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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
m2o3
COO
NiO
CdO
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cao
200
300 P-tYpe
A1203 ZnO Ti02
MgO Ca02
400
500 insulators
600
Ga203 700
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homgeneous
800°C
n-type
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.
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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
A
- 2.21
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
Desired*
Undesired*
pi-ally1 Pi p i -a1 l y l Pi
P’
1.98 2.15 2.92 3.26
2.51 2.51 1.45, 3.84 1.91. 2.13,
;;-a1 l y l pi-a1 l y l
3.84,
4.33.
* 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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3.80 3.88 3.92
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L a t t i c e pdrmeters o f t r a n s i t i o n metdl5 far 1 imJni.nc productlan.
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and underired values
rrelated to desired +)
(----I
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
1.9
*a??! E:nerp-!!ltz-fN cuzo y-nno2 6-M2
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anion distrnccs.1n A. of axldes correlated to desired (-)
undrslred valwr (-------)
and
far 11mnene pmduction.
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
+
+
+
+
+
+
+
+
+
+
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 <1,5 1,5 to 50 50 t.o 7500 P o r e volume c m 3 /g between p o r e r a d i i nm 1,5 to 50 50 t o 7500 Average p o r e r a d i u s (nm) between p o r e r a d i i 0 t o 7500 1,5 to 50 50 t o 7500
14,8
5,5 6,7 2,6
2 2 5 212 2 0,4
0,017 0,424
60 4,6 326
Catalyst C
12,o
I
2 2
2,3 2 2,l 7,1 5 012 2,6
0,014 0,447
77 3,8 344
A c t i v i t y and s e l e c t i v i t y measurements h a v e b e e n p e r f o r m e d w i t h t h e two t y p e s of c a t a l y s t by means of an i n t e r n a l r e c y c l e r e a c t o r . R e a c t i o n t e m p e r a t u r e was v a r i e d between 233 and 335°C and p r e s s u r e between 5 and 15 b a r s . D e t a i l e d i n f o r m a t i o n on e x p e r i m e n t a l t e c h n i q u e s and r e s u l t s
43
are outlined in [ 5 ] . Selected data on activity and selectivity which are characteristic for the two catalysts are specified in Tables 2 and
TABLE 2 Average selectivity 2 of catalyst A and C of hydrocarbon of carbon number n x n s = moles moles of CO converted to hydrocarbons
I
I
OC E?H,
1 P(C0) I X(C0)
I
Catalyst A
233-271
Catalyst
c
309-335
3,6-3,7
3,7-3,8
bar bar
I
3,5-3,7
%
I
ca. 1 5
I I
3,4-3,6
ca. 2 5
Although the average selectivities as given in Table 2 refer only to a limited range of conversion X(C0) it could be shown that they are independent of X(C0) up to values of about 6 5 %: furthermore it should be mentioned that the selectivities for both the catalyst types A and C were only very slightly effected by reaction temperature, thus, the selectivity change of the two catalyst types is mainly due to the different pretreatment [ 4 ] . The activities shown in Table 3 have been calculated on the basis of the experimental data given in Table 4 for comparable temperatures. TABLE 3 Activities of catalyst A and C; (P,? r Temperature Catalyst A Catalyst c
OC
= 3,6
bar, Pco = 3,7 bar)
1 0 - 3 moles/g-cat. h 250
270
3,7
8,9
0,4+)
1,l')
31 0 3 O+) 6,7
330 88+' 15,3
"extrapolated, since no tests were performed at this temperature
44
It is obvious that catalyst A exhibits a greater activity than C. From table 4 it can be derived that the activation energies corresponding to catalyst A are lower than those of C. The different activities and selectivities as outlined above certainly cannot be explained by the slight differences in the structural data of catalysts A and C. TABLE 4 Apparent activation energies of CO-consumption and hydrocarbon formation r = r o * G x p ( - ~ a /,~r~0) Cataly t A
0
I
Catalvst c 2)
Component rnl
co CH4 C2H4 C2H6 C3H6 C3H8 ‘qH8 ‘qH1 0
8 , 9 10’
121
4,O l o 8 7 1,2 10 6 7,O 1 0
144
2,8 lo7
122
1,2 lo5
114
xco
% OC
118 119
8,O l o 6
119
6,8 lo4
118
Pco bar PH2 bar T
‘a
’)mol/h g-cat
3,6
-
12
-
19
22
233
-
271
309
3,5
3,7
3,4
3,7
3,7
-
2 2 2 2 2 2 2 2
6 12 6
7 10
10 10 10
3,6 3,8 28 335
2 , kJ/mol
In order to obtain an insight into the different behaviour of catalyst types A and C their adsorption properties were investigated. For adsorption hydrogen and carbon monoxide were used as reactants and ethylene as an important intermediate which can be further hydrogenated to ethane or incorporated into chain growth. The primary objective of the adsorption was to determine the strength of adsorption in terms of heat of adsorption and the number of active sites on the surface and hence, tl-ie turn-over-nunkerfor CO consumption as a measure of catalysk activity. EXPERIMENTAL The adsorption measurements were performed with a vacuum microbalance (Sartorius No. 4 4 3 3 ) with a maximum sensitivity up to 0 , 0 0 1 mg. The balance was connected to a vacuum system where an ultimate vacuum of torr was attainable with an oil diffusion pump which was backed up by a mechanical rotary pump . The residual gas analysis in the system
45 was made with the help of a quadrupole mass spectrometer (Balzers QMG 311) being directly attached to the microbalance. This gas analysis was designed to check on the concentration of gases in the system. No compounds other than hydrogen and traces of nitrogen and water were detected. Before starting the adsorption experiments the catalyst in its reduced state was treated with hydrogen at 523 K and 1 bar for approximately 15 hours. Hereafter the system was evacuated to a pressure of torr and the catalyst was heated up to 568 K for
few hours till its
weight was constant. The gases used showed the following purity:99,999 vol % H2, 99,995 vol % CO and 99,950 vol % C2H4. Adsorption isotherms were measured up to final pressure of 800 torr. For hydrogen the isotherms were taken in the temperature range from 77 K to 523 K , for carbon monoxide from 77 K to 473 K , and for ethylene from 195 K to 523 K. -1 Adsorption measurements were not possible between 5'10 torr due to Knudsen forces, which interfered with the balance in this pressure range. At constant pressure and temperature the increasing weight of the catalyst was noted with time. Two adsorption processes were observed which could clearly be differentiated into a fastand a slow process. The fast process came to an end within seconds while the slow process went on for hours without reaching an equilibrium. In the present work, only the fast process which came to equilibrium is dealt with in detail. RESULTS The primary results of the adsorption measurements are shown in Figures la to lc in form of the isobars for carbon monoxide, ethylene and hydrogen for the two types of catalysts.At low temperatures the amount of gas adsorbed decreases in all cases with increasing temperature. Partiallay this decrease is followed by an increase at higher tern peratures which again may be followed by another decrease at a constant pressure. For catalyst A the isobars for carbon monoxide have a corresponding minimum at about 330 K while for C it occurs at about 380 K. For ethylene only catalyst A shows a minimum at about 473 K while catalyst C exhibits no minimum up to 523 K . The hydrogen isobars reach a minimum at 330 K for both the catalysts. For catalyst A a maximum is obtained at 473 K while for catalyst 523 K .
c
there is no such maximum up to
The heats of adsorption which are given in Table 5 for the three gases have been determined by using Clausius-Clapeyron equation for the descending parts of the isobars. The numerical values have an accuracy Of about t 1 0 %. It should be mentioned that within experimental accu-
46
0,6];0VER4GE
imp/pl CO-ADSORPTION
roo Tom w Tom 170TM
0
, 200
I
TIKI
XXI
500
LbO
I
\ \ \ b \ b
TlKl
500
CATALYST
c
0.2
300 Torr 200 Tor, 100 Torr
OJ 3w
;OVERAGE
ODs
Lbo
lmp/gl H.- ADSORPTION
M1 M3
om 0.01
0 200
3w
LOO
500
TIKI
M O
100
5W
TlKl
600
CATALYST C
OD5 ML
OD3
om 0.01
0
200
300
Fig. l a to I c : Adsorption isobars f o r
CO,
C 2 H 4 and H2
5bo
TlKl
47
racy the heat of adsorption is independent of coverages between 0 1 9 and 2 5 - 1 0 1 3 molecules/cm . With respect to the heat of adsorption of carbon monoxide and hydrogen no significant difference between types A and C
Adsorbent
Catalyst-Type A
co C2H4
H2
C A C
Temperature Range (K) 195 - 273 195 - 273 273 - 473 273
A
77 473
C
77
-
Heat of Adsorption ( kJ/mo le ) aI9
913 14,7
523
715
325 523
3 35
325
415
heats of carbon monoxide adsorption are comparable to the values of about 1 0 kJ/mole obtained earlier by Kolbel and Roberg [ 6 1 on iron catalysts; however, their values of about 8 kJ/mole for hydrogen are high as compared to the present data. The difference may be due to the presence of various oxide compounds besides iron. The active sites defined as the number of molecules adsorbed per surface area have been determined for the various gases by extrapolation of the observed isotherms to higher pressures corresponding to maximum coverage of the surface. This could be done for hydrogen by applying Langmuire equation for non-dissociative adsorption for all temperatures and pressures. For carbon monoxide this method was only applicable at 423 K. Freundlich isotherms have been used for ethylene adsorption. The results are given in Table 6; within experimental accuracy no significant difference in the numbers of active sites is observed for the three adsorbed species. TABLE 6 Number of active sites per surface area ( 1 0 1 4 * c m - 2 )
co Catalyst Type A 0 , 7 2 0,2 C 0,8 2 0,2
C2H4 2 t 1
H2 1,4 5 0,3
I t 1
1,5
t
O,?
Turn-over-numbers for CO-consumption corresponding to the reaction rates as listed in Table 3 have been calculated using an average number
48
of active sites of 1,5.1014 ~rn-~.The results shown in Table I indicate that the different reduction temperatures of catalysts A and C resulted into different activities of a single site; i.e. the activity decrease of the catalyst by higher reduction temperature is not due to a change of the number of sites per surface area but to their different activities.
Temperature OC Catalyst A Catalyst C
250
270
310
330
0,03
0,07
0,23+)
0,66+'
0,004+)
0,01+)
0,06
0,14
DISCUSSION Adsorption behaviour of the two catalyst types A and C is different in so far as at higher temperatures additional adsorption processes may occur as shown by the various isobars. If such a second adsorption is observed, its onset temperature is always lower for catalyst A as compared to C . The second process of carbon monoxide adsorption starts at about 3 3 0 K for cabalyst A and only at 3 8 0 K for C. Similarly for ethylene it begins at 470 K for catalyst A and no such process takes place in catalyst C up to 5 2 0 K, while for hydrogen it occurs at 330 K for both the catalysts. These results indicate that at comparable temperatures adsorption activity of catalyst A is greater than that of C. In accordance herewith the turn-over-numbers show that the activity of an individual site is less for catalyst C than for A. This is in agreement with the facts that the activation energies for the various reactions are less lowered by catalyst C than by A. The selectivity difference of the two catalysts with respect to olefin formation may be related to the different heats of ethylene adsorption obtained. The stronger adsorption of ethylene on catalyst A most probably leads to greater activation of the adsorbed molecule and hence, chain formation and further hydrogenation to ethane is facilitated. This is experimentally verfied for catalyst A in comparison to C by the formation of less olefins and the greater probability of chain growth which has been shown elsewhere (4). CONCLUDING REMARKS Applying two different reduction temperatures to the oxide precursor
49
of a Fischer-Tropsch-catalyst r e s u l t e d i n d i f f e r e n t s e l e c t i v i t y and
a c t i v i t y b e h a v i o u r a s w e l l a s a d s o r p t i o n p r o p e r t i e s . The c a t a l y t i c and a d s o r p t i o n p r o p e r t i e s are i n c e r t a i n r e s p e c t complementary t o one a n o t her. A s a supplementary i n d i c a t i o n f o r t h e d i f f e r e n t c a t a l y t i c performance p r e l i m i n a r y ESCA-studies w e r e p e r f o r m e d , which d i d n o t r e v e a l any s i g n i f i c a n t d i f f e r e n c e s i n s u r f a c e c o m p o s i t i o n o f c a t a l y s t t y p e s T e n t a t i v e l y i t i s t h e r e f o r e s u g g e s t e d t h a t t h e two r e d u c t i o n
A and C .
t e m p e r a t u r e s may r e s u l t i n t h e f o r m a t i o n o f d i f f e r e n t c a t a l y t i c s u r f a c e compounds i n between t h e i n d i v i d u a l components. ACKNOWLEDGEMENTS Thanks a r e due t o D r . H a r t i g f o r p r o v i d i n g t h e ESCA s p e c t r a . F i n a n c i a l s u p p o r t f o r t h i s i n v e s t i g a t i o n s h a s been g i v e n by Ruhrchemie AG within
a
p r o j e c t s p o n s o r e d by t h e M i n i s t r y of R e s e a r c h and Technology
of t h e F e d e r a l R e p u b l i c of Germany. REFERENCES Anderson, C a t a l y s i s Vol. I V , Ed. P.H.
1 R.B.
Emmett, N e w York, 1956,
p. 1 1 9 2 H.
Kolbel, G. L e u t e r i t z , Z.
E l e k t r o c h e m . 6 4 ( 1 9 6 0 ) 437
3 H. K b l b e l , D . S c h n e i d t , E r d b l Kohle 3 (1977) 139 4 A.
Zein E l Deen, J. J a c o b s , M.
B a e r n s , ACS Symp. S e r i e s 6 5 , Ed.
W. Weekmann j r . , Dan L u s s ( 1 9 7 8 ) p . 26 (Minor d i f f e r e n c e s between some o f t h e r e s u l t s g i v e n i n [ 4 ] and t h o s e o f T a b l e s 1 t o 4 of t h i s work a r e d u e t o a d d i t i o n a l d a t a which have become a v a i l a b l e )
5 A. Zein E l Deen, J . J a c o b s , M.
B a e r n s , Chem. I n g . T e c h n . , sub-
mitted f o r publication 6 H. K o l b e l , H.
Roberg, B e r . Bunsenges. Phys. Chem. 81 ( 1 9 7 7 ) 634
DISCUSSION
S.P.S.
ANDREW
: Fischer-Tropsch
c a t a l y s t is a complex f o r m u l a t i o n
w i t h a complex r e a c t i o n p r o d u c t i n c l u d i n g t h e p r o d u c t i o n o f h i g h b o i l i n g p o i n t w a x e s . Could a d i f f e r e n t a c t i v i t y o f t h e K 2 C 0 3
as a
r e s u l t o f d i f f e r e n t p r e p a r a t i o n p r o c e d u r e s c a u s e a d i f f e r e n t format i o n of w a x e s and h e n c e a d i f f e r e n t c a t a l y s t a c t i v i t y ? Have y o u made s i m i l a r m e a s u r e m e n t s b u t i n t h e a b s e n c e o f K2C03 ?
M.
BAERNS
:
The r e s u l t s r e p o r t e d i n o u r p a p e r h a v e b e e n p e r f o r m e d
w i t h a c a t a l y s t o f c o n s t a n t c o m p o s i t i o n : t h e amount o f K 2 0 was n o t
50
changed.
We have however, experimental evidence that the absence
of K 2 0 in a catalyst of otherwise similar composition results in shorter chain length but also in a higher proportion of saturated hydrocarbons.
J.F.
:
LEPAGE
With respect to the distribution of products, what
i s the relative importance of additives to your iron based catalyst,
especially Mn and K20. M. B A E R N S : To our experience the addition of manganese, which may
be considered as a promotor, to an iron-based F.T. catalyst results into a change of the product distribution towards shorter chain length of the hydrocarbons.
The addition of K 2 0 increases the ole-
fins but simultaneously the chain length i s also increased. has been outlined before, for instance by Dry et al.
This
(J. Catal.
Is,
190, 1 9 6 9 ) . R.
KIEFFER
M.
BAERNS
:
:
What i s the importance of
C02
in your F.T. reactions ?
C02 is most probably not formed by a primary reaction
but by the water shift conversion as a secondary reaction between HZO, produced by the F.T.
synthesis, and CO.
To a lesser degree
C02 may also originate from the Boudouard-reaction.
However, the
deposition of carbon on the catalyst i s not appreciable as compared to the amount of C02 formed. It should be mentioned that the selectivity of the CO-conversion towards C 0 2 amounted to about 45 to 50% under all conditions applied to the catalyst in this study.
51
T H E INFLUENCE OF I I I P R E G N A T I O N , D R Y I N G A N D A C T I V A T I O N O N THE A C T I V I T Y A N D D I S T R I B U T I O N OF C U O O N a-ALUMINA
N.
K o t t e r and L .
Riekert
I n s t i t u t f u r Chemische V e r f a h r e n s t e c h n i k
der U n i v e r s i t a t Karlsruhe,
K a r l s r u h e (G.F.R.)
ABSTRACT The i n f l u e n c e o f t h e d i s t r i b u t i o n o f t h e a c t i v e component o n t h e a c t i . v i t y o f impregnation-type alumina,
p o r o u s c a t a l y s t s i s d e m o n s t r a t e d f o r CUD o n
a c t i v i t y b e i n g measured f o r o x i d a t i o n o f CO.
The v i s c o s i t y o f
t h e impregnating s o l u t i o n a f f e c t s the d i s t r i b u t i o n o f t h e catalystp r e c u r s o r ( C U ( N O ~ ) ~a )f t e r i m p r e g n a t i o n a n d d r y i n g . T h e u n i f o r m i t y o f t h e d i s t r i b u t i o n o f t h e a c t i v e component and t h e r e b y t h e a c t i v i t y o f t h e f i n i s h e d c a t a l y s t depend c o n s i d e r a b l y on t h e p r o c e d u r e o f impregnat i o n and d r y i n g .
INTRODUCTION I d e a l l y an impregnation-type
c a t a l y s t shou d be a porous s t r u c t u r e
c o n s i s t i n g o f an i n e r t c a r r i e r - m a t e r i a l , w i t h t h e a c t i v e component.
whi h i s covered everywhere
Such p o r o u s c a t a l y s t s a r e g e n e r a l l y t r e a t e d
a s homogeneous p s e u d o - c o n t i n u a i n t h e o r e t i c a l w o r k ( e . g .
/ I / ) , a i m i n g a1
a n u n d e r s t a n d i n g o f s i m u l t a n e o u s r e a c t i o n a n d mass t r a n s f e r i n t h e porous s o l i d .
I t appears d o u b t f u l ,
however,
i f the assumption o f u n i -
f o r m i t y i s generally j u s t i f i e d w i t h respect t o r e a l impregnation-type catalysts,
w h i c h s o m e t i m e s f a l l somewhat s h o r t o f
the ideal,
since the
a c t i v e component does n o t u n i f o r m l y c o v e r t h e porous s t r u c t u r e , a c t i v e surface being smaller than t h e t o t a l surface.
the
Two t y p e s o f i n -
homogeneous d i s t r i b u t i o n o f t h e a c t i v e c o m p o n e n t c a n b e d i s t i n g u i s h e d : (a) macroscopic inhomogeneity,
t h e average d e n s i t y o f t h e a c t i v e
component b e i n g d i f f e r e n t f o r d i f f e r e n t l a y e r s o f t h e p e l l e t , f o r e x a m p l e when t h e a c t i v e c o m p o n e n t i s c o n c e n t r a t e d a t t h e core o r near the outer surface;
52
(b) microscopic o r surface inhomogeneity,
t h e pore walls o f t h e
c a r r i e r m a t e r i a l b e i n g n o t homogeneously covered everywhere by t h e a c t i v e component. Both types o f inhomogeneity w i l l s i m u l t a n e o u s l y i n f l u e n c e t h e dynamics o f a chemical r e a c t i o n t a k i n g p l a c e i n t h e c a t a l y s t . Macroscopic inhomogeneity i s encountered frequently i n c a t a l y s t s t h a t have been p r e p a r e d b y i m p r e g n a t i o n , d r y i n g and a c t i v a t i o n and
mi-
has been documented i n t h e l i t e r a t u r e f o r a f e w c a s e s /2,3,4/;
c r o s c o p i c i n h o m o g e n e i t y seems t o p r e v a i l i n g e n e r a l i n s u c h c a t a lysts;
r e a l impregnation-type c a t a l y s t s are thus
the ideal.
n general f a r from
I n t h e p r e s e n t p a p e r some e x p e r i m e n t a l m e t h o d s t h a t c h a r a c -
t e r i z e t h e d i s t r i b u t i o n o f t h e a c t i v e component w 11 be r e v i e w e d b r i e f l y and t h e r e s u l t s o f experiments aimed a t i m p r o v i n g t h e method o f p r e p a r a t i o n w i l l be r e p o r t e d . CHARACTERIZATION OF THE D I S T R I B U T I O N OF THE ACTIVE COMPONENT V i s u a l c h a r a c t e r i z a t i o n o f t h e d i s t r i b u t i o n o f t h e a c t i v e component i s p o s s i b l e i f i t ' s r e f l e c t a n c e o r morphology i s d i f f e r e n t from t h a t o f the c a r r i e r , e i t h e r by inspection, light-microscopy o r electronmicroscopy.
It i s d i f f i c u l t ,
however, t o q u a n t i f y t h e r e s u l t s o f v i s u a l
inspection i n detail. The o b s e r v a t i o n o f e q u i l i b r i a between a porous c a t a l y s t and a f l u i d s u r r o u n d i n g i t (e.g.
adsorption, mercury-penetration) w i l l always
average over t h e e n t i r e space o f t h e p e l l e t and can t h e r e f o r e n o t g i v e i n f o r m a t i o n a b o u t t h e s p a t i a l d i s t r i b u t i o n o f s o l i d components.
For
example t h e d e t e r m i n a t i o n o f t o t a l s u r f a c e area ( f r o m nitrogen-adsorpt i o n ) and o f t h e s u r f a c e area o f t h e a c t i v e component ( f r o m chemis o r p t i o n ) w i l l g i v e a measure o f t h e f r a c t i o n o f t h e s u r f a c e which i s covered b y t h e a c t i v e component, b u t n o t a b o u t i t ' s d i s t r i b u t i o n i n space. Certain q u a n t i t a t i v e information about t h e d i s t r i b u t i o n o f the a c t i v e component ( o r r a t h e r i t s a c c e s s i b i l i t y )
can be obtained from observa-
t i o n s o f t h e k i n e t i c s o f a c a t a l y t i c r e a c t i o n i n t h e porous s o l i d , we h a v e show:
previously-/5/.
as
An a p p a r e n t d i f f u s i v i t y o f t h e r e a c t a n t s
w i t h r e s p e c t t o t h e a c t i v e component can be o b t a i n e d f r o m t h e temperature-dependence o f t h e r a t e o f r e a c t i o n /6/
and compared t o 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 t h e reactant i n t h e porous structure,
the ratio
o f t h e two q u a n t i t i e s b e i n g a measure o f t h e a c c e s s i b i l i t y o f t h e act i v e component r e l a t i v e t o t h a t o f t h e porous s t r u c t u r e i n g e n e r a l . However,
k i n e t i c d a t a which c h a r a c t e r i z e c a t a l y s t performance can
53
n o t be p r e d i c t e d e x a c t l y from o t h e r o b s e r v a t i o n s d e s c r i b i n g t h e d i s t r i b u t i o n o f t h e a c t i v e component i n a m o r e o r l e s s q u a l i t a t i v e way. CuO O N ALUMINA : PREPARATION A N D P R O P E R T I E S CuO a n
y-alumina
CuO was d e p o s i t e d o n y - a l u m i n a i n t h e c o n v e n t i o n a l w a y : t h e c a r r i e r 2 ( s p h e r e s o f y - a l u m i n a , d i a m e t e r 6 mm, s p e c i f i c s u r f a c e a r e a 2 4 0 m / g , p o r o s i t y 4 5 % ) was s o a k e d i n 3 m C ~ ( N O ~ ) ~ - s o l u t if oo nr 1 4 h , d r i e d a t 350 K f o r 14 h and s u b s e q u e n t l y h e a t e d t o 820
C U ( N O ~ )t ~o CuO. T h e r e s u l t i n g c a t a l y s t 142
m2/g,
K f o r 24 h t o
convert
has a BET-surface area o f
however, t h e s u r f a c e a r e a o f t h e a c t i v e component amounted
t o only about 7 % o f t h i s value,
a s shown b y C O - c h e m i s o r p t i o n .
The k i n e -
t i c s o f t h e o x i d a t i o n o f C O ( l e s s t h a n 0 . 1 m o l % i n a i r ) was s t u d i e d o n t h i s c a t a l y s t i n a g r a d i e n t ess r e c i r c u l a t i o n r e a c t o r . The r e a c t i o n f i r s t o r d e r i n CO;
S
i t s r a t e i s described by a r a t e c o e f f i c i e n t k ( i n 1 Vk
dnCO 7 = k cco
where Vk i s t h e volume o f t h e c a t a l y s t p e l l e t and cco t h e volume-concent r a t i o n ( i n mol m-3) o f C O i n t h e gas phase a t t h e s u r f a c e o f t h i s p e l 1e t . The t e m p e r a t u r e - d e p e n d e n c e o f t h i s r a t e - c o e f f i c i e n t i s shown i n f i g u r e 1. T h e p o i n t o f i n t e r s e c t i o n ( a ) o f t h e h i g h - t e m p e r a t u r e a n d t h e low-temperature Arrhenius-line indicates t h e onset o f a d i f f u s i o n l i m i t a t i o n a t a h i g h e r r a t e ( o r t e m p e r a t u r e ) as would be expected f r o m an i n d e p e n d e n t d e t e r m i n a t i o n o f 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 CO i n t h e 2 s t r u c t u r e (oeff = 1.9 cm / s e c ) , a s s u m i n g a m a c r o s c o p i c a l l y u n i f o r m d i s t r i b u t i o n o f CuO ( d o t t e d l i n e ) / 5 / . T h e a c t i v e c o m p o n e n t w h i c h c o v e r s o n l y a b o u t 7 % o f t h e s u r f a c e i n t h e p o r e s m u s t t h e r e f o r e be more a c c e s s i b l e t o t h e r e a c t a n t s t h a n t h e porous s t r u c t u r e on t h e average. The r a t h e r h i g h s u r f a c e a r e a o f t h i s p a r t i c u l a r c a r r i e r i s t h u s n o t u t i l i z e d c a t a l y t i c a l l y a n d a m a t e r i a l o f much s m a l l e r s u r f a c e a r e a should be e q u a l l y s u i t a b l e as a c a r r i e r f o r a c a t a l y s t o f comparable activity,
p r o v i d e d a u n i f o r m d i s t r i b u t i o n o f t h e a c t i v e component i n
t h e p o r o u s s t r u c t u r e can be o b t a i n e d . S u c h a c a r r i e r w o u l d be a - a l u m i n a which a l s o has t h e advantage o f h i g h e r thermal and mechanical s t a b i l i t y .
54
-T
700
600
,
500
K450
400
370
0105-
QOl+
I
74
I
16
,
-1
2.0
2.2
, \
2.4
2.6
103~ T
Fig. 1. Rate c o e f f i c i e n t k of C O - o x i d a t i o n o f CuO/y-A1203 a s f u n c t i o n o f t e m p r a t re. D o t t e d l i n e : computed values o f k f o r Deff = 1.9 l o - ? c m y / s e c , assuming uniform d i s t r i b u t i o n o f C U O CuO on a-alumina T h e c a r r i e r material used f o r t h e preparation of C u O - c a t a l y s t s o n a-A1203 was characterized by the f o l l o w i n g data : c y l i n d e r s , 6 m m d i a 2 m e t e r , specific s u r f a c e a r e a 6 m / g , porosity 60 % , pore s i z e d i s t r i bution s e e f i g u r e 2. According t o its x - r a y - d i f f r a c t i o n - p a t t e r n the ideal hexagonal s t r u c t u r e o f a-A1203 w a s n o t y e t f u l l y d e v e l o p e d , t h i s t y p e o f high temperature alumina is s o m e t i m e s d e s i g n a t e d @ - A 1 2 0 3 .
55
pore radius em F i g . 2.
P o r e v o l u m e d i s t r i b u t i o n o f cl-A1203 f r o m m e r c u r y - p o r o s i m e t r y
P r e p a r i n g a c a t a l y s t o n t h i s c a r r i e r b y t h e same s e q u e n c e o f i m p r e g nation,
d r y i n g and c a l c i n a t i o n as d e s c r i b e d above leads t o a r a t h e r
d i s a p p o i n t i n g r e s u l t . T h e a c t i v e c o m p o n e n t ( 3 . 5 w t % CuO) i s c o n c e n t r a t e d i n a s h e l l on t h e o u t s i d e s u r f a c e o f t h e s t r u c t u r e ( f i g u r e 3a). A m u c h b e t t e r d i s p e r s i o n o f t h e a c t i v e c o m p o n e n t was o b t a i n e d b y i m p r e g n a t i n g w i t h a n aqueous s o l u t i o n c o n t a i n i n g 3 m o l / l o f CU(HCOO)~ s a t u r a t e d w i t h NH3,
d r y i n g a t 350 K a n d c a l c i n a t i o n a t 800 K ( f i g u r e 3b).
The two methods o f p r e p a r a t i o n ( a ) a n d ( b ) a r e d i f f e r e n t i n t h e f o l lowing respect : ( a ) i m p r e g n a t i o n w i t h s o l u t i o n o f C U ( N O ~ ): ~ v i s c o s i t y o f i m p r e g n a t i o n s o l u t i o n 1 . 5 cP; d e c o m p o s i t i o n o f c a t a l y s t p r e c u r s o r C U ( N O ~ )t o~ CuO o c c u r s a r o u n d 500 K a n d i s endothermic;
i n t e r m e d i a t e C U ( N O ~ )i ~s v o l a t i l e / 7 / .
( b ) i m p r e g n a t i o n w i t h a m m o n i a c a l s o l u t i o n o f C U ( H C O O ) ~: v i s c o s i t y o f i m p r e g n a t i o n s o l u t i o n 3.5cP;
decomposition o f
c a t a l y s t p r e c u r s o r C U ( H C O O ) ~ t o CuO o c c u r s a r o u n d 4 7 0 K a n d i s exothermic;
C U ( H C O O ) ~i s l e s s v o l a t i l e t h a n C U ( N O ~ ) ~ .
56
. F i g . 3 . C u O on a-A1 O3 ( a ) i m p r e g n a t i o n w i f h 3 m C U ( N O ~ ) ~( ,b ) i m p r e g n a t i o n w i t h 3 m C U ( H C O O ) ~ , s a t u r a t e d w i t h NH3 The c a t a l y s t p r e c u r s o r must be u n i f o r m l y d i s t r i b u t e d i n t h e p o r e s i m m e d i a t e l y a f t e r i m p r e g n a t i o n when t h e m a t e r i a l i s s t i l l w e t i n e i t h e r case ( a ) o r (b), exchange,
s i n c e a-A1203 i s i n e r t and has no c a p a c i t y f o r i o n -
c o n t r a r y t o y-AlZO3.
The n o n u n i f o r m d i s t r i b u t i o n m u s t t h e r e f o r e a r i s e d u r i n g d r y i n g a n d / o r d u r i n g d e c o m p o s i t i o n o f t h e d i f f e r e n t p r e c u r s o r s t o CuO. E x a m i n a t i o n o f t h e d i s t r i b u t i o n o f t h e c r u s h e d c y l i n d e r s a f t e r d r y i n g showed t h a t i n c a s e ( a ) t h e p r e c u r s o r C U ( N O ~ )was ~ already concentrated near t h e s u r f a c e o f t h e porous s o l i d b e f o r e decomposition t o CuO and t h a t t h i s e f f e c t was m o r e p r o n o u n c e d a t l o w e r d r y i n g t e m p e r a t u r e s ( l o w e r e v a poration rates). A similar distribution o f the precursor-salt after i m p r e g n a t i o n and d r y i n g has a l r e a d y been observed b y Boreskov / 8 / . I t appears t h a t t h e n o n u n i f o r m d i s t r i b u t i o n o f t h e a c t i v e component
i n case ( a ) i s m o s t l y due t o nonuniform d i s t r i b u t i o n o f t h e p r e c u r s o r C U ( N O ~ ) ~w,h i c h i s a l r e a d y e s t a b l i s h e d a f t e r d r y i n g ;
although
a n a d d i t i o n a l m i g r a t i o n o f Cu i n t h e f o r m o f C U ( N O ~ )d ~u r i n g c a l c i -
57
n a t i o n i s a l s o conceivable.
A nonuniform d i s t r i b u t i o n o f t h e
i n i t i a l l y d i s s o l v e d and u n i f o r m l y d i s t r i b u t e d s a l t can o n l y a r i s e due t o f l o w o f t h e s o l u t i o n i n t h e porous s t r u c t u r e o r due t o d i f f u s i o n i n t h e s o l u t i o n . The d r i v i n g f o r c e s o f t h e m i g r a t i o n s m u s t be a consequence o f t h e e v a p o r a t i o n o f s o l v e n t and t h e y w i l l be c a p i l l a r y f o r c e s
i n t h e f i r s t case,
concentration gradients i n
t h e second case. Viscous f l o w due t o c a p i l l a r y f o r c e s i s p r o b a b l y r e s p o n s i b l e f o r t h e o b s e r v e d m i g r a t i o n o f d i s s o l v e d C U ( N O ~ )t o~w a r d s t h e surface o f t h e p e l l e t i n o u r case. Rates o f viscous f l o w i n t h e impregnating.so1ution as
well as o f d i f f u s i o n i n the l i q u i d are
r o u g h l y i n v e r s e l y p r o p o r t i o n a l t o t h e v i s c o s i t y o f t h e l i q u i d . The difference i n t h e v i s c o s i t y o f t h e impregnating solutions therefore appeared i m p o r t a n t w i t h r e s p e c t t o t h e d i s t r i b u t i o n o f t h e c a t a l y s t precursor a f t e r drying,
since a higher v i s c o s i t y should lead t o a
m o r e u n i f o r m d i s t r i b u t i o n , a s was o b s e r v e d . I n o r d e r t o v e r i f y t h i s h y p o t h e s i s a s e r i e s o f c a t a l y s t s was p r e p a r e d b y i m p r e g n a t i o n w i t h s o l u t i o n s o f 2 m Cu(N03)*,
where d i f f e r e n t s m a l l amounts o f Hydroxy-
e t h y l c e l l u l o s e (HEC) h a d b e e n a d d e d i n o r d e r t o i n c r e a s e t h e v i s c o s i t y 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 . An a - a l u m i n a c a r r i e r w i t h e x c e p t i o n a l l y low surface area (<
2
0.1 m /g)
and hence a h i g h average
was us-ed f o r t h e s e e x p e r i m e n t s . T h e i m p r e g n a -
pore-radius (
t e d p e l l e t s w e r e d r i e d a n d c a l c i n e d f o r 1 h a t 500
K and heated t o
800 K t o r e m o v e t h e H E C . T h e r e s u l t s a r e s h o w n i n f i g u r e s 4 a n d 5 . A l l c a t a l y s t s c o n t a i n e d 0 . 5 w t % CuO. The v i s c o s i t y o f t h e s o l u t i o n used f o r i m p r e g n a t i o n i n f l u e n c e s t h e macroscopic homogeneity and a l s o t h e c a t a l y t i c a c t i v i t y ( F i g . 5 ) I t i s i n t h i s way p o s s i b l e t o a c h i e v e r o u g h l y t h e same c a t a l y t i c
a c t i v i t y a s was o b s e r v e d f o r 1 0 % CuO o n h i g h - s u r f a c e t h e more rugged
a-A1203 c a t a l y s t c a r r i e r ( F i g .
It should be mentioned,
however,
y-A1203 w i t h
6).
t h a t t h e macroscopic d i s t r i b u -
t i o n o f t h e a c t i v e component i s n o t t h e o n l y p r o p e r t y which changes w i t h t h e c a r r i e r and t h e method o f p r e p a r a t i o n .
The a c t i v a t i o n e n e r g y
o f t h e C O - o x i d a t i o n i s d i f f e r e n t b y 3 5 % f o r CuO o n c r A 1 2 0 3 a n d o n y-A1203,
r e s p e c t i v e l y , a s c a n be s e e n f r o m f i g u r e 6. A d d i t i o n o f HEC
t o t h e i m p r e g n a t i n g s o l u t i o n a l s o changes t h e h a b i t o f t h e d e p o s i t s o f CuO i n t h e p o r o u s s t r u c t u r e ( F i g .
7).
The method o f p r e p a r a t i o n i n f l u e n c e s s i m u l t a n e o u s l y t h e m a c r o s c o p i c and t h e m i c r o s c o p i c inhomogeneity o f t h e r e s u l t i n g c a t a l y s t s and i t i s not possible effects
-
a t least a t present
-
t o d i f f e r e n t i a t e between b o t h
w i t h respect t o c a t a l y s t performance.
58
.Fig.
4 . P h o t o g r a p h s o f c r o s s s e c t i o n o f CuO o n a-A1 0 p r e p a r e d b y i m p r e g n a t i o n w i t h 2 m C U ( N O ~ )s~o l u t i o n o f 8 i 3 f e r e n t v i s c o s i t i e s : (1) no HEC added; v = 1 . 5 cP; ( 2 ) 1 w t % HEC; v = 1 5 cP; ( 3 ) 2 w t % HEC; v = 100 CP
10 8
6
2
F i g . 5. V o l u m e - s p e c i f i c r a t e c o e f f i c i e n t k o f C O - o x i d a t i o n on C u O l a - A l 2 0 3 c a t a l y s t s as f u n c t i o n o f t e m p e r a t u r e ; d e s i g n a t i o n as i n f i g u r e 4
59
‘4 J
\
10 -
8 4
5-
-
UUI
’
I
1.4
26
I
I
I
18
2.0
-LK
2.2
I
2.4
26
T
F i g . 6 . V o l u m e - s p e c i f i c r a t e c o e f f i c i e n t k o f C O o x i d a t i o n o n CuO : A : 3 . 5 w t % CuO o n a - A 1 2 0 3 ( 6 m 2 / g ) e x f o r m i a t e , d r i e d a n d a c t i v a t e d a t 520 K 0:lO w t % CuO o n v - A 1 2 0 3 ( 2 4 0 m 2 / g ) e x n i t r a t e
60
F i g . 7. D e p o s i t s o f CuO in porous a-A1203 as observed by scanning electron microscopy. (a) impregnation with 2 m C U ( N O ~ )with ~ 2 wt% HEC added; (b) impregnation with 2 m C U ( N O ~ ) ~ ,no HEC added
REFERENCES 1 R. Aris, "The Mathematical Theory of Diffusion and Reaction in Permeable Catalysts", Oxford University Press, Oxford 1975 2 R.C. Vincent and R.P. Elerrill, J.Catalysis 3 5 , (1974), 206 3 H.C. Chen and R. Anderson. J. Catalysis 43 ?J976), 200 4 J . Cervellb, J.F. Garcia de la Banda, E . T e r m a n a and F. Jim@nez, Chem.Ing.Tech. 48 (1976), 520 5 M. Kotter, P. L z e r a and L. Riekert, B e r . B u n s e n g e s . p h y s . C h e m . 80 (1976), 6 1 6 Kadlec, J . Ilich'alek and A. Simecek, Chem.Eng.Science 3 (1970) 319 7 K. Stern, J.Chem .Ref.Data, 1 (1972), 757 8 G. Boreskov, i n "Preparation-of Catalysts" (D. Delmon, P. Jacobs and G. Poncelet, eds.), p. 222, Elsevier Scientific Publishing Company, Amsterdam-Oxford-New York, 1976 DISCUSSION
B. DELMON
: C o n c e r n i n g Fig.
6 , h a v e y o u a n e x p l a n a t i o n f o r t h e appa-
r e n t l y d i f f e r e n t a c t i v a t i o n e n e r g i e s ( e s p e c i a l l y in t h e low t e m p e r a ture region) ?
Is t h i s s t i l l r e l a t e d t o d i f f u s i o n a l p r o c e s s e s , or
61 do you attribute the difference to chemical modifications of the active species by some interaction with the support L. RIEKERT
:
?
The different activation energies of CO-oxidation ob-
served with different carriers ( a vs. y-A1 0
2 3
)
indicate that besi-
des the macroscopic distribution other properties of the active component (e.g. texture, incipient compound formation) depend on the type of carrier and/or the method of preparation.
See also
Fig. 7 of the paper. E. MATIJEVIC
:
The addition of nonionic polymers affects the nu-
cleation stage of the precipitation of metal oxide.
As
a result
the primary particle size of the precipitated solid will decrease as the polymer concentration increases.
A
minimum concentration
of the additive is needed to initiate this effect, which indeed may be due to a change in viscosity.
Entirely different observa-
tions are made if ionic polyelectrolytes are used. This was clearly demonstrated in our studies of the effects of nonionic and ionic Dextrans on the formation of monodispersed chromium hydroxides. L. RIEKERT
:
Any addition to the impregnating solution can possibly
have an effect on nucleation, crystallization and also on solidstate-reactions taking place during calcination. This possibility can never be excluded and should be kept in mind Fig. 7 in the paper.
-
compare also
However, we believe that our experiments
show conclusively that the macroscopic distribution of the active component (ex-nitrate) on a-A1203 is critically affected by the addition of HEC.
The most obvious and simple explanation seems to
be that the increase in viscosity hinders capillary flow during drying. H.W.J.
NEUKERMANS
:
When you change the viscosity of the dipping
solution, do'nt you change the volume of liquid absorbed, or did you take special precautions to always have the same copper content? L. RIEKERT
:
We took care to exclude this effect in our experiments
(Figs. 4 and 5 ) by using a carrier with rather wide pores.
The
same amount of liquid was taken in all cases, independent of the viscosity of the impregnating solution.
62 W.D.
R O B I N S O N : H a v e y o u c o n s i d e r e d or used f a s t drying
r e d u c e d pressure of m i c r o w a v e
-
-
using
t o m i n i m i z e t h e t r a n s f e r of C u
from t h e inside o f t h e p e l l e t ? W a s i t , if used, s u c c e s s f u l ?
L. R I E K E R T : W e d i d o b s e r v e a m o r e h o m o g e n e o u s m a c r o s c o p i c d i s t r i b u t i o n of t h e a c t i v e c o m p o n e n t as t h e r a t e of drying w a s increased. T h i s effect w a s t o b e expected b e c a u s e t h e d i s t r i b u t i o n m u s t d e p e n d o n t h e i n t e r a c t i o n o f t w o rate-processes
: r e m o v a l of t h e s o l v e n t
b y e v a p o r a t i o n and c a p i l l a r y - f l o w t o w a r d s n a r r o w p o r e s at t h e pellet-surface.
I t i s t h e r a t i o b e t w e e n t h e r a t e s of t h e s e t w o
p r o c e s s e s w h i c h s e e m s t o b e important w i t h r e s p e c t t o t h e r e s u l t i n g d i s t r i b u t i o n of t h e a c t i v e c o m p o n e n t o n inert carriers.
H.
S C H A R F : Why is Cu0-a-A1203 m o r e a c t i v e t h a n Cu0-y-Al2O3 d e s p i t e
t h e l o w e r surface a r e a o f 0-A1203 ?
L. R I E K E R T : B e c a u s e t h e s u r f a c e a r e a of t h e active c o m p o n e n t the total surface area
-
-
not
d e t e r m i n e s c a t a l y s t activity.
J. S C H E V E : I s i t p o s s i b l e t h a t w e t t i n g of t h e surface h a s a m a i n e f f e c t o n y o u r p r o c e s s , b e c a u s e f o r m a m i d e c a n b e formed d u r i n g annealing of copper formate with ammonia ?
T h i s s u b s t a n c e is w e l l
k n o w n by c a t a l y s t m a n u f a c t u r e r s t o i m p r o v e w e t t i n g of surfaces.
In
t h e s a m e way t h e o t h e r o r g a n i c s y o u used f o r i m p r e g n a t i o n m a y act.
L. R I E K E R T : Any c h a n g e of t h e c o m p o s i t i o n 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 w i l l of c o u r s e c h a n g e all its p h y s i c a l properties.
However,
s i n c e t h e a l u m i n a c a r r i e r s a r e c o m p l e t e l y wetted by a l l s o l u t i o n s w h i c h h a v e b e e n u s e d and s i n c e t h e contact-angle
c a n not f a l l b e l o w
z e r o , I would not e x p e c t wetting of t h e s u p p o r t t o b e o f c r i t i c a l influence.
J.J.F.
S C H O L T E N : I n m a n y c a s e s w h e r e i m p r e g n a t i o n m e t h o d s are ap-
p l i e d , a d s o r p t i o n o f i o n s or m o l e c u l e s f r o m t h e s o l u t i o n t o t h e s u r f a c e m a y b e u s e d a s well. o r e s p e c i a l l y Cu(NH3):'
I s t h e r e any a d s o r b a b i l i t y of C u
i o n s w i t h r e s p e c t t o t h e s u r f a c e of
a-
++ and
y-alumina ?
L. R I E K E R T : T h e r e w a s n o a d d i t i o n a l u p t a k e o f C U ( N O ~ ) o n~ our a - A l 2 O 3 S u p p o r t s beyond pore-volume filling.
T h u s t h e CL-A1203
c a r r i e r s c a n b e C o n s i d e r e d a s c h e m i c a l l y inert t o w a r d s t h e c a t a l y s t
63 precursor C U ( N O ~ ) ~Also . in the case of I C U ( N H ~ (HCOO)2 )~~ the amount of Cu on the finished catalyst corresponded to the amount expected for pore-volume filling of the a-A1203 support.
G.H.
van den BERG
:
What is the influence of increasing the viscosi-
ty of the impregnation solution on the penetration of the micropores? Or does your carrier not contain any micropores ? L. RIEKERT : There are no micropores in the a-A1203-carriers which we used, as shown from physisorption-isotherms and electron-microscopy.
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65
A S T U D Y OF THE CHEMISORPTION O F CHROMIUM(VI), MOLYBDENUM(V1) AND T U N G S T E N (VI) O N T O f - A L U M I N A
A. I A N N I B E L L O l , S.MARENGO
1
,
F. TRIFIRO12 and P. L. V I L L A
3
'Stazione sperimentale p e r i Combustibil i, San Donato Milanese, I t a l y 2 . l s t i tuto d i Tecnologie chimiche special i del I l U n i v e r s i t h , Bologna, I t a l y 31stituto di C h i m i c a i n d u s t r i a l e del P o l i t e c n i c o , Milano, I t a l y
ABSTRACT
In the p u l s e and step a d d i t i o n o f ammonium dichromate, ammonium paramolybdate and ammoniumparatungstate s o l u t i o n s to y-alumina a l a r g e i n c r e a s e o f the p H was o b s e r v e d in the l i q u i d phase w h i z h suggests a common mechanism o f i n t e r a s t i o n w i t h the b a s i c s i t e s o f 1-alumina.
T h i s mechanism i n v o l v e s the decondensg
t i o n o f the isopolyanion in the presence o f alumina f o l l o w e d by the c h e m i s o r p t i o n o f monomeric species.
The most r e l e v a n t d i f f e r e n c e between the three metal ions
i s the l i m i t i n g amount o f the chemisorbed oxospecies on i - a l u m i n a : C r O
3
3. 28;
19. 2 wt. %. 3 3 The e l e c t r o n i c and Raman s p e c t r a o f the chemisorbed molybdenum(V1) and
MOO 7 . 72; W O
tungsten(V1) at low coverage can be i n t e r p r e t e d as due t o oxospecies in t e t r a h e d r a l co-ordination.
The r e l e v a n c e o f the a d s o r p t i o n phenomena in the p r e p s
r a t i o n of the commercial c a t a l y s t i s f i n a l l y p o i n t e d out.
1 INTRODUCTION The oxides o f the s i x t h g r o u p of elements promoted w i t h o t h e r components and supported onto
y -alumina
r e p r e s e n t an important c l a s s of commercial catalysts.
The most f r e q u e n t method o f i n c o r p o r a t i o n of these elements i s impregnation; in t h i s technique of p r e p a r a t i o n a d s o r p t i o n phenomena a r e involved, but t h e i r r e f a t i v e importance compared to the 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 thermal treatments o f the c a t a l y s t s has not been e x t e n s i v e l y i n v e s t i g a t e d [l-6,
141. On
the o t h e r hand the study of these a d s o r p t i o n phenomena may g i v e u s e f u l i n f o r m s
66 tion f o r the r a t i o n a l i z i n g of the catalyst preparation. Moreover the presence of
chemisorption phenomena,
i f relevant, o f f e r s the p o s s i b i l i t y of s e l e c t i v e l y p r g
p a r i n g catalysts, i. e. of p r e p a r i n g samples o f catalysts containing only s u r f a c e phases at v a r i o u s degrees of coverage; this may be useful also f o r the understanding of the c a t a l y t i c p r o c e s s [5c].
In this paper we r e p o r t the r e s u l t s of
an investigation on the chemisorption processes which o c c u r when 8-alumina i s impregnated w i t h ammonium dichromate, ammonium paramolybdate and ammonium paratungstate aqueous solutions. tographic technique [5a,
In the experimental approach the l i q u i d Thromg
5b] was used to point out the chemistry of the i n t e r a c
tion a l s o f r o m a quantitative point of view, together w i t h a c h a r a c t e r i z a t i o n of the chemisorbed species by Raman and e l e - t r o n i c spectroscopies.
2 EXPERIMENTAL SECTION 2. 1 M a t e r i a l s g-alumina A K Z O Chemie CK 300 ( p a r t i c l e diameter 0. 1-0.3 mm); o t h e r chemic a l s a r e C a r l o E r b a , p u r e reagent grade.
2. 2 Experimental p r o c e d u r e P u l s e addition experiments w e r e c a r r i e d out as d e s c r i b e d elsewhere [5a]
.
T h e c o l u m n w a s f i I l e d w i t h 4 . 2 g o f l - a l u m i n a ; b i d i s t i l l e d water was used as a c a r
r i e r (20-40 mL/h); 30- l O O p L pulses of ammonium paratungstate, ammonium p a r a molybdate, ammonium dichromate, chromium n i t r a t e and chromic a c i d aqueous s o l ~ -6 tions containing 6-50 * 10 mol Me(V1) w e r e i n j e c t e d at 20-80°C. The composition of the effluent was r e - o r d e d w i t h a r e f r a c t i v e index monitor,
LDC Model 1107, followed b y a n U V detector, L D C M o d e I 1520, operating at 254 nm, followed b y a flowpH-meter,
MetrohomModeI E 5 3 2 w i t h a 5 - m m o u t s i d e diam-
e t e r g l a s s electrode plugged inside a teflon c e l l w i t h f r e e volume o f 300 S t e p addition experiments [5b,
5c]
/uL.
w e r e c a r r i e d out at 20, 60 and 80°C on
a column 200 mm long, 4 mm diameter, f i l l e d w i t h 1. 5 g o f b/-alumina.
The i n l e t
f l o w r a t e was 25mL/h; b i d i s t i l l e d w a t e r w a s i n i t i a l l y f e d , f o l l o w e d b y theisopolyanion ammonium sal t solution containingo. 038mol Me(VI)/L. A f t e r each run the column w a s w a s h e d w i t h 4 0 m L o f b i d i s t i I l e d w a t e r at thesame temperatureof the a d sorptionexperiments,
toeliminate non adsorbed Me(VI).
Adsorptionexperiments w e r e c a r r i e d out at 20 and 6OoC p o u r i n g 4. S g o f
y-
alumina i n t o a 500 m L flask containing 450 m L o f v i g o r o u s l y s t i r r e d aqueous s o l u tion of the isopolyanion ammonium s a l t (IO-'rnol
Me(Vl)/kg).
67
2. 3 A n a l y s i s Mo(VI) and W ( V I ) in l i q u i d and s o l i d phases w e r e determined by X - r a y f l u o r e s cence using a P h i l i p s Model P W 1540 spectrophotometer.
C r ( V I ) was determined
b y atomic adsorption u s i n g a P e r k i n - E l m e r Model 306 spectrophotometer. The e l e c t r o n i c s p e c t r a w e r e determined by a C a r y 15 spectrophotometer w i t h a Model 151 1000 attachment f o r diffuse r e f l e c t a n c e spectra.
The Raman s p e c t r a
w e r e r e c o r d e d by a C a r y 83 spectrophotometer.
3 RESULTS AND D I S C U S S I O N 3. 1 P u l s e addition experiments Fig.
1 shows the apparent retention volume, the p H and the a r e a of the peak
(as r e v e a l e d b y t h e U V m o n i t o r ) o f the outlet pulses vs. the number of pulsesofam_ monium dichromate and ammonium paramolybdate.
I 10 20 30 40 50 60 70 80
*O
Number OF pulses
F i g . 1. Retention volume, peak a r e a (UV monitor) and p H of the outlet p u l s e vs. the number o f equimolecul a r pulses (57prnol Me(Vl))of ammonium dichromate and ammoniumparamolybdate solutions onto 8-alumina in neut r a l environment
Number OF pulses
Fig. 2. Retention volume and peak area ( U V m o n i t o r ) vs. the number of equimg l e c u l a r pulses ( 5 7 p m o l M e ( V I ) ) of ammonium dichromate and ammoniumpara molybdate solutions in a c i d i c environment ( p H 2. 7)
N o signal in the o u t l e t was observed f o r the f i r s t 2-3 pulses suggesting the corn_ p l e t e chemisorption of the sample injected. A p a r t f r o m differences in the r e t e n t i o n volumes, common features of the subsequent pulses were: i ) the high value of the pH of the i n i t i a l outlet pulses compared to the i n l e t val-
68 ue (ammonium dichromate solution: p H 3. 7; ammonium paramolybdate solution: pH
5. 4);the i n i t i a l value decreases in the subsequent pulses; i i ) the presence o f o x y anions in the outlet also at low value of coverage of x-alumina and in increasing quantity; i i i ) the decrease of the retention volume. The presence of C r ( V I ) and Mo(VI) i n the outlet basic solution, or, in other words,
the p a r t i a l i r r e v e r s i b l e chemisorption of these species also at low value i-
of coverage, indicates an equilibrium adsorption controlled by the H 0 depletion 3 -3 i n the solution and an acid-base mechanism of interaction. Indeed by using a 10 M HN03o r HCI solution as a c a r r i e r a complete adsorption of C r ( V I ) and Mo(V1) was recorded together w i t h the presence in the outlet pulse of the ammonium ion (Fig. 2). The variation of the response of the adsorption system to the successive pulses r e f l e c t s the surface heterogeneity of alumina.
In table 1 the limiting amount of C r ( V I ) and Mo(VI) i r r e v e r s i b l y adsorbed onto -alumina i s reported. In this table data on the chemisorption of chromium nitrate and chromic acid (frequently used in the preparation of chromia-alumina catalysts) are also included. In the pulse addition of these species in thepresence of b i d i s t i l l e d water complete adsorption was observed. Recalling that chromium n i t r a t e i s an acid, also the adsorption of this species can be described in terms o f acid-base interaction, though the i r r e v e r s i b l y adsorbed quantity i s smaller (1. 13 wt.
% C r O ). 3
TABLE 1 Me(VI) content of Y-A1 0 at maximum coverage in pulse addition 2 3 (NH4)2Cr207
Pulse Carrier wt.
% Me03
p m o l Me(VI)
HNO,IO-~M
HO ,
H2C r 0 4
Cr(NO3I3
( NH4)6M~7024 HCI 1 0 - 3 ~
H O ,
HO ,
HO ,
2.32
1. 7
3. 28
1. 13
7. 19
7. 72
1. 29
0. 94
1. 82
0. 63
2. 77
2. 98
m2AI 203
In considering the different amount of C r ( V I ) and Mo(VI) i r r e v e r s i b l y adsorbed onto y-alumina,
i t must be kept in mind that pulse addition was not c a r r i e d out up
to the point of maximum coverage (i. e. complete recovery in the outlet pulse of the sample injected). In fact in proximity of the surface saturation aslowdesorption from weaker sites, p a r t i c u l a r l y evident in the chromium incorporation, i s observed. The difference in the amount of transition metal ions chemisorbed can b e p a r t i a l l y
69 a t t r i b u t e d to these facts. ence of HNO
3
However the low amount of Cr(V1) adsorbed in the p r e g
p r o b a b l y r e f l e c t s a l s o a competing adsorption of
N O - anion. 3
The r e s u l t s of the pulse addition of W ( V I ) w e r e not r e p o r t e d in F i g .
1. In the
f i r s t pulses of W(V1) at 6OoC an increase of the p H of the l i q u i d effluent f r o m the column was observed, i n d i c a t i n g a common i n t e r a c t i o n mechanism f o r W(VI), C r ( V I ) and Mo(V1). However the r e l a t i v e high response of the U V monitor at 254 nm in-
[ 71 , w h i c h
dicates the presence in the outlet pulse o f W(V1) in a condensed f o r m
must be considered an unstable species a t the pH measured ( 8 . 9). We may deduce that the e q u i l i b r i u m conditions w e r e not reached in the l i q u i d phase, to the s l o w decondensation o f the isopolytungstate species
mainly due
[ 81 .
As the response o f the adsorption system i s under k i n e t i c control, we conside r e d i t of l i t t l e significance to c a r r y out pulse addition up to the maximumcoverage.
3. 2 S t e p addition experiments F i g . 3 shows the t r a n s i e n t value of the p H in the effluent o f the column vs. the hexavalent species added/m
2
of i-alumina.
In these experiments increases o f p H
higher than 9. 5 w e r e detected. The p r o f i l e o f the p H c u r v e may be r e l a t e d to the spectrum o f basic s i t e s of y-alumina.
In table 2 the amount o f C r ( V I ) , Mo(VI) and
W ( V I ) i r r e v e r s i b l y adsorbed i s reported.
We have n o data on the thermodinamics
of these systems; f r o m the r e s u l t s obtained i t appears that in changing f r o m C r ( V I ) to W(V1) the e q u i l i b r i u m i s s h i f t e d in the sense of the adsorption.
We a l s o observed
[ 71
that in the step addition of ammonium paratungstate the
outlet signal was s t r o n g l y dependent on temperature. 10PH
F i g . 3. p H meter response in the step addition of ammonium dichromate, a 3 monium paramolybdate and ammonium paratungstate t o I-alumina. Tempera ture: 6OoC
I
0
I
I
I
,
I
,
,
,
60
40 80 Me (Vl) in the eluted solution, p rnoVrn2
20
3. 3 B a t c h adsorption experiments Fig.
4 shows the change of the p H and of the content of the t r a n s i t i o n metal ions
in the l i q u i d phase when 450 m L of a solution of ammonium dishromate,
ammonium
-2
paramolybdate and ammonium paratungstate (10
mol Me(Vl)/kg) a r e brought i n t o
contact w i t h 4. 5 g of x-alumina at 2OoC. A d s o r p t i o n of W ( V I ) at 6OoC i s a l s o r e -
70 p o r t e d . I t appears that a t 2OoC e q u i l i b r i u m i s o b t a i n e d in a few minutes f o r C r ( V I ) and Mo(VI) w h i l e the r a t e o f a d s o r p t i o n of W(VI) i s much l o w e r in agreement w i t h the p u l s e experiments.
In f a c t the a d s o r p t i o n of W(VI) i s f a r f r o m complete at 2OoC
a f t e r 6 h : indeed t h e o b s e r v e d quantity a t e q u i l i b r i u m s h o u l d be g r e a t e r at 2OoC than a t 60°C. The s m a l l e r b u t s i g n i f i c a n t i n c r e a s e o f the p H o b s e r v e d in the b a t c h a d s o r p t i o n e x p e r iments f u r t h e r substantiates an a d s o r p t i o n m e c h a n i s m w h i r h i n v o l v e s the decondensat i o n o f p o l y a n i o n s f o l l o w e d b y the a d s o r p t i o n o f monomeric species:
Al
- OH + H30++ Me0;-
==
+ 2H20
At-0-MeO;
T h e metal i o n zontent o f i - a l u m i n a i s r e p o r t e d in table 2.
l t l
lor
-be@----
V
4
3 0
'
Mo
'
n
'
*
'
a
'
2
4 6 8 Elapsed fime, s - ~ O - ~
'
I
"
F i g . 4. Me(VI) c o n c e n t r a t i o n in the l i q u i d phase and pH o f the s o l u t i o n vs. time in b a t c h a d s o r p t i o n e x p e r i m e n t s f r o m ammonium dichromate, ammonium p a r a molybdate and ammonium p a r a t u n g s t a t e s o l u t i o n s
71 TABLE 2
Cr(V1) wt.
% Me0
3 p m o l Me(V1)
Mo(V1)
W(VI)
3
7. 7
19. 2
1. 7 0
2. 96
4. 60
Cr(V1) Mo(V1) W(VI) 2OoC W ( V I ) 6 0 ° C 1. 97
5. 75
9. 31
1. 10
2. 20
2. 20
11.95 2. 86
m2AI 0 2 3 See text 3.
4 E l e c t r o n i c and Raman s p e c t r a
Fig. 5 shows the e l e c t r o n i c spectra of C r ( l l 1 ) and Cr(V1) o n 1-Al 0 In a l l the 2 3' c a l c i n e d samples the absence of the 600 nm band showed that n o Cr(ll1) was p r e sent: thus alumina s t a b i l i z e s the highest valence state of chromium as already known f r o m the l i t e r a t u r e
[ 91.
Moreover samples obtained by addition of Cr(V1) solution onto #-alumina and calcined at 55OOC show only minor modifications in respect to the same samples only d r i e d at 1 10°C. S u c h a t r e n d was a l s o observed in the e l e c t r o n i c and Raman s p e c t r a of a l l Mo(VI)-AI 0 and W ( V I ) - A I 0 samples, thus evidentiating the st= 2 3 2 3 b i l i t y of the chemisorbed species. Mo(VI)-AI 0 and W(VI)-AI 0 samples w i t h low content of t r a n s i t i o n metal ion 2 3 2 3 showed s p e c t r a w i t h a maximum of absorption at about 250 nm. A t higher contents a shoulder at 320 nm i s observed which c o u l d be a t t r i b u t e d to another type o f centres (Fig. 6). The high fluorescence and the low signal d i d not allow the r e c o r d i n g of the Raman s p e c t r a of Cr(V1) chemisorbed on #-A1 0 w h i l e f o r Mo(V1) and W(V1) 2 3' significant s p e c t r a w e r e obtained only f o r contents not smaller than 57, MOO and 3 2 % WO (Fig. 7). The s p e c t r a of a l l the samples display the absence of c r y s t a l l o 3 g r a p h i c a l l y we1 I defined phases [ 7 ] as we a l s o checked by X - r a y patterns; they a r e Characterized b y a s t r o n g band(at 950 c m - l f o r Mo(VI) and 960 c m - l f o r W(V1)) shifted to higher wavelengths w i t h the increase of Mo(V1) and W(VI) content. These s p e c t r a r e m i n d of the s p e c t r a of aqueous solutions. U n l i k e the solutions, Mo(V1) and W(V1) adsorbed on Y-A1203 display a broader band which r e f l e c t s the surface heterogeneity, and a shift of the maximum to somewhat higher wavenumber. The p o s i t i o n of the maximum of the most intense band, p a r t i c u l a r l y at a low cover-
72
.c 5
I 2a
I
1
700
800
600
I
I
500
400
I
3OOnm
P-
I
200
Fig. 5. E l e c t r o n i c spectra of Cr(V1)-AI 0 2 (3% C r O d r i e d at l l O ° C (a) and c a l c i n e d 3 at 600°C (b); of C r ( l l l ) - A I 0 (1. 13% C r 0 3 ) 2 3 d r i e d at. 1 10°C (c)andcalcinedat700°C (d)
-----
Dried 1 1 O D c
500
400
I
300
200
A , nm
Fig. 6. E l e c t r o n i c spectra. ( 1 ) 7. 7% MOO on A l 0 ; ( 2 ) 19.2% WO on A l 0 ; (3) 3 9.3 I T o on A1203; (47 3. 05T0 $do3on
h03
A1203
Fig. 7. Raman spectra. ( 1 ) 5. 75% MOO on 3 A1203 d r i e d at 1 10°C; (2a) 7. 7% M o O 3 o n A I p 3 d r i e d a t llO°C; (2b) 7. 7% MoOgonA1203catcined at 550OC; (3a) 19. 2% WO3 onA1203 d r i e d at 1 10°C; (3b) 19. 2% W 0 3 0 n A1203calcinedat 550OC; (4a) 9. 31 WO3on A1203dried at 1 10°C; (4b) 9. 3170 W 0 3 0 n A1203 calcined at 550OC; (5) 3.05% WO3 on A1203 calcined at 55OOC
73 age, Tould suggest an octahedral c o - o r d i n a t i o n o f the t r a n s i t i o n metal i o n (aqug
ous s o l u t i o n s o f polymolybdate and t e t r a h e d r a l molybdate e x h i b i t a s t r o n g band at 940 cm-'
[ 101
and 897 c m - l [ l l ]
tungstate at 952 c m - l
r e s p e c t i v e l y ; polytungstate and t e t r a h e d r a l
[ I 2 1 and 931 [ l l ] r e s p e c t i v e l y ) . However t h i s c o n c l u s i o n
c o n t r a s t s w i t h the r e s u l t s of the a d s o r p t i o n experiments, of the e l e c t r o n i c s p e c t r a and f i n a l l y w i t h the l i t e r a t u r e data w h i c h suggest the f o r m a t i o n o f aluminium m o l y b date and tungstate-like species in t e t r a h e d r a l c o - o r d i n a t i o n
[13,
141 .
T h e r e f o r e we a t t r i b u t e the intense band t o the symmetric s t r e t c h i n g o f Me(VI)O= 4 in t e t r a h e d r a l co-ordination, w h i l e the s h i f t to h i g h e r wavelength c o u l d r e f l e c t the d i s t o r t i o n f r o m T D s y m m e t r y and an i n c r e a s e o f the Me(VI)=O double bond character
[ 71
.
4 CONCLUSION T h e pulse and s t e p a d d i t i o n experiments c l e a r l y p r o v e the i n t e r a c t i o n of the s i x t h g r o u p o f t r a n s i t i o n element oxides w i t h f - A l
0 and evidence the r o l e o f 2 3
b a s i c i t y o f r-A1 0 surface. 2 3 These experiments suggested a common i n t e r a c t i o n mechanism i n v o l v i n g the decondensation of isopolyanion in the p r e s e n c e o f alumina f o l l o w e d by the chemi s o r p t i o n o f monomeric species.
The most s i g n i f i c a n t d i f f e r e n c e s a r e o b s e r v e d in
the q u a n t i t i e s o f t r a n s i t i o n metal i o n chemisorbed. The r e l e v a n c e of the chemisorption phenomena on the p r e p a r a t i o n o f commercial chromia-alumina c a t a l y s t s i s moderate because o f the h i g h e r chromium content o f these commercial catalysts. Instead in the MOO -WO -1-Ai 0 systems the c h e m i s o r p t i o n p r o c e s s e s w h i c h 3 3 2 3 o c c u r d u r i n g the impregnation may be d e t e r m i n i n g f a c t o r s in the p r e p a r a t i o n o f i n d u s t r i a l catalysts.
In the case o f Mo(VI) the amount o f i r r e v e r s i b l y chemisorbed M o ( V I ) (about 8
%
MOO must be compared w i t h the content o f a t y p i c a l commercial HDS 3 c a t a l y s t (10-1270 MOO 1. T h i s o b s e r v a t i o n becomes m o r e p r e s s i n g in the WO 3 3 #-A1 0 system when i t i s c o n s i d e r e d that the amount of c h e m i s o r b e d W ( V I ) ( 1 9 - 2 0 2 3 wt. % W O ) i s in the r a n g e o f WO content o f a commercial c a t a l y s t (20-2170). 3 3 The p r e s e n c e o f s u r f a c e phases, i. e. o f phases h i g h l y d i s p e r s e d and n o t r e -
wt.
vealed by X - r a y s ,
was d i r e c t l y detected f o r chromia-alumina c a t a l y s t s and i s
p r a c t i c a l l y e s t a b l i s h e d f o r molybdena-alumina.
As r e g a r d s the WO -Al 0 system,
3 2 3 the general c o n s i s t e n c e of a d s o r p t i o n experiments, of s p e c t r o s c o p i c data and of
14 X - r a y pattern suggests that i n a typical commercial 8-alumina the limiting amount of W(VI) which can be dispersed as a surface phase i s i n the range of
19-2070 WO 3' ACKNOWLEDGEMENTS Dr. Paolo T i t t a r e l l i c a r r i e d out X-ray determinations.
Prof. A l b e r t o G i r e l l i
i s thanked f o r helpful suggestions and f o r the discussion of our work. Oneof the authors (P. L. V i l l a ) also thanks the Consiglio Nazionale delle Ricerche f o r finan cia1 support.
REFERENCES 1 G. P. Poole and D. 5. Mclver, Advances in Catalysis, 17 (1967) 223. 2 (a) J. H. Ashley and P. C. H. Mitchell, J. Chem. SOC.(A), (1968) 2821; (1969) 2730; (b) N. P. Martinez, P. C. H. Mitchell and P. Chi plunker, in P. C. H. Mitzhell (Ed), Proc. Climax 2nd Int. Conference on the Chemistry and Uses of Molybdenum, Climax Molybdenum Co. Ltd., London, 1977, p. 164. 3 J. Sonnemans and P. Mars, J. Catal., 31 (1973) 209. 4 G. C. A. Schuit and B. C. Gates, A. I. Ch. E. J., 19 (1973) 417. 5 (a) A. lannibello and F. T r i f i r b , 2.Anorg. Allg. Chem., 413 (1975) 293; (b) A. lannibello, S. Marengo and F. T r i f i r b , Chimica e Industria, 57 (19751676; ( c ) A. lannibello, S. Marengo, V. B e r t i , P. L . V i I l a and F. T r i f i r b , Can. J. Chem. Eng. 55 ( 1 977) 747. 6 F. T r i f i r b , Annali Chimica, 64 (1974) 377. 7 A. lannibello, P. L. V i l l a and S. Marengo, J. Phys. Chem., submitted f o r pub Iicat ion. 8 P. Souchay, Ions Min6raux Condens&, Masson 0 CIE, Paris, 1969, p. 54. 9 J. Deren, J. Haber and J. Siechowski, i n W. M. H. Sachtler, G. C. A. Schuit and P.Zwietering (Eds. ), Proc. 3 r d Int. Congr. Catal., Amsterdam, July 20-25, 1964, North-Holland Publishing Co., Amsterdam, 1965, p. 993. 10 J. Aveston, E. W. Anacker and J. S. Johnson, Inorg. Chem., 3 (1964) 735. 1 1 W. P. Griffith, J. Chem. SOC.(A), (1970) 286. 12 J. Aveston, Inorg. Chem., 3 (1964) 981. 13 (a) P. Biloen and G. I. Pott, J. Catal., 30 (1973) 169; (b) G. I . P o t t and W. M. J. Stork, in 6.Delmon, P. A. Jacobs and G. Poncelet (Eds. ), Preparation of Catalysts, Elsevier, Amsterdam, 1976, p. 537. 14 A. lannibello and P. C. H. Mitchell, 2nd Int. Symp. Scientific Bases f o r the Preparation of Heterogenous Catalysts, Louvain-La-Neuve, September 4-7, 1978, in press.
,
DISCUSSION
R.
GALIASSO
:
1 ) Do you observe the formation of a white precipitate
during your pulse experiment with ammonium hepta-molybdate solution ?
2 ) How d i d y o u m e a s u r e t h e precipitate is formed ?
%
o f Moo3 i r r e v e r s i b l y a d s o r b e d if t h i s
76
A. IANNIBELLO : 1) We did not observe any precipitate in pulse addition experiments. In step addition experiments, we observed the presence of A13+ cations in the last fraction of the effluent solution; these cations gave rise, by standing, to the precipitation of aluminium molybdate species. 2 ) Surface area and pore volume of the original y-alumina were little changed after pulse and step addition
of Mo (VI) (y-alumina
:
2
180 m /g and 0.55 ml/g; 7 . 7 % MooS on Y-AlZ03:
1 7 2 m2/g and 0 . 5 1 ml/g); moreover, X-ray diffraction pattern indica-
ted that in the Mo03-A1203 sample no other phase was present but y-Al2O3.
Therefore we concluded that all Mo (VI) is present as chemi-
sorbed species. K. HABERSBERGER : I suppose that you have extended these studies also
to the simultaneous adsorption of two or more components of this type, e.g. molybdate and vanadate, in order to obtain a better insight into the preparation of supported mixed oxide catalyst. A.
IANNIBELLO
:
We are not yet dealing with this problem in a systema-
tic fashion. Our preliminary data suggest that in adsorption systems in which competing oxoanions are present, kinetic factors can seriously affect the time at which a true adsorption equilibrium is reached. Compare, for instance, the equilibrium distribution of Cr(VI), Mo(VI) and W(V1) between liquid and solid phases with the respective adsorption rates. K.R. KRISHNAMURTHY
:
The limiting amount of chemisorbed 0x0 species
in y-alumina is in the order Cr03>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
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271,
1952, 1961,
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1966.
13,
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22, 264,
1974,
51. K. Nakada, M. Tanaka, S. Aizawa, M. Kasuga: Asahi Garasu Kogyo Giztsu Shoreikai Kenkyu Hokoku 5, 221-32 ( 1 9 7 4 ) 52. G. Parravano: JACS 1952, 1194-8 53. K. Hauffe & A. Rahmel: 2. Physik. Chem. (Frankfurt) 1954, 104-128 54. G.A. Martin, A. Renouprez, G. Dalmai-Imelik and B. Imelik J. Chim. Phys. 5, 1147-60 ( 1 9 7 0 ) 5 5 . R.H. Tien & E.T. Turkdogan: Metallurgical Transaction 3, 2039-48 ( 1 9 7 2 ) 56. J . FranGois-Rosetti, M. Th. Charton, B. Imelik, Bull.Soc.Chim.Fr. ( 1 9 5 7 ) 614-5 57. A. Roman and B. Delmon: C.r. 273, 1310-13 ( 1 9 7 1 ) 5 8 . A. Roman and B. Delmon: J . Catalysis 30, 333-42 ( 1 9 7 3 ) 5 9 . E.J. Nowak: J. Phys. Chem. 3,3790-4 ( 1 9 6 9 ) 60. V.A. Telipko, G.A. Skorokhod, V.M. Vlasenko and A.G. Fefer: Kinatika i. Kataliz 759-64 ( 1 9 7 0 ) 6 1 . V. Zapletal, K. Kolomasnik, J. Soukup, V. Ruzicka, J. Tikalova: Coll. Czech. Chem. Communic. 37, 2544-8 ( 1 9 7 2 ) 6 2 . M. van den Tempel: Coll. C.N.R.S. 9 3 8 , Physicochimie des composes amphiphiles Bordeau ( 1 9 7 8 ) 6 3 . G.S. Levinson: A.C.S. Chicago meeting, Div. Petr. Chem. 47-55 ( 1 9 6 7 )
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.
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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-
W
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
-
r 0
3
14 HOURS
0.14M Ni(NO&
0.28M Ni(NO,),
--
0.42M UREA 7.6 k g S i O p / m 3 25% TOTAL Ni
2 0 HOURS
- 0 . 4 2 M UREA
0.84M UREA 10.0kg Si02/m3 4 0 % TOTAL Ni
3 6 % TOTAL Ni
E
d
W
0 v)
m
a z
W W
0
__
/ 0 - 1
0
I
I
I
I I I 0.5
1
I
1
0
I
I
I
I
0.5
I
I
I
I
I
I
I
I
0
I
0.5
I
I
I
I
P/po
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
I
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
n
t
I1
08 FRESH
- 04 I
u
$ 0.4 5m a $
L
0
z
0
F 0.4
4 HOURS
@5
a 0.4
25 0.4
W
W
k 1
t -1
'U 80 HOURS
8
0.4
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
n
0 I
0
I
I
I
I
I
I
I
I
I
\ 1
N
E
FRESH AND REDUCED, 673 K
U
/
1073 K
/
I
I
I
I
I
I
I
I
I
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.
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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.
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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. <0.4
1
Adsarltira CAT10NS
<0.5 10 2 0
----- . ------4 3.9-4.5 'v 5.5 -6
C AT I0NS
or
5.7 -6.7 6.5 -6.9 ?/
87
ANIONS
-u6.75
8 5 -7.5 7.0 9.0
-
t
---- ----'v 8.9 8.4-9.0 8.7-9.7 w 10.4 12.1 -12.7
A NlONS
T h u s , w i t h t h e s e v a l u e s , w e can h o p e t o p r e d i c t r o u g h l y t h e b e h a v i o u r of an o x i d e when it i s impregnated by a s o l u t i o n o f a m e t a l c o m p l e x salt.
3.6.
LIMITS AND SECONDARY PHENOMENA
I n practice,the p r e p a r a t i o n of m e t a l c a t a l y s t s by c o m p l e x a d s o r p t i o n may b e l i m i t e d or c o m p l i c a t e d by m a n y f a c t o r s o r s e c o n d a r y phenomena. T h r e e of them a r e d i s c u s s e d b e l o w :
225
3.6.1.
Texture of the oxide
W e h a v e seen t h a t a n important a d s o r p t i o n capacity r e q u i r e s t h e c h o i c e o f a d e q u a t e p H conditions. H o w e v e r , t h i s c o n d i t i o n i s n e c e s s a r y , b u t not s u f f i c i e n t in itself.Thus,if t h e o x i d e in o p e r a t i o n h a s a low s u r f a c e a r e a f t h a t i s t o say, a s m a l l n u m b e r of a d s o r p t i o n s i t e s , i t s a d s o r p t i o n c a p a c i t y w i l l b e n e c e s s a r i l y low. F o r instance, w e c a n n o t h o p e to f i x m o r e than 0.2 w t % o f ulatinum o n 2
an a l p h a a l u m i n a o f 1 0 m /g f r o m a c h l o r o p l a t i n i c acid solution.In t h e 2
s a m e conditions, a g a m m a a l u m i n a o f 200 m / g f i x e s about 3 w t % of plat inum. 3.6.2.
Stability of metal complexes
T h e f o r m u l a s o f s e v e r a l c h l o r o m e t a l and a m i n e c o m p l e x e s h a v e b e e n l i s t e d in t a b l e 2 and t a b l e 3 to i l l u s t r a t e t h e p o s s i b i l i t i e s of m e t a l a n i o n and c a t i o n adsorption. T h e s t a b i l i t y o f t h e s e c o m p l e x e s i s not a l w a y s excellent. F o r i n s t a n c e , in an aqueous m e d i u m , s o m e c h l o r o m e t a l anions are subj e c t t o a q u a t i o n , h y d r o l y s i s , r e d u c t i o n (if t h e m e t a l h a s s e v e r a l valences), and even p o l y m e r i z a t i o n r e a c t i o n s (45). T h e s e r e a c t i o n s lead t o d i f f e r e n t c o m p l e x e s w h o s e
f o r m u l a s are g i v e n
in t a b l e 5. TABLE 5 A q u a t i o n , h y d r o l y s i s , r e d u c t i o n and p o l y m e r i z a t i o n o f c h l o r o m e t a l a n i o n s (MC1,)
.
X-
Chloro Aquo Complmxmm
.
Chloro hydroxy Complrxrm
.
Chloro Aquohydroxy Complmxmm
.
Reduord Chloro Aquohydroxy Cornplrxrm
.
Polymeric
Spaaimm
226
The kinetics of these reactions depends on a number of parameters such as pH, chlorine concentration or temperature. Concerning the adsorption of these anions on oxides,the reactions of aquation, hydrolysis, reduction and polymerization modify the charge of the coordination sphere of the anion, and this modification may have a significant influence on the thermodynamics of the adsorption equilibria. In this case, there are as many exchange equilibria as there are species in solution. It is therefore important to know the metal species in operation during impregnation,and,if possible,to maintain conditions where only one metal species exists. Solubility of the oxide
3.6.3.
The apparent solubility of mineral oxides or hydroxides depends on their acid-base character (figure 7 ) .
2
0 -2 -4 I
L
0
2
6
4
I
1
I
8.
10
12
b
4 2
0
-2 -4 1
0
I
2
1
4
8
1
I
I~
1
I
6
0
1
2
I
PH Fig. 7 . Variation of the solubility of alumina and silica versus pH.
227 For instance,the solubility of an acid oxide or hydroxide,such as silica,increases very rapidly in a basic medium. On the other hand, a basic oxide or hydroxide dissolves better in acid medium. For an amphoteric oxide, the solubility increases as well for acid pH as for basic PH * Anionic or cationic exchanges on basic or acid oxides often imply using solutions that are respectively acid or basic. Consequently, the support may dissolve if the apparent solubility is high at the pH of the impregnation solution.In this case,it involves solubilization of ions containing the specific metal of the support. This phenomenon of dissolution has been referred to many times in literature for an alumina support that has been brought into contact with a solution containing a strong acid such as HC1 or H2PtC16 (8,9, 10,15,46).
Figure 8 shows for example that when gamma alumina is in contact with a hydrochloric acid solution containing much more chlorine than that which corresponds to the adsorption capacity of the support, then a part of the hydrochloric acid is adsorbed on the alumina,and the excess HC1 is neutralized by surface dissolution of alumina into aluminium chloride (13).
2.5
---_
CI adsorbed
_-
Fig. 8 . Chlorine adsorption and aluminium dissolution for the HC l-y-AlZ03 system.
228 4.
CONCLUSIONS I n analyzing the p h e n o m e n a o c c u r r i n g w h e n an o x i d e p a r t i c l e i s p u t
i n t o a n a q u e o u s s o l u t i o n , it i s p o s s i b l e t o b r i n g out the i m p o r t a n c e o f a s p e c i f i c p r o p e r t y of t h e oxide: i t s i s o e l e c t r i c p o i n t
(I.E.P.S.)
o r z e r o p o i n t o f c h a r g e (Z.P.C.). I n d e e d , an o x i d e in c o n t a c t w i t h a s o l u t i o n w h o s e pH is b e l o w i t s I.E.P.S. anions.0n
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 b e s u r r o u n d e d b y c o m p e n s a t i n g t h e o t h e r hand, t h e s a m e o x i d e in a s o l u t i o n w i t h a p H a b o v e
i t s I.E.P.S.
t e n d s t o p o l a r i z e negatively and b e s u r r o u n d e d by c o m p e n -
sating cations. W i t h t h i s s i m p l i f i e d schema, t h e p H r a n g e t h a t h a s t o b e used and t h e k i n d o f m e t a l c o m p l e x e s (anions o r c a t i o n s ) t h a t m u s t b e s e l e c t e d c a n b e estimated. T h u s , t h e I.E.P.S.
o f a n o x i d e and t h e p H o f t h e im-
p r e g n a t i n g s o l u t i o n seem t o b e the m o s t important p a r a m e t e r s t h a t r e g u l a t e a d s o r p t i o n o f m e t a l complexes. H o w e v e r , k n o w i n g o n l y t h e I.E.P.S. o f an o x i d e is n o t enough.
It m u s t b e k e p t i n mind,for i n s t a n c e , t h a t
t h e c a p a c i t y o f c a t i o n i c a d s o r p t i o n of s i l i c a b e c o m e s s i g n i f i c a n t a b o v e p H 6, w h i c h i s f i v e p H u n i t s from i t s isoelectric point. C o n s e q u e n t l y , in o r d e r t o k n o w w i t h a g o o d p r e c i s i o n t h e a d s o r p t i o n c a p a c i t y of an o x i d e at a g i v e n p H , i t i s p r e f e r a b l e t o know t h e evolut i o n o f t h e p o l a r i z a t i o n a s a f u n c t i o n o f pH. T h i s c a n b e done, for e x a m p l e , either b y m e a s u r i n g t h e e l e c t r o p h o r e t i c v e l o c i t y , o r by neutral i z a t i o n e x p e r i m e n t s at c o n s t a n t p ~ . N o t w i t h s t a n d i n g t h e c o m p l i c a t i o n r a i s e d b y s e c o n d a r y p a r a m e t e r s such a s s m a l l s u r f a c e a r e a o f t h e oxide, i n s t a b i l i t y of t h e m e t a l c o m p l e x e s or solubilization of the oxide, we think that all these considerations w h i c h seem t o b e valid. in 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 on a l u m i n a o r s i l i c a , a r e a l s o valid for o t h e r oxides. F u r t h e r m o r e , o t h e r p o s s i b i l i t i e s m a y be c o n s i d e r e d , s u c h as t h e d o p i n g o f a c a t a l y t i c a c t i v e o x i d e by s u r f a c e a d s o r p t i o n o f a f o r e i g n m e t a l ion, or e v e n t h e s e l e c t i v e d e p o s i t i n g of a m e t a l on o n e o f t h e c o n s t i t u e n t s of an o x i d e mixture. T h i s i s t o b e c o n f i r m e d , h o w e v e r , t h r o u g h f u r t h e r o r i g i n a l experiments.
ACKNOWLEDGEMENTS W e t h a n k Pr. R. M O N T A R N A L
(I.F.P.)
and Drs. M. M I C H E L and R. P O I S S O N
(RHONE-POULENC) f o r h e l p f u l discussions.
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1. v.S. B o r o n i n , V . S . N i k u l i n a a n d 37 ( 1 9 6 3 ) 626-629. 2. H . A . B e n e s i s , R . M . 328-335.
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C u r t i s a n d H.P.
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S u g i e r and J.F.
7. R.W.
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Elektrochemie, Kulling,
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23. J . S t a n t o n a n d R . W .
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25. D . B r i g g s a n d Y . M . 194-196.
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Duval and M . J .
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40. J. Stanton and R.W. Maatman, J. Colloid. Sci., 18 (1963) 132-146. 41. A. Kozawa, J. Elec. SOC., 106 (1959) 552-556. 42. M. Michel, personal communication. 43. G.A. Parks, Chem. Reviews, 65 (1965) 177-198. 44. J.F. Lepage, Catalyse de contact, Technip, Paris (1978). 45. W.P. Griffith, The Chemistry of the rarer platinum metals Interscience Publishers, London (1967). 46. J.M. Vergnaud, B. Rey-Coquais, 8 . Buathier and R. Neyron, Bull. SOC. Chim. Fr., (1968) 3881-3885.
DISCUSSION MATIJEVIC : The picture given by Dr. Brunelle i s greatly oversimplified. Assuming that electrostatic attraction is solely responsible for adsorption is not correct (i.e. that anionic adsorption is going to take place below I.E.P.
and cationic adsorption above I.E.P.).
Indeed, opposite charges will attract solute species into the interfacial layer, but it will not make them adsorb. For the latter to happen a reaction has to take place, such as surface precipitation, condensation, complexation, etc. A s a matter of fact uncharged solutes may show higher adsorption density on the same substrate than the OPPositelY charged species of similar composition. J.P. BRUNELLE
:
First of all, we must remind that our topic is not
the "fixation" of any solute on any substrate.
In front of the com-
231 plexity o f i n t e r a c t i o n p h e n o m e n a b e t w e e n s o l u t e s and c a r r i e r s , w e h a v e excluded f o r e x a m p l e t h e c a s e o f t h e f i x a t i o n o f h y d r o l y z a b l e c a t i o n s on oxides.
I n t h e same m a n n e r , w e h a v e not c o n s i d e r e d t h e
c a s e of f i x a t i o n o f c o m p l e x i n g or p r e c i p i t a t i n g a n i o n s s u c h as o x a l a t e , c i t r a t e , s e l e n a t e o r p h o s p h a t e o n alumina. I n fact, w e h a v e limited our s p e e c h t o t h e a d s o r p t i o n p h e n o m e n a 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 and a m i n e
c o m p l e x e s o f g r o u p s 7 a , 8 and lb
m e t a l s on m i n e r a l o x i d e s s u c h as a l u m i n a o r s i l i c a , and t h i s , f o r t w o reasons: it i s t h e f i e l d w h i c h seems t h e best k n o w n
(or r a t h e r t h e
least u n k n o w n ) and it i s o f g r e a t i n t e r e s t for t h e c a t a l y s i s searchers. I n t h i s v e r y c a s e , w e h a v e attempted t o p r e s e n t a s i m p l i f i e d a n a l y s i s o f t h e p a r a m e t e r s w h i c h g o v e r n t h e a d s o r p t i o n phenomena. T h i s a n a l y s i s , t h o u g h s i m p l i f i e d , h a s b e e n t i l l now c o n f i r m e d experim e n t a l l y , and f u r t h e r work w i l l s h o w us t h e l i m i t s o f our presentation.
L. R I E K E R T : C o m m e n t : B e c a u s e o f t h e m a n y c h e m i c a l a n d p h y s i c a l intera c t i o n s t a k i n g p l a c e in t h e s e s y s t e m s any exact q u a n t a t i v e
descrip-
t i o n w i l l b e h e l p f u l , a l t h o u g h d i f f e r e n t p h e n o m e n a m a y b e . lumped i n s u c h a description.
I n t h i s r e s p e c t t h e c o n t r i b u t i o n by Dr. B r u n e l l e
a p p e a r s w e l c o m e and essential. P e r h a p s i t w o u l d b e a d v i s a b l e t o disting u i s h exactly in t e r m s of e x p e r i m e n t a l l y m e a s u r e d q u a n t i t i e s
...
the
t e r m s " a d s o r p t i o n " , "fixation", "ion-exchange'' and " r e a c t i o n w i t h t h e carrier".
J.P.
B R U N E L L E : F i x a t i o n and i n t e r a c t i o n are g e n e r a l t e r m s w h i c h may
i n c l u d e m a n y t y p e s o f p h e n o m e n a s u c h a s p r e c i p i t a t i o n , exchange, a d s o r p t i o n , n e u t r a l i z a t i o n , c o m p l e x a t i o n (or s p e c i f i c adsorption)
...
I n a s c h e m a t i c m a n n e r , t h e p o l a r i z a t i o n in an acidic m e d i u m of a s u r f a c e h y d r o x y l g r o u p may p r o c e e d b y t h e t w o f o l l o w i n g w a y s (1)
M-OH
+
H+A-
:
+ M-OH~A
= ,-
or (2)
M-OH
+
"+A-
M+A-
r -
+
H ~ O
I n t h e s a m e w a y , t h e p o l a r i z a t i o n in a basic medium o f a s u p e r f i c i a l hydroxyl group may be written : (3)
M-OH
+
B+ O H -
-
B+
M(OH);
or (4)
M-OH
+
B+ OH-
+
MO-B+
+
I n o u r opinion, e q u a t i o n s ( 1 ) and cesses.
Equations
(2)
and
H ~ O
( 3 ) c o r r e s p o n d t o a d s o r p t i o n pro-
( 4 ) c o r r e s p o n d t o n e u t r a l i z a t i o n processes.
232 Exchange reactions correspond in fact to the following cases (5)
+ M A~
(6)
MO B1
+ A;
- + +
4
+
B2
-
A
M+A;
- +
M O B2
+
:
A;
+
:B
For example, polarization of alumina in hydrochloric acid with chloride fixation may occur via an adsorption process or a neutralization process On the other hand, fixation of chloroplatinate anions on chlorinated alumina proceeds via an exchange reaction between adsorbed chloride anions and chloroplatinic anions of the solution. V.
F A T T O R E : The average crystallite dimensions of platinum on a carrier
are dependent not only on the impregnation technique used (see your first figure). It is possible to vary and to obtain very low and similar crystallite size starting from different impregnation methods just chasing
the proper controlled drying and calcining conditions like,for
instance, oxidizing or reducing atmosphere, fast or slow rate of drying. J.P. BRUNELLE : We agree with you when you say that the choice of opti-
mal activation treatment may also lead to small crystallites. But, one may also recognize that it is generally
easier
to obtain a great
dispersion of a metal on a carrier when an adsorption or exchange procedure is used.
233
ANALYSIS OF STEPS OF IlVI?REGNATION AND DRYING IN PREPARATION OF SUPPORTED CATALYSTS
V.B. Fenelonov, A.V. Neimark, L.I. Kheifets, A.A. Institute of Catalysis, Novosibirsk, U S S R
Samakhov
ABSTUCT The steps of impregnation and drying and their influence upon the uniformity of distribution of catalyst components on supports have been analyzed. The component adsorption during capillary impregnation of mono- and &dispersed supports as well as con?poner_t redistribution on drying have been exznined. Several parameters determining the distribution of precipitated component within the catalyst particles nave been proposed. INTRODUGTI O N Distribution of the active components of supported catalysts, introduced into the support from solution, may depend significantly upon the conditions of imgregnation and drying (ref. l-G).The final distribution of components adsorbed from solutim may be determined by sorp tion-diffusion nechaiiism before drying (adsorption catalysts) (ref. 2-4). If the extent of adsorption of ccrnponents is negligibly small as cornparad to their total concsntration, then distribution depends primarily on drying conditions (impregnated catalysts) (ref. 5 , 7 - 9 ) . The best known works ir, this field (partially reviewed by J.R.Anderson, ref.1) orfer extensive consideration on the details of precipitation of adsorbed components. However, only limited studies have been carried out o n the role of drying and the combined effects 3f capillary transfer and adsorption duri3g drying. The objective of this paper is to systematize a l l known factors effecting the distribution of an active component during impregnation and drying. RESULTS 1 . Analysis of eulibrium conditions The ratios between adsorbed and dissolved component in
234
the impregnated support under equilibrium conditions is determined by the adsorption isotherm, equilibrium concentration of solution
Ceq, and support structural characteristics (pore volume V, and surface area S ) . For the general case, during the contact of m grams of the support with a solution volume Vo , at initial concentration C o , the following equations are true:
Ceg =
6
c, f f - am/v*co i - (f"-fo 1 V vm o
(3)
where Q and are values of adsorption and total component concentration in one gram of the supporting substance; fo is the fraction of pore volume filled with pure solvent before contact with the solution; fv is the fraction of the pore volume inaccessible to the component. Introduction of parameter fv permits the cases of "negative" adsorption and molecular-sieve effects (when fv>O) to be taken into account. As follows from equation (3), the decrease in the component concentration after impregnation does not necessarily point to adsorption and may be observed, for example, at a=O and fo > 0. At f, > O the increase in C is possible as well. P Parameter P is the ratio between the adsorbed and the dissolved component in the support volume: =
where
K
a g-a -
a Ce$V&jV)
and K
="
co
=
--
K"
v, (! - h)
Ks S (4 -
=
fS)
'v,(r--fYI
(4)
are characteristics of the adsorption
s *
isotherm (in the linear region of the isotherm, Kv and Ks are values of the Henry constant); fS is the fraction of the surface inaccessible to the component.
At P>>1.0 the component is primarily in an adsorbed form ("adsorption" catalyst) and at PC! 1.0 adsorption is negligibly small ("impregnated" catalysts). Figure 1 shows the change in parameter P during adsorption of H2PtC16 on three typical supports (ref. 4). For these cases adsorption isotherms are described by the Langmuir equation. In the Henry region of the isotherm, parameter P is constant but decreases subsequently down to the value corresponding to the saturated solution (Cs). 2. Impregnation The concentration and distribution of an active component depends
235
Fig. 1. Plot of parameter P versus equilibrium concentration C in adsorption of H2PtC16 from aqueous solutions on activated eq carbon ( I ) , active alumina (2) and alunio-silicate (3) (data taken from ref. 3). significantly upon t h e conditions and duration of impregnation.If the initial support is saturated with a pure solvent (fo=l.O), then impregnation reduces to the diffusion of the component into the support pores, accompanied by adsorption. Hereafter this method will be called "diffusional impregnationt1.The characteristic time Qf diffusional impregnation (ref. 1,2) is found from the equation: td = R~(I+P)/D (51 where R is the grain radius and D is an effective coefficient of the component diffusion in the pore medium. The uniform distribution is reached only at t > 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 dA<<1.0 we may assume f o r mono?(u) = u. I n this case
[C0/Cs] P:, (1
(20)
+ 7; 1
and c o n d i t i o n f o r the s o r p t i o n t o be n e g l e c t e d w i l l be p r e s e n t e d as:
PA en
5
4
(21 1
1.0
cO
If c o n d i t i o n s r e v e r s i b l e t o eq. ( 1 9 ) and ( 2 1 ) a r e f u l f i l l e d , then
the whole component i s bound upon drying. Furthermore, a t 4 ( ( 1.0 t h e s u r f a c e c o n c e n t r a t i o n i n c r e a s e s w i t h movement along the g r a i h r a d i u s while a t ~ 4 ~ 3 1 . 0the component i s c o n c e n t r a t e d a t the mouth o f s m a l l pores coming e i t h e r t o the e x t e r n a l g r a i n s u r f a c e or' t o the s u r f a c e o f macropores. F i n a l l y , the c o n d i t i o n f o r p r e p a r a t i o n of a f i n e l y - d i s p e r s e d d e p o s i t , r e s u l t i n g from the component p r e c i p i t a t i o n a f t e r the s o l u t i o n s a t u r a t i o n , transforms t o
( provided t h a t
v(u) = u )
,
and
241
(provided that
7 (u)
= 1.0
I n b o t h c a s e s ucr = VI1/Vr
1.
. A s f o l l o w s from eqs.
( 2 2 ) and ( 2 3 ) , I
t h e p o r t i o n of a f i n e l y - d i s p e r s e d d e p o s i t i n c r e a s e s w i t h Ps
.
CONCLUSIONS The s t e p of i m p r e g n a t i o n c o n t r i b u t e 6 s i g n i f i c a n t l y t o the d i s t r i b u t i o n of an a c t i v e component for the c a t a l y s t o f t h e a d s o r p t i o n type a t P * l o o I n t h i s c a s e t h e u n i f o r m i t y of d i s t r i b u t i o n depends on i m p r e g n a t i o n time. To o b t a i n t h e uniform d i s t r i b u t i o n (termed a s d i s t r i b u t i o n of t h e first t y p e ) , t h e time of impregn a t i o n must be l o n g e r t h a n t h e c h a r a c t e r i s t i c time , td. The maxinum amount of t h e i n s e r t e d component i s deterrnined by s o r p t i o n isotherm. A t s h o r t e r p e r i o d s o f time of i m p r e g n a t i o n t h e d i s t r i b u t i o n i s c h a r a c t e r i z e d by t h e i n c r e a s e d c o n c e n t r a t i o n of t h e component n e a r the e x t e r n a l s u r f a c e of the g r a i n ( d i s t r i b u t i o n of the second t y p e ) . I n the c a s e of impregnated c a t a l y s t s (Pd 1.0) , a f t e r the s t e p of i m p r e g n a t i o n ( p r o v i d e d t h a t time of impregnation exceeds t cap. 1, an e q u i l i b r i u m d i s t r i b u t i o n i s e s t a b l i s h e d which changes s i g n i f i c a n t l y a t the s t e p of d r y i n g , The c a t a l y s t s of t h e f i r s t type a r e o b t a i n e d i f dJ>1.0, o l g L ~ 1 . 0 , H < 1.0 ( b $ CsVh may be i n t r o d u c e d ) o r i f ~ ~ & l . O ,d,P)1.0 and
.
any H ( b 4CsVr
may be i n t r o d u c e d ) . The c a t a l y s t s of t h e second
>
type a r e o b t a i n e d i f d ~ S 1 . 0 and 0'4 (1.0, H 1, especially i f o(qb 1.0. C o n d i t i o n d3.31.0, c ~ $ < ( l . O l e a d s t o the c a t a l y s t s o f t h e t h i r d type c h a r a c t e r i z e d by h i g h c o n c e n t r a t i o n a t t h e g r a i n c e n t e r . I n the i n t e r m e d i a t e c a s e s , b o u n d a r i e s between t h e s e p r i n c i p a l t y p e s of d i s t r i b u t i o n become l e s s d i s t i n c t . T h i s a n a l y s i s should be c o n s i d e r e d o n l y a s a f i r s t approximation. I t does n o t take i n t o account p o s s i b l e changes i n t h e s u p p o r t s t r u c t u r e d u r i n g i m p r e g n a t i o n , n o r does i t 2 o n s i d e r the d e t a i l s of i n t e r a c t i o n between a c t i v e componer-t and s u p p o r t . The d i s t r i h u t i o n of an a c t i v e component may a l s o chauge d u r i n g h e a t i n g . A c t u a l l y the s i g n i f i c a n t p r o c e s s f a c t o r s which determine d i s t r i b u t i o n a r e more complicated. We b e l i e v e , however, t h a t o u r a n a l y s i s may be u s e f d l f o r comparative e s t i m a t i c n of the e f f e c t s o f p r i n c i p a l p a r a m e t e r s on the d i s t r i b u t i o n of an a c t i v e component.
242
It may also pro?iids assistance to x f i d w s t a n d the p:iysico-chmica.l S ~ G details of the ~ ~ Y J C ~ S invoived. REFERENCES 2
2
3 4
5
6 7 8
9 10
11 12
J.R, Anderson, Structure of Netallic Catalysts, Acad. Press, London - K.Y., 1975. P.B. Wej-sz, Trans. Pareday SOC., 63 (1967) 1801, 1807. I?. Harriott, J. Catalysis 14 (1969) 43. ROW. Maatmaii and C.D. Prater, Ind. Eng. chern., 49 (1957) 253. R.W. Maatman, Ind. Eng. chern., 51 (1959) 913. R.C. Vincent and P.P. Merrill, J. Cat., 35 (19743 206., A.V. Weimark, V.B. Ponelonov and L . I . Heifets, React. Kinet. Catal. Lett., 5 (1976) 67. V.B. Fenelonov, Kinetika i Kataliz, 16 (1975) 732; i 7 (1976) 1582. G.K. Boreskov, Proc. 1st Int. Symp. Catalysts Preparation, Brussel, 1976, p . 223. V.B. Fenelonov in sb. Adsorption and Porosity, tWauk811,Moscow, 1916, P. 55. A.V. Neimark and L.1, Hei.fetn,, Doklady Akademii Nauk SSSR, 228 (1976) 135. A.V. Neimark, L.M. P i s men, V.E. Babenko and L . I . Heifets, Teor. Osnovy Khimicheskoi Tekhnologii, 9 (1975) 369.
Note added i n p r o o f :
I t can b e added t h a t i n t h e c a s e of
"adsorption" c a t a l y s t s , intro-
d u c t i o n of adsorbed competitive substances allows t o r e g u l a t e t h e d i s t r i b u t i o n o f t h e main component w i t h i n wide i n t e r v a l . T h i s r e g u l a t i o n i s b a s e d on t h e c h r o m a t o g r a p h i c e f f e c t s r e s u l t i n g
i n t h e d i s p l a c e m e n t o f w e a k l y a d s o r b e d c o m p o n e n t s by s t r o n g l y a d s o r b e d ones.
I n c o n t r a s t w i t h t h e c l o s e a n a l o g o u s method
graphy
-
-
p a p e r chromato-
component motion i s a s s o c i a t e d w i t h t h e m i g r a t i o n from t h e
e x t e r n a l s u r f a c e i n t o t h e g r a i n and c a n b e accompanied by t h e a d d i t i o n a l c o n c e n t r a t i n g o f t h e d i s p l a c e d component. F o r e x a m p l e , s i m u l t a n e o u s i m p r e g n a t i o n o f A 1 20 3 c a t a l y s t s w i t h s o l u t i o n s H 2 P t C 1 6 , a n d H 2 C 2 0 4 o r H2S04 p e r m i t s t h e p r e p a r a t i o n o f Pt/A1203 c a t a l y s t s w i t h a n a c t i v e component d i s t r i b u t e d e i t h e r i n t h e c e n t r a l zones o r i n t h e a n n u l a r form i n s i d e t h e g r a i n .
I n t h i s case
t h e c o m p e t i t i v e a g e n t i s a d s o r b e d more s t r o n g l y t h a n H 2 P t C 1 6 . taneous impregnation of A 1 2 0 3 HCOOH,
HN03
with s o l u t i o n s of H2PtC16
Simul-
and CH3COOH,
o r HC1 a i d s i n the distribution of P t a t the external
boundary of t h e g r a i n .
These c o m p e t i t i v e a g e n t s a r e adsorbed i n a
w e a k e r f o r m than H 2 P t C I 6 . V a r i a t i o n s i n t h e c o n c e n t r a t i o n o f components, t i o n of
t h e i r types, dura-
t h e i r i m p r e g n a t i o n as w e l l a s a p p l i c a t i o n of
successive
t r e a t m e n t s by a c o m p e t i t i v e a g e n t o r a n a c t i v e component s i g n i f i c a n t l y
243 enlarge the possibilities of regulating the distribution of an active component.
DISCUSSION
R. GALIASSO
In my opinion, your model for support structures is
:
oversimplified
1 ) How do you take into account that during the
:
drying processes the a 3 function changes due to the increasing influence of diffusional gradients, mainly during the period the 2)
rate decreases ?
Could you comment on the form of the il function,
which appears in equations 16 and 17 for bidispersed catalysts ? V. FENELONOV
: 1)
Our analysis should be considered only as a first
approximation which is necessary for a physico-chemical analysis of processes, responsible for formation of structures on catalyst supports.
We have taken into account that the value o f the parameter
a 3 may be increased during the fast drying, as a result of a displacement of the evaporation zone in the depth of the pellet and an increase in the vapor diffusion resistance (in the text "fall in the value a 3 " must be replaced by "increase in the value a,").
In this
case, in the central part of the pellet, conditions may be produced similar to those obtained during slow drying. 2 1 During the slow drying regime a bidispersed structure is obtained :
at first liquid is removed from the large pores between aggregates and only afterwards from the narrow pores in the .aggregates. So we may represent the drying processes in this simplified way: first of all processes of removal of liquid from the secondary structure between aggregates should be considered. porous material.
The aggregates are viewed as non
Then the independent processes of removal of
liquid from the interior structure of the aggregates has to be taken into account.
This approach permits the use of our equations for
bidispersed structures too. H. CHARCOSSET
:
What do you know about the particle size of platinum
in the catalysts prepared from different ion competitors (last slide in the communication) ?
V.B.
FENELONOV
:
Generally speaking, the dispersion of the active
component depends on its surface concentration.
It may be defined
as the amount of component per unity of support surface and as the number of component crystals per unit of support surface.
It is
244
c o n n e c t e d t o s i n t e r i n g and t o o t h e r r e l a t e d p r o c e s s e s o f r e d i s t r i b u tion.
S o , in our e x p e r i m e n t s t h e c r y s t a l s i z e i s about 10-15
A
in
t h e c a s e of a uniform d i s t r i b u t i o n ; if t h e c o m p o n e n t i s c o n c e n t r a t e d i n a n a r r o w z o n e of t h e pellet, t h e c r y s t a l s i z e i s increased.
245
SOME MECHANISTIC CORRELATIONS BETWEEN IMPREGNATION AND ACTIVATION OPERATIONS FOR THE PREPARATION OF HIGH-SELECTIVITY SUPPORTED METAL CATALYSTS
and T. INUI
H. SHINGU
Department of Hydrocarbon Chemistry, Faculty of Engineering, Kyoto ( 3 8 Higashikishimoto, Shimogamo, University, Kyoto, 606 Japan Sakyo, Kyoto, 606 Japan)
SUMMARY From reaction-engineering analysis of catalyst performances, it is concluded that the essential structural requirements for the highselectivity (i.e., demonstrative of 100% purity of catalytic function) supported metal catalysts, particularly, for the oxidation catalysts o n silver and copper bases, comprise three classes of uniformity principles concerning the distribution, the size or the microsurface structure, and the microscopic environment of catalyst materials in structural matrices of the catalyst system, which are respectively related to (1) the porous support, ( 2 ) the dispersed metal particles or agglomerates, and ( 3 ) the situation of supporting the latter. According to these uniformity principles, the selection of suitable supports having the prerequisite uniform pore and surface structure and the correct impregnation and activation procedures for the preparation of high-selectivity catalysts are discussed, and some demonstrative examples are illustrated together with interpretations of mechanistic correlations between the operations.
1. INTRODUCTION For the establishment of scientific bases for the preparation of heterogeneous catalysts it is imperative that adequate reaction engineering studies should first be made to ensure the clarification of the engineering objectives, particularly, the essential structural requirements for a genuinely heterogeneous catalyst which may be definable, by analogy to enzyme systems, as a chemical “reactingapp a r a tu s ’‘
.
These structural requirements could be concluded from surfacekinetic or certain mechanistic studies of the real working state
of the catalyst, i.e., from reaction engineering analysis of the
catalyst performances, namely, the selectivity, stability, and activity.
As
a result, these three fundamentals of catalyst
performance are found to be, functionally as well
RS
structurally,
in close connection with each other from the reacting-apparatus viewpoint. The present paper shows firstly, that with the supported metal catalysts, for some selective oxidations, the structural requirements for an ideal selectivity are eventually almost the same as those for high stability and activity (except in the last case, one additional scalar parameter, namely, the bulk concentration of active sites in structural matrices of the catalyst system). essential requirements may be summarized
The
:
(1)
Uniform distribution of the in-pore surface in the structural matrices of a uniform macro- and/or micro-pore system of the support.
(2)
Uniform distribution of reduced metal particles or agglomerates of suitable uniform size, eventually supported in dispersion through the in-pore surface.
(3)
Uniform physical and chemical situation of supporting the dispersed metal particles or agglomerates on the in-pore surface which renders them capable of satisfactory reproduction and/or reversibility of the redox transformation of the individual metal particle or the microscopic surface layer of the metal agglomerates under the reaction conditions.
In view of the uniformity principles which occur in all the above items, especially item ( 1 1 , it is evident that the non-uniformity of the pore structure of the conventional preformed supports precludes the application of various measures for uniform impregnation.
There-
fore, it has been necessary to select or eventually to synthesize suitable supports having the prerequisite uniform pore and/or surface structure for the investigation of impregnation and activation operations for the purpose of preparing high-selectivity metal catalysts. It has also been found that even with supports of sufficient uniformity the ordinary impregnation and activation procedures failed to produce a final uniform distribution of impregnants and dispersed metal support particles at both macro- and micro-porous levels. In particular, certain mechanistic correlations between impregnation and activation processes, depending upon the chemical and physical nature of the impregnating compounds, should be taken into consideration. Thus, with bimodal supports which possess a three-dimensionally uniform, isotropic and continuous, pore structure, the macroporous uniformity control could be effected by filling the macroporous space
o r s u r f a c e u n i f o r m l y w i t h a commensurate q u a n t i t y o f
impregnant
l i q u i d o r solution of appropriate concentration using wetting o r spraying techniques,
provided t h a t t h e impregnants could be deposited
i n a n amorphous o r a homogeneous s t a t e and/or If
( s u c h a s g l a s s y ) on e v a p o r a t i o n
activation. t h e c r y s t a l l i n e impregnant d e p o s i t s a r e t o m e l t p r i o r t o t h e
activation process,
such as thermal decomposition,
c o n t r o l may b e n e g l e c t e d ,
t h e above
and a c o r r e c t p r o c e d u r e w i t h t h e a c t i v a t i o n
p r o c e s s under microporous uniformity
c o n t r o l becomes t h e main s u b j e c t .
The m i c r o p o r o u s u n i f o r m i t y c o n t r o l i s e s s e n t i a l l y r e l a t e d t o t h e activation process,
but it a f f e c t s t h e impregnation procedure
t h a t it a l s o corresponds t o t h e formation of structure o r the active sites. of
in
the f i n a l microsurface
The m i c r o p o r o u s
surface structure
t h e support a s such does n o t always m e e t t h e requirements of
above i t e m s
(2)
and
the
( 3 ) , and c e r t a i n chemical a c t i v a t o r s and/or
s t r u c t u r a l promoters should be used a s necessary according t o t h e a c t i v a t i o n mechanism, procedures,
t o modify t h e a c t i v a t i o n as w e l l a s impregnation
so t h a t t h e u s u a l c o n t r o l o f
exchange e f f e c t s i n micropores,
t h e a d s o r p t i o n and/or
much l e s s o f
ion-
t h e chromatographic
e f f e c t s i n m u l t i p o r o u s s y s t e m s , may s e e m t o b e a l m o s t i m m a t e r i a l i n the present context.
All t h e s e c o n t r o l s over c a t a l y s t p r e p a r a t i o n parameters have been e f f e c t e d together with the corresponding systematic reactionengineering a n a l y s e s of
c a t a l y s t performance
proper use of one-dimensional bed f l o w s y s t e m s
d a t a o b t a i n e d by t h e
isothermal integral reactors i n fixed-
f o r laboratory as w e l l a s p i l o t tests,
which a t t h e
same t i m e h a v e s y s t e m a t i c a l l y r e v e a l e d t h e e s s e n t i a l s t r u c t u r a l requirements
for high-selectivity
c a t a l y s t s and t h e underlying
uniformity principles. I t i s t h e r e f o r e considered as a p p r o p r i a t e t o p r e s e n t a t t h e o u t s e t
some d e s c r i p t i o n o f macro-
fundamental c o n c e p t s and d e f i n i t i o n s ,
and m i c r o - k i n e t i c
aspects,
of
from b o t h
t h e experimental foundation of
chemical r e a c t i o n engineering i n heterogeneous c a t a l y s i s . In the course of o f macro-
the systematic presentation of
and m i c r o - k i n e t i c
examples of
c o n t r o l s of
t h e p r e p a r a t i o n of
high-selectivity
i l l u s t r a t e d which a r e i n d i c a t i v e of
the principles
c a t a l y s t performances,
some
c a t a l y s t s may b e
100% p u r i t y of
catalytic function,
on copper o r s i l v e r b a s e s f o r vapour phase o x i d a t i o n s .
248 2.
FUNDAMENTALS OF REACTION-ENGINEERING ANALYSIS OF CATALYST PERFORMANCES In view of the developments of chemical engineering in recent years,
it is surprising that, until now in the description of rate processes the fundamental difference between homogeneous and heterogeneous catalysis has been overlooked or even deliberately neglected.
The
classical chemical kinetics (which deal essentially with mathematical treatment of ideally homogeneous reactions on macroscopic or thermodynamic basis and may therefore, be renamed, after A. Skrabal (ref. 11, as homogenkinetics) have always formed the basis for describing the rate process of heterogeneous catalysis, leading to various expressions for the homogenkinetic approximation to heterogeneous kinetics such as the Langmuir-Hinshelwood
(LH) or Hougen-Watson
+
(HW) treatment,
in combination with the semi-empirical pseudo-equilibrium assumption about the gas-solid interactions, namely, the Langmuir adsorption isotherm.
The HW treatment could cause further speculation about
the detailed surface mechanism by applying the transition-state (TS) theory which is an empirical, statistical-thermodynamic, theory
and is applicable only to homogenkinetics (ref. 2
&
3).
Consequently,
these versions of homogenkinetic approximation could have afforded at best correlation data of limited applicability which could not generally be rationalized, especially from the microscopic point of view.
Since the kinetics of heterogeneous catalysis must include
the essential part of the catalyst surface, certain molecular, microscopic description of the active, i.e., reacting surface is required in order to characterize the heterogeneous reaction system or the surface phase kinetically.
But, owing to the lack of physical know-
ledge of the active surface or the working state of the catalyst, we could only define the catalyst surface in a chemical, phenomenological way.
According to such chemical definition, the concept of the active
surface may be substantiated in quantity and quality by assuming that a macroscopic description of the catalyst surface may represent a uniform reaction phase which should be microscopically analogous to that o f homogeneous system, so that a kinetic expression such as the bulk concentration of active sites or centers within the extent, i.e.,
the
volume, of the catalyst system may be assumed as rational (ref. 4 ) on condition that the uniformity of the catalyst surface and the preclu-
+ In
contrast to homogenkinetics, we may here introduce the term heterogenkinetics as a theoretical representation for heterogeneous kinetics, which characterizes in general terms the microscopic restrictions due to heterogeneity in chemical kinetics.
249
sion of diffusional (mass and heat transfer) problems, both external and internal to the catalyst system, are ideally assured.
Obviously
in this chemical approach to the homogenkinetic approximation, there can be no reason for assuming a two-dimensional, macroscopic, unit extent of the reaction system, and the concept of the active surface or sites merges with that of the adsorption surface si'tes, as is implicitly indicated by the LH or HW treatment, where the microscopic characterization of the heterogeneous surface or site is deliberately neglected and the physical descriution o f the extent and nature Of the surface is replaced with a chemical, empirical one by use of such an intensive parameter as the limiting (saturation) capacity for adsorption or the specified rate for surface reaction in concentration units.+ Thus, the neglect of heterogenkinetic aspects of the surface process has obscured the kinetic distinction between homogeneous and heterogeneous catalysis in that the surface active site should represent an activated state associated with a certain unit surface ensemble in which microscopically, many complex elementary-stochastic processes are involved.
As
a consequence, the heterogenkinetic fea-
tures of the elementary surface processes, including adsorption and desorption as necessary steps in the reaction, should be confirmed phenomenologically in the chain nature of each and all processes involved, as well as in the mechanistic complexity of the definition of the active site in comparison with the molecular simplicity of that inherent to homogeneous catalysis.
There is already much experimental
+The conventional representation of the specific surface rate or intrinsic activity of the catalyst on the basis of a two-dimensional unit extent of the catalyst surface (e.g., g-mole cm-* sec-l) should be coQ8idered as physically ill-defined, since the extent of the adsorption surface or site bears, microscopically, no direct relation to the intensive parameter of the active surface or sites, although the BET surface area may be regarded as a certain measure of the adsorption capacity of unit mass of the catalyst. -1
the term turnover number or frequency (e.g., molecules site sec ')may be regarded as of no exact physical meaning, because the active site, as a kinetic term, should represent the chemical reaction center, macroscopically, in terms of intensive parameter (i.e., concentration in the catalyst system) or, microscopically, in reference to a certain activated state of the catalyst surface. Its quantity and quality could only be determined experimentally through rate measurements, so that the evaluation of turnover number would depend solely upon the mechanistic definition of the active site itself. If the active sites are assumed to be equivalent to or, at least, proportional in quantity to the adsorption sites, the mechanism of the surface reaction should correspondingly become too simple and one-sided to be realistic for heterogeneous surface catalysis, except for the extremes such as quasi-homogeneous or "immobilized" homogeneous catalysis. Al_so,
evidence f o r these heterogenkinetic c h a r a c t e r i s t i c s of catalysis
:
the
heterogeneous
"sigmoid k i n e t i c s " which i s i n d i c a t i v e o f t h e
p r e v a l e n c e of a c e r t a i n s e t of e l e m e n t a r y c h a i n p r o c e s s e s h a s been o b s e r v e d f o r a number o f
surface processes
such a s adsorption
(including not only i r r e v e r s i b l e chemisorption but a l s o reversible physi-
and c h e m i s o r p t i o n ) ; d e s o r p t i o n
( t h e f i r s t o b s e r v a t i o n of
a u t o c a t a l y s i s i n thermal decomposition of G.N.
Lewis
( r e f . 5)
may b e r e g a r d e d a s t h e p r o t o t y p e ) , s u r f a c e
o x i d a t i o n and r e d u c t i o n on s u p p o r t s
s o l i d s i l v e r o x i d e by
( o f m e t a l s and o x i d e s u n i f o r m l y d i s p e r s e d
( r e f . 6 ) , and numerous s o l i d - c a t a l y s e d r e a c t i o n s
(especially i n r e l a t i o n t o the t r a n s i e n t behaviours of retardation or inhibition (ref. 7).
Also,
t h e commonly o b s e r v e d l o w v a l u e o f
s a t u r a t i o n p r e s s u r e f o r c h e m i s o r p t i o n s of Langmuir-type h e a t of
adsorption
a t solid catalyst surfaces
( r e f . 9 ) may b e c o n s i d e r e d a s i n d i c a t i v e
of t h e e x t e n t of mechanistic complexity of active sites,
w i t h low
( r e f . 8 ) and t h e chemical r e l a x a t i o n measurements
t h e a d s o r p t i o n and t h e
respectively.
The i n c o n s i s t e n c i e s d u e t o t h e n e g l e c t o f h e t e r o g e n k i n e t i c s seem
t o have been r e f l e c t e d i n t h e r e a c t i o n e n g i n e e r i n g of heterogeneous c a t a l y s i s , so t h a t , macrokinetics,
i n c o n t r a s t t o t h e w e l l d e f i n e d and e l a b o r a t e d
t h e m i c r o k i n e t i c s h a s been almost d i s r e g a r d e d w i t h o u t
proper definition'
s i n c e van K r e v e l e n
(1957; r e f .
10) m i s d e f i n e d it
a s i d e n t i c a l t o " c l a s s i c a l molecular" chemical k i n e t i c s . been r e a l i z e d
I t has
( r e f . ll),however, t h a t f o r reaction engineering controls
o f c a t a l y s t p e r f o r m a n c e s i t i s m o s t i m p o r t a n t t o make a c l e a r d i s t i n c t i o n b e t w e e n macro-
and m i c r o - k i n e t i c
factors i n designing
t h e e x p e r i m e n t a l r e a c t o r s and o p e r a t i n g c o n d i t i o n s , as w e l l a s i n e v a l u a t i n g t h e observed d a t a ,
+M i c r o k - i n e t i c s
and t h a t ,
as a result,
the essential
may b e d e f i n e d a s a b r a n c h o f a p p l i e d o r e n g i n e e r i n g k i n e t i c s which, complementary t o m a c r o k i n e t i c s , i s c h a r a c t e r i z e d a s encompassing t h e regime of molecular-microscopic d e s c r i p t i o n of r a t e phenomena r e l e v a n t t o c h e m i c a l e n g i n e e r i n g . For t h e r e a c t i o n engineering of heterogeneous c a t a l y s i s , i n p a r t i c u l a r , m i c r o k i n e t i c s i s t o d e s c r i b e m o l e c u l a r t r a n s p o r t p h e n o m e n a , a s w e l l as m i c r o s c o p i c mass and e n e r g y t r a n s f e r w i t h o r w i t h o u t s u r f a c e c h e m i c a l t r a n s formations, i n t h e realm of c a t a l y s t s u r f a c e s t r u c t u r e i n o r d e r t o accomplish t h e r e a c t i o n engineering a n a l y s i s of c a t a l y s t performance o r c a t a l y t i c f u n c t i o n i n g of t h e c a t a l y s t system ( r e f . 1 1 ) . In s h o r t , m i c r o k i n e t i c s , d e a l i n g with engineering a s p e c t s of heterogenk i n e t i c s , may p l a y on p h e n o m e n o l o g i c a l b a s i s a m o s t i m p o r t a n t r o l e i n t r e a t i n g t h e e n g i n e e r i n g problems of t h e c a t a l y s t performance c o n t r o l concerning t h e heterogeneous s u r f a c e p r o c e s s e s which a r e i n c l u s i v e o f mass and h e a t t r a n s f e r i n t h e s u r f a c e e n v i r o n m e n t o r i n t h e m i c r o p o r e s , under e x c l u s i o n o f t h e m a c r o k i n e t i c mass and h e a t t r a n s f e r limitcations.
p a r t of t h e c a t a l y s t performance c o n t r o s c o n s i s t s of t h e ana y s i s and c o n t r o l of m i c r o k i n e t i c r a t h e r t h a n m a c r o k i n e t i c f a c t o r s . t h i s way,
considering t h a t microkinetic
In
f a c t o r s may r e p r e s e n t t h e
crucial heterogenkinetic characteristics of the active surface
site or reaction center, and heterogeneous
t h e k i n e t i c d i s t i n c t i o n b e t w e e n homogeneous
c a t a l y s i s may f u r t h e r b e d e l i n e a t e d ,
and i t f o l l o w s
from r e a c t i o n e n g i n e e r i n g e x p e r i m e n t a t i o n t h a t fundamental c o n c e p t s and d e f i n i t i o n s c o n c e r n i n g t h e k i n e t i c d e s c r i p t i o n of h e t e r o g e n e o u s c a t a l y s i s should b e re-examined 2.1
and,
i f need, be revised.
t h e rate i s only v a l i d f o r heterogeneous
I n t e g r a l r e p r e s e n t a t i o n of catalysis Firstly,
from t h e f o r e g o i n g h e t e r o g e n k i n e t i c arguments,
it i s
e v i d e n t t h a t t h e d i f f e r e n t i a l r e p r e s e n t a t i o n of t h e r a t e such a s specific rate o r catalytic constant,
i.e.,
concentration of c a t a l y s t per u n i t time, cannot apply i n heterogeneous
cases,
m o l e s converted per u n i t
i n homogeneous c a t a l y s i s
although formally a similar
d i f f e r e n t i a l r a t e e x p r e s s i o n may b e a s s u m e d t h r o u g h h o m o g e n k i n e t i c approximation i n t h e macrokinetic regime of heterogeneous Furthermore,
catalysis.
i t i s commonplace i n e x p e r i m e n t a l p r a c t i c e f o r b a s i c
r a t e information t o c o n s i s t of
i n t e g r a l o r average r a t e s
b u t t h e d i f f e r e n t i a l treatment of i n heterogeneous
(ref.
12),
the i n t e g r a l r a t e data obtained
c a t a l y s i s i s a t variance with t h a t obtained i n
homogeneous s y s t e m s i n t h a t f o r t h e f o r m e r case t h e r e i s a n i r r e d u c i b l e d i s c o n t i n u i t y b e t w e e n t h e t i m e d i m e n s i o n s f o r macro-
a n d micro-
k i n e t i c regimes due t o t h e complexity o f h e t e r o g e n e o u s s u r f a c e s t r u c t u r e , whereas f o r t h e l a t t e r t h e m i c r o k i n e t i c time dimension
i s assumed t o b e i n c l u d e d i n t h e t i m e d i m e n s i o n o f m o l e c u l a r - k i n e t i c continuity.
P r a c t i c a l l y , d i s c o n t i n u i t y l i m i t s may c o r r e s p o n d t o t h e
l i m i t e x t e n t of t h e c a t a l y s t system which i s a l m o s t e q u a l t o t h e c a t a l y s t p a r t i c l e s i z e of
l e s s t h a n 1 mm d i a m e t e r f o r a f l u i d i z e d o r
s u s p e n s o i d c a t a l y t i c r e a c t o r of
complete-mixing
porous c a t a l y s t p e l l e t diameter of t u b u l a r r e a c t o r of
batch type o r the
l e s s t h a n 6-8mm'
steady flow type.
Therefore,
f o r a fixed-bed
i t may b e c o n c l u d e d
t h a t i t i s more c o r r e c t f o r h e t e r o g e n e o u s c a t a l y s i s n o t t o make u s e + N a t u r a l l y , t h e s e l i m i t i n g f i g u r e s may b e m o d i f i e d w i t h c e r t a i n f a c t o r s d e p e n d i n g on t h e p o r o u s s t r u c t u r e o f t h e c a t a l y s t p e l l e t s , a s w e l l a s on t h e r e a c t i o n , e s p e c i a l l y f l u i d f l o w , c o n d i t i o n s . As f o r l a r g e r p a r t i c l e o r p e l l e t d i a m e t e r s , t h e f i g u r e s s h o u l d be m u l t i p l i e d with a macrokinetic c o r r e c t i o n f a c t o r such a s T h i e l e modulus o r " e f f e c t i v e n e s s " f a c t o r .
252 of
t h e d i f f e r e n t i a l r e p r e s e n t a t i o n of t h a r a t e , on t h e analogy of
homogenkinetics,
such a s s p e c i f i c r a t e
and o t h e r d i f f e r e n t i a l terms,
(constant), i n i t i a l rate,
especially differential reactor,
b u t t o r e f e r t o i n t e g r a l terms s u c h a s t h e s p e c i f i c a c t i v i t y ( f o r s p e c i f i c r a t e ) a s e x p r e s s e d by t h e i n t e g r a l o r a v e r a g e r a t e s p e r u n i t e x t e n t of t h e c a t a l y s t system (space-time
expressions are not
only p r a c t i c a l l y but s c i e n t i f i c a l l y v a l i d a l s o f o r heterogeneous c a t a l y s i s : Space-Time-Yield,
STY, a n d S p a c e - T i m e - C o n v e r s i o n ,
i n moles p e r l i t e r c a t a l y s t space p e r hour)
STC,
t h e q u a n t i t y of which can
be a c c u r a t e l y e v a l u a t e d d i r e c t l y from measurements w i t h a p r a c t i c a l isothermal i n t e r n a l r e a c t o r of e i t h e r batch o r flow type.
2.2
Contact t i m e vs,
t r u e r e a c t i o n time problem i n heterogeneous
catalysis Now t h a t t h e i n t e g r a l r a t e c h a r a c t e r o f t h e m i c r o k i n e t i c r e g i m e of h e t e r o g e n e o u s c a t a l y s i s i s acknowledged as above, t o explain that the contact t i m e ,
d e f i n e d as t h e r e c i p r o c a l of space v e l o c i t y , r e s p e c t t o t e m p e r a t u r e and p r e s s u r e , t h e t r u e time,
it is possible
tc, i n heterogeneous c a t a l y s i s ,
SV, and c o r r e c t e d w i t h
may become l i n e a r m e a s u r e f o r
a s w e l l a s t h e r e s i d e n c e t i m e i n homogeneous c a t a l y s i s ,
with the condition t h a t the macrokinetic factors are w e l l controlled
so t h a t d i f f u s i o n t o and from t h e e x t e r n a l b o u n d a r y of t h e c a t a l y s t system does n o t i n f l u e n c e t h e r e a c t i o n rate o r t h e s p e c i f i c a c t i v i t y . This c o n d i t i o n can be f u l f i l l e d experimentally, efficient stirring
(>600-800 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
(<6mm) a s shown i n F i g .
1.
e s s e n t i a l point of the macrokinetic
c o n t r o l of c o n t a c t time i s t h e production of an e f f i c i e n t s t e a d y flow c o n d i t i o n w i t h t u r b u l e n c e a t t h e c a t a l y s t boundary and t h a t , therefore,
t h e r e i s a d e f i n i t e p o s s i b i l i t y of modifying t h e s p e c i f i c
a c t i v i t y by i n f l u e n c i n g m i c r o k i n e t i c f a c t o r s , macrokinetic control
(ref.
2 . 3 The s t a t i o n a r y - r e a c t i n g the r e a l working-state
through p a r t i c u l a r
13). s t a t e of t h e c a t a l y s t system r e p r e s e n t s of catalyst surface
Knowing t h a t t h e a b o v e - m e n t i o n e d
condition of macrokinetic control
of c o n t a c t time corresponds t o providing t h e homogenkinetic approximation i n t h e macrokinetic regime of heterogeneous
catalysis,
i t becomes c o n c e i v a b l e t h a t , u n d e r t h e s e m a c r o k i n e t i c c o n t r o l s w i t h m i c r o k i n e t i c f a c t o r s remaining p r a c t i c a l l y unchanged, reactions,
the catalytic
a s r e p r e s e n t e d c o l l e c t i v e l y by t h e s p e c i f i c a c t i v i t y ,
are
253 proceeding within the external boundary of the catalyst system, integrally, in quasi-stationary state, and microscopically, in a diffusional steady state which may be contrasted phenomenologically with the homogenkinetic steady state.
The basic conditions for
providing this category of homogenkinetic approximation in the microkinetic regime consist firstly of uniformity and stability, i.e., the reproducibility of the catalytic function of the active catalyst surface or sites and then of a proper microkinetic control over reaction engineering parameters such as temperature, pressure, reactant compositions including diluents and especially inhibitors, in order to obtain sets of reproducible data for specific activities.
Naturally,
these microkinetic controls relate precisely to the whole subject except that of macrokinetic controls of the reaction engineering analysis of catalyst performance, as well as, in another version, to that of catalyst evaluation, so that the stationary-reacting state of the catalyst system as a unit integral reactor should uniquely represent the real working state of the catalyst surface (ref. 14). Experimentally, this state of the real catalyst system or bed can best be realized in a fixed-bed flow system by the use of an isothermal one-dimensional integral reactor under proper macrokinetic control and be most comprehensively investigated with respect to the axial profiles of temperature and reaction figures as shown in Fig. 1. With this type of tubular reactor which is applicable not only to laboratory but also to industrial practice, the isothermicity, in macrokinetic measures, is ensured by applying efficient external cooling (by use of a molten salt bath with high speed stirring) to maintain the wall temperature of the reactor tube (of smaller diameter < 2 0 mm) almost equal to the constant bath temperature throughout the
catalyst-bed, and may be confirmed by the observed linear proportionality between the average temperature rise, AT, in the catalyst-bed and the integral conversion (of oxygen), A(02-STC), in the range of temperature rises up to as large as 20-50°C 2.4
:
A~~~~=c0nst.xb(0~-STC).
The Linear Reciprocal Space Time Relationship (LRSTR) presents basic correlation data for microkinetic analysis of catalyst performance and also for a posteriori design of catalytic reactors That the homogenkinetic approximations both in macro-and micro-
kinetic regimes may be prcctically established in a continuous flow reaction with the isothermal one-dimensional reactor, can be domonstrated conclusively by examining the linear correlation between the reciprocals of integral rates, STY or STC, and space velocity, S V , that is, LPSTR (ref. 1 1
&
15)
:
254
Constant Bath Temperatue TB = 176.5OC.
(Tg)
(TC)
160
k
40
(EOSTY) 1*5*.o
11.0
Operation Types t
PQ/PC~H~
A
: 611
B 111.3
C
1/3
D
116 (P*-Oit$)
Surface Kinetic : Oxidative 4-• Reductive Conditions
The real working state of the catalyst surface as represented stationary-reacting state of the cataiyst bed in an isothermal one-dimensional integral reactor : Axial profiles for catalyst bed temperature, T-, selectivity of ethylene oxide formation, EC-Selec.%, and reciprokal space-time-yield of ethylene oxide, (EO-STY) I , in vapour phase oxidation, under ordinary pressure, of ethylene-oxygen mixture with silver catalyst supported on preformed spherical alpha alumina supports of 2mm in diameter. Fig. 1.
- by-the
(STY);'
where tc=3600x(SV)-',
= (STY);'
contact'time
+ atc
(I),
in sec, and a is a coefficient
which, as the gradient for reciprocal integral rates, comprises a homogenkinetic factor for stoichiometric deceleration with reaction time and certain retardation ( a > 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
(<3.75
3
x
lo2
and
2
x 10
3
nm, and micropores
nm), wh ic h cannot be p e n e t r a t e d by mercury a t t h e maximum pressure a p p l i e d .
The m ic r o p o r e s a lm o st e x c l u s i v e l y c o n t r i b u t e t o t h e s u r f a c e area o f t h e c a r r i e r . From t a b l e
4
one can
see t h e p ore volume o f t h e f i n a l p r o d u c t t o be
0.33
ml/g
l ower t h a n t h e p o r e volume o f t h e c a r r i e r . T h i s volume corresponds w i t h t h e volume o f z i n c a c e t a t e .2H20 i n t h e c a t a l y s t . The volume of t h e macropores i s h a r d l y affected.
Supported by t h e decrease o f t h e s u r f a c e area observed,
i t may
be concluded t h a t t h e metal compound i s m a i n l y s i t u a t e d i n t h e micropores o f the carrier.
269
12.
T
pore volume comer
irnl/g cool1
cotalyst
08-
pore mdlus inm )
+
fig 3 Hg-PENETRATION CURVES OFACTIVE CARBON CARRIER AND ZINCACETATE CATALYST
COPPER ( 1 1 )
3.2.
CHLORIDE ON
8-ALUMINA
The r e q u i r e d me t a l l o a d i n g o f t h i s c a t a l y s t system i s a p p r o x i m a t e l y 12 gram CuCl
2
p e r 100 gram o f c a r r i e r . The pore volume o f t h e alumina c a r r i e r amounts t o
0 . 4 0 ml/g.
The s o l u b i l i t y o f CuCI2 i n w a t e r i s h i g h enough t o a l l o w a one-step
d r y impregnation.
By s o a k in g e x pe ri men t s i t has been found, t h a t the a d s o r p t i o n o f CuCI2 on %-alumina i s v e r y weak and t h e r e f o r e we m i g h t expect a m o b i l i t y o f t h e metal compound on d r y i n g . T h i s has been c o n f i r m e d e x p e r i m e n t a l l y . Fig . 4a. shows t h e homogeneous copper d i s t r i b u t i o n o v e r t h e c r o s s s e c t i o n o f a s l o w l y d r i e d p a r t i c l e as measured by e l e c t r o n m i c r o p r o b e ( c a t a l y s t I ) . On f a s t d r y i n g an uneven copper d i s t r i b u t i o n i s o b t a i n e d as shown i n f i g . 4b ( c a t a l y s t 11).
fig L COPPER DISTRIBUTION ON SLOW AND FAST DRYING
The 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 and d r i e d c a t a l y s t s a r e g i v e n i n t a b l e The mercury p e n e t r a t i o n cu rves a r e g i v e n i n f i g . 6 .
All d a t a a r e based on 1 g o f
carr ier.
TABLE 5.
C h a r a c t e r i s t i c s o f c a r r i e r and d r i e d c a t a l y s t s Catalyst I
Catalyst I I
-
slow
122 0.40
99
fast 105 0.38
Carrier Drying Surfa c e a r e a (m2/g) T o t a l p o r e volume ( m l / g ) CuC12 c o n t e n t (wt % )
-
0.37 11.3
11.3
5.
270 These d a t a show, t h a t t h e r e l a t i v e decrease o f t h e s u r f a c e area on impregnating &-alu m in a
w i t h copper ( I I ) c h l o r i d e i s more severe than t h e decrease o f t h e p o r e
volume. T h i s i n d i c a t e s , t h a t t he s m a l l e r pores a r e p r e f e r e n t i a l l y f i l l e d w i t h t h e m etal compound. The l owest s u r f a c e a rea i s found f o r t h e s l o w l y d r i e d p r o d u c t . The f a s t d r i e d p r o d u c t shows a s u r f a c e a rea between those o f t h e c a r r i e r and t h e s l o w l y d r i e d p r o d u c t . The mercury p e n e t r a t i o n curve o f t h i s p r o d u c t i s s i t u a t e d between t h e c u r v e s o f t he c a r r i e r and t h e s l o w l y d r i e d p r o d u c t too. The d a t a i l l u s t r a t e t h a t upon f a s t d r y i n g , copper ( \ I )
c h l o r i d e m i g r a t e s towards
t h e l a r g e r p o r e s a t t he o u t e r l a y e r o f t h e p a r t i c l e s , g i v i n g r i s e t o an uneven copper d i s t r ibu t ion o v e r t h e p a r t i c l e s
.
By exposure t o w a t e r vapour, 5% o f w a t e r has been adsorbed on c a t a l y s t I I . A f t e r s l o w l y d r y i n g , t h e copper d i s t r i b u t i o n has become homogeneous and t h e s u r f a c e a r e a was decreased t o
98 m2/g.
We may conclude t h a t a weakly adsorbed metal compound i s m o b i l e i n a c a r r i e r p a r t i c l e i f a s u i t a b l e t r a n s p o r t medium i s p r e s e n t . T h i s phenomenon makes a r e d i s t r i b u t i o n o f t h e metal compound o v e r t he c a r r i e r p a r t i c l e p o s s i b l e .
lig 5 Hg-PENETRATION CURVES OF ALUMINA CARRIER AND
3.3 PLAT I NUM ON An
x-ALUMINA -x
CUPRIC CHLORIDE CATALYSTS
example o f s t r o n g i n t e r a c t i o n between metal compound and c a r r i e r i s t h e c h l o -
roplatinic acid
-alumina system, whi ch has been i n v e s t i g a t e d by many i n s t i t u t e s .
I n t h i s paper t h e b eh avi o ur o f t h e P t compound i n s o l u t i o n and t h e c a r r i e r a r e di s c u s s e d s e p a r a t e l y . A model o f t h e c o m p e t i t i v e a d s o r p t i o n o f c h l o r o p l a t i n i c a c i d and o t h e r a c i d s i s gi ve n.
3.3.1.
C h l o r o p l a t i n i c a c i d (CPA)
The b e h a v io u r o f c h l o r o p l a t i n i c a c i d i n s o l u t i o n has been s t u d i e d by Davidson and Jameson
121 . H 2 P t C I 6 i s a s t r o n g a c i d whi ch upon d i s s o c i a t i o n y i e l d s two
p r o t o n s p e r molecule. From s p e c t r o s c o p i c measurements t he y concluded t h a t t h e remaining complex may undergo I i g a n d exchange w i t h w a t e r mol e cul e s: PtCI6’-
+
H20
(PtC15H20)-
+
C1-
271 T h i s mono-aquo-chloro-complex can undergo h y d r o l y s i s : (PtC15H20)-
zH (PtC150H)2-
+
H+
The d i s s o c i a t i o n c o n s t a n t f o r t h i s complex has been determined t o be pKa = 3.8. T h i s v a l u e i n d i c a t e s t h a t (PtCI H 0)- behaves as a weak a c i d . 5 2 The phenomenon o f a q u a t i o n and h y d r o l y s i s o f d i l u t e d CPA s o l u t i o n can be demons t r a t e d t o be t i m e dependent by p o t e n t i o m e t r i c t i t r a t i o n w i t h a l k a l i . The t i t r a t i o n c u r v e s o f f r e s h and aged 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 s we found a r e g i v e n i n fig. 6
119.6 TITUATION OF CHLORORATINIC ACID SOLUTION AFTER AGEING
The c u r v e s i n d i c a t e , t h a t t h e a q u a t i o n r e a c t i o n does n o t proceed instantaneous-
l y . A f t e r 2 h t h e e q u i l i b r i u m s i t u a t i o n i s al most reached. S in c e im p r e g n a ti on proceeds l a r g e l y a t pH values o f approx. 4 and because impregn a t i o n o f a b a t c h t ake s s e v e r a l h ou rs i n p r a c t i c e (2-4 h r s ) ,
i t may be assumed, 2-
t h a t a s u b s t a n t i a l p a r t o f t he p l a t i n u m i s p r e s e n t as ( P t C I OH)
5
3.3.2
.
Carrier
The 8 - a l u m i n a c a r r i e r used i n these i n v e s t i g a t i o n s undergoes some i n t e r a c t i o n
w i t h t h e a c i d s o l u t i o n . T h i s c o u l d be demonstrated d u r i n g a t y p i c a l soaking e x p e r i ment. The pH o f t h e so aki n g s o l u t i o n i n crea sed from 1.2 t o a c o n s t a n t l e v e l o f 4 w i t h i n 2 h.
A t t h e low i n i t i a l pH va l ue , an a c i d a t t a c k o f t h e alumina occurs: Al
-
0- H
+
H+ 4
Al+
+
H20
The c a t i o n i c a lu m in a i s b e l i e v e d t o y i e l d a s i t e f o r a d s o r p t i o n ( i o n i c i n t e r a c t i o n ) 2o f P t C l OH T h i s i s su pp ort ed by o u r o b s e r v a t i o n , t h a t on impregnation o f alumina
5
.
u s i n g a K2PtC16 s o l u t i o n w i t h o u t any f r e e a c i d a homogeneous p l a t i n u m d i s t r i b u t i o n o v e r t h e p a r t i c l e and a low P t i mpre gn at i on y i e l d a r e o b t a i n e d . An e q u i v a l e n t c o n c e n t r a t i o n o f H2PtC16 does y i e l d a
s u r f a c e l a y e r o f p l a t i n u m under i d e n t i c a l
c o n d i t i o n s . The r e c e n t l y p u b l i s h e d a c i d a t t a c k model o f Santacesaria s l i g h t l y d i f f e r e n t , b u t does g i v e t h e same r e s u l t s
133.
C.S.
is
272
3.3.3
Competitive adsorption o f CPA and H C I
When g - a l u m i n a i s impregnated w i t h a CPA s o l u t i o n , the protons from c h l o r o p l a t i n i c a c i d a t t a c k the alumina surface on p e n e t r a t i o n o f the extrudates by the l i q u i d . The s i t e s created by t h i s a t t a c k are s i t u a t e d i n the o u t e r l a y e r o f the 2extrudates and serve as strong adsorption s i t e s f o r P t C l OH
5
.
A homogeneous P t d i s t r i b u t i o n can be obtained when a d d i t i o n a l a c i d i s present i n
t h e impregnation 1 i q u i d 141. This a c i d creates a d d i t i o n a l adsorption s i t e s o n t o 2t h e alumina b o t h for P t C l OH and the a c i d anions throughout the p a r t i c l e . Due
5
t o the adsorption competition between both anions a homogeneous P t d i s t r i b u t i o n can be obtained. Impregnation experiments o f H 2 P t C l
6
w i t h v a r y i n g amounts o f h y d r o c h l o r i c a c i d
have been c a r r i e d out. The p e n e t r a t i o n depth and impregnation y i e l d a r e given i n f i g . 7.
mmolmalumina
0 04 08 12 fig 7 PLATINUM PENETRATION AND IMPREGNATION YIELD ON ADDITION OF HCI
0
The P t p e n e t r a t i o n increases on increasing the H C I concentration i n the impregnat i o n s o l u t i o n . A second e f f e c t o f the c o m p e t i t i v e adsorption i s the decrease o f the platinum impregnation y i e l d on increase o f the HCI concentration. The c o m p e t i t i v e adsorption o f P t C l OH2- and C 1 - can a l s o be demonstrated by recor-
5
d i n g t h e separate adsorption isotherms o f H C I and CPA on )S-alumina ( f i g . 8 ) . The a d s o r p t i o n isotherms have been recorded under the c o n d i t i o n s given i n t a b l e 3 . ~mceq/g~
.x/'-oxa''c hydrochloric
I.
0
--chIoroplatlnc
03
02
:", 0 fig 8
005
,
]
010
SORPTION OFACIOS ON 1-ALUMINA
_
conc s o h Imol/lJ
015
f 25'Ct
213 The adsorption curves f o r H C l and CPA i n d i c a t e t h a t t h e adsorption s t r e n g t h s o f C I - and P t C l OH2- are s i m i l a r . Therefore H C I i s a good impregnation a i d t o o b t a i n
5
a homogeneous P t d i s t r i b u t i o n . The incomplete p l a t i n u m p e n e t r a t i o n obtained w i t h o u t impregnation a i d i s caused n o t o n l y by the s t r o n g i n t e r a c t i o n b u t a l s o by the low amount o f platinum a p p l i e d i n our experiments. On a d d i t i o n o f CPA t o an impregnation s o l u t i o n i n s u f f i c i e n t q u a n t i t y t o reach t h e equi 1 i b r i u m loading (0.38 maeq/g), a homogeneous P t - d i s t r i b u t i o n w i l l be obtained w i t h o u t impregnation a i d .
3.3.4.
Competitive a d s o r p t i o n o f CPA and o x a l i c a c i d
Adsorption phenomena are a l s o e x p l o i t e d i n the technique f o r o b t a i n i n g a subsurface metal l a y e r , as described i n the patent l i t e r a t u r e [5,61 and r e c e n t l y by Nuttall
171
: A d d i t i o n o f a small q u a n t i t y o f an a c i d t o the impregnation s o l u t i o n
which shows a very s t r o n g adsorption on alumina as compared t o CPA, y i e l d s an o u t e r p l a t i n u m - d e f i c i e n t s h e l l i n the c a r r i e r p a r t i c l e . The thickness of t h i s l a y e r can be c o n t r o l l e d by the added amount o f such an a c i d . All,
a t l e a s t d i b a s i c , organic acids a r e claimed t o y i e l d t h i s e f f e c t ( l i k e
oxal i c , t a r t a r i c and c i t r i c a c i d ) . Aromatic acids c o n t a i n i n g a hydroxylgroup adj a c e n t t o the c a r b o x y l i c group, l i k e s a l i c y l i c a c i d are claimed as w e l l . The d i f f e r e n t behaviour o f e.g. CPA,
i s drawn schematically i n f i g .
h y d r o c h l o r i c and o x a l i c a c i d on impregnating
9.
increasing qty a1 acid
fig 9 PLATINUMOISTRIBUTION IN ALUMINA EXTRUDATES A 1 FOUR LEVELS
OF
HYDROCHLORIC AND OXALIC ACID CONCENTRATION
On a d d i t i o n o f small q u a n t i t i e s o f o x a l i c a c i d t o the CPA s o l u t i o n , the o u t e r l a y e r o f the extrudates i s attacked by protons and o x a l a t e anions a r e s t r o n g l y adsorbed on these s i t e s . The platinum compound i s deposited i n a subsurface l a y e r , j u s t under the l a y e r o f more s t r o n g l y adsorbed o x a l a t e ions. The r e l a t i v e thickness o f the s h e l l has been determined a t v a r y i n g a d d i t i o n s o f o x a l i c a c i d and c i t r i c a c i d ( f i g . 1 0 ) . These r e s u l t s have been compared w i t h the data reported by N u t t a l l f o r c i t r i c acid. The l a t t e r r e s u l t s do not agree w i t h our observations.
274
1
re1 she1 1
801 thickness
oio
yo oio 050 oio fig10 RELATIVE THICKNESS OF PLATlhUM-DEFICIENT LAYER ON ADDITION OF ORGANIC ACIDS The a d s o r p t i o n i s o t h e r m o f o x a l i c a c i d on alumina has been determined ( f i g . 8 ) . I n t h e c o n c e n t r a t i o n range a p p l i e d i n p r a c t i c e ( 0.01 m o l / l ) ,
t h e curve indeed
shows a v e r y s t r o n g a d s o r p t i o n as compared t o t he a d s o r p t i o n o f CPA o r H C I . The phenomena observed have t o be a t t r i b u t e d t o chromatographic e f f e c t s based on t h e d i f f e r e n t a d s o r p t i o n s t r e n g t h s and t h e p e n e t r a t i o n o f the impregnation solut i o n from o u t s i d e towards t h e p a r t i c l e c e n t r e . T h i s c o u l d be demonstrated by separ a t i o n o f t h e compounds i n v o l v e d ,
u s i n g l o n g e x t r u d a t e s (10 cm), p l a c e d i n a v e r -
t i c a l p o s i t i o n . A f t e r a p p l i c a t i o n o f a m i x t u r e o f CPA and o x a l i c a c i d s o l u t i o n , c h l o r o p l a t i n i c a c i d moves under t h e i n f l u e n c e o f a 10% H C I s o l u t i o n as m o b i l e phase; o x a l i c a c i d h a r d l y migrates.
A second e f f e c t , u n d e r l i n i n g t h e chromatographic c h a r a c t e r o f t h i s behaviour i s i t s t i m e dependency; we found a broadening o f t h e p l a t i n u m - r i c h zone w i t h time. The more s t r o n g l y adsorbed o x a l i c a c i d does n o t m i g r a t e n o t i c e a b l y d u r i n g a p e r i o d o f 2-20 min.
ACKNOWLEDGEMENT The a u t h o r s w i s h t o thank Hr. A.L.L.
Sidkamp f o r h i s v a l u a b l e c o n t r i b u t i o n
i n t h e e x p e r im e n t s and d i s c u s s i o n s .
1. G. B e r r e b i and Ph. Bernusset, P r e p a r a t i o n o f c a t a l y s t s , ed. Delmon, Jacobs and Po nce l et , E l s e v i e r , Amsterdam (1976) pag. 24 2. C.M.
Davidson and R.F.
Jameson. Trans Faraday SOC.
3. E. S a n ta c e s a r ia , S . Carra and I . Adam: Dev. no. 1 (1977) 41.
5
4. R.W.
Maatman,
5. U.S.
P a t e n t 3.388.077
Ind.Eng.
Chem.
2
(1965) 2462.
Ind. Eng. Chem. Prod. Res.
(1959) 913.
(1968).
6. Neth. Pat. Appl. 291777 (19 63 ). 7. T.A. N u t t a l l C S l R r e p o r t CENG 182. P r e t o r i a 1977.
275
DISCUSSION V.B. FENELONOV
:
Is it possible that in your experiments there exist
big temperature gradients, which could be an explanation for the experimental data from the fast drying ?
In that case liquid can
move in a cold zone of the pellet, particularly in the external surface domains. G.H. van den BERG : I agree with you that heat transport into the particles is a very important factor in intra-particle metal compound transport.
However, measurements of temperature gradients in these
particles i s impossible.
Calculations are difficult to make because
reliable data are lacking. A. O Z A K I
:
What is your definition of fast or slow drying in view
of the apparent contradiction with the paper of Kotter and Riekert ? E.H. van den BERG : I cannot give a quantitative criterion for the difference between fast and slow drying.
In our experiments, the
fast dried product has been submitted to a forced flow of hot air (large surplus amount). J.P.BRUNELLE
:
on y-alumina ?
What kind of mechanism occurs in the fixation of CuCl
2
Do you think it is an "hydrolytic adsorption", in
other words a mechanism which consists of an anion adsorption and a precipitatio.1 of the cation ? G.H. van den BERG
:
Apart from its weak nature, we do not know the
character of the interaction between copperlI1) chloride and yalumina. R.
GALIASSO : I would like to point out that we have observed that
when Y-Al2O3 impregnated with ammonium paramolybdate solution, or nickel nitrate solution is slowly dried, a homogeneous M o [or N i ) distribution is obtained in the alumina pellets. Fast drying on the other hand, leads to inhomogeneous distribution which is in agreement with your results in the case of the CuC12/yA1203 system.
It is
surprising that you do not have a higher degree of surface reduction in the fastly dried samples compared to slowly dried ones.
Would
this mean that the macropore-micropore migration during drying is the same under both sets of conditions ?
276 G.H. van den BERG
:
1) Your comment indicates that particle size is
not the source of the differences between Maatmans' and our results, obtained on slow and fast drying respectively. 2) Upon fast drying the copper compound in the porous system tends to migrate towards the larger pores.
The minor effect on the
surface area and pore volume is caused by the rather high metal compound loading. C.J. WRIGHT and M.V. TWIGG
:
We are perturbed that the result presented
in Figure 1 for the copper distribution produced by fast drying conditions might be an artefact of the experimental technique.
The
use of the electron probe on a 4011 particle will be influenced by the method of particle sectioning (or sputtering) and by the corrections used to compensate for the escape depth of the X-rays. A further complication will surely arise from the irregular shape of your particles. G.H. van den BERG
:
The special sample preparation technique we
applied diminishes the risk of migration of the copper species to a large extent. The penetration depth o f the electron beam is 2.5p
at the 4 0 kV
applied; therefore we do not need large corrections. It is clear that the results obtained cannot be interpreted quantitatively, due to the effects you mentioned and other ones. Nevertheless, the effect of different drying techniques on the copper compound distribution mentioned in the paper has been found to be qualitatively reproducible. P.G. MENON : There is a good agreement between the results on PtyA1203 in this paper and those in the paper
by Sivasanker et al.,
particularly in the competitive adsorption of HC1 and H2PtC16 from a solution mixture (e.g., compare Fig. 7 ) and 8 of this paper with Fig. 4 of the paper of Sivasanker). E.A. IRVINE : 1) Have you any comparative data for different types (forms) of aluminas, i.e. is your gamma alumina well defined as such and, i f not, is there the possibility that different forms of alumina will give a different picture in terms of the copper distribution ?
G.H.
van d e n BERG : W e c a n n o t
p r o v i d e c o m p a r a t i v e d a t a for o t h e r
t y p e s of a l u m i n a t h a n t h e g r a d e E a p p l i e d ( T a b l e 2). Due t o t h e d i f f e r e n t i n t e r a c t i o n c h a r a c t e r i s t i c s o t h e r a l u m i n a s will s h o w a d i f f e r e n t p i c t u r e u n d e r i d e n t i c a l p r e p a r a t i o n c o n d i t i o n s .
L. R I E K E R T : a) T h e d i s t r i b u t i o n s o f t h e a c t i v e c o m p o n e n t w h i c h y o u found in t h e c a s e o f w e a k i n t e r a c t i o n w i t h t h e c a r r i e r for s l o w a s c o m p a r e d t o f a s t d r y i n g are a t v a r i a n c e w i t h t h e p r e d i c t i o n s just m a d e by F e n e l o n o v and w i t h w h a t o n e w o u l d expect from simple q u a l i t a t i v e considerations.
I s t h e r e a n explanation ?
b ) H o w have t h e s e d i s t r i b u t i o n s b e e n o b t a i n e d e x p e r i m e n t a l l y ?
G.H.
v a n d e n BERG : a ) I n m y o p i n i o n t h e effect found d o e s fit in
Fenelonov's m o d e l , if t h e p r o p e r v a l u e s of t h e v a r i o u s p a r a m e t e r s a r e p u t i n t o t h e formulae.
Because of lack o f sufficiently reliable
d a t a , w e d i d not succeed in d e m o n s t r a t i n g t h i s so far. b ) T h e s e d i s t r i b u t i o n s h a v e b e e n o b t a i n e d v i a a n electron m i c r o p r o b e technique.
This page intentionally left blank
279
STUDY OF THE INTERACTION OF Fe203-Mo03 WITH SEVERAL SUPPORTS
L. CAIRATI I, M. CARBUCICCHIO 2, 0. RUGGERI Euteco Istituto Istituto Istituto
and F. TRIFIRO 4
Milano di Fisica UniversitB Parma di Metallurgia Bologna di Tecnologie Chimiche Speciali UniversitB Bologna
ABSTRACT Fe203-Mo03 have been supported on Si02 and A1203 of different surface areas in order to prepare catalysts with high mechanical strength, for the selective oxidation of methanol to formaldehyde. MEssbauer, Raman, X-ray dispersion, electron microscopy and catalytic measurements have been carried out in order to characterize the catalysts. A decreasing activity and selectivity in CH20 was found both for Fe203-Mo03 and Moo3 catalysts with the increase of surface area of silica. By increasing the surface area of the supports a decrease of the amount of Fe2(Mo04)3 was observed with formation of a new compound of Fe(II1) different in the case of Si02 and A1203. Formation of surface compounds of the type FeMoSi and FeMoAl has been hypothesized.
1
INTRODUCTION
The oxidation of methanol to formaldehyde is normally carried out in a fixed bed reactor with a catalyst based on Fe203-Mo03 (1-2). In order to obtain the oxidation of methanol in a fluidized bed reactor, supported Fe203-Mo03 on Si02 dnd A1203 has been prepared so as to increase the mechanical strength of the catalyst. In fact unsupported Fe203-Moo3 presents very low mechanical strength (3) and therefore it cannot be used in fluidized bed reactors. With a view to optimizing the preparation and to understanding the difference in activity and selectivity observed with supports of different nature and surface area, Raman, Mbssbauer, X-ray and electron microscopy measurements have been carried out.
280
2 EXPERIMENTAL SECTION 2.1. Preparation of catalysts Silica (Grace) and A1203 (Saint Gobain 125 m 2/g) with different surface areas have been prepared by calcination at high temperature (llOO°C) for different durations. The grain size of Si02 and A1203 used was 200-270 mesh. The active components Fe203 and Moo3 were supported by impregnation from soluble salts of Fe(II1) and Mo(V1) in the presence of citric acid ( 4 ) . A solution of (NH4)6 Mo7 OZ4 4H20, citric acid, Fe(NO3I3 9H20 was sprayed on the support-grains kept in vacuum under rotation in a rotavapor; after drying at llO°C the catalyst was calcined in a fluidized bed reactor in a flow of air at 36OoC for 10 hours; afterwards the catalyst was calcined at 4 3 O o C for 16 hours in an oven. The amounts of Fe and Mo as Fe203 and Moo3 were in all the catalysts, 2 . 3 % by weight, with a molar ratio Mo/Fe=Z. Molybdenum oxide catalysts were prepared by impregnation of the support with the solution of (NH4) Mo7 OZ4 4H20 (as before). The analyzed samples are labeled in the following way by indicating the kind of support and the surface area between brackets; for example, Si02 (100) indicates an Si02 supported catalyst with surface area of 100 m 2 /g. 2.2. Measurements of catalytic activity The reactor was made of Pyrex with 0 2 8 nun and 1000 mm long. The reagent was distributed from a porous disc with 10-20 pm perforations. The length of the bed was 450 mm with a flow-rate of 125 Nl/h. The concentration of methanol was 5.5% in air. The analysis of products was carried out according to ref. 3. 2.3. Mksbauer, Raman, X-ray and SEM measurements The MSssbauer absorption spectra were measured by means of a standard spectrometer consisting of a constant acceleration electromechanical driving system and a multichannel analyser working in multiscaler time mode. Both the source (25 mCi CoS7 in Rh matrix) and the absorbers were kept at room temperature. The powder of catalysts on which A1 film was evaporated were observed with a Jeol 15 kV electronic microscope. The chemical nature of the surface was analyzed by X-ray energy dispersion electron microprobe (Montedel). The Raman spectra were recorded by a Cary 8 3 spectrophotometer.
281
3 RESULTS AND DISCUSSION 3.1 Measurements of activity Figs.1 and 2 show the conversion of methanol, the selectivity to formaldehyde and the yield of CO, C02, HC02CH3 vs the silica surface for T=260' and 300'C respectively. For Si02 (300) the values of selectivity and activity are only slightly less than those obtained for Si02 (100). The table shows the same values obtained at T=300°C for Mo03;Si02 catalysts at two levels of surface area of Si02. From Figs. 1 and 2 and the table it is possible to deduce that with increasing surface area of the support a decrease in activity and selectivity occurs. The decrease in activity was not observed with Mo03-Si02 catalysts. The values of selectivity in CH20 are the same for both groups of catalysts and are independent of temperature.
0
50
n;L/a
50
0
100
100
mz/s
Fig. 1. Activity pattern of Fe203-Mo03 on Si02 at 260'C.
Fig. 2. Activity pattern of Fe203-Mo03 on S102 at 300'C.
Activity pattern of Moo3 on Si02 at 300'C Catalyst
X%
S%
CO 7.5% Moo3 on Si02 (2.3)
40.0
7 . 5 % Moo3 on Si02 ( 1 0 0 )
52.0
90.0 73.0
Yield C02
%
HC02CH3
2.3
0.0
1.0
7.2
1.4
5.4
282
3.2. Raman spectra Raman spectroscopy allows characterization of the supported catalysts as the Si02 and the A1203 are Raman inactive. Therefore it was possible to observe the characteristic bands of molybdenum oxygen bonds of pure and mixed oxides (5)(6). Fig. 3 records the spectra of Fe203-Mo03 on SiOz ( 2 1 3 ) A1203 ~ ( 0 . 5 ) and Moo3 on Si02 (2.3). In the case of samples with higher surface area no more bands were detected. Fig. 3D is the spectrum of Moo3 (10%) on Si02. Notwithstanding the high amount of Moo3 present in the catalysts its most intense band was only detectable with the highest gain of the intensity of the instrument. The noise of the instrument covers the other bands. In Fig. 3C and Fig. 3B it is possible to recognize the bands at 800 and 960 cm-‘ respectively,as those of the Mo-0-Mo and Mo=O groups of Fe2(Mo04)3 ( 6 ) . In Figs. 3A and 3C the bands at 8 2 0 and at 1000 cm-’ are respectively those of the Mo-0-Mo and Mo=O groups of Moo3. In Fig. 3B the bands of Moo3 are more difficult to recognize.
-
Fig. 3 . Raman spectra A Moo3 on Si02 ( 2 . 3 ) B Fe203-Mo03 on A1203 ( 0 . 5 ) C Fe203-Mo03 on Si02 (2.3) D Moo3 on Si02 (100)
t
to00
800
RAMAN SHlFT
600
ACM-’
283
3.3
SEM and X-ray analysis measurements SEM does not show any significant difference in the morphology of
the analysed silica-supported Fe203-Mo03 catalysts, either before or after their use. Fig. 4 illustrates a characteristic image of these catalysts obtained by means of SEM. The qualitative analysis carried out by X-ray electronic microprobe on Si02-supported Fe203-Moo3 catalysts with high surface area allows the supposition that the active component is distributed rather uniformly on the support and does not accumulate on preferential zones. Fig. 5a shows a typical spectrum of this catalyst; it is possible to detect the presence of small quantities of Fe and Mo. For samples with areas lower than 2.3 m 2/gr where considerable numbers of grains are present, a segregation of hexagonal plates having constant size (8-10 pm side - see Fig. 5b); the said plates are made of a coherent material while the surrounding catalysts have an irregular and incoherent form. The most important datum resulting from these observations has been obtained by X-ray microanalyses; as a result of these we can now assert that the plates are made exclusively of Mo; therefore they can be attributed to Moo3 (Fig. 5c). Moreover an X-ray map of a plate-containing grain has been produced by preselecting on the microanalyser the energy band of Xray emission relative to Mo. Fig. 5d shows the characteristic Mo spots, which are particularly concentrated in the plate zones,while appearing rarefied i n the surrounding areas.
Fig. 4.
Scanning electron micrograph of silica supported Fe203-Mo03 catalyst X. 1300.
284
Fig. 5a Fig. 5b Fig. 5c Fig. 5d 3.4
X-ray chemical analysis at energy dispersion of catalyst Si02 without plate. Scanning electron micrography of an hexagonal plate on catalyst Si02 (2.3)X.300. X-ray chemical analysis at energy dispersion of catalyst containing plate. X-ray Mo distribution map.
Mgssbauer measurements
The MSssbauer spectra, carried out on samples with surface areas lower than 2.3 m 2/g and on both the Si02 and A1203 supports,display a well-defined single-peak with the same isomer shift and line-width as the one measured for the pure Fe(II1)-molybdate (7). Figs. 6a and 7a show the spectra for the samples Si02(1) and A1203(0.5) respectively.These spectra, in agreement with the Raman, are evidence that the supported Fe(II1)-molybdate can form for low areas independently of the support. AS the MSssbauer measurements can provide short range information, this technique can be utilized for continuously following the modificatiorsof the iron surroundings, independently of the crystalline 2 state of the compound. For 2 m /g of sample surface area the MSssbauer spectra display a broadened peak; by increasing the area the broadening increases further. It is worth noticing that for both the silica 3+ and alumina supports,the isomer shift remains that typical for Fe
285
F i g . 6 Room temperature Mtlssbauer spectra for s i l i c a supported c a t a l y s t s : a ) SiOz(l), b ) SiO (100) and C ) Si02(%O) 2
1
d 1
-2
1
1
1
0 I 2 velocity (mm/sec) -I
516
0
1725
1715
'
F i g . 7 Room temperature HBssbauer spectra f o r alumina supported
2878
c a t a l y s t s : a ) A1203(0,5), b) A1203(5,4) and
t
28721
U
c ) A1203(125)
1 -2 -I 0 I 2 velocity (mm /set)
286
ions: the line-width increase is linear up to 40 m2 in the case of Si02 and up to 10 m2 for A1203 with a greater slope for alumina. Figs. 6b and 7b record the spectra for the Si02(100) and A1203(5.4) samples, respectively. As these spectra are peaked corresponding to the line position of the Fe,(MoOd),, they can be interpreted assuming the superposition of at least two spectra; (i) a single-line due to Fe3+ ions of Fe(II1) -molybdate, (ii) an unresolved spectrum due to Fe3+ ions located in distorted sublattices. This second contribution could be due to iron located at the Fe2(MoOq)3 surface in lattice sites which would be highly distorted. However, this assumption implies that the support plays only the role of providing high surface areas and cannot explain the different behaviour for alumina and silica. Therefore, a chemical reaction between the supports and the precipitated complex salt must be assumed. The observed behaviour of the line-width as a function of the sample surface, suggests that the Fe(II1)-molybdate contribution reduces with increasing area while the other contribution to the spectra increases. If this assumption is correct one can reasonably expect to resolve, for very high surface areas, this second contribution, i.e. the one due to iron with distorted surroundings. It must be indicated that,due to the broadening of the lines, the spectra of high area samples need very long counting times in order to reduce sufficiently the statistical error. Figs. 6c and 7c record the spectra for SiO2(3O0) and A 1 2 0 3 ( 1 2 5 ) respectively. They can be interpreted as a superposition of two quadrupole doublets whose positions are indicated in the figures: the position of the single line of the Fe(II1)-molybdate which is no longer present i s indicated with an arrow. The spectra of SiO2(3O0) and A 1 2 0 3 ( 1 2 5 ) are clearly different, in agreement with the hypothesized reaction between the precipitate salt and the supports. They could be reasonably interpreted as due to the formation of some of the following compounds : Fe3+ substituted alumina or silica, i) ii) a-Fe203 small particles, iii) Fe-Mo-Si or Fe-Mo-A'1 compounds. In the case of the A1203(125) sample, the comparison between the Mljssbauer parameters obtained and the data available from the literature suggests (8) that (i) no Fe3+ substituted alumina is formed, (ii) growth of a-Fe203 small particles cannot be excluded and (iii) Fe-Mo-A1 compound at the support is likely to form. A similar analysis of the S i 0 2 ( 3 0 0 ) sample indicates (9) that (i) it is possible to exclude the L.'
281
f o r m a t i o n of i r o n s i l i c a compounds, w h i l e ( i i ) a-Fe203 Small P a r t i c l e s and (iii) Fe-Mo-Si compounds a r e b o t h l i k e l y t o form. I t i s worth n o t i n g t h a t t h e s m a l l a-Fe203 p a r t i c l e s growing s h o u l d l i b e r a t e molybdenum o x i d e . A s SEM photography r e v e a l s t h e p r e s e n c e of molybdenum c r y s t a l l i t e s f o r low b u t n o t f o r h i g h sample s u r f a c e a r e a , it i s r e a s o n a b l e t o assume t h a t t h e MBssbauer s p e c t r a measured f o r h i g h a r e a s a r e e s s e n t i a l l y due t o Fe3+ i o n s i n Fe-Mo-Si and Fe-Mo-A1 mixed compounds.
4
CONCLUSION A s b o t h Raman and MSssbauer measurements r e v e a l e d t h e p r e s e n c e o f
Fe2(Mo04)3 and confirmed i t f o r low s u r f a c e a r e a , w e can s t a t e t h a t method o f p r e p a r a t i o n used i n t h i s p a p e r l e a d s t o t h e f o r m a t i o n of t h e same a c t i v e s p e c i e s i n t h e absence o f s u p p o r t . T h e r e f o r e a l l chemical changes o b s e r v e d on h i g h s u r f a c e a r e a must be a t t r i b u t e d t o s u p p o r t r e a c t i v i t y . With low s u r f a c e a r e a s , t h e o n l y d i f f e r e n c e w i t h respect t o t h e p r e p a r a t i o n w i t h o u t s u p p o r t i s t h e s e g r e g a t i o n of Mooj observed by SEM. T h i s i n d i c a t e s t h a t S i 0 2 , a s i n t h e c a s e o f low s u r f a c e a r e a , m o d i f i e s t h e c a t a l y s t morphology. The absence of bands i n Raman s p e c t r a and t h e absence of p l a t e s of Moo3 i n h i g h s u r f a c e area c a t a l y s t s c a n n o t e x c l u d e t h e f o r m a t i o n o f Moo3 and Fe2(Mo04)3. The MBssbauer s p e c t r a demonstrated t h e d e c r e a s e o f t h e amount o f Fe2(Mo04)3w i t h i n c r e a s i n g s u r f a c e a r e a o f t h e s u p p o r t s and t h e f o r m a t i o n o f a new F e ( I I 1 ) compound. The n a t u r e o f t h i s F e ( I I 1 ) compound depends on t h e n a t u r e o f supp o r t . From t h e c a t a l y t i c p o i n t o f view t h e v e r y low p e r c e n t a g e o f C 0 2 formed may be c o n s i d e r e d a s a s i g n o f absence o f Fe203; i n f a c t i t i s w e l l known t h a t Fe203 i s p a r t i c u l a r l y a c t i v e i n methanol o x i d a t i o n , g i v i n g
288
Also in this case, a promotor effect of the iron on Moo3 must be assumed. As Mijssbauer measurements revealed the absence of Fe2(MoO4l3 the formation of a new surface compound with molybdenum and very probably with the support must be hypothesized. The different nature of Fe(II1) revealed in A1203 catalysts, confirms the r61e of the support in determining the nature of supported active species. The decrease in activity with increasing surface area, can be attributed to the formation of this new compound and/or to a diffusion effect that decreases the efficiency of the catalyst (11). The fact that in the case of Moo3 a slight increase in activity was observed can be an indication that in the case of mixed oxides the diffusion effect cannot play a major r6le in decreasing the activity. On the contrary, the analogous behaviour in selectivity of the two classes of catalysts confirms that the diffusion phenomena can be contributory. The over-oxidation of CH20 can occur as a consecutive reaction within the pores. REFERENCES 1 M. Dente, I. Pasquon, Chim. Ind. (Milano), 47 (1963) 359. 2 N. Pernicone, J. Less Common Metals, 36 (1974) 289. 3 G . Alessandrini, L. Cairati, P. Forzatti, P.L. Villa and F.Trifiro, J. Less Common Metals, 54 (1977) 373-386. 4 C. Marcilly, B. Delmon, Comptes Rendus, 268 (1969) 1795-1805. 5 P.L. Villa, F. Trifiro and I. Pasquon, Reactor Kinetics and Catalysis Letters, 1 (1974) 341-344. 6 I. Pasquon, F. Trifiro and G. Caputo, Chim.Ind. (Milano), 55 (1973) 168-175. 7 M. Carbucicchio and F. Trifiro, J. Cata1.,43 (1975) 77-83. 8 Ch. Janot and P. Delcroix, J. de Physique, 35 (1974) 557-561. 9 A.M. van der Knaun, Phys.Stat Sol (0),18 (1973) 215-226. 10 F. Trifiro and I. Pasquon, J. Catal, 12 (1968) 412-416.
11 C.N. Satterfield, Mass Transfer in Heterogeneous Catalysis, M . 1 . T . press Massachusetts (1970). DISCUSSION
L. GUCZI
:
If I u n d e r s t o o d c o r r e c t l y , y o u c o r r e l a t e d t h e d i s a p p e a r a n c e
o f t h e m i d d l e peak t o t h e d e s t r u c t i o n of t h e F e - m o l y b d a t e compound. I t h i n k t h i s i s not n e c e s s a r i l y so if w e a s s u m e a l a r g e q u a d r u p o l e
289
splitting, due to the very large increase in surface area and thus in dispersion of active components. M. CARBUCICCHIO
:
btain d
From the analysis of the Mdssbauer spectra
2
for supported catalysts with intermediate areas (up to 120 m /g for 2 it is possible to exclude that the
S i 0 2 and up to 1 5 m /g for A 1 2 0 3 )
broadening of the lines observed by increasing the support areas is due to the increase of the quadrupole splitting of only one kind of iron.
The spectra are always due to two contributions
line" and a "double line" spectrum.
:
allsingle
By increasing the surface area,
the single line contribution decreases and the double line contribution to the spectra increases while its quadrupole splitting remains constant.
This suggests that no evolution of the iron sur-
roundings occurs.
It is a possible hypothesis that this double line
contribution is due to iron ions located at the surface of Fe2(Mo04)3. However the double line contribution is not the same for A1203 and S i O l , as one must expect if the supports are inert.
Moreover, also
the smallest quadruplar splitting measured should imply such a high electric field gradient at the Mbssbauer nucleus that, in our opinion, it is not reasonable to attribute this doublet to iron ions of Fe2(MoO4I3 even located at the surface. catalytic measurements
:
This is also supported by the
the observed decrease of the catalytic
activity by increasing the catalyst area is in agreement with a destruction of the active species rather than with the increase of the active component area. B. DELMON
:
There might be much mutual benefit if FeMo catalysts
could be compared to CoMo.
In relation to this, I wish to refer to
our recent results (1,2) with the CoMo system in its oxidic form; we studied this system when supported on y-A1203 and SiO
2'
The main feature o f the CoMo/Y-A1203 system is the existence of a COMO "bilayer", namely a mixed cobalt-molybdenum oxidic species, extremely well dispersed on the surface of the support. know at the moment the stoichiometry of this species.
We do not I suggest
this mixed species could be the equivalent of the "distorted" iron molybdate described by the authors of the present paper.
The formation
of the monolayer strongly promotes the dispersion of cobalt oxide. One could speculate that, if analogies really exist between FeMo/AlZ03 and CoMo/AlZ03, Fe203 could segregate when a "bilayer" (as a "distorted" iron molybdate) is formed, as does co (as cobalt oxide), if
290
iron is in excess with respect to the stoichiometry of the bilayer. Actually, this speculation seems to be supported by the authors' data. When the species are deposited on Si02, in contrast with the case of A1203, there is less affinity between Moo3 and the support, and normal molybdates can form.
We observe CoMo04 on Si02.
If the
analogy I suggest really exists, normal Fe (Mo0413 should form on 2 Si02. Have the authors any data supporting this prediction ? With low surface area catalysts, it is always possible to form the normal molybdate because only a small proportion of the deposited phases can come into strong interaction with the support. A s a result, segregation of Moo3 will occur in your samples.
This
kind of Moo3 is representing the excess of the composition (Mo/Fe=2) with respect to the stoichiometry of the normal molybdate (Mo/Fe=3/2). It should be noticed that this segregation effect gives MOO 3' whereas the one caused by the formation of the "bilayer" gives group VIII metal oxide.
1 1 ) B. Delmon, P. Grange, M.A. Apecetche, P . S . Gajardo, F. Delannay C.R. Acad. Sci, in press. (2) P.S. Gajardo, P. Grange, B. Delmon, submitted for publication. M. CARBURCICCHIO
:
The existence o f possible analogies between
supported Co-Mo and Fe-Mo in the oxidic form, should be further investigated on alumina and on silica.
The proposed mechanism of
the formation of the "bilayer" can be very useful in order to interpret the nature of the compounds formed in our systems.
With
respect to your question we can affirm that silica has a much lower reactivity towards Fe2(MoOq)3 than alumina. with your prediction.
This is in agreement
Indeed, in the case of silica, Fe2(MoO4I3 is
2 still present at 120 m /g while it was found to be absent in the
case of alumina having much lower surface areas.
However, for
very ~ i g hsurface areas and for long calcination times also in the case of the silica support Fe2(Mo04)3
is absent.
On the supported
catalysts with areas lower than these very high values, Fe2(Mo04)3 coexists with the products of the reaction between the support and the active species.
However, at this moment, we cannot say if
Fe2(MoOq)3 is in the distorted or in the normal form.
J. SCHEVE
:
You should strengthen the information from the y-
resonance spectra by drawing in addition spectra made at 7 7 K.
The
computed doublets now can only be seen as a shoulder in the original spectra,
291 M. CARBUCICCHIO : You are right.
The Mdssbauer spectra at 7 7 K for
catalysts supported on high area silica and alumina confirm the fact that no a-Fe203, in the form of very small particles, grows as a consequence of the interaction between support and active species. However, the doublets observed at room temperature are not expected to be transformed in a shoulder at 7 7 K.
a-Fe203 small particles
have at room temperature a superparamagnetic behaviour due to the fast relaxation of their magnetic moments
:
the room temperature
paramagnetic doublet transforms in a six-line pattern at liquid nitrogen temperature.
In any case the low temperature measurements
we performed both on silica and on alumina supported catalysts with high areas (unpublished data), confirmed the information (i.e. the absence of a-Fe203 small particles) we obtained by comparing our room temperature Mdssbauer data both with the ones available in literature and with our catalytic measurements. S.P.S.
ANDREW
:
Was an analysis made for carbon on the used catalyst ?
Could an involatile by-product be the explanation of the different activities of the high and low area catalysts F. TRIFIRO
:
?
We do not exclude, as you suggest, that the decrease
of the activity with the increase o f the surface area of silica can be due to a strong absorption of products of side reactions.
However
we can exclude that this by-product of carbon is formed since an excess of oxygen is present inside the reactor.
N. PERNICONE
:
Have you any data about the influence of the iron
molybdate concentration on both activity and selectivity of your catalysts ? F.
T R I F I R O : We have data only for SiOz with low surface area.
We
can say that by increasing the amount of the active species, the activity increases but the selectivity decreases.
It is necessary
to take into account that the contact time in our experiment was very high. A. KUBASOV
:
You showed that catalytic activity decreased after
the formation of a chemical bond between the catalyst and support. But there are a lot of cases where activity increases.
For example,
we found out that in the case of boronphosphate on silica the
292 a c t i v i t y increased a f t e r c h e m i c a l i n t e r a c t i o n b e t w e e n components. So m y q u e s t i o n i s : what i s t h e n a t u r e o f t h e i n t e r a c t i o n in y o u r case ?
W h a t i s changed, t h e electronic s t a t e , t h e c o o r d i n a t i o n
number or something else ?
F. T R I F I R O : I n i t i a l l y , w e w e r e a l s o s u r p r i s e d t h a t t h e a c t i v i t y d e c l i n e d w i t h a n i n c r e a s e o f s u r f a c e area.
A f t e r w a r d s from t h e
a n a l y s i s o f M d s s b a u e r d a t a we r e v e a l e d t h a t d u r i n g t h e p r e p a r a t i o n o f t h e c a t a l y s t s not o n l y an i n c r e a s e o f s u r f a c e area of a c t i v e c o m p o n e n t s o c c u r s , b u t a l s o an i n t e r a c t i o n w i t h t h e support.
This
s t r o n g l y c h a n g e s t h e n a t u r e of t h e s t a r t i n g c a t a l y s t s , at least w h e n t h e e l e c t r o n i c p r o p e r t i e s o f i r o n a r e considered.
However, it is not
known whether the coordination of molybdenum, that must be a more i m p o r t a n t element f o r t h e i n v e s t i g a t e d r e a c t i o n , a l s o changes.
P.R.
C O U R T Y : You showed t h a t w i t h s i l i c a s u p p o r t e d i r o n m o l y b d a t e
c a t a l y s t s , t h e amount o f m e t h y l f o r m i a t e f o r m e d i s o f t h e s a m e o r d e r
of m a g n i t u d e a s t h e a m o u n t o f C Q formed.
On the other hand, with
b u l k i r o n m o l y b d a t e t h e amount o f m e t h y l f o r m i a t e f o r m e d is v e r y low. T h i s s u g g e s t s t h a t t h e s i l i c a c a r r i e r , even w i t h a l o w s u r f a c e a r e a , strongly modifies the specific catalytic properties of the active phase.
Also, the formation of dimethylether which occurs with the
b u l k c a t a l y s t , is n o t m e n t i o n e d f o r t h e s u p p o r t e d ones.
Can you
comment on this ?
F. T R I F I R O : W e h a v e o b s e r v e d d i m e t h y l e t h e r a n d n o m e t h y l f o r m i a t e w i t h p u r e i r o n molybdate.
We b e l i e v e t h a t t h e f o r m a t i o n o f m e t h y l -
f o r m i a t e w i t h t h e s u p p o r t e d c a t a l y s t s c o u l d b e d u e t o t h e acid s i t e s o f s i l i c a r a t h e r than t o a m o d i f i c a t i o n o f t h e i r o n molybdate. F o r m i c acid formed a s i n t e r m e d i a t e in t h e p r o d u c t i o n o f CO, c a n r e a c t w i t h m e t h a n o l in t h e p r e s e n c e o f a c i d s i t e s of Si02.
293
RELATIONSHIP BETWEEN AVERAGE PORE DIAMETER AND SELECTIVITY I N I R O N CHROMIUM
-
-
POTASSIUM DEHYDROGENATION CATALYSTS.
Ph. COURTY and J.F.
LE PAGE
I n s t i t u t F r a n g a i s du P B t r o l e
, Ruei 1 - M a l m a i s o n ,
France.
P r o c a t a l y s e , P a r i s , France.
ABSTRACT The d e h y d r o g e n a t i o n o f e t h y l b e n z e n e i n t o s t y r e n e i s c a r r i e d o u t w i t h b u l k c a t a l y s t s g e n e r a l l y composed o f i r o n , chromium and potassium o x i d e s . Beside t h i s d e h y d r o g e n a t i o n , s i d e r e a c t i o n s producing t o l u e n e and benzene
are a l s o observed. A s f a r as t h e performances o f t h e c a t a l y s t a r e c o n c e r n e d , it h a s been esta-
b l i s h e d t h a t t h e main s t e p of t h e c a t a l y s t p r e p a r a t i o n w a s t h e f i n a l c a l c i n a t i o n . The s e l e c t i v i t y t o w a r d s s t y r e n e p r o d u c t i o n i s d i r e c t l y c o r r e l a t e d t o t h e s p e c i f i c area which i n t u r n depends on t h e c a l c i n a t i o n c o n d i t i o n s .
I t h a s a l s o been o b s e r v e d t h a t a I50
hrs
s t a b i l i z e t h e c a t a l y s t ; during t h i s t i m e , improved w i t h o u t change i n a c t i v i t y .
running period w a s necessary t o s e l e c t i v i t y is progressively
Surface rebuilding of t h e c r y s t a l l i t e s
have been invoked t o e x p l a i n t h i s i n c r e a s e o f s e l e c t i v i t y i n t h e o p e r a t i n g conditions.
TRANSFORMATION ANALYSIS S t y r e n e i s e s s e n t i a l l y o b t a i n e d by d e h y d r o g e n a t i o n of e t h y l b e n z e n e . The r a n g e o f o p e r a t i n g c o n d i t i o n s c o n v e n t i o n a l l y used i s mentioned i n t a b l e I. The r e a c t i o n which i s h i g h l y endothermic i s thermodynamically l i m i t e d and t h e c o n v e r s i o n i s i n c r e a s e d when i n c r e a s i n g t h e a p p l i e d t e m p e r a t u r e . The f e e d i s much d i l u t e d w i t h steam f o r t h e f o l l o w i n g p u r p o s e s : l i m i t a t i o n o f t h e coking r a t e o f t h e c a t a l y s t , d e c r e a s e o f t h e t e m p e r a t u r e f a l l due t o t h e r e a c t i o n e n d o t h e r m i c i t y and improvement of t h e c o n v e r s i o n a t t h e e q u i l i b r i u m . I n d u s t r i a l r e a c t o r s a r e e i t h e r a d i a b a t i c o r i s o t h e r m and t h e v a r i o u s r e . a c t i o n s QCCUrrkgduring t h e o p e r a t i o n are p o i n t e d o u t i n t a b l e I ; w e c a n o b s e r v e t h a t t h e e t h y l b e n z e n e d e h y d r o g e n a t i o n i s accompanied by s e c o n d a r y r e a c t i o n s which a r e d e t r i m e n t a l t o t h e s e l e c t i v i t y : t o l u e n e , benzene b u t a l s o s m a l l q u a n t i t i e s of heavy a r o m a t i c s ( d i p h e n y l
, naphtalene,
a n t h r a c e n e ) are produced
294 whereas t h e l i g h t i n t e r m e d i a t e p a r a f f i n s a r e mainly transformed i n t o CO 2 ' TABLE I
I n d u s t r i a1 Operating Condi t i ons T
: 580
OC
- 630
P ( b a r s ) abs.
: 1
-
: 0.3
L.H.S.V?
OC
- -1 5
1 h
-4
w t H20/wt C H C H 1.5 6 5 2 5 -
&Hiq8
Main Reaction : C H C H +C 6 5 2 5
+
H CH=CHZ
6 5
H2
= 29 Kcal.mo1
-1
Side R e a c t i o n s :
+
C H C H 6 5 2 5
H2---)C
H CH
6 5
3
+
CHk
C H C H + 2 H 2 - ) C b +2CHq 6 5 2 5 6 6
C H C H 6 5 2 5
+
CH4 CO
+
+
H20
H20
+
H2+C6H6 CO
C02
+ 3 +
C2H6
H2
H2
*For e x p r e s s i n g L.H.S.V., CATALYST
only ethylbenzene i s considered.
DESCRIPTION
Commercial c a t a l y s t s a s w e l l a s c a t a l y s t s being p r e s e n t l y s t u d i e d i n r e s e a r c h l a b o r a t o r i e s seem t o be prepared from f e r r i c i r o n oxide a s t h e major a c t i v e agent (60
-
80 w t
%), potassium oxide a s a s t a b i l i z e r (10
-
20 w t % ) and
chromium o x i d e as a promoter ; t h e y c a n a l s o p o s s i b l y c o n t a i n v a r i o u s addit i v e s as t e x t u r e o r s t r u c t u r e s t a b i l i z e r s . S o l i d s t a t e i n t e r a c t i o n s occur between t h e s e i r o n , chromium and potassium oxides, when t h e y a r e heated at t e m p e r a t u r e s ranging from 700 t o 1000 OC.
The mixture o f i r o n and chromium
o x i d e s , a c t i n g probably a s a s o l i d s o l u t i o n , would be t h e e s s e n t i a l a c t i v e phase, whereas t h e potassium oxide e f f e c t , i n presence of steam, would c o n s i s t i n avoiding t h e b u i l d i n g up of coke. C a t a l y s t s A, B, C , D a r e l a b o r a t o r y c a t a l y s t s prepared by wet mixing of f e r r i c o x i d e , potassium c a r b o n a t e , chromium o x i d e , b i n d e r s and t e x t u r e promoters.
The r e s u l t i n g p a s t e i s transformed i n t o 5 mm diameter and 5 mm
l e n g t h e x t r u d a t e s ; t h e product i s d r i e d , t h e n c a l c i n e d f o r 2 hours a t temper a t u r e s ranging from 850 t o 1000 O C .
F i g u r e 1 shows t h e change of t e x t u r e
c h a r a c t e r i s t i c s a s a f u n c t i o n of t h e c a l c i n a t i o n temperature.
Between 900 and
950 OC, t h e r a p i d i n c r e a s e o f t h e average pore diameter i n d i c a t e s a v e r y i m p o r t a n t s i n t e r i n g e f f e c t ; a t t h e same t i m e , t h e s p e c i f i c s u r f a c e a r e a decreas e s , t h e t o t a l pore volume remaining almost c o n s t a n t . Table 2 summarizes t e x t u r a l p r o p e r t i e s of A,
B,
C and D c a t a l y s t s .
C a t a l y s t E i s an i n d u s t r i a l
295 c a t a l y s t h a v i n g t h e same c o m p o s i t i o n a s t h e above mentioned c a t a l y s t s .
Figure 1 Styrene catalysts (serie A
- D)
Textural properties VS. calcination temperature.
I
,
air cokinotion of
tT‘C
Duration C r y s t a11i t e s S i z e (h) (scanning microscopy)
f4&
2 hrs.
I
wo
TABLE 2
-
Catalyst A B C D
950
b PXJ0.C
Textural properties
Calcination Temp. ( “ C ) 890 920 940 970
2 2 2
2000
2
4000
-
3000
4800
5000
2 -1
c s
1
4
2400 2700 3200
nd nd
-
s.sp.
3.2 2.5 2.2
EXPERIMENTAL CONDITIONS The a c t i v i t y of t h e c a t a l y s t s
is
d e t e r m i n e d i n a 11G60m&anique1’c a t a - t e s t
working at a t m o s p h e r i c p r e s s u r e ; non s t o p r u n s a t 2 b a r s h a v e been performed i n t h e same equipment t o s t u d y t h e c a t a l y s t
s t a b i l i t y . Operating conditions
are summarized i n t a b l e 3 . Mention h a s t o be made t h a t a c t i v i t y t e s t s a r e r e a l i z e d w i t h c a t a l y s t s which have been aged f o r 150
hrs
i n the conditions
of t h e s t a b i l i t y t e s t s . TABLE 3
-
Test conditions
A c t iv i t y P (bars) L.H.S.V? (h’l) W t H20/Wt C H C H 6 5 2 5 T OC 580 *For e x p r e s s i n g L.H.S.V.,
1
0.4 2.2 630
-
Aging and S t a b i l i t y 2
0.4 2.2 580 630
-
only ethylbenzene i s considered.
296 Crushed c a t a l y s t s ( 5
-
6 mesh) a r e p r e h e a t e d under n i t r o g e n atmosphere up
t o 500 OC ; steam i s f i r s t i n j e c t e d a t 500
550
OC.
O C
E t h y l b e n z e n e i s p u r c h a s e d from C.D.F.
and f i n a l l y e t h y l b e n z e n e a t Chimie.
P r o d u c t s a r e a n a l y z e d by G.P.C.
EXPERIMENTAL RESULTS We s h a l l examine s u c c e s s i v e l y t h e e x p e r i m e n t a l r e s u l t s p e r t a i n i n g t o t h e a c t i v i t y and t h e n t o t h e s t a b i l i z a t i o n o f t h e c a t a l y s t s . p e r f o r m a n c e s w i l l be e x p r e s s e d a s f o l l o w s :
Catalyst
Ethylbenzene conversion (C) i n
% =
Product ( i ) S e l e c t i v i t y ( S i ) i n
(qi1
Product ( i ) Yield i n Catalyst
%
mol.
%
E.B
mol.
inlet E.B
E.B o u t l e t inlet
loo
-
Product ( i ) E.B outlet
= E.B inlet
Product ( i )
= E , B inlet
-
with
Ri
I
x 100
c.si
activity
Figures 2 t o c a t a l y s t s A,
5 show t h e r e s u l t s o b t a i n e d from a c t i v i t y t e s t s , w i t h t h e
B , C , D t r e a t e d f o r 150 h o u r s , i n t h e aging c o n d i t i o n s .
When
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 of t h e c a t a l y s t s i s i n c r e a s e d , t h e c o n v e r s i o n p r o g r e s s i v e l y d e c r e a s e s ( f i g . 2 ) as t h e s u r f a c e a r e a d e c r e a s e s .
A strong
e f f e c t o f 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 on t h e s t y r e n e y i e l d i s o b s e r v e d , as shown i n f i g u r e
3
j
f o r i n s t a n c e , t h e maximum o f s t y r e n e y i e l d g o e s from
64 t o 71 % when 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 i n c r e a s e s from 890 t o 970 Figure
OC.
4 p r e s e n t s t h e benzene p l u s t o l u e n e y i e l d s as a f u n c t i o n o f t h e
r e a c t i o n temperature f o r t h e various A,
B , C , D c a t a l y s t s ; one c a n s e e i n
t h e figure t h a t t h e dealkylating a c t i v i t y of t h e c a t a l y s t decreases a s t h e c a l c i n a t i o n temperature i s increased.
A l l t h e s e e x p e r i m e n t a l r e s u l t s a r e summarized by t h e s e l e c t i v i t y - c o n v e r s i o n curves presented i n f i g u r e
5 : t h e h i g h e r t h e t e m p e r a t u r e of c a l c i n a t i o n ,
t h e h i g h e r t h e c a t a l y s t s e l e c t i v i t y towards t h e dehydrogenation. X-ray
d i f f r a c t i o n s t u d y o f t h e d i a g r a m s of t h e A ,
f a i l s t o show any
difference.
B , C , D and E c a t a l y s t s
I n a l l o f them, a w e l l c r y s t a l l i z e d Fe 0
phase and a p o o r l y c r y s t a l l i z e d K 2 C r 0 4
3 4
phase a r e d e t e c t e d as shown i n
f i g u r e 11. B e s i d e s , t h e K2FeZ2O3& p h a s e , p r e v i o u s l y mentioned might be considered as
C61
and
which
t h e a c t i v e s p e c i e s h a s n o t been o b s e r v e d .
The i n c r e a s e of t h e c a t a l y s t s s e l e c t i v i t y t o w a r d s t h e d e h y d r o g e n a t i o n c a n be c o r r e l a t e d w i t h t h e v a r i a t i o n o f t h e p o r e d i a m e t e r and, i n a b e t t e r way, w i t h t h e d e c r e a s e o f t h e s p e c i f i c s u r f a c e a r e a , as shown i n f i g u r e s
A s t h e c a t a l y s t s u s e d i n t h i s s t u d y a r e non s u p p o r t e d c a t a l y s t s ,
6 and 7 .
it is not
s u r p r i s i n g t h a t t h e a c t i v i t y s h o u l d be p r o p o r t i o n a l t o t h e s u r f a c e a r e a a l l t h e more as i r o n o x i d e which c o n t r i b u t e s f o r an e s s e n t i a l p a r t t o t h e development o f t h i s s u r f a c e i s a l s o t h e a c t i v e c o n s t i t u e n t o f t h e c a t a l y s t .
297
.-D
Ethylbenzene
conversion
TOC
TOC
c )
600
620
600
640
Figure 2 : Conversion VS. r e a c t i o n temperature. 25
F
620
c
640
Figure 3 : Styrene y i e l d V S . react i o n t emgerature
.
Benzene +Tolume yield
Figure 4 : B + T y i e l d s V S . r e a c t i o n temper at ure
.
Figure 5 : Styrene s e l e c t i v i t y conversion.
VS.
298
s
( C - 75%
specific orgo I n L p - ' )
0P(h 80
b
3000
N)
5000
4000
F i g u r e 6 : S e l e c t i v i t y ( a t 75 %,, c o n v e r s i o n ) VS. p o r e d i a m e t e r ( A )
The o t h e r components, K2C03,
I
3
2
4
5.
F i g u r e 7 : S e l e c t i v i t y ( a t 75 % c o n v e r s i o n ) v s . s p e c . a r e a (m2g-I)
offer a negligible surface
C r 0 3 , K2Cr04
a f t e r b e i n g s e p a r a t e l y c a l c i n e d a t t e m p e r a t u r e s r a n g i n g from 800 t o 1000 OC. The main s i d e r e a c t i o n s which r e d u c e t h e c a t a l y s t s e l e c t i v i t y c o n c e r n t h e a t t a c k o f t h e e t h y l r a d i c a l C-C bonds by steam a c c o r d i n g t o t h e f o l l o w i n g reactions :
+
C H C H 6 5 2 5 C H CH 6 5 3
+
2 H20 + C - H
2 H20 4
CH
6 5
C H 6 6
3
+
+ C02
C02
+ 3
+ 3
H2
H2
T h i s i s s u p p o r t e d by t h e a n a l y s i s o f t h e r e a c t i o n g a s e s which g e n e r a l l y c o n t a i n 10 t o 15
%
C 0 2 , about I% CO and o n l y It o 2
%
CH4
; toluene t o
benzene r a t i o i s c l o s e t o 2 . Steam d e a l k y l a t i o n o f a l k y l b e n z e n e s h a s i n d e e d been r e c e n t l y e x p l a i n e d a s a s e l e c t i v e steam r e f o r m i n g o f s i d e c h a i n s
[7]
; t h e s e l e c t i v i t y of a c t i v e
s i t e s c a n be c o r r e l a t e d w i t h t h e a v e r a g e s i z e o f c r y s t a l l i t e s .
The s m a l l e s t
c r y s t a l l i t e s corresponding t o t h e highest s u r f a c e s e x h i b i t t h e highest activ i t i e s ; when t h e c r y s t a l l i t e s i z e is d e c r e a s e d , c o n v e n t i o n a l steam r e f o r m i n g i n c r e a s e s t o t h e d e t r i m e n t o f C-C bond r u p t u r e t h r o u g h an hydrogen a t t a c k . Such an o b s e r v a t i o n a g r e e s w i t h t h e r e s u l t s o f M a s l i a n s k i and R a b i n o v i t c h [8] who have o b s e r v e d t h a t , w i t h group VIII metals other than iron, steam
299 d e a l k y l a t i o n r e q u e s t s h i g h d e n s i t y o f s i t e s f o r water a c t i v a t i o n . Figures
8 and 9 show t h e e l e c t r o n s c a n n i n g m i c r o p h o t o g r a p h s o f
A and D
c a t a l y s t s , t h e a v e r a g e p o r e d i a m e t e r o f which w a s r e s p e c t i v e l y e q u a l t o 0
0
2400 A and 4800 A .
I t c a n be o b s e r v e d t h a t t h e a v e r a g e c r y s t a l l i t e s s i z e s
r e a c h about t h e same v a l u e (2000
-
3000
i for
A , 4000
-
5000
i for
D)
: cata-
l y s t D e x h i b i t i n g t h e l a r g e s t a v e r a g e c r y s t a l l i t e s i z e i s t h e most s e l e c t i v e ,
as f a r as d e h y d r o g e n a t i o n o f e t h y l b e n z e n e i n t o s t y r e n e i s c o n c e r n e d ( f i g . 5 ) .
-
-
F i g u r e 9 : Cat. D Electron sc a n n i ng microphotograph (G x 20 000).
Figure 8 : C a t . A Electron scanning microphotograph (G x 20 000).
Catalyst A 300
stabilization
t e s t i n v e r y s e v e r e c o n d i t i o n s h a s been c a r r i e d o u t w i t h a
hrs
c a t a l y s t made a t an i n d u s t r i a l s c a l e ; t h i s c a t a l y s t
(E) h a s t h e same compo-
s i t i o n as t h e c a t a l y s t s A , B , C , D p r e v i o u s l y mentioned.
F i g u r e 10 shows
t h e e v o l u t i o n o f t h i s c a t a l y g t performances (conversion,
s t y r e n e y i e l d and
s e l e c t i v i t y , benzene and t o l u e n e y i e l d s ) as a f u n c t i o n o f t h e r u n n i n g t i m e . may be o b s e r v e d t h a t , f o r 200
hrs
t h e s e l e c t i v i t y o f t h e c a t a l y s t does not
c e a s e t o i n c r e a s e even though t h e c o n v e r s i o n r e m a i n s c o n s t a n t v a r i e s from
: styrene yield
55 t o 6 3 % t o t h e d e t r i m e n t o f l i g h e r a r o m a t i c s y i e l d s which
d e c r e a s e from
It
17 % down t o about 10 %.
300
A
T O I U ~ ~A
%(mot)
90. 80
-_.-.
f.
-\:,
70.-
/.a,-’
l,_.__-.
-
-..-.-
........_. y..,
-
20.
d
\,
......
40 .
F i g u r e 10 P i l o t test (Cat. E ) performances VS. running time
yield -Styrene -__._-..----.-->C.____.---._--.__ _.___--.
--- ...___.__ -.-* 50 60
_._.-.-
yitld
-,-,.
Styrena rkctivi9,. .-..-.-. ..-. d Ethylbeturn conversion
.
10 -
....
...... ...... ..- .........
.,
Toluene yield .......................
u
30 5-
20 10 .
Bnum yi$d ~L
-
300
200
100
[104].100
0101 .!
CAT. E AFTER
300 +RS. PILOT TEST
F i g u r e 11
X[llQ 60
1
X-ray diagrams
X [026 40
f o r EN and EW (aged) c a t a l y s t s .
{L 60
50
40
.“I]
1)
100
x,
CAT.
E
10
20
NEW
hematite(FezO,cO
[4221 10
2
60
50
40
-
30
20
e degree, 10
-
F i g u r e 13 : C a t . EW Electron F i g u r e 12 : C a t . EN Electron s c a n n i n g microphotograph ( G x 20 0 0 0 ) s c a n n i n g microphotograph (G x 20 0 0 0 )
301 C a t a l y t i c c h a r a c t e r i s t i c s o f c a t a l y s t E e i t h e r new ( E ) o r a f t e r u s e i n t h e N s t a b i l i z a t i o n t e s t EW have been d e t e r m i n e d . T e x t u r e c h a r a c t e r i s t i c s ( s u r f a c e a r e a , p o r e s s i z e s , t o t a l pore volume) r e m a i n t h e same f o r EN , c a l c i n e d f o r 2 h o u r s w i t h a i r a t 500
OC
EW and even E
W
a s shown i n t a b l e 4 ; c h a n g e s o f
t e x t u r a l c h a r a c t e r i s t i c s c a n n o t be invoked t o e x p l a i n t h e o b s e r v e d change i n s e l e c t i v i t y during t h e non-stop run. X-ray d i f f r a c t i o n d i a g r a m s have a l s o been d e t e r m i n e d ; a s shown i n f i g u r e 11 t h e o n l y d i f f e r e n c e between EN and EW l i e s i n a smooth r e d u c t i o n o f Fe 0 2 3 i n t o Fe304 d u r i n g t h e s t a b i l i z a t i o n p e r i o d ; c r y s t a l l i t e s i z e s do n o t change
as i t i s c o n f i r m e d by t h e e l e c t r o n s c a n n i n g m i c r o p h o t o g r a p h s p r e s e n t e d i n f i g s 1 2 and 13.
TABLE 4
new E
-
Catalyst E
N
aged EW
- Textural
properties
Bulk density
Structural density
(g/ml)
(g/ml )
Total pore volume (m1/100 g
Cumulated Average pore pore volume diameter (0-2000 b a r d
(i)
Speci f ic
area (m2/g
1
1.84
3.86
28.4
23-3
3 200
2.7
1.90
3.86
26.7
26.1
3 500
2.9
1.96
3.85
25.0
24.1
3200
2.9
(316 h r s p i 1o t - p l a n t aged)
a i r - c a l c i ned (500 OC/2 hr)
CONCLUSION The s t a b i l i z a t i o n o f t h e c a t a l y s t s u s e d f o r t h e d e h y d r o g e n a t i o n o f e t h y l benzene i n t o s t y r e n e i n s i d e t h e r e a c t o r c o r r e s p o n d s t o a slow i n c r e a s e o f sel e c t i v i t y w i t h o u t change i n a c t i v i t y . A f t e r a 200
hrs
period, t h e c & t a l y s t
is s t a b i l i z e d b u t t h e s e l e c t i v i t y i s c o n s i d e r a b l y i n c r e a s e d . Although a r e d u c t i o n o f Fe 0 i n t o f e r r o m a g n e t i c Fe304 g o e s w i t h t h e 2 3 s t a b i l i z a t i o n , t h i s does not j u s t i f y t h e observed i n c r e a s e o f t h e s e l e c t i v i t y ;
as a m a t t e r o f f a c t , t h i s r e d u c t i o n i s v e r y r a p i d and r e q u i r e s o n l y a few h o u r s i n t h e o p e r a t i n g c o n d i t i o n s , t o be a c h i e v e d .
A s the specific surface
a r e a , t h e p o r e s i z e s and t h e t o t a l p o r e volume do n o t v a r y d u r i n g t h e s t a b i l i z a t i o n period, only a progressive rebuilding of t h e c a t a l y s t surface can be invoked t o e x p l a i n t h i s phenomenon. M o d i f i c a t i o n s o f t h e d i s t r i b u t i o n o f s t e p s , k i n k s and t e r r a c e s on t h e s u r f a c e o f t h e a c t i v e c r y s t a l l i t e s d u r i n g t h i s s t a b i l i z a t i o n p e r i o d might account f o r t h i s i n c r e a s e o f s e l e c t i v i t y
(91 ; s u c h r e c o n s t r u c t i o n mechanisms
are c u r r e n t l y mentioned n o t o n l y w i t h o x i d e c a t a l y s t s , b u t a l s o w i t h metal
302 catalysts
[14611 .
The i n f l u e n c e o f 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 on t h e performances h a s been t h e r e f o r e s t u d i e d a f t e r a p r i o r a g i n g o f t h e c a t a l y s t s f o r I50
hrs
r e a c t i o n c o n d i t i o n s . I t h a s been o b s e r v e d t h a t t h e s e l e c t i v i t y i n c r e a s e d up t o an optimum when 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 a n d , c o n s e q u e n t l y , t h e c r y s t a l l i t e s s i z e s i n c r e a s e d . For c a t a l y s t s having t h e above mentioned c o m p o s i t i o n , 0
t h i s optimum i s o b t a i n e d f o r an a v e r a g e c r y s t a l l i t e s i z e round about 5000 A ; beyond t h i s v a l u e , t h e s e l e c t i v i t y , i n i s o - c o n v e r s i o n
conditions, begins t o
d e c r e a s e b e c a u s e o f t h e r e a c t i o n t e m p e r a t u r e i n c r e a s e , which i s n e c e s s a r y t o compensate t h e a c t i v i t y l o s s , and which r e l a t i v e l y f a v o r s t h e d e t r i m e n t a l thermal dealkyl at ion. These r e s u l t s f i n a l l y show t h a t i t i s o f t e n d a n g e r o u s t o s e l e c t c a t a l y s t s t a k i n g i n t o account o n l y t h e i r i n i t i a l a c t i v i t y . Moreover i n t h e p r e s e n t s t u d y , i t h a s been o b s e r v e d t h a t t h e c a t a l y s t s which are t h e most s e l e c t i v e a f t e r t h e s t a b i l i z a t i o n p e r i o d , have not n e c e s s a r i l y t h e h i g h e s t s t a b i l i t y .
REFERENCES
-
1 E.H. Lee, C a t a l y s i s Review, 1973, 8, (21, P. 285 305. 2 S h e l l O i l U.S.P. 3 360 579, 2 461 147, 2 414 585. 3 C a t a l y s t and Chemicals I n c . U.S.P. 2 971 927. 4 Dow Chemical U.S.P. 3 849 339, 3 839 478, 3 703 593, 3 205 179. 5 P r o c a t a l y s e . B R I T . P. 1 504 627. 6 K. S h i b a t a , T. K i y o u r a , J. C a t . , 1969, (l), P. 103 105. 7 G.L. R a b i n o v i t c h , G.N. M a s l i a n s k y , L.N. T r e i g e r , Symposium o n t h e mechanisms of hydrocarbon r e a c t i o n s , 5 7 j u n e 73, S i o f o k ( H u n g a r y ) , P. 48, P. 97. 8 D.C. G r e n o b l e , J. C a t . , 1978, P. 203 211. 9 D.W. B l a k e l y , G.A. S o m o r j a i , J. C a t . , 1976, P. 181 196. 10 R. B r i l l and J. Kurzidim, " S t r u c t u r e e t p r o p r i b t b s d e s s u r f a c e s d e s s o l i d e s " C o l l . C.N.R.S., P a r i s , j u i l l e t 1969, P. 99, C.N.R.S. P a r i s 1970. 11 A.G. F r i e d l a n d e r , Ph. C o u r t y , R. M o n t a r n a l , J. C a t . 77, 48, P. 312 321.
-
2,
-
1,
-
5,
-
-
DISCUSSION 2.
PAAL : T h e phenomenon o f c a t a l y s t s t a b i l i z a t i o n r e m i n d s me o f
a
s i m i l a r phenomenon d e s c r i b e d f o r p o t a s s i u m - c h r o m i a - a l u m i n a c a t a l y s t s (Karansky, B.A.,
Rozengart,
M.I.
et al.,
Kinetika i Kataliz
The s t a b i l i z a t i o n p r o c e s s l a s t e d f o r a b o u t 3 0 m i n .
768(1963)).
w a s a t t r i b u t e d t o t h e reduction of C r ( V 1 ) t o C r ( I I 1 ) to Cr(I1)). activity.
Reduction proceeded under t h e e f f e c t of t h e hydrocarbon
cluded here,
COURTY
and
(or eventually
The l a t t e r h a s b e e n c l a i m e d t o p o s s e s s d e h y d r o g e n a t i o n
f e e d , a s shown b y t h e a p p e a r a n c e o f C O z .
P.R.
5,
Can t h i s p r o c e s s b e e x -
c o n s i d e r i n g t h a t F e may h i n d e r r e d u c t i o n ?
: According t o
t h e above mentioned r e f e r e n c e ,
the
s t a b i l i z a t i o n p e r i o d which c o r r e s p o n d s t o a r e d u c t i o n of Cr6+
into
303 Cr
3+
or Cr2+ is a short phenomenon
(about 30 min.).
The stabilization
of the styrene catalyst requires more than 150 hrs and cannot be correlated with a simple reduction phenomenon. always some C02 production
Anyway, we observe
(due to steam attack of the side chains)
during the whole life of the catalyst. Since the particle size of the crushed catalysts used
E.J. NEWSON:
in your experiments was as large as 5-6 mesh (about 3-4 mm diameter), it is just possible that “diffusion-confusion‘‘ could equally well explain your Figures 2 and 3 .
Focus attention on curve
0
(catalyst
A 0
pore diam. 2400 A ) and curve D (catalyst pore diam. 4800 A).
From
Figure 2, starting at low temperature and showing intrinsic kinetics 0
presumably, it is clear that the conversion over cat
A
(2400 A) is
0
higher than over cat B (4800 the opposite is observed,
A).
At high temperatures (640’C) just
A simi ar comment applies to Figure 3,
the selectivity data. Do you have the same data for the same batch of catalysts in small
particles size (e.9. 0.3 mm diam.
?
If yes, this new data should
be used the reply. P.R.
COURTY
:
The catalyst granulometry (5-6 mesh/3.4-4 mm) for the
test procedure was chosen for reducine the intraparticle diffusion limitation and not for supressing it.
It is well known that the
rate of the dehydrogenation reaction for such catalystg ir: diffusion limited.
The effectiveness factor we evaluated is 1 for an eq.
diam. of dp = 0.4 mm and about 0 . 8 for dp = 3.7 mm.
Nevertheless
we have selected this granulometry since the smallest granule dimensions used in industrial reactors are dp = 3.2 mm ( 1 / 8 ” ) 1 = 5 - 6 mm.
and
This corresponds to an eq. diam. of about 3.7 mm (our
average granulometry).
Studies carried out with smaller particles
cannot be directly related to an improvement of industrial catalysts. With respect to your observation, we agree that some diffusion limitation occurs, but the diffusion effect is far from being sufficient to explain the selectivity differences.
vs temperature
The
conversion
curves (Fig. 2) are parallel (curves B,C,D) and
correspond to catalysts which also exhibit different pore sizes. This suggests that the effectiveness factor should have approximate values.
Its value is slightly higher for
A.
This is suppor-ced
by a similar study carried out with smaller particles. same effect is also observed
:
The
the selectivity increases with
304
t h e i n c r e a s e of p o r e d i a m e t e r a n d d e c r e a s e o f t h e s p e c i f i c area. J.R.H.
ROSS : U n d e r t h e c o n d i t i o n s o f t h e p r o c e s s e s under c o n s i d e r a t i o n ,
t h e r e a c t i o n F e 2 0 3 + 3 H 2 * 2Fe + 3 H 2 0 may occur(1). W h e t h e r t h e oxide or the metal is the stable phase depends on the ratio Metallic i r o n is a n a c t i v e steam r e f o r m i n g and hydroge-
PAz0/PH2.
nation/dehydrogenation
catalyst.
Have t h e a u t h o r s c o n s i d e r e d t h e p o s s i b i l i t y t h a t t h e i r a c t i v a t e d c a t a l y s t m a y c o n t a i n s o m e metallic iron ?
Could metallic iron
h a v e b e e n re-oxidized b e f o r e t h e x-ray m e a s u r e m e n t s w e r e m a d e o n t h e used c a t a l y s t s ? 1. J.R.H. R O S S , in M.W. R o b e r t s and J.M. T h o m a s (Eds.) " S u r f a c e and D e f e c t 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 R e p o r t s , C h e m i c a l Society, L o n d o n 1975, p. 34.
P.R.
C O U R T Y : U n d e r p r o c e s s c o n d i t i o n s ( T > 6 0 0 ° C and P H
/ PH20 2 t h e r e a c t o r o u t l e t ) t h e m o r e stable iron c o m p o u n d is t h e m a g n e t i c oxide,
O.1,at
T h e f o r m a t i o n o f m e t a l l i c i r o n , from a t h e r m o d y n a m i c p o i n t
o f v i e w , s e e m s d i f f i c u l t ( s e e either C h a u d r o n o r Ellingham d i a g r a m s f o r t h e s y s t e m Fe30q-Fe-H2-H20).
O n t h e o t h e r hand, t h e f o r m a t i o n
of m e t a l l i c iron would b e a c c o m p a n i e d by a n i n c r e a s e o f t h e hydro-
g e n o l y s i s activity.
I n d e e d d u r i n g t h e whole c a t a l y s t l i f e , w e d o
not o b s e r v e c r a c k i n g o f t h e a r o m a t i c r i n g s (less t h a n 0.5%).
During
t h e s t a b i l i z a t i o n p e r i o d we d o not o b s e r v e a n i n c r e a s e o f t h e m e t h a n e yield, w h i c h would b e a r e s u l t o f h y d r o g e n o l y s i s o f t h e s i d e c h a i n s o n r e d u c e d iron.
C o n v e r s e l y , t o l u e n e and b e n z e n e yield d e c r e a s e s
.in p a r a l l e l w i t h t h e C 0 2 yield, s u g g e s t i n g t h a t t h e c o r r e s p o n d i n g r e a c t i o n is steam dealkylation.
Finally, we have never observed
m e t a l l i c i r o n in o u r so-called "stabilized catalysts". M.V.
T W I G G : H a v e t h e steam r e f o r m i n g d e a l k y l a t i o n r e a c t i o n s o f
t o l u e n e and e t h y l b e n z e n e
been studied separately ?
If t h e y h a v e ,
could d e t a i l s b e g i v e n ? P.R.
C O U R T Y : I n t h e c o n d i t i o n s o f steam d e a l k y l a t i o n r e a c t i o n s
(T7600'C;
L H S V = 0.4
-
1 h-I; H 2 0 / H C > 1 0 m o l e / m o l e ) , t h e r e a c t i v i t y
o f e t h y l b e n z e n e and t o l u e n e o v e r t h e s t y r e n e c a t a l y s t are very different.
W i t h t o l u e n e a s a feed, steam d e a l k y l a t i o n t a k e s place,
b u t t h e b e n z e n e yield is v e r y low ( l e s s t h a n 5%).
With ethylbenzene
a s a f e e d , t h e m a i n r e a c t i o n i s d e h y d r o g e n a t i o n ; via steam d e a l k y l a t i o n
305
r e a c t i o n s , e t h y l b e n z e n e d a m e t h y l a t i o n ( w h i c h p r o d u c e s t o l u e n e ) and d e e t h y l a t i o n ( w h i c h p r o d u c e s benzene) a r e observed.
T h e m o s t important
r e a c t i o n r e s u l t s i n t h e f o r m a t i o n o f toluene. K. K O C H L O E F L : 1) B y - p r o d u c t s e.g.
b e n z e n e and t o l u e n e in t h e
d e h y d r o g e n a t i o n o f e t h l y b e n z e n e a r e formed e x c l u s i v e l y by steam dealkylation.
T h i s c o n c l u s i o n r e s u l t e d from our e x p e r i m e n t s w h e r e
D 2 w a s added t o t h e feed ( H 2 0 + ethylbenzene).
In this case no
b e n z e n e o r t o l u e n e c o n t a i n i n g d e u t e r i u m w e r e found. 2 ) A p a r t from t h e p r o c e s s e s described
in t h i s p a p e r , a l s o K-migration
in t h e p a r t i c l e s o f t h e Fe-Cr-K c a t a l y s t p l a y s a n i m p o r t a n t r o l e d u r i n g t h e a c t i v a t i o n period. P.R.
COURTY
:
T h e K-migration into c a t a l y s t p a r t i c l e s i s a w e l l
known phenomenon ( I ) .
A l t h o u g h t h e n e w c a t a l y s t e x h i b i t s a n uniform
r a d i a l d i s t r i b u t i o n f o r c h r o m i u m and p o t a s s i u m , t h e used c a t a l y s t (20.000 h r s a g i n g ) s h o w s t h e p o t a s s i u m t o b e largely c o n c e n t r a t e d n e a r
t h e center o f t h e p e l l e t s and s o m e e n r i c h m e n t o f c h r o m i u m n e a r t h e p e r i p h e r y o f t h e pellets.
S i n c e t h e d e c r e a s e of t h e s u p e r f i c i a l
c o n c e n t r a t i o n of p o t a s s i u m i s a c c o m p a n i e d by a d e c r e a s e i n a c t i v i t y (the activity r e m a i n s c o n s t a n t d u r i n g t h e s t a b i l i z a t i o n p e r i o d )
,
the
p h e n o m e n o n o f K-migration a l o n e c a n n o t b e invoked t o explain t h e stabilization.
O n t h e o t h e r h a n d , t h e s t a b i l i z a t i o n p h e n o m e n o n is
r a p i d (200 h r s ) w i t h r e s p e c t t o t h e e x p e c t e d d u r a t i o n of l i f e o f t h e c a t a l y s t (20.000 h r s o r more). ( 1 ) E.H.
L E E , C a t a l y s i s Review 8 ( 2 )
,
285-305 (1973).
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307
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 Exxon Research and Engineering Company, Linden, New Jersey 07036
ABSTRACT The difficulty of preparing highly-dispersed ruthenium catalysts on basic oxide supports is related to hydrolysis of the support by the aqueous phase.
Use of
anhydrous, aprotic solutions prevents the hydrolysis reactions with the hydroxyl groups of the support, allowing selective adsorption of ruthenium chloride onto the support. An adsorption preparation is not possible from aqueous solution due to the high pH values of the solutions. The preparation of well-dispersed ruthenium on MgO-washcoated monoliths was accomplished by a non-aqueous adsorption preparation.
Aqueous solution preparations in contrast lead to ruthenium mal-
distribution with the metal poorly dispersed on this type of monolith support. Determination of the metal dispersion for supported ruthenium catalysts is complicated by a number of factors. One of the most important of these factors is multiple adsorption of CO on Ru catalysts. Ruthenium on alumina metal dispersions measured with H2 chemisorption do not agree with those measured with CO. We have, however, found for many ruthenium catalysts on MgO and on MgO/MgA1204 monoliths that there is good agreement between the metal dispersions obtained by
H2 and CO. We propose that the CO/H2 ratio is a probe of the metal-support interaction of Ru with MgO.
INTRODUCTION Ruthenium catalysts are of interest in the catalytic reduction of NO to N2 without formation of undesirable byproducts, such as NH3.
It has been observed
that supported ruthenium is a uniquely selective catalyst for the direct reduction of NO to N2 (ref. 1). Ruthenium supported on MgO powder has recently been reported to have excellent stability to oxidizing conditions at 9OOOC (ref. 2). In comparison, ruthenium on alumina readily volatilizes as Ru03 and Ru04 under oxidizing conditions at 900°C (ref. 2).
Although MgO powder is not a practical
support under conditions where reaction of the support with water can occur, such as high temperature automotive exhaust environments, a stable ruthenium catalyst can be prepared by reaction of an alumina washcoated cordierite monolith with
308
excess MgO (refs. 3-4). Therefore, the preparation of highly-dispersed ruthenium catalysts on MgO-washcoated monoliths is of practical importance. The preparation of supported ruthenium catalysts of high metal dispersion is markedly dependent on the catalyst preparation step where the ruthenium salt is contacted with the support surface. The advantages of aprotic, non-aqueous solutions will be described in detail. Comparison of the ruthenium studies will be made with iridium and platinum supported on MgO. A model of heterogeneous catalyst preparation from non-aqueous solution will be described, and contrasted with, conventional aqueous catalyst preparations. The carbon monoxide and hydrogen chemisorption values for ruthenium on magnesium oxide provide evidence for a strong metal-support interaction between ruthenium metal and the basic MgO. Comparisons will be made with the chemisorption behavior of H2 and CO on ruthenium supported on alumina (refs. 5 - 7 ) . Recognition of the importance that the support surface chemistry can play in heterogeneous catalyst preparation (ref. 8) is a necessary starting point to obtain highly-dispersed ruthenium on basic oxide supports. A key aspect of the surface chemistry of an oxide in an aqueous solution is the dissociation of the hydroxyl groups (ref. 9).
The equilibria is:
+H
A M-OH2+ -MM-OH-M-0
\
-
+
(decreasing p~
increasing PIS
This expression indicates that by appropriate adjustment of pH the surface may carry either a positive or a negative charge (ref. 10). Also, the above equilibria implies that a given oxide surface when in an aqueous solution will spontaneously react to give a particular pH value. This has in fact been observed in our work (ref. 11). The situation during heterogeneous catalyst preparation is much more complex, however, than implied by the above equilibria expression. Reaction of the support itself with the aqueous solution can buffer (ref. 12) the pH at a constant value regardless of attempts to externally adjust the solution pH. This buffering capacity of a high surface area oxide support is particularly relevant to the preparation of ruthenium catalysts on alumina washcoated cordierite monoliths, on MgO powder, and on MgO-reacted cordierite monoliths. The implications of this work to other heterogeneous catalyst preparations where highly acidic metal salt solutions are employed will be considered in subsequent discussions. The advantages of non-aqueous solutions in the preparation of highly-dispersed ruthenium catalysts will be described in detail. These catalysts can be prepared from non-aqueous solution on MgO/MgAl 0 cordierite monoliths (refs. 3 - 4 ) which 2 4 have 60% metal dispersion, i.e. 60% of the metal atoms are surface atoms in the
309
supported phase. This is about a factor of five improvement in ruthenium dispersion over the best aqueous catalyst preparation on the analogous monolith. EXPERIMENTAL The gas adsorption measurements were carried out as previously described (refs. 13-14). A description of the gas adsorption cell used for the monolith chemisorption studies will be described in detail in a future publication. Ruthenium chloride trihydrate, chloroiridic and chloroplatinic acid were obtained from Engelhard Industries Inc.
The MgO powder was Baker reagent grade and was
found to have a surface area of 59 m2/gm.
The gamma-A1203 of 190 m2/gm was
obtained from Engelhard Industries Inc. The alumina washcoated monoliths were obtainei from the Corning Glass Works. Catalyst preparations with monolith supports Catalyst preparations in this paper are f o r cordierite monolith cylinders (refs. 15-16) 1" in diameter by 3" long. The monoliths were air dried at 5OOOC for 16 hrs, then cooled to ambient temperature in a dry atmosphere. The monoliths, while still in the dry box, were put in a tightly fitting glass vessel containing 30 cc of a non-aqueous solution of ruthenium chloride, of a concentration to give 0.03 wt% Ru.
This allows an equal volume of solution to be in contact with the
interior monolith channels as is in contact with the broken, irregular exterior surface of the monolith. Approximately 10% by weight of the monolith was a tightly adhering alumina washcoat. The area of this washcoat was 120 m2/gm of alumina, or about 12 m2/gm of monolith. The adsorption From the intensely black ruthenium chloride solution required about 30 min from anhydrous acetone or acetonitrile. The rate of adsorption was rapid in the first 5 min and complete adsorption then required a further 25 min, leaving a colorless liquid. Evaporation of the decanted acetone from these cores left no residue.
In agreement with these visual observations, X-ray fluorescence
examination did not detect any Ru in the decanted liquid after adsorption. The aqueous adsorption preparation was done in identical fashion, except that an aqueous solution of ruthenium chloride was used. A quasi-incipient wetness impregnation was attempted by flowing an acidified aqueous solution through the monolith channels, the volume being sufficient to just wet the alumina washcoat surface. Similar methods of aqueous preparations were also attempted with MgO/MgA1204 monoliths. The MgO/MgAl 0 washcoat had a surface area of 60 m2 per 2 4 gm of washcoat or 6 m 2 /gm of monolith. RESULTS AND DISCUSSION Ruthenium chloride chemistry in aqueous phase Before discussing the non-aqueous preparations of highly-dispersed ruthenium catalysts, the aqueous chemistry associated with such preparations will be
310 reviewed. The hydrated chloride salts of the second and the third row Group VIII transition metals are standard materials for the preparation of highlydispersed supported metal catalysts (ref. 17).
Aqueous solutions of these metal
chlorides are usually highly acidic (refs. 18-19).
For example, an aqueous
solution of ruthenium chloride of 0.06M concentration has a pH of 1.5. An increase in the pH of a solution, either 0.06 or 0.006M in ruthenium chloride, to 1.7 by the addition of 0.1N NaOH results in quantitative precipitation of ruthenium from solution. When this solution is reacidified to pH 1.5 with 0.1N HC1, the precipitated salt is completely solubilized. It is interesting that allowing a ruthenium chloride solution of pH 1.7 to stand overnight results in an insoluble product, even after reacidification to a pH of less than one. Analogous results are obtained for aqueous solutions of other second and third row Group VIII metal chlorides but, naturally, with different pH and metal concentration dependencies. The implications of the ready precipitation of ruthenium chloride from aqueous solution with a relatively small increase in the pH is of importance in the preparation of Ru catalysts supported on A1203 and MgO. Catalyst support reactivity with acidic metal salt solution Alumina is one of the most commonly employed heterogeneous catalyst supports. Alumina has been reported recently (ref. 8) to be quite reactive in aqueous solution. This reactivity is due to the amphoteric nature of the hydroxyl groups terminating the lattice of the support (ref. 10).
For example, we found that
the addition of 5 m of pure A1203 to 100 gm of distilled water rapidly changed the pH to 7.8 from 6.0. In addition, titration of the above suspension with aliquots of 0.1N HC1 results in the alumina buffering the system to a pH of 4.2 (as long as any undissolved alumina is present). Continuous hydrolysis of the alumina support is responsible for the buffering behavior of the suspension. A similar effect is observed for a silica-alumina cracking catalyst with 14% A1203 content (ref. 17).
Quite different behavior is observed upon titration of
aqueous slurries of silica, titania and carbon (refs. 18,20) with 0.1N HC1. The implications of the hydrolysis characteristics of alumina and silicaalumina to aqueous heterogeneous catalyst preparations are: Acidic metal chloride solutions may result in metal salt precipitation when using salts which are insoluble at a pH of 4.2. Metal salt precipitation must be taken into account, especially for excess aqueous solution preparations in which adsorption from the aqueous phase may be confused with precipitation from solution. Hydrolysis of alumina rapidly occurs on contact with water, giving a pH of 7.8. This high pH level may lead to even more complete salt precipitation with incipient wetness catalyst preparations. Such preparations necessarily contain relatively small amounts of water. Alumina containing a 0.2 wt% sodium impurity was found to buffer the slurry to a pH of 8.6, compared to 7.8 for very pure alumina (ref. 11).
Therefore the
311 purity of the alumina is also an important factor to consider in preparations where acidic metal salt solutions are used. Monolith catalvst DreDarations Aqueous solutions of ruthenium chloride when contacted with either A1203 coated monoliths or MgO/MgAl 0 coated monoliths gave instantaneous precipitation 2 4 from solution as observed visually. Prior acidification of the solution with either HC1 or HNO failed to prevent this precipitation of the ruthenium due to 3 the buffering action of the support. Gross mal-distribution of ruthenium on the monolith interior channels could not be avoided because of the ruthenium precipitation in these aqueous preparations. With the quasi-incipient wetness preparation, precipitation occurred to such a degree that the metal salt was only deposited on the top third of the monoliths with the solution flowing down the monolith channels. With no satisfactory method of preparing either a well-dispersed or uniformly distributed Ru monolith catalyst from aqueous solutions, we investigated non-aqueous methods. Use of polar, aprotic, non-aqueous solvents provided a completely satisfactory solution to the problem. Tremillon (ref. 2 1 ) .
Such solutions are defined by
Initial studies were with dehydrated monoliths, but
subsequent work demonstrated that rehydrated monoliths could also be used satisfactorily. Completely uniform distribution of Ru on the monolith was achieved for adsorption from either acetone or acetonitrile. This adsoFption method is clearly a major improvement over earlier aqueous techniques. Ruthenium dispersion of about 60%, corresponding to 1.8 nm particles, could reproducibly be obtained by our new non-aqueous procedure, see Table 1. The ruthenium dispersion calculated from H2 chemisorption on a MgO/MgAl 0 monolith 2 4 was 20% less than that calculated from the adsorption of CO. Subsequent work (to be published) on MgO powders indicates that multiple CO chemisorption on surface ruthenium atoms is responsible for the differences in the metal areas derived from H2 and CO chemisorption. Another advantage of preparations using polar, aprotic solutions is that the usually employed hydrated metal chloride salts can be used (refs. 1,7,19).
This avoids the requirement for difficult to
handle and expensive metal alkoxides, acetylacetonates, or organometallics in non-aqueous preparations. The use of protic, non-aqueous solvent to deposit Ru on MgO/MgAl 0 coated 2 4 monoliths was unsatisfactory. Methanol and ethanol were used to dissolve the RuCl3*xH20. Apparently, the protic solutions interact strongly with the dehydrated surface by hydrogen bonding to form charged polar layers (ref. 10) which inhibit uniform adsorption of the ruthenium salt. The final monoliths made by this method always showed incomplete adsorption and very spotty, non-uniform deposition.
312 The rate of ruthenium chloride adsorption from acetone and the ruthenium dispersion of the catalyst were not influenced by rehydration of the MgO/MgA1204 monoliths. Rehydration was accomplished by allowing the calcined monoliths to stand overnight at ambient temperature in an atmosphere saturated with water. Also, acetone-water mixtures can be employed for the ruthenium chloride adsorption if the acetone content is 60 volume %, or greater. Qualitatively, the rate of adsorption is identical for an acetone-water solution containing 60 vol % acetone to that found with anhydrous acetone. Use of acetone-water solutions where the acetone is less than 60% resulted in precipitation of ruthenium and "spotted" adsorption. A mixed aqueous-non-aqueous solution may be of particular advantage for salts of limited solubility in the non-aqueous solvent itself. The precipitation of full-sized (3.75" x 4.5") MgO/MgA1204 washcoated automotive monoliths (ref. 3) presented no problems with ruthenium chloride-acetone solutions. The procedure used was scaled up from that used for the 1" x 3" cores. Excellent distribution of the metal on the large monoliths was demonstrated by Ru analysis, using X-ray fluorescence of a sectioned core. Hydrogen and carbon monoxide chemisorption studies on the 1" x 3" monoliths show (Table 1) improved Ru dispersion by a factor of five for acetone adsorption preparation compared to an aqueous preparation. Dispersion values calculated from both gases were found to be in fair agreement for these samples. For example, the dispersions for 0.03% Ru on a MgO/MgA1204 monolith by H2 and CO were 61 and 74%, respectively. This relatively good agreement between the H2 and CO chemisorptions for well-dispersed ruthenium is not generally found (refs. 5 , 7 ) . Apparently a strong metal-support interaction between Ru and MgO significantly retards multiple CO chemisorption, compared with Ru of similar dispersion supported on Al 0 (refs. 5,7). This point will be discussed in more detail in 2 3 a subsequent section where comparisons are made between Ru supported on A1 0 2 3 and MgO, both for aqueous and non-aqueous preparations. It is of interest that, regardless of the chemical nature of the support, poorly dispersed Ru does not show anomalous chemisorption behavior (refs. 5,7,13). Aqueous and non-aqueous preparations on A120 3 and MgO powders It has been shown by Benesi, Curtis and Studer (ref. 22) that adsorption catalyst preparations can have advantages over incipient wetness preparations. Both acetone adsorption and aqueous incioient wetness Preparations were carried out using RuC13*xH20 for A1 0 and MgO powders. Aqueous adsorption preparations were 2 3 unsatisfactory, as discussed earlier. The alumina extrudates were crushed and calcined overnight at 500'C in air, then transferred rapidly to a dry box to prevent readsorption of water. An aqueous iscipient wetness procedure was used to prepare 0 . 3 % Ru on A1 0
2 3'
Chemisorptions on this sample were performed in the usual manner and are reported in Table 2.
As observed in other experiments with alumina supports
313 TABLE 1 The dispersion of Ru on monolith supports as measured by CO and H2 chemisorption Method of % Disp. by CO (6)
Preparation(4s5)
Catdlyst 0.1% Ru'l) 0.1% Ru (1) 0.03% Ru (2) 0.04% Ru (3) 0.04% Ru (3)
Aq. F Aq. 0 Ac Ac Ac
.
. .
% Disp. by H? ( 7 )
13 13 76 80
-
74
61
(1) The alumina coated monolith (ref. 4 ) was further coated with magnesium nitrate by a quasi-incipient wetness procedure. After calcination at 6OO0C (ref. 4 ) , the monolith contained 1.8% MgO.
(2) MgO was added in two steps to the monolith. First, the monolith was immersed in molten magnesium nitrate (90°C), the excess liquid removed. It was dried for 4 hrs at TIDOC, then calcined for 4 hrs at 60OoC. This was followed by an incipient wetness impregnation as in 1. The monolith after this contained 12% MgO. (3) Preparation as for 2, but 16% MgO was added. ( 4 ) Aq. F denotes aqueous incipient wetness impregnation, followed by freeze-drying. Aq. 0 denotes aqueous incipient wetness impregnation, followed by air drying at 110'C. Ac. denotes adsorption from acetone solutions. In all cases ruthenium chloride was used.
(5) After drying or evaporation of acetone, all samples were reduced at 5OO0C for 2 hrs. ( 6 ) Dispersion values calc. on the assumption of solely linear CO adsorption, i.e. one CO molecule per Ru surface atom. (7) Dispersion values calc. on the assumption that H2 dissociates on adsorption and that one H atom adsorbs on one surface Ru atom. TABLE 2 Chemisorption studies on Ru, Ir and Pt Metal Preparation PreContent 1. 0.3% Ru 2. 0.3% Ru 3. 0.3% Ru 4 . 0.3% Ru 5. 0.3% Ru 6. 1.0% Ru 7 . 1.0% Ru 8. 2% Ir 9. 2% Ir 10. 2% Pt
Support
it& MgO MgO MgO MgO MgO MgO
MgO MgO
Method Aq *
. Aq Ac . Ac . Ac . Ac . Ac . Aq Ac .
AC
*
treatment (2) None None None None A None
B None None None
% M Disp.
% M Disp.
by CO
by H2 31 32 38
79
76 43 100 50
18 15
-
-
-
57 67 19
CO Disp.1 H2 Disp. 2.5 2.4
1.1
-
-
(1) Aq. denotes aqueous incipient wetness impregnation. Ac. denotes an acetone adsorption preparation.
(2) Pretreatment A consisted of calcining catalyst No. 4 in flowing air at 850'C for 16 hrs.B-was a calcination of catalyst No. 6 in flowing air at 500'C for 2 hrs. In all cases, the catalysts were reduced at 5OO0C in flowing H2 for 2 hrs after the pretreatment.
314 (refs. 5,7), the H2 and CO dispersions are not the same, due to the presence Of multiple CO chemisorption on surface ruthenium atoms. For a sample with 31% dispersion (based on H2 chemisorption) a CO/H2 ratio of 2.5 was found. This dispersion corresponds to a particle size of 2.3 nm, if the particles are spheres or cubes. For similar size Ru particles (2.5 nm) Dalla Betta (ref. 7 ) reported a CO/H ratio of 2.3. His catalyst contained 5% Ru on A1203 and was prepared by 2
an aqueous procedure. The analogous anhydrous catalyst was prepared in a dry box using rapid stirring of the acetone slurry to insure uniform adsorption of the ruthenium chloride from solution. The Ru dispersion of this catalyst was essentially the same as that of its aqueous counterpart (cf. samples 1 and 2, Table 2).
This is an interesting example where essentially the same metal
dispersions result from completely different metal salt addition procedures. The reduction procedures were, of course, identical. The similarity of the dispersions suggest that migration of either the metal complex or of the metal particles occurring at some stage during reduction is the key factor in determining the ultimate metal dispersion. In contrast, quite substantial differences in Ru dispersions on MgO were obtained when the solvents were changed (lines 3 and 4, Table 2).
The MgO powder
was precalcined and the ruthenium chloride added by procedures identical to those used with A1 0 2 3' As expected from the results on the monoliths, the acetone preparation on powdered MgO gave much higher ruthenium dispersions than did the aqueous preparation by a factor of 2.4 (samples 3 and 4, Table 2).
If one assumes no
multiple CO adsorption, the dispersion of sample 4 is 100% or atomic dispersion. Unfortunately, H2 chemisorption of this atomic dispersion could not be obtained on sample 4, as the H2 isotherms were steeply sloping. This makes uncertain the monolayer capacity. However, for somewhat less well-dispersed Ru on MgO/MgA1204 monoliths, good agreement in Ru dispersions between H2 and CO has been found (sample 5 , Table 1).
A possible explanation is that MgO powder interacts more
strongly with the Rff than does the MgO/MgA1204 surface.
It is also possible that
there is a change in the mechanism or the stoichiometry of H2 adsorption. The most surprising result for Ru on MgO was the relatively high dispersion (43%) found for the incipient wetness preparation (sample 3, Table 2). Based on the previous discussion of the aqueous chemistry of ruthenium chloride, there is no doubt that precipitation occurs on contact of the aqueous phase with the MgO. Nevertheless, the ruthenium dispersion is a little higher than that obtained for the analogous catalyst prepared using alumina. This surprisingly high dispersion suggests that the basic oxide surface of MgO has a favorable support interaction with ruthenium (or its complexes) leading to small metal particle sizes after reduction. An intermediate metal chloride or metal chloro-hydrate complex could migrate on the MgO during the reduction process, giving the good Ru dispersion.
315 This is an example where precipitation of the metal salt during the aqueous impregnation step does not result in thn expected poorly-dispersed metal phase after reduction. For comparison, 2% Ir and 2% Pt were deposited on the same MgO by the adsorption of the hexachlorides from acetone. After reduction, Table 2 shows that their dispersions were 57% and 19%, as measured by hydrogen chemisorption. A catalyst consisting of 2% Ir on MgO was prepared by aqueous incipient wetness.
Surprisingly
it was even better dispersed (67%) than the acetone preparation (57%). Here again we have another instance where a catalyst prepared by a method involving salt precipitation gives an unexpectedly high dispersion. In a fashion similar to Ru, this observation implies a strong metal-support interaction for Ir on MgO. Major decreases in iridium particle size in an oxidizing atmosphere have been reported recently (ref. 2 3 ) .
The Ir was supported on barium-doped alumina and
the mechanism of particle size diminution was postulated to be related to the formation of BaIr03.
Although neither bulk magnesium ruthenates or bulk magnesium
iridates have been reported (refs. 2 , 2 3 ) , our data shows that MgO surfaces have an unexpectedly beneficial effect on Ru and Ir dispersions. As the Ru on MgO systems are intended for automotive exhaust purification, it
is of interest to study their stability in high temperature oxidizing environments (refs. 2 , 2 4 ) .
Calcination of the 0.3% Ru catalyst in air at 85OoC for 1 6 hrs
gave a sample that was still about 50% dispersed after reduction (samples 4 and 5, Table 2). This was achieved despite the fact that the MgO decreased in area to 30 m2/gm. Calcination of a 1% Ru sample at 500'C for 2 hrs showed almost no l o s s of dispersion (samples 6 and 7 , Table 2 ) .
The retention of good Ru dispersion
after high temperature calcination is consistent with the formation of a surface ruthenate (ref. 2 ) .
Studies of the effect of calcination on Ru dispersion using
a MgO support of invariant surface area will be described elsewhere (ref. 25). SUMMARY A combination of low metal dispersion and mal-distribution of Ru coated monoliths prepared from aqueous solutions led us to use non-aqueous, aprotic solutions for these unique systems. Excellent Ru dispersion and uniform distribution was obtained from both acetone or acetonitrile solutions. The agreement between metal areas derived from CO and H2 chemisorptions for Ru on MgO/MgAl 0 monoliths indicates only minor multiple CO chemisorption. This 2 4
agreement indicates the presence of a strong interaction between Ru and MgO in contrast to the behavior of well-dispersed Ru on A1203.
Studies of similarly
prepared Pt and Ir on MgO showed that the dispersions are in the following order Ir >> Pt % Ru. With Ir the metal dispersions were the same, for both aqueous and non-aqueous solutions. With MgO a considerable improvement in Ru
316 dispersion was obtained by the use of non-aqueous solvents. The high dispersion of Ru on MgO was retained after a high temperature air calcination.
REFERENCES 1. T. P. Kobylinski and B. W. Taylor, J. Catal., 33 (1974) 376. 2. S. J. Tauster, L. L. Murrell and J. P. DeLuca, J. Catal., 48 (1977) 258. 3. M. W. Pepper, J. P. DeLuca, R. P. Rhodes and L. S. Bernstein, SAE Paper #750684, presented at Fuels and Lubricants Meeting, Houston, Texas, June 3, 1975. 4. L. L. Murrell and D. J. C. Yates, U.S. Patent 3,980,589 and J. P. DeLuca, G. B. McVicker and L. L. Murrell, U . S . Patent 3,990,998. 5. R. A . Dalla Betta, J. Catal., 34 (1974) 57. 6 . K. C. Taylor, J. Catal., 38 (1975) 299. 7. R. A. Dalla Betta, J. Phys. Chem., 79 (1975) 2519. 8. J. R. Anderson, Structure of Metallic Catalysts, Academic Press, New York, 1975, 33. 9. G. D. Parfitt, Pure and Appl. Chem., 48 (1976) 415. 10. C. B. Amphlett, Inorganic Ion Exchangers, Elsevier, New York, 1964, Chapter 5. 11. Five grams of a given oxide were calcined overnight at 500'C and transferred to a dry box. It was then contacted with 100 cc of distilled water with a pH of 6.0. Reforming grade A1203 gave a pH of 7.6. An A1203 sample contaminated with 0.2% Na results in a pH of 8.6. A BaTi03 support gave a pH of 8.5, silica (Cabosil) 4.8, titania (Caboti) 5.3 and silica-alumina (DA-1 Davison) 4.2. 12. D. D. Perrin and B. Dempsey, Buffers for pH and Metal Ion Control, Chapman and Hall, London, 1974. 13. J. H. Sinfelt and D. J. C. Yates, J. Catal., 8 (1967) 82. 14. D. J. C. Yates and J. H. Sinfelt, J. Catal., 8 (1967) 348. 15. Reference 8, p. 88. 16. J. J. Burton and R. L. Garten (Eds.), Advanced Materials in Catalysis, Academic Press, New York, 1977, Chapter 10. 17. Reference 8, Chapter 4. 18. Reference 8, p. 181. 19. J. Escard, C. Leclerc and J. P. Contour, J. Catal., 29 (1973) 31. 20. The oxides were treated as described in reference 11, except for the carbon (Carbolac) which was dried in flowing helium at 300'C. For silica, titania and carbon the pH values as a function of volume of 0.1N HC1 added were identical to those obtained by addition of the same volumes of 0.1N HC1 to distilled water alone. 21. B. Tremillon, Chemistry in Non-aqueous Solvents, D. Reidel, Pub. Co. Dordrecht, Holland, 1974, Chapter 1. 22* H. A. Benesi, R. M. Curtis and H. P. Studer, J. Catal., 10 (1968) 328. 23. G . B. McVicker, R. L. Garten and R. T. K. Baker, Fifth Canadian Symposium on Catalysis, October 26-27, 1977, Calgary, Alberta, p. 346. 24. Y. Ti Yu Yao and J. T. Kummer, J. Catal., 46 (1977) 388. 25. L. L. Murrell and D. J. C. Yates, in press.
DISCUSSION H.T. RIJNTEN
:
You indicated in your paper that after impregnation
of RuC13 on A I Z O J from an aqueous solution, low dispersion and
maldistribution occur.
A
low dispersion might be understood from the
polymeric RU species in aqueous solution. maldistribution occurs ?
Can you indicate why
317 L.L.
MURRELL : M a l d i s t r i b u t i o n o c c u r s d u e t o t h e d e p l e t i o n of
r u t h e n i u m in s o l u t i o n , d u e t o t h e . p H c h a n g e s from t h e h y d r o l y s i s o f t h e A 1 2 0 3 support b e f o r e u n i f o r m c o n t a c t i n g o f the m o n o l i t h s u p p o r t w i t h t h e s o l u t i o n c a n be accomplished.
F.S.
S T O N E : T h e r e s u l t s showing very h i g h d i s p e r s i o n o f RU o n M g O
p o w d e r a r e c e r t a i n l y v e r y i n t e r e s t i n g and c l e a r l y o n e would l i k e t o be a b l e to d e f i n e b e t t e r t h e p r o p e r t i e s w h i c h g i v e rise t o it.
It
w o u l d h e l p i n u n d e r s t a n d i n g t h e o r i g i n o f t h i s b e h a v i o u r if o n e could d i s c r i m i n a t e b e t w e e n t h e effects o f s u r f a c e O H g r o u p s o n t h e o n e hand a n d basic c e n t r e s o n t h e other.
F o r M g O o u t g a s s e d at 5 O O 0 C ,
a s i n t h e p r e s e n t work, t h e r e i s a m a r k e d o v e r l a p o f t h e s e t w o effects. M g O p o w d e r o u t g a s s e d b e l o w 5 O O 0 C r e m a i n s v e r y hydroxylated.
However,
if outgassed above 50OOC the oxide reveals its intrinsic surface b a s i c i t y i n c r e a s i n g l y a s t h e o u t g a s s i n g t e m p e r a t u r e i s raised.
Thus
a s e r i e s o f p r e p a r a t i o n s o f R u / M g O c a t a l y s t s using M g O p o w d e r s w h i c h h a v e b e e n pre-outgassed
at, say, 3 5 0 ° C ,
5OO0C, 65OoC, 8OO0C
and 9 5 0 O C respectively before the non-aqueous impregnation of the r u t h e n i u m would s h o w w h e t h e r t h e h i g h d i s p e r s i o n in t h e f i n a l Ru/MgO c a t a l y s t d e p e n d s o n b o t h t h e a b o v e e f f e c t s o r o n o n l y o n e of them. Provided t h e M g O p o w d e r is slowly h e a t e d in a g o o d d y n a m i c vacuum d u r i n g o u t g a s s i n g , t h e M g O s u r f a c e area could b e m a i n t a i n e d at a s i m i l a r h i g h value for all t h e p r e p a r a t i o n s , t h e r e b y m i n i m i z i n g any c h a n g e s in texture.
L.L.
MURRELL : I w o u l d a g r e e t h a t t h e s e e x p e r i m e n t s would be
i n t e r e s t i n g t o do.
S t u d i e s showed t h a t t h e r u t h e n i u m d i s p e r s i o n w a s
not g r e a t l y i n f l u e n c e d f o r a fully-hydroxylated M g O s a m p l e d r i e d a t l l O ° C c o m p a r e d t o t h e 5 O O 0 C c a l c i n a t i o n r e p o r t e d in our paper.
A.
O Z A K I : I am in favor o f y o u r i n t e r p r e t a t i o n t o form a s u r f a c e
r u t h e n a t e of Mg.
I n t h i s c a s e s o m e p a r t o f MgO i s d i s s o l v e d i n t o
s o l u t i o n t o form t h e precipitate.
D.E.
W E B S T E R : R u t h e n i u m c o n t a i n i n g c a t a l y s t s w h i c h a r e stable under
p r o l o n g e d o x i d 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 a r e k n o w n , b u t m u c h m o r e d i f f i c u l t y is experienced w i t h p r o d u c i n g stable r u t h e n i u m c a t a l y s t s t o o p e r a t e u n d e r c o n t i n u o u s short o x i d a t i o n and r e d u c t i o n cycles.
I s t h e r e any r e a s o n t o expect t h e h i g h d i s p e r s i o n c a t a l y s t s
p r e p a r e d by t h e n o n - a q u e o u s r o u t e w i l l p e r f o r m better in s u c h
318 conditions 7
L.L.
M U R R E L L : T h i s c o m m e n t is c e r t a i n l y i n k e e p i n g w i t h o b s e r v a t i o n s
in o u r laboratories.
S h o r t o x i d a t i o n - r e d u c t i o n c y c l e s do lead t o
s i g n i f i c a n t sintering o f t h e ruthenium p h a s e for a c e t o n e p r e p a r e d catalysts.
T h e m e c h a n i s m of s i n t e r i n g of ruthenium f o r r e d u c t i o n -
o x i d a t i o n c y c l e s i s c o n s i s t e n t w i t h t h e o b s e r v a t i o n s o f G.B. V i c k e r , R.L.
G a r t e n , and R.T.K.
MC
B a k e r , for I r 0 2 m i g r a t i o n o n
BaO-doped a l u m i n a f o r c y c l e d reducing-oxidizing
conditions (To be
P u b l i s h e d in J. Catalysis).
H. C H A R C O S S E T : 1.
Have you considered the possibility of an
i n c o m p l e t e reduction o f ruthenium t o the m e t a l l i c s t a t e i n y o u r c a t a l y s t s , in p a r t i c u l a r w h e n s u p p o r t e d by A 1 2 0 3 7
2) H a v e y o u
c o m p a r e d t h e c h e m i s o r p t i o n data w i t h e l e c t r o n m i c r o s c o p y d a t a , i n typical examples at least ? L.L.
M U R R E L L : I n c o m p l e t e r e d u c t i o n o f r u t h e n i u m o n a l u m i n a and on
M g O i s not g r e a t l y d i f f e r e n t t h a n found f o r r u t h e n i u m o n s i l i c a b a s e d o n t h e s i m i l a r i t y of t h e infrared s p e c t r a o f C O c h e m i s o r b e d o n R u o n t h e s e t h r e e supports. T r a n s m i s s i o n e l e c t r o n s p e c t r o s c o p y h a s b e e n a t t e m p t e d b u t i s e x p e r i m e n t a l l y q u i t e d i f f i c u l t d u e to t h e o p a q u e n e s s o f t h e support.
C.J.
W R I G H T : Y o u p r o p o s e d a m e c h a n i s m f o r t h e s t a b i l i z a t i o n of
r u t h e n i u m u p o n t h e MgO s u r f a c e u n d e r o x i d i z i n g conditions.
Under
r e d u c i n g c o n d i t i o n s d o e s t h e ruthenium h a v e t h e s a m e r e s i s t a n c e t o sintering and are you able t o propose a physical mechanism for any s t a b i l i z a t i o n t h a t o c c u r s 7
L.L.
M U R R E L L : Ruthenium o n m a g n e s i u m o x i d e s h o w e v i d e n c e for a
s t r o n g metal-support
i n t e r a c t i o n w h e n r u t h e n i u m is r e d u c e d t o t h e
m e t a l i n t h a t t h e CO c h e m i s o r p t i o n s h o w s l i t t l e e v i d e n c e o f m u l t i p l e c h e m i s o r p t i o n , i.e., ruthenium phase.
M(CO)x, x = 2 - 4 ,
even f o r a highly dispersed
Ruthenium o n A 1 2 0 3 in c o m p a r i s o n , has M ( C O ) x
c o m p l e x e s w i t h v a l u e s o f x = 2-4.
The model that we propose is
i n t e r a c t i o n of Ruthenium r a f t e o r s m a l l c l u s t e r s w i t h t h e basic o x i d e i o n s o f t h e m a g n e s i u m o x i d e surface.
M.V.
T W I G G : You have d i s c u s s e d r e s u l t s o b t a i n e d u s i n g a c e t o n e and
acetonitrile.
Why w e r e t h e s e s o l v e n t s s e l e c t e d , a n d would you c a r e
319 t o comment o n t h e a d v a n t a g e s and d i s a d v a n t a g e s a s s o c i a t e d w i t h o t h e r non-aqueous
L.L.
solvents.
MURRELL
:
A c e t o n e and a c e t o n i t r i l e w e r e selected b e c a u s e o f the
excellent s o l u b i l i t y o f h y d r a t e d r u t h e n i u m c h l o r i d e in t h e s e t w o solvents.
A l c o h o l s o l v e n t s a l t h o u g h e f f e c t i v e for d i s s o l v i n g t h e
m e t a l s a l t lead t o p o o r adsorption characteristics.
An e x h a u s t i v e
study o f high diaelectric solvents was not undertaken a s the two s o l v e n t s d i s c u s s e d in t h e p a p e r p r o v e d e f f e c t i v e in s o l v i n g t h e l i m i t a t i o n s of a q u e o u s preparations.
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321
CATALYST A C T I V A T I O N BY R E D U C T I O N :: N.PERNICONE
and F.TRAINA
Montedison S.p.A.,
D I P I / A t t i v i t h C a t a l i z z a t o r i , Novara,
Italy
ABSTRACT A f t e r h a v i n g s t r i c t l y d e f i n e d t h e c o n c e p t o f a c t i v a t i o n by reduct i o n , t h e i n d u s t r i a l p r o c e d u r e s f o r t h e r e d u c t i o n of t h e c a t a l y s t s for h i g h and low t e m p e r a t u r e CO c o n v e r s i o n and f o r ammonia s y n t h e s i s a r e r e p o r t e d and t h e b a s i c p r i n c i p l e s on which t h e y a r e based a r e d i s c u s sed. Emphasis is g i v e n t o t h e d i f f i c u l t y o f o b t a i n i n g l a b o r a t o r y - s c a l e d a t a t h a t can be e a s i l y connected w i t h s u c h i n d u s t r i a l p r o c e d u r e s . The e x p e r i m e n t a l t e c h n i q u e s u s e d f o r b a s i c and a p p l i e d r e s e a r c h o n c a t a l y s t r e d u c t i o n a r e b r i e f l y d i s c u s s e d , i n p a r t i c u l a r thermogravimetry and t e m p e r a t u r e programmed r e d u c t i o n . F i n a l l y t h e now a v a i l a b l e knowledge a b o u t t h e r e d u c t i o n mechanisms
o f t h e above-mentioned i n d u s t r i a l c a t a l y s t s i s summarized.
1. THE CONCEPT O F A C T I V A T I O N The manufacture of a n i n d u s t r i a l c a t a l y s t ( r e f s . 1 - 2 )
i n v o l v e s so-
m e u n i t o p e r a t i o n s , which can be grouped i n t o t h r e e main c l a s s e s (Chemical P r e p a r a t i o n ,
Thermal Treatments and Forming). The c l a s s
ttThermal T r e a t m e n t s t t i n c l u d e s t h r e e main u n i t o p e r a t i o n s : Drying, C a l c i n a t i o n and A c t i v a t i o n .
There i s no u n i v e r s a l l y a c c e p t e d d e f i n i -
t i o n f o r t h e s e u n i t o p e r a t i o n s . For i n s t a n c e , t h e r e is some c o n f u s i o n between d r y i n g and c a l c i n a t i o n : some p e o p l e d i s t i n g u i s h them o n l y on t h e b a s i s of t h e t e m p e r a t u r e l e v e l , w h i l e o t h e r s pay a t t e n t i o n t o t h e purpose of t h e o p e r a t i o n . Other m i s u n d e r s t a n d i n g s may o c c u r about c a l c i n a t i o n and a c t i v a t i o n .
I n a n a t t e m p t t o r e a c h a more uniform ter-
minology among c a t a l y s i s s p e c i a l i s t s , w e p r o p o s e t h e f o l l o w i n g d e f i :: P u b l i s h e d i n P u r e A p p l . Chem.,
50,
9-10 (19781, p . 1169.
322 nitions. By d r y i n g w e s t r i c t l y mean a t h e r m a l t r e a t m e n t performed w i t h t h e aim of e l i m i n a t i n g t h e a b s o r b e d w a t e r , though w e admit t h a t a l s o t h e h y d r a t i o n water may be p a r t i a l l y or c o m p l e t e l y e l i m i n a t e d . Drying c a n be performed i n a i r or i n e r t g a s atmosphere or under vacuum a t t e m p e r a t u r e s u s u a l l y n o t much h i g h e r t h a n 100°C. By c a l c i n a t i o n a t h e r m a l t r e a t m e n t c a r r i e d o u t i n a n o x i d i z i n g atmosphere s h o u l d be s t r i c t l y understood.
However w e p r e f e r t o mean
by c a l c i n a t i o n any t h e r m a l t r e a t m e n t c a r r i e d o u t w i t h t h e f o l l o w i n g purposes:
t o decompose some compounds p r e s e n t i n t h e c a t a l y s t ,
gene-
r a l l y w i t h e v o l u t i o n o f gaseous p r o d u c t s , and/or t o a l l o w s o l i d s t a t e r e a c t i o n s among d i f f e r e n t c a t a l y s t components t o o c c u r and/or t o make t h e c a t a l y s t s i n t e r . The i m p o r t a n c e o f c a l c i n a t i o n is s t r a i g h t f o r w a r d , a s it can i n f l u e n c e a l l t h e c a t a l y s t p r o p e r t i e s ,
e s p e c i a l l y porous
s t r u c t u r e and mechanical s t r e n g t h . Having s o d e f i n e d d r y i n g and c a l c i n a t i o n ,
it is p o s s i b l e t o re-
s t r i c t t h e d e f i n i t i o n of a c t i v a t i o n t o t h e r m a l t r e a t m e n t s c a r r i e d o u t i n t h e r e a c t o r i t s e l f , e i t h e r i n d u s t r i a l , p i l o t or l a b o r a t o r y s c a l e , u s u a l l y i n s p e c i a l gaseous atmospheres. Such t r e a t m e n t s a r e r e q u i r e d o n l y f o r some c a t a l y s t s . As a f u r t h e r r e s t r i c t i o n w e do n o t c o n s i d e r
a k i n d o f a c t i v a t i o n t h e r e g e n e r a t i o n of poisoned c a t a l y s t s , even i f
it i s c a r r i e d o u t " i n s i t u " . T h e r e f o r e t h e t e r m l l a c t i v a t i o n l l i s due t o f r e s h c a t a l y s t s only.
2.
ACTIVATION BY REDUCTION
It i s a matter of f a c t t h a t i n most c a s e s t h e a c t i v a t i o n c o n s i s t s i n a r e d u c t i o n by s u i t a b l e gaseous r e d u c i n g a g e n t s . This i s needed because it is v e r y t r o u b l e s o m e f o r t h e m a n u f a c t u r e r t o p r e p a r e c a t a l y s t s where t h e o x i d a t i o n s t a t e s of t h e m e t a l l i c elements a r e lower t h a n t h o s e e x i s t i n g i n a i r atmosphere.
I n t h e few c a s e s where t h e
r e d u c t i o n i s c a r r i e d o u t by t h e m a n u f a c t u r e r , a p a s s i v a t i o n procedu-
r e i s t h e n needed t o h a n d l e t h e c a t a l y s t i n a i r and f i n a l l y a n a c t i v a t i o n by r e d u c t i o n , though s h o r t e r , must be performed.
323
The most i m p o r t a n t c a t a l y t i c p r o c e s s e s , from an i n d u s t r i a l p o i n t o f view, i n v o l v i n g c a t a l y s t a c t i v a t i o n by r e d u c t i o n , a r e summarized i n Table 1. TABLE 1 Some c a t a l y t i c p r o c e s s e s i n v o l v i n g c a t a l y s t a c t i v a t i o n by r e d u c t i o n Catalytic process
Catalyst reduction
Steam r e f o r m i n g o f hydrocarbons High t e m p e r a t u r e CO c o n v e r s i o n Low t e m p e r a t u r e CO c o n v e r s i o n Methanation Ammonia s y n t h e s i s #.
-
NiO Fez03 c CUO NiO Fe3O4-
3C
Ni Fe3O4 cu N i Fe
Only t h e main compound t o be reduced i s r e p o r t e d h e r e .
It s h o u l d be s t r e s s e d t h a t s u c h a c t i v a t i o n , though n o t performed by t h e manufacturer,
is i n f a c t t h e f i n a l s t a g e of c a t a l y s t p r e p a r a t i o n
and t h a t its i n c o r r e c t performance may have d r a m a t i c e f f e c t s on c a t a l y s t efficacy.
To c o n s i d e r o n l y one example, i f exothermic r e d u c t i o n s
a r e c a r r i e d o u t a t t o o h i g h r a t e s , t h e c a t a l y s t t e m p e r a t u r e may exceed t h e maximum a l l o w a b l e , w i t h consequent d e c r e a s e o f c a t a l y s t s u r f a c e a r e a and/or u n d e s i r a b l e s o l i d s t a t e r e a c t i o n s .
To a v o i d s u c h d a n g e r s ,
u s u a l l y t h e manufacturer s p e c i f i e s t h e reduction procedure i n d e t a i l , h a v i n g t a k e n i n t o a c c o u n t t h e c h a r a c t e r i s t i c s of t h e p l a n t . W e w i l l g i v e h e r e some examples of t h e s e p r o c e d u r e s , a f t e r h a v i n g d i s c u s s e d t h e p r i n c i p l e s on which t h e y a r e based.
3.
PROCEDURES FOR THE REDUCTION OF SOME INDUSTRIAL CATALYSTS
3.1 High t e m p e r a t u r e CO c o n v e r s i o n c a t a l y s t Taking i n t o a c c o u n t t h e chemical composition of v a r i o u s commercial c a t a l y s t s , t h e r e a c t i o n s t h a t may o c c u r d u r i n g r e d u c t i o n i n t h e conver-
t e r a r e r e p o r t e d i n T a b l e 2, t o g e t h e r w i t h t h e i r s t a n d a r d e n t h a l p y values.
A s it i s n e c e s s a r y t o a v o i d c a t a l y s t o v e r h e a t i n g d u r i n g r e d u c t i o n ( o t h e r w i s e a d e c r e a s e of c a t a l y t i c a c t i v i t y , due t o s i n t e r i n g , and/or of mechanical s t r e n g t h w i l l r e s u l t ) , t h e f o l l o w i n g g e n e r a l c o n c l u s i o n s can be drawn from Table 2 about t h e i n f l u e n c e of c a t a l y s t composition
324
TABLE 2 R e a c t i o n s o c c u r r i n g d u r i n g r e d u c t i o n of h i g h t e m p e r a t u r e CO conversion catalysts.
AH
Reaction
2 d-Fe00H 2 8-FeOOH 30(-Fe203 + H2 3d,-Fe203 CO 3J-Fe203 H2 CO 3J-Fe203 2CrO3 3H2 2Cr03 3co
+ +
+ + +
= a(-Fe2O3 = /-Fez03 = 2Fe3O4
= 2Fe304 = 2Fe3O4 = 2Fe304
= Cr2O3 = Cr2O3
a t 25OC,
+ H20 + H20 + H20 + + + + +
Kcal/mole
+ 9.3 + 5.0
-
C02 H20 H20 3H20 3C02
-
2.3 12.1 20.3 30.1
-163.6 -193.1
t h e reduction process : t h e c o n t e n t o f C r 0 3 s h o u l d be a s low a s p o s s i b l e ( i n p r a c t i c e ,
a
maximum c o n t e n t of 0.5% can be e a s i l y t o l e r a t e d ) ; t h e presence of
(y-Fe203 Y
hence o f
J-FeOOH,
i n h i g h amounts s h o u l d
be avoided: t h e p r e s e n c e of O(-FeOOH ( g o e t h i t e ) is v e r y u s e f u l , due t o i t s endot h e r m i c decomposition i n a r a n g e of t e m p e r a t u r e where F e 0
2 3
reduc-
t i o n has a l r e a d y s t a r t e d . From t h e same T a b l e it i s a l s o p o s s i b l e t o c o n c l u d e t h a t t h e r e d u c t i o n
of t h i s c a t a l y s t s h o u l d be c a r r i e d o u t more by H
t h a n by CO. 2 However, about t h e composition of t h e r e d u c i n g gas, t h e r e is a n o t h e r
i m p o r t a n t argument t o be t a k e n i n t o a c c o u n t , namely t h e p o s s i b i l i t y of Fe 0
3 4
r e d u c t i o n t o Fe ( t h i s s h o u l d be c a r e f u l l y avoided, a s m e t a l l i c
Fe c a t a l y z e s t h e h i g h l y e x o t h e r m i c r e a c t i o n o f m e t h a n a t i o n ) . To e l i m i n a t e s u c h r i s k , a s u f f i c i e n t amount o f s t e a m must be added t o t h e reduc i n g gas t o have a H O/H2 r a t i o h i g h e r t h a n t h e e q u i l i b r i u m one a t t h e 2 = 0.09 a t 40OoC). Steam working t e m p e r a t u r e ( f o r i n s t a n c e ( H O/H ) 2 2 eq h a s a l s o t h e f u n c t i o n o f b a l a n c i n g t h e h i g h e r r e d u c i n g power of CO w i t h r e s p e c t t o H2 for Fe 0
3 4
( t h i s i s a s e c o n d r e a s o n f o r h a v i n g a low
CO c o n t e n t i n t h e r e d u c i n g g a s ) . I n a d d i t i o n t o t h e above c o n s i d e r a t i o n s , based o n thermodynamic data,
t h e r e d u c t i o n p r o c e d u r e s recommended by c a t a l y s t manufactures r e l y
a l s o on o t h e r f a c t o r s , l i k e o p e r a t i n g r e s t r a i n t s t y p i c a l o f t h e p l a n t
325
and e a s e o f o p e r a t i o n . I n p r a c t i c e , t h e y s t r o n g l y depend on t h e reduct i o n conditions of t h e reforming c a t a l y s t s .
W e w i l l report here t h e
main s t a g e s of t h e r e d u c t i o n p r o c e d u r e of t h e Montedison MHTC c a t a l y s t i n a s h i f t c o n v e r t e r f o l l o w i n g a modern steam reformer. A f t e r h a v i n g purged t h e c o n v e r t e r w i t h n i t r o g e n , t h e c a t a l y s t i s h e a t e d f i r s t l y i n n i t r o g e n s t r e a m up t o 1 5 O o C , t h e n i n s u p e r h e a t e d steam up t o 300°C ( b o t h t h e s e f e e d s come from t h e r e f o r m e r ) . The reduct i o n s t a r t s a t t h i s t e m p e r a t u r e when a H2-steam m i x t u r e comes from t h e r e f o r m e r and c o n t i n u e s w i t h p r o c e s s gas (H -CO-CO -N ), always con2 2 2 t a i n i n g much steam, up t o 350°C ( s t e a m h a s a l s o t h e f u n c t i o n o f i n c r e a s i n g h e a t t r a n s f e r ) . I f d u r i n g r e d u c t i o n sudden i n c r e a s e s o f t e m p e r a t u r e occur,
it i s n e c e s s a r y t o lower t h e p r o c e s s gas c o n t e n t , or,
e x c e p t i o n a l l y , t o s t o p it completely
u n t i l t h e temperature decreases
below t h e recommended l i m i t (500°C can be t o l e r a t e d € o r s h o r t p e r i o d s ) . However, w i t h p r o p e r o p e r a t i o n t h e maximum t e m p e r a t u r e is u n l i k e l y t o exceed 4OO0C,
w i t h b e n e f i t f o r c a t a l y s t performances.
O t h e r r e d u c t i o n p r o c e d u r e s i n v o l v e t h e use o f w e t p r o c e s s g a s s t a r t i n g a t lower t e m p e r a t u r e s . This may cause steam c o n d e n s a t i o n i n t h e c a t a l y s t bed, w i t h p o s s i b l e damage f o r mechanical s t r e n g t h .
3.2 Low-temperature CO c o n v e r s i o n c a t a l y s t The r e a c t i o n s t h a t may o c c u r d u r i n g r e d u c t i o n i n t h e c o n v e r t e r a r e r e p o r t e d i n Table 3, t o g e t h e r w i t h t h e i r s t a n d a r d e n t h a l p y v a l u e s ( t h e main components of commercial c a t a l y s t s a r e CuO and ZnO, t o g e t h e r w i t h some amount o f amorphous a l u m i n a ) . TABLE
3
P o s s i b l e r e a c t i o n s d u r i n g r e d u c t i o n of low-temperature CO c o n v e r s i o n catalyst React i o n CUO CUO
ZnO ZnO
+ H2 + co + H2 + CO
= CU
= cu = Zn = Zn
+ H20 + c02 + H20 + CO2
AHat
25OC,
Kcal/mole
- 20.7 - 30.6 + 25.4 f 15.5
The r e d u c t i o n o f ZnO s h o u l d n o t o c c u r (AG = + 19.2 kcal/mole a t 200°C); however, a t t e n t i o n has been drawn t o t h e p o s s i b l e f o r m a t i o n of b r a s s
326 ( r e f s . 3-4).
I n f a c t , t h i s s o l i d s t a t e r e a c t i o n has a n e g a t i v e AG
v a l u e t h a t can compensate, e s p e c i a l l y a t low Zn c o n c e n t r a t i o n s , t h e p o s i t i v e O G o f ZnO r e d u c t i o n . The low-temperature CO c o n v e r s i o n c a t a l y s t r e q u i r e s v e r y c a r e f u l r e d u c t i o n t o g i v e i t s b e s t performances.
T h e r e f o r e t h e r e d u c t i o n pro-
c e d u r e does n o t a p p r e c i a b l y depend on t h e c h a r a c t e r i s t i c s o f t h e p l a n t , c o n t r a r y t o t h a t of t h e h i g h t e m p e r a t u r e c o n v e r s i o n c a t a l y s t .
I n fact,
it i s performed i n d e p e n d e n t l y of t h e s t a r t - u p of both r e f o r m i n g and h i g h t e m p e r a t u r e s h i f t c o n v e r t e r s . W e w i l l r e p o r t h e r e t h e main f e a t u r e s of t h e r e d u c t i o n p r o c e d u r e s recommended by some m a n u f a c t u r e r s . A f t e r h a v i n g purged t h e c a t a l y s t f r e e from a i r w i t h a n i n e r t gas ( n i t r o g e n or n a t u r a l g a s ) , t h e t e m p e r a t u r e is r a i s e d , gas s t r e a m ( s p a c e v e l o c i t y 500-1000 h-1)
i s g r a d u a l l y added up t o 0.3-0.5
mole
t o 7O-12O0C.
always i n i n e r t Then hydrogen
% and l a t e r t h e t e m p e r a t u r e is
s l o w l y i n c r e a s e d up t o 1 8 O O C . A t t h i s t e m p e r a t u r e t h e hydrogen c o n t e n t
is r a i s e d t o 1.5-2
mole
% i n 3-5 hours. The t e m p e r a t u r e i s s l o w l y i n -
c r e a s e d up t o 20O-22O0C and f i n a l l y t h e hydrogen c o n t e n t is r a i s e d t o about
10 mole%.
A l l m a n u f a c t u r e r s stress t h a t it is n e c e s s a r y t o d e c r e a s e t h e h y b gen f e e d or even t o s t o p it completely a t any t i m e , s h o u l d any suddesl i n c r e a s e of c a t a l y s t t e m p e r a t u r e occur.
I t may be u s e f u l t o remark t h a t
t h e u s e o f steam, t o o b t a i n a b e t t e r t e m p e r a t u r e c o n t r o l , is n o t recommended, b e c a u s e i t s e e m s t o h a v e a n e g a t i v e i n f l u e n c e o n c a t a l y t i c a c t i v i t y .
-
3 . 3 Ammonia s y n t h e s i s c a t a l y s t The main r e a c t i o n o c c u r r i n g d u r i n g t h e r e d u c t i o n of t h e ammonia synthesis catalyst is
+
+
+
Fe 0 4 H 2 = 3Fe 4H 0 AH = 35.8 Kcal 3 4 2 S m a l l e n t h a l p y d i f f e r e n c e s , due t o t h e p r e s e n c e o f f o r e i g n c a t i o n s (Al, Mg, Ca) i n t h e m a g n e t i t e l a t t i c e and of s m a l l amounts o f o t h e r i r o n o x i d e s i n t h e c a t a l y s t s (4-Fe 0 2
3,
FeO, f e r r i t e s ) , can be neglec-
t e d , a s a f i r s t approximation. Owing t o i t s e n d o t h e r m i c i t y , t h i s r e d u c t i o n is performed i n t h e p l a n t s f o l l o w i n g r a t h e r d i f f e r e n t g u i d e l i n e s w i t h respect t o t h e two CO c o n v e r s i o n c a t a l y s t s .
A powerful e l e c t r i c h e a t e r i s needed t o
s t a r t u p t h e r e d u c t i o n and t o s u s t a i n it almost t i l l i t s end, T o de-
327 c r e a s e power consumption, t h e r e d u c t i o n i s u s u a l l y performed w i t h s y n t h e s i s gas ( H -N2 2
3:1),
not with pure H2,
t h e e x o t h e r m i c i t y of ammonia s y n t h e s i s .
t o t a k e a d v a n t a g e of
After t h e synthesis reaction
h a s s t a r t e d , a c a r e f u l t e m p e r a t u r e c o n t r o l i s o b v i o u s l y needed. The most i m p o r t a n t f a c t o r t h a t i n f l u e n c e s t h e f i n a l c a t a l y t i c prop e r t i e s i s t h e w a t e r c o n c e n t r a t i o n i n t h e gas i n c o n t a c t , d u r i n g t h e reduction process, with t h e already reduced c a t a l y s t .
I n f a c t , owing
t o t h e h i g h r e a c t i v i t y of i r o n s u r f a c e towards oxygenated compounds, repeated oxidation-reduction r e a c t i o n s occur i n water-containing hydrogen atmosphere w i t h consequent r e c r y s t a l l i z a t i o n and growing of i r o n c r y s t a l l i t e s . To minimize s u c h phenomena, t h e f o l l o w i n g o p e r a t i n g c r i t e r i a a r e adopted, when c g m p a t i b l e w i t h p l a n t ' s c h a r a c t e r i s t i c s :
-
low r e d u c t i o n r a t e
- high space v e l o c i t y - low p r e s s u r e -
e f f e c t i v e d r y i n g of f e e d gas
- small
c a t a l y s t granules
I n any c a s e , t h e measurement o f t h e w a t e r c o n t e n t of t h e e x i t g a s e s h a s t o be done, a s c a t a l y s t m a n u f a c t u r e r s u s u a l l y s p e c i f y t h e maximum v a l u e allowed. A s a n example, w e w i l l r e p o r t h e r e t h e main d i r e c t i o n s f o r t h e r e d u c t i o n of t h e Montedison MSFN c a t a l y s t i n a m u l t i b e d converter. A f t e r h a v i n g purged t h e c o n v e r t e r f r e e from a i r , t h e c a t a l y s t t e m p e r a t u r e is s l o w l y i n c r e a s e d by means o f t h e h e a t e r i n a s t r e a m o f s y n t h e s i s gas a t 70-100 A t m . creased over 3OO0C,
The t e m p e r a t u r e o f t h e f i r s t bed is i n -
w h i l e t h o s e o f t h e o t h e r beds a r e k e p t below t h i s
v a l u e . When t h e c a t a l y s t i n t h e f i r s t bed has s t a r t e d i t s r e d u c t i o n , t h e o p e r a t i n g c o n d i t i o n s (mainly s y n t h e s i s gas s p a c e v e l o c i t y and h e a t s u p p l y ) a r e c o n t r o l l e d i n s u c h a way a s t o have a w a t e r c o n t e n t between 2000 and 3000 ppm i n t h e e x i t gas. A s a n a d d i t i o n a l r e s t r a i n t , t h e bed t e m p e r a t u r e s h o u l d n o t exceed 4804C. When t h e f i r s t bed c a t a l y s t i s a l m o s t c o m p l e t e l y reduced ( 8 0 - 9 0 % ) , t h e o t h e r beds a r e reduced i n t u r n i n t h e same way. A t t h e end o f t h e r e d u c t i o n s t a g e t h e h e a t e r
i s t a k e n o u t of s e r v i c e and t h e p r e s s u r e is s l o w l y i n c r e a s e d
s reached. t h e normal working v a l u e i
until
During t h i s o p e r a t i o n t h e reduc-
328 t i o n of t h e r e s i d u a l unreduced m a g n e t i t e s l o w l y o c c u r s .
3.4 Relation with applied laboratory-scale research From t h e above-reported
d e s c r i p t i o n of t h e r e d u c t i o n p r o c e d u r e s of
some i n d u s t r i a l c a t a l y s t s , it is c l e a r t h a t it is v e r y d i f f i c u l t , i f not impossible, t o s t r i c t l y d e f i n e t h e o p e r a t i n g conditions a t t h e catalyst level.
To j u s t i f y t h i s s t a t e m e n t , l e t us summarize t h e main
f e a t u r e s of t h e r e d u c t i o n o f t h e above-mentioned c a t a l y s t s . I n t h e more complex c a s e ( h i g h t e m p e r a t u r e CO c o n v e r s i o n ) t h r e e d i f f e r e n t chemical p r o c e s s e s ( w i t h o u t c o n s i d e r i n g t h e p h y s i c a l o n e s ) a r e simultaneously involved:
-
-
hydroxide decomposition oxide reduction CO c o n v e r s i o n
I n t h e i n t e r m e d i a t e c a s e (ammonia s y n t h e s i s ) w e have s i m u l t a n e o u s l y :
- oxide reduction - ammonia s y n t h e s i s Even i n t h e s i m p l e r c a s e (low t e m p z r a t u r e CO c o n v e r s i o n ) , where o n l y r e d u c t i o n t a k e s p l a c e , t h e r e d u c t i o n i s performed w i t h both temperat u r e and hydrogen p a r t i a l p r e s s u r e v a r i a b l e o v e r a wide range. I n f a c t , t h e r e d u c t i o n s of t h e i n d u s t r i a l c a t a l y s t s a r e o f t e n performed under c o n d i t i o n s v a r y i n g v e r y i r r e g u l a r l y , w i t h t h e aim e n s u r i n g t h a t t e m p e r a t u r e and/or w a t e r c o n c e n t r a t i o n i n t h e bed a r e under control.
An a d d i t i o n a l c o m p l i c a t i o n is g i v e n by t h e use of h i g h p r e s s u r e s .
The d i f f i c u l t i e s o f r e p r o d u c i n g on a l a b o r a t o r y s c a l e s u c h complex p r o c e s s e s and of o b t a i n i n g meaningful d a t a have caused a marked l a c k o f a p p l i e d r e s e a r c h on t h i s s u b j e c t . What i s u s u a l l y done i s t h e comp a r i s o n of t h e c a t a l y s t s i n t h e same e x p e r i m e n t a l c o n d i t i o n s , o b v i o u s l y chosen i n s u c h a way a s t o o b t a i n r e s u l t s t h a t can be e a s i l y i n t e r preted.
Now w e w i l l b r i e f l y d i s c u s s t h e e x p e r i m e n t a l t e c h n i q u e s u s e d
f o r these investigations.
4.
EXPERIMENTAL TECHNIQUES FOR RESEARCH O N CATALYST REDUCTION
4.1 Thermogravimetry ( T G ) As c a t a l y s t r e d u c t i o n i n v o l v e s u s u a l l y oxygen a b s t r a c t i o n ,
a p a r t from
o t h e r s i m u l t a n e o u s decomposition p r o c e s s e s , l i k e d e h y d r a t i o n , thermo-
329 g r a v i m e t r y i s t h e most s u i t a b l e t e c h n i q u e , and t h e most w i d e l y used i n d e e d , t o g e t i n f o r m a t i o n u s e f u l both i n a p p l i e d and b a s i c r e s e a r c h . The modern commercial t h e r m o g r a v i m e t r i c b a l a n c e s have e l i m i n a t e d most of t h e e x p e r i m e n t a l d i f f i c u l t i e s commonly e n c o u n t e r e d by res e a r c h e r s u n t i l a b o u t 2 0 y e a r s ago. ( u p t o 10-7
Ease o f u s e and h i g h s e n s i t i v i t y
g) make now f e a s i b l e i n v e s t i g a t i o n s p r e v i o u s l y v e r y d i f -
f i c u l t . However one m u s t be c o m p l e t e l y aware o f t h e many e x p e r i m e n t a l v a r i a b l e s t h a t may s t r o n g l y i n f l u e n c e t h e r e s u l t s o b t a i n a b l e w i t h a t h e r m o g r a v i m e t r i c balance. Such e x p e r i m e n t a l v a r i a b l e s can be summarized a s f o l l o w s :
- geometry - shape of -
of t h e b a l a n c e t h e pan
mass and s h a p e of t h e sample f l o w r a t e of t h e r e d u c i n g gas ( p u r e or mixed with i n e r t gas)
-
r a t e of temperature i n c r e a s e
- way
of r e a c h i n g c o n s t a n t t e m p e r a t u r e ( f o r
isothermal runs).
It may be s e e n t h a t o n l y v a r i a b l e s c o n n e c t e d w i t h t h e a c t u a l experiment have been c o n s i d e r e d , w h i l e o t h e r i m p o r t a n t f a c t o r s can be d e t e c t e d i n t h e p r e v i o u s h i s t o r y of t h e sample. Obviously, w e do n o t i n t e n d t o d e e p l y d i s c u s s h e r e s u c h t h e r m o g r a v i m e t r i c problems; however, some t y p i c a l r e s u l t s w i l l be r e p o r t e d l a t e r . W e would r e c a l l h e r e some d i f f i c u l t i e s connected w i t h t h e p o s s i b i l -
i t y o f u s i n g thermogravimetry i n c o n d i t i o n s s i m u l a t i n g t h e a c t u a l r e d u c t i o n of some i n d u s t r i a l c a t a l y s t s ,
namely t h a t commercial thermo-
g r a v i m e t r i c b a l a n c e s a r e u s u a l l y n o t q u a l i f i e d f o r work under p r e s s u r e and/or w i t h h i g h w a t e r c o n c e n t r a t i o n s i n t h e r e d u c i n g gas. Very o f t e n it is n e c e s s a r y t o o b t a i n a d d i t i o n a l i n f o r m a t i o n a b o u t t h e p r o c e s s e s o c c u r r i n g d u r i n g c a t a l y s t r e d u c t i o n ; t h i s g o a l can be r e a c h e d by p h y s i c a l methods of a n a l y s i s of t h e g a s e s l e a v i n g t h e thermob a l a n c e . Connections of thermobalances w i t h quadrupole mass s p e c t r o -
meters,
gas chromatographs,
already effected ( r e f .
i n f r a r e d s p e c t r o m e t e r s , e t c . have been
5 ) . They a r e s t r o n g l y recommended when proces-
ses d i f f e r e n t from r e d u c t i o n and from d e h y d r a t i o n may s i m u l t a n e o u s l y
330 occur. W e w i l l l i m i t o u r d i s c u s s i o n t o t h e c o l l e c t i o n of t h e r m o g r a v i m e t r i c
d a t a , which can be o b t a i n e d by e i t h e r dynamic h e a t i n g or i s o t h e r m a l method.
I n t h e c a s e of c a t a l y s t r e d u c t i o n s t u d i e s , t h e former i s usu-
a l l y employed f o r f a s t c h a r a c t e r i z a t i o n of t h e r e d u c t i o n p r o c e s s , mainly on a comparative b a s i s . Mathematical methods of d a t a t r e a t m e n t have been developped f o r t h e dynamic h e a t i n g method ( r e f . 6 ) ; a l l t h e s e a r e based on t h e e q u a t i o n
where
= degree of transformation,
factor,
q = r a t e of temperature increase,
T = temperature
( O K ) ,
2 = frequency
E = a c t i v a t i o n energy,
n = reaction order. However, t h e p h y s i c a l meaning of t h e k i n e t i c p a r a m e t e r s o b t a i n e d by s u c h t r e a t m e n t s of t h e e x p e r i m e n t a l d a t a is v e r y u n c e r t a i n , s o t h a t
it is p r o b a b l y p r e f e r a b l e t o r e l y on t h e d a t a immediately a v a i l a b l e from t h e thermogram, namely s t a r t i n g r e d u c t i o n t e m p e r a t u r e ( T , ) , maximum r e d u c t i o n r a t e (R ) and i t s c o r r e s p o n d i n g t e m p e r a t u r e ( T ). m m The b e s t d a t a can be o b t a i n e d i f t h e d i f f e r e n t i a l t h e r m o g r a v i m e t r i c (DTG) curve i s a l s o a v a i l a b l e ,
a s i n most modern i n s t r u m e n t s . W e
stress t h a t , i f a c o r r e c t comparison among d i f f e r e n t c a t a l y s t samples i s t o be done, t h e p r e v i o u s l y mentioned e x p e r i m e n t a l f a c t o r s m u s t be kept s t r i c t l y constant. The i s o t h e r m a l method is more f r e q u e n t l y u s e d f o r b a s i c s t u d i e s on r e d u c t i o n mechanisms.
The e x p e r i m e n t a l r e s u l t c o n s i s t s i n a c u r v e of
d e g r e e of t r a n s f o r m a t i o n v e r s u s t i m e a t c o n s t a n t t e m p e r a t u r e . The e x p e r i m e n t a l c u r v e m u s t be f i t t e d t o a t h e o r e t i c a l one o b t a i n e d on t h e b a s i s of r e a s o n a b l e h y p o t h e s e s c o n c e r n i n g t h e r e d u c t i o n mechanism. Various models a r e p r e s e n t l y a v a i l a b l e f o r t h e mathematical t r e a t ment of i s o t h e r m a l t h e r m o g r a v i m e t r i c d a t a ( r e f . 7 ) . Though t h e y a r e v a l i d f o r any g a s - s o l i d r e a c t i o n o c c u r r i n g w i t h l o s s or i n c r e a s e of weight, w e w i l l b r i e f l y d i s c u s s them o n l y i n t h e a s p e c t s c o n c e r n i n g o x i d e r e d u c t i o n . All t h e s e models assume t h a t t h e n u c l e a t i o n p r o c e s s , namely t h e i n i t i a l f o r m a t i o n o f germs of t h e m e t a l l i c or r e d u c e d
331 oxide phase,
i s v e r y f a s t w i t h r e s p e c t t o t h e p r o g r e s s of t h e reduc-
t i o n , s o t h a t t h e presence of an i n d u c t i o n p e r i o d cannot be explained. The s i m p l e s t model, t h e r e f o r e more f r e q u e n t l y used,
i s t h e unrea-
c t e d s h r i n k i n g c o r e model. It assumes t h a t t h e sample p a r t i c l e is completely non-porous,
t h a t t h e reduction occurs a t t h e s h a rp in te r-
f a c e between a l r e a d y reduced and unreduced s o l i d and t h a t i n t h e par-
t i c l e , of simple geometric shape, t h e r e a r e no temperature g r a d i e n t s . I n t h i s very f a v o u r a b l e c a s e one can d i s t i n g u i s h between r e d u c t i o n s c o n t r o l l e d by r e a c t i o n a t t h e phase boundary or by a s h d i f f u s i o n (however a s h i f t from one c o n t r o l l i n g s t e p t o a n o t h e r u s u a l l y occurs when temperature i s changed, w i t h p o s s i b l e e x i s t e n c e of a wide range of intermediate s t a t e s ) . I f t h e c a t a l y s t p a r t i c l e s a r e porous and i n t e r n a l d i f f u s i o n is v e r y f a s t w i t h r e s p e c t t o r e d u c t i o n , t h e homogeneous model s h o u l d be a p p l i e d , where r e d u c t i o n is v i s u a l i z e d a s o c c u r r i n g throughout t h e s o l i d phase.
However t h e mathematical t r e a t m e n t i s very complex, s o
t h a t some s i m p l i f i c a t i o n s have been introduced.
I n t h e b e s t known
s i m p l i f i e d model t h e p a r t i c l e is c o n s i d e r e d l i k e a n a g g r e g a t e of s m a l l s p h e r e s , each of which i s reduced a c c o r d i n g t o t h e u n r e a c t e d s h r i n k i n g c o r e model. I n t h e c a s e s where t h e p a r t i c l e i s porous and t h e r e d u c t i o n is f a s t w i t h r e s p e c t t o a s h d i f f u s i o n , a m o d i f i c a t i o n of t h e unreacted s h r i n k i n g c o r e model has been developped, assuming t h a t t h e r e a c t i o n zone has a f i n i t e t h i c k n e s s . For a l l t h e s e models more complex c a s e s have been considered, assuming, f o r i n s t a n c e , i r r e g u l a r shape of t h e p a r t i c l e ,
temperature
g r a d i e n t s i n s i d e and o u t s i d e t h e p a r t i c l e , v a r i o u s r e a c t i o n o r d e r s , p a r t i c l e c o n t r a c t i o n or expansion d u r i n g r e d u c t i o n , v a r i o u s a d s o r p t i o n mechanisms of t h e reducing gas, more t h a n one reducing a g e n t , i n f l u ence of p a r t i c l e s i z e d i s t r i b u t i o n , tion,
formation of c r a c k s d u r i n g reduc-
etc.
The i n c r e a s i n g complexity of t h e d a t a t r e a t m e n t t y p i c a l o f t h e s e more s o p h i s t i c a t e d models may sometimes discourage t h e c a t a l y s i s r e s e a r c h e r ; furthermore, he i s aware t h a t t h e c o n d i t i o n s f o r t h e v a l i d i t y of t h e v a r i o u s t h e o r e t i c a l models a r e o f t e n very f a r from
332 t h e a c t u a l r e d u c t i o n procedures used f o r t h e i n d u s t r i a l c a t a l y s t s ( f o r i n s t a n c e , t h e f r e q u e n t l y o c c u r r i n g r e t a r d i n g e f f e c t of water
i s n o t t a k e n i n t o account). However, t h i s seem t o be t h e main road a l o n g which one can hope t o reach,
soon or l a t e r , a comprehensive
p i c t u r e of t h e complex phenomena o c c u r r i n g d u r i n g t h e r e d u c t i o n o f i n d u s t r i a l c a t a l y s t s , w i t h consequent b e n e f i t f o r t h e r a t i o n a l o p t i m i z a t i o n of t h i s process.
4 . 2 Temperature programmed r e d u c t i o n (TPR) This t e c h n i q u e has been r e c e n t l y developped by S h e l l r e s e a r c h e r s (ref.
8 ) . The p r i n c i p l e , on which TPR is based, can be summarized
as follows. I f a hydrogen-inert u s u a l l y about
5%)
gas (except helium) mixture ( H
2
i s allowed t o pass, s u c c e s s i v e l y , over t h e f i r s t
d e t e c t o r of a thermal c o n d u c t i v i t y c e l l , through t h e c a t a l y s t sample placed i n a f u r n a c e connected t o a l i n e a r t e m p e r a t u r e programmer, through a n a b s o r p t i o n t r a p a b l e t o c o l l e c t any gaseous product of c a t a l y s t r e d u c t i o n , f i n a l l y over t h e second d e t e c t o r of t h e T.C.
c e l l , and t h e sample temperature is l i n e a r l y i n c r e a s e d , when hydrogen
w i l l be consumed owing t o c a t a l y s t r e d u c t i o n , t h e T.C.
c e l l w i l l be
unbalanced and a s i g n a l , p r o p o r t i o n a l t o t h e d i f f e r e n c e of hydrogen c o n c e n t r a t i o n between i n l e t and o u t l e t of t h e sample, w i l l be r e c e i v e d
by t h e r e c o r d e r . I f t h e gas flow is c o n s t a n t , s u c h d i f f e r e n c e i s p r o p o r t i o n a l t o t h e r a t e of hydrogen consumption, namely t o t h e r a t e of c a t a l y s t reduction. I n p r a c t i c e , t h e TPR p r o f i l e i s q u i t e s i m i l a r t o t h a t a r i s i n g
f r o m DTG,
but with t h e g r e a t advantages t h a t t h e e f f e c t of any o t h e r
r e a c t i o n simultaneous w i t h r e d u c t i o n i s e l i m i n a t e d and t h a t , a s t h e r e d u c i n g gas p a s s e s through t h e sample, not over, l i k e i n thermogravimetry, t h e e l i m i n a t i o n of e x t e r n a l d i f f u s i o n e f f e c t s i s much e a s i e r , Moreover t h e s e n s i t i v i t y of TPR measurements is o f t e n h i g h e r t h a n t h a t of thermogravimetric measurements and, l a s t but not l e a s t ,
the
r e q u i r e d equipment i s r e l a t i v e l y inexpensive. On t h e o t h e r hand, some d i f f i c u l t i e s may a r i s e i f a c c u r a t e quant i t a t i v e measurements a r e needed, a s t h e c a l i b r a t i o n procedures a r e
s t i l l open t o o b j e c t i o n s ( a c c o r d i n g t o S h e l l r e s e a r c h e r s an accuracy of
2
10% can be reached). Moreover, l i k e thermogravimetry, r u n s a t
333 h i g h p r e s s u r e s and h i g h w a t e r c o n c e n t r a t i o n s a r e h a r d l y f e a s i b l e .
A s TPR i s a r e l a t i v e l y r e c e n t t e c h n i q u e , t h e i n f l u e n c e o f t h e v a r i o u s o p e r a t i n g v a r i a b l e s on t h e TPR d a t a i s s t i l l t o be i n v e s t i g a t e d . When a d e e p e r knowledge of t h e c a p a b i l i t i e s of t h i s t e c h n i q u e
w i l l be a v a i l a b l e , it w i l l s u r e l y r e p r e s e n t a powerful t o o l t o obt a i n i n f o r m a t i o n on c a t a l y s t r e d u c t i o n mechanisms.
4 . 3 Other t e c h n i q u e s While a t p r e s e n t thermogravimetry and t e m p e r a t u r e programmed red u c t i o n a r e undoubtedly t h e most w i d e l y used t e c h n i q u e s i n r e s e a r c h on c a t a l y s t r e d u c t i o n , i n s p e c i a l c a s e s t h e t e c h n i q u e s h e r e a f t e r mentioned may f i n d a p p l i c a t i o n . D i f f e r e n t i a l t h e r m a l a n a l y s i s (DTA). T h i s well-known t e c h n i q u e may g i v e q u a l i t a t i v e i n f o r m a t i o n i n a d d i t i o n t o t h e t h e r m o g r a v i m e t r i c d a t a , i f one c a n t a k e advantage of t h e modern i n s t r u m e n t s s i m u l t a n e o u s l y m o n i t o r i n g b o t h t h e r m o g r a v i m e t r i c and DTA c u r v e s (however, when p r e c i s e DTA d a t a ar'e r e q u i r e d , t h e use o f a n i n d i p e n d e n t DTA equipment i s recommended).
I n f a c t , when complex s i m u l t a n e o u s phe-
nomena o c c w d u r i n g c a t a l y s t r e d u c t i o n , t h e comparison o f thermograv i m e t r i c and DTA c u r v e s may h e l p t o g i v e a n i n s i g h t i n t o t h e whole process
.
DTA c o u l d be u s e f u l a l s o t o d e t e c t t h e t o t a l t h e r m a l e f f e c t of t h e r e d u c t i o n p r o c e s s i n i n d u s t r i a l c o n d i t i o n s , however, a s i n t h e c a s e o f thermogravimetry,
o n l y a p p r o p r i a t e l y b u i l t equipment can
withstand high pressures. Magnetic measurements. When t h e non reduced and/or t h e r e d u c e d phases i n c l u d e f e r r o m a g n e t i c or f e r r i m a g n e t i c m a t e r i a l s (Fe, C o , Ni,
Fe 0
3 4'
etc.),
one c a n t a k e a d v a n t a g e o f t h e p o s s i b i l i t y o f mea-
s u r i n g t h e s a t u r a t i o n m a g n e t i z a t i o n a s a f u n c t i o n of t i m e t o f o l l o w t h e r e d u c t i o n p r o c e s s w i t h h i g h s e n s i t i v i t y . However t h i s t e c h n i q u e has been s c a r c e l y used. Mdssbauer s p e c t r o s c o p y .
The u s e of t h i s powerful t e c h n i q u e i n
c a t a l y s t reduction is p r a c t i c a l l y confined t o iron-containing lysts.
cata-
The main a d v a n t a g e o f Mdssbauer s p e c t r o s c o p y w i t h r e s p e c t t o
thermogravimetry and TPR is t h a t it a l l o w s one t o d i s t i n g u i s h s i m u l -
334 t a n e o u s r e d u c t i o n p r o c e s s e s due t o d i f f e r e n t o x i d e s . On t h e o t h e r hand, a s t h e r e c o r d i n g of each MBssbauer s p e c t r u m t a k e s some h o u r s ,
i t i s n e c e s s a r y t o c o o l t h e sample f o r t h e measurement, s o t h a t t h e reduction process i s discontinuous.
5.
REDUCTION MECHANISMS OF SOME I N D U S T R I A L CATALYSTS The t i t l e o f t h i s c h a p t e r i s p r o b a b l y t o o a m b i t i o u s , a s , owing t o
t h e p r e v i o u s l y mentioned i n a d e q u a t e knowledge on c a t a l y s t r e d u c t i o n c a r r i e d o u t i n i n d u s t r i a l c o n d i t i o n s , o n l y t h e i n f o r m a t i o n coming from b a s i c r e s e a r c h , c a r r i e d o u t v e r y o f t e n on s i n g l e c a t a l y s t compon e n t s and i n e a s y e x p e r i m e n t a l c o n d i t i o n s ,
is a v a i l a b l e a t present.
Y e t some i n t e r e s t i n g c o n c l u s i o n s can be drawn from s u c h b a s i c knowledge. I n a d d i t i o n , w e w i l l r e p o r t h e r e a f t e r t h e r e s u l t s o f r e d u c t i o n c h a r a c t e r i z a t i o n of some i n d u s t r i a l c a t a l y s t s .
5.1 High t e m p e r a t u r e CO c o n v e r s i o n c a t a l y s t A s w e have p r e v i o u s l y s e e n i n Chapter 3.1,
t h e a c t i v a t i o n of t h i s
c a t a l y s t i n v o l v e s mainly t h e r e d u c t i o n s of j - F e Fe 0
3 4'
0
2 3
and 0(-Fe203 t o
The mechanisms o f t h e s e r e d u c t i o n s have been s t u d i e d i n Mon-
t e d i s o n some y e a r s ago ( r e f . 9 ) . I s o t h e r m a l t h e r m o g r a v i m e t r i c r u n s
w e r e performed i n t h e t e m p e r a t u r e r a n g e 2 5 O - 3 5 O 0 C i n p u r e H
2
and
H -He
m i x t u r e s . S a m p l e w e r e i n t h e form o f f i n e powders, so t h a t t h e 2 i n f l u e n c e of a s h d i f f u s i o n was minimized ( a c c o r d i n g t o r e f . 10 it b e g i n s above 8000
8
p a r t i c l e s i z e ) . Q u a l i t a t i v e and q u a n t i t a t i v e
a n a l y s e s o f t h e c r y s t a l phases i n samples t a k e n a t v a r i o u s d e g r e e s of r e d u c t i o n w e r e c a r r i e d o u t by X-ray
diffraction.
A s concerns t h e r e d u c t i o n of b(-Fe203, it o c c u r s t h r o u g h t h e s o l i d 1
s o l u t i o n Fe
0 (OQK<-) w i t h o u t phase change. The k i n e t i c e q u a t i o n 3-x 4 3 i s o f t h e t y p e l n ( 1 - d ) = - K t , w i t h a n a c t i v a t i o n energy o f 29 Kcal/mole. Two s i m u l t a n e o u s r e d u c t i o n mechanisms a r e i n v o l v e d :
a ) Fez+ i o n s formed on t h e s u r f a c e m i g r a t e t h r o u g h t h e l a t t i c e t o occupy v a c a n t o c t a h e d r a l s i t e s ; b) e l e c t r o n t r a s f e r from Fe2+ i o n s on t h e s u r f a c e t o Fe3+ i o n s i n t h e bulk, t h e r e f o r e r e g e n e r a t i n g F e allowing t h e reduction t o continue.
3+
i o n s on t h e s u r f a c e and
335
More i m p o r t a n t f o r o u r purposes i s t h e r e d u c t i o n of d-Fe 0 which 2 3’ i s u s u a l l y t h e main component of t h e h i g h t e m p e r a t u r e CO c o n v e r s i o n catalysts.
It o c c u r s w i t h d i f f e r e n t mechanisms, depending on t h e
amount of l a t t i c e d i s o r d e r p r e s e n t ‘ i n t h e sample. I n h i g h l y - d i s o r d e r e d samples 6-Fe203 i s f i r s t l y completely r e d u c e d t o Fe 0
3 4’
l a t e r r e d u c e d t o Fe.
which is
For o r d e r e d samples ( n a t u r a l or t r e a t e d a t h i g h
t e m p e r a t u r e ) t h e r e d u c t i o n s of O(-Fe 0
2 3
t o Fe 0
3 4
and o f F e 0
3 4
t o Fe
s i m u l t a n e o u s l y o c c u r . I n both c a s e s a t t h e b e g i n n i n g of t h e r e d u c t i o n an o u t e r s h e l l o f Fe 0
3 4
i s formed, w i t h a d e g r e e of d i s o r d e r s i m i l a r
t o t h a t of t h e p a r e n t d-Fe 0 phase. AS t h e r e d u c t i o n o f O(-Fe 0 2 3 2 3 proceeds t h r o u g h t h e m i g r a t i o n o f F e z + i o n s and e l e c t r o n s from t h e o u t e r s u r f a c e t o t h e Fe 0 -Fe 0
2 3
3 4
i n t e r f a c e and a s t h i s m i g r a t i o n i s
e a s i e r i n t h e c a s e of d i s o r d e r e d l a t t i c e , i n w e l l - o r d e r e d samples t h e r e d u c t i o n o f t h e o u t e r s p i n e l phase b e g i n s when a l a r g e amount of (-Fe
0
2 3
i s s t i l l t o be reduced. I n t h e c a s e of t h e h i g h t e m p e r a t u r e
CO c o n v e r s i o n c a t a l y s t s t h e l a t t i c e d i s o r d e r i s o b v i o u s l y h i g h ( t h e 2 s u r f a c e a r e a i s of t h e o r d e r o f 100 m / g ) , s o t h a t t h e r i s k of F e
f o r m a t i o n i s minimized. I n f o r m a t i o n a b o u t t h e i n f l u e n c e o f w a t e r vapour on t h e r e d u c t i o n o f D(-Fe203 t o F e 0
3 4’
though v e r y i m p o r t a n t f o r i n d u s t r i a l r e d u c t i o n s ,
i s s c a n t y . According t o a r e c e n t paper ( r e f .
l l ) , t h e reduction r a t e
i s lowered by w a t e r vapour f o r Mg- and Li-doped O(-Fe 0 p u r e O(-Fe 0
2 3
while f o r 2 3, it i s f i r s t l y d e c r e a s e d , t h e n i n c r e a s e d . On t h i s sub-
j e c t r e s e a r c h work c o n c e r n i n g t h e h i g h t e m p e r a t u r e CO c o n v e r s i o n
c a t a l y s t i s c l e a r l y needed.
Now w e w i l l d i s c u s s some r e s u l t s o b t a i n e d d u r i n g o u r i n v e s t i g a t i o n s on t h e r e d u c t i o n b e h a v i o r of t h e Montedison MHTC c a t a l y s t .
I n Table
4 w e r e p o r t some TPR d a t a c o n c e r n i n g l a b o r a t o r y p r e p a r a t i o n s of t h e h i g h t e m p e r a t u r e CO c o n v e r s i o n c a t a l y s t , a s f u n c t i o n o f b o t h C r 0 2 3 c o n c e n t r a t i o n and method o f p r e p a r a t i o n . The method of p r e p a r a t i o n of a l l t h e samples r e p o r t e d i n Table
4 i s based on p r e c i p i t a t i o n from FeSO w h i l e chromium i s added a s CrV1. be drawn from T a b l e
4
4
and Na2C03 aqueous s o l u t i o n s ,
The f o l l o w i n g main c o n c l u s i o n s can
(samples A-B-C):
336 TABLE 4
TPR d a t a of h i g h t e m p e r a t u r e CO conversion c a t a l y s t s ( l a b o r a t o r y preparations).
-
A
-
7.5 7.5 7.5
B C D
264 270
-
195 196 197
335 338 3 25
221
350
342 374 373 43 3
The d e f i n i t i o n s of Ts and Tm have been given i n c h a p t e r 4.1 a ) t h e presence o f C r i n t h e c a t a l y s t has p r a c t i c a l l y no i n f l u e n c e on t h e r e d u c t i o n of d-Fe203 t o F e 0
3 4'
Fe 0
3 4
w h i l e t h e r e d u c t i o n of
t o F e is s l i g h t l y hindered (samples A and B have been pre-
pared by t h e same method); b) a s t h e s h o u l d e r on t h e d-FeZ03 r e d u c t i o n peak i s p r e s e n t only i n Cr-containing samples, it has been a t t r i b u t e d t o t h e r e d u c t i o n o f CrV1 t o C r l I 1 t h a t gives a C r
( t h i s has been confirmed by chemical a n a l y s i s , VI
i n samples B and C ) .
c o n c e n t r a t i o n of 0.5-1$
Moreover, t h e d a t a o b t a i n e d f o r sample D show
t h a t it i s p o s s i b l e ,
through s p e c i a l methods of c a t a l y s t p r e p a r a t i o n , n o t o n l y t o e l i m i n a t e t h e harmful presence of CrV1, but a l s o t o o b t a i n such a good I11 d i s t r i b u t i o n of C r i n t h e Fe o x i d e s a s t o d e l a y both 4-Fe 0 and
Fe 0
3 4
r e d u c t i o n s , t h e l a t t e r by about 9OoC.
b e t t e r d i s t r i b u t i o n of C r l I 1
2 3 It i s expected t h a t a
i n t h e c a t a l y s t should appreciably i m -
prove i t s l i f e . Some d a t a concerning t h e r e d u c t i o n behavior of t h e Montedison i n d u s t r i a l c a t a l y s t MHTC a r e r e p o r t e d i n Table
5.
The good agreement of TPR and TG d a t a is f i r s t l y t o be n o t i c e d (comp a r e Tm O(-Fe203 red. b u t i o n of t h e O(-FeOOH
and Ts F e 0
3 4
red. of r u n s 1 and 2 ) .
The a t t r i -
decomposition peak comes immediately from
comparison o f r u n 1 w i t h r u n 2 and of r u n s 2-3 w i t h r u n 4. t e r m i n a t i o n of Ts O(-Fe 0
2 3
red.
o v e r l a p p i n g with t h e 4 - F e O O H of Ts 4-Fe 0 red. 2 3
The de-
i n TG runs was not p o s s i b l e owing t o
decomposition peak ( s e e t h e TPR v a l u e
and t h e TG v a l u e s of T
m
4-FeOOH dec.).
The i n -
337 TABLE
5
Reduction d a t a of t h e Montedison c a t a l y s t MHTC ( s i z e 0.2-0.3 Run
Technique
1 2
TPR TG TG TG
3 4
Gas
d-FeOOH dec. O(-Fe203 r e d .
H 2 5% N2 HZ 5% N2 H2 H2
95% 95%
195 125 125 130
235 225 225
325 330 290
mm)
Fe 0
3 4
red.
370 380
640
315
385
The r a t e of t e m p e r a t u r e i n c r e a s e was Z°C/min f o r a l l t h e r u n s . The d e f i n i t i o n s o f T- and T have been g i v e n i n c h a p t e r 4.1. B m c o n c e n t r a t i o n on t h e r e d u c t i o n o f d-Fe 0 t o Fe 0 2 3 3 4 and e s p e c i a l l y on t h e r e d u c t i o n o f Fe 0 t o Fe is q u i t e e v i d e n t f l u e n c e of H
2
3 4
( t h e same phenomenon was r e p o r t e d i n r e f . 9 ) . From t h e r e l a t i v e l y low v a l u e s o f Ts Fe 0
3 4
r e d . it is c l e a r t h e need o f e x c e s s w a t e r
vapour d u r i n g i n d u s t r i a l r e d u c t i o n s , where c a t a l y s t t e m p e r a t u r e may w e l l exceed 400°C. F i n a l l y , w e w i l l n o t i c e t h e s t r i c t l y s i m i l a r reduction behavior of l a b o r a t o r y and i n d u s t r i a l h i g h t e m p e r a t u r e CO c o n v e r s i o n c a t a l y s t s , when p r e p a r e d by same method (compare sample B of T a b l e 4 w i t h r u n 1 of Table
5 ) . The o n l y d i f f e r e n c e i s t h a t no s h o u l d e r
due t o t h e p r e s e n c e of CrV1 was d e t e c t e d i n t h e i n d u s t r i a l c a t a l y s t .
5.2 Low t e m p e r a t u r e CO c o n v e r s i o n c a t a l y s t As it was shown i n c h a p t e r 3.2,
t h e a c t i v a t i o n of t h i s c a t a l y s t
i n v o l v e s t h e r e d u c t i o n o f CuO t o Cu. CuO and ZnO a r e p r a c t i c a l l y t h e o n l y c r y s t a l l i n e phases d e t e c t e d i n t h e commercial Cu-Zn-A1 oxide catalys ts.
However,
i n some o f them t h e p r e s e n c e of s m a l l
amounts of t h e f o l l o w i n g c r y s t a l l i n e compounds c a n be d e t e c t e d by X-ray d i f f r a c t i o n :
d -alumina,
z i n c o r copper a l u m i n a t e , b a s i c z i n c
c a r b o n a t e , b a s i c copper c a r b o n a t e ( t h e p r e s e n c e of t h e two l a s t compounds i s c l e a r l y due t o low c a l c i n a t i o n t e m p e r a t u r e ) . The r e d u c t i o n d e g r e e of ZnO a t t e m p e r a t u r e s below 2 3 0 ° C s h o u l d be n e g l i g i b l e ; i n f a c t , t h e f o r m a t i o n of b r a s s r e p o r t e d i n r e f . o c c u r s i n a p p r e c i a b l e amount a t t e m p e r a t u r e s h i g h e r t h a n
3
338
2 5 O 0 C , w h i l e a t lower t e m p e r a t u r e s it r e q u i r e s a v e r y l o n g t i m e . Therefore during c a t a l y s t a c t i v a t i o n , unless t h e c a t a l y s t is incaut i o u s l y o v e r h e a t e d , t h e r e d u c t i o n o f ZnO can be n e g l e c t e d . The r e d u c t i o n of p u r e CuO has been s t u d i e d by many Authors;
Bond
( r e f . 29) and Schoepp ( r e f . 3 0 ) s e e m t o have performed t h e most comprehensive thermogravimetric i n v e s t i g a t i o n s .
The method o f p r e p a r a -
t i o n was found t o have a s t r o n g i n f l u e n c e on t h e r e d u c t i o n behavior. The r e d u c t i o n k i n e t i c s w e r e s a t i s f a c t o r i l y i n t e r p r e t e d i n t e r m s of both i n s t a n t a n e o u s and c o n s t a n t - r a t e n u c l e a t i o n s ( r e f . 31). Though t h e b a s i c knowledge c o n c e r n i n g t h e r e d u c t i o n of p u r e CuO may be a n u s e f u l s t a r t i n g - p o i n t ,
w e c l e a r l y need i n f o r m a t i o n about
t h e r e d u c t i o n o f CuO d i s p e r s e d o v e r a ZnO-A1 0 mixed c a r r i e r . T h i s 2 3 k i n d o f i n f o r m a t i o n i s a l m o s t c o m p l e t e l y l a c k i n g . W e a r e aware o n l y o f a B u l g a r i a n DTA s t u d y ( r e f . 3 2 ) on t h e r e d u c t i o n o f two commercial c a t a l y s t s n o t c o n t a i n i n g alumina (CuO-ZnO and CuO-ZnO-Cr The s t a r t i n g r e d u c t i o n t e m p e r a t u r e s w e r e ,
respectively,
0 )
2 3
.
130°C and
150°C w i t h hydrogen c o n c e n t r a t i o n 2.6% and h e a t i n g r a t e 60C/min. The CuO r e d u c t i o n was found t o o c c u r i n two s t a g e s : f i r s t l y t o Cu 0, 2 t h e n t o Cu. However, from o u r TG and TPR e x p e r i m e n t s we n e v e r obt a i n e d any e v i d e n c e o f a two-stages
reduction process.
W e a r e i n v e s t i g a t i n g t h e i n f l u e n c e of various f a c t o r s concerning
c a t a l y s t composition and p r e p a r a t i o n p r o c e d u r e on t h e r e d u c t i o n beh a v i o r . Some p r e l i m i n a r y r e s u l t s a r e r e p o r t e d i n Table 8.
TABLE 8 TPR d a t a o f p u r e and s u p p o r t e d CuO c a t a l y s t s . Sample S LTC 4 LTC 6 LTC 1
Chemical composition CUO CUO CuO-ZnO Cu0-Zn0-A1 0 2 3
The d e f i n i t i o n s of T
Ts( " C ) 180 110 140 157
Tm("C) 2.50
198 200 20 2
and Tm have been g i v e n i n c h a p t e r 4.1
While sample S h a s been p r e p a r e d by t h e r m a l decomposition of C U ~ ( O H ) ~ C O t~h, e p r e p a r a t i o n of sample LTC
4 h a s been performed
339
w i t h t h e p r o c e d u r e u s e d f o r t h e Montedison c a t a l y s t .
It may be n o t i -
c e d t h a t t h e s t r o n g i n f l u e n c e of t h e p r e p a r a t i o n method on t h e reduct i o n b e h a v i o r o f CuO, a l r e a d y r e p o r t e d by Bond ( r e f . 2 9 ) , confirmed.
has been
By comparison of t h e TPR d a t a of t h e samples o f t h e LTC
s e r i e s , a l l p r e p a r e d w i t h t h e same p r o c e d u r e , it i s p o s s i b l e t o conc l u d e t h a t t h e r e t a r d i n g e f f e c t of ZnO a d d i t i o n concerns o n l y t h e n u c l e a t i o n of m e t a l l i c Cu. A f u r t h e r r e t a r d i n g e f f e c t on Cu nuclea t i o n is g i v e n by t h e a d d i t i o n of A 1 0 2 3' The r e d u c t i o n b e h a v i o r of i n d u s t r i a l c a t a l y s t s i s a l s o under i n v es tigation i n our laboratories.
Some p r e l i m i n a r y r e s u l t s a r e re-
p o r t e d i n Table 9.
TABLE 9 TPR d a t a of some low t e m p e r a t u r e CO c o n v e r s i o n c a t a l y s t s . Catalyst
218
Mont e d i s o n
A
177 158 195
B C The d e f i n i t i o n s o f Ts and T
m
have been g i v e n i n c h a p t e r 4.1.
A p p r e c i a b l e d i f f e r e n c e s e x i s t among t h e v a l u e s of T
and T of t h e m a s t h e p r e p a r a t i o n method of c a t a S
examined c a t a l y s t s . l y s t s A,
Obviously,
B and C is n o t known, w e c a n n o t draw any c o n c l u s i o n from
t h e above-reported
d a t a , which o n l y s u g g e s t t h e need o f f u r t h e r
s t u d i e s . However, w e were s u c c e s s f u l i n p r e p a r i n g a c a t a l y s t of lower r e d u c i b i l i t y ;
w e t h i n k t h a t t h i s f e a t u r e s h o u l d make e a s i e r
t h e temperature c o n t r o l during i n d u s t r i a l reduction.
5 . 3 Ammonia s y n t h e s i s c a t a l y s t A s w e have s e e n i n c h a p t e r 3 . 3 ,
t h e a c t i v a t i o n of t h i s c a t a l y s t
o c c u r s by t h e endothermic r e d u c t i o n of F e 0
3 4
t o Fe, which p r o c e e d s
w i t h o u t i n t e r m e d i a t e p h a s e s (FeO is n o t s t a b l e below 570°C).
A s t h e c a t a l y s t (promoted f u s e d m a g n e t i t e ) i s p r a c t i c a l l y nonporous, t h e u n r e a c t e d s h r i n k i n g c o r e model c o u l d be used t o h a n d l e t h e isothermal thermogravimetric data. Unfortunately t h e c a t a l y s t
340 p a r t i c l e shape i s very i r r e g u l a r , while t h e model i s s t r i c t l y v a l i d only f o r s p h e r i c a l p a r t i c l e s of uniform s i z e ; a mathematical t r e a t ment f o r v a r i o u s s p h e r i c 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 s i s however a v a i l a b l e ( r e f . 12). The c l a s s i c a l u n r e a c t e d s h r i n k i n g c o r e model has been a p p l i e d t o t h e r e d u c t i o n of a commercial ammonia s y n t h e s i s c a t a l y s t by some Polish researchers ( r e f . used.
13). Cube-shaped c a t a l y s t s p a r t i c l e s w e r e
It was found t h a t t h e r e d u c t i o n process proceeds i n t h e i n t e r -
mediate r e g i o n between chemical r e a c t i o n and a s h d i f f u s i o n c o n t r o l s . However, t h e temperature range i n v e s t i g a t e d (450-550"C)
is a t t h e
upper l i m i t of t h e u s u a l r e d u c t i o n temperatures of t h e ammonia synt h e s i s c a t a l y s t s (some of them begin t o reduce below 35OOC). The same P o l i s h group ( r e f . 14) developped a mathematical model f o r t h e c o n s t a n t - r a t e r e d u c t i o n performed a t temperatures slowly i n c r e a s i n g . Though t h e s t a r t i n g temperature chosen is r a t h e r high ( 4 O O O C ) and t h e t o t a l r e d u c t i o n time t o o s h o r t ( 1 5 h o u r s ) , and though t h e i n t e r a c t i o n s among neighbouring p a r t i c l e s w e r e not considered, t h i s work r e p r e s e n t s t h e f i r s t a t t e m p t towards t h e mathematical modeling of c a t a l y s t r e d u c t i o n i n i n d u s t r i a l conditions. However, it is our o p i n i o n t h a t t h e b a s i c knowledge of t h e reduct i o n mechanisms of promoted magnetites is s t i l l t o o incomplete t o allow a sound development of t h e a p p l i e d r e s e a r c h . With t h i s concept i n mind, we have c a r r i e d o u t some i n v e s t i g a t i o n s with t h e aim of c l a r i f y i n g t h e b a s i c phenomena o c c u r r i n g d u r i n g magnetite r e d u c t i o n . The problem was a t t a c k e d through a n i s o t h e r m a l thermogravimetric study ( r e f . 15) of pure magnetite r e d u c t i o n i n t h e low-temperature range (220-330°C)
t h a t s h o u l d allow both t o minimize t h e i n f l u e n c e
of a s h d i f f u s i o n and t o work w i t h t h e low water c o n c e n t r a t i o n s req u i r e d i n i n d u s t r i a l r e d u c t i o n s . The experiments were c a r r i e d o u t by means of a s e n s i t i v e e l e c t r o b a l a n c e ; p a r t i c l e s i z e i n t h e range 0.5-3bm
powdered magnetite with
was used ( a c c o r d i n g t o r e f .
16
t h e i n f l u e n c e of a s h d i f f u s i o n should be n e g l i g i b l e with p a r t i c l e s i z e below 200Am). It was found t h a t t h e r e d u c t i o n r a t e decreases with t h e i n c r e a s i n g of sample mass i n t h e range 0-30 mg; it a l s o decreases when
341 w a t e r vapour (1500 vpm) i s added t o t h e r e d u c i n g gas ( p u r e hydrogen). T h i s b e h a v i o r c a n be e x p l a i n e d i n t e r m s o f i n t e r p a r t i c l e d i f f u s i o n phenomena.
I n f a c t , a s i n TG e x p e r i m e n t a l c o n d i t i o n s t h e r e i s no
hydrogen flow t h r o u g h t h e powder,
a water concentration g r a d i e n t
may e a s i l y a r i s e between t h e i n t e r i o r of t h e sample and t h e hydrogen stream.
I n o t h e r words, t h e m a g n e t i t e heap t e n d s t o r e a c t a s a whole T h i s i s confirmed
g r a i n from t h e o u t e r l a y e r s towards t h e i n t e r i o r .
by t h e p r e s e n c e o f many u n r e a c t e d p a r t i c l e s i n samples r e d u c e d up t o 6 5 % . However, t h i s i n t e r p a r t i c l e d i f f u s i o n s h o u l d be c o n s i d e r e d more p r o p e r l y , a s s u g g e s t e d i n r e f .
17, a s u r f a c e d i f f u s i o n consi-
s t i n g , owing t o t h e h i g h r e a c t i v i t y of f i n e l y d i v i d e d i r o n towards oxygenated compounds, o f r e p e a t e d o x i d a t i o n by w a t e r - r e d u c t i o n by hydrogen s t e p s . T h i s is a l s o i n accordance w i t h t h e r e l a t i v e l y h i g h v a l u e o f t h e a c t i v a t i o n energy ( 1 4 . 5 k c a l / m o l e ) .
It may be o b s e r v e d
t h a t t h e s i t u a t i o n o c c u r r i n g i n t h e s e e x p e r i m e n t s ( i n t e r a c t i o n of r e d u c e d p a r t i c l e s w i t h w a t e r vapour coming from t h e n e i g h b o u r s ) reminds what o c c u r s , on a l a r g e r s c a l e , d u r i n g r e d u c t i o n i n indus t r i a l c o n d i t i o n s , where, however, i n t o account.
a l s o a s h d i f f u s i o n i s t o be t a k e n
U n f o r t u n a t e l y no mathematical model i s a v a i l a b l e , a s
y e t , t o d e s c r i b e s u c h a complex behavior.
It was a l s o found t h a t a n i n d u c t i o n p e r i o d i s always p r e s e n t i f d r y hydrogen is u s e d and t h a t it d i s a p p e a r s i f w a t e r vapour is added.
AS mixing o f m a g n e t i t e w i t h P t b l a c k h a s no i n f l u e n c e on t h e induct i o n period, t h e rate-determining s t e p i n t h e i n i t i a l s t a g e of t h e r e d u c t i o n s h o u l d n o t be t h e f o r m a t i o n o f m e t a l l i c germs,
but r a t h e r
t h e h y d r o x y l a t i o n of t h e o x i d e s u r f a c e . S u r f a c e hydroxyls s h o u l d i n d e e d make e a s i e r t h e m i g r a t i o n of d i s s o c i a t e d hydrogen ( r e f .
18).
The s u b s t i t u t i o n o f Fe3+ w i t h A13+ i o n s i n m a g n e t i t e d e c r e a s e s t h e r e d u c t i o n r a t e and makes t h e i n d u c t i o n p e r i o d t o d i s a p p e a r ( r e f . 1 9 ) . The l a t t e r f a c t c o u l d be r e l a t e d t o a h i g h e r amount o f h y d r o x y l s connected t o Al, t h e former t o t h e f o r m a t i o n , d u r i n g red u c t i o n , o f FeAl 0 groups, u n r e d u c i b l e i n o u r e x p e r i m e n t a l condi2 4 t i o n s , t h a t c o u l d d e c r e a s e t h e advancing o f t h e r e a c t i n g i n t e r f a c e . I n f a c t , it was d e m o n s t r a t e d t h a t FeAl 0
2 4
groups i n c l u d e d i n t h e
l a t t i c e o f ? ( - F e o c c u r i n t h e r e d u c e d samples ( r e f .
20).
342 W e have a l s o c a r r i e d o u t i s o t h e r m a l t h e r m o g r a v i m e t r i c r u n s t o
measure t h e r e d u c t i o n r a t e of some commercial c a t a l y s t s on samples of d i f f e r e n t p a r t i c l e s i z e
(1-5fim
and 0.3-0.4
mm).
The i n f l u e n c e
o f sample mass on t h e r e d u c t i o n r a t e was much lower i n t h e c a s e o f
0.3-0.4
It means t h a t for t h e s e samples i n t e r p a r t i c l e
mm samples.
d i f f u s i o n i s n e g l i g i b l e w i t h r e s p e c t t o t h e i n t r a p a r t i c l e one.
The
i n d u c t i o n p e r i o d , l i k e i n t h e A l - c o n t a i n i n g m a g n e t i t e s , is n o t pres e n t . Values of t h e r e d u c t i o n r a t e K
0.5
( s l o p e of t h e r e d u c t i o n CUT
v e a t 50% c o n v e r s i o n ) of some commercial c a t a l y s t s ( p a r t i c l e s i z e
0.3-0.4
mm) a r e r e p o r t e d i n Table 6.
TABLE 6 Reduction r a t e of some commercial c a t a l y s t s f o r ammonia s y n t h e s i s . Cat a 1ys t
K
0.5
Ms FN A B C
-103 (min-l)
Experimental c o n d i t i o n s
1.63 0.68 1.40 3.15
Temperature Sample mass Reducing gas
345°C 20 mg pure H
2
It may be s e e n t h a t a p p r e c i a b l e d i f f e r e n c e s e x i s t among t h e examined c a t a l y s t s , though t h e chemical composition i s p r a c t i c a l l y t h e same. T h i s f a c t shows t h e importance of t h e p r e p a r a t i o n method, whose d e t a i l s o b v i o u s l y a r e n o t d i s c l o s e d by m a n u f a c t u r e r s , w i t h r e s p e c t t o t h e r e d u c t i o n p r o c e s s . From a p r a c t i c a l p o i n t o f view, a v e r y low r e d u c i b i l i t y makes e a s i e r t h e c o n t r o l of w a t e r c o n c e n t r a t i o n i n t h e bed,
b u t may p r o l o n g t o o much t h e r e d u c t i o n p e r i o d , w h i l e t h e
o p p o s i t e h o l d s i f t h e r e d u c i b i l i t y is v e r y high.
F o r a b e t t e r c h a r a c t e r i z a t i o n of t h e r e d u c t i o n b e h a v i o r of comm e r c i a l c a t a l y s t s f o r ammonia s y n t h e s i s , w e have a l s o c a r r i e d o u t dynamic h e a t i n g t h e r m o g r a v i m e t r i c r e d u c t i o n r u n s .
The r e s u l t s o b t a i n e d
a r e r e p o r t e d i n Table 7. There a r e some c a t a l y s t s (MSFN, F, H ) t h a t b e g i n t o be reduced, i n 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 , a t a b o u t 3 5 0 ° C , w h i l e f o r o t h e r s (D,
E,
G ) t h e s t a r t i n g r e d u c t i o n t e m p e r a t u r e is
about 38OOC. I n t h e DTG p r o f i l e s o f t h e former group a s h o u l d e r cent e r e d a t a b o u t 420°C i s p r e s e n t , which i s c o m p l e t e l y a b s e n t i n t h o s e of t h e l a t t e r . As i n c a t a l y s t s MSFN,
F and H a s m a l l amount of FeO
343 TABLE 7
TG r e d u c t i o n d a t a of some commercial c a t a l y s t s f o r ammonia s y n t h e s i s . Catalyst
MSFN D E F G H
3 45 3 85
590 575
375
570 5 80
345 3 80 355
9.9 12.3
13.8 9.5 9.9 9.8
595 600
Experimental c o n d i t i o n s : Rate o f temperature i n c r e a s e s0C/min Reducing gas pure H 2 The d e f i n i t i o n s of Ts, Tm and R have been given i n c h a p t e r 4.1.
m
is present (estimated
1-5% from X-ray d i f f r a c t i o n powder p a t t e r n )
and a s FeO i s u s u a l l y reduced a t t e m p e r a t u r e s lower t h a n Fe 0
3 4'
lower T
v a l u e s of c a t a l y s t s MSFN,
the
F and H can be explained. This
is confirmed by t h e r e s u l t s o b t a i n e d by o t h e r Authors ( r e f .
21),
who s t u d i e d t h e r e d u c t i o n of FeO-containing ammonia s y n t h e s i s c a t a l y s t s by Mdssbauer spectroscopy.
These Authors found a l s o d i f f e r e n t
r e d u c t i o n mechanisms depending on FeO c o n t e n t I f the R
m
v a l u e s r e p o r t e d i n Table
(4% and 27%).
7 a r e considered, it may be
n o t i c e d t h a t t h e c a t a l y s t s D and E a r e more e a s i l y r e d u c i b l e t h a n t h e o t h e r s ( t h e same h o l d s f o r c a t a l y s t C i n Table 6 ) . W e have found t h a t i n t h e l e s s e a s i l y r e d u c i b l e c a t a l y s t s FeO and CaFe 0
3 5
are
p r e s e n t ( t h e s e two compounds a r e always p r e s e n t t o g e t h e r , a s shown a l s o i n r e f . 2 2 ) , w h i l e i n c a t a l y s t s C,
D and E two u n i d e n t i f i e d
c r y s t a l phases (probably complex f e r r i t e s ) have been d e t e c t e d . The same two phases have been found i n a c a t a l y s t prepared i n o u r labor a t o r y by very f a s t c o o l i n g o f f u s e d promoted magnetite.
This i s a
c l e a r i n d i c a t i o n of t h e i n f l u e n c e t h a t t h e method of p r e p a r a t i o n may have on t h e r e d u c t i o n behavior of ammonia s y n t h e s i s c a t a l y s t s . The f i n a l degree of r e d u c t i o n h a s been r e p o r t e d ( r e f .
23) t o be
a c r u c i a l p o i n t for t h e a c t i v i t y of ammonia s y n t h e s i s c a t a l y s t s . For promoted c a t a l y s t s t h e i r o n s p e c i f i c a c t i v i t y always i n c r e a s e s w i t h t h e r e d u c t i o n degree,
but,
due t o t h e p o s s i b l e o c c u r r i n g of
s i n t e r i n g phenomena, t h e c a t a l y t i c a c t i v i t y may r e a c h a maximum va-
344
l u e , t h e n decrease. For a b e t t e r understanding of t h i s problem, a d i s c u s s i o n of t h e s t r u c t u r e of t h e reduced c a t a l y s t i s c e r t a i n l y useful.
According t o our ( r e f s . 20 and 24) and Hosemann and coworkers
( r e f s . 25-26)
r e s u l t s , i n alumina-containing reduced c a t a l y s t s
FeAl 0 groups a r e e n d o t a c t i c a l l y b u i l t i n t o t h e d,-Fe l a t t i c e (one 2 4 FeA1204 group t a k e s t h e p l a c e of seven Fe atoms), causing paracrys t a l l i n e distortions.
Other Authors ( r e f . 2 7 ) suggest t h a t A1203,
not FeA1204, is p r e s e n t i n samples reduced over 4OOOC.
However, t h e
X-ray d i f f r a c t i o n l i n e s of F e A l 0 w e r e c l e a r l y d e t e c t e d by us a f t e r 2 4 high temperature a n n e a l i n g i n i n e r t atmosphere of samples reduced Moreover, t h e presence of A 1 0 groups was excluded on 2 3 t h e b a s i s of SIMS experiments ( r e f . 2 8 ) . a t sOO°C.
Another debated argument is t h e c o n s t i t u t i o n o f t h e F e A l 0 groups 2 4 ( a r e s i n g l e groups or c l u s t e r s , and i n t h e l a t t e r case what is t h e i r s i z e ? ) . Hosemann has r e c e n t l y demonstrated ( r e f .
2 6 ) t h a t t h e pre-
s e n c e of p a r a c r y s t a l l i n i t y can be e x p l a i n e d only i f no more t h a n two FeAl 0 groups a r e j o i n e d t o g e t h e r . On t h e c o n t r a r y , o t h e r Authors 2 4 ( r e f . 2 7 ) , on t h e b a s i s of Mbssbauer spectroscopy data, s t a t e d t h a t t h e s i z e of t h e i n c l u s i o n s i n t h e d-Fe l a t t i c e , e i t h e r F e A l 0 o r 2 4 0 A1203, should be a t l e a s t 30 A. W e t o o suggested t h e presence of s m a l l c l u s t e r s of FeAl 0 ( r e f . 2 0 ) . However, if according t o 2 4 Hosemann's d a t a ( r e f . 2 6 ) , we assume a n average s i z e of 15 for
w
each (FeA1 0 )
2 4 2
cluster, i n t h e c a s e of our alumina-promoted samples
about 2 5 0 c l u s t e r s a r e included i n each q-Fe c r y s t a l l i t e . I n t h i s circumstance it may be c a l c u l a t e d t h a t about 3% of t h e Fe atoms a r e c o n t a c t i n g t h e ( F e A l 0 ) c l u s t e r s , while t h e remaining 97% is 2 4 2 i n t h e same s t r u c t u r a l c o n f i g u r a t i o n a s i n pure q-Fe. Such a s m a l l percentage could not have a d e t e c t a b l e i n f l u e n c e on both Curie point and Mbssbauer parameters of d-Fe.
Moreover, Hosemann q u e s t i o n s ( r e f .
2 6 ) t h e p a r a l l e l i s m between Fe-A1 a l l o y s and reduced alumina-promot e d F e c a t a l y s t s , on which o t h e r Authors ( r e f .
2 7 ) have based t h e i r
conclusions. I f we t a k e i n t o account t h e s t r u c t u r e of Al-containing magnetites, where t h e A13+
i o n s a r e randomly d i s t r i b u t e d i n t h e s p i -
n e l l a t t i c e , and a l s o t h e r e d u c t i o n mechanism ( s h e l l by s h e l l from t h e e x t e r n a l s u r f a c e towards t h e i n t e r i o r ) , f o r r e d u c t i o n tempera-
345 t u r e s below 500°C it does not s e e m r e a s o n a b l e t o assume t h a t a group l i k e FeAl 0
2 4
can e a s i l y d i f f u s e i n t h e O(-Fe l a t t i c e t o r e a c h o t h e r
groups and form l a r g e c l u s t e r s . This may be another argument i n supp o r t of Hosemann's r e s u l t s . The d i s o r d e r i n t r o d u c e d i n t h e alumina-cointaining
catalysts
through t h e i r r e d u c t i o n has a l s o some r e l e v a n c y w i t h t h e i r c a t a l y t i c activity.
I n f a c t , w e have found ( r e f .
24) t h a t t h e s p e c i f i c a c t i v i t y
of i r o n i n c r e a s e s i n t h e presence of alumina, which is n o t t h e r e f o r e a s t r u c t u r a l promoter only. While t h e alumina-containing
c a t a l y s t s have been s t u d i e d r a t h e r
deeply, t h i s is not t h e c a s e of t h e i n d u s t r i a l c a t a l y s t s , which cont a i n a l s o K,
Ca, Mg and S i oxides.
Clausen e t a l .
( r e f . 21) found
evidence o f some unreduced Fe compound i n t h e i r Mtlssbauer s p e c t r a o f c a t a l y s t s reduced a t 550°C and s u g g e s t e d t h a t it could i n v o l v e Ca. I n f a c t , w e have d e t e c t e d CaFe 0
3 5
catalysts.
i n some reduced commercial
It i s t o be expected t h a t i n t e r e s t i n g i n f o r m a t i o n could
be o b t a i n e d on t h e mechanisms of t h e r e d u c t i o n process and i t s corr e l a t i o n s with c a t a l y t i c a c t i v i t y i f thorough s t u d i e s w e r e accomp l i s h e d on magnetites promoted w i t h t h e v a r i o u s o x i d e s used i n t h e industrial practice. CONCLUSIONS A f t e r having d i s c u s s e d t h e i n d u s t r i a l procedures f o r t h e r e d u c t i o n of some comnercial c a t a l y s t s ( h i g h and low t e m p e r a t u r e CO conversion and ammonia s y n t h e s i s ) , t h e experimental t e c h n i q u e s f o r l a b o r a t o r y s c a l e i n v e s t i g a t i o n s on t h e r e d u c t i o n p r o c e s s and t h e c u r r e n t l y a v a i l a b l e b a s i c knowledge on t h e r e d u c t i o n mechanisms, w e w i l l summarize h e r e t h e p r e s e n t s t a t u s o f t h e problem. The degree of knowledge of t h e r e d u c t i o n mechanisms of t h e abovementioned i n d u s t r i a l c a t a l y s t s should be d e f i n e d i n s u f f i c i e n t . ever,
How-
it i s s l i g h t l y b e t t e r f o r t h e ammonia s y n t h e s i s c a t a l y s t . As
concerns t h e r e d u c t i o n mechanisms of t h e pure main compounds (a(-Fe 0 2 3' CuO, F e 0 ), t h e degree of knowledge i s f a i r l y good, however t h e r e
3 4
i s wide s p a c e f o r improvement. For i n s t a n c e , more i n f o r m a t i o n on t h e i n f l u e n c e of t h e composition o f t h e v a r i o u s reducing gas mixtures
346
used i n t h e i n d u s t r i a l p r a c t i c e on t h e r e d u c t i o n r a t e of t h e s e pure compounds should be obtained. For t h e complete knowledge of t h e r e d u c t i o n behavior o f many comm e r c i a l c a t a l y s t s much i s s t i l l t o be done.
Information i s l a c k i n g
about t h e i n f l u e n c e of t h e o t h e r c a t a l y s t components on t h e reduct i o n of t h e main compound ( o n l y t h e i n f l u e n c e of A 1 0
2 3
t i o n of F e 0
3 4
is f a i r l y known).
on t h e reduc-
A l s o t h e i n f l u e n c e of t h e gaseous
phase composition is n o t s u f f i c i e n t l y known, except f o r t h e e f f e c t of water vapour on t h e r e d u c t i o n of Fe 0
3 4'
W e have remarked t h e experimental d i f f i c u l t i e s t h a t hamper t h e
c o l l e c t i o n of meaningful d a t a about c a t a l y s t r e d u c t i o n . Most o f t h e p u b l i s h e d thermogravimetric d a t a , though r e p r e s e n t i n g t h e main s o u r c e of i n f o r m a t i o n , s h o u l d be c o n s i d e r e d w i t h much c a u t i o n , a s n o t a l l t h e experimental f a c t o r s t h a t may have a s t r o n g i n f l u e n c e on t h e obtained results w e r e previously c l e a r l y
recognized.
Temperature-
programmed r e d u c t i o n seems t o be a very promising technique,
but it
r e q u i r e s deeper s t u d i e s b e f o r e a r e l i a b l e e v a l u a t i o n of i t s capabi-
l i t i e s can be done. Many mathematical models a r e a v a i l a b l e t o handle t h e k i n e t i c d a t a concerning oxide r e d u c t i o n i n g a s - s o l i d systems, but t h e i r l i m i t a t i o n s , due t o t h e need of a v o i d i n g mathematical d i f f i c u l t i e s , a r e s t i l l t o o many. However, some p r o g r e s s i n t h i s f i e l d
i s t o be expected i n t h e n e a r f u t u r e . I n t h e g e n e r a l remarks on t h e previous Symposium on c a t a l y s t prep a r a t i o n t h e e d i t o r s of t h e proceedings n o t i c e d t h e almost complete l a c k of r e p o r t s about c a t a l y s t a c t i v a t i o n .
The p r e s e n t paper prob-
a b l y e x p l a i n s some r e a s o n s of such a d i s a p p o i n t i n g f a c t .
It i s t o
be hoped t h a t , s t a r t i n g w i t h t h i s Symposium, and s t i l l more i n t h e forthcoming ones, c a t a l y s t a c t i v a t i o n w i l l r e c e i v e i n c r e a s i n g a t t e n t i o n i n c a t a l y s i s research.
347
REFERENCES 1 F. T r a i n a and N. Pernicone, C h i m . Ind. ( M i l a n ) , 5 2 ( 1 9 7 0 ) 1 . 2 G. B e r r e b i and P. B e r n u s s e t , i n B. D e l m o n , P.A. Jacobs and G.Poncelet (Eds.), 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 , A m s t e r d a m , 1976, p. 1 3 . 3 T. V a n H e r w i j n e n and W.A. de Jong, J. C a t a l . , 3 4 ( 1 9 7 4 ) 2 0 9 . 4 G . L e h e r t e , R . D e r i e and P.H. D u v i g n e a u d , i n B . D e l m o n , P.A. Jacobs and G . P o n c e l e t ( E d 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 , A m s t e r d a m , 1 9 7 6 , p. 3 0 3 . 5 I. B u z a s ( E d . ) , T h e r m a l A n a l y s i s , v o l . 3 , s e c t . 6 , H e y d e n Son L t d . , L o n d o n , 1975. 6 A. B l a z e k , T h e r m a l A n a l y s i s , V a n N o s t r a n d R e i n h o l d Co., L o n d o n , 1973, p. 6 1 . 7 B.S. S a m p a t h and R. H u g h e s , C h e m i c a l . E n g i n e e r , ( 1 9 7 3 ) 4 8 5 . 8 J.W.Jenkins, B.D, M c N i c o l and S.D. R o b e r t s o n , Chem. T e c h . , ( 1 9 7 7 )
316. 9 U . C o l o m b o , F. G a z z a r r i n i and G. L a n z a v e c c h i a , M a t e r . S c i . E n g . , 2(1967)125. 10 J.J.
Heizmann,
P.
B e c k e r and R.
Baro, Mem.
Sci.
R e v . Metallurg.,
70(1973)625. 1 1 R.P. V i s w a n a t h , B. V i s w a n a t h a n and M.V.C. Sastri, React. K i n e t . C a t a l . L e t t e r s , 2(1975)51. 1 2 H.G. M c I l v r i e d and F.E. Massoth, Ind. E n g . Chem. F u n d a m . , 12( 1973) 225. 1 3 A. B a r a n s k i , A. B i e l a n s k i and A. P a t t e k , J. C a t a l . , 2 6 ( 1 9 7 2 ) 2 8 6 . 1 4 A . B a r a n s k i , A. F u l i n s k i , A. P a t t e k and A. R e i z e r , B u l l . A c a d . P o l . S c i . Ser. S c i . C h i m . , 24(1976)729. 15 G. L i b e r t i , G. S e r v i , A. G u a g l i o , C. R u f f i n o and N. Pernicone, J. T h e r m a l A n a l . , 6(1974)183. 16 S.M. L o k t e v , I . P . M u k h l e n o v , I . F . D a r o v s k i k h , L . I . Z v e z d k i n a , G.L. Y a k o v l e v a and E . I . D o b k i n a , Khim. P r o m . , (1970)108. 17 Z.P. P o p o v i c h and M.V. T o v b i n , U k r . Khim. Z h u r . , 3 6 ( 1 9 7 0 ) 7 2 6 . 18 J . E . B e n s o n , H.W. K o h n and M. Boudart, J. C a t a l . , 5 ( 1 9 6 6 ) 3 0 7 . 1 9 N. P e r n i c o n e , G. L i b e r t i and G. S e r v i , i n C. E y r a u d and M. E s c o u b e s P r o g r e s s i n V a c u u m Microbalance T e c h n i q u e s , v o l . 3 , (Eds.), H e y d e n , L o n d o n , 1975, p. 304. 2 0 G. 'Fagherazzi, F. G a l a n t e , F. G a r b a s s i and N. Pernicone, J. C a t a l . ,
26( 1 9 7 2 ) 3 4 4 . 21 B . S . C l a u s e n , S. M b r u p , H. T o p s b e , R. C a n d i a , E.J. Jensen, A. B a r a n s k i and A. P a t t e k , J. P h y s . , 37(1976)C6-245. 2 2 P.D. R a b i n a , T.Y. M a l y s h e v a , L.D. K u z n e t s o v and V.A. Batyrev, K i n e t i c s C a t a l y s i s ( E n g l i s h transl.), 11(1970)1030. 23 I . V . U v a r o v a , M.T. R u s o v and N.P. S a m c h e n k o , K i n e t i c s C a t a l y s i s ( E n g l i s h transl.), 10(1969)456. 24 N. P e r n i c o n e , G. F a g h e r a z z i , F. G a l a n t e , F. G a r b a s s i , F. L a z z e r i n and A. M a t t e r a , i n J.W. H i g h t o w e r (Ed.), Catalysis, North-Holland P u b l . Co., A m s t e r d a m , 1973, p. 1 2 4 1 . 25 R. H o s e m a n n , A . P r e i s i n g e r and W. V o g e l , Ber. B u n s e n g e s . P h y s i k . Chem., 7 0 ( 1 9 6 6 ) 7 9 6 . 26 H. L u d w i c z e k , A . P r e i s i n g e r , A. F i s c h e r , R, H o s e m a n n , A. Schoenfeld and W. V o g e l , J. C a t a l . , 5 1 ( 1 9 7 8 ) 3 2 6 .
348
J.
H. Topsbe, J.A. Dumesic a n d M. Boudart, C a t a l . , 28(1973)477. R. Buhl a n d A. P r e i s i n g e r , S u r f a c e Sci., 47(1975)344. W.D. Bond, J. Phys. Chem., 66(1962)1573. R. Schoepp a n d I. H a j a l , B u l l . SOC. chim. F r a n c e , (1975)1965. 31 B. Delmon, I n t r o d u c t i o n a l a c i n e t i q u e h e t e r o g e n e , E d i t i o n s Technip, P a r i s , 1969, p. 393. 3 2 D. S h i s h k o v , D. I v a n o v a n d G. Radoeva, J. Appl. Chem. URSS ( E n g l i s h t r a n s l . ) , 44(1971)263.
27 28 29 30
D I SCU S S ION Z.G. SZABO : You have projected the activity of ammonia synthesis catalysts as a function of the degree of reduction.
Several years
ago, we have found that the maximum activity of nickel reduced from NiO was also not at 100% reduction. supported on its own oxide.
Ni was more active if it was
Even this maximum changed if the NiO
was doped with foreign ions, pointing to an electronic interaction between the metallic Ni and its supporting oxide. tional Congress on Catalysis, Paris, 1960).
(cf. I1 Interna-
Can the unreduced Fe304
be considered as a kind of support ?
N. PERNICONE
:
I think that small amounts (max
Q
5%) of unreduced
oxide can hardly be regarded as a support for the metal.
Though
your remark about the Ni-NiO system is very interesting, in the case of Fe catalysts the activity maximum found at incomplete reduction is to be attributed to subsequent sintering phenomena.
S.P.S.
ANDREW : You suggested that it is important that a low partial
pressure of water is employed during reduction of NH3 synthesis catalyst in order to obtain a high activity.
There are important
theoretical consequences depending on whether what is important is the absolute partial pressure of water or its relative partial pressure with respect to that of hydrogen. N. PERNICONE
:
I favour the latter.
I agree completely with you.
In fact, I was speaking
in terms of vpm of water in the gaseous phase (synthesis gas). A.V.
KRYLOVA : What do you think about the practical use of adding
quantities of transition metal oxides to accelerate the reduction of the ammonia synthesis catalyst
?
We added a same small quantity of
NiO (0.8%) to the precipitated promoted iron catalyst prepared by two methods
:
1) impregnation (on the surface) and 2 ) coprecipitation
349
(in the bulk).
We found (figure below) that in the first case,the
rate of the reduction (curve 2 ) and the activity of the catalyst are higher, while in the second case they are lower (curve 3 ) than for a sample without NiO (curve 1 ) .
In conclusion we think that
the addition of NiO and some other transition metal oxides (for example C O O ) may be useful but it is necessary to use a well-defined method for the addition of these oxides.
R%
t
N. PERNICONE : I f industrial catalysts are considered, there is no need for an acceleration of the reduction, because difficulties could arise to maintain a low water concentration.
If desired,
reduction can be initiated at lower temperatures in the presence The results you have
of small amounts of FeO, as shown in Table 7 .
reported for Ni-promoted catalysts are very interesting, but are for a method of preparation (precipitation) which is not practiced industrially.
++
A s Ni++ ions can substitute Fe
ions in the spinel
lattice, both the extent and the order of this substitution may be influenced by the way of Ni addition and influence both reduction rate and activity of the promoted catalyst. N. B A E R N S
: 1) A s
you mentioned, temperature programmed reduction
is not feasible at high pressures.
Could you comment on the main
experimental obstacles to high pressure application of TPR 7 2)
Can you comment on the rate law on which the rate constant is
based, that you gave for the reduction of the ammonia catalyst ? N. PERNICONE
: 1)
The difficulty of performing high pressure T P R
is mainly due to the need of both strictly constant gas flow rate
and uniform conditions at the two thermal conductivity detectors.
350
These requirements are not easy, though not impossible, to be fulfilled at pressure of, for instance, 100 atm.
2) The definition of K O a 5 is given in ref. 1 5 .
It is not a rate
constant, but the experimental reduction rate at 5 0 % conversion. B. DELMON : You quite rightly pin-pointed the difficulties in
studying the reduction of catalysts during activation.
I comple-
tely agree with your wish that catalyst activation should receive increasing attention in catalysis research. You suggested that investigators might hesitate to study the corresponding phenomena, because several reactions occur simultaneously. However, it becomes increasingly clear that there is much interaction between simultaneously occurring reactions, namely a "coupling" of the processes ( 1 ) . In a field quite similar to catalyst activation, namely the dehydration-reduction of goethite to magnetite, we conclusively showed how the occurrence of one reaction has an effect on the other, the ultimate products having quite different structures and textures ( 2 ) . There are coupling phenomena in the reaction of CaC03 with O2
SO2
and
( 3 ) . We more than suspect that several coupling effects occur
during the activation of cobalt-molybdenum hydrodesulfurization catalysts ( 4 ) . Because of the powerful action that we can exert on the texture and structure of the final catalyst by creation or suppression of coupling, I think that research in this area should be strongly encouraged. ( 1 ) V.V.
Boldyrev, M. Bulens, B. Delmon, The Control of the Reacti-
vity of Solids, Elsevier, Amsterdam, in press.
(2) M.L. Garcia-Gonzalez, P. Grange, B. Delmon, in "Reactivity of Solids" (J.Wood, 0. Lindqvist, C. Helgessan, N.G. Venneberg, Editors), Plenum Press, New York, 1 9 7 7 ,
755-759.
( 3 ) G. Van Houte, B. Delmon, Several papers submitted for publica-
tion. (4) J.M.
Zabala, P. Grange, B. Delmon, C.R. Acad. Sci. Paris,
Serie C , 2 7 9 , N. PERNICONE
:
725-728
(1974).
I am very glad for such forthcoming increase of
research work on simultaneous chemical processes.
351 M. HOUALLA : You have reported that the influence of H 0 and A 1 2 0 3 2 is to decrease the induction period for the reduction of F e 0 2 3' which you relate to "spillover" phenomenon by the OH groups.
Other studies on the reduction of NiOx attribute an opposite role to H 2 0. Did you try to assess your hypothesis by reducing Fe203 with carbon monoxide, for example ? x
See paper of B. Delmon and M. Houalla, in these Proceedings.
N. PERNICONE
From a comparison of the reduction behavior of Fe304
:
(refs. 15 and 19 of our paper) and NiO ( R . Frety, L. Tournayan and H. Charcosset, Ann. Chim., 9 (1974), 3411, the following conclusions can be drawn
:
a) in both cases, water has an inhibiting effect on the progression of the reaction interface; b ) the initial reduction rate is increased by H 0 in the case of 2
Fe30q, but decreased in the case of NiO. It is probable that the nucleation stage depends very much on the experimental conditions. We have not yet studied the reduction of Fe304 by CO. H. CHARCOSSET
:
When studying the reduction of unsupported nickel
oxide by hydrogen we found it very useful to carry out electron microscopic examination of partially reduced samples, having submitted them to selective extraction of the metal phase by means of a solution of Br2 in .2H30H. Have you applied such a method to iron catalysts and could you give some detail about the results, for instance, where was the iron located in a partially reduced iron oxide particle N. PERNICONE
:
?
The metal phase in partially reduced magnetites was
dissolved in 0.01 N HC1. This method gives as good results as the one you have mentioned. More details are reported in ref. 1 5 .
We
have shown that the reduction of magnetite particles proceeds in an irregular way from a macroscopic point of view (see Fig. 7b of ref. 1 5 ) .
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353
THE ACTIVATION OF I R O N CATALYST FOR AMMONIA SYNTHESIS B A R A i S K I , M. LAGAN, A . PATTEK and A. R E I Z E R J a g e l l o n i a n U n i v e r s i t y , Cracow, Poland and L . J . C H R I S T I A N S E N and H . TOPSOE Haldor TopsGe Research L a b o r a t o r i e s , LyncJby, Denmark
A.
SUMMARY
The a c t i v a t i o n o f ammonia c a t a l y s t s i s e s s e n t i a l l y a r e d u c t i o n o f non-porous m a g n e t i t e p a r t i c l e s w i t h hydrogen t o y i e l d t h e i n t e r n a l porous s t r u c t u r e o f t h e i r o n c a t a l y s t . The r e d u c t i o n i s s t u d i e d by means o f a "core-and-shell" model where t h e g a s - s o l i d r e a c t i o n proceeds on t h e i n t e r f a c e between t h e c o r e and s h e l l . E f f e c t i v e d i f f u s i o n c o e f f i c i e n t s a r e c a l c u l a t e d from e x p e r i m e n t a l p o r e s i z e d i s t r i b u t i o n measurements. D i f f e r e n t r e a c t i o n r a t e e x p r e s s i o n s w e r e t e s t e d on e x p e r i m e n t a l r e d u c t i o n c u r v e s f o r c a s e s w i t h and w i t h o u t w a t e r a d d i t i o n , and i t i s shown t h a t i t i s n e c e s s a r y t o i n c l u d e t h e a d s o r p t i o n o f water i n t h e r a t e e x p r e s s i o n . F i n a l l y , some l i m i t a t i o n s i n t h e model a r e d e s c r i b e d . INTRODUCTION
The p r e p a r a t i o n and a c t i v a t i o n of an i r o n ammonia s y n t h e s i s c a t a l y s t i n v o l v e e s s e n t i a l l y two s t e p s [ I ] . I r o n o x i d e s a r e f i r s t f u s e d t o g e t h e r w i t h promoters such a s A 1 2 0 3 , CaO, and K20, t o y i e l d a nonporous "magnetite" w i t h n e g l i g i b l e s u r f a c e a r e a . This s t r u c t u r e i s s u b s e q u e n t l y a c t i v a t e d by r e d u c t i o n t o y i e l d t h e a c t i v e , h i g h l y po2 r o u s s t r u c t u r e w i t h a s u r f a c e a r e a i n t h e range 5-20 m /g. I n an e a r l i e r s t u d y 121 a model f o r t h i s r e d u c t i o n p r o c e s s was proposed. According t o t h i s model a p a r t l y reduced c a t a l y s t i s assumed t o c o n s i s t o f an unreduced c o r e o f o x i d e surrounded by a por o u s s h e l l o f reduced m a t e r i a l , and t h e r e a c t i o n proceeds on t h e i n t e r f a c e between t h e c o r e and s h e l l . The r e a c t a n t H 2 and t h e p r o d u c t H 2 0 d i f f u s e i n and o u t through t h e s h e l l . Recent s t r u c t u r e i n v e s t i g a t i o n s [ 3 , 4 ] show t h a t ammonia s y n t h e s i s c a t a l y s t s a r e n o t s i n g l e phase systems and t h a t t h e core-and-shell
p i c t u r e f o r t h e r e d u c t i o n p r o c e s s is n o t r e p r e s e n t a t i v e f o r a l l c a t a l y s t c o m p o s i t i o n s . However, f o r c a t a l y s t s w i t h low w f i s t i t e c o n t e n t s t h e core-and-shell
model was found a p p l i c a b l e .
I n t h e p r e s e n t s t u d y , t h e n e c e s s i t y of i n c l u d i n g t h e a d s o r p t i o n o f w a t e r i n t h e k i n e t i c e x p r e s s i o n i s shown. The c o r e - a n d - s h e l l
mode l is f u r t h e r extended t o i n c l u d e f i l m d i f f u s i o n b e s i d e s d i f f u s i o n
of w a t e r and hydrogen i n t h e s h e l l . I n o r d e r t o t e s t t h e model, k i n e t i c measurements a r e performed o v e r a range o f t e m p e r a t u r e s and w a t e r c o n c e n t r a t i o n s , and p o r e s i z e d i s t r i b u t i o n s a r e measured d i -
rect on reduced c a t a l y s t s . These l a t t e r measurements a l l o w c a l c u l a t i o n s of e f f e c t i v e diffusion coefficients. Another aim of t h i s t y p e of s t u d y i s t o c a l c u l a t e t h e r a t i o o f t h e p a r t i a l p r e s s u r e s o f w a t e r and hydrogen a t t h e i n t e r f a c e where t h e reduction t a k e s place. It i s l i k e l y t h a t t h i s r a t i o determines t h e p a r t i c l e s i z e o f t h e r e s u l t i n g m e t a l l i c i r o n phase and, t h e r e f o r e ,
w i l l be d e t e r m i n i n g a l s o f o r t h e a c t i v i t y o f t h e f i n a l c a t a l y s t . EXPERIMENTAL
mm) o f t h e m u l t i p l e promoted Topsq5e K M I c a t a l y s t w e r e used i n t h e e x p e r i m e n t s . The H 2 - N Z g a s m i x t u r e w a s Regular g r a i n s ( 1 . 0 0 - 1 . 1 9
o b t a i n e d by c a t a l y t i c decomposition o f ammonia followed by d r y i n g a t l i q u i d n i t r o g e n t e m p e r a t u r e . I n some e x p e r i m e n t s w a t e r was added t o t h i s g a s m i x t u r e . The k i n e t i c e x p e r i m e n t s were c a r r i e d o u t u s i n g a s p r i n g m i c r o b a l a n c e a s d e s c r i b e d i n 121. Pore s i z e d i s t r i b u t i o n c u r v e s
w e r e o b t a i n e d by means of a C a r l o Erba p o r o s i m e t e r , model 1500. The c a t a l y s t s used f o r t h e s e e x p e r i m e n t s had f i r s t been reduced i n a spec i a l l y b u i l t r e a c t o r . T h i s r e a c t o r allowed t h e reduced c a t a l y s t s t o be p l a c e d i n a c o n t a i n e r which c o u l d be s e a l e d o f f i n s i d e t h e r e a c t o r . I n t h i s way t h e reduced c a t a l y s t s c o u l d be weighed and t r a n s f e r r e d t o t h e p o r o s i m e t e r w i t h o u t b e i n g exposed t o a i r . RESULTS The k i n e t i c c u r v e s f o r r e d u c t i o n i n d r y N 2 : H 2
gas a t temperatures
between 450°C and 5 5 0 ° C a r e shown i n F i g . 1 . Adding w a t e r t o t h e red u c t i o n g a s d e c r e a s e s t h e r a t e of r e d u c t i o n a s shown i n F i g . 2 f o r red u c t i o n a t 550'C. Pore s i z e d i s t r i b u t i o n c u r v e s f o r c a t a l y s t s reduced a t d i f f e r e n t temperatures i n dry H2-N2
a r e shown i n F i g . 3 . The p o r e volumes a r e
r e f e r r e d t o t h e mass o f reduced c a t a l y s t . The c a t a l y s t had i n a l l c a s e s been reduced t o Rz0.8.
Two d i s t i n c t maxima a r e observed i n
355
4
12 16 20 24 28 32 36 40 44 48 52
8
tk F i g . 1 . Reduction i n d r y H2-N2 a t d i f f e r e n t t e m p e r a t u r e s . The p o i n t s r e p r e s e n t t h e e x p e r i m e n t a l d a t a , and t h e c u r v e s a r e c a l c u l a t e d by t h e model.
f
100 R [%I
60 50
Lot 30t
iuiuuppm -*% 8
0 0
0
2olloo 0
0
10
L 1
2
---+
3
4
5
t-
6
7
8
9
1011
1213
t [ S. F i g . 2 . Reduction a t 550'C
i n H2-%with d i f f e r e n t water c o n t e n t s .
356
[ ml/g reduced catalyst 1
pore radius rp,A F i g . 3 . Pore s i z e d i s t r i b u t i o n c u r v e s f o r c a t a l y s t s reduced a t d i f f e r e n t t e m p e r a t u r e s i n d r y HZ-N2.
t h e c u r v e s . The maximum around 100-120 A i s i n agreement w i t h t h e r e s u l t s o f N i e l s e n 151; it c a n b e s e e n t h a t t h e p o s i t i o n o f t h e maximum i n c r e a s e s somewhat w i t h i n c r e a s i n g t e m p e r a t u r e . The second maximum around 260-430 A h a s n o t been r e p o r t e d p r e v i o u s l y . The i n t e n s i t y and t h e p o s i t i o n i n c r e a s e somewhat w i t h r i s e i n temperature. On t h e b a s i s o f t h e measured p o r e s i z e d i s t r i b u t i o n t h e e f f e c t i v e d i f f u s i o n c o e f f i c i e n t s , D, o f hydrogen and w a t e r i n t h e porous s h e l l a r e c a l c u l a t e d by u s e o f t h e formula : n D = L - C
j=1
t
(J+ ->
jB''
1
DK,j
-1
(1)
The symbols are e x p l a i n e d i n t h e l i s t i n c l u d e d a t t h e end. The b u l k d i f f u s i o n c o e f f i c i e n t s , D B , were c a l c u l a t e d a c c o r d i n g t o
357
S a t t e r f i e l d and Sherwood 161 and t h e l a b y r i n t h f a c t o r , L, e q u a l t o 0 . 4 5 was t a k e n from N i e l s e n 111. The e f f e c t i v e d i f f u s i o n c o e f f i c i e n t s f o r hydrogen and water a r e shown a s a f u n c t i o n o f t e m p e r a t u r e i n F i g . 4 f o r r e d u c t i o n i n d r y H2-N2. The d a t a f o r r e d u c t i o n a t 550'C i n H2-N2 c o n t a i n i n g d i f f e r e n t amounts of w a t e r a r e a l s o shown. The a p p a r e n t a c t i v a t i o n energy i s 4 . 0 kcal/mol f o r w a t e r .
-1 .oo
t
i
0 . 7 kcal/mol f o r hydrogen and 2 . 6
f
1.1
A
n,, - n2u
- 1.75
0
-2.001 I
1.24
1.28
1.32
1.36
1.40
1.44
w ,K-1 T
F i g . 4 . The e f f e c t i v e d i f f u s i o n c o e f f i c i e n t s o f w a t e r and hydrogen a t d i f f e r e n t t e m p e r a t u r e s and w a t e r c o n c e n t r a t i o n s . 0: d r y gas, 0 : 6 0 0 0 ppm H20, A : 1 0 1 0 0 ppm HZO.
MATHEMATICAL MODEL FOR THE REDUCTION PROCESS
The mathematical model f o r t h e r e d u c t i o n p r o c e s s t a k e s i t s s t a r t i n g p o i n t i n t h e work of S p i t z e r e t a l . 171. I t i s assumed t h a t t h e r e d u c t i o n o f a s i n g l e p a r t i c l e c a n b e d e s c r i b e d a s a c o r e - a n d - s h e l l model, t h a t t h e r e a c t i o n p r o c e e d s
on t h e i n t e r f a c e between t h e c o r e and t h e s h e l l , and t h a t t h e e f f e c t i v e d i f f u s i o n c o e f f i c i e n t s a r e independent of t h e reduction d e g r e e . Two g a s p h a s e components t a k e p a r t i n t h e r e a c t i o n : hydrogen as r e a c t a n t , and w a t e r a s p r o d u c t . The t r a n s p o r t o f hydrogen t h r o u g h t h e s h e l l i s governed by
t h e b a s i c d i f f u s i o n e q u a t i o n . The amount o f hydrogen t h a t d i f f u ses through t h e s h e l l i n u n i t t i m e can be found by i n t e g r a t i o n o f t h i s e q u a t i o n from t h e p a r t i c l e s u r f a c e t o t h e i n t e r f a c e . By u s i n g t h e i d e a l gas law t o c o n v e r t from c o n c e n t r a t i o n t o p r e s s u r e , t h e f o l l o w i n g e q u a t i o n i s found :
The gas f i l m r e s i s t a n c e w i l l a l s o be i n c l u d e d h e r e , s i n c e t h e flow r a t e s encountered o f t e n a r e s m a l l . The flow o f hydrogen from t h e bulk gas phase t o t h e s u r f a c e o f t h e p a r t i c l e is given by t h e f i l m t r a n s p o r t e q u a t i o n :
'x~2'(pH20-
(3)
'H2
Hydrogen is consumed a t a r e d u c t i o n r a t e , V , a t t h e i n t e r f a c e . This g i v e s t h e e q u a t i o n (4)
= 4n.xi2.V
-NH
Combining e q u a t i o n s ( 2 ) and ( 4 1 , and e q u a t i o n s ( 3 ) and ( 4 ) , e l i m i n a t e s t h e s u r f a c e c o n c e n t r a t i o n and t h e mole flow, i f t h e two r e s u l t i n g e q u a t i o n s a r e combined. The r e s u l t i n g e q u a t i o n i s :
The r e d u c t i o n degree i s now i n t r o d u c e d . I t i s d e f i n e d through the equation : R = 1
-
(2)3
and i n s e r t i o n o f t h i s e q u a t i o n i n ( 5 ) g i v e s :
The e q u a t i o n f o r w a t e r can be d e r i v e d i n a s i m i l a r way
:
These t w o e q u a t i o n s form t h e b a s i c e q u a t i o n s , s i n c e no assumptions about t h e a c t u a l form of t h e r a t e e x p r e s s i o n have been
359
made, and since they express the partial pressure at the interface as a function of the bulk pressures and the reduction degree. The rate of reduction, V, can be expressed by the consumption of oxygen, i.e. by the equation : dxi
v = c O
dt
Insertion of (6) in this equation gives
.dR
co xo
v=
:
3(1-R)2/3
(10)
dt
The variables in this equation can be separated and the equation integrated as :
The expression for V is a function of the partial pressures of hydrogen and water at the interface. Insertion of (7) and (8) in the rate expression gives after some rearrangements an expression which can be used direct in the integral in eq. (11). Langmuir-Hinshelwood rate expression The following rate expression takes into account the adsorption of water, but not that of hydrogen:
Insertion of equations (7) and (8) in this equation gives after some rearrangements the following quadratic equation : AV2
+
BV
+
C = 0
where
B =
-
KA,H20
1 {(I+ -.R T Kr
g
X
p H : , ) +
:(l-R)
m+ ”(a
1 -R)m- 1 -RIB)}
360
_1 - - + a
1
1
KG,H2
Ke K G , H 2 0
_1 -- - I
'
+
DH2
1 Ke D H 2 0
Given t h e p a r t i a l p r e s s u r e s i n t h e b u l k phase and t h e reduct i o n d e g r e e t h i s e q u a t i o n c a n be s o l v e d as a s i m p l e q u a d r a t i c eq u a t i o n , where o n l y t h e p o s i t i v e r o o t i s used. If no a d s o r p t i o n o f H 2 0 i s c o n s i d e r e d (KA,H
= 01, a s i m p l e
2
f i r s t o r d e r r e a c t i o n i n hydrogen r e s u l t s and t h e i n t e g r a l i n eq u a t i o n ( 1 1 ) can be s o l v e d a n a l y t i c a l l y . The r e s u l t i n g e x p r e s s i o n
i s i n t h a t c a s e i d e n t i c a l t o t h a t d e r i v e d by S e t h and R o s s [ 8 ] w i t h t h e e x c e p t i o n t h a t t h e p a r a m e t e r s have a d i f f e r e n t p h y s i c a l meaning. However, i f t h e k i n e t i c d a t a a r e a n a l y z e d u s i n g t h e rev e r s i b l e f i r s t o r d e r k i n e t i c s , t h e p a r a m e t e r s have no p h y s i c a l l y meaningful v a l u e s .
The a c t i v a t i o n energy (21 kcal/mol)
for the
e f f e c t i v e d i f f u s i o n c o e f f i c i e n t f o r water i s unacceptably high. T h e r e f o r e , i t seems t h a t t h e a d s o r p t i o n o f w a t e r must be c o n s i d e r e d and, i n t h e f o l l o w i n g , t h e k i n e t i c s g i v e n by e q u a t i o n (12) w i l l be t e s t e d . The e f f e c t i v e d i f f u s i o n c o e f f i c i e n t s w e r e c a l c u l a t e d from t h e p o r o s i m e t e r d a t a , and t h e mass t r a n s f e r c o e f f i c i e n t s w e r e c a l c u l a t e d by u s i n g t h e c o r r e l a t i o n s g i v e n by S z e k e l y e t a l . p a r a m e t e r s Kr and K
A,H2O
191. The
a r e t h e n determined by a n o n - l i n e a r l e a s t
s q u a r e s f i t . The i n t e g r a l i n e q u a t i o n (11) c a n n o t b e s o l v e d anal y t i c a l l y and an o r d i n a r y Gauss method h a s t h e r e f o r e been used t o solve t h e i n t e g r a l numerically. DISCUSSION The v a l u e s o f t h e p a r a m e t e r s Kr and KA,H20 o b t a i n e d by f i t t i n g t h e d a t a i n F i g . 1 a r e shown i n F i g . 5 . The k i n e t i c c u r v e s c a l c u l a t e d from t h e s e p a r a m e t e r s a r e shown i n F i g . 1. The agreement between t h e measured and t h e c a l c u l a t e d v a l u e s i s q u i t e good, es p e c i a l l y f o r t e m p e r a t u r e s above 450°C.
The mean d e v i a t i o n ex-
p r e s s e d a s p e r c e n t a g e o f t h e r e d u c t i o n t i m e s , amounts t o 2 . 2
%
f o r t h e c u r v e a t 4 5 0 ' C and a b o u t 0 . 6 0 % f o r t h e remaining reduct i o n c u r v e s . I t s h o u l d b e n o t e d t h a t t h e d e v i a t i o n s between t h e measured and t h e c a l c u l a t e d v a l u e s seem t o be s y s t e m a t i c r a t h e r t h a n random.
361
701 I
.
1.20
I
1.24
1.28 1.32
1.36
1.40 71000 . K
1.20
Temperature dependence of log K A ,
1.24
1.28
1.32 1.36
1.40
y, K”
- l
H20
(Fig.5A) and log K,
(Fig.5B).
In order to test the physical significance of the two parameters, K, and KA,H20r the calculated values have been drawn in Arrhenius type plots in Fig.s 5 A and 5 B. Although there is some scatter in the points, they seem to form straight lines with reasonable values for the slopes. The activation energy for the rate constant, Kr, is equal to 12.6 f 6.3 kcal/mol. This value is quite close to the one found by Quets et al. [lo]. The heat of adsorption of water is estimated to be 39.0 f 10.8 kcal/mol, which seems quite reasonable. Consequently, it can be concluded that for reduction in dry H2-N2 the model seems both to represent the data well and to give reasonable values of the estimated parameters. In the following, the values of the water concentration at the interface and the importance of the water adsorption term will be discussed briefly. The partial pressure of water, pHioI at the interface increases with increasing temperature and reduction deatm at 550’C gree from 5x10-4 atm at 450’C for R = 0.1 to The adsorption term in the denominator of equation for R = 0.8. ( 1 2 ) increases with increasing reduction degree, but decreases with increasing temperature. At 450’C the term varies from 1.3 for R = 0.1 to 4.5 for R = 0.8. At 550’C the variation is from 0.2 for R = 0.1 to 1.0 for R = 0.8. So it seems that the inhibiting influence of the water is significant, even for the very high reduction temperatures. The application of the model to explain the data for reduction in HZ-NZ with added water shows again that the agreement is better for the higher reduction temperatures. However, the results obtained until now are rather uncertain, and further experiments will be needed.
362
This might also indicate that some of the assumptions in the evaluation of the model are not fulfilled. It was mentioned in the Introduction that the catalyst encountered might be multiphase, and the assumption of a single core-and-shell model should therefore be further investigated. This investigation should include a multiple core-and-shell model [111 and, preferably, a l s o take into account the irregular shapes of the grains and the variation of the pore size distribution with reduction degree [ 1 2 ] . LIST OF SYMBOLS A, B, C Parameters in equation (13) Oxygen concentration in core, mol/cm3 cO D Effective diffusion coefficient, cm*/sec Bulk diffusion coefficient, cm2/sec D13 Knudsen diffusion coefficient for radius x. DK, j 3 Film mass transfer coefficient, cm/s KG Adsorption constant, cm3/mo1 KA Chemical equilibrium constant Ke Rate constant, cm3/mol Kr L Labyrinth factor N Flow, mol/sec n Number of experimental points Partial pressure in bulk phase Pb Partial pressure at interface Pi Partial pressure at particle surface PO R Reduction degree, defined in equation (6) R Gas constant, atm cm3/mol/'K g T Temperature, 'K V Reaction rate, mol/cmz/sec X Core radius, cm i X Particle radius, cm 0 Parameter defined in equation ( 1 7 ) a Parameter defined in equation ( 7 8 ) P Porosity in j-th pore volume fraction & j
363
REFERENCES 1 A.Nielsen, An Investigation on Promoted Iron Catalysts for the Synthesis of Ammonia, 3rd edn., Jul. Gjellerups Forlag, Copenhagen, 1968. 2 A.Barafiski,A.Bielafiski and A.Pattek, J.Cata1.,26(1972)286. 3 B.S. Clausen, S. Marup, H. Topsple, R.Candia, E.J. Jensen, A. Barafiski and A. Pattek, J.Physiquer37(1976)C6-245. 4 E.J. Jensen, H. Topsde, 0. Sdrensen, R. Candia, F. Krag, B.S. Clausen, S. Mdrup, Scand.J.Metallurq-y,6(1977)6. 5 Reference 1, p.205. 6 C.N. Satterfield and T.K. Sherwood, The Role of Diffusion in Catalysis, Addison-Wesley Publishing Cornp., Reading-Paolo Alto - London, 1969, p.21. 7 R.H. Spitzer, F.S. Manning and W.O. Philbrook, Trans.Met.Soc. AIME,236 ( 1966)726. 8 B.B.L. Seth and H.M. ROSS, Trans.Met.Soc.AIME, 233(1965)180. 9 J.Szekely, J.W. Ewans and H.Y. Sohn, Gas-Solid Reactions,Academic Press, New York - San Fransisco - London, 1976, p.13. 10 J.M. Quets, M.E. Wadsworth and J.R. Lewis, Trans.Met.Soc.AIME, 221 (1961)1186. 1 1 J.Y.Park and 0. Levenspiel, Chern.Eng.Science,30(1975)1207. 12 A. Barafiski, R. Candia and H. Topsde, Unpublished results. DISCUSSION N. P E R N I C O N E : T h e t e m p e r a t u r e r a n g e y o u h a v e c h o s e n for y o u r i n v e s t i g a t i o n s i s 450-550OC.
T h e s e a r e r a t h e r high t e m p e r a t u r e s
w i t h r e s p e c t t o t h o s e used for t h e i n d u s t r i a l r e d u c t i o n o f a m m o n i a s y n t h e s i s catalysts.
Did you prefer a higher temperature range
t o h a v e h i g h e r r e d u c t i o n rates, w h i c h a r e t h e r e f o r e m o r e e a s i l y m e a s u r a b l e , o r for o t h e r r e a s o n s ?
A. B A R A N S K I : E x p e r i m e n t s h a v e b e e n c a r r i e d o u t at t e m p e r a t u r e s l o w e r t h a n 4 5 O o C , b u t t h e y a r e n o t i n c l u d e d here.
However, the
r a t e o f r e d u c t i o n of i n d u s t r i a l a m m o n i a c a t a l y s t s is q u i t e l o w , b e l o w a r o u n d 4 O O 0 C , a n d it i s t h e r e f o r e m a i n l y t h e t e m p e r a t u r e r e g i o n a b o v e t h i s v a l u e w h i c h is o f i n d u s t r i a l importance.
We
h a v e included d a t a o v e r a r a t h e r w i d e t e m p e r a t u r e range, s i n c e i n t h i s way t h e t e m p e r a t u r e d e p e n d e n c e of t h e p a r a m e t e r s in t h e k i n e t i c e q u a t i o n c a n b e o b t a i n e d a n d c h e c k e d for p h y s i c a l significance.
F u t h e r m o r e , during t h e i n d u s t r i a l r e d u c t i o n o f a
c a t a l y s t , t h e t e m p e r a t u r e a l s o c h a n g e s o v e r q u i t e a w i d e range.
S.P.S.
ANDREW
:
I h a v e had t h e e x p e r i e n c e o f using t h e a u t h o r s '
form o f m a t h e m a t i c a l a n a l y s i s to t h e r e a c t i o n of a n u m b e r of i m p u r e solid substances. promotors
-
(NB3 synthesis catalyst
m a y be c a l l e d impure m a g n e t i t e ) .
-
because of the
I have a l w a y s f o u n d
t h a t t h e e x p e r i m e n t a l c u r v e followed t h e t h e o r e t i c a l u p to a b o u t
364 80% c o n v e r s i o n b u t t h e e x p e r i m e n t a l rate w a s m a r k e d l y l e s s t h a n the theoretical above this
conversion.
Did y o u f i n d a n y t h i n g
similar 7 A. B A R A N S K I : Yes, we h a v e t h e s a m e e x p e r i e n c e and o n l y t h e d a t a u p t o 80% r e d u c t i o n h a v e b e e n reported here.
The discrepancy above
t h i s r e d u c t i o n degree w a s r e p o r t e d in a n e a r l i e r p a p e r t h e paper).
(ref.2 in
T h e lower r e d u c t i o n rate at h i g h r e d u c t i o n d e g r e e s i s
m o s t likely d u e t o a d i f f i c u l t l y reducible. p r o m o t e r - r i c h p h a s e w h i c h f o r m s a s t h e r e d u c t i o n o f t h e mixed m a g n e t i t e s p r o g r e s s e s (see ref. 3 i n t h e paper).
J.W.E.
COENEN : In the discussion of your paper you proceed to test
t h e p h y s i c a l s i g n i f i c a n c e o f t w o p a r a m e t e r s : t h e a d s o r p t i o n equil i b r i u m c o n s t a n t f o r w a t e r a d s o r p t i o n and t h e r a t e constant. From t h e i r t e m p e r a t u r e d e p e n d e n c e y o u d e r i v e t h e a d s o r p t i o n heat and t h e a c t i v a t i o n e n e r g y , a n d y o u c o n c l u d e t h e i r v a l u e s to b e reasonable.
B u t y o u c o u l d go o n e s t e p f u r t h e r a n d check a l s o
t h e a b s o l u t e value o f t h e s e c o n s t a n t s for reasonableness.
In other
w o r d s , w h a t about entropy e f f e c t s 7 F o r i n s t a n c e , t h e a d s o r p t i o n c o n s t a n t may b e w r i t t e n a s : K = exp
(hSz/R) exp(-AHz/RT).
S i n c e t h e v a l u e o f K and t h e energy f a c t o r i s k n o w n , y o u c a n c a l c u l a t e ASo and c o n s i d e r w h e t h e r i t s v a l u e i s acceptable.
Since
you find a high heat of adsorption, the adsorption should be i m m o b i l e w h i c h m e a n s t h a t t h e e n t r o p y l o s s would i n v o l v e c o m p l e t e
loss o f t r a n s l a t i o n a l e n t r o p y
:
for a s t a n d a r d s t a t e o f 1 atm
[ t h a t i s p a r t i a l p r e s s u r e expressed
AS:
should b e at least -30 c a l / K m o l ,
in atmospheres).
A.
B A R A N S K I : T h e c a l c u l a t e d value o f t h e a d s o r p t i o n entropy a m o u n t s
t o -14.6 cal/K mol.
A d s o r p t i o n o f m o l e c u l e s from t h e g a s p h a s e
o n t o a s u r f a c e r e s u l t s i n a d e c r e a s e in entropy.
If t h e a d s o r p t i o n
i s m o b i l e l i k e f o r xenon o n m e r c u r y , o n l y o n e d e g r e e o f freedom o f t r a n s l a t i o n a l m o t i o n i s l o s t and b e t w e e n -2.0 and -5.4 found f o r t h e a d s o r p t i o n entropy
(I).
e.u.
T h e c a l c u l a t e d value o f
-14 e.u. f o r t h e w a t e r a d s o r p t i o n entropy d o e s not seem unreasonable. ( 1 ) J.H. d e BOER, T h e D y n a m i c a l c h a r a c t e r o f A d s o r p t i o n , Oxford a n d C l a r e n d o n P r e s s , 1968, p p , 112-117.
is
366
PREPARATION AND CHARACTERIZATION OF SMALL IRON PARTICLES SUPPORTED ON MgO H . T O P S ~ E ~J.A. ,
.
S M!3RUPd
a
,
DUNE SIC^ , E . G. D E R O U A N E ~
B.
s . C L A U S E N ~,
J. VILLADSENa and N . TOPS@Ea
Haldor Topsfle Research L a b o r a t o r i e s , DK-2800 Lyngby (Denmark).
bDepartment o f Chemical E n g i n e e r i n g , U n i v e r s i t y o f FJisconsin , Madison, Wisc. 5 3 7 0 6 (U.S.A.). C F a c u l t 6 s U n i v e r s i t a i r e s N.D.
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 , B-5000 Namur ( B e l c J i u m ) . dLaboratory o f Applied P h y s i c s 11, T e c h n i c a l U n i v e r s i t y o f Denmark, DK-2800 Lyngby (Denmark)
.
ABSTRACT The p r e p a r a t i o n o f s m a l l m e t a l l i c i r o n p a r t i c l e s from i o n exchanged magnesium hydroxycarbonate and c o p r e c i p i t a t e d s t a r t i n g materials i s r e p o r t e d . The e f f e c t o f d i f f e r e n t a c t i v a t i o n p r o c e d u r e s p r i o r t o red u c t i o n i s s t u d i e d , and t h e n a t u r e o f t h e s t a r t i n a m a t e r i a l i s found t o p l a y a more i m p o r t a n t r o l e t h a n t h e a c t i v a t i o n p r o c e d u r e . The small e s t i r o n p a r t i c l e s w e r e p r e p a r e d s t a r t i n g from c o p r e c i p i t a t e d s t r u c t u r e s . These r e s u l t s a r e d i s c u s s e d i n t e r m s o f a model f o r t h e s t r u c t u r e and g e n e s i s of t h e v a r i o u s c a t a l y s t s . The s t r u c t u r a l i n f o r m a t i o n i s o b t a i n e d mainly from Plsssbauer s p e c t r o s c o p y and X-ray d i f f r a c t i o n . The s t u d y l e a d s t o a p o s s i b l e e x p l a n a t i o n o f t h e n a t u r e o f t h e metals u p p o r t e d i n t e r a c t i o n i n t h e s e c a t a l y s t systems.
1 INTRODUCTION The s e a r c h f o r means o f p r e p a r i n g s m a l l , t h e r m a l l y s t a b l e i r o n p a r t i c l e s h a s f o r many y e a r s been m o t i v a t e d both by p r a c t i c a l and t h e o r e t i c a l i n t e r e s t . From a p r a c t i c a l v i e w p o i n t , one h a s s e a r c h e d f o r c a t a l y s t s w i t h a h i g h s p e c i f i c i r o n s u r f a c e a r e a . Such c a t a l y s t s would n a t u r a l l y be of i n t e r e s t , f o r example, i n t h e ammonia o r F i scher-Tropsch s y n t h e s i s . T h e o r e t i c a l l y , s m a l l c a t a l y s t p a r t i c l e s a r e a l s o o f g r e a t c u r r e n t i n t e r e s t owing t o t h e i r s p e c i a l s u r f a c e i
s t r u c t u r e s and e l e c t r o n i c p r o p e r t i e s . Small m e t a l l i c i r o n p a r t i c l e s a r e , however, d i f f i c u l t t o s t a b i l i z e , s i n c e i r o n s i n t e r s e a s i l y a t
366
hydrogen reduction temperatures (- 700 K). The stabilization can be achieved by textural promotion as is done, for example, in the classical ammonia synthesis catalyst [l] or by dispersion on a thermostable high surface area support, as will be discussed in the present paper. In order to stabilize metallic iron particles, the support interaction should be optimum, i.e. neither so strong that the reduction to the metallic state fails to occur, nor so weak that high mobility results in rapid sintering. Boudart et aZ. [2] found that magnesium oxide is an ideal support for the preparation of small iron particles with sizes below 10 nm. Reducticn to metallic iron was possible, and the resulting small iron particles showed little tendency toward sintering. A large fraction of the metallic iron surface area was accessible to gases. The catalysts were used in an investigation of the particle size dependence of the ammonia synthesis reaction [31. They were prepared by exchange of magnesium cations with iron ions at the surface of magnesium hydroxycarbonate (KHC) crystals, followed by drying and direct reduction in hydrogen [2]. Bussisre et a l . 141 and Dutartre e t aZ. (51 also supported iron on magnesium oxide. However, their catalysts were prepared by coprecipitation of iron and magnesium nitrates, followed by calcination and reduction at high temperatures. The metallic iron particle size was reported to be larger than those in the P1HC-based catalysts. Also, the observed properties of these samples were rather different from those of the samples prepared from the PMC precursor [2,6]. Ferromagnetic resonance (FPIR) was used by Derouane and Monseur [71 to show that different thermal treatments of ion exchanged, PmC-based structures resulted in different magnetic phases. In view of the above studies, the present paper deals with a further examination of the influence of both the nature of the starting material and its activation on the ultimate structure of iron supported on magnesium oxide. We have used different physical and chemical techniques to provide information about the chemical state and dispersion of the various iron- and magnesium-containing phases present during the catalyst genesis. In particular, the results of Plijssbauer spectroscopy, X-ray diffraction, CO chemisorption, and N2 physical adsorption (BET), reveal a complex interaction among these different phases which appears essential for the preparation of small, metallic iron particles.
367 2 EXPERIMENTAL
2 . 1 Sample p r e p a r a t i o n and nomenclature
The magnesium hydroxycarbonate (PTHC) c a t a l y s t s were p r e p a r e d acc o r d i n g t o t h e method d e s c r i b e d i n [ 2 1 . The d r i e d i o n exchanged MHC
w i l l be denoted x%Fe-MHC, where x d e n o t e s t h e weight p e r c e n t o f i r o n i n t h e reduced c a t a l y s t . These samples w i l l be r e f e r r e d t o a s p r e p r e c u r s o r s . x%Fe-MHC-y l a b e l s t h e same c a t a l y s t a f t e r t h e p r e t r e a t ment o r a c t i v a t i o n p r o c e d u r e d e s c r i b e d by "y". Some c a t a l y s t s w e r e p r e p a r e d as d e s c r i b e d i n 121 by d i r e c t r e d u c t i o n ( y = H2) o f t h e p r e - p r e c u r s o r s a c c o r d i n g t o t h e program: 2-5 h a t 370 K ;
12-16 h a t
520 K ;
2-5 h a t 6 0 0 K ;
2-5 h a t 650 K , f o l l o w e d by a t l e a s t 20 h a t
700 K.
O t h e r c a t a l y s t s were made by f i r s t decomposing t h e pre-pre-
c u r s o r s t o g i v e d i f f e r e n t o x i d i c p r e c u r s o r s b e f o r e t h e f i n a l reduct i o n . These decompositions c o n s i s t o f slow a c t i v a t i o n i n e i t h e r a i r (y = a i r ) o r vacuum ( y = vac) a c c o r d i n g t o t h e above time-temperat u r e program, o r a d i r e c t c a l c i n a t i o n a t 770 K i n a i r f o r 2 4 h ( y =
c a l c ) . The c a t a l y s t s r e s u l t i n g from r e d u c t i o n o f t h e s e p r e c u r s o r s w i l l b e denoted: y = a i r , H 2 ;
y = vac,H2;
etc.
For t h e s u p p o r t e d i r o n c a t a l y s t s p r e p a r e d by c o p r e c i p i t a t i o n (CP) o f t h e magnesium and i r o n ( I I 1 ) n i t r a t e s w i t h ammonium h y d r o x i d e , an analogous nomenclature w i l l b e used having t h e g e n e r a l form x%Fe-CP-y. 2 . 2 Mgssbauer s p e c t r o s c o p y
The MEssbauer s p e c t r a w e r e o b t a i n e d w i t h a c o n s t a n t a c c e l e r a t i o n s p e c t r o m e t e r u s i n g a s o u r c e o f 5 7 C 0 i n Rh. The e x p e r i m e n t s on t h e reduced samples w e r e performed i n situ i n an u l t r a h i g h vacuum compati b l e c e l l a l l o w i n g t e m p e r a t u r e s between 7 8 K and 1 0 0 K and a p p l i c a t i o n o f magnetic f i e l d s . 2.3 X-ray d i f f r a c t i o n X-ray r e s u l t s w e r e o b t a i n e d w i t h a P h i l i p s v e r t i c a l d i f f r a c t o meter equipped w i t h a p r o p o r t i o n a l c o u n t e r and g r a p h i t e monochromator.
CuKa r a d i a t i o n was employed a t a s c a n speed o f f 0 ( 2 @ )p e r minute.
2 . 4 CO c h e m i s o r p t i o n and t o t a l s u r f a c e a r e a measurements CO c h e m i s o r p t i o n a s d e s c r i b e d i n [21, and BET s u r f a c e a r e a meas-
urements were c a r r i e d o u t u s i n g a c o n s t a n t volume system.
368
3 RESULTS 3.1 P r e - p r e c u r s o r s The X-ray diagrams o f t h e MHC p r e - p r e c u r s o r s w e r e a l l t y p i c a l o f t h e MHC s t r u c t u r e . Chemical a n a l y s i s and TGA r e s u l t s showed t h a t f o r t h e d i f f e r e n t i r o n l o a d i n g s t h e f o r m u l a s c a n be a p p r o x i m a t e l y des c r i b e d a s 3 (Mg,Fe) C 0 3 (Mg,Fe) (OH) 2'3%H20, a l t h o u g h t h e i d e a l f o r mula o f t h e p u r e Mg compound h a s been d e f i n i t i v e l y shown t o be 4MgCO3-Mg(OH)2 * 4 H 2 0 [81. The s t r u c t u r e o f t h e CP p r e - p r e c u r s o r s d i d , however, seem t o v a r y w i t h t h e i r o n c o n c e n t r a t i o n . The O%Fe-CP samp l e was q u i t e p u r e Mg(OHI2. However, a s t h e i r o n c o n t e n t i n c r e a s e d , t h e s t r u c t u r e s c o n t a i n e d NO3 and c r y s t a l w a t e r i n i n c r e a s i n g amounts. The X-ray diagram o f t h e lG%Fe-CP showed t h a t t h e system i s mainl y single-phased with a s t r u c t u r e ( s j d g r e n i t e type) n o t unlike t h e v a r i o u s MHC samples. detected.
I n a d d i t i o n , minor amounts o f Mg(OH)2 were
3.2 P r e c u r s o r s I n F i g . 1 w e show t h e M8ssbauer s p e c t r a o f t h e 8%Fe-MHC-vacr t h e IG%Fe-hTHC-air, and t h e IG%Fe-CP-calc p r e c u r s o r s a t 293 K and 78 K. The 293 K s p e c t r a ( F i g s . l a , l c , and l e ) a l l c o n t a i n a paramagnetic Fe 3+ ( 2 - l i n e ) component. Such a component c o u l d o r i g i n a t e from b o t h s u p e r paramagnetic and p a r a m a g n e t i c i r o n s p e c i e s . The 8%Fe-MHC-vac sample shows a s i g n i f i c a n t , m a g n e t i c a l l y o r d e r e d component a t room t e m p e r a t u r e (spectrum l a ) ; t h i s component d o e s n o t s i g n i f i c a n t l y i n c r e a s e i n r e l a t i v e i n t e n s i t y as t h e t e m p e r a t u r e i s lowered ( s p e c t r u m l b ) . These s p e c t r a can be i n t e r p r e t e d i n t e r m s o f a p a r a m a g n e t i c component and a magnetic ( 6 - l i n e ) component: t h e former component i s t y p i c a l of F e ( I I 1 ) i n ElgO [9-111, and t h e l a t t e r compon e n t has q u i t e broad l i n e s and shows " s t r u c t u r e " a t 70 X i n d i c a t i v e o f Fe on s e v e r a l d i f f e r e n t s i t e s . T h i s may b e i n t e r p r e t e d a s i r o n p r e s e n t i n MgxFe3-x04 f e r r i t e p h a s e s o f v a r i o u s compositions. The X-ray r e s u l t s are i n agreement w i t h t h e s e f i n d i n g s , s i n c e b o t h a s p i n e l phase w i t h l a t t i c e c o n s t a n t ( a = 5.398 A) t y p i c a l o f Fe304/MgFe 0 2 4, and an i r o n c o n t a i n i n g MgO phase ( a = 4.236 A) a r e observed t o g e t h e r w i t h a q u i t e p u r e 14gO p h a s e ( a = 4.218 A ) . T h i s l a t t i c e parameter i s t y p i c a l o f s m a l l p a r t i c l e s o f MgO. The p r e s e n c e o f s e v e r a l magneti c a l l y o r d e r e d p h a s e s was also confirmed by F14R [71. The 16%Fe-MHCa i r sample ( s p e c t r a l c and I d ) shows s u p e r p a r a m a g n e t i c b e h a v i o r . T h i s i s s e e n from t h e appearance o f a m a g n e t i c a l l y s p l i t component a s t h e t e m p e r a t u r e i s lowered t o 78 K . The IG%Fe-CP-calc sample p r e p a r e d
369
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F i g . 1. K6ssbauer s p e c t r a o f p r e c u r s o r s o b t a i n e d a t 293 K and 78 K. ( a ) and ( b ) 8%Fe-MHC-vac; (c ) and ( d ) 16%Fe-MHC-air; (e) and ( f ) 1 6 %Fe-CP-calc
.
by c o p r e c i p i t a t i o n ( s p e c t r a l e and I f ) i s , however, t h e o n l y sample t h a t d o e s n o t show any magnetic o r d e r i n g even a t 78 K. The X-ray p a t t e r n s show a s i n g l e NgO t y p e phase w i t h l a t t i c e c o n s t a n t a = 4 . 2 1 9 A . A 12%Fe-MHC-calc sample showed a M6ssbauer spectrum q u i t e s i m i l a r t o t h a t of t h e 168Fe-PTHC-air sample. However, some magnetic s p l i t t i n g was p r e s e n t a l r e a d y a t 293 K. 3.3 Reduced c a t a l y s t s
The NEssbauer spectra o b t a i n e d in situ a t 670 K of t h e p r o g r e s s i v e l y reduced p r e - p r e c u r s o r s and o f t h e reduced p r e c u r s o r s , a r e shown i n Fig.2. These s p e c t r a a l l show t h e p r e s e n c e o f i r o n i n two forms: m e t a l l i c i r o n ( s u p e r p a r a m a g n e t i c and/or f e r r o m a g n e t i c ) and Fe2+ i n magnesium o x i d e . The r e l a t i v e a r o u n t o f i r o n which i s reduced t o t h e m e t a l l i c s t a t e i s q u i t e s i m i l a r i n t h e d i f f e r e n t samp l e s and a p p e a r s t o b e independent o f b o t h t h e n a t u r e o f t h e prep r e c u r s o r and t h e i r p r i o r a c t i v a t i o n p r e t r e a t m e n t s . The f r a c t i o n o f reduced i r o n h a s a l s o been observed t o depend o n l y s l i g h t l y on metal loading [21.
370
Fig. 2. In s i t u Mksbauer spectra of reduced MHC- and CP-based cata l y s t s in H2 at 670 K. (a) 16%Fe-NHC-air,H2; (b) 16%Fe-IWC-H2; (c) 8%Fe-MHC-vac,H2; (d) 16%Fe-CP-calc,Hg; (e) 16%Fe-CP-H2.
371 I t i s , however, i n t e r e s t i n g t h a t t h e m e t a l l i c i r o n p a r t i c l e s i n
t h e CP-based c a t a l y s t s a r e much s m a l l e r t h a n t h o s e i n t h e PmC-based o n e s . T h i s i s s e e n from t h e FBssbauer s p e c t r a , where o n l y t h e 16%FeCP-calc,H2
( s p e c t r u m 2d) and t h e 16%Fe-CP-H2 (spectrum 2 e ) samples
show s m a l l p a r t i c l e b e h a v i o r , i . e . a l a r g e s u p e r p a r a m a g n e t i c metall i c i r o n peak ( a t a p p r o x i m a t e l y - 0.2 mm s-') i s p r e s e n t , and a magn e t i c h y p e r f i n e f i e l d smaller t h a n t h a t o f b u l k a-Fe i s o b s e r v e d due t o t h e p r e s e n c e o f c o l l e c t i v e magnetic e x c i t a t i o n s [12,131. These two e f f e c t s a r e n o t dominant i n t h e PWC c a t a l y s t s ( s p e c t r a 2 a , 2b, and 2 c ) , where t h e m e t a l l i c i r o n s p e c t r a l p a r a m e t e r s a r e t y p i c a l o f b u l k a-Fe. I n o r d e r t o o b t a i n i n f o r m a t i o n c o n c e r n i n g t h e Fez+ i o n s , s p e c t r a ( n o t shown h e r e ) w e r e a l s o r e c o r d e d a t 78 K. A t t h i s temDerature two q u a d r u p o l e s p l i t Fe2+ d o u b l e t s w e r e o b s e r v e d . The r e l a t i v e i n t e n s i -
t i e s of t h e s e two d o u b l e t s were a b o u t e q u a l f o r t h e 168Fe-MHC-H2 samp l e . However, f o r CP-based c a t a l y s t s , t h e d o u b l e t w i t h t h e s m a l l quadr u p o l e s p l i t t i n g was t h e dominant one, i n d e p e n d e n t of t h e a c t i v a t i o n p r o c e d u r e . The s p e c t r a can be i n t e r p r e t e d a s Fe2+ b e i n g p r e s e n t i n c l u s t e r s i n t h e Ma0 r a t h e r t h a n i n s o l i d s o l u t i o n [ 1 4 1 . The d o u b l e t s w i t h t h e l a r g e and s m a l l quadrupole s p l i t t i n g s o r i g i n a t e from t h e " i n t e r i o r " and " e x t e r i o r " atoms o f t h e c l u s t e r , r e s p e c t i v e l y . Thus t h e c l u s t e r s i z e a p p e a r s t o b e s m a l l e r f o r t h e CP-based c a t a l y s t s and
i s o n l y s l i g h t l y a f f e c t e d by v a r i o u s p r e t r e a t m e n t s . The m e t a l l i c i r o n p a r t i c l e s i z e c o u l d n o t be measured d i r e c t l y from t h e X-ray d i f f r a c t i o n p a t t e r n s , s i n c e t h e s e c o u l d n o t be recorded in s i t u . The number o f exposed s u r f a c e i r o n atoms c a n , howe v e r , be d e t e r m i n e d from CO c h e m i s o r p t i o n u s i n g a 2 : l Fe:CO s u r f a c e s t o i c h i o m e t r y 1151. T h i s s t o i c h i o m e t r y may n o t h o l d f o r v e r y s m a l l i r o n p a r t i c l e s a s shown r e c e n t l y [ 1 6 ] , b u t w i l l s u f f i c e f o r t h e p r e s e n t estimates of p a r t i c l e s i z e . In order t o c a l c u l a t e the p a r t i c l e s i z e from t h e c h e m i s o r p t i o n measurements, t h e f r a c t i o n o f i r o n reduced t o t h e metallic s t a t e must be known. These f r a c t i o n s can be d i r e c t l y determined from t h e i n s i t u MBssbauer s p e c t r a o b t a i n e d i n hydrogen, and t h e r e s u l t i n g p a r t i c l e s i z e s a r e g i v e n i n F i g . 3 . The r e s u l t s show t h a t f o r b o t h t h e CP- and t h e MHC-based c a t a l y s t s t h e m e t a l l i c i r o n p a r t i c l e s i z e i n c r e a s e s w i t h i n c r e a s i n g metal loading. I t i s s e e n t h a t t h e i r o n p a r t i c l e s i z e i n t h e CP-based c a t a l y s t s i s much s m a l l e r t h a n i n t h e NHC-based c a t a l y s t s w i t h s i m i l a r i r o n loadi n g , and i s a l s o much s m a l l e r t h a n t h a t r e p o r t e d by o t h e r i n v e s t i g a tors [41.
I t i s i n t e r e s t i n g t h a t upon r e d u c t i o n o f t h e d i f f e r e n t p r e -
372
0 Fe IN REDUCED CATALYSTS ( w t %)
Fiq. 3 . M e t a l l i c i r o n p a r t i c l e s i z e a s f u n c t i o n o f i r o n loadi n g and a c t i v a t i o n p r o c e d u r e f o r MHC- and CP-based c a t a l y s t s . The a c t i v a t i o n p r o c e d u r e s ( “ y ” ) a r e specified i n the figure.
Fe
0
5
10
15
20
IN REDUCED CATALYSTS ( w t %)
Fig. 4 . Total s u r f a c e a r e a s of x%Fe-PlHC-H2 ( 0 ) and x%Fe-CP-H2 (A) c a t a l y s t s a s f u n c t i o n o f i r o n loading.
c u r s o r s t r u c t u r e s (y = c a l c , H 2 , y = vaCIH2, o r y = a i r , H 2 ) o r by d i r e c t r e d u c t i o n ( y = H 2 ) o f t h e p r e - p r e c u r s o r , t h e r e s u l t i n g metall i c iron p a r t i c l e sizes are rather similar. The dependence of t h e t o t a l s u r f a c e a r e a s on i r o n l o a d i n g i s dep i c t e d i n Fig.4. I t i s s e e n t h a t t h e p a r t i c l e s i z e o f t h e (Mg,Fe)O p a r t i c l e s ( t h e dominant p h a s e ) i n c r e a s e s g r e a t l y i n t h e p r e s e n c e o f iron. F i g u r e 5 shows a slow s c a n o f t h e ( 2 2 0 ) - r e f l e c t i o n s o f t h e (Mg,Fe)O phase i n 1 6%Fe-MEC-H2 ( F i g . 5 a ) and 16%Fe-CP-H2 ( F i g . 5b) The b a r s r e p r e s e n t t h e l i n e w i d t h , IIBET, which s h o u l d be e x p e c t e d from t h e a v e r a g e p a r t i c l e s i z e c a l c u l a t e d from t h e measured s u r f a c e a r e a s . The f i g u r e shows t h a t t h e o b s e r v e d l i n e w i d t h i s much l a r g e r f o r t h e 1WC-based c a t a l y s t t h a n t h a t c a l c u l a t e d from t h e s u r f a c e a r e a , wherea s a good agreement i s o b t a i n e d i n t h e CP-based c a t a l y s t i n d i c a t i n g a s i n g l e (Hg,Fe)O phase. The s o u r c e o f e x c e s s i v e l i n e b r o a d e n i n g i n t h e MHC-based c a t a l y s t i s undoubtedly t h e e x i s t e n c e o f a d i s t r i b u t i o n o f (E”,g,Fe)Op h a s e s w i t h d i f f e r e n t composition and c r y s t a l l i t e s i z e s . The above t y p e o f b e h a v i o r i s q u i t e g e n e r a l f o r a l l t h e d i f f e r e n t c a t a l y s t s investigated.
.
373
I
I
I
I
I
64
63
62
61
D
DIFFRACTION ANGLE (0213)
F i g . 5. X-ray d i f f r a c t i o n p r o f i l e s o f t h e ( 2 2 0 ) - r e f l e c t i o n s o f t h e (Mg,Fe)O phase i n reduced c a t a l y s t s . (a) 16%Fe-1HC-H2; ( b ) 16%FeCP-H2. The e x p e c t e d l i n e w i d t h s , rBET, c a l c u l a t e d on t h e b a s i s o f measured t o t a l s u r f a c e a r e a s a r e a l s o shown.
4 GENESIS OF CATALYSTS
On t h e b a s i s o f t h e r e s u l t s , s c h e m a t i c models are proposed f o r t h e g e n e s i s o f t h e &HC- and CP-type c a t a l y s t s . F i g u r e 6 i l l u s t r a t e s t h e s i t u a t i o n where t h e c a t a l y s t i s produced v i a a c a l c i n e d p r e c u r s o r s t r u c t u r e . A more g e n e r a l d i s c u s s i o n w i l l be g i v e n i n s e c t i o n 5. 4 . 1 MHC c a t a l y s t s
The p r e p a r a t i o n o f MHC c a t a l y s t s c a n b e d e s c r i b e d a c c o r d i n g t o t h e sequence p r e s e n t e d i n F i g . 6 , a s f o l l o w s : - During t h e i o n exchange of t h e i n i t i a l l y l a r g e ( a p p r o x . 3 0 0 ~ 2 0nm [21) c r y s t a l s o f ImC, o n l y t h e s u r f a c e r e g i o n o f t h e s e c r y s t a l s be-
comes "exchanged" w i t h i r o n ( F i g . 6a)
-
.
The s t r u c t u r e which forms upon decomposition (Fig.6b) o f t h e " s u r f a c e exchanged" MHC c r y s t a l s r e t a i n s some o f t h e c h a r a c t e r i s t i c s o f t h e p r e - p r e c u r s o r s t r u c t u r e . R a t h e r l a r g e i r o n r i c h MgO p a r t i c l e s
form a t t h e e x t e r i o r r e g i o n s o f t h e p r e - p r e c u r s o r p a r t i c l e s . A t t h e regions corresponding t o t h e i n t e r i o r of t h e pre-precursor, very s m a l l MgO p a r t i c l e s f r e e o f i r o n a r e formed.
3 74
GENESIS OF CATALYSTS MHC-TYPE
m-
mHc CRYSTAL
-
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--c
"SURFACE" EXCHANGE
CALCINATION
REDUCTION
a
b
C
Mi0
CP-TYPE
-
COPRECIPITATIONd MIXED HYDROXIDE
d
Feo
-
CALCINATION
REDUCTION
e
f
F i g . 6 . Schematic models f o r t h e g e n e s i s o f t h e NHC- and CP-type catalysts.
- During t h e r e d u c t i o n o f t h i s p r e c u r s o r s t r u c t u r e ( F i g . 6 ~ )o~r t h e d i r e c t r e d u c t i o n of t h e p r e - p r e c u r s o r , m e t a l l i c i r o n p a r t i c l e s a r e produced. These p a r t i c l e s a r e , however, q u i t e l a r g e due t o t h e v e r y h i g h l o c a l i r o n c o n c e n t r a t i o n , which i s much h i g h e r t h a n t h e a v e r age c o n c e n t r a t i o n . A s a r e s u l t of t h e r e d u c t i o n p r o c e s s , t h e c a t a l y s t produced from t h e MHC . p r e - p r e c u r s o r c o n s i s t s of s e v e r a l t y p e s o f p h a s e s , t h e most i m p o r t a n t of which a r e :
-
a q u i t e p u r e MgO p h a s e w i t h s m a l l p a r t i c l e s i z e MgO p h a s e ( s ) w i t h a h i g h c o n t e n t o f Fe2+ p r e s e n t a s l a r g e
-
clusters m e t a l l i c i r o n p a r t i c l e s with 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 and l a r g e a v e r a g e s i z e .
4 . 2 CP c a t a l y s t s
The p r e p a r a t i o n of t h e c o p r e c i p i t a t e d c a t a l y s t s i s t h o u g h t t o proceed as f o l l o w s :
- The c o p r e c i p i t a t i o n p r o c e d u r e e n s u r e s t h a t t h e mixed h y d r o x y n i t r a t e p r e - p r e c u r s o r w i l l have a homogeneous i r o n d i s t r i b u t i o n ( F i g . 6 d )
375
w i t h a much lower l o c a l i r o n c o n c e n t r a t i o n t h a n i n t h e MHC p r e p r e c u r s o r o f t h e same o v e r a l l m e t a l l o a d i n g .
-
S i m i l a r l y , t h e p r e c u r s o r s t r u c t u r e ( F i g . 6 e ) w i l l form a s i n g l e
iron-magnesium o x i d e p h a s e w i t h i r o n c o n c e n t r a t i o n and p a r t i c l e s i z e l y i n g between t h e i r o n r i c h and i r o n poor (llg,Fe)O r e g i o n s formed i n t h e MHC-based p r e c u r s o r s .
-
A s t h i s homogeneous p r e c u r s o r i s reduced ( F i g . G f ) , q u i t e s m a l l
p a r t i c l e s should r e s u l t a s compared t o t h a t o f WIG-based c a t a l y s t w i t h s i m i l a r m e t a l l o a d i n g . A l s o , it i s e x p e c t e d , due t o t h e homogeneous s t r u c t u r e o f t h e o x i d i c compounds b e f o r e r e d u c t i o n , t h a t t h e r e s u l t i n g m e t a l l i c i r o n p a r t i c l e s i z e d i s t r i b u t i o n o f t h e reduced c a t a l y s t s h o u l d be q u i t e narrow. The reduced c a t a l y s t s w i l l t h e r e f o r e contain t h e following phases :
-
a MgO phase o f medium p a r t i c l e s i z e w i t h l o w c o n t e n t o f Fez+ p r e s e n t i n s m a l l c l u s t e r s
-
m e t a l l i c i r o n p a r t i c l e s w i t h narrow p a r t i c l e s i z e d i s t r i b u t i o n and s m a l l a v e r a q e s i z e .
5 DISCUSSION The p r s d u c t i o n o f s m a l l m e t a l l i c i r o n p a r t i c l e s u s i n g llg0 a s a s u p p o r t may be u n d e r s t o o d i n terms o f t h e v a r i o u s i r o n - and magnes i u m - c o n t a i n i n g compounds e n c o u n t e r e d i n t h e g e n e s i s o f t h e reduced c a t a l y s t s . The f a c t t h a t t h e p r e c u r s o r s s t a r t i n g from CP pre-prec u r s o r s a r e much more homogeneous and have lower l o c a l i r o n concent r a t i o n i s s e e n from b o t h t h e Mgssbauer and t h e X-ray r e s u l t s . The X-ray p a t t e r n s of t h e C P p r e c u r s o r s , i n c o n t r a s t t o t h o s e o f t h e P%lC p r e c u r s o r s , showed s i n g l e phase b e h a v i o r and l i n e w i d t h s e q u a l
rBET. The l o w local i r o n c o n c e n t r a t i o n i n t h e CP p r e c u r s o r s w a s s e e n from t h e Mgssbauer s p e c t r a , where magnetic o r d e r i n g was a b s e n t
to
even a t 78 K , i n c o n t r a s t a g a i n t o t h e s i t u a t i o n e n c o u n t e r e d f o r t h e MHC p r e c u r s o r s . These f a c t s imply t h a t t h e decomposition of t h e p r e - p r e c u r s o r s t r u c t u r e o c c u r s b e f o r e a d i s t r i b u t i o n of i r o n can be a t t a i n e d . A s a r e s u l t , t h e EHC-based c a t a l y s t s formed a r e q u i t e inhomogeneous, whereas t h e CP-based c a t a l y s t s w i l l be r a t h e r honogeneous. I n t h i s way t h e systems w i l l r e t a i n a "memory" of t h e n a t u r e o f t h e p r e p r e c u r s o r which c a n n o t be " e r a s e d " by s u b s e q u e n t t r e a t m e n t s , s i n c e i n t e r - p a r t i c l e e q u i l i b r i u m w i l l n o t be e s t a b l i s h e d . The s i m i l a r i t y i n s t r u c t u r e and p a r t i c l e s i z e o f t h e reduced c a t a l y s t s a c t i v a t e d .
376 i n d i f f e r e n t ways s u g g e s t s t h a t under r e d u c i n g c o n d i t i o n s , intrap a r t i c l e e q u i l i b r i u m seems t o be e s t a b l i s h e d , hence e r a s i n g d i f f e r ences i n t h e previous precursor s t r u c tu r e s . Thus t h e two main f a c t o r s a f f e c t i n g s t r u c t u r e and metallic i r o n p a r t i c l e s i z e i n t h e reduced c a t a l y s t s a r e t h e m e t a l l o a d i n g and t h e type of pre-precursor. Both c h e m i s o r p t i o n and 1 8 s s b a u e r r e s u l t s show t h a t v e r y s m a l l
m e t a l l i c i r o n p a r t i c l e s c a n be produced from t h e c o p r e c i p i t a t e d pre-precursors.
For example, t h e m e t a l l i c i r o n p a r t i c l e s i z e i n t h e
16%Fe-CP c a t a l y s t s i s much s m a l l e r t h a n t h o s e o f t h e 16%Fe-MHC cata l y s t s . Indeed, from t h e r e s u l t s o f Boudart e t aZ. [ 2 , 6 ] it a p p e a r s t h a t t h e m e t a l l o a d i n g must be reduced t o c a . 1 % i n t h e MIC-based c a t a l y s t system i n o r d e r t o produce t h e s m a l l a v e r a g e m e t a l l i c i r o n p a r t i c l e s i z e r e p o r t e d h e r e f o r t h e 16%Fe-CP c a t a l y s t s . From a comparison between t h e IGssbauer s p e c t r a o f t h e l%Fe-?THCH2 c a t a l y s t r e p o r t e d i n [ 2 ] , a n d o f t h e 16%Fe-CP-calc,H2 shown i n F i g . 2 d I w e can g e t i n f o r m a t i o n r e g a r d i n g t h e p a r t i c l e s i z e d i s t r i b u t i o n . Both samples have a b o u t t h e same a v e r a g e p a r t i c l e s i z e and f r a c t i o n o f s u p e r p a r a m a g n e t i c m e t a l l i c i r o n a t 6 7 0 K . From a s t u d y o f t h e magnitude o f t h e h y p e r f i n e f i e l d e x h i b i t e d by t h e f r a c t i o n of t h e m e t a l l i c i r o n t h a t is magnetically s p l i t , t h e p a r t i c l e s a t t h e upper end o f t h e p a r t i c l e s i z e d i s t r i b u t i o n can be s t u d i e d . A s mentioned i n s e c t i o n 3 . 3 , t h e M6ssbauer l i n e s o f t h e CP c a t a l y s t have s h i f t e d due t o t h e p r e s e n c e o f c o l l e c t i v e magnetic e x c i t a t i o n s . Thus, t h e l a r g e m e t a l l i c i r o n p a r t i c l e s i n t h i s sample a r e s t i l l q u i t e s m a l l . T h i s i s i n c o n t r a s t t o t h e b e h a v i o r o f t h e 1%Fe-fitHC-H2 c a t a l y s t where t h e r e s u l t s do n o t i n d i c a t e t h e p r e s e n c e o f c o l l e c t i v e magnetic e x c i t a t i o n s . The p a r t i c l e s a t t h e upper end o f t h e p a r t i c l e s i z e d i s t r i b u t i o n must, t h e r e f o r e , b e q u i t e l a r g e , and t h e p a r t i c l e s i z e d i s t r i b u t i o n must be r a t h e r broad. I n t h e p r e v i o u s s t u d y 1 2 1 o f d i r e c t l y r e d u c e d YHC-based c a t a l y s t s (x%Fe-NHC-H2), t h e f r a c t i o n o f t h e i r o n reduced t o t h e metall i c s t a t e was found t o b e a l m o s t i n d e p e n d e n t o f i r o n l o a d i n g . The p r e s e n t r e s u l t s c o n f i r m t h e s e f i n d i n g s , b u t show a l s o t h a t t h e f r a c t i o n o f reduced m e t a l i s i n d e p e n d e n t of b o t h t h e t y p e of pre-prec u r s o r and p r e c u r s o r . T h i s b e h a v i o r i s q u i t e i n c o n t r a s t t o t h e beh a v i o r o f , f o r example, s i l i c a - and alumina-supported c a t a l y s t s ( f o r a review see [ 1 7 ] ) , where t h e f r a c t i o n o f reduced m e t a l v a r i e s w i t h m e t a l l o a d i n g . The p r e s e n c e o f s t a b l e Fe2+ c l u s t e r s i n F q O may be t h e r e a s o n f o r t h e r a t h e r c o n s t a n t d e g r e e o f r e d u c t i o n o f
371
the metal in the Fe/FgO system and may furthermore be the cause of the similarity in structure and metallic iron particle size of catalysts reduced from different precursors. The strong interaction of part of the iron with the support, forming clusters, may also explain why small, thermostable iron particles are easily formed when MgO is used as a support. In this way MgO may provide a unique and highly desirable support-interaction for producing small metallic iron particles.
REFERENCES
1 A-Nielsen, An Investigation on Promoted Iron Catalysts for the Synthesis of Ammonia, 3rd edn., Jul. Gjellerups Forlag, Copenhagen, 1968. 2 M-Boudart, A.Delbouille, J.A.Dumesic, S.Khammouma and H.Tops@e, J.Cata1. , 37 (1975)486. 3 J.A.Dumesic, H.Topsge, S.Khammouma and ILBoudart, J.Cata1. 37 (1 975)503. 4 P.Bussi&re, R.Dutartre, G.A.Martin and J-P.Mathieu, C.R.Acad. Sci. (Paris), sec.6, 280C, (1975)1133. 5 R-Dutartre, 1I.Primet and G.A.Martin, Reac.Kinet.and Catal.Letters, 3 (1 975)249. 6 M.Boudart, J.A.Dumesic and H.Topsfie, Proc.Natl.Acad.Sci.(USA), 74(1977)806. 7 E.G.Derouane and P.Monseur, Chem.Phys.Letters, 44(1976)114. 8 M.Akao, F.Marumo and S.Iwai, Acta Cryst. B30(1974)2670. 9 H.Wiedersich, in "Proceedings of the Second Symposium on Low Energy X- and Gamma Sources", (P.S.Baker and JLGerrard, Eds.) Austin, TX, 1967. 10 V.G.Bhide and B.R.Tambe, J.Plater.Sci., 4(1969)955. 1 1 N.R.Large, R.W.Wilkinson and R.J.Bullock, U.K.Atomic Energy Authority Research Group Report, AERE-R 5580, Chem.Division, Atomic Energy Research Establishment, Harwell, Berkshire,l968. 12 S.M@rup and H.Tops@e, Appl.Phys., 11(1976)63. 13 S.Pl@rup, H.Topsfie and J.Lipka, J.Phys. (Paris), 37(1976)C6-287. 14 V.V.Kurash, V.I.Gol'danskii, T.V.Malysheva, V.S.Urusov, L.M. Kuznetsov and L.A.Moskovkina, 1norg.Materials (USSR), 7 (1971)1395. 15 P.H.Emmett and S.Brunauer, J.Amer.Chem.Soc. 56(1934)35. 16 N.Tops@e, H.Tops@e and J.A.Dumesic, Unpublished results. 17 J.A.Dumesic and H.Tops@e, Advan.Cata1. 26(1977)121. DISCUSSION L.
GUCZI
: You
s a i d t h a t Fe 2+ c l u s t e r s a r e r e s p o n s i b l e t o p r e v e n t
t h e m i g r a t i o n of Feo.
Can t h e i n t e r a c t i o n b e t w e e n MgO a n d F e 2 +
c l u s t e r s he t h e r e a s o n f o r t h e h i n d r a n c e o f t h i s m i g r a t i o n ?
H. TOPS$E port,
: In order
t o s t a b i l i z e s m a l l m e t a l p a r t i c l e s on a s u p -
one n e e d s a s t r o n g i n t e r a c t i o n between t h e m e t a l and t h e
support.
T h i s i n t e r a c t i o n may he so s t r o n g t h a t p a r t o f t h e a t o m s
are not reduced to the metallic state.
In the present case, the
fraction of the iron atoms which are not reduced to the metallic 2+ state, is present as Fe -rich clusters in the MgO. We believe, as illustrated in Fig. 6, that the metallic iron particles are interacting with these clusters and in this way are prevented from migration.
L.L. VAN REYEN : In view of the similarity in the crystal structures of the magnesium-iron-hydroxy nitrates and carbonates used as preprecursors for the preparation of iron/magnesia catalysts and the corresponding nickel-aluminium layer compounds from which nickel/ alumina catalysts are made, do you have experience about the relative merits of carbonates or nitrates as starting materials with respect to the activity and stability of the iron/magnesia catalysts prepared therefrom ? H. TOPSdE : In an earlier investigation (ref. 3 in the paper) the ammonia synthesis activity of magnesium hydroxycarbonate-based catalysts was studied.
It was found that the turnover number de-
creased with decreasing particle size. found for the coprecipitated catalysts.
A
similar behavior has been However, some differences
were observed between the two types of catalyst. These differences can be understood taking into account the differences in particle size distribution found,as discussed in the present paper. With respect to the stability of these catalysts, they appear to
be quite thermostable, since no significant changes in chemisorption or catalytic properties occurred after prolonged exposure to the reaction mixture. M. CARBUCICCHIO
:
Have you made a computer fit to the spectra for
1 6 % Fe-CP-ca1c.-H2 and 16% Fe-CP-H2 (Fig. 2 d and e) in order to
individualise, among other things, the position at -0.2 mm/sec of the superparamagnetic iron peak 7
From the figures, this is not
apparent. H. TOPSdE
:
Yes.
The spectra in Fig. 2 have been computer-fitted,
but the fits are not shown.
The positions of the superparamagnetic
as well as those of the hyperfine split metallic iron peaks have shifted from zero velocity due to the second order Doppler shift (the spectra were obtained at 670'K).
379 E.J.
NEWSON
:
1 ) W e r e t h e co-precipitated c a t a l y s t s ( C P ) p r e p a r e d
by h e t e r o g e n e o u s o r h o m o g e n e o u s c o - p r e c i p i t a t i o n ?
Can you give
a c t u a l p H r a n g e s used in the p r e p a r a t i o n ? 2 ) What w e r e t h e c o r r e s p o n d i n g activity r e s u l t s for t h e MHC c a t a l y s t s and t h e CP c a t a l y s t s ?
W h i c h r e a c t i o n w a s used ? P r e s u m a b l y
a c t i v i t y t e s t s w e r e c a r r i e d out at i d e n t i c a l l y s i m i l a r r e a c t i o n conditions ?
H. T O P S d E : T h e p r e c i p i t a t i o n b e h a v i o u r o f iron and m a g n e s i u m i s quite different, which makes homogeneous precipitation methods difficult. T h e p r e c i p i t a t i o n w a s carried o u t w i t h excess o f NH40H w i t h o u t f o l l o w i n g t h e pH.
H o w e v e r , t h e X-ray and M B s s b a u e r d a t a
s h o w e d t h a t t h e r e s u l t i n g c a t a l y s t s w e r e very h o m o g e n e o u s in struct u r e and not just a m i x t u r e o f components. W i t h r e g a r d t o t h e s e c o n d p a r t of t h e q u e s t i o n , * w e r e f e r t o t h e a n s w e r t o L.L.
J.W.
VAN REYEN.
H I G H T O W E R : Do y o u o b s e r v e t h e f o r m a t i o n o f t h e very s t a b l e
f e r r i t e MgFe204 in any o f y o u r w o r k ?
A. T O P S 0 E : I n some o f t h e oxidic p r e c u r s o r s , MgFeZ04 m a y b e present. i n Fig.
A s m e n t i o n e d i n t h e paper, the a n a l y s e s o f t h e s p e c t r a s h o w n 1 are q u i t e difficult.
s p i n e l p h a s e s b e i n g present.
H o w e v e r , t h e y point t o o n e o r m o r e T h e m a g n e t i c c o m p o n e n t s could b e d u e
t o a m i x t u r e o f M g F e 2 0 4 and F e 3 0 q o r to t h e p r e s e n c e of m i x e d p h a s e s of t h e c o m p o s i t i o n MgxFe3-x04.
In the reduced catalysts,
M g F e 2 0 4 i s clearly not p r e s e n t , s i n c e
t r i v a l e n t iron i s n o l o n g e r
observed.
A. K U B A S O V : It i s k n o w n t h a t t h e s u r f a c e o f M g O is usually c o v e r e d w i t h c a r b o n a t e - l i k e species.
S u c h s p e c i e s c h a n g e t h e s u r f a c e pro-
p e r t i e s rather d r a s t i c a l l y , b u t it i s d i f f i c u l t t o r e m o v e them from t h e surface.
T h e r e f o r e , I s u p p o s e t h a t such s p e c i e s exist o n M g O
in y o u r c a s e too.
W h a t d o y o u t h i n k about t h e p o s s i b l e i n f l u e n c e
of t h e m o n t h e p r e p a r a t i o n o f y o u r c a t a l y s t s ?
H. TOPS$E
:
We h a v e d o n e T G A s t u d i e s o f t h e d e c o m p o s i t i o n of b o t h
t h e MHC- and CP-based catalysts.
T h e s e r e s u l t s show t h a t t h e
c a r b o n a t e d e c o m p o s e s at a q u i t e h i g h t e m p e r a t u r e and, as y o u suggest, y o u m a y h a v e c a r b o n a t e s p e c i e s left o n t h e surface. b e e n s h o w n t o b e t h e c a s e by I R studies.
This has also
However, t h e d i f f e r e n c e in
380 the nature of the decomposition behavior of
the MHC- and CP-pre-
cursors does not seem to influence greatly the structure of the final catalysts. A. O Z A K I : I s there any difference in particle size of Fe when you
reduce by a N2-H2 mixture H. TOPSdE
:
?
We have tried to reduce the catalysts in both very pure
hydrogen and in N2-H2 mixture and found no significant differences in the metallic iron particle size.
However, the initial activity
toward NH3 synthesis varied greatly, depending on whether or not nitrogen was present in the reduction gas (see ref. 3 in the paper). The results were interpreted in terms of a nitrogen induced reconstruction of the surface of the metallic iron particles (see also : J.A.
Dumesic, H. Topsbe, and M. Boudart, J. Catal.
37, 513-622
(1975), and M. Boudart, J.A. Dumesic, and H. Topsbe, Proc. Natl. Acad. Sci. U . S . A .
74, NO3
(1977), Chemistry p. 806-810.
381
RUTHENIUM CATALYSTS FOR AMMONIA SYNTHESIS PREPARED BY DIFFERENT METHODS
A.
OZAKI, K.
URABE, K. SHIMAZAKI and S. SUMIYA
Research Laboratory of Resources U t i l i z a t i o n , Tokyo I n s t i t u t e of Technology, Nagatsuta, Midori-ku, Yokohama(Japan)
ABSTRACT c a t a l y s t s promoted by a l k a l i were prepared by impregnation of A1203 2 3 w i t h 1) RuCl -KN03(or CsN03) mixture, o r 2 ) K4Ru(CN16 followed by reduction with 3 hydrogen. The n i t r a t e anion was mostly coverted t o NH giving h i g h l y e f f i c i e n t 3 and w a t e r - r e s i s t a n t ammonia c a t a l y s t s . I n p a r t i c u l a r , t h e cesium oxide-promoted Ru/A1 0
c a t a l y s t was a s a c t i v e a s t h a t promoted by m e t a l l i c potassium.
I n t h e hydrogen
treatment of K4Ru(CN)6/A1203, t h r e e o r more CN l i g a n d s were converted t o CH4 and N H 3 , suggesting t h a t K ( 1 ) i n a d d i t i o n t o Ru(I1) i s reduced t o metal by hydrogen,
which i s supported by hydrogen evolution upon water treatment as w e l l a s d r a s t i c poisoning by water vapor.
INTRODUCTION I t has been shown t h a t t h e t r a n s i t i o n metals a r e remarkably promoted by a d d i t i o n
o f m e t a l l i c potassium f o r t h e ammonia s y n t h e s i s ( r e f . 1 and 2) as w e l l as f o r t h e i s o t o p i c e q u i l i b r a t i o n of n i t r o g e n ( r e f . 2 and 3) and t h a t ruthenium is t h e most a c t i v e element f o r t h i s c a t a l y s t system ( r e f . 1 and 3 ) .
The r o l e of potassium
has been shown t o be an e l e c t r o n donor, a s evidenced by remarkable i n c r e a s e i n t h e a c t i v i t y with decrease i n t h e i o n i z a t i o n p o t e n t i a l of a l k a l i metal a s the promoter, i . e . C s > 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.
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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.
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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
<
c
u W 2
LL W
CL
40
30
20
10
1 o-3
F /ern-'
Fig.3. Reflectance spectra of HSA NiO-MgO solid solution (MN 5). After admission of H20 vapour (20 Torr); 1 - Outgassed 17 h at 1000°C; 2 After admission of O2 at 0.25 Torr (4) 3 Outgassed 17 h at 1000°C; 4 and 5 As 5, but with scale shifted vertically; 6 - Outgassed 3 h and 180 Torr (5); 5' at 50OoC; 7 - Outgassed 17 h at 800OC. Reflectance scales at right-hand side.
-
-
-
-
0.75
-
R 0.50
-
0.25
-
40
I
I
38
20
1
10 1 o-% /cm -I
Fig.4. Reflectance spectra at stages during the preparation of HSA Coo-MgO (MCo 5). Sequence of spectra from hottom to top: Co-impregnated Mg(OH)2 outgassed 17 h at 30O0C, further 17 h at 40O0C, further 5 h at 500°C, further 6 h at 6OO0C, respectively. Prior to recording top spectrum, sample was given further 17 h at 700°C, 17 h at 8OO0C and 17 h at 1000°C, always in vacuo.
427
recalling Table 1, the surface area is fully developed.
At 5OO0C and above,
however, a large general decrease in absorption (increase in reflectance) occurs and a spectrum develops which exhibits four +-A bands, the one at highest
being
For the 1000°C-treated oxide, which was pale blue in colour, the -1 -1 -1 bands (reflectance minima) occur at 7,000cm , 9,000cm (shoulder), 13,500 cm a triplet.
This spectrum is certainly not that and 16,800, 18,300, 19,500 cm-' (triplet). 2+ -1 expected for octahedral Co , which exhibits bands at 8,500 and 19,800 cm , but 2+ contains instead features typical for tetrahedrally-coordinated Co [30]. Exposure to H 0 vapour at 2OoC (Fig.5, Curves 1 and 2) produced a dramatic 2 change towards a typically octahedral Co2+ spectrum, and the solid became pink, Outgassing and heating very readily regenerated the blue colour, and the prominent features of the original spectrum were almist completely recovered on outgassing CO produced the changes shown in Fig.5, Curves 3 and 4. At 1 Torr, 2 -1 -1 there was decrease in absorption at 7,000cm , at 9,000cm and in the triplet -1 -1 region, and the 13,500 cm band was replaced by a shoulder at 15,000 cm ,
at 40OoC.
Further exposure (not illustrated in Fig.5) continued to decrease the original -1 bands and a shoulder developed at 8100 cm . Even at high exposures (p > 200 Torr for several hours), however, the effect was less pronounced than with H 0 vapour at 2
R
1"
0.6 /2
/'
04
04
t
I
I
1
1
LO
30
20
10
04
I 0-33/cm-l
Effect of H 0 vapour and CO on reflectance spectrum of MCo 5. 2 2 Outgassed 24 h at 1000°C; 2 After admission of H 0 vapour at 2OoC (2 T o r r ) ;
Fig.5.
3 1
-
Outgassed 17 h at lO0OoC; 4
-
2
After admission of C02 at 2OoC (1 Torr).
428 low exposures (p
-
2 Torr for a few minutes).
Progressive outgassing retraced
the spectral changes exactly, but a high temperature (8OOOC) was necessary to recover completely the original features, notably the transformation of the 15,000 cm-'
-1
shoulder into the 13,500 cm
band.
The effect of oxygen at 2OoC on the spectrum of 1000°C-outgassed MCo is shown in Fig.6.
As with MN, the principal effect was the development of a broad back-
ground absorption.
Stepwise outgassing led to a complete recovery of the original
spectrum; the colour, which had become dark brown in oxygen, reverted to blue.
40
30
20
10
10-~i/~;n-~
Fig.6. Effect of oxygen at 2OoC on reflectance spectrum of MCo 5. 1 - Outgassed 17 h at 1000°C; 2 - 0.08 Torr 02; 3 - 0.040 Torr 02; 4 - 1.20 Torr 0 2; 5 - 4.0 Torr 02; 6 - 12 Torr 02; 7 - 5 0 Torr 02;8 - 160 Torr 02 , after 17 h.
The effect of 0 on the 7,000cm-' band was examined in detail using a very slow 2 scanning speed; this was undertaken because of an apparent discrepancy with our previously reported result [23] which indicated a decrease in absorption at 7.000 cm-l on exposure to oxygen.
The slow-scan results, transposed into a
Kubelka-Munk function, now lead us to the conclusion that oxygen does
not
decrease
the 7 , 0 0 0 cm-l band, any apparent effect in experiments using normal scanning being an artefact caused by the complicating influence of the change in background absorption. The effect of air on 1000°C-outgassed MCo was to give spectral changes which
429 were a combination of those reported for H 0, CO 2
colour changed to reddish-brown.
and 02, as expected, and the blue
2
Outgassing did not produce marked spectral changes
until the temperature exceeded 4OO0C, but thereafter the broad background absorption decreased progressively. Adsorption calorimetry Air-exposed samples of HSA NiO-MgO were outgassed at 50O0C and the adsorptions
of NO and CO were studied volumetrically and calorimetrically at O°C.
*
oxide coverage on MN 10 at O°C tended to a limiting value of with
-
1% for pure MgO (Fig.7).
-
Nitric
4%, as compared
MN 5 showed an intermediate value.
The heat
of adsorption of NO, however, was not greatly affected by the presence of nickel. -1
MgO, MN 5 and iUN 10 all show heats between 2 0 and 12 kcal mol
(Fig.B), although
there is a trend whereby the heat falls less rapidly with coverage as nickel content increases.
A fraction of the adsorbed gas could be removed by evacuation
at O°C, and readsorption occurred with heats of adsorption equal to those of the -1
previous limiting values (12-15 kcal mol
).
Carbon monoxide coverages were much less than those of NO. where NO coverages on MN 5 and MN 10 were
-
At p = 0 . 5 Torr,
1%, CO coverages were
-
0.2%.
Heats
of adsorption were also somewhat lower, with initial values of 16 kcal mol-I in both cases. Heats of adsorption of NO and CO were also studied on MSA MN 5 and MN 10 2 -1
(surface areas of 46 and 42 m g
Interaction of NO and CO was studied calorimetrically on MSA
to those on HSA MN. MN 10.
respectively), with generally similar results
After outgassing for 15 h at 5OO0C, NO was adsorbed to saturation at O°C
and then the reversible fraction was removed by pumping for 15 h at O°C.
CO was
then dosed to the surface carrying presorbed NO and the uptake and the heat of The total uptake amounted to a coverage increase of -1 0.1% and the heat of adsorption was 95 kcal mol This is to be compared with
adsorption were measured.
.
only 16 kcal mol
-1
for the surface without presorbed NO.
After outgassing again
at 5OO0C, the experiment was repeated in the reverse sequence, NO being dosed to the surface carrying presorbed CO. The initial dose of NO registered a heat of -1 -1 , as compared with 15-20 kcal mol for the surface
adsorption of 36 kcal mol without presorbed CO. -1 mol 1.
Subsequent doses registered normal heats (15 and 12 kcal
This clear evidence of interaction of CO and NO at O°C manifests itself as a true catalysis of the CO
+
NO reaction at higher temperatures.
temperature for sustained catalysis of the CO
*
+
The threshold
NO reaction over HSA MN 10 in a
The definition of 100% coverage is taken as 1 molecule adsorbed per anion-cation pair in the surface.
430
0
I
I
PRESSURE/Torr
10
Adsorption isotherms of NO at O°C on HSA MgO, MN 5 and MN 10.
Fig.7.
H E A T OF ADSORPTION
: J rno1-l
o M g O 1270rn2g‘11 a M N 5 ( 1 5 0 rn2g’ll
k c a l rn01-l
00
20
a 0.
@
a
a
8
0
10
0
1
1
I
CO V E R A G E
Fig.%.
%
Heats of adsorption of NO at O°C on HSA MgO, MN 5 and MN 10.
flow system* was found to be
-
the products being CO and N20. N2° 2 and above 5OO0C N was the principal nitrogenous
3OO0C,
production was maximum at 4OO0C,
2
product.
*We
are indebted to Mr.J.Greenwood for carrying out these experiments.
431 DISCUSSION The X-ray diffraction results, supplemented by the electron diffraction results in the case of Coo-MgO, afford good evidence that the HSA oxides are true solid solutions.
The lattice parameter variations (Fig.2) are the same as those
reported for LSA oxides prepared in air at 1000°C. lines converge at zero solute concentration to a of well-crystallized MgO [29).
Moreover, the two straight 0
= 4.2116 A, the lattice parameter
Although Co and Ni have the same metallic radii,
their divalent ionic radii in 6-coordination are significantly different. 2+ 8 Substitution of Ni for Mg2+ in MgO leads to a decrease of . a because of the d 2+ electronic configuration of Ni and consequent high crystal field stabilization when octahedrally coordinated.
By the same token, the increase of . a
on
substituting Co2+ shows that the d7 ion must be in the high spin state 5 2 [(tZg) (eg) 1. A more exacting test for true solid solution is the magnetic behaviour.
Here
again it was important to study material which had been outgassed at high temperature and maintained in vacuo or an inert atmosphere.
When this was done (Table 3), the
(3.25 to 3.59 B.M. for Ni Mg 0 and 5.14 to 5.42 B.M. for Co Mg 0) eff x 1-x x 1-x were entirely consistent with expectations for octahedrally-coordinated Ni2+ and 2+ octahedrally-coordinated spin-free Co respectively. This agrees with the X-ray values of
)1
results, where the a
values implied simple substitutional solid solution with good
distribution of solute in the bulk.
Magnetic data give additional information on
solute distribution through the magnitude of the Weiss constant.
The low values
of - 9 confirm that the magnetic ions are well dispersed and that clustering (if any) must be on a very minor scale. Table 4 shows the effects on the magnetic properties of hydration in air and subsequent dehydration.
The MN results are those one would expect:
Geff remains
essentially unaltered since Ni2+ is still octahedrally coordinated in the hydroxide but the exchange interaction (as reflected in the Weiss constant) is weakened because of the expanded and lower symmetry structure. outgassing.
The normal value is regained on
The MCo results, however, reveal a surprising effect.
very low values on hydration, especially with the very dilute MCo 3.
i.leff
falls to
It is not
possible to account for ~c
values of 3.73 and 4.22 B.M. on the basis of a change eff 2+ in coordination, e.g. tetrahedral Co , which would in any case be unexpected on 2+ hydration. Moreover, 3.73 B.M. is below the spin-only value for spin-free Co , 2+ and the OH- anion does not have a strong enough field to give spin-pairing of Co .
We interpret this result as showing that hydration is accompanied by oxidation of . some of the cobalt to Co3+, with increased Dq leading to spin-pairing in the d6ion
(Cceff
= 0).
The overall magnetic moment is therefore drastically reduced.
clear that the oxidation is more complete the more dilute the solid solution.
It is The
changes are reversed on outgassing, but it is noteworthy that an outgassing at 4OO0C, although more than adequate to regenerate the surface area (cf.Table l), does
432
not regenerate
2+
the original conditions of wholly divalent Co
highlights the fact that even minor contamination of Co Mg
x
1-x
.
This result
0 with atmospheric
gases will lead to values of Ueff rather below the octahedral value for high spin Co2+ due to superficial oxidation.
The lattice parameter and magnetic results reflect primarily the properties Of the bulk.
The data so far discussed are in complete accord with the HSA oxides 2+
being true solid solutions with the solute ions (Ni
2+
and Co
)
well distributed in
the bulk and correctly substituted there for Mg2+ ions.
To obtain information more specifically on the surface region of the crystallites, we turn to the reflectance spectroscopy and adsorption results.
The spectrum of
1000°C-outgassed HSA MN (Fig.3) shows d-d absorption at 5,000 cm-l which is additional to that expected for Ni2+ in 0 symmetry. This absorption is h selectively reduced by contact with H 0 vapour, as also is the broad absorption 2
at ca. 24,000 cm-'.
-1
We believe that the absorption at 5,000cm
and at ca.
24,000 cm-' must be due to Ni2+ species specifically associated with the surface. A tentative assignment is that the species contributing are recessed Ni2+ ions in ) symmetry on the {loo) faces. By analogy with the 4v spectra of 5-fold Ni2+ complexes C31.321, a CllVfit is consistent with high spin
5-fold square pyramidal ( C -1
Ni2+ and Dq = 1000 cm
,
the apical angle caused by recession of the cation from
the cube face [33] being taken as
looo
[34];
-1
the 5,000 and 24,000cm
absorptions
can then be assigned as the transitions from 3B to the lower-lying 3E(F) and to 1 3 the E(P) levels respectively. Additional bands expected between these absorptions [32] are overlapped by the octahedral spectrum.
Other coordinations (4-fold Td,
3-fold D ) have been considered, but they give less satisfactory fits than 5-fold 3h Oxygen contact does not significantly change either the 5,000 or the 24,000 CqV. cm-l bands, but produces general absorption. This we ascribe to limited 3+ cations, and thereby chemisorption producing 02- or 0- adsorbed anions and Ni 2+ 3+ generating intense intervalence absorption (Ni /Ni ). Some oxygen is certainly bound quite strongly, since the original spectrum is not fully regained on outgassing at 5OO0C (Fig.3). The 500°C-outgassed surface exemplified by Fig.3, Curve 6 is the type of surface
on which the NO and CO adsorptions were carried out.
Fig.7 shows that the presence
of Ni2+ ions gives rise to a specific NO adsorption, since replacement of only 5% Of the Mg2+ ions in MgO leads to a threefold increase in coverage, and the further increase on going to MN 10 indicates that the surface concentration of Ni2+ is 2+ concentration. The calorimetry
indeed changing broadly in parallel with total Ni
experiments (Fig.8), however, show that the heat of adsorption of NO on MN 5 and MN 10 is not significantly greater than on MgO.
Whilst this does not exclude the
interpretation that the additional adsorption is restricted to the nickel ions (or Ni plus adjacent oxide ions), it suggests that the effect of Ni ions may be to activate NO for surface diffusion on to the MgO matrix.
Cimino et al. [3,35]
433 have postulated a rather similar role for the transition metal ion in N 0
2
decomposition over LSA solid solutions, with oxygen migrating over the surface of the matrix after break-up of ti 0 . The results with CO were broadly similar 2 to those with NO, except that coverages and heats were smaller, so the same conclusions follow. A further important result to emerge from the calorimetry was the clear evidence for strong chemical interaction between CO and NO in the adsorbed state, even at O°C.
The relatively lower heat in the case of NO dosed to CO ads (as compared to CO dosed to NO ) is due to the chemical reaction heat being diluted ads by that of the simple adsorption of NO on the free surface, NO limiting coverages being much higher than those of CO. threshold temperature for the CO
+
The nickel ions are able to lower the
NO reaction, as compared to MgO, and also they
lead to sustained decomposition of the N 0 intermediate once the temperature for 2
clearing adsorbed oxygen is exceeded. In sharp contrast to HSA MN, the reflectance spectrum of 100O0C-outgassed HSA 2+ MCo (Fig.5, Curve 1 ) is not recognisable as the spectrum of octahedral Co ions known to be present in the solid from the magnetic study. are expected for spin-free Co2+ in Oh symmetry viz. 4T
4T (F)
-#
4
-1
19,800cm
Two main
(F)
+ 4T
11-11
absorptions
(F) and
h3 2g T (P), and in single crystal Co Mg 0 these occur at 8,500 and Ig x 1-x respectively [36]. Absorption at these values is present, but is
evidently dominated by more intensely absorbing chromophores.
The bands at
7,000cm-l and at 16,000-20,000cm-l (triplet) are characteristic of Co2+ in T 4
4
4
d
(F) + T (F) and A (F) 4 T (P) transitions respectively 2 1 2 1 Tetrahedral coordination [30,37], with the latter the source of the blue colour.
symmetry. being the A'
produces intense absorption. The band at 13,500 cm-' is not expected for either 2+ octahedral or tetrahedral Co . Let us note that all the 'non-octahedral' absorption is greatly reduced on exposure to H 0 vapour and a recognizably 2 CO also attenuates the non-octahedral bands. 2 3+ Oxygen develops a very intense general absorption, obviously producing Co ions 2+ 3+ and intervalence (Co /Co ) absorption, but does not affect the non-octahedral
octahedral spectrum results,
bands. 2c Bearing in mind the magnetic evidence of octahedral Co in the bulk, and the
entirely normal values for a at all the Co concentrations studied, we conclude 2+O that the non-octahedral Co is principally present in the surface layers of the HSA crystallites.
It is able to dominate the octahedral spectrum because the
centres responsible are non-centrosymmetric and hence have high intrinsic absorption.
The tetrahedral spectrum could arise in various ways, either from
the near-tetrahedral sites expected on (110) faces [33], or from 'disproportionation' of 5-coordinated surface cations on ~ l O O f faces [23], or from occupancy of the actual tetrahedral holes of the cubic close-packed anion structure. Bielanski et al. [Zl,22] propose that Schottky defect formation leads to
434 tetrahedral Co2+ throughout the bulk of the crystallites of their Coo-MgO.
Our
vacuum-prepared HSA MCo appears to have a conventional Octahedral distribution for the Co2+ ions in the bulk, but to have tetrahedral Co2+ in the surface region.
A
mechanism for developing tetrahedral Co2+ in the ccp anion structure of the surface region of Co Mg
x
1-x
0 is shown schematically in Fig.9.
Schottky defects generated
in the surface region during the treatment at 1000°C lead to an extension of the ccp structure (Fig.9b).
The cations then redistribute in the anion array (Fig.9~)
so as to give cation occupation partly in octahedral holes and partly in tetrahedral
holes, always with the proviso that adjacent face-shared oxygen polyhedra would not be occupied.
This process requires no valency change.
Such an expanded anion
SURFACE R E G I O N
I
I
I
I
I
I
B
A
C
Schematic representation of formation of tetrahedral Co2+ in surface region of HSA Coo-MgO solid solution. A Cubic close packed (ccp) anions; cations in octahedral holes, no cations in tetrahedral holes (NaC1 structure). B - Expanded ccp structure with Schottky defects (anion and cation vacancies) in surface region. Cations still in octahedral holes but some octahedral holes now unoccupied (cation vacancies, shown shaded). No tetrahedral holes occupied. C Expanded ccp structure but with cations reorganized in surface region. Anion vacancies (open squares) as in 8, but cations redistributed among a mixture of octahedral and tetrahedral (T) holes, cf.spinel structure. Fig.9.
-
-
array would convert rapidly to hydroxide on exposure to H 0 vapour, anion vacancies
( 0 )being
2-
destroyed by the process 0
+ 0 +
2
H20
+ OH-
+
OH-:
would remain divalent but would become octahedrally coordinated [cf.Mg(OH) C0(0ff)~].
If water vapour
and
2-
2+
If oxygen were admitted
alone, penetration
)
would be oxidized
to fill the anion vacancies
would probably be limited at room temperature to one or two layers only. 2+
or
, as in an oxyhydroxide [cf.CoO(OH)]:
all cations would become octahedral and oxidizable cations (Co
limited number of Co
2
oxygen were admitted, the array would again be
completed but in this case by both OH- and 0
(C03+).
all cations
A
ions would be oxidized, commensurate with the uptake of
oxygen to form 02-: 4c02+
+ o2 +
2 0
+
4C03+
+
202-
435 The oxidized cations in this case would be preferentially those Co2+ ions in octahedral sites, because of the greater gain in crystal field stabilization energy (by analogy with the fact that CogOq is a normal and not an inverse spinel). 2+ tetrahedral Co ions would be unaffected.
The
H20 exposure This model is sufficient to explain our observations on HSA MCo. 3+ Atmospheric leads to an octahedral Co2+ spectrum with no evidence for Co 3+ 2+ exposure (H20 + 0 ) generates Co and destroys tetrahedral Co , as evidenced by 2 the magnetic results and the intervalence absorption. Exposure to oxygen does not 2+ 3+ affect tetrahedral Co , but creates Co (intervalence absorption). The motion
.
of a Co2+ ion from an octahedral site to a tetrahedral site which is inherent in the model requires the passage through a 'squeeze' position of Dgh symmetry and then an approach in CgV symmetry.
The band at 13,500 cm-',
not previously assigned,
seems likely to be associated with the Co2+ ion in one of these symmetries, other bands from the same chromophore being overlapped by the tetrahedral spectrum. The model is consistent with the esr results reported by Kuznetsova et al. [24] Boreskov refers to Co ions of Types I and 11. Type I ions 2+ are regarded as surface tetrahedral Co and are not affected by oxygen; Type I1
and Boreskov [19].
ions are octahedral and are readily oxidized and reduced. these are now components of the above model. advanced by Bielanski et al.[21,22],
Following Boreskov [38],
Our picture is also Close to that
except that we do not consider the defect
structure in our HSA MCo to involve the bulk at all significantly. reflectance spectra and those of Dyrek et a1.[20]
Their
appear to have been recorded only
in air (or after exposure to air) and thus Co3+ will always be present.
Also a
band at 5,260cm-l in these circumstances [21] could be due to a H 0 combination 2 2+ band from adsorbed atmospheric water rather than tetrahedral Co . Similarly in their magnetic work inference about tetrahedral Co2+ is difficult if Co3+ ions are present.
In our work we hope to have shown that the intriguing complexity of the
HSA Coo-MgO system can be rationalized further by conducting spectroscopic and
magnetic studies in a controlled environment and one which includes the ability to measure reflectance spectra in high vacuum.
REFERENCES 1
E.G.Vrieland and P.W.Selwood, J.Catalysis, 3 (1964) 539.
2 A.Cimino, M.Schiavello and F.S.Stone, Disc.Faraday SOC., 41 (1966) 350. 3 A.Cimino, Chim.Ind.(Milan), 56 (1974) 27. 4 F.S.Stone, J.Solid State Chem., 12 (19751 271. 5 A.W.Sleight and W.J.Linn, Ann.N.Y.Acad.Sci., 272 (1976) 22. 6 R.J.H.Voorhoeve, J.P.Remeika and L.E.Trimble, Ann.N.Y.Acad.Sci., 272 (1976) 3; R.J.H.Voorhoeve, D.W.Johnson, J.P.Remeika and P.K.Gallagher, Science, 195 (1977) 827. 7 M.G.Lofthouse, Thesis, Univ.Bristo1 (1972); A.P.Hagan, Thesis, Univ.Bristo1 (1974); C.Otero Arean, Thesis, Univ.Bath (1976); Univ.Bath (1977).
M.A.Trevethan, Thesis,
436
8
9 10
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
A.Cimino, R.Bosco, V.Indovina and M.Schiavello, J.Catalysis, 5 (1966) 271; A,Cimino,V.Indovina, F.Pepe and M.Schiavello, J.Catalysis, 14 (1969) 49; A.Cimino, V.Indovina, F.Pepe and F.S.Stone, Gazz.Chim.Ital., 103 (1973) 935. A.Cimino and F.Pepe, J.CatalYSiS, 25 (1972) 362. A.Cimino, M.Lo Jacono, P.Porta and M.Valigi, Z.Phys.Chem.(Frankfurt), 55 (1967) 14. A.Cimino, M.Lo Jacono, P.Porta and M.Valigi, Z.Phys.Chem.(Frankfurt), 70 (1970) 166. V.Indovina and F.Pepe, personal communication. F.Pepe and F.S.Stone, J-Catalysis,in press. P.J.Anderson and R.F.Horlock, Trans.Faraday Soc., 58 (1963) 1993. P.J.Anderson and P.L.Morgan, Trans.Faraday SOC., 60 (1964) 930. S.Z.Roginskii and V.A.Seleznev, Kinetika i Kataliz, 8 (1967) 1342. N.P.Keier, 1.S.Sazonova and R.V.Bunina, Kinetika i Kataliz, 10 (1969) 1036. V.V.Popovskii, G.K.Boreskov, V.S.Muzykantov, V.A.Sazonov, G.I.Panov, V.A.Rotchin, L.M.Plyasova and V.V.Malakhov, Kinetika i Kataliz, 13 (1972) 727. G.K.Boreskov, Proc.6th Int.Congr.Catalysis,London, July 12-16, 1976, Chem.Soc., London, 1977, p.204. K.Dyrek, Bull.Acad.Polon.Sci., Ser.Sci.Chim., 21 (1973) 675; K.Dyrek and V.A.Shvets, Bull.Acad.Polon.Sci., Ser.Sci.Chim., 22 (1974) 315. A.Bielanski, Z.Kluz, M.Jagiello and L.Waclawska, Z.Phys.Chem.(Frankfurt), 97 (1975) 207. A.Bielanski, Z.Kluz and A.Wojtaszczyk, Bull.Acad.Polon.Sci., Ser.Sci.Chim., 25 (1977) 721. A.P.Hagan, C.Otero Arean and F.S.Stone, Proc.8th 1nt.Symp.Reactivity of Solids, Gothenburg, June 14-19, 1976, Plenum Press, New York, 1977, p.69. L.I.Kuznetsova, G.K.Boreskov, T.M.Yurieva, V.F.Anufrienko and N.G.Maksimov, Doklady Akad.Nauk, S.S.S.R., 216 (1974) 1323; N.G.Maksimov, L.I.Kuznetsova, V.F.Anufrienko and T.M.Yurieva, 1zv.Akad.Nauk Feorg.Materialy, 12 (1976) 1219. C.Otero Arean and F.S.Stone, to be published. T.A.Egerton, A.Hagan, F.S.Stone and J.C.Vickerman, J.C.S.Faraday I, 68 (1972) 723. A.Zecchina, M.G.Lofthouse and F.S.Stone, J.C.S.Faraday I, 71 (1975) 1476. R.L.Gale, J.Haber and F.S.Stone, J.Catalysis, 1 (1962) 32. A.Cimino, P.Porta and M.Valigi, J.Amer.Ceram.Soc., 49 (1966) 152. R.Pappalardo, D.Wood and R.C.Linares, J.Chem.Phys., 35 (1961) 2041. M.Ciampolini, Inorg.Chem., 5 (1966) 35; M.Ciampolini, N.Nardi and G.P.Speroni, Coord.Chem.Rev., 1 (1966) 222. M.Gerloch, J.Koh1, J.Lewis and W.Urland, J.Chem.Soc.(A), (1970) 3269. J.Haber and F.S.Stone, Trans.Faraday SOC., 59 (1963) 152. L.Sacconi, Coord.Chem.Rev., 8 (1972) 351. A.Cimino, F.Pepe and M.Schiavello, Proc.5th Int.Congr.Catalysis,Florida, Aug.20-26, 1972, North Holland /American Elsevier, 1973, p.125. W.Low, Phys.Rev., 109 (1958) 256. F.Pepe, M.Schiavello and G.Ferraris, J.Solid State Chem., 12 (1975) 63. G.K.Boreskov, Proc.6th Int.Congr.Catalysis, London, July 12-16, 1976, Chem.Soc., London, 1977, p.214.
DISCUSSION K. J O H A N S E N : In Figure 2 you have a linear relationship for lattice
parameters as a function of the concentration of N i and Co. this continue for higher concentrations 2
Does
Or, in other words, is it
an ideal solution ? F.S. STONE
:
The linear relationship does continue for higher
concentrations.
This has been shown (1-3) for low surface area
437 (LSA) solid s o l u t i o n s and t h e i n f e r e n c e is t h a t t h e s o l u t i o n s are ideal.
I n o u r w o r k w i t h high s u r f a c e a r e a (HSA) s o l i d s o l u t i o n s
we have explored a s f a r as 15-20 m o l e p e r cent N i o r C o , and w e find n o d i s c r e p a n c y w i t h this.
By 15-20 m o l e p e r c e n t t h e a r e a s h a v e
d e c r e a s e d t o v a l u e s o u t s i d e t h e arbitrarily-defined 2 -1 t h e y are b e l o w 100 m g
HSA r a n g e , i.e.
.
1. J. R o b i n , Ann. Chim. A c t a , 10(1955), 389. 2. W.C. Hahn and A. M u a n , J. Phys. Chem S o l i d s , 1 9 ( 1 9 6 1 ) , 339. 3. A. Cimino, M.Lo J a c o n o , P. P o r t a a n d M. V a l i g i , Z. Phys. Chem. (Frankfurt), 5 5 ( 1 9 6 7 ) 14; 7 0 ( 1 9 7 0 ) 166. L.
G U C Z I : Your f i n d i n g o f t e t r a h e d r a l c o b a l t a t t h e s u r f a c e by
reflectance spectroscopy strengthens our previous hypothesis e x p l a i n i n g the b e h a v i o u r o f a - C r ( I I 1 ) o x i d e u n d e r r e d u c i n g c o n d i t i o n s
(J. K a l m a n and L. Gucai, J. Catal.,
4 7 ( 1 9 7 7 ) 371).
We have found a
d e c r e a s e o f c a t a l y t i c activity a f t e r long t r e a t m e n t w i t h hydrogen. T h e activity, h o w e v e r , could be r e s t o r e d i n s t a n t a n e o u s l y by c o n t a c t i n g t h e catalyst w i t h a i r at room temperature.
We e x p l a i n e d i t by
a s s u m i n g t h e f o r m a t i o n o f C r ( I 1 ) in t e t r a h e d r a l p o s i t i o n s , w h i c h r e q u i r e d large energy.
T h e r e v e r s e t r a n s i t i o n , from t e t r a h e d r a l
Cr(I1) to octahedral Cr(II1)
,
t a k e s p l a c e v e r y easily.
Your
e x p e r i m e n t a l d a t a seem t o m e t o v e r i f y our assumption.
F.S.
S T O N E : T h e r e i s indeed s o m e a n a l o g y b e t w e e n Cr(I1) and C o ( I I ) ,
s i n c e b o t h a r e k n o w n , for i n s t a n c e , t o p r e f e r t e t r a h e d r a l c o o r d i n a t i o n i n t h e s p i n e l structure.
However, as regards ease of conversion to
o c t a h e d r a l c o o r d i n a t i o n , it i s w o r t h n o t i n g t h a t for o u r t e t r a h e d r a l C o ( I 1 ) i o n s in MgO, m e r e exposure t o o x y g e n at room t e m p e r a t u r e d o e s n o t d e s t r o y t h e t e t r a h e d r a l spectrum.
~ x p o s u r e t o a i r d e s t r o y s it,
and we s h o w c l e a r l y t h a t t h e key c o m p o n e n t in a i r r e s p o n s i b l e for t h i s i s w a t e r vapour.
T h e r e s u l t is t h e n o c t a h e d r a l c o o r d i n a t i o n for t h e
c o b a l t , and t h e o x y g e n p r o d u c e s s o m e o x i d a t i o n t o C o ( I I 1 ) . c o r r e s p o n d i n g p r o c e s s w i t h C r ( I 1 ) m a y be easier.
The
If t h e r e i s tetra-
h e d r a l C r ( I 1 ) in t h e surface o f r e d u c e d a - C r 2 0 3 , o x y g e n m a y b e able o n i t s own to p r o d u c e o c t a h e d r a l C r ( I I 1 ) : certainly t h e r e is a l a r g e g a i n in c r y s t a l field s t a b i l i z a t i o n energy t o a s s i s t it.
The
e x i s t e n c e o f c h r o m i u m i n d i f f e r e n t c o o r d i n a t i o n s on t h e s u r f a c e of t h e corundum s t r u c t u r e ( a - A 1 2 0 3 and a - C r 0 ) i s a t o p i c w h i c h h a s 2 3 i n t e r e s t e d u s a l s o (1,2). 1. F.S. Stone, C h i m i a , 23 ( 1 9 6 9 ),490. 2. F . S . S t o n e and J.C. V i c k e r m a n , Proc. Roy. SOC. (19771, 331.
( L o n d o n ) , A 354
438 J.W.
H I G H T O W E R : During t h e NO a d s o r p t i o n e x p e r i m e n t s , w a s t h e r e
any e v i d e n c e of NO d e c o m p o s i t i o n such a s i s f r e q u e n t l y o b s e r v e d w h e n N O r e a c t s s t o i c h i o m e t r i c a l l y with a r e d u c e d t r a n s i t i o n m e t a l atom t o l i b e r a t e N 2 / N 2 0 ?
F.S.
S T O N E : W e h a v e not o b s e r v e d NO d e c o m p o s i t i o n t o g i v e N
2
Or
N 2 0 at O O C , the t e m p e r a t u r e o f t h e a d s o r p t i o n experiments.
L. R I E K E R T
:
If I u n d e r s t o o d c o r r e c t l y , i t i s assumed t h a t t h e
d i f f e r e n t adsorption c a p a c i t i e s o f t h e d i f f e r e n t s o l i d s [ M g O , MN 5 and MN 10) are d u e to k i n e t i c s , s i n c e the h e a t o f a d s o r p t i o n i s e s s e n t i a l l y t h e same for a 1 1 t h e solids. (a)
W o u l d t h i s m e a n t h a t t h e i s o t h e r m s o b s e r v e d are not e q u i l i b r i u m
i s o t h e r m s , even t h o u g h t h e y e x h i b i t s a t u r a t i o n ? tb)
W o u l d not t h e heat o f a d s o r p t i o n d e t e r m i n e only t h e s l o p e o f
t h e i s o t h e r m n e a r t h e o r i g i n , w h e r e a s t h e a m o u n t a d s o r b e d at s a t u r a t i o n i s i n d e p e n d e n t o f the h e a t o f a d s o r p t i o n and should only depend on the stoichiometry or on the structure of the a d s o r p t i o n layer ?
F.S.
S T O N E : T h e N O a d s o r p t i o n i s o t h e r m s a r e n o t e q u i l i b r i u m iso-
t h e r m s , s i n c e they are not r e v e r s i b l e at O O C . t h e y are activAtion-energy
limited, a s y o u say.
u n c o m m o n t o find in non-equilibrium
We assume that However, it is not
c h e m i s o r p t i o n that " s a t u r a t i o n "
coverages do relate to heats of adsorption, higher coverages being a s s o c i a t e d w i t h h i g h e r m o l a r heats.
T h e f a c t t h a t t h i s d o e s not
h a p p e n in t h e p r e s e n t c a s e t h e r e f o r e s e e m e d t o j u s t i f y t h e s p e c u l a t i o n a b o u t n i c k e l i o n s f a c i l i t a t i n g c h e m i s o r p t i o n o n t h e solvent m a t r i x , but it i s c e r t a i n l y n o m o r e t h a n speculation.
J.A.
RABO
:
I n t h e c a s e o f a d s r o b i n g N O o n Ni2+-containing
MgO
s u r f a c e d i d y o u o b s e r v e b y I R or E S R t h e n a t u r e o f NOi n a m e l y , d o e s i t f o r m t h e NO
F.S.
+
cation ?
S T O N E : We h a v e o b s e r v e d a specific E S R spectrum t o d e v e l o p
when NO i s contacted w i t h t h e n i c k e l - c o n t a i n i n g M g O s p e c i m e n s , b u t we h a v e not identified t h e a d s o r b e d s p e c i e s responsible.
The
I R s p e c t r a o f N O adsorbed o n t h e s e MgO-based o x i d e s are c o m p l e x
(I), and s e v e r a l s p e c i e s are c e r t a i n l y present.
However, in view
of t h e basic nature o f t h e s e o x i d e s , I would not t h i n k it likely t h a t t h e NO+ c a t i o n i s o n e o f them. 1. L. C e r r u t i , E. Modone, E. G u g l i e l m i n o t t i a n d E. Borello, J.C.S. F a r a d a y I , lO(19741, 729.
439
TENTATIVE CLASSIFICATION OF THE FACTORS INFLUENCING THE REDUCTION STEP IN THE ACTIVATION OF SUPPORTED CATALYSTS
B. DEMON, M. HOUALLA Groupe de Physico-Chimie Mindrale et de Catalyse Universitd Cathofique de Louvain, Louvain-la-Neuve (Belgium)
ABSTRACT The communication is an attempt to classify the various parameters which affect the reduction step in the activation of supported oxides.
Discussion will
mainly be conducted with reference to hydrodesulfurization catalysts. The following groups of parameters will be discussed
:
dispersion, interaction between
active oxides and support, reduction conditions, additives. Extensive reference will be made to mechanisms known in the field of the reaction of solids.
1. INTRODUCTION Before it begins to work in a catalytic reactor, a catalyst has undergone a succession of transformations, first,during its preparation, starting from raw materials, and ending with a product which can be shipped to the user, and second, while converting the transportable material into the form which is active in the reactor. Activation, which usually designates the final steps in the conversion of the catalyst into the working form, may correspond to different transformations, according to cases, for example dehydration, decomposition, reduction or sulfidation, either isolated or combined. All correspond to chemical changes occuring during reactions which involve solids. The science of the chemical transformations of solids has now gained the status of a new branch of chemistry.
It is therefore logical to approach the chemical reac-
tions involved in the activation of catalysts by using, as a background, the large body of the existing knowledge in the field of the chemistry of solids in general. The present communication examines, from the standpoint of solid state chemistry, the factors which influence the reduction step in activation, namely the reduction of supported oxides into metals or lower oxides.
The factors
controlling reactions of solids, in general, begin to be discerned ( 1 - 3 1 . Reductions of oxides are one of the groups of reactions of solids which have been the object of the most intensive study.
It is felt that time comes where
the knowledge gained on the reduction of oxides may really help understand the reduction of the special class of oxides which supported catalysts constitute. The main objective of the present communication is to propose a d U b A i 6 i C a -
t i o n of the factors which control the reduction of supported oxides. Science needs such attempts to ordering, especially when basic knowledge has still to be fortified, because such rational groupings help distinguish between alternative hypotheses. While presenting this classification, some typic& indoma-
atiom will be examined. Thus, in addition to reaching the main objective, we hope to delineate cases where a systematic study has contributed important pieces
Of
inEormation with respect to the reactivity of supported oxides in reduction. Too few examples of in-depth studies have already been published for a successful methodology to begin to shape out.
However, we believe that the present
communication can give some hints in this last respect. The Organizing Committee of this Symposium very rightly indicated that the preparation and activation of catalysts constitute a whole interacting sequence of steps. The behaviour of the catalyst at one step is conditioned by all previous steps, and, in turn,alters all subsequent steps and, ultimately, the active form. The present authors have kept this remark in their minds.
Because
this Symposium was centered on impregnation and activation, impregnated samples have been considered. Many situations can exist as a consequence of the reactions taking place during impregnation, drying and calcination. One major point of discussion in this communication will be the parameters which depend on the pmpOJLation btepb which precede reduction. The communication will discuss results obtained by ourselves and other investigators in the Groupe de Physico-Chimie Minerale et de Catalyse (GPCMC), as well as data reported by other investigators. Because of our special interest in hy&ode~uR6wLizatian and hydrroa%eating Cataeyb.tn, illustrations will be taken preferentially from that field, although other examples may be occasionnaly discussed, when arguments and observations can be readily extrapolated to HDS catalysts.
One feature which distinguishes hydrotreating catalysts from others,
e.g., hydrogenation catalysts, with respect to reduction, is the simultaneous presence of
two oxides on
the surface of the carrier. The situation is more
complicated, and hence, more difficult to analyke but, on the other hand, this complexity unveils new phenomena, possibly having some general bearing, which deserve discussion. In the case of hydrotreating catalysts, the sequence of preparation steps is the following
:
(a) Adsorption or deposition of the precursor or precursors on the carrier. (b) Drying, which, in addition to the removal of water, may lead to crystallization of the precursors in the pores and on the outer surface of the carrier
441 (c) Decomposition of the precursor salts (dehydration,decomposition to oxides), namely "calcination". (d) Possibly, if double impregnation is used, again a sequence of steps similar, respectively, to (a), (b) and (c). (e) Activation, namely reduction and sulfidation. Our discussion will rest on the assumption that the catalysts are prepared according to this general sequence. We will examine, successively, the influence of the following groups of parameters on the reduction of the supported oxides
:
(i) the parameters which depend on the starting supported oxides, namely the solid resulting from steps (a) through (dl; (ii) the parameters which depend on the reduction conditions, with some reference to the role of H
2
S
in the reduction-sulfidation process (step
e) i (iii) the role of additives with reference to their influence on the para-
.
meters mentioned under (i) and (ii)
2.
INFLUENCE OF THE VARIOUS PREPARATION STEPS ON THE CHARACTERISTICS OF THE STARTING SUPPORTED OXIDE This first section intends to summarize the specific bearing of each prepara-
tion step (i.e. impregnation, drying, calcination) on the characteristics of the supported oxide which will be reduced. Two subsequent sections will examine in detail the corresponding parameters with respect to dinpennion and to
intenaction between active oxide and buppoht. 2.1. Influence of the impregnation step To a large extent, impregnation determines the hepahtition of the deposited substance in the body of the carrier (in pores as on the outer surface, uniformly all through the pellet or bead, or only in the porosity of the outer parts of the pellet).
At a small scale, it determines the d i n p W i o n (uniform depo-
sition of isolated ions, uniform layer o f deposited substance on the walls of the pores, or isolated clusters; small or large clusters).
Impregnation also
influences the nature of inte,'ulction existing between the support and the impregnating precursor which, in turn, may determine the kind of interaction ultimately taking place between the oxide and the support. Moreover, the bUh&lCe 06
t h e b u p p ~ h t ,and even, in principle, its t e x t ~ ~ may e , be & a e d ,
sequence of the contact with the impregnating solution. titute a considerable trauma for the support.
as a con-
Impregnation can cons-
Impregnating solutions, because
of their content in various ions, may have a strong corrosive power and can bring about ion exchange as well as partial dissolution and redeposition.
442
The influence of impregnation on repartition and dispersion of the precursor, and the nature of the interaction between precursor and support are discussed in various contributions to this Symposium.
One will mention,here with refe-
rence to HDS catalysts, that Mo from ammonium paramolybdate can adsorb on the support, either through an interaction with surface hydroxyl groups (as MOO:-)
(4-71, or, very likely, through mere crystallization on the surface. Similarly salts of group VIII metals can either exchange on the surface or crystallize ( 4 , 8, 9, 101.
Modification of the surface of the support and of its texture
during impregnation has been reported (11, 1 2 ) .
In other instances, it can be
inferred from data obtained in other fields, e.g. the destruction of structures of crystalline alumino-silicates in solution of moderately low pH (131, or the fact that cation exchange capacity measurements (C.E.C.) lead to values well exceeding those obtained by less destructive methods [14,15). 2.2. Influence of the drying step The effects of drying on repartition and dispersion of the active phase begin to be well documented (16, 171.
The contributions to the present Symposium
dealing with impregnation often describe effects which are due to impregnation
p l u &ying. A
factor which, in our opinion, would deserve more attention is the ability
of the partially dried or dehydrated salt to wet the support. We believe that the choice of the impregnating salt and, possibly, of the drying procedure may have a strong effect on the characteristics of the catalyst. 2.3. Influence of the calcination step Calcination of the deposited precursor brings about several transformations and solid state reactions which can be summarized as follows
. decomposition of the dried precursor; . solid state reactions of the supported oxide
:
(e.g. crystallographic chan-
ge) or oxides (e.g. reactions between oxides);
. solid state transformations of the support; . reactions between deposited oxides and the support; . sintering of the various phases (supported phases, support). The product resulting from this last step in catalyst preparation is the reactant of the reduction step. We mustnow examine in detail the characteristics of the solids and their bearing on the reduction.
A
rather satisfactory classification is to consider
separately the parameters related to dinpe,Uifln and those describing the type of h & ! A a d f l n between the supported oxides and the solid carrier.
443 3 . TNFLUENCE OF DISPERSION ON REDUCTION
3.1. Dispersion characteristics of supported oxides Generally speaking, one should use a broad definition of dinpUtA,iiOn, namely the distribution, at a large and small scale, of the active phase over the carrier, and also the shape and orientation of the crystallites or aggregates of the deposited oxide. Fig. 1 suggests the various situations, either with respect to repartition in the depth of each pellet or bead (Fig. la) or to the dispersion on the walls (Fig. lb).
In the present section, we shall not be concerned with the nature
of the interaction between active oxide and support (Fig. lc). 3.2. Active phase loading The previous sections have suggested how the various stages of catalyst preparation may influence dispersion when catalysts of identical composition are considered. At this point, it is necessary to examine the effects of the loading in active phase on the dispersion.
Indeed for a given carrier and depen-
ding on active phase-support interaction, increasing the active phase loading
will modify the degree of dispersion of the oxide. The final repartition of
a
r-l Interact ion
oxide- support
C Fig. 1
:
Repartition of the active phase over the carrier.
444
the active phase might vary from a "monolayer" type arrangement to aggregate formation. Direct observation of the evolution of the active phase disperseness can be made by differents techniques (electron microscopy, X-Ray-diffraction lines broadening, E.S.C.A.)
-
3.3. Kinetic consequences of dispersion
In principle, the reactivity of a solid is strongly dependent upon its dispersion. The reactivity of supported oxides is known to be profoundly altered, in comparison to that of the bulk oxide.
In order to understand the reduction
step in the activation of supported catalysts, it is necessary to distinguish what may be attributed to dispersion and what is due to the interaction between the active phase and the support. We shall examine, in the present section, some consequences of the mere 3.3.1.
dispUL5iOrZ.
Theoretical consideration. The decrease of reactivity often observed,
when supported oxides are compared to unsupported ones, seems contradictory to common sense, because one speculates that dispersed solids should be more reactive.
We shall intend to clarify this problem in the present paragraph, sup-
posing that no special interaction between active phase and support takes place. One can imagine that the effect of dispersion is mainly to lessen the possibility of nucleus formation either because of the small size of the aggregates of the active phase, or because of a lack of nucleus forming sites on many of these aggregates[l91.
Thus, when the nucleation stage in reduction is a
rate-limiting step, it is to be expected that modifications of nucleation brought about by the disperseness of the active phase will have considerable kinetic consequences. The effect of particle size on reduction is actually complex. We have shown in Fig. 2 the results of a mathematical analysis, which describes the evolution of the kinetics of the transformation, as a function of particle size a tin re-
otherwise identical experimental conditions. The values of parameter A
s (0)
flects the relative rates of nucleus formation to nucleus growth[20).
It turns
out to be proportional to the cube of the radius of the particles, aO. ase of the particle size by a factor of 10 will, then, increase As(o) tor of 1,000.
An incre-
by a fac-
Fig. 2 shows that decreasing the particle size (starting from
As(o) = 1,000, for example) will first increase the reduction rate until a cer-
tain particle size threshold is attained,after which the reaction is slowed down. We do not believe that the slow reduction of finely dispersed oxides can -be
explained according to the preceding picture.
&-
However, this should be
445
taken into account before looking for other interpretations, or unless the assumption of an inhibition of reaction by carrier-active phase interaction is confirmed by suitable physico-chemical investigations.
0
25
75
5
ti/4nkbok: Fig. 2 : Theoretical evolution of the kinetics of reactions where nucleus formation is rate limiting, k' . rate of nucleus formation; 90
k.
.
:
rate of nucleus growth (ref. 201
Fig. 3 : Comparison of reduction of a bulk and silica supported nickel oxide at 265OC (a:extent of the reduction to metal. Dotted lines : experimental results) [ref. 21). Solid lines : theoretical curves for different value of
3.3.2. Experimental results in relation with HDS catalysts.
As indicated in
Fig. 3, the previous picture could very satisfactorily account for the difference of reactivitlin reduction between bulk and silica supported nickel oxide
[21).
A reduction of the size of the NiO particles on Si02, in comparison to
unsupported Ni0,by a factor of 7 (from % 0.25 pm
for bulk NiO [2ll),would be
quite sufficient for explaining the difference. The situation of NiO/Si02 is formally very similar to that of unpromoted HDS type catalysts in their oxidic form.
The case of MOO /Si02 can be cited as 3
an example. Fig. 4 shows clearly a decrease of the reducibility of Moo3 supported on SiO when Moo3 content is lowered (e.y. smaller particles of Moo3) [221. Ta2 king into account the picture proposed above, it seems likely that the phenomenon
can be related to the modifications of the reactivity of the oxide with
dispersion. If this interpretation is true, one should not attribute the variations of the degree of reduction of Moo3 supported on A1203 only to the interaction with the support (23-261.
446
0.6
I
1
' 1
I I
I
I c(
0.4
a2
I
0
1
I 24
12 Wt%
..
'" 100
MOO3
Fig. 4 : Effect of Moo3 content on the extent of reduction reached in 10 hours by a series of Mo03/Si0 catalysts (T = 400'C; pH = 1 atm.) (ref. 2 2 ) . 2
2
4. INFLUENCE OF THE INTERACTION ACTIVE OXIDE-SUPPORT
4.I. Overall picture The interaction between the supported oxide and the carrier, although determined by all steps in the preparation, depends more strongly on the phenomena which have taken place during calcination. We shall therefore refer to the phenomena mentioned in the above corresponding paragraph for attempting a classification of the possible situations; these situations may C O t X h t on a given catalyst, because different chemical reactions may occur simultaneously. The following cases will be considered (Fig. 5)
:
. weak forces between supported oxide and carrier; . aggregates of deposited oxide, electronically interacting with the carrier; . monolayer of deposited oxide;
. aggregates covalently or ionically bound with the support; . solid solution of active phase in the support present either on all the surface of the carrier or in patches;
. new compound present on the whole surface of the carrier or in domains.
447
a
Weak forces
1-
b
Junction
,
c
Monolayer
O M O M O
d Chemically bound aggregate
Fig. 5
:
e
Solid solution in support
f
New compound w i t h support
Various types of active oxide-support interaction.
4.1.1. Very Weak forces.
Ideally, when the interaction is due to weak forces
(VAN DER WAALS forces), the role of the support is only to disperse the catalyst. In principle, weak forces could also be sufficient to bring about some preferential orientation of the lattice of the supported oxide (Fig. 5a).
Graphite [ 2 7 1
and, perhaps, sometimes silica [ 2 8 1 could be examples of carriers which exert only a weak action on the supported substance. 4.1.2. Electronic interaction. In principle, semi-conducting or conducting substances deposited on a semi-conducting support may form a j u n c t i o n with the latter, even if no chemical bond (in the conventional sense) is formed, by tunnelling or similar phenomena (Fig. 5b).
Electron transfer across the bounda-
ry would change the electron density of deposited aggregates, with the effect being more pronounced for small aggregates; this could change the reactivity. 4.1.3. "Adhesion" type interaction. There is certainly a whole spectrum of possible situations between the weak interaction discussed above and strong chemical interaction. For example, certain oxides, like Mn 0 and Moo3, are reported to wet spon2 3 taneously each other, while others (MOO and Co304) tend to form aggregates 3 (2 9) . A possible explanation of the wetting phenomenon may well be found in relatively weak forces, similar to those mentioned for interpreting adhesion phenomena.
Obviously, stronger interaction can also bring about wetting.
448 4.1.4.
Chemical interaction
:
one monolayer or less.
This category of situa-
tions comprises those where all the atoms or molecules of the supported substance are exposed on the surface.
One case is when the deposited oxide acts as a
dope in very small concentration on the surface. The upper limit is encountered when the active phase combines with the carrier as a continuous monolayer (Fig. 5c).
A typical example of the latter is the formation of a monolayer of
Moo3 on the surface of y-A1203 as follows :
HO
OH
-t
OH
A1203
HO
Many evidences of this phenomenahave been presented ( 5 , 30-341.
We shall take
here another example which, although not completely elucidated, may well illustrate this section. Silica is generally considered as an inert carrier and is expected to act merely as a dispersing agent.
However, recent evidences (12, 351, including re-
sults obtained in GPCMC, seem estimated.
to indicate that its inertness was probably over-
E.S.C.A studies of a series of MOO /SiO 3
2
catalysts containing up to
20 wt % Moo3, prepared by pore volume impregnation, followed by drying at 110"
and calcination at 500"C, show a shift towards higher values of the binding line, with increasing active phase loading (Fig. 6). energy (B.E.) of the Si 2P
100
0
16
8
w t "1. Fig. 6
:
2L
MOO3
Evolution of the Si binding energy as a function of M a 3 content in 2P a series of MOO /sio2 catalysts (ref. 221 3
449
These results, which cannot be deduced from previously reported data (281 on a MOO /Si02 catalyst containing 12 wt % Moo3, suggest an electron transfer from 3 silica to molybdena. Unfortunately a direct verification from the Mo lines (e.g. Mo 3d3l2 and 3d5l2) is impossible, because of their broadening, in compaThis broadening, however, also indicates that MOO
rison with those of Moo3.
3
supported on silica suffers an interaction from the latter. Such an electronic effect would be similar to what has been observed between Moo3 and y-A1203 (36-411, where a slight shift (<0.5 ev), towards hiaher bindino energies, of the Ho 3d3’2,
3d5’2
4.1.5. Chemical interaction
levels has been reported. :
aggregates.
When the active phase loading ex-
ceeds the amount which corresponds to a monolayer, it is likely that the excess oxides will present features more similar to those of bulk oxides,since the possible interaction with the support will exert relatively less and less importance when the ratio (active phase volume)/(contact area between active phase and support) decreases (Fig. 5d).
4.1.6. Chemical interaction
:
solid solutions. Depending on the temperature
of calcination, the oxide and the carrier, when their structure are compatible, can dissolve in each other to different extents, ranging from an ionic exchange confined to the interface to a multiple layer thick solid solution and, ultimately a homogeneous one (Fig.5e).
Occurence of the different situations and
their bearing on reactivity have been discussed in a case not related to catalysts, but quite illustrative, namely that of the NiO-CuO system [421. Examples of formation of homogeneous solid solution involving substances known as supports are abundant in literature Cr 0 - A 1 0 (431 Coo-MgO (441. The changes in 2 3
2 3
reactivity which may occur when solid solutions are formed are discussed in detail in a general frame ( 3 1 .
While the possibility that solid solutions for-
mation in the case of hydrotreating catalysts has not been studied, such a phenomenon cannot be disregarded. For example Co304 and CoA1204, having both a spinel structure, may form a solid solution in the usual conditions of preparation of catalyst. 4.1.7. Chemical interaction
:
new compounds. The formation of a new compound
can be either restricted to the near surface domains or extended to the bulk of the solid (Fig. 5f). in
1i;erature. 2 i
tal ions (Ni
Examples illustrating the first situation are not lacking 2 i
Many authors have reported that Co 2+
2 i
, Mn , Cu
)
and other transition me-
can combine with transition alumina to form cobalt
pseudo aluminate or a surface spinel (45-481. In other instances, one may observe the formation of a well defined solid.
It has, in effect, been re-
450 ported that, at high loading of Moo3 on A1203 (20 wt
%
Moo3) A12(Mo04)3 crys-
tals can be detected L6, 49).
4.1.8. Mixed situations. A major difficulty, in describing the supported oxide which undergoes activation,is the fact that many of the previously described situations C O Q X h t in a given catalyst. This can be illustrated by taking again the example of Moo3 deposited on SiO 2' which is considered to be a relatively simple one (121. Depending on the composition, at least 4 different species have been detected (Fig. 7).
I
(360 '440
1.5
1 15
30 wt
Fig. 7
:
"la
Moo3
Product distribution in MOO /Si02 catalysts as determined from acidi3
metric titrations : SMA ( 0 ) silicomolybdic acid, DMA ( 0 ) dimolybdates; PMA (A) polymolybdates. The estimation of Moo3 (n) is derived from the intensity ratio of the reflectance bands at 360 and 4 4 0 cm-l (ref. 121.
The following products, ranging from free Moo3 to silica bound molybdena, have been detected, namely
:
. MOO3' . dimolybdates (DMA) , . polymolybdates (PMA) , . silicomolybdic acid (SMA), . molybdenum bound to the surface of
the support, in a manner similar to
the monolayer model proposed for MOO /A1203 system (311. 3
451 The situation for the oxidic form of HDS catalysts supported on alumina is still more complicated ( 6 , 45, 50, 511. Another additional complication arises when two or several different oxides are deposited on a support, namely the possibility that they combine tOg&h&h. Although not perfectly fitting into the classification which we have adopted in this communicationkhis phenomedmhas to be mentioned here.
In some HDS cata-
lyst precursors, it has been indicated that Co and Mo can combine in the form of cobalt molybdate [22, 50, 51, 521.
4.2.- Influence of the interactions between deposited oxide and support on reduction
:
general comment
When only weak interactions are at stake, the major action of the carrier is simply to disperse the supported oxide.
In addition, the presence of the sup-
port may, as a consequence of a narrow porosity, impede the access of the reducing gas to the active oxide, or modify the thermal stability and resistance to sintering of the supported phase. The situation is obviously more complicated if a chemical interaction takes place.
However, the behaviour, in reduction, of solids which exhibit characte-
ristics similar to those of supported catalysts,begins to be known [ 3 ) .
This
is true for the behaviour of solids, the surface of which has been modified by ion exchange or by formation of an adhering layer, or which have partially reacted to form a solid solution near the surface 12, 3 , 531.
The relative reacti-
vities of solid solution, compound oxides (oxides associating in a given stoichiometry two or several metals) and pure oxides have also been discussed (2.31, and the various aspects of the modifications that the presence of a foreign phase (i.e. the carrier) can bring about on the reactivity of oxides have been examined and classified [ I , 31.
It may thus be hoped that the reduction of
supported oxides is amenable to a full interpretation.
4.3. Influence of the interactions between deposited oxide and support on the reduction of HDS catalysts precursors In the present paragraph, we shall attempt to analyze the phenomena which may occur during the reduction of the oxidic form of HDS catalyst.
The complexity
of the CoMo/Ai2O3 systems practically reflects all possibilities.
The discus-
sion will therefore be centered on this case, but simpler situations, namely those of oxides supported on Si02, will also be commented. Evidence has been presented [ 6 , 45, 51, 521 for the existence of at least six different species in the CoMo/A1203 system in its oxidic form (Fig. 8 ) :
. free cobalt oxide, . CoAl204 or "cobalt aluminate",
452
..MOO monolayer, 3 free MOO 3' A12 (Moo4)3 ,
. . . CoMo04.
Fig. 8 :Species eventually present in the CoMo/Al 0 system. 2 3 It is suspected that, among all the preparation parameters which determine the presence and amount of these species, the 0hde.h of impregnation (if double impregnation is used) and the active phase h l d h l g are of prime importance.
We
shall conduct the discussion in steps, considering first the Co/support and the MOO /support systems, and then the complete CoMo/support catalyst. 3
4.3.1. Reduction of cobalt oxide/support.
The effectsof disperseness of
group VIII metal oxides on supports have been fully discussed above, with NiO as an example. The only additional feature is the formation of a compound with the carrier, cobalt aluminate in the present case (not reducible at moderate temperature (5411. The fact that all the deposited oxide cannot be reduced, even when nucleation is induced in some way, reflects the presence of a compound with the carrier. Let us take again the simple example of NiO/SiOZ [19) for illustrating this point. 10).
Part of nickel oxide seems to be withdrawn from reduction (Figs. 9 and The amount of unreducible nickel seems to be almost independant of nickel
loading (Fig. 10). It seems that the effect cannot be attributed to dispersion, as copper, which has been said to very efficiently promote nucleus formation [191, cannot make the reaction complete (Fig.91.This result suggests that even quite inert carriers may combine in some way with supported oxides and prevent them from reacting. Although apparently not forming during impregnation (191, a nickel silicate might be the unreactive compound (551. However an alternative explanation, such as the presence of a fraction of the active oxide encapsulated in the pores of the carrier, cannot be dismissed in principle.
453
' I
1
NiO
a 0
0 2 U
I Y
I
SiO,
.4 g
without addition fcuiNr = 6 3 n w t )
0
t' 0 .z
+
325%
a5
e
3
0 2 50
300
350
400
w t NiO in sample
T OC
Fig. 9
Of the max degree of reduction vs. temperature, for pure and promoted NiO/Si02
:
Apparent maximum
ti
g
Fig. 10 : Influence of NiO content 'on the amount of spontaneously reducible, initiable and unreducible NiO at 325OC (ref. 191.
(ref. 191. 4.3.3. Reduction of molybdenum oxide/support. occur in the
Three different situations may
system (Fig. 8 ) .
Concerning the reduction of "free" Mo03-an explanation has been proposed, which takes into account the effect of dispersion (23, 241. One salient feature of the reduction of MOO
is that it proceeds according to the sequence 3 Mo6+ -+ Mo4+ + Mo
The reduction of a monolayer will obviously have completely different kinetics, in comparison with that of the bulk oxide.
This aspect has been investi-
gated when Moo3 is deposited on various supports (23, 561, with the case of y-AlZ03 having been the object of the most extensive research ( 2 3 , 2 5 , 26, 34, 57, 581.
Results of reduction experiments have been interpreted by assuming an
epitaxial monolayer of aluminium molybdate (34, 561, or a surface interaction compound having a distribution of bonding strengths ( 2 6 ) .
It must however,
be noted that a comparison of the reduction results of MOO /Si02 (221 and 3 MoO /A1 0 (26) suggests that there are no essentially different features, when 3 2 3 account is taken of the i.ncompk?&te presence of a monolayer in the former case (Fig. 11). Actually, the most conspicuous feature in the reduction of the MOO
is the stabilization of Mo"
(23-26. 591.
monolayer 3 It should, however, be remarked that
in RDS catalysts, this stabilization is not strong enough, and the reduction can go further, either because of the presence of sulfur containing molecules (RZ/H2S)
or, in
"complete" catalysts (containing Co or Ni, in addition to Mo) ,
454
because of the action of these elements in promoting reduction (1, 2, 3, 23, 251.
Q5C d
Q25
1
5
0I 0
1
t
hrs
t
hrs
11
Q50I o(
Q25
I 1 1
Fig. 11
: Effect of MOO content on the reduction of supported Mo03 ( a : fraction 3 of Moo3 reduced to Mo; figures on the curves refer to wt % Moo3; MOO curve 3 correspond to reduction of bulk Moo3 at the same temperature.
a. Mo03/Si02, T = 4OO0C, = 1 atm. [ref. 221 pH2 b. Mo03/A1203, T = 5OODC, pH = 1 atm. [ref. 26 1 2
Few results are available concerning the reduction of A12
MOO^)^.
Massoth [261
has reported that the ease of reduction of bulk A12(Mo0413 is somewhat intermediate between that of MOO and of 10 3
%
Mo/A1203.
456
Consequently, taking into consideration the possible influence of dispersion on the reduction of A12(Mo04)3,we could infer that the presence of this compound might, in principle, account for the difficulty to reduce the Mo/A1203 system. 4.3.3. Reduction of the Co-Mo oxide/support system. Two new problems have to be discussed, with reference to the complete system, where cobalt and molybdenum oxides are present. The first concerns the presence of cob&
moLy6datc.
The formation of a well
defined CoMo04 on the surface of A1 0 is likely to modify the reducibility of 2 3 cobalt promoted catalyst Co-Mo/Al 0 since X.P.S. studies [281 indicate a dee2 3 per reduction of the former. The other complication which arises when cobalt and molybdenum oxides are simultaneously present on the surface of the carrier is the possible m & d
eddeot in reduction. We have already indicated the promoting effect of cobalt (or nickel) on the reduction of MOO monolayer. This effect is also exerted on 3 "free" MOO [23, 251 , as it is on bulk Moo3 [60, 61). On the other hand, molyb3 denum oxides seem not to affect the reduction of groupe VIII metal oxides [621. 5 . INFLUENCE OF REDUCTION CONDITIONS
Generally speaking, the assessment of the influence of the parameters related to the reduction conditions requires a consideration of the different steps of the reduction process, namely
:
(i) coming into contact of the reducing agent with the solid; (ii) adsorption; (iii) nucleation; (iv) reaction (interface progress). On the other hand, many parameters are necessary for describing the reduction conditions :
. composition of the reducing gas
(nature of reducing agent, impurities,
presence of the gaseous product of reaction, etc
. temperature; . time sequence of various conditions and/or temperature). Not all aspects can be discussed here.
...) ;
(progressive variation of composition We shall mainly restrict our review
to the case of the activation of HDS catalysts and to parameters which have been shown to influence this activation.
456 5 . f . Role of 5.1.1.
H2g
Inhibition of nucleation.
Charcosset et a1 [ 6 3 1 , from their studies
of the influence of water on copper catalyzed reduction of NiO, have inferred that H 2 0 inhibits the reduction by blocking nucleus forming sites. Similar effects probably take place when Co/A1 0 and Mo/Al 0 are reduced 2 3
with a mixture of H -H 0 ( 4 . 5 torr H20) I 6 4 1 . 2
H -H
2
2
2
0 is always lower than with pure H2
2 3
The degree of reduction with
(Fig. 1 2 ) .
When, in the course of the
reaction, H -H 0 is changed by hydrogen, the reduction exhibits an induction 2
2
period, which is absent when the reduction stage is initiated with pure H
2’
The appearance of an induction stage would possibly be related to a progressive restoration of the reactivity of nucleus forming sites previously inhibited by the water.
1 d
0 12
24
36 t
41
hrs
Fig. 12 : Reduction of Mo/Al 0 and Co/A1203 in dry and wet H2 2 3 (T = 4OO0C, pH2 (ref. 6 4 1 .
1 atm.; x : fraction of MOO or CO 0 reduced to metal) 3 3 4
457
5.1.2.
Transfer of active species. The water may be indirectly responsible
for the acceleration of reduction, by providing a "transport vehicle" for the reducing species activated over a promoter (65, 661.
Water, indeed, plays an
important role in the spill-over of hydrogen activated on a group VIII metal onto WO
3
in the partial reduction of the latter ( 6 5 , 661 and similar effects
probably occur with MOO 3' 5.1.3. Change of the disperseness of the product. quite different way.
The water may also act in a
One can envisage that, depending on temperature, nature
of the metal, and H /H 0 ratio,water may influence the disperseness of the re2 2 duced phase, by modifying the interaction metal-support. The following situations might be considered ( 6 7 )
:
(i) the water is adsorbed on the carrier.
In this case, H 0 will eventu2 ally modify the roughness of the support surface. The water will also increase the mobility of the metal, as the polarizability of the water is usually less than that of the ions of the carrier, thus
leading to an easier sintering;
(ii) the water can oxidize the metal at the interface,increasing, generally, the adherence of the metal to an oxidic support. The discussion can be developed along similar lines, when oxygen, rather than water, is present at the surface or in the gas phase. 5.2. Role of H S in reduction-sulfidation 2 The present paragraph is a little outside the scope of the communication, as it deals with more than merely reduction, namely reduction-sulfidation. However, we believe that the corresponding results should be mentioned here, because of their direct relevance to the activation of HDS catalysts. The results of a series of temperature programmed reduction or reduction-sulfidation on MOO shown in Fig. 13 (curves 1, 2) indicate clearly an easier re3 duction of Moo3 in the presence of H2S (681. In effect, the reduction starts at a much lower temperature than when it is carried out with pure H2; it then proceeds until the formation of Moo2 with only traces of MoS2. tion to MoS
2
Total sulfida-
occurs at higher temperatures.
5.3. Influence of the succession in time of different reduction conditions It has been emphasized that the reaction of solids has a different course if the succession of reduction conditions (the activation program) is different. This can be demonstrated theoretically when nucleation is partially rate-determining (691, and experiments in the reduction of NiO fully substantiate the conclusion (70, 711.
458
I
I
I
I
10
E.100 P
0
-1 0
\
-20
-30r
250
0
750
500 T
800
'C
Fig. 13 : Reduction and reduction-sulfidation of Moo3 and Moo2 (ref. 681 : curve 1 : Moo3 + H2 ; curve 3 : Moo2 + H2-H2S curve 2
:
Moo3
+
H -H S 2 2
;
curve 4
:
Mo
+
H2S.
Results obtained in the preparation of uauppohted HDS cata'lysts suggest how critical the activation cycle may be. first reducing MoO
If reduction-sulfidation is realized by
to M o O
with a El -H 0 mixture, and only then introducing 3 2 2 2 the sulfidation begins at temperatures, similar to those observed with Moo3,
H S, 2 but the sulfidation does not go to completion and stops when an approximately equimolecular mixture of Moo2 and MoS2 (both identified in X-ray diffraction) is observed (Fig. 13 curve 3) ( 6 8 ) . that phenomenon.
There is no undisputable explanation for
It might simply be attributed to the fact that reduction of
in the presence of H s takes place at lower temperature and, hence, gives 3 2 a more reactive Moo2. Alternatively, one might speculate that nucleation of MOO
takes place more easily from Moo3 than from Moo2. A third explanation is 2 that we have an example of a coupfing of reactions, namely the fact that two dif-
MoS
ferent reactions (here
:
reduction and sulfidation) promote themselves mutually,
when occuring simultaneously. Some instances of such coupling exist in other fields of solid state chemistry (3, 72, 731.
459 6 . INFLUENCE OF ADDITIVES
Additives (so called "minor components") are'widely used in catalysts. The fact that they so often play an important role has fascinated investigators since catalysis has begun to develop as a science. When trying to assess their possible effects, one is led to suspect that some explanations have been overemphasized and others, equally reasonable ones, almost forgotten. Modification of the electronic properties of metals or semiconductors by foreign elements dispersed in the bulk or on the surface has been the object of innumerable discussions in academic laboratories. On the other hand, catalyst manufacturers are faced with less fashionable effects of accidental impurities, the action of which has rarely been investigated, and the effect of which, if deleterious, has to be corrected more or less empirically, by the use of additives. A first problem, which is far from being solved, is to distinguish between the effects which correspond to a p o m o ~ o f lof the active sites (by changing the local atomic arrangement, the electronic density, etc...)
by additives, and tho-
se which merely reflect the fact that the additives have influenced in some way the phtpcZl~V%Ofl b t t p b .
In the first cases, one has to take, as a background,
the theories of catalysis, while in the second, the science of the reactions of solids will serve as a reference. The present paragraph will try to consider the second category of effects. It is probably not possible to enumerate exhaustively the various effects that additives may exert on the preparation steps. The classification made for the reaction of solids ( 3 1 , broadly speaking, certainly constitutes an excellent frame. We shall only cite here some typical effects, with relation to HDS catalysts and the additives used or mentioned in relation to these catalysts. The best classification is the one which considers the possible effects of additives on the three categories of parameters examined above, namely those related to dispersion, to active phase-support interaction, and to the development of the activation reaction. 6.1.
Dispersion
The role of soluble additives in modifying dispersion during impregnation is discussed in other contributions. Let us only speculate on the role that insoluble acidic or basic sites on the support of the catalyst might have in helping dispersion or, conversely, in promoting nucleation of a deposited cluster. We do not believe that this effect has received sufficient attention. Additives may also play an important role during the decomposition of the precursor salt, by changing the kinetics of the reactions and, also its b
p
~
d
course (because of more or less numerous nuclei are being formed) and consequently, the dispersion and texture of the product ( 3 , 74 1. In a similar respect,
460
additives are known in ceramic science and technology to prevent (role of "spacers" [ 751.) or promote sintering according to cases. 6.2. Active phase-support interaction It is believed that the role of certain additives in HDS catalyskis to change the reactivity of the surface of the support with the active oxide. This belief is supported, for example, by studies indicating a change in the distribution of sites of different symmetries in alumina, when it is doped with 2+ Zn2+ or Ga ( 76 I. Additives may inhibit interaction between active phase and support. An example can be found in the reported influence of fluorine and boron on the interaction between NiO and A1203 (771. Conversely, the additives may promote active phasecarrier interaction.
The influence of the incorporation of a small amount of
A 1 0 in a SiO matrix, on the interaction between NiO and support can be consi2 3 2 dered as an example of the promoting effect of the additive (Fig. 14) (781.
Another possibility is that additives may simply act as a "glue" between the active phase and the carrier. The corresponding phenomena belong to the field of adhesion, and are far from clear presently. 1
1
NiO
a(
i
t
hrs
Fig. 14 : Reduction of NiO supported on silica and silica alumina; Si/Si+Al=95wt.%); 1 0 wt % NiO; T=400°C;pH = 1 atm; a : fraction of NiO reduced to metal rref.78). 2
461
6.3. Development of the reactions taking place during activation The role of additives in the reduction of oxides has been extensively studied (1, 2, 3, 20, 661, and special attention has been focussed on the oxides which are present in the oxidic form of HDS catalysts [ 2 3 , 25, 79, 80, 811. Many effects have been discussed in the previous sections. Let us simply indicate that Cu, in NiO/Si02,(Figs. 9, 10) may constitute a typical example of the effect of additives, otherwise inert, on the reduction step. 6.4.
Mixed situations
Although the preceding paragraphs have attempted to classify the various effects of additives, it should be remarked that many of them may modify several features of the catalyst to be activated and the activation reaction. For example, addition of an alkali metal ion on an A1 0 carrier can vary the poro2 3 sity distribution of the carrier, modify the acidity, change the dispersion of the active phase during impregnation, affect the interaction oxide-support, influence the surface reactivity with respect to hydrogen and, possibly, provoque an alteration of the texture of the reduced solid. 7. CONCLUSION
The factors which influence the reduction step of supported catalysts are multiple.
Elaborating a simple and exhaustive classification which accounts
for all the different situations is a difficult task.
However, we have sugges-
ted a possible approach. We have restricted the discussion to the major factors likely to have a pronounced effect on reduction and which are relatively well documented. This means that some special factors have been either disregarded or only briefly mentioned.
Others, like spill-over processes,have been tho-
roughly reviewed several times and no further discussion was necessary. Concluding this communication, it seems that one remark may emerge, namely that much information is available in the field of the science of the reactivity of solids which can be used almost directly for explaining the reaction of AUppUhted solids. The opinion has been often expressed that the making of catalysts is an art, much like cooking. This is certainly not the opinion of the scientists interested in the bases of preparation of catalysts. Transdisciplinary co-operation can help making the activation of catalysts more scientifically footed. ACKNOWLEDGEMENTS Parts of the experimental work carried out in GPCMC, and the postdoctoral fellowship of one of us (M.H.) were supported by the "Services de la Programation de la Politique Scientifique" in the frame of the "Actions Concertees In-
.
teruniversitair% Catalyse"
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and N.G. Vannerberg (Eds.), Reactivity of Solids, Plenum Press, New York, 1977, pp.69-74. M. Lojacono, A. Cimino and G.C.A. Schuit, Gazz. Chim. Ital 103(1973) 12811295. M. Lojacono, M. Schiavello and A. Cimino, J. Phys. Chem. 75(1971) 1044-1050. M. Lojacono, M. Schiavello, D. Cordischi and G. Mercati, Gazz. Chim. Ital. 105 ( 1975) 1165-1 176. M. Lojacono and M. Schiavello, in B. Delmon, P.A. Jacobs, G. Poncelet (Eds.), Preparation of Catalysts, Elsevier, Amsterdam, 1976, pp. 473-484. G.N. Asmolov and O.V. Krylov, Kinet. Katal. ll(1970) 1028-1033. J. Medema, C. Van Stam, V.H.J. De Beer, A.J.A. Konings and D.C. Koningsberger, submitted to publication. V.H.J. De Beer, M.J.M. Van der Aalst, C.J. Machiels and G.C.A. Schuit, J. Catal. 43(1976) 78-89. M, Lojacono, M. Schiavello, V.H.J. De Beer and G. Minelli, J. Phys. Chem. 81 (1977) 1583-1588. M. Bulens, Ann. Chim. l(1976) 13-25. P. Ratnasamy, A.V. Ramaswamy, K. Bannerjee, D.K. Sharma and N. Ray, J. Catal. 38(1975) 19-32. J.W.E. Coenen, B.G. Linsen, in B.G. Linsen (Ed.), Physical and Chemical Aspects of Absorbants and Catalysts, Academic Press, New York, 1970, p.479. T. Fransen, P.C. Van Berge and P. Mars, React. Kinet. Catal. Lett. 5(1976) 445-452. P. Sontag, D.Q. Kim and F. Marion, C.R. Acad. Sci. Ser. C 259(1964) 47044707. T. Kabe, S. Yamadaya, M. Oba and Y. Miki, Int. Chem. Eng. lZ(1972) 366. N. Giordano, J.C.J. Bart, A. Castellan and A. Vaghi, in P.C.H. Mitchell (Ed.), Chemistry and Uses of Molybdenum, Climax, Reading, 1973, pp.194-200. J.M. Zabala, P. Grange and B. Delmon, C.R. Acad. Sci. Ser. C 279(1974) 561-563. D.K. Lambiev, T.T. Tomova and G.C. Samsonov, Powder Metallurgy Int. 4(1972) 17. J.M. Zabala, M. Mainil, P. Grange and B. Delmon, C.R. Acad. Sci. Ser. C 280 (1975) 1129-1 132. H. Charcosset, R. Frety, P. Grange and Y. Trambouze, C.R. Acad. Sci. Ser. C (19681 1746-1748. P. Gajardo, P. Grange and B. Delmon, to be published. J.E. Benson, H.W. Kohn and M. Boudart, J. Catal. 5(1966) 307-313. H. Charcosset, B. Delmon, Ind. Chim. Belg. 38(1973) 481-495. J. Geus, Communication EUCHEM Meeting, Namur,(1976). J.M. Zabala, P. Grange and B. Delmon, C.R. Acad. Sci. Ser. C 279(1974) 725728. B. Delmon, ref.[20), pp.414-436. B. Delmon, ref.[20), pp. 485-498 and 258-269.
464 71 B. Delmon and A. Roman, J. Chem. SOC. 69(1973) 941-948. 72 M.L. Garcia-Gonzalez, P. Grange and B. Delmon, C.R. Acad. Sci. Ser. C 280 (1975) 1439-1441. 73 M.L. Garcia-Gonzalez, P. Grange and B. Delmon, in J. Wood, 0. LinqViSt, C. Helgesson and N.G. VannerberqfEds.), Reactivity of Solids, Plenum Press, New York, 1977, pp.69-74. 74 V.V. Boldyrev, Wissenshaft U. Fortshritt 28(1978) 17-21. 75 D.L. Trimm, in B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Eds.), 2nd International Symposium on Preparation of Catalysts, Louvain-la-Neuve, 1978. 76 A. Cimino, M. Lojacono and M. Schiavello, J. Phys. Chem. 79(1975) 243-249. 77 H. Lafitau, E. Nee1 and J.C. Clement, in B. Delmon, P.A. Jacobs and G. Poncelet ( Eds.), Preparation of Catalysts, Elsevier, Amsterdam(l976), pp.393-401. 78 M. Houalla, B. Delmon, unpublished results. 79 W. Verhoeven and B. Delmon, C.R. Acad. Sci. Ser. C 262(1966), 33-35. 80 W. Verhoeven and B. Delmon, Bull. SOC. Chim. Fr. (1966) 3065-3073 and 3073-3079. 81 H. Charcosset, R. Frety, A. Soldat, Y. Trambouze, J. Catal. 22(1971) 204-212.
DISCUSSION J.W.
H I G H T O W E R : Your bi-layer model for t h e optimally active 0.25
Co/Mo catalyst suggests that the Mo i s covered by a C o layer and m a y not be readily available to t h e g a s phase.
T h i s picture may be
naive i n view of recent work we have d o n e with some Mo-containing Haldor Tops$e catalysts both with and without Co for t h e m e t a t h e s i s o f p r o p y l e n e near room temperature.
Mo is the active ingredient in
t h i s system, and the a c t i v i t y at constant MO loading is independent o f t h e p r e s e n c e o f Co.
Furthermore, NO p o i s o n s t h i s reaction and
can be used a s a titrating a g e n t to determine t h e active s i t e concentration.
T h e infrared spectra o f N O a d s o r b e d o n these
m a t e r i a l s show bands that c a n be attributed to NO o n both M O ( 2 bands) and Co
(1 band).
Hence, both C o and M o m u s t be readily
available to the g a s phase.
B. DELMON
: We are not yet
i n a position to p r o p o s e a p r e c i s e model
of t h e bilayer, and o u r answer, presently, i s o n l y speculative. Measurements of the intensity of t h e XPS l i n e s suggest that C o associated with this bilayer lies closer to t h e surface a s opposed to C o i n CoA1204.
O n the other hand a r o u g h estimate indicates
that t h i s bilayer associates approximately 1 Co to several Mo a t o m s (3 t o 4).
If this picture i s correct, one may conceive an arrangement
i n which C o of t h e bilayer d o e s not shield Mo and we arrive a t t h e conclusion that both a t o m s should be readily accessible.
J.T.
R I C H A R D S O N : I have a q u e s t i o n and a comment.
The question is :
what is t h e evidence that b u l k C03O4 exist o n t h e surface in the
465 p r e s e n c e o f Moo3 ? 10-15 wt
%
It h a s a l w a y s b e e n m y i n t e r p r e t a t i o n t h a t w i t h
Mooj, c o b a l t t h a t is not p r e s e n t a s C o A 1 2 0 4 or C o X M o
1 -x03
i s COO. T h e comment : m a g n e t i c m e a s u r e m e n t s i n d i c a t e t h a t c o m m e r c i a l c a t a l y s t s h a v e d i f f e r e n t c o b a l t s t a t e s after c a l c i n i n g and a f t e r s u l f i d i n g p l u s regeneration.
P e r h a p s w e should t u r n our a t t e n t i o n t o
r e g e n e r a t i o n s y s t e m s s i n c e t h e s e s t i l l r e t a i n their activity.
B. DELMON : A s c o n c e r n s y o u r q u e s t i o n , t h e r e are m a n y e v i d e n c e s t h a t c o 3 0 q is p r e s e n t o n t h e c a t a l y s t c o n t a i n i n g a h i g h p r o p o r t i o n of Co (r = Co/(Co+Mo) = 0.75 1.00). W e c a n c i t e f i r s t t h e blackening
-
o f t h e s a m p l e w h i c h i s c h a r a c t e r i s t i c o f t h e p r e s e n c e o f Co304. A n o t h e r e v i d e n c e s t e m s from t h e s e n s i t i v i t y of t h e U.V.
reflectance
s p e c t r u m o f t h e s a m p l e s t o t h e p r e s e n c e o f C o 3 0 4 (ref. 45 o f our p a p e r ) i l l u s t r a t e d by t h e p r e s e n c e o f a shoulder n e a r 7 5 0 nm. Moreover electron diffraction measurements performed on these Another
s a m p l e s g i v e a f u r t h e r p r o o f o n t h e e x i s t e n c e o f Co304.
r e a s o n c o m i n g f r o m XPS m e a s u r e m e n t s l i e s in t h e h i g h i n t e n s i t y r a t i o o f t h e p r i n c i p a l t o t h e s a t e l l i t e p e a k s o f Co 2 p 3 / 2 (p-I /(I +Is))
P
i t i s pZO.7
for r
5
0.75-1.00,
o b t a i n s , ~ ‘ 0 . 5 8 f o r Co(I1)
(e.g.
i n C o A 1 2 0 4 and b.CoMo04).
t h e v a l u e s o b s e r v e d d o not c o r r e s p o n d t o p u r e C o 3 0 4
However,
( ~ ~ 0 . 8 2 ) .O n e
m i g h t e s t i m a t e t h a t , in t h e c a t a l y s t w i t h r=1.0, o n l y 5 0
%
of Co i s
W e cannot i n d i c a t e i n w h i c h form e x a c t l y is t h e
p r e s e n t a s Co304. r e m a i n i n g Co.
P
B u t w e p a r t l y a g r e e w i t h your o b s e r v a t i o n w h e n w e
c o n s i d e r c a t a l y s t s w i t h smaller p r o p o r t i o n o f rC0.55.
C o is
e s s e n t i a l l y in t h e form o f Co(II), b u t p r o b a b l y n o t a s COO. C o n c e r n i n g y o u r c o m m e n t , w e c o m p l e t e l y a g r e e w i t h you.
We think
t h a t m e a s u r e m e n t s m a d e o n f r e s h c a t a l y s t s , or o n c a t a l y s t s having w o r k e d o n l y a f e w h o u r s a r e blurred b y t h e p r e s e n c e o f s p e c i e s w h i c h have nothing t o d o w i t h t h e c a t a l y s t activity.
Long t i m e
w o r k i n g , a n d , s t i l l better, r e g e n e r a t i o n , would r e v e a l t h e r e a l l y i m p o r t a n t p h a s e s , b e c a u s e they p r e s u m a b l y d e v e l o p at t h e e x p e n s e o f t h e o t h e r ones.
F.P.J.M.
KERKHOF
:
1) We h a v e studied t h e binding energy o f t h e
S i ( 2 p ) e l e c t r o n for a s e r i e s of W 0 3 / S i 0 2 c a t a l y s t s and found a c o n s t a n t value
(1).
:
w h e r e a s o n e should o b t a i n , a n d a c t u a l l y
M o r e o v e r , t h e Si(2p)/O(ls) intensity r a t i o w a s
c o n s t a n t and t h e s a m e a s i n p u r e S i 0 2 , w h i c h f a c t s u p p o r t s t h e “ b u l k “ c h a r a c t e r o f t h e XPS t e c h n i q u e for high s u r f a c e area s u p p o r t s
So, in our v i e w , i t i s very d i f f i c u l t t o d e t e c t t h e p r e s e n c e o f
466 a s u r f a c e l a y e r o f Mo-Si-0
compound by X P S .
C o u l d y o u comment
on t h i s 7
2 ) Is i t p o s s i b l e t h a t t h e maximal r e d u c t i o n f o r t h e
2
15
%
Moo3/
S i 0 2 c a t a l y s t must be a t t r i b u t e d t o t h e w a t e r evolved d u r i n g reduction, poisoning the reduction
B. DELMON
: It
?
i s t r u e t h a t the v a r i a t i o n of t h e binding energy of
the Si(2p) level i s q u i t e important,
e s p e c i a l l y i f one t a k e s i n t o
account t h a t s e v e r a l s i l i c a l a y e r s c o n t r i b u t e t o t h e Si(2p)XPS signal.
w e have s t r e s s e d i n o u r communication
Having t h i s i n mind,
t h e need o f a d i r e c t v e r i f i c a t i o n o f t h e h y p o t h e s i s o f a n e l e c t r o n t r a n s f e r from s i l i c a t o molybdena by c h e c k i n g t h e s h i f t i n t h e Mo(3d)
lines.
U n f o r t u n a t e l y s u c h a t t e m p t s were f r u s t r a t e d ,
i n the case of
t h e ~ o o ~ / S i sO a m~p l e s e x a m i n e d , b y t h e e x t e n s i v e b r o a d e n i n g o f t h e Mo(3d) s p e c t r u m . C o n c e r n i n g t h e d i f f i c u l t y t o d e t e c t t h e p r e s e n c e o f a Mo-Si-0 compound b y XPS,
surface
we think t h a t i n p r i n c i p l e i t i s possible t o observe s u r f a c e l a y e r when i t s o c c u r r e n c e l e a d s
t h e formation of t h i s kind of
t o a s h i f t i n t h e Mo(3d) b i n d i n g e n e r g y and o b v i o u s l y it r e p r e s e n t s
a c o n s i d e r a b l e f r a c t i o n o f t o t a l Mo c o n t e n t . 2 ) Your r e m a r k r e f e r s t o o u r F i g .
4.
W e must remark t h a t t h e
maximum d e g r e e o f r e d u c t i o n w e r e p o r t i s q u i t e a n e m p i r i c a l v a l u e . The r e d u c t i o n a f t e r 1 0 h r s d e p e n d s o n t h e k i n e t i c s o f t h e p r o c e s s , and,
among o t h e r p a r a m e t e r s ,
on t h e n a t u r e o f
the species (bulk
Moo3, Moo3 i n i n t e r a c t i o n w i t h S i 0 2 ) , o n t h e d i s p e r s i o n o f t h e s p e c i e s , a n d o n d i f f u s i o n phenomena H20).
(especially with respect t o
Hence, t h i s v a l u e o n l y i n d i r e c t l y and i m p e r f e c t l y c h a r a c t e -
r i z e s t h e d e p o s i t e d s p e c i e s on t h e d e g r e e o f d i s p e r s i o n .
I n our
a b o v e m e n t i o n e d i n v e s t i g a t i o n s on Mo03/Si02 w i t h a n a l y t i c a l e l e c t r o n microscopy,
t h e u p p e r l i m i t o f Moo3 i n i n t e r a c t i o n w i t h S i 0 2 w a s
e s t i m a t e d t o b e a b o u t I %. or, at least,
The s h a p e o f t h e c u r v e b e t w e e n 7 a n d 2 0 %
t h e a s c e n d i n g b r a n c h between I and 1 5 % , c o u l d t h u s The d e c r e a s e b e t w e e n 1 5 a n d 2 0 % may
be a t t r i b u t e d t o dispersion.
be a t t r i b u t e d , e i t h e r t o t h e i n v e r s i o n of t h e e f f e c t of p a r t i c l e s i z e growth i n d i c a t e d i n F i g s . a r i s i n g a s a consequence of
2 and 3 ,
o r t o d i f f u s i o n phenomena
t h e c o n s t r i c t i o n of
the pores or of the
c o m p l e t e b l o c k i n g of p o r e s .
J.F.
LEPAGE : On t h e b a s i s o f y o u r m o d e l s ,
c a n you e x p l a i n t h e
c h a n g e s i n r e l a t i v e a c t i v i t y o b s e r v e d i n h y d r o g e n a t i o n of o l e f i n s o r aromatics, i n HDS,
when t h e c a r r i e r s o f t h e Co-Mo
catalysts are
467 I S it possible as
changed, the Co/Mo ratio remaining "the same" ? w it h me ta 11 ic cat a 1y s ts to invoke
"
fa c i 1e " or
"
d i f fi ci 1e " r e a c ti on s
with respect to the form, the size or the composition of the active "sulfided deposists" 7
8.
DELMON
We do not know whether it will turn out that certain
:
re actions i n hydro d e s u 1fur i zat io n are
"
fac i 1e " and other
"
d i f f i c i 1e "
.
But, whichever the model used for explaining the activity of HDS catalysts, we can expect the selectivity to change with the form and size of the sulfide particles, with the composition of the catalysts, and with the nature of the support ( 1 ) .
If we forget the acidic
properties of the support, and consider only the reactions supposed to be catalyzed by the sulfides, the problem which is still not solved is whether the support is directly involved in the architecture of the active phase, or if it mainly plays a role in achieving a given dispersion] repartition in size, contact between sulfides, etc.. during preparation.
It is clear that A1203 brings about a good
dispersion because it favors the formation of a CoMo "bilayer" and that SiOg is much less efficient in this respect.
The difference
between the two carriers may lie in the fact that the CoMo "bilayer" is interacting with A1203, while, on Si02 it is only weakly interacting and consequently Co and Mo tend to form CoMo04.
The
"bilayer" would be a good precursor of a finely (interldispersed active sulfided phase, whereas CoMo04 is a very poor precursor (2,3).
So, in our opinion at least part of the difference in cataly-
tic behaviour must be attributed to differences in the genesis of the active systems, rather than to specific interaction between the final sulfided species and supports. B. Delmon : A.C.S. meeting Petroleum Div. Preprints New Orleans (1977)I p.503. 2. G. Hagenbach, Ph. Courty and B . Delmon, J. Catal. 3 (1971), 295 and 3 . Catal. 31 (1973), 264. 3 . P. Grange and B. Delmon, J. Less Comm. Metals. 36 (1974), 353. 1.
V. K O S T R O V
:
What do you think about the theoretical evolution of the
inhibition of the kinetics of nucleation ?
For example, what is the
influence of steam on kinetics of reduction of Co-Mo/alumina catalysts
?
B. DELMON
:
We were referring to the kinetic consequences of the
intervention of two quite different processes in many reactions of solids, namely nucleation and interface progress (as growth of
nuclei).
Taking a s e r i e s of
samples w i t h d i f f e r e n t g r a i n s i z e s , b u t
c h a r a c t e r i z e d by i d e n t i c a l r a t e s o f n u c l e u s f o r m a t i o n k
on t h e 9 s u r f a c e and i d e n t i c a l r a t e s o f i n t e r f a c e p r o g r e s s , t h e r a t e - l i m i t i n g
f o r small o n e s . This explains the s t e p i s ki f o r l a r g e p a r t i c l e s , k 9 v a r i a t i o n of t h e o v e r a l l r a t e s w i t h p a r t i c l e s i z e ( p r o p o r t i o n a l t o
1'3 i n d i c a t e d i n F i g . 2 ( 1 ) . Our s u g g e s t i o n i s t h a t t h e d e c r e a s e AS (0) i n r e d u c t i o n r a t e s observed with supported o x i d e s , i n comparison t o unsupported ones, can,
i n c e r t a i n cases, be a t t r i b u t e d t o t h e f a c t
t h a t n u c l e a t i o n becomes more r a t e - l i m i t i n g . B u t y o u a r e q u i t e r i g h t i n i n d i c a t i n g t h a t some f a c t o r s c o u l d
intervene, (e.g.
such a s i n h i b i t i o n by H 2 0 ,
etc...
Both ' a c t i v a t i o n e f f e c t s
by group V I I I m e t a l s ) and i n h i b i t i o n c a n b e o b s e r v e d i n t h e
r e d u c t i o n of o x i d e s
(2, 3, 4 ) .
1 . B. Delmon, I n t r o d u c t i o n a l a c i n e t i q u e h e t e r o g e n e , E d i t i o n T e c h n i p , P a r i s , 1969; K i n e t i k a g e t e r o g e n n y k h r e a k t s i i , E d i t i o n s Min., MOSCOU, 1972. 2 . J. M a s s o n , 8 . D e l m o n , J. N e c h t s c h e i n , C . R . A c a d . S c i . , P a r i s , S e r i e C , 266, ( 1 9 6 8 ) , 428-430. 3 . A . Roman, 8 . Delmon, J . C a t a l . , 30, ( 1 9 7 3 ) , 3 3 3 - 3 4 2 . 4 . H. C h a r c o s s e t , B . Delmon, I n d . Chim. B e l g . , 38, ( 1 9 7 3 ) , 4 8 1 - 4 9 5 .
469
PREPARATIVE CHEMISTRY OF COBALT-MOLYBDENUM/ALUMINA CATALYSTS A. IANNIBELLO and P.C.H. MITCHELL Stazione Sperimentale per i Combustibili, S. Donato Milanese, Italy and Department of Chemistry, The University, Whiteknights, Reading, U.K.
ABSTRACT The catalytic properties of COO-MOO3/A1203 catalysts are affected by their method of preparation. As a basis for understanding these effects we have investigated the chemical changes in the preparation of the catalysts and the structures of the cobalt and molybdenum 0x0-species formed. Our main results are : (a) Adsorption of molybdate by alumina involved exchange of surface -OH groups 2for [Moo4] ions.
(b) Some aluminium molybdate was formed in the preparation of MOO /A1203 3 catalysts PlOg Mo03/100g A1203) and remained in unwashed catalysts. (c) The predominant oxomolybdenum(V1) species in the MOO /A1203 catalysts 3 depended on the molybdenum concentration and were: strongly bound tetrahedral [Moo4] (<5g Mo03/100g A1203), less strongly bound tetrahedral [Moo,,] (>5g Moo3/ lOOg A1203), and octahedral [Moo6] (>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 <0.05 g MOO3 ), which was formed by reaction of molybdate with alumina at the higher molybdate concentrations, probably contained aluminium molybdate (i.r. spectrum, shoulder at ca. 920 cm-l, cf. aluminium molybdate peak at 915 cm-';
amorphous to X-ray diffraction).
A similar reaction
is the formation of insoluble aluminium phosphates during adsorption of phosphate
on alumina from solutions of Na2HP04[7].
Although the preceeding observations
were incidental to our main work they are of some significance in catalyst preparation. Formation of the finely divided solid implies some breakdown of the alumina surface in contact with the aqueous molybdate solution. If this solid is retained in the catalysts (as in the PF procedure, see below) it could cause
inhomogeneity in their compositions and properties. The analytical data give the total amount of molybdenum converted to an insoluble form, i.e. the total amount reacted, and not necessarily the amount adsorbed by the alumina. (iii) Effect of washing
The effect of washing with distilled water on the molybdenum content of catalysts prepared by the EA and PF methods was studied in order to discover how much molybdenum was irreversibly bound to the alumina (Fig. 2 ) .
For the FA catalysts,
there was no loss of molybdenum from the first three samples (which contained up to ca. 7X Moo3) on washing; but part of the molybdenum was washed from the other samples and so these contained some less strongly bound molybdenum. A portion of each PF catalyst was washed with water after equilibration but
before drying (and
so
the pores still contained the impregnating solution).
The
pH of the wash liquid was determined as a function of time; for all samples the pH increased (by ca. 0.7 units) to a constant value during ca. 1 min presumably because of diffusion of OH- ions from the pores into the bulk of the solution, The final value of the pH which is determined by the equilibria in solution was lower at higher molybdenum and ammonium ion concentrations as in the equilibrium adsorption experiments. The concentrations of MOO in the catalyst before and after washing are shown 3 in Fig. 2. The amount of molybdenum retained in the EA series catalysts after washing is greater than in the PF series catalysts. Thus the amount of strongly bound molybdenum is less for the PF series catalysts. We can understand this result on the basis of exchange of molybdate ions with surface hydroxyl groups of alumina [equations (la) and (lb)]. In the EA preparative procedure hydroxide ions diffuse from the pores into the bulk of the impregnating solution but this is not possible in the PF procedure since
of the impregnating solution is in
474
the pores. Thus the local pH in the pores will be greater in the PF procedure and so the uptake of molybdate by reaction with the alumina will be less.
(In
general the uptake of molybdate decreases as the pH of the impregnating solution increases[6,g]).
lor c lo[ ‘z8
A U
6
I
1
I
14
16
18
20
Fig. 2. Effect of washing on MOO /A1 0 catalysts. Amount of molybdenum in the 2 3 catalysts after washing(C ’ ) plotzed verms amount before washing (Cs) for Catalysts prepared by thesequilibrium adsorption (EA) method (curve A) and the pore-filling (PF) method (curve B) determined by X-ray fluorescence. (iv) Drying and calcination and structures of the molybdenum species The Mo03/A1203 catalysts were dried in air at llO°C for 24 h and calcined in
air at 55OoC for 24 h. Their
U.V.
diffuse reflectance spectra (Fig. 3) showed
the following features.
EA and PF catalysts: washed; calcined at 55OoC The spectra were characteristic of tetrahedral oxomolybdenum species. The U.V. absorbances were the same for those FA and PF catalysts which contained the same amount of molybdenum. Thus both series contained the same strongly adsorbed [Moo4] species with the same degree of dispersion.
FA and PF catalysts: unwashed; calcined at 55OoC The PF series catalysts with Moo3 > 10 g showed an additional
U.V.
peak at ca. 32,000 cm-’ and so
475
contained some octahedral oxomolybdenum species formed presumably by thermal decomposition of bulk ammonium molybdate deposited in the pores when the catalysts were dried.
The lower-molybdenum PF catalysts and the EA catalysts had
spectra characteristic of tetrahedral [Mo04] species. The spectra showed one peak at
EA and PF catalysts: unwashed; dried llO°C ca. 37,000 cm-';
thus the main species is tetrahedral molybdenum(VI),
[Mo04],
[9]. The U.V. absorbance was greater for EA than for PF catalysts with the same amount of molybdenum which suggests that the oxomolybdenum species is more dispersed in the EA catalysts[lO].
This agrees with our observation that the amount of strongly bound molybdenum is greater in the EA catalysts. The PF catalysts with the highest molybdenum concentrations (16, 20 g Moo3) showed tetrahedral 2oxomolybdenum spectra; evidently the tetrahedral [Moo4] ion is deposited (as ammonium molybdate) from the solution in the pores when the catalysts are dried. This agrees with our suggestion (see above) of a high (alkaline) local pH in the pores the effect of which would be to shift the molybdate solution equilibria 2over to the [Moo4] ion.
PF20
J
45
I
I
I
I
I
I
I
I
I
1
40 35 V/10 3cm-1
1
1
I
I\'
30
j
45
I
I
I
I
I
I
1
I
I
1
40 35 3 V/10 cm' 1
I
t
I
I\,
30
Fig. 3. Representative U.V. reflectance spectra (vs. MgO blank) of Mo03/A1203 unwashed, dried llO°C; Bd washed, dried llO°C; C, unwashed, catalysts: calcined 550 C; D, washed, calcined 550 C. Catalysts prepared by equilibrium adsorption (EA) and pore-filling (PF) methods. Numbers on the curves are the Mo(V1) concentrations [(g Mo03)/(100 g A1203)1.
6,
476 EA and PF c a t a l y s t s :
washed; c a l c i n e d a t 55OoC
of t e t r a h e d r a l [Moo4] s p e c i e s .
The s p e c t r a were c h a r a c t e r i s t i c
Thus washing removes t h e s p e c i e s which o t h e r w i s e
would form o c t a h e d r a l oxomolybdenum(V1) i n t h e high-molybdenum PF c a t a l y s t s . (b) CoO-Mo03/A120_3 C a t a l y s t s The c a t l a y s t s contained 8 g Mo03/100 g A1203 and up t o ca. 4 g COO. prepared by t h e p o r e - f i l l i n g
(PF) method i n t h r e e d i f f e r e n t ways:
They were
(1) simultaneous
impregnation w i t h a s o l u t i o n c o n t a i n i n g b o t h molybdate(V1) and c o b a l t ( I 1 ) i o n s , ( i i ) adding c o b a l t ( I 1 ) s o l u t i o n t o Mo03/A1203 c a t a l y s t s d r i e d a t l l O ° C , ( i i i ) t o c a t a l y s t s c a l c i n e d a t 55OoC.
The u . v . - v i s i b l e
and
r e f l e c t a n c e s p e c t r a and
magnetic moments of 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 d r i e d a t l l O ° C and c a l c i n e d a t 55OoC were measured. (i>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.
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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 << kS
-
governed rvo'ution by k s
r '
/
evolution governed by k's
:
reversible at 7OO0C, for
.
-
The stationary level is established at R or I , according to the values of Ks, while the time required for sulfur elimination depends mainly on the rate constant of desorption kIS. at 7 0 0 ° ,
For steam-reforming
stabilization corresponds to an inhibition and partial
poisoning.
By eliminating H 2 S , the initial activity can be regained
after a time having the same degree of magnitude as for deactivation. For benzene hydrogenation at 150°C, stabilization corresponds to practically complete poisoning due to the huge value of K S .
By eliminating H2S, the initial activity would be regained but only after a time of infinite length due to the quite small value of k'
Of course the high value of K value of kIS.
S
=
S'
kS/kIS is connected with the low
To speed up sulfur elimination, k I S can be increased
by using high temperatures, but the risk of Ni sintering then appears.
To conclude on Ni
:
-For steam-reforming, selective poisoning is partial and reversible, and a reversible stabilization is attained. -For benzene hydrogenation, selective poisoning is practically complete and irreversible.
So just very weak quantities of sulfur
compounds are tolerated in the feed.
Anyway this leads to a
progressive and complete deactivation as described in section 4 - 2 . In the case of platinum all these phenomena tend to produce less poisoning and more reversibility.
~ the case of At 5 0 0 " ~ in
Catalytic Reforming, the reversibility makes it possible to eliminate any accidental excess of sulfur.
Of course reality is more complex,
534 and for example, the last traces of sulfur can be quite difficult to eliminate due to the site heterogeneity of the metal.
By the
use of so-called thio-resistant platinum we again move in the direction of less poisoning and more reversibility due to the decrease in KS.
Thio-resistance is obtained by deposition of highly
dispersed Pt on an acid support (ref. 17).
The accepting power of
the support decreases the electronic density of Pt, which is then unable to produce such strong bonding with S, for Pt acts as a donor in the formation of PtS (ref. 18). 3.4. Stabilization of the catalyst in its efficient structure By this we mean the addition to the feed, of chemical species, which do not contribute to the stoichiometry of the catalysis, but which are necessary to maintain the efficient composition and structure of the catalyst.
Two illustrative examples concern an
intrinsic effect on the catalyst itself.
The first is the
addition of HC1 to the feed of a catalytic reforming unit in order to maintain the acidity of the support.
Under the influence of the
temperature and water in the feed, this acidity decreases but is restored by this addition of HC1.
The second is the addition of
HZS, to the feed of a hydrodesulfurization unit in order to produce a PH2s/PH2 ratio capable of maintaining the cobalt molybdenum in the sulfidation state (see Figure 7 for a comparative illustration). On the other hand hydrogen is often added, not to modify the catalyst itself, but to modify the selectivity of the reaction by hydrogenating the coke precursor.
In this way this stabilization
process belongs rather to the section on stabilization by transient parasitic coke formation. 4. DEACTIVATION DURING WORKING While the final state is obviously the important point for stabilization, it is the kinetic evolution which is important for deactivation phenomena, in order ot define the variation of operating conditions needed to maintain the performances and to determine the moment when the catalyst has to be regenerated or discarded.
Among
the causes of deactivation described above, we shall not consider coke formation, which is the continuation of phenomena leading in a first phase to a pseudo-stabilization; a more or less similar mechanism can operate but is considerably slower on the catalyst which is deeply modified by the first and rapid coke deposition.
535 4.1.
Deactivation, relevant to the catalyst itself (the case of sintering)
The underlying cause of this deactivation is the thermodynamic metastable state of a catalyst, if we consider its possible evolution, independently from catalysis, which, fortunately is greatly slowed down by kinetic factors.
The simplest example concerns the large
surface area of porous catalysts which tends to decrease, diminishing the overall free energy of the system.
Among the numerous methods
of textural or structural deactivation we shall consider only the sintering of metal deposited on a support.
This deactivation process
becomes significant for high temperature reactions such as steam reforming or post-combustion of exhaust gases. During catalyst regeneration by coke combustion, the high local temperatures can also lead to significant sintering. Two essential sintering mechanisms are generally invoked (ref. 1 9 )
:
i) sintering by crystallite migration, followed by their coalescence after collision ii) sintering by migration, towards large crystallites, of atomic o r molecular compounds extracted from small crystallites. For the first mechanism (ref. 20) it seems that once crystallites collide the coalescence is rapid, if we consider the ease of sintering of bulky metals at temperature as low as 2 0 0 ' C . step will usually be the diffusion step.
Hence the limiting
The diffusion rate depends
mainly on the bonding strength, between crystallite and carrier. Since the value of the overall bonding increases with crystallite size, it is only for small crystallites that this mechanism appears likely to cause sintering, even if we consider the surface migration of a particle by its change in shape, through a type of creeping process (ref. 21).
Moreover, the redispersion phenomenon is
difficult to explain by this mechanism. The second mechanism (ref. 2 2 ) ,
called Ostwald Ripening, proceeds
according to the three successive steps shown in Figure 11. i) From crystallites, atomic or molecular species escape to produce, in the gas or in the adsorbed phase, highly dispersed atoms, not necessarily in a metallic form. ii) The migration from small to large crystallites proceeds under the influence of a concentration gradient, which will be justified below. iii) The capture of diffusing species can occur in two ways either by collision and in incorporation into a large crystallite, hence leading to crystallite growing and sintering
:
536
Extroction
Deposition
Migration
x: sites permitting strong interactions.
high rate for weak interactions.
towards chemisorbed phase.
-
7 Q 7 X A
towards homogeneous gas phase diffusion phase. Fig. 1 1
:
on the crystallite. i o n the support.
Ostwald Ripening mechanism (growing of large crystallites
at the expense of small crystallites). or by settling on some free support sites, with which the bonding strength may be quite high.
Such a phenomenon is the possible
source of redispersion. The concentration gradient from a surrounding of small crystallites, towards a surrounding of large crystallites has been explained by thermodynamic considerations (ref. 19).
If the thermodynamic
equilibrium is achieved, large crytallites are in equilibrium with a lower concentration of dispersed atoms than small crystallites (this is the two-dimensional analog of Ostwald Ripening, by the application of Kelvin's law).
Of course thermodynamic equilibrium
may be far from being attained during sintering, but it remains that the rate of loss of atoms is smaller than the rate of capture for large crystallites, while for small crystallites the rate of loss is greater than the rate of capture.
The Ostwald Ripening mechanism
can be used to interpret more experimental data than the Crystallite Migration mechanism. some examples.
We shall try to show its potential by examining
An important general parameter which governs the
sintering rate is also the interaction strength of catalytic atoms, with the bulk of the metal crystallite, with the support, or with the homogeneous phase surrounding the crystallite.
But depending on
whether this parameter affects one or the other of the three above mentioned steps, it can enhance or hinder the sintering rate, or even induce a redispersion. From such considerations, it is possible to attempt interpretation, for example, of the apparently contradictory influences of an oxygen
537 p a r t i a l p r e s s u r e on t h e m e t a l d i s p e r s i o n .
The m o s t g e n e r a l l y o b s e r v e d
r e s u l t i s t h a t oxyge'n e n h a n c e s s i n t e r i n g r a t e a s c o m p a r e d w i t h t h e c a s e o f a r e d u c i n g medium ( r e f .
However i t h a s b e e n p o s s i b l e t o
19).
o b s e r v e a c r y s t a l l i t e r e d i s p e r s i o n by u s i n g a n oxygen atmosphere
(ref.
Besides, a redispersion i s claimed, although without decisive
23).
p r o o f , when u s i n g o x y g e n a n d c h l o r i d e compounds f o r t h e p l a t i n u m o f a r e f o r m i n g c a t a l y s t f o r example. phenomena,
I t i s possible t o reconcile
u s i n g i d e a s d e v e l o p e d by d i f f e r e n t w o r k e r s
these
( r e f . 24).
e x t r a c t i o n s t e p i s r e l a t i v e l y d i f f i c u l t i n a r e d u c i n g medium,
t h e h i g h bonding s t r e n g t h o f a m e t a l l i c atom w i t h t h e c r y s t a l l i t e h e a t of s u b l i m a t i o n c a n be h i g h e r t h a n 100 kcal/mole
The
due t o (the
i s many c a s e s ) ,
a n d t o t h e weak b o n d i n g s t r e n g t h o f z e r o m e t a l w i t h a c a r r i e r s u c h a s alumina.
I n a n o x y g e n medium,
t h e e x t r a c t i o n s t e p i s made much
e a s i e r , b o t h s u p e r f i c i a l c r y s t a l l i t e o x i d a t i o n a n d by t h e f o r m a t i o n o f more s t a b l e c o m p o u n d s a f t e r e x t r a c t i o n . v o l a t i l i z a t i o n towards t h e gas phase, p l a t i n u m o x y c h l o r i d e w i t h 02 chemisorbed phase,
+
For e x t r a c t i o n by
we o b t a i n P t O f o r e x a m p l e ( o r For e x t r a c t i o n towards a
C12).
w e o b t a i n Pt2'/Al2O3,
f o r example.
Then, a f t e r
t h e e x t r a c t i o n and m i g r a t i o n s t e p s :
-
e i t h e r t h e d i f f u s i n g s p e c i e s a r e c a p t u r e d by a l a r g e c r y s t a l l i t e ;
sintering occurs,
-
or,
a n d so i s e x p l a i n e d i t s e n h a n c e m e n t by o x y g e n ,
t h e d i f f u s i n g s p e c i e s c a n b e s t r o n g l y h e l d by some p a r t i c u l a r
s i t e s of t h e s u p p o r t ,
and t h i s i n d u c e s r e d i s p e r s i o n .
For s p e c i e s
d i f f u s i n g i n t h e a d s o r b e d phase such scavenging c o u l d r e s u l t from t h e i n t e r v e n t i o n of p a r t i c u l a r s i t e s of t h e support : c o r n e r , edges, various defects,
r e s p o n s i b l e f o r t h e w e l l known " d e c o r a t i n g e f f e c t " .
For d i f f u s i o n i n t h e gas phase,
a mechanism of 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 must be i n v o k e d . Of c o u r s e t h e r e a c t i n g m o l e c u l e s t h e m s e l v e s c a n c o n t r i b u t e t o crystallite evolution.
S t r o n g bonding between r e a c t a n t and m e t a l
can promote s u r f a c e r e c o n s t r u c t i o n ,
as seen p r e v i o u s l y b u t can a l s o
f a c i l i t a t e t h e atom e x t r a c t i o n t o w a r d s t h e a d s o r b e d o r homogeneous phase,
t h u s l e a d i n g t o s i n t e r i n g , t o metal e l i m i n a t i o n , o r t o
redispersion.
Examples a r e g i v e n o f s i n t e r i n g enhanced by r e a c t a n t s
( r e f . 2 5 ) , a n d t h e p o s s i b i l i t y o f m e t a l loss by r e a c t i o n w i t h r e a c t a n t i s w e l l known
(Ru02 i n p o s t combustion c a t a l y s t s ) .
T h e s e few e x a m p l e s i l l u s t r a t e t h e i n t e r p r e t i n g p o w e r o f t h e Ostwald Ripening mechanism.
However, a c c o r d i n g t o t h e a u t h o r s
t h e m s e l v e s , t h e h y p o t h e s e s have n o t been proved d i r e c t l y , and c h o i c e b e t w e e n t h e two b a s i c t h e o r i e s i s s t i l l t h e s u b j e c t o f discussion.
The k i n e t i c e v o l u t i o n i t s e l f d o e s n o t make
538
discrimination possible (ref. 19). In conclusion, a huge amount of experimentation has been performed which has led to practical results and to the establishment of intellectual concepts of the sintering mechanism.
A
more systematic
experimental approach now seems needed to progress our understanding of these phenomena. Irreversible poisoning
4.2.
For selective poisoning, an intrinsic modification of the catalytic species occurs.
For non-selective poisoning the decrease in activity
results mainly from the covering of the catalytic sites and sometimes from pore plugging.
In any case, location of the poison has a great
influence on the downward drift of performances and can be discussed from Figure 12.
The phenomenon depends on the competition between
the penetration rate of poisons (or poison precursors) and their deposition rate on the catalyst surface.
'
Two extreme cases can be
considered.
rp penetration
f
equivalent 'O a boundory layer
rate of deposit precursors m
d
I(,-
e
p
o
s
i
t
o
Fig. 1 2
:
Different
aspects of irreversible poisoning ( X : active sites).
t h
llpl
' P B rD
rate deposition on pore walls
uniform deposition (slight effect for
small -rp >> 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 << rD. Deposition takes place on the pore mouths, with quite P different consequences from the previous case. The bead periphery
539
becomes inactive.
For fast reactions a diffusion
-
limitation
appears (or is greatly increased), and this limitation is of the same nature as that created by a larger immobile boundary around Hence, the activity varies as Ta with temperature,
a catalyst bead.
which produces apparently very low activation energies.
Of course
the metal distribution through the catalyst bead has a great influence on such poisoning.
If the metal has been deposited on the periphery
to avoid diffusion-limitation (in the case of a fast reaction), the decrease in activity may be drastic.
So a compromise has to be sought
Several authors have published theoretical or experimental papers for achieving such a compromise (ref. 26). The poisoning of nickel hydrogenation catalyst by sulfide compounds (at low temperature) is an example of selective irreversible poisoning
(see Figure 10).
Consideration of the influence of lead
on the activity of post-combustion catalysts is particularly interesting, although it would be difficult to distinguish between selective or non-selective poisoning.
The poison precursors consist
mainly of more or less volatile lead halides probably diffusing in both the gaseous and adsorbed phase.
Their penetration rate
into the pore increases extensively with the temperature.
If
poisoning has taken place at low temperature it mainly affects the pore mouth.
Hence due to the apparently weak activation energy,
conversion will increase very slowly with temperature, for such a catalyst, and may remain inadequate, as illustrated by Figure 13 (ref. 2 7 ) . 100
Fig. 13 Fresh (0hrl.
:
Laboratory evaluation of
a lead-poisoned Pt-A1203 catalyst GHSV = 90,000/hr-1 lead accumulation = 0.1 g Pb/gal; converter *Hourson test OlgPbIgalin fuel
=
2300 cm2 radial flow
at 59OOC (poisoning at low temperature) (ref. 27a), and for
200
300
400
500
600
Temperature ('C)
different times.
If poisoning occurs at high temperature, the lead compounds penetrate statistically into the entire pore, and deactivation is less, even for a higher total lead percentage
(1.5% of uniformly dispersed lead
can be less harmful than 1% on pore mouths).
540
A
Fig.
14
C a t a l y s t w i t h 0 . 0 0 % Pd on
:
alumina c a r r i e r .
Activity after
aging during 30hI i n a burner a t d i f f e r e n t temperatures, Pb;
1 C1
-
0.5 B r l .
(0.25 g / l
Laboratory
a c t i v i t y t e s t f o r G H S V = 3 0 , 0 0 0 h-' ( r e f . 27b).
---------- A c t i v i t y
for the fresh
catalyst. Activity a f t e r aging. F i g u r e 14 shows t h a t ,
f o r p o i s o n i n g a t 7OO0C, d e a c t i v a t i o n may b e much
l e s s t h a n f o r p o i s o n i n g a t 400.C.
a t 400.C
Moreover,
it t h e c a t a l y s t poisoned
i s h e a t e d t o 7OO0C, f o r e x a m p l e , t h e a c t i v i t y may i n c r e a s e a s
the r e s u l t of b o t h
A
b e t t e r d t o t r i b u t i o n o f the l e a d a n d of the e l i -
m i n a t i o n o f some l e a d b y i t s d e s o r p t i o n
from t h e c a t a l y s t ' s s u r f a c e .
We c a n n o t e t h a t t h e p r o o f o f a s e l e c t i v e c o n t r i b u t i o n f o r l e a d p o i s o n i n g i s g i v e n by t h e i n t e r e s t i n g c o m p l e x e v o l u t i o n o f a c t i v i t y i n CO o x i d a t i o n v e r s u s m e t a l c o v e r a g e by l e a d .
U n t i l h a l f of t h e metal i s
c o v e r e d t h e a c t i v i t y i n c r e a s e s , a phenomenon w h i c h i s i n t e r p r e t e d by an i n t r i n s i c e f f e c t (ref. 28).
( e l e c t r o n i c o r l i g a n d e f f e c t ) o f l e a d on p l a t i n u m
More p r e c i s e l y ,
t h e o x i d a t i o n r a t e can be w r i t t e n ,
for
P t o r Pd c a t a l y t i c s p e c i e s :
r = k
b o 2 po2 +
bC0
;
w i t h bCO pc0
>> 1
( r e f . 28 e t 2 7 b ) .
pco
b a n d bCO t h e a d s o r p t i o n c o e f f i c i e n t s o f 02 By a d d i n g a s m a l l p e r c e n t a g e o f l e a d , t h e g r e a t d e c r e a s e
k being the r a t e constant, O2
and CO.
o f bCO h a s much more i n f l u e n c e t h a n t h e s m a l l d e c r e a s e o f k b r e a c t i v i t y ) a n d o f t h e a c t i v e s i t e number o f P t o r P d .
(the 02 By i n c r e a s i n g
t h e l e a d ' p e r c e n t a g e a g a i n , t h e a c t i v i t y g o e s t h r o u g h a maximum a n d d e c r e a s e s a g a i n when t h e d e c r e a s e o f k b
02
a n d t h e a c t i v e s i t e number
becomes g r e a t e r t h a n t h a t o f b
CO' A n o t h e r e x a m p l e o f p o r e mouth p o i s o n i n g i s o b s e r v e d i n t h e c a s e
of hydrodesulfurization
c a t a l y s t s , o f vacuum r e s i d u e s
o n Co-Mo/A1203
c o n t a i n i n g vanadium and n i c k e l compounds. F i g u r e 1 5 shows t h a t a vanadium d e p o s i t i o n o c c u r s a t t h e bead p e r i p h e r y vanadium s u l f i d e ) w h i l e n i c k e l p a s s e s
( i n t h e form o f
(also i n t h e form o f n i c k e l (ref. 29). The r p / r D The d e p o s i t i o n i n v o l v e s
s u l f i d e ) deeper i n t o t h e h e a r t of t h e bead r a t i o i s d i f f e r e n t f o r t h e two p o i s o n s .
t h e i n t e r v e n t i o n of t h e c a t a l y t i c s p e c i e s .
The a r o m a t i c a s p h a l t e n i c
541
compounds in which the vanadium, for example, is combined, are hydrogenated into naphthenic structures, which are then hydrogenolysed, leading thus to vanadium sulfide deposition.
,
Moreover geometrical
factors certainly have an important influence on the penetration rate
Fig. 15
rnlration
:
Hydrodesulfurization
of vacuum residues.
Ni and V
concentration all along a bead ,,,,............- .............................
diameter Alumina Vanadium
Nickel
; if
-w----- . ',
1: 1jj
100
-
i -
'\ ----__-__ .
i
(microprobe analysis).
A1203
"
---
._....__._..
Ni 200
p
through the pore of the large molecules or micelles of asphaltenic compounds. In the case of pore mouth poisoning, a means of couteracting deactivation consists of using a catalyst with bimodal pore size distribution (or a spread out pore size distribution).
Due to the
hi.gh deposition rate for this poisoning mode, the deposit occurs immediately in the large pores which operate like a filter, thus providing protection for small pores, responsible for the high catalytic surface area and activity.
The large pores act as the
guard case often located at the entrance of a reactor to provide protection for the catalytic bed. The general concept of competition between transportation and deposition inside the pores as described above, can be transposed for phenomena occuring in the reactant flow along the catalytic bed.
1.3. Catalyst defects Here we shall describe some examples of parasitic reactions due to bad design or manufacture of the catalyst. be included i n an "ideal" catalyst.
"Foreign" species can
Thus the presence of group VIII
metals in a catalyst for methanol synthesis can catalyze methane formation accompanied by high heat emission.
Besides a decrease
i n yield, this causes sintering of the catalyst.
Another more
general case of catalyst defects concerns the parasite activity
of the support.
A characteristic example is the selective hydro-
genation of diolefins or acetylenic compounds or, more practically, of steam-cracking gasoline, on catalysts such as Pd or Ni/A1203. Due to the reactivity of these highly unsaturated compounds, their polymerization can occur even for a small acidity of the alumina, leading to polymer deposition on the support.
The way to counteract
this deactivation is of course to decrease the acidity of the carrier and also to use the best technology.
By operating for example,
in the liquid phase, the catalyst can be cleaned of the accumulated liquid polymers.
5. REGENERATION general definition of regeneration can be the recovery
A
(restoration) of activity after irreversible damage.
In this sense,
the intrinsic stabilization is not, of course, affected by the regeneration concept.
Reversible poisoning (inhibition), which
is cancelled out by the elimination of poison from the feed, is likewise not invoked in regeneration. Elimination of coke by burning with oxygen is a true regeneration. The well-known precaution to be taken consists in feeding in a progressively growing oxygen percentage in order to avoid high temperatures leading to sintering (and eventually to explosion). Sometimes the presence of sulfur is claimed to enhance sintering during coke burning
(case of reforming catalyst), and sulfur has
to be eliminated by previous H 2 treatment according to the reaction (ref. 30).
n2
+
Pts < r Pt
+
H ~ S
The damage induced by sintering is much more difficult to restore. For deposited metal however, we have tried to interpret the redispersion of platinum by oxygen or rather by the O 2 claimed by various patents.
+
C12 mixture
Much more systematic experimental
investigation is needed, firstly to prove that redispersion does occur effectively, and secondly to establish its mechanism.
Another
recently proved possibility for example, for the decrease of accessible platinum for a reforming catalyst by hydrogen treatment at high temperature, is the formation of Pt-A1 alloys (ref. 31).
In this
case, of course, regeneration by oxygen is quite different (ref. 32). Regeneration outside of the reactor, rejuvenation, is generally achieved by chemical treatment and will not be discussed here.
643 6.
GENERAL CONCLUSION The variety and complexity of the phenomena involved in the
stabilization, deactivation and regeneration of catalysts should be clear from what has been said.
We have only been able to give a
slight glimpse of the types of variations that we feel to be important and with which we are most familiar.
However, we feel able
to conclude that the enormous number of experimental results obtained from fundamental research or industrial achievements can now form the basis for general laws and mechanisms which are valid as an effective working hypothesis in the study of these phenomena. Furthermore, experimental techniques are becoming more and more effective in characterizing the state of catalysts at different stages of their evolution and in following catalytic performances in the steady or transient state. Therefore, we feel that systematic, well directed and properly exploited research should enable rapid advances in the understanding of these phenomena while at the same time providing practical applications from the industrial standpoint.
REFERENCES
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808-814.
F. Avendano Thesis Paris ( 1 9 7 5 ) . Editions Technip Paris. P. Boutry, P. Courty, J.C. Daumas and R. Montarnal, Bull. SOC. Chim. Fr. 10 ( 1 9 6 8 ) 4 0 5 0 - 4 0 5 6 . 5. G. Rambeau and H. Amariglio, J. Chim. Phy. 7 5 ( 1 9 7 8 ) 1 1 0 - 1 1 5 . 6 . R. Brill and J. Kurzidim, Colloques Internationaux du C.N.R.S. Editions du C.N.R.S. Paris ( 1 9 7 0 ) 1 8 7 Paris (Juillet 1 9 6 9 ) 9 9 - 1 0 1 . 7 . J.A. Dumesic, H. Topse and M. Boudart J . Catal. 3 7 ( 1 9 7 5 ) 5 1 3 - 5 2 2 . 8 . A.G. Friedlander, Ph. Courty and R. Montarnal J . Catal. 48 ( 1 9 7 7 ) 3. 4.
312-321. 9.
W.#, Sachtler and R.A. Van Santen Advances in Catalysis 2 6 , 6 9 - 1 1 9 Academic Press N.Y. ( 1 9 7 7 ) 10. R.L. Moss and L. Whalley Advances in Catalysis 2 2 , 1 1 5 - 1 8 5 Academic Press N.Y. ( 1 9 7 2 ) V. Ponec Catalysis Review Sc and Eng. 11, 4 1 - 7 0 Marcel Dekker N.Y. ( 1 9 7 5 ) . 11. A. Takeuchi, K.1- Tanaka and K. Miyahara J. of Catal. 40 ( 1 9 7 5 )
-
101-107. 12.
Ph. Courty and J.F. Le Page Second International Symposium on the Scientific Bases for the Preparation of Heterogeneous Catalysis Louvain (Sept. 1 9 7 8 ) . 1 3 . J.R. Rostrup-Nielsen "Steam Reforming Catalyst" Danish Technical Press Copenhagen ( 1 9 7 5 ) . J.R. Rostrup-Nielsen and D.L. Trimm J. Catal. 48 ( 1 9 7 7 ) 1 5 5 - 1 6 5 . 1 4 . C. Portier, G. Thomas, J.F. Le Page and R. Montarnal C.R. Acad. Sc. Paris Serie C , 2 8 4 ( 1 9 7 7 ) 8 6 5 - 8 6 7 .
544 15. 16.
C.E. Alvarez Herrera Thesis Poitiers 1 9 7 7 Editions Technip Paris. R. Montarnal and G. Martino Franco-Sovietic Seminar on Catalysis Kiev (Sept. 1 9 7 4 ) and Rev. Inst. Francais d u Pet. Vol XXXII N O 3 (1977)
367-391.
J.A. Rabo, V. Schumaker and P.E. Pickert Proc. 3rd Int. Congr. Catalysis North Holland Pub. CY Amsterdam ( 1 9 6 5 ) Vol. 2 , 1 2 6 4 . Proc. Sixth Int. Cong. on Catalysis London 1 8 . G.D. Chukin and A1 (July 1 9 7 6 ) . The Chemical Society London ( 1 9 7 7 ) 6 6 8 - 6 7 6 . 1 9 . S.E. Wanke and P.C. Flynn Catal. Rev. Sci. Eng. 1 2 ( 1 ) ( 1 9 7 5 ) 9 3 - 1 3 5 Marcel Dekker ( 1 9 7 5 ) . 2 0 . E. Ruckenstein and B. Pulvermacher AICHE J. 1 9 ( 1 9 7 3 ) 3 5 6 . 2 1 . R. Kern, A. Masson and J.J. Metois Surface Sci. 2 7 ( 1 9 7 1 ) 4 8 3 . 2 2 . P.C. Flynn and S.E. Wanke J. Catal. 3 4 ( 1 9 7 4 ) 3 9 0 . 2 3 . R.M.J. Fiederow and S.E. Wanke J . Catal. 4 3 ( 1 9 7 6 ) 3 4 - 4 2 . R.M.J. Fiederow, B.S. Chahar and S.E. Wanke J. Catal. 5 1 ( 1 9 7 8 ) 17.
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DISCUSSION H. SHINGU
As to your definitions and classifications about
:
stabilization or catalyst evolution during catalysis, particularly referring to Fig. 1, I would like to point out that 1)
:
the steady state terminology for the stationary level of
activity (you mention e v e n true state in Fig. 1) should be correctly referred to as stationary state, because the kinetic term steady state signifies generally that the overall rates are changing with
c
2
2
# 0, whereas i n the stationary state the rates 2 2 are time independent, i.e. C d x/dt = 0, so that the theoretical time, i.e.
d X/dt
true steady state in Fig. 1 may seem to be self-contradictory.
The
currently prevailing misuse of the term steady state, especially in reaction engineering fields, seems to originate from a confusion between stabilization and deactivation during working time, according to your classification and analysis.
545 2)
the transient period of catalyst stabilization usually involves
much complicated surface processes, and may be better analyzed with respect to the reversibility or reproducibility and the time required for the stabilization. Essentially, the intrinsic stabilization should be reversible or more generally reproducible and require only short time for the transient period to be reached (e.g., less than several minutes with a high selectivity catalyst).
Irreversible stabilization usually requires
much longer time, and cannot be referred to as intrinsic, because the catalyst surface structure is always essentially modified by this treatment, as you have concluded about the complex stabilization. R. MONTARNAL : As far as my paper is concerned, the concept of steady-
state is presented for an open flow reactor, in the absence o f any deactivation process.
In this case, a steady state is attained
after the time at which the concentrations in a given slice of catalyst remain constant, and so, the reaction rate remains constant in each slice.
This true steady state refers to a reversible
intrinsic phenomenon which is due to the mechanism of the catalytic It corresponds to what I have called intrinsic stabiliza-
reaction. tion.
This must be distinguished from the pseudo-stabilization due
to coke formation, which is an irreversible phenomenon and leads to new catalyst different from the initial one.
Once this pseudo-
stabilization by deactivation is stopped,the coked catalyst works with its own steady state, depending on its own intrinsic catalytic properties. D.J.C.
YATES
:
I should like to draw your attention to several U.S.
Patents issued in my name in the last few years, where you will find precise experimental evidence that noble metals can easily be redispersed to atomic dispersion, starting from particles in the 2 0 - 3 0 nm region, by the addition of halogens. R. MONTARNAL 1,099,233
:
As you say, as early a s 1 9 5 4 ,
in French patent
it is claimed, on the basis of X-ray diffraction results,
that a redispersion occurred of platinum crystallites of about 0
0
2 0 0 A into crystallites less than 5 0 A in size.
again taken up in US patent 3 , 1 3 4 , 7 3 2
Such claims are
for platinum.
They are
also taken up, for example, in your French patents 2 , 2 4 0 , 7 6 2 2,257,337
and
“for iridium- and halogen-based catalysts on a refractory
546
support, using sophisticated techniques in the case of iridium". What I meant to say was that I have not' round any scientific publication concerning this redispersion.
I feel that the intimate
mechanism of this redispersion still remains to be determined, even though mention can be made of an interaction between volatile species and the OH sites of the support, as I suggested in my reply to Charcosset. J.W.E. COENEN
In the process you call Oswald ripening, often
:
intermediate formation of a volatile or soluble compound which acts as the transport agent is an important step with nickel catalysts.
I shall come accross two examples, both involving
crystallite growth caused by unfavorable transient situation during processing. One concerns methanation.
If the catalyst is brought into contact
with a carbon monoxide containing gas stream under conditions where Ni(CO),
formation is thermodynamically favoured, a situation may
occur where small Ni crystals are "eaten" and at the same time, or later Ni(CO),
is decomposed and Ni deposited on larger crystals.
The second example refers to fatty acid hydrogenation.
Nickel drop
formation is possible and occurs at low hydrogen pressures again with loss of the smaller crystals.
When later hydrogen pressure is
built up, nickel metal is deposited but not in very active form (autoclave walls, large nickel crystals).
R. MONTARNAL
:
You gave two quite nice and characteristic examples
of the general idea which I paraphrased in my paper in the following terms
:
"Of course the reactional molecules themselves can contribute
to crystallite evolution.
Strong bonding between reactant and metal
can promote surface reconstruction, as seen before, but also make easier the atom extraction towards the adsorbed or homogeneous phase, thus leading either to sintering, to metal elimination or why not to redispersion". H. CHARCOSSET
In your communication, you consider the redispersion
:
of sintered Pt by O 2 or by O 2 and C12 as a rather questionable
phenomenon.
Could you comment in more detail on this
R.E. MONTARNAL
:
?
The platinum redispersion by means of O 2 and C12
finally appears to be rather well proved.
It seems to proceed by
formation of volatile compounds from Pt crystallites (complexes of
547
Pt-Al-chlorides or Pt-oxychlorides, at the temperature used for redispersion).
The migrating species are trapped by sites such as
surface OH groups.
The strong interaction with these sites allows
deposition of these species in a highly dispersed state leading to small crystallites after reduction. R. PRICE
:
Catalysts with bimodal pore size distributions were
mentioned as a possible means of reducing the extent of poisoning by metals.
In our experience with this type of catalyst the
greatest amount of carbon and metals deposition occurs in the smaller 0
pores ( < 5 0 A) and the resultant loss of surface area is not recovered on regeneration.
Can this be interpreted in terms of the poisoning
mechanism which you describe o r does it suggest, perhaps, a difference in the metal-containing poisons present in our coal-derived feedstock compared to the metallic components of petroleum bqsed feedstocks
?
R. MONTARNAL : By using catalyst with a uniform distribution of rather small pores, we observe coke and metal deposition.
This
agrees with your result that these deposits can occur in the small, pores of a catalyst with a bimodal distribution of pore sizes.
In
.
our experiments, the metal accumulation was,as presented in the paper, at the periphery of the pellet.
This result only concerns
the macro-distribution and does not provide any information on the microdistribution between large and small pores.
I would like to
emphasize that in our eqperiments, the deposition a t the periphery
of the pellet is indicative of a deposition rate which is faster than the penetration rate.
So, if added macropores have a sufficient-
ly high catalytic surface area to allow fast reactions to occur, they may be able to achieve a sort of "scavenging" o f metal compounds (deposited as sulfides), leading then to a protection of the micropores.
This explanation has been advanced by different authors,
especially in patents, for petroleum based feedstocks with V and Ni. The possibility that such a protection occurs, depends on many factors as the nature of the charge, of the catalyst, and of the operating conditions.
So it is psssible that, for coal derived
feedstocks containing for example fe and Ti compounds and minerals, the characteristics of the deposition could be different. N.
:
I agree that a more systematic experimental approach
to thexmal sintering would be beneficial.
These experiments should
cover a wide range of catalytic metals, metal concentrations and support types with different atmospheres to give a more complete picture.
However, it seems likely that both Oswald ripening and
crystallite growth migration may operate together, one phenomenon predominating under a given set of experimental conditions.
Could
you comment on the usefulness of crystallite particle size measurements, particularly at the early stage of sintering experiments, attempting to differentiate between the possible mechanisms
?
R. MONTARNAL : The answer to your question i s given in the review
of Wanke and Flynn (1).
These authors state that if support
surfaces were energetically homogeneous, a number of tests could distinguish between crystallite and atomic migration.
The crystallite
migration model predicts that metals on catalysts with unisized or narrow pore size distribution will sinter readily, while the atomic migration model predicts such catalysts would sinter extremely slowly. However, real surfaces are energetically heterogeneous; if one includes strong trapping sites and preferred adsorption areas, either model can account for a wide variety of phenomena. will be extremely difficult for this case.
Model discrimination Moreover specific
mathematical kinetic laws have been developed by different authors for the evolution of crystallite sizes, in order to be able to differentiate between the involved mechanisms.
But due to the
complexity of a real catalyst, it seems that till now "the experimental identification of the sintering mechanism(s) is a problem that awaits solution".
However, in spite o f this negative conclusion
by the authors mentioned, it seems to us that crystallite migration is able to explain sintering of small crystallites, but only in the initial stages and cannot explain redispersion. 1.
S.
Wanke and P.C. Flynn, Catal. Rev. Sci. Eng.
J.W. HIGHTOWER
:
(1)
12, 93
(1975).
In your lecture you mentioned the surface migration
and the gas phase migration models in the sintering of supported metal catalysts.
Could you comment o n the relative importance o f
these two effects in the case of supported P t catalysts either in reforming or in auto exhaust gas purification ?
R. MONTARNAL
:
In the case of supported Pt for catalysts of auto
exhaust purification, sintering is observed in an oxidative atmosphere and by using unleaded gasoline.
In this case, no chloride
compounds need to be added, and the formation of rather volatile
649
compounds such as platinum oxychlorides or complexes of platinumaluminium chlorides cannot be invoked.
It is then generally admitted
that the migration of Pt species occurs in the adsorbed phase, by diffusion of ionic platinum compounds.
But the detailed mechanism
of extraction from the Pt crystallite and of the migration remains to be established.
Considering now the case of a reforming catalyst,
two recent publications (1) ( 2 ) have shown that in the presence of 02,
redispersion first occurs from 5 0 0 to 600"C, at still higher
temperatures sintering occurs again.
The authors give a detailed
discussion of the different modes of transfer for platinum compounds. They conclude ( 1 ) that "migration of PtO
2 (s)
(adsorbed) is restricted
to individual support grains and vapor phase transport is required for transfer of P t from one grain to another".
However for reforming
catalysts, we must also consider that chloride compounds are present and that, at the catalytic temperature, the phenomena are more complex In conclusion, in spite of the possibility of migration in the adsorbed phase, each individual case requires a specific discussion. (1) R.M.J. Fiedorov and S.E. Wanke, J. Catal. 43, 34 (1976). (2) R.M.J. Fiedorov, B.S. Chahar and S.E. W a n k c J. Catal. (1978).
s,193
E.J.
NEWSON
:
In reference to Fig. 15 on HDS vacuum residues and
deposition of Ni and V from the oil in the pores of the alumina based catalyst, it would be more appropriate to call this phenomenon pore-plugging deactivation and not pore-mouth poisoning.
This is
easily shown by the drastic decrease in pore volume of the spent catalyst, say from 0.5 to0.2 cc/g. The questions really refer to the ultimate life-time of the HDS catalyst.
Using a catalyst with a bimodal pore size distribution
would probably give a slight gain in initial HDS activity but a higher demetallation rate is a necessary consequence.
Since some
of the small pores have probably been given up to make "large pores which operate like a filter, it does not necessarily follow (remember the higher demetallation rate) that ultimate life of the catalyst will be increased.
Which data do you have to substantiate
the claim for increase in ultimate life, say 1000 to 2000 hours ? Which feed analyses and process conditions were applicable R. MONTARNAL
:
?
Of course, pore plugging can occur but I think it
often occurs after a rather long period of pore mouth poisoning as the result o f the progressive growing of the volume of the deposits
a t t h e p o r e mouth. 0.2
The d r a s t i c d e c r e a s e i n p o r e v o l u m e f r o m 0 . 5
c c / g d o e s n o t seem t o b e a d e f i n i t e p r o o f
f o r plugging,
to
since
t h e p o r e volume i s d e t e r m i n e d a t low t e m p e r a t u r e , o f t e n w i t h i n t h e c a t a l y s t on which a d s o r b e d h y d r o c a r b o n s
s u f f i c i e n t washing of
( o r m i c e l l e s ) of high molecular weight can cause an apparent plugging. A t higher
be
temperature,
under t h e c a t a l y t i c c o n d i t i o n s , m i c e l l e s can
" d i s s o c i a t e d " o r hydrocarbons desorbed,
l e a d i n g t o a decrease of
plugging, while pore poisoning is maintained. I don't
e n t i r e l y agree with your i d e a t h a t l a r g e pores a r e obtained
a t t h e expense of
small ones,
f o r a m a c r o p o r o s i t y c a n be a d d e d t o
a c a t a l y s t p e l l e t while maintaining a high f r a c t i o n of microporosity. the ultimate l i f e t i m e of a
Concerning t h e comparison between
c a t a l y s t w i t h b i m o d a l a n d monomodal p o r e s i z e d i s t r i b u t i o n , answer i s n o t simple. catalysts,
It is d i f f i c u l t t o build
I t a l s o seems t r u e t h a t m a c r o p o r o s i t y i n c r e a s e s t h e
"chemical" l i f e - t i m e
of
the catalyst,
of t h e mechanical s t r e n g t h of
u n f o r t u n a t e l y a t t h e expense
the catalyst pellet.
D E L M O N : You h a v e q u i t e a u t h o r i t a t i v e l y p r e s e n t e d t h e m a j o r c a t e -
g o r i e s of d e a c t i v a t i o n p r o c e s s e s . t i o n a l category, I
two "identical"
d i f f e r i n g o n l y by t h e p r e s e n c e o r t h e a b s e n c e o f
macropores.
B.
the
However,
w h i c h you d i d n o t m e n t i o n
I wonder w h e t h e r
one a d d i -
s h o u l d n o t be a l l u d e d t o .
r e f e r t o what c o u l d be c a l l e d " c a t a s t r o p h i c " d e a c t i v a t i o n which
occurs q u i t e o f t e n a t t h e s c a l e of a l s o i n t h e whole bed.
as follows.
individual pellets,
Catastrophic a-activation
Imagine a c a t a l y s t f o r p a r t i a l
t h a t on-stream
a s o l i d s t a t e phenomenon
a s a complete oxidation c a t a l y s t . w i l l i n c r e a s e enormously,
transformations.
oxidation.
Suppose
forms another phase a c t i n g
The t e m p e r a t u r e o f t h e p e l l e t
thus triggering further solid state
S i n c e c o m p l e t e o x i d a t i o n c o n s u m e s more o x y g e n ,
a highly reducing product stream consequently i n t e n s e r e d u c t i o n of t h i s way,
b u t sometimes
i s best examplified
(CO,
etc
...)
i s formed, and
t h e s o l i d p h a s e s may o c c u r .
In
some k i n d o f a u t o c a t a l y t i c d e s t r u c t i o n o f t h e a c t i v e
phase t a k e s p l a c e . I n my o p i n i o n ,
it i s t h i s kind of a u t o c a t a l y t i c c a t a s t r o p h i c
d e a c t i v a t i o n p r o c e s s which c o u l d j u s t i f y t o invoke a d i s t i n c t category of d e a c t i v a t i o n processes.
Indeed,
the other categories
you h a v e m e n t i o n e d c o r r e s p o n d more o r l e s s t o p r o g r e s s i v e c h a n g e s many t i m e s w i t h a u t o - a d j u s t m e n t . W i t h c a t a s t r o p h i c p h e n o m e n a , p r e p a r a t i o n o f c a t a l y s t s i s s t i l l more
critical than with the other categories of deactivation.
Any nucleus
of the unwanted phase could trigger catastrophic nucleation if it is inadvertently formed during preparation. I believe that the above remarks are far from being mere speculation, especially in the case of partial oxidation processes. R. MONTARNAL
Your remark illustrates that it is nearly impossible
:
to consider all cases of such a broad topic as catalyst stabilization and deactivation.
However your example of "catastrophic" deactiva-
tion could be inserted under my heading
:
"Catalyst Defects".
In
this part, I have only mentioned "Foreign and Parasite Species included in an Ideal Catalyst, due to Poor Design or Manufacture of the Catalyst, and thus leading to some Parasite Reactions".
By
parasite reactions, I mainly refer to catalytic parasite reactions. You are quite right in emphasizing the importance of catalyst preparation when such autocatalytic "Catastrophic Deactivation" could occur, as it is in the case of mixed oxides used for partial oxidation processes. N. PERNICONE
You have reported some results concerning transient
:
phenomena in kinetic runs of ammonia synthesis catalysts.
I would
like to stress that both Amarijlio and Brill's experiments (refs. 5-6 of your paper) have been performed at atmospheric pressure and low temperatures (below 3OOOC).
During one activity test, carried
out in the pressure and temperature ranges currently used in industrial practice, we never detected such phenomena.
They are
certainly interesting from a basic point of view, but not relevant to the industrial conditions. R. MONTARNAL
:
My purpose was to give some examples of "intrinsic
stabilization".
It is possible that under industrial conditions
transient periods would not exist.
However, it is also possible
that the response time of the performance test would be too long
to allow the detection of the transient period observed by the authors invoked in my paper.
2.
PAAL
:
I should like to add two short comments as far as pseudo-
stabilization due to coke formation is concerned.
I completely agree with you that the severity influences coke formation.
In addition to the amount, the quality of coke also can be
affected.
For example, coking of platinum may leed to the formation
552 of carbonaceous s p e c i e s
( o n e c a r b o n atom p e r P t atom,
I n t h e p r e s e n c e of
dissociated form).
l o w e r hydrogen
e x h i b i t s h i g h e r t o t a l a c t i v i t y i n t h e p r e s e n c e of ( i. e .
pressure
probably i n
such d e p o s i t s platinum
u n d e r more s e v e r e c o n d i t i o n s )
(1)
.
Considering
a l s o s e l e c t i v i t y c h a n g e s under t h e combined e f f e c t o f hydrogen a n d c a r b o n a c e o u s d e p o s i t s we b e l i e v e t h a t t h e s e c o n d e x p l a n a t i o n o f f e r e d by y o u
(1) P a u l Z.,
R.
t h e l i g a n d e f f e c t ) may b e v a l i d i n t h i s c a s e .
(i.e.
Dobrovolosky M . ,
T L t L n i j r P..
5,6 5
Catal.
J.
(1977).
MONTARNAL : I a m l i t t l e s u r p r i s e d b y y o u r r e s u l t a c c o r d i n g t o
i n t h e c a s e of t h e h i g h e s t
w h i c h t h e d e a c t i v a t i o n seems h i g h e s t hydrogen p r e s s u r e ,
w i t h less severe c o n d i t i o n s .
i.e.
What i s t h e
c a t a l y t i c reaction used i n your experiments ? 2. PAAL
:
The d e c r e a s e o f
a c t i v i t y i n h i g h e r h y d r o g e n p r e s s u r e was
observed i n 3-methylpentane
reactions
(isomerization
c y c l i s a t i o n ) a n d was e x p l a i n e d by c o m p e t i t i o n of carbon for sites l e f t a f t e r p a r t i a l
R.
-
hydrogenolysis
hydrogen and hydro-
carbonization.
MONTARNAL : I a g r e e o n t h e e x p l a n a t i o n a c c o r d i n g t o w h i c h h y d r o g e n
a c t s as an i n h i b i t o r n-hexane
(1)
on iridium,
(2).
This leads t o the hydrogenolysis of
the reaction rate being
:
r = [(bHPH2)2
+ (b P
1 + Kl(b P
+ bpPHC [ ( b P
)3’2
H2
H2
)1’2
+
K1K2])
H2
T h i s e q u a t i o n i s s i m i l a r t o t h e o n e you h a v e also c o n s i d e r e d i n y o u r
most r e c e n t p a p e r
(3)
:
KCH ’CH
V K H ’CH
r = k
l+rn
K
~
~
P
’CH ~
~
+
B o i t i a u x , G. M a r t i n o a n d R . M o n t a r n a l , C . R . Acad. SC. t 280 ( 2 3 J u i n 1975). S B r i e C , p . 145. (2) J . P . B o i t i a u x , T h e s e P a r i s 1976. ( 3 ) Z. P a a l , K. Mausek a n d P . T e t e n y i , A c t a . Chim. Acad. S c i . Hung. Tomus 94(2), p - 119 (1977).
(1) J . P .
Z. PAAL
:
Under more s e v e r e c o n d i t i o n s a s e v e r e c o k i n g o c c u r s l e a d i n g
t o “bulk” coke formation.
I n t h i s r e s p e c t , n-hexane
and n-hexene
-
553
which may form coke via polycondensation of their all-trans polyene dehydrogenation products (1) or via unsaturated C5-cyclic polymers (21,
cause more severe deactivation than any C6-cyclic species
(cyclohexane, cyclohexene or benzene). of the structure of the reactant.
This points to the importance
A systematic study in this respect
might reveal the pathways of coking. (1) Paal Z., Fyer J.R., Thomson S.J., Mechanisms of Hydrocarbon Reactions. A symposium Akad. Kiado. Budapest, 1975, p. 137. ( 2 ) Myers C.G., Lang W.H., Wein P.B., Ind. Eng. Chem. 53, 299 (1961). R. MONTARNAL : In the case o f the hydrogenofysis o f n-hexane, we have
observed a rather different type of result.
The deactivation caused
by the presence o f small quantities of benzene, cyclohexene and cyclohexane was strong and of the same order of magnitude.
But it
was for the hydrogenolysis of n-hexane on Ir instead of on Pt, and for quite different operation conditions (1) bar and P
= 0.05 bar.
c6 (1) Boitiaux J.P., Thesis, Paris 1 9 7 6 .
:
T = 180°C,
= 0.95
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665
PREPARATION AND PROPERTIES OF MONODISPERSED COLLOIDAL METAL HYDROUS OXIDES::
MATIJEVICI
EGON Institute of Colloid and Surface Science and Department of Chemistry, Clarkson College of Technology, Potsdam, New York 13676 (U.S.A.)
ABSTRACT The methods for preparation of colloidal dispersions consisting of exceedingly uniform particles of hydrous oxides of a number of metals are described. Some of the sols contain spherical amorphous particles, while others are well crystallized submicronic or micronic sol ids of different morphologies. These novel systems have been used to establish the chemical mechanisms of metal (hydrous) oxides formation and growth. Various properties of such uniform particles (surface charge, magnetism, color, etc.) are illustrated, and their use in the studies of interactions with different solutes (adsorption), with other particulate matter (heterocoagulation), and with surfaces of different materials (adhesion) are discussed. INTRODUCTION Few families of inorganic compounds are as significant as metal (hydrous) oxides". These materials appear in the nature as different minerals and ores and they are produced by hydrolytic corrosion of metals. In addition, metal hydrous oxides in various forms find numerous uses, such as pigments, catalysts, catalyst carriers, fillers, coatings, etc. Thus, it comes as no surprise that much work has been done on these compounds, both in terins of their preparation and characterization; nevertheless, relatively little is still known about the mechanism of formation of any of the metal oxides or hydroxides. This lack of knowledge is easily understood if one recognizes the complexity of the processes involved in the precipitation of such solids. For example, a minor change in pH can result in the formation of entirely different species in terms of chemical composition as well as of morphology. Equally important are the roles of other parameters, primarily of the temperature and o f the nature of the anions in the solution in which the precipitation takes place. The latter can exercise a pro*Metal (hydrous) oxide designation is taken here in a rather general way; it includes oxides, hydroxides, hydrated oxides, oxyhydroxides, etc. ~
~~
:: P u b l i s h e d i n P u r e A p p l .
Chem.,
50, 9-10 (19781 , p .
1193.
556
found e f f e c t on t h e s o l i d phase formed, even though t h e respective anions may n o t appear as c o n s t i t u e n t species o f t h e p r e c i p i t a t e ,
A l l t h e f a c t o r s mentioned above
(pH, temperature, anions) a f f e c t t h e complexation o f t h e solutes which a c t as precursors t o t h e nucleation o f t h e metal hydrous oxides and which l a t e r determine t h e i r growth. The s e n s i t i v i t y o f t h e described p r e c i p i t a t i o n processes explains t h e poor r e p r o d u c i b i l i t y u s u a l l y encountered i n t h e studies o f metal hydrous oxide formation.
It a l s o accounts f o r t h e f i n d i n g t h a t t h e r e s u l t i n g s o l i d s are mostly ill
defined i n shape and polydisperse. Over t h e past few years we have succeeded i n t h e preparation o f c o l l o i d a l d i s persions o f a number o f metal hydrous oxides c o n s i s t i n g o f p a r t i c l e s exceedingly uniform i n s i z e and shape (1).
Such sols can be repeatedly obtained by r e l a t i v e l y
simple procedures and, as one would expect, t h e morphology, chemical composition, and o t h e r c h a r a c t e r i s t i c s o f t h e p r e c i p i t a t e s depend s t r o n g l y on t h e experimental conditions.
The r e p r o d u c i b i l i t y i n t h e generation o f w e l l defined suspensions
makes t h e e l u c i d a t i o n o f t h e chemical formation mechanisms possible.
I n order t o
accomplish t h e l a t t e r aim, t h e knowledge o f t h e composition o f a l l species i n solution, i n which t h e s o l i d phase i s formed, and p a r t i c u l a r l y o f t h e complexes containing t h e metal i o n which i s t h e major c o n s t i t u e n t o f t h e p r e c i p i t a t e , i s implied. The task i n o b t a i n i n g t h i s i n f o r m a t i o n i s by no means easy. The monodispersed s o l s can a l s o be employed i n studies o f various phenomena, such as i n adsorption, adhesion, heterocoagulation, c o l o r determination, t o mention a few. I n t h i s presentation an overview o f t h e a u t h o r ' s program i n metal hydrous oxides w i l l be o f f e r e d w i t h examples i n t h e f o l l o w i n g areas o f research: a. Preparation o f monodispersed sols. b. Mechanism f o r formation. c. Characterization o f t h e p a r t i c u l a t e matter. d. I n t e r a c t i o n s w i t h solutes. e. I n t e r a c t i o n s w i t h o t h e r p a r t i c l e s . f. Adhesion. PREPARATION OF MONODISPERSED SOLS OF METAL HYDROUS OXIDES If uniform p a r t i c l e s a r e t o be obtained from a s o l u t i o n i n which t h e p r e c i p i t a t i n g components a r e continuously generated, secondary n u c l e a t i o n must be avoided. This c o n d i t i o n implies t h a t , upon reaching t h e c r i t i c a l supersaturation l e a d i n g t o t h e b u r s t o f nuclei, t h e r a t e o f c r y s t a l growth must exceed t h e r a t e o f n u c l e i formation. I n t h e case o f systems studied i n t h i s work, complexes o f metal ions w i t h hydroxyls, and o f t e n w i t h o t h e r anions, a r e the precursors t o t h e s o l i d phase separation. Thus, h y d r o x y l a t i o n i s t h e c o n t r o l l i n g step, which w i l l determine t h e nature o f t h e f i n a l product. The usual procedure o f adding base leads
557
t o l o c a l supersaturations and, consequently, t o p o o r l y defined stages o f nucleat i o n and p a r t i c l e growth.
As a r e s u l t t h e produced s o l i d s are i r r e g u l a r i n shape and o f broad s i z e d i s t r i b u t i o n s . H y d r o x y l a t i o n i s g r e a t l y accelerated w i t h temperature; depending on t h e metal and on t h e temperature o f aging, hydrolyzed metal i o n complexes may form i n solut i o n s o f various degrees o f a c i d i t y .
Thus, i t i s p o s s i b l e t o r e g u l a t e t h e r a t e
of complex f o r m a t i o n i n an aqueous s o l u t i o n of metal s a l t s by proper adjustment
of pH and temperature.
Once t h e s o l u t i o n becomes supersaturated i n t h e c o n s t i t -
uent species of a g i v e n metal hydrous oxide, t h e n u c l e a t i o n occurs.
Further
aging a t a convenient temperature w i l l continue t o produce t h e p r e c i p i t a t i n g complexes which, assuming a proper r a t e o f t h e i r generation, a r e consumed i n p a r t i c l e growth.
Consequently, no secondary n u c l e a t i o n w i l l t a k e place, and a
u n i f o r m uptake o f t h e c o n s t i t u e n t species by t h e e x i s t i n g primary p a r t i c l e s y i e l d s a monodispersed s o l . The chemical composition o f t h e s o l i d w i l l s t r o n g l y depend on t h e n a t u r e o f t h e complex s o l u t e s produced on aging. dominant r o l e .
I n t h i s r e s p e c t t h e anions p l a y a
I n some cases w e l l d e f i n e d s t o i c h i o m e t r i c species form which
i n c l u d e one o r more hydroxyl and n e g a t i v e l y charged ions, whereas i n o t h e r cases t h e anions may cause condensation o f h y d r o l y s i s products i n t o polynuclear solutes o f d i f f e r e n t degrees o f complexity.
As a r e s u l t t h e p r e c i p i t a t e d p a r t i c l e s from
s o l u t i o n s o f d i f f e r e n t s a l t s o f t h e same metal i o n may have d i f f e r e n t composition and s t r u c t u r e . To i l l u s t r a t e t h e above described s i t u a t i o n s i n F i g u r e 1 a r e given e l e c t r o n micrographs o f f o u r d i f f e r e n t systems, a l l p r e c i p i t a t e d by aging a c i d i f i e d f e r r i c s a l t solutions.
The scanning e l e c t r o n micrograph l a shows s p h e r i c a l p a r t i c l e s o f
hematite (a-Fe203) obtained by aging a s o l u t i o n 0.0315 M i n FeC13 and 0.005 M i n HC1 f o r 2 weeks a t 100°C (2), whereas l b i s a transmission e l e c t r o n micrograph o f 6-FeOOH p a r t i c l e s generated i n s o l u t i o n 0.27 M i n FeC13 and 0.01 M i n HC1 heated a t 100°C f o r 24 hr.
F i g u r e l c represents submicron a l u n i t e t y p e c r y s t a l s o f t h e
composition Fe3(0H),(S04),-2
H20 which formed on aging a t 98OC f o r 2 h r a solu-
t i o n 0.18 M i n Fe(N03)3 and 0.32 M i n Na2S04 (3).
F i n a l l y , F i g u r e I d shows
p a r t i c l e s obtained on h e a t i n g f o r 20 min a t 100°C a s o l u t i o n which was 0.0038 M i n FeC13, 0.24 M i n
H3PO4
w i t h NaOH added t o a d j u s t t h e pH t o 1.86.
The chemical
a n a l y s i s o f these p a r t i c l e s gave a composition c o n s i s t e n t w i t h FeP04. The f o u r examples show t h a t i n t h e presence o f c h l o r i d e ions, under r a t h e r s i m i l a r c o n d i t i o n s , two e n t i r e l y d i f f e r e n t s o l s a r e generated i n terms of chemical composition and p a r t i c l e morphology (Figures l a and b).
The s o l i d B-FeOOH con-
t a i n e d considerable amounts o f c h l o r i d e ions when f r e s h l y prepared, b u t these anions c o u l d be removed by repeated washing w i t h water w i t h o u t any apparent change i n p a r t i c l e s i z e o r shape.
On t h e o t h e r hand, s o l i d s formed on h e a t i n g
558
C
d
Fig. 1. Scanning and transmission e l e c t r o n micrographs o f p a r t i c l e s p r e c i p i t a t e d from various f e r r i c s a l t solutions. (a) Hematite, a-Fe203 ( s o l u t i o n 0.0315 M i n FeC13 and 0.005 M i n HC1 aged f o r 2 weeks a t 100°C). (b) B-FeOOH ( s o l u t i o n 0.27 M i n FeC13 and 0.01 M i n HC1 aged f o r 24 h r a t 100°C). ( c ) F e r r i c basic s u l f a t e Fe3(OH)5(S!++)2-2H20( s o l u t i o n 0.18 M i n Fe(N03)3 and 0.32 M i n Na2S04 aged f o r 2 h r a t 98 C). (d) F e r r i c phosphate, FePOtt ( s o l u t i o n 0.0038 M i n FeC13 and 0.24 M i n H ~ P O L , a t pH 1.86 aged f o r 20 min a t 100 C).
559 f e r r i c s a l t s o l u t i o n s i n t h e presence of s u l f a t e i o n s c o n s i s t of s t o i c h i o m e t r i c a l l y s t a b l e and s t r u c t u r a l l y w e l l d e f i n e d b a s i c f e r r i c s u l f a t e s .
F i n a l l y , no d e t e c t a b l e
amounts o f h y d r o x y l l i g a n d s a r e found i n t h e systems p r e c i p i t a t e d as d e s c r i b e d above f r o m aged s o l u t i o n s i n t h e presence of phosphate i o n s .
The r e s u l t i n g f e r r i c
phosphate r e d i s o l v e s on c o o l i n g . The f o u r d i f f e r e n t d i s p e r s i o n s shown i n F i g u r e 2 were o b t a i n e d by a g i n g a c i d i f i e d aluminum s a l t s o l u t i o n s .
Again t h e a n i o n s p l a y an e s s e n t i a l r o l e .
The per-
f e c t spheres ( F i g u r e 2a) were generated on h e a t i n g aluminum s u l f a t e s o l u t i o n s
M) f o r 48 h r a t 97OC ( i n i t i a l pH 4.0). The p r e c i p i t a t e c o n t a i n e d a con(2 x s i d e r a b l e amount o f s u l f a t e i o n s , b u t t h e s e anions were r e a d i l y l e a c h e d o u t by r i n s i n g t h e s o l i d s w i t h w a t e r y i e l d i n g p u r e amorphous aluminum h y d r o x i d e p a r t i c l e s w i t h o u t change i n shape (4).
H e a t i n g a t t h e same t e m p e r a t u r e f o r 20 h r a s o l u t i o n
which was 0.050 M each i n Al(NO,),
and Na,HPO,
and 0.035 M i n HN03 ( i n i t i a l pH
o f t h e m i x t u r e b e i n g 2 . 0 ) gave a l s o s p h e r i c a l p a r t i c l e s ( F i g u r e 2b) b u t t h e X-ray and chemical a n a l y s i s i d e n t i f i e d t h e s o l i d s t o be c o n s i s t e n t w i t h t h e c o m p o s i t i o n o f t h e m i n e r a l v a r i s c i t e , AlP0,.
Scanning e l e c t r o n micrographs (2c and d ) show
two unusual morphologies o f boehmite, o b t a i n e d by a g i n g o f aluminum c h l o r i d e (0.0050 M A l C l 3 ) and aluminum p e r c h l o r a t e (0.0030 M A1(C10,+)3) s o l u t i o n s , r e s p e c t i v e l y , a t 125OC f o r 12 hr ( 5 ) . Again, i t i s c l e a r l y demonstrated t h a t r a t h e r d i f f e r e n t , y e t q u i t e u n i f o r m c o l l o i d a l metal o x i d e d i s p e r s i o n s , can be prepared by homogeneous p r e c i p i t a t i o n o f d i f f e r e n t s a l t s o l u t i o n s i n which d i f f e r e n t a n i o n s have a p r o f o u n d e f f e c t on t h e p r o p e r t i e s o f t h e s o l i d formed. As a l a s t example, F i g u r e 3 g i v e s a t r a n s m i s s i o n e l e c t r o n m i c r o g r a p h and a r e p l i c a o f Co304 p a r t i c l e s w h i c h have a s p i n e l s t r u c t u r e .
The s i g n i f i c a n t f i n d -
i n g i s t h a t such systems a r e formed on h e a t i n g a t 100°C o f c o b a l t ( I 1 ) s a l t s o l u t i o n s i n oxygen o r a i r onZy i n t h e presence o f a c e t a t e i o n s .
No p r e c i p i t a t e was
o b t a i n e d when, under o t h e r w i s e i d e n t i c a l c o n d i t i o n s , t h e s u l f a t e , c h l o r i d e , p e r c h l o r a t e , o r n i t r a t e s a l t s o f c o b a l t ( I 1 ) were aged. Many more monodispersed s o l s o f d i f f e r e n t hydrous metal oxides, i n v o l v i n g t h o s e of chromium (6,7) copper ( 8 ) , t i t a n i u m ( 9 ) , and o t h e r m e t a l s , have now been prepared f o l l o w i n g s i m i l a r procedures. MECHANISM OF METAL HYDROUS O X I D E FORMATION
The examples i n t h e p r e c e d i n g s e c t i o n c l e a r l y show t h a t i t i s necessary t o know t h e c o m p o s i t i o n o f t h e s o l u t i o n i n terms o f t h e n a t u r e and o f t h e concentrat i o n o f a l l complexes i f one i s t o e x p l a i n t h e chemical processes i n homogeneous p r e c i p i t a t i o n o f m e t a l hydrous oxides.
U n f o r t u n a t e l y such i n f o r m a t i o n i s n o t
always a v a i l a b l e , p a r t i c u l a r l y f o r s o l u t i o n s a t h i g h e r temperatures.
Thus, one
has no c h o i c e b u t t o d e t e r m i n e t h e c o m p o s i t i o n o f t h e d i f f e r e n t s o l u t e s under t h e a c t u a l c o n d i t i o n s o f t h e s o l i d phase f o r m a t i o n .
660
C
d
Fig. 2. Transmission and scanning e l e c t r o n micrographs o f p a r t i c l e s p r e c i p i t a t e d from various aluminum s a l t solutions. (a) Amorphous aluminum hydrous oxide (solut i o n 0.0020 M i n A12(S0 ) 3 , pH 4.0 aged f o r 48 h r a t 97OC). ( b ) V a r i s c i t e , A1P04 ( s o l u t i o n 0.050 M i n AliN03)3 and Na2HP04 and 0.035 M i n HN03 aged f o r 20 h r a t 97OC). ( c ) and (d) Boehmite, a-Al00H ( s o l u t i o n s o f 0.0050 M i n AlC13 and 0.0030 M i n A1(C104)3, respectively, aged f o r 12 h r a t 125OC).
561
a
b
Fig. 3. Electron micrograph and a replica of Co301, spinel particles obtained by heating a 0.010 M Co(I1)-acetate solution for 4 hr at 100 C. Until recently no substantiated mechanisms on metal hydrous oxide formation have been available (10). Using the monodispersed sols it was possible to develop a better understanding of the essential processes in,the generation of chromium hydroxide (11 ,lZ), titanium dioxide (9), and ferric basic sulfates (13). In all these cases the particles formed in the presence of sulfate ions, but the role of these anions was quite different in each system. Thus, in solutions containing chromium and sulfate ions solid chromium basic sulfate precursor precipitates first, which acts as a heterogeneous nucleating material for the amorphous chromium hydroxide (11). An application of the Nielsen's chronomal analysis showed that the particles grow via surface reaction which involves polynuclear layer growth (12). In the highly acidic solutions of titanium salts, sulfate ions bind the titanium(1V) ions into solute complexes, which on prolonged aging at elevated temperatures slowly decompose. These species act as a reservoir for the titanium ions; on decomplexation they hydrolyze and precipitate. Finally, in solutions containing ferric and sulfate ions at low pH, well defined monomeric and dimeric solute hydroxy and sulfato complexes of iron(II1) form, giving crystals of fixed stoichiometric composition (13). The role of chloride ions in the precipitation of different ferric hydrous
662 oxides has n o t been explained y e t , b u t t h e work on t h e c h l o r o - f e r r i c complexes i n a c i d i c solutions, which i s p r e s e n t l y c a r r i e d o u t i n our laboratory, may shed l i g h t on t h e f i n d i n g s t h a t a r e l a t i v e l y small v a r i a t i o n i n conditions n o t o n l y y i e l d p a r t i c l e s o f d i f f e r e n t morphologies, b u t a l s o o f e n t i r e l y d i f f e r e n t chemic a l compositions. CHARACTERIZATION OF MONODISPERSED METAL HYDROUS OXIDE SOLS Depending on a p p l i c a t i o n , t h e c h a r a c t e r i z a t i o n o f c o l l o i d a l p a r t i c l e s may take I n t h i s review o n l y t w o p r o p e r t i e s w i l l be discussed:
many forms.
surface charge
and magnetism. Surface Charqe Owing t o t h e nature o f t h e s t a b i l zing species (OH-,H+) any metal hydrous oxide i s s t r o n g l y pH dependent. p o i n t o f zero charge ( i s o e l e c t r i c point, i.e.p.)
t h e surface charge o f
F u r t h e m r e , the electrokinetic v a r i e s not o n l y w i t h the consti-
t u e n t metal, b u t a l s o w i t h t h e composition o f i t s hydrous oxides. f a c t t h e reported values f o r t h e i.e.p. by several u n i t s i n pH (14).
As a matter o f
f o r apparently the same m a t e r i a l s d i f f e r e d
For example, the l i t e r a t u r e data f o r the i.e.p.
of
s y n t h e t i c t i t a n i u m d i o x i d e range from pH 2.6 t o 7.3, whereas f o r a very pure sample o f TiO,
i t was found t o be between 4.5 and 5.2 (15).
I n another example,
the cubic Co304 p a r t i c l e s i l l u s t r a t e d i n Figure 3 show an i.e.p. a considerably higher value o f 11.4 was reported f o r a Co(I1,III)
a t pH 5.4 (16); oxide by Tewari
and Campbell (17). Since t h e i s o e l e c t r i c p o i n t defines t h e pH below which t h e p a r t i c l e s a r e p o s i t i v e l y charged and above which they a r e n e g a t i v e l y charged, t h i s property of a metal hydrous oxide i s e s s e n t i a l i n considering t h e sol s t a b i l i t y , p a r t i c l e adhesion, heterocoagulation with o t h e r p a r t i c l e s , and i n t e r a c t i o n s with d i f f e r e n t lsolutes. F i g u r e 4 gives t h e e l e c t r o k i n e t i c m o b i l i t y data o f f o u r aluminum hydroxide s o l s as a f u n c t i o n o f pH. Three o f these curves r e f e r t o systems i l l u s t r a t e d i n Figure 2. The i n t e r e s t i n g f i n d i n g i s t h a t t h e l a r g e s t d i f f e r e n c e i n the m o b i l i t y curves i s f o r two amorphous spherical p a r t i c l e s prepared by aging aluminum s u l f a t e solutions.
The lower values o f t h e i.e.p.
i s f o r t h e sol, as d i r e c t l y generated
(diamonds), whereas t h e higher value i s f o r t h e same s o l from which t h e s u l f a t e This example shows
ions were removed by repeated r i n s i n g w i t h water ( t r i a n g l e s ) .
t h a t anionic " i m p u r i t i e s " have a pronounced e f f e c t on the surface charge characteri s t i c s o f metal hydrous oxides. Obviously, the discrepancies i n many reported data may be due t o s i m i l a r causes.
563
0 FROM AI(ClO&
PLUS SULFATE
2
4
6
a PH
10
12
Fig. 4. 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 f u n c t i o n o f pH o f f o u r d i f f e r e n t c o l l o i d a l aluminum hydrous oxides as i l l u s t r a t e d i n Figure l c (O), I d (0,l a (O), and t h e same p a r t i c l e s as shown i n l a from which s u l f a t e ions were removed by leaching (A). Magnetism It i s w e l l known t h a t magnetic p r o p e r t i e s o f d i f f e r e n t (hydrous) oxides o f a
given metal s t r o n g l y depend on t h e i r composition.
Much l e s s well understood i s t h e r e l a t i o n s h i p between various types o f magnetism and the shape o f the p a r t i c l e s o f t h e same chemical composition o r o f t h e m a t e r i a l s having t h e same chemical compos i t i o n and shape b u t d i f f e r e n t p a r t i c l e size. The monodispersed metal hydrous oxides lend themselves exceedingly w e l l f o r t h e i n v e s t i g a t i o n o f magnetic p r o p e r t i e s as a f u n c t i o n o f various parameters. Figure 5 shows t h e magnetization as a f u n c t i o n o f applied magnetic f i e l d f o r n e a r l y spherical a-Fe203 p a r t i c l e s (30-40 nm modal diameter a t 298'K).
I t was
suggested t h a t hematite p a r t i c l e s o f t h i s s i z e should be i n the antiferromagnetic s t a t e (18).
Thus, t h e u vs H p l o t should e x h i b i t some hysteresis.
Within t h e
experimental e r r o r no hysteresis i s seen and t h e curve i s t y p i c a l l y S-shaped, c h a r a c t e r i s t i c o f pure superparamagnetism ( p o s s i b l y i n coexistence w i t h weak ferromagnetism).
This observation would suggest t h a t the proposed r e l a t i o n s h i p
o f the magnetic s t a t e s t o p a r t i c l e s i z e (18) f o r Fe203 systems may n o t be correct. Heating the same hematite p a r t i c l e s a t high temperatures (960'K)
i n the helium
atmosphere o f 500 t o r r d r a s t i c a l l y a l t e r s t h e magnetic properties o f a-Fe203
564
Fig. 5. Magnetization at 298'K as a function of applied field of spherical a-Fe20s particles having modal diameters o f 30-40 nm.
Fig. 6. Magnetization at 298'K as a function of applied magnetic field of spherigal a-FenO3 particles having modal diameters of 30-40 nm after heat treatment at 960 K in a He atmosphere of 500 torr. (Figure 6), although little change could be observed with respect to particle size and shape. The high u values indicate the presence o f ferromagnetic components, presumably Fe304 or y-Fe203, in these particles. Obviously, the crystallinity of the solids must have been affected by the heat treatment.
565
INTERACTIONS WITH SOLUTES I t is well known t h a t the surfaces of hydrous metal oxides are rather reactive, i.e. various molecular or ionic species adsorb on such solids from aqueous solutions. The interfacial processes depend on the pH, since the l a t t e r i s the factor c o n t r o l l i n g the surface charge. I t i s important t o note t h a t the opposite charge between the adsorbent and the adsorbate i s not a sufficient condition f o r adsorption. The e l e c t r o s t a t i c attraction will only f a c i l i t a t e the approach of the solute species to the p a r t i c l e surface; other forces are needed t o keep them a t t h e interface. These forces may be hydrogen bond or other types of bindings (such as chelation), surface precipitation etc. I t i s the a b i l i t y of the constituent metal ions and of the potential determining species t o interact with a variety of solutes t h a t make metal hydrous oxide surfaces so reactive. Two examples t o be given here deal w i t h the interaction o f monodispersed,
Fig. 7. The total amount of aspartic acid sorbed on spherical chromium hydroxide particles (modal diameter 350 nm) as a function of pH using different i n i t i a l concentrations of aqueous solutions of the amino acid.
566 s p h e r i c a l , amorphous chromium h y d r o x i d e p a r t i c l e s w i t h a s p a r t i c a c i d and o f c r y s t a l l i n e r o d - l i k e 6-FeOOH p a r t i c l e s w i t h e t h y l e n e d i a m i n e t e t r a a c e t i c a c i d (EDTA). F i g u r e 7 g i v e s t h e t o t a l amount o f a s p a r t i c a c i d sorbed on a chromium h y d r o x i d e s o l h a v i n g p a r t i c l e s o f a modal d i a m e t e r o f 350 nm as a f u n c t i o n o f pH u s i n g d i f f e r e n t i n i t i a l concentrations o f t h e solute.
Analogous measurements showed
t h a t a t 90°C t h e adsorbed amounts a r e c o n s i d e r a b l y h i g h e r (19). o b v i o u s l y due t o t h e charge on t h e s o l i d .
The i.e.p.
The pH e f f e c t i s
o f t h e used s o l i s a t pH
8.5 and l i t t l e u p t a k e o f a s p a r t i c a c i d was observed above t h i s pH value.
'L
It i s
e v i d e n t t h a t o p p o s i t e charges o f t h e adsorbent and adsorbate a r e needed t o b r i n g about t h e i n t e r a c t i o n . The s i g n i f i c a n t f i n d i n g i s t h a t t h e amounts o f t h e amino a c i d t a k e n up by t h e m e t a l h y d r o x i d e a r e a p p r o x i m a t e l y f o u r o r d e r s o f magnitude h i g h e r t h a n t h e q u a n t i t i e s c a l c u l a t e d on t h e b a s i s o f monolayer a d s o r p t i o n assuming g e o m e t r i c s u r f a c e areas o f t h e s p h e r i c a l adsorbent p a r t i c l e s .
The obvious c o n c l u s i o n i s t h a t
t h e m a j o r f r a c t i o n o f t h e a s p a r t i c a c i d molecules i s absorbed i n t h e i n t e r i o r o f
(uf
1 1 I
E
p-
Q,
5ooc
-
I
I
FeOOH-EDTA
0
E 1 3
Y
n W m
a 0 cn
2
n
a a k
0 1 W
cj
z
0 0
C
i
CONC. EDTA IN SOLN.,mM Fig. 8. A d s o r p t i o n i s o t h e r m s a t 5OoC f o r EOTA on 6-FeOOH c o n s i s t i n g o f r o d - l i k e p a r t i c l e s w i t h an average l e n g t h o f 1 um and a w i d t h o f 0.3 um. Each c u r v e i s f o r a d i f f e r e n t pH value.
567
2
4
6
8
12
10
PH
Fig. 9. Solubility o f 8-FeOOH (expressed as released i r o n ) i n t h e absence and i n the presence o f EDTA i n d i f f e r e n t concentrations as a f u n c t i o n o f pH a t 5OoC. the adsorbent, which presumes t h a t the amorphous chromium hydroxide i s permeable t o t h i s amino acid.
The absorption seems t o be due e i t h e r t o c h e l a t i o n o r t o
c o o r d i n a t i o n o f chromium ions w i t h the a s p a r t i c a c i d molecules.
The much stronger
uptake o f these s o l u t e species a t higher temperature supports t h e suggested chemis o r p t i o n mechanism. Contrary t o t h e f i n d i n g s w i t h a s p a r t i c acid, the same chromium hydroxide sol The l a t t e r i s a l a r g e r
did not adsorb any measurable q u a n t i t i e s of tryptophan.
molecule w i t h a stronger aromatic (hydrophobic) character; i t s monocarboxylate n a t u r e (as d i s t i n g u i s h e d from the dicarboxyl i c a s p a r t i c a c i d ) precludes chelate formation by the carboxyl groups only. The amount o f EDTA adsorbed on 6-FeOOH i s a l s o pH dependent and the uptake decreases s t r o n g l y w i t h increasing pH (Figure 8).
The area per Fe atom i n 8-FeOOH,
c a l c u l a t e d from the c r y s t a l s t r u c t u r e o f t h i s s o l i d , i s 27
i2, and t h e area per
l i g a n d molecule a t t h e c o n d i t i o n o f maximum adsorption a t pH 2.8 ( a t 25'C),
as
c a l c u l a t e d from t h e corresponding adsorption isotherm and t h e BET s p e c i f i c sur-
face area o f t h e ferricoxyhydroxide, i s 33 O A2
.
The l a t t e r value i s i n reasonable
agreement w i t h t h e estimated cross-sectional area o f t h e EDTA molecule, which
568
i n d i c a t e s t h a t a t s a t u r a t i o n a monomolecular l a y e r i s formed based on a Fe:EDTA i n t e r f a c i a l complex o f 1:1.
The surface complexation i s f u r t h e r supported by t h e
value of t h e free energy of adsorption, which was found t o be A G ' ? ~ ~= -9 kcal/mole
-
(20) I t was o f special i n t e r e s t t o i n v e s t i g a t e i f t h e i n t e r a c t i o n o f EDTA w i t h t h e same B-FeOOH was accompanied by d i s s o l u t i o n o f some o f t h e oxyhydroxide.
Figure 9
i s a p l o t o f t h e amount o f f e r r i c ions released as a f u n c t i o n o f pH i n t h e absence
o f EDTA and i n t h e presence o f t h r e e d i f f e r e n t concentrations o f t h i s c h e l a t i n g agent.
A very pronounced maximum i s observed.
A t lower pH values, a t which con-
s i d e r a b l e adsorption takes place, t h e r e l e a s e o f f e r r i c ions i s r a t h e r small; t h e d i s s o l u t i o n i s i n h i b i t e d by t h e surface complexation o f l a t t i c e f e r r i c i o n s w i t h t h e o r g a n i c ligand. The maximum d i s s o l u t i o n o f f e r r i c ions i s over t h e pH range 8 these c o n d i t i o n s no adsorption takes place.
-
10.
Under
Partial dissolution o f particles
y i e l d s p o s i t i v e l y charged f e r r i c h y d r o l y s i s products (Fe(OH),+,
FeOH",
Fe2(0H)24+
which r e a c t w i t h EDTA anions and, thus, enhance f u r t h e r r e l e a s e o f f e r r i c i o n s from t h e s o l i d B-FeOOH. F i n a l l y , a t t h e h i g h e s t pH values ( > 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
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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
Generally catalysts obtained from alumina pretreated at 75OOC
have a smaller concentration of basic centres than those with at 550'C.
precalcined 2 3 The highest concentration of basic sites is achieved on the alumina A1 0
pretreated at 550OC and then doped with potassium ethoxide. On the surface of this catalyst two types of basic centres (at strengths of 26.5
TABLE 2
Basic and aoidic sites concentrations on investigated oatalysts surfaces (calcination after
i:
Basic centres concentration at various (=ol/g) strength ( H - ) 12.2 15.0 17.2 18.4 22.3 24.6 26.5
atalyst
1.17
0.99
0.99
0.99
0.91
1.02
1.02
1.02
1.02
1.02
0.91 0.81
Li Li Na Na
1.60 0.93 1.82 1.42
1.60 0.93 1.82 1.42
0.91 0.93
0.
1.60 93 1.82
1.31 0.93 1.19
0.93 1.19
0.93 1.19
2.22
2.03
1A 2 2.03
1.42
K IZ
1.42 2-03
1.69 2.42 1.86
1.69
1.69
1.31 0.93 1.82 1.42 2.03 1.69
2.22
2.22
2.02
1.86
1.86
1.86
cs CS
~
aAl
0
2 3
~
~
~
~
1.42 2.03
1.69 1.81 1.86
1 .oo
2.03 1.69 1.81 1.86
33.0
1.69
0 0 0 0 0 0 0 0
1.81
0
1.86
0
1 .oo
~
(550) = pretreatment temperature of alumina 55OoC
1 Aoidio centres conoentration at various strength (Ho)(mmol/g) +4.8 +1.5 -3.0 -5.6 -8.2
2.73 2.79 2.77 0.59 7.64 2.89
1.92 2.79
0.96 0.81
2.11 0.42
2.11 0.21
7.64 2.89
7.64 2.89
7.00
7.00
7.00
3.k9 ' 4.50 13.97
3.49 4.50 3.97
0.63 0.40 0.23 0.09 6.47 2.02
6065 2.99 2.52 4.19 3.27 3-09 3.09
0.56 0.40 0 0 0 0 0 0
0 0
centres were observed for systems obtained from alumina calcined at lower temperatures, i.e. at 550°C. Specific surface area Specific surface areas of alumina doped with lithium, sodium, potassium and caesium ethoxides are presented in Table 3.
Introduction of alkali alkoxi
des on A1 0 surface resulted in an increase of specific surface area (about 2 3 lo%), the differences between preparations doped with various ethoxides are insignificant. Hydroxyl group concentration Surface hydroxyl group concentrations found for the investigated catalysts are given in Table 3.
Introduction of alkali ethoxides on alumina surface cau-
ses a decrease of the amount of OH groups.
Only alumina doped with caesium
alkoxide has a concentration of surface hydroxyl groups similar to that of the pure alumina. Alkali ion content The quantity of alkali cations deposited on alumina surface was measured by titration with HC1 (Table 3). In the series of catalysts investigated the number of lithium ions deposited during the reaction of the alkoxide with A1 0 2 3 was the greatest among alkali cations. The decrease of hydroxyl group concentration, as compared with pure alumina, was the greatest in the case of the lithium ions doped alumina.
The number of Cs'
ions deposited on alumina surface
was the smallest in the series and simultaneously after reaction of caesium alkoxide the minimal change in OH groups concentration was observed. All alumina preparations have a y-Al2O3 structure, which has been checked Assuming that 100 faces dominate on the surface,the ratio 02 of the number of alkali cations per 100 A of catalyst to the number of surface by X-ray analysis.
oxygen anions on the same area of surface was calculated. One-electron donor and one-electron acceptor properties The concentrations of one-electron donor centres on the surfaces increase with an increase of the calcination temperature after impregnation, reaching a maximum in the case of preparations calcined at 250°C.
Calcination at higher
temperatures causes a rapid fall of the one-electron donor centre concentration. The greatest number of centres of this type occurs on the surface of A1 0 2 3 doped with caesium alkoxide (Fig. 1). Catalysts which have not been calcined after impregnation do not exhibit any one-electron acceptor properties at all. Among the preparations calcined at 550°C only the catalyst
doped with lithium ions had one-electron acceptor pro-
perties comparable to pure alumina. Minute concentrations of one-electron
600
TABLE 3
Specific surface area, alkali ion content and hpdroxyl group concentration on investigated systems surfaces (oaloination after impregnation at 55OoC ) Specific OH group, Aur. ion surface ooncentration content
Catalyst
(=ol/g
area (m2//g )
a
1
(=ol/g)
b
d
0
a = alkali naphthenide titration b Ichromatographic method c = nlknli ion number per 100H2 of catalyst surface '2 d = oxygen ion6 number to W i ions number ratio en 1OOA of catalyret surface
& l
9
0 20
400
450
100
2%
359
5%
oc
Fig. 1. One-electron donor aites ooncentration as a funotion of the oaloination temperature after alumina impregnation
601 acceptor centres occurred on the surfaces of all the other catalysts (Table 4).
TABLE 4 One-electron donor and one-eleotron aooeptor oentree ooncentration on investigated oatalysts surfaces (catcfnatfon after impregnation at 55OoC ) he-eleotron aooeptor One-eleotron donor oentrea ooncentration mntres oonoentration atalyet
-
(S P W g 1
(a p w g ) 2.1 x 4.4 x 1.1 x 2.2 I 1.2 x
550 750 550 750 550
10;: 10,-
1o;i 10 15 lol5
750
4.5 x lol4
550 750 550
5.2 X 1015 2.7 x 1014
<5 x 1.7 x 10
750 I
DISCUSSION
The addition of alkali metal ethoxides to alumina causes considerable changes in the physicochemical properties of the oxide. The type of apparent change depends on the pretreatment temperature of alumina and on the calcination temperature of the catalyst after impregnation. It is quite clear that the properties of the catalytic systems obtained depend mainly on the pathway of the reaction between alkoxide and surface active sites on A1203. At room temperature it is possible that alkali metal ethoxides undergo reactions with Surface hydroxyl groups and with Lewis type acidic centres, presumably with the naked surface aluminum cations. A s a result of the reaction with hydroxyl groups the new bond between alkali metal and surface oxygen would be formed z ~ i -+ 0RO-M+ ~
-+
-AL-O-M+
:
+ ROH
In the case of reaction with Lewis sites the equation may be envisaged as follows
:
Reaction (1) causes a decrease of the surface OH group concentration. As a result of reaction (2) the blocking of most acidic surface centres (Lewis type centres with strength Ho < -8.2)
occurs.
The basic strength of catalysts not calcined after the introduction of
alkoxide is much smaller than for pure alumina. This fact seems to be a result 2- . of suppressing the surface oxygen anions 0 with alkali cations (Eq. 2). The surface alkoxide anions may act as electron pair donors and are probably responsible for the basicity in the range of H- = 17.2 - 18.2.
According to
McEwen (ref. 20) the ethoxide anions in solution have the same basic strength. The relatively low concentration of one-electron donor sites on the surface of catalytic systems not calcined after impregnation may be a result of the destruction of part of the surface OH groups and the suppresion of surface oxygen anions.
It is well known that both types of active sites can act as
one-electron donors.
In this same reaction pathway the one-electron acceptor
sites (aluminum cations) (ref. 21) are blocked by alkoxide anions. The calcination of the impregnated catalysts causes considerable changes in the properties and structure of the surfaces.
As
a result of reaction (3) the formation of new hydroxyl groups and new
naked oxygen anions may take place.
The two surface sites formed are respon-
sible for the increase of the concentration and strength of the basic sites. An increase in the concentration of basic and one-electron donor centres is probably due to the formation of new naked oxygen anions after the evolution of ethylene.
Secondary hydroxyl groups formed in reaction ( 3 ) may also act as
new one-electron donor centres. Differences in the properties of alumina doped with alkoxides of different alkali metals point to the role played by the properties of the cation introduced (electron affinity and ionic radius).
The electron affinity and ionic
radius of the cation in the respective alkali metal alkoxide determine the basicity and the reactivity of the anion.
It seems likely that the reaction
of the alkoxide molecule with the surface hydroxyl group of alumina begins with the attack of the anion at the acidic proton of the hydroxyl group as in analogous homophase reactions.
The second step is the addition of the cation to
2the oxygen anion 0 released. Such a mechanism is supported by the observed
differences in the reaction of alkoxides and sodium hydroxide with the A 1 0 2 3 surface. Obviously, the structure of the anion is important so far as it determines the type of product formed during the calcination of the impregnated material. Larger cations such as potassium and caesium possess greater or comparable radii in relation to oxygen anions. It cannot be ruled out that their coordination number is higher than that of Li+ or N a ' .
The presumption is supported 02 by the ratio of the alkali metal ions to the oxygen anions per 100 A of
603 alumina surface (Table 3 ) .
One must presume that
the ratios close to the value
of the coordination number in the sequence of the cations under discussion may
be an explanation of the fact that various quantities of different cations are retained on the same alumina surface
under unaltered conditions.
Moreover the
ionic radius determine the hardness of the cation. The small lithium cation is much harder than the other alkali metal cations. As the oxygen anion is one of the hardest anions, its affinity for a cation will grow with an increase of
cation hardness and should be highest for the lithium cation. It is interesting to note that catalysts doped with alkali metal alkoxides (calcination after impregnation at 550°C) possess higher overall concentrations
of acidic centres than the initial alumina preparations.
We think that the
alkali cations are responsible for the formation of new acidic sites on the surface.
It is possible that the titration with n-butylamine leads to side
reactions in which complexes of the amine with surface alkali cations are formed.
The number of coordinated titre molecules may be greater than one. A
similar phenomenon is observed during the formation of stable complexes of polycyclic amines with alkali metal cations in solutions (ref. 2 2 ) .
Thus,
Benesi's method would provide results allowing a qualitative comparison of acidity rather than the obtention of true, absolute values of the acid sites concentration.
REFERENCES
1. S. Malinowski, W. Grabowski,W.J. Palion and B. Zielinski, Bull. Acad. Polon. Sci., Ser. Sci. Chim., 2 1 (1973) 737. 2. M. Marczewski, S. Malinowski, Bull. Acad. Polon. Sci., Ser. Sci. Chim., 24 (1976) 1. 3 . M. Marczewski, S. Malinowski, Bull. Acad. Polon. Sci., Ser. Sci. Chim., 24 (1976) 187. 4. J. Kijenski, S. Malinowski, Bull. Acad. Polon., Sci., Ser. Sci. Chim., 25 (1977) 329. 5. J. Kijenski, S. Malinowski, J . C . S . , Faraday I, 74 (1978) 250. 6. N.S. Figoli, S.A. Hillar, M. Parera, J. Catal., 20 (1971) 230. 7. L.D. Scharme, J. Phys. Chem., 20 (1974) 2070. 8 . T.T. Chuang, I.G. Dalla Lona, J.C.S., Faraday I, 68 (1972) 777. 9 . H. Bremer, K.H. Steinberg, K.D. Wendlant, 2. Anorg. Allgem. Chem., 366 (1969) 130. 10. H. Pines, W.O. Haag, J. Am. Chem. SOC., 82 (1960) 2471. 11. K.V. Topchieva, K. Yun Pin, I.V. Smirnova, Adv. Catal., 9 ( 1 9 5 7 ) 799. 12. S. Santhangopalan, C.N. Pillai, Indian J. Chem., 11 (1973) 957. 13. M. Iato, T. Kanbayashi, N. Kobayashi, Y. Shima, J. Catal., 7 (1967) 342. 14. H.A. Benesi, J. Phys. Chem., 6 1 (1957) 970. 15. K. Tanabe, Solid Acids and Bases, Academic Press, New York, 1970. 16. J. Kijenski, R. Hombek, S . Malinowski, J. Catal., 50 (1977) 186. 17. L. Nondek, React. Kinet. Catal. Lett., 2 (1976) 283. 18. M. Che, C. Naccache, B. Imelik, Trans. Faraday SOC., 63 (19671 2254. 19. B.D. Flockhart, J.A.N. Scott, R.C. Pink, Trans. Faraday SOC., 62 ( 1 9 6 6 ) 730. 20. W.K. McEwen, J. Am.Chem.Soc., 58 (1936) 1124. 21. J.B. Peri, J. Phys. Chem., 69 (1965) 220. 22. J.J. Christensen, D.J. Eatough, R.M. Izatt, Chem.Rev. 74 ( 1 9 7 4 ) 351.
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605
MECHANISM OF FORMATION OF A CATALYTICALLY ACTIVE PHASE IN THE REACTION OF Cr02C12 WITH SILICA GEL xx
D. Mehandjie?, S. AngelovX and D. Damyanov x Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria xx High School for Chemical Technology, Bourgas, Bulgaria
ABSTRACT A new method for the preparation of supported chromium oxide catalysts for ethylene polymerization has been proposed. It is based on the reaction of chromium dioxydichloride vapour with the silanols on the silica gel surface. Hydrolysis of the surface compound obtained leads to the formation of "secondary" hydroxyl groups,which can again interact with the volatile halide. The chromium content in the surface phase and its properties were controlled by the initial concentration and distribution of silanol groups and the number of "halogenation-hydrolysis" cycles. The structure and composition of the active phase on completion of the halogenation or of the cycle "halogenation-hydrolysisx was studied. The catalytic activity of the samples with respect to the ethylene polymerization was studied in a system which contained molecularly deposited chromium oxide, diethyl aluminium chloride and diphenyl magnesium.
1. INTRODUCTION
In the industrial production of polyethylene under low and moderate pressures, Philips Petroleum & Co chromium oxide catalysts containing up to 5 wt.% Cr on various carriers are widely applied.The usual technology of preparation of such catalysts consists of three stages:l) impregnation of the carrier (mostly, Si02 or Si02/A1203) with a water solution of Cr03 or a salt of Cr(V1); 2 ) drying, and 3 ) activation under fluidized conditions in a stream of dry air at temperatures of 400' to 80OoC. The catalysts obtained are kept in an atmosphere of dry air or inert gases at room temperature (ref. 1) The active phase deposition from a water solution has a number of
606
disadvantages. E.g.,on removal of the water (drying) and heat-treatment (activation), the metal oxide particles are aggregated, which means that part of the active surface and perhaps some useful properties of the isolated metal ions are lost. For this reason, in a recent investigation attempts have been made to develop new methods for depositing the active phase, which would ensure maximum utilization of the catalytic properties of each deposited transition-metal ion, e.g. (ref. 2 ) . The purpose of the present paper was to show the possibility of preparation of catalysts for polymerization of ethylene using a new method of chromium oxide deposition on silica gel surfaces. The method is based on the so-called reactions of “molecular” deposition proposed by Kol’tsov and Aleskovskii (ref. 3) for modifying the silica gel surface. In our case, the active phase was deposited using the reaction of chromium dioxydichloride (chromyl chloride) with the silanols on the silica gel surface. * ZSi - 0 - Cr O 2 C1 + HC1 ESi - OH + Cr O 2 C12 (la) or Esi - OH ?Si - 0 .
+ Cr
\ Cr
O 2 + 2 HC1 (Ib) ESi - OH ZSi Hydrolysis of the surface coumpound obtained FSi - 0 - Cr O2 C1 + H20 * ESi - 0 - Cr O2 OH + HC1 (2) leads to the formation of “secondary” hydroxyl groups which can again interact with the volatile halide. Reactions ( I ) and ( 2 ) form a cycle of molecular deposition during which one layer of hydrated chromium oxide is formed on the surface. Bienert et al. (ref. 4 ) , Smirnov et al. (ref. 5 ) and D.Damyanov et al. (ref. 6 ) pointed out the possibility of preparing catalytically active phases on the surface of silica gel using reactions with some transition metal halides and oxyhalides. O2 C l q
*
- 0/
EXPERIP4ENTAL 2.1. Samples Commercial silica gel with a specific surface area of 2 5 7 m 2 /g and pore volume of 0 . 8 5 cm3 /g at a relative pressure P/Po=0.95 was used as a carrier. The iron impurities were preliminary removed by repeated treatment of the silica gel with hydrochloric acid and washing until negative reaction for chlorine ions. The chromyl chloride was obtained by a laboratory method described in ref. 7, then distilled several times. Two series of catalysts were obtained. The silica gel of the first series of a samples was heated under high vacuum /about 10-5Torr/ at 4 7 0 K for 8 h. After that, chromyl chloride vapours were allowed to come into contact with the carrier for 2.
607 1 2 h at 440 K. On completion of the halogenation, the hydrogen cnloride evolved and the residual chromyl chloride were removed by heat-treatment under high vacuum. The hydrolysis was also carried out at 440 K by water vapours in a flow of spectrographically pure nitrogen for 12 h. Samples symbolized as MD n, where n=1,2,3,4,5 denotes the number of the cycles “halogenation-hydrolysis”,were obtained. By carrying out, only once, the cycle of molecular deposition on silica gel previously dehydroxylated in air, the samples MD 300, PID 400, YD 500, !4D 600 and YD 700 were obtained. The numbers denote the temperatures of preliminary dehydroxylation in ‘C. 2.2. Sample characterization The catalysts were analyzed for chromium and chlorine as described in ref. 8 . The amount of structural water (surface OH groups and intraglobular water) (ref. 9 ) in silica gel was determined thermogravimetrically before the heat treatment. The specific surface areas and the pore size distributions of the samples were obtained from the adsorption isotherms of nitrogen at its boiling temperature. 2.3. IR studies The IR spectra of the samples were recorded with a UR-20 apparatus (Karl Zeiss, DDR). The powdered silica gel was pressed into thin plates of about 30 mg with dimensions 10 x 30 mm. In order to avoid contact with air and water vapours, the heat-treatment under vaccum and the reactions of molecular deposition were carried out in the same quartz cell where the spectra were recorded (ref. 10). 2.4. Investigation of the magnetic properties The magnetic susceptibility was measured by the Faraday method. The apparatus constructed in the Institute of General and Inorganic Chemistry of the Bulgarian Academy of Sciences enabled one to carry out the measurements in vacuo or in the atmosphere of various gases. Before each measurement the samples were heat-treated in this apparatus for 1 h at 440 K. Spectrographically pure nitrogen under a pressure of 1-2 Torr was introduced and the measurement was carried out. The susceptibility change depending on temperature was studied between 77.4 and 440 K. The magnetic susceptibility of three samples (YD 1, :ID 5 and MD 600) was measured within the range 4-300 K by collaborators of Prof. M . Pouchard in Laboratoire de Chimie du Solide du C.N.R.S., Bordeaux. The EPR spectra of the samples were recorded within the range of 77.4 400 K with a commercially available spectrometer JES-3BX. 2.5. Determination of the catalytic activity The sample activity towards ethylene polymerization was studied in a
608 three component Ziegler-Natta system containing, along with the chromium oxide molecularly deposited on silica gel, diethyl aluminium chloride (A1 (C2H5l2 C1, 1.2 g/l) and diphenyl magnesium ((C H ) Mg, 0.3 6 5 2 9/11. The first co-catalyst was a preparation of the firm Schering-AG, the second one was synthetized as a 2% solution in chloro-benzene. The ethylene has a purity of 99.85%. Isooctane pre-dried on a molecular sieve 3A was used as polymerization medium. The reaction proceeded at a constant ethylene pressure of 10 atm and a temperature of 360 K. The activity was judged by the yield of polyethylene 30 min after the beginning of the process. 3. RESULTS AND DISCUSSION 3.1. Composition and structure of the chromium-oxide layers
Table 1 contains the results of the analytical determinations of the catalyst compositions. It is evident that with increasing number of the cycles, the chromium amount and the Cr/C1 ratio increase whereas the specific surface area and the pore volume decrease. This may be explained with the gradual filling of the pores during the formation of each subsequent layer. With the increase of the temperature of pre-dehydroxylation (samples MD 300 to !ID 700) the structural water amount decreases, TABLE 1 ~~
OH m mol/g
~
Cr/C1
surfase area mean pore pore vQlume 3 m /g radius A cm / g before* after**before after before after MD-1 1.26 3.84 1.97 257 203 0.85 0.70 54 58 MD-2 2.22 3.96 203 187 0.70 0.67 58 58 MD-3 2.78 4.15 187 150 0.67 0.51 58 58 MD-4 3.50 5.83 150 119 0.51 0.43 58 58 MD-5 4.40 5.79 119 105 0.43 0.33 58 58 MD300 1.37 3.55 1.87 263 236 0.89 0.69 54 56 MD400 1.39 3.13 1.82 256 233 0.88 0.69 53 53 MD500 1.50 1.69 2.08 275 223 0.88 0.75 51 48 MD600 1.24 1.37 1.48 268 225 0.88 0.73 48 49 MD700 0.40 1.00 1.41 256 51 227 0.88 0.84 45 *before the cycle of molecular deposition ** after the cycle of molecular deposition whereas that of chromium, at first negligible, increases and shows a sharp decrease at dehydroxylation temperature of 70OoC. This is probably due to the dehydroxylation mainly in the narrower pores and to the scarifying of the surface as a result of the removal of the intraglobular water. This is confirmed by the relatively small change of the initial specific surface area.
609
The table shows also,in the samples pre-heated up to 500'C and treated after,that only once with chromyl chloride, the Cr/C1 ratio is almost the same (%2) and does not correspond to the stoichiometry of halogenation according to the reaction (la) in which one Cr O2 C12 molecule reacts with one OH group of the surface. This "anomaly" can be explained by assuming that nearly half ?f the Cr O2 C12 molecules reacts with two hydroxyl groups each (reaction 1b ) . The proceeding of a reaction of the type la and/or lb is confirmed by the IR spectra. Fig. 1 shows the IR spectrum of the initial silica gel and the spectra after various treatments in the process of molecular deposition. As it is known (ref.9), the silica gel spectrum consists of a band at 3745 cm-1 which is ascribed to the "free" surface hydroxyl groups, a wide band between 3500 and 3700 cm-l which is due to hydroxyl groups connected by hydrogen bonds (bound groups) and a band at 3680 cm-l caused by the intraglobular water. These bands can be seen in the figure (spectra la and 1b ) . The band at 3680 cm-l appeared after heat-treatment under vacuum for 6 h at 470 K (spectrum 1b1. On treatment with chromyl chloride vapour, the band at 3745 cm-l disappeared and the intensity of that between 3500 and 3700 cm-l decreased. However, this is obscured by the band of the intraglobular water. With a view to obtaining more precise information, the samples were subjected to preliminary deuteration by treatment with heavy water vapour at 298 K for 12 h. This led to the formation of OD groups on the surface. The silica gel spectrum after deuteration is shown in Fig. lC.The free OD groups gave a band at 2760 cm-', whereas a band with a maximum at 2600 cm-l corresponded to the bound OD groups. The band at 3680 cm-l corresponding to the intraglobular water did not change. The reaction of molecular deposition (treatment for 12 h at 440 K with chromyl chloride vapour) led to a substantial change in the IR spectrum. The band of the free hydroxyl groups, OD, at 2670 cm-l disappeared and the band around 2600 cm-l strongly decreased. The fact that the latter band did not completely disappear indicates that chromyl chlorida cannot react with the bound OD groups in the narrowest pores. A new band at 1750 cm-l appeared in the spectrum after the reaction. A second deuteration and treatment under high vacuum produced no change in the spectrum (spectra le and 1f) . It is evident that during the treatment with chromyl chloride vapour, (la) and ( 1b ) type reactions proceed and cause the disappearance of the bands for the OD groups. The appearance of the new band at 1750 cm-l is due to a surface complex containing chromium, oxygen and silicon. The study of the sample porous structures showed that the specific
610 s u r f a c e a r e a and t h e p o r e volume d e c r e a s e d a s a r e s u l t o f d e p o s i t i o n ( T a b l e 1 ) . The d i s t r i b u t i o n c u r v e s of t h e p o r e s w i t h r e s p e c t t o t h e i r r a d i i p r e s e r v e d t h e i r c h a r a c t e r , a s m a l l s h i f t o f t h e maxima t o w a r d s
1400 1800 F i g . 1.
2600
3000 3500 cm-I
I R s p e c t r a o f t h e s i l i c a g e l u s e d as s u p p o r t a f t e r v a r i o u s
t . r e a t m e n t s : a , i n i t i a l s a m p l e ; b , a f t e r h i g h vacuum t r e a t m e n t a t 4 7 0 K f o r 6 h ; c , a f t e r d e u t e r a t i o n a t 298 K f o r 1 2 h and a s u b s e q u e n t t r e a t m e n t u n d e r h i g h vacuum a t 4 7 0 K f o r 4 h ; a f t e r c o n t a c t w i t h chromium d i o x y d i c h l o r i d e v a p o u r a t 4 4 0 K f o r 3 h and p o s t - t r e a t m e n t u n d e r h i g h vacuum a t t h e same t e m p e r a t u r e ; e , a f t e r c o n t a c t w i t h D 2 0 a t 298 K f o r 1 2 h and t r e a t m e n t u n d e r h i g h vacuum a t 4 4 0 K f o r 3 h ; f , a f t e r c o n t a c t
w i t h chromyl c h l o r i d e a t 4 4 0 K and pumping a t t h e same t e m p e r a t u r e f o r 25 h . I f a l l t h e chromyl c h l o r i d e m o l e c u l e s r e a c t w i t h t h e s i l i c a g e l s u r f a c e a c c o r d i n g o n l y t o e q u a t i o n s ( l a ) and ( 1b ) , t h e v a l e n c y of t h e chromium a n c h o r e d t o t h e s u r f a c e s h o u l d r e m a i n t h e s a m e a s i n C r O 2 However, w i t h i n t h e r a n g e 77-
el2.
4 0 0 K , t h e s a m p l e s showed EPR s i g n a l s .
They were a n a l o g o u s t o t h e s i g n a l s of t h e s o - c a l l e d y- and 4 - p h a s e s ( r e f . 1 1 ) o b s e r v e d w i t h chromium o x i d e d e p o s i t e d by t h e t r a d i t i o n a l
611
technology of impregnation on silica and alumina-silica carriers. In our case, the y- phase signal ascribed to CrV ions predominates immediately after halogenation. This signal was rather narrow ( s 2 0 G ) and slightly asymmetric. The maximum difference between gI and gII was found to be 0.045 with gI = 1.975 0.002 and gII = 1.928 k 0.004. A signal of this type was also observed in the initial chromyl chloride even after distillation. Here it was completely symmetric with g = 1.986 0.002 and indicated a resolved hyperfine structure due to the nuclear spin of Cr53. The fact that the signal is preserved after distillation shows that the molecules containing CrV ions are stable and can react with the surface hydroxyl groups, the valency state of chromium in them remaining the same. A comparison of the signal intensities of the y- phase in the chromyl chloride and on the surface after halogenation showed, however, that the CrV amount per gram chromium on the surface was 20 - 40 times as large as that in chromyl chloride. This indicates that during halogenation part (about 0.1%) of the chromium ions are reduced to CrV probably by the HC1 evolved. The y- phase signals described in literature for silica gel samples impregnated with CrV1 - containing solutions are rather asymmetric (re€.ll). Thus, there is a greater similarity between the signals obtained by us and those observed by L.L. van Reijen et al.(ref.l2) and Yu.I.Pecherskaya and V.B.Kazanskii (ref.13) with chromium oxide deposited on alumina supports. These authors ascribed the more symmetrical y- phase signal to chromyl ions with CrV in strongly distorted square pyramidal coordination. By analogy one can assume that in our case the CrV ions obtained during the halogenation have higher coordination numbers than the CrV ions stabilized on a silica gel surface by the traditional technology usually have. After hydrolysis of the samples, a considerable decrease of the yphase signal and a substantial increase of the 8- phase signal, especially of the BN - type one, is observed. According to the nomenclature adopted in ref.11, BN is a phase containing Cr'I1 - ions which are aggregated and exchange coupled. Table 2 contains the widths of the B, - phase lines measured at room temperature. It is evident that they are smaller than those usually obtained with chromium oxyde layers deposited by impregnation. This can be explained by a higher homogeneity of distribution of the CrlI1 ions on sites with different symmetry of the crystal field. The amount of chromium reduced after the hydrolysis to CrrI1 was established on the basis of the experimentally determined effective magnetic moments of the samples. As evidenced from Table 2, the magnetic moments (peff) calculated from the atomic magnetic
*
*
612
TABLE 2 Samples
~
MD-1 MD-2
AH
Cr
'+ Cr content
Yield
Yield
G
Peff BM
330 270
2.00 2.13
2.7 2.3
6.1 10.3
91 136
1441 1214
104 63
745 357
55 31
250 418 601
~~
c,3+
(wt
%)
( 9 PE / g Cr)
( 9 PE / g catalyst
~~
~
~~
MD-3
300
2.26
1.9
12.6
MD-4 MD-5 MD300 MD400 MD500
310 330
2.39 2.42
1.6 1.6
267 280
2.47 2.45
1.4 1.5
15.4 18.6 6.6 6.7
330
2.30
1.9
7.2
77
926
MD600 MD700
310 330
2.30
1.9
6.1
91
2.23
2.0
2.0
82
1477 3832
42
PE = polyethylene susceptibilities of deposited chromium are smaller than the spin only value typical of CrI11 (pIII = 3.87 BM). This shows that in addition to the paramagnetic ion species established with EPR in the hydrolyzed samples, there remains also some unreduced CrV1. Of course, it is not excluded that some quantities of CrIV and even Cr" could also be present, but one can assume that the amounts of these intermediate forms are negligibly small, as in the amount of CrV estimated from the EPR spectrum of the y-type. On this basis, we determined the CrV1/Cr I11 ratio using the formula: 2 2 P eff = A P + (~1 - A ~) P vI~ ~ where A is the relative amount of Cr3+ in the sample. As pv1=O, A = (p/3.87) ' . Thus, the ratio between the two predominating paramagnetic ion species was calculated (Table 2). It is evident that with the increase of the number of the layers, the relative amount of CrI11 increases. On a single treatment with chromyl chloride with increasing degree of pre-dehydroxylation, the relative amount of CrV1 increases. Since the chromium amount in the samples is rather high, the possibility of formation of sufficiently large clusters of exchange coupled CrlI1 ions is not excluded. It was interesting to see whether some kind Of magnetic ordering could appear under these conditions. As established by measurements of the magnetic susceptibility at temperatures up to 4 K, the Curie-Weiss law is obeyed up to 50 K; around 50 K, the x (T) curve shows a small maximum: after that the susceptibility sharply increases with further decrease of temperature. This result is in
613
agreement with the conclusion maae on the basis of the EPR studies: the main part of the CrlI1 produced in the hydrolysis reaction is included in a B, - phase in which chromium (111) ions are antiferromagnetically coupled. The antiferromagnetic exchange interaction leads to some magnetic ordering at around 5 0 K. Only below this temperature one observes paramagnetism of isolated centres which are considerably less numerous. 3.2. Catalytic properties Table 2 shows the characteristics of the catalytic activity with respect to polymerisation of ethylene in g polyethylene per g catalyst, and g polyethylene per g Cr, 30 min after the beginning of the process. It is evident that there is a correlation between the amount of chromium reduced on hydrolysis to Cr'", and the activity: the larger the relative amount of Cr'", the lower the catalyst activity. Since the number of reduced ions is larger when the density of the surface hydroxyl groups is higher, one may conclude that samples with larger mean distances between the chromium ions are more active and less able to form clusters of exchange coupled Cr (111) ions. The treatment of a freshly halogenated sample with diethyl aluminium chloride at 360 K leads to a monotonous decrease of the intensity of the y- phase signal. Simultaneously, the catalytic activity increases and reaches its maximum. This shows that the Cr" accumulated during the halogenation is not catalytically active in the reaction of polymerisation. In addition to this, no other kind of the y- type signal appear during the treatment described, i.e. no other kind of Cr" are produced, The present study showed that by the method of the molecular deposition of chromium oxide on silica gel one could prepare catalysts for ethylene polymerization. Its advantages consist of the possibility of regulating the thickness, structure and composition of the active phase by varying the number of cycles of molecular deposition and the density of hydroxyl groups on the carrier surface. ACKNOWLEDGEMENTS Thanks are due to Prof.M.Pouchard (Laboratoire de Chimie du Solide du C.N.R.S., Bordeaux, France) for the low temperature measurements of the magnetic susceptibility. We are also indebited to Mr.D.Panayotov f o r his assistance in running the IR spectra.
614
REFERENCES 1 A.Clark, Catalysis Rev. 3(2), 145 (1969) 2 Yu.1. Yermakov, Catalysis Rev.-Sci.Eng. E(1),77 (1976) 3 S.I. Kol'tzov, G.N. Kuznezova and V.B. Aleskovskii, Zh.prik1. Khimii, 40, 2774 (1967) 4 XBienert, W.Hanke, U.Illgen, H.-G.Jerschkewitz, G.Lischke, G.bhlmann, and I.W. Schulz, USSR Conference on the Mechanism of the Heterogeneous - Catalytical Reactions, September 1974, MOSCOW, Rep.P 78 5 V.M. Smirnov, S.I. Kol'tzov and V.B. Aleskovskii, Izvestiya Visshykh Uchebnykh Zavedenii Khimiya i Khim.Techn., 17, 1759 (1974) 6 D.Damyanov, D.Mehandzhiev and Tzv.Obretenov, 3rd International Symp. on Heterogeneous Cat., October 1975, Varna, Rep.p 95 B.11, Ferdinand Enke Verlag, 7 G.Brauer, Hdb.d.Praparat.Anorg.Chem., Stuttgart, 1962, p.1211 8 D.Damyanov and D.Mehandjiev, Commun.Dept.Chem.Bulg.Acad.Sci., 9, 294 (1976) 9 v.Ya. Davydov, L.T. Zhuravlev and A.V. Kiselev, Trans.Faraday SOC. , 60, 2254 (1964) lO-.Damyanov, D.Panayotov, D.Mehandjiev and Tzv.Obretenov, Commun.Dept. Chem.Bulg.Acad.Sci., 2, 394 (1976) 11 Ch.P.Poole, Jr. and D.C.Mac Iver, in D.D.Eley, J.H. de Boer, M.Boudart (eds.) Adv.Catalysis, Vo1.17, Academic Press, N.-Y. and London, 1967, p.223 12 L.L. van Reijen, P.Cossee and H.J. van Haren, J.Phis.Chem., 38 572 (1963) 13 Yu.I.Pecherskaya and V.B. Kazanskii in O.V. Krylov (Ed.) Problemy kinetiki i kataliza, Vo1.13, I-vo "Nauka", Moskva, 1968, p.236
615
THE USE OF TRANSPORT REACTIONS FOR DISPERSING SUPPORTED SPECIES RI HAASE, He-G, JERSCHKEWITZ, G. GHLblANN, J. RICHTER-MENDAU and J. SCHEVE VEB Chemiekombinat Bitterfeld, DDR-44 Bitterfeld Zentralinstitut ftir physikalische Chemie der Akademie der Wissenschaften der DDR, DDR-1199 Berlin-Adlershof ABSTRACT
For manufacturing CatalyEtS a main object is to disperse the active material on the support. Methods of fixing the catalytically active substances only on the surface and not to place them in the bulk are favourable, By using methods of ma88 transport a dispersing in the reaction vessel before and during the reaction is imaginable, Therefore we investigated the spreading of “moltenttV205 on and the reacting of vanadiumoxichloride with the surface of SiOrTi02, and mordenite at various conditions, These processes are similar to those in industrial kilns. It could be demonstrated that the spreading rate is predominantly determined by the surface roughness while the effect itself is strongly influenced by the stoichiometry of the vanadium oxide, According to the melting point V204 does not disperse on these conditions. Because the host lattices of the support and the ambient atmosphere influence the V5+/V4+ relation they also determine the dispersing of the initial vanadium oxide. These findings enable u8 to choose a p p r o p r i a t e c o n d i t i o n s f o r getting highly dispersed coats of catalytically active material. INTRODUCTION
From an economic point of view the preparation of induetrial c a t a lysts i s f a v o u r e d by d r y mixinq o f t h e s u p p o r t and the c a t a l y t i c a l l y
active components. By means of this only one operation is necessary for getting the green pellets of the catalyst. But a special problem arises in this case, the homogeneous and high dispersion of the
616
active material. There are three methods of mass transport for meeting this goal; diffusion, viscous flow or spreading and evaporation and nucleation. An additional amount of HC1 in the firing gas or wetting the green pellets with liquid hydrochloric acid can form volatile or liquid oxichlorides of transition metals, which can easily diffuse, or spread over the support. The redox conditions of the system determine the reaction and transport rates. This method is the only one for dispersing a molten layer. We modelled the industrial process using V205 and V204 and different supports. EXPERIMENTAL
Smallcylinders of pressed powder of V205 or V204 2 mm in diameter and 2 mm high are formed and annealed. These pieces are put on to the surface of the support which has been pressed as a wet paste in a combustion boat and dried. This sample is placed in a quartz tube and heated in a special furnace. One half of this tube furnace can be lifted and the sample is visible during the heating operation. Gas, one liter per hour,flows through the quartz tube, consisting of air, air saturated with HC1 or air loaded with a mixture of HC1 and CH30H. Diatomite, anatase and mordenite, pure and impregnated with K2S04 serve as supports. In a second set of runs, discs 5 mm thick are cut from a fused silica rod, 16 mm in diameter. These discs are ground with sandpaper of different mesh to obtain varying roughness of the surface. Then they are used as supports for the spreading experiments of V205. The roughness is measured with the stylus method. RESULTS
The experiments describe the basic processes used in manufacturing the DS-type catalysts of the VEB Chemiekombinat Bitterfeld Chemical Company. Therefore diatomite is preferred as support but nevertheless anatase and mordenite are a l s o investigated because they are mixed or impregnated with V 0 in industrial catalysts, too. 2 5 The first surprising observation is that V205 spreads over the surface of fused silica at 710 K, that means 250 K lower than its melting point. The roughness of the three quartz discs is demonstrated in fig. 1 by the depth of the grooves, formed by grinding
617
with sandpaper. The depth should be s m a l l e r than 3 x 10” promoting t h e spreading of powder ( r e f . 1).
cm f o r
C
Fig. 1. Roughness of d i s c s c u t from a fueed s i l i c a rod,ground with sandpaper of a ) 280 mesh, b) 150 mesh, c ) 100 mesh. The covered a r e a s t r o n g l y depends on t h e roughness of t h e surface. The s u r f a c e of t h e d i s c a i n fig. 2 has been ground w i t h 280 mesh s i l i c a sand, d i m b with 150 mesh and d i s c c with 100 mesh.
Fig. 2. V205 c o a t s on t h e eurfacee of d i f f e r e n t rough s i l i c a s u r f a c e s , spreaded from 2 mm p e l l e t s i n dry a i r a t 710 K. The e f f e c t is i d e n t i f i e d a8 spreading became t h e i n c r e a s e of t h e a r e a with time mainly follows a square r o o t law, which is only v a l i d f o r a spreading mechanism ( r e f . 1) ( f i g . 3 ) .
618
ij
m z
150
1
Fig. 3. Time dependence of the spreading of V205 over silica surfaces ground with sandpaper of different mesh. Continuing our experiments we measured the dispersing of the starting V205 or V204 pellets in various gas atmospheres.
Fig. 4. Dispersing of vanadium oxides on the surface of diatomite. Similar to the manufacturing conditions, where gas-fired kilns are used, a stream of gas flows over the sample. For getting Uniform flat surfaces of the different samples, we smoothed the surface with
619
a spatula. Because the combustion boat was to small no circular coat of vanadium oxide was available. Therefore the diameter was used for comparing the different effects, In the case of diatomite they are demonstrated in fig. 4, We got the highest dispersion when air wa8 bubbled through conc. HC1. We observed two different forms of vanadium oxide Coat8 on the surface: a red non-transparent spot and a yellow one which is transparent. The diameter of the latter one was 5 times larger. After three hours the red spot did not grow further, The differences of two runs are also drawn in fig. 4, In air V205 only yielded the red spot, The first three hours its diameter was equal to those resulting in an air/HCl mixture, and then it further increased slower. V204 did not coat the surface of diatomite when using air as ambient atmosphere. The spot of V205 on diatomite increased in a stronger reducing atmosphere, i.e. CH30H/ HCl/air, a little bit quicker than in HCl/air. The variation of spreading with temperature is shown in fig. 5.
Fig. 5. Temperature dependence of the spreading of V 20 5 diatomite surface at 7 h runs.
Over
a
At 673 K no dispersion was observed but already at 723 K V205 crept over the silica surface, that means 240 K below the melting point, This agrees with other observations, for example at 723 K the electrical conductivity changes, This is discussed to be due to surface melting (ref. 2), In order to meet manufacturing conditions as real as possible we used a diatomite, mixed with K2S04 in the same 1 . ~ 8 ratio 8 as in SO2 converting catalysts. The results are given in fig. 6. It is to be seen, that V205 and V204 are not apreading on the surface in a non oxidizing atmosphere.
620
Fig. 6 . Variation of the surface area of V,05, which has been soaked in diatomite.
solved in K2S04,
Ambient air/HCl partly oxidizes V204 and it is solved in K2S04, yielding a red coloured spot of V205. The diameter of V205 mixed with carbon black is a little bit larger than that of the V204 spot. Fig. 7 demonstrates the dispersion of vanadium-V-oxide and vanadium-IV-oxide on the surface of anatase at 873 K. The runs with V205 on anatase and a mixture of air and HC1 yielded the same results as on diatomite. We observed also two different sorts o f vanadium oxide spots on the surface of Ti0,. A red spot, which was nontransparent and a yellow one, nearly ten times larger than the red spot and transparent. V204 did not coat the surface neither in air nor in the mixture of gaseous HC1 and air. Nevertheless on a mixed Ti0,/K,S04 sample vanadium-IV-oxide is actually better solved than .5'2' The third material investigated as core material for vanadium oxide coated catalyst was mordenite. This support exhibited a special behaviour. V205 covered a three times larger part of the surface if mordenite was mixed with K2S04 than on the pure powder. In both cases, the spots decreased after three hours. In this set of runs the effect of the concentration of the transport medium was observed. The mass dispersed per unit time increased with the concentration of HC1 (fig. 8). From the preceding results we assumed the supports to control strongly the dispersing of the vanadium oxide. This was confirmed bY the experiments shown in fig. 9. The three different supports, anatase, guhr, and mordenite dispersed V,05 using standard conditions, in significantly various extents. Anatase was most active
621
10
!&/K,SO4 /air
10
5
A -A
i(hJ
.
Fig, 7. Spreading and dissolving of vanadium oxides on anatase surf ace
I
Fig. 8. The effect o f mordenite/K2S04 and the concentration of H C 1 on the dispersion o f V2O5.
622
0mm
VjOs /HCl/3h
Fig. 9. Comparision of l?i02, Si02 and alumosilicate with regard to the mass transport of V 0 2 5' and mordenite was less effective. Guhr took a position between the other two. A reverse sequence was observed when the powders of the support material were mixed with K2S04. DISCUSSION
The two forms of V205 coatings observed on the surface of diatomite demonstrate, that when firing a mixture of vanadium oxide with a support in a stream of air, containing hydrochloric acid, two different proceases of mass transport take place (fig. 10).
Fig. 10. Diatomite covered with transparent and non-transparent V205 coats.
623
The first one occured in ambient air and is due to a spreading of V205 (fig. 1 and 2). According to the melting point of the different VOx phases only V 0 pure or better slightly reduced is expected to 2 5
exhibit such behaviour (ref. 3). V204 (mp 2240 K) should not spread. This prediction is confirmed (fig. 3 and 6 ) . Because a slight reduktion decreases the melting point this effect should increme the spreading (fig. 5). Furthermore the surface is more coated by a spreading process if the annealing temperature is raised (fig. 4 ) . The transparent yellow spot observed in an air/HCl gas stream in addition to the non-transparent red one results from transporting V 0 by means of VOC13. This is supported by the dependence of this 2 5 effect from the concentration of the transport medium (fig. 7). A second argument is the decrease of the red spot with time and the observation of condensed V205 on the cold wall of the silica tube. The dispersion via gaa phase is also confirmed by the ssymmetrical form of the transparent spot, resulting from the dragging of gaseous VOC13 in the direction of the gas stream. The two forms of mass transport differ also in the w a y of penetration of the bulk. The volatile V0Cl3 diffuses through the sample till to the bottom of the combustion boat. The molten part of the V205 or V205,x spreads only across the surface if the viscosity remains high i.e. at moderate temperatures, At high temperatures (about 1000 K ) the versatile fluid penetrates the bulk too. This effect depends from the diameter of the pores. At a largegrained support only bulk impregnation is obtained. The pellet drops into the support, The results obtained from mixtures of the oxides with K2S04 can be explained by assuming the corresponding vanadium oxide to form a melt with K2S04. If pure V204 or V205 are used in a reducing atmosphere, no dispersing on the surface is observed (fig. 5 ) . Under slightly oxidizing conditions or in air V205 is quasi extracted by the K2S04. According to ref. 4 a V204/K2S04 mixture melts at 728 K. The great effect of K2S04 mixed with mordenite can be due t o the great capillary tension operating on the melt in the small pores of the mordenite. Resuming our results we can state that dispersing of vanadium oxide on supports is enhanced by using hydrochloric acid. A alight reduction is also favourable. Both processes are already used for manufacturing catalysts.
624
REF'ERENCES
1
da. E. Geguzin, Physik des Sinterns, VEB Deutscher Verlag fur Grundetoffindustrie, Leipzig, 1973 Berets, Advances in Catalysis 2 (1957) 204 2 H. Clark and D.J. 3 H. Oppermann, W, Reichelt, G. Krabbes and E. Wolf, Krietall und Technik 2 (1) (1977) 717-728 4 S. Hghle and A. Meisel, Kinetika i Kataliz 2 ( 5 ) 1276 (1971) DISCUSSION B. DELMON : In the field of ceramics we have observed that the
impregnation of alumina-silicates (e.g. kaolin) with nitrates gives a substantially better dispersion of the mineralizers (e.g. alkali metals, Mg, Ca, Cu, Zn as oxides) than other salts.
Should this Nitrates, while
effect be related to the phenomena you describe ?
decomposing, might provide a more oxidizing atmosphere, thus explaining the difference, into account
J. SCHEVE
:
Or should additional factors be taken
?
Of course the impregnation is improved by using nitrates
but in this case you get also penetration of the bulk.
Using a gas
phase process such as oxychlorination you get only surface contamination.
The dispersion of a coating on a surface is enhanced by
a slight reduction.
Therefore a more oxidizing atmosphere, when
using nitrates is not a favourable one.
The dropping of the melting
point, due to nonstoichiometry is the major factor to spread the coating material on the surface of the support.
S.P.S.
ANDREW
:
The subject of the spreading of one oxide over a
metal o r over another oxide has been much studied by technologists interested in enamelling.
What has been found is that spreading
is enhanced when there is a chemical likeness or affinity between the two materials or an intermediate compound forms between the two layers.
The oxidative power of the atmosphere is most important.
Have you compared your findings with this theory
J. SCHEVE
:
?
According to our knowledge (Op. cit. ( 1 ) )
spreading
theory tells us that transport on the surface is favoured by two phenomena
:
physically by the bending of the surface and chemically
by decreasing the melting point.
Chemical reaction with the support
material prevents such spreading as shown in the case of V 2 04
On
625
anatase.
Chemical affinity is only necessary if you use a flux
material, but this has not been the aim of our experiments.
The
influence of the oxidative power of the atmosphere has been demonstrated. The basic model we used is that of Thomson i.e. decreasing the vapour pressure by bending of the surface.
According to this theory
lattice defects created by the ambient atmosphere also influence the melting point.
Our experiments are not analogous to enamelling
operations, because in the case flux material is used for enhanced spreading, and the coated surface is fused.
Chemical likeness
should exist between flux material and the powder to be spread. If the porous support can react with the coating sample, it does not spread but is soaked into the pores or the space between the grains as shown in our paper. F. ROOZEBOOM
:
From your experiments concerning the spreading
behaviour as a function of surface roughness, I get the impression that you used the geometric surface, instead of the actual surface area.
The latter surface will be larger than the former by some
roughness factor.
Did you take into account the roughness factors
of your samples in plotting the surface areas in Fig. 3 ? If,for example, the roughness factors of the 280 mesh and 150 mesh sandpaper-treated samples are 10 and 5 times larger than the 100 mesh sample, respectively, the lines in Fig. 3 would even coincide.
This
might suggest that, by sandpaper-treating, you mainly introduced a macroscopic surface change, which will have no effect on the interaction between the vanadium oxide and the support.
J. S C H E V E : The surface area in Fig. 3 is indeed a geometrical one,
because we used fused silica rod.
According to theory (Op. cit. (1))
even macroscopic roughness influences strongly the spreading. is what we proved in this experiment.
This
The lines of Fig. 3 would
be much more disconnected it we take into account the roughness factor.
The 280 mesh sample is smoother and the real surface area
therefore lower than in the case of the 100 mesh sample, which even from a geometrical point of view is much more covered by the V205
(Compare Figs. 1 and 2 with Fig. 3 in the paper).
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627
SYNTHESIS OF NEW CATALYTIC MATERIALS: METAL CARBIDES OF THE GROUP V I ELEMENTS
L. LECLERCQ*, K. IMURA, S. YOSHIDA',
B
T. BARBEE and M. BOUDART
Deportment of Chemical Engineering, Stanford University, Stanford, California 94305 (U.S.A.)
ABSTRACT In a study of the surface reactivity of tungsten carbide powders, the role of surface carbon 2nd oxygen has been pointed out.
The presence o f graphitic carbon on the surface as revealed
by Auger electron spectroscopy and electron spin resonance, suppresses hydrogen chemisorption. By contrast, the latter can be enhanced by hydrogen spillover occurring t o the oxide surface region from the surfoce carbide. To avoid free carbon cantaminatian,molybdenum oxycarbide samples, with a deficiency i n carbon content compared to the theoretical value for M o O C or M o C, have been synthesized
2
from molybdenum hexacarbonyl M O ( C O ) by ~ condensation of vapors i n a low pressure gas.
The resulting high specific area powders exhibit a higher activity i n the synthesis o f hydrocarbons from CO-H
2
mixtures than molybdenum metal.
Moreover a further carburizatian o f the initial
oxycarbide powder increases its catalytic activity which becomes closely comparable to that reported by different authors for ruthenium catalysts.
INTROD UCT 1 0N The incentive t o find substitute materials to replace platinum group metals i s growing rapidly with the number and amplitude of industrial processes which require their use.
What are the
essential properties that new catalytic materials should possess to simulate the surface reactivity o f Group V l l l metals o f the lost two transition series? Perhaps the answer could be formulated by Sabatier's dictum:
a most important property of
an excellent catalyst i s its ability to bind many-organic and inorganic molecules strongly enough but not too strongly. element, e.g.
This leads to the simple idea that, by dissolving a nan-metallic
carbon, i n a metal of the Group VI
B
(tungsten or molybdenum which are strong
adsorbers of molecules l i k e CO) i t i s possible t o temper the surface reactivity of the host metal by increasing the electron to atom ratio of the host. * C n leave from University o f Poitiers. XOn leave from Kyoto University.
The net result might be that carbides of
628 Group V I B metals behove l i k e Group V l l l metals i n their surface reoctivity.
This has been
observed by Levy and Boudart (ref. I) who reported that tungsten carbide exhibits platinuml i k e behavior for hydrogen chemisorption ond isomerization o f 2,2-dimethylpropone.
This
latter isomerizotion i s carried out at appreciable rates only b y platinum and iridium (ref. 2). There are not only chemical but also structural analogies between platinum and tungsten corbide surfaces (refs.
3,4).
Numerous studies deal p i t h tungsten carbide since tungsten carbide
electrodes have been reported to function well ond durably i n a methanol fuel cell (ref.
5).
Another desirable property of WC surfaces i s their. possible resistance t o sulfur (refs. 6,7). the nature of active sites i n WC i s still a matter of controversy (refs. 8 , 9 ) .
But
Thus we have
re-examined the tungsten carbide surface b y Auger electron spectroscopy (AES) and electron paramognetic resonance (EPR).
The influence o f carbon and oxygen on surface reactivity for
hydrogen chem isorpt ion has been c l ori fi ed
.
It seems that the Golden Rule o f Catalysis formulated by Sabatier i s quite generally applicable to carbides.
Thus, carbided nickel surfaces behove l i k e capper in their selectivity for the
decomposition of formic acid (ref. 10). In addition, Sinfelt (ref.
II) reported an increase o f
the a c t i v i t y o f molybdenum far the hydrogenalysis of ethane, due t o the formation of a bulk molybdenum corbide Mo2C.
The analogy of activity between Mo2C and Ru has been noted
previously (ref. I). Such an analogy might be interesting i n the Fischer-Tropsch (FT) synthesis of hydrocarbons from C O and
H2
mixtures since ruthenium i s an active FT catalyst.
Thus, we
hove synthesized molybdenum carbides of high specific surface areas and investigated their cotalytic activity and selectivity i n the FT reactions, i n relation t o their chemisorptive properties.
The latter have been improved by an activation treatment consisting of further carburizatian of the originol preparation.
PREPARATION OF CARBIDE CATALYSTS Traditional methods of preparation (ref. 12) do not look very promising i n the case o f refractory metals such as tungsten (ref. 13). i t was shown (ref.
modified.
Besides i n the case of molybdenum on alumina,
14) that the metal interacts strongly with the support and have i t s properties
During the impregnation o f the support, A I - 0 - M o complexes occur which are very
difficult to reduce (ref. 15). The preparation of metal carbides has been thoroughly reviewed (refs. 16-19) and it can
be summarized i n the following Table, from reference 16, used t o describe the nomenclature o f the different samples studied in this paper.
629
TABLE 1 General Preparation of Metal Carbides Met hod
Reaction
(I) Direct reaction either by
Symbols Studied
WC-AEG commercial Iy obtained at AEG Telefunken
Me+ C - MeC
melting or sintering o f the elements or metal hydrides i n o protected atmosphere or i n vacuum.
(preparation not available).
(2) Direct reaction o f the metal oxide and excess carbon i n a protective or reducing atmosphere.
M e O + C - M e C + CO
WC-PWA from Pratt and Whitney Aircraft (MeO=W03)
(3) Reaction o f the metal with a
M e + C O -4 MeC + C 0 2
Sample S 3, M e = M o C H = C H 410 x y Sample S 2 (Mo2C) Me=Mo,C H =CH x v 4
carburizing gas
M e + C H -MeC+H2 X
Y
MeCI4 + C H
(4) Precipitation from the gas phase by reacting the metal halide or metal carbonyl i n hydrogen or inert gas.
+
H2
X Y
-
MeC + HCI + CmHn M e carbanyl
+ H2
C H = C H 410 x y
-
M o oxycarbides
M e C + (CO, Cog, H2,
H20)
WC-CMR,MeCI4 = WCI6
...
S 208 220 M e carbonyl = M o ( c 0 )
6 MeC = (MoOC, Mo2C)
By necessity, catalytic materials must exhibit specific surface areas i n the 50 range corresponding t o particle sizes i n the v i c i n i t y of 100
- 100 m2g -I
1. Although tungsten carbide i s
now available with a surface area of 30 m2g-' from highly dispersed tungstic acid (refs. 20,21), no one had reported the preparation o f carbide particles in the vicinity o f 100
1 until our
recent work at Stanford i n the central facilities o f Stanford's Center for Material Research (CMR) The effectiveness o f fine particles synthesis by vapor condensation i n reactive gases, has been demonstrated at CMR i n a series of experiments i n which a range of compounds including tungsten and molybdenum carbides have been produced.
I n this process, metal compounds
(tungsten chloride WCI or molybdenum hexocarbonyl MO(CO)~ are evaporated into hydrogen
6
or C O at law pressure (10-50 Tarr).
A resistance heated baffle box vapor source was used at
low pressure o f hydrogen and at a controlled temperature o f about 623-773 the metal compounds to be vaporized.
K
depending upon
The vapor of the metal compound passed over a hot
tungsten filament at 1400 t o 2100 K where it reocted with the ambient gas (isobutane and hydrogen, or simply hydrogen or CO for molybdenum carbonyl, with or without argon).
This technique has been availoble since the work of Pfund i n 1933 (ref. 22), though i t has not been widefy used.
However, several investigators (refs. 23-27) i n Japan have studied
this process i n more details.
630
TUNGSTEN CARBIDE Characterization I n additionto the WC-CMR sample, different kinds of tungsten carbide had been obtained from various sources (Table
I).
Specific surface area was measured
by the nitrogen BET method.
The crystal structures of these sample were obtained by X-ray diffraction. TABLE 2 Characteristics o f the tungsten carbide samples studied
wc
Surface Area
AEG PWA CMR
(I) The
H yd rage n
BET
Samples
Uptake
(I)
m2g-1
mol m-2
6.3 5.5 15
5.0 0.4 0. I
Crystal Structure Crystal System Lattice Constant
hexagonal hexagonal cubic
an
co (nm)
0.2901 0.2905 0.2901
0.2884 0.2836 0.2830
Sample was pre-exposed ta oxygen and then evacuated at 423 K for
I n order to investigate the surface properties of WC, at room temperature.
I h.
EPR and AES studies were carried out
I n the EPR work, the magnetic field was calibrated with DPPH powder.
A l l WC samples gave symmetrical singlet signals whose g value were between 2.002 and 2.003 with width between 5 and 7 Gauss.
The line shape and parameters indicated that the signals
could be assigned to radicals associated with carbonaceous compounds similar to carbon black, whose origin o f spins i s believed t o be unpaired electrons stabilized i n the condensed aromatic
A quantitative analysis was not attempted i n this study, but spin density for
rings (ref. 28).
the CMR sample could be estimated t o be ten times longer than i n the other WC samples.
It
was thus obvious that the WC-CMR sample contained a large amount o f carbonaceous compounds. I n the AES work, the most interesting feature i s the line shape of the carbon KLL transitions
(270 eV). The WC-AEG sample gave a triplet signal and the PWA and CMR samples gave a singlet signal.
Many
AES studies for metal carbides (refs.9,
gives a characteristic signal similar to the one of WC-AEG, carbon gives a singlet signal l i k e the one found on
29) agree that carbidic carbon while i t i s well known that graphitic
PWA and CMR samples with no significant
difference i n line shape between the carbon KLL signals.
Thus the latter two are covered with
graphitic carbon. Moreover, a
AES triplet signal due to surface oxygen was found for all tungsten carbides
and the signal was very stable.
Hydrogen treatment at room temperature and a pressure of
2 q kPa followed by evacuation at 423 K caused l i t t l e change i n the oxygen peak of the AEG sample,
In addition, hydrogen treatment at 573 K hardly affected hydrogen adsorption i n
this sample.
These results suggest that the main part o f the oxygen species existed in a n
oxide layer on the surface,
631 Hydrogen adsorption on
WC-AEG
Hydrogen adsorption was carried out ot room temperature on the exhibited the highest surface reactivity. with the hydrogen onto the at
423 K.
WC-AEG sample which
Water vapor was introduced i n various amounts
AEG sample after the latter had been exposed to air and evacuated
I t was shown (ref. 30) that the adsorption rates of hydrogen increased greatly with
the amount of preadsorbed water.
But water accelerates the reduction of
WO to H W 0 3 at
3 x room temperature i n the presence of platinum by hydrogen spillover from platinum t o WO (refs.
31,32).
Besides, Levy and Boudart (ref.
the formation o f
H WO at room temperature.
3 WO also catalyzes 3 Similarly, the surface oxide o f WC-AEG
I) found
that WC mixed with
x 3 appears t o react at room temperature with spillover hydrogen from the WC part of the surface, with the formation o f H WO increasing the total hydrogen uptake on the AEG sample in the x 3 presence of water. Conclusion
This study pointed out the influence of graphitic carbon which can make a surface inactive for hydrogen chemisorption while the carbide region could adsorb hydrogen i n the dissociative form t o cause spillover t o the oxide region and the formation of H WO at room temperature. x 3 Therefore i n discussing the catalytic activity of tungsten carbide one should be careful t o consider the actual surface composition consisting not only of tungsten and carbidic carbon but also of oxygen and graphitic carbon.
Surface composition influences considerably the
surface reactivity as w i l l be illustrated again i n the next part i n the case of molybdenum carbides.
MOLYBDENUM CARBIDES Preporation and C haracterizat ion
As has been seen, the synthesis of fine particles by vapor condensation i n reactive gases was not successful for
WC-CMR samples, giving uncontrollable carbon deposits. The above
described preparation o f molybdenum carbides by decomposition of M o ( C 0 ) that the carbon supply i s already included i n the starting material.
6
has the advantage
The conditions of
temperatures (source and spiral) and pressures were sought to give lower content of carbon than the stoichiometric value for M o C.
2
In addition, a molybdenum carbide (sample a flowing mixture of CH4:H2 (3:2),
S 2)
has been obtained by carburizing M o i n
with an excess of CH during 3 h at 973
4
prepared i n situ by successive reductions of MOO i n H
3
(ref.
11)
2’
K.
The M o was
The method of Sinfelt. e t at.
was also used to make a sample to be compared with the different molybdenum carbides.
By decomposition of M o ( C 0 )
6
at low pressure and high temperature followed by vapor
condensotion, several molybdenum oxycarbide samples (S 202 t o
S 220)
have been obtained
632 exhibiting o f.c.c.
l o t t i e 0s shown
I./ X-roydiffroction.
TABLE 3 Composition and Particle Size of Molybdenum Oxycorbides 0s o Function of Experimental Conditions Samples Gas Composition Pressure (Tom) Ar H2 CO
Source Spiral
K
T
s 202
0 20 0 573 573 20 20 0 0 10 0 773 209 0 773 10 0 214 873 0 150 0 873 215 50 45 0 873 217 50 0 50 s 220 20 0 80 873 -~~~~ S S S S S S
203 208
T K
1373 1373 1373 1373* 2073 2073 1773 2073
Atom Ratio X-ray Crystollite C : Mo : 0 Size from integral 1/2 height width
0.40: 0.33: 0.32: 0.31 : 0.35: 0.31 : 0.33: 0.35:
I I I I I I I I
: 2
:1.8 :1.7
:1.1 :1.1
:0.5 :0.6 : 1.7
-
48 73
60 72 86 90 70 68 130 140
from BET surface
-
-
I60 113
-
-
-
-
-
~
*Change of the shape of the spiral. The molybdenum carbide structure when first reported (ref. 33) was prepared by treating metallic molybdenum with o mixture of C O and H
2'
30.35 atom $ o f carbon.
lattice (4.14
It hod a f.c.c.
The formulo Mo2C wos ossigned to this carbide.
1)with
But other outhors
(ref. 34) prepared molybdenum oxycorbides by pyrolysis o f molybdenum hexacorbonyl M o ( C 0 )
6
under a low pressure of hydrogen, t o give at 573 K, a range of oxycorbides isostructurol with Mo2N, with also f.c.c.
lattice (4.17-4.18
61,.
Molybdenum oxycorbide i s on interstitial
compound where oxygen ond corbon occupy interstitial sites according to Htigg's rule (ref. 39, i n o complex arrangement vorying between the two limiting formulae M o C and MoOC.
It
2 i s well known that transition metal carbides, particularly of the Group V I metals, con dissolve considerable amounts o f oxygen by substitution for carbon (ref. 17). O n l y the somple S 2 "overcarburized" b y on excess of methane i n hydrogen, exhibits o hexagonal structure corresponding to Mo2C.
Its carbon content (9.81 weight
$ is
much larger
than the theoretical value for M o C (6.25 weight $).The presence o f graphitic carbon on i t s 2 surface hos been evidenced by AES. Particle Size
As shown i n Table 3, the crystollite sizes were determined b y the X-ray diffraction line broadening method from the measurement of half height peaks or from t h e integral widths. They hove been quolitotively verified by electron microscopy.
The results, though i n acceptable 0
agreement, have t o be token 0 s rough volues i n the vicinity o f 100 A, os also shown by volues determined from the BET surface area.
Note that X-my l i n e brocdening gives o volume
overage diometer while gas adsorption gives o surfoce overage diometer, the two volues generally not ogreeing with each other.
633 Adsorption Measurements Adsorption measurements were performed with a conventional Pyrex glass constant volume adsorption system provided with a Texas Instrument quartz spiral Boudon gauge.
Prior t o
adsorption measurements, the samples were heated i n hydrogen at 623 K for 2 h and then evacuated at 623
K
for another 2 h or sometimes overnight.
Adsorption isotherms of C O were
measured at room temperature (except for Mo metal at 195 K i n a Dry Ice and acetone bath
3
4
t o avoid CO dissociation) between 5.4 x 10 Po and 4.3 x 10 Pa. isotherm was extrapolated t o zero pressure t o obtain
CO was measured on new or used samples (i.e.
CO
The linear portion o f the
the chemisorption value.
after the FT catalytic tests).
Adsorption of
Chemisorption
o f CO was taken as the measure of active sites on the catalytic surface and a l l activity data were based on the C O uptake value with the only assumption that one C O molecule per site. Surface areas by BET were also determined with N or Ar at liquid nitrogen temperature.
2
The hydrogen uptake at room temperature i s very small on the different molybdenum carbides studied.
This seems t o be a general property of carbides including tungsten carbides (refs. 36,37).
It was found that hydrogen adsorption i s greatly reduced on carburized tungsten because the hydrogen molecules are not dissociated at room temperature.
Adsorption o f CO and H2 i s
much more abundant on molybdenum metal than on molybdenum carbides (Table 4).
TABLE 4 Comparative Adsorption Measurement and A c t i v i t y i n Methanation Reaction of CO, H 2 Mixtures on Molybdenum Oxycarbides, M o C and Mo. Samples
Bulk Composition (I)atomic C/Mo (2) atomic O/Mo New Used
(I)0.32
2 H2 Uptake CO Uptake (RT) -I pmol g New Used
S 208
(211.7
0.65 1.3
-
1.4
S 215 s 217
(1)0.32 (2)0.55
0.5 0.9
-
0.6
S 220
(I)0.35
-
0.35
1.7
(I)0.31 (211.10 . .
--
0.25
(I) 0.9
-
0
-
-
Oxycarbides
(2) 1.65
S 209 hi022 52
Mo s4
-
-
p mol g -1 N e w Used
BET (c) CH4 Formation Sur Area E Yx 10 3 S-I m2 9-1 kcal NewUsed mol-l
(4
(a)
31.721.7
42 36.5
14
38.5
(a)
13.3
-
18
13
24
15.2
20
-
II
41.5
59 33
-
22.5
1.75
-
6
II
19.5
105
-
24
16
2.5
-
(a)
25
0.4
I0
(0)
39 12.6 (0)
-
(b)
-
(a) Co uptake at room temperature (RT)
(b) Co uptake at 195 K (c) Turnover number i n methane formation measured at Pt = I atm., (d) Idem, at 573 K obtained from Arrhenius plots. (e) Idem, at 571.5 K.
P~,/Pc0=3
(4
(4
(4 (dl
(dl
634 Catalytic Tests: Synthesis of Hydrocarbons from CO and H2 After the same pretreatment of the catalysts used for adsorption measurements, reaction
The
rates were measured either in a flow reactor or in a differential batch recycle reactor. specific rate i s expressed as a turnover frequency
(N)i n units of
reciprocal second (s
-I
), i.e.
molecules of product formed per unit surface active site per second. As shown i n Table 4, molybdenum carbides I r e more active by a factor of 10 to 20 than molybdenum metal i n methane
The FT synthesis seems to modify the surface properties of the formation from CO and H 2' oxycarbide catalysts. The carbon content determined by chemical analysis i s significantly increased while specific surface arms and CO adsorption decreased. Activation Treatments: Carburizat ion'
A futther carburization, conceived as an activation treatment has been performed on both samples S 209 (oxycarbide) and S 4 (Mo metal) by a recycling mixture of n-butane (nC4) and hydrogen (PH/P
-9) during 3 h at 773 K, controlled by analyzing the products formed i n
c4-
the hydrogenolysis of n-butane, until only methane and hydrogen were left in the gas phase. Moreover, S 209 was activated i n another way by recycling an original mixture of CO and H
2
(PH2/PCO'3)
over the catalyst during 2 h at 773 K.
The results are shown i n Table 5.
TABLE 5 Comparison of Activity of Mo Catalysts in the CO + H2 Reaction with that of Ru Catalysts Catalysts
Activation
B.E.T.
Treatment
Sur. Area (used) m2 g-I
Oxycarbide s 209
H2, 2 h 623 K
S 209 after carburization
nC4+
S 209 after carburization
CO+ H2
S 209 after (I) carburizction
CO+ H2
S 209 after (2) carburizction M o Powder
s4
CO uptake
p o l ,-I
Experimental
Turnover Frequency
Conditions
NCH4
NCn(3)
to3
lo3
2
or4* Tco K
59 (new) 33
39(new) 12.6
760
3
572
22.5
31.5
33
760
3
578
186
34.5
40
760 0.6
575
39.5
59
33.5
29
750
3
564
45
55
25
21
750
3
546
28.5
31
24
105
760 3
502
2.5
3.3
29
H2 ~
773 K
773 K
773 K
22 I
C O + H,
L
773 K Mool+ H2, 16h 873 K
635
s4
nC4+ H2
after carburization
773 K
5%
H2
Ru/AI 203
723 K Hq, 2 h
0.5 to 5% R u/A I 0
7.5
12.5
-
35(new)
760 3 584
I79
221
760 3 548
I81
(ref,
38)
28.5( used)
-
-
750 3 553
25-4
(ref. 39)
723 K
I%
H2/9
Ru/S i 0 2
-
20.4(used)
80 3 600
57.3
(ref.
40)
4h 6 7 3 ~
(I) Carburization of a new sample during 3 h. (2) Extensive carburization following (I) during 6 h after adsorption (3) NC =turnover frequency for total hydrocarbons formation. n The activity for the reaction of methanation ( N c H ) i s greatly increased over that of Mo
4
metal by about two orders o f magnitude, after carburization by n-butane and hydrogen, on the carburized samples S 209 and S 4 which then exhibit identical activity.
These improved activities
are of the same order o f magnitude as those found by some authors for Ru catalysts (refs. 38-40). I t can be noted from Table
5, that the carburization o f molybdenum metal (sample S 4) decreases
by a factor o f fifteen the number of active sites while the specific surface area o f the material
i s only halved.
By contrast, the number of active sites for CO adsorption does not change very
at 773 K . I n a certain way, it seems 2 that the activation treatment stabilized the active surface of the catalysts. But the carburization much after carburization either by nC4-H2 or CO-H
must be controlled because extensive carburization with C O and H at 773 K leads t o asensible 2 decrease of the number of active sites and of the specific surface area. The activation treatment
by CO and H2 at 773 K i s less efficient than the one b y nC4 and H2 at 773 K , probably due t o the influence o f surface oxygen after
breaking o f the CO bond.
Selectivity Selectivity i s defined as the ratio of the quantity of a given product (extrapolated t o zero conversion) to the total quantity of hydrocarbons formed, also at zero conversion.
All the
molybdenum carbides have a selectivity resembling that o f ruthenium i n favor of methane formation. I n the same way, this selectivity can be monitored, i n a short range, by variation of pressures ratio (PH /P ) or temperature. For instance, with S 209 sample carburized by CO and H 2’ 2 co the selectivity i n methane varies from 66.38 (PH /Pc0=0.6, at 574 K) t o 92.446 (PH2/Pc0=3,
2
T=546
K). The
starting material (S 209) has a selectivity pattern very close to that of
molybdenum metal.
Both give hydrocarbons up to C8.
This suggest that the molybdenum
oxycarbide (S 209) could have some molybdenum sites favoring chain growth because o f its carbon deficiency. toward methane.
The carburization could correct this deficiency and change the selectivity
636
C O NCL US 1 0N Molybdenum carbide powders with high specific surface area exhibit favorable alteration of their chemisorptive properties in CO adsorption as compared to those of Mo, W and WC.
Consequently they can be classified among the best catalysts for hydrocarbons synthesis from CO-H2 mixtures.
Such catalytic materials could compete favorably with ruthenium which i s #
rare
and more expensive than molybdenum by a factor of more than twenty five.
ACKNOWLEDGMENT This work was initiated with support from the Center for Materials Research at Stanford University under the NSF-MRL Program. REFERENCES
I M. Boudart and R. Levy, Science, 181 (1973)547. 2 M. Boudart and L. D. Ptak, J. Catal., 16 (1970) 90. 3 D. F. Ollis and M. Boudart, The Structure and Chemistry of Solid Surfaces, G. A. Somorjai, i n J. Wiley (ed.), N e w York, 1969, paper 63. 4M. Boudart and D. F . Ollis, Surface Sci., 23(1970)320. th 5 L. Baudendistel, H. B&m, J. Leggler, L. Louis and F. A. Pohl, 7 Intersoc. Energy Conversion Eng. Cong., 1972, p. 20. 6 G. Schulz-Ekloff, D. Baresel and W. Sarholz, Conference on Catalysis, Roermond, 1974. 7 G. A. Mills and F. W. Steffgen, Catalysis Reviews 8(1973) 159-210. 8 P. N. Ross and P. Stonehart, J. Catal., 39(1975)298-301. 9 P. N. Ross and P. Stonehart, J. Catal., 48(1977)42. 10 R. J. Madix, Catal. Rev.-Sci. and Engrg., I5(1977)293. II J. H. Sinfelt and D. J. C. Yates, Nature Physical Science, 229(1971)27. 12 J. H. Sinfelt, Ann. Rev. Materials Science, 2(1972)641. 13 P. Biloen and G. T. Pott, J. Catal., 30(1973)169. 14 A. Cassi Faustin, J. Grimblot, M. Guelton and J. P. Beaufils, C. R. Acad. Sci., Paris, serie C t. 281 (1975) 487. 15 F. E. Massoth, J. Catal., 30(1973)204. 16 P. Schwarzkopf and R. Kieffer i n collaboration with W. Leszynski and F. Benesovsky, Refractory Hard Metals, Macmillan (ed.), N e w York, 1953. 17 E. K. Storms, The Refractory Carbides, Academic Press, N e w York, 1967. 18 L. E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York, 1971. 19 T. Yo. Kosolapova, Carbides, Plenum Press, N e w York, 1971. 20V. Sh. Palanker, D. V. Sokolsky, E. A. Mazulevsky and N. Baibatyrov, J. of Power Sources, I (1976/77) 169. 21 E. A. Mazulevsky, V. Sh. Palanker, E. N. Baibatyrov, A. M. Khisametdinov and E. I. Damanovskaya, Kinetika i Kataliz, 18(1977) 767-775. 22 A. H. Pfund, J. Opt. SOC. Amer., 23(1933)375. 23 K. Kimoto, Y. Kamiya, M. Nonoyama and R. Uyeda, Japan J. afAppl. Phys., 2(1963)702. 24 A. Tnsaki, S. Tomiyama, S. lido, N. Wada and R. Uyeda, Japan J. of Appl. Phys., 4 (1965) 707. 25 S. Yatsuya, R. Uyeda and Y. Fubana, Japan J. of Appl. Phys., II (l972)408. 26 N. Wada, Japan J. of Appl. Phys., 7(1968) 1287. 27 N. Wada, Japan J. of Appl. Phys., 6(1967)553. 28 R. L. Collins, M. D. Bell and G. Kraus, J. of Appl. Physics, 30(1959)56. 29 T. W. Haas, J. T. Grant and G. J. Dooley, J. Appl. Physics, 43(1972) 1853. 30 S. Yoshida, K. lmura and M. Boudart, Communication presented at the Fall Meeting of the California Catalysis Society, 1977. 31 J. E. Benson and M. Boudart, J. Catal., 4(1965) 704.
637
32 M. A. Vannice, J. E. Benson and M. Boudart, J. Catal., 16(1970)348. 33 J. J. Lander and L. H. Germer, Amer. Inst. Mining and Metallurgical Engineers Tech. Pub., 2259 (1947). 34 I. F. Ferguson, J. B. Ainscough, D. Morse and A. W. Miller, Nature, 202 (1964) 1327. 35 G. Hagg, Z. Phys. Chem., 12(1931)33. 36 H. Froitzheim, H. Ibach and S. Lehwald, Surface Sci., 63(1977)56. 37 J. B. Benziger, E. I. Ko and R. J. Madix, J. Catal., in press, 1978. 38M. A. Vannice, J. Catal., 37(1975)449. 39 R. A. Dalla Betta, A. G. Piken and M. S. Shelef, J. Catal., 35(1974)54. 40 G. C. Bond and B. D. Turnham, J. CataL, 45 (1976) 128. DISCUSSION MONTARNAL :
1)
It seems to me that for Fischer-Tropsch synthesis
there is an inhibition of the reaction by CO.
Your results show
that the CO chemisorption is much lower on molybdenum carbide than on the metal
(Tables 4 and 5).
So could it not be possible that
the activity increase you observe when going from metal to carbide, could be due to the decrease of inhibition by CO ? In other words,
the rate increase would be due to a decrease of the
adsorption of CO,
rather than to an increase of the reaction rate
constant. 2)
It also seems to me that Ru gives high molecular weights in F.T.
synthesis, when used under high pressure.
Do you think that your
analogy between molybdenum carbide and Ru could be extended also to this characteristic property of Ru ?
L. LECLERCQ:
1)
I think your suggestion is very interesting.
It
will need additional kinetic studies to determine the influence of CO partial pressure before and after the carburization of our catalysts. 2)
It was our intention to verify if, under high pressure,
the
analogy between molybdenum carbide and ruthenium could be extended to high molecular weight hydrocarbon formation in F.T.
synthesi~.
We have still to put our catalyst in sULtable texture and to solve some more technological problems. K.
KOCHLOEFL : Can you give more details concerning methanation
activity of molybdenum oxycarbide catalysts)
(comparison with usual methanation
?
L. LECLERCQ: Nickel is the most usual industrial catalyst for methanation reaction although Ruthenium has a higher activity. According to Vannice's paper
(ref.
38), the turnover frequency of
638 3S-1 compared to Ruthenium for methanation is 32 1020 3 3s-1 181 10(at 275°C, H = 3). Molybdenum oxycarbide can be 2!CO ranked within the activity range of metals such as Co, Rh, Pd between
Ni!AI
to 20 10-3s-~ MoOC S 209 has initially a turnover frequency of 10- 3s-1 (299°C ,H = 3). 2!CO After the activation treatment via a further carburization, the 3s-1, resulting Mo carbide has a turnover of 221 10quite identical 12
23
to the Ruthenium activity.
J. LEMAITRE
: You mentioned that carbon deposited on the surface of
carbides was responsible for their poor catalytic properties.
Do
you know some specific treatment that could activate a carbide catalyst poisoned with carbon ?
L. LECLERCQ : Classical regeneration treatments with oxygen on WCCMR exhibiting carbon deposition on the surface were attempted but gave oxide formation like W0 As shown by different authors 3. (Somorjai et al. and Wise et al.) carbonaceous compounds can react with hydrogen to produce methane.
Reductive regeneration seems
more appropriate for such materials with W or Mo.
P.A. SERMON: Since butane and C
1C8
hydrocarbons in the Fischer-
Tropsch synthesis almost double the carbon content of your molybdenum oxycarbides,
is it possible that you are seeing retention
of hydrocarbon-carbonaceous residues on the carbide surface. catalytic activity of such residues on metals G.
The
(e.g. A.S.A!Ammar and
Webb, J. Chem. Soc Faraday Trans. 1.,23..,195
(1978»might well
explain the substantial improvement in activity you observe after hydrocarbon treatment.
Have you examined the structure of the
additional carbonaceous material deposited by hydrocarbons ?
L. LECLERCQ : A retention of hydrocarbon-carbonaceous residues on the catalyst surface is certainly a possible explanation to the improvement in activity observed after carburization by hydrocarbon treatment.
Another explanation could be found.
For instance,
the carburization could result in a change of the surface composition leading to surface carbon content approaching the theoretical value for M0
Our Auger experiments based on the carbon peak has not 2C. made i t possible to determine the nature of carbonaceous species before or after the carburization treatment.
639 P.C.H.
MITCHELL:
One method which was used to prepare molybdenum
carbide was to expose molybdenum powder, in H
to CH
at 973 K.
prepared by reducing M00 3 Do you consider i t probable that whenever
4 2, Mo metal is exposed to a hydrocarbon above a certain temperature,
a layer of molybdenum carbide will form and that the real catalyst in reactions involving H2/hydrocarbon mixtures will be molybdenum carbide rather than Mo metal ?
L.
LECLERCQ:
Yes,
i t is quite probable.
Whenever Mo metal is exposed
to hydrocarbons in the presence of hydrogen above a certain temperature, molybdenum carbide is formed as demonstrated by Sinfelt (ref.
11).
He has shown that the specific activity of molybdenum
for ethane hydrogenolysis significantly increases during carbiding. The carburization seems to involve more than carbiding of the surface since the catalytic activity does not develop fully untill bulk carbide is formed.
We have also shown that carburization could
simultaneously lead to carbon deposit.
Anyway,
at low temperature
and low conversion Mo metal can be tested since i t exhibits a stable activity.
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641
A NOVEL NETHOD FOR THE PREPARATION AND PRODUCTION OF SKELETON
CATALYSTS
J. PETR6 Organic Chemical Technology Department, Technical University Budapest /Hungary/ ABSTFiACT A new method has been developed for the preparation of skeleton
c a t a l y s t s , the essence of which i s t h a t t h e a l l o y is decomposed in t h e s o l i d phase and t h e aluminium oxide hydrate formed remains partly as Ni-support in the catalyst. In this way metal catalysts on oxide support can also be prepared. Fundamental research work by metal microscopy, X-ray d i f f r a c t i o n , microprobe analysis, SII1IS, c r y s t a l l i t e s i z e d i s t r i b u t i o n invest i g a t i o n s i n conjunction with the development o f t h e c a t a l y a t are i l l u s t r a t e d by examples.
1. INTRODUCTION
More than 50 years have elapsed since t h e discovery o f skeleton c a t a l y s t s /1925/, but t h e i r importance and significance did not decrease. This i s indicated a l s o by s e v e r a l hundred publications and patents relevant t o the theme. However, i n t h i s period t h e e s s e n t i a l s t e p s of technology, s u i t a b l e for i n d u s t r i a l production, remained the same, and t h e s t a r t i n g alloy of commercial skeleton-Ni was a l s o mostly of t h e same composition, consisting i n about 50 : 50 % o f A 1 and N i . 2. New skeleton-Ni c a t a l m t k recent years we developed a skeleton-Ni catalyst,the activity of which i s considerably higher / i n a given case two- t o thre&old/ than t h a t o f t h e c a t a l y s t used up t o t h e present, p a r t i c u l a r l y i n t h e reduction o f t h e carbonyl n i t r i l e and n i t r o groups o f organic molecules, and can replace t h e said commercial c a t a l y s t . The s t a b i l i t y o f t h e new c a t a l y s t s i s a l s o high. I n t h e i r development t w o f a c t o r s played an important role: t h e elaboration of a new
642
process f o r the preparation of the c a t a l y s t s a n d t h e promotion of nickel.
2.1 New process f o r t h e preparation o f skeleton c a t a l y s t s (ref. 1) Conventional skeleton-nickel preparation involves t h e feeding o f t h e nickel-aluminium a l l o y i n t o a l a r g e quantity of a l k a l i n e solution, where t h e d i s s o l u t i o n of aluminium from the a l l o y proceeds i n an exothermal process, accompanied by vigorous hydrogen evolution and foaming. After t h e termination of t h e introduction of t h e a l l o y and of foaming, after-leaching o r leachings f o l l o w , which can take another 3 t o 1 2 hours. The feeding o f t h e a l l o y powder i n t o t h e a l k a l i n e solution is time consuming and cumbersome process. Moreover, t h e r e s i d e n t i a l time of t h e first batches of t h e a l l o y i n the a l k a l i n e s o l u t i o n i s considerably longer than t h a t o f t h e l a s t ones, which involves the hazard of a non-uniform product. The essence of t h e process developed by u s i s t h a t t h e a l l o y powder i s decomposed i n s o l i d phase a s follows. The a l l o y powder is homogenized i n a kneading device with a small quantity of NaOH, and, under continuous s t i r r i n g of t h e powder, water i s added a t a r a t e t o moisten u n i f o m i l y t h e substance without formation of a l i q u i d phase. Decomposition is performed by water, NaOH works as a c a t a l y s t , so f r o m aluminium mainly aluminium hydroxide i s formed a t t h e end o f t h e process. \%en decomposition i s terminated, giving about equal volume of water t o t h e mass, two s o l i d phases a r e separated, t h e lower of which contains t h e c a t a l y s t , while aluminium hydroxide i n t h e f o r m o f a white p r e c i p i t a t e i s i n t h e upper phase, and can be e a s i l y separated. The operation i s terminated by a b r i e f treatment i n a hot a l k a l i n e s o l u t i o n where hydrogen evolution i s already insubstantial. The aluminium hydroxide p a r t l y remains i n t h e c a t a l y s t p r e p a r e d a s described above, and serves as a supportfoPthe nickel. Accordingly, t h e s p e c i f i c weight o f t h e c a t a l y s t s i s lower than t h a t of t h e conventional c a t a l y s t , so by mixing it can be e a s i l y dispersed i n a l l the volume o f the r e a c t i o n mixture and even it i s e a s i l y settled and f i l t e r e d . The preparation process i s generally applicable for skeleton c a t a l y s t s and, r e f e r r e d t o t h e metal content, t h e a c t i v i t y of these c a t a l y s t s i s usually higher than t h a t o f prepared i n conventional way. Since t h e whole quantity of t h e a l l o y r e a c t s f o r an i d e n t i c a l period, t h e substance i s uniform. Technical advantages a r e as f o l l o w s : There i s no foaming during
643
t h e l e a c h i n g process, The volume of t h e necessary equipment and t h e time of o p e r a t i o n a r e about half of t h a t needed f o r conventional c a t a l y s t preparation. The s p e c i f i c m o u n t of NaOH i s a l s o lower. On introducing t h e new process, e x i s t i n g equipment can be used, only a n a d d i t i o n a l kneading device is needed. In conjunction w i t h t h e process discussed above, another one has a l s o been developed f o r p r e p a r a t i o n of c a t a l y s t s . C a t a l y s t s on oxide support a r e g e n e r a l l y prepared by t h e j o i n t p r e c i p i t a t i o n o f t h e hydroxides. This is followed by t h e washing and c a l c i n a t i o n of t h e hydroxides and t h e r e d u c t i o n of t h e metal oxide. Another p o s s i b i l i t y i s t h e impregnation of t h e support , where t h e i n s o l u b i l i z a t i o n i n water of t h e substance applied, washing, drying and repeated reduction a t high temperature a r e t h e u s u a l s t e p s . Whichever way is chosen, both involve s e v e r a l s t e p s , have a high volume demand and comprise various d i f f i c u l t operations. The process developed by us has t h e following e s s e n t i a l f e a t u r e s : From t h e given metal o r metals a n a l l o y is prepared i n t h e way usual f o r s k e l e t o n c a t a l y s t s e.g. with aluminium o r o t h e r a l k a l i - s o l u b l e metal o r metal combination, depending on t h e kind of support t o be prepared The a l l o y i s decomposed i n s o l i d phase, as described i n 2.1, e s s e n t i a l l y by water. Thus a metal on oxide support i s obtained i n a s i n g l e s t e p , I n a d d i t i o n t o t h e s i m p l i c i t y of t h i s process t h e c a t a l y s t prepared i n t h i s w a y i s considerably more a c t i v e t h a n t h a t of prepared by high-temperature reduction g e n e r a l l y i n t h e case o f as i n t h e process mentioned above t h e n i c k e l e.g. a t 620-670 K h i g h e s t temperature i s about 370-390 K (ref. 2). On t h e basis of t h e new procesees, a family of c a t a l y s t s can be b u i l t up from a l l o y s . S t a r t i n g possibly from t h e same a l l o y , e i t h e r an a c t i v e "supported" s k e l e t o n c a t a l y s t f o r liquid-phase hydrogenation t e r m i n a t i r g t h e p r e p a r a t i o n with a l k a l i n e s o l u t i o n treatment , o r an oxide supported metal c a t a l y s t f o r high-temperature gaseous phase r e a c t i o n s can be prepared according t o requirement.
.
,
Pig. 1 summarizes what has been said above.
2.2 Increasing t h e a c t i v i t y of skeleton-Ni by promotion (ref. 3 ) It h a s already been established that the activity of a nickel
c a t a l y s t i s enhanced by c e r t a i n promotors e.g. chromium, c o b a l t , manganese, molybdenum, e t c . Actually, s k e l e t o n n i c k e l c a t a l y s t s have been prepared w i t h some of t h e promotors, but t h e s e d i d not f i n d wide i n d u s t r i a l a p p l i c a t i o n .
.
644
aloy powder in solid phase leaching washing neutral New type of skeleton- catalysts for reactions in liquid phase
elimination pyrophority
0 of
washin neutra? drying
@a
New type of metal catalyst with high metal content /for li uid phase reactions/ 0
BO
New type of metal cotalyst on oxid-support with medium or small metal content for gasphase reactions @
Fig. 1. New w a y s f o r c a t a l y s t s preparation. A , B, C , D see ref.
I n r e c e n t y e a r s we have developed v a r i o u s skeleton-nickel types containing s e v e r a l promotors, which a r e adequately stable, while t h e i r a c t i v i t y i s under i d e n t i c a l c o n d i t i o n s 2-3-fold of t h a t o f skeleton-nickel of good q u a l i t y , p a r t i c u l a r l y i n t h e hydrogenation o f carbonyl and n i t r i l e groups. T h i s h a s been proved a l s o by the r e s u l t s of plant-scale experiments i n Hungary. 3 . Some results of fundamental r e s e a r c h work received d u r i n g c a t a l y s t s development. The a l l o y of s k e l e t o n c a t a l y s t s includes t h e potential catalytic properties, so that research work will begin with the investigation
of these alloys. For simple optical investigation of metal alloys metal microscopy is used. Etching of the polished surface with an alkaline or acidic
s o l u t i o n , v a r i o u s contours can be developed. Fig. 2 shows t h e micrograph of an A l - N i a l l o y , containing 180 and C r promotors, taken i n p o l a r i z e d l i g h t a f t e r a l k a l i n e etching. The f e a t u r e d s t r u c t u r e c l e a r l y shows t h e s e p a r a t i o n o f phases of d i f f e r e n t composition. Light s p o t s on t h e p i c t u r e presumably r e p r e s e n t #-phases, r i c h i n n i c k e l N i g l ; t h e d a r k e r i s t h e colour, t h e r i c h e r i s t h e phase i n A l . uch phases 3, may be e.g. t h e p-phase ( N U 1 3 ) , t h e Al-alloys of t h e promotors, e t c .
!
645
Fig. 2. O p t i c a l micrograph of skeleton-Ni a l l o y containing Mo and C r promotors a f t e r a l k a l i n e etching. X - r a y d i f f r a c t i o n can g i v e information on t h e metal o r a l l o y phases p r e s e n t i n t h e substance. Fig. 3/a shows t h e diffractograms o f a skeleton-Ni a l l o y containing t h r e e promotors Mo, C r , Co and Fig. 3/b o f t h e c a t a l y s t prepared f r o m it by t h e process described i n S e c t i o n 2.1. It can be seen t h a t t h e diffractogram o f t h e a l l o y i s very r i c h i n l i n e s . Only a few phases, mainly t h o s e present i n l a r g e r q u a n t i t i e s can be unequivocally i d e n t i f i e d . The i n t e r f e r e n c e o f t h e bands of a series of p o s s i b l e a l l o y phases does not permit a detailed analysis. On t h e diffractogram of t h e c a t a l y s t prepared from t h e given a l l o y (Fig. 3/b) t h e broadening o f t h e Wi-peak is i n d i c a t i v e o f t h e small
646
s i z e of t h e c r y s t a l l i t e s , and t h e r e i s presumably a l s o an i n t e r f e r e n c e between t h i s peak and those o f t h e phases containing promotors. Anoreover, a non-identificable peak appear6 on t h e p i c t u r e .
d ................................................... m s 40 1 2u
60
b+
ol
r CKU
.................................................... (0
M
u)
30
m
10
Reflection angle 2 8 ( 3
Fig. 3 . X - r a y diffractograms o f skeleton-Ni a l l o y containing Mo, C r and Co promotors 2 and of t h e c a t a l y s t made f r o m it according t o (ref. 1)
.
More d e t a i l e d but s t i l l q u a l i t a t i v e information can be obtained by microprobe analyser. F i g 4 shows t h e p i c t u r e s of a n a l l o y containing 50 w,t % of Al, 50 w t % of N i and no promotor, and o f t h e c a t a l y s t prepared f r o m it by t h e process described i n 2.1. The l i g h t e r is a spot on t h e "electron" p i c t u r e the higher of t h e atomic number or/and t h e concentration of t h e given element, and v i c e versa. The -phases of l i g h t colour a r e separated f r o m one another by ando( -phases, t h e l a t t e r containing only a f e w % of N i . The q u a n t i t a t i v e d i s t r i b u t i o n can be o f t h e phases r i c h i n A 1 ( b a n a d ) and r i c h i n Ni(7) estimated on t h e b a s i s o f t h e microprobe scans showing t h e Al and N i d i s t r i b u t i o n , r e s p e c t i v e l y , t o about 50-50 %. The microprobe scans o f t h e c a t a l y s t prepared from t h i s a l l o y were taken on p i l l s . It can be seen on t h e "electron" p i c t u r e t h a t t h e predominant p a r t o f t h e g r a i n s c o n t a i n s A 1 besides N i (mainly i n t h e form of t h e hydroxide o r oxide hydrate, see s e c t . 2.11, and t h i s i s supported a l s o by t h e microprobe scans o f A 1 and N i d i s t r i b u t i o n .
p-
1'
647
648
Using Mo and C r promotors t h e number of phases increases, as can be observed on t h e "electron" p i c t u r e i n Fig. 5 . The a n a l y s i s of t h e scans shows t h a t promotors have a t l e a s t double e f f e c t s , On one hand, they s h i f t t h e "equilibrium" o f the A 1 - N i phases t o t h e debit of the 7-phase towards t h e 6-phase, and on t h e other hand, new phases a r e formed with the promotors. The d i s t r i b u t i o n o f Mo is uneven, it i s alloyed by C r , and mainly w i t h aluminium. A Mo-Cr-Ni phase can also be observed, i n which A 1 i s present i n a small quantity or i t i s t o t a l l y absent. The C r f r a c t i o n unalloyed with hlo i s uniformly d i s t r i b u t e d i n t h e A1 and N i . The s t r u c t u r e o f t h e a l l o y mentioned above becomes more complicated consisting of a small amount o f s i l i c o n (Fig. 6). A l m o s t t h e t o t a l quantity of No i s alloyed with S i , a f r a c t i o n of C r i s a l s o present i n t h i s a l l o y , and t h i s Mo-Si-Cr a l l o y i s t o be found i n t h e phase r i c h i n Al, i n which _ t h e quantity o f N i i s minimal. Some o f C r and S i is present i n the nickel-aluminium phase. Fig. 7 shows the curves of a microprobe l i n e analysis f o r a skeleton c a t a l y s t containing Mo and C r promotors where Mo-Cr phase can be detected. In a c a t a l y s t containing Mo-Cr-Co promotors t h e changes i n composition going from t h e surface i n t o t h e bulk o f the g r a i n s has been investigated by SIMS. Results i n Fig. 8 show t h a t i n t h e external zone of about 5-15 nm o f t h e g r a i n s t h e concentration o f N i , C r and Co i s somewhat lower, while t h a t of Mo about t h e same as i n t h e bulk. This r e s u l t can be explained by t h e p e r i t e c t i c formation o f t h e grains. The crystallite s i z e d i s t r i b u t i o n and t h e thermal s t a b i l i t y of c a t a l y s t s containing s e v e r a l promotors and prepared according t o section 2.1 have been investigated on t h e b a s i s of X - r a y l i n e broadening. The r e s u l t s of these investigations a r e shown i n Fig. 9. Accordingly, the average diameter of t h e c r y s t a l l i t e s i s about 3 nm, and does not exceed even a f t e r heat treatment for t w o hours a t 870 K 6-7 nm, thus, thermal s t a b i l i t y is a l s o good, which presumably can be a t t r i b u t e d t o t h e residual aluminium oxide hydrate contained i n the catalyst.
649
Fig. 5 .
650
Fig. 6 .
651
Fig.
7.
652
SlMS curws of o promoted Skeleton Ni eotolyrl ( C K M )
CPS toe..
I : : : : : : :tinuend Current density-l@ A em’ 1min 50-x)O 1(depth)
Fig. 8. SIMS curves o f skeleton N i containing Mo, C r and Co promotors. The catalyst has been prepared according t o (ref. lj.
Y 100
80
Go
t
123 1. Room temperature 2 After annealing at 873 K, in Ar, 2 hours 3.After annealing at 873K, in He+ 10% H,, 2 hours
Lo 20 L
nm
600 1200 1800 2 4 0
Fig. 9. Crystallite s i z e distribution and thermal s t a b i l i t y o f skeleton-Ni catalyst containing Mo Cr and Co promotors, prepared according t o ( r e f 1).
,
653
ACKNOWLEDGEMENT We thank D r . A. KdlmBn, C e n t r a l Chemical Research I n s t i t u t e o f t h e Hungarian Acadelny o f Sciences for t h e X-ray d i f f r a c t i o n analysis, D r . J. Giber, P h y s i c a l Department of t h e Technical University o f Budapest f o r t h e SINS i n v e s t i g a t i o n s , and t h e Company BASF f o r t h e determination of c r y s t a l l i t e s i z e d i s t r i b u t i o n s .
WERENCES 1 Hung. Pat. US p a t . GFR Pat. 2 Hung. P a t . US Pat. GFR O f f . 3 Hung. P a t . US P a t . W R Off. D Hung. P a t . GFR P a t .
162 281
B
s e e Fig. 1
169 494 A 4 021 3 n 2 553 660 170 253 C a p p l i e d for 2 544 761. 1 6 1 915 22 09 000
s e e Fig. 1
3 809 658 2 209 004
s e e Fig. 1 s e e Fig. 1
DISCUSSION C.S. BROOKS : This paper is further indication of the unexploited potential for devising innovative variations on Raney and skeletal type catalysts even though it has been 51 years since their introduction by Marray-Raney.
I have a specific comment on the nature of
the distribution of the promotor elements Mo and Cr, as shown in the electron microprobe flgures in the paper.
There would appear to be
considerable segregation and heterogeneity in the distribution of the promotor elements in the nickel-aluminum alloy.
Equilibrium
phase diagrams are not immediately available to me, but I would inquire whether the concentration
of these promotor elements is not
exceeding the solubility limit in nickel for the conditions of solidification.
In our laboratory we have had considerable success in
obtaining high orders of homogeneity in the nickel phase by using controlled solidification of eutectic alloys.
We have produced pure
phases of NiA13 in fibrous configurations from directional solifidication of nickel-aluminum alloys. J. PETRO : Phase diagrams for systems like A1-Ni-Cr-Mo are not avai-
lable, but it is known that in solid phase Ni dissolves 37.5 wt%
MO
and (at least at higher temperatures, e.g. 1250°C) there exists a phase with composition of 35 wt%
Mo
+
42 wt%
Cr + 23 wt% Ni
This phase is in equilibrium with some Co-Mo phase.
(no A1 1).
So, at least in
654
simple alloys, the solubility of M O and Cr in Ni seems to be much higher than the concentration used in our alloys. J.W.E. COENEN
:
Could you give some indications on the method of
obtaining crystallite size distributions from X-ray diffraction lines
J.
?
PETRO
The ideal line profile of a (200) reflection has been
:
calculated by Fourier analysis (and Stokes method).
The measured
and calculated curves have been fitted using different crystallite size distributions until a good agreement for the two curves has been obtained.
J.F.
L E PAGE
:
Are your promotors M o , C r , Co efficient to improve
the thermal stability of your catalysts in the liquid phase in presence of small amounts of water J. P E T R O
Generally speaking, yes.
:
?
But in order to give a more
precise answer, you ought to tell the temperature and H 2 pressure these types of catalysts can be used until about 150-18QOC if the pressure is at least several times 10 atm. B. DELMON
:
There are two ways of considering aluminium (whichever
its chemical form) remaining in skeleton catalysts.
As alumina,it
could play essentially the role of a support or, more specifically
of a dispersing substance and of an inhibitor towards sintering. Aluminium could also play the role of an electronic promotor, with micro-domains of alumina or aluminium atoms being the electronically active agents. The picture one can also make of the ideal preparation procedure, i.e. preparation of the starting alloy and alkali leaching, depends a lot on the real reason of the activity of skeleton catalysts. If the first role is predominant, the best is probably to allow an incipient segregation of aluminium in the alloy thus "preshaping" the spacers, by a process similar to dispersion hardening, at the boundaries of the individual nickel crystallites. If the second role is predominant the maximum electronic effect is obtained for the maximum dispersion of aluminium. In relation to your preparation method, what do you think is the role of aluminium or alumina
?
J. PETRO : After our investigations the predominant part of the A 1
r e m a i n i n g i n t h e c a t a l y s t is a s a l u m i n a a n d s e r v e s a s a s u p p o r t
for the N i
( m e t a l l i c A 1 a s f r e e m e t a l c o u l d not b e in t h e c a t a -
l y s t , at most
i n N i - r i c h p h a s e s it c a n o n l y b e p r e s e n t i n v e r y
small quantities).
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MINISYMPOSI UM ON CATALYST NORMAL IZAT ION
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659
NORMALIZATION OF CATALYST TEST METHODS L. MOSCOU
Akzo Chemie Nederland bv, Research Centre Amsterdam, P.O.
Box 15,
1000 AA Amsterdam, The N e t h e r l a n d s .
ABSTRACT
A r e p o r t i s g i v e n o f o r g a n i z a t i o n a l developments i n t h e f i e l d o f n o r m a l i z a t i o n o f c a t a l y s t t e s t methods and 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 i n Europe s i n c e 1975. The f o l l o w i n g t o p i c s a r e discussed: ( I ) r e s u l t s o f t h e i n q u i r y i n 1975 on t h e need i n Europe f o r n o r m a l i z e d c a t a l y s t t e s t methods, ( 1 1 ) a r o u n d - t a b l e m e e t i n g i n 1976 and t h e f o r m a t i o n o f an ad-hoc Committee on c a t a l y s t t e s t i n g , ( 1 1 1 ) c o o r d i n a t i o n between t h e v a r i o u s c a t a l y s t groups by t h e IUPAC commission 1.6 on S u r f a c e and C o l l o i d Chemistry.
1. R e s u l t s o f 1975 i n q u i r y D u r i n g t h e 1 s t I n t e r n a t i o n a l Symposium on " S c i e n t i f i c Bases f o r t h e P r e p a r a t i o n o f Heterogeneous C a t a l y s t s " ,
t h a t was h e l d i n B r u s s e l s 3 y e a r s ago, a s p e c i a l
s e s s i o n on t h e s t a n d a r d i z a t i o n o f c a t a l y s t t e s t i n g was o r g a n i z e d b y D r . R. Cahen. The two m a j o r events d u r i n g t h a t s e s s i o n were a p r o g r e s s r e p o r t on c a t a l y s t s t a n d a r d i z a t i o n a c t i v i t i e s as o r g a n i z e d i n t h e U.S.A. C a t a l y s t s " 1 ) and i n t h e U.S.S.R.
by "ASTM committee D-32 on
and t h e i s s u e o f a q u e s t i o n n a i r e t o t h o s e
p r e s e n t a t t h e s e s s i o n . The purpose o f t h a t i n q u i r y was t o f i n d o u t how much need i s f e l t i n Europe f o r d e v e l o p i n g s t a n d a r d i z e d c a t a l y s t testmethods. The c o n c l u s i o n s drawn a r e based on 29 i n q u i r y forms r e t u r n e d o u t o f 100: a. A d e f i n i t e i n c e n t i v e e x i s t s t o develop s t a n d a r d i z e d testmethods f o r c a t a l y s t s . b. A number o f p e o p l e a r e w i l l i n g t o c o n t r i b u t e p e r s o n a l l y t o develop t h e s e methods e i t h e r b y s u b m i t t i n g t h e i r own method, by t e s t i n g proposed methods o r by a t t e n d i n g t e c h n i c a l meetings. c. No f i n a n c i a l s u p p o r t i s o f f e r e d . d. The most u r g e n t l y needed t e s t s a r e t h o s e t o measure physicochemical p r o p e r t i e s o f catalysts. e. C o l l a b o r a t i o n s h o u l d be e s t a b l i s h e d w i t h ASTM c o r n i t t e e D-32 on a c o l l e c t i v e and n o t on an i n d i v i d u a l b a s i s .
660
2 . The Round Table Meeting and the formation o f "ad-hoc committee"
The next step was the organization o f a Round Table Meeting. Hereto, D r . Cahen contacted the BCR t o advise on such organization; the BCR i s the Community Bureau o f Reference, being p a r t of the Commission o f the European Communities. As a r e s u l t , the "Round Table Meeting on C a t a l y s t Normalization" was h e l d i n the BCR-Offices i n Brussels on June gth,
1976, under the chairmanship o f the l a t e
D r . K.F. Lauer; here several experts gave b r i e f accounts on c a t a l y s t normalization a c t i v i t i e s t h a t were i n progress already i n many European Countries. The f o l l o w i n g experts accepted nomination i n an "ad-hoc committee on c a t a l y s t testing":M. K.S.W.
Baerns, R. Cahen, E.G.
Sing and
Derouane, W. van der E i j k , M. Michel, L. Moscou,
F. T r i f i r B .
During a f i r s t meeting o f t h i s committee on October 14th, 1976, i t became c l e a r how d i v e r s i f i e d the actual normalization a c t i v i t i e s i n Europe were already; i t became c l e a r a l s o t h a t the "committees primary task should be t o coordinate the e x i s t i n g a c t i v i t i e s i n t h i s f i e l d , more than organizing new a c t i v i t i e s " . The f o l l o w i n g aims o f the ad-hoc comnittee were s p e c i f i e d : The "ad-hoc comnittee should provide a basis a. f o r t h e cooperation between the various bodies concerned w i t h the normal z a t i o n o f c a t a l y s t t e s t i n g and f o r the p r o v i s i o n of reference materials; b. f o r the c r i t i c a l assessment o f t e s t i n g procedures and f o r the s e l e c t i o n o f appropriate reference materials; c. f o r the c o l l e c t i o n and dissemination o f information gained i n these a c t i v i t i e s . I t was a l s o concluded t h a t , t o support these objectives, i t i s d e s i r a b l e t h a t an i n t e r n a t i o n a l organization w i l l promote, coordinate o r patronize f u r t h e r
a c t i v i t i e s , and t h a t these o b j e c t i v e s a r e beyond the scope o f the BCR. 3. IUPAC The I n t e r n a t i o n a l Union o f Pure and Applied Chemistry ( I U P A C ) appears t o be an organization t h a t i s w i l l i n g and able t o p l a y such a coordinating r o l e . During i t s general assembly i n August 1977, "IUPAC comnission 1.6. on c o l l o i d and surface chemistry" has shown general i n t e r e s t t o become a c t i v e i n t h e area o f heterogeneous c a t a l y s i s " 2 ) . During t h i s session two members o f the ad-hoc committee (R. Cahen and L. Moscou) reported on the a c t i v i t i e s of normalization bodies. A formal request from t h e ad-hoc committee t o IUPAC lead t o t h e decision t h a t IUPAC comnission 1.6. w i l l s t a r t t o coordinate the normalization a c t i v i t i e s by dessimination o f information between a l l bodies, a c t i v e i n the f i e l d . This w i l l be achieved by r e c e i v i n g minutes and progress r e p o r t s from each body and by submitting copies o f them t o a l l o t h e r bodies. For address of the secretary o f IUPAC comnission 1.6 see r e f . 3).
661 Each group which i s a c t i v e i n the f i e l d of normalization o f c a t a l y s t methods o r on c a t a l y s t reference materials, and which i s n o t y e t contacted by IUPAC, i s i n v i t e d t o contact the secretary. 4. Present symposium
The present I n t e r n a t i o n a l Symposium i s a good opportunity t o present the progress made both i n the organizational area as given i n the above paragraphs, and i n the development o f normalized c a t a l y s t methods. We expect representatives o f normalization bodies i n Belgium, Great B r i t a i n , I t a l y , U.S.A.
and U.S.S.R.
t o r e p o r t on t h e i r aims, a c t i v i t i e s and r e s u l t s obtained
so f a r .
S c i e n t i f i c aspects o f normalization w i l l be presented i n papers by Broekhoff and by Scholten; they w i l l discuss backgrounds t h a t p l a y a r o l e i n normalization o f pore s i z e d i s t r i b u t i o n and chemisorption methods respectively. We hope and expect t h a t t h i s symposium w i l l c o n t r i b u t e t o a r a p i d development o f normalized c a t a l y s t methods and t h a t i t w i l l s t i m u l a t e f u r t h e r i n t e r n a t i o n a l cooperation and encourage l i a i s o n between the various groups. REFERENCES 1) J.R. Kiovsky, "Progress Towards Standardized C a t a l y s t Testing i n the U.S.A.
ASTM-committee D-32 on Catalysts". Symp. on t h e S c i e n t i f i c Bases f o r the Preparation o f Heterogeneous Catalysts , Brussels , October 1975. 2) IUPAC commission 1-6, Subcommittee on c a t a l y s t a c t i v i t y , r e p o r t o f meeting h e l d 2.15 p.m., 12th August 1977 i n the Palac K u l t u r y i Nauki, Warsaw. 3) Prof. D r . J . Lyklema, Secretary o f IUPAC 1.6, address: Lab. f o r Physical and C o l l o i d Chemistry, State A g r i c u l t u r a l University, De D r e i j e n 6, Wageningen, The Netherlands.
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663
MESOPORE DETEREIINATION FROM NITROGEN SORPTION ISOTHERMS: FUNDAMENTALS, SCOPE, LIMITATIONS J.C.P. BROEKHOFF Unilever Research Vlaardingen, P.O. Box 1 1 4 , 3130 AC Vlaardingen The Netherlands
SUMMARY
The thermodynamic basis for obtaining pore-size distributions from nitrogen sorption isotherms is reviewed. It is shown that pore volume and pore surface area can be determined from sorption isotherms, but only in the simplest of cases. The basic irreversibility of sorption in the hysteresis region is even then a complicating factor. In actual practise, irregularity of pore shape and interdependence of pore domains in the porous structure necessitates the use of rather drastic simplifications. A s a result, the outcome of the analysis is greatly dependent upon the set of approximations used, and this should be borne in mind in drawing conclusions from the obtained data.
INTRODUCTION Nitrogen sorption isotherms are widely used for obtaining information about the porous structure of heterogeneous catalysts in the mesopore-size region. The relative simplicity of the experimental determination and the apparent ease of interpretation of the experimental data account sufficiently for the popularity of the method. In this paper we will not discuss the experimental methods as these have been described in detail elsewhere [ I ] ,
but we will restrict ourselves
to a general discussion of the scope and limitations of the information on porous structures which can be obtained from the experimental data. A porous structure can be considered as a collection of elements in pore space, each characterised by shape, size and relative allocation with respect to other elements in pore space. In practice, our information on pore structure will seldom be as detailed as that. In most cases we will have to be content with information concerning size and number of elements in ?ore snace. The shape of the pores for actual porous structures is often difficult to define and the information concerning pore shape that can be obtained from nitrogen sorption data, is rather limited. It is common practice to assume a predominant pore
664
shape rather than to establish it independently. Yet, it is to realised that the quantitative data on pore size obtained from nitrogen sorption data are often critically dependent upon the assumed pore shape. Information on the relative location of different elements in pore space can only be obtained in exceptional cases. Again, it is common practice to assume independent pores; i.e. the pores are assumed to be in direct communication with the external environment. If this condition is not obeyed in an actual case, it will render the common procedures for obtaining pore size data from nitrogen sorption data invalid. Even in the simplest case of independent pores of simple well-defined shape, there are difficulties in interpreting nitrogen sorption data in terms of porous structure. Pore size analysis is traditionally based upon the application of Kelvin's relation between vapour pressure of a capillary condensed phase and pore size. Kelvin's relation was not intended for pores in the mesosize range, as it basically ignores physical sorption of the adsorbate on the pore walls. The latter phenomenon necessitates additional correction terms to Kelvin's relation, as was first pointed out by Deryagin [Z]
as early as 1940 on the basis of
thermodynamic reasoning. This has largely been ignored by subsequent authors in the field. Evidently, there is ample reason to examine the basis for obtaining pore size data from nitrogen sorption isotherms. DEFINITION OF PORE SIZE Pore size is an intuitive concept, with obvious meaning for simple geometric pore shapes such as slits, cylinders and spherical cavities. Actual pores may have highly irregular forms with "sizes" quite different in different directions, best to be described by the stereological method [3].
Unfortunately, the concepts
of stereology are difficult to translate to the analysis of nitrogen sorption
isotherms. As will be shown subsequently, nitrogen sorption data basically yield information on volume and surface area of elements in pore space. Thus, it is most convenient in the present context to define pore size F$
as the ratio
between volume and surface area of a certain element in pore space:
% = v/s The
F$
thus defined would equal d/2 for a slit of width d, r/2 for a cylinder of
radius r, and r/3 for a spherical cavity of radius r. For an arbitrary pore shape, % may be taken as a characteristic size. One may consider this type of information to be insufficient for certain purposes, but one cannot hope to obtain more detailed information from nitrogen sorption isotherms. The BET-theory will readily yield an estimate of the total surface area of the porous system from the nitrogen sorption isotherm, SBET. One may argue over the theoretical significance of the BET-equation [ 4 ] , but its practical validity
665
for obtaining a reliable estimate of the total surface area is only debated in cases where micropores (with sizes
153.
near saturation becomes asymptotical, and V lacks clear definition. Common P practice of defining V as the volume adsorbate taken up at a certain relative P pressure (e.g. p/po = 0 . 9 6 ) is then potentially dangerous and should be exercised with care. A somewhat better criterion may be the uptake at the upper closure point of the sorption hysteresis loop, but one should then be certain that this volume does not depend upon the final uptake around saturation reached experimentally. In the simplest case of a clearly defined V
P’
one may define a mean
pore size R * m‘ Rm = Vp/SBET It should be realized that SBET in principle also contains, the surface area present in macropores (\
> 100
nm), pores which in practice will not be filled
.
around saturation and will not contribute to V In many cases, the contribution P of macropore surface area will be small, and thus will not lead to serious errors in the estimation of R but in the absence of a clearly defined mesopore system, m’ R as derived from nitrogen sorption data becomes meaningless, (for an illustram tion, see Fig. 2 ) . In some cases, e.g. that of pelletised catalysts, there is a
.
very simple way of obtaining V The total catalyst volume can then be obtained P either by measuring accurately the dimensions of the catalyst o r by determining its apparent density by imbibition in mercury at reduced hydrostatic pressure [ 6 ] . A determination of the volume of the solids forming the skeleton of the
pore system, e.g. by helium or by imbibition into a wetting liquid, then leads to an estimate of the total V including macropores. The method again is liable P’ to errors: helium adsorption at room temperature may not always be neglected, imbibition liquids such as water may chemically react with the porous solids. Thus, a measure of the mean pores size R
is obtained with relative case under m certain restrictions. For some purposes, R will suffice as a characterisation m of the porous system, e.g. in routine quality control in the production of
heterogeneous catalysts. Often, more detailed information is required on the distribution of pore sizes. Then, one has to take recourse to interpretation of the complete nitrogen sorption isotherm by means of thermodynamic methods.
THERMODYNAMICS OF VAPOUR SORPTION IN CAPILLARIES Nitrogen sorption in mesopores around 78
OK
generally shows hysteresis: it
1.o
1.o
VAIVSAT
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
VA I V SAT
P I PO I
0
I
0.2
I
I
0.4
I
I
0.6
I
I
0.8
1:o
FIG. 1. Nitrogen sorption isotherm for a porous silica: well-defined mesopore system.
P IP O I
I
0
I
0.2
I
I
0.4
1
1
1
0.6
1
0.8
1
J
1.0
FIG. 2. Nitrogen sorption isotherm on chemically corroded glass spheres of around 50 pm diameter :
no clearly defined mesopore system.
667 is found that less nitrogen is taken up at a certain nitrogen relative pressure after increase in relative pressure from a low value than after a decrease in pressure from saturation. This hysteresis is both a fundamental difficulty in interpreting sorption measurement and a source of information on the size and even shape of the porous structure elements. It is usual practice to interpret hysteresis in terms of Kelvin's relation for the capillary condensate in the pores [7].
A s stated before, this is not permissible, as Kelvin's equation ignores
physical adsorption in the mesopore system. Yet, without physical adsorption capillary condensation below saturation can be shown to be extremely unlikely [ 8 ] , at variance with general experimental evidence. Kelvin's relation is most
easily derived from thermodynamic considerations. It is now clearly necessary to include physical adsorption into the thermodynamic treatment of capillary condensation and evaporation, in order to see in what sense Kelvin's relation should be modified. Different lines of reasoning are possible, but as long as they start from the same assumptions concerning the properties of the porous system, they should, and indeed do, lead to the same final relations, as only the first two laws of thermodynamics are invoked. A general treatment can be based upon the Gibbs Maximum Work principle, as follows: Consider the experimental set-up for measuring nitrogen sorption isotherms depicted in Fig. 3 : An adsorbent at temperature T is in equilibrium with a vapour phase at a certain (low) pressure p in a volume V, composed of a vapour phase volume V and a g g The complete nitrogen sorption isotherm can volume of the solid adsorbent V
.
now be traced by compressing the vapour phase with a frictionless piston, thereby increasing the pressure and reducing V until the saturation pressure g' ps is reached. In the absence of adsorption or capillary condensation, the volume
V
at each pressure p can be calculated, e.g. in the case of an ideal vapour g g from:
where N
-
t
is the total number of moles vapour in the system and R is the gas
constant. In practice, V
g
is found to be smaller, as N
moles are adsorbed in
the porous system:
pg Vg
=
(Nt - Na)
T
(4)
A simultaneous measurement of p
and V g
not only yields the com?lete sorption g
isotherm, but also the work done on the system U, in reaching saturation:
668
FIG. 3. Illustration of the relation
between Work of Adsorption and the adsorption isotherm.
. pgat PS .... ... ..... . .. ..**
.*..a
....... ............. ..... . ~ . . . . ... . . . .. ... FIG. 4. Change in state of a porous system accompanying capillary condensation. The decrease in interfacial tension at the pore wall from usg to ultimately uSl is the driving force for capillary condensation.
669 PS
w = -j
pg d V
g
= N
t
ET
In (ps/pi) +
Pi
ET
p’
(Ns-Ni) -
Na
k
T d In p
(5)
g
Pi
.
where N
and N signify the values of Na at pi and p For simplicity, the molar i volume of the adsorbate has been neglected in comparion with that of the vapour.
E q . (5) illustrates that an adsorption measurement in essence is a measurement
of the work of adsorption, represented by the last term of Eq. ( 5 ) . The latter
quantity is readily determined from any experimental isotherm. Gibbs Maximum Work principle now simply states that W is expended in increasing the free energy F of the system:
(6)
W)F s - F .1 As illustrated in Fig. 4 , F
and F. may simply be enumerated in terms o f the
fundamental thermodynamic properties of the porous system and that of the vapour phase. At pi, the porous system contributes an amount u this is u
S towards F
sl
.A
sg
S to Fi, at saturation
detailed calculation of F from first principles leads
to:
Here, a stands for the interfacial free energy per unit surface area. Its magnitude depends upon the amount adsorbed at the interface, and thus upon p
g’
Combi-
nation of E q s . (5), (6) and (7) yields the completely general relation:
Pi during adsorption
S>,
u
sg
- u
(8)
sl
Conversely, during desorption gradual lowering of the pressure causes the system to perform work W
\<
F
s
- F.. 1
This work stems from the decrease in free energy of
the system, and an integration along the desorption branch of the hysteresis loop results in: PSRT,(
s<-
Nadlnp Pi u - u sg sl
g during desorption ( 9 )
670 A s S is the true mesopore surface area, it appears that the adsorption branch of
the hysteresis loop leads to estimates of the pore surface area which tend to be low, whereas the desorption branch of the hysteresis loop results in estimates of S which tend to be high. Only in the (exceptional) case where there is no hysteresis do the two estimates coincide with the true mesopore surface area. Eqs (8) and (9) may be compared to the well-known relationship which is due to Kiselev [9]
and applied to the calculation of mesopore size and surface area
by Brunauer et al. [lo] :
s =
It is then clear that (10) is a special case of (8), (9) if? sorption in the
b u - a equals u the liquid sg sl kz' surface tension under all conditions. A s stated before, reversibility is excep-
mesopore system is completely reversible and
tional. It can further be demonstrated that condition b is only satisfied if physical adsorption in the multilayer region is negligible. This is certainly not the case for physical adsorption of nitrogen at 78
OK.
For a single pore emptying at a well-defined pressure pd, and of pore volume
V and pore surface area S , equation (9) may be simply integrated to:
(Osg - u s l ) v1
v .t s
>, .
ET
In (pS/pd)
Obviously, again Eq. (9a) reduces to the wellknown Kelvin-equation if one substitutes u
Ig
for u
sg
-
usl. Eqs. (8) and (9a) may thus be viewed as "corrected"
Kelvin-relations. The practical app ication of Eqs. (8) or (9) requires a quantitative knowledge of u
sg
- usl.
We will not indulge in attempts to calculate this quantity from
as u - a can be sg sl obtained directly from experimental data for solid surfaces with small curvature.
molecular models of physical adsorption (e.g. in Ref.[]]]),
The t-curve of multimolecular nitrogen sorption for oxides [12],
measured on non-
porous particles, essentially is a direct determination of the number of moles adsorbed per unit surface area N / S as a function of p /p in the case that no a g s pore filling takes place even at high relative pressures. A line of reasoning similar to the one given above then leads to the expression:
671
- u sl at any pressure pi clearly is a function of pi and can be disg rectly found from a t-curve by integration along the isotherm. This integration
Thus, u
is facilitated by using a numerical expression for the t-curve, such as the (see Fig. 5 ) :
empirical one [13,14],
In (p /p ) = -0.371/t2 + 0.387 exp(-1.14 g s
t)
(12)
An expression of a somewhat simpler form was proposed by a number of authors, of the general appearance:
[15,161
In these equations t represents the statistical thickness of the adsorbed layer, defined as: (14)
t = Na vl/S
where v1 is the volume of one mole of liquid adsorbate. Substituting (13) and (14) into ( 1 1 ) leads to:
u
sg
- u
sl
+
= u
g '
c n ET v1 (n-1)
where ti is calculated from (13) at the pressure pi. Eq. (15) demonstrates that at high pressures p. (and thus at large values of t or for large values of I$, i u - u tends to u and thus the thermodynamically correct expressions (8) sg sl Ig and (9) tend to the Kiselev-Brunauer form of Kelvin's relation (10). For nitrogen 3 sorption on oxides at 77.6 OK, reasonable estimates for C and n are 0 . 2 5 nm and 3 respectively. This means that e.g. at a relative pressure of 0.5 the last term in Eq. (15) exceeds the value of u
l8
by approximately a factor of 2 , and
thus serious errors are incurred when using Eq. (10) instead o f the rigorous Eqs. ( 8 ) or (9).
CALCULATION OF PORE SIZE FROM RELATIVE PRESSURE OF CAPILLARY DESORPTION The treatment of the preceding paragraph gives the formal thermodynamic framework for estimating pore size from the experimental pressure of capillary desorption, and shows how t h e key thermodynamic quantity u - usl can be calsg culated from the experimentally determined t-curve of multimolecular nitrogen sorption. It is essential to point out, however, that negligible curvature of the pore walls forms the basis for the existence of the t-curve. It seems reasonable to assume that the same values of u
sg
-
usl will hold for slit-shaped pores with
4.0 t nm
40
10-3 Nlm
3.6 3.2 30
2.8
2.4
2.0
20
1.6 1.2 10
0.8
0.L PI PO
C
PI PO
I
I
I
I
J
I
I
I
I
02
0.4
06
0.8
1.0
0.2
0.4
0.6
0.8
FIG. 5 a. t-curve of multimolecular nitrogen adsorption on oxides at 78OK.
I
1.0
FIG. 5 b. Change of u - usl with pressure, sg calculated from a. with the aid o f Eq. (11).
673 sufficient width, but there is no a priori reason that the same values will hold for cases where the pore wall curvature is of the same order of magnitude as the reciprocal of the characteristic dimension of the pore. Thus, while Eqs. (8) and
(9) are still rigorously valid, lack of knowledge of the key quantitity u sg - u sl will bar any simple direct application. It is at this point that any modelless method will break down, and we will have to adopt lines of reasoning which are semi-thermodynamic at best, the use of which can only be justified by their ultimate practical success. Three assumptions have to be made:
-
Pore geometry can simply be described by either of the following geometrical models: slits, cylinders, spheroidal cavities.
-
Desorption from any individual pore takes place at a single well-defined
-
pressure pd, leaving behind an adsorbed layer of thickness t d' The adsorbed layer can be treated as an adsorbed layer of the same thickness t as that on a non-curved pore surface, except for the influence of the curvature of the interface between adsorbed layer and vapour, which is supposed to possess an interfacial tension u
1g' For the sake of generality, the experimental t-curve will be represented by:
In (pg/pS) = -F(t)
(16)
It can then be shown that for the pressure of desorption the following
relations will hold: [17,18]
where k = 1 for slits, k = 2 for cylinders, k = 3 for spheroidal cavities. Uninviting as these equations may appear, they readily demonstrate that:
-
the relation between pore size and pressure of capillary desorption is a complicated function of the t-curve of multimolecular nitrogen adsorption as
-
well as of the pore geometry, in this case the pore wall curvature. even if the equations are correctly solved (e.g. by numerical methods) then the estimates of R will tend to be lower limits of the actual values. This is a consequence of Eq. (9) which states that any intrinsic irreversibility
674 in the process of capillary evaporation will lead to estimates of the pore surface area which are too high.
-
for all but the slit geometry, even the thickness of the adsorbed layer in an otherwise empty pore will deviate from its experimental value, in this case the t-curve on non-porous surfaces.
IRREVERSIBILITY: O R I G I N S AND CONSEQUENCES OF HYSTERESIS The existance of a hysteresis loop in the nitrogen sorption isotherm, even for the most careful and accurate determinations, points to the fact that either the adsorption branch or the desorption branch or both are essentially irreversible in nature. We have already discussed the consequence of irreversibility for the accuracy of determination of pore sizes from vapour sorption measurements. A general treatment of the origin of hysteresis has been given by Everett [ l g ] . Here we are faced with the problem of asserting which of the branches of the hysteresis loop is reversible and thus is best used for the determination of pore sizes. It has since long been realised that scanning of the hysteresis loop, i.e. reversal of the sorption direction at any point on either the adsorption or the desorption branch leads to valuable insight into the origins of hysteresis. The theoretical framework for interpretation of scanning behaviour has been impressively developed by Everett et al. [ZO].
Here it suffices to recall that most of
the evidence available [21] strongly points to the view that in general both adsorption and desorption branch are irreversible in nature, and thus that application of equilibrium thermodynamics to either of these branches will lead to over- or underestimates of pore sizes for fundamental rather than experimental reasons. Again, further progress can only be made by adopting specific simple pore models. For uniform cylinders open at both ends, irreversibility during filling by capillary condensation, may be viewed as the difficulty associated with bridging the gap between a situation where the pore is empty apart from the presence of an adsorbed film, and a completely filled pore. The filling process entails a number of intermediate stages with higher free energies than either the initial or the final stage, and thus spontaneous filling is retarded. It may be shown that ultimately these free energy barriers vanish once the thickness of the adsorbed film exceeds a critical value t
cr ’
formally given by: [I71
The pressure at which this occurs again depends upon the pore size and can be calculated from (18). For slits (k
= l),
there is obviously no critical thickness
of the adsorbed layer, and pore-filling is viewed as the process of merging of opposite adsorbed layers once their thickness approaches half the pore width, or
676
at a somewhat earlier stage [22]. (k
=
For cylinders (k
=
2) and spheroidal cavities
3 ) pore-filling may be described by Eqs. (20) and (18). Everett and Haynes
[23] have described hydrodynamic rather than thermodynamic criteria for spontaneous pore-filling. As they did not explicitly account for adsorption phenomena, their treatment seems to be less suitable for the analysis of mesopore sizes. A more detailed analysis of the desorption process for uniform cylinders leads to the concept that desorption essentially is a reversible process, [21] except for the very last stage where the two menisci approaching each other from opposite sides of the cylinder finally merge, resulting in breaking of the meniscus. If this picture is correct, then Eqs. (19) and (18), after omission of the inequality sign, can be used for calculating pore distributions from the desorption branch in the case of open cylinders. The same holds for slits (k = I ) , but for spheroidal cavities (k
=
3 ) the status of Eq. ( 1 8 ) is uncertain to say the least.
No picture has as yet been proposed for the reversible evaporation from spheroidal cavities. For pores of irregular shape, one can seldom expect evaporation to be a reversible process. Therefore, according to Eq. (9), application of equilibrium thermodynamics to the calculation of pore sizes from the desorption branch fundamentally leads to underestimates of pore size. Conversely, the adsorption branch will lead to overestimates, unless a relation of the type of Eq. (20) is employed
.
The next step is to remove the constraint that all pores are independent, viz. that they communicate independently with the external environment. One may e.g. simply view the porous system as a mutually interconnected network of pores of different sizes. In such a network, emptying of an individual element in pore space can only occur if there is at least one connecting pathway to its external environment containing pores which have already emptied. According to Ksenzhek [24],
emptying of e.g. a porous grain or a tablet of catalyst, will take place
by first emptying the outermost shells of the network before evaporation from the interior of the catalyst can take place. The wider the range of pore sizes and the larger the size of the network, the larger will be deviations from reversibility for evaporation from any pore in the interior of the network. In actual pore systems, this latter situation may be rule rather than exception. Relatively little work seems to have been done yet on this aspect of determining pore sizes in real systems. Notice that the actual position of the elements in pore space with respect to each other now becomes of predominant importance, and that not much is known about that in actual systems [25]. There is reason to believe that for pore filling during adsorption, network effects are much less important. Next comes the question of the lower limit in hysteresis. It was noticed long ago that for all adsorbates there seems to be a critical relative pressure below
676
which hysteresis in amechanically rigid and chemically inert pore system is not OK this seems to be around p /p = 0 . 4 2 . It is thus g s generally accepted that hysteresis does not occur in pores below a certain size.
observed. For nitrogen at 78 [26].
But the real situation seems to be less simple than that. There is good
reason to believe that a true capillary condensed state cannot exist below a critical relative pressure, and that around that relative pressure, all pores, independent of their size, are emptied of capillary condensate, although adsorbed films on the pore walls remain stable. Around that relative pressure the equilibrium thermodynamics relations for capillary condensation and evaporation do break down. The fundamental cause does seem to be the assumption of an incompressible fluid for the capillary condensed phase [ 2 7 , 2 8 ] . Real liquids are compressible and can be brought in a state of hydrodynamic tension up to a certain limit. Below pg/ps
=
0.42 the tension acting on the capillary condensed state is believed to
have exceeded its limiting value. Thus, around that relative pressure a steep descent in the desorption branch
is erroneously attributed to a peak in the pore size distribution around 2.5 nm. Actually, pores wider as well as smaller than that size may be show to contribute to the desorption phenomenon 1223, and assignment on the basis of equilibrium thermodynamical equations is fundamentally invalid.
OBTAINING THE PORE S I Z E DISTRIBUTION From the foregoing discussion, the necessity of proceeding with the utmost caution in any attempt to obtain pore size distributions from nitrogen sorption data, will be obvious. The earliest, and still most frequently used, method is that of Barret, Joyner and Halenda [ 2 9 ] , based on the cylindrical pore model. It is in essence a numerical approximation of the basic integral equation of Wheeler [30]. Let us call Lk the total length of pores with radius between 5, - A% and 5, + AR,, mean size R,, empyting over any interval p k-Ak to 'k+Ak' We may then write:
where V. is the total volume of adsorbate at a point on the isotherm pi/p
S'
the
lower bound of the i-th pressure interval, and ti the thickness of the adsorbed film at that pressure. Eq. ( 2 1 ) may be solved for Li:
By dividing the isotherm into suitable pressure intervals and working downwards from saturation, the complete distribution of L. (R.) and thus of pore volume and 1
1
677
surface area may be obtained, provided pressure intervals are taken small enough (usually ranging from 0.005 to 0.02 relative pressure units). The equation is specific for cylinderical pores, although the equation may be easily reworked for slit-geometry [31]. its application: The value of
%
A number of remarks can be made concerning
is the size of pores emptying over an assigned pressure interval.
Its magnitude is usually calculated from the Kelvin-equation. As we have seen, this is not permitted for mesopores. For the desorption branch of the isotherm, a solution of value of
%
% must be
found from Eqs. (18) and ( 1 9 ) . For adsorption, the
from Eqs. (18) and (20) may be adopted.
According to Eq. (181, the value of ti is not only determined by pi/ps but also by %, and is thus different for each group of pores under consideration. This
, as calculated from Eq. 'k (la), involving a considerable amount of additional work.(This may be done once and the results used subsequently, if a standard division of the isotherm in
means that in Eq. (22) ti has to be replaced by ti
intervals is adopted. A computing device is indispensable). An accurate estimate of V is essential. As discussed before, this may pose P problems in cases where the isotherm approaches asymptotically towards saturation. Evaporation has to be basically reversible, otherwise incorrect values of will be used and incorrect values of 4,will be obtained. Any incorrect
s
4,
assignment will influence the value of L. calculated in a subsequent step: This is a basic (and somewhat disturbing) property of the pore size distribution calculation. If for some reason the values found for 4, from preceding steps were e.g. too low, then Eq. (22) will "generate" in subsequent steps values for
L. which are too high and thus pore volume which is totally unrealistically assigned to the smaller pore sizes. It has not been u n c o m n to sum i = k : l
71
Rk 4,
and to proceed from saturation downward until an interval is found for which 'cum = SBET. This is a dangerous procedure, as in all cases where L. initially was underestimated, extending the pore size distribution will finally lead to equality between S and SBET by the said "generation" of pore surface area, CUm
but to physically unrealistic results. It is better to end the calculation at the lower closing point of the hysteresis loop, but in any case around values of ps/po = 0.5 due to the final breakdown of the capillary condensed state.
An even more fundamental problem is the proper assignment of pore shape. All the equations depend heavily upon the assignment of regularly shaped cylindrical pores, and actually are rather sensitive to the choice of geometry. Some authors have argued that a model of regular sphere packing will be more realistic in many
678 cases [32,33].
This does not seem to solve the problem, as we are still faced
with the problem of accounting for the influence of pore geometry on capillary equilibria, a problem which seems rather more difficult to solve than in the case of simple pore geometry. Therefore, we prefer to adopt the latter approach for the time being. ANALYSING THE NITROGEN SORPTION ISOTHERM In practice, evaluation of nitrogen sorption data in our laboratory proceeds roughly as follows: 1. General inspection of the isotherm. We assume that the measured isotherm is
smooth and shows no unexpected irregularities. Otherwise, a repeat determination is imperative. Permanent hysteresis at relative pressures below pg/po = 0.4 is highly suspect of experimental error and renders further interpretation hazardous.
2. Determination of SBET from a BET-plot, ideally over the pressure range 0.05
< p /p < g o
0.3. Restricted linearity over this pressure range is a warning
to proceed with caution and may e.g. be due to the presence of micropores (see under 4b). 3 . Determination of the BET C-value.
For inorganic oxides this should be around 100. Values far in excess of 100 often point to the presence of micropores, which render the physical significance of SBET doubtful. Values substantially lower than 100 may point to nonoxidic surfaces. This means that the t-curve of De Boer et al. [I21 may not be valid for the system under consideration and t-curves should be determined separately for the class of materials under investigation [34].
4. Making a t-plot of the adsorption branch of the isotherm [35], Fig. 6. This should be done over the pressure range 0.08 < p /p < 0.86. g
o
In principle, three
different types of t-plot should be distinguished (see Fig. 6). Type a., the straight line, either points to macropores only (see also under
51, or to the presence of slitshaped pores (see under 4b). Type b., sloping downwards, points to micropores. In fortunate cases, a second straight line is found at higher
relative pressures. This may be taken as
representing adsorption in slit-shaped mesopores or in macropores. The surface area present in these pores, Sw, may be obtained from the slope of this second straight line: 6 Sw = 1.547 x 10 dV /dt
3 2 (t in nm, Va in Nm /kg, S in m /kg).
The t-value belonging to the intercepts of the extrapolation of the two linear parts of the t-plots, gives a rough, theoretically incorrect, measure of half
679
B
A Va
C
FIG. 6. T-plots of the adsorption branch of nitrogen sorption isotherms; three fundamental cases: A. Macropores only or wide slit shaped pores B. Narrow slit shaped pores or micropores C "Cylindrical" mesopores
.
.
A
8
C
FIG. I. Classification of hysteresis loop shapes according to de Boer: A. Type A behaviour; "cylindrical pores" B. Type B behaviour; slit shaped pores C. Type E behaviour; indication of bottleneck constrictions.
680
the mean diameter of the micropores filling during the adsorption process. If sloping down of the t-curve is discernible below p /p = 0.3 then there is g o reason to suspect the value for SBET obtained under 2 . Type c., shows an upward curvature in the t-plot with ascending t-values. This is a strong indication of the presence of curved pore surfaces, and may, with due caution, be taken as an indication of the presence of cylindrical or spheroidal pores. The pressure corresponding to the steepest slope in the t-plot may be used to obtain an indication of the radius of the most frequently occurring mesopore size from Eqs. (17,18 and 20). 5 . Assessing the mesopore volume. I f the isotherm shows saturation adsorption in
the vincinity of p /p = 1 then complete mesopore filling may safely be assumed g o If the isotherm shows asymptotical behaviour around p /p = 1, then the upper g o closing point of the hysteresis loop may be used for assessing V but one P' should be aware of the possibility of the presence of a non-negligible macropore surface area, interfering with subsequent attempts to calculate pore size distributions. If no clearly developed hysteresis loop is found, then the macropore system may be predominant and no mesopore volume can be calculated. Attempts to obtain a mesopore distribution will lead to false conclusions, and the analysis ends here. 6. Analysis of the shape and the width of the hysteresis loop. Here the shape analysis of De Boer may be adopted [36], Fig. 7 . A-type hysteresis loop points to "cylindershaped" pores, then the t-plot should show type c. behaviour. It should be borne in mind that this is only a model description. B-type hysteresis may point to the presence of slit-shaped pores: the t-plot should show b- or a-type behaviour. E-type hysteresis: often associated with "ink-bottle" pores, more generally suspect of severe irreversibility during desorption. The t-plot can be used to obtain information on the curvature of the pore walls. 7. Adoption of a "working model" for the pore shape. For the sake of pore size distribution calculation one has to choose between either "cylindrical" or "slitshaped" pores. It is wise to substantiate the choice by independent information, e.g. from electron microscopy (EM), bearing in mind that we are dealing with a model description. In cases where we are tempted to choose the slit pore model, EM often points to sheet- or layerlike structures. "Cylindrical pore'' models are often adopted in cases where EM really points to agglomeration of approximately spherical particles (see under 8). 8 . Selection of a pore size distribution calculation procedure. If we have
chosen, under 7.,
a "cylinder pore" model, then the adsorption branch of the
hysteresis loop should be analysed on the basis of Eqs. (18) and (20), followed by an analysis of the desorption branch on the basis of Eqs. (18) and (19).
681 Ideally both distribution curves should coincide. Substantial discrepancies may point to misinterpretations in an earlier stage of the analysis and thus are a diagnostic tool in their own right. In both cases the total cumulative pore surface area S should be about cum equal to SBET, unless micropores are present, in which case S cum < SBET should
hold. If Scum >> 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
<2
run) are totally missed, whereas very large 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.
This page intentionally left blank
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.
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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
736 PART I C I PANTS
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.,
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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
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I m p e r i a l Chemical I n d u s t r i e s Ltd. Agricultural Division P.O. Box 6 Billingham C l e v e l a n d TS23 1LD
SWEDEN
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E u t e c o S.p.A. V i a M o n t e s a n t o 23 Sesto S . Giovanni Milano
ANTONELLI G.
ANTOS
KUWAIT
Dept. of Chemical Technology Kemic e n t r u m Box 7 4 0 S-22007 Lund 7
ANDERSSON A .
ANDREW
W . GERMANY
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
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ITALY
UOP, I n c . Corp. Research C e n t e r Algonquin & Mt. P r o s p e c t Rds. D e s P l a i n e s , I l l . 60016
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JAPAN
.
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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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Jagellonian University Institute of Chemistry Krupnicza 41 30-060 Cracow
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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
BARRAL R.
Produits Chimiques Ugine Kuhlmann Centre de Recherches de Lyon F-69310 Pierre-Benite
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BECKER E.R.
BEECROFT T.
BERETS D.J.
BERREBI G.
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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737 BETTAHAR M . ,
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BLANCHARD G .
BLASER H . U .
C i b a - G e i g y AG R-1060 3124002 B a s e l
BLINDHEIM U . ,
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Unilever Research O l i v i e r van N o o r t l a a n 120 V l a a r d i ng e n
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BRADSHAW D . I . ,
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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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BREMER H., Prof.
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BROEKHOFF J.C.P.,
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BRUNELLE
J
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I
Unilever Research P.O. Box 114 3130 AC Vlaardingen United Technologies Research Center East Hartford, Conn.06108
BUKOWIECKI S., Dr.
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CHABOT J., Mrs.
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Institut F r a n q a i s du P B t r o l e Av. de Bois-Preau 4, B.P. 311 F-92506 Reuil-Malmaison
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Universite d e Compiggne B.P. 233 60200 Compiegne
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Institut F r a n q a i s du P e t r o l e Av. de Bois-PrBau 4, B.P. 311 F-92506 Reuil-Malmaison
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Rhbne-Poulenc Industries 12, r u e d e s Gardinoux 93308 Aubervilliers
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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.,
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Dr.
Eindhoven University of T e c h n o l o g y Dept. Inorg. Chem. and C a t a l y s i s P.O. Box 513 Eindhoven T H E NETHERLANDS Solvay et Cie. 3 1 0 r u e de Ransbeek B-1120 Bruxelles
BELGIUM
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Groupe de Physico-Chimie Mindrale et de Catalyse Place Croix du Sud 1 B-1348 Louvain-la-Neuve
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Laboratoire de Catalyse E.N.S.C.L. B.P. 40 59650 Villeneuve d'Ascq
DELOBEL R .
DEMMERING G . I Dr.
DEROUANE E .G. I Prof.
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Kemisk Reaktionstekn. Chalmers Tekn. Hdgsk. Fack. S - 4 0 2 2 0 Gdteborg
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DIJKHUIS C.G.M.. Dr.
DISTELDORF J., Dr.
DJALALI M., Dr.
SWEDEN
Universitaire Instellingen Antwerpen Department Scheikunde Universiteitsplein 1 B-2610 Wilrijk BELGIUM DSM-Central Laboratory Department of Catalysis P.O. Box 1 8 Geleen
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DODWELL C.H.
.. .
.
W GERMANY
I RAN
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DOESBU RG E B M
Technische Hogeschool Delft Lab. voor Anorg. en Fysische Chem. Julianalaan 136 Delft THE NETHERLANDS
DU ROCH E R
BASF AG Ammonlabor 6 7 Ludwigshafen
DVORAK 3. , Dr
.
.
W GERMANY
Technical University Praha Department of Organic Technology 166.28 Praha 6 CZECHOSLOVAKIA
741 EDERER H.J., Dr.
EDMONDS T., Dr.
ENGELBACH H., Dr.
Inst. fiir Angew. Physikalische Chemie IM Neuenheimer Feld 253 D-6900 Heidelberg
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British Petroleum Co. Ltd. BP Research Centre Chertsey Road, Sunbury-on-Thames Middx. TW16 7LN BASF AG D-6700 Ludwigshafen
ENGLAND W. GERMANY
Institut far Anorganische Chemie und Analytische Chemie Johannes Gutenberg Universitat D-6500 Mainz W GERMANY
ENGLER M.
.
FATTORE V., Dr.
FENELONOV B., Dr.
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.
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,
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,
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GAT1 G.,
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Min6rale BELGIUM
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H i g h Pressur R e s . I n s t . 11-2443 Szazhalombatta Pf 3 2
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Dr.
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G R Z E C H O W I A K J.
GUBITOSA G . .
GUCZI L.
,
GUIL J.M.,
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HABERSBERGER K., D r .
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HEGEDUS L . L . ,
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BENRION
P.N.,
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HIGHTOWER J . W . ,
Prof.
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Dr.
I
IRGANG M., Dr. IRVINE E.A.,
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Imperial Chemical Industries Ltd. Petrochemicals Division HQ Wilton, P.O. Box 90 Middlesbrough Cleveland
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Institute of Chemical Physics Academy of Sciences of USSR 117334 Vorob'evskoe Shosse 2-6 Moscow
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Katholieke Universiteit Leuven Laboratorium voor Oppervlakte en Colloidalescheikunde de Croylaan 42 B-3030 Heverlee
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JSOVER B., Dr.
JUHASZ J.
KAWAGUCHI T.
Dept. of Chem. Techn. Kemicentrum Box 740 S-22007 Lund
SWEDEN
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U.S.A.
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Norton Company P . O . Box 350 Akron, Ohio 44309
U.S.A.
KIOVSKY J.R., Dr.
KLOCKENKEMPE R
KOCH
n.
KOCHLOEFL K., Dr.
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Universitat Mdnster Inst. fdr Physikal. Chemie Schlossplatz 4 44 Mdnster
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Institut fiir Technische Chemie Technische UniversitZt Berlin Strasse des 17 Juni 135 1000 Berlin 12
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Chem. & Eng. Institute of Ivanovo 1 Engels St!., 153460 Ivanovo ' I . S . S .R.
KOTTER M.
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KRAUS M.,
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Institute of Chemical Process Fundamentals Czechoslovak Academy of Sciences Suchdol CZECHOSLOVAKIA 16502 Praha 6
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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
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Chem. Eng. Mendeleev's Institute of Moscow MOSCOW A -47
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,
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L A U R E R P.
Moscow S t a t e University Moscow
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Dr.
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LECLQUX A.,
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Dr.
,
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Pro-Catalyse rue Jean G o u j o n 21 F-75008 P a r i s
FRANCE
H.P.C. Produits Chimiques 540 B.P. 87011 L i m o g e s C e d e x
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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
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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
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via
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PIVOT J.C.
POISSON R. I Dr.
PONCELET G.l Dr.
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HUNGARY
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BELGIUM
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Dr.
Dr.
W .GERMANY
ENGLAND
SNEA- (P)-France Centre de Recherches de Lacq B.P. 34 Lacq F-64170 Artix
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I
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SING K.S.W.,
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ENGLAND
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State University Department of Chemistry Lenin's K i l l s
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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