e n e r g i e s (Ea)

.

TABLE 2

C a t a l y t i c p r o p e r t i e s of c a t a l y s t s N-2

N-3

K20

Cs20

none

H2m1STP/g.cat

0.68

1.08

0.31

0.13

NH mlSTP/g.cat.hr

1.9

8:3

0.076*

0.039*

28

28

17

17

*

c-2

c-3

-

0.98

1.08

1.22

1.35

4.3

1.9

0.67

0.70

1.1

27

27

28

26

26

x K

N-1

3 Ea K c a l / m l

c-1

X none

Catalyst Promoter

c-4

- cyanide -

e s t i m a t i o n by e x t r a p o l a t i o n

Since l i t t l e d i f f e r e n c e i s observed i n Ea on those alkali-promoted c a t a l y s t s , t h e observed d i f f e r e n c e i n t h e c a t a l y t i c a c t i v i t y seems t o come from t h e e x t e n t of dispersion.

However t h e v a r i a t i o n i n hydrogen chemisorption i s t o o small t o

explain the difference.

The hydrogen chemisorptions on t h e cyanide s e r i e s of

c a t a l y s t s a r e g e n e r a l l y l a r g e r , whereas t h e c a t a l y t i c a c t i v i t i e s are lower those on Cat. N - 1 and N - 2 .

than

I f m e t a l l i c potassium i s formed during t h e decomposition

386

450

400

I

I

I

1.5

1.6

250

300

I

I

..4

TEMP. ("C) 350

I

I

1

I

1.7

1.8

1.9

10 OO/T

F i g . 2 . Arrhenius plots of r a t e of ammonia s y n t h e s i s .

387 of Cat. C-2 o r C-3 a s suggested by t h e stoichiometry, hydrogen can be chemisorbed on K i n a d d i t i o n t o on Ru, giving r i s e to l a r g e r chemisorptions.

In f a c t , as

described l a t e r , t h e chemisorption value on Cat. C-2 decreased by about 30% a f t e r a t r e a t m e n t with water vapor.

The l a r g e r chemisorption on C a t . C-3 o r C-4 than on

Cat. C-2 can be due t o l a r g e r amount of potassium a s suggested by t h e stoichiometry. Thus t h e r e a l chemisorption on Ru would be l e s s than t h a t observed on t h e cyanide s e r i e s of c a t a l y s t s .

i s remarkable.

The high a c t i v i t y on Cat. N - 1 o r N-2

I n comparison with Cat. N-3

which has no promoter, Cat. N - 1 and N-2 a r e r e s p e c t i v e l y , 11 and 31 t i m e s more a c t i v e i n terms of t h e r a t e p e r chemisorption. When Cat. X was a c t i v a t e d by a d d i t i o n of m e t a l l i c potassium (K/Ru=2.9mol / m o l )

,

t h e e x t e n t o f promotion a t 3OOOC w a s 110

t i m e s a s shown i n Table 1. Since t h e potassium a d d i t i o n may give r i s e t o an increase i n d i s p e r s i o n of ruthenium on alumina as was t h e case with unsupported ruthenium ( r e f . 2 ) , t h e d i f f e r e n c e i n promoting e f f i c i e n c y between m e t a l l i c potassium and cesium oxide would be

smaller.

Thus cesium oxide is a promising promoter which i s

s t a b l e i n t h e presence of water vapor. K > C s 0 >K 0 , i s i n accord with 2 2

The e f f i c i e n c y sequence of promoters,

t h e o r d e r of e l e c t r o - p o s i t i v i t y ,

the e l e c t r o n donation t o ruthenium as has been suggested ( r e f . 4 ) .

i n conformity with The high a c t i v i t y

o f C a t . N-2 a r i s e s f i r s t l y from t h e high e l e c t r o p o s i t i v i t y of C s 0 and secondly from 2 t h e increased d i s p e r s i o n of ruthenium. Presumably t h e low temperature reduction which

i s r e a l i z e d i n t h e presence of n i t r a t e i s r e s p o n s i b l e f o r t h e higher d i s p e r s i o n . Although m e t a l l i c potassium i s e f f i c i e n t as t h e promoter and i s l i k e l y formed i n Cat. C-2 o r C-3, t h e s p e c i f i c a c t i v i t y p e r chemisorption f o r Cat. C-2 o r C-3 i s much l e s s than t h a t on Cat. N-2.

On t h e o t h e r hand it i s t o be noted t h a t t h e

decomposition of K R u ( W /A1203 made i n H2-N2 mixture r e s u l t s i n a higher a c t i v i t y 4 6 than t h a t i n H a , a s i s c l e a r form t h e comparison of Cat. C - 1 o r C-4 w i t h C-2 o r C-3. This r e s u l t seems t o be caused by a cooperative i n c o r p o r a t i o n of K and N2 i n t o Ru metal ( r e f . 111, which g i v e s r i s e t o a higher potassium content.

Hence t h e lower

a c t i v i t y of Cat. C-2 o r C-3 seems t o be caused by a l e s s extensive i n t e r a c t i o n o f K w i t h Ru o r a p o s s i b l e loss o f K during t h e decomposition of K Ru(CN) 4 6' 3. E f f e c t of water vapor treatment.

The c a t a l y s t N-1 was t r e a t e d with a c i r c u l a t i n g C O D (=3/1) mixture containing s a t u r a t e d water vapor a t 45OOC f o r 24

pr,

2 during which C n 4 and CO

were formed. 2 A f t e r evacuation a t 400°C f o r 2 h r , t h e ammonia s y n t h e s i s a c t i v i t y was found t o be 64% of t h e i n i t i a l value, while it was recovered t o 86% a f t e r hydrogen treatment a t 45OOC f o r 40 h r . treatments.

The amount o f hydrogen chemisorption changed l i t t l e i n t h e above

In c o n t r a s t , t h e Cat. X w i t h K was found t o s u f f e r a d r a s t i c poisoning

by water vapor. A f t e r a treatment with c i r c u l a t i n g water vapor (15 t o r r ) a t 45OoC

f o r 14 h r ( u n t i l t h e f i n i s h o f hydrogen e v o l u t i o n ) followed by evacuation a t 400'C f o r 2 h r , t h e ammonia s y n t h e s i s a c t i v i t y was found t o be 6% of t h e i n i t i a l value and recovered t o 15% a f t e r hydrogen treatment a t 45OoC f o r 40 h r , with the f i n a l

388 a c t i v i t y being comparable t o t h a t of Cat.N-1.

I n t e r e s t i n g l y t h e amount of hydrogen

chemisorption on Cat. X i n c r e a s e d by a f a c t o r of 2 . 8 i n t h e above treatments.

The

a d d i t i o n of potassium followed by t h e armnonia s y n t h e s i s runs presumably gave r i s e t o a c o r r o s i v e chemisorption of nitrogen t o form a compound ( r e f . ll), which was decomposed by water r e s u l t i n g i n t h e increased d i s p e r s i o n . n i t r o g e n uptake has been observed on Ru-K/Al

In fact, a large

c a t a l y s t s ( r e f . 1 2 ) , whereas l i t t l e

0

2 3

uptake on Cat. N-1. Since m e t a l l i c potassium i s l i k e l y formed i n t h e decomposition of cyanide s e r i e s of c a t a l y s t s , t h e e f f e c t of water vapor would be r e v e a l i n g .

Cat. C-2 w a s t r e a t e d

w i t h c i r c u l a t i n g water vapor a t 44OOC u n t i l t h e f i n i s h of p r e s s u r e i n c r e a s e .

The

evolved gas was confirmed t o be hydrogen (0.67 m l H /mol Ru), suggesting t h a t 2 1.3 mol K/mol Ru i s formed i n C a t . C-2, which i s near t o 1 moleK/moleRu as estimated from t h e hydrogen consumption during t h e decomposition.

The amount of

hydrogen chemisorption decreased t o 72% of t h e i n i t i a l v a l u e , i n d i c a t i n g t h a t t h e i n i t i a l value involved t h e chemisorption on K.

I t may be concluded from t h e above r e s u l t s t h a t m e t a l l i c potassium ( o r aluminum)

is formed by hydrogen reduction o f K R(CN) / A 1 0 4

promotion of Ru.

6

2 3

r e s u l t i n g i n an extensive

I t i s r e c a l l e d t h a t an i r o n c a t a l y s t

was used f o r a m o n i a s y n t h e s i s ( r e f . 1 3 ) .

derived from ferrocyanide

Although t h e formation of potassium

was n o t d e t e c t e d a t t h a t t i m e ( r e f . 1 4 ) , it might b e t h e reason of high a c t i v i t y .

REFERENCES

1 A. Ozaki, K. Aika and H . Hori, Bull. Chem. SOC. J a p . , 44, 3216 (1971) 2 K. Urabe, K. Aika and A. Ozaki, J. Cat., 42, 197 ( 1 9 7 6 r 3 K. Urabe, A. Ohya and A. Ozaki, t o be published 4 K. Aika, H. Hori and A. Ozaki, J . Cat., 21, 424 (1972) 5 K. Urabe, T. Yoshioka and A. Ozaki, J . Cat., i n p r e s s 6 F. Krauss, 2. Anorg. Allg. Chem., 59 (1927) 7 K. Masuno, S . Waku, Nippon Kagaku Zasshi 83, 161 (1962) 8 J . L . Howe, J. Am. Chem. SOC., I s 9 8 1 (1896) 9 V.A. Pospelov and G.S. Zhdanov, Zh. Fiz. a i m , SSSR, 21, 405 (1947) 1 0 I. Nakagawa and T. Shimanouchi, Spectrochim. A c t a , 2, 101 (1962) 11 A. Ohya, K. Urabe and A. Ozaki, Chem. L e t t . , 233 1 2 K. Urabe, K. S h i r a t o r i and A. Ozaki, J. Cat., submitted 13 A. Mittasch and E . Kuss, 2. Elektrochem., 34, 159 (1928) 14 A. Mittasch, E. Kuss and 0. E m e r t . 2 . Anorg. Allg. Chem., 193 (1928)

165,

1978,

170,

DISCUSSION J.G.

v a n OMMEN : I t i s n o t s t r a n g e t h a t C s z O a n d K m e t a l h a v e a

c o m p a r a b l e p r o m o t i n g e f f e c t o n Ru ?

Couldn't

i t be, t h a t y o u

(when C s 0 r e a c t s w i t h w a t e r i t 2 w i t h K 0 o r KOH. I n that c a s e , i n m y o p i n i o n it i s 2 e a s i e r t o understand t h e comparable promoter e f f e c t of CsOH and a c t u a l l y compare C s 2 0 o r C s O H forms CsOH)

KOH b e c a u s e

they are both s t r o n g bases.

YOU

a l s o showed t h a t p o i s o n i n g by w a t e r v a p o r w a s i r r e v e r s i b l e

b e c a u s e p o t a s s i u m i s c o n v e r t e d t o KOH.

But couldn't

it be possible

t h a t w a t e r v a p o r a l s o d e s t r o y e s t h e r u t h e n i u m s u r f a c e by r e c o n s t r u c t i n g i t , and so t h a t t h e l o w e r a c t i v i t y i s c a u s e d by t h i s effect ? In your paper you also mention an activity sequence of promotors, t h a t d i f f e r s from t h e sequence p r e s e n t e d in Fig. 2.

From this

f i g u r e , t h e s e q u e n c e is C s 2 0 > K > K 2 0 , w h i l e in t h e t e x t t h e s e q u e n c e

is K > C s 2 0 > K 2 0 .

Can you explain this ?

C a n y o u a l s o a g r e e t h a t if o n e c o m p a r e s t h e o v e r a l l a c t i v i t y of t h e C s 0 p r o m o t e d c a t a l y s t w i t h t h e K m e t a l promoted one, t h e 2 first o n e g i v e s t h e b e t t e r catalysts.

A. O Z A K I : K e x h i b i t s definitely a h i g h e r p r o m o t i n g e f f e c t t h a n K 2 0 a n d , after p o i s o n i n g w i t h H20, t h e p r o m o t i n g e f f e c t a p p r o a c h e s T h e s e q u e n c e K > C s 2 0 > K 0 is based o n t h e a c t i v i t y 2 p e r s u r f a c e a t o m , t h e latter being d e t e r m i n e d by H 2 chemisorption. t h a t o f K20.

A n activity i n c r e a s e d u e t o d e s i n t e g r a t i o n o f p a r t i c l e w a s f o u n d w i t h K-promoted Ru.

(J. Cat.

S,

430(1975)).

I f it t a k e s p l a c e ,

a n i n c r e a s e in a c t i v i t y i s o b s e r v e d in t h e second run.

P.E.H.

NIELSEN

: H a v e y o u o b s e r v e d a n y s u p p o r t effect i n g o i n g from

c a r b o n s u p p o r t e d Ru-K c a t a l y s t t o a l u m i n a s u p p o r t e d Ru-K c a t a l y s t ? A. O Z A K I

: T h e s e t w o catalysts, s h o u l d not b e c o m p a r e d i n t e r m s o f t h e

a c t i v i t y p e r s u r f a c e Ru.

But e v a l u a t i o n o f Ru d i s p e r s i o n o n c a r b o n

r e m a i n s d i f f i c u l t so t h a t n o c o m p a r i s o n h a s b e e n made.

K. J O H A N S E N : 1) W h i c h is t h e s p a c e v e l o c i t y used i n y o u r a c t i v i t y m e a s u r e m e n t s in T a b l e 2 ?

2) Have y o u any h i g h p r e s s u r e a c t i v i t y

measurements ?

A. O Z A K I : 1) 2 0 0 0 - 3 0 0 0 hr-'.

2) T h e r a t e o f N H 3 f o r m a t i o n on

Ru-K./Ac t e n d s t o a p p r o a c h a p l a t e a u v a l u e at h i g h pressures.

J.W.

H I G H T O W E R : F r o m T a b l e 2 and Fig. 2, I n o t i c e t h a t t h e l e a s t

a c t i v e u n p r o m o t e d c a t a l y s t s ( X and N-3) h a v e t h e l o w e s t a p p a r e n t a c t i v a t i o n energy.

B e c a u s e t h e r a t e s in t h e s e c a s e s a r e q u i t e low,

I d o not t h i n k t h a t d i f f u s i o n l i m i t a t i o n s c a n be r e s p o n s i b l e for t h e s e results.

T h e p r e - e x p o n e n t i a l f a c t o r m u s t be s o m e h o w g r e a t l y

390 altered by t h e promotor.

C a n you comment o n t h e s e r a t h e r s u r p r i s i n g

observations ?

A.

OZAKI

:

T h e p r o m o t o r effect of K on R u a p p a r e n t l y r e s u l t s in a

r e m a r k a b l e i n c r e a s e in t h e pre-exponential

factor.

T h i s must be

c a u s e d by a structural c h a n g e in Ru s u r f a c e , b u t we c a n n o t g i v e any f u r t h e r comment o n t h i s at present.

Z.G.

SZABO

: You have p u t e m p h a s i s o n t h e n i t r a t e effect, and not

w i t h o u t reason.

During t h e r e d u c t i o n , N H 3 i s first formed and

afterwards NHqN03. NZO.

U n d e r y o u r e x p e r i m e n t a l c o n d i t i o n s it p r o d u c e s

We a l s o u s e d t h i s d e c o m p o s i t i o n for m a k i n g h i g h l y active c a t a -

lysts.

It i s blowing u p t h e precipitate, r e s u l t i n g in very high

s u r f a c e development.

A s the activation energies are nearly the

same, it p o i n t s to a n increased p r e - e x p o n e n t i a l factor.

But t h i s

effect must operate at the most appropriate stage, during the preparation.

T h i s i s p e r h a p s t h e r e a s o n why p r e v i o u s l y added

i s n o t advantageous.

A.

OZAKI

:

T h a n k y o u for y o u r comment.

NH4N0

3

391

L I M I T I N G FACTORS ON THE STRUCTURAL CHARACTERISTICS OF Ru/SiOp AND RuFe/Si02 CATALYSTS

L . G U C Z I , K . MATUSEK, I . MANNINGER, J . K I R b Y and M . ESZTERLE I n s t i t u t e of I s o t o p e s o f t h e Hungarian Academy of S c i e n c e s , H-1525 Budap e s t , P.O.Box

7 7 , Hungary

ABSTRACT The e f f e c t of d i f f e r e n t t r e a t m e n t s on t h e s u r f a c e c h a r a c t e r i s t i c s of s u p p o r t e d ruthenium c a t a l y s t s h a s been i n v e s t i g a t e d . I t w a s e s t a b l i s h e d t h a t n o t o n l y a t c a l c i n a t i o n f o l l o w e d by hydrogen t r e a t m e n t , b u t a l s o a t the reduction a t

773 K , hydrogen i s s t r o n g l y bonded t o t h e s u r f a c e and

i n c o r p o r a t e d i n t o s u b - s u r f a c e l a y e r s . Repeated oxygen-hydrogen t r e a t m e n t removes t h i s hydrogen and a l o n g w i t h t h e d i s i n t e g r a t i o n of r u t h e n i u m p a r t i c l e s t h e a c t i v i t y of c a t a l y s t s i s enhanced. On i r o n a d d i t i o n m e t a l l i c i r o n i s n o t formed. N e v e r t h e l e s s , t h e a g g l o m e r a t i o n of ruthenium p a r t i c l e s

i s h i n d e r e d a t t h e p r e p a r a t i o n and t h e c a t a l y s t i s s t a b i l i z e d by t h e p r e s e n c e of Fe 0

2 3'

1. INTRODUCTION I n r e c e n t y e a r s much e f f o r t h a s been s p e n t on s u p p o r t e d r u t h e n i u m c a t a l y s t s f o r t h e i r b e i n g a c t i v e i n d i r e c t c o n v e r s i o n of NO

i n t o n i t r o g e n [I].

I n v e s t i g a t i o n s on t h e p r e p a r a t i o n of t h e s e c a t a l y s t s were m a i n l y f o c u s e d on t h e m e t a l l i c d i s p e r s i o n and t h e c a t a l y s t s t a b i l i z a t i o n under h i g h temper a t u r e oxidizing condition. When t h e c a t a l y s t p r e p a r e d from RuC13.3H20 s u p p o r t e d on alumina o r on s i l i c a was c a l c i n e d a t h i g h t e m p e r a t u r e t h e f o r m a t i o n of small Ru02 p a r t i c l e s h a s been o b s e r v e d by Mossbauer s p e c t r o s c o p y [2].

However, t h e

f o l l o w i n g r e d u c t i o n by hydrogen y i e l d e d m e t a l s w i t h l a r g e c r y s t a l l i t e s i z e . S i m i l a r e f f e c t was a l s o found by D a l l a - B e t t a

131.

Since a t h i g h temperature

c a l c i n a t i o n t h e f o r m a t i o n of h i g h l y v o l a t i l e RuOk may n o t be e x c l u d e d s t a b i l i z a t i o n and f i x a t i o n of ruthenium i n form of r u t h e n a t e ( i n c o m b i n a t i o n w i t h

B a , La 141 and w i t h Mg [ 5 ] ) was p r o p o s e d . However, Mossbauer d a t a o b t a i n e d on t h i s s t a b i l i z e d c a t a l y s t a f t e r r e p e a t e d n e t r e d u c i n g and o x i d i z i n g c y c l e show a l o s s i n r u t h e n i u m s t a b i l i z a t i o n which i s l i k e l y due t o t h e s e p a r a t i o n of ruthenium metal from t h e s t a b i l i z i n g BaO phase

161.

The m e t a l l i c d i s p e r s i o n and t h e c a t a l y t i c a c t i v i t y of s u p p o r t e d ruthenium

392 c a t a l y s t s , however, c a n be a l t e r e d n o t o n l y by c a l c i n a t i o n . A t h i g h temper a t u r e r e d u c t i o n i n hydrogen s t r e a m t h e s i n t e r i n g of m e t a l l i c phase was observed

171

tarnal e t al.

b u t t h i s was n o t t h e s o l e r e a s o n f o r d e c r e a s e d a c t i v i t y . Mon-

181

have shown t h a t Ru/A1203 reduced i n hydrogen a t 623 K

r e v e a l e d much lower i n i t i a l a c t i v i t y i n ammonia d e c o m p o s i t i o n t h a t t h e one c a l c i n e d a t 773 K . However, on r e d u c e d c a t a l y s t t h e a f t e r t h e f i r s t c y c l e s t a t i o n a r y a c t i v i t y became h i g h e r t h a n on t h e o x i d i z e d form. T h i s phenomenon p r e d i c t s a n e f f e c t d i f f e r e n t from t h a t due t o s o l e l y p a r t i c l e s i z e v a r i a t i o n , o t h e r w i s e i n i t i a l a c t i v i t y f o r reduced c a t a l y s t s h o u l d have been h i g h e r . O x i d a t i o n i t s e l f a t medium t e m p e r a t u r e range a f f e c t s t h e c a t a l y t i c a c t i v i t y e i t h e r by p e r t u r b i n g t h e s u r f a c e Ru atoms [9,10]

o r s i m p l y by removing t h e

carbonaceous m a t e r i a l formed a t exposure t o carbon monoxide

1111.

The p r e s e n t p a p e r i s concerned w i t h t w o problems. F i r s t , we wish t o s t u d y t h e h y s t e r e s i s o b s e r v e d i n . . t h e i n i t i a l a c t i v i t y of r e d u c e d ruthenium by u s i n g d i f f e r e n t t r e a t m e n t s and a p p l y i n g a combination of d i f f e r e n t methods f o r d e t e r m i n a t i o n of m e t a l l o a d , d i s p e r s i o n e t c . Second, s i m i l a r l y t o a n e a r l i e r s t u d y 1121 a t t h e a d d i t i o n of i r o n a s a second m e t a l t o Ru/Si02 t h e i n f l u e n c e of i r o n on t h e s t a b i l i z a t i o n of ruthenium will be i n v e s t i g a t e d . A s t e s t r e a c t i o n H2-D2

exchange, ethane-D2 exchange and t h e h y d r o g e n o l y s i s

o f e t h a n e w i l l be used. 2, EXPERIMENTAL

2.1.

Catalyst

RuC13 s o l u t i o n a t pH = 1 i s used f o r i m p r e g n a t i o n of s i l i c a g e l (SAS, 2 s p e c i f i c s u r f a c e i s 560 m g - l ) . A f t e r d r y i n g i t a t 393 K o v e r n i g h t t h e c a t a l y s t i s c a l c i n e d a t 573 K f o r 1 hour f o l l o w e d by r e d u c t i o n i n hydrogen

a t 7 7 3 K f o r 3 hours. I n t h e f o l l o wi n g s e c t i o n s , i f otherwise n o t s t a t e d , t h i s s t a n d a r d t r e a t m e n t i s used. F o r t h e p r e p a r a t i o n of b i m e t a l l i c c a t a l y s t RuC13/Si02 sample b e f o r e t h e c a l c i n a t i o n s t e p i s i m p r e g n a t e d w i t h Fe(N0 )

3 3

s o l u t i o n i n d i f f e r e n t c o n c e n t r a t i o n s a t pH = 1. Metal c o n c e n t r a t i o n i s det e r m i n e d by X-ray f l u o r e s c e n c e method u s i n g RuKa l i n e .

(1 C i 241Am

source

and S n - t a r g e t i s u s e d . ) Average p a r t i c l e s i z e i s measured by X-ray d i f f r a c t i o n . 2.2.

Mossbauer s p e c t r o s c o p y

57Fe i n 88% e n r i c h m e n t h a s been i n c o r p o r a t e d i n t o t h e c a t a l y s t and t h e Mossbauer s p e c t r a a r e r e c o r d e d a t c o n s t a n t a c c e l e r a t i o n mode u s i n g m u l t i c h a n n e l a n a l y z e r . 57C0

i n C r m a t r i x as s o u r c e and s t a i n l e s s s t e e l a s

r e f e r e n c e sample a r e a p p l i e d . Deconvolution of s p e c t r a a l s o c a r r i e d o u t u s i n g l e a s t s q u a r e f i t t i n g program [ l f .

393 2 . 3 . A d s o r p t i o n and c a t a l y t i c r e a c t i o n s A l l a d s o r p t i o n measurements a r e c a r r i e d o u t i n dynamic a p p a r a t u s u s i n g

n i t r o g e n and h e l i u m c a r r i e r g a s e s . TPD method a p p e a r s t o be s u i t a b l e t o m o n i t o r t h e n a t u r e of hydrogen a d s o r b e d on t h e s u r f a c e . I s o t o p e exchange between d e u t e r i u m and e t h a n e a s w e l l a s between hydrogen and d e u t e r i u m i s a n a l y s e d by AEI MS 1 0 C 2 mass s p e c t r o m e t e r c o n n e c t e d d i r e c t l y t o t h e c a t a l y t i c a p p a r a t u s . The d e u t e r a t e d peaks a r e c o r r e c t e d f o r n a t u r a l l y o c c u r i n g carbon-13 and f o r f r a g m e n t a t i o n . C a t a l y t i c hydrogenolysis of ethane i s i n v e s t i g a t e d i n a g l a s s c i r c u l a t i n g a p p a r a t u s c o n n e c t e d t o g a s chromatograph.

3 . RESULTS AND DISCUSSION I n t a b l e 1 t h e t r e a t m e n t , m e t a l c o n c e n t r a t i o n and X-ray d i f f r a c t i o n d a t a a r e p r e s e n t e d . I t i s obvious from t h e d a t a t h a t p a r t i c l e s i z e i n c r e a s e s under oxygen t r e a t m e n t and a t t h e same t i m e some l o s s i n Ru m e t a l c o n c e n t r a t i o n occurs.

TABLE 1. Change of metal c o n c e n t r a t i o n and c r y s t a l l i t e s i z e on d i f f e r e n t t r e a t m e n t s

Cat. No

Treatment

Ru, c o n t e n t

wt% Ru [O]

impregnation pH = 1

0.52

RulH3001

H2 a t 573 K

0.52

Ru [H5 0 O]

H2 a t 773 K

0.52

standard

0.45

Ru [O 40 01

O 2 a t 673 K and

Ru IS]

H2 a t 773 K spent

RuFe IS] (I) RuFe IS] ( I 1 ) RuFe IS] (11) RuFeCS] (111) RuFe [S] (111)

standard standard spent standard spent

Ru

PI

Fe, c o n t e n t wt%

-

X-ray

(d

amorphous crystalline i n trace

0.45

-

+ .O 21.5-5

0.41

-

amorphous

0.1 0.2 0.2 0.7 0.7

amorphous amorphous amorphous amorphous amorphous

0.49 0.49

0.35 0.49

0.35

18. 5 2 2 . 5

A t h i g h t e m p e r a t u r e hydrogen t r e a t m e n t t r a c e of X-ray d i f f r a c t i o n l i n e can

be o b s e r v e d , which means t h a t s i n t e r i n g of m e t a l p a r t i c l e s o c c u r s under t h i s c o n d i t i o n . When Ru/Si02 and RuFe/Si02 c a t a l y s t s a r e compared t h e main char a c t e r i s t i c i s t h a t ruthenium p a r t i c l e s i z e d r a s t i c a l l y d e c r e a s e s on i r o n a d d i t i o n ( s e e l i n e 4 and e.g.

l i n e 7 ) . On s p e n t c a t a l y s t s r e g a r d l e s s of

whether i t c o n t a i n s i r o n o r n o t t h e ruthenium c o n c e n t r a t i o n f u r t h e r d e c r e a s e s (compare l i n e s 4 and 6 ) and s i p u l t a n e o u s l y t h e l a r g e c r y s t a l l i t e s of ruthenium a r e d i s i n t e g r a t e d and t h e sample becomes amorphous.

The change i n m e t a l

394 c o n c e n t r a t i o n a r e i l l u s t r a t e d by X-ray

fluorescence spectra taken a t

d i f f e r e n t r e a t m e n t s as shown i n f i g u r e 1.

X-ray f l u o r e s c e n c e s p e c t r a of c a t a l y s t s a f t e r d i f f e r e n t treatment

( a ) : 0 . 5 2 w t $ Ru standard, ( b ) : SiOz impregnated with RuCl

3

solution,

( c ) : A s ( b ) a f t e r standard treatment, ( d ) : A s ( b ) a f t e r reduction

with H2 a t 573 K , ( e ) : A s ( c ) a f t e r ethane exchange, ( f ) : RuFe ( 0 . 7 w t k ) a f t e r ethane exchange.

When c a t a l y s t s p r e p a r e d e i t h e r by s t a n d a r d t r e a t m e n t o r by r e d u c t i o n w i t h hydrogen a t 773 K a r e t e s t e d i n e t h a n e h y d r o g e n o l y s i s i t i s found t h a t i n b o t h c a s e s t h e c a t a l y t i c a c t i v i t y measured a t 523 K u s i n g 1 O : l hydrogen-1 -1 mol s gcat). S i n c e t h e - e t h a n e m i x t u r e i s v e r y low ( i n t h e r a n g e of c r y s t a l l i t e s i z e i s d i f f e r e n t f o r t h e t w o c a t a l y s t s we may assume t h a t t h e

l o w c a t a l y t i c a c t i v i t y i s d e v o t e d t o some e x t e n t t o t h e h i g h t e m p e r a t u r e hydrogen t r e a t m e n t b u t t h e main e f f e c t i s n o t t h e s i n t e r i n g . F u r t h e r m o r e , a c t i v i t y enhancement of a b o u t two o r d e r s of magnitude i s e x p e r i e n c e d on t h e r e p e a t e d oxygen (20 Torr) and hydrogen ( 4 0 Torr) c y c l e s between h y d r o g e n o l y s i s r u n s and t h e a c t i v i t y l e v e l s o f f a f t e r a b o u t 10 c y c l e s . For t h e e l u c i d a t i o n of t h i s e f f e c t i t i s assumed a s working h y p o t h e s i s t h a t b e s i d e t h e d i f f e r e n c e i n c r y s t a l l i t e s i z e between Ru/di02 and RuFe/Yi02 t h e p r e s e n c e of hydrogen s t r o n g l y bonded t o t h e c a t a l y s t i s t h e r e a s o n f o r t h e low i n i t i a l a c t i v i t y . TPD d a t a shown i n t a b l e 2 i n d i c a t e t h a t i f t h e t r e a t m e n t i n hydrogen i s c a r r i e d o u t a t 773 K , t h e i n t e n s i t y of a second peak desorbed a t 773 K i n c r e a s e s and t h i s s o r t of hydrogen c a n n o t be removed a t lower t e m p e r a t u r e , i . e .

a t h i g h t e m p e r a t u r e hydrogen t r e a t m e n t p a r t of m e t a l

395 s i t e s remains c o v e r e d .

TABLE 2 Oxygen a d s o r p t i o n and hydrogen TPD d a t a

0 C a t . No

Treatment

adsorbed

2

/urn01

cat g-l

TPD (Hydrogen) Peak a k S 3 K

Peak a 773 K

Ru

Standard

Ru

H2 a t 573 K

Ru

4.17

-

-

0

Ru

Repe a t e d 0 -H cycl. 2 2 H~ a t 573 K~ b H2 a t 773 K

-

4.6

RuFe(1)

H2 a t 573 K

7.8

RuFe ( I )

Re pe a t e d 0 2 -H 2 c y c l .

7.2

Ru

trace 8.2

RuFe(1)

H~ a t 573 K~

RuFe ( I )

H2 a t 773 If

b

-

-

-

2.4

1.6

-

3.5

7.0

RuFe(I1)

Standard

2.6

RuFe(I1)

H2 a t 573 K

9.9

RuFe (11) RuFe ( I1 )

H~ a t 573 K~ b H2 a t 773 K

-

RuFe (111)

H2 a t 573 K

4 4

RuFe ( I I1 )

Spent

4.1

.

-

.

-

4 1

2.4

5.2

10.9

-

-

a ) C a t a l y s t t r e a t e d i n H2 a t 573 K and c o o l e d down i n H2. b) C a t a l y s t t r e a t e d i n H2 a t

773 K and c o o l e d down i n H2.

Oxygen a d s o r p t i o n g i v e s o n l y a p p r o x i m a t i o n t o t h e number of s u r f a c e s i t e s and i t i s n o t a s s e n s i t i v e as o t h e r methods. N e v e r t h e l e s s , i n some c a s e s i t i s used b e a r i n g i n mind t h a t t h e e x a c t s t o i c h i o m e t r y i s s t i l l n o t c l e a r .

Kubicka r14J h a s found a s t o i c h i o m e t r y O/Ru i s 2 , w h i l e Gonzales 1151 was u s i n g O/Ru e q u a l t o 1. I n t a b l e 2 t h e d a t a a r e summarized from which i t a p p e a r s t h a t oxygen a d s o r p t i o n g e n e r a l l y f o l l o w s t h e p a r t i c l e s i z e change d e t e r m i n e d by o t h e r methods. Ru/SiO,

-

t r e a t e d by s t a n d a r d p r o c e d u r e r e v e a l s

l a r g e c r y s t a l l i t e s i z e and o u r method i s n o t s e n s i t i v e enough t o t r a c e oxygen a d s o r p t i o n ( l i n e 1 ) . When RuFe/Si02 i s t r e a t e d i n t h e s i m i l a r way oxygen a d s o r p t i o n c a n be measured b u t t h i s amount i s a b o u t f o u r t i m e s l e s s t h a n t h e one d e t e r m i n e d a f t e r hydrogen t r e a t m e n t o n l y a t 573 K ( s e e l i n e 1 1 ) . T h a t i s , i r o n e x e r t s s t a b i l i z a t i o n e f f e c t on Ru p a r t i c l e s d u r i n g t h e f o r m a t i o n of

396 m e t a l from RuC13. T h i s e f f e c t i s s i g n i f i c a n t o n l y a t c a l c i n a t i o n b e c a u s e when r e d u c t i o n t a k e s p l a c e under m i l d c o n d i t i o n s (573 K i n hydrogen) t h e r e i s o n l y a l i t t l e d i f f e r e n c e i n oxygen a d s o r p t i o n (compare l i n e s 2 and 6 ) .

I t i s , however, s i g n i f i c a n t t h a t a t 0 . 2 w t $ Fe a d d i t i o n n o t o n l y t h e oxygen a d s o r b e d b u t t h e amount of hydrogen r e c o v e r e d by TPD measurements p a s s t h r o u g h maximum.

I n o r d e r t o f u r t h e r c l a r i f y t h e e f f e c t of hydrogen, r e d u c t i o n of Ru/Si02 c a t a l y s t was c a r r i e d o u t in d e u t e r i u m a t 773 K. A f t e r e v a c u a t i o n 50 T o r r hydrogen g a s was a d m i t t e d t o t h e c a t a l y s t and a t 473 K t h e d e u t e r i u m removal from t h e s u r f a c e w a s measured by t h e i n c r e a s e o f HD peak i n t e n s i t y . The r a t e -8 of D atoms e n t e r i n g t h e hydrogen i s 6 . 6 ~ 1 0 m o l s-l g c i t . The r a t e of e t h a n e h y d r o g e n o l y s i s measured a f t e r e v a c u a t i o n i s 6 . 6 ~ 1 0 - l mol ~ s-l gcat.

A t the

f o l l o w i n g oxygen t r e a t m e n t t h e r a t e o f t h e e n t e r of d e u t e r i u m r e m a i n s t h e same b u t t h e r a t e of h y d r o g e n o l y s i s i n c r e a s e up t o 1 . 4 3 ~ 1 0 - m ~ o l s-l

The

i n t e g r a l amount of d e u t e r i u m removed w a s a b o u t t w i c e as h i g h as can be e x p e c t e d o n t h e b a s i s of m e t a l s u r f a c e a r e a . C o n s i d e r i n g t h e s i m u l t a n e o u s i n c r e a s e i n c a t a l y t i c a c t i v i t y i t seems r e a s o n a b l e t o assume t h a t d e u t e r i u m i s i n c o r p o r a t e d i n t o t h e sub-surface

l a y e r s and a t t h e s u r f a c e " c o r r o s i o n "

under t h e e f f e c t of c y c l i c oxygen t r e a t m e n t i t becomes a c c e s s i b l e f o r hydrogen and f o r t h e r e a c t i o n m i x t u r e . I n t h e l i g h t of t h e s e e x p e r i m e n t s t h e r e s u l t o b t a i n e d by Montarnal [8J can be u n d e r s t a n d . During t h e r e d u c t i o n p r o c e d u r e hydrogen i s i n c o r p o r a t e d i n t o a few s u b - s u r f a c e l a y e r s and under t h e e f f e c t o f r e a c t i o n m i x t u r e p a r t i c l e s a r e d i s i n t e g r a t e d w i t h t h e s i m u l t a n e o u s removal of hydrogen atoms s t r o n g l y bonded t o ruthenium atoms. T h i s i d e a i s f u r t h e r s u p p o r t e d by t w o o t h e r e x p e r i m e n t a l f a c t s .

When oxygen-hydrogen t r e a t m e n t

i s c a r r i e d o u t i n X-ray d i f f r a c t o m e t e r and t h e Ru l i n e a t 44.0'

i s measured,

t h e l i n e p r a c t i c a l l y d i s a p p e a r s a f t e r a b o u t 1 0 c y c l e s of t h i s t r e a t m e n t . T h i s i s i n agreement w i t h t h a t we found when f r e s h and s p e n t Ru/SiO

are

compared ( s e e t a b l e 1 ) . I t means t h a t t h e i n c r e a s e i n p a r t i c l e s i z e i s r a t h e r due t o t h e oxygen-hydrogen t r e a t m e n t t h a n t o t h e c a t a l y t i c r e a c t i o n . The i n c r e a s e i n t h e number of s u r f a c e Ru atoms i s proved i f c a t a l y t i c A -D and ethane-D2 exchange a r e s t u d i e d d u r i n g t h i s i n i t i a l s t a b i l i z a t i o n 2 2 p e r i o d . I n t a b l e 3 t h e r e s u l t s a r e p r e s e n t e d . Although H-D exchange p r o c e e d s much f a s t e r t h a n ethane-D2 r e a c t i o n , t h e t w o r e a c t i o n s change p a r a l l e l a f t e r e a c h oxygen-hydrogen t r e a t m e n t . S i n c e H-D exchange presumably does n o t have s t r u c t u r e s e n s i t i v i t y and d i r e c t l y p r o p o r t i o n a l t o t h e number of s u r f a c e s i t e t h e p a r a l l e l i s m i s s u r e l y a s c e r t a i n e d t o t h e i n c r e a s e of working metallic surface. We have shown t h a t on i r o n a d d i t i o n n o t o n l y t h e i n i t i a l p a r t i c l e s i z e of t h e c a l c i n e d and reduced c a t a l y s t i s s m a l l e r b u t t h e oxygen a d s o r p t i o n on s p e n t c a t a l y s t i s p r a c t i c a l l y n o t a l t e r e d , e i t h e r . To s t u d y t h e e f f e c t of

397 i r o n i n RuFe/Si02 c a t a l y s t Mossbauer s p e c t r o s c o p y on c a t a l y s t c o n t a i n i n g i r o n 57Fe form h a s been a p p l i e d . I n f i g u r e 2 t h e r e s u l t s a r e p r e s e n t e d .

TABLE 3 The r a t e s ( i n m o l s-l g c a t ) o f ethane-D2 and H 2 -D2 exchange on Ru/Si02 and RuFe/YiO c a t a l y s t s a s a f u n c t i o n o f t h e number o f runs 2 0 w t $ Fe

No.

runs

0 . 2 w t $ Fe

0.1 w t $ Fe

C2HS/D2

H/D exch.

1

4.2xlO-'l

7 . Z X ~ O - ~ 5 . 6 ~ 1 0 ~ 1~. 2 ~ 1 O - ~ 5 . 8 ~ 1 0 - ~

2

8 . 1 ~ 1 0 - ~ l~. l ~ l O - ~

C2H6/D2

4.9~10-~

H/D exch.

C2H6/D2

H/D exch. 6.6~10-~

1 . 4 ~ 1 0 - ~ 8.7x10-*

3.4~10-~

3

l . l x l ~ - l o 2.lX10-3

2.4~10-~

1 . 4 ~ 1 0 - ~ 2.1~1O-~

6.6~10'~

4

1 . 6 ~ 1 0 - ~ O 2.2x10-3

3,gxl~-6

2. O X ~ O - ~ 3 . 2 x N 7

1. ~ x I O - ~

5

1.8~10 - ~ ~

-

2.0~10-5

7. kX1o-7

2. lX10-5

F i g u r e 2. Mossbauer s p e c t r a ( a ) 0.5 w t $ FeC13/Si02; ( b ) 0.5 w t $ Fe(N03)3/Si02; ( c ) 0.5 w t $ Fe(NO3l3 + 0.52

w t $ RuC13/Si02; ( d ) 0.7 w t $ Fe(N03)3

w t $ RuC13/Si02;

+ 0.52 reduced a t

773 K i n H2; ( e ) 0.7 w t $ Fe(N03)3

w t $ RuC13/Si02;

+ 0.52 reduced a t

773 K i n H2; (f) 0.7 w t $ Fe(NO3)? + 0.52 w t $ RuC13/Si02;

reduced a t

773 K i n H2; ( g ) 0.2

w t $ Fe(N03)3 + 0.52

w t $ RuC13/Si02;

reduced a t

773 K i n €I2. ( d ) - ( e ) measured i n a i r , ( f ) - ( g ) i n hydrogen.

398 Comparing t h e upper 3 s p e c t r a i t c a n be e s t a b l i s h e d t h a t t h e s i x p e a k s i n t h e spectrum a t

77 K i s a r e s u l t o f weak s p i n - s p i n i n t e r a c t i o n due t o

t h e Fe7+ i o n s s i t t i n g i n l a r g e d i s t a n c e from e a c h o t h e r ; t h i s i s n o t e f f e c t e d by t h e a n i o n s and by t h e p r e s e n c e of ruthenium. S i n c e a t 298 K no peak was found a f t e r t w o m i l l i o n c o u n t s i n e a c h channel we may assume t h a t t h e Debye-Waller f a c t o r i s s m a l l , c o n s e q u e n t l y Fe"

i o n s a r e weakly bonded t o

t h e s u r f a c e . A f t e r r e d u c t i o n i n hydrogen t h e m o s t c h a r a c t e r i s t i c i s t h e d e c r e a s e of i n t e n s i t y of peaks which a p p e a r s a t h i g h v e l o c i t y , i . e .

which

r e s u l t s from t h e paramagnetic r e l a x a t i o n . I t shows a s t r o n g e r s p i n - s p i n interaction,

t h e r e f o r e m e t a l l i c p a r t i c l e s a r e a g l l o m e r a t e d t o some e x t e n t .

When meafurements a r e c a r r i e d o u t i n hydrogen (sample h a s n o t been exposed

t o a i r a f t e r r e d u c t i o n ) m e t a l l i c i r o n i s n o t formed, o n l y some amount of Fe2+ ( i n a b o u t

6% c o n c e n t r a t i o n ) . Formation of RuFe b i m e t a l l i c phase h a s n o t

been d e t e c t e d a t a l l , a l t h o u g h t h e Mossbauer p a r a m e t e r s f o r t h i s a l l o y a r e well known

(161.

We may s a y , t h e r e f o r e ,

[17]

t h a t u n l i k e t h e P t F e system

i t i s n o t t h e s u p e r - s t r u c t u r e p h a s e which i n c r e a s e s t h e m e t a l l i c d i s p e r s i o n . S i n c e on c a l c i n a t i o n i n oxygen Ru02 and Fe 0

2 3

a r e formed and t h i s l a t t e r

c a n n o t be r e d u c e d i n t o m e t a l l i c phase a s i n d i c a t e d by Mossbauer s p e c t r a , t h e s t r o n g i n t e r a c t i o n between Fe203 o x i d e phase and r u t h e n i u m formed a f t e r r e d u c t i o n i s t h e r e a s o n why t h e m i g r a t i o n of m e t a l l i c ruthenium p a r t i c l e s t o form l a r g e c r y s t a l l i t e s i s h i n d e r e d .

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

a t oxygen-hydrogen t r e a t m e n t i s n o t changed, e i t h e r . I n t a b l e 4 i t i s shown t h a t a t oxygen t r e a t m e n t a t

573 K t h e Fe2+ c o n t e n t i s reduced and i t

remains t h e same a t t h e f o l l o w i n g hydrogen t r e a t m e n t .

The r e d u c t i o n a t

e l e v a t e d t e m p e r a t u r e r e s t o r e s t h e o r i g i n a l Fe2+ c o n t e n t .

TABLE 4 Mossbauer p a r a m e t e r s f o r RuFe/Si02 c a t a l y s t a t d i f f e r e n t t r e a t m e n t s Fe Samp- C le Fe2+

2+

6,mm s -1 A , mm s-l

Fe

r,

mm s

-1

6,mm s -1 A , mm s -1 ~~

1 2

3

0.067 (0.015)

0.88 (0.04)

'+ r,mm

dx2 8-l

df

~~~

1.93

0.69

(0.08)

(0.09)

0.30 (0.06)

0.44 (0.08)

(0.01)

0.30

0.026

1.09

(0.01)

(0.03)

2.22 (0.05)

0.068 (0.01)

1.00 (0.02)

1.97

0.58

0.31

(0.05)

(0.06)

(0.01)

1.04

0.93 (0.01)

1.22

(0.01)

1.04 (0.01)

1.02 (0.01)

1.07

0.91 (0.01)

1.10 (0.02)

1.69

Sample 1: S t a n d a r d t r e a t m e n t ; Sample 2: 02-Hp t r e a t m e n t a t 523 K ; Sample 3: H2 t r e a t m e n t a t 873 K ; C a l c u l a t i o n : c o n s t r a i n e d d o u b l e t f o r Fe and Fe3+ w i t h e q u a l l i n e w i d t h and a m p l i t u d e ; C

2+

a+ - [Fez+] / ( [Fe2+] + rFe3+1 ) ;

f i g u r e s i n p a r a n t h e s i s a r e t h e e r r o r s of c a l c u f k t e d .

399 S i n c e Mossbauer d a t a g i v e c l e a r e v i d e n c e t h a t i r o n i n m e t a l l i c phase i s n o t formed d i r e c t metal-to-metal

i n t e r a c t i o n c a n n o t be o p e r a t i v e . T h i s i s

shown by t h e d i s t r i b u t i o n of d e u t e r a t e d p r o d u c t s of e t h a n e which a r e t h e same r e g a r d l e s s of t h e i r o n c o n t e n t . The s o l e e f f e c t i s t h e i n c r e a s e i n t h e number of a c t i v e s i t e s on t h e e f f e c t of i r o n . F u r t h e r i m p o r t a n t e v i d e n c e :18]

f o r t h i s s t a t e m e n t i s t h a t i n e t h a n e and n-butane h y d r o g e n o l y s i s no

change i n e n e r g y of a c t i v a t i o n and s e l e c t i v i t y h a s been o b s e r v e d , t h e o n l y e f f e c t i s t h e i n c r e a s e i n a c t i v i t y . Unlike t h e PtFe/Si02,

where t h e f o r m a t i o n

of b i m e t a l l i c system was a l s o proved by t h e change i n a c t i v a t i o n e n e r g y and s e l e c t i v i t y , h e r e o n l y t h e morphology and s t r u c t u r e a r e a f f e c t e d i n d i r e c t l y by i r o n a d d i t i o n by h i n d e r i n g t h e m i g r a t i o n of m e t a l l i c p a r t i c l e s d u r i n g t h e p r e p a r a t i o n and r e a c t i o n . Consequently ruthenium i s s t a b i l i z e d by t h e p r e s e n c e of i r o n w i t h o u t any e l e c t r o n i n t e r a c t i o n .

4 . CONCLUSIONS

Two main c o n c l u s i o n s c a n be o b t a i n e d from t h e r e s u l t s : i ) Hydrogen a f t e r t h e r e d u c t i o n p e r i o d i s i n c o r p o r a t e d a l s o i n t o t h e sub- s u r f a c e l a y e r s of Ru p a r t i c l e s and i t i s s t r o n g l y bonded. Repeated oxygen-hydrogen c y c l e c a n be used n o t o n l y t o d i s i n t e g r a t e t h e p a r t i c l e s b u t t o remove t h e hydrogen bonded s t r o n g l y and t h i s p r o c e d u r e r e s u l t s i n t h e enhancement of c a t a l y t i c a c t i v i t y r e g a r d l e s s of t h e n a t u r e of r e a c t i o n . i i ) I r o n a d d i t i o n h i n d e r s t h e a g g l o m e r a t i o n of p a r t i c l e s , a l t h o u g h i r o n i t s e l f i s n o t r e d u c e d i n t o m e t a l l i c p h a s e . T h i s e f f e c t c a n be i n d i c a t e d by t h e c o n s t a n c y of k i n e t i c p a r a m e t e r s i n a l l r e a c t i o n s s t u d i e d and t h e i n c r e a s e i n a c t i v i t y i s e x c l u s i v e l y due t o t h e l a r g e r number of s u r f a c e r u t h e n i u m atoms.

REFERENCES 1. M . S h e l e f and H.S.

Gandhi, I n d . Eng. Chem. Prod. Res. Develop. _1_1(1972)2

2. C.A.

Clausen I11 and M.L.

3. R.A.

Dalla-Betta,

Good, J. C a t a l . 3 ( 1 9 7 5 ) 9 2

J. C a t a l . 2 ( 1 9 7 4 ) 5 7

4 . M . S h e l e f and H.S.

Gandhi, P l a t . M e t a l s . Rev. = ( 1 9 7 4 ) 2

5 . S . J . T a u s t e r , L . L . M u r r e l l and J . P . DeLuca, J . C a t a l . %(1977)258 6. C.A. Clause I11 and M.L. Good, J . C a t a l . 9 ( 1 9 7 7 ) 5 8 7 . A . A . Davydov and A . T . B e l l , J . C a t a l . 3 ( 1 9 7 7 ) 3 3 2 Courty and R.E.

8. A . G .

F r i e d l a n d e r , Ph.R.

9. K.C.

Taylor, J. C a t a l . 2 ( 1 9 7 3 ) 4 7 8

1 0 . M.F.

Brown, and R.D.

11. R.A.

Dalla-Betta

M o n t a r n a l , J. C a t a l . %(1977)712

Gonzales, J . Phys. Chem. =(1976)1731

and M. S h e l e f , J . C a t a l . 5 ( 1 9 7 7 ) 1 1 1

1 2 . L . G u c z i , K . Matusek and M . E s z t e r l e , p a p e r s u b m i t t e d , J . C a t a l .

13. I . D g z s i , D . L .

Nagy, L . Guczi and M . E s z t e r l e , R e a c t i o n K i n e t i c s and

C a t a l y s i s L e t t e r s , Vo1.8(1978)301

400

14. H . Kubicka, Reaction Kinetics and Catalysis Letters, 5(1976)223 15. R.D.

Gonzales, personal communication

16. H . Ohno, J. Phys. SOC. Japan, 2 ( 1 9 7 1 ) 9 2 17. L. Guczi, K. Matusek, M. Eszterle, Paper presented at EUCHEM Conference, Namur, 1977

18. L. Guczi, K. Matusek, M. Eszterle, F. Till and D.L.

Nagy, to be pub-

lished in Acta Chim. Acad. Sci. Hung.

DISCUSSION H. C H A R C O S S E T : 1 ) W i t h r e s p e c t t o y o u r H2-TPD e x p e r i m e n t s ( T a b l e 2 )

,

w e h a v e o b s e r v e d o v e r P t / S i 0 2 t h e s a m e p h e n o m e n a a s d e s c r i b e d by y o u for Ru/SiOZ

: 2 p e a k s a r e o b t a i n e d a f t e r c o o l i n g d o w n from

7 7 3 K i n s t e a d o f o n e a f t e r c o o l i n g from a b o u t S O O K .

The same

e x p e r i m e n t s p e r f o r m e d o v e r a P t w i r e in UHV c o n d i t i o n s o f d e s o r p t i o n did n o t s h o w t h i s effect.

We arrived to the conclusion that the

s u p p o r t o r c a n d ) i t s i m p u r i t i e s w e r e i n v o l v e d in t h e e x p e r i m e n t s o n Pt/Si02.

H a v e y o u c o n s i d e r e d a p o s s i b l e e f f e c t o f t h e s u p p o r t in

y o u r case, instead o f p e n e t r a t i o n o f H in t h e sub-surface l a y e r s ? 2)

You suppose that 0 2 - H 2

c y c l e s d i s i n t e g r a t e t h e RU particles.

c o u l d b e p r o v e d by m i c r o s c o p i c examinations.

This

Have you experimental

data with regard to that possibility ?

L. G U C Z I : W e h a v e m e a s u r e d in t h e T P D exp'eriments o n P t / S i 0 2 o n l y o n e p e a k at 4233.

T h e b l a n k S i O z showed a s m a l l p e a k at 9 2 3 3 b u t

t h e o n e o b t a i n e d in case o f Ru/Si02 i s h i g h e r t h a n o n t h e s u p p o r t itself. Si02.

Moreover, t h e t e m p e r a t u r e i s l o w e r f o r R u / S i 0 2 t h a n f o r Nevertheless,

t h e p e a k c o m i n g at 7 7 3 K f o r Ru/Si02 i s

characteristic for the strength of the hydrogen sorption rather than for t h e a b s o l u t e amount.

However, you are right that we need

additional experiments concerning the deuterium exchange with the s u p p o r t itself. T h e d i s i n t e g r a t i o n o f R U p a r t i c l e s h a s b e e n p r o v e d by X-ray diffraction.

T h e spent

Ru/Si02 a f t e r having u n d e r g o n e s e v e r a l

0 2 - H z c y c l e s , d o e s not s h o w X-ray l i n e s and it b e c o m e s amorphous. At t h e s a m e t i m e t h e rate o f n2/Dz e x c h a n g e i n c r e a s e s indicating t h a t t h e n u m b e r o f active s i t e s r e s p o n s i b l e f o r t h e c a t a l y t i c a c t i v i t y a l s o increases. B. D E L M O N : A t t h e end o f y o u r c o n c l u s i o n , y o u w r i t e "Iron a d d i t i o n hinders the agglomeration of particles, although iron itself is not r e d u c e d i n t o m e t a l l i c phase".

I w o n d e r w h e t h e r t h i s "although" i

401 should not be "because".

What is your opinion concerning the

interpretation I suggest, namely that o x i d e s quite often efficiently play the role of "spacers" o r o f grain growth inhibitors 7 L. GUCZI : Since there i s no metal-metal interaction a s a result o f the fact that iron cannot b e reduced beyond the Fe2+ state, w e believe that Fe203 particles which are very finely dispersed interact with ruthenium atoms and prevent their agglomeration t o form larger crystals.

Thus the substitution o f the word "although" by "because" is

correct. At t h e present stage w e still do not know if this effect starts at the nucleation t o form particles.

However, the migration o f small

ruthenium clusters are prevented by iron oxide. M. CARBUCICCHIO : Have y o u interpreted t h e paramagnetic central contribution o f the Mdssbauer spectra obtained for unreduced catalysts (Fig. 2a, b, c) 7 L. GUCZI : Thesespectra are typical f o r isolated paramagnetic Fe

3+

ions (probably in the form of some ferric aquo-complex) with long spin-relaxation times. outmost lines is 5 7 2 P.A.

The hyperfine field corresponding t o t h e

2 2 kG.

SERMON : T P D results (Gombeis e t al.,

Naturwisenechaffen,

(1977))

have shown that H 2 spillover onto A1203 from P t may be desorbed at high temperature.

Were your results for Pt/Si02 (mentioned t o

H. Charcosset) obtained under identical reduction and dcsorption conditions as those for Ru/Si02, o r might the H2 taken u p by Ru/Si02 at 600-700K and desorbed at 773K be spillover rather than subsurface 7

If t h i s i s not true, are there sufficient "internal"

Ru atoms to accommodate the additional 2H atoms per "surface" Ru atom (suggested by T P D and deuterium work) in a subsurface state, particularly in view o f the high RU dispersion indicated by

O2

chemisorption and XRD 7 L. GUCZI : We may exclude the effect of spillover in the case Of Pt/Si02 and Ru/Si02.

T h e experimental facts are a s follows : 1) o n

Pt/Si02, H/Pt i s equal to 1.3

if the O/Pt value i s equal t o 1.

It means that considerable amount o f hydrogen is not taken up by the support.

2)

If we reduce a 5 % FePt/Si02 having a Fe/Pt ratio

%

2.2,

t h e amount o f i r o n a b o v e t h e s t o i c h i o m e t r i c r a t i o c a n n o t

b e reduced.

T h e r e a s o n i s p r o b a b l y t h a t h y d r o g e n activated at P t

c a n r e d u c e t h e iron w h i c h i s in c o n t a c t w i t h p l a t i n u m and h y d r o g e n c a n n o t m i g r a t e f a r away. W e b e l i e v e in our i d e a based upon t h e f a c t s t h a t 0 2 - H 2 increases the number

titration

of s u r f a c e s i t e s ( p a r a l l e l i s m between H/D

a x c h a n g e C H 6-D2 e x c h a n g e and t h e d i s a p p e a r a n c e o f t h e RU line at 4 4 " ) t h a t d i s i n t e g r a t i o n o f t h e p a r t i c l e s occurs.

R. M O N T A R N A L : You m e n t i o n e d o u r r e s u l t s (your ref.8)

on NH 3 d e c o m p o s i t i o n (30OOC) w i t h r e s p e c t t o t h e need f o r a c t i v a t i o n by

N H 3 at r e l a t i v e l y h i g h t e m p e r a t u r e s ( 7 0 O - 8 0 O 0 C ) , o f a R U c a t a l y s t reduced in H

2'

O n t h e c o n t r a r y , simple c a l c i n a t i o n o f p r e c u r s o r s o f t h e R u catal y s t s i m m e d i a t e l y g i v e s c a t a l y t i c activity f o r N H 3 d e c o m p o s i t i o n a t 300'C. If I h a v e well u n d e r s t o o d , y o u a t t r i b u t e t h e need o f a c t i v a t i o n f o r t h e r e d u c e d c a t a l y s t t o t h e p r e s e n c e o f strongly i n h i b i t i n g a d s o r b e d a t o m s w h i c h are eliminated by a c t i v a t i o n by N H g i v i n g s i m u l t a n e o u s l y d i s i n t e g r a t e d small R u particles.

3

at 7 O O 0 C , It s e e m s

to us that we can agree with your interpretation which is a more p r e c i s e d e s c r i p t i o n o f what w e called r e c o n s t r u c t i o n p h e n o m e n o n u n d e r t h e i n f l u e n c e o f reactants.

L. G U C Z I : T h e term r e c o n s t r u c t i o n used in y o u r p a p e r c a n b e i n f l u e n c e d b y t w o f a c t o r s : o n e i s t h e r e m o v a l o f hydrogen s t r o n g l y bound to R U p a r t i c l e s and t h e o t h e r i s t h e d i s i n t e g r a t i o n o f R U p a r t i c l e s i n t o s m a l l e r crystallites.

J. S C E E V E : C a n y o u e x p l a i n t h e loss o f r u t h e n i u m d u r i n g c a l c i n a t i o n and r e a c t i o n ?

I f d u e t o s u b l i m a t i o n o f r u t h e n i u m o x i d e it

s h o u l d a l s o o c c u r during t h e 0 2-H2 cycle,

L. GUCZI : We did not look d e e p l y for t h e r e a s o n o f t h e loss o f RU from t h e surface.

P r o b a b l y t h e m a i n loss o c c u r s during t h e

c a l c i n a t i o n s t e p b e c a u s e o f t h e h i g h t e m p e r a t u r e t r e a t m e n t in o x y g e n atmosphere.

P.G.

A d d i t i o n a l l o s s s e e m s t o be o f m i n o r importance.

MENON : Y o u r r e s u l t s o n RU c a t a l y s t s are v e r y s i m i l a r t o our

observations on Pt-A1203 catalysts, which I mentioned

earlier in

connection with the paper of Scholten.

Exposure of the catalyst

to high temperature (500°C) in H2 results in a stronger chemisorption of H2 on the catalyst.

But a few H2-02 cycles at 20'C

to anneal or smooth out the heterogeneity of the catalyst.

seem It is

remarkable that this effect i s seen for both supported P t and Ru catalysts.

Perhaps this phenomenon is of a more general occurrence

in supported metal catalyst than hitherto suspected.

L. GUCZI

:

Just one remark: the 02-H2 cycle was carried out at the

reaction temperature. A. OZAKI : Deuterium displacement with SiOH must be taken into

account when you work on metal/Si02 catalyst.

L. GUCZI : It might be possible that the excess amount of deuterium

entering the gas phase is partly due to the SiOH groups.

However

according to TG-MS data after treatment at high temperature for a long time, the major part of the structural OH groups is destroyed.

So their contribution seems to be negligible.


405

PREPARATION ASPECTS OF Ru-SUPPORTED CATALYSTS AND THEIR INFLUENCE ON THE FINAL PRODUCTS A. Bossi, F. Garbassi, A. Orlandi, G. Petrini and L. Zanderighi'

"G. Donegani" Research Institute, Montedison SPA, Novara (Italy) 'Universitl

di Milano, Istituto di Chimica Fisica, Milano (Italy)

ABSTRACT A wide investigation of the influence of some preparation aspects, such as the nature of the support, the acidity of the precursor solution and the activation temperature, was carried out on Ru catalysts supported on Si02, A1203, Si02-A1203 and MgO. For this purpose several bulk and surface techniques, such as thermal analysis, chemisorption, transmission electron microscopy, X-ray photoelectron spectroscopy and Auger electron spectroscopy, were employed. The dependence of the metal dispersion on the homogeneity of the metal distribution is discussed, together with the connection between the latter and the precursor solution acidity.

A bimodal distribution of Ru-supported particles, massive

metal crystallites and bidimensional sheets, was found. Binding energy measurements demonstrated that only on silica is Ru in a metallic state, while on the other supports an oxidized ruthenium species is prevalent, suggesting an interaction of the bidimensional sheets with the support.

INTRODUCTION The increased yield in a metal-catalyzed reaction is in principle related to an increase of the specific surface of the metal.

When a high surface area

is obtained by dispersing the metal on a suitable support, changes in activity and selectivity ascribed to the

so

called "crystallite-effect" and/or to the

"support effect" can be observed (ref. 1 ) . Due to the dispersion, defined as the ratio between surface and total amount of the metal atoms, particles of different size and shape can be present on the support surface. According to their physical behaviour such particles might be roughly classified in four different species, with different chemical and catalytic properties : metal crystals, metal crystallites (refs. 2 and 3 ) , metal clusters (refs. 4 and 5 ) , sheets or rafts (refs. 6-8).

Some of them give rise to char-

406

ge transfer or chemical bonding with the support (refs. 9 and 10).

The procedure

followed during the preparation of the dispersed metal can influence the presence of one or more of such species on the support surface. Thus it is important to characterize the physico-chemical state of the metal in such catalysts and to connect these properties with the preparation procedure. In this work, we have investigated the influence of some aspects of preparation on the physico-chemical characteristics of supported ruthenium, which is known to be a catalyst for many chemical reactions (refs. 11-16). EXPERIMENTAL Sample preparation 2 Commercial microspheroidal supports like Si02 ( S samples, 680 m /g), A1203 (A samples, 320 m 2 /g) and Si02-A1 0 (SA sample, 620 m 2 /g) were used, while MgO (M 2 3 sample, 230 m 2/g) was prepared by decomposition of in vacuo. The precursor salt was a commercial ruthenium trichloride (Rudi-Pont), containing an excess of C1- (ref. 16)'. Aqueous solutions (sol. I ) ,

1/10 diluted (sol. 2) and concenbrated (sol. 3)

acetic solutions and 1 / 1 0 diluted (sol. 4 ) nitric solutions were used for the impregnation. MgO was impregnated with an aqueous solution of RUNO(NO~)~,where x

3.

Impregnations were made by the incipient wetness method, followed by

12 hrs drying at 383 K after overnight equilibration. The catalysts were then

activated with H2 in a fluidized bed (10 l/h) according to the following procedure : 4 hrs heating from room temperature to the activation temperature (673, 773 or 873 K), 2 hrs rest at this temperature and then discharging in air, after cooling. Only S1 and A1 samples were discharged and kept in an inert atmosphere. Chemical analysis The Ru content in the supported samples was determined by atomic absorption spectroscopy after dissolution of the metal as Ru

3+

(ref. 1 7 ) .

content was determined by potentiometric titration with AgNO

3

The chlorine (ref. 18).

Surface chemical compositions and binding energies The surface chemical analysis was performed on all samples both with Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS, MgKa radiation) in a commercial PHI instrument. Targets were prepared from powder samples by insertion in pure In foil (ref. 19).

Quantitative analysis was based

on XPS peak areas, corrected according to the relative photoelectric cross-sections +The precursor analysis has given a Cl/Ru ratio of 3.77, with 2.85 H20.

407

(ref. 20). rather than on AES peak-to-peak heights, where empirical sensitivity factors must be used (ref. 21).

Auger analysis however proved useful in

revealing contaminant traces. and Ru The following peak areas, O l s , AlZp, Si2p, MggP, C1 3 ~ 3 1 2were 2P measured. The last was choosen instead of the stronger Ru 3d5,,2 transition because of its overlap with the C I S contamination peak.

Electrostatic charging

occurred during measurements on insulator samples, affecting the experimental binding energy values. A flood gun was used to remove this effect and the C l s peak at 285.0 eV was assumed as reference. In such determinations, the R u ~ transition ~ ~ / ~was used, after subtraction of the carbon contribution.

Total and metal surface area Total surface area and pore size distribution measurements were carried out by N

2

adsorption at 77 K in a static volumetric apparatus. Metal surface areas were determined by means of O2 chemisorption at room temperature, both by the static and pulse (ref. 22) technique, with good agreement between the two methods. Heating for 4 hrs at 6 7 3 K in H2 flow, followed by a degassing in vacuum for 45 min and a cooling to room temperature preceded all static measurements.

In the case of pulse experiments the degassing was

carried out in He. Thermal analysis The reduction of pure RuC13 and of ruthenium chloride supported on Si02 and A1203 was carried out in a horizontal Linseis L-81 thermobalance.

100-150 mg of the previously dried (at 383 K) samples were put in a dry H2 flow (10 l/hr) at

room temperature until a constant weight was reached. Then the sample was heated st a rate of IOK/min up to 800 K in H while flow recording the weight loss as 2 a function of the temperature. Under the same experimental conditions, DTA runs carried out on the supported samples did not reveal any thermal effect corresponding to the halide reduction, due to the very low Ru content. Transmission electron microscopy (TEM) TE micrographs were obtained using a Philips EM-300 microscope, supplied with the high resolution stage. Direct and extractive replica observations were performed (ref. 23).

Electron diffraction experiments proved the presence of

ruthenium on the target.

408

RESULTS AND DISCUSSION Table 1 lists some qualitative observations of phenomena occuring during impregnation and drying. The dark-red aqueous solution of ruthenium chloride does not change its colour on contact with S and SA supports, but becomes black with A support.

By increasing the acidity of the precursor solution no colour change

is observed. This seems to indicate that only alumina gives a strong interaction with the ruthenium salt, with the precipitation of hydrolysis products. By washing the samples after drying the extractability of Ru from the catalyst increases with the acidity of the precursor solution and appears greater on silica than on alumina samples. TABLE 1 Behaviour of the Ru-supported catalysts during impregnation and drying Sample

Colour after impregnation

Colour after drying at 383 K

Ru extractability

s1 s2 s3 s4

dark-red red-violet red-violet dark-red

black brown brown black

low high high low

A1 A2 A3 A4

black dark-violet black black

black black-violet black black

absent high low very low

SA1

dark-red

black

low

black

gray

absent

M1

The influence of the support and of the reduction temperature on the metal distribution into the catalyst particle is shown in Tables 2 and 3, where bulk and surface chemical analysis data are collected. In all S samples the amount of chlorine is very low, not detectable on the surface by XPS and decreases as a function of the reduction temperature. On the contrary on A samples it seems that all the initial chlorine is present on dried samples, with a slight lowering due to subsequent reduction. Moreover the surface Cl/Ru ratio decreases remarkably suggesting the formation of C1-poor hydrolysis products. Assuming the ratio between the concentrations of external and total ruthenium (Ruex and Rut respectively) as a measure of the homogeneity of the metal distribution in a single catalyst pellet and calling it DIRS (Distribution Index Relative to the Surface), we can note that DIRS values greater than 1, as for A and SA samples, indicate an interaction between the support and the precursor.

409

TABLE 2 Influence of the support on the metal distribution Sample

Bulk chemical analysis' Ru C1 Cl/Ru

S1-673 A1-673 SA1-673 M1-673

3.0 2.9 3.1 3.2

0.3 3.4 0.3

0.9 9.9 0.9

-

-

Surface chemical analysis' Ru c1 Cl/Ru 2.5 27.5 13.2 3.0

-

-

10.1

0.4

-

-

-

DIRS 0.8 9.5 4.2 0.9

TABLE 3 Influence of the reduction temperature on the metal distribution ~~~

Sample A1-383 A1-673 A1-773 AI-873

Bulk chemical analysis' Ru C1 Cl/Ru 2.8 11.1 4.0 2.9 9.9 3.4 3.2 7.1 2.2 3.2 6.7 2.1

S1-383 S1-673 S1-773 S1-873

3.1 3.0 3.3 3.4

'Data

8.2 0.9 0.7 0.5

2.7 0.3 0.2 0.1

~

Surface chemical analysis' Ru c1 Cl/Ru 25.7 27.5 26.6 22.2 3.0 2.5 3.9 3.0

16.1 10.1 9.6 10.7

-

0.6 0.4 0.4 0.5

-

-

-

-

DIRS 9.2 9.5 8.3 6.9 1 .o 0.8 1.2 0.9

expressed as atoms x 104/g. cat.

The interaction of the ruthenium solution with the basic sites of alumina is likely to determine the precipitation of hydrolysis products, as insoluble oxychlorides o r hydroxides, on the external surface of the particle. These compounds inhibit the further diffusion of Ru within the particle pores with consequent increase of the metal concentration at the external particle shell. By increasing the acidity of the solution the basic sites are neutralized and the DIRS value decreases to about 1 in the sample A2 (Table 4). TABLE 4 Influence of the precursor acidity on the metal distribution ~~

~~

Sample

Bulk chemical analysis' Ru C1 Cl/Ru

A1-673 A2-673 A3-673 A4-673 S1-673 S2-673 S3-673 S4-673

2.9 3.2 2.9 2.4 3.0 3.1 3.3 3.2

9.9 8.5 7.6 6.3 0.9 0.9 0.9 0.8

3.4 2.6 2.7 2.7 0.3 0.3 0.3 0.3

Surface chemical analysis' Ru c1 Cl/Ru 27.5 4.4 11.2 6.8 2.5 2.5 3.3 2.5

10.1 8.9 8.9

-

0.4 2 .o 0.8

-

DIRS 9.5 1.4 3.9 2.9 0.8 0.8 1 .o 0.8

The interaction between unreduced ruthenium and support is proved by the thermo-

410 gravimetric runs reported in fig. 1 .

The experimental results for ruthenium chlo-

ride, ruthenium dioxide and samples S1 and A1 are expressed as mg/100 mg Ru.

A

correction has been made for the catalysts due to the weight l o s s of the support according to the equation : "cat u R ' *

=

"

-

Awsxs

where A Wcat and A W

are the weight losses referred to 100 mg of catalyst and sup-

port respectively; X

and XRu are the weight fractions of support and metal in

the catalyst. The reduction temperature of Ru in the samples S1 and A1 is lower than in the pure trichloride, but similar to Ru02. The weight loss of the sample S1 at 520 K indicates the total elmination of C1 and complete reduction of the precursor to metal. at 7 7 3 K.

On the sample A1 the chlorine loss is still incomplete

A l l these results are in good agreement with the chemical analysis data.

Fig. 2 shows the binding energy values corresponding to the Ru 3d5,2 transition as a function of the reduction temperature of the samples. As reference, values corresponding to some pure compounds are reported (ref. 2 4 ) . The analysis of these results produces some interesting findings. S samples, when reduced at high temperature, become similar and show binding energy values between Ru and Ru02, indicating that the most abundant species of ruthenium on the surface is not in a metallic state. Conversely the sample kept in an inert atmosphere after reduction shows only metallic Ru on the surface. It seems that the highly dispersed ruthenium is particularly active with oxygen at room temperature. On A samples binding energy values near to those of Ru02 are always observed in the sample kept in an inert atmosphere. As a reaction between ruthenium and residual water cannot be postulated for thermodynamic reasons, an interaction between the metal and the support must take place (ref. 2 5 ) .

This suggests that a

measurable fraction of Ru may be present in a physical state able to produce charge transfer effects between metal and support, like clusters, sheets or dispersed atoms. The sample SA1 has a binding energy very similar to those o f the A series whereas that of sample M is higher.

In both cases Ru is therefore present in a high

oxidation state. N

2

adsorption isotherms have shown that the preparation parameters have only a

little influence on the total surface area, pore volume, pore shape and pore size distribution. The metal dispersion data obtained by O2 chemisorption and TEM are collected in Tables 5 , 6 and 7.

411

TABLE 5 Influence of the support on the metal dispersion Sample

TEM D (A)

Oxygen chemisorption Dispersion d (A)

S1-673 A1-67 3 SA1-673 M1-673

0.43 0.26 0.33 0.37 ~~~

22 34 28

86 207 210

-

24

~~

The apparent particle size, d, was calculated by oxygen chemisorption at room temperature assuming a spherical model and an adsorption stoichiometry O/Ru = 2

~~~~~

A pure RuClx o S1 sample

x A 1 sample

300

400

500

600

700

8( Temp. ( K )

Fig. 1. Weight loss curves of ruthenium dioxide, ruthenium chloride and samples S1 and A1 after drying at 383 K.

412 A 1 o A2 a A3 A A4 v

54 v

M1 A

F a

SAl A

I

T

Estimated error

-

IV

RuC13

282.0

. c-

A ~u03 (In vacuum) RuO2. xH2O

2

281.0

(inert atm)

s10

(inert atm)

t

c1

280.0 J

a

0

279.0t

Ru

o

I

V

-0

I

I

II

I

Fig. 2 . Ru3i5,2 binding energy in function of the thermal treatment. Binding energy values for u metal and compounds are reported. (ref. 2 6 ) .

The ruthenium site area ( 9 . 0 3

2

) was calculated as the average value

for the (OOI), (110) and (100) planes (ref. 2 7 ) . A s a consequence of the influence of acidity on the metal distribution of the A samples there is a dispersion increase with the precursor acidity. A n influence of the anion is apparent from the results of the samples A2 and A4.

Nevertheless this

influence disappears after high temperature treatment. TABLE 6

Influence of the precursor acidity and of the reduction temperature on the dispersion of Al,,O,-supported ruthenium Sample

Oxygen chernisorption Dispersion d (A)

TEM D (1)

A1-673 A1-773 AI-a73

0.26 0.26 0.26

34 34 34

207 221

A2-673 A2-773 A2-873

0.15 0.26 0.35

62 34 26

300 87

A3-673 A3-773 A3-873

0.19 0.21 0.33

48 44 2a

280 82

A4-673 A4-7 7 3 A4-873

0.42 0.42 0.45

22 22 20

253 76

-

-

-

-

413 The significant variation of the particle size against reduction temperature, particularly evidenced by TEM (Tables 6 and 7 ) , must be attributed to the different heating rates during the activation process.

A competition between the reduction

and agglomeration rates occurs, so that with high heating rates small particles are obtained. Finally, the particle size data from TEM and oxygen chemisorption differ greatly in all samples. This behaviour is in agreement with other observations (ref. 8 ) . Bearing in mind that X-ray diffraction spectra did not give rise to definite metal patterns, we suggest that ruthenium is present in the examined samples in a bimodal distribution, as massive crystallites and bidimensional sheets of atoms causing a high dispersion degree. CONCLUSIONS From the analysis of the experimental results, some conclusions concerning the preparation of Ru-supported catalysts can be drawn. When the support surface exchanges the hydroxyl groups with the precursor solution anions, the formation of a film of Ru hydroxide or oxychloride occurs on the external shell of the support particles, inhibiting the successive diffusion of TABLE 7 Influence of the precursor acidity and of the reduction temperature on the dispersion of Si0,-supported ruthenium ~

Sample

Oxygen chemisorption Dispersion d (A)

TEM D (A)

S1-673 S1-773 S1-873

0.43 0.44 0.40

22 20 22

a6 168 80

S2-673 52-773 S2-873

0.45 0.42 0.48

20 22 18

-

S3-673 S3-773 s3-a73

0.37 0.32 0.35

24 2a 26

210

S4-673 s4-773 s4-873

0.35 0.37 0.37

26 24 24

152

ruthenium into the pores.

-

-

110 75

-

This phenomenon results in a large inhomogeneity of the

metal distribution, with DIRS values greater than 1.

By increasing the acidity of

the precursor solution, it is possible to neutralize the basic sites, producing a more homogeneous catalyst (DIRS = I ) . The final metal particle size depends on the kind of support and on the acidity of the precursor solution. When an interaction between solution and support occurs,

414 a low dispersion degree is obtained, as a consequence of a high metal concentration

on the external shell of the catalyst particle. With acidification, a higher degree of dispersion is reached, together with a more homogeneous metal distribution. Thus, control of the above parameters allows the desired dispersion degree to be achieved. On the supports considered, a bimodal distribution of Ru particles is present :

metal crystallites and bidimensional sheets. On alumina, silica-alumina and magnesia the highly dispersed ruthenium appears in an oxidized state, while on silica the most abundant species is metallic Ru.

The presence of an oxidized form

of Ru suggests an interaction between the metal and the semiconductor like a charge transfer or a weak chemical bond. The nature of such oxidized Ru species and their dependence on the preparation parameters has not yet be ascertained. ACKNOWLEDGEMENTS We are indebted to Prof.

S.

Pizzini for his interest in this work and to R.

Berts, G. Mittino, G. Morelli and L. Pozzi for the technical assistance in the experimental work. REFERENCES 1 R.L. Moss, in R.B. Anderson and P.T. Day.sor. (Ed.), Experimental Methods in Catalytic Resezrch, Vol. 111, Academic Press, New York, 1976,pp.43-94. 2 F.L. Williams and M. Boudart, J. Catalysis, 30(1973)438-443. 3 R. Bouwman, G.J.M. Lippits and W.M.H. Sachtler, J. Catalysis, 25(1972)350-361. 4 R.P. Messmer, S.K. Knudson, K.H. Johnson, J.B. Diamnnd 2nd C.Y. Yang, Phys. Rev. B, 13(1976)1396-1415. 5 P. Fantucci and P. Balzarini, EUCHEM Conference, Namur, 1977. 6 L. Spenadel and M. Boudart, J. Am. Chem. SOC., 64(1960)204-207. 7 P. Debye and B. Chu, J . Am. Chem. S O C . , 66(1962)1021-1027. 8 E.B. Prestridge, G.H. Via and J.H. Sinfelt, J. Catalysis. 50(1977)115-123. 9 F. Solymosi, Catal. Rev., 1(1967)233-255. 10 J . Escard, C. LeclPre and J.P. Contour, J. Catalysis, 29(1973)31-39. 11 G.C.A. Schuit and L.L. van Reijen, Adv. Catalysis, 10(1958)242-317. 12 J.H. Sinfelt and D.J.C. Yates, Nature, 229(1971)27. 13 J.L. Carter, J.A. Cusumano and J.H. Sinfelt, J. Catalysis, 20(1971)223-229. 14 P.H. Emmett, in E. Drauglis and R.I. Jaffee (Ed.), The Physical Basis for Heterogeneous Catalysis, Plenum Press, New York, 1974, p.3. 15 H.H. Storch, N. Golumbic and R.B. Anderson, The Fischer-Tropsch and Related Syntheses, Wiley, New York, 1951, p.309. 16 W.P. Griffith, The Chemistry of the Rarer Platinum Metals, Interscience, London, 1967, p.136. 17 N. Bottazzini, L. Fenoggio and A. Gozzi, Montedison Internal Rep. 11/77 (1977). 18 N. Bottazzini and A. Gozzi, Montedison Internal Rep. 13/77 (1977). 19 G.E. Theriault, T.L. Barry and M.J.B. Thomas, Anal. Chem., 47(1975)1492-1493. 20 J.H. Scofield, Lawrence Livermore Laboratory Rep. UCRL-51326 (1973). 2 1 P.W. Palmberg, Anal. Chem.,45(1973)549A-556A. 22 N.E. Buyanova, A.P. Karnaukhov, N.G..Koroleva, I.D. Rotner and O.N. Chernyavskaya, Kinet. Catal., 13(1972)1364-1369. 23 G. Dalmai-Imelik, C. Leclercq and I . Mutin, J . Microscopie, 20(1974)123-132. 24 K.S. Kim and N. Winograd, J . Catalysis, 35(1974)66-72. 25 C.A. Clausen and M.L. Good, J. Catalysis, 46(1977)53-64.

415 26 H. Kubicka, React. Kinet. Catal. Letters, 27 H. Kubicka, J. Catalysis, 12(1968)223-237

5(1976)223-228

DISCUSSION

L.L.

M U R R E L L : T h e o b s e r v a t i o n of t h e a p p a r e n t o x i d i z a t i o n o f

ruthenium o n A 1 2 0 3 c o m p a r e d t o ruthenium o n S i 0 2 i s a p p a r e n t l y n o t a d r a s t i c d i f f e r e n c e b e c a u s e of t h e v e r y s i m i l a r i n f r a r e d s p e c t r a o f C O c h e m i s o r b e d on R u on A 1 2 0 3 , Si02, and MgO.

Could

y o u c o m m e n t on t h e extent of t h e d e g r e e o f " o x i d a t i o n " o f t h e r e d u c e d R U r a f t s o n A 1 2 0 3 o b s e r v e d in y o u r w o r k ?

A. B O S S I : We h a v e not yet m a d e infrared s p e c t r a o n Ru o n Si02 a n d MgO.

From t h e p r e l i m i n a r y r e s u l t s o n A 1 2 0 3 i t a p p e a r s t h a t

t h e amount of non m e t a l l i c ruthenium d e p e n d s o n t h e h i s t o r y o f t h e sample.

A b o u t t h e d e g r e e of " o x i d a t i o n " o f non m e t a l l i c

ruthenium, X P S d a t a g i v e obviously an a v e r a g e value for b o t h t y p e s o f R u particles.

B i n d i n g energy v a l u e s on A 1 2 0 3 are s i g n i f i c a n t -

ly d i f f e r e n t from t h o s e o n S i 0 2 , w i t h a c h e m i c a l s h i f t s i m i l a r t o t h a t of "non-defective" R u 0 2 ( s e e K . S . Catalysis 3 5

Kim

&

N. W i n o g r a d , J.

(19741, 66).

H. C H A R C O S S E T : I n a g e n e r a l way y o u r r e s u l t s a r e in a g r e e m e n t w i t h o u r s ( p a p e r B 8): e.g.

w i t h r e g a r d t o t h e d i f f e r e n c e in

r e d u c i b i l i t y of Ru d e p o s i t e d either on A 1 2 0 3 o r o n S i 0 2 .

Could

y o u c o m m e n t on t h e c h e m i s o r p t i o n s t o i c h i o m e t r i e s o f d i f f e r e n t gases on RU ?

H a v e y o u c a r r i e d o u t H 2 o r CO c h e m i s o r p t i o n m e a s u r e -

m e n t s on y o u r c a t a l y s t s and compared t h e r e s u l t s w i t h t h o s e o b t a i n e d w i t h O 2 as a d s o r b a t e ?

A.

B O S S I : A c c o r d i n g t o our e x p e r i m e n t a l r e s u l t s , on p u r e r u t h e -

nium t h e a d s o r p t i o n s t o i c h i o m e t r i e s a p p e a r t o be H / R U = 1 and O/Ru = 1 ( s e e a l s o R.A. and K.C.

D a l l a Betta, J. C a t a l y s i s ,

T a y l o r , J. C a t a l y s i s

0

(1973), 478).

2,(19741,

57,

O n the o t h e r h a n d ,

o n w e l l dispersed s u p p o r t e d r u t h e n i u m , t h e O / R u r a t i o a p p e a r s t o b e r e l a t e d t o the b i n d i n g energy d a t a p r e s e n t e d in t h e p a p e r

(Fig. 2 ) .

We c a r r i e d o u t C O a d s o r p t i o n e x p e r i m e n t s on R U / A ~ ~atO d ~ifferent t e m p e r a t u r e s in a s t a t i c system and w e f o u n d t h a t t h e a m o u n t o f C O a d s o r b e d s i g n i f i c a n t l y increases in a t i m e r a n g e of 16 hours. It is o u r o p i n i o n t h a t an interaction b e t w e e n C O and b u l k r u t h e n i u m

416 takes places.

Work is in progress in order to obtain detailed in-

formation on this phenomenon. A. OSAKI

:

If you have any data on H2 consumption during reduction,

it would be helpful. A. BOSS1

:

We have not made such measurements.

J.W. GEUS : Can you explain in some detail how you distinguish by means of bright and dark field electron micrographs globular and sheet-like (denoted rafts by the ESCA workers) particles ? This cannot be done by comparing the contrast of the particles in bright-factor micrographs as the contrast is mainly due to the particles being oriented in a Bragg reflection position or not. S.R. TENNISON

:

We have found "high transparency" crystallites

similar to those found by the authors when studying platinum and ruthenium supported on graphite, a system where there is no possibility for support interaction.

The transparency was such that

the support structure could be observed through up to 5 or 6 superimposed 4 0

Pt crystallites in the high resolution transmission

micrographs (x 1.600.000).

-+

30'

However, when the sample was tilted by

the results showed the crystallites to be approximately

spherical.

This demonstrated the risks involved in assuming raft

formation simply on the basis of transparency in the HRTEM results. A comparison of crystallite sizes determined by X-ray line broadening and electron microscopy also demonstrated the problems involved in a reliable detection of supported metal particles smaller 0

than approximately 15 A.

For highly dispersed catalysts, XRD

consistently gives significantly lower average crystallite sizes than HRTEM.

417

HIGH SURFACE A R W OXIDE SOLID SOLUTION CATALYSTS

++

'

A. P.HAGAN, M .G.LOFTHOUSE,

F,S .STONE and M.A.TREVETHAN"'

School of Chemistry, University of Bath, Bath BA2 7AY, England

SUMMARY NiO-MgO and Coo-MgO catalysts (1 to 10 mole % NiO or COO) have been prepared in high surface area form (40 to 300 m2g-I) by impregnation of Mg(OH)2

with Ni

or Co nitrate, followed by thermal decomposition and annealing in vacuo at 1000°C. The resulting solids have been characterized by a combination of X-ray diffraction, electron diffraction and magnetic susceptibility studies for their bulk properties and by UV-visible reflectance spectroscopy and adsorption for surface properties. Lattice parameters, magnetic moments and Weiss constant values, all determined after high temperature outgassing and without exposure to the atmosphere, are fully consistent with the vacuum-annealed catalysts being true substitutional solid solutions Ni Mg

x

1-SG

0 and Co Mg

x

1-x

0, with Ni2+ and Co2+ in octahedral coordination.

Reflectance studies, however, reveal the presence of Ni and Co ions in other 0, and these x 1-x ions are considered to be located in the surface regions of the crystallites. The

coordinations, notably Co2+ in tetrahedral coordination in Co Mg effects of adsorbing 02, CO and H 0 vapour on Ni Mg described and discussed.

0 and Co Mg 0 are 2 2 x 1-x x 1-x A calorimetric study of the adsorption of NO and CO

and their chemical interaction on Ni Mg

x

1-x

0 is also reported.

INTRODUCTION Oxide solid solution catalysts are important for fundamental studies of the relationship between electronic properties and'catalysis.

This is especially the

case when the solute is a transition metal oxide and the solvent an insulating, diamagnetic oxide,

The value o f such solid solutions in catalytic studies was

first recognized [1,2] in regard to identifying how electronic interactions between

+

++

+++

Present address: Shell U.K. Oil Ltd., Shell Haven Refinery, Stanford-le-Hope, Essex, England Present address: DuPont (U.K.) Ltd., Instruments Products Division, 64 Wilbury Way, Hitchin, Herts., England Present address: Unilever Research Ltd., Port Sunlight Laboratories, Wirral, England

418 cations affect catalytic action.

The scope is now much wider c 3 - 6 3 , and includes

studies of the effects of solvent ionicity and the use of more complex solvent oxides

spinels, scheelites, perovskites) in which site symmetry and point defect

concent ations can be varied by appropriate substitutions. The majority of solid solution catalysts so far prepared and characterized have 2 -1 * been oxides of low surface area (LSA). typically 1 to 20 m g Catalytic

.

studies can readily be carried out on such solids, but quantitative chemisorption work and surface spectroscopy are difficult.

Thus there is a need to prepare and 2 -1 characterise oxide solid solutions of higher surface area (20 to 300 m g 1.

Secondly, with such areas solid solution catalysts would in principle become competitive with technically-used supported mixed oxide catalysts, The relevance of this is that supported mixed oxides are often difficult to stabilize on account of chemical and physical changes in the active component (e.g. irreversible oxidation or reduction, crystallization, compound formation with the support) during use at conventional catalytic temperatures, typically 250-750°C.

With solid solution

catalysts these undesirable characteristics are less likely to occur.

Moreover,

high temperature applications of transition metal oxide catalysis (T > 75OoC) become feasible. The research to be described relates to Ni Mg

x

1-x

0 and Co Mg

and covers work carried out over several years [7].

x

1-x

0 (0

< x < 0.1)

The choice was influenced by

the fact that both NiO-MgO and COO-MgO had already been studied as catalysts for

N20 decomposition by Cimino et al. [2,3,8,9]using low surface area solid solutions 2 -1 (SA < 20 m g ). Those studies, which had been accompanied by thorough work on characterization by X-ray, magnetic and spectroscopic methods [lO,ll], had shown that 2+ 2+ the catalytic activities of both Ni and Co were enhanced if they were present as dilute solid solutions of NiO or COO in MgO. reactions has since followed [12,13].

Related catalytic work on other

Our choice of NiO-MgO and Coo-MgO was also

influenced by the fact that Anderson and co-workers [14,15] had shown that Mg(0H) 2 2 -1 could be decomposed to give MgO in a high surface area (200-300m g ) form, stable 2+ 2+ at 1000°C, provided water vapour was rigorously excluded. Since Ni and Co ions are known to have high diffusion rates in bulk MgO at 1000°C, these studies affordec? a very good background for an attempt to prepare high surface area solid solutions.

We adopted as a major premise the desirability of high temperature

preparation to ensure that cation diffusion rates should not be a limiting factor in dispersing the solute ions in the MgO structure. Although there has been other work on high surface area NiO-MgO and Coo-MgO

*In this paper, we divide the surface area range arbitrarily into three groups: 2 -1 (1) low surface area, LSA, < 20 m g ; (2) medium surface area, MSA, 20-100 2 -1. m g , (3) high surface area, HSA, 100-300 m2g-I.

419 [16-191, the catalyst characterization has not been reported in sufficient detail to judge whether the oxides were in fact solid solutions or merely mixtures containing incompletely dispersed transition metal ions.

Recently, however, a

comprehensive characterization study on HSA, MSA and LSA Coo-MgO has been underThis work has overlapped our

taken by Dyrek, Bielanski and co-workers [20-22]. own studies [23],

and the important conclusion from both groups is that tetrahedral

Co2+ ions are present in both MSA and HSA Co Mg

x

1-x

0.

Bielanski et al. [21,22]

believe that these ions are distributed throughout the bulk of the crystallites, whereas we have linked them with the surface region.

Also within the last few

years there has been an esr study by Kuznetsov et al. [24] identifying the presence of tetrahedral Co2+ in Coo-MgO.

No comparable work has yet appeared on MSA and

HSA NiO-MgO.

EXPERIMENTAL Preparation of high surface area (HSA) solid solutions Solid solutions Ni Mg

x

1-x

0 (designated MN) and Co MglwxO (designated MCo) were X

prepared by decomposition of Mg(0H)

which had been impregnated with an aqueous 2 solution of Ni(N03)2 or CO(NO~)~. Mg(0H) was precipitated from Mg(N03)2 2 solution with NH OH, dried at 120°C and then ground and sieved t o 60 mesh. The 4 hydroxide was impregnated with an approximately equal volume of Ni or Co nitrate solution, whose concentration depended on the nickel or cobalt concentration ultimately required in the oxide solid solution.

The impregnated Mg(OH)2

was

dried at 12OoC (Ni impregnation) or at 5OoC (Co impregnation), the lower temperature for cobalt being necessary to avoid premature decomposition of nitrate. mixture was then ground and reslurried in water.

The

This procedure was repeated

twice more in order to achieve complete mixing through the hydroxide mass. The dried, nitrate-impregnated Mg(0H)

2 assembly shown in Fig.1 and evacuated.

was placed in a platinum boat in the With Trap 1 at 77 K, the furnace

temperature was raised slowly to 350°C with continuous evacuation and these conditions were maintained for 12 h.

The sample was isolated by closing the

stopcock and the furnace was cooled; Trap 1 was then warmed and condensed water Trap 1 was cooled again to 77 K, the whole system was returned

was pumped away.

-6

to high vacuum (10

Torr, as monitored by the ionization gauge) and the furnace

temperature was increased in stages to 70OoC. temperature raised to 1000°C;

Trap 2 was cooled to 77 K and the

Trap 3 was then cooled to 77 K.

These conditions

(lOOO°C, continuous pumping, all 3 traps at 77 K) were maintained for 12 h in order to allow time for ionic diffusion to homogenize the oxide solid solution. With the temperature still at 1000°C, the sample was magnetically withdrawn into the copper block in vacuo and rapidly cooled,

The essential requirement for

producing a high surface area oxide is adequate removal of water vapour when the precursor material is undergoing decomposition.

Development of high surface area

420

in a NiO-MgO or Coo-MgO mixture can be achieved merely by heating to 30OoC.

The

purpose of the 1000°C vacuum anneal is to complete internal dehydroxylation and to form true oxide solid solution (whilst still maintaining the surface area).

This

is a much more demanding requirement than mere production of high surface area.

13

2

1

3

7

Fig.1. Apparatus for preparation of high surface area solid solutions. 1,2,3 Traps; 4 - Stopcocks; 5 - Ionization gauge; 6 Magnetic follower; 7 Receiver tube for magnetic follower; 8 O-ring seal; 9 - Copper block; 10 - Furnace; 11 - Sample in platinum boat; 12 Mullite tube; 13 - Trolley for Thermocouple. furnace; 14

-

-

-

-

-

-

High purity is not a pre-requisite for preparing the HSA solid solutions, but because of the requirements of the magnetic and spectroscopic studies, the present research has been conducted with very pure starting materials, including the preparation of Mg(N0

) by dissolving specpure Mg in Aristar HNO and use of 3 2 3 specpure Ni and Co nitrate. Chemical analysis of the solid solutions was made by

dissolving them in HNO followed by EDTA titration and either atomic absorption 3 (Ni) or spectrophotometric analysis ((20). In this paper we report on NiO-MgO and Coo-MgO solutions with nominal nickel or cobalt concentrations of 1, 3 , 5, 7.5 and LO mole per cent.

They are designated

for convenience as MN 1, MN 3 , MN 5, MN 7.5 and MN 10 for NiO-MgO and MCo 1, MCo 3, MCo 5, MCo 7.5 and MCo 10 for Coo-MgO.

The actual molar percentages (always known

from chemical analysis) differ slightly from the nominal percentages, but they are only listed when the precise values are relevant.

421 Techniaues and methods of study 1. Surface area determination.

Surface areas were determined by the BET method (N2, 77 K) after outgassing at 50OoC. 2. X-ray diffraction and electron diffraction. XRD was carried out with Ni-filtered Cu KU radiation.

For powder photographs a

Philips 114.6 mm diameter Debye-Scherrer camera was used. were studied conventionally.

Air-exposed specimens

To study 800°C or 1000°C outgassed specimens, a

special device was used [25] which permitted grinding and transfer to capillaries in vacuo, prior to sealing. X-ray diffractometry of air-exposed solid solutions was carried out using a Philips goniometer spectrometer with a flat sample.

For X-ray diffractometry in

vacuo a Rigaku-Denki high-temperature attachment was used with a Pt sample holder: the specimens were outgassed at 800°C and cooled in vacuo. Electron diffraction was carried out using an A.E.I. EM802 electron microscope fitted with a high temperature attachment.

After installation of the HSA oxide

specimen and evaporation of a gold film (as internal standard) the oxide was heated briefly at 7OO0C in the vacuum of the microscope before recording the diffraction pattern. 3 . Gravimetry and magnetic susceptibility.

A vacuum-mounted Cahn RG microbalance was used for TG studies and for magnetic susceptibility measurements by the Gouy method [26].

The specimen was contained

in a cylindrical Pt gauze bucket which facilitated removal of water vapour during high temperature outgassing, thereby avoiding loss of surface area.

4. Diffuse reflectance spectroscopy. UV-visible reflectance spectra were measured with a Pye Unicam SP 700C spectrometer using a sample cell attached to a vacuum manifold [27].

Gases could be dosed

in situ and the sample could be outgassed in the cell up to loOO°C. 5. Adsorption calorimetry, Heats of adsorption were determined using an all-glass resistance thermometer calorimeter of a design previously described [28].

The oxide could be outgassed

at 5OO0C in situ.

RESULTS Surface areas Surface areas of solids prepared as described in the preceding section ranged 2 -1 from 275-300 m g for MN 1 and MCo 1 to 100-125 m2g-l for MN 10 and MCo 10. The high surface area develops already at 25OoC, as exemplified by the data for the formation of MCo 3 in Table 1.

These results refer to Co-impregnated Mg(OHI2

which had been heated in a silica tube under conditions closely matching those in the assembly of Fig.1. but with interruptions at various temperatures and cooling

422

to 77 K to determine the surface area. Mg(0H)

2

decomposes (250OC);

The surface area rises abruptly as

some decrease in area occurs at 400-6OO0C, but there-

after it is constant to ~ O O O ~ C . The high surface area is essentially stable in vacuo at lO0OoC.

If cooled and

exposed to air at 2OoC, the oxides readily take up water vapour, oxygen and CO

2'

the high surface area oxide can be regained by a subsequent outgassing.

but

However,

if heated in open air, permanent loss of area occurs. After 15 h at 1000°C, areas 2 -1 which were in the HSA range 300-125 m g (depending on Ni or Co content) become 2 -1

typically 100-20 m g

(MSA range).

The present paper deals mostly with HSA solid

solutions, but occasional reference will be made to MSA solid solutions,

TABLE 1 Development and maintenance of surface area in the formation of HSA MCo 3 Temp.of heating (OC)

20

200

250

300

350

400

500

600

800

1000

Surface area ( m2g-1)

34

40

237

243

245

234

207

179

174

180

Lattice parameters X-ray analysis of the 100O0C-prepared HSA materials revealed a set of lines all of which could be assigned to a single cubic MgO-type phase.

Short contact with

the atmosphere produced no extra lines, but gravimetric measurements showed that water vapour was readily adsorbed.

As a skin of hydroxide can affect the lattice

parameter of a small oxide crystal [29], it was considered important for correct determination of a

to take the X-ray pattern in vacuo after a high-temperature

outgassing in situ.

For MN this was done at 2OoC in the high-temperature

diffractometer after cooling from an 800°C-outgassing.

For MCo it was done in

two ways: (a) by a Debye-Scherrer photograph after a 1000°C-outgas, grinding in vacuo and transfer to a capillary which was then sealed, (b) by electron diffraction after a short 700OC outgas in situ.

Nelson-Riley extrapolations were used for the

X-ray determinations of ao: the accuracy of the determination is necessarily limited by the breadth of the high-angle reflections.

Results are shown in Table 2 and

compared graphically with those for highly-sintered LSA solid solutions in Flg.2. Magnetic measurements The temperature dependence of the magnetic susceptibility X is given by XM

= Xo

+

C/(T

+ e), where Yo is the Van Vleck temperature independent para-

magnetism (TIP), C the Curie constant (related to the effective magnetic moment 2 the equation C = p eff/8),and 8 the Weiss constant. An assumption that

ueff by X

= 0 will not greatly affect the determination of p

affect 8.

but may significantly eff' We therefore used an iterative procedure t o evaluate 8 and Xo eff'

423

TABLE 2 Lattice parameters (a ) of HSA solid solutions determined in situ after hightemperature outgassing

~~~

MN MN MN MN MN

~

~~

1 3 5 7.5 10

~~~~

1.19 3.34 5.55 7.66 10.06

Sample MCo MCo MCo MCo MCo

Lattice parameter ao/A0

[Nil by chemical analvsis/mol%

Sample

4.2114 4.2097 4.2089 4.2081 4.2074

[Co] by chemical analysis/mole %

1 3 5 7.5 10

-

0.94 3.14 4.50 7.63 9.54

*

o * Lattice parameter a /A Debye-Scherrer ElectronOdiffraction

-

4.212 4.213 4.215 4.217

4.213 4.214 4.216 4.217

We are indebted to Dr.C.Otero Arean for these measurements

~

4.220 COO-MOO .

4 . L IU 6 0

NiO-MgO

Fig.2. Variation of lattice parameter of Coo-MgO and NiO-MgO with Co and Ni content. Triangles HSA MCo, by X-ray diffraction; Squares HSA MCo, by electron diffraction;1 Solid small circles LSA MCo, from ref.11; Large open circles HSA MN, by X-ray diffraction; Small open circles LSA MN, from ref.10.

-

-

-

-

-

from determinations of X at about 20 temperatures between 77 and 373 K.

Samples

of HSA MN and MCo were outgassed in situ on the magnetic balance at 800°C and cooled in vacuo.

Helium was then admitted for the susceptibility measurements. Results 2+ are given in Table 3. peff has values typical of Ni2+ and Co in octahedral coordination, and

e

is quite low.

424

As already mentioned, the HSA oxides readily take up water vapour from the atmosphere.

If left in air, bulk rehydration will occur.

Specific study of

this was made by allowing MN and MCo to stand in air over saturated NH C1 4 XRD of the product showed the presence of both the cubic

solution for 30 days.

oxide and the hexagonal hydroxide, and TG showed a weight l o s s of 25-27% on heating

TABLE 3 Magnetic properties of HSA MN and MCo outgassed at 8OO0C

Peff/B .Me

Sample

-8/K

1 05 Xo/erg gauss-2 mol -1

MN 1 MN 3 IN 5 MN 7.5 MN 10

3.42 3.59 3.41 3.30 3.25

40 35 45 50 60

50 30 35 40 40

MCo MCo MCo MCo MCo

5.40 5.14 5.42 5.18 5.39

70

30 120 60 30 20

1 3 5 7.5 10

from 200'

50 50 55 70

to 800° in vacuo, suggesting that the solid was about 80% hydroxide.

This bulk hydration of HSA oxide on contact with moist air affects the magnetic parameters, as shown in Table 4.

These hydrated oxides were then outgassed in

situ on the magnetic balance in a series of stages, with cooling for a sequence of magnetic susceptibility measurements (77 to 373 K) at each stage. how the true oxide values are gradually recovered.

Table 4 shows

This effect with MCo shows the

importance of measuring the magnetic properties of HSA oxide in the outgassed state. Diffuse reflectance spectra HSA MN outgassed at 1000°C (Fig.3, Curve 1 ) shows four principal regions of

2-C

absorption with band maxima (reflectance minima) at 23,000, 14,400, 8,300 and 5,000 -1 Absorption in the first three of these regions is expected for Ni2+ in octacm , 3 hedral symmetry (transitions from 3A (F) to Tlg(P), 3T (F) and 3T (F) 2g 1g 2g respectively), but the band at 5,000 cm-l (at the spectrometer limit) is unexpected. Dosing H 0 vapour at 2OoC (Fig.3, Curve 2) did not significantly affect the 2 -1 octahedral bands at 14,400 and 8,300 cm but greatly decreased the absorption at 5,000 cm-l.

-1

Simultaneously the broad absorption at 23,000 cm

leaving a band at higher

23,000cm-'

i,

is a composite absorption, one part (at higher

band,the other (at lower

markedly decreased,

This shows clearly that the broad absorption at

i) being

i) being

the octahedral

a band which, like that at 5,000 cm-1- is destroyed

by H 0 vapour. An outgassing recovered the initial spectrum (Fig.3, Curve 3). 2 c02 -1 also decreased preferentially the 5,000cm band, but less effectively than H20. In this case there was very little effect on the other bands.

The change

425 TABLE 4 Magnetic properties of HSA MN and MCo on hydration and dehydration

*

Sample

Condition

MN 3 MN 3 MN MN MN MN

5 5 5 5

MN MN MN MN

10

OG 800°

n OG 800'

n

n

& OG 4o0° H & OG 8OO0

OG 800°

10 10

n

n

& OG 4o0° H & OG 800°

10

MCo 3 MCo 3

OG 800'

5 5 5 5 MCo 5

OG 800°

n

MCo MCo MCo MCo

n H & OG 250° n & OG 4o0° H & OG 800°

MCo 10

OG 800°

MCo 10 MCo 10 MCo 10

H & OG 400° n & OG 800'

n

-B/K

Cleff/B.M.

5

-2

10 X /erg gauss

3.59 3.21

35 5

30 5

3.41 3.38 3.30 3.38

45 5 50 55

35 35 45 50

3.25 3.32 3.20 3.24

60 -5 80 90

40 15 55 50

5.14 3.73

50 35

120

5.42 4.22 4.45 4.78 5.05

50 35 35 55 60

60 15 20

5.39 4.50 4.92 5.10

70 40 60 55

20

mol

-1

10

10 5

10 0

25

-

*oG 800° - outgassed at 800°C; H hydrated; H & OG 250° (40O0, 800O) - hydrated and then outgassed at 25OoC (400°C, 8OO0C)

was ultimately reversible on outgassing, but a feature of the CO

2

spectra was a

small fluctuation of the background absorption. On desorbing C02 the background - I . . . at 1 5 , 0 0 0 - 3 5 , 0 0 0 cm initially increased, but by a 600°C-outgas this trend had been reversed. Oxygen adsorption (Fig.3, Curves 3-5) developed a broad background absorption, -1 most intense between 1 5 , 0 0 0 and 3 5 , 0 0 0 cm This made difficult any conclusion

.

about the influence of O2 on the small.

d-d

bands, but if there is any effect it must be

Outgassing in stages (Fig.3, Curves 5 ' , 6 , 7 ) showed that the oxygen effect

was still significant after a 500°C desorption, but could be completely reversed Exposure to air gave the expected effects of a combination -1 band (the effects of H 2 0 of 02, H 0 and COz, namely a decrease in the 5 , 0 0 0 cm 2 -1 and CO ), a change in the 2 4 , 0 0 0 cm band so as to shift the maximum and produce

at high temperature.

2

a shoulder (the effect of H O), along with a general increase in absorption between 2 1 7 , 0 0 0 and 3 2 , 0 0 0 cm-l (the effect of 0 2 ) .

Turning to MCo, Fig.4 shows a series of spectra taken at stages during the formation of the solid solution.

Little change occurs below 5OO0C,

even though,

426

T

a W V

L

12g Mo03/100g A1203). Washing the catalysts with water before calcining removed octahedral and less strongly bound tetrahedral oxomolybdenum species. ( d ) Catalysts prepared by the pore-filling (PF) technique contained a smaller proportion of strongly bound molybdenum than those prepared by the equilibrium

adsorption (EA) technique. The local pH in the pores is greater in the PF technique and so the extent of reaction of molybdate with alumina is less than in the EA technique. (e) Addition of a cobalt(I1) nitrate solution to a Mo03/A1203 catalyst caused part of the cobalt to be adsorbed and part to be precipitated as a surface hydroxide. (f) The Co0-Mo03/A1203 catalysts contained tetrahedral and octahedral oxocobalt species. The proportion of the octahedral species was greater in catalysts prepared by cobalt-impregnation of calcined MOO /A1203 catalysts. 3 (9) There was interaction of the oxocobalt and oxomolybdenum species in the catalysts.

470 INTRODUCTION Catalysts comprising cobalt (or nickel) and molybdenum 0x0-species on y-alumina are used in hydrodesulphurisation (abbreviated hds) of petroleum and other fossil fuels[l].

Their activities are affected by the method of preparation (impreg-

nation, washing, drying, calcining, and sulphiding)[2]. For example, the catalytic activity in hds depends on the method of impregnation. The most active catalysts are prepared by double impregnation of y-alumina, first with ammonium molybdate solution and then with cobalt(I1)

[or nickel (1111 nitrate solution

Catalysts prepared by kneading boehmite with solutions of molybdenum(V1)

[2a]. and cobalt(I1) salts are less active apparently because migration of promoter ions into the alumina lattice is favoured during the phase transformation of boehmite to y-alumina which occurs when the catalyst is calcined [2c,d,e].

The presence of cobalt in COO-MOO /A1203 catalysts causes a synergic increase 3

of hds activity (promoter effect) [1,2].

To understand the effect of cobalt we

must distinguish between active and inactive cobalt[3].

Active cobalt is at

the catalyst surface and in the oxide form of the catalyst is in octahedral co-ordination by oxide ions[3,4].

Active cobalt is sulphided under hds

conditions and intercalates in MoS2 in the active catalyst[5].

The inactive

cobalt is locked in the alumina lattice and is tetrahedrally co-ordinated by oxide ions in both the oxide and sulphided forms of the catalysts. We expect therefore that the most active catalysts are those which are prepared in such a way that the amount of octahedral cobalt is optimised. As a basis for understanding how preparative variables affect properties of the catalysts we have investigated chemical changes in the preparation of the catalysts and the structures of the cobalt and molybdenum 0x0-species formed.

(We follow the convention of expressing the concentrations of cobalt and molybdenum in the catalysts in terms of oxides (COO and Moo3) but we do not thereby imply that these oxides are present as well-defined phases).

We show in this

paper that impregnation of calcined MOO /A1203 with cobalt gives the greatest 3 proportion of octahedral cobalt. EXPERIMENTAL Gamma-alumina (Ketjen 000-1.5E;

2 -1 surface area, 300 m g ; water pore volume,

0.72 cm3 g-l; particle diameter, 0.15

-

0.30 mm) was impregnated with aqueous

solutions of ammonium heptamolybdate at room temperature (ca. 2OoC) by: (a) 3 equilibrium adsorption (EA series) [ammonium heptamolybdate solution (100 cm ) stirred with y-alumina (1 g) until no further uptake of molybdenum or pH change and then filtered], (b) pore-filling (PF series) [ammonium heptamolybdate solution 3 ( 0 . 7 cm ) added to y-alumina (1 g) and left for 48 h]. Catalysts of both series were washed with distilled water (3 x 75 em

3

,

45 min) (some samples only), dried

471 at llO°C, and calcined in air at 55OoC.

The Co0-M0O3/A1 0 catalysts were 2 3

prepared by the pore-filling method from solutions of cobalt(I1) nitrate and the various Mo03/A1203 catalysts dried at llO°C and calcined at 55OoC. Molybdenum was determined by atomic absorption (changes of concentration of the impregnating solutions, EA series catalysts) and by X-ray fluorescence (solids). U.v.-visible spectra of the solids were measdred by the reflectance technique against a magnesium oxide standard, and i.r. spectra in KBr discs. Magnetic susceptibilities were determined by the Gouy method at room temperature. RESULTS AND DISCUSSION We have prepared catalysts containing various amounts of cobalt and molybdenum supported on y-alumina in order to determine the effect of variables in the preparation (method of impregnation, washing, drying, and calcining) on the composition of the catalysts and the structures of the cobalt and molybdenum species. (a) E ~ ~ / & 1 ~Catalysts g~ - (i) Preparation Two methods were used to impregnate the y-alumina support with ammonium heptamolybdate: equilibrium adsorption (EA series) and pore-filling (PF serdes). In the EA method y-alumina was left in contact with a solution of the molybdate until there was no further change of the molybdate concentration and the liquid phase was removed by filtration. In the PF method, catalysts of desired compositions were prepared by adding to y-alumina solutions containing the requisite amount of molybdate in a volume just sufficient to fill the pores; any excess of moisture was removed by evaporation, not filtration. We describe first our results with the EA method. When y-alumina was added to a solution of amoniumheptamolybdate the pH (ca. 7.0) rapidly decreased (pH 4-5) and then increased slowly to a steady value (pH 5-6). The pH changes were a convenient means of following the reaction to equilibrium (constant final pH). We attribute the fast initial decrease to exchange of ammonium ions with protons of the alumina (the presence of ammonium ions in the dried catalysts was shown by their i.r. spectra) and the slow subsequent increase to exchange of molybdate ions with surface hydroxyl groups[6]. Al-OH Al0-H

+ +

2[Moo4]

=

A1-OMo03[MoO4I2- = AlO-MoO;

+ OH+ OH-

The increase of pH was slower (ca. 0.5 h) than we would expect for bulk diffusion in the alumina pores (see below) or for the establishment of acidbase and molybdate-polymerisation equilibria in solution (fast reactions). The final constant pH was lower the greater the initial (and equilibrium) concentration of molybdate becuase of release of protons in the equilibrium

472

[Mo~O~~+ ] ~ 4H20 = 7[MoO4I2- + 8H+ (2) The species which adsorbs on the alumina is the [MoO4I2- ion (see below). A s this species adsorbs, the above equilibrium shifts to the right so releasing protons the concentration of which increases with the amount of molybdate adsorbed, i.e. with the total molybdate Concentration. The isotherm of molybdenum concentration change ('adsorption isotherm') (Fig. 1) was similar to that determined previously with more points[3]. Uptake of molybdenum tailed off at ca. 12 g Mo0,/100 g A1203. There was an inflexion at ca. 5 gMo03 corresponding to the plateau in the more detailed isotherm[3]. d

-Fig. 1. Reaction of ammonium heptamolybdate with y-alumina. Amount of Mo(V1)

reacted (adsorbed) (Cs) from change of concentration of the impregnating solution determined by atomic absorption (curve A) and from analysis of calcined Mo03/A1203 catalysts by X-ray fluorescence (curve B) plotted versus equilibrium concentration (C ) of Mo(V1) in the impregnating solution. aq (ii) Composition of the samples

The concentration of molybdenum in the Mo03/A1203 samples was assessed in two ways: from the change of concentration of molybdenum in the solution during the adsorption experiments (determined by atomic absorption, abbreviated AA, on filtered and centrifuged solutions) and by analysis of solid samples (by X-ray fluorescence, abbreviated XRF). The results by the two methods agreed at low

473 molybdenum concentrations, but at higher concentrations XRF gave lower analyses than AA (Fig. 1). The difference is due to the loss of molybdenum as a finely divided solid which passed through the filter. Before the AA determination of molybdenum in the filtrate, suspended solid was removed by centrifuging. This solid (equivalent in weight to E f f e c t of c o b a l t on t h e molybdenum s p e c t r a

Fig. 4.

S e l e c t e d s p e c t r a a r e shown i n

For t h e l l O ° C d r i e d c a t a l y s t s prepared by simultaneous impregnation,

and by a d d i n g c o b a l t t o l l O ° C d r i e d Mo03/A1203 samples, t h e e f f e c t of c o b a l t is t o s h i f t t h e Mo-0 peak t o lower wavenumbers.

The reason f o r t h i s i s n o t c l e a r

b u t i s a p p a r e n t l y n o t due t o a c o b a l t c o n t r i b u t i o n t o t h e spectrum s i n c e t h e e f f e c t i s n o t observed f o r c a t a l y s t s prepared by adding c o b a l t t o 55OoC c a l c i n e d Mo03/A1203 c a t a l y s t s .

For t h e 55OoC c a l c i n e d c a t a l y s t s t h e e f f e c t of c o b a l t is

t o reduce t h e i n t e n s i t y of t h e Mo-0 peak and s h i f t i t t o lower wavenumbers.

The

changes i n t h e s p e c t r a i n c r e a s e w i t h i n c r e a s i n g c o n c e n t r a t i o n of c o b a l t i n t h e catalysts.

Our i n t e r p r e t a t i o n i s t h a t c o b a l t is i n t e r a c t i n g w i t h t h e oxomolybde-

num s p e c i e s i n t h e c a t a l y s t presumably by forming Co-0-Mo

bonds.

The i n t e r a c t i o n

as judged from peak broadening is less i n t h e c a t a l y s t s prepared by double impregn a t i o n and by adding c o b a l t ( I 1 ) t o l l O ° C d r i e d Mo03/A1203 c a t a l y s t s t h a n i n t h o s e prepared by adding c o b a l t (11) t o 55OoC c a l c i n e d Mo03/A1203 c a t a l y s t s .

The

broadening of t h e molybdenum s p e c t r a t o lower wavenumbers could i n d i c a t e an i n c r e a s e d p r o p o r t i o n of o c t a h e d r a l oxomolybdenum s p e c i e s i n t h e c o b a l t - c o n t a i n i n g c a t a l y s t s b u t t h e change i s small and t h e major molybdenum s p e c i e s remains tetrahedral.

( i i ) S p e c t r a and magnetic p r o p e r t i e s of c o b a l t

Selected cobalt spectra a r e

The s p e c t r a of a l l p r e p a r a t i o n s of l l O ° C d r i e d Co0-Mo03/A1203 samples were i d e n t i c a l (peaks a t 17.7, 7.2, 6.2 x 1 03 cm-1). I n t h e s p e c t r a of shown i n Fig. 4.

t h e high-cobalt

samples t h e r e was an a d d i t i o n a l s h o u l d e r a t c a . 20,000 cm-'

i n d i c a t i n g t h e presence of some o c t a h e d r a l c o b a l t ( I 1 ) .

The s p e c t r a of t h e b l u e

l l O ° C d r i e d c a t a l y s t corresponded t o t h e spectrum of b l u e c o b a l t (11) hydroxide i n which c o b a l t ( I 1 ) is o c t a h e d r a l l y co-ordinated by o x i d e r a t h e r t h a n t o t e t r a h e d r a l l y co-ordinated c o b a l t ( I 1 ) .

Thus i t a p p e a r s t h a t a t least p a r t of t h e

411

cobalt in our catalysts is deposited as a surface precipitated hydroxide[ll]. Variations in the room temperature magnetic moments were only just outside the experimental precision (+ 0.1 BM).

For the catalysts prepared by simultaneous

impregnation of alumina, and by the addition of cobalt to llO°C dried Mo03/A1203 catalysts, the magnetic moments decreased slightly when the dried catalysts were calcined ( 4 . 6 to 4 . 4 and 4.9 to 4.8 BM respectively).

The magnetic moments of

the catalyst prepared by adding cobalt to calcined Mo03/A1203 catalysts increased significantly after calcining (4.7 to 5.1 BM) in agreement with spectroscopic indications that these catalysts contained more octahedral cobalt.

A

-

L5

40

35

30

ABf r d 5

Fig. 4 . U.V. and visible reflectance spectra (vs. MgO blank) of Mo03/A1203 (broken lines) and Co0-Mo03/A1203 (full lines) catalysts ( X 4 g COO, 8g Moo3, U.V. spectra assigned to oxoMo(V1) species a1.d visible spectra to 100 g A1 0,). Catalysts prepared by double impregnation of crystal lield transitions of Co(I1). y-A120 (A), adding Co(I1) to llO°C dried Mg03/A1203 (B) and 55OoC calcined Moo3/ A1203 fC), all calcined at 55OoC except 110 C dried Co0-Mo03/A1203 (C').

478

REFERENCES 1 P.C.H. Mitchell, in 'catalysis', C. Kemball (Ed.), Chemical Society, London, Vol. I, 1977, p.204, and references therein. 2 (a) J.B. McKinley, in 'Catalysis', P.H. Emmett (Ed.), Reinhold, New York, Vol. 5, 1957, p.405; (b) V.H.J. de Beer and G.C.A. Schuit, in 'Preparation of Catalysts', B. Delmon, P.A. Jacobs and G . Poncelet (Eds.), Elsevier, Amsterdam, 1976, p.343; (c) Y. Kotera, K. Ogawa, M. Oba, K. Shimomura, M. Yonemura, A . Ueno, and N. Rodo, ibid, p.371; (d) W. Ripperger and W. Saum, in 'Proceedings of the Climax Second International Conference on the Chemistry and Uses of Molybdenum', P.C.H. Mithcell (Ed.), Climax Molybdenum Co. Ltd., London, 1977, p.175; (e) M. Inoguchi, T. Mizutori, K. Tate, Y. Satomi, K. Inaba, Y. Kaneko, R. Nishiyama, and T. Nagai, Bull. Japan. Petrol. Inst. 13 (1976) 19. 3 N.P. Martinez, P.C.H. Mitchell, and P. Chiplunker, in 'Proceedings of the Climax Second International Conference on the Chemistry and Uses of Molybdenum', P.C.H. Mitchell (Ed.), Climax Molybdenum Co. Ltd., London, 1977, p.164. 4 J.T. Richardson, Ind. and Eng. Chem. (Fundamentals), 3 (1964) 154; J.H. Ashley, Thesis, University of Reading, 1969, p.104; N.P. Martinez and P.C.H. Mitchell, Paper 15, Chemical Society, University of Bath, 1977. 5 A.L. Farragher and P. Cossee, 'Proceedings of the Fifth International Congress on Catalysis', J.W. Hightower (Ed.), North-Holland, Amsterdam, 1973, p.1301. 6 A. Iannibello and F. Trifiro, Z. anorg. Chem., 413 (1975) 293; A. Iannibello, S. Marengo and F. Trifiro, Chimica e Industria 57 (1975) 676. 7 H.P. Boehm, Discuss. Faraday SOC., 52 (1971) 264. 8 J. Sonnemans and P. Mars, J. Catalysis, 31 (1973) 209. 9 J.H. Ashley and P.C.H. Mitchell, J. Chem. SOC. ( A ) , (1968) 2821; (1969) 2730. 10 J.M.J.G. Lipsch and G.C.A. Schuit, J. Catalysis, 15 (1969) 174. 11 P. Tewari and W. Lee, J. Colloid Interface Sci., 52 (1975) 77; H.C. Yao and M. Bettman, J. Catalysis, 41 (1976) 349.

DISCUSSION E. MATIJEVIC

:

I am somewhat worried about the validity of equations

la or lb in the preprint. While they may stoichiometrically describe the situation (if the titration curves are in agreement with the reaction shown), they cannot mechanistically represent the true process. At pH 4 - 5 alumina is strongly positively charged (precipitation is at Q 9 ) . Thus a protonated aluminum hydroxide surface species should be involved in the condensation reaction with the molybdate ion. A. IANNIBELLO and P.C.H. MITCHELL

:

We agree with the comment.

The

following equations describe the interaction of metamolybdate with alumina under acidic conditions

+ 4H20 A1-OH + H Al-OH'

2

3

+ 0

+ [MOO4] 2-

:

+ -+

7 ( M o 0 4 )2- + 8H+

+

A ~ O H + ~H ~ O

+

A~OMOO;+ H ~ O

The observed rise of pH is then correctly described as due to removal of protons from the solution rather than to release of hydroxide ions.

479

THE INFLUENCE OF THE SUPPORT ON CO-MO HYDRODESULFURIZATION CATALYSTS H. TOPSQE',

B.S. CLAUSEN+, N. BURR!CESCI+',

R. CANDIA'and

S.M@RUP+

Haldor Topsae Research Laboratories, DK-2800, Lyngby, Denmark. + Laboratory of Applied Physics 11, Technical University of Denmark, DK-2800, Lyngby, Denmark. x

SUMMARY The effect of support on the structural properties of cobaltmolybdenum hydrodesulfurization catalysts has been investigated by means of MEssbauer spectroscopy. The study shows that the interaction of Co and Mo with the support plays an essential role in determining the resulting phases, both in the oxidic and in the sulfided state.

1 INTRODUCTION

The continuous scientific attention paid to hydrodesulfurization (HDS) catalysts is undoubtedly inspired by the need for preparing more efficient catalysts for sulfur removal especially from difficult feedstocks. The great research effort has yielded a better understanding of this difficult catalyst system. There still exist, however, diverging views regarding the structure of the catalysts, the relation between the structural and catalytic properties, and the changes resulting from different preparation conditions. It is clear that the nature of the support plays an important role for the catalyst properties. (For recent reviews, the reader is referred to [ 1 , 2 ] ) . Most of the published works deal with alumina-supported catalysts. Less attention has been paid to unsupported catalysts and to carbon- and silica-supported catalysts. In the present work we have studied the influence of the sup'Permanent Italy.

address: Montedison Research Laboratories, Novara,

480

p o r t on t h e s t r u c t u r e and p r o p e r t i e s o f t h e c a t a l y s t . For t h i s p u r p o s e w e have used MBssbauer s p e c t r o s c o p y , which e a r l i e r h a s been shown t o b e v e r y u s e f u l f o r t h e s t u d y of Co-Mo HDS c a t a l y s t s [3-51 and o t h e r c a t a l y s t s [61. I n t h i s s t u d y , w e have a p p l i e d HGssbauer s p e c t r o s c o p y mainly a s a " f i n g e r p r i n t " t e c h n i q u e . By comparing MBssbauer s p e c t r a o f c a t a l y s t s on d i f f e r e n t s u p p o r t s w i t h t h o s e o f model compounds and a l s o by s t u d y i n g t h e s h a p e s o f t h e i n d i v i d u a l s p e c t r a one can g e t s t r u c t u r a l and chemical i n f o r m a t i o n which i s n o t n e c e s s a r i l y l i m i t e d t o t h e MGssbauer atoms. The f a c t t h a t t h e s t u d i e s can be c a r r i e d o u t in situ and t h a t t h e d e g r e e o f c r y s t a l l i n i t y d o e s n o t p l a y any r o l e , makes t h i s t e c h n i q u e e s p e c i a l l y u s e f u l f o r s t u d i e s o f HDS c a t a l y s t s .

2 EXPERIMENTAL 2.1 P r e p a r a t i o n o f samples. A l l samples w e r e doped w i t h 5 7 Co.

2.1.1 coPloo4 was p r e p a r e d by p r e c i p i t a t i o n a t b o i l i n g p o i n t H ~ ONa2Mo04, f o l u s i n g s t o i c h i o m e t r i c s o l u t i o n s o f C O ( N O ~ ) ~ . ~ and

lowed by washing, d r y i n g and c a l c i n a t i o n a t 500°C i n a i r . X-ray d i f f r a c t i o n a n a l y s i s showed t h a t t h e r e s u l t i n g phase was p u r e CoMo04. 2.1.2 Co:MoS2 was p r e p a r e d by impregnation o f MoS2 powder (Riedel-De Haen AG) w i t h a s o l u t i o n o f Co(N03) 2 - 6 H 2 0 r f o l l o w e d by h e a t t r e a t m e n t i n H2/H2S ( 2 % H2S) a t 325'C.

The sample c o n t a i n e d

a p p r o x i m a t e l y 1 ppm Co. 2.1.3 Cogs8 was p r e p a r e d by s u l f i d i n g C O ( N O ~ ) ~ * ~a Ht ~6 0O0 ' C i n H2/H2S ( 2 % H2S).

-.2.1.4

The c a t a l y s t samples w e r e p r e p a r e d by i m p r e g n a t i n g t h e

s u p p o r t s w i t h an ammoniacal s o l u t i o n o f c o b a l t n i t r a t e and ammonium paramolybdate.

The s u p p o r t s used were: y-A1203 (230 m2/g) ;

S i 0 2 Davison Grade 950 (950 m2/cJ);

and a c t i v e carbon ( 9 0 0 m2/g).

Samples c o n t a i n i n g o n l y Co ( w i t h o u t 110) w e r e a l s o p r e p a r e d . The impregnated samples w e r e d r i e d i n a i r a t room t e m p e r a t u r e and a f t e r w a r d s c a l c i n e d i n a i r f o r 24 h o u r s a t 500'C

(for the

carbon-supported samples t h e c a l c i n a t i o n t e m p e r a t u r e was 230'C). 2.1.5

Another sample was p r e p a r e d by e v a p o r a t i o n o f t h e i m -

p r e g n a t i o n l i q u o r t o d r y n e s s a t room t e m p e r a t u r e , f o l l o w e d by c a l c i n a t i o n i n a i r a t 500'C

f o r 24 hours.

481 S u l f i d e d c a t a l y s t samples were p r e p a r e d by t r e a t i n g t h e oxid-

i c samples a t 325°C i n H2/H2S ( 2 % H2S) f o r 2 4 h o u r s . The nomenclature used i n t h i s p a p e r f o r t h e samples w i l l become c l e a r from t h e f o l l o w i n g example: Col-l!06/A1203 d e n o t e s an alumina-supported sample c o n t a i n i n 9 1 % Co and 6 % Mo. 2.2 Mksbauer spectroscopy

The samples w e r e p r e s s e d i n t o t h i n w a f e r s and p l a c e d i n an in s i t u PlEssbauer Pyrex c e l l connected t o a g a s h a n d l i n g system 151. The samples w e r e used a s s o u r c e s w i t h a moving s i n g l e - l i n e a b s o r b e r o f K4Fe (CN) 6 *3H20 e n r i c h e d w i t h 57Fe. Zero v e l o c i t y c o r r e s p o n d s t o t h e c e n t r o i d o f t h e spectrum o b t a i n e d a t room t e m p e r a t u r e w i t h a source of 57C0 i n n a t u r a l i r o n . P o s i t i v e v e l o c i t y corresponds t o t h e s o u r c e moving away from t h e a b s o r b e r .

3 RESULTS AND DISCUSSION F i g u r e 1 shows room t e m p e r a t u r e llijssbauer s p e c t r a o f model compounds and o f t h e c a l c i n e d c a t a l y s t s . F i g u r e 2 shows t h e room t e m p e r a t u r e s p e c t r a a f t e r s u l f i d a t i o n a t 325'C. The s p e c t r a o f some model compounds a r e a l s o i n c l u d e d h e r e . I n t h e f o l l o w i n g w e w i l l g i v e a b r i e f i n t e r p r e t a t i o n o f t h e v a r i o u s s p e c t r a . The main emphasis w i l l be on t h e g r e a t d i f f e r e n c e s observed among t h e c a t a l y s t s on d i f f e r e n t s u p p o r t s . A q u a n t i t a t i v e a n a l y s i s of t h e M6ssb a u e r p a r a m e t e r s w i l l be p u b l i s h e d e l s e w h e r e . 3.1

CoMo04 F i g u r e l a shows t h e M k s b a u e r spectrum o f b u l k CoFlo04. An ana-

l y s i s o f t h i s s p e c t r u m h a s been p u b l i s h e d e a r l i e r [71.

3.2

co304

Co30q was n o t p r e p a r e d , b u t i t s I G s s b a u e r spectrum i s w e l l known 181 and l o o k s q u i t e s i m i l a r t o F i g . l e , e x c e p t f o r t h e s m a l l s h o u l d e r s a p p e a r i n g t o t h e r i g h t and l e f t o f t h e two a b s o r p t i o n lines.

3.3 c o g s g The Mzssbauer spectrum o f t h i s model compound i s shown i n F i g . 2a. T h i s spectrum h a s been d i s c u s s e d i n a p r e v i o u s p u b l i c a t i o n 191.

482

z

P c

n K

5m4 4

w

2 c

4w K

-4

-2 0 2 VELOCITY (MM/S)

4

1. Room temperature Mksbauer s p e c t r a of samples i n t h e o x i d i c state.

Fig.

483

a.

.*....... . .. . t

*

9

:*

.*

-4

-2

0

2

4

VELOCITY (MM/S)

Fig. 2 . Room temperature MEssbauer s p e c t r a of samples i n the sulfided s t a t e .

484

3.4 Co:MoS2 Co:MoS2 g i v e s t h e t w o - l i n e spectrum shown i n F i g . 2b 141. T h i s phase w i l l be f u r t h e r d i s c u s s e d i n c o n n e c t i o n w i t h t h e s u l fided catalysts.

3.5 Col-Mo6

(unsupported o x i d i c c a t a l y s t ) The sample p r e p a r e d by e v a p o r a t i o n o f t h e i m p r e g n a t i o n s o l u -

t i o n followed by c a l c i n a t i o n g i v e s a ItEssbauer spectrum ( F i g . I b ) which i s q u i t e s i m i l a r t o t h e s t o i c h i o m e t r i c CoMo04 ( F i g . l a ) . T h i s r e s u l t i s e x p e c t e d , s i n c e CoMo04 i s t h e thermodynamically s t a b l e compound under t h e c a l c i n a t i o n c o n d i t i o n s used f o r t h e c a t a l y s t s . 3.6 Alumina-supported samples 3.6.1

O x i d i c form

Samples: C o 0.25/A1203 and Col-Mo6/A1203. The s p e c t r a o f Co O.25/Al2O3 ( F i g . I c ) and Col-Mo6/A1203 ( F i g . I d ) i n t h e oxid-

i c s t a t e a r e both t y p i c a l f o r c o b a l t d i f f u s e d i n t o alumina 1 4 , 1 0 1 . The broad l i n e s i n d i c a t e t h a t t h e c o b a l t i s n o t p r e s e n t i n a s i n g l e t y p e o f s i t e ; r a t h e r , a d i s t r i b u t i o n o f Co s u r r o u n d i n g s e x i s t s . I t i s i n t e r e s t i n g t h a t t h e c o b a l t i o n s i n t h e two samples are p r e s e n t i n q u i t e s i m i l a r s u r r o u n d i n g s . Thus, i n t h e c a l c i n e d c a t a l y s t , Col-Mo6/A1203, t h e p r e s e n c e o f molybdenum does n o t i n f l u ence

t o any l a r g e e x t e n t t h e s t r u c t u r e of t h e c o b a l t i o n s . C e r -

t a i n l y , a c o b a l t molybdate phase d o e s n o t form. T h i s phase h a s p r e v i o u s l y [ I l l been proposed t o be p r e s e n t f o r c a t a l y s t s c a l c i n e d a t r e l a t i v e l y low t e m p e r a t u r e s such a s t h o s e (500°C) used i n t h e p r e s e n t s t u d y . I f t h e s u p p o r t i n t e r a c t i o n s w i t h Co and Mo a r e weak, c o b a l t molybdate would b e e x p e c t e d t o form upon c a l c i n a t i o n . T h i s

w a s i n f a c t observed f o r t h e unsupported o x i d i c c a t a l y s t (Col-Mo6) ( F i g . I b ) a s d e s c r i b e d above. I n s t u d i e s o f o t h e r Co/A1203 samples w i t h h i g h e r c o b a l t conc e n t r a t i o n s it was found t h a t , above a c e r t a i n l e v e l , p a r t o f t h e c o b a l t d i d n o t i n t e r a c t w i t h alumina. I n s t e a d , a s e p a r a t e Co304 phase was formed i n a c c o r d a n c e w i t h t h e f i n d i n g s o f L o Jacono e t a l . 1121. The p r e s e n c e o f molybdenum i n f l u e n c e d t h e c o n c e n t r a t i o n l e v e l o f c o b a l t n e c e s s a r y f o r t h e Co30q f o r m a t i o n , b u t n o t t h e n a t u r e o f t h i s phase. I t i s i n t e r e s t i n g t o n o t e t h a t under t h e s e c i r c u m s t a n c e s where t h e Co304 phase is formed, t h e c o b a l t molybd a t e f o r m a t i o n d i d n o t o c c u r . T h i s s u g g e s t s a h i g h s t a b i l i t y of t h e molybdenum on t h e alumina s u r f a c e .

485

With r e s p e c t t o t h e v a r i o u s models proposed f o r t h e s t r u c t u r e o f t h e o x i d i c c a t a l y s t , t h e monolayer model a s o n g i n a l l y proposed by S c h u i t and Gates [ I 3 1 seems t o r e p r e s e n t q u i t e c l o s e l y t h e act u a l c a t a l y s t s t r u c t u r e f o r c a t a l y s t s w i t h a composition t y p i c a l o f t h a t used i n d u s t r i a l l y . I t s h o u l d b e s t r e s s e d t h a t t h e c a t a l y s t s t r u c t u r e i s dependent on many p a r a m e t e r s ( s u r f a c e a r e a o f t h e a l u mina; c a l c i n a t i o n t e m p e r a t u r e ; m e t a l l o a d i n g s ; i m p r e g n a t i o n proc e d u r e , e t c . ) , and o t h e r models may d e s c r i b e t h e s t r u c t u r e encount e r e d when t h e p a r a m e t e r s a r e changed. 3.6.2

S u l f i d e d form

Samples: Co O.25/Al2O3

and Col-M06/A1~0~.Upon s u l f i d a t i o n ,

t h e Col-Mo6/A1203 c a t a l y s t ( F i g . 2d) behaved q u i t e d i f f e r e n t l y from t h e sample c o n t a i n i n g no molybdenum; i n t h i s sample, p a r t o f t h e c o b a l t i s t r a n s f o r m e d i n t o a Cogs8 phase (compare s p e c t r a 2 c and 2 a ) , whereas i n t h e o t h e r sample a t l e a s t two c o b a l t pos i t i o n s can b e d i s t i n g u i s h e d , none of which i s a Cogs8 phase. On t h e o t h e r hand, one o f t h e s e p o s i t i o n s g i v e s a Mzssbauer spectrum q u i t e s i m i l a r t o t h a t of Co:MoS2 ( F i g . 2 b ) . I n f a c t , a d e t a i l e d i n v e s t i g a t i o n o f t h e s p e c t r a of Co:MoS2 and Col-Mo6/A1203 v e r s u s t e m p e r a t u r e s t r o n g l y s u g g e s t s t h a t c o b a l t i n Col-Mo6/A1203 i s p r e s e n t i n a MoS2-like phase 1 4 1 . These r e s u l t s a r e i n t e r e s t i n g i n view o f t h e r e c e n t r e s e a r c h on alumina-supported c a t a l y s t s which h a s been d i r e c t e d toward t h e p o s s i b l e p r e s e n c e o r absence o f c o b a l t i n MoS2. The Co:MoS2 s t r u c t u r e i n t h e A1203-supported c a t a l y s t h a s most l i k e l y a two-dimensional c h a r a c t e r . T h i s i s s e e n from t h e a c c e s s i b i l i t y o f t h e c o b a l t atoms t o g a s e x p o s u r e 141. I t s h o u l d be n o t e d t h a t t h e Col-Mo6/A1203 c a t a l y s t s s t u d i e d

h e r e have a c o m p o s i t i o n t y p i c a l o f t h o s e used i n d u s t r i a l l y . For s u c h c a t a l y s t s it a p p e a r s , t h e r e f o r e , t h a t a s e p a r a t e p h a s e o f Cogs8 i s n o t p s e s e n t ; r a t h e r , t h e s t r u c t u r e resembles t h a t proposed i n t h e " p s e u d o - i n t e r c a l a t i o n " model [ 1 4 I . C o g s 8 was, howe v e r p r e s e n t when t h e c o n c e n t r a t i o n o f c o b a l t w a s h i g h . I t i s i n t e r e s t i n g t h a t previous Mksbauer spectroscopy r e s u l t s i n d i c a t e t h a t t h e Co:MoS2 p h a s e i s c a t a l y t i c a l l y a c t i v e [ 4 1 .

3.7 S i l i c a - s u p p o r t e d samples 3.7.1 O x i d i c form Samples: Col/Si02 and Col-Mo6/Si02.

The Col/Si02 sample

( F i g . l e ) shows t h a t c o b a l t i s mainly p r e s e n t a s C o 3 0 4 .

I n addi-

486 tion, a small part of the cobalt seems to have formed a compound with the silica. This shows that the interaction of cobalt with silica is present, but is much weaker than in the case of alumina. The spectrum of the Co1-Mo6/Si0 2 sample in the oxidic state (Fig. 1f) is quite complicated and very different from the spectrum of the sample without molybdenum. This indicates the formation of a Co-Mo-O-containing surface compound, the exact nature of which is not clear.

The presence of a poorly crystallized cobalt molybdate

phase cannot be excluded. It is, however, not the only phase formed on the silica-supported catalyst. The results for the silica-supported samples are significantly different from those of the alumina-supported samples, because neither the cobalt nor the molybdenum seems to interact appreciably with silica. 3.7.2

Sulfided form

Samples: co1/Si0 2 and C01-M06/Si0 2• Upon sulfidation, the C0 30 4 present in the oxidic Co1/Si0 2 sample is observed to transform into COgS S' whereas both cOgS S and the Co in the MOS 2-like phase are present in the sulfided Co1-Mo6/Si0 catalyst (Fig.2d). 2 The CO:MoS 2 type phase can form upon sulfidation of a Co-Mo-O precursor structure. This is observed also for the carbon-supported catalyst as seen below. Formation of the CO:MoS

structure 2 does not seem, therefore, to require the type of precursor structures which were observed in the alumina-supported catalyst. 3.S Carbon-supported catalysts A cobalt molybdate-like phase is observed when the carbon-

supported catalyst is calcined at 230·C (Fig. 1d). The crystallinity of this phase is probably quite poor as seen from the broad lines and from the intensity ratios, which are somewhat different from those observed in bulk CoM00 4 (Fig.1a). After sulfidation at 32S·C the spectrum (Fig. 2e) shows that almost all the cobalt ends up in a MOS 2-like phase, whilst sulfidation of bulk CoMo0 gives 4 [1]. The above discussion of the silica-supported catS alysts with respect to the support interactions for the cobalt

also cOgS

and the molybdenum holds also in the case of carbon-supported catalysts.

487

4 CONCLUSIONS The present results show that the support plays an essential role for the formation of the cobalt-containing phases, both in the oxidic and in the sulfided states of the catalyst. In the alumina-supported catalysts the support interaction is quite strong. This is in particular evident from the Mhsbauer spectra of the oxidic catalyst which indicate that cobalt is present in the alumina lattice. In the sulfided state, some of the cobalt atoms are present in a MoS2-like surface structure which is probably the catalytically active phase. At high cobalt contents (higher than that corresponding to maximum catalytic activity) a Cogs8 phase is also found. In the silica-supported catalysts, the weaker support interaction leads to the formation of new phases both in the oxidic state and in the sulfided state. In the latter, cobalt is present both as Cogs8 and in a MoS2-like structure. The M6ssbauer spectra of the oxidic carbon-supported catalysts are similar to those of the unsupported catalysts. This indicates a very weak support interaction. In the sulfided state, the cobalt seems to be present mainly in a MoS2 phase.

REFERENCES 1 V.H.J.de Beer and G.C.A. Schuit, in Preparation of Catalysts, B. Delmon, P.A. Jacobs and G. Poncelet (Eds.), Elsevier, Amsterdam, 1976,p.343. 2 For a recent review of the"contact synergy model", see B.Delmon, American Chem.Soc.Petroleum Div. Preprints,22(1977)503. 3 H. Topsde and S. M@rup, in A.Z. Hrynkiewich and J.A. Sawicki (Eds.) Proc. Int. Conf. MEssbauer Spectrosc., Akademia G6rniczo-Hutnicza Im. S . Staszica W. Krakowie, Cracow, Poland, 1 (1975)305. 4 B.S. Clausen, S. Mmrup, H. Topsme and R. Candia, J.Physique, 37(1976)C6-249. 5 B.S.Clausen, Thesis, LTF 11, Technical University of Denmark, 1976, unpublished. 6 J.A. Dumesic and H. Topsee, Adv-Catal., 26(1977)121. 7 B.S. Clausen, H. Topsqje, J. Villadsen and S. M@rup, in D.Barb and D. Tarina (Eds.) , Proc. Int. Conf .MEssbauer Spectrosc., Bucharest, Romania, 1977, p.155. 8 C.D. Spencer and D. Schroeer, Phys.Rev.B,9(1974)3658. 9 B.S.Clausen, H. Tops@e, J. Villadsen, S.MQrup and R. Candia, in D. Barb and D. Tarina (Eds.) , Proc. Int. Conf. MEssbauer Spectrosc., Bucharest, Romania, 1977, p.177. 10 G.K.Wertheim and D.N.E.Buchanan, in A.H.Schoen and D.M.J. Coryston (Eds.), Proc. 2nd Int. Conf. Mksbauer Effect,Saclay, France, 1961, John Wiley & Sons, Inc. New York, 1962,p.130.

488

1 1 R. MOn6 and L. Moscou, Am. Chem. Soc., Symp.series, 20(1975) 150. 12 M. Lo Jacono, K. Cimino and G.C.A. Schuit, Gazz. Chim. Ital. 103(1978) 1281. 13 G.C.A. Schuit and B.C. Gates, AIChE J. 19(1973)417. 14 A.L. Farragher and P. Cossee, in J.W.Hightower (Ed.), Proc.5th Int. Congr. on Catalysis,North-Holland, Amsterdam ,1973,p.1301. DISCUSSION M.

: You a f f i r m

CARBUCICCHIO

Si02. (Fig.

t h a t n e i t h e r Co n o r Mo i n t e r a c t w i t h

T h i s i s b a s e d on t h e Mdssbauer s p e c t r u m f o r C o ( l ) - M o ( 6 ) / S i 0 2

-

If)

a q u i t e complicated spectrum

-

w h i c h you d o

not interpret.

On t h e o t h e r h a n d f r o m t h e s p e c t r u m f o r C o ( l ) / S i 0 2 ( F i g . l c )

,

a

c o n t r i b u t i o n a p p e a r s which could be n o t n e g l i g i b l e due t o t h e p r o d u c t o f t h e r e a c t i o n b e t w e e n Co a n d S i 0 2 .

H.

TOPS#E

: We d o n o t

s t a t e t h a t t h e r e i s no i n t e r a c t i o n o f

a c t i v e component w i t h S i 0 2 .

We d o , h o w e v e r ,

the

say t h a t t h i s interac-

t i o n i s n o t so i m p o r t a n t a s f o r a l u m i n a - s u p p o r t e d

catalysts.

This

i s s e e n f r o m t h e l a r g e f r a c t i o n o f t h e Co t h a t i s p r e s e n t a s Co30q i n t h e c a l c i n e d , and a s Cogs8 i n t h e s u l f i d e d C o ( l ) / S i 0 2 sample. Besides t h e above Co phases w e observe,

a s discussed i n the paper,

s m a l l " s h o u l d e r s " w h i c h m o s t l i k e l y , a s you s u g g e s t ,

a r e owing t o

Co reacting with the Si02 surface.

For c a l c i n e d s i l i c a - s u p p o r t e d c a t a l y s t s c o n t a i n i n g b o t h C o and Mo, compound i s o b s e r v e d .

a Co-Mo-containing

t h e alumina-supported

T h i s i s i n c o n t r a s t to

c a t a l y s t s a n d shows t h a t f o r s i l i c a - s u p p o r t e d

c a t a l y s t s a l a r g e f r a c t i o n o f t h e Mo i s i n t e r a c t i n g o n l y w e a k l y w i t h the silica.

J.

SCHEVE : How c a n you b e s u r e t h a t t h e s t r o n g gamma-Co-source

in

your samples does n o t i n f l u e n c e s o l i d s t a t e r e a c t i o n s between t h e oxides during calcination ? oxides !)

H.

( r e m e m b e r t h e Tamman t e m p e r a t u r e o f t h e

.

TOPS#E : We h a v e s t u d i e d w i t h o t h e r p h y s i c a l t e c h n i q u e s s a m p l e s

w i t h a n d w i t h o u t r a d i o a c t i v e Co a n d h a v e f o u n d n o e v i d e n c e t h a t t h e s o l i d s t a t e r e a c t i o n s o c c u r r i n g d u r i n g c a l c i n a t i o n a r e a f f e c t e d by t h e r a t h e r weak gamma r a y s o u r c e 5 7 C 0 Mbssbauer

(61 m C i ) .

The m a i n p r o b l e m i n

source experiments i s associated with the electron

capture process.

The i n f l u e n c e o f

s u c h p r o c e s s e s on t h e Mdssbauer

s p e c t r a o f Co-Mo c a t a l y s t s h a s r e c e n t l y b e e n d e a l t w i t h B.S.

C l a u s e n and H .

Topsde,

t o appear i n J . Physique).

(S. Mmrup,

489

E.V. LUDENA : I would like to make a comment with regard to some molecular orbital calculations we have performed in order to determine the effect of cobalt on the reaction of thiophene over a MOO The main result is that for given configurations, there

catalyst.

is a narrowing of the gap between the highest occupied and the lowest unoccupied molecular orbitals.

Hence, it seems that there

is a strong electronic effect of cobalt on molybdenum oxide favoring the hydrodesulfurization reaction. H. TOPSdE : In the active state of our catalysts the molybdenum is

sulfided.

We observe a valence change of the promotor atoms located

in the Co-Mo-S structure during the catalytic reaction.

Since Mo

and Co are closely associated in the Co-Mo-S structure, this would undoubtedly affect the electronic structure of the Mo atoms.

It

would indeed be interesting to compare the present Mdssbauer results with the type of calculations you have performed. B. DELMON : 1 ) With respect to the spectrum of Co(l)-Mo(6)/Si02

in

Fig. 1 (spectrum f) have you any interpretation for the line at -1 -0.7 mm s , which shows up in no other spectrum ? 2) Concerning the sulphided catalysts, your results quite impressively show that a new cobalt species is formed.

But this finding raises

problems I With unsupported systems Farragher ( I ) , as well as ourselves (2) put a very low limit to the maximum number of group VIII metal atoms which can be intercalated or pseudo-intercalated in M o S p or WS2.

The

totality of the C o signals in b and g (Fig. 2) must be attributed to the cobalt associated with MoS2, as no other cobalt signal is visible. In contradiction to the above recalled results and calculations, one should thus admit that we have one intercalated cobalt atom for six molybdenum atoms: this is really much more than it ever has been assumed.

What explanation could you propose ? Is there an extremely

high dispersion of MoS2 ? What do you think about some special feature related to the model of Schuit for sulfided catalysts (3) ? 3 ) Also in Fig. 2, curves c and d: it is clear that there is much

less CoA1204 in catalysts containing only Co than in those containing also Mo. This agrees with our recent results ( 4 ) . (1) A . L .

Farragher, P. Cossee, in "Catalysis", Proc. 5th 1nt.Cong.

Catalysis (J.W. Hightower, ed.), North Holland, Amsterdam, 1973, 1316-1317.

,

490

(2) F. D e l a n n a y , D.S.

T h a k u r , B. D e l m o n , J. L e s s

Common Metals,

in press. ( 3 ) W.H.J.

d e Boer, G.C.A.

(B. D e l m o n , P.A.

S c h u i t , in " P r e p a r a t i o n o f C a t a l y s t s "

J a c o b s , G. P o n c e l e t , eds.).Elsevier,

Amsterdam,

1976, 343. (4) B .

Delmon, P .

C.R.

G r a n g e , M.A.

Acad. Sci.,

Ser. C ,

A p e c e t c h e , P. G a j a r d o , F. D e l a n n a y ,

287

( 1 9 7 8 ) , 401.

H. TOPSI6E : 1 ) We h a v e at t h e m o m e n t n o d e t a i l e d e x p l a n a t i o n o f t h e l i n e at a b o u t -0.7 mm s - l

in Fig. 1 f.

It s e e m s t o c o r r e s p o n d t o a

c o m p o n e n t w i t h very low i s o m e r shift w h i c h m a y h a v e formed a s a r e s u l t o f t h e A u g e r C a s c a d e f o l l o w i n g t h e e l e c t r o n c a p t u r e process. 2 ) W e d o not b e l i e v e t h a t t h e p r e s e n t r e s u l t s are in c o n f l i c t w i t h t h e r e s u l t s y o u refer t o , o b t a i n e d o n u n s u p p o r t e d systems.

These

s y s t e m s w i l l have a low d e g r e e of d i s p e r s i o n o f t h e M o S 2 phase. O u r r e s u l t s o n s u p p o r t e d c a t a l y s t s p o i n t t o C o b e i n g p r e s e n t at t h e s u r f a c e of a M o S 2 - l i k e s t r u c t u r e w i t h a h i g h d e g r e e o f dispersion. W i t h r e s p e c t t o Fig. 2 b w e r e c a l l t h a t the u n s u p p o r t e d Co:MoS2 s a m p l e c o n t a i n e d o n l y 1 ppm Co.

P.G.

M E N O N : T h e s t r u c t u r e o f virgin, r e d u c e d s u l f i d e d c a t a l y s t m a y

c h a n g e s t i l l f u r t h e r o n e x p o s u r e of t h e c a t a l y s t t o a c t u a l r e a c t i o n conditions.

For silica-supported multi-component molybdate catalysts

c o n t a i n i n g B i , M o , F e , C o and N i , w e h a v e f o u n d recently

( 1 ) that

exposure of the catalyst to ammoxidation of propylene results in a s u b s t a n t i a l enrichment o f t h e s u r f a c e with B i and F e and, c o n s e q u e n t ly,

'burying" o f Mo, Co, S i r etc.

T h e M d s s b a u e r s p e c t r a of t h e

c a t a l y s t a l s o c h a n g e s significantly.

No s u c h c h a n g e s o c c u r on an

i d e n t i c a l t h e r m a l t r e a t m e n t o f t h e c a t a l y s t w i t h o u t reactants.

Have

y o u f o u n d any s u r f a c e e n r i c h m e n t o r d e p l e t i o n o n exposing y o u r catalyst t o a c t u a l h y d r o d e s u l f u r i z a t i o n r e a c t i o n , instead o f s u l f i d i n g them o n l y ?

( 1 ) T.S.R.

P r a s a d a Rao and P.G.

M e n o n , J. C a t a l y s i s ,

G , (Jan. 1 9 7 8 ) .

H. T O P S d E : F i r s t , it should b e m e n t i o n e d t h a t t h e s t a t e o f t h e c a t a l y s t d i d not c h a n g e s i g n i f i c a n t l y going from H 2 / H 2 S t o H 2 / t h i o phene reaction mixtures

( s e e ref. 4 in t h e paper).

H o w e v e r , very

large c h a n g e s were o b s e r v e d w h e n t h e sulfided c a t a l y s t s w e r e exposed to air.

It i s t h e r e f o r e e s s e n t i a l t h a t s u l f i d e d c a t a l y s t s a r e exami-

ned in situ.

T h e c h a n g e s y o u m e n t i o n occur g o i n g from calcined t o

used catalysts.

In the case of hydrodesulfurization catalysts you

would expect from t h e r m o d y n a m i c s g r e a t c h a n g e s b e t w e e n t h e s t r u c t u r e s o f calcined and s u l f i d e d catalysts.

Indeed, the expected changes

w e r e o b s e r v e d f o r t h e u n s u p p o r t e d catalysts.

The changes we observe

for s u p p o r t e d c a t a l y s t s depend very m u c h o n t h e i n t e r a c t i o n o f t h e C o and M O w i t h t h e support.

I n g e n e r a l , t h e c h a n g e s m a y alter t h e

f r a c t i o n o f v a r i o u s a t o m s exposed at t h e surface. F o r example, in t h e case of t h e C o - M o c a t a l y s t s s u p p o r t e d o n a l u m i n a , w e d i d obs e r v e a s u r f a c e e n r i c h m e n t of t h e Co atoms u p o n s u l f i d a t i o n , s i n c e C o a t o m s m o v e d f r o m p o s i t i o n s in t h e a l u m i n a t o p o s i t i o n s a t t h e s u r f a c e o f a Co-Mo-S phase.

C.J.

W R I G H T : F r o m y o u r M d s s b a u e r s p e c t r a it i s p r e s u m a b l y p o s s i b l e

t o estimate the ratio of molybdenum atoms t o "intercalated" cobalt a t o m s in t h e s a m p l e o f s u l p h i d e d Co(l)-Mo(6)/A1203.

What is the

m a g n i t u d e of t h i s r a t i o , and d o y o u h a v e any m e a s u r e m e n t s o f t h e c r y s t a l l i t e s i z e of t h e m o l y b d e n u m s u l p h i d e in y o u r s a m p l e w h i c h would enable y o u t o r a t i o n a l i z e t h i s r a t i o in t e r m s of t h e "intercalation" model ?

H. T O P S @ E : F o r t h e s u l f i d e d C o ( l ) - M o ( 6 ) / a l u m i n a c a t a l y s t m o r e t h a n

30% of the C o a t o m s a r e p r e s e n t in a M o S Z - l i k e phase.

Our results

p o i n t t o t h e C o a t o m s being p r e s e n t at t h e s u r f a c e o f t h i s phase. A s a c o n s e q u e n c e t h e M o S Z - l i k e s a m p l e m u s t b e p r e s e n t in a high s t a t e of d i s p e r s i o n and m a y h a v e r a t h e r two- t h a n t h r e e - d i m e n s i o n a l character.

P.C.H.

M I T C H E L L : R e g a r d i n g t h e p r e s e n c e o f C o 3 0 q i n t h e catalyst,

I would like t o s u g g e s t t h a t f o r "good" c a t a l y s t s t h e f o r m a t i o n o f Co.,Oq

should b e avoided o r m i n i m i z e d and t h a t t h i s c a n b e d o n e by

p r e p a r i n g t h e c a t a l y s t s in such a way t h a t c o b a l t and m o l y b d e n u m interact in t h e o x i d i c catalyst.

&

I notice also that the authors

p r e p a r e d t h e i r c a t a l y s t by a c o - i m p r e g n a t i o n method.

Possibly with

t h i s p r o c e d u r e i n t e r a c t i o n o f m o l y b d e n u m and c o b a l t s e p a r a t e l y w i t h t h e support r a t h e r t h a n w i t h each o t h e r i s most likely. W e p r e p a r e d o u r c a t a l y s t s b y adding c o b a l t ( I 1 ) s o l u t i o n t o M o 0 3 / A 1 2 0 3 and t h e n found e v i d e n c e o f a C O O - M o o 3 interaction.

H. T O P S 6 E : W e a g r e e that in o r d e r t o m a k e g o o d c a t a l y s t , C o 3 0 q form a t i o n s h o u l d b e avoided in calcined catalysts, s i n c e t h i s will b e

492 c o n v e r t e d i n t o Cogs8 u p o n s u l f i d i n g .

Co304

f o r m a t i o n can b e avoided

i f C o i n t e r a c t s w i t h t h e a l u m i n a o r i f , a s you s u g g e s t , C o a n d Mo

interact.

From t h e p r e s e n t r e s u l t s i t d o e s , h o w e v e r , a p p e a r t h a t t h e

i n t e r a c t i o n o f C o w i t h a l u m i n a i s more i m p o r t a n t t h a n t h e i n t e r a c t i o n b e t w e e n C o a n d Mo.

C o i s t h e r e f o r e l o c a t e d i n t h e alumina b u t ,

as

r e c e n t M B s s b a u e r r e s u l t s s h o w , i n i m m e d i a t e v i c i n i t y o f t h e Mo s u r face layers. (e.9.

We h a v e f o u n d t h a t c h a n g i n g t h e p r e p a r a t i o n p r o c e d u r e s

changing t h e o r d e r of

the catalyst.

i m p r e g n a t i o n ) c h a n g e s t h e s t r u c t u r e of

493

STUDY OF SOME VARIABLES INVOLVED IN THE PREPARATION OF IMPREGNATED CATALYSTS FOR THE HYDROTREATMENT OF HEAVY OILS 0. OCHOA, R. GALIASSOX and P. ANDREUX

Instituto Venezolano de Investigaciones Cientificas,Caracas,Venesuela

SUMMARY In the present investigation it was found that the most significant variables for the hydrotreatment of heavy oils were the following: use of additives, drying and activating conditions and active metal content. The metals deposited over alumina base supports were oxides of molybdenum and nickel. The activities were tested with deasphalted Venezuelan heavy oils diluted with gasoil. The best additives modified the solute-support interactions and gave homogeneous distribution profiles for Mo through the catalyst pellets, as evidenced by electron microprobe analysis. The effect of the drying rate was found to be critical in attaining a good molybdenum distribution and activity. Low heating rates and the use of a stream of hot air during calcination yielded better catalysts. The activity increased with Moo3 concentration up to 15%. A maximum was obtained for a NiO/Mo03 relation of about 0,3. When impregnating nickel first, the hydrodemetallation activity was improved.

INTRODUCTION During the hydrodesulfurization (HDS) and hydrodenetallatlon (HDM) reactions on NiMo/A1203 catalysts, a deactivation phenomenon takes place due to carbon and vanadium deposition (10% of C and 1% V ) during the first 24 hours, The kinetics of these reactions is not well established for resines and asphaltenes present in the heavy ends. Furthermore, strong diffusion control of big molecules into the narrow pores takes place. The reactions which occur are the following :

*

Present address : Instituto Tecnologico Venezolano del Petroleo, Caracas, Venezuela.

494

organosulfur compound + H2

4.

H2

----- ' -----

~

desulfurized organic compound + H2S V on the catalyst

+

RH

There are few systematic studies about the influence of the multiple variables involved in the preparation of catalysts by impregnation of porous alumina base supports for light hydrocarbon and for heavy o i l s desulfurization (1-4). There is no open literature mentioning hydrodemetallition catalysts preparation. This work attempts to clarify which of the parameters found as important through the routes of support impregnation are important for the activity and selectivity and specially for the stability, when working with heavy molecules. The mean objective is the control of the homogeneity in the metal distribution profile into the catalyst particle. Crust catalysts have been previously prepared (5). EXPERIMENTAL Catalysts were prepared using commercial y-alumina as support and ammonium heptamolybdate (AHM) and nickel nitrate (NN) as impregnating ealts. The dry techniques was generally used. Additives employed were an ammonium salt (AS), a peroxide compound (P) and an a m h e (A), among others. Chromatographic plates were prepared with suspensions of the support in a pulverized form. Adsorption isotherms studies were carried out using powders and pellets. The activity was tested in a trickle bed reactor, the charge being fed through a pump, and preheated and mixed with a H2 stream. Some tests were performed in a batch reactor. The feeds employed were diluted Boscan and Morichal o i l s with 500 and 130 ppm of V respectively, 2.5% S and 22'-24' API. For the drying experiments a McBain balance was employed when working with single pellets. For bed drying a thermostated reactor was used. The hydrothermal treatment of the support was carried out in a tubular reactor also employed in the calcination of the catalysts. The support was subjected to a stream of steam diluted with nitrogen. The metal contents on the catalyst were determined by neutron activation analysis and X ray fluorescence. The Mo distribution along the catalyst pellets was performed by microprobe analysis. Vanadium in oil was determined by neutron activation and sulfur by titulation of SO2 in

496

a LECO apparatus. Physicochemical characterization of the catalysts was achieved by standard methods. For thermogravimetric studies a Du Pont instrument and the McBain thermobalance were employed. RESULTS AND DISCUSSION IMPREGNATION Solute-support interactions To prevent the formation of a white precipitate when AHM solutions are placed in contact with the support, several additives were assayed. Ammonium salts, peroxides and amines insure stable solutions. Other additives lead to poor results. Yamagata et al. (6) have explained the appearance of the precipitate as due to the interaction between p-molybdic acid and acidic OH of the support. A comparative technique by modified thin layer chromatography was used to assay different additives. Further analysis of impregnated pellets by electron microprobe gave the distribution profiles shown in fig. 1.

o'08 0.05

5 T

0.05 0

0

0

0.02

L

O

wj

1.0

0.5

0

,131

0.021 0

,

,1 I

I 0

I .o

0.5

4 1

0

0

I

1

0.5

I

Fig. 1. Electron microprobe analyses of Mo . 1) P; 2 ) As; 3 ) A 4) without additive Additive A gave the best result by TLC (greater Rf of molybdenum). The mechanical strength of the support was damaged by additive P. In order to select among A and AS, catalysts were prepared with the same concentration of Mooj (18%) and either of the two additives. Since the activities were almost equal, the final selection was done on the basis of the favourable effect of modifying the pore radius distribution which was exhibited more pronouncely by AS, and on the basis of the toxicity

496

of A , which was h i g h e r t h a n t h a t of AS. N e v e r t h e l e s s , t h e mechanical s t r e n g t h i s a f u n c t i o n of t h e c o n c e n t r a t i o n of AS. By employing t h e above mentioned TLC, e l u t i o n experiments were performed t o o b t a i n q u a l i t a t i v e information of t h e minimum amount of AS r e q u i r e d . F i g . 2 is a photograph corresponding t o a p l a t e on which s o l u t i o n s of d i f f e r e n t c o n c e n t r a t i o n s of AHM and of a f i x e d c o n c e n t r a t i o n of AHM and d i f f e r e n t ones of A S were d e p o s i t e d . Addition of i n c r e a s i n g volumes of water allowed d e t e c t i o n of d i f f e r e n t t y p e s of i n t e r a c t i o n s . I n t h e p r e s e n c e o f A S , t h e Rf v a r i e d w i t h c o n c e n t r a t i o n , whereas f o r d i f f e r e n t c o n c e n t r a t i o n s o f t h e s a l t i n t h e absence of a d d i t i v e , t h e Rf was t h e same. These r e s u l t s s u g g e s t t h a t , above some c o n c e n t r a t i o n of A S , most of t h e i n t e r a c t i o n i s e l i m i n a t e d and more s a l t is d i s p l a c e d by t h e s o l v e n t . These r e s u l t s are i n agreement w i t h e l e c t r o n microprobe determinations. I n order t o study t h e adsorption isotherms, t h e pulverized support was p l a c e d i n c o n t a c t w i t h e x c e s s s o l u t i o n of AHM d u r i n g 1 2 h o u r s , i n t h e p r e s e n c e of A o r AS. F i g . 3 shows t h e c o n c e n t r a t i o n s r e t a i n e d by t h e s u p p o r t . Three c u r v e s a r e seen. The f i r s t corresponds t o t h e t o t a l c o n c e n t r a t i o n . The second i s o b t a i n e d by s u b s t r a c t i n g from t h e f i r s t t h e i n i t i a l porous volume e q u i l i b r i u m c o n c e n t r a t i o n . T h i s r e s u l t must be c o r r e c t e d t a k i n g i n t o account t h e d e c r e a s e i n volume w i t h c o n c e n t r a t i o n o f t h e s a l t . I n t h i s way, a t h i r d c u r v e i s o b t a i n e d which e x h i b i t s a s a t u r a t i o n c o n c e n t r a t i o n of 6-7% Moo3. T h i s c o r r e c t i n g procedure i s an approximation, which does n o t t a k e i n t o account o t h e r p o s s i b l e e f f e c t s such as pore plugging and e x c l u s i o n of molybdenum s p e c i e s from t h e narrower pores. Using t h e Langmuir i s o t h e r m , t h e r e l a t i o n among t h e c o n c e n t r a t i o n adsorbed i n t h e e q u i l i b r i u m (ca( e q ) ) and t h e e x t e r n a l c o n c e n t r a t i o n (c,) can be expressed a s (7) :

ca ( e q ) ‘e

= - K ca(eq)

+

sK

where K i s t h e a d s o r p t i o n - d e s o r p t i o n e q u i l i b r i u m c o n s t a n t and s i s t h e number o f a c t i v e c e n t e r s . The v a l u e s o b t a i n e d f o r AHM were t h e f o l l o w i n g w i t h a d d i t i v e AS: K = 0.53 lt/Mol and s = 1.71 Mol/Kg w i t h a d d i t i v e A: K = 0 . 6 1 lt/Mol and s = 1.23 Mol/Kg These r e s u l t s show that when u s i n g A , t h e number of a c t i v e c e n t e r s a v a i l a b l e f o r a d s o r p t i o n i s reduced, s u p p o r t i n g t h e TLC o b s e r v a t i o n s . When u s i n g AS, t h e amount of Mo r e t a i n e d a s Mooj, a f t e r t h e impregnated s u p p o r t was washed f o r 2 4 h o u r s , w a s n e a r 2 % . With A as a d d i t i v e , t h i s

497

value dropped to 0.5%. This also implies in this case a lesser degree of irreversible adsorption. For NN the results also fit the Langmuir isotherm. The values of K and s where, respectively, of 1.7 lt/Mol and 2.5 Mol/Kg. These values are lower than those reported previously (7). Irreversible adsorption was not detected by washing.

.

Fig.2 TLC at different AHM and AS concentrations. 5% AHM

1.-

10% AHM 3.- 15% AHM

2.-

4.- 10% AHM; 0.5% SA 5.- 10% AHM; 2.5% SA 6.- 10% AHM; 5.0% SA

'C (Mol/lf. ) 0 .I

0.3

0.5 1.0

.

Fig.3 Adsorption isotherm of AHM on y-alumina. AS as additive.

i?

i

*

0.5

0

0.5

I .o

1.5

CE (Mol/lt. )

Using only 120% of pore volume for the solution, the metals were carried into the pores of the dry support by capillary forces and the concentration in the catalyst was quickly raised to 90-95 % of the total ___ concentration attainable. The remaining 5-10% was reached by diffusion following partial adsorption of the salt initially present in the pores,

498

Using a mathematical model similar to that developed by Vincent and Merrill (8), it was found that adsorption of AHM was the limiting step during the establishment of the equilibrium.The values of the constants K l l = 0.35 Klll = 3.0 (see addendum) were: K1 = 0.15

.

DRYING Kinetics: These experiments were performed with single pellets at varicw conditions, as summarized in table 1. The drying curves are shown in fig 4 . They show two typical periods, the first being the so called constant rate and the second the falling rate periods. Two models were assayed in order to find out the possible mechanism of drying: the diffusion model and the capillary flow model (9). The best fit was obtained with the latter. TABLE 1. Drying conditions for Mo/A1203 catalysts 0.80

CAT

0.60

so-2

so-1

s k

SO-3 SO-4

TEMPERATURE GAS FLOW LT/H 30 30 50 100

2

10 10

10

0.40

0.20

SO 100 Time (min. )

0.00

150

.

Fig.4 Moisture content of impregnated pellets as a function of time Influence of drying conditions on the molybdenum distribution and catalyst activity: In this case three drying conditions were used and experiences were performed with catalyst beds, Fig, 5 gives the distribution profiles obtained; the activities for these catalysts can be found in table 2 . The mildest drying conditions obviously give better macroscale distribution and activity. The remarkable fact is that with reduction

499

i n s i z e t h e d i f f e r e n c e i n a c t i v i t y p e r s i s t , suggesting t h a t not only t h e m a c r o s c a l e molybdenum d i s t r i b u t i o n i s s i g n i f i c a n t , b u t a l s o i t s d i s t r i b u t i o n a c r o s s t h e microstructures along t h e p e l l e t .

I t seems

t h a t t h e d e m e t a l l a t i o n r e a c t i o n i s more s e n s i t i v e t h a n t h e d e s u l f u r i z a t i o n o n e t o t h e c a t a l y s t s i z e , a f a c t which c o u l d be e x p l a i n e d on t h e b a s i s of d i f f u s i o n a l l i m i t a t i o n s . A c t i v i t y (T=380°C, VHA=0.32 h-l atm.) a s a f u n c t i o n of d r y i n g conditions

TABLE 2 .

P a r t i c l e diameter CATALYST

0.36 mm

5mm

HDS %

HDM %

HDS %

HDM %

38 30

30 27

50

TO-2

45

47 43

TO-3

27

22

40

38

TO-1

F i g . 5. Molybdenum d i s t r i b u 0.25

t i o n a s a f u n c t i o n of d r y i n g conditions. 1. TO-1: 3OoC, 10 l t / h

i

2. TO-2:

5OoC, 1 0 1 t / h

3. TO-3:

5O0C,1O0 l t / h

9 0.25 2 aio 0

1

When t h e r e i s no i n t e r a c t i o n between s o l u t e and s u p p o r t , t h e s o l u t i o n f l o w s from t h e macropores s u c t i o n e d by t h e m i c r o p o r e s , o r i g i n a t i n g a l a c k of s o l u t e i n t h e f o r m e r , when d r y ( 1 0 , l l ) . When d r y i n g b e g i n s , a l l t h e p o r e s a r e f i l l e d w i t h l i q u i d , and t h e t e m p e r a t u r e o f t h e s o l i d c o r r e s p o n d s t o t h a t of t h e w e t b u l b . As d r y i n g p r o c e e d s , t h e macropores

w i l l p a r t i a l l y empty, and t h e s u r f a c e t e m p e r a t u r e w i l l r i s e . The concent r a t i o n w i l l grow i n t h e narrower m i c r o p o r e s . When r e a c h i n g t h e c r i t i c a l s u c t i o n r a d i u s (rcs) s u c t i o n w i l l no l o n g e r t a k e p l a c e . The s o l u t e w i l l p r e c i p i t a t e when a t t a i n i n g t h e s u p e r s a t u r a t i o n o f t h e s o l u t i o n o r by s u r f a c e e f f e c t s , The f i n a l r e s u l t w i l l be a marked h e t e r o g e n e i t y i n t h e micro and macropore d i s t r i b u t i o n a l o n g a l l t h e s o l i d , i n d i s c r e t e

500

microregions. Nothing can be said at this stage about the radial distribution. When there exists reversible and irreversible interactions, part of the material will stay adsorbed on the walls when drying proceeds. This material corresponds to the irreversible concentration, and a equilibrium will be established at any moment between the solute remaining in solution and that adsorbed. In this way, besides the micro-macropore heterogeneity, heterogeneity among the micropores from the external and the internal regions of the pellet may exist. If the drying rate is much larger than the desorption rate, the final result would be lesser heterogeneity as there will be no significant migration of the adsorbed species. When an additive is present whose vapour tension is different from that of the solvent, their evaporation rates will differ, and this situation could change the intensity of the adsorption and the mechanism of migration inside the pores. The elimination of stabilizer would produce early precipitation, or concomitant change in pH modifications in the solute-support interactions. With respect to the influence of the distribution on the activity,in the case of a homogeneous radial distribution with heterogeneity among micro and macropores, two situations can be possible : a. The high molecular weight molecules are excluded from the narrower pores, as is the case for the resins and asphaltenes. The activity will be low since most of the M o is not available. b. Light molecules like tiophene would reach much more active Mo since diffusional restrictions are smaller. Microscale heterogeneity is not significant. ACTIVE METALS CONCENTRATION -3 Moo content : The relative activity of Mo/A1203 catalysts was plotted against the trioxide concentration and depicted in fig. 6. Activity was referred to a commercial NiMoAl catalyst (Moo3 148, NiO 3 % ) . HDS and HDM reached a maximum about 15% Moo3. Higher concentration lead to a l o s s in HDS activity. For HDM a flat maximum is apparently obtained, and due to the limited range of concentration studied, it is not clear whether over 20% the activity decreases. The decrease in activity has been explained by other authors and by us (12,2) as due to the detrimental effect of clusters formed over the initial active centers, In addition, another explanation valid for heavy molecules is given by the micropore plugging during drying stages (shorter time for oversaturation and precipitation). As can be seen, nickel affected the activity for the HDS reaction, because the relative activities for this reaction were less than one.

501

The HDM values were, on the other hand, approximately 1. NiO content : Changing the NiO content at a fixed Moo3 concentration (14%), a maximum was obtained for the ratio of about 0.3 NiO/Mo03. This is in agreement with the reported values (13,14). These maxima have been explained by different theories which in general, are based on the probability of having.molybdenum surrounded by accesible nickel (15). Total metal content : When the total amount of metals was changed from 0 . 1 2 ~ 1 0 -to ~ 0.17x10-* Mol/g support, but for a fixed oxides relation, the activity remained almost constant. At the higher concentration it slightly increased. In order to prepare industrial catalysts the minimum content in this range should be used since after 2 4 hours on stream, the equilibrium value of the activity was the same for all these catalysts. Impregnation order : When molybdenum was impregnated first or coimpregnated with nickel the HDS and HDM activities were almost the same. When impregnating Ni first, the activity was still the same for the HDS but higher for HDM (see fig. 8). The explanation could lie in the possibility that NN could adsorb on the active centers excluding Mo and giving a different adsorption for the latter ( 5 ) . A similar effect was obtained by Mitchell and coworkers (16) for Co. Ni/A1203 has been claimed as a good demetallation catalyst (17,5). Probably accesible nickel sulfide compounds are more acidic than those of Mo and crack the heavy molecules.

-

- 0.90.$

0.30

20,

8

- 0.70 .L

z I

4

12

I

20

Mo4 %

Fig. 6. Relative activity as a function of Moo3 content.

0 0.1 0.2 0.3 0.4 0.5 NiO/Mo&

Fig. 7. HDS activity as a function of NiO/Mo03 ratio.

A few experiments were carried out in this field to confirm previous results obtained with thiophene as testing molecule (3,4).

502

Fig. 8. Activation as a function of impregnation order.

300

400

350 T OC

Catalysts were activated from a drying temperature of 5OoC to a calcining one of 500-600°C, using two rates for the increase in temperature ( 5 0 and 100°C/h). The activities can be found in table 3 . TABLE 3 . Activity of catalysts calcined at various conditions Temp.

Air

(rate)

g/cm2 h

Catalyst R 107 R 103 R 106 R 105 Commercial A

HDS 50 100 50 50

%

HDM

3.11

70

48

3.11

65 62 60 70

46

3.11 3.11

43

45

%

Tcalc 550 550 500 600

49

Higher activity was observed with the lower rate of increase of temperature. To study this phenomen, thermogravimetric experiments were performed with AHM and NN on alumina. The slow thermogram (50°C/h) is similar to those published (18), and shows for the AHM two well defined peaks due to water l o s s (l0O/l2O0C) and ammonia loss (20O/25O0C). Complete decomposition occuxs at higher temperatures. This suggests that polymeric molybdates are absent. The higher rate gives Less defined peaks (see figs. 9 and 10). DSC measurements demonstrated exothermic processes taking place at

503

20O/25O0C. The narrowest peaks were obtained for the lower rate. Finally, XRD showed slightly lower cristaanity for the higher rate. More detailed studies on crystal configuration with rate of decomposition of the salt are needed to explain the activity results. Calcining temperature is important. The activity has a smooth maximum at 55OoC. Mone and coworkers (19) explain this effect as due to reaction of Ni with alumina to give inactive Ni aluminate. ESCA measurements are in progress and could confirm this theory. In the same way, using a stream of hot air through the catalyst during calcination, more controlled heat transfer, oxidation of ammonia and sweeping of the gaseous products were achieved. Betteractivity was also observed.

3 Fig. 9. Weight decrease as a function of temperature for a Mo/A1203 catalyst. 100

200

300 TOC

300

500

T OC B) NN/A1203 Fig. 10. TGA, DSC of: A) A1203 D ) AHM/A1203 1°C/min

Hydrothermal tion. At 620°C 200A. Surface dependence was

C) AHM/AI-z03 E ) AHM/A1203

0.5'C/min 5'C/min.

treatment of the support changed the micropore distributhe average micropore radius was raised from 66 to about l o s s was of 50% at the 24 hours, and a exponential time found for this reduction. Lower temperaturesdid not

504

modify micropore distribution appreciably, and higher ones started thermal sintering with pore volume loss. The activity did not change for the HDS and HDM reactions with these modifications. Mass transfer control rate for heavy molecules (neff = 0.2-0.6) originated that only pores of diameter higher than 400 were used. These results support the conclusions drawn from the drying stage results. REFERENCES 1. Y. Kotera, N. Todo, K. Muramatsu, M. Kurita, T. Sato, M. Ogawa and T. Kabe, Int. Chem. Eng. 11, 752 (1971). 2. M. Inoguchi, R. Nishiyama, Y. Satori, T. Misutori, N. Inaba, H. Kagaya, K. Tate, H. Hosaka, K. Niume and T. Ota, Bull. Japan Petrol. Inst. 3, 13 (1971). 3. R. Gallasso, 5th Iberoam. Congress of Catalysis, Lisboa (1976). 4. J. Laine, Doctoral Thesis, Imperial College, London (1971). 5. A. Cuenca, R. Galiasso and P. Andreu, PACHEC'77, Denver, Colorado (1977). 6. N. Yamagata, Y. Owada, S. 0kazaki.and K. Tanabe, J. Cat. 47,358 (1977). 7. J. Cervello, Thesis, Universidad Complutense, Madrid (1974). 8. R.C. Vincent and R.P. Merrill, J. Cat. 35, 206 (1974). 9. R. Galiasso, 0. Ochoa and P. Andreu, to be presented in the 6th Iberoamerican Congress on Catalysis, Rio de Janeiro, Brasil 1978. 10. R.W. Maatman and C.D. Prater, Ind. Eng. Chem., 49, 253 (1957). 11. Ch. Chen and R.B. Anderson, Ind. Eng. Chem. Prod. Res. Dev., 12, 122 (1973). 12. N. Todo, X. Muramatsu, M. Kurita, K. Ogawa, T. Sato, M. Ogawa and Y. Kotera, Bull. Japan Petrol. Inst., 14, 89 (1972). 13. S . P . Ahuja, M.L. Derrien and J.F. Le Page, Ind. Eng. Chem.Prod.Res. Dev., 9, 272 (1970). 14. J.T. Richardson, Ind. Eng. Chem. Fund., 3, 154 (1964). 15. V.H.J. De Beer, in preparation of Catalysts (B. Delmon Ed.) 1976, p. 343. 16. M. Martinez, P.C.H. Mitchell and P. Chiplunker, Climax 2nd Conf., Oxford 1976. 17. A. Vargas, M. Sc. Thesis, IVIC 1977. 18. E. Ma, Bull. Japan Chem. SOC., 37, 171 (1964). 19. R. M o d , ref. 15 p. 381.

505

ADDENDUM -df/dt

+

:

0 . 5 t - ~ 'df/dR=-klf(l-e)+klle; ~ f=C/Co; t = time

d(eC,)/dt

R = r/Ri

klf (l-O)/klll + klle/klll k dimensionless

=

AKNOWLEDGEMENT. To Prof. B. Delmon, Dr.Sc. and Analytical Service of I V I C for analyses. DISCUSSION J.F. LEPAGE

:

As far as the impregnation of your catalyst is concerned,

what do you recommend if demetallation is aimed at

:

first MO then

Co (or Ni) or first Co then Mo or Co and MO together. R.E. GALIASSO

:

After 10 or 12 hours on stream the catalysts'you

mentioned have the,same activity due to carbon and vanadium deposition. Mo or Ni/A1203 catalysts have better initial activity and thermal stability

.

R. PRICE : A substantial variation in hydrodemetallization activity has been observed for different catalyst preparations.

Would the

authors care to speculate on the nature of the active sites, and support properties R.E. GALIASSO

:

involved in the hydrodemetallization reaction ?

With the present status of our knowledge of the

hydrodemetallization process it is very difficult to speculate about active sites, because it is not known if there is a catalytic or adsorption process. L. MOSCOU

:

Metals and support, both play an important role.

Fig. 5 of your paper shows molybdenum concentration

profiles over the particle as a function of drying conditions.

Can

you clarify the very different average Mo-contents between experiments

1 and 3 ?

Did you suggest that this difference in average molybdenum

content is a l s o due to changed drying conditions

?

R.E. GALIASSO

:

The to'tal arount of metals is the same for all drying

experiments.

The difference in the MO content from sample TO 1 to

TO 3 is due mainly to the gravity forces during static drying and the heterogeneity between particles.

To obtain qualitative results of

Mo distributions a great deal of sample needed to be considered. R.

BADILLA-OHLBAUM

:

Can you give the characteristics of your trickle

506 b e d r e a c t o r and t h e f u l l c o n d i t i o n s of o p e r a t i o n ? from your r e s u l t s

( F i g , 81,

f o r HDM b u t n o t f o r H D S .

I t appears,

t h a t you s e e a d i f f e r e n c e i n c a t a l y s t s

One w o u l d e x p e c t t h e r e v e r s e c o n s i d e r i n g

t h a t HDM i s m o r e a f f e c t e d b y d i f f u s i o n i n t h i s k i n d o f

R.E.

G A L I A S S O : The s m a l l t r i c k l e b e d r e a c t o r u s e d

c a t a l y s t d i l u t e d with ceramics were

: T

support.

h a s 4 1 6 cm3 o f

( l : l ) , and t h e o p e r a t i o n c o n d i t i o n s

= 38OoC, VVH = 0 , 3 3 h - ' ,

= 80 a t m .

P H2

Some r e s u l t s were

o b t a i n e d i n a 1 0 0 cm3 r e a c t o r i n s i m i l a r c o n d i t i o n s .

The d i f f e r e n c e

i n HDM a c t i v i t y c o u l d b e d u e t o t h e p r e s e n c e o f a c c e s s i b l e S N i adsorbed on A1203

i n a d d i t i o n t o " c o n v e n t i o n a l " Mo a c t i v e s i t e s .

507

PREPARATION AND PROPERTIES OF THIOMOLYBDATE GRAPHITE CATALYSTS G.C.

STEVENS and T. EDMONDS

The B r i t i s h Petroleum Company Limited, BP Research Centre, Sunbury-on-Thames,

Middlesex, England

ABSTRACT

T h i s study aims t o t a k e fundamentalinfor?@ition

determined by ESCA following t h e

a c t i v a t i o n o f a CoMo/A1203 c a t a l y s t and use it t o prepare a more a c t i v e form of t h a t c a t a l y s t system.

The requirements t o be met i n t h e p r e p a r a t i o n of a more a c t i v e

c a t a l y s t were i s o l a t e d as:1 Molybdenum must be deposited e x c l u s i v e l y as sulphide. 2

The d e p o s i t i o n o f MoS2 must maximise t h e a r e a of b a s a l plane t o favour

hydrogenolysis.

3 A promoter f o r t h e b a s a l plane of MoS2. 4 Elimination o f A1203 t o avoid t h e formation o f t h e MoO3-Al203 w n o l a y e r . The development o f t h e p r e p a r a t i o n of t h i s c a t a l y s t w i l l be described, and w i l l i n c l u d e monitoring i t s s u r f a c e s t a t e s , elemental d i s t r i b u t i o n and a c t i v i t y .

The

f i n a l result w a s a highly active catalyst.

INTRODUCTION A s e r i e s of p u b l i c a t i o n s h a s described our fundamental s t u d i e s on t h e CoMo-Al203

system ( r e f s 1

-

3).

ESCA a n a l y s i s showed t h a t t h e a c t i v e CoMo c a t a l y s t contained

a s t a t e similar t o Mo4+-sulphide i n MoS2 on t h e s u r f a c e ( r e f 2).

Although t h i s w a s

t h e predominant s t a t e , d e t a i l e d a n a l y s i s of t h e s p e c t r a a l s o revealed t h e presence of a Mok+-oxide component and t h e magnitude of t h e t o t a l Mo4+ s i g n a l was r e l a t e d t o t h e r a t e s of r e a c t i o n of t h e c a t a l y s t with thiophene and carbon disulphide.

I n recent

p u b l i c a t i o n s o t h e r a u t h o r s using d i f f e r e n t techniques have subsequently reported r e s u l t s which a r e c o n s i s t e n t with our i n t e r p r e t a t i o n ( r e f s 4 & 5). These r e s u l t s drew a t t e n t i o n t o t h e a c t i v i t y of MoS2 i t s e l f and we examined i n p a r t i c u l a r some of t h e s t r u c t u r a l i n f l u e n c e s o f t h e b a s a l and edge p l a n e s o f t h e disulphide ( r e f 3 ) .

The r e s u l t s suggested t h a t with thiophene t h e b a s a l plane of

MoS2 was t h e r e a c t i o n s i t e giving mainly d e s u l p h u r i s a t i o n s e l e c t i v i t y . I n t h i s paper we d e s c r i b e work which set out t o develop a new formulation of t h e CoMo system designed t o e x p l o i t t h e r e s u l t s we have reported.

We aimed t o produce

508

a n e n t i r e l y s u l p h i d i c system i n which t h e b a s a l p l a n e o f MoS2 predominated without i n f l u e n c e o f o t h e r s u r f a c e s , p a r t i c u l a r l y c a t a l y s t support s u r f a c e s .

We were

e s p e c i a l l y concerned t o avoid formation o f a n o x i d i c M003-Al203 monolayer a t any stage (ref 6).

F i n a l l y we wanted t o have a supported system with a s l a r g e a s u r f a c e

a r e a as p o s s i b l e t o encourage molybdenum s u l p h i d e d i s t r i b u t i o n a s i f e l l a s t o maximise t h e p o s s i b l e i n t e r a c t i o n between t h e molybdenum and t h e promoter.

SYSTEM CHOSEN FOR STUDY

A h i g h p u r i t y carbon b l a c k w a s chosen as support.

The m a t e r i a l w a s g r a p h i t i s e d

a t Z700°C t o remove any oxygen c o n t a i n i n g f u n c t i o n s such a s hydroxyl o r carbonyl

groups l e a v i n g a h i g h s u r f a c e a r e a m a t e r i a l , predominantly b a s a l plane g r a p h i t e . Ammonium thiomolybdate w a s used as t h e source of molybdenum and t h i s w a s impregnated o n t o t h e support p r i o r t o c o n t r o l l e d decomposition t o produce supported MoS2.

In

o t h e r p r e p a r a t i o n s c o b a l t i o n s were a l s o impregnated t o promote MoS2. I n t h i s paper we d e s c r i b e : -

-

t h e c o n d i t i o n s f o r decomposition o f thiomolybdate development of impregnation method and p h y s i c a l p r o p e r t i e s of c a t a l y s t s produced c a t a l y t i c p r o p e r t i e s o f t h e promoted c a t a l y s t system

PREPARATION AND DECOMPOSITION OF AMMONIUM THIOMOLYBDATE

Ammonium thiomolybdate was prepared by p a s s i n g H2S i n t o ammonium molybdate. MoS3 i s formed from t h e thiomolybdate by decomposition a t 2OOOC under vacuum ( r e f 7 )

and f u r t h e r decomposition t o MoS2 o c c u r s under hydrogen ( r e f

8).

The decomposition o f ammonium thiomolybdate was s t u d i e d i n a n atmospheric p r e s s u r e m i c r o r e a c t o r equipped with a n a u t o m t i c sampling valve and g a s chromatograph on t h e reactor effluent.

A s t h e s a l t t h e r m a l l y decomposed hydrogen s u l p h i d e was l i b e r a t e d .

The r a t e of decomposition a s a f u n c t i o n of temperature could t h e r e f o r e be examined by r e c o r d i n g t h e change i n H2S GC peak h e i g h t as t h e f u r n a c e temperature was r a i s e d . Under hydrogen no H2S peaks were observed below 150°C i n t h e samples t a k e n by t h e GC system.

A t h i g h e r temperatures H2S began t o appear i n t h e e f f l u e n t samples which

were t a k e n at one minute i n t e r v a l s throughout t h e experiment.

A t 1 8 0 0 ~t h e r e was a

s t e a d y e v o l u t i o n o f H2S and t h i s reached a maximum r a t e a t about 200OC. These experiments were r e p e a t e d u s i n g ammonium thiomolybdate under n i t r o g e n and a l s o with a sample o f g r a p h i t i s e d carbon impregnated with ammonium thiomolybdate s o l u t i o n so a s t o d e p o s i t about 8 p e r cent weight molybdenum on t h e support.

Table1

s u m a r i s e s t h e temperatures a t which H2S f i r s t appeared and t h e t e m p e r a t u r e s a t which maximum H2S evolved.

With t h e supported thiomolybdate sample, H2S appeared o n l y

a f t e r s t a n d i n g a t 225OC f o r 30 minutes and a l l H2S w a s given o f f a t t h i s temperature.

609 TABLE 1 Decomposition of ammonium thiomolybdate

Gas

Sample

“4

)2MOSl+

(NH4)2MoSq/graphitised C

N2 H2 H2

Temperature

OC

for

H2S Appearance

Max H2S E v o l u t i o n

160 160

200 200 225

225

The chemical a c t i v i t i e s o f t h e samples of MoS2 prepared by t h e r m a l decomposition were compared with t h o s e o b t a i n e d by g r i n d i n g commercially a v a i l a b l e MoS2 ( r e f 3 ) . The conversion o f thiophene vapour i n a s t r e a m of hydrogen g a s was measured a t atmospheric p r e s s u r e and 325OC.

0.2 g samples of MoS2 were used w i t h hydrogen

flowing a t 30 m l min-1 and thiophene b e i n g i n j e c t e d c o n t i n u o u s l y a t t h e r a t e o f

0.05 nil h-l.

The c o n v e r s i o n s o f thiophene were s t e a d y a f t e r 30 minutes a t r e a c t i o n

t e m p e r a t u r e and a r e l i s t e d i n Table 2.

The t a b l e a l s o i n c l u d e s t h e r a t i o of butane

t o mixed b u t e n e s i n t h e m i c r o r e a c t o r e f f l u e n t .

TABLE 2 Conversion o f thiophene vapour i n atmospheric p r e s s u r e m i c r o r e a c t o r a t 325°C Sample

Butane

Stable

% Thiophene

Ratio

Conversion MoS2 n-heptane ground a i r ground by (NH4)2MoS4 decomposition MoS2/graphitised C (8% w t Mo from (NH4)2MoS4)

80 76 41 50

Butenes

0.8 2.9

1-3 0.7

Table 2 shows t h a t t h e samples of ground MoS2, prepared as d e s c r i b e d p r e v i o u s l y (ref

3 ) , were more a c t i v e and had c h a r a c t e r i s t i c a l l y d i f f e r e n t product r a t i o s .

Of

t h e m a t e r i a l s d e r i v e d by decomposition o f ammonium thiomolybdate, t h e supported sample was t h e more a c t i v e though having o n l y

8 p e r c e n t weight Mo.

The supported

m a t e r i a l a l s o gave l e s s b u t a n e analogous t o t h e MoS2 sample ground i n heptane which contains a high proportion c f basal plane surfac e a r e a ( r e f 3 ) . These r e s u l t s show t h a t supported and unsupported amonium thiomolybdate t h e r m a l l y decomposes above 225’C.

When t h e thiomolybdate i s supported on g r a p h i t i s e d carbon,

t h e d e s u l p h u r i s a t i o n a c t i v i t y is improved, probably because t h e t o t a l s u r f a c e i s greater.

The product p a t t e r n s u g g e s t s t h a t t h e MoS2 produced by decomposition on

g r a p h i t i s e d carbon h a s a h i g h e r p r o p o r t i o n o f b a s a l p l a n e s u r f a c e a r e a t h a n t h a t produced by decomposition o f unsupported ammonium thiomolybdate.

610 DEVELOPMENT OF IMPREGNATION TECHNIQUE FOR THIOMOLYBDATE GRAPHITE CATALYSTS

Our f i r s t attempts a t making a n a l l sulphide, uniformly d i s t r i b u t e d c a t a l y s t system employed d i r e c t evaporation.

C a t a l y s t samples were prepared by impregnation

of t h e g r a p h i t i s e d carbon support using aqueous s o l u t i o n s of c o b a l t n i t r a t e and ammonium thiomolybdate.

The c o b a l t n i t r a t e s o l u t i o n was made up t o contain

s u f f i c i e n t cobalt t o g i v e about 3 p e r cent weight on t h e support and was dissolved i n a similar volume of water t o that of t h e support.

Water was r e m v e d under

vacuum on a steam b a t h and t h e deposited s a l t w a s decomposed by heating t o 150°C f o r about 2 hours i n a i r .

The c o b a l t containing support was then impregnated with

a s e r i e s o f ammonium thiomolybdate s o l u t i o n s t o b r i n g t h e molybdenum content t o

8 p e r cent weight.

about

ESCA s p e c t r a from samples prepared by t h i s procedure were compared with s p e c t r a Table 3 l i s t s t h e binding e n e r g i e s of t h e main

from (NH4)2MoS4, MoS2 and MoO3. peaks.

The standard f o r MoS2 and Moo3 was Au(4f7/2) = 83.8 eV.

The carbon

support was c a l i b r a t e d a g a i n s t gold and t h e measured value C ( l s ) = 284.3 eV was used as a secondary standard f o r t h e c a t a l y s t and thiomolybdate.

No charging

e f f e c t s were observed with t h e c a t a l y s t samples.

TABLE 3 ESCA d a t a based on Au(4f7/2) =

83.8 eV o r C ( l s ) = 284.3 eV

~~

~~

Sample

'

MO S2 Moo3

L

Binding e n e r g i e s

-r

3% C0/8% Mo/graphitised

C

284.1 284.5 284.3

531.5 531.6 531.4

161.7

228.9 232.5 232.3

-

The r e s u l t s i n Table 3 show, s u r p r i s i n g l y , t h e complete absence of molybdenum d i s u l p h i d e on t h e c a t a l y s t s u r f a c e .

Although t h e Mo(3d) s i g n a l s were simple

g i v i n g no i n d i c a t i o n of more t h a n one s t a t e o f molybdenum, t h e binding e n e r g i e s c l e a r l y show t h e presence of t h e f u l l y o x i d i s e d Mo6+-oxide state as i n MoO3. An unexpectedly small sulphur (2p) s i g n a l was d e t e c t e d on t h e f r e s h l y prepared c a t a l y s t by ESCA

On t h e o t h e r hand X-ray fluorescence, which has a g r e a t e r sampling depth, did show t h e presence of sulphur.

Taken t o g e t h e r t h e s e r e s u l t s

i n d i c a t e almost complete o x i d a t i o n o f t h e s u r f a c e of t h e c a t a l y s t during p r e p a r a t i o n . Samples containing thiomolybdate on g r a p h i t i s e d carbon p e l l e t s were examined by e l e c t r o n microprobe t o confirm t h a t c o b a l t and molybdenum were d i s t r i b u t e d evenly

on both t h e s u r f a c e and over t h e c r o s s - s e c t i o n of t h e p e l l e t .

C a t a l y s t s containing

3 p e r cent weight cobalt showed a s l i g h t l y higher c o b a l t concentration a t t h e s u r f a c e of t h e p e l l e t but a t 2 p e r cent weight t h e r e was an even d i s t r i b u t i o n over t h e cross-section.

However, d e s p i t e good d i s t r i b u t i o n a c r o s s i n d i v i d u a l p e l l e t s ,

511 t h e i n t e n s i t i e s of c o b a l t v a r i e d widely between p e l l e t s .

To examine t h i s problem f u r t h e r , samples o f g r a p h i t i s e d carbon p e l l e t s were impregnated with 2 p e r c e n t weight c o b a l t u s i n g t h e technique d e s c r i b e d and t h e d i s t r i b u t i o n of c o b a l t c o u n t s measured by t h e microprobe on 20 p e l l e t s s e l e c t e d by chance.

The r e s u l t s showed t h a t c o b a l t l o a d i n g s on i n d i v i d u a l p e l l e t s could

vary by up t o t e n f o l d .

Subsequent impregnation of ammonium thiomolybdate d i d not

affect the distribution. I n an a t t e m p t t o overcome t h e problems o f o x i d a t i o n and uneven d i s t r i b u t i o n o f a c t i v e components t h e p r e p a r a t i o n method was modified by u s i n g a n i t r o g e n swept r o t a r y evaporator t o remove water from s o l u t i o n s used f o r impregnation.

A s before,

c o b a l t n i t r a t e w a s impregnated f i r s t , decomposed and t h e n ammonium thiomolybdate w a s deposited. F i g u r e 1 shows t h e ESCA s p e c t r a from a sample prepared using t h i s a l t e r n a t i v e method.

It can be s e e n t h a t t h e Mo(3d) s i g n a l s ( F i g u r e l ( a ) ) are still complicated

and suggest a s i g n i f i c a n t amount o f MoG+-oxide i s s t i l l p r e s e n t a l t h o u g h t h e l a r g e r The o x i d a t i o n o f t h e

s i g n a l can be a s s i g n e d t o a n '(NH4)2MoS4' l i k e s t a t e . c a t a l y s t s u r f a c e h a s been s u b s t a n t i a l l y reduced.

The d i s t r i b u t i o n of c o b a l t was a l s o checked w i t h t h e e l e c t r o n microprobe.

The

sample prepared u s i n g r o t a r y evaporation showed a much more uniform d i s t r i b u t i o n

of a c t i v e component among t h e twenty p e l l e t s examined. up t o 2 p e r cent weight c o b a l t and

I n c a t a l y s t s containing

6 p e r c e n t weight molybdenum t h e r e was no s i g n

o f s h e l l impregnation.

A s a f i n a l check o n t h e composition o f t h e s u r f a c e o f t h e a c t i v e c a t a l y s t , ESCA s p e c t r a were measured a f t e r r e a c t i o n with thiophen i n hydrogen a t 325'C.

The

Mo(3d) spectrum i n F i g u r e l ( b ) shows t h e presence of j u s t one molybdenum s t a t e

a t b i n d i n g e n e r g i e s very similar t o t h o s e from MoS2.

T h i s r e s u l t confirms t h a t

t h e thiomolybdate on g r a p h i t i s e d carbon decomposes under t h e experimental c o n d i t i o n s t o form, e x c l u s i v e l y , a s u r f a c e similar t o t h a t o f MoS2.

F u l l binding energies

from both t h e f r e s h and used c a t a l y s t s a r e l i s t e d i n Table

4.

TABLE 4 ESCA d a t a from c a t a l y s t s prepared by r o t a r y e v a p o r a t i o n

Sample

Binding Energies (eV) *

Fresh c a t a l y s t (2 w t % Co; 6 w t % Mo) Catalyst a f t e r reaction

232.5 229.3 162.9

778.6

531* 0

162.0

778.1

531.5 0.2

M0(5d5/2)

*

228.8

S(2p3/2) C0(2p3/2)

I O(ls) O(ls)

I M0(3d5/2)

Standard C ( l s ) = 284.3 eV Apart from changes i n t h e Mo(3d) spectrum a l r e a d y d e s c r i b e d , t h e S(2p) s i g n a l

i s s i m p l i f i e d t o g i v e t h e s i n g l e s u l p h i d e sulphur spectrum found i n MoS2 i t s e l f .

512

T. 7589/FCF

(a) FRESH CATALYST

I

I

I

I

I

I

I

I

I

1

1

I

I

1

I

I

I

I

(b) CATALYST AFTER REACTION

I

1

238

I

236

I

I

234

I

232

230

228

I

226

I

I

224

BINDING ENERGY

FIG 1. ESCA ANALYSIS OF CoMo CATALYST SUPPORTED ON GRAPHITIZED CARBON - M O (3d ) SPECTRA

613 The oxygen (1s) s i g n a l i s reduced t o very low l e v e l s and t h e r a t i o O ( l s ) : M 0 ( 3 d ~ / ~ ) approaches t h e very low value found i n n-heptane ground MoS2 (cf Table

5, r e f 3 ) .

Oxygen containing s p e c i e s p r e f e r e n t i a l l y absorb on t h e MoS2 edge plane ( r e f 3 ) so t h e l o w value of t h e r e l a t i v e O ( l s ) i n t e n s i t y i n d i c a t e s t h a t on t h e g r a p h i t i s e d carbon c a t a l y s t t h e r e i s a high proportion of MoS2 b a s a l plane s u r f a c e a r e a . On both f r e s h c a t a l y s t and sulphided c a t a l y s t only one major Co(2p3/2) s i g n a l i s observed a t a binding energy similar t o t h a t i n a c o b a l t sulphide ( c f cobalt oxide 781.0 eV ( r e f 3 ) ) .

This i n d i c a t e s a c l o s e i n t e r a c t i o n between c o b a l t i o n s

and MoS2 and an exchange of l i g a n d s during preparation. These r e s u l t s have shown t h a t c a r e i s needed i n preparing a n evenly d i s t r i b u t e d f u l l y sulphided system.

Given adequate technique, ammonium thiomolybdate and

c o b a l t i o n s can be evenly d i s t r i b u t e d by impregnation onto g r a p h i t i s e d carbon pellets.

There i s ready l i g a n d exchange with Co2+ i o n s and, a f t e r a c t i v a t i o n ,

t h e MoS2 b a s a l plane makes up a l a r g e p a r t of t h e c a t a l y s t s u r f a c e a r e a .

CATALYTIC PROPERTIES OF THE PROMOTED MoS2-GRAPHITISED CARBON SYSTEM

Cobalt promoted MoS2 on g r a p h i t i s e d carbon prepared by t h e r o t a r y evaporation method was t e s t e d f o r conversion of thiophene vapour using t h e atmospheric p r e s s u r e microreactor t e s t described f o r use with MoS2 samples.

Table 5 shows d a t a f o r

thiophene conversion and compares t h e novel c a t a l y s t with a conventional commercial CoMo A1203 c a t a l y s t .

TABLE 5 Conversion of thiophene vapour i n atmosphere p r e s s u r e microreactor a t 325OC Sample

Stable

% Thiophene Conversion CoMo A1203 (2.4% b i t Co 7.5% w t Mo) MoSz/graphitised C (8% w t Mo) Co/MoS2/graphitised C (3%w t Co, 8%w t Mo)

Ratio Butane/ Butene

42

0.5

50

0.7

100

A l l butane

It i s c l e a r that c o b a l t promotion h a s a l a r g e e f f e c t on t h e MoS2/graphitised carbon c a t a l y s t and produces a c a t a l y s t of exceptional a c t i v i t y . Four compositions of c o b a l t promoted MoS2 on g r a p h i t i s e d carbon c a t a l y s t were prepared as i n d i c a t e d i n Table 6 and thiophene conversions were measured a t 260, 280, 9 0 and 32OOC using 0.13 g of c a t a l y s t i n each case. flow w a s a t

P

I n t h i s t e s t hydrogen

ml/minute and thiophene was i n j e c t e d a t 0.05 ml/h a s before.

The

r e s u l t s a r e p l o t t e d i n Figure 2 and temperatures f o r 50 p e r cent thiophene conversion were estimated and recorded i n Table 6.

Comparison with e a r l i e r r e s u l t s i n Table 2

a t 325°C emphasises t h e improvement i n a c t i v i t y obtained.

FOR JHIOMOLYBDATE GRAPHITE CATALYSTS

-

FIG 2 CONVERSION OF MIOPHENE AT VARIOUS TEMPERATUFiES

-

RG 3 ACllVlTY AND SELECnVlTY OfTHlOMOLYBaATE GRAPHITE CATALYSTS

z

.-

8

z w

Y

lA K

P

515 TABLE 6

E f f e c t of c a t a l y s t composition on thiophene conversion Sample

% w t Cobalt

% w t Molybdenum

Temp OC

(50% conversion) A B

1

C

1

D

2

3 3 6 6

2

2 61 9 3 272

246

I n a l l experiments t h e r a t i o of butane t o butenes i n t h e product was measured and i n Figure 3 t h e s e r a t i o s a r e p l o t t e d a g a i n s t p e r cent conversion f o r thiophene. A s can be seen a l l t h e p o i n t s l i e on t h e same s t r a i g h t l i n e up t o 80 p e r cent

d e s u l p h u r i s a t i o n and t h e very low value of butane/butenes a t t h i s conversion (0.33) emphasises t h e very s e l e c t i v e behaviour of t h e c a t a l y s t s .

The f i g u r e i n c l u d e s a

l i n e f o r a t y p i c a l commercial c a t a l y s t f o r comparison. The r e s u l t s have shown t h e very s t r o n g promotional power of c o b a l t on a supported MoS2 c a t a l y s t which h a s m i n l y b a s a l plane s u r f a c e exposed.

The promotional. e f f e c t

v a r i e s with Co:Mo r a t i o and appears t o be l a r g e s t a t a weight r a t i o of about

1:3.

RELEVANCE TO MODELS OF THE COMO SYSTEM

Three main models of t h e CoMo system have been described

-

t h e monolayer model

a r g u e s f o r c o b a l t promotion v i a i n c o r p o r a t i o n i n t h e alumina surface.

I n the

g r a p h i t e supported system t h e promotion caused by c o b a l t i n t h e absence of alumina shows t h a t t h i s model i s not t h e only way i n which c o b a l t promotion can occur.

In

t h e i n t e r c a l a t i o n model promotion occurs v i a t h e i n t e r c a l a t i o n o f t h e c o b a l t i n t h e edge s i t e s of MoS2. much lower than t h e

This model, however, r e q u i r e s very low l o a d i n g s o f c o b a l t

l:3 weight r a t i o o f c o b a l t t o molybdenum observed h e r e and

would not be favoured i n systems with a high p r o p o r t i o n o f b a s a l plane s u r f a c e a r e a . The synergy model assumes a very c l o s e i n t e r a c t i o n between c o b a l t and molybdenum sulphides.

There i s c l e a r evidence f o r t h i s i n t h e p r e s e n t study from t h e exchange

o f l i g a n d s between c o b a l t and molybdenum a t t h e p r e p a r a t i o n s t a g e and t h e ESCA data showing t h e a c t i v e phases t o be MoS2 and a sulphide o f cobalt.

Thus, t h e p r e s e n t

results favour t h e synergy model.

SUMMARY OF CONCLUSIONS

A very a c t i v e , h i g h l y s e l e c t i v e d e s u l p h u r i s a t i o n c a t a l y s t can be prepared by

c o b a l t promotion of t h e MoS2 b a s a l plane.

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

h a s been described and t h e a c t i v i t y shown t o depend on t h e r a t i o of c o b a l t t o molybdenum.

516 ACKNOWLEDGEMENT

Permission t o publish t h i s paper has been given by The B r i t i s h Petroleum Company Limited. REFERENCES

1 G.C. Stevens and T. Edmonds, J C a t a l , 44(1976)488 2 G.C. Stevens and T. Edmonds, J C a t a l , 37(1975)544 3 G.C. Stevens and T. Edmonds, J Less Common Metals, 54(1977)321-330 4 P. Grange, B. Delmon e t a l l J Less Common Metals, 54(1977)311-320 5 M.J.M. van der Aalst and V.H.J. de Beer, J Catal, 49(1977)247-253 6 G.C.A. Schuit and B.C. Gates, AIChEJ, 19(1973)417 7 J.C. Wildervanck and F. J e l l i n c k , 2. Anorg Allg Cjem, 328(1964)39 8 E. Furimskv and C.H. Amberg, Can J Chem, 53(1975) 3567 9 A.L. FarraKher and P. Cossee, Proc 5 t h I n t e r n a t i o n a l Cong C a t a l y s i s , M i a m i Beach, (1972)lWl 10 G. Hagenbach, P.H. Courty and B. Delmon, J C a t a l , 31(1973)264

DISCUSSION F.

KERKHOF

:

A r e t h e MoS2 c r y s t a l l i t e s s m a l l e r when c o b a l t i s u s e d ?

We s u g g e s t t h a t t h i s may b e c o n c l u d e d f r o m t h e Mo/C XPS i n t e n s i t y ratio,

which s h o u l d be h i g h e r i n case o f smaller c r y s t a l l i t e s

(or a

" m o n o l a y e r " o f MoS2).

T.

EDMONDS : W e d i d n o t s t u d y t h e u n p r o m o t e d c a t a l y s t u s e d i n T a b l e

2

b y xps.

Our XPS s t u d i e s w e r e c o n f i n e d t o t h e p r o m o t e d c a t a l y s t s .

Consequently w e c a n n o t g i v e an answer d i r e c t l y t o t h i s i n t e r e s t i n g question.

However,

if

the butane

: butene

r a t i o f o r t h e unpromoted

c a t a l y s t i n T a b l e 2 i s compared w i t h t h i s r a t i o f o r t h e promoted c a t a l y s t a t t h e same t h i o p h e n e c o n v e r s i o n Figure 3 ,

(50 p e r c e n t ) , given i n

a marked d e c r e a s e i s n o t e d f o l l o w i n g p r o m o t i o n .

This

could i n d i c a t e t h a t t h e promotor i n c r e a s e s t h e r a t i o of b a s a l plane edge s i t e s .

B. DELMON : Your a p p r o a c h i s q u i t e o r i g i n a l a n d f r u i t f u l . I would

2nd p a r a g r a p h o f y o u r p a p e r

: "This

p r e pa r a t ion "

EDMONDS

:

ligands during

. The a r g u m e n t i s a s f o l l o w s : c o b a l t i s d e p o s i t e d a s

n i t r a t e and decomposed t o o x i d e . molybdate

7,

indicates a close interaction

between c o b a l t i o n s and MoS2 and a n exchange o f

T.

Therefore,

l i k e t o a s k y o u some c l a r i f i c a t i o n o f y o u r s e n t e n c e , p .

Molybdenum i s d e p o s i t e d a s t h i o -

( a n d u l t i m a t e l y decomposed t o s u l p h i d e ) .

Y e t following

c a t a l y s t p r e p a r a t i o n w e f i n d t h a t c o b a l t h a s a b i n d i n g e n e r g y which

617 c a n o n l y b e i n t e r p r e t e d by t h e c o b a l t c a t i o n b e i n g i n c l o s e p r o x i m i t y t o sulphide anions;

t h e molybdenum s p e c t r u m i n d i c a t e s a s s o c i a t i o n

w i t h b o t h oxygen and s u l p h u r a n i o n s .

Consequently,

oxidation has occurred during preparation,

even i f

some

a n i o n exchange must e x p l a i n

the presence of cobalt i n association with sulphur anions.

This

e x c h a n g e c a n o n l y o c c u r i f t h e c o b a l t i o n s a n d MoS2, o r t h e t h i o molybdate anion,

L.

have i n t e r a c t e d .

MOSCOU : I h a v e t w o q u e s t i o n s o n t h i s p a p e r .

1 ) D i d y o u make a n y a t t e m p t t o look f o r t h e p r e s e n c e of s e p a r a t e MoS2 p a r t i c l e s u n d e r t h e h i g h r e s o l u t i o n e l e c t r o n m i c r o s c o p e ? 2 ) What h a p p e n s i f

t h e s e c a t a l y s t s a r e c o n t a c t e d w i t h a i r a t room

t e m p e r a t u r e and t h e n c o n t a c t e d w i t h t h i o p h e n e under t e s t c o n d i t i o n s ? W i l l t h e s t r u c t u r e and c a t a l y t i c a c t i v i t y of

T.

the c a t a l y s t be restored ?

EDMONDS : 1 . The c a t a l y s t was n o t s t u d i e d by h i g h r e s o l u t i o n

e l e c t r o n microscopy. 2.

O n l y m i n o r c h a n g e s o c c u r when t h e s e c a t a l y s t s a r e e x p o s e d t o a i r

a t room t e m p e r a t u r e o v e r a t i m e s c a l e o f w e e k s o r e v e n m o n t h s . major change which is n o t e d

SO:-

( b y ESCA) i s t h e c o n v e r s i o n o f S

2-

The to

but our evidence suggests t h a t the sulphate anion i s readily

re-reduced

t o sulphide i n hydrogen.

o x i d a t i o n of MoSZ t o Moo3.

T h e r e may b e a n e x t r e m e l y l i m i t e d

C o n s e q u e n t l y t h e r e i s l i t t l e c h a n g e when

t h e c a t a l y s t i s r e a c t e d with thiophene under t e s t c o n d i t i o n s again s i n c e t h e chemical form of t h e c a t a l y s t i s e s s e n t i a l l y unchanged.


619

CATALYST STABILIZATION AND DEACTIVATION COMPARED WITH CATALYST PREPARATION. R.E.

MONTARNAL

Institut Fransais du Petrole, Rueil-Malmaison, France.

ABSTRACT Once a catalyst has been prepared and placed in its reactor, a crucial problem concerns its different ways of evolution.

This

paper attempts to make as rational an analysis as possible, rather than an exhaustive study of the possible transformations, which can be classified under two essential headings

:

i) stabilization, to

achieve a steady state performance and ii) deactivation during the working time of the catalyst.

INTRODUCTION Once inside a catalytic reactor, a heterogeneous catalyst does not remain in the same fixed state, as assumed by the academic definition of catalysis, but may go through various, extreme transformations. 1. First of all, the final stages of preparation are sometimes pre-

formed within the reactor, following the introduction of a catalyst precursor. 2.

At the beginning of catalysis the fresh catalyst undergoes an

evolution leading more or less quickly tq performance stabilization. 3 , Once the stabilization period has been achieved, performances

generally decrease slowly under the influence of deactivation phenomena : aging period. 4.

The deactivated catalyst can be regenerated inside or outside

the reactor, or may be discarded if regeneration is impossible or uneconomic. Until a few years ago researches undertaken in these areas were of a rather empirical nature due to the complexity of the phenomena. More progress has been made recently, e.g. by the Berkeley Symposium (ref. l)., and the present paper aims to demonstrate some general parameters governing these transformations.

Of course, due to the

vastness and complexity of the topic, some over-simplifications have been necessary, but hopefully excessive divergence from reality has been avoided.

Some classifications and some interpretations certainly

may be debatable and some cases will also be omitted.

Moreover the

material o f the text and the examples chosen will belong rather to the area of Refining and their generalization to cover all heterogeneous catalysis may require some variations.

1. F I N A L STEPS OF PREPARATION WITHIN THE REACTOR These last stages of preparation are indeed of the same type a s those performed outside the reactor.

We indicate here the main

reasons for finishing the preparation inside the reactor. The main reason is the risk of degradation of some of the final catalyst in contact with the atmosphere during transportation or handling.

Thus many nickel catalysts are pyrophoric and must be

introduced into the reactor as the oxides (or massic alloys, for Raney nickel).

The reduction step may lead to troublesome compli-

cations if, for example, the catalytic reaction can be performed at low temperature (hydrogenation at 150°C) while the reduction must be at 400OC.

Hence the use of passivation techniques aids

performance of the reduction in milder conditions.

For a reforming

catalyst, platinum compounds are also reduced in situ, although i t would be possible to handle a reduced catalyst.

Hydrodesulfuriza-

tion catalysts are introduced in the form o f Co and Mo oxides and are sulfided inside the reactor by a combination H S 2

-t

H2.

Some

degradation of the sulfide could occur by exposure to air, leading to a longer transient period of stabilization or even to a lower steady state of performances.

Moreover we must note that, for

catalysts subjected to regeneration by burning coke with oxygen, a reduction step has to be performed inside the reactor to obtain the desired zero valency state of the metallic catalyst. A more fundamental reason concerns the usefulness o f performing

the final stages of preparation by means of the reactions themselves, which can prepare the catalyst surface to produce the best performance. The classic example i s of Fe30q reduction by N 2 iron for N H 3 synthesis.

+

H2,

to obtain

But such a process belongs to both the

final stages of preparation and to catalyst stabilization by reconstruction phenomena, as seen below.

Indeed this raises the

problem of knowing the nature of the fresh catalyst. The answer to this question is complicated and can only be considered after catalyst stabilization has been discussed.

521 2.

GENERALITIES ON CATALYST EVOLUTION DURING CATALYSIS This paper essentially concerns the case of an open flow reactor,

either for gradient concentration technology (piston flow), or uniform concentration technology.

As a simplification, it can

sometimes be useful to consider a differential reactor for which concentrations and temperature around the catalyst are perfectly defined.

Generally, the evolution of activity as a function of time

has the form presented in Fig. 1. transient period

During a relatively short

(a few hours, a few days) the activity decreases

or increases rapidly, until a stationary level is reached.

Indeed

a true steady state corresponding to the dashed line in the figure is not obtained.

During the second period, the activity decreases

slowly due to deactivation (also called aging).

The distinction between the two periods, which lies essentially in their length

\

results from the actions for each different processes

Theoretical true steady state

\-.J

that we shall try to charac-

___---

terize according to the following classification.

Time Fig. 1 Transient period of stabilization

Working period with slow deactivation

:

General evolution of

activity during catalysis.

Stabilization period We shall distinguish

-

:

An intrinsic stabilization, e.g. inherent to the nature of

the catalyst-reaction system itself.

-

A stabilization due to the transient influence of a parasite

reaction which will essentially be coke formation.

-

A stabilization due to a reversible selective poisoning, in

other words due to a kinetic inhibition by some compounds in the feed that are irrelevant to the catalytic reaction. Aging period or deactivation period during working The slow downward drift of performances can result

-

:

from the continuation of coke formation, but according to kinetic

characteristics and even mechanisms different from those which act

522

during stabilization;

-

from the evolution of the cata yst itself independently of any

catalytic reaction;

-

from irreversible poisoning by some compounds in the feed; from parasite reactions due to a bad catalyst conception or

manufacture;

-

from various other causes

:

attrition, breaking, etc

The problem of regeneration will also be mentioned. 3. STABILIZATION PERIOD The general concept of a catalyst evolving when placed in contact with reactants is rather old.

Many authors have invoked the action

of reactants on the catalyst as a counterpart of the action of the catalyst on the reactants, according to schematic reaction (1) occuring during the transient period of stabilization Fresh

(So)

+

Reactants

catalyst

+ +catalyst

(S1) + Products (1) (during stabilization)

The problem relevant to stabilization is first to determine the nature and kinetics of the reactions leading from So to S 1 , and secondly (and mainly) to characterize the final state of the catalyst and the final performance, for these govern the reactor design. The possibility of reaching the final state either by a reversible or an irreversible process should first be considered.

The steady

state will be reached by a reversible path if, once this state has been obtained following transient modification of the operating conditions, by returning to the original conditions, we can again obtain the original performance.

In another form, the steady state

is reversibly obtained if it depends only on operating conditions and not on the path followed to reach it. Remark

:

The steady state of equilibrium for the.catalyst

-

reac-

tant system does not correspond to a thermodynamic equilibrium between reactants, as appears evident for differential reactors.

Equilibrium

results from the dynamic play o f antogonistic reactions between the homogeneous phase and the solid. 3.1. Intrinsic stabilization We shall try to move from simplicity to complexity, or rather, from well established t-e 3.1.1.

more

centroversial.

Stabilization related only to the catalytic mechanism.

By this

523

we mean the case where stabilization is attained by the involvement of elementary steps of the catalysis itself and thus has a reversible character.

Different examples can be given.

The classical mechanism of LAllGMUIR-HINSHELWOOD involves, as does any mechanism, a transient period evolution, since the stationary surface coverage in different species cannot be established immediately.

But this period is often very short, due to the high values of

adsorption

-

desorption rate constants, and so can be neglected.

It

is interesting to consider simple cases for which this period is Let us, for example, look at the two

sufficiently long to be observed.

reaction schemes in Figure 2 , with the indicated values of rate conatants, which lead to a slow accumulation of the inhibitor UI. initial time, the adsorption established for

-

and gives for u

A

A0

the value indicated in the

Figure, because uI is still equal to zero. the different values of

At the

desorption equilibrium is quite rapidly After an infinite period,

uAm and uIm can be easily obtained from the

stationary-state equations, and are also indicated in the Figure.

,

In other words, the uA fraction decreases from UAO to the

U I fraction increases from zero to U

rate of B decreases from kS is greater as k

UAo

to kS

(5

1-

Am'

-

UAOO

while

Hence the formation

The period of stabilization

is weak, and it is easy to understand that the

i stabilization time can be measured.

Fig. 2

:

Transient period

for a LANGMUIR-HINSHELWOOD mechanism

(OD

represents

reaction rate constants of quite high values).

1

Fractions occupied by the

differant mokcutm

Ki*bi

k'i

left

A A ~ Y I

right AJB 0

represents the surface

fraction occupied by a

4

Stobilirotion

time

-

given compound.

Another characteristic example is the case of a protonic acid catalyst used for the isomerization of n-paraffins (ref. 2).

:

RH+?iRH

The complexity of the phenomena leads us to give only

the essential aspects. catalysts SH',

Beginning with the protonic form of the

its superficial structure goes towards a steady state

624

-

under the influence of several reactions, which are elementary steps of the catalysis and can be expressed overall as ( 2 ) .

+

SH+

ki

Reactants >(RH)

SiR+

+ Products

(2)

( H a, iRH)

In the steady state the catalyst surface is shared between SH+ and SiR+ (SR'

being much less stable than SiR+ appears only in a very

weak proportion).

Such an evolution is presented in Fig. 3a.

On

the other hand, the catalytic transformation of RH is governed by the following two elementary reactions RH

+

SH+

RH

+

SiR

, iRH

kl

generally with

i RH Fig. 3

t

Catalyst atabilization

S H' + Reactontr (RH)

:

SiR+ + P

r

Superficial fractiona

o

d

u

c

t

a

'

w

:

Stabilization

process for a protonic @

*

acid catalyst, during n

a f ~ R(Hz,iRH) ~

paraffin isomerization n R H S i R H (ref. 2 ) Tim Catalytic

RH RH

reaction

+ SH A + S i R k5_

(ref. 33).

Reaction rate

Products ( iRH) products

ksrkc r,= kt ( R H )

Time

Thus the RH transformation rate goes from the initial equation r

= k l [RHI [SH

+l o

= kl

to the final equation r,

=

{kl

[SH+ 1,

+ k3

[RH],

since

[SH+l0

=

1

:

[SiR+],]

[RH]

Since kj >> k l , the reaction rate increases from r in Fig. 3b.

:

Indeed, in the steady state

active form of the catalyst.

+ SiR

to r-,

as shown

is practically the

The length of the transient period

increases as k l weakens relative to k3.

The relevant experimental

results have been established for liquid super-acid catalysts, but the same phenomena can occur for example with zeolites.

For xylene

isomerization on Y zeolite such a transient period of one hour has been observed (ref. 3).

526 A

third example is the oxidation of hydrocarbons proceeding by

a redox mechanism on oxide catalysts.

The two elementary reactions

govern both the superficial evolution of the solid, and the conversion of reactants leading to a steady state where the solid surface remains in an intermediary state of oxidation (ref. 4).

In fact, the

real mechanism is more complex, for the bulk evolution of the solid goes in parallel with the superficial evolution. Complex stabilization

3.1.2.

a) N H 3 synthesis or decomposition. Some authors have shown the existence of transient periods of several hours before reaching a new steady state, after modification of experimental conditons, for NH3 synthesis (ref. 5).

Thus on

Fe at 25OoC, by increasing N 2 from 5 to 2 5 % , the NH3 formation rate increases, but it takes more than 10 hours to reach the new steady state.

we again obtain the initial performance,

By returning to 5 % N 2 ,

again after a long transition period.

The reversibility of the

phenomenon permits invoking the intervention of one or several slow steps of the catalytic mechanism.

It is well-known that the nitrogen

activation by chemisorption is a slow step in N H 3 synthesis. Besides, others experiments have resulted in some superficial structural modifications of the catalyst by the reactants (ref. 6). Thus fig. 4 shows that Fe304 reduced by the mixture N2 + H 2 produces an iron which is immediately active when catalytic conditions are established.

On the contrary, Fe304 reduced by H 2 is initially

inactive for synthesis and only becomes active after a prolonged period in contact with N 2 not very different.

+

H2.

The two final steady-states are

The interpretation of these results stemmed

from the need for a restructuring, by chemisorbed N2, of the iron surface produced by H 2 reduction, to create the crystallographic This plane is obtained

plane ( 1 1 1 ) which is active in catalysis.

H2.

directly in the case of reduction by N 2 +

1

t

NH3 1

;* I I I

2 30min

Fig. 4

:

Time dependence

of ammonia yield at 2 7 5 O C 1)

catalyst reduced in

N2-H2

mixture 2 ) catalyst

reduced in pure H

I j

(ref. 6 ) .

-

Time 10 min

Such a reconstruction

24h.

2

526

is intrinsic, since it is created by the reactant N2. may not be reversible.

It may or

Other authors also invoke reconstruction

phenomena to interpret the effect of pretreatments on the transient period and on the final state for NH3 synthesis on Fe/Mg 0 (ref. 7). For NH3 decomposition also, the catalyst Ru/A1203 obtained by reduction with H2, is not immediately active and must be activated by NH3 at 5 O O 0 C , which can also be interpreted as requiring a surface reconstruction (ref. 8 ) .

In conclusion, the superficial

modifications occurring during the stabilization of catalysts for NH3 synthesis can involve the elementary steps of the catalysis, but also some superficial reconstruction phenomena induced by the reactants.

Moreover these recontructions can be of a purely

structural nature, or imply strongly bonded nitrogen species, going from chemisorbed nitrogen to nitride, i.e. the incorporation of N in a crystallographic lattice.

The reversibility of transformations

is questionable. b) Alloys or complex catalysts. The reconstruction concept used for a pure metal must be extended for the case of more complex catalysts.

For alloys, the superficial

enrichment with metal having a weaker heat of sublimation is well known, by thermal treatment in a vacuum or inert atmosphere (ref. 9). Besides, in the presence of reactants the superficial enrichment concerns the metal giving the stronger bonding with these reactants, if of course, the segregation is kinetically possible (sufficient temperature).

Thus while for Pd-Au, this enrichment concerns gold

in a vacuum, it concerns palladium in the presence of oxygen (ref. 10).

Hence, during the catalytic reaction, the superficial

composition (and structure of course) can change either by a purely thermic effect or under the influence of strongly bonded reactants. The two effects may be opposed.

The reversibility of the final state

depends on the experimental conditions.

Almost similar reconstructions

have been observed for other complex catalysts.

They can be used,

for example, to interpret the evolution of nickel sulfide properties during the selective hydrogenation of acetylene (ref. 11).

Under the

influence of acetelyne, the authors invoke the migration of sulfur atoms according to the reversible superficial reaction ( 3 ) .

These migrations induce activity modifications through transient

527

periods of stabilization.

In a paper presented in this Symposium

(ref. 12), the catalyst stabilization also seems to be governed by a reconstruction phenomenon, in the general sense.

For the ethyl-

benzene dehydrogenation into styrene on Fe30j promoted by Cr-K, the decrease in a few hours, of the parasitic reaction of dealkylation can be interpreted by some structural catalyst modification; possibly the migration of the promotors. In conclusion, the intrinsic catalyst stabilization due to the interaction between the solid and the reactants is an unavoidable phenomenon which leads to the true steady state of the catalystreactant system, at least if it is permitted by the kinetics.

We

can consider that the fresh catalyst is defined by this steady state and that its characterization would have to be performed in this state.

3.2. Pseudo-stabilization under the influence of transient parasite coke formation We must first recall that coke is generally formed of polycondensed aromatics with a wide variety of possible structures ranging from undesorbable “liquid“ products to graphite.

The rapid

deactivation induced by parasite coke formation can lead to a practical stabilization of performances.

The decrease of performances stops,

for a coke level which remains constant and depends on the severity of the operating conditions.

Once this stabilization is achieved,

if the severity of the operating conditions is increased, the coke level is also increased for the new state of stabilization which is reached.

On returning to the previous conditions, the performances

are lower than previously. nature.

This evolution is then of an irreversible

It is indeed a pseudo-stabilization, because the nature of

the catalyst is greatly modified by parasitic irreversible coke deposition.

Due to this irreversibility it is impossible to invoke

a stabilization process such as in Figure 2 , for which product,

rsI, would represent the coke.

The coke results from an irreversible

reaction ( 4 ) such as (for hydrocarbons reactions) Reactants

+

kc n

Fresh catalyst

:

Coked catalyst

?

+ Products

(4)

(H2 and light products)

The rate of coke formation can be written in the general form r

C

=kc

n

(Cons)

=

k

c

n

c

(‘Huns

)

:

represents the concentration of highly unsaturated products, ‘Huns which are coke precursors. The rate of coke formation declines until it is cancelled-out by the decomposition of this coke itself on the coking sites, so as to decrease their number (n (kc).

)

or their activity

This schematic process is generally accepted, but can operate

according to different kinetic forms.

The simplest form of decrease

of n

But the decrease can be linear,

and r

is an exponential curve.

or even non-existent during a prolonged period (ref. 13).

Beyond,

it must be explained that in spite of the total self poisoning for coke formation, there is only partial self poisoning for the catalytic reaction, whose activity decreases to a finite value. -We can invoke a surface heterogeneity with two types of site. Only the more active sites (n the total sites (cokinq lytic reaction. sites, n

.

+

)

would be able to perform coking, while

non coking = nc

+

n) would promote the cata-

The coke deposition would eliminate only the coking

This over-simplified hypothesis can be improved by

invoking an activity site distribution. -We can consider a single type of highly active site on the fresh catalyst, able to perform both the catalytic reaction and the more difficult coking.

The coke deposition would decrease the site

activity, which is relevant to the decrease in k

instead of n

.

The activity decrease of a site can result from the coke deposition on the site itself, but also and even more probably from the ligand effect (or electronic effect) of the coke on a neighbouring site. Thus there only remains the reaction activity. -We can also consider a demanding nature for the coking, which is a bimolecular reaction, since it involves a polycondensation, as opposed to a non demanding nature for the catalytic reaction. formation would require many more sites than the reaction.

Coke

So, the

sole effect of site dilution, due to coke deposition, could decrease coking drastically, until it is eliminated, while the catalytic reaction would be reduced only in proportion to dilution. A

demonstrative example is the evolution of n-heptene on a protonic

acid heterogeneous catalyst according to the reaction path of Figure

5 (ref. 1 4 ) . The coke percentage stabilizes itself at a given level where its formation is cancelled out, while n-heptene transformation decreases to a finite level.

Why ?

Coking is caused by the hydrogen

transfer reaction between heptenes and isobutene, producing isobutane, and heptadienes which are coke precursors.

This reaction i s

difficult in the conventional sense, for it involves a difficult

Fig. 5 350° 210-2 bars of %HI4 in N g

Reaction rotes n heptene rote

:

n-heptene reaction

on acid silica-alumina catalyst.

Pseudo-stabiliza

tion by coke formation (ref. 1 4 ) .

-

coke %

Time

Heptenes

+ isobutene -c Heptadienes + isobutane

+

coke

hydride abstraction and is demanding in the BOUDART sense for it involves at least two sites.

This explains why self poisoning coke

formation leads drastically to its own cancellation.

On the contrary

catalytic isomerization and cracking are performed through carbonium ions, i.e. intermediates which are easily obtained by proton addition to n-heptene, and isomerization involves only one site for the skeletal rearrangement.

An important point in this example is that

the coke formation rate here is particularly easy to characterize from the parallel isobutane formation rate. The self poisoning is particularly clear and well known for the reaction of paraffins and H2 on transition metals at around 200' 4OO0C,

Conversion decreases quickly to a stable level.

to

Moreover,

because the reaction is complex (with hydrogenolysis, isomerization, cyclization, dehydrogenation), the different paths are selectively reduced.

Basically, the most drastic decrease concerns the most

demanding reactions; either they need special sites (corner, edges or a collection of several sites. done to characterize

Very little research has been

the final state of the pseudo stabilized

catalyst. Hydrogenation reactions at low temperature can themselves be self poisoned by parasitic coke deposition.

It is known that in

the absence of hydrogen, an olefin is chemisorbed on a transition metal and undergoes a self-hydrogenation, giving both the relevant paraffin in the gas phase, and a highly dehydrogenated chemisorbed complex which is a coke precursor.

Of course the absence of H

2

...

)

530

corresponds to extreme Conditions.

With the olefin + H2 mixture,

as feed, for a catalytic hydrogenation, the presence of H2 reduces coke deposition drastically.

However, the fact remains that the

steady-state of the working catalyst can be different from the state of the fresh catalyst and to a certain extent may depend on the experimental conditions.

A

good Qharacterization of the final

catalyst would also be very useful. Catalytic reforming is a more complex case due to the bifunctionality of the catalyst and to the complexity of the feed. Bearing this in mind, with the example of n-heptane as a model reactant, Figure 6 shows the deactivation observed for a fresh catalyst (ref. 15).

1

Fig. 6

Toluene formation rate from n heptane

P~c.O,l4b P b .0,86b

I

:

Pseudo-stabilization

by coke formation, for a reforming catalyst (ref. 15).

coke% before run:OX after run: 2,6%

0.t Benzene 4 10-CHydrogenation rate-0,l mole h'l g'l 10O0C

I

10

20

30

-

Time(h)

The coke level goes to 2 . 6 % , while the chloride is maintained at around 1 % .

We observe in correlation a decrease from 4 to 0.1 in

the hydrogenation activity of benzene.

A l l this illustrates the

modification of the catalyst. Conclusions

:

Catalysts can undergo rapid, profound and irreversible

transformations by coke deposition during the first hours of operation.

Hence several consequences must be emphasized

:

- The only well-defined catalyst, for both fundamental investigations and industrial uses, is the stabilized catalyst.

-

The catalyst characterization should be done on this final

catalyst.

This is hardly ever the case because of the difficulties

encountered.

It would be very useful to adapt physical methods

for characterizing a coked catalyst by measurement of free metallic sites, acidity, etc...

This is an indispensable requirement for

531 t h e c o r r e c t d e f i n i t i o n of t h e T.O.N. deductions.

I n t h e same way,

systematically,

and f o r a l l t h e consequent

i t would b e f r u i t f u l

i n d i v i d u a l f u n c t i o n s o f t h e coked c a t a l y s t , i . e . reaction, 3.3.

t o use

c a t a l y t i c t e s t r e a c t i o n s c a p a b l e of

characterizing

hydrogenation

r e a c t i o n c o n n e c t e d w i t h a g i v e n t y p e of a c i d i t y , e t c . . .

S t a b i l i z a t i o n by s e l e c t i v e r e v e r s i b l e p o i s o n i n g

Among t h e i n f i n i t e v a r i e t y o f c a s e s o f p o i s o n i n g ,

it i s possible

t o i n t r o d u c e two t y p e s o f d i f f e r e n t i a t i o n . - D i f f e r e n t i a t i o n b e t w e e n two e x t r e m e p o l e s w h i c h a r e

:

(1) a

s e l e c t i v e poisoning concerning t h e c a t a l y t i c s p e c i e s themselves, and

(2)

a non s e l e c t i v e p o i s o n i n g c o n c e r n i n g p o i s o n d e p o s i t i o n on

any p o i n t of t h e c a t a l y s t s u r f a c e .

Indeed, the t e r m poison should

b e a p p l i e d o n l y t o t h e f i r s t c a s e , b u t i t w i l l be e x t e n d e d h e r e t o a l l t y p e s o f a c t i v i t y m o d i f i c a t i o n s by d e p o s i t i o n o f

f o r e i g n compounds

p r e s e n t i n t h e f e e d and n o t i n v o l v e d i n t h e c a t a l y t i c r e a c t i o n . - D i f f e r e n t i a t i o n between r e v e r s i b l e o r n o n - r e v e r s i b l e

poisoning.

N o n - s e l e c t i v e p o i s o n i n g i s g e n e r a l l y i r r e v e r s i b l e and w i l l be examined i n the section concerning deactivation.

Selective poisoning can

b e r e v e r s i b l e i n which c a s e i t l e a d s t o a t r u e s t a b i l i z a t i o n , and t h e poison i s then an i n h i b i t o r .

I t c a n be i r r e v e r s i b l e ,

i n which

case it leads rather t o a progressive deactivation. C o n s i d e r i n g t h e e x a m p l e o f t h e i n f l u e n c e o f H S on G r o u p V I I I 2 t h e problem o f s e l e c t i v e p o i s o n i n g w i l l be examined f o r

metals,

t h e c a s e o f r e v e r s i b i l i t y , b u t w e w i l l a l s o s e e how i t c a n b e compared w i t h t h e c a s e o f i r r e v e r s i b i l i t y .

,

a ) S e l e c t i v e p o i s o n i n g o f group V I I I m e t a l s by H 2 S Isotherms of

chemisorption.

The s t r o n g b u t v a r i a b l e a f f i n i t y

o f g r o u p V I I I m e t a l s f o r s u l f i d e c o m p o u n d s i s c l e a r l y shown i n t h e thermodynamic d i a g r a m s o f F i g u r e 7 f o r two e x t r e m e m e t a l s Pt,

r e s p e c t i v e l y h a r d and s o f t

and

: N i

(ref. 16).

The s u l f i d e a r e a i s r e a c h e d f o r a much w e a k e r H S / H 2 2

t h e c a s e o f N i t h a n i n t h e case o f P t .

ratio in

For both c a s e s , H 2 S

c h e m i s o r p t i o n on t h e m e t a l c o r r e s p o n d s t o t h e l i n e AB i n F i g u r e 7. T h i s c h e m i s o r p t i o n p r o c e e d s a c c o r d i n g t o t h e e q u a t i o n s i n F i g u r e 9. I f t h e thermodynamic e q u i l i b r i u m i s c o n s i d e r e d f o r H2 w e e a s i l y obtain the equations for fractions Us e q u a t i o n r e p o r t e d i n F i g u r e 9 ,

g o v e r n s t h e s u r f a c e c o v e r a g e by S .

i s much h i g h e r f o r N i t h a n f o r P t . temperature increases,

for

0

",

UH,

Us.

chemisorption, From t h e

we s e e t h a t t h e K /K r a t i o S H For a given temperature,

KS/KH

B e y o n d , K S d e c r e a s e s when t h e

( 5 ) i s e x o t h e r m i c due t o t h e e a s y H S 2

532

B

B

I

* Thermodynamic diagrams, -Fiq. for the bulk of the solids. 7 :

Fig. 8

:

Isotherms for the dissocia-

tive chemisorption of H 2 S on Ni and P t (reversibility and apparent irreversibility).

3Ni+HzS

ks Kr -NiS+PNiH+Q

(6)

k'r (01)( 2 0 H ) (With 2 steps : N i S H L) NiS)

(3Ov)

2 N i H(2On)

Ni + H2 S

dissociation.

2Ni+Ho

4

K"

Fig. 9

:

Dissociative

chemisorption of H2S and

(6)

H 2 on Ni.

Uv

free frac-

tion of metal; on fraction

( 2 bv)

covered by hydrogen;

KO

3 Ni S + H t

(7)

us

fraction covered by sulfur

Hence the isotherms giving u s versus PHZs provide the

forms presented in Figure 8 , for Ni o r for Pt, and at low or high temperature. b) Reversibility and practical irreversibility Strictly speaking, H S chemisorption, then poisoning, is reversible 2

for both Ni and P t since ( 5 )

( 6 ) ( 7 ) are reversible.

Let us consider

however the H S pressure at point P in Figure 8 for Ni, which at 2 points R and I respectively, cuts the isotherms for 7 0 0 ' or 150OC. The temperature of 1 5 0 " can correspond to industrial benzene

533 hydrogenation on Ni, and 700" to methane Steam Reforming. H S

2

Introducing

at this pressure gives, for both reactions, the evolutions

presented in Figure 10.

After a certain time H 2 S is discarded from

the feed and the evolution of the activity is again given in Figure 10.

f

Fig. 10

r

with

M rithout

H2

-

without

s

HIS

kIS

-

:

Ni poisoning by H 2 S

2

kS

apparently irreversible at

150°, for kIS > rD.

O/O)

deposition on pore mouths ( diffusion limitation1 TM law

Deposition takes place uniformly all along the pore.

If

the poisoning is non-selective the rate decrease results from statistic covering and may be weak.

Let us consider e.g.

that 1%

of metal is placed on a support and covers 1 % of the surface area; for

1% of statistic poison deposition (having the same textural characteristics as the metal crystallites),

only a small fraction of these

crystallites will be covered. If the poisoning i s selective, i f 6 effect will be greater and could be more complex, considering mainly srlectivities, exactly as in the case of reversible selective poisoning. r 12), t h e complete i n h i b i t i o n o f t h e d i s s o l u t i o n can be understood, i f one considers t h a t t h e r e a c t i o n FeOOH + EDTA4- + H 2 0 e F e E D T A -

+

3 OH-

i s strongly shifted t o the l e f t . The two examples o f f e r e d c l e a r l y i l l u s t r a t e t h e important r o l e of s p e c i f i c i n t e r a c t i o n s between a s o l i d and a s o l u t e r e s u l t i n g i n e n t i r e l y d i f f e r e n t behavior o f d i f f e r e n t systems. INTERACTIONS OF METAL HYDROUS OXIDES WITH OTHER PARTICULATE MATTER Heterocoagulation Most o f t h e c o l l o i d s t a b i l i t y s t u d i e s have been c a r r i e d o u t w i t h s i n g l e systems, p r e f e r a b l y monodispersed sols, such as polymer latexes.

Yet, by f a r a m a j o r i t y o f

t h e n a t u r a l l y o c c u r r i n g dispersions, o r those used i n various a p p l i c a t i o n s , c o n s i s t s o f mixed t y p e p a r t i c l e s , which may vary i n composition, size, shape, and o t h e r properties.

I t i s , t h e r e f o r e , o f fundamental i n t e r e s t t o study heterocoagulation

phenomena, which i m p l i e s s t a b i l i t y o f systems c o n t a i n i n g d i s s i m i l a r suspended solids. I n o r d e r t o apply t h e t h e o r e t i c a l a n a l y s i s i t i s necessary t o work w i t h s p h e r i c a l p a r t i c l e s o f known size, p o t e n t i a l , and s p e c i f i c a t t r a c t i o n c h a r a c t e r i s t i c s .

In

a d d i t i o n , t h e s t a b i l i t y o f such mixed systems depends on t h e r a t i o o f t h e p a r t i c l e number concentrations as w e l l as on t h e i o n i c composition o f t h e suspending media.

569

2

I

I

6

8

AI(OHl3- PS LATEX KNO-: 0.01M

i ( 3 '

0

J

4

10

PH Flg. 10. Total stability ratio, WT, as a function of pH for a pure alumfnum hydroxide sol (modal diameter 570 nm) (O), a pure polystyrene latex (PSL, modal diameter 380 nm) (O),and for mixtures of the two sols containing 25% (V), 50% (A) and 75% (0) PSL particles (in terms of number concentration). Monodispersed spherical metal hydrous oxide sols are particularly suitable for the study of interactions between unlike particles, because the necessary parameters can be experimentally determined. A comprehensive investigation was carried out with a binary system consisting of a polymer (polyvinyl chloride, PVC) latex stabilized with sulfate ions and spherical chromium hydroxide particles (21). The advantage of such a combination is that changing pH affects little the surface potential of the latex, whereas the metal hydroxide particles not only undergo a change in this quantity, but the sign of the charge can be reversed. The obtained data showed that excellent qualitative agreement existed between the experimental results and the theoretical calculations for dispersions of these two colloids. To illustrate the effects of interactions in a mixed system, the stability ratios of sols consisting of spherical aluminum hydroxide particles (4) having a modal diameter of 570 nm and of a polystyrene latex (PSL, modal diameter 380 nm) stabilized by sulfate ions will be given. The rate of coagulation was followed by means of laser light scattering at a low angle (8 = .)'5 Figure 10 is a Plot

570

1.5b AI(OH13- PS LATEX KNO,: 0.01M

M

;.o 1

Q

0

J

0.5

0 -

6

4

2

8

10

PH Fig. 11. Analogous p l o t as F i g u r e 10 except t h a t t h e aluminum hydroxide p a r t i c l e s contained some s u l f a t e ions. o f t h e t o t a l s t a b i l i t y r a t i o , WT,

d e f i n e d as

where PI11 and W z 2 a r e t h e homocoagulation s t a b i l i t y r a t i o s f o r aluminum hydroxide and t h e l a t e x , r e s p e c t i v e l y , and W12

n l and

i s t h e heterocoagulation s t a b i l i t y r a t i o .

n2 a r e t h e primary p a r t i c l e number f r a c t i o n s o f the two d i s s i m i l a r p a r t i c l e s .

As seen i n Figure 10, t h e l a t e x i s l e a s t s t a b l e a t low pH ( s 3) whereas p u r e These pH values a r e c l o s e t o aluminum hydroxide i s unstable a t h i g h pH (Q 9.5). the i s o e l e c t r i c p o i n t s o f t h e two c o l l o i d a l systems.

Depending on t h e p a r t i c l e

number r a t i o and pH t h e b i n a r y systems may be e i t h e r l e s s o r more s t a b l e t h a n i n d i v i d u a l sols.

For example, a t pH 6 a l l mixed dispersions a r e l e s s s t a b l e t h a n

the s i n g l e systems.

This i s understood i f one considers t h a t t h e p a r t i c l e s o f

and o f aluminum hydroxide c a r r y opposite charges.

PSL

A t pH 8 the system c o n t a i n i n g

25% aluminum hydroxide p a r t i c l e s i s more s t a b l e than t h e pure aluminum hydroxide

sol.

I n t h e s t u d i e d case, aluminum hydroxide sol was c a r e f u l l y washed t o e l i m i n a t e

a l l s u l f a t e ions which are present i n the p a r t i c l e s as they are prepared (4).

571

Figure 11 gives an anologous p l o t t o the previous one except t h a t the aluminum The difference i n the behav-

hydroxide p a r t i c l e s s t i l l contained some sulfate ions.

i o r o f the p u r i f i e d and unpurified aluminum hydroxide sol i s q u i t e dramatic. I n addition, the r e p r o d u c i b i l i t y o f the r e s u l t s i n the l a t t e r case i s rather poor. This study exemplifies the s e n s i t i v i t y o f such systems t o anionic contaminations which often tend t o be disregarded. P a r t i c l e Adsorption Mixed systems which contain p a r t i c l e s g r e a t l y divergent i n size cannot be analyzed i n the same manner as those having p a r t i c l e s o f comparable size. I n the former case, the more f i n e l y dispersed systems may coagulate selectively, a heterof l o c may form, o r the small p a r t i c l e s may adsorb on the l a r g e r ones, causing a change i n the properties o f the l a t t e r . A l l o f these phenomena were observed i n a binary system containing negatively charged polyvinyl chloride (PVC) l a t e x It was shown t h a t ( p a r t i c l e diameter 7020 nm) and s i l i c a (diameter % 14 nm).

under c e r t a i n conditions s i l i c a adsorbs on l a t e x enhancing i t s s t a b i l i t y toward e l e c t r o l y t e s (22). The adsorption o f small p a r t i c l e s on larger ones i s o f p a r t i c u l a r interest, as

2

0

-2 NONO,: 0.01M

0 --4

2.O

I

2.5

3.0

3.5

4 .O

PH Fig. 12. Electrokinetic InobilCties (umlseclVlcm) as a function of ptl of PVC l a t e x (0.003% by wt, modal p a - t i c l e diameter 320 nm) i n the absence (0)and i n the presence of Fe(N03)3: 5 x 10-5 (v),7 x lom4 (0), 2.4 x lo-$ (A), and 5 x lom4M (0).

572

o t h e r properties

-

i n addition t o s t a b i l i t y

-

can be altered, such as t h e surface

charge, r e a c t i v i t y , w e t t a b i l i t y , adhesivity, pigment c h a r a c t e r i s t i c s , etc. Figure 12 shows the m o b i l i t y o f a (PVC) l a t e x as a f u n c t i o n o f pH and o f t h e same l a t e x i n the presence o f f o u r d i f f e r e n t concentrations o f Fe(N03)3. I n a l l cases t h e a d d i t i o n o f the f e r r i c s a l t had a strong e f f e c t on t h e p a r t i c l e charge causing the charge reversal from negative t o p o s i t i v e a t appropriate pH values. Independent measurements showed t h a t the sharp change i n the e l e c t r o k i n e t i c m o b i l i t y was associated w i t h t h e p r e c i p i t a t i o n o f f e r r i c hydroxide.

Consequently,

t h e m o d i f i c a t i o n o f t h e l a t e x surface was due t o t h e deposition o f t h e metal hydrous oxide on the polymer p a r t i c l e s .

Needless t o say the so coated l a t e x

showed d i f f e r e n t s t a b i l i t y , adhesion c h a r a c t e r i s t i c s , etc. than t h e untreated material. It i s expected t h a t reactants which a f f e c t t h e p r e c i p i t a t i o n o f f e r r i c hydroxide

would a l s o i n f l u e n c e i t s i n t e r a c t i o n w i t h t h e l a t e x .

This i s c l e a r l y evident i n

Figure 13, which gives t h e m o b i l i t y data o f t h e same l a t e x i n the presence o f Fe(N03)3 t o which NaF was added i n d i f f e r e n t concentrations.

F l u o r i d e ions a r e

known t o complex w i t h t h e f e r r i c i o n and, as a r e s u l t , the p r e c i p i t a t i o n phenomena as w e l l as t h e surface charge groups o f f e r r i c hydroxide are a l t e r e d by these

4

2

0

-2

-4

2

3

4

5

6

7

PH Fig. 13. E l e c t r o k i n e t i c m o b i l i t i e s _as a f u n c t i o n o f pH o f t h e same PVC l a t e x as i n Figure 12 i n t h e presence o f 1 x 10 M Fe(NO3l3 (0) t o which 7 x (01, M (0) NaF was added, respectively. 5 x (A), and 1 x

573

1

I

I

I

I

>

t

3 -

\

m 0

=E

INIT. pH:3.9 FINAL pHt1.8

1

2

0

4

-#A 8

6

10

30

DAYS Fig. 14. The change of e l e c t r o k i n e t i c m o b i l i t i e s (pm/sec/V/cm) as a function o f time (da.ys) of-the same PVC l a t e x as i n Figure 12 on a c i d i f i c a t i o n o f a sol cont a i n i n g 1 x 10 M Fe!N03),. Circles and squares represent systems t o which HC1 and HN03, respectively, were added t o lower the pH i n the presence o f 0.01 M NaN0,; t r i a n g l e s represent systems the pH of which was lowered by HNO, i n the presence o f 0.0010 M NaF p r i o r t o aging. anions.

Indeed, w i t h increasing f l u o r i d e concentration the charge reversal occurs a t higher pH values and the magnitude o f the p o s i t i v e charge decreases. Acidification o f the sols containing p a r t i c l e s with adsorbed metal oxides

should bring about d i s s o l u t i o n of the coating and, consequently, a change i n the surface charge t o less positive, o r even negative. Figure 14 shows t h a t i n the presence o f HNOJ and HC1 very long times are needed t o dissolve, a t l e a s t i n part, the metal oxide coating.

However, even a f t e r one month the charge i s not reversed

back t o negative; obviously, the metal hydrous oxide must be polymerized a t the surface t o which i t adheres t i g h t l y .

Addition o f only 0.0010 hf NaF g r e a t l y

accelerates the removal o f the oxide l a y e r and the p a r t i c l e s eventually become uncharged.

The l a t t e r observation imp1i e s t h a t the negatively charged potential

determining groups o f the o r i g i n a l l a t e x surface are neutralized by the metal counterion complexes. Apparently, the bonds formed between the s t a b i l i z i n g s u l f a t e ions and the adsorbed f e r r i c species are not broken by a c i d i f i c a t i o n even i n the presence o f F-.

574

PARTICLE ADHESION The d e p o s i t i o n o f c o l l o i d a l p a r t i c l e s on o t h e r s o l i d s and t h e i r removal from these s u b s t r a t e s depend on t h e chemical and p h y s i c a l f o r c e s a c t i n g between the I n t h e absence o f chemical bonds t h e adhesion can be treated

adhering surfaces.

ta

as heterocoagulation o f d i s s i m i l a r p a r t i c l e s , t a k i n g t h e r a d i u s o f one t o infinity.

I n o r d e r t o compare t h e t h e o r e t i c a l p r e d i c t i o n s w i t h experimental d a t a

i t i s necessary t o have s u f f i c i e n t l y w e l l d e f i n e d systems w i t h known parameters.

Again, monodispersed metal hydrous oxides, p a r t i c u l a r l y those o f s p h e r i c a l p a r t i c l e s , can serve as e x c e l l e n t models f o r p a r t i c l e adhesion and removal studies. Using t h e packed column procedure (23), t h e i n t e r a c t i o n s o f such sols w i t h glaSS and s t e e l have been s t u d i e d as a f u n c t i o n of v a r i o u s c o n d i t i o n s (pH, different e l e c t r o l y t e s , temperature, e t c . ).

I

I

I

Cr(OHI3ON GLASS

I

' I

Ca (NO&:

1 .o

n

' I

.8

0 Y

z

n

-

.6

c

Y

z .4

.2

0 0

2

4

6

TIME ( x I o - ~sec )

-1(

50

100

-

150

DISTANCE, x (A)

Fig. 15. L e f t : F r a c t i o n o f monodispersed s p h e r i c a l chromium hydroxide wrtfcles (modal diameter 280 nm) desorbed from glass on repeated e l u t i o n w i t h rinse s o l u t i o n s and i n t h e presence o f 10-5 (0), lo-% (0), and of pH 11.5 i n t h e absence (0) 1 0 - j M Ca(N03),. The p a r t i c l e s were adsorbed on t h e glass from a s o l a t pH 3. R i g h t : C a l c u l a t e d p o t e n t i a l energy curves as a f u n c t i o n o f d i s t a n c e using the sphere-plate model (Eqs. 2 and 4) f o r t h e same systems shown l e f t .

575

Figure 15 i l l u s t r a t e s the r e s u l t s obtained w i t h spherical chromium hydroxide p a r t i c l e s (diameter 280 nm) on glass (24). A t pH 3 glass beads placed i n a column r a p i d l y and q u a n t i t a t i v e l y remove these metal hydroxide p a r t i c l e s from an aqueous suspension by adhesion.

Under these conditions t h e sol i s p o s i t i v e l y charged and

s t a b l e whereas the glass beads are n e g a t i v e l y charged.

Precautions are taken t h a t

no f i l t r a t i o n takes place i n the course o f the, deposition process.

The subsequent A t pH

removal o f p a r t i c l e s depends on t h e composition o f the r i n s i n g s o l u t i o n . 11.5,

a t which both s o l i d s a r e n e g a t i v e l y charged, r a p i d desorption of t h e

p a r t i c l e s i s observed (Figure 15 l e f t ) .

However, i f the r i n s e s o l u t i o n contains

Ca(N03)2, desorption can be reduced o r completely i n h i b i t e d depending on the conc e n t r a t i o n o f the e l e c t r o l y t e .

The higher t h e charge o f the added cation, the

lower i s the concentration needed t o prevent p a r t i c l e removal (24). The r e s u l t s can be i n t e r p r e t e d i n terms o f t h e t o t a l i n t e r a c t i o n energies.

The

a t t r a c t i v e energy, $A, as a f u n c t i o n of distance (23 was c a l c u l a t e d using the equation:

A 1 3 2 i s the o v e r a l l Hamaker constant f o r t h e system sphere-mediumplate,

which can

be approximated by:

where subscript 1 a p p l i e s t o chromium hydroxide, 2 t o glass, and 3 t o water. The f o r chromium hydroxide was taken as 14.9 kr, A 2 2 f o r glass as 20.9 M y value and

f o r water as 10.3 W . For double l a y e r r e p u l s i v e energy, $R, t h e plate/sphere expression o f Hogg,

Healy and Fuerstenau (25) was taken:

i n which

and $2 are t h e surface p o t e n t i a l s (equated w i t h t h e corresponding

6-potentials),

for t h e p l a t e and t h e p a r t i c l e s , respectively,

constant, and

K

E

i s the dielectric

t h e r e c i p r o c a l Debye-Hickel thickness.

-

The r i g h t s i d e o f Figure 15 gives t h e c a l c u l a t e d t o t a l i n t e r a c t i o n energy curves, $(r)

+

$ R ( ~ ) , as a f u n c t i o n o f distance o f separatiafl f o r the

chromium hydraxide/glass systems i n t h e absence and i n t h e presence of the Same concentrations o f Ca(N03)2 as used i n t h e desorption experiments.

A t the highest

s a l t concentration e s s e n t i a l l y o n l y a t t r a c t i o n p r e v a i l s and indeed no p a r t i c l e removal i s observed.

When no calcium n i t r a t e i s added, a number o f p a r t i c l e s

516

ADS.

pH:5.0

n

0 Y

z \

n

c

Y

z

0

40

00

120

160

200

RINSE SOLUTION (cm3) Fig. 16. Fraction of monodispersed spherical chromium hydroxide particles (modal diameter 280 nm) desorbed from steel on rinsing with an aqueous solution o f fl 11.7. Number of particles originally deposited on 3 g of steel at pH 5.0 was 2.9 x lo9 (0)and 1.8 x 1O1O (A). Circles give the analogous data for desorption of spherical hematite particles (modal diameter 140 nm) from the same steel. The number of originally deposited hematite particles: 3.0 x lo9.

.appears to be at sufficient distance which enables them to overcome the barrier and desorb; however, particles which escaped cannot readsorb due to the high potential barrier. Thus, the double layer theory, albeit in its oversimplified form, explains at least semiquantitatively the adhesion phenomena in the described system. Similar observations were made with chromium hydroxide on steel (Figure 16). The reproducibility o f data is best shown by triangles and squares which represent two separate runs made with beds of steel beads on which the number o f originally adsorbed particles differed by nearly one order of magnitude. In the same diagram is also included the desorption curve of spherical hematite particles from the same steel under otherwise identical conditions. The large difference in the desorption rate is primarily due to the variation in the attractive forces, as determined by the Hamaker constants. This illustrates the sensitivity of the adhesion phenomena to the physical properties of the adsorbent/ adsorbate system.

577

CONCLUDING REMARKS I t was not the i n t e n t i o n o f t h i s review t o g i v e an i n depth analysis o f each

problem discussed.

Readers are r e f e r r e d t o i n d i v i d u a l l y c i t e d r e p o r t s f o r more

d e t a i l e d information o f the experimental techniques, t h e r e s u l t s obtained, and the t h e o r e t i c a l analyses. However, i t i s hoped t h a t the d i f f e r e n t cases described i n t h i s presentation c l e a r l y show the usefulness o f t h e monodispersed metal hydrous oxide systems i n t h e studies o f various i n t e r f a c i a l phenomena. ACKNOWLEDGMENTS This a r t i c l e i s based on work done w i t h t h e support o f t h e NSF grants CHE77 02185 and ENG 75-08403 as w e l l as E P R I c o n t r a c t RP-966-1. The author i s g r e a t l y indebted t o h i s many collaborators, and s p e c i f i c a l l y t o R. Brace, B. Gray, E. Katsanis, J. Kolakowski, H. Kumanomido, R. Kuo, J. Rubio, R. Sapieszko, H. Sasaki, P. Scheiner, W. Scott, T. Sugimoto, and R. Wilhelmy, on

whose c o n t r i b u t i o n s was based t h i s presentation. REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

E. Matijevic!, Progr. C o l l o i d Polymer Sci., 61(1976)24. E. M a t i j e v i d and P. Scheiner, J. C o l l o i d I n t e r f a c e Sci., 63(1978)509. E. M a t i j e v i d , R. Sapieszko and J.B. M e l v i l l e , J. C o l l o i d I n t e r f a c e Sci., 50 (1975)567. R. Brace and E. Matijevic', J. Inorg. Nucl. Chem., 35(1973)3691. W.B. Scott and E. Matijevi(, J. C o l l o i d I n t e r f a c e Sci. ( i n press). R. Demchak and E. M a t i j e v i c , J. C o l l o i d I n t e r f a c e Sci., 31(1969)257. E. Matijevid, A.D. Lindsay, S. K r a t o h v i l , M.E. Jones, R.I. Larson and N.W. Cayey, J. C o l l o i d Inteyface Sci., 36(1971)273. P. McFadyen and E. M a t i j e v i c , J. Inorg. Nucl. Chem., 35(1973)1883. E. Matijevit?, M. Budnik and L. Meites, J. C o l l o i d I n t e r f a c e Sci., 61(1977)302. K.H. Lieser, Angew Chem. , I n t . Ed. , 8(1969)188. A. B e l l and E. M a t i j e v i d , J. Inorg. Nucl. Chem., 37(1975)907. A. B e l l and E. Matijevic', J. Phys. Chem. 78(1974)2621. R.S. Sapieszko, R.C. Patel, and E. M a t i j e v i d , J. Phys. Chem., 81(1977)1061. G.A. Parks, Chem. Rev., 65(1965)177. M. Visca and E. M a t i j e v i d , J. C o l l o i d I n t e r f a c e Sci. ( i n press). T. Sugimoto and E. M a t i j e v i d , J. Inorg. Nucl. Chem. ( i n press). P.H. Tewari and A.B. Campbell, J. C o l l o i d I n t e r f a c e Sci., 55(1976)531. Yu.F. Krupyanskii and I.P. Suzdalev, Sov. ,Phys.-JETP, 38(1974)859. H. Kumanomido, R.C. Patel and E. M a t i j e v i c , J. C o l l o i d I n t e r f a c e Sci. ( i n press). J. Rubio and E. Matijevic', J. C o l l o i d I n t e r f a c e Sci. ( i n press). A. B l e i e r and E. M a t i j e v i d , J. C o l l o i d I n t e r f a c e Sci., 55(1976)510. A. B l e i e r and E. Matijevic!, J . Chem. SOC., Faraday Trans. I, 74(1978)1346. E.J. C l a y f i e l d and E.C. Lumb, ,Disc. Faraday SOC., 42(1966)285. J. Kolakowski and E. M a t i j e v i c , J. Chem. SOC., Faraday Trans. I ( i n press). R. Hogg, T.W. Healy and D.W. Fuerstenau, Trans. Faraday SOC., 62(1966)1638.

DISCUSSION R. POISSON : T h B n e P o u l e n c h a s p a t e n t e d p r o c e s s e s f o r c o n t r o l l i n g

t h e s i z e o f z e o l i t e , for e x a m p l e , b e t w e e n t h e m i c r o n and 10 m i c r o n s

i n c o n c e n t r a t e d medium.

578 My questions

:

1 ) You have been talking about polymer sulfated cations.

they are

How large

What kind of polycondensation they have (Linear

?

?).

In general, how do you see the polycondensation of cations (geometrically speaking).

What is the location of the counter ions

(first or second hydration shell

?),

their degree of freedom compared

to a normal salt.

2) when the polycondensation is conducted at sufficiently high concentration, a slight neutralisation (homogeneous transition from the sol to the gel state.

permits the

!)

Is this compulsory salt

accompanied by a neutralisation of the ions to form an insoluble salt or is this fact independent of the sol or gel state; in other words, can we have polycations in the gel state

?)

E. MATIJEVIC : 1) There is no information available on the distri-

bution of molecular weights or conformation of polynuclear complexes of chromium containing sulfate ions. SOC.

72, 782,

Hall and Eyring ( J . Am. Chem.

1950) have shown that chromium hydroxylated species

undergo continuous polymerization on heating.

Thus, it is not

possible to define a given polymer in solution of such salts aged at elevated temperatures.

In dilute solutions there should be no

considerably greater restriction in the motion of such polymeric ions and their counterions than one would expect in electrolyte solutions of highly charged ionic species. The chemical mechanism involved in the formation of spherical chromium hydroxyde particles has been discussed in greater detail by my associate, J. Phys. Chem.

A.

Bell (J. Inorg. Nucl. Chem.

3,2 6 2 1 ,

31,

909, 1 9 7 5 ;

1974).

2 ) None of the work presented here has yielded gels.

Gel formation

has been studied in great detail, among other by Sing (Brunel University) or Teichner (Lyon).

However, it is indeed possible to

have polycations in the gel state. Re comment

:

My presentation dealt only with metal (hydrous) oxides.

Except

for some earlier work by Heller,who prepared monodispersed 6-FeOOH

sols, little progress has been made in the production of monodispersed metal (hydrous) oxides of well defined morphologies.

In this

respect our work is unique. Monodispersed dispersions of various other materials such as organic latexes, elements (sulfur, selenium, gold, etc.), different inorganic salts, silica, tungstic acid and others have been frequently reported

in the literature. Since zeolites cannot be properly described as metal oxides this family of compounds has not been considered in my lecture.

B. DELMON

:

There are many questions that scientists interested in

catalysis could ask you in relation to possible applications of your preparation techniques of catalytic materials, well defined both in structure and in texture; these questions could especially concern the possibility of making mixed oxides (compound oxides with a given stoichiometry, or solid solutions) or sulphides. But my question is related to the solid state chemistry aspect of the preparation of powders with uniform size.

I agree that the

control of nucleation, and, more precisely, of the number of nuclei, is crucial.

But concerning the second step involved, the growth

of nuclei, one could remark that there is always a factor working against uniform size.

It is the lower stability and growth rate o f

smaller nuclei, as a consequence of the changes of the free energy of the surface with crystallite size.

Larger nuclei have a

tendency

to grow at the expense of the smaller ones, thus progressively

increasing the dispersion in size.

The only way I know to suppress

this tendency and to restore the equality in growth rate is to introduce diffusion limitations in the supply of material with which nuclei are fed. It has been observed, that this kind of limitation plays a crucial role in the growth of X-zeolite crystallites.

In the

latter case, the diffusion limitations can te made to work by dispersing the growing nuclei in a gel. crystallites grow in a liquid.

In your case, the

To what factor do you attribute the

uniformity in size ? E. M A T I J E V I C

:

The procedure described apparently yields nuclei of

uniform size and, consequently, the Ostwald Ripening (the growth of larger nuclei at the expense of the smaller ones) does not take

place. The most important condition for the uniform growth of particles is the proper rate of production of solute species on which nuclei feed.

In the systems illustrated in this work the rate is controlled

by the concentration of the metal salt, pH, temperature, and in some cases, by the concentration of specific anions.

Thus, as long as

the rate of formation of the constituent species which are built onto the particles (rate of crystal growth), is sufficiently high, secondary nucleation will not take place. uniform particle size.

This assures a reasonably

Thus, only one burst of nuclei occurs,

which then grow uniformly. Finally, during the entire growth process particles are charged which prevents their aggregation.

O.P.

KRIVORUCHKO

:

What is the main reason for formation of amorphous

or crystal sols of metal hydrous oxides ? What is the reason for the influence of metal concentration and the anion nature

According to the considerations developed by us,

?

the intermediates at the sol particles formation are the polynuclear hydroxocomplexes of the corresponding metal. The direction of polycondensation of metal aquo-ions and the nature of complexes drastically depend on the total metal concentration in the solution as well as on the nature of the initial salt anion.

Theses circumstances

determine the most important properties of s o l s and gels of hydroxides.

Are the sols formed continuous or through some limited

set of species

?

It was shown experimentally that for our systems the sol particle formation proceeded through a limited set of polymeric hydroxocomplexes.

So for definite conditions, the particles of aluminum

hydrous-oxide sol were formed through consecutive stages of formation of complexes, which contained 2 and 13 A 1 (111) ions.

E. MATIJEVIC

:

It depends on the system and on the conditions

whether the particle formation is a result of continuous polymerization or crystal growth of well defined species.

For example,

in the case of the precipitation of basic ferric sulfates in acidic solutions, only well defined monomeric and dimeric complexes are involved in particle nucleation and growth.

On the other hand,

polymeric hydroxo (and substituted hydroxo) complexes are precursors to chromium hydroxide sol formation.

Thus, it is not possible to

offer a generalized scheme for the preparation of the uniform hydrous oxides.

However, if continuous polymerization takes place,

the resulting particles will most likely be amorphous.

Well

defined solute complexes of low degree of polymerization will yield cry'stalline materials. from case to case.

In some systems the anions (such as sulfate or

phosphate) may promote chromium hydroxide.

The role of the anions will also differ polymerization, as in the case with

In other cases well defined coordinated metal-

anion species (with or without hydroxyl ions) may form (e.g. ferric sulfate solutes).

Obviously in each case the resulting materials

581 will differ for reasons given above. O.P. KRIVORUCHKO

:

What do you think about the mechanism of transfor-

mation of amorphous aggregates of primary particles into the crystals

?

In the mechanism of formation and growth of the metal

hydroxide crystals which you discuss,it is supposed that primary amorphous particles are dissolved.

We showed that the transition

of hardly solved hydroxides from the amorphous to the

crystalline

phase is the result of the crystallization in the bulk of the primary particles rather than dissolution.

After the achievement

of the determined degree of crystallinity, formation and growth of the secondary crystals by means of oriented growth of one type of primary particles to another one at the same facet become possible. E. MATIJEVIC

:

I never stated that amorphous particles must dissolve

in order for crystals to form.

on the contrary, we showed in the

case of spherical amorphous aluminium hydroxide particles, that removal of sulfate ions and subsequent heating of the sol, resulted in direct crystallization of the particles into boehmite (Brace and Matijevic, J. Inorg. Nucl. Chem.

15, 3691,

1973).

Obviously,

removing the sulfate ions was essential to crystal formation. V. FENELONOV

:

Your lecture is very interesting for catalyst pre-

paration but the sizes of the particles which you have shown to us on the slides are very large.

If I try to use your sols for

support preparation, I would get a support which surface area would be only a few square meter per gram.

The sols with particle size

D

of about 30-40 A are more useful and important for catalyst preparation.

Therefore, I am interested to know whether it should be

possible to use your conclusions for the preparation and the examination of small particle systems ?

Are you sure that there

is no difference in the genesis of small and large sized particles E. MATIJEVIC

:

We have tried to grow larger particles because they

find various applications, such as pigments or corrosion product models.

Furthermore, particles of spherical shape can be analyzed

for size distribution in situ by light scattering, as long as they behave as Mie scatterers. In some instances we prepared much smaller particles, such as spherical hematite sols.

It is quite conceivable that some of the

?

582

dispersions could be obtained in particles sizes as small as a few nanometers, whereas in some other cases this may not be possible. One important parameter in this respect would be the initial concentration of metal salt solution.

Futhermore, heterogeneous nucleation could

be employed to control the particle size.

K.S.W.

SING

1 ) Can you remove sulphate ions from the spherical

:

particles of amorphous alumina without changing the particle shape ? It is possible to remove sulphate from the surface layer (eq. by ion exchange or restructuring the surface) without changing the bulk composition ? 2 )

Does the electrophoretic mobility curve for

pure aluminium hydroxide

(or oxide-hydroxide) depend on the chemical

or physical structure of the surface ?

E. MATIJEVIC : 1 .

I t is quite possible to remove all of sulfate ions

from spherical colloidal amorphous aluminium hydroxide without changing the particle shape (Brace and Matijevic, J. Inorg. Nucl. Chem.

31, 3 6 9 1

(1973).

It is conceivable that incomplete washing

would remove the surface sulfate groups, while leaving some of these anions in the bulk of the particle.

However, it is quite likely

that a redistribution of the sulfate ions throughout the particle would take place with time. 2. Different physical structures of pure aluminium hydroxide could have some effect on the electrokinetic

properties of these systems.

However, I believe that the average values of mobilities are not sufficiently sensitive to demonstrate such differences.

The electro-

kinetic properties are determined by the charged surface groups; in the case of aluminium hydroxide these are-AlO- and-AlOH;.

Even

highly charged colloidal particles carry relatively few charges (e.9. o n e charge per 500-1000 A L ) .

This low density of charge

groups would make the effects of structure differences difficult to detect.

We have shown, for example, that pure aluminium hydroxide

spherical particles, and latex particles coated with a monolayer of aluminium hydroxide showed essentially id-entical electrokinetic behaviour as a function of pH.

L. R I E K E R T :

What is the space-time yield for the preparation of

monodisperse alumina ? In other words how many kg can be made per hour and m3 of reaction volume ?

E. MATIJEVIC case.

:

The yield on monodispersed s o l s varies from case to

Some systems have been prepared only in very small quantities

in dilute dispersion.

Specifically in the case of monodispersed

alumina, our procedure was somewhat modified at Unilever Research Laboratories at Port Sunlight, England (Dr. D. Barby), so that kilogram quantities cou'ld be prepared in concentrated slurries over short periods of time (hours). M.V. TWIGG

:

An important difference between systems containing

chromium(II1) and aluminium(JII),

iron(III), etc. is that ligand

substitution reactions of chromium(II1) are relatively very slow. Is this kinetic inertness reflected in the times needed for the preparation of your products ? E. MATIJEVIC

:

I t is not possible to relate times necessary to

generate monodispersed hydrous oxides of different metals to their liquid substitution reactions.

A

more important parameter seems

to be the temperature; the higher the aging temperature, the shorter is the reaction time.

However, one cannot expect that

monodispersed sols will form at any temperature.

To obtain uniform

particles, the particle growth must be faster than particle nucleation (after the initial burst of nuclei). temperature polydisperse.

If above a given

secondary nucleation takes place, the s o l s will become


585

PROCESS FOR THE PRODUCTION OF SPHERICAL CATALYST SUPPORTS

CAHEN; J.M.

R.M.

Labofina S.A.,

ANDRE; H.R.

DEBUS*

chaussee de Vilvorde 98-100,

B-1120 B r u x e l l e s

SUMMARY S p h e r i c a l alumina c a t a l y s t s u p p o r t s a r e produced by t h i s p r o c e s s i n which a m i x t u r e of an aluminum hydroxide hydrosol and o r g a n i c monomers i s i n t r o d u c e d dropw i s e , through c a l i b r a t e d o r i f i c e s , i n t o a column of h e a t e d o i l .

I n t h e c o u r s e of

t h i s p r o c e d u r e , each drop forms a s p h e r e ; t h e monomers c o n t a i n e d t h e r e i n polymerise and g i v e s u f f i c i e n t r i g i d i t y t o t h e s p h e r e s so t h a t t h e y may b e handled w i t h e a s e during t h e ensuing steps.

Excess w a t e r and t h e o r g a n i c polymer a r e e l i m i n a t e d d u r i n g

t h e f i n a l c a l c i n a t i o n s t a g e l e a v i n g v e r y h a r d alumina s p h e r e s , t h e p u r i t y of which l i e s above 99 %. a r e described.

The p r o p e r t i e s of t h e s t a r t i n g m i x t u r e and of t h e o b t a i n e d s p h e r e s Examples of c a t a l y s t s made from t h e s e s u p p o r t s a r e given and t h e i r

d e s u l f u r i z a t i o n a c t i v i t y i s compared t o t h a t of commercial c a t a l y s t s .

INTRODUCTION The shapes and s i z e s of t h e c a t a l y s t s u s u a l l y loaded i n f i x e d bed r e a c t o r s a r e :

-

p e l l e t s o r c y l i n d e r s (diameter 3-7 mm; l e n g t h 3-7 nun)

- extrudates -

( d i a m e t e r 1.5-5 mm; l e n g t h 2-8 mm)

s p h e r e s ( d i a m e t e r 1.5-5 mm). P e l l e t s a r e produced by t a b l e t t i n g ; t h e i r p r i c e i s r a t h e r h i g h a s each c y l i n d e r

i s formed i n d i v i d u a l l y i n a s e t of punches and d i e s and because t o o l wear i s important.

The p a r t i c l e s produced i n t h i s way have e x c e l l e n t mechanical p r o p e r t i e s .

The major p a r t of f i x e d bed c a t a l y s t s a r e i n t h e form of e x t r u d a t e s . manufacturing c o s t s a r e low due t o t h e g r e a t c a p a c i t y of t h e e x t r u d e r s . d i s a d v a n t a g e of t h e s e c a t a l y s t s i s t h e i r moderate mechanical s t r e n g t h .

Their The main Neither

p e l l e t s n o r e x t r u d a t e s can be used i n moving beds. Alumina s p h e r e s , which a r e mainly used i n c a t a l y t i c reforming a s c a r r i e r s f o r noble m e t a l s such a s platinum have t h e double advantage of h i g h mechanical s t r e n g t h *This work was c a r r i e d o u t under t h e d i r e c t i o n of t h e l a t e D r . Henri R. Debus; t h e p r e s e n t paper i s d e d i c a t e d t o h i s memory.

586 ( l i t t l e l o s s e s of p r e c i o u s m e t a l s due t o a t t r i t i o n and b r e a k a g e ) and i d e a l r e a c t o r packing (good c o n t a c t between c a t a l y s t and r e a g e n t s ) . S p h e r o n i z i n g p r o c e s s e s may be d i v i d e d i n t o methods which r e l y upon t h e b u i l d - u p of s m a l l e r p a r t i c l e s i n t o s p h e r e s by means of a r o l l i n g , o r "snow-ball" t e c h n i q u e , and t h o s e which form s p h e r e s by i n d i v i d u a l l y s h a p i n g p a r t i c l e s .

The p r o d u c t i o n c o s t s

a r e low f o r t h e f i r s t t e c h n i q u e , however t h e s p h e r e s a r e i r r e g u l a r and t h e i r p a r t i c l e s i z e i s randomly d i s t r i b u t e d .

The s p h e r e s manufactured by i n d i v i d u a l s h a p i n g

a r e more e x p e n s i v e , however t h e i r shape and c a l i b r a t i o n a r e very r e g u l a r .

In

g e n e r a l , c a t a l y s t s of t h i s t y p e a r e produced by so c a l l e d " o i l - d r o p " i . e . by suspending d r o p s of an aqueous gel-forming l i q u i d i n a w a t e r i n m i s c i b l e l i q u i d such a s m i n e r a l o i l so a s t o form s p h e r i c a l d r o p s . a b o u t by a change of pH.

The c o a g u l a t i o n of t h e g e l i s b r o u g h t

The s p h e r e s a r e t h e n aged f o r a s u f f i c i e n t t i m e s o t h a t

t h e d r o p s can be withdrawn, washed, d r i e d and c a l c i n e d .

DESCRIPTION OF THE PROCESS The p r o c e s s developed by Labofina and p a t e n t e d i n a number of c o u n t r i e s b e l o n g s t o t h e " o i l - d r o p " group; i t s o r i g i n a l i t y l i e s i n t h e i n c o r p o r a t i o n of a n o r g a n i c monomer i n t o an aluminum h y d r o s o l which i s then i n t r o d u c e d dropwise, through c a l i b r a t e d o r i f i c e s , i n t o a column o f h e a t e d o i l . forms a s p h e r e .

Each d r o p of o i l - i n s o l u b l e m i x t u r e

Under t h e i n f l u e n c e of t h e h e a t e d o i l , t h e p o l y m e r i s a t i o n of t h e

monomer i s i n i t i a t e d .

G e l a t i o n by m o d i f i c a t i o n of t h e pH i s n o t n e c e s s a r y a s t h e

s p h e r e s m a i n t a i n t h e i r form through t h e e f f e c t of t h e polymer.

The r e s u l t i n g

s p h e r e s can be d i r e c t l y u n l o a d e d , washed, d r i e d and c a l c i n e d w i t h o u t f u r t h e r a g e i n g . The o r g a n i c m a t t e r i s c o m p l e t e l y removed d u r i n g t h e c a l c i n a t i o n s t e p .

A simplified

flow-sheet i s shown i n f i g u r e 1. F i g u r e 1 shows t h a t t h e r e a c t i o n m i x t u r e i s c o n s t i t u t e d of an i n o r g a n i c p a r t (alumina p r e c u r s o r s ) and an o r g a n i c p a r t (monomer, p o l y m e r i s a t i o n i n i t i a t o r s and reticulation agents). I t i s beyond t h e scope of t h i s paper t o g i v e t e c h n o l o g i c a l d e t a i l s such a s t h e

n a t u r e of t h e o i l , d i a m e t e r and l e n g t h of t h e c a l i b r a t e d o r i f i c e s , l e n g t h of t h e o i l column which a r e key f a c t o r s , b u t which may v a r y from c a s e t o c a s e and a r e a d a p t e d t o the individual circumstances.

I t i s however i m p o r t a n t t o g i v e more d e t a i l s

c o n c e r n i n g t h e components of t h e r e a c t i o n m i x t u r e , which c o n s t i t u t e s t h e e s s e n t i a l f e a t u r e of t h e p r o c e s s .

A s t h e alumina s p h e r e s w i l l be used a s c a t a l y s t s u p p o r t s , a number of c o n d i t i o n s must be r e s p e c t e d , so t h a t t h e f i n a l p r o d u c t w i l l have t h e d e s i r e d p r o p e r t i e s of p u r i t y , s t r u c t u r e and porous t e x t u r e .

YJF HYDROXIOE

l

MIXTURE VESSEL

CALIBRATED ORIFICES

WASHING HEATED OIL

DRYlNG

I CALCINATION

3 or 8 A1203 SPHERES Fig. 1

Simplified flow-sheet

The inorganic compounds The aluminas most commonly used as catalysts and catalyst supports are of the h or y form.

The preparation and characterization of "active aluminas" has been

extensively described by Lippens and Steggerda (ref. 1).

Such aluminas may be

obtained by low temperature calcination (300-6OO0C) of gelatinous boehmite(pseud0boehmite).

This is the starting material used in the process described here.

The first difficulty encountered was the necessity to incorporate a gel into a reaction mixture which must remain sufficiently fluid so as to pass the calibrated orifices and take the form of droplets. A suitably fluid mixture is obtained when an alkaline monomer is used; in this case the pH is adjusted to a sufficiently high value by means of ammonia.

The alumina spheres made in alkaline medium have

very interesting properties; however acid type monomers give a product which is by far superior.

The introduction of an acid monomer into the reaction mixture consisting of a fluid suspension of pseudoboehmite in water brings about its immediate gelification into a stiff unusable paste. This difficulty has been overcome by the addition of highly acid products to the reaction mixture

so

as to form a hydrous alumina sol. The choice of the acid

constituents is however critical. Suitable hydrosols are produced by dissolving aluminum metal in aluminum trichloride solution, or by adding aluminum salt solutions, nitric acid or nitric and perchloric acid mixtures to the pseudoboehmite gel. The second problem is to avoid inorganic constituents that remain in the alumina after calcination and that may impair the activity of the finished catalyst.

Impu-

rities may also inhibit the polymerisation of the monomer so that the spheres leaving the oil column will collapse. For these reasons, polymerisation initiators must be carefully chosen (this will be the subject of the next chapter) and, at this stage of the process, it is not always possible to introduce any type of catalytically active compound at any concentration. It should however be stressed that fluid reaction mixtures containing variable concentrations of catalyst precursors have been made and have successfully been "oil-dropped"; among the compounds that have been added to the pseudoboehmite the following can be mentioned : salts of nickel, molybdenum, chromium, iron, cobalt, zinc, silicium, titanium, germanium, lanthanum, platinum, palladium, rhodium, gold; this list is not exhaustive. Each addition to the initial reaction mixture is a particular case and the technology must be adapted. Catalyst precursors can more easily be introduced in the subsequent steps of the process; as the spheres are very firm and cohesive in the wet stage after leaving the hot oil column, they can be readily handled, washed, impregnated, transported to dryers and ovens.

The organic components and polymerisation initiators The process is based on the incorporation of an easily polymerisable organic monomer to the aluminum hydrosol; this implies that the monomer as well as the polymer must be water soluble. Good results have been obtained with the principal monomers that comply with this requirement. For economical reasons it is preferable to use the cheapest monomers available because the polymer is finally burnt off during the calcination step. Furthermore, no solid residue must be left by the polymer after its decomposition. These reasons have led to the choice of acrylic acid and acrylamide; more sophisticated monomers, such as N-hydroxymethylacrylamide and N-hydroxymethylmethacrylamide for example, have been used to confirm the results. The polymerisation of the monomer within the alumina hydrosol droplet is not

689 s u f f i c i e n t t o make a r i g i d s p h e r e a t t h e o u t l e t of t h e o i l column.

Two f a c t o r s , one

n a t u r a l , t h e o t h e r a r t i f i c i a l , c o n t r i b u t e t o b r i n g about t h e r e q u i r e d r i g i d i t y ,

The

f i r s t one i s due t o t h e tendency of t h e aluminum hydroxide t o bond w i t h t h e polymer c h a i n ; under c e r t a i n c o n d i t i o n s , very s m a l l amounts of monomer, a s low a s 0.5 % w t . based on A1203,

a r e r e q u i r e d t o c r e a t e t h e t r i d i m e n s i o n a l network r e s p o n s i b l e f o r

t h e good c o h e s i o n of t h e s p h e r e s .

T h i s can a l s o be brought a b o u t by adding a

c e r t a i n amount of r e t i c u l a t i o n a g e n t s t o t h e monomer.

An amount of 5 % (based on

monomer) of dihydroxyethylene-glycol-bis-acrylamide i s s u f f i c i e n t t o r e t i c u l a t e t h e s t r a i g h t - c h a i n polymers.

This product

- made

from a c r y l a m i d e and e t h y l e n e g l y c o l

-

c a n under c e r t a i n c o n d i t i o n s be made i n s i t u by a d d i n g b o t h components d i r e c t l y t o t h e a c r y l i c a c i d monomer. F i n a l l y , i n o r d e r t o have an e f f i c i e n t l y working system, i t i s n e c e s s a r y t h a t a l l of t h e monomer c o n t a i n e d i n e a c h d r o p l e t of r e a c t i n g m i x t u r e p o l y m e r i s e s d u r i n g t h e span of time i t d r o p s down t h e column of o i l .

The u s u a l p o l y m e r i s a t i o n i n i t i a -

t o r s f o r t h e s e t y p e s of monomers a r e o x i d a t i o n - r e d u c t i o n c o u p l e s ,

A s already stated

above, c a r e must b e t a k e n i n c h o s i n g t h e components t h a t a r e added t o t h e alumina, so a s t o f i n i s h w i t h a v e r y pure p r o d u c t .

A s t h e q u a n t i t y of p o l y m e r i s a t i o n i n i -

t i a t o r s a r e s m a l l , f o r c e r t a i n alumina g r a d e s i t i s p o s s i b l e t o u s e c o u p l e s t h a t l e a v e some r e s i d u e on c a l c i n a t i o n , such a s p e r s u l f a t e s - b i s u l f i t e s .

I f the purity

r e q u i r e m e n t s f o r t h e alumina s p h e r e s a r e v e r y s t r i n g e n t , o x i d a t i o n - r e d u c t i o n c o u p l e s such a s hydrogen peroxide-hydroquinone o r h y d r a z i n e a r e p r e f e r r e d .

For c e r t a i n

p a r t i c u l a r r e a c t i o n m i x t u r e s , t h e w e l l known i n h i b i t i n g e f f e c t of oxygen on polymer i s a t i o n h a s been a p p l i e d t o monitor t h e r a t e o f t h i s r e a c t i o n t o a c e r t a i n degree.

PROPERTIES OF THE ALUMINA SPHERES The f l o w - s h e e t r e p r e s e n t e d i n f i g . 1 shows t h a t d r y i n g and c a l c i n a t i o n s t e p s are required,

The w e t s p h e r e s c o n t a i n l a r g e amounts of w a t e r and some polymer

which must be e l i m i n a t e d .

The pseudoboehmite must f u r t h e r m o r e be transformed t o

" a c t i v e alumina", h o r Y A1203.

During t h i s f i n a l phase of t h e p r o c e s s , t h e s p h e r e s

s h r i n k i n s i z e and g a i n h a r d n e s s which i s b r o u g h t about by t h e development of t h e A 1 0 s t r u c t u r e . The range of p h y s i c a l p r o p e r t i e s of t y p i c a l alumina s p h e r e s 2 3 produced by t h i s p r o c e s s a r e g i v e n i n t a b l e 1. I t i s p o s s i b l e t o widen somewhat t h e range of t h e p r o p e r t i e s r e p o r t e d i n t a b l e

1; however t h i s w i l l have an i n f l u e n c e on s e v e r a l p r o p e r t i e s s i m u l t a n e o u s l y .

Thus

f o r example, t h e l o w e r i n g of a p p a r e n t b u l k d e n s i t y i n e v i t a b l y w i l l b r i n g about some

loss of mechanical s t r e n g t h , The alumina s p h e r e s d e s c r i b e d i n t a b l e 1 a r e comparable and v e r y o f t e n s u p e r i o r t o commercially a v a i l a b l e o n e s ,

They a r e q u i t e e q u i v a l e n t by t h e i r p u r i t y , s i z e ,

590 d e n s i t y , s u r f a c e a r e a and pore volume; t h e y a r e s u p e r i o r by t h e i r c r u s h i n g s t r e n g t h and low a t t r i t i o n .

T y p i c a l o i l - d r o p p e d commercial alumina s p h e r e s have c r u s h i n g

s t r e n g t h s of 3-4 kgf and a t t r i t i o n s around 0 . 5 % w t . ; compared t o e x t r u d a t e s and t o t h e more i r r e g u l a r alumina s p h e r e s made by r o l l i n g t e c h n i q u e s , t h e s u p e r i o r i t y of t h e s p h e r e s d e s c r i b e d h e r e i s even more s t r i k i n g ,

TABLE 1 P h y s i c a l p r o p e r t i e s of t h e alumina s p h e r e s Properties Alumina t y p e P u r i t y , Yo w t . A1203 D i a m e t e r , nun Apparent b u l k d e n s i t y , g/ml Crushing strength i n d i v i d u a l , kgf bulk , kglcm2 Attrition losses, % w t . S p e c i f i c s u r f a c e a r e a , m2/g Pore volume, d / g

Range

Typical ( a )

99 -99.9 1-3 0.6-0.8

99.9 1.8 0.705

7-10 2040 < 1 200-300 0.4-0.7

8.3 40 0.03 224 0.55

h

'

( a ) Sample produced from commercial type pseudoboehmite and a c r y l i c a c i d i n aqueous s o l u t i o n of n i t r i c and p e r c h l o r i c a c i d s ; hydrogen p e r o x i d e / h y d r o quinone were used a s p o l y m e r i s a t i o n a c t i v a t o r s .

TESTING OF CATALYSTS MADE FROM ALUMINA SPHERES I n t h e p r e c e d i n g c h a p t e r t h e alumina s p h e r e s were shown t o have t h e p h y s i c a l p r o p e r t i e s r e q u i r e d f o r a good c a t a l y s t s u p p o r t .

The s p h e r e s were t h e r e f o r e

impregnated w i t h c o b a l t and molybdenum s a l t s ; t h e c a t a l y s t o b t a i n e d was used t o d e s u l f u r i z e g a s o i l and was compared i n i d e n t i c a l t e s t s t o t h e most r e c e n t commercial hydrodesulfurization catalysts. F i g u r e 2 r e p r e s e n t s t h e r e s u l t s of t h e s e t e s t s which were c a r r i e d o u t on a 60 m l c a t a l y s t r e a c t o r a t 350°C, 40 kg/cm2 p r e s s u r e , hydrogen t o f e e d r a t i o of 150 1 (NTP)/1 and l i q u i d h o u r l y space v e l o c i t i e s (LHSV) comprised between 2 and 6 ; LHSV i s d e f i n e d a s volume of f e e d per volume c a t a l y s t and hour.

The g a s o i l

f e e d had a b o i l i n g range of 200-350°C and a s u l f u r c o n t e n t of 1 . 3 % w t .

I t can

be s e e n t h a t t h e a c t i v i t y of t h e Labofina sample compares w e l l w i t h t h a t of commercial c a t a l y s t s , a s b o t h commercial samples can b e r a t e d among t h e b e s t ones available. The p h y s i c a l p r o p e r t i e s of t h e c a t a l y s t s u p p o r t a r e n o t s o i m p o r t a n t f o r naphtha and g a s o i l d e s u l f u r i z a t i o n a s compared t o s u l f u r removal from p e t r o l e u m r e s i d u e s ; i n t h i s c a s e p o r e - s i z e d i s t r i b u t i o n p l a y s a major r o l e ( r e f . 2 ) .

The

591 efficiency of the spherical alumina support has also been tested for this application. A cobalt-molybdenum oxides catalyst has been prepared by impregnation of a spherical alumina support with the metal salts. Catalyst properties are reported in table 2 .

95

90 LABOFINA

85 COM. 2

80 2

3

4

5

6

L.H.S.V ( h - 1 ) 2

Gasoil hydrodesulfurization

TABLE 2 Properties of residue desulfurization catalyst Moog, YD wt.

coo,

% wt. Si02, % wt.

15.2 2.8 1.4

A1203

type h

Surface area, m2/g Apparent bulk density, g/ml Total pore volume, ml/g Volume of pores smaller than 100 A diameter, ml/g

2 00 0.768 0.56 0.42

592 T h i s c a t a l y s t h a s been i n c l u d e d i n a t e s t i n g p r o j e c t aimed a t t h e s e l e c t i o n of s u i t a b l e c o m e r c i a l c a t a l y s t s f o r r e s i d u e d e s u l f u r i z a t i o n .

Each t e s t l a s t e d

two weeks and covered a c t i v i t y a s w e l l a s s e l e c t i v i t y w i t h r e s p e c t t o m e t a l s removal, g a s o i l p r o d u c t i o n , l o w e r i n g of t h e v i s c o s i t y and s p e c i f i c g r a v i t y of t h e 350'C

+

product,

The f e e d s t o c k used was 'Kuwait a t m o s p h e r i c r e s i d u e (350°C

+

f r a c t i o n ) c o n t a i n i n g 4 . 1 5 % s u l f u r , 59 ppm vanadium and 1 7 . 5 ppm n i c k e l . The c a t a l y s t s (300 ml) were f i r s t p r e s u l f i d e d and t h e n t e s t e d a t t h e f o l l o w i n g conditions : Temperature

: 400°C

Pressure

: 100 kglcm2

H2/feed

: 1000 1 (NTP)/1

LHSV

: v a r i a b l e between 0 . 3 and 1 . 5

To compare t h e performances of t h e d i f f e r e n t samples, a given c a t a l y s t h a s

been a r b i t r a r i l y chosen a s r e f e r e n c e and i t s r e l a t i v e v o l u m e t r i c a c t i v i t y (RVA)

set a t

RVA

=

00; RVA i s d e f i n e d by t h e f o l l o w i n g r e l a t i o n s h i p :

100 x

[

LHSV unknown LHSV r e f e r e n c e

]

a t 1 % s u l f u r r e m a i n i n g i n t h e 350°C

Tab e 3 summarizes t h e r e s u l t s o b t a i n e d . activity.

+

product.

High v a l u e s f o r RVA mean b e t t e r

The m e t a l s removal a c t i v i t y i s g i v e n f o r c o n s t a n t s u l f u r c o n v e r s i o n .

High v a l u e s mean s h o r t c a t a l y s t l i f e expectancy.

The s u l f u r removal a c t i v i t y of

t h e c a t a l y s t made from t h e s p h e r i c a l alumina s u p p o r t i s e q u i v a l e n t t o t h a t of t h e b e s t commercial samples t e s t e d ; i t s m e t a l removal s e l e c t i v i t y needs however t o b e improved.

TABLE 3 D e s u l f u r i z a t i o n and m e t a l s removal of Kuwait a t m o s p h e r i c r e s i d u e Catalyst

RVA

% m e t a l s removed a t 75 %

su 1f u r remova 1

Comnercial 1 ( R e f e r e n c e ) Commercial 2 Commercial 3 Commercial 4 Commercial 5 Commercial 6 Commercial 7 Commercial 8 Labofina 101864

100 142 99 171 125 60 215 149 175

Nickel

Vanadium

48 58 41 39 52 59 66 65 52

58 72 59 50 64 69 77 78 59

593

CONCLUSIONS The production of spherical alumina catalyst supports according to the new technique described in this paper has a number of definite technological and economical advantages. The range of finished products can be quite large. The alumina spheres have physical properties that meet the very stringent specifications required for the production of catalysts. Performance tests of hydrodesulfurization catalyst samples made from these supports showed that they ranked among the best commercial ones.

ACKNOWLEDGMENT This work was subsidized by the "Institut pour 1'Encouragement de la Recherche Scientifique dans 1 'Industrie et 1 'Agriculture (I .R.S.I.A.) I t .

REFERENCES

1. B.C. Lippens and J.J. Steggerda, in B.G. Linsen (Ed.), Physical and Chemical Aspects of Adsorbents and Catalysts, Academic Press, London and New York 1970, ch. 4 . 2 . R.L. Richardson and S.K. Alley, in J.W. Ward (Ed.) and S.A. Qader (Ed.), Hydrocracking and Hydrotreating, ACS Symposium Series 20, American Chemical Society, Washington DC, 1975, ch. 9. DISCUSSION R. POISSON

Why do you say 17 or y A1203 ?

:

I thought that every-

body agreed that boehmite and crystalline boehmite (pseudoboehmite) conduct to the y phase and not to 17 alumina ? R.M. CAHEN : The authors cited in reference ( 1 ) of our paper indicate that pseudo-boehmite".

..

decomposes at about 3OOOC into

an alumina for which a doubling of the 1 . 9 8 no longer observed"...

and 1 . 4

t

diffraction

Many authors refer to this form as 0

17-alumina.

However, as the 4.6

A

band is extremely broad and does

not show any sign of a sharp peak, we prefer to call it a y-alumina

..."

In accordance with the above authors' definition of y-alumina, we can state that this is the product we obtain; as however, the same form is called 17-alumina by other, we gave both designations in our paper. E.R. BECKER

:

Does the .porosity or pore size distribution vary as

a function of particle radius in your supports prepared by the methods you described R.M. CAHEN

:

?

Pore-size distribution has only been measured on average

samples of the alumina spheres; we do not therefore know if there is a variation in function of particle radius. P.R. COURTY

:

Can you give information on the method you have used

for the determination of the attrition properties of your alumina carrier ( 0 . 0 3 % attrition).

A SOCONY test measurement would be

helpful for this type of carrier, because of its great severity.

R.M. CAHEN

:

Attrition is measured by tumbling a weighed sample of

spheres in a tube which turns around an axis normal to its length for a given amount of time.

The quantity of powder collected is

weighed and attrition is reported as percentage powder found per initial weight.

A more severe test method, as suggested by Courty,

would be helpful in this case, in view of the strength of the particles.

V. FATTORE

:

Can you control your process in order to obtain

particles with lower apparent bulk density, for instance 0.4-0.5 g/cc? Going up with the dimensions of the particles can you still produce a product with almost spherical shape ?

R.M.

CAHEN

:

1. Spherical particles of low apparent bulk density

have been produced, however at the cost of some mechanical strength. 2.

We have not tried to increase particle diameter above 2 mm; the

spherical shape of the 2 mm particles has been maintained.

595

METHODS OF SATURATION WITH ALKALI IONS.

INFLUENCE OF THE PROPERTIES OF OXIDES

R. HOMBEK, J. KIJENSKI and S. MALINOWSKI Institute of Organic Chemistry and Technology Technical University (Politechnika), Warsaw (Poland)

INTRODUCTION

Many industrial catalysts, especially for dehydrogenation and cracking processes, are modified by the introduction of alkali metal ions.

The problem

of the influence of alkali ions on the properties of oxide catalysts has been investigated by us (ref. 1-5) since several years.

Numerous papers concerning

this subject have been published by other authors (ref. 6-15). Alumina is the most thoroughly investigated catalyst from this point of view.

Parera and

Figoli (ref. 6 ) and Scharme (ref. 7) reported that the addition of sodium to alumina involved a decrease of surface acidity.

Chuang and cowork. (ref. 8 )

and Bremer and cowork.(ref. 9) have found using IR spectroscopy that sodium ions react with the most acidic surface OH groups only.

According to Pines

(ref. 10) it is possible that sodium hydroxide undergoes a reaction with Lewis acid centres :

- A1 + NaOH

+

-

A1-OH

-

+

Na

On the other hand our recent results (ref. 2) and those obtained by other authors (ref. 12) support the supposition that after impregnation with small amounts of alkali hydroxide the acidity of the surface increases, so the studied process is much more complicated than one might assume.

The common method of

introducing alkali ions onto oxide surfaces lies in the saturation of the solid oxide with an alkali hydroxide aqueous solution. In the present work the influence of various alkali ions, introduced from ethanolic solutions of corresponding alkoxides, on the physicochemical properties of alumina was investigated. Our new impregnation procedure permits avoiding the reaction of water with dehydrated alumina surface.

Water being a

source of considerable changes in the properties of catalysts prepared by the hydroxide addition method.

We have taken into consideration the higher reacti-

vity of the alkoxides than that of the respective hydroxides, assuming that the reaction with the A1203 surface will be more effective and cover a greater number of surface sites.

596

EXPERIMENTAL Alumina has been obtained by precipitation of A1(0Hl3 with water from a benzene solution Al(iso-C3H7)3. The hydroxide was then washed with distilled water and dried at 12OoC for 24 hr.

The resulting preparation was calcined

at 550 or 75OoC for 24 hr in a stream of dry and oxygen-free nitrogen. Lithium, sodium, potassium and caesium ethoxides were obtained by dissolving the corresponding alkali metals in absolute alcohol at room temperature. Alkali cations were introduced onto the A1203 surface during adsorption of alkoxides from absolute alcohol solution. Impregnation procedure was as follows

:

alumina calcined at 550 or 750°C was cooled down to room temperature

and was soaked in a known volume of corresponding alkali metal alkoxide. After 20 hr the catalyst was filtered-off, washed with absolute alcohol and then

calcined at 100, 150, 2 0 0 , 220, 250, 350 and 55OOC in a stream of dry and deoxidized nitrogen for 3 hr. The number of alkali cations deposited on the A1 0 2 3 surface was determined by titration of the filtered-off alkoxide excess with a hydrochloric acid solution in the presence of phenolphtalein.

The titer of

the alkoxide used for the impregnation was determined by the same method. Surface basicity and acidity of the catalysts were determined by the titration method with benzoic acid or n-butylamine using a series of Harnmett indicators (ref. 14,151.

The specific surfaces areas of the catalysts were mea-

sured by the BET method. Hydroxyl group concentrations were determined by titration with sodium naphthenide (ref. 16) and by chromatography using the reaction with Zn(CH3)2. 2THF (ref. 17). One-electron donor and one-electron acceptor properties were determined on the basis of adsorption on the catalyst surface of appropriate acceptor and donor and by recording the signals of newly-formed anion- or cation-radicals using ESR spectroscopy (ref. 18,19).

Perylene was used as the electron donor

and tetracyanoethylene was used as the electron acceptor. Quantitative results were obtained by comparing the signal intensity of the investigated sample with that of the standard

- DPPH solution in NaC1.

Adsorption of perylene was

carried out in the presence of molecular oxygen. ESR measurements were performed on an X-band spectrometer (modulation 100 kHz) at room temperature. RESULTS Basic and acidic properties The values of maximum basic strength (H of catalysts under study are -max For comparison the values of basic strength of sodium

given in Table 1.

hydroxide doped alumina were investigated in the same conditions.

597 TABLE 1. Basic and acidic strength (H and H ) of alkali alkoxides doped alumina -max omax

Alkali oation

Na+ a

Caloination em*~rature H--HemaxH-( c)

18.4 % 18.4 ig.4 a t8.4 v 22.3 22.3 -3.0 24.6 -3.0 24.6 -3.0

20

100 150

200 220

250 350 550

Li+

Na'

Ti+

CS+

I3omarI-max HomaxH-max HomaxH-maxHomax

17.2 18.4 17.2 18.4 "0. $8.4 22.3 3 18.4 V 26.5 V 22.3 26.5 2.8 22.3 -3.0 26.5 -5.6 24.6 -5.6 26.5 -5.6 26.5 -5.6 26.5 -5.6

18.4 22.3 26.5 26.5 26.4 26.5 26.5 26.5

18.4 22.3 v 26.5 2.8 26.5 -3.0 26.5 -5.6 26.5 -5.6 26.5 -5.6 26.5

2v

2.8 -3.0 -5.6 -5.6 -5.6

The basic strength of catalysts doped with alkoxides and sodium hydroxide increases with the rise of calcination temperature after alumina impregnation. H-max of preparations containing alkali alkoxide is in all cases smaller or equal to H- of pure alumina. Caesium ethoxide doped alumina reaches the basic strength 26.5> SBET for the desorption branch but not for the adsorption branch, then this may well point to severe irreversibility of the desorption branch, and thus to bottle-neck effects. If the slit-pore model is adopted, then only the desorption branch can be used for the analysis on the basis of E q . (19) with k = 1, which mathematically should be about equal to cum SBET or Sw, depending on where the downward deviation in the t-plot of the adsorption branch is noted. Use of the cylinder pore model in these cases

very much simplifies the analysis. In this case

S

should lead to Scum > SBET or Sw. If part of the desorption takes place in the region 0.42 < p /p < 0.5, then g o the analysis is of restricted applicability only. 9. Finally, a balanced judgement should be made on the basis of all the information acquired during the analysis described, leading to a qualified picture of the pore structure of the analysed sample, bearing in mind the approximate nature of the analysis method itself. CONCLUSIONS

Mesopore Structure determination from nitrogen sorption isotherms, is based upon a series of simplifications of reality. It is inevitable, to adopt simplified geometric models for the pore shape. Pore interconnectivity has largely to be ignored. It is difficult to account for irreversibility of sorption in the hysteresis region in a rigorous manner. The relations governing capillary condensation as well as capillary desorption are complicated functions of capillary effects, sorption effects and pore shape effect, and again can only be formulated quantitatively on the basis of simplifying assumptions, An elaborate analysis of the sorption isotherm in favourable cases can lead to a block of reasonably consistent information, leading to a semi-quantified picture of the mesopore structure. The method is hardly suitable for standardisation and routine analysis. If one is not primarily interested in the physical significance of the data obtained, as e.g. for comparative purposes in production qualirj control, any convenient simplified routine for analysis can be adopted, dictated by considerations of convenience.

682

REFERENCES 1

2 3 4 5 6 7 8 9 10

S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, London 1967, Chapter 8. B.V. Deryagin, Acta Physicochim U.R.S.S. 12, (1940) 139; Proc. Intern. Congress Surface Activity, 2 nd, London 1957, Vol 11, 112. F.A. Dullien, in S. Modry (Ed.), Proc. Int. Symp. on Pore Structure and Properties of Materials, Prague 1973, Vo l. I, C 173. J.H. de Boer, in Surface Area Determination, D.H. Everett and Ottewill (Eds.), Butterworth & Co, Ltd., London 1970. L. Gurvitch, J. Phys. Chem. SOC. Russ. 47, (1915) 805. H.W. Haynes, Thesis Univ. of Colorado, Boulder, Colo., 1969. S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, London 1967, Chapter 3. J.C.P. Broekhoff, Thesis Delft 1969, Ch. I. A.V. Kiselev, The Structure and Properties of Porous Materials, D.H. Everett and F.S. Stone gds.), Butterworth, London, 1957, p. 130. S. Brunauer, R.Sh. Mikhail and E.E. Bodor, J. Colloid Interface Sci., 25, (1967) 353.

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

B.V. Deryagin, J. Colloid Interface Sci. 54, (1977) 157. J.H. de Boer, B.G. Linsen and Th.J. Osinga, J . Catalysis 4, (1965) 643. J.C.P. Broekhoff and J.H. de Boer, J. Catalysis 9, (1967) 15. J.C.P. Broekhoff and B.G. Linsen, Physical and Chemical Aspects of Adsorbents and Catalysts, B.G. Linden (Ed.), Academic Press, London 1970, Ch. I. J . Frenkel, Kinetic Theory of Liquids, Oxford 1946. T.L. Hill, J. Phys. Colloid Chem. 54, (1950) 1186. J.C.P. Broekhoff and J.H. de Boer, J. Catalysis, 9, (1967) 8; 10, (1968) 391. A. Lecloux, in S. Modry (Ed.), Proc. Int. Symp. on Pore Structure and Properties of Materials, Prague 1973, Vol. 4, p. C 43. D.H. Everett, in E.A. Flood (Ed.), The Solid-Gas Interface, Marcel Dekker, New York 1967, V o l . 11, 1055. D.H. Everett and F.S. Smith, Trans. Fara. S O C . 50, (1954) 187, D.H. Everett, Trans. Fara. SOC. 50, (1954) 1077. J.C.P. Broekhoff, L.F. Brown and W.P. van Beek, in S. Modry (Ed.), Proc. Iupac-Rilem Int. Symp. Pore Struct. and Prop. Mat., Prague 1973, Part IV, C 85. J.C.P. Broekhoff and W.P. van Beek, submitted to Trans. Fara. SOC. D.H. Everett and J.M. Haynes, J . Colloid Interface Sci. 38, (1972) 125. 0. Ksenzhek, Russ. J. Phys. Chem. 37, (1963) 691. D.H. Everett, The Structure and Properties of Porous Materials, D.H. Everett and F.S. Stone (Eds.), Butterworth, London 1958, p. 118. R.M. Harris and G. Whitaker, J . Appl. Chem. 13, (1963) 349. C.G.V. Burgess and D.H. Everett, J. Colloid Interface Sci. 33, (1970) 6 I . R.G. Avery and J.D.F. Ramsay, J. Colloid Interface Sci. 42, (1963) 597. E.P. Barret, L.G. Joyner and P.H. Halenda, J . Amer. Chem. SOC. 73, (195 ) 373. A. Wheeler, Catalysis, Vol. II., Rheinhold, New York 1955, p. 118. W.B. Innes, Anal. Chem. 29, (1957) 1069. D.R. Dollimore and G.R. Heal, in S. Modry (Ed.) Proc. Intern. Syrnp. on ore Structure and Properties of Materials, Prague 1973, Vol. I, A-73. D.C. Havard and R. Wilson, J. Colloid Interface Sci. 57, (1976) 276. A. Lecloux, in Surface Area Determination, D.H. Everett and Ottewill (Eds.), Butterworth & Co, London 1970. B.C. Lippens and J.H. de Boer, J. Catalysis 4, (1965) 319. J.H. de Boer, in D.H. Everett and F . S . Stone (Eds.), The Structure and Properties of Porous Materials, Butterworth, London 1958, p. 68.

683 DISCUSSION K.S.W.

S I N G : I n t h i s excellent s u r v e y of t h e p r o b l e m s involved in

t h e d e t e r m i n a t i o n o f m e s o p o r e s i z e d i s t r i b u t i o n , y o u h a v e pointed o u t that it i s important t o t a k e i n t o account t h e e f f e c t o f any m i c r o p o r e f i l l i n g c o n t r i b u t i o n o n t h e s h a p e o f t h e n i t r o g e n absorpt i o n isotherm.

It is u s e f u l t o r e c a l l t h a t t h e f o l l o w i n g classifi-

c a t i o n o f p o r e s i z e w a s p u t f o r w a r d b y t h e IUPAC i n 1972: microp o r e s , o f w i d t h l e s s t h a n about 2 nm; m e s o p o r e s , o f w i d t h b e t w e e n Q

2 nm and

Q

50 nm; m a c r o p o r e s of w i d t h g r e a t e r t h a n

Q

50 nm.

One

m a y r e g a r d m i c r o p o r e filling a s a p r i m a r y s t a g e o f p h y s i s o r p t i o n , i.e.

taking p l a c e o v e r t h e s m a l l " m o n o l a y e r " r a n g e o f t h e adsorp-

t i o n i s o t h e r m , and capillary c o n d e n s a t i o n as a s e c o n d a r y p r o c e s s , w h i c h o c c u r s o n l y at h i g h e r r e l a t i v e p r e s s u r e and a f t e r t h e format i o n o f a n a d s o r b e d l a y e r o n t h e p o r e walls. I n p r a c t i c e , m i c r o p o r e filling i s u s u a l l y a s s o c i a t e d w i t h t h e a p p e a r a n c e o f a r e v e r s i b l e T y p e I i s o t h e r m , w h e r e a s c a p i l l a r y cond e n s a t i o n i s o b s e r v e d a s a T y p e I V i s o t h e r m , w h i c h g e n e r a l l y exhib i t s a h y s t e r e s i s loop.

You have drawn attention to the difficulty

c r e a t e d w h e n t h e s t e e p part o f t h e d e s o r p t i o n b r a n c h i s located at t h e c h a r a c t e r i s t i c m i n i m u m r e l a t i v e pressure.

I would a g r e e t h a t

in t h i s c a s e t h e u s u a l a n a l y s i s o f t h e d e s o r p t i o n b r a n c h provides a m i s l e a d i n g p i c t u r e o f t h e p o r e s i z e distribution. this behaviour

(i.e.

I n our v i e w ,

t h e t y p e E h y s t e r e s i s loop) i s i n d i c a t i v e of a

w i d e d i s t r i b u t i o n o f p o r e s i z e , w h i c h e x t e n d s from t h e m i c r o p o r e into t h e m e s o p o r e range.

U n f o r t u n a t e l y , it i s t h e n i m p o s s i b l e t o

o b t a i n a u n i q u e s o l u t i o n for t h e p o r e s i z e d i s t r i b u t i o n f r o m a s i n g l e a d s o r p t i o n isotherm and a r a n g e o f a d s o r p t i o n s m u s t b e used.

T o identify t h e s e and other p r o b l e m s in t h e i n t e r p r e t a t i o n of p h y s i s o r p t i o n i s o t h e r m s , it i s s t r o n g l y r e c o m m e n d e d t h a t t h e adsorpt i o n isotherm d a t a should a l w a y s b e g i v e n in any p u b l i c a t i o n d e a l i n g w i t h r e f e r e n c e m a t e r i a l s , i.e.

J.C.P.

s t a n d a r d c a t a l y s t s or adsorbents.

B R O E K H O F F : T h a n k y o u for y o u r v a l u a b l e c o n t r i b u t i o n t o t h e

discussion.

N. B A E R N S

:

I n v a r i o u s c a s e s it is n e c e s s a r y t o u s e low t e m p e r a t u r e

n i t r o g e n a d s o r p t i o n as well as m e r c u r y p o r o s i m e t r y t o o b t a i n the p o r e s i z e d i s t r i b u t i o n of a p o r o u s m a t e r i a l .

W h e n t h e p o r e sizes

r a n g e from 1.5 to 1 0 0 0 nm, c a n you m a k e any r e c o m m e n d a t i o n w i t h r e s p e c t t o t h e m e t h o d o f combining t h e t w o m e t h o d s ?

Should one u s e

684

a) the adsorption or desorption branchs of the nitrogen isotherms or b) the penetration or retraction data of the mercury porosimetry respectively 7 J.C.P.

BROEKHOFF

:

Calculated pore size distributionsfrom nitrogen

sorption as well as mercury penetration are seldom identical to the "true" pore size distributions,but are nearly always deformed. This deformation is dependent on the method of determination, so it will not always be possible to join smoothlypore size distributions from mercury penetration to those from nitrogen sorption. no simple solution for this problem.

There is

For A and E type isotherms an

analysis of the adsorption branch of the nitrogen isotherm is probably to be preferred, but compatibility with mercury penetration is o n l y to be expected in exceptional cases (e.g. for smooth cylinders

open at both ends).

685

METAL SURFACE AREA AND METAL DISPERSION IN CATALYSTS J.J.F.

SCHOLTEN

Central Laboratory, DSM, P.O. Box 18, Geleen, The Netherlands and Department of Chemical Technology, Delft University of Technology, Julianalaan 136, Delft-2208, The Netherlands

ABSTRACT A survey is presented of a number of methods for the determination of freemetal surface areas. The following metals are dealt with: iron, copper, silver, gold, nickel, cobalt, palladium and platinum. Alloy catalysts are not considered. Recommendations are made with respect to the choice of the adsorbate, the measuring technique and data interpretation.

1. INTRODUCTION Besides the well known methods developed for the determination of the total surface area and the pore volume distribution of heterogeneous catalysts and other porous technical materials, special procedures are devised for measuring the extent of the metallic part of the surface ("free-metal surface area").

In

industrial catalysis research these methods play a role in the following areas:

-

In the development of new metal-on-carrier catalysts. In most (not in all) cases, our final goal is to reach a dispersion as high

as possible, and this dispersion depends i.a. on the metal-load, the time and temperature of reduction, the type of carrier material and its texture. Determination

of the free-metal surface area, after and in between various preparation steps, has proved to provide a good guide for optimalization of the catalyst preparation method.

-

In laboratory process development work. It is often very instructive to measure the free-metal surface area of a

catalyst before. during and after a laboratory test run. Sometimes it appears that the decline of catalytic activity with time is related to the decline of

686 the free-metal surface area with time, and this might be caused by poisoning or by sintering of the metal crystallites. By combining the measurements with the determination of the mean metal particle size from X-ray line broadening, poisoning effects can be separated from sintering effects. Of course, the change of the total surface area and the pore size distribution of the carrier have also to be taken into account.

-

In industrial practice Here, determinition of the free-metal surface area plays a role in trouble

shooting. Furthermore, it is a good precaution to control, by free-metal surface area determinations, the quality and reproducibility of new catalyst batches, before loading the reactor.

In this article we first present a short survey of various receipts recommended in the literature, without striving for completeness. After that, the various pitfalls encountered in practice will be reviewed. Finally a number of recommendations will be made with respect to the choice of the adsorbate, the measuring technique and data interpretation. One general statement, however, has to be made already at the very start of this survey: determination of the free-metal surface area is only one aspect of a more full characterization of the catalyst and of its metallic part of the surface. The catalytic behaviour of the metallic part of the surface depends on at least three variables, viz. the texture, the structure and the surface composition. Some methods available to study these aspects are shortly summarized in table 1. TABLE 1 ~~~

~~~

Survey of some methods necessary for a full characterization of a metal-on-carrier catalyst, besides free-metal surface area determination. TEXTURE

BET method. Total accesible surface area. Hg penetration. Pore size distribution. Capillary condensation. Pore size distribution from Kelvin law. EM. Electron microscopy for i.a. structural details. X ray line broadening. Mean metal particle size. SAXS. Small angle X ray scattering. Pore size distribution and metal particle size distribution.

STRUCTURE

X ray diffraction. Bulk crystallographic structure. applicable to technical samples.

LEED. Surface crystallography. Not SURFACE ANALYSIS XPS AES

-

Qualitative and semi-quantitative elemental analysis. of the surface. Analysis 'in depth'.

An introductory treatment of these techniques may, for instance, be found in Anderson's book, 'Structure of Metallic Catalysts' (1).

687 2. METHODS

2.1

FOR FREE-METAL SURFACE AREA DETERMINATION

Iron To our best knowledge, Emmett and Brunauer were the pioneers in the field. In

1937 they published a paper (2) in which for the first time a very elegant method was introduced for measuring the free-iron surface area of industrial ammonia synthesis catalysts. It has not yet losed its value, and is still daily practice in many laboratories (Fig. 1). Volume CO adsorbed

I

(cm3 NTP/g) 30

20

I/

I

lo

I

2o

I

I

30

40

--c CO

pressure (kPa)

Fig. 1. Adsorption of carbon monoxide on a singly promoted ammonia synthesis catalyst. From lit. 3 . A. Total adsorption at 90 K. C. Volume CO chemisorbed. B. Physical adsorption, after evacuation at 195 K. First carbon monoxide is adsorbed at 90 K; at this temperature the gas is adsorbed both physically and chemically. At 195 K the physisorbed part is pumped away very easily. When, after pumping at 195 K, a second isotherm is measured, this isotherm proves to run at a constant distanceunder the first. After repeated pumping at 195 K this second isotherm is reproducible; it obviously represents the physical adsorbed carbon monoxide. From the distance between the two isotherms the volume of chemically chemisorbed carbon monoxide may be derived, and this monolayer volume has to be "translated" in the free-iron surface area. The pumping time chosen for desorption of the physisorbed part may, however, influence the final result. This is due to the fact that on pumping at 195 K also a small part of the weakly chemisorbed carbon monoxide is removed (3). This difficulty may be circumvented by subtracting the physical nitrogen adsorption isotherm from the total carbon monoxide adsorption isotherm A, instead of subtracting the poorly defined physical CO adsorption isotherm B ( 4 ) . In this case we have to multiply the adsorbed quantities of nitrogen by a factor of 1.05, as appears from the almost constant ratio of the physical adsorption of nitrogen and carbon monoxide

688

of 1.05 on different inert adsorbents ( 3 ) . The factor 1.05 corresponds with the difference of 5 % in the surface area of a nitrogen and a carbon monoxide molecule. The "translation" of the extent of the amount of carbon monoxide, chemisorbed in a monolayer, in a free-metal surface area, is hampered by a number of still unresolved fundamental questions, and this difficulty is encountered in all other free-metal surface area determinations as well. First, the distribution of the various crystallographic planes at the iron syrface is unknown, and different 2 For alpha-iron planes contain different numbers of iron atoms per cm , n Fe' instance: (111) face: (110) face: (100) face:

15 = 0.71 x 10 15 nFe = 1.7 x 10 15 %e = 1.2 x 10

%e

Second, the number of carbon monoxide molecules that can be chemisorbed per iron atom on the various planes (the surface stoichiometry) is unknown. Various binding forms of carbon monoxide have to be considered, and the ratio of these binding forms may be a function of the iron crystallite diameter. The best way of circumventing these difficulties would be a calibration of the method by using pure iron samples, the BET surface area of which is determined beforehand from the physical adsorption of methane or nitrogen. For iron crystallites larger than 100 nm in diameter, both Brunauer and Emmett (5) and Westerik and Zwietering (6) found that the ratio of the volume of CO chemisorbed in a monolayer to the volume of nitrogen physisorbed in a monolayer is about 1.20 to 1.25. A reastablishment of this ratio for various iron samples, applying modern ulta high vacuum techniques would be highly desirable. 2.2 Copper The first publication dealing with the determination of the free-copper surface area of technical catalysts was by Emmett and Skau (7); exactly the same method was applied as advocated for iron samples. Owing to the low heat of carbon monoxide chemisorption on copper (ca. 71.2 kJ/mole at low coverage) it is difficult to differentiate between physically and chemically bonded gas. Therefore, the separate determination of the nitrogen adsorption isotherm (the trick also applied in the iron case) is here an absolute necessity. In many cases the metallic part of the catalyst surface is small as compared to the total surface area. Than the above method fails, as the physical isotherm B for these samples lies very close to the total adsorption isotherm A (see Fig. l),

so that the result is given by the difference between two large numbers and, in consequence, is not very accurate. An alternative method is the adsorptive decomposition of nitrous oxide on the metallic part of the catalyst (8):

689

From a calorimetric determination of the reaction enthalpy, Dell, Stone and Tilly (9) found that, for various values of the oxygen coverage, this enthalpy is equal to half the heat of chemisorption of oxygen plus the heat of decomposition of nitrous oxide, which amounts to -83.74 kJ/mole N20. From this it follows that the oxygen atoms are adsorbed on the same sites and in the same form from nitrous oxide as from pure oxygen, Hence there is thermodynamic correspondence between the two surface reactions; their kinetics are different, however. The lower reactivity in both surface and bulk reactions of nitrous oxide as compared with that of oxygen (9, 10, 11) is related, we believe, to the difference in electronic structure between the two molecules. The linear nitrous oxide molecule, being stabilized by resonance, has the lower reactivity, whereas oxygen is the more reactive molecule, owing to its pseudo-radical character, caused by the presence of two unpaired electrons in different p-levels.

As no pressure change is involved in reaction (l), progress of the reaction has to be measured from the nitrogen enrichment as a function of time, either mass-spectrometrically or by freezing out the nitrous oxide at regular time intervals (12). Oxygen chemisorption cannot be used, notwithstanding the lower heat of reaction; the rate of gas uptake is relatively insensitive to temperature and pressure (Rhodin (13) ), and extremely fast, even at very low temperature. The applicability of nitrous oxide is based on the activated nature of the process, the activation energy being a function of coverage and increasing with coverage (8). This can be seen from Fig. 2, in which results are plotted for a copper-on-magnesium oxide catalyst, used for the dehydrogenation of cyclohexanol. Figure 2 shows that the higher the temperature chosen, the higher the coverage reached, in accordance with the activation energy increasing with coverage. Finally, around 363 K, full coverage is reached. From experiments with pure copper samples, the BET surface area of which was known, it was found that one oxygen atom is adsorbed per two copper atoms at full coverage, taking the number of surface copper 15 2 atoms 1.7 x 10 /cm

.

Care should be taken that no break-through of oxygen occurs (oxidation of the sub-surface layers), as observed at 393 K and at 413 K (see Fig. 2). Therefore it is safe to chose 293 K as the temperature of reaction (12), taking into account that no full coverage is reached in that case. The amount of N 0 required to cover 2 2 of copper surface area is 0.176 2 0.010 cm3 (NTP) at 293 K.

1m

A method of determining the free-copper surface area from nitrous oxide

adsorption-decomposition at 293 K by means of a chromatographic technique is described by DvorAk and Pagek (14).

690

0 atoms

6

5

4 3

2

1

Fig. 2. Adsorptive decomposition of nitrous oxide as a function of time, on a copper-on-magnesium oxide catalyst, at various temperatures. Nitrous oxide pressure: 26.6 kPa (lit. 8 ) .

2 . 3 Silver

Silver is an important catalyst f o r the epoxidation of ethylene and it may also be applied for oxidizing methanol to formaldehyde. Just like with copper, the free-metal surface area can be determined from the adsorption-decomposition of nitrous oxide (15). Full oxygen coverage is arrived at around 443 K, and no break-through difficulties like with copper occur. As may be seen from Figure 3 the course of the reaction is analogous to that on copper, but in a higher temperature range. Above 443 K, correction of the total amount

of nitrogen evolved is necessary, due to the start of the quasi-homogeneous decomposition of nitrous oxide, according to:

It is likely that this reaction is catalyzed by the surface silver oxide.

691

Q atoms

x 10'~~

silver (g)

5b

160

150

reaction time (hr)

Fig. 3. Oxygen chemisorption via nitrous oxide adsorption-decomposition on a silver on alpha-alumina catalyst at various temperatures as a function of time of reaction. Initial nitrous oxide pressure 19.95 kPa. The numbers in the figure refer to the reaction temperature in K. Line 5 indicates full coverage adsorption, after correction for reaction (2). Like with copper, the increase of coverage with temperature is due to the increase of activation energy with coverage. Contrary to copper, adsorption of oxygen can very well be applied for measuring free-silver surface areas. In accordance with Kholyavenko's results (17), we found that the chemisorption isotherms at 373, 432 and 443 K coincide; the adsorption at 13.6 kPa oxygen pressure corresponds with full coverage at these temperatures (16). Interestingly, chemisorption at still higher temperature (473 K) corresponds with lower coverage, as indicated in Fig. 4. In Fig. 5 the amount of nitrogen evolved per gram of silver during decompositionadsorption of nitrous oxide at 423 K is plotted as a function of the BET surface area. The measurements were performed on four silver powders of high purity (15).

It can be concluded from this figure that 0.4 cm3 of evolved nitrogen corresponds to one square meter of silver. This result can be used as a basis for determining the free-silver surface area of silver-on-carrier catalysts.

692

-19 number of oxygen atoms x 10

silver surface area (mA)

24

373, 423 and 443 K

-oxygen

pressure (kPa)

Fig. 4. Oxygen chemisorption on silver powder (lit. 16). x: 323 K A : 373 K a: 423 K 0: 443 K

473 K

0 :

It is important to note that chemisorption of oxygen leads to a result deviating from the nitrous oxide findings; more oxygen is taken up starting from O2 (see 3 Fig. 5 ) . Whereas on exposure to nitrous oxide a maximum oxygen coverage of 0.2 cm 3

per square meter was arrived at, exposure to oxygen gave 0.27 cm

per square meter.

It is known that oxygen is adsorbed on silver in both atoaic and molecular forms (17), and that it is the molecular form to which the epoxidation activity is to be described. Nitrous oxide, however, gives rise to only the atomic species and, indeed, a very low epoxidation activity is found with this gas (18). From the difference of 0.07 cm3 oxygen per square meter silver it follows that, starting from oxygen gas, about 2 5 % is in the molecular form. 2.4

Gold Sometimes gold is used as alloying component in group VIII catalysts. It is

noteworthy to mention the work by Schrader (19) in which chemisorption of oxygen was observed on (111)-oriented gold films in the temperature range of 473 to 873 K. Complete receipts for measuring free-gold surface areas are not worked out. In view of the low sintering temperature of gold, careful1 surface oxidation via adsorptive decomposition of nitrous oxide at not too high a temperature might be a good proposal for further research. 2.5 Nickel Probably the best method to measure the free-nickel surface area of dispersed nickel specimens is hydrogen chemisorption in the temperature range 273 and at pressures up to about 10

- 2 0 kPa

-

300

K

(1). An adsorption isotherm, measured

by Sinfelt, Taylor and Yates (20) is presented in Fig. 6, and a result for hydrogen

693 on cobalt is included. chemisorbed 02 (cm3 (NTP))/g

chemisorbed O2 (cm3 (NTP))/g A g

from 0

2

0.100

- 0.100

0.015

- 0.075

0.050

. 0.050

0.025

. 0.025

0

-

0.1

0.3 0.4 0.5 2 BET surface area (m /g)

0.2

Fig. 5. Oxygen chemisorption from N 0 adsorption-decomposition, and from 0 2 2 chemisorption, on four pure silver samples, as a function of the BET surface area. 0: oxygen results nitrous oxide results.

.:

Above 5 kPa the adsorbed volume increases very little with pressure. Backextrapolation of the isotherms in this pressure region to zero pressure, in order to find the extent of the chemisorbed monolayer, is perhaps a good standard procedure. Assuming 1.54 x 1015 nickel surface atoms per square centimeter, a surface stoichiometry of about one is found in that case. volume adsorbed

I

I

10

I

20

1

30

Pressure (kPa)

Fig. 6. Adsorption isotherms for hydrogen on silica-supported nickel and cobalt at 291 K. The samples are prereduced in flowing hydrogen, to 643 K. Evacuation at the same temperature (lit. 20).

Isosteric heats of adsorption on the three most densely packed nickel planes were calculated from isotherms by Christmann, Schober, Ertl and Neumann (21). Only small differences were found between the (111), (110) and (100) planes, the heat of chemisorption amounting to 94.2 kJ/mole and remaining constant up to about 0 = 0 . 5 . At higher coverages this heat levels of to about 67 kJ/mole at full coverage, due to increasing mutual repulsion of the hydrogen atoms with increasing coverage. It is interesting to compare the above results with those of Sweet and Rideal (22) for polycrystalline nickel films. They arrive at practically the same result, but at coverages below about 0.03 the isosteric heat of adsorption increases up to 132 kJ/mole. These high heats may be ascribed to chemisorption on edges and corners of the crystallites. At very high coverage a weak type of hydrogen chemisorption is detected (29.3 kJ/ mole).

A s the full-coverage definition in Sweet and Rideal's work was fully

arbitrary, it might be that we are dealing here with a separate type of weakly chemisorbed hydrogen in excess of the strongly held monolayer. Such weakly bound "hydrogen in excess", the existence of which is very well described by Lynch and Flanagan (23) for the case of palladium, might play an important role in the catalytic action of nickel, and deserves separate detection. Benndorf and Thieme (24), in a TPD study of hydrogen desorption from polycrystalline nickel, detected a small first order desorption peak at 120 K, corresponding with a heat of adsorption of about 25 kJ/mole. This peak was described by them as being due to physical hydrogen adsorption, but in our view the heat of adsorption would be 11 kJ/mole at the utmost in that case. A further study of this question is highly desirable. In principle carbon monoxide can serve as the adsorbate gas for the determination of nickel surface areas, but the adsorption behaviour is much more complicated as found for hydrogen: at least four states of CO are detected (25). Carbon monoxide can react to Ni(C0)4,

especially when small nickel crystallites are present. For

instance, nickel carbonyl is readily formed .by passing carbon monoxide at a pressure of 101 kPa over finely divided nickel at 350-370 K (1). Of course freenickel surface area determinations are strongly disturbed by this side-reaction. 2.6 Palladium Hydrogen may be used as the adsorbent, but conditions have to be avoided where hydrogen absorption into the metal occurs to an undesirable extent. At 343 K and a hydrogen pressure of 133 Pa the equilibrium concentration of absorbed hydrogen does n o t exceed about 0.2 atom %, and these are the conditions used by Aben ( 2 6 ) for surface area measurements on a series of palladium/alumina, palladium/silica and palladium black specimens. Of course the seriousness of hydrogen absorption in relation to the monolayer uptake is dependent on the palladium dispersion.

For the absorption component not to exceed (say) 10 % of the monolayer uptake, we need the ratio of the total number of surface palladium atoms to the total number

of palladium atoms present to be higher than 0.02. Aben's samples were mostly reduced in hydrogen at 670 K followed by evacuation (16 h) at this temperature. It was shown that 670 K was the minimum temperature at which removal of the hydrogen was sufficiently complete, about 3 % of the surface remaining covered. This residual hydrogen can be readily removed by evacuation at 850 K, but only at the expense of significant sintering. These conclusions are in agreement with those of Konvalinka and Scholten (27) who measured the TPD spectrum of hydrogen desorbing from 9 . 5 wt% Pd-on-activated carbon; indeed the last traces of hydrogen desorb above 670 K, the remaining coverage being only a few % of total coverage (see Fig. 6).

1 (arbitrary units)

1

Fig. 6. TPD spectrum for hydrogen on palladium (9.4 wt% Pd-on-activated carbon) Heating rate 10 K/min. Peak maxima at 293, 407, 643 and 780 K. A: weakly chemisorbed hydrogen ("hydrogen in excess") B: normally dissociatively chemisorbed hydrogen. Besides sintering and the dissolution of hydrogen in the bulk, the possibility of chemisorption of weakly bound "hydrogen in excess" has to be taken into account (23). A method to separate this weakly bound hydrogen from the normal dissociatively chemisorbed hydrogen was introduced by Scholten and Konvalinka (27). (For platinum this procedure was described earlier by Free1 (33) ) . Hydrogen is pulsewise adsorbed from a hydrogedargon mixture at 233 K and a partial hydrogen pressure of 0.3 kPa, and the hydrogen uptake is plotted as a function of the number of pulses (Fig. 7). In the range where the chemisorbed monolayer is formed, all hydrogen is withdrawn from the argon. After a certain number of pulses less hydrogen is withdrawn and than we enter the region of weak hydrogen chemisorption and of palladium hydride

696 formation. By the extrapolation method indicated in Fig. 7, the kinkpoint is found which corresponds with the hydrogen monolayer.

Hydrogen uptake (cm3 H /g cat.) 1'34 2

1

-Number

of pulses

Fig. 7 . Pulse-wise hydrogen chemisorption on 9.4 wt% Pd-on-activated carbon. The hydrogen uptake is catharometrically detected.

In the calculation of the free-palladium surface area from hydrogen chemisorption up to the kinkpoint it was assumed that, according to Sundquist (281, 7 0 % (1111, 25 % (100) and 5 % (110) planes are exposed by the palladium crystallites, from which a mean surface concentration of 1.45 x lo1'

Pd sites/m2 is calculated.

Furthermore, Aben's (26) surface stoichiometry of one hydrogen per surface palladium atom was accepted. In this way a surface area of 6.78 m2 Pd/g of catalyst was arrived at. A carbon monoxide chemisorption isotherm at 293 K was measured with a parallel

sample from the same batch which underwent exactly the same pretreatment procedure. This method is described by Scholten and van Montfoort (29), and is based on a calibration with pure palladium samples, the BET surface area of which is known. A free-palladium surface area of 7.5 m2 Pd/g was arrived at, a value much higher

than found from the hydrogen pulse method. If, however, in this last method the distribution of exposed planes is taken to be 33.3 % (111), 33.3 % (110) and 33.3 % (110), the result is 8.19 m

2

Pd/g, which is in much better agreement with

the CO result. Anderson (1) rightly states that the carbon monoxide method has the difficulty that the chemisorption stoichiometry is variable because the proportion of chemisorbed species in the linear and bridged forms can vary, the former offering a chemisorption stoichiometry of one, the latter of two. Because the linear and bridged forms are bound to the surface with different energies and because their

697 chemisorption stoichiometries are different, the relative proportions of the two forms are temperature and pressure dependent. Furthermore, the proportions appear also to depend on the metal particle size (1).

In view of the above, we agree with Anderson (1) that when dealing with transition metals which readily chemisorb hydrogen dissociatively, this should be the first choice for surface area measurement, provided complications due to hydrogen absorption are absent or can be eliminated. We now return to Aben's method (26), where hydrogen chemisorption is used to determine the surface areas of supported samples. The samples were pretreated in hydrogen (101 kPa) and then in vacuo at 673 K. The combined amount of adsorbed and absorbed hydrogen was then determined under chosen conditions (343 K, 0.133 kPa). This was corrected for the hydrogen uptake of the support, measured from a blank run, and also for the amount of absorbed hydrogen (H/W = approx. 0.002) using absorption data for palladium foil. Aben's technique is not suitable for high surface area palladium blacks, because the conditions of pretreatment and measurement of hydrogen adsorption would result in a substantial loss in surface area from sample sintering. Then Sermon's technique 130) may be applied, in which the samples are contacted with hydrogen at room temperature, measuring both the amount of hydrogen chemisorbed and the amount of water generated from the reduction of surface oxygens

.

2 . 7 Platinum

Numerous publications are devoted to the problem of the determination of freeplatinum surface areas. This is caused by the fact that especially alumina-supported catalysts are frequently used in industrial processes, involving hydrogenation, dehydrogenation, oxidation and naphta reforming. A good survey of methods published up to 1974 is presented by Anderson (1). He concludes that in general hydrogen chemisorption provides the best method, but optimum conditions for various systems need to be established experimentally. When it can be done without an unreasonably large contribution from adsorption on the support, the measurement of the surface area of dispersed platinum by hydrogen chemisorption is preferably done at 273-300 K and at pressures up to about 0.2 kPa. Further Anderson believes that it is currently best to work on the basis of one hydrogen atom adsorbed per surface platinum atom for the entire platinum size range, with this assumption being regarded as tentative for sizes less than about 1.0 nm diameter. The separation of the strong rapid chemisorption from the weak slow chemisorption is one of the main problems. We recently tried to solve this problem by applying the hydrogen pulse method (271, mentioned already in the discussion of palladium. A 1 % Pt-0.08 % Zn-on-silica catalyst, prereduced in hydrogen at 773 K and treated in vacuo at the same temperature, was pulse-loaded with hydrogen in argon at 233 K. All further conditions were equal to those mentioned in lit. 27. Results are given

698 in Fig. 8.

3 cm H2

1

g cat.

8

j / i

monolayer

5

10 -number

15

20

of pulses

Fig. 8. Pulse-loading of 1 % Pt-on-silica catalyst at 233 K. Unpublished results by Konvalinka and Scholten. The platinum was alloyed with 0.08 % Zn.

The first eight pulses are rapidly and totally adsorbed, but at the ninth pulse less hydrogen is taken up, and here we enter the region of weak adsorption. By the extrapolation method indicated in the figure a standard method is introduced to find a measure for the extent of the monolayer. Of course this extrapolation method is an arbitrary one, but more information is gained on the extent and types of weak and strong chemisorption when the pulse run is immediately followed by a TPD run. The TPD diagram for the 1 % Pt-0.08 % Zn-on-silica catalyst is plotted in Fig. 9. The first desorption trace (top of the peak at 295 K) corresponds to the desorption of weakly chemisorbed hydrogen and Amenomiya (31)).

( Y - and

( p -hydrogen,

in the terminology of Cvetanovic

At least two forms of strongly chemisorbed hydrogen are found

6 -hydrogen, according to lit. 31); top of the peaks at 460 and 580 K.

Above 640 K there is still a substantial amount of desorbing hydrogen. This is partly hydrogen desorbing from platinum, as follows from separate runs with platinum black as the adsorbent

( E

-hydrogen, top of peak at 720 K ) .

.There are, however, strong indications that above 640 K hydrogen not only desorbs from platinum but also from the silica carrier (spilled-over hydrogen). That spillover occurs indeed follows from the fact that the integrated amount of desorbed hydrogen was only 65 % of the amount pulse-loaded on the catalyst. The foregoing illustrates the limited significance of measuring the free-metal surface area only, especially when trying to correlate

this quantity with the

activity of the catalyst. This also follows from the work by Aben, van der Eijk and Oelderik (32), who demonstrated that in the hydrogenation of benzene over

699

alumina-supported-platinum it is likely that only

p -hydrogen is involved in the

hydrogenation, the benzene hydrogenation rate being a linear function of the population of the

p

-peaks.

conc. desorbing hydrogen (arbitrary units)

I

1

300

1

I

400

500

I

600

-Temperature

I

1

700

800

(K)

Fig. 9. Temperature programed desorption of hydrogen from a 1 % Pt-0.08 % Zn-onsilica catalyst. Heating rate 10 K/min. Apparatus and method are described in lit. 27. Various other methods for the determination of free-platinum surface areas are available. Carbon monoxide chemisorption is compared with hydrogen chemisorption by J. Free1 (33). The ratio between the number of carbon monoxide molecules and hydrogen atoms taken up at full coverage is about 0.87, but for platinum dispersions with H/Pt

>

0.25 much lower values are found. Therefore, as argued alrady in the

discussion of palladium, hydrogen chemisorption must be recommended as the best determinant of platinum surface areas. Hydrogen-oxygen titration, the reaction between a monolayer of chemisorbed oxygen and gaseous hydrogen, is very popular as an alternative method for the estimation of surface area in dispersed platinum samples (see for instance lit. 33). This method will not be fully discussed in this paper, and only some small remarks will be made.

From the stoichiometry:

+ H20

(3)

it follows that the sensitivity of the method is three times as high as for hydrogen chemisorption. This increase in sensitivity can even as well be attained by taking a sample three times as large, and applying normal hydrogen chemisorption, and in doing so the complications of the titration method are avoided. It was pointed out by various authors that the results of the titration method are strongly influenced by pretreatment. Samples given a "mild" pretreatment in hydrogen have to be distinguished from those given "prolonged" pretreatment in hydrogen (33) (34) (35). This phenomenon might be related to the fact that alumina in the immediate surrounding of the platinum crystallite is partly reduced during "prolonged" pretreatment, whereas the heat of solution of aluminum in platinum is gained, and a Pt/Al-alloy is formed: platinum

+

Y -alumina

+

hydrogen

-

less Y -alumina

+

water

+

Pt/Al-alloy

As a result, more oxygen is taken up as corresponds to the free-platinum surface area due to the re-oxidation of the aluminum, and hence the stoichiometry of the titration method is out of balance. Another interpretation is possible as well. During "prolonged" pretreatment oxygen-deficient alumina patches are formed to which the platinum crystallites become strongly bound:

Pt

+ Y

-A1 0 2 3

+

x H2

Pt/A1203-x + x H20

(4)

Then, more oxygen is consumed during the O2 chemisorption run as part of it is used for the re-oxidation of the alumina. If this is true, it is likely that the chemical bonds of the platinum to the carrier may act as "bridges", by which hydrogen spillover from the platinum to the alumina surface is enhanced (36).

-

If the above interpretation is correct, there are two reasons for a deviation from the expected stoichiometry: to much oxygen is taken up, and hydrogen is spilled over to the support.

3. GENERAL CONSIDERATIONS AND RECOMMENDATIONS 3-.1 First of all, in judging the value of free-metal surface area determinations, one should be aware of the fact that in many cases the condition of the metal surface area "in situ" (during catalytic action) deviates considerably from the condition arrived at after the pretreatment procedure which always precedes the measurement. During catalytic action the metallic part of the catalyst surface is often covered with strongly chemisorbed atoms or carbonaceous deposits, and only a small part of the metal surface is catalytically active. As a result of the

701

pretreatment (oxidation, reduction, followed by evacuation at higher temperature) the condition of the surface is seriously altered, whereas at the same time sintering and reaction of the metal particles with the carrier may occur. This is one of the reasons that in many cases a direct relation between freemetal surface area and catalytic activity is absent. 3.2 Sometimes a linear relation between free-metal surface area and catalytic

activity is found. For instance, Mars and coworkers (371, studying phenol hydrogenation over four different types of nickel catalysts differing in free-nickel 2

surface area from 3 to 100 m /g, arrived at an equal activity per m2 Ni in the temperature range from 308 K to 363 K for all samples investigated. In other cases, the activity seems to be related to a special type of chemisorbed hydrogen, and it is to be expected that weakly chemisorbed groups are more likely to be involved in catalysis than the strongly chemisorbed ones. A good example may be found in the work by Aben, van der Eijk and Oelderik (see 2 . 8 ) . 3.3 After pretreatment, including evacuation of the catalysts, the hydrogen coverage

of the metal is not always zero (26) ( 2 9 ) . This may influence the results in two ways: in applying hydrogen as the adsorbent too low a surface area is found, and using O2 as the adsorbent too high a value may be arrived at, due to reaction with the remaining hydrogen. Another unfavourable side-effect of hydrogen is that a number of metals either take up hydrogen in true solution as dissolved atoms, or form hydrides. Metals in which hydrogen forms a true solution are, for instance, aluminium, chromium, molybdenum, tungsten, iron, cobalt, nickel, manganese, copper, silver and platinum. Solubility data are summarized by Anderson (1). It appears that, except for manganese which behaves exceptional, the hydrogen solubility in all -3 these metals extrapolated to 273 K is less than about 10 atom% at a hydrogen pressure of 101 kPa. There is a substantial number of transition metals which form hydrides, often of variable stoichiometry, and these include titanium, zirconium, hafnium, thorium, vanadium, niobium, tantalum, cerium, lanthanum, rare earth metals, and palladium. With these methals the amount of hydrogen absorbed can often be substantial, and it decreases with increasing temperature. Again, data are summarized by Anderson (1). In the vicinity of room temperature and at a hydrogen pressure of 101 kPa

the uptake can reach the region of 30-75 atom%.

3.4 Small amounts of impurity metals, often undetected by normal analytical procedures, may accumulate at the surface of metal crystallites. As a rule, metals with the lower surface energy strongly accumulate at the surface (38). If they chemisorb hydrogen, their presence remains unnoticed; if they do not, too low a free-metal

surface area is found. An extension of the measurement with temperature programmed desorption of hydrogen can, in favourable cases, provide us with further information. For instance, Scholten and Konvalinka ( 2 7 ) , in studying hydrogen chemisorption on pure palladium, arrived at an enthalpy of adsorption of normal dissociatively adsorbed hydrogen of

-

(90

2

5) kJ/mole. from an analysis of the TPD spectra.

However, contaminated samples, containing zinc, lead, calcium and carbon as surface impurities (this followed from ESCA analysis) produced a decrease in the enthalpy of adsorption of about 15 %, the enthalpy being of the order of

-

(65

2

5) kJ/mole.

Hence, the shift in peak positions in TPD spectra may give a first indication for the presence of impurities. A palladium-on-carbon sample, intentionally contaminated with 5 at.% germanium in the Pd, gave a shift in the peak position of

-

24 K, corresponding with a

decrease of the enthalpy of adsorption of about 4.6 kJ/mole. Due to the presence of germanium, the Pd(3d 5/2) line in the ESCA spectrum is shifted from 334.6 (Pd on carbon) to 335.4 eV. Hence, a chemical shift of the order of 1 eV is found, indicating an increase in the electron density on the Pd sites (27). In this respect it is important to note that Pd/Ge-on-carbon differs essentially in its catalytic behaviour from pure Pd-on-carbon (in the reduction of HNO

3 to NH OH). However, from surface area measurements with hydrogen or carbon monoxide 2

as the adsorbates, the difference between both samples can hardly be detected, and additional measurements (TPD, AES, ESCA) are necessary. 3.5 As argued already in 2 . 7 , application of the pulse method, followed by the determination of the hydrogen TPD spectrum, has the additional advantage that information is gained about the extent of "hydrogen in excess" (weakly adsorbed hydrogen) and on the occurence of hydrogen spillover. In principle CO-TPD may be applied a s well. 3.6 Important extra information is obtained by measuring X-ray diffraction line broadening (1). When the metal crystallites are partly poisoned, or partly inaccesible due to chemical fixation to the carrier surface, very large differences are found between the mean particle size calculated from chemisorption and from line broadening. However, the absolute accuracy with which the average particle diameter can be obtained by line broadening should not be overestimated, and the influence of particle shape and size distribution factors probably limits this to an accuracy of about 30 %. Furthermore, there may be sensitivity limitations with supported catalysts if the metal loading is low, and difficulties will in general be encountered at a metal content

(0.5 wt.% in the case of platinum. Since the intensity of diffracted

radiation is proportional to the square of the atomic number, this problem becomes more severe for lower atomic number elements.

Depending on the type of diffractometer applied, very small crystallites (say

50 nm)

do not

contribute to the line broadening. Of course, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) may be used to determine mean particle diameters and to discover the detailed form of the crystallites and other structural details (11, but these methods are very laborious. 3.1 Supports for metallic catalysts are not necessarily inert. The consequence of

this is twofold: chemisorption may occur on the support, and hence disturb the surface area determination, and gases chemisorbed on the support may play a role in the catalytic reaction itself. For example, the palladium crystallites in a palladium-on- Y-alumina catalyst are active for H -D exchange. After evacuation above 123 K, Y-alumina becomes 2 2 active for this reaction as well, and the support starts to chemisorb some hydrogen. Another exmple. Recently we found that the nitrous oxide method, recommended for the determination of copper and silver free-metal surface areas in 2.2 and 2.3, may also be applied with palladium and platinum catalysts (39), and the same

general reaction behaviour as outlined in Fig. 2 and 3 was found. With platinum, maximum oxygen coverage is reached at 393 K and 25 kPa pressure. However, under these conditions the silica carrier appeared to be moderately active for N 0 2 decomposition, and corrections for this effect had to be made by carrying out a separate run with the plain carrier. It goes without saying that corrections have to be made for physical adsorption, both on the carrier and on the chemisorbed layer. 3.8 Free-metal surface areas may be calculated by calibration with unsupported

metal powders of known BET surface area. However, such powders pose a difficult problem for surface cleaning, as they are very susceptible to sintering and particle growth. Moreover, in applying such method, it is tacitly assumed that the distribution of crystallogrphic planes at the surface of the metal powders equals that

of the crystallites on the carrier, whereas in reality this distribution is influenced by particle size and by the interaction of the metal particles with the support (see J.R. Anderson, lit. 1, chapter 5 ) . Calibration with UHV evaporated metal films of known BET surface area (from Xe or CH

4

isotherms) is probably the best method, but this is rarely done.

3.9 An alternative for the calibration method is calculating the free-metal surface

area from the extent of the chemisorbed monolayer. This requires a knowledge of the chemisorption stoichiometry at monolayer coverage, and of the number of metal atoms per unit surface area,

7 04 In many cases the chemisorption stoichiometry is unknown or variable (CO), and the number of metal atoms per unit area of surface is mostly approximated by assuming that the surface is formed from equal proportions of the main low index planes. There is no scientific basis for this last supposition (see 3.8), and from the observation of the actual equilibrium shapes of f.c.c. crystals by Sundquist (28), a distribution 70 % (ill), 2 5 % (100) and 5 % (110) seems to be a better approximation. Of course the shape of the crystallites is dictated by the anisotropy of the surface energy, and hence will be influenced by the interaction with the support; therefore a general answer to this question can not be formulated. Perhaps the best advice is to combine the methods indicated in 3.8 and 3.9.

3.10 Dispersion and mean particle size. It is convenient to define the state of subdivision of the metal in terms of the ratio of the total number of surface atoms to the total number of metal atoms present (1): D = n /nt, where n m s number of surface atoms and nt the total number of metal atoms.

is the

Sometimes it is found from the extent of the chemisorbed layer that D is m nearly one, its maximum value. This does not necessarily mean that the metal is dispersed atomically. For instance, palladium crystallites with a diameter of

1 nm and containing about 63 Pd atoms, have nearly 9 0 % of their atoms at the surface. A low Dm value, on the other hand, does not always mean that the metal crystallites are large. Sometimes, a large part of the metal atoms is inaccesible due to strong binding with the carrier or reaction with the carrier. For instance, nickel catalysts, prepared by nickel precipitation, and particularly those prepared by co-precipitation, are very difficult to reduce, and sometimes only half of the nickel is present as metallic nickel. Another example: in the preparation of Pt-on-alumina catalysts, starting from H PtC16 and HC1, it appears that after 2

calcining,part of the platinum is dissolved in the alumina surface as Pt(II1). Therefore, in the calculation of D from n and nt, one has to differentiate m between the metallic and the non-metallic part of nt, the total number of metal atoms in the starting compound. Otherwise n

may be low due to irreversible surface poisoning; in such case

the mean crystallite size found from X-ray line broadening points to a much higher dispersion.

It goes without saying that the dispersion D does not give any information m about the particle size distribution: this distribution follows from electron microscopy, or, in the case of ferromagnetic particles, from a magnetochemical investigation. The mean particle size may be calculated from the free-metal surface area S and the total volume of the dispersed metal, V. For spherical particles d

V.S.'

the mean volume-surface diameter is given by:

705

E x a c t l y t h e same e x p r e s s i o n i s found f o r cubes: t h a n

d‘V . S . is

t h e mean edge of t h e

cube.

d

V.S.

i s d e f i n e d by:

i s t h e number of p a r t i c l e s of d i a m e t e r d I’ i X-ray l i n e broadening, however, g i v e s t h e weight-mean d i a m e t e r d

where n

*

W’

w

=

zi

n l. d l. 4 /

1

(7)

ni di 3

From e l e c t r o n microscopy, b o t h (6) and ( 7 ) may be c a l c u l a t e d , and, of c o u r s e , a l s o o t h e r t y p e s of mean d i a m e t e r s . One s h o u l d be aware t h a t such mean v a l u e s a r e n o t merely comparable, and n o t o n l y f o r mathematical r e a s o n s . The upper and lower l i m i t s of p a r t i c l e d e t e c t i o n

in X-ray and EM s t u d i e s are d i f f e r e n t , and i n t h e a p p l i c a t i o n of eq. ( 5 ) a l a r g e error is i n t r o d u c e d i f p a r t of t h e s u r f a c e is i n a c c e s i b l e .

REFERENCES 1. J . R . Anderson, “ S t r u c t u r e of M e t a l l i c C a t a l y s t s ” , Academic P r e s s , London, New York, S a n F r a n c i s c o , 1975 2 . P.H. Emmett, S . Brunauer, J. Am. Chem. SOC. 59, 310, 1937 3 . J.J.F. S c h o l t e n , T h e s i s D e l f t U n i v e r s i t y of Technology, 1959 4 . R.B. Anderson e t a l . , J. Am. Chem. SOC. 70, 2465, 1948 5. S . Brunauer and P.H. Emmett, J . Am. Chem. SOC. 59, 310, 1937 6 . P. Z w i e t e r i n g and R. Westrik, Proc. Kon. Akad. Wet. 56, 492, 1953 7 . P.H. Emmett and Nis Skau, J . Am. Chem. SOC. 65, 1029, 1943 8 . J . J . F . S c h o l t e n and J . A . Konvalinka, T r a n s . Faraday SOC. 65, 2456, 1969 9 . R.M. D e l l , F.S. S t o n e and P.F. T i l e y , T r a n s . Faraday SOC. 49, 201, 1953 10. G. E r t l , S u r f a c e S c i e n c e 5, 208, 1967 11. G. E r t l , Z. Phys. Chem. 50, 46, 1966 1 2 . Th.J. Osinga, B.C. L i n s e n and W.P. van Beek, J . C a t a l y s i s 1, 277, 1967 1 3 . T.N. Rhodin, Adv. C a t a l y s i s , 2, 39, 1953 108, 1970 1 4 . B. Dvorak and J . Pagek, J. C a t a l . 1 5 . J.J.F. S c h o l t e n , J . A . Konvalinka and F.W. Beekman, J. C a t a l . 28, 209, 1973 16. K.M. Kholyavenko and 1. Ya. Rubanik, Kinet. Katal. 2, 505, 1964 17. W.M.H. S a c h t l e r , C a t a l . Rev. 4, 27, 1970 1 8 . W. Herzog, B e r . Bunsenges. Phys. Chem. 74. 216; 1970 1 9 . Malcolm E. S c h r a d e r , J . o f C o l l o i d and I n t e r f a c e Sc. 59, 456, 1977 69, 95, 1965 20. J . H . S i n f e l t , W.F. Taylor and D . J . C . Yates, J. Phys. =em. 2 1 . K. Christmann, 0. Schober, G. E r t l and M. Neumann, J. Chem. Phys. 60, 4528, 1974 22. F. Sweet and Sir E r i c R i d e a l , Actes du Deuxit’me Congrbs I n t e r n a t i o n a l d e C a t a l y s e , P a r i s 1960, page 175, E d i t i o n Technip, P a r i s 1960 2 3 . J . F . Lynch and T.B. Flanagan, J . Phys. Chem. 2628, 1973 2 4 . C . Benndorf and F. Thieme, 2. f u r Phys. Chem. N.F. 87, 40, 1973 25. N. Hayashi and K . Kawasaki, J . C a t a l . 48, 243, 19772 6 . P.C. Aben, J . Catal. lo, 224, 1968 S c h o l t e n , J . C a t a l . 48, 374, 1977 27. J . A . Konvalinka and J.J.F. 28. B.E. S u n d q u i s t , Acta. Met, 67, 1964 and 2,585, 1964

Is,

12,

706 29. 30. 31. 32.

33. 34. 35. 36. 37. 38. 39.

J.J.F. Scholten and A. van Montfoort, J. Catal. 1, 85, 1962 P.A. Sermon, J. Catal. 24, 460, 1972 R.J. Cvetanovic and Y. Amenomiya, Advan. in Catal. l7, 103, 1967 P.C. Aben, H. van der Eijk and J.M. Oelderik, Proceedings of the fifth international congres on catalysis, West-Palm Beach, Florida, Aug. 1972 North-Holland Publishing Company, Amsterdam 1973, page 717 J . Freel, J. Catal. 25, 139, 1972, and 25, 149, 1972 J. Prasad, K.R. Murthy and P.G. Menon, J. Catal. 1978, to be published F.M. Dautzenberg and H.B.M. Wolters, J. Catal. 1978, to be published 211-39, 1973 P.A. Sermon and G.C. Bond, Catal. Rev. P. Mars, J.J.F. Scholten and P. Zwietering, Actes du deuxihme congres international de catalyse, Paris 1960, Edition Technip, Paris, 1961, page 1245 W.M.H. Sachtler and R.A. van Santen, Adv. in Catalysis 3, 69, 1977 J.J.F. Scholten, J.C. Rasser and Ming Lee, unpublished results

a,

NOTE ADDED IN PROOF P la t inum T h e study by Dautzenberg and coworkers (our reference 35) is now published. See

:

Dautzenberg, F.M. and Wolters, H.B.M.,

J. Catal.

u, 26

den Otter, G.J.

J. Catal.

53,

and Dautzenberg, F.M.,

116 (1978).

According to these authors our equation ( 3 ) should read O

( 5 )

+

2H2(g)

-P

2H (S)

(1978).

:

+ H20.

After reduction of Pt-on-y-alumina catalysts at relatively high temperatures (500 and 65OoC), the hydrogen chemisorption capacity decreases.

This is rationalized by Dautzenberg and coworkers in

terms of an alloy model, by assuming that hydrogen chemisorption, which does not take place on Al, is dissociative and needs two adjacent P t sites.

On alloy particles, surface enriched with

alumina, the number of P t dual sites is very low and hence the hydrogen chemisorption is suppressed. Hydrogen spillover is not considered by these authors as a possible reason for deviating stoichiometries. DISCUSS ION

M. BAERNS

:

You mentioned the difficulty of reaching agreement

on the definition of monolayer coverage to determine the active metal surface, that is to say whether one should use a definite przssure

(for instance 50 Torr) or whether one should extrapolate

the chemisorption data to zero pressure, (the latter even implying that there is in fact no equilibrium).

Would it not be useful to

compare maximum coverage in order to have a measure independent of

707

pressure ? J.J.F. SCHOLTEN

:

The difficulty lies in the fact that the nature

of weak chemisorption (also called "hydrogen in excess" in the literature) is unknown.

This type of weak adsorption is reported

on Pd (ref. 23 and 27) and on Pt (see Fig. 9, the peak at 273 K). We also found it with N i (not yet published). According to ref. 23, we are dealing with "interstitial hydrogen" in between the strongly chemisorbed hydrogen atoms. In ref. 27, we found evidence in the case of Pd that this is subsurface hydrogen in the octahedral interstices just below the surface. The difficulty lies in the precise separation of these two forms of sorbed hydrogen. den Otter and Dautzenberg (J. Catal. (1970)

to saturation. Then H/Ptsurface occurs

53,

116

take 6 = 1 at the point where the strong chemisorption comes = 1.

The pressure at which this

is not exactly known.

On the basis of this definition of full coverage, the particle size calculated from it is in reasonable accordance with electron microscopy results. P.G. M E N O N : One reason for the inconsistent results for H2-02 titrations of supported platinum catalysts, reported in the past, arises from taking the very first H2 chemisorption measurement on a freshly reduced catalyst as the basis for the stoichiometry calculation. Recently we have found (1) that if the reduced catalyst is first given a few H2-02 cycles at room temperature to anneal or "homogenize" it, and if the calculations are based on H2 titration (and not on the first H2 chemisorption) then the stoichiometry HC:OC:HT is consistently 1:1:3, independent of the pretreatment of the catalyst and independent of Pt crystallite size. On exposure of a Pt-alumina catalyst to temperatures above 500'C

in

H2, the stronger chemisorption of H2 occurring seems to be responsible for the apparent "loss" or "inaccessibility" of a part of the surface Pt and the sharp decrease in the hydrogenolysis activity of the catalyst.

Temperature programmed desorption (TPD) of H2 and the de-

sorption-readsorption of H2 in a 5% H2-Argon stream during programmed heating (TPR) of Pt-A1203, as also the very similar results obtained for Pt-A1203 and platinum black, support this hypothesis of a stronger chemisorption of H 2

(and spill-over on to A1203) at higher temperature

708 ( s e e ref. 2).

1.

J. P r a s a d ; K.R.

M u r t h y and P.G.

Menon, J. C a t a l y s i s ,

g,

515

(1978).

2. P.G.

J.J.F.

M e n o n and G.F.

SCHOLTEN

F r o m e n t , J. C a t a l y s i s

(submitted).

: O b v i o u s l y your r e s u l t s a r e not in a c c o r d a n c e w i t h

t h o s e o f D a u t z e n b e r g and coworkers.

I have no personal experience

w i t h t h i s m e t h o d and h e n c e c a n n o t g i v e a f i n a l judgement.

H. C H A R C O S S E T : I have t w o q u e s t i o n s and t w o r e m a r k s o n y o u r c o m m u n i c a tion.

( 1 ) I a g r e e t h a t (Pt, Al) f o r m a t i o n t a k e s p l a c e easier t h a n it

w a s t h o u g h t before.

We h a v e shown t h a t r e d u c t i o n o f A 1 2 0 3 t o A 1 i s

i n h i b i t e d , d u e t o a l l o y i n g o f P t w i t h Re or Ir. 2 )

I recall that hydrogen-oxygen titrations of bimetallic catalysts

give

no

t r u e d a t a o n t h e d i s p e r s i o n o f t h e t w o m e t a l s nor o n t h e i r

i n t e r a c t i o n ( s e e our p a p e r a t t h i s symposium). 3 ) W h e n m e a s u r i n g t h e f r e e m e t a l l i c a r e a in m o n o m e t a l l i c c a t a l y s t s , a phenomenon to be considered is a possible partial reoxidation of the m e t a l by t h e s u p p o r t d u r i n g t h e e v a c u a t i o n of H2 at h i g h temperature. Could you comment on this ?

4) H2-TPD i s a v e r y s u i t a b l e m e t h o d t o c h a r a c t e r i z e m e t a l catalysts. I w o n d e r if t h e r e i s n o e x p e r i m e n t a l a r t e f a c t d u r i n g d e s o r p t i o n u n d e r i n e r t g a s flow.

F o r i n s t a n c e in Fig. 9 o f y o u r p a p e r , I would t h i n k

t h a t p a r t of t h e h y d r o g e n d e s o r b e d at low t e m p e r a t u r e is r e a d s o r b e d , g i v i n g r i s e t o H 2 d e s o r p t i o n at h i g h e r t e m p e r a t u r e s ,

What d o y o u

t h i n k about t h i s e f f e c t ?

J.J.F.

S C H O L T E N : T h a n k y o u for y o u r comments.

A s t o y o u r second q u e s t i o n , I know t h a t s u r f a c e h y d r o x y l g r o u p s from t h e s i l i c a s u p p o r t m a y m i g r a t e t o t h e n i c k e l s u r f a c e and o x i d i z e it. T h i s w a s shown long a g o by S c h u i t and v a n R e i j e n ( s e e t h e i r c h a p t e r o n t h i s subject in “ A d v a n c e s in C a t a l y s i s , 10).

However

,

t h i s is a

v e r y s l o w p r o c e s s , and t h e lower t h e h y d r o x y l c o v e r a g e o f t h e silica t h e l o w e r i t s rate. O u r N i catalyst w a s r e d u c e d in a 3 0 l/h h y d r o g e n s t r e a m , p u r i f i e d v i a a deoxo-purifier

and m o l e c u l a r sieves.

at 773 K i n u l t r a p u r e a r g o n for 1 2 hrs.

F i n a l l y it w a s t r e a t e d T h e a r g o n first p a s s e s

709

t h r o u g h a c a r t r i d g e f i l l e d w i t h a l a r g e amount o f a h i g h l y r e d u c e d N i c a t a l y s t . Under t h e s e c i r c u m s t a n c e s t h e OH-coverage o f t h e s i l i c a b e c o m e s v e r y l o w . M o r e o v e r , t h e TPD r u n w a s s t a r t e d i m m e d i a t e l y a f t e r p u l s i n g , and p u l s i n g i s d o n e i m m e d i a t e l y a f t e r p r e t r e a t m e n t . A s t o your l a s t q u e s t i o n ,

I d o n o t a g r e e w i t h y o u r p o i n t of v i e w .

Our

TPD m e a s u r e m e n t s a r e p e r f o r m e d u n d e r s u c h c o n d i t i o n s t h a t t h e r a t e o f h y d r o g e n d e s o r p t i o n i s o n l y a l i t t l e b i t h i g h e r t h a n t h e r a t e of a d s o r p t i o n (see t h e explanation i n r e f . ding it).

27,

and t h e a r t i c l e p r e c e e -

H e n c e , we m e a s u r e d t h e r a t e o f t h e a d s o r p t i o n - e q u i l i b r i u m

Once t h e p o p u l a t i o n of a w e a k l y a d s o r b e d t y p e of h y d r o g e n i s

shift.

t o t a l l y d e s o r b e d , it is i m p o s s i b l e t h a t it should r e a d s o r b i n a h i g h e r temperature range 2.

P A A L : I h i g h l y a p p r e c i a t e y o u r i d e a s of

and TPD.

combining p u l s e t i t r a t i o n

T h e r e a r e however o t h e r f a c t o r s making t h e s i t u a t i o n more

complicated.

T h i s is h y d r o g e n r e t e n t i o n s t u d i e d t h o r o u g h l y b y Thomson

e t a l . ( 1 - 4 ) . M e t a l s a f t e r r e d u c t i o n c o n t a i n some h y d r o g e n e v e n a f t e r a l o n g e x p o s u r e t o a r g o n . S u p p o r t e d c a t a l y s t s may e x h i b i t s p i l l - o v e r . A v e r y s t r o n g l y h e l d h y d r o g e n was o b s e r v e d w i t h p l a t i n u m b l a c k w h i c h

was i d e n t i f i e d a s a b s o r b e d h y d r o g e n . T h i s may b e s l o w l y e x c h a n g e d b y g a s phase hydrogen even a t e l e v a t e d temperature

(several hours a r e

r e q u i r e d f o r i t s t o t a l exchange by t r i t i u m ) and v e r y p r o b a b l y d o e s n o t desorb even a t high temperatures. T h e d e f i c i t i n y o u r h y d r o g e n b a l a n c e may w e l l b e e x p l a i n e d b y s u c h a n absorbed

( i n t e r s t i t i a l ) hydrogen.

i t s "upper layer".

S u b s u r f a c e h y d r o g e n may o n l y b e

So i t s p r e s e n c e a n d e v e n t u a l s l o w e q u i l i b r i u m w i t h

adsorbed s u r f a c e hydrogen should b e considered. ( 1 ) T a y l o r , G.F.,

Thornson, J . J . ,

Webb, G . , J .

Taylor, G.F.,

Thornson, J . J . ,

Webb, G . ,

(2)

( 3 ) A l t h a m , J . , Webb, G . , (4) Paal,Z.,

J.J.F.

Thomson,

J.

S.J.,

Catal.

Catal. 8,388 (1967). J. Catal.

G,

191 ( 1 9 6 8 ) .

g ,1 3 3 ( 1 9 6 8 1 . 30, 9 6 ( 1 9 7 3 ) .

J. C a t a l .

SCHOLTEN : I a l w a y s have had d i f f i c u l t i e s

i n accepting the

e x i s t e n c e o f s u c h v e r y t i g h t l y bound hyd ro g e n i n m e t a l s , b u t I w i l l s t u d y t h e p a p e r s c i t e d b y you o n t h i s s u b j e c t a g a i n .

I don't think

t h a t t h e d e f i c i t i n o u r h y d r o g e n b a l a n c e c a n b e made u n d e r s t a n d a b l e from t h e o c c u r r e n c e of v e r y s t r o n g hydrogen bonding.

The r e a s o n is

t h a t i n a s e r i e s o f m e a s u r e m e n t s (pulse-TPD-pulse-TPD-pulse e t c . ) ,

710 t h e d e f i c i t i s found a f t e r each p u l s e . s t o o d o n l y i n t e r m s of H 2

L.

: I

GUCZI

T h i s can probably b e under-

spill-over.

highly a p p r e c i a t e Dautzenberg's i d e a about t h e double

bonded H atom t o two P t a t o m s . T h i s a g r e e s w i t h E l e y ' s d a t a on d o u b l e bonded H atom on N i w i t h a h i g h h e a t o f f o r m a t i o n .

H o w e v e r , i t would

mean t h a t w i t h i n c r e a s i n g d i s p e r s i o n t h e p r o b a b i l i t y t o f i n d a p r o p e r double P t s i t e w i l l decrease.

Consequently,

t h e H/O

ratio i n the

t i t r a t i o n will d e c r e a s e and t h i s i s i n c o n t r a d i c t i o n w i t h d a t a who f o u n d i t i n t h e r e v e r s e way.

Hall's

Would you comment t h i s ?

SCHOLTEN : H i g h l y d i s p e r s e d P t p a r t i c l e s s t i l l h a v e a h i g h

J.J.F.

number o f d u a l s i t e s I F o r i n s t a n c e a 10

i Pd

p a r t i c l e contains about

5 0 Pd a t o m s a t i t s s u r f a c e , o n a t o t a l number o f 6 3 Pd a t o m s . t h e r a t i o Pd s u r f a c e / P d t o t a l

Then,

(very high d i s p e r s i o n ) , but

i s n e a r l y one

t h e number o f d u a l s i t e s h a s h a r d l y d e c r e a s e d .

A.

: Why H 2

OZAKI

c h e m i s o r p t i o n on Fe and C O c h e m i s o r p t i o n on N i a t

-196OC

i s n o t a d o p t e d s i n c e even Fe c a n form c a r b o n y l

J.J.F.

SCHOLTEN : C O c h e m i s o r p t i o n i s t r a d i t i o n a l l y u s e d .

?

It is a

v e r y v a l u a b l e m e t h o d , w h i c h was d e s i g n e d b y Emmett a n d c o - w o r k e r s . I h a v e no e x p e r i e n c e w i t h h y d r o g e n on i r o n , b u t i n p r i n c i p l e i t m i g h t

be useful a s well.

N.

PERNICONE

: With

r e s p e c t t o t h e r a t i o R of

s o r b e d t o t h a t of N2 monolayer i n p u r e F e ,

t h e volume of CO c h e m i -

I would remark t h a t

it

d e p e n d s on t h e e v a c u a t i o n t e m p e r a t u r e a f t e r t h e f i r s t CO a d s o r p t i o n (ref. ref.

1 ) and a l s o on t h e r e d u c t i o n t e m p e r a t u r e of Fe304

2 , R i s a b o u t 1 . 2 when F e 3 0 4 i s r e d u c e d a t 3 0 0 ' C

(ref. 2 ) .

In

while the usual

r e d u c t i o n t e m p e r a t u r e of A1203 promoted Fe c a t a l y s t s a r e i n t h e r a n g e 400-500OC.

i f F e 3 0 4 i s r e d u c e d a t 45OoC, t h e R

We h a v e f o u n d t h a t ,

v a l u e i s i n t h e range 0.5-0.6,

i n good agreement w i t h t h e v a l u e ex-

t r a p o l a t e d form t h e d a t a of r e f .

2.

W h i l e t h e r e may b e some d o u b t s

about t h e p u r i t y of Fe304 used i n r e f .

2,

t h i s i s not t h e case f o r a

very pure sample. This problem i s very important f o r t h e c o r r e c t determination of t h e Fe s u r f a c e a r e a in o x i d e- p r o mo t ed

Fe c a t a l y s t .

Therefore a reexami-

n a t i o n o f t h e whole p r o b l e m would b e t i m e l y .

( 1 ) F.V.

K o z e n e v s k a y a a n d A.Y.

Rozovski,

Kinetics Catalysis

711 (English Transl.),

( 2 ) R.

7

(19661, 951.

W e s t r i k a n d P. Z w i e t e r i n g , P r o c .

Koninkl.

Ned.

Akad.

Wet.

B 56 ( 1 9 5 3 1 , 4 9 2 .

SCHOLTEN : T h a n k y o u f o r y o u r i n t e r e s t i n g r e m a r k .

J.J.F.

H.

SHINGU

: A s

t o the determination of t h e free-metal

s u r f a c e area

of c o p p e r and s i l v e r e i t h e r by t h e n i t r o u s o x i d e o r t h e oxygen ads o r p t i o n method,

I would

d e f i n i t i o n "free-metal

l i k e t o point out the dubiousness of t h e

s u r f a c e area".

Indeed t h e a d s o r p t i v e behaviour

( t o oxygen) and t h e r e f o r e , t h e s u r f a c e s t r u c t u r e a n d / o r

microscopic

t e x t u r e o f t h e m e t a l l i c l a y e r , a r e so much v a r i a b l e i n w i d e r a n g e s , d e p e n d i n g upon t h e p r e p a r a t i o n p a r a m e t e r s o f

t h e metal,

especially

s u p p o r t e d m e t a l s a m p l e s , t h a t e v e n t h e " a c t i v a t e d n a t u r e " o f t h e ads o r p t i o n of N 0 cannot b e used t o d i s t i n g u i s h between t h e s e s u r f a c e 2

s t r u c t u r e s w h i c h a r e so much r e l a t e d t o t h e i r c a t a l y t i c b e h a v i o u r s .

SCHOLTEN : On p a g e 2 o f o u r p a p e r w e s t a t e d a l r e a d y t h a t t h e

J.J.F.

determination of t h e free-metal

s u r f a c e a r e a i s o n l y one aspect of

a more f u l l c h a r a c t e r i z a t i o n o f a c a t a l y s t .

Of c o u r s e ,

the surface

c r y s t a l l o g r a p h y may i n f l u e n c e t h e c a t a l y t i c b e h a v i o u r , b u t n o m e t h o d s

are a v a i l a b l e t o study t h i s a s p e c t f o r m e t a l s on supports. Above d i a m e t e r s o f 5 nm, t i o n of

t h e v a r i a t i o n of t h e surface-plane

supported metal c r y s t a l l i t e s a s a f u n c t i o n of t h e diameter

is very limited.

P e r h a p s t h e m e t h o d o f p r e p a r a t i o n may h a v e a n

i n f l u e n c e on t h i s d i s t r i b u t i o n .

It i s , however,

epoxidation of ethylene over t h e

(ill), (100) a n d

silver,

distribu-

known t h a t i n t h e

(110) p l a n e s o f

t h e f i n a l a c t i v i t y and s e l e c t i v i t y i s e x a c t l y t h e s a m e f o r

a l l three planes. It is likely t h a t during the reaction recrystallization occurs,

s u l t i n g i n an e q u i l i b r i u m s u r f a c e p l a n e d i s t r i b u t i o n S a c h t l e r i n " C a t a l y s i s Reviews",

A.

BOSS1

:

Q ( 1 ) , 27-52

re-

( s e e W.M.M.

(1970)).

Concerning t h e d e t e r m i n a t i o n of copper s u r f a c e a r e a I

t h i n k i t c o u l d b e i n t e r e s t i n g t o m e n t i o n t h e work o f V a s i l e v i c h e t al.

( K i n e t i c s and C a t a l y s i s , E ( 6 ) , 1571, R u s s i a n e d . ) .

These

a u t h o r s d e t e r m i n e d t h e o x y g e n a d s o r p t i o n f r o m room t e m p e r a t u r e t o t h e l i q u i d n i t r o g e n temperature by u s i n g a s t a t i c volumetric apparatus.

T h e y f o u n d a minimum i n t h e a d s o r p t i o n c a p a c i t y o f t h e c o p p e r

a t a b o u t -136OC.

A t higher

temperatures a b u l k o x i d a t i o n of copper

712 takes place while at lower temperatures chemisorption and physical adsorption were observed.

By comparing oxygen adsorption results

and nitrous oxide decomposition data they suggest a method for the determination of the copper surface area in Cu/ZnO/A1203

catalysts

based on the oxygen chemisorptibn at -136OC. J.J.F.

SCHOLTEN

:

In the paper by A.A.

Vasilevich and co-workersl

I did not find a real experimental proof that the oxygen uptake at -136'C

is only chemisorption and nothing else. It might be right,

but I am not yet convinced. The equality of Smetal, from oxygen chemisorption, and SBET, from krypton physisorption, as claimed by Vasilevich, has to be checked with a

really pure cu surface, and not

with Cu prepared from a "analytical grade" copper compound,whatever this may be.

Moreover, the comparison between both results is not

very clearly and quantitatively demonstrated in Vasilevich's paper. Conclusion; reinvestigation is highly desirable. P.A. SERMON

:

You have described weak chemisorption (in excess of

monolayer capacity) for CO and H2 upon various metals, which complicates chemisorption measurements of metal areas but enhances catalytic activity. (i) What physical or chemical model do you have for chemisorbed CO and H2 (particularly in view of the paper of Guczi, this symposium, describing subsurface hydrogen as strongly chemisorbed).

( i i ) What thermal or basic conditions can we use to m i n i -

mise weak chemisorption in adsorption experiments, and what conditions will allow us to maximise it in catalytic reactions ? J.J.F.

SCHOLTEN

excess".

:

I have no experience with CO adsorption "in

With hydrogen, however, I found this phenomenon for the

case of palladium and platinum, whereas for nickel strong indications for its existence are demonstrated during the presentation of my paper. It is well known that the position of hydrogen atoms in the bulk of metals is quite difficult to study (neutron diffraction). Hydrogen on a metal surface is even much more difficult and we have mainly to rely on indirect evidence. Arguments for the subsurface position of "hydrogen in excess" may be found in my paper with J.A.

Konvalinka, J. Catal. 48,374 (1977),

but no definite proof is available. A hydrogen atom in an octahedral subsurface interstice can have a chemical interaction with a

713 c h e m i s o r b e d h y d r o g e n atom o n t h e surface.

Therefore, I think the

s u b s u r f a c e p o s i t i o n of t h e h y d r o g e n atom is d i s t i n c t from t h e p o s i t i o n in a b u l k o c t a h e d r a l hole, and i s e n e r g e t i c a l l y m o r e favourable. A c c o r d i n g t o L y n c h and F l a n a g a n ( J . Phys. Chem.

77,

2628 (1973))

h y d r o g e n t a k e n u p in e x c e s s o f a m o n o l a y e r is a d s o r b e d in i n t e r s t i t i a l s u r f a c e s i t e s , and t h i s h y d r o g e n should b e a p r e c u r s o r t o i n t e r s t i t i a l l y a b s o r b e d hydrogen.

I have n o e x p e r i e n c e w i t h extremely strongly bound s u b s u r f a c e hydrogen.

F o r t h e c a s e o f palladium w e k n o w f o r s u r e ( n e u t r o n diffrac-

t i o n ) t h a t H a t o m s are absorbed in t h e o c t a h e d r a l i n t e r s t i c e s only. T h i s dissolved h y d r o g e n is m o r e w e a k l y bound t h a n t h e c h e m i s o r b e d hydrogen

(

A H

24 K J / m o l

o f H 2 chemisorption).

v e r s u s 90 kJ/mol f o r t h e s t r o n g e s t form

F o r m e i t is d i f f i c u l t t o a c c e p t t h a t t h e r e

should exist a s p e c i a l t y p e o f v e r y strong b o n d i n g in t h e o c t a h e d r a l holes. Indeed it i s t o b e expected t h a t t h e w e a k l y c h e m i s o r b e d f o r m s o f h y d r o g e n p l a y a n i m p o r t a n t r o l e i n catalysis. A g o o d e x a m p l e o f t h i s m a y b e found i n a s t u d y o f b e n z e n e h y d r o g e n a t i o n o v e r p l a t i n u m b y O e l d e r i k and c o - w o r k e r s (Proc. o f t h e V t h Int. Congr. o n C a t a l y s i s , F l o r i d a , Aug.

1972, p a p e r n o 49). T h e extent of w e a k l y a d s o r b e d

h y d r o g e n is i n c r e a s e d b y i n c r e a s i n g t h e h y d r o g e n pressure.

Weakly

bound h y d r o g e n i s not p e r c e p t i b l e in L E E D s t u d i e s , p r e s u m a b l y due t o t h e l o w p r e s s u r e s applied in t h i s t e c h n i q u e .

V.D.

Y A G O D O V S K I : I would like t o m a k e s o m e g e n e r a l r e m a r k s o n t h e

B E T equation.

It is w e l l k n o w n t h a t B E T e q u a t i o n i s not q u i t e sui-

t a b l e t o d e t e r m i n e t h e s u r f a c e a r e a o f t h e catalysts. t w o r e a s o n s t o this.

T h e r e are

F i r s t l y , in t h e c o u r s e of t h e m o s t c o r r e c t

s t a t i s t i c a l d e r i v a t i o n o f t h e BET e q u a t i o n (for e x a m p l e , b y T. H i l l ) we cannot substitute the ratio P/P tion.

i n t o t h e X v a l u e o f t h i s equa-

T h i s d o e s n o t allow u s t o o b t a i n r i g o r o u s l y t h e ordinary form

o f t h i s e q u a t i o n , w h i c h i s u s e d for t h e c a l c u l a t i o n o f t h e s u r f a c e a r e a and t h e e n e r g e t i c p a r a m e t e r C from e x p e r i m e n t a l isotherms. S e c o n d l y , e v e n if w e neglect t h i s c i r c u m s t a n c e , w e w i l l have t o t a k e into a c c o u n t t h a t in t h e B E T m o d e l t h e h o m o g e n e i t y o f t h e s u r f a c e a s w e l l as t h e a b s e n c e o f t h e i n t e r a c t i o n between adsorbed m o l e c u l e s is assumed. But t h e p o r o u s c a t a l y s t s h a v e h e t e r o g e n e o u s surfaces. F o r t h i s r e a s o n t h e BET- e q u a t i o n is not c o r r e c t f o r s u c h catalysts. S i n c e t h e e x p e r i m e n t a l a d s o r p t i o n i s o t h e r m s o f t e n o b e y t h e B E T equation, we s h o u l d c o n s i d e r them only a s e m p i r i c a l and t h e r e f o r e its

714 p a r a m e t e r s have n o t t h e p h y s i c a l meaning which i s u s u a l l y a t t r i b u t e d t o them.

However, t h e s i t u a t i o n i s improved i n t h e c a s e o f

n o b l e g a s e s a d s o r p t i o n a t low t e m p e r a t u r e s , b e c a u s e i t i s s l i g h t l y s e n s i t i v e t o t h e inhomogeneity of t h e s u r f a c e .

Therefore,

it would

b e b e t t e r t o measure t h e i s o t h e r m s a t t h r e e o r f o u r t e m p e r a t u r e s t o be s u r e t h a t t h e

dependence o f i s o s t e r i c h e a t of a d s o r p t i o n on

a d s o r b e d amount i s n e g l i g e a b l e . I n my o p i n i o n , t h e BET e q u a t i o n i s s u i t a b l e o n l y f o r s e m i q u a n t i t a t i v e e s t i m a t i o n of t h e s p e c i f i c s u r f a c e a r e a of a s e r i e s of

catalysts

o f t h e same n a t u r e , b u t i s n o t c o r r e c t f o r a c c u r a t e c a l c u l a t i o n s o f t h e a b s o l u t e v a l u e of t h e s u r f a c e a r e a .

715

PROGRESS REPORT ON THE WORK OF THE SCI/IUPAC/NPL

WORKING PARTYON

CATALYST REFERENCE MATERIALS C.C.

BOND

',

R.L.

MOSS b, R.C.

PITKETHLEY

',

K.S.W.

SING

and R. WILSON

a

Department of I n d u s t r i a l Chemistry, Brunel U n i v e r s i t y , Uxbridge, Middlesex(UK)

b

Warren S p r i n g Laboratory, Stevenage, H e r t f o r d s h i r e (UK)

c

ex BP Research Centre, Sunbury, Middlesex (UK)

d

School of Chemistry, Brunel University, Uxbridge, Middlesex (UK)

e

Division of Chemical Standards, National Physical Laboratory, Teddington, Middlesex (UK)

INTRODUCTION A f t e r t h e s u c c e s s f u l conclusion of t h e Society of Chemical I n d u s t r y (SCI)/IUPAC/ National Physical Laboratory (NPL) p r o j e c t on s u r f a c e a r e a r e f e r e n c e m a t e r i a l s ( r e f . I ) , it w a s suggested by IUPAC Commission I 6 on Colloid and Surface Chemistry t h a t SCI should i n i t i a t e a similar p r o j e c t on c a t a l y s t r e f e r e n c e m a t e r i a l s .

This

was agreed by t h e SCI Colloid and Surface Chemistry Group and a working p a r t y c o n s i s t i n g of t h e a u t h o r s with R o f e s s o r Sing as chairman, Dr. Wilson as s e c r e t a r y and Professor Kemball ( U n i v e r s i t y of Minburgh) r e p r e s e n t i n g IUPAC Commission I 6

was s e t up and t h e f i r s t meeting took place i n January 1976, when i t was decided t h a t t h e p r o j e c t should be based at t h e NPL which already had a g e n e r a l programme

on r e f e r e n c e m a t e r i a l s . The d e c i s i o n t o e s t a b l i s h a s e t of heterogeneous c a t a l y s t r e f e r e n c e m a t e r i a l s

was j u s t i f i e d f o r t h e following reasons:( A ) The v a r i e t y of methods used i n d i f f e r e n t l a b o r a t o r i e s f o r t h e p r e p a r a t i o n o f c a t a l y s t s makes i t d i f f i c u l t t o compare published r e s u l t s . The a v a i l a b i l i t y of c a t a l y s t r e f e r e n c e m a t e r i a l s would g r e a t l y improve t h e s i t u a t i o n and should thereby a i d progress i n r e l a t i n g t h e a c t i v i t y of c a t a l y s t s t o t h e i r composition and s t r u c t u r e .

(B) Readily a v a i l a b l e c a t a l y s t r e f e r e n c e m a t e r i a l s would f a c i l i t a t e t h e v a l i d a t i o n of procedures f o r c a t a l y s t c h a r a c t e r i s a t i o n and performance t e s t i n g and permit t h e comparison of t h e a c t i v i t y and e f f i c i e n c y of o t h e r c a t a l y s t s . ( C ) For t h e t r a i n i n g of research s t u d e n t s and experimental workers. Eight c a t a l y s t s were chosen as being p o t e n t i a l l y s u i t a b l e as r e f e r e n c e m a t e r i a l s .

Three of t h e s e , ( i ) 13$ A 1 0 /Si02 acid-type cracking c a t a l y s t , ( i i )C O / M O / A ~ ~ O ~ 2 3

716 desulphurisation catalyst and (iii) 10% Ni/Si02 hydrogenation catalyst where selected for immediate attention. Others under consideration included (iv) bismuth molybdate, (v) chromium oxide gel, (vi) 0.5% or 0.75% Ft on A1203, (vii)2% Pd on Y-A1203, (viii) 0.5

-

0.75% Pt/Re/Al2O3.

Selection of the catalyst

was based on industrial relevance and commercial availability and in drawing up detailed specifications for catalysts (i), (ii) and (iii), the requirements of chemical engineers in addition to those of chemists were considered.

PROGRESS Approximately 100 kg quantities of catalyst (i), (ii) and (iii) have been donated by Joseph Crosfield and Sons Ltd, Laporte Industries Ltd and Akzo Chemie Nederland bv respectively. Representative, approximately 100 g, samples have been abstracted using a specially constructed 20 litre rotating riffle. Such a sampling procedure was considered essential as it cannot be assumed that the bulk materials are completely homogeneous with respect to their physical and chemical properties; the catalysts are in the form of powders or granules with mean particle sizes of approximately 90

for (i), 1 nun for (ii) and 80 Lrn f o r (iii) and there is a tendency for the finer and coarser material to separate. Questionnaires asking for advice on characterisation and test procedures on

catalysts (i), (ii) and (iii) have been distributed to over sixty experts, mainly in the UK.

About one third of those contacted have offered advice and practical

assistance, and agreed to participate in the physical characterisation and testing of the materials. After careful consideration of this advice, the Working Party is writing characterisation and test programmes. Details of the procedures to be used, including pretreatment Conditions, will be specified and representative samples of the catalysts are being distributed to the participating laboratories. In the case of Ni/Si02, it was necessary to ask a few of the participating laboratories to make a preliminary assessment of the "ease of reduction" before the pretreatment conditions could be speoified. Routine measurements of chemical analysis, surface area and pore size distribution, have been specified for all three catalysts and will be made by a number of the participating laboratories. It was more difficult to specify methods f o r the measurement of catalyst activity. However, after careful consideration, it was agreed that cumene cracking should be specified for (i),

.

thiophene desulphurisation for (ii) and benzene hydrogenation for (iii)

Other methods to be used by some of the participants will include selective chemisorption, electron microscopy, X-ray line broadening, Auger and photoelectron spectroscopy and secondary ion mass spectroscopy. When the Working Party is satisfied that the selected catalysts are suitable

717 f o r use as r e f e r e n c e m a t e r i a l s , d e t a i l e d r e s u l t s w i l l be made a v a i l a b l e as soon

as p o s s i b l e and t h e c e r t i f i e d samples w i l l be made a v a i l a b l e v i a t h e NPL. Further o f f e r s of a c t i v e p a r t i c i p a t i o n would be welcome.

The Working Party

would a l s o be g r a t e f u l f o r a s s i s t a n c e i n o b t a i n i n g bulk q u a n t i t i e s o f p o t e n t i a l c a t a l y s t r e f e r e n c e m a t e r i a l s , p a r t i c u l a r l y supported noble metal c a t a l y s t s .

ACKNOWLEDGMENTS We a r e very g r a t e f u l t o t h e t h r e e companies, Joseph C r o s f i e l d and Sons Ltd, Laporte I n d u s t r i e s Ltd and Akzo Chemie Nederland bv f o r t h e i r k i n d donations of bulk q u a n t i t i e s of c a t a l y s t m a t e r i a l s .

We a r e a l s o indebted t o t h e l a b o r a t o r i e s

t h a t a r e p a r t i c i p a t i n g i n t h e measurement programme.

RFPERENCE 1 D.H. E v e r e t t , G.D. P a r f i t t , K.S.W. 24 (1974) 199-219-

Sing and R. Wilson, J.appl.Chem.

Bioteohnol.


719

ORGANIZATION AND FUNCTIONS OF ASTM COMMITTEE D-32 ON CATALYSTS ARTHUR H. NEAL Exxon Research and Development Laboratories, Baton Rouge, LA

(U.S.A.)

ABSTRACT ASTM Committee D-32 on Catalysts was organized in 1975 with the objectives of developing standard test procedures for characterizing catalysts and related materials and of stimulating research in these areas. Members of the committee represent a broad cross-section of the catalyst industry. The technical activities of the committee are carried on in four technical subcommittees covering the major areas of catalyst characterization: Physical Chemical; Physical-Mechanical; Chemical Analysis and Catalytic Properties. Task groups within each of these subcommittees are working on a broad variety of individual test methods which are beginning to evolve as new ASTM standard procedures.

OBJECTIVES AND ORGANIZATION ASTM Committee D-32 on catalysts was officially organized on January 14, 1975. At that time officers were elected, objectives were decided upon, and a working organization was developed.

Some of the steps leading to the formation of this

committee were reviewed at the first symposium in this series, in 1976, by Dr. J. R. Kiovsky of the Norton Company. Although only three years old, this committee has nearly 100 members from the United States and several other countries. Membership has been drawn from catalyst manufacturing companies, petroleum refining companies, the chemical process industries and government and academic laboratories. The objectives of the committee are to develop standard test methods for the characterization of catalysts and related materials and to stimulate research in these areas. For all practical purposes, "catalysts" refers to the usual heterogeneous catalysts broadly used in industry. Consideration has already been given, however, to other forms of catalysts such as supported enzymes and the organometallics. It is likely that, in time, committee activities will be broadened to consider standards for these and other new materials

as

well.

In order to meet its objectives, it was decided to organize the committee into technical subcommittees, each dealing with major areas of catalyst characterization. The four technical subcommittees are the following.

720 0

Physical Chemical

0

Physical-Mechanical

0

Chemical Analysis

0

Catalytic Properties

Other ways of organizing the technical work of the committee were considered, for instance, dividing it into areas of application such as petroleum refining and petrochemical processing.

It was felt, however, that the organization chosen was

the most effective, since it allows each member to concentrate his efforts in those areas where his expertise is used to best advantage. In addition to these technical subcommittees, there are other supporting subcommittees, which assist the technical subcommittees in the collection, handling and presentation of data, in the details of writing standard procedures and in interfacing and cooperating with other committees. Four of these supporting subcommittees are presently active as follows. 0

Editorial

0

Nomenclature and Definitions

0

Statistics and Data Handling

0

Liaison

It should be pointed out that it is the intention of Committee D-32 to see that

its evolving standard procedures are written in metric terms using SI units. TECHNICAL ACTIVITIES Within each of the technical subcommittees, the development of individual test methods is largely carried on through "task groups". Typically such a group will contain 10 or 15 individuals, particularly interested in and skilled in a particular area of characterization. The current activities within Committee D-32 are best illustrated by recounting the task groups which are active in each of the subcommittees. First, however, it might be of interest to consider the criteria by which the current projects have been selected from the many areas of testing which were considered. In deciding among the many types of tests which might be worked on, two questions are usually asked. These are:

1. Is a standard test really needed at the present time? 2.

Can a standard procedure be developed with reasonable effort?

This tends to eliminate procedures which are of interest only in catalyst research, and not really necessary for the buying and selling of commercial catalysts. This also gives low priority to very sophisticated analytical and characterization procedures which would be difficult to standardize, and where the required capabilities might exist in a limited number of laboratories or installations. Essentially all of the areas of current activity, enumerated below, clearly meet these criteria. Of course, some of the tests currently being developed will require a great deal of

work and it could be several years before they lead to standard procedures.

721

The present projects under study by the Physical Chemical Subcommittee are the following. One group, working on surface area determination by nitrogen adsorption has completed development of a multi-point method, and only the full society ballot is required for final approval of this as an ASTM standard. It is likely that this will be the second published standard from D-32. This group has now turned its attention to a single-point method and ~ $ 1 1soon begin to consider dynamic techniques. A second group is working on methods for pore size distribution and is very close to writing a draft of the proposed method. Another group is working on the measurement of zeolite content of cracking catalysts by x-ray techniques. Finally, chemisorption techniques for the determination of metals dispersion have been studied both for nickel and for noble metal catalysts. The Subcommittee on Physical-Mechanical Properties has active task groups working on methods for bulk density determination, particle size measurement, attrition and abrasion. Another group is working on both single-particle and bulk crush strength methods. In this area it is quite obvious that standardized techniques are highly desirable, since specifications on these properties are commonly used in the purchase of most industrial catalysts. The Chemical Analysis Subcommittee has produced the first actual published standard procedure arising from D-32 activities. This is a method for the determination of cobalt in cobalt-alumina catalysts by a potentiometric technique. The same task group is now working on a draft for a procedure for the measurement of molybdenum which may be applied to the widely used CoMo and NiMo catalysts. Task groups have also been active in the analysis of noble metals as found in reforming and selective hydrogenation catalysts and in the oxidation catalysts on many late model automobiles. Under development at the present time are wet chemical procedures for assay type analyses, as well as rapid x-ray fluorescent procedures which might be used for plant control. It is especially important in the area of chemical analysis that liaison be maintained with other ASTM Committees, and with other bodies interested in chemical analysis, since many of the analytical procedures already developed by other groups can be easily adapted to the special problems of catalyst analysis. Finally, the Catalytic Properties Subcommittee is interested in developing standardized procedures for measuring the actual performance of catalysts. Describing the activity, selectivity or other performance characteristics of a catalyst obviously involves testing it f o r a particular process application. This is a very difficult area technically, because of the many variables which must be standardized in any given test. Also, the details of such tests often are considered proprietary by the companies using such tests. In this area, it has been necessary to take a much longer range viewpoint, in expecting to come up with actual standardized procedures. This group, however, has already selected one catalyst activity test

722

which obviously meets the criteria mentioned before, of need and standardizability. This is a microactivity test for fluid cracking catalysts. A test of this type is already broadly used in the petroleum industry throughout the world. This has been a particularly appropriate place to start trying to develop broadly useful standard catalyst activity tests.

The successful development of such a microactivity

test for cracking catalysts should lead to a succession of other such standard procedures. Certainly there are many types of catalysts in common industrial use where standardized performance tests are necessary and achievable. In addition to the efforts toward developing standard activity tests, other groups within this subcommittee are concentrating on developing standard reactor design criteria and also standard approaches to analyzing and reporting performance data. To date, ASTM Committee D-32 has been fortunate to have the participation of several individuals and companies from Europe.

In addition, we have had contact

with, and offers of cooperation from, several national and international groups with similar interests in developing standard testing procedures.

ASTM D-32

welcomes the participation of any individuals who are willing to lend their efforts toward the objectives of ASTM D-32,

and will strive to cooperate with any groups

working along similar lines. DISCUSSION J.W. GEUS

:

Can you give any details about the method(s) ASTM is using

to calculate the size-distribution of mesopores from the experimental nitrogen sorption data ? A.H. NEAL

:

Work toward standardizing pore size distribution by nitro-

gen sorption is in an early stage of development.

Circulation of a

single 82-point isotherm among several laboratories with each calculating pore size distribution by their own method showed considerable disagreement. This indicated the need to reach consensus on data treatment, before proceeding with the standardization of the experimental procedures.

This will be the next step, but this is not present-

ly being worked o n , since the same task group is concentrating on mercury penetration methods.

723

B6EASUREMENT OF THE ACTIVITY OF SOLID STATE CATALYSTS G.K.

BORESKOV

Institute of Catalysis, Novosibirsk

,U

S S R

This communication is devoted to a consideration of catalyst test methods for quality control of industrial catalysts, the improvement of test methods and the development of new catalysts. It is not expedient and, in most cases, quite impossible to test the catalysts under conditions duplicating their industrial application. Customarily the procedure is to measure activity under isothermal conditions. This is then used as a basis for estimation of results to be obtained in large scale use. The great variety of catalytic proceeses precludes any possibility of creating allpurpose installations. However, for certain groups of catalytic reactions common ideas and similar designs of individual units may be employed. Most catalytic processes are carried out under steady state conditions with essentially constant catalytic properties. Therefore, during tests the steady state reaction conditions conforming to a given temperature and reaction mixture composition must be achieved. For these processes we recommend a gradientless method for measuring the activity with rapid circulation of the reaction mixture. This method permits direct measurement of the reaction rate, excluding the effects of external diffusion and non-uniform flow of the reaction mixture through the catalyst bed. Furthermore, variations in the temperature of the catalyst bed as a result of the reaction (even for processes with sizeable heat effects)may be reduced to a minimum. Electromagnetic pistons or membrane pumps may be used for circulation. It is thus poasible to test randomsized catalyst grains and in the case of individual large grains to study the internal diffusion of reaction mixture components. These results may be used directly to calculate the operating parameters of the catalyst in industrial reactors. When developing catalyst preparation methods it is useful to measure catalytic activity in the kinetic region as well. This is done with finely

724

divided particles of the catalyst under study. Such permits estimation of the effect of porous structure , thus determining optimal pore size distribution. For catalyst evaluation, measurements should be carried out at average rates of conversion , sufficiently far from equilibrium. Regulation of component feed, temperature, and the choice of sample for analysis should be automated. Computers should be used for the calculation of results. In the case of complex multi-path reactions the most advantageous scheme involves feed-back between the computer and the operating unit to control variations in parameters. For a limited number of steady state catalytic processes, the gradientless method is difficult. Problems arise because of sidereactions, e.g. polymerization in the reactor, oxidation of ammonia to nitrogen oxides, or methanol oxidation on silver catalyste, proceeding via external diffusion. Any attempt to carry out these reactions in the kinetic region results in a decrease in selectivity. Results cannot then be characterietic of the catalyst working under industrial conditions. For these reactions the catalyst should be te sted in flow-circulation installations operating via external diffusion. For catalysts working under non-steady state conditions, the testing procedure is much more complicated. In this case it is necessary to determine the catalytic activity for different catalyst states reached in the course of operation as well as the rate of catalyst-state variation when changing the reaction mixture composition and temperature. Here the beet approach is a pulse method involving determination of the catalytic activity and calculation of the change in catalyet composition from the material balance. If analysis of the reaction mixture proceeds very rapidly, the change in activity may be investigated using the gradientlees and f l o w methods. It is extremely important t o avoid a lag between variations in the reaction mixture composition and variations in the catalyst properties. For this reason the volume of the reaction system including the circulation loop must be minimal. Tests of catalyst thermostability and duration of operation reduce to a determination of the change in catalytic activity after exposure to the conditions in question.

725 W.J.

THOMAS

c o m m e n t : I would l i k e . t o c o n c u r w i t h and e m p h a s i z e o n e

r e m a r k m a d e in P r o f . Boreskov's c o n t r i b u t i o n o n t h e m e a s u r e m e n t o f c a t a l y s t activities.

S o m e important i n d u s t r i a l p r o c e s s e s p r o c e e d

v i a c o n c u r r e n t o r c o n s e c u t i v e p a t h s and a r e a l s o affected by intraa n d inter-particle t r a n s p o r t resistances.

Any attempt to standardise

c a t a l y s t s for s u c h r e a c t i o n s should t a k e c o g n i z a n c e o f t h e e f f e c t of t r a n s p o r t r e s i s t a n c e s o n r e a c t i o n selectivity.

At the University of

B a t h w e h a v e r e c e n t l y completed some w o r k w h i c h d e m o n s t r a t e s t h a t r e a c t i o n s e l e c t i v i t y m a y b e either s u p p r e s s e d or e n h a n c e d f o r c e r t a i n c l a s s e s o f r e a c t i o n s by c h a n g e s in inter- and intra-particle r e s i s t a n c e s to h e a t and m a s s transfer.

T h u s a c t i v i t i e s s h o u l d b e compared

for c l o s e l y p r e s c r i b e d c o n d i t i o n s in t h e d i f f u s i o n limited r e g i o n r a t h e r than t h e k i n e t i c r e g i o n w h e n n o r m a l i s i n g c a t a l y s t s f o r c e r t a i n reactions.


727

THE COUNCIL OF EUROPE RESEARCH GROUP ON CATALYSIS E.G. DEROUANE FacultEs Universitaires de Namur, rue de Bruxelles, 61, B-5000 Namur. Belgium. At the beginning of 1975, under the auspices of the Committee on Science and Technology of the Parliamentary Assembly of the Council of Europe, the Research Group on Catalysis (of the Study Group on Surface Chemistry and Colloids) was founded. Its aim was to generate an efficient link between individuals and laboratories willing to share information and cooperate in the fields related to the "surface reactivity and catalytic properties of highly dispersed metals and alloys". Contacts with the Council of Europe (CE) Research Group on Catalysis can be made either through: Professor Eric G. Derouane, Doctor J.P. Massus, Facultas Universitaires de Namur, Laboratoire de Catalyse, or Rue de Bruxelles, 61, B-5000-Namur. Belgium

Conseiller Scientifique, SecrEtariat de la Commission de la Science et de la Technologie, Conseil de l'Europe, F-67006-Strasbourg Cedex. France.

Members of the CE Research Group on Catalysis are: Prof. H. Gruber (Austria), Prof. B. Delmon, Prof. E. Derouane, Dr. A. Frennet, Prof. G. L'Homme (Belgium), Prof. J.J. Fripiat, Prof. G. Gault, Prof. B. Imelik, Prof. R. Maurel, Dr. C. Naccache, Dr. J.C. VEdrine (France), Dr. J.K.A. Clarke (Ireland), Prof. J.W. Coenen, Prof. J.W. Geus, Prof. V. Ponec, Prof. J. van Hooff (Netherlands), Prof. D.L. Trimm (Norway), Prof. R. Larsson, Dr. H. Lervik (Sweden), Prof. G.C. Bond, Prof. C. Kemball, Dr. R. Joyner, Prof. M.W. Roberts, and Dr. P.B. Wells (United Kingdom). Among the common programmes of the CE Research Group on Catalysis is the European Reference Catalysts project (Eurocat project). The aims of this joint effort are as follows: i. to have a number of common reference catalysts that will be distributed among the laboratories affiliated to the Group and eventually made available to other laboratories upon request and according to their availability. All the reference catalysts are supported metals. A 6% Pt on silica and a 20% Ni on silica are presently available and their characterization is in progress. At a later date, the Group intends to study a 2% Pt on carbon and a 2% Pt on alumina.

728

ii. to characterize in depth these reference catalysts by all the methods which are currently available in the laboratories participating in the project. Among others, the following characterization tests and methods are used:

-

Analysis by various chemical and physical methods : metal and impurities

-

Adsorption : physisorption, chemisorption, titration, and porosity measure-

contents. ments.

- Physical

techniques: electron microscopy (TEM, SEM), electron spectroscopies

( U P S , X P S , AES), magnetic measurements (static and FMR absorption), EXAFS.

-

Catalytic activity and selectivity tests for hydrogenation (mainly benzene) and exchange (olefins and paraffins) reactions.

iii. to compare and discuss critically the results obtained in the various laboratories in order to evaluate and compare the accuracy and reliability of the techniques and testing conditions that are used and optimize these. iv. to use these catalysts as reference materials, i.e., as standardizing catalysts (common reference points) for a better and quantitative evaluation of catalytic data pertaining to a limited and selected number of catalytic reactions; the attention will be most particularly focussed on reactions and mechanisms pertaining to the synthesis and conversion of hydrocarbons. R e s u l t s and their discussion will be made available to the catalytic community

by publication in the relevant journals and through a detailed report to be issued

by the Committee on Science and Technology of the Council of Europe.

129

CONCLUDING REMARKS A

scientific symposium has the primary function to convey to the

participants, as directly and efficiently as possible, the scientific information detained by the authors of the lectures and communications and by the other participants, and to promote, during the discussions and conversations, the development of new concepts, new interpretations and new lines of research.

It also has other less obvious and visible

functions, which reach, well beyond purely scientific problems, more general questions related to the social and economical role of fundamental and applied research.

Mgr E. Massaux, Rector of the

Universite Catholique de Louvain, Professor G. Smets, President of the Union of Pure and Applied Chemistry (IUPAC), and Dr. A. Lecloux, President, SocietB Chimique de Belgique mentioned various aspects of these functions.

The very lively Round Table Discussion which took

place at the end of the Symposium put into light many other interesting ideas. The present concluding remarks will be an attempt to speculate on what has been, or what could ideally have been, the "other" functions of the Second International Symposium on the Scientific Bases for the Preparation of Heterogeneous Catalysts. In many countries (and, most probably, in

countries), a major

problem is to foster the contact between fundamental science and applied or industrial science.

Innumerable methods were imagined,

on the one hand, to help that industry benefits completely of the progress of science and, on the other hand, for encouraging scientists to study problems of interest for industry.

When he took the IUPAC

presidential charge, Professor G. Smets said that "there should not be a separation of the applied field from the fundamental". "Today, there is a much greater overlap and interplay among the different branches of chemistry".

If the distinction, or the

border, between the scientific and technological domains becomes less clear, the real problem is to foster contacts between scientists working in universities or academic institutes, and scientists and engineers working in industry.

In principle, a

symposium can provide such contacts. The Rector of the Universite Catholique de Louvain, in his welcoming address, cited the sentence describing the general philosophy of the Symposium, emphasizing three words

:

the Symposium

730 concerned "selected scientific domains involved in the preparation of

+,

i.e. industrial heterogeneous catalysts".

With the subject

described this way, the Symposium attracted investigators working in pure science as well as engineers.

The half-day session on

"Catalyst Normalization" had the same effect.

And, indeed, a good

balance in the number of scientists an(3. engineers among the registrants, and in their respective contributions (in the form of lectures, communications or discussions) was achieved, with slightly more than half the participants coming from industry and only slightly less than half of the contributions being presented by them. This approximately equal representation tends to characterize many meetings on catalysis nowadays. A meeting like our Symposium may serve as a forum for contact between fundamental science and applied or industrial science. But there is no guarantee that making the people meet together will really initiate communications between them. On a purely scientific and rational basis, there are some reasons why communication might be difficult.

The main reason is

the pluridisciplinarity of applied science, namely the fact that each field of technology makes use of scientific facts coming from many domains.

Enumerating the scientific domains involved in the

preparation of heterogeneous catalyst might be long, with inorganic and possibly organo-metallic chemistry, surface chemistry, colloid chemistry and solid-state chemistry being some of the major headings.

There is some paradox in the fact that University,

which is naturally inclined to investigate all the directions of human thinking and to protect and develop all the disciplines of science, which, in summary, strives to universality, actually houses and produces primarily monodisciplinary minds.

It is one

task of meetings like our Symposium to initiate mutual communication between different disciplines.

The various discussions during the

Symposium, and especially during the Round-Table, indicated how imperfect the result still is, except possibly for some aspects of solid state chemistry and colloid chemistry, where fruitful contributions begin to appear. The role of University is to teach.

One participant said that

"Industry had come [to the Symposium] more to listen than to tell". Although scientists should be modest vis

vis their colleagues

in industry, there is some feeling that a meeting like our Symposium

"may contribute to the diffusion of knowledge among people absorbed by the demanding tasks of industrial activity".

Teaching supposes

that the teacher assembles disparate science in self-consistent, logically organized bodies of knowledge (theories or interpretations); indeed, real teaching is not only teaching how to reproduce results, but mainly teaching the facts and c m c e p t s which will serve a s new starting points for obtaining new results, for the attainment of more distant goals.

In catalyst preparation, the need of such a

teaching is immense, because, in this field, as a participant said "there is very little in terms of rational quantitative science and [this] reminds one of the Middle Ages", adding humoristically that, "even then, there was more theory in the Middle Ages!" There are other reasons, of human or sociological nature, for which communication might be difficult between participant to interdisciplinary meetings on an applied field of science. There are first the prejudices that scientists and engineers nourish against each other.

They are common-place : research in

the University is "academic", the making of catalysts is "no science", etc.

What might be surprising for an outsider is that

the prejudices are not the privilege of the "other" category. University scientists sometimes feel that engineers are not multidisciplinary enough, and during the Symposium Round-Table, an equal number of scientists from University and from Industry expressed the view that (let us keep the past tense) "there was many recipes found over many years, and many facts and pseudo-facts which may or which may not be true" in catalyst formulation and manufacture, or

:

"catalysis is, on the whole, empiricism still".

Some discussion concerning these prejudices was made in the General Remarks of the First Symposium of this young series and during the Round-Table, but we believe the mutual imperfect appreciation of each other contribution is not a major factor against contact between basic research and industrial practice. The participants to the present Symposium, as those to all meetings on catalysis, know each other quite well, and generally recognize the value of the various kinds of contributions. Much more of a nuisance might be the problem of secrecy, which was already mentioned in the General Remarks of the First Symposium, and which gave way to lively intervention during the Round-Table of the second one.

Some quite interesting ideas were

expressed, and, conspicuously, they all came from Industry.

First,

there is the general feeling that "the real knowledge of catalyst preparation is presumably located in industry, with some research also going on in universities, and may be expanding in recent years..

.

A lot of [that knowledge] cannot be protected by patents,

and giving it away free would help the competition possibly more than the Company which was holding it out".

Even keeping in mind

that "industrial people have not told everything about their best catalyst" that "they have not told how to do and what to do". one participant remarked that "perhaps they have told how not to do and what not to do [to their University Colleagues] and that is enough to have a meeting".

Another correction to these view is that

knowledge detained by industry is of another nature than the one which is normally transmitted in scientific meetings

:

"when we

are talking about industrial know how, in the field of catalysis,

...

what we are often talking about is formulations which work, as opposed to an organized body of knowledge

...

There is a lot of work

which can be shared with the universities on obtaining an organized body of knowledge.

I do not say we are hiding it in industry.

I do not think it is hidden in industry in the field of catalysis. It may be hidden in some other subjects, but not catalysis". Even supposing that a large amount of knowledge is hidden by industry, the participants rightly recalled that the extent to which knowledge is hidden is necessarily limited.

On one hand,

a catalyst manufacturing company must "publish as much knowledge as possible, for two reasons.

One reason is that the customers,

in order to u s e [our.] catalysts, have to know a lot of what we could call proprietary knowledge.

Another [reason], which should

not be forgotten, is the value of public relation by distributing knowledge, by publishing papers, preferably of a scientific nature". On the other hand, a senior scientist of a major oil company reminded his "academic colleagues that there is in fact a tremendous amount [of information] available on this or that company is doing.

...

the latest thing that

Unfortunately, this is in the

patent literature, and is not the accepted kind of literature, that one gets in the mail, as a scientific journal

...

It is almost a

necessity for having a valid patent to have things in there in such a way that anyone that is knowledgeable in the field can repeat it".

733 Most gratifying for the organizers of the Symposium was the following remark from Industry, which related

directly to the

functions of a Symposium grouping University scientists and Industry engineers

:

“If you did not have a conference of this

nature, there is no way to flush out the information that we have“. The idea emerged from the Round-Table discussions that when considering specific problems concerning a specific catalyst, and more general ones, common to many systems, the secrecy constrainsts apply only to the former.

It is our personal feeling that

scientific research in University is profitably inspired by the practical problems of Industry, but that only problems having a reasonable degree of generality are of interest. from Industry confirmed

:

A

scientist

“all our problems [in Industry] are,

basically, not specific problems.

I feel that I do not learn

relatively much out o f another paper on a specific catalyst

. . . ‘ I .

If this is true, there is much hope that communication could function satisfactorily.

But we have the impression that University

scientists and Industry engineers need cooperation for extracting the general problems from the specific scientific studies and specific practical difficulties. We might conclude this part of our considerations by saying that the discussions confirmed, more clearly than we had thought, the central role o f a meeting like our Symposium in helping communication between Industry and University. But this kind of Symposium might well have, in addition, a quite different function.

There are several reasons, pertaining

to the structure of our societies (here, eastward or westward) which accentuate the natural predilection of University scientists to stick to his or her scientific domain, to keep away of pluridisciplinarity to avoid new budding fields and to ignore the industrial aspects of science.

Working in well established

fields is comfortable, in many respects

-

too comfortable!

Intellectually, this obviously demands less effort than grasping new areas, new relationships between branches of science.

For

the financial welfare of a laboratory, long-established frames and circuits of money allocation are often more secure than hypothetical and timid new programs, unless the new field is obviously part of a fashionable subject of national or international interest.

On psychological grounds, a scientist will feel more

secure in his field or micro-field than when discussing with

specialists of other disciplines with a different jargon, and his convivial spirit will be more gratified by meeting again and again his old fellow-specialists, who usually do not forget to cite them in communications and discussions. describe ironically this situation.

It is easy to

But, is it sure that the system

the scientists are part of is not responsible for that situation ? The career of a scientist largely depends on the recognition of his work by his peers.

Let u s assume that his value on the job

market is measured by this Recognition, e.g. by his Science Citation Index Number, that we shall simply note R; R is approximately given by

:

R = N x A x V where N is the Number of people working in the field (the fellowspecialists),

A

the time the field has been established, e.g., its

Age, and V the Value of the cited publication.

Two factors out

of three are maximalized if the scientist keeps working in his relatively old, monodisciplinary field. we feel that one of the major function of a meeting like our Symposium is to help create recognition for work done outside the well established fields.

It is to attract more interest from the

University scientists in multidisciplinary fields and in fields highly related to industrial activities.

It is to raise these

fields from the level of practical knowledge to hopefully fashionable domains, ultimately recognized as "fully scientific". Scientific meetings on multidisciplinary topics which, sometimes. look little scientific, because they are inspired by practical or industrial problems, certainly contribute to the recognition of the corresponding investigations as truely scientific.

These

meetings can incite University laboratories to make the high quality research which the respective countries have the right to expect, in view of the amount of money they spend. This is the point of view of an University scientist.

We

presume that a meeting like our Symposium also has additional functions, specific for fulfilling the needs of Industry investigators and engineers. In view of the above considerations and of the clearly expressed

hope of the participants, the organizers of the Symposium feel that a third one should be added to the series, probably in 1982. They will try to retain what was estimated to be good in 1978. But clearly, a continued effort should be made for ensuring maximum pluridisciplinarity to the future Symposium. B. DELMON

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Petrobras I l h a d o Fundao Q u a d r a 7 (CENPES)

ADAMIS V .

Degussa Wolfgang Technical Service Catalysts Division P.O. Box 602 6450 Hanau 1

A L B E R T R.

AL-CHALABI

n.,

Dr.

Jr.,

ALLEY S.K.,

Dr.

s.P.s.,

Dr.

G.J.,

Dr.

ANUNDSKAS A . ,

AOMURA K.,

ARNTZ D . ,

U n i o n O i l Company o f C a l i f o r n i a Research Center Box 76 B r e a , CA 92621

U.S.A.

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ANTOS

KUWAIT

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ANDERSSON A .

ANDREW

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Kuwait I n s t i t u t e f o r S c i e n t i f i c Research P.O. B O X 1 2 0 0 9 Safat

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BRASIL

Dr.

Prof.

Dr.

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.

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736 BACHELIER J.

BADIE P.

BADILLA-OHLBAUM, R.

BAERNS M., Prof.

BANKS, R.G. S

.

BARANSKI A., Prof.

BARBY D., Dr.

Universite de Caen Ensica Avenue d'Edembourg F-14000 Caen

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Unilever Research Lab. Port Sunlight, Wirral Merseyside

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BARRACLOUGH R.N., Dr.

National Res. and Dev. Corporation P.O. Box 236, Kingsgate House, 66/74 Victoria Street ENGLAND London SW1

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Produits Chimiques Ugine Kuhlmann Centre de Recherches de Lyon F-69310 Pierre-Benite

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British Petroleum Co. Ltd. Research Center Chertsey Road, Sunbury-on-Thames Middx. Air Products & Chem. Inc. P.O. Box 538 Allentown, Pa. 18104 Laporte Industries Ltd. Moorfield Road Widnes, Cheshire, WA8 OJU

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American Cyanamid Co. 1937 Westmain Street Stamford, Conn. 06903

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Pro-Catalyse 21, rue Jean Goujon 75008 Paris

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737 BETTAHAR M . ,

.

0 . N . R. S Abri-Hydra Alger

Dr.

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ALGERIA

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BLASER H . U .

C i b a - G e i g y AG R-1060 3124002 B a s e l

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Montedison S.p.A. I s t i t u t o Donegani V i a G. F a u s e r , 4 Novara

Prof.

BRADSHAW D . I . ,

Dr.

ERANDENBURG J . A . ,

B R A U N G.

Stanford University D e p t . o f Chem. E n g i n e e r i n g Stanford University S t a n f o r d , CA 94305 B r i t i s h G a s Corporation London R e s e a r c h S t a t i o n M i c h a e l RD, Fulham London SW6 2 A D

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Dr.

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BRUNELLE

J

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Dr

.

O . ,

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.

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BUSCH-PETERSEN B.

I

Unilever Research P.O. Box 114 3130 AC Vlaardingen United Technologies Research Center East Hartford, Conn.06108

BUKOWIECKI S., Dr.

CAHEN R .

D.D.R.

Prof.

CHABOT J., Mrs.

CHADWICK D., Dr.

CHARCOSSET H., Dr.

Labof ina S .A. Chaussee de Vilvorde 98-100 1120 Bruxelles

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Prof.

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Haldor T o p s d e A/S Nymdllevej 55 DK-2800 Lyngby

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COSGROVE L.A.,

Dr.

FRANCE

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P.O.

C O S Y N S J.

COURTINE P.

COURTY Ph.

DAUMAS

J.c.,

Dr.

DE BEER v.H.J.,

Dr.

DECLERCK C1.

DECLIPPELEIR G.,

DELLER, Dr.

Dr.

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.

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DEWOLFS R . ,

Dr.

DIJKHUIS C.G.M.. Dr.

DISTELDORF J., Dr.

DJALALI M., Dr.

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.. .

.

W GERMANY

I RAN

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.

.

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ENGLER M.

.

FATTORE V., Dr.

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FERINO I. , Dr.

FERRARIO M.

FIGUEIREDO C.

Snamprogetti S.p.A. Via Fabiani 1 S. Donato Milanese Institute of Catalysis Academy of Sciences of USSR Siberian Branch Novosibirsk

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-

FISCHER L., Dr.

FISHEL N.A.

,

Dr.

FONTANA P., Dr.

FOUQUET G., Dr. FRENNET A.

,

Dr.

Chemische Werke Hills GB i/Anorg. Abt. D-437 Marl Monsanto 800 North Lindbergh S t Louis, Mo. 63166 Alusuisse Forschung & Entwicklung CH-8212 Neuhausen BASF AG 6700 Ludwigshafen Labo. Catalyse FNRS Chimie G6nBrale-ERM 30 Av. de la Renaissance Bruxelles

BRASIL

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U.S.A.

SWITZERLAND

W. GERMANY

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742 Shell Laboratory Badhuisweg 3 1 0 3 1 CM A m s t e r d a m

GAAF J .

Facultes U n i v e r s i t a i r e s N o t r e D a m e de l a P a i x L a b o r a t o i r e de C a t a l y s e r u e de B r u x e l l e s B-5000 N a m u r

G A B E L I C A Z., D r .

GALIASSO R . ,

THE NETHERLANDS

BELGIUM

Intevep

Dr.

Sede C e n t r a l I n t e v e p A p a r t a d o Postal 149 Los T e q u e s GAMAURY J . F .

GAT1 G.,

,

Dr

.

U n i v e r s a l Matthey Rue E p i n a l C a l a i s 62100

GEMBICKI S.A.

GEORGE Z.M.

G E U S J.W.

Dr.

U.S.A.

Alberta Research Council 11315-87 A v e n u e E d m o n t o n A l b e r t a T6G 2C2

CANADA

State U n i v e r s i t y U t r e c h t A n a l . Chem. Lab. C r o e s e s t r . 77A Utrecht

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C i b a - G e i g y AG R e f . PE 3.32/CH/mc Pos t f a c h CH-4002 B a s e l S I C C CIMS/4 Shell Centre London S E I 7 P G

GROENENDAAL W.

Min6rale BELGIUM

SW I T Z E R L A N D

ENGLAND

N e s t e Oy Research Center

GRONSTRAND M .

SF-06850 GRYAZNOVA Z . V . ,

HUNGARY

UOP P r o c e s s D i v i s i o n 2 0 UOP P l a z a A l g o n g u i n & M t Prospect R o a d D e s P l a i n e s , 111. 6 0 0 1 6

G r o u p e de P h y s i c o - C h i m i e e t de C a t a l y s e P l a c e C r o i x du Sud 1 B-1348 Louvain-la-Neuve

Dr.

GREMMELMAIER C . ,

FRANCE

H i g h Pressur R e s . I n s t . 11-2443 Szazhalombatta Pf 3 2

Dr.

GRANGE P . ,

VENEZUELA

Dr.

Kulloo

FINLAND

Moscow U n i v e r s i t y , Chemical Department Lenin's H i l l MOSCOW

U.S.S.R.

743 P o l i s h Chem. S O C . Gdanska 719 s t r . 50-344 Wroclaw

G R Z E C H O W I A K J.

GUBITOSA G . .

GUCZI L.

,

GUIL J.M.,

POLAND

M o n t e d i s o n S.p.A. I s t i t u t o Donegani V i a G. Fauser, 4 Novara

Dr.

I n s t i t u t e of I s o t o p e s P.O. Box 7 7 H-1525 B u d a p e s t 1 1 4

Prof.

ITALY

HUNGARY

I n s t i t u t o Rocasolano S e r r a n o 119 Madrid-6

Dr.

SPAIN

HABERSBERGER K., D r .

Heyrovsky I n s t i t u t e of P h y s i c a l Chemistry and E l e c t r o c h e m i s t r y C z e c h o s l o v a k Academy o f S c i e n c e s Machova 7 CS-121 3 8 P r a h a 2 CZECHOSLOVAKIA

HAEGH G.S.

Central Institute for Industrial Research Forskningsv. 1 P.O. Box 3 5 0 Blindern, Oslo 3 B a y e r AG Z B ZF P e t r o c h e m i e u n d Verfahrensentwicklung Q 18 D-5090 L e v e r k u s e n

HEGEDUS L . L . ,

Dr.

BENRION

P.N.,

Dr.

HERMANN W .

HIGHTOWER J . W . ,

Prof.

U.S.A.

W . GERMANY

CEN-SCK B-2400 M O l D e g u s s a Wolfgang Gruenaustr. 3 D-645 Hanau 9

HENSEL J . , D r .

.

W GERMANY

G e n e r a l M o t o r s Co. Research L a b o r a t o r i e s W a r r e n , Mich. 4 8 0 9 0 C h e m i s c h e Werke H D l s AG 2BFE/33 PB 1 5 P o s t f a c h 1180 D-4370 M a r l

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NORWAY

BELGIUM

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E l e k t r o k e m i s k a AB Fack 445 0 1 S u r t e

SWEDEN

G e o r g e R. Brown S c h o o l o f Engineer ing Rice U n i v e r s i t y Houston, Texas 77001

U.S.A.

744 HOLT A.

HUTCHINGS G., Dr.

IANNIBELLO A., Dr.

INACKER 0 .

Dr.

I

IRGANG M., Dr. IRVINE E.A.,

Dr.

ISAEV O., Dr.

JACOBS P .A. I Dr.

ENGLAND

Imperial Chemical Industries Ltd. Petrochemicals Division HQ Wilton, P.O. Box 90 Middlesbrough Cleveland

ENGLAND

Stazione Sperimentale Combustibili Via A. De Gasperi 3 S. Donato M.

,

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BASF AG D-6700 Ludwigshafen

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Imperial Chemical Industries Ltd. Agricultural Division Q. Buildings (Catalysis and Chem. Group) Billingham, Cleveland

ENGLAND

Institute of Chemical Physics Academy of Sciences of USSR 117334 Vorob'evskoe Shosse 2-6 Moscow

U.S .S .R.

Katholieke Universiteit Leuven Laboratorium voor Oppervlakte en Colloidalescheikunde de Croylaan 42 B-3030 Heverlee

BELGIUM

JANSSEN M.M.,

Dr.

JENNINGS J.R., Dr.

JENSEN E.J.

JOHANSEN K.

FRANCE

CERIA/IIF-IMC Avenue Gryson 1 B-1070 Bruxelles

Dr.

ITALY

Dornier System , NTF Postfach 1360 D-7990 Friedrichshafen

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JACQUES JANNES G.

Universal Matthey Products Ltd. Underbridge Way, Brimsdown Enfield, Middx.

Shell Research Postbus 3003 Amsterdam

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Imperial Chemical Industries Ltd. Agricultural Division Billingham, Cleveland

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Haldor Topsbe A / S Nymbllevej 5 5 DK-2800 Lyngby

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Haldor Topsbe A/S Nymbllevej 55 DK-2800 Lyngby

DENMARK

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JOHNSON M.M., Dr.

JOUSTRA A . ,

DrS.

JSOVER B., Dr.

JUHASZ J.

KAWAGUCHI T.

Dept. of Chem. Techn. Kemicentrum Box 740 S-22007 Lund

SWEDEN

Phillips Petroleum Co. 347 RB # 1 Bartlesville, Okl. 74003

U.S.A.

Shell Research P.O. Box 3003 Amsterdam

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Elte TTK Slervetlen es Anal. Kem. Tan 52 P.O. Box 123 1143-Budapest

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Elte TTK Slervetlen es Anal. Kem. Tan 52 P.O. Box 123 1143-Budapest

HUNGARY

Department of Chemistry Tokyo Gakugei University Koganei-Shi , Tokyo 184

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KERKHOF F.P.J.M.

Laboratorium voor Chemische Technologie P1. Muidergracht 30 1010 TV Amsterdam THE NETHERLANDS

KIEFFER, Dr.

Institut de Chirnie rue Blaise Pascal, 1 F-67008 Strasbourg

FRANCE

Norton Company P . O . Box 350 Akron, Ohio 44309

U.S.A.

KIOVSKY J.R., Dr.

KLOCKENKEMPE R

KOCH

n.

KOCHLOEFL K., Dr.

KOHLER M.

Universitat Mdnster Inst. fdr Physikal. Chemie Schlossplatz 4 44 Mdnster

W ,GERMANY

Institut fiir Technische Chemie Technische UniversitZt Berlin Strasse des 17 Juni 135 1000 Berlin 12

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Technische Hogeschool Delft Lab. voor Organische Chemie Julianalaan 136 Delft THE NETHERLANDS

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Siemens AG Bau 32 Giinter Scharowski-str. D-8520 Erlangen

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KOSTROV V.V., Dr.

Chem. & Eng. Institute of Ivanovo 1 Engels St!., 153460 Ivanovo ' I . S . S .R.

KOTTER M.

Institut fdr Chem.Verfarenstechnik Universitat Karlsruhe P.O. Box 6380 D-75 Karlsruhe W.GERMANY

KRAUS M.,

Dr.

Institute of Chemical Process Fundamentals Czechoslovak Academy of Sciences Suchdol CZECHOSLOVAKIA 16502 Praha 6

KREMENIC G., Dr.

KRENZKE L.D.,

Dr.

KRICSFALUSSY Z.. Dr. KRISHNAMURTHY K.R., Dr.

KRISTIANSEN L.A., Dr.

KRIVORUCHKO O.P., Dr.

Inst. Catalisis y Petroleoquim. Serrano 119 Madrid 6 CNRS Centre de Recherche sur les Solides a Organisation Cristalline Imparfaite rue de la Ferollerie 1B F-45045 Orleans Cedex Bayer AG D-5090 Leverkusen Indian Petrochemicals Corp. Ltd. Research Center Baroda 391346 Norsk Hydro A / S Research Center 3901 Porsgrunn Institute of Catalysis pr. Nayki, 5 630090 Novosibirsk 90

SPAIN

FRANCE W .GERMANY

INDIA

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U.S.S.R.

KRUISSINK E.C., Drs.

Techn. Univ. Delft Lab. Inorgan. and Phys. Chem. Julianalaan 136 Delft THE NETHERLANDS

KRYLOVA A.V., Dr.

Chem. Eng. Mendeleev's Institute of Moscow MOSCOW A -47

U . S . S .R.

741 KUBASQV A.A.,

Dr.

K U C K M.A.

U.S.S.R.

Stauffer C h e m i c a l Co. Eastern Research C e n t e r Dobbs Ferry, N.Y. 10522

U.S.A.

Central Research Laboratory showa D e n K O 2-24-60 2-Chome, T a m a g a w a Q h t a k u Tokyo

KUNO K.

,

L A M B E R T P.

L A U R E R P.

Moscow S t a t e University Moscow

,

LECLERE C.

Dr.

LECLERCQ L.

Dr.

,

LECLQUX A.,

L E I G H D.,

BASF AG D-6700 L u d w i g s h a f e n

Dr.

,

UCB, S.A. Secteur Chimique C h a u s s B e d e Charleroi 4 B-1060 B r u x e l l e s

Dr.

Dr.

Dr.

Dr.

BELGIUM

.

W GERMANY

C o m p a g n i e d e s MBtaux P r B c i e u x bd P. Vaillant Couturier 74 F-94200 Ivry

FRANCE

UniversitB de P o i t i e r s rue du H a u t d e s S a b l e s 174 F-86000 P o i t i e r s

FRANCE

Solvay & Cie r u e de Ransbeek 3 1 0 B-1120 Bruxelles

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Engelhard Industried Ltd. Valley RD, Cinderford GlOUs. GL14 2PB

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L E P A G E J.F.,

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Rhone P o u l e n c Industries 12, Rue des Gardinoux 9 3 3 0 8 Aubervilliers

FRANCE

UniversitB d e L i e g e Chaire de G B n i e C h i m i q u e r u e A. S t B v a r t 2 B-4000 L i e g e

BELGIUM

AV.

LE P A G E ,M.

L ’ H Q M M E G.,

Prof.

L I D E F E L T J .Q.

L O H R E N G E L G.

Kemi sk Reaktions teknik Chalmers Tekn. Hogskola S-40220 Gdteborg-9

SWEDEN

Ruhr-Universitbt Bochum D-4630 Bochum

W GERMANY

.

148 LOPEZ F . J . ,

Dr.

Dr.

IVIC Apartado Caracas

.

McCULLOCH A .

MALINOWSKI S . , P r o f .

W . GERMANY

T H E NETHERLANDS

I m p e r i a l Chemical I n d u s t r i e s L t d . Mond D i v . R & D D e p t . P.O. Box 8 , The H e a t h Runcorn C h e s h i r e WA7 4QD

ENGLAND

I n s t i t u t e of O r g a n i c C h e m i s t r y and Technology Technical University Politechnike 0 0 - 6 6 2 Warszawa Kozykowa 75

POLAND

High P r e s s u r e R e s . I n s t . H-2443 S z a z h a l o m b a t t a P. Box 3 2

MANDY T.

MARION J .

MARSDEN C . E . ,

Dr.

. .

MARTINEZ N P

MARTOS J .

VENEZUELA

A K Z O Chemie

Box 1 5 Amsterdam - N

M A R T A N M.,

FRANCE

1827

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LUDWIG G.

MAAT H .J

VENEZUELA

Soci6t6 Shell Frangaise C e n t r e d e R e c h e r c h e s d e Grand couronne 76530 Grand Couronne

LUCIEN J.

LUDENA E . ,

E s c u e l a de Quimica F a c u l t a d de I n g e n i e r i a U n i v e r s i d a d de Carabobo Valencia

Dr.

HUNGARY

P r o d u i t s C h i m i q u e s U g i n e Kuhlmann C.R.L. r u e Henri Moissan F-69310 P i e r r e - B 6 n i t e

FRANCE

I m p e r i a l Chemical I n d u s t r i e s L t d . Mond D i v i s i o n The H e a t h , R u n c o r n C h e s h i r e W A 7 4QD

ENCLAND

Chemical P r o c e s s e s Weizmann I n s t i t u t e P l a s t i c Department Re h o v o t

ISRAEL

I.U.T. Cumana A p t d o . 255 Cumana, Edo. S u c r e

VENEZUELA

Enpe t r o 1 Centro de I n v e s t i g a c i o n de Enpetrol E s c o m b r e r a s 183 Cartagena (Murcia)

SPAIN

149 MATERNOVA J . ,

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Dr.

1 6 5 0 2 Praha G - S u c h d o l MATHEWS J . F . ,

Prof.

CZECHOSLOVAKIA

U n i v e r s i t y of S a s k a t c h e w a n Dept.

of C h e m i c a l E n g i n e e r i n g

Saskatchewan S a s k a t o o n S I N OW0 Prof.

MATIJEVIC E . ,

CANADA

I n s t i t u t e of C o l l o i d a n d Surface Science Clarkson College Potsdam, N.Y. 13676

MEIDER H . ,

U.S.A.

Institute Rudjer Boskovic B i j e n i c k a 54 41000 Z a g r e b

Dr.

YUGOSLAVIA

U.S. Army DARCOM S t i t S U r . A b r a m s B l d g . Rm 7 4 0 D-6000 Frankfurt/M

MEIKLE J .

MENON P . G . ,

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Universiteit Gent L a b o r . P e t r o c h e m i s c h e Techniek Krijgslaan 271 B-9000 G e n t

Dr.

BELGIUM

K F A J U l i c h GMBH

MENZER R.

P o s t f a c h 1913 D-5170

MILBERGER

E.

I

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J U l i c h IRB

T h e Standard O i l Co. 4440 Warrensville C t r . Cleveland, Ohio 44128

Dr.

U.S.A.

Shell Research Ltd. Thornton Research Centre

M I L E S R.

P.O. Box 1

MITCHELL P . C . H . ]

MOL J . C . ,

Dr.

MONE R .

Dr.

C h e s t e r CH1 3SH

ENGLAND

D e p t . of C h e m i s t r y U n i v e r s i t y of R e a d i n g Whi t e k n i g h t s R e a d i n g RG6 2AD

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C h e m i c a l Technology P l a n t a g e Muidergracht 30 1010 T V A m s t e r d a m

Lab.

AKZO C h e m i e

P o s t b u s 15 A m s terdam MONTARNAL R .

T H E NETHERLANDS

THE NETHERLANDS

I n s t i t u t FranGais du P 6 t r o l e Avenue Bois-Pr6au 4 B . P . 311 F-92500 Rueil-Malmaison

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7 50 MOORE

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C.L.

MORAIS-ANES

J.M.,

MORAVSKY A . P . ,

Dr.

Dr.

CNP Rua A r t i l h a r i a , Lisboa

UM 7 9 - 7 '

Moskowskaja O b l a s t Chernogolovka, Otdelenie

PORTUGAL

ICP

U .S.S . R .

Moscow MORIMOTO T .

MORMINO V . ,

Dr.

MOSCOU L .

U.S.A.

Chiyoda Chemical Engineering and C o n s t r u c t i o n C o m p a n y 3-13 Moriya-cho, K a n a g a w a - k u Yokohama 221

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Snamprogetti S.p.A. V i a Fabiani 1 2 0 0 9 7 S. D o n a t o Milanese

ITALY

AKZO C h e m i e N e d e r l a n d Research Centre Amsterdam P.O. B o x 15 1 0 0 0 AA A m s t e r d a m

THE NETHERLANDS

Soci6t6 S h e l l Fransaise

MOU RE AU X

C e n t r e d e R e c h e r c h e s de G r a n d Couronne F-76530 G r a n d C o u r o n n e

MROSS W . ,

Dr.

MULLER W.H.,

B A S F AG Wak, M310 D-6700 L u d w i g s h a f e n DT.

Exxon P . O . B o x 45 Linden, N.J.

D.L.

NEAL A . H . ,

N E U K E R M A N S H.,

NEWSON E . J . ,

NICLAES H . J .

07036

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Dr.

Dr.

Dr.

.

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H o e c h s t AG

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&

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U.S.A.

Chemicals Europe bte 3

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H a l d o r Topsde A / S N y m d l l e v e j 55 DK-2800 L y n g b y

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BELGIUM

761 NIELSEN P .E .H.

NIERLICH F.

I

Dr.

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DENMARK

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C e n t r a l Institute for Industrial Research

NILSEN B.P.

Forskningsv. P . B . 350 Blindern, NOWECK K . ,

Dr.

1 NORWAY

Oslo 3

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Postfach

C C A C

OZAKI A.,

PAAL Z . ,

V e n e z o l a n o de I n v e s t i g a c i o n e s i e n t i f icas a r r e t e r a P a n a m e r i c a n a K m . 11 p t d o . 1827 aracas VENEZUELA

Inst.

OCHOA DE 0 .

ODENBRAND,

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L.

Dr.

Dr.

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Inst.

PASSERIEUX R.

HUNGARY

Pro-Catalyse rue Jean G o u j o n 21 F-75008 P a r i s

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H.P.C. Produits Chimiques 540 B.P. 87011 L i m o g e s C e d e x

FRANCE

Intevep

PAZOS J . M .

BP R e s e a r c h C e n t e r Process B r a n c h Sunbury-on-Thames Middx. P E P E F.,

JAPAN

Isotopes

P . O . B o x 77 H-1525 B u d a p e s t P A P E E D.

SWEDEN

Dr.

U n i v e r s i t a d i Roma di Chimica Generale Roma

ENGLAND

Istituto

PERNICONE N.

ITALY

Montedison S . p . A . D I P I / A t t i v i t i catalizzatori

via

G.

FaUSer

28100 N o v a r a

4 ITALY

752 PETRO J. I Dr.

PIVOT J.C.

POISSON R. I Dr.

PONCELET G.l Dr.

Organic Chem. Technology Dept. Miiegye tem H-1521 Budapest

HUNGARY

Universite de Liege Inst. de Chimie Industrielle rue A. Stevart 2 B-4000 Liege

BELGIUM

Societe A.D.G. B.P. No 1 69230 Saint Genis Lava1

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Rhone-Poulenc Industries 1 2 , Rue des Gardinoux 93308 Aubervilliers

FRANCE

Groupe de Physico-Chimie Minerale et de Catalyse Place Croix du Sud 1 B-1348 Louvain-la-Neuve

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POSPISIL M., Dr.

Czech. Technical University Dept. of Technical and Nuclear Engineering Brehova 7 11519 Praha 1 CZECHOSLOVAKIA

PRESCHER G., Dr.

Degussa Wolfgang Postfach 602 Abt. FC-0 D-6450 Hanau 1 N.C.B. C.R.E. Stoke Orchard Che ltenham Glos.

PRICE R.

QUET C., Dr.

RABO J.A.,

Dr.

RAO V.N.P.,

REES L.V.C.,

Dr.

Dr.

W .GERMANY

ENGLAND

SNEA- (P)-France Centre de Recherches de Lacq B.P. 34 Lacq F-64170 Artix

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Union Carbide Corp. Tarrytown Technical Center Old Saw Mill River Road Tarrytown, N.Y. 10591

U.S.A.

Dupont Company Chemicals, Dyes & Pigments Dept. Experimental Station Wilmington, Delaware 19898

U.S.A.

Imperial College Chemistry Dept. London SW7 2AY

ENGLAND

7 53

REID R .

Imperial Chemical Industries Ltd. Corporate Lab. P.O. Box 1 1 The Heath I Runcorn Cheshire

RICHARDSON J.T., Prof.

RICHARDSON P .J. , Dr.

RIEKERT L., Prof.

RIESSER G.H., Dr.

RIJNTEN H.Th.

ROBINSON E.

I

ROBINSON W.D.

ROOZEBOOM F., Drs.

Dr.

ROTERUD P.T.

RUIZ PANIEGO A., Dr.

RUPPERT W.

U.S.A.

Imperial Chemical Industries Ltd. Organics Division Hexagon Housel Manchester M9 3DA Universitat Karlruhe Inst. Chem. Verfarenstechnik P.O. Box 6380 D-7500 Karlsruhe

ENGLAND

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Shell Development Co. Box 1380 Houston, Texas 77001

U.S.A.

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Lambeg Industrial Research ASSOC. Lambeg, Lisburn Co Antrim NORTHERN IRELAND Monsanto Co. 800 N Lindbergh Blvd St. Louis, Mo. 63166

ROONEY J.J., Dr.

R O S S J.R.H.,

University of Houston Dept. Chem. Eng. Houston, Texas 77004

AKZO Chemie BV Stationstraat 48 3818 LW Amersfoort Dr.

ENGLAND

Queens University Chemistry Department David Keir Building Delfast Techn. University Twente P.O. Box 217 Enschede

U.S.A.

NORTHERN IRELAND

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School of Chemistry University of Bradford Bradford BD7 LDP W. Yorks.

ENGLAND

Sintef Applied Chemistry Division N-7034 Trondheim NTH

NORWAY

Insti tuto Rocasolano Serrano 119 Madrid 6 Institut far Techn. Chemie I Egerlandstr. 3 8 5 2 Erlangen

SPAIN

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754 RUSEK M.

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R Y N D I N J.,

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U.S.S.R.

I s t i t u t o d i C h i m i c a Fisica

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Dr.

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SIEG R.P.,

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Dr.

SING K.S.W.,

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SOKOLOF V.l Dr.

SPEAKMAN J.G.I Dr.

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761

AUTHOR INDEX Alzamora, L.E. 1 4 3 Andre, J.M. 5 8 5 Andreu, P. 4 9 3 Angelov, S. 605 Baerns, M. 4 1 Baranski, A. 3 5 3 Barbee, T. 6 2 7 Becker, E.R. 1 5 9 Blanchard, G. 1 7 9 Bond, C.C. 7 1 5 Boreskov, G.K. 7 2 3 Bossi, A. 4 0 5 Boudart, M. 6 2 7 Broekhoff, J.C.F. 6 6 3 Brunelle, J.P. 2 1 1 Burriesci, N. 4 7 9 Cahen, R.N. 5 8 5 Cairati, L. 2 7 9 Cale, T.S. 1 3 1 Candia, R. 4 7 9 Carbucicchio, E. 2 7 9 Charcosset, H. 1 9 7 Chenebaux, M.T. 1 9 7 Chou, T.S. 1 7 1 Christiansen, L.J. 3 5 3 Clausen, B.S. 3 6 5 , 4 7 9 Coenen, J.W.E. 8 9 Courty, Ph. 2 9 3 Crump, J.G. 1 3 1 Damyanov, D. 6 0 5 Dass, M.R. 4 1 Debus, H.R. 5 8 5 Delmon, B. 4 3 9 Derouane, E.G. 3 6 5 , 7 2 7 Desai, P. 1 3 1 Doesburg, E.B.M. 143 Dubus, R.J. 1 3 1 Dumesic, J.A. 3 6 5

Galiasso, R. 4 9 3 Garbassi, F. 4 0 5 Geus, J.W. 1 1 3 Guczi, L. 3 9 1

Iannibello, A. 65, 4 6 9 Imura, K. 6 2 7 Inacker, 0. 2 9 Inui, T. 2 4 5 Jerschkewitz, H.-G. Kerkhof, F.P.J.M. 77 Kheifets, L.I. 2 3 3 Kijenski, J. 5 9 5 Kiraly, J. 3 9 1 Kotter, .!I 51 Kruissink, E.C. 1 4 3 Lagan, M. 3 5 3 Leclercq, L. 6 2 7 Le Page, J.F. 2 9 3 Lofthouse, K.G. 4 1 7 Lohrengel, G. 4 1 Malinowski, S . 5 9 5 Manninger, I. 3 9 1 Marengo, S. 6 5 Matijevic, E. 5 5 5 fiatusek, K. 3 9 1 Mehandjiev, D. 6 0 5 Mitchell, P.C.H. 4 6 9 Montarnal, R.E. 5 1 9 Mgkup, S. 365, 4 7 9 MOSCOU, L. 6 5 9 Moss, R.L. 7 1 5 Moulijn, J.A. 7 7 Murrell, L.L. 3 0 7 Neal, A.H. 7 1 9 Neimark, A.V. 2 3 3 Nuttall, T.A. 1 5 9

Edmonds, T. 5 0 7 Engler, M. 2 9 Eszterle, M. 3 9 1 Fenelonov, V.B.

Haase, R. 6 1 5 Hagan, A.P. 4 1 7 Hegedus, L.L. 1 7 1 Hermans, L.A.M. 1 1 3 Hombek, R. 5 9 5 Houalla, M. 4 3 9

233

Ochoa, 0. 4 9 3 bhlmann, G. 6 1 5 Orlandi, A. 4 0 5 Orr, S . 1 4 3 Osterwalder, U. 1 3 1 Oudejans, J.C. 7 7 Ozaki, A. 3 8 1

615

762

Pattek, A. 3 5 3 Pernicone, N. 3 2 1 Petrini, G. 405 Petro, J. 6 4 1 Pitkethley, R . C . 715 Plog,

c.

29

Potter, N.M. 171 Primet, M. 1 9 7 Ramaswamy, A.V. 1 8 5 Ratnasamy, P. 1 8 5 Reizer, A. 3 5 3 Richardson, J.T. 1 3 1 Richter-Plendau, J. 6 1 5 Riekert, L. 5 1 Rijnten, H.T. 2 6 5 ROSS, J.R.H. 1 4 3 Ruggeri, 0. 2 7 9 Samakhov, A.A. 2 3 3 Scheve, J. 6 1 5 Scholten, J.J.F. 6 8 5 Seidl, M. 2 9 Shimazaki, K . 381 Shingu, H. 2 4 5 Sing, K . S . W . 715 Sivasanker, S. 185 Stevens, G . C . 507 Stone, F.S. 4 1 7 Sumiya, S. 381 Summers, J . C . 171 Thomas, R. 7 7 Topsqie, H. 353, 3 6 5 , 4 7 9 Topsqie, N . 365 Traina, F. 3 2 1 Trevethan, M.A. 417 Trifiro, F. 65, 2 7 9 Trim, D.L. 1 Unger, K. Urabe, K .

29 381

Van den Berg, G.H. 2 6 5 Van Reijen, L . L . 143 Van Veen, G. 1 4 3 Villa, P . L . 65 Villadsen, J . 3 6 5 Wilson, R. Yates, D.J.C. Yoshida, S .

715 307 627

Zanderighi, L .

405

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